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Leahey, Matthew
REPLACEMENT OF HARD CHROME ELECTROPLATING BY
TUNGSTEN CARBIDE BASED HIGH VELOCITY OXYGEN
FUELED THERMAL SPRAY
By
Matthew H. Leahey
A Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
In Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANICAL ENGINEERING
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, CT
December 2009
CONTENTS
LIST OF TABLES............................................................................................................ iv
LIST OF FIGURES ........................................................................................................... v
ACKNOWLEDGMENT .................................................................................................. vi
ABSTRACT .................................................................................................................... vii
1. INTRODUCTION ....................................................................................................... 1
1.1
PROBLEM DEFINITION ................................................................................. 1
1.2
PLANNED METHODOLOGY ......................................................................... 2
1.3
PROCESS (ELECTROPLATING & HVOF).................................................... 3
1.4
PROPERTIES .................................................................................................... 4
1.5
SUMMARY COMPARISON............................................................................ 4
2. HARD CHROME ELECTROPLATING.................................................................... 5
2.1
APPLICATIONS ............................................................................................... 5
2.1.1 COATINGS SETTINGS & ENVIRONMENTS ................................... 5
2.1.2 AEROSPACE ........................................................................................ 5
2.1.3 AUTOMOTIVE ..................................................................................... 7
2.1.4
2.2
OIL & GAS ............................................................................................ 8
HARD CHROME ELECTROPLATING PROCESSING................................. 9
2.2.1 PROCESS DESCRIPTION ................................................................... 9
2.2.2 PLATING FACTORS.......................................................................... 11
2.3
PROPERTIES .................................................................................................. 12
2.3.1
HARDNESS......................................................................................... 13
2.3.2 OXIDATION AND CORROSION PREVENTION ........................... 15
2.3.3 THERMAL EFFECTS......................................................................... 16
2.3.4 COEFFICIENT OF FRICTION........................................................... 17
2.3.5
OIL RETENTION................................................................................ 18
2.3.6 STRESS & FATIGUE ......................................................................... 19
ii
2.3.7 OTHER DESIGN CONSIDERATIONS ............................................. 20
2.4
ENVIRONMENTAL HEALTH & SAFETY ISSUES AND RISKS.............. 21
2.4.1
HUMAN EFFECTS ............................................................................. 21
2.4.2 ENVIRONMENTAL EFFECTS ......................................................... 21
2.4.3 FEDERAL REGULATIONS............................................................... 23
3. HIGH VELOCITY OXYGEN FUEL ....................................................................... 28
3.1
APPLICATIONS ............................................................................................. 28
3.1.1 COATINGS SETTINGS & ENVIRONMENTS ................................. 28
3.1.2 AEROSPACE ...................................................................................... 29
3.1.3
OIL & GAS .......................................................................................... 30
3.1.4 AUTOMOTIVE ................................................................................... 30
3.2
TUNGSTEN-CARBIDE HIGH VELOCITY OXYGEN FUEL THERMAL
SPRAY PROCESSING ................................................................................... 30
3.2.1 PROCESS DESCRIPTION ................................................................. 30
3.2.2
EQUIPMENT DESCRIPTION............................................................ 31
3.2.3 THERMODYNAMIC CONCEPTS .................................................... 33
3.2.4 KINETIC CONCEPTS ........................................................................ 34
3.3
PROPERTIES .................................................................................................. 35
3.3.1
HARDNESS & WEAR........................................................................ 35
3.3.2
OIL RETENTION................................................................................ 37
3.3.3 OXIDATION AND CORROSION PREVENTION ........................... 38
3.3.4 STRESS AND THERMAL EFFECTS................................................ 40
3.4
ENVIRONMENTAL HEALTH & SAFETY ISSUES AND RISKS.............. 41
3.4.1 NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND
HEALTH.............................................................................................. 42
3.4.2 PROCESS RISKS ................................................................................ 43
4. CONCLUSIONS ....................................................................................................... 45
5. REFERENCES .......................................................................................................... 46
iii
LIST OF TABLES
Table 1: Typical Properties of Electroplated Chromium (Inwood)................................ 13 Table 2: Chromium Concentrations in Abiotic Materials. ............................................. 23 Table 3: Properties of Typical Fuels for HVOF Flame Spraying (BACH).................... 34 iv
LIST OF FIGURES
Figure 1: Worn turbine shaft from gas turbine engine ..................................................... 7
Figure 2: Schematics of an electrolytic cell for plating metal "M" from a solution of the
metal salt "MA". ............................................................................................................ 11
Figure 3: Hardness as a function of bath temperature and current density .................... 14
Figure 4: Hardness profiles of different Hard Chrome Electroplate Before and After
Different Thermal Cycles ................................................................................................ 17
Figure 5:
SEM micrographs EHC coatings cross-section:
(A) double plating on
polished substrate and (B) flash plating on grit-blasted surface...................................... 19
Figure 6: Aircraft aileron actuating assembly. HVOF has been commonly sprayed on
the running surface of the hydraulic piston and/or cylinder. ........................................... 29
Figure 7: Cross section of HVOF gun (Hybrid Nozzle Diamond Jet 2600), Courtesy of
Sulzer-Metco.................................................................................................................... 33
Figure 8: Evolution of the weight loss of coated samples. .............................................. 36
Figure 9: SEM samples of worn HVOF sprayed WC-12Co .......................................... 37
Figure 10: SEM micrograph of WC-17Co coating cross-section after the polarization
test in 0.1 N HCl showing an extensive crevice corrosion phenomenon. ....................... 40
v
ACKNOWLEDGMENT
The author wishes to thank Professor Ernesto Gutierrez-Miravete at Rensselaer
Polytechnic Institute for the encouragement and structure he provided on completing this
class. I am grateful to Dr. Sudangshu Bose for his insight and expertise on coatings
surface treatments. Also, thank you to my co-workers at Pratt & Whitney Aircraft
Engines for their support and guidance on completion of this project and my graduate
work. Finally, thank you to my family for their support, patience and allowing me time
away so I could write this.
vi
ABSTRACT
Tungsten carbide high velocity oxygen fuel has recently been identified as a viable
replacement for hard chrome electroplating. Hard chrome electroplating has been a
valuable surface treatment for parts in high wear and erosion environments due to its
high hardness, reactivity of chromium and low coefficient of friction. Its use has grown
considerably in the aerospace and automotive industry for several decades. In the last
two decades, environmental concerns have risen due to chrome electroplating’s
hexavalent chromium. During this time, high velocity oxygen fueled (HVOF) thermal
sprays have been developed which provide exceptional coating quality because of their
low porosity and oxides. A traditional coating available for HVOF is tungsten-carbide
(WC). WC based HVOF coatings provide hardness which surpasses that of hard chrome
electroplating with minimal and controlled environmental impact. This report surveys
and evaluates the critical factors that have made hard chrome electroplating so popular
and substantiates the use tungsten carbide HVOF as an ideal successor.
vii
1. INTRODUCTION
Hard chromium plating has been a number one choice of several industries for several
decades seeking a protective coating to protect parts from harsh factors found in certain
environments. It has been a stalwart of protecting parts in high wear environments
because of its superior hardness and corrosion inhibiting qualities due to chromium’s
natural ability to react with elements such as oxygen.
Recently the dangers of chromium have been recognized by the public and
government agencies and as a result these have begun to enact laws protecting the
general public and workers involved with handling chromium (i.e. chromium platers).
As a result of these laws and the high cost of complying with them, industry technology
has begun searching for a hard chromium electroplating replacement.
The consequence of dumping manufacturing wastes and byproducts into
uncontrolled surroundings and the subsequent health and environmental effects have
been well documented in both everyday media as well as popular culture with movies
and books. As a result, hard chrome plating materials such as hexavalent chromium are
highly regulated and in some cases banned. Europe, in particular, has lead the way
against hexavalent chromium, and other harmful materials such as lead, mercury and
cadmium, with Directive 2000/53/EC published by the European Parliament and the
Council on end-of-life vehicles in the Official Journal of European Union which must be
implemented as national law by all Member States of the European Union.
This
directive now prohibits the use of the above mentioned chemicals in all vehicles sold
after January 7, 2003 with hexavalent chrome limited to 2 grams per vehicle.
1.1 PROBLEM DEFINITION
This report will review the increased exposure of the environmental risks of chromium
and the related regulation to justify an increase in the move for a more environmentally
friendly, yet equally functioning and economically capable coating. The report will
target the tungsten carbide (W-C) based high velocity oxygen fuel thermal sprayed
coating as a viable replacement.
The metal finishing industry, and chrome electroplating especially, has become a
prime target for environmental regulators because they are the majority contributors to
1
the metal discharging into waste streams and most of their wastes are considered toxic.1
The primary sources of hexavalent chromium in the atmosphere are chromate chemicals
used as rust inhibitors in cooling towers and emitted as mists, particulate matter emitted
during manufacture and use of metal chromates.2
A study developed and presented by the Rowan Technology Group at the Thermal
Spraying and Surface Engineering Association Spring Conference3 provided a set of
guidelines for a viable replacement of electroplated hard chrome coatings. This report
plans to discuss several of these topics to justify a W-C based HVOF as the viable
replacement for the hard chrome plating: including hardness, friction, corrosion and, in
the case of chrome plating, hydrogen embrittlement.4
1.2 PLANNED METHODOLOGY
This report will describe hard chrome electroplating and tungsten-carbide high velocity
oxygen fuel thermal spray coating by comparing their applications, processes,
characteristics, properties and environmental issues and associated risks. This overview
for each coating will be done on the generic process and property attributes for both the
hard chrome electroplating and tungsten carbide high velocity oxygen fueled thermal
spray. The reason for this is because, after comprehensively researching both coatings
and how they are applied amongst various companies and applications, it is understood
that there is significant variability amongst the different processes owned by each
respective company and amongst the solutions and powder compositions employed by
these companies.
Also, the data presented here will be cited from other reports and papers done on
similar subjects. Data will be from certain variations of the coatings and the best
possible conclusions will be made from these.
1
Hubal, Elaine A. Net-waste reduction analysis applied to zero-water discharge systems for chromic acid
electroplating
2
United States; Agency for Toxic Substances & Disease Registry
3
Legg, Keith; “Alternatives to Hard Chrome Plating for the Aerospace Industry”
4
http://www.hazmat-alternatives.com/Documents/Briefings&Presentations/TS&SEA-Himley_UK-4-20-
05.pdf
2
Following this methodology for identifying a WC HVOF thermal spray will ensure
that a sound and reasonable justification can be made to substantiate the replacement of
hard chromium electroplating.
1.3 PROCESS (ELECTROPLATING & HVOF)
The two coatings could not differ more in terms of how they are applied: hard chrome
electroplating being performed via an electrolysis process; tungsten carbide HVOF being
applied via a thermally based process. However, the end results both provide a coating
capable of providing wear and corrosion protection in harsh, high temperature
environments.
Hard chrome electroplating is performed by placing the work piece (usually a part
made of stainless steel, nickel, copper or plastic) entirely into a chromium based
solution. A wire is run from the work piece (cathode), to the negative terminals of a
battery and then back into the tank to another object made of alloyed lead (anode). As
soon as power is supplied at the battery and a direct current is supplied to the anode,
oxidation occurs which forms positively charged cations. The positively charged cations
combine with negatively charge anions and deposit on the work piece. Detailed steps of
the process will be discussed further on in the report as well as process requirements
including cleaning and preparation and other unique process parameters.
High velocity oxygen fuel thermal spray is a completely different process that
employs different physiochemical processes such as combustion rather than electrolysis.
The process uses internal combustion to melt and propel a powdered material onto the
work piece. A gaseous or liquid fuel, such as kerosene or propylene, and oxygen flow
into a chamber with an ignition source that creates a continuous flame. The powder - in
the case of this repair, a tungsten-carbide based powder, is injected into the flame. The
flame melts and propels the powder particles onto the work piece where it plastically
deforms, cools and solidifies.5
5
Bose, Sudhangshu; High Temperature Coatings
3
1.4 PROPERTIES
This report will discuss the properties of both coatings with respect to the environments
they are used in. A similar study developed and presented by the Rowan Technology
Group provided a set of guidelines for a viable replacement of electroplated hard chrome
coatings. This report plans to discuss several of these topics to justify a W-C based
HVOF as the viable replacement for the hard chrome plating: including hardness,
friction, corrosion and, in the case of chrome plating, hydrogen embrittlement.6 As
previously mentioned, different specifications of each coating usually have different
property values that each coating source must meet. The coating source also might have
their own techniques to help them meet the quality standards imposed on them by their
customer and the specification. The property section that will be discussed in detail for
each coating later will discuss their general property advantages and disadvantages.
1.5 SUMMARY COMPARISON
This introduction has presented two coatings that meet the industry demands needing an
anti-wear, corrosion inhibiting, dimensional restoring and/or oil retaining coating. The
plating and HVOF processes also provide a robust, repeatable and cost feasible method
for applying them.
6
Legg, Keith; Alternatives to Hard Chrome Plating for the Aerospace Industry
4
2. HARD CHROME ELECTROPLATING
2.1 APPLICATIONS
2.1.1
COATINGS SETTINGS & ENVIRONMENTS
Hard chrome plating has found a home in several industries for its unique and high
performing capabilities. From large scale industries such as the petroleum and aerospace
industries to smaller scale industries such as the biomedical industry, hard chrome has
found its niche.
A significant advantage of hard chrome electroplating is that its
properties cannot be easily obtained in a cast or wrought material and can easily be
applied to inexpensive steel or cast iron hardware at rubbing interfaces. The cost benefit
is that it may not be necessary to utilize a material that already has anti-wear features to
it but may not be cost effective to employ.7
2.1.2
AEROSPACE
The aerospace industry is primarily responsible for applying hard chrome plating on a
massive scale. Hard chrome plating owes a lot to the aerospace and defense industries
maturation from the industrial boon to World War II arms proliferation. Because of this,
hard chrome plating success amongst the aerospace and defense and later on other
industries such as automotive.
Both the original equipment manufacturer (OEM) and aftermarket aerospace
industries have utilized hard chrome plating for its anti-wear capabilities and corrosion
prevention. Jet engines have utilized the process repeatedly for decades amongst several
core components, particular rotating hardware such as bearings, shafts, rotors and
smaller hardware like fasteners. Figure 1 shows a shaft of a gas turbine engine heavily
worn due to contact stresses and fretting. The large amount of parts located within the
oil wetted bearing areas within jet engines require coatings to provide exceptional
protection for parts and must do so for long periods of time and under exceptional stress.
Aerospace bearings in general are made of high hardness materials, such as steel, to
provide a durable and robust design. Nevertheless, parts still nevertheless wear and for
years, chrome plate has been utilized as a solution to prevent and fix wear both in the
7
Inwood, B.C.; “Electroplated Coatings for Wear Resistance”
5
OEM and aftermarket industries. The bearings are also manufactured to very high
tolerances and the fits among the bearing parts are created to prevent any imbalances by
the rotors attached to the bearings. These tight interference fits also escalate an already
highly strained wear couple.
In addition to the highly strained wear couples by these bearings, the addition of
high temperature oil requires hard chrome plating to prevent corrosion, degradation and
ultimately, failure. The existence of this oil prevents other traditional hard coatings from
being applied in these areas. Coatings such as air plasma thermal spray are limited due
to their high porosity and natural occurrence of oxides which allow the permeation of
coating into the microstructure leading to degradation and excessive wear.
Airframes have also utilized hard chrome plating within various hardware outside of
the propulsion system. It is utilized by landing gear seal systems and shock absorbers
because of its exceptionally long lasting anti wear abilities. Aircraft landing gears use
powerful hydraulic systems to both extend and retract the gear as well as providing
shock absorption to the aircraft upon landing. The situation in which the hard chrome
plating comes in useful is on the seal surface of the stroke mechanisms of the hydraulic
systems. Considering the amount of stress a landing gear system is put under whether
it’s an 18-ton fighter jet landing on an aircraft carrier at high speeds and stopping within
hundreds of feet or a 175-ton passenger jet landing on a routine landing cycle, the
landing gear hydraulic system is highly stressed. The seal at the interface between the
landing gear piston and the hydraulic cylinder as well as the shock absorbing unit require
this anti wear property as well as a coating that can provide a high lubricity to minimize
the wear couple.8
8
Monaghan, K. J.; “Comparison of Seal Friction on Chrome and HVOF Coated Rods Under Conditions of
Short Stroke Reciprocating Motion”
6
Figure 1: Worn turbine shaft from gas turbine engine9
2.1.3
AUTOMOTIVE
The automotive industry has used chrome plating for many decades due to its superb
wear and corrosion resistance. Also decorative chrome plating has been used for more
aesthetic like bumpers, trim and even external engine parts. Decorative chrome plating
is the same as hard chrome plating with the exception of the thickness at which it is
applied:
decorative is applied at fractions of the thickness of hard chrome and
sometimes requires a nickel plated underlay coating which supports bonding and a
unique decorative shine.
The functional automotive applications that hard chrome electroplating is used in
are centered on the moving parts of an engine drive train, transmission, steering and
differential components. In these scenarios, there is always a part moving near or
attached to a stationary part whether it is a rotating part such as a bearing or two mating
static parts that are subjected to vibratory modes and relative motional wear.
Some of these settings also have harsh aspect to their environments aside from the
wear. The engine can be subjected to chemicals such as the motor oil and fuel that may
9
Sahraoui, Tahar, et al; “Structure and Wear Behaviour of HVOF Sprayed Cr3C2-NiCr and WC-Co
Coatings
7
contain some ingredients that may be harmful over time to some materials within the
engine components. Also several of the parts mentioned above can be exposed to
external elements that in some geographic areas of the United States can be quite
harmful as well. Body pieces of the car are protected with other rust and corrosion
inhibitors but the parts on the inside of the car may be susceptible to corrosion damage
caused by water and road salt.
It should also be mentioned that the lifespan of several of the parts in the
categories discussed above will equal, if not surpass, the life of the car or truck.
Therefore a durable coating is required and hard chrome plating has met these
performance requirements.
Chrome plating facilities rely on the automotive industry for a large portion of their
revenue. This is particularly true for locations where the automotive manufacturing
industry is prominent. In some cases, nearly a third of operating revenues is from the
automotive industry.10
2.1.4
OIL & GAS
Oil and gas can be separated into two categories when it comes to hard chrome plating:
extraction (or drilling) and transporting. These two settings and the reasons for needing
anti-wear and corrosion resistance are very similar to the automotive reasons.
Powerful motors and drive trains with high strength bearings, usually consisting of
hard steels, are required to transmit heavy torques over long distances. Therefore the
risk wear couples and interfaces formed by these configurations can be reduced by
adding the hard chrome plating to the hardware.
In the case of offshore drilling, hardware used for drilling, extraction, transportation
and storage are constantly under the attack of extreme temperatures and corrosive salt
water. Hard chrome plating on vital wear interfaces serves to prevent damage and
possible part failure by averting harmful corrosion.
10
www.modernhc.com/capabilities.htm
8
2.2 HARD CHROME ELECTROPLATING PROCESSING
2.2.1
PROCESS DESCRIPTION
Before the electroplating process begins, a rigorous preparation process is involved to
ensure adequate cleaning of the part before plating. Any surface contaminants will
significantly affect the bond with the coating. Bond quality affects all aspects of the
coating: adhesion, appearance, corrosion resistance, and composition. If there is poor
preparation, there’s risk of bad adhesion, porosity and non-uniform coatings which may
all ultimately lead to failure.
Critical preparation must be done by both chemical and mechanical means and is
vital in creating an adequate coating. The first portion of the process is an abrasive
surface treatment to remove any build up of material which is very common during a replating when a part may come in with large amounts of oxidation (i.e. rust) or corrosion
from use. Depending on the material of the substrate, a silicon oxide or aluminum oxide
is sprayed on the material at high velocity to remove this outer surface layer of
contaminants. This not only removes a surface layer but will also roughen up the
surface to help with bonding.
The next portion of the preparation process involves more chemical means. This
helps in dissolving contaminants that reach the surface externally from the environment
such as oils, dust and grease that usually come from prior processing or handling. Other
cleaning steps will remove intrinsic layers, such as protective oxides that form naturally
as a result of contact with air.11 The methods for cleaning these normally are either
halogenated or non-halogenated solvents by way of a vapor degreasing or cold cleaning.
A popular and effective solvent is trichloroethylene (TCE) but it is controlled by OSHA
as it is a carcinogen and irritant of mucous membranes.
More aqueous cleaning is used after the solvent step. These employ more harsh
chemicals (such as hydrofluoric, boric or sulfuric acid) that actually etch the part and
remove the intrinsic oxide layer. As the oxide layer is removed, the part is activated for
the electrodeposition process because the oxide acts as a barrier for proper deposition.
These acids are very dangerous due to their corrosive nature and can lead to serious
11
Dennis, J.K.; Nickel and Chromium Electroplating
9
injury and/or death. Their exposure quantities are limited by the government and proper
apparatus are required to limit exposure by involved in the process.12
The plating process involves bonding a metal onto a surface through electrolytic
means. The part is placed in a bath of a solution called an electrolyte, in the case of
chrome plating, the electrolyte is primarily chromic acid (CrO3) along with other
additives (SO4 and F) which help as catalysts and can affect coating post-plate.13 The
part to be plated is connected to the negative end of a power source (battery or rectifier)
and acts as a cathode. An anode is attached to the positive side and is placed in the tank.
As a current is run through the circuit, cations from the plating solution become
positively charged and travel to the negatively charged cathode and bond. It provides
electrons to reduce the positively charge ions to metallic form.14 Figure 2 shows a
general schematic of the electroplating process. In the case of the chrome plating, the
anode is lead based and the solution is consumed as opposed to other electroplating
where the anode is consumed such as nickel plating. This increases the need to move
harmful chromic acid around to replenish the plating solution.
12
Rondi, Carrie-Anne; Silver Electroplating: EH&S Perspectives
13
Dennis, J.K.; Nickel and Chromium Electroplating
14
http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm
10
Figure 2: Schematics of an electrolytic cell for plating metal "M" from a solution of the metal salt
"MA".
15
2.2.2
PLATING FACTORS
Several factors contribute to the plating final product. The concentration and ratio of the
solution are identified as critical factors in the plating process.
Because of the
importance of SO4 and F as catalysts in some solutions, their presence with respect to the
chromic acid end ups resulting in coating qualities, such as brightness, dullness,
15
http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm
11
deposition rates and hardness, being affected. Through testing, hard chrome plating’s
ideal ratio between the chromic acid and the sulfuric acid is approximately 100:1 in
order to obtain the hardest deposits.16
Bath temperature and current density are also critical factors that play an important
role in coating resultant. High solution temperatures are necessary to prevent burning
and formation of rough deposits likely to form with higher current densities. A ratio
system has been set up for bath temperature and current that will optimize the hardness
of the coating.17
Temperature plays an important role by itself as it contributes to difficult coating
outcomes and plays a health and safety role to the worker. Too high a temperature
reduces the viscosity of the solution, quickly draining work piece and reducing the bath
drag-out. This may, in turn, result in breakdown of solution additives, carbonate build
up on work piece and drying of the solution on work piece. High temperatures lead to
worker risks as well because of evaporation of harmful chemicals such as hexavalent
chromium and risk of skin contact leading to burns and acid attack.18 Increasing the
temperature does contribute to lower solution concentrations, which minimizes solution
cost. Simply put, higher temperatures require lower solution concentrations and vice
versa. These positive and negative relationships are important to be considered when
selecting the proper coating quality.19
2.3 PROPERTIES
The properties of hard chrome plating vary due to several factors including plating
processing parameters, testing environment and plating solution components. Table 1
lists a range of hard chrome plating properties and the tolerancing in the results are likely
caused by these factors.
16
http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm
17
http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm
18
Rondi, Carrie-Anne; Silver Electroplating: EH&S Perspectives
19
Dennis, J.K.; Nickel and Chromium Electroplating
12
Table 1: Typical Properties of Electroplated Chromium20
Tensile strength, MN/m2
100-350
Young’s modulus of elasticity, MN/m2
100-200
Hardness as plated, VPN
450-1100
Hardness on heating, VPN:
400°C
450-800
600°C
420-490
800°C
250
Crack density, per cm:
Normal
10-100
High
>250
Stress, as plated, MN/m2:
Normal
-120 to +415
Thin, crack free
Up to 1400
Density, g/cm3
6.9-7.2
Thermal conductivity at 18°C, W/m °C
51
Melting point+, °C
1890 ±10
Coefficient of linear expansion
8.1 x 10-6
Electrical conductivity, µΩcm
15-50
2.3.1
HARDNESS
Hard chrome electroplating’s general resistance to wear can be contributed to its extreme
hardness. The reason for its high hardness is due to chromium’s high density and the
method in which it is process as per Figure 3. Aside from chromium’s natural high
density which affects hardness, process parameters can also affect hardness.
20
Inwood, B.C.; Electroplated Coatings for Wear Resistance
13
Figure 3: Hardness as a function of bath temperature and current density21
The chrome electroplating process yields a coating with minimal porosity which
reduces the chances of contaminants or voids, which can risk a reduction in density,
adhesion and ultimately, hardness. Temperature of the plating solution is the prime
factor in affecting the coating hardness: generally, an increase of temperature will result
in an increase of hardness. However, exceeding approximately 70°C will produce a
reduction in density.
The general range of hardness for hard electroplated chrome ranges from 450 –
1100 VPN, depending on process factors and external operational temperature after
plating.
21
Dennis, J.K.; Nickel and Chromium Electroplating
14
2.3.2
OXIDATION AND CORROSION PREVENTION
Chromium, regardless of form or condition, has a natural ability to prevent oxidation and
corrosion. This is due to how it attracts oxygen and forms a protective oxide layer on its
external boundary. The oxygen layer that forms is Cr2O3. This process of passivation
allows the chromium - oxygen layer to form on the outside of the chromium coating and
prevent any other harmful agents, such as acids or more oxygen, from attacking the
chromium metal or any underlying metal.
Chromium’s natural ability to form this Cr2O3 layer also has helped it become a
valuable alloying element amongst metal alloys and other coatings. Most common
example is stainless steel because of its high chromium content.
Oxide layer formation is common among many metals, alloys and coatings.
Frequently this oxide layer is detrimental (i.e. steel and rust) but often it provides
protection (i.e. aluminum – anodizing). An index ratio has been created to define the
level of protectiveness or non-protectiveness.
The Pilling-Bedworth Ratio (PBR)
roughly defined as,
.
A PBR of much less than or much greater than 1 denote a detrimental oxide formation
With PBR << 1, the oxide coating does not consume enough of the parent material
compared to the volume of oxide and is too porous to be protective. In the case of a
PBR >> 1, the oxide consumes too much of the base metal resulting in coating
frequently falling off and exposing more base metal. Many other factors contribute to a
materials oxide protectiveness that PBR does not factor in but the PBR for Cr2O3
formation on a chrome coating has been calculated at 1.99 confirming its
protectiveness.22
As discussed in the section on application, hard chrome plating can be employed in
some rather corrosive environments. The presence of harmful salts, sulfates, chlorides
and other corrosive compounds can be blocked out by the oxide protection layer. The
reaction of the Cr2O3 with the environmental components such as oxygen makes it a
22
Bose, Sudangshu; High Temperature Coatings
15
valuable alloying element because of chromium’s passivating ability. However, the
common applications of hard chrome plating make it uncommon for it to be subjected to
these types of corrosion because of the high temperature which it must occur.
2.3.3
THERMAL EFFECTS
Hard chrome plating does handle thermal changes well due to its high thermal
conductivity value.23 A localized increase in friction may result in the increase of
material or coating temperature. The chrome plating’s ability to effectively dissipate this
heat through the material and surroundings will reduce the risk of its failure by a
decrease in its hardness and excessive wear.
Hard chrome plating, like most other materials, can be negatively affected by
thermal cycling. Studies performed on various hard chromium coatings have been
performed with uncoated steel used as a control to test and understand the effects of
thermal fatigue on the coating with respect to its hardness. The results in the change of
hardness are shown below in Figure 4. Reasons for the change in hardness due to
thermal fatigue were speculated to be due to a loss of valuable hydrogen from the
coating and thereby causing a phase change and reducing the hardness. Also, thermal
cycling allowed a diffusion of oxygen further into the material causing a porous, more
brittle material less able to satisfactorily withstand damage such as fretting, erosion and
wear.24
23
Inwood, B.C.; Electroplated Coatings for Wear Resistance
24
Hadavi, S.M.M.; The Efffect of Thermal Fatigue on the Hardness of Hard Chromium Electroplatings
16
Hardness, GNm -2
20
16
No Coat
HC
12
Low Crack
LC
8
4
0
50
100
200
# of Thermal Cycles
Figure 4: Hardness profiles of different Hard Chrome Electroplate Before and After Different
Thermal Cycles25
Other effects thermal cycling can have on the hard chrome plating are on the
protective oxides that are formed on the external layer. Oxidation layers generally have
lower Coefficients of Thermal Expansions (CTE) and become overly stressed during
significant increases and decreases in temperature as the base metal expands and
contracts. The resultant effect is the spallation of the oxide layer, exposing more coating
to the environment.26
2.3.4
COEFFICIENT OF FRICTION
The coefficient of friction is a dimensionless scalar value which describes the ratio of the
force of friction between two bodies and the force pressing them together. The wear
coupling that is formed has a coefficient friction that can also be greatly altered when
factoring in materials, lubrication, temperature and other wear system parameters.27
25
Hadavi, S.M.M.; The Efffect of Thermal Fatigue on the Hardness of Hard Chromium Electroplatings
26
Bose, Sudangshu; High Temperature Coatings
27
http://depts.washington.edu/nanolab/ChemE554/Summaries%20ChemE%20554/Introduction%20Tribolog
y.htm
17
Tests have been performed to understand the coefficient of friction and its
tribological effect on a wear system. Hard chrome plating has a natural low coefficient
of friction which helps in minimizing the wear due in a sliding or fretting environment.
This is especially true when the plating is applied to a steel substrate since the steel like
likely to have coefficient of friction 20%-40% greater than the chrome plating.28
2.3.5
OIL RETENTION
Hard chrome electroplating and other electroplating has been used in oil wetted
environments because of its superior ability to withstand the effects the oil may cause.
Whether a plating procedure requires a post plate machining operation to obtain a
specific surface finish of thickness or not, the plating sometimes does not have porous or
a rough surface in which oil can easily penetrate. This characteristic allows the plating
to be used in an oil wetted environment. High wear areas where an anti-wear surface
treatment is required, such as bearing journals, piston sealing surfaces or other moving
parts of an internal combustion engine (i.e. cam surfaces), hard chrome has been utilized
for a long time to withstand both wear and oil attack.
Figure 5 below shows a smooth surface of hard chrome plating which limits oil
penetrating ability into the plate to help lubricate the surface. Comparing the surface of
the substrate to the surface of the plating, it shows the plating tends to mimic the surface
of the substrate.
28
http://www.electro-coatings.com/wear-resistant/hard-chrome.php
18
Figure 5: SEM micrographs EHC coatings cross-section: (A) double plating on polished substrate
and (B) flash plating on grit-blasted surface.29
The addition of microcracks into the surface will add both benefits and weaknesses
with regards to oil retention and other characteristics of the plating. Microcracks can be
formed during the deposition process. With the microcracks, the coating will be much
harder increasing the wear resistance but corrosion resistance and fatigue life will be
reduced.
The microcracks will allow oil to penetrate the surface increasing the
lubrication to extend life of coating and substrate. However, without these microcracks,
coating life will be limited because it will be a softer coating with increased wear rates
and less oil can be retained to aid in lubrication and ultimately extending life. This
presents a difficult scenario but analysis has shown that a combination of both
microcracks and smooth surfaces provide a good compromise of fatigue, oil retention,
corrosion protection and wear life.30
2.3.6
STRESS & FATIGUE
Electroplated hard chrome has been used in high wear and erosion situations where
temperature changes are evident because of its coefficient of thermal expansion and high
oxidation resistance. When subjected to a cyclic temperature setting, the expansion and
contractions of the material lead to crack initiation and growth. The hard chrome plating
has a coefficient of thermal expansion that will minimize crack initiation and growth
29
Bolleli, Giovanni; Corrosion Resistance of HVOF-sprayed Coatings for Hard Chrome Replacement
30
http://www.nhml.com/resources/1999/7/1/hard-chromium-plating
19
The oxide layer that forms becomes brittle over time and highly stressed. This
results in its possibility to flake off, taking with it some of the chromium plating coating
required to protect the base material.
2.3.7
OTHER DESIGN CONSIDERATIONS
Hydrogen embrittlement is a concern with hard chrome electroplating and is heavily
process dependent. The secondary processes within the chrome electroplating process,
and in some case the plating procedure itself, involves a relatively large amount of
atomic or molecular hydrogen. The high temperature involved with all of the procedures
increase hydrogen’s solubility within the base metal which allows the hydrogen to
ingrain itself within voids of the part, possibly leading to hydrogen embrittlement or
hydrogen induced cracking.31
In order to relieve the part of the hydrogen embrittlement, parts are subjected to
high temperatures to allow the hydrogen to diffuse out.32
Another concern that must be considered during design of a part with hard chrome
plating on the surface is the possibility of crack growth in the coating at the bond layer
between the coating and substrate. During both the plating process and operation of the
part, a considerable amount of residual stress is put into the work piece because of its
inability to expand at the same rate as the thin coating layer. Testing has shown that
specific modifications to the coating process noticeably reduce the possibility of thermal
fatigue in the coating-substrate system.33
31
http://www.nhml.com/resources/1999/7/1/hard-chromium-plating
32
Jewett, R.P.; Hydrogen Embrittlement
33
Hadavi, S.M.M; Comparison of Seal Friction on Chrome and HVOF Coated Rods Under Conditions of
Short Stroke Reciprocating Motion
20
2.4 ENVIRONMENTAL HEALTH & SAFETY ISSUES AND RISKS
2.4.1
HUMAN EFFECTS
Chromium exists in abundance in nature and various forms and conditions.
It is
considered an essential trace element in the human body and chromium deficiency is a
condition for lack of the element in the diet.
The United States Food and Drug
Administration has even set guidelines for the daily intake of the element as it
contributes to an inability to process glucose, weight loss, confusion and in extreme
case, nerve damage.34
The most abundant form of chromium, trivalent chromium, exists in all forms of
nature and has been identified as a dietary requirements. Tests have shown that it may
contribute to the processing of sugars and insulin as well as the overall metabolic
process of the human body.35
The rare and man-made version of chromium is hexavalent chromium. It is a
byproduct of the hard chrome electroplating process and has grave affects on those in
proximity to it. Aside from chrome electroplating, it is a staple of several manufacturing
industries including textiles, tanning and stainless steel industries. Effects of contact or
inhalation exposure to the compound can result in anything from skin irritation,
headaches and nosebleeds to developing cancer in the lungs, kidneys and other vital
organs.36
2.4.2 ENVIRONMENTAL EFFECTS
The negative effects of chromium on the environment are usually the result of
anthropogenic actions such as manufacturing. Chromium is typically given off in the air
with water vapor in the chromate (Cr04) and dichromate (Cr2O7) anion states due to
industrial output. This vapor can be transported via wind or after falling amongst the
soil or water which can be transported through solid means. Once in the soil or water,
transport can be complex and surpasses the scope of this report. However, hexavalent
34
Freund, H.; Chromium Deficiency During Total Prenateral Nutrition
35
United Kingdom; Review of Chromium
36
Salnikow, K.; Genetic and Epigenetic Mechanisms in Metal Carcinogenesis and Cocarcinogenesis.
21
chromium travels easily through soil and can leach out into water supplies affecting
those living within that watershed area.37
The risks of hexavalent chromium are not limited to humans and can affect other
mammals, as well as marine life (plant and animal), avian and other animals living close
to, or in, the soil. However, how the hexavalent chromium affects the plant or animal is
highly dependent on the surroundings. Possible factors that may affect the toxicity
include, but are not limited to, the species of animal/plant, composition of water or soil
(including pH, temperature, oxygen levels and the presence of other chemicals or
organic matter).38
Table 2 below is an abridged table from the Handbook of Chemical Risk
Assessment: Health Hazards to Humans, Plants and Animals which presents study
results of the concentration of chromium found in the earth, water and air in and around
the specified areas.39
37
IETEG 2005; Chromium (VI) Handbook
38
IETEG, Eisler, R.; Chromium Hazards to Fish, Wildlife and Invertebrates: A Synoptic Review
39
Eisler, R.; Handbook of Chemical Risk Assessment: Health Hazards to Humans, Plants and Animals
22
Table 2: Chromium Concentrations in Abiotic Materials.40
Sample
Concentration
TERRESTIAL (mg/kg)
Earth’s Crust
100-300
Soils
Trace to 300
Serpentine Materials
1800
Sewage sludge (USA)
428
WATER (µg/L)
Drinking Water (USA)
1.8 (0.4-8.0)
Seawater
0.3 (0-0.5)
Streams
0-112, mean 9.7
Untreated industrial effluents
5,000,000
Irrigation Water
160,000-780,000
AIR (µg/L)
2.4.3
Urban
0.01-0.03
Chromate Plants
1000
Rural Areas
<0.01-0.01
FEDERAL REGULATIONS
2.4.3.1 ENVIRONMENTAL PROTECTION AGENCY
The Environmental Protection Agency (EPA) is a government agency reporting directly
to the Executive Branch of the United States government and is charged with
establishing and governing the environmental policies of the United States. The hard
chrome plating industry was the first categorically regulated industry in the United
States and all portions of the process are closely regulated by government agencies
including the EPA.41
The EPA, as part of their Toxicological Review of Hexavalent Chromium, August
1998 (CAS 18540-29-9) provides substantiation for their data listed in the Integrated
40
Eisler, R.; Handbook of Chemical Risk Assessment: Health Hazards to Humans, Plants and Animals
41
www.finishing.com/faqs/chrome.html
23
Risk Information System which presents information on possible harmful materials with
respect to their oral and inhaled risks and health effects as a carcinogen.
2.4.3.2 OCCUPATIONAL SAFETY AND HEALTH ADMINISTRATION
The Occupational Safety and Health Administration (OSHA) is a division of the United
States Department of Labor and was created to protect injuries, illnesses and fatalities
due to work place events and does so by enforcing certain laws and regulations. OSHA
has labeled hexavalent chrome as a harmful chemical and strictly regulated the
compound. OSHA regulation OSHA 1910.134 has reduced the permissible exposure
limit (PEL) for Hexavalent Chromium to greater than one fifth the previous PEL at 5
micrograms per cubic meter (µg/m3) on an 8 hour time-weighted average. Secondary
provisions also exist as part of this regulation that protect workers and those close to the
chromium process such as exposure determination and preferred exposure control
methods.
Certain situations exist where meeting the PEL is not feasible and the
regulation accounts for that with requirements for respiratory protection, protective
clothing and equipment, hygiene areas and practices, medical surveillance,
recordkeeping of up to four years of implantation of these engineering controls.
(71:10099-10385).42
Other OSHA sub-standard regulations exist for specific industries that provide
regulation similar the general requirements listed above, but also go more in-depth on
aspects unique to the specific industries which the regulation affects.
2.4.3.3 CLEAN AIR ACT
With hexavalent chromium being primarily an inhalation risk, the air quality of sites
working with the chemical have become increasingly regulated and governed. The
United States has submitted a series of Clean Air Acts which regulate hazardous and
toxic emissions. The Act lists over 170 items as identified hazardous air pollutants
including chromium compounds as one. Unless otherwise specified, these listings are
42
http://www.osha.gov/pls/oshaweb/searchresults.category?p_text=chromium&p_title=&p_status=CURRE
NT
24
defined as including any unique chemical substance that contains the named chemical as
part of the chemical’s infrastructure.
The Clean Air Act released by the United States declares that any metal finishing
operation with processes that could emit hazardous air pollutants or volatile organic
compounds are required to obtain an operation permit and comply with other regulatory
requirements. Since the EPA requires 189 air toxins to be regulated, and has authority to
require pollution prevention measures, it has become costly to regulated air toxics.43
Therefore, waste reduction incentives have increased.
2.4.3.4 CLEAN WATER ACT
The Clean Water Act regulates the amount of chemicals and toxins released directly
(into a body of water) or indirectly (publicly owned facilities) in the water. Standards
set concentration limits for the discharge of specific chemicals. Title 40 of the Code of
Federal Regulation sets the maximum drinking waters standard for chromium at 0.1
parts per million (ppm) to protect against the adverse health effects discussed earlier (40
CFR 141.32).44
The risks of hexavalent chromium have been publicized in popular media recently
in such cases in Hinkley, California in areas surrounding a Pacific Light and Gas plant
and in Midland, Texas where the amount of hexavalent chromium present in the
groundwater exceeded the federally regulated and enforced limit of 0.1 ppm. The
Hinkley measurement was identified at 0.58 ppm and the Midland case measured at
5.250 ppm.45 46
2.4.3.5 RESOURCE CONSERVATION RECOVERY ACT
The Resource Conservation and Recovery Act (RCRA) is a principal federal law created
by the United States which governs the disposal and handling of solid and hazardous
wastes. It was initially created in 1976 to amend the Solid Waste Disposal Act of 1965
43
www.wmrc.uiuc.edu/manuals/finishing.htm
44
http://www.epa.gov/lawsregs/laws/index.html#env
45
http://en.wikipedia.org/wiki/Hexavalent_chromium
46
http://www.youtube.com/watch?v=mJ3KhaIATlA
25
to set objectives for protecting human and environmental health due to waste disposal
risks, conserving limited resources, reducing amount of waste and ensuring sound waste
management to combat the rise of municipal and industrial wastes.
This legislation, most recently amended through 2002, provides strict governance of
disposal of hazardous waste, including chromium VI in amounts exceeding 500 mg/l.
This document governs, among other restrictions, the use of the following practices:
•
Recordkeeping practices that accurately identify the quantities of such
hazardous waste generated, the constituent which are significant in quantity
or in potential harm to human/environmental health and disposition of
wastes
•
Labeling practices of containers used for storage, transport or disposal of
hazardous waste
•
Proper use of specific hazardous waste container.
•
Provision of information on specific hazardous waste for those transporting,
treating, storing or disposing.
•
Employment of proper manifest system to control waste treatment, storage
or disposal
•
Effort of waste minimization by hazardous waste generator to reduce general
volume of hazardous waste and proposed method of treatment, storage or
disposal to reduce present and future threat to human health and
environment.
•
Report submittal to proper government administrator disclosing compliance
of above information and any changes in volume and toxicity of waste
compared to previous years.47
2.4.3.6 EMERGENCY PLANNING & COMMUNITY RIGHT-TO-KNOW ACT
The Emergency Planning and Community Right-To-Know Act was created in 1986 by
the United States that aimed to mandate awareness amongst the general public in the
case of chemical releases and to provide the public and local governments with
47
United States, Solid Waste Disposal Act 2002.
26
information concerning potential chemical hazards present in their communities. This
act along with OSHA enforces the, now common use of Material Safety Data Sheets
(MSDS). Its three point directives were:
1. Self-identification of facilities that store or use hazardous or toxic chemicals
2. Reporting and public dissemination of information on these chemicals.
3. Preparing plans for mitigating accidental releases of these chemicals.48
2.4.3.7 FEDERAL REGULATION SUMMARY
The regulations created and enforced by the federal government reinforce the need to
determine an environmentally friendly surface treatment for hard chromium plating.
Real life instances were caused by hexavalent chromium in uncontrolled ecological
settings and created limits and restrictions to protect people and the rest of the
environment.
48
http://www.campuserc.org/resources/EHSguide/EPCRA/Pages/default.aspx
27
3. HIGH VELOCITY OXYGEN FUEL
3.1 APPLICATIONS
3.1.1
COATINGS SETTINGS & ENVIRONMENTS
High velocity oxygen fuel has just recently become on par with thermal spraying
technologies after nearly a quarter of a century under development. However, coating
quality and cost effectiveness improvements still remain. The coating evolved from the
detonation gun (D-Gun) that was common place for applying hard, wear-resistant
coatings. HVOF is recognized as valuable in applications commonly requiring wear
resistance for abrasive conditions, partially superimposed with corrosion and have
become increasingly successful for protection against liquid erosion and cavitation.49
Common applications include:
•
Ball and plate valves
•
Components for plastic extruders
•
Rotors and pistons for compressors and pumps
•
Treads of large scale engines
•
Wire-drawing rolls
•
Torch nozzles
•
Circumferential and running surfaces of hydraulic cylinders
•
Paper and foil rollers
•
Sieves in salt industry
•
Sliding surfaces for railway switches
•
Surfaces of water turbines and large scale ventilators
•
Different spindles50
With superalloys commonly used amongst high temperature applications, HVOF
coatings have been used for temperature settings considered for both wear and erosion
situations (WC-based) as well as corrosion and oxidation resistance. HVOF technology
was used in this development primarily due to its low oxygen content and negligible
49
Bach, Friedrich-Wilhelm; Modern Surface Technology
50
Bach, Friedrich-Wilhelm; Modern Surface Technology
28
coating porosity. Both criteria are prerequisites for subsequent heat treatment of the
coating, which then shows mechanical properties comparable to the corresponding base
metal.51
3.1.2
AEROSPACE
Aerospace applications of HVOF coatings have mostly been seen as wear-protection
coatings on running surfaces of hydraulic cylinders for landing–gear components or
wing control surface actuation and adjusting systems. Figure 6 shows a hydraulic
actuation assembly. The sliding arm on the right connects to a piston which commonly
wears and has recently begun using HVOF for anti-wear OEM and aftermarket purposes.
WC-Co and WC-Co-Cr HVOF based hard alloys dominate these settings for their
exceptional wear resistance. The most important selection criteria are favorable intrinsic
stresses after coating and excellent fatigue resistance.
Due to the extremely long life cycle of aerospace components, the aftermarket
repair industry is a large business which looks to repair parts rather than replace. HVOF
have helped this business model and some of these coatings have been able to go more
than one overhaul cycle with replacement (HCAT GTE Test). For those coatings that
cannot exceed more than one overhaul cycle, they allow removal and re-coating with a
relatively very small turnaround time compared to other coating systems.
Figure 6: Aircraft aileron actuating assembly. HVOF has been commonly sprayed on the running
surface of the hydraulic piston and/or cylinder.52
51
Bach, Friedrich-Wilhelm; Modern Surface Technology
52
Legg, Keith; Alternatives to Hard Chrome Plating for Aeropsace Industry
29
3.1.3
OIL & GAS
Apparatus engineering for the petrochemical and gas industry is a further large field of
applications of thermal spray coatings. Experience has been gained with HVOF coatings
in boilers, heat exchangers, pipes, valves, sliders and similar components in combustion
engineering. In certain cases, WC containing coatings are used for lower operating
temperatures.
The main wear conditions involve gas erosion superimposed with
corrosion and oxidation.53
3.1.4
AUTOMOTIVE
High velocity oxygen fuel applications within the automotive industry have been
limited, with the use of hard chrome electroplating still very common since it has been
used for decades. Considering the automotive industry’s natural tendency to stay with
mature, repeatable processes which have economically benefitted greatly due to years of
process development, it has been challenging for HVOF to invade the market. HVOF, at
the moment, restricts itself to smaller batch processing because of the specialized setup
of spray booth and guns. However, with cost reduction one of the few remaining
improvements that is left with HVOF, it may not be long before an HVOF coating has
usurped a hard chrome plating in that industry.
Until now, brake disk coatings have begun to use HVOF in specialized applications
and experience has been gained from a number of long-term tests, but real series
applications have not yet been disclosed.54
3.2 TUNGSTEN-CARBIDE HIGH VELOCITY OXYGEN FUEL
THERMAL SPRAY PROCESSING
3.2.1
PROCESS DESCRIPTION
The general processes of thermal spray coatings are all very similar. They rely on a very
high temperature to turn a powder, wire or other solid state material into molten droplets
and project them onto a surface at extremely high velocity. Thermal spray coatings are
53
Bach, Friedrich-Wilhelm; Modern Surface Technology
54
Bach, Friedrich-Wilhelm; Modern Surface Technology
30
generally classified based on their ignition source. In the case of plasma sprays, it is an
arc generated between a cathode and anode and in the presence of a plasma gas, creates a
plasma jet which melts and projects powder out and onto a surface at temperatures
approaching 15,000K at a very high velocity.55 In the case of a wire spray, wire, in place
of a powder, is fed into a jet stream.
High velocity oxygen fuel is similar to these two mentioned thermal sprays with
just a different combustion source. The HVOF spray technique was derived from the
detonation gun thermal spray technique which used a cycling combustion to ignite and
project a powder fed into the jet stream. HVOF uses a continuous burning operation
rather than a cycling operation which uses a lower temperature but high particle velocity,
creating a coating with reduced porosity and exceptionally high bond strength.
Before a base material can be sprayed, the part must go through certain preparation
to ensure a proper surface is created for a good bond to the coating. Initially a part is
usually cleaned and blasted with an abrasive material to roughening up the bond surface
prior to spray. This is achieved by a propelling a fine, hard material (i.e. Al2O3, SiC) by
way of a pneumatic propulsion. With a roughened surface, the coating interface will be
formed by an interlocking mechanical bond. The industry standard for applying a
coating after an abrasive grit blast is approximately 2 hours. This is to ensure the
coating surface does not get contaminated by impurities in the air.
When the part is set up within the coating system, the main parameters that affect
the coating result are stand-off distance of the nozzle and gun travel speed. These values
are usually determined by a thermal spray shop and refined during development of a
specific process.
3.2.2
EQUIPMENT DESCRIPTION
The main part of the High Velocity Oxygen System is the gun itself. The basic
overall design of the gun consists of an internal combustion chamber and a
converging/diverging nozzle. A combustible mixture of fuel and oxygen under high
55
Bose, Sudangshu; High Temperature Coatings
31
pressure is ignited in the chamber to create the continuous flame. Figure 6 shows a cross
section of an HVOF gun with various critical components identified.
The design and dimension play a very large part in the type of HVOF system that is
used and in fact it determines the type of fuel to be used, the gas temperature and
resulting particle velocity. Typical fuels include propylene, acetylene, propane and
methyl acetylene-propadiene mixture (MAPP), and hydrogen.
These fuels are
commonly gaseous but liquid fuels can be used such as kerosene. Air can be used in
place of the oxygen to create a High Velocity Air Fuel thermal spray.56
The velocity of the jet stream exiting the gun achieves supersonic speeds which can
be visible by the supersonic rings in the jet. To create this velocity, the gun uses a
design similar to supersonic jet engines with a converging/diverging nozzle.
The
diverging exit of the HVOF nozzle is a distinguishing feature of the gun compared to
other thermal spray guns.
As a result of the intense combustion that takes place in the chamber of the gun,
high temperatures are created which carry through the gun.
In order to maintain
functionality with the hardware, the gun is cooled normally with water, but on open
combustion system guns, air is used as well.
The powder is injected into the system either just after the combustion chamber or
within it. The powder is forced into the system at pressure and with a carrier gas to
shield powder from other surroundings as it goes through combustion and aids in coating
quality after part has been sprayed as it greatly minimizes chances of oxide.57,58
The Sulzer-Metco Hybrid Nozzle Diamond Jet 2600 HVOF gun cross section is
shown in Figure 7.
It shows the various areas of the gun that are critical to the
combustion, powder inject and jet exhaust/nozzle configuration.
56
Bose, Sudangshu, High Temperature Coatings
57
Bose, Sudangshu, High Temperature Coatings
58
Bach, Friedrich-Wilhelm; Modern Surface Technology
32
Figure 7: Cross section of HVOF gun (Hybrid Nozzle Diamond Jet 2600), Courtesy of SulzerMetco59
3.2.3
THERMODYNAMIC CONCEPTS
As discussed previously, the HVOF system uses a variety of fuels to achieve different
combustions. Propane, propylene or acetylene are just some of the possible choices for
fuels. The results of this decision are critical for heat input into the combustion and
spray velocity which ultimately affect the coating quality and are dependent on such
factors as HVOF gun system (open vs. closed) and powder.
The ratio of fuel to
air/oxygen mixture can also help in fine tuning the resultant.
In certain instances, the temperature must be reduced to minimize the risk of phase
transformations particularly within carbide powders and in some cases reduce changes of
decarburizing coatings containing significant amounts of carbon. To lower temperatures
without changing fuels, frequently nitrogen or other compressed gas might be injected
into the jet to lower the temperature.
Table 3 below is an example of different fuels used and their respective maximum
temperatures and specific fuel ratios:
59
Bose, Sudhangshu; High Temperature Coatings
33
Table 3: Properties of Typical Fuels for HVOF Flame Spraying60
Fuel
Maximum
Oxygen/Fuel Ratio
Temperature
Max Surf Temp HVOF App.
Propane
2828
4.5
3-8
Propylene
2896
3.7
3.5-7
Hydrogen
2856
0.42
0.3-0.6
Ethene
2924
2.4
2-5
Acetylene
3160
1.5
1.3-4
Kerosene
~2900
1.9
2.8-4.8
3.2.4
KINETIC CONCEPTS
Powder particles will travel supersonically at about 1570-3350 ft/s. As particles exit the
nozzle at these supersonic speeds, it is also in a jet stream that is well past the melting
point of the powder (approximately 3000-5000°F). Given the high temperature and
velocity, the coating left on the surface will have a very low porosity and low oxide
content. The reason for this superior quality coating is the particle spends less time in an
uncontrolled environment which minimizes contamination and because it will strike the
surface in a molten state allowing it to better form around the surface and other previous
laid particles, this also reduces the ability for contaminants and voids to get into the
coating structure which in turn, increases coating density due to more uniform makeup.61
Because of this high kinetic energy, a smoother coating surface results which
reduces roughness as opposed to that obtained with plasma and other thermal sprays.
The benefit of this, especially with very hard WC based HVOF coatings, is it will
minimize spraying to larger sizes and having to machine away a portion to achieve a
certain surface finish.62
60
Bach, Friedrich-Wilhelm; Modern Surface Technology
61
Bose, Sudangshu; High Temperature Coatings
62
Bach, Friedrich-Wilhelm; Modern Surface Technology
34
3.3 PROPERTIES
3.3.1
HARDNESS & WEAR
Carbide based materials are commonly used in situations for their hardness.
For
decades, carbides have been the main material for cutting tools, from saws to drilling.
Carbide based cermet coatings have also been employed for surface treatment of parts
that require a hard material in wear interfaces.
The properties and performance of tungsten carbide coatings is attributed to a
complex function of carbide size, shape and distribution, matrix hardness and toughness,
and a solution of carbon in the cobalt matrix. A coating must retain a large volume
fraction of finely distributed tungsten monocarbide (WC) to achieve the optimum wear
properties. This is largely dependent on the minimizing of decarburization of WC,
which can readily occur at high temperatures associated with the thermal spray process.
Minimizing decarburizing is essential and is one thing that makes the HVOF process an
ideal choice for applying the wear coatings since it does mitigate that decarburizing
phenomenon.
In tests comparing HVOF coatings WC-12Co to Cr3C2-25NiCr, the WC based
coating contained a very high amount of the tungsten monocarbide crystal (WC) which
was confirmed both by microscopy and x-ray diffraction. This increase in the amount of
WC can be achieved due to a higher flame velocity and lower flame temperature of the
HVOF process, which would limit the carbon decomposition process.
Wear testing also conducted under the same test was performed to understand the
HVOF coatings ability to withstand excessive wear under a severe load. The tests were
performed without lubrication and with increasing load. Both microstructural evaluation
of the coating surface and weight tests showed that the WC-12Co was exceptionally
capable under high wear conditions due to its minimal amount of weight loss and the
lack of pitting and scratches on the surface of the coating which can ultimately lead to an
exponential failure rate of the coating. Figures 8 and 9 below presents both these results.
35
Figure 8: Evolution of the weight loss of coated samples.63
63
Sahraoui, Tahar; Structure and Wear Behavior of HVOF Sprayed Cr3O2-NiCr and WC-Co Coatings
36
Figure 9: SEM samples of worn HVOF sprayed WC-12Co64
3.3.2
OIL RETENTION
In a test conducted by the U.S. Hard Chrome Alternatives Team, a WC-12Co HVOF
coating was applied to several surfaces on a Pratt & Whitney TF33 engine. All surfaces
were located in high wear, oil wetted areas, including bearing housings, and compressor
and turbine hubs. The test was conducted to gather empirical test results on the HVOF
coatings to understand their corrosion wear and fatigue characteristics. The engine test
was conducted by running the engine for approximately 4500 simulated flight hours and
tested both for performance and endurance.
With regards to the HVOF coatings ability to withstand an oil wetted environment,
an oil analysis by spectrometry was performed on the TF33 lubrication system and found
no amounts of tungsten or cobalt in the oil, meaning no coating was liberated during
64
Sahraoui, Tahar; Structure and Wear Behavior of HVOF Sprayed Cr3O2-NiCr and WC-Co Coatings
37
engine operation.65 This substantiates not only its use in an oil wetted environment
where oil can penetrate its microstructure to possible liberation of coating but also its
durability in a high wear environment. A risk of coating liberation into the oil system
not only can be trouble for the coating but also some softer material that is within the
same oil system. Tungsten and cobalt is likely to be harder than some nickels, titanium
and possibly some steels within the system.
The test does not draw any conclusion as for the reason why the coating is able to
withstand the oil wetted environment but it is speculated that coating, as-sprayed, has
good powder splatter, minimal porosities, and superior hardness which add to the
coatings hardness, durability and toughness as well as from oil penetrating the coating.
Other results and conclusions drawn from this test include that the coating passes
both visual, florescent penetrant inspection and met all serviceable limits after the 4500
simulated flight hours and repeated assembly and disassemblies. This is uncommon for
hard chrome electroplate because of its ability to easily become scored after assembly
and disassembly cycles. Because of this, it is industry standard to strip and recoat hard
chrome plating to inspect base material, which drives lengthy and expensive overhaul
cycles compared to not having to remove the HVOF coating: if the base material fails
(i.e. crack) under the HVOF coating, the coating will also show similar failure.66
3.3.3
OXIDATION AND CORROSION PREVENTION
Tungsten-carbide based high velocity oxygen fueled spray has shown, both in real world
and laboratory tests, that it is a capable coating for oxygen and corrosion prevention in
high wear settings.
Tests were performed comparing the corrosion prevention of
tungsten-carbide based HVOF coatings and hard chrome plating by performing
electrochemical polarization tests in 0.1 N HCl and 0.1 N HNO3 and a Corrodkote test.67
These tests test both the corrosion and oxidation prevention ability of a material.
The chlorine from the HCl will tend to attack protective oxide layers that passivating
materials forms for protection while the nitrate (NO3) in the nitric acid very quickly form
65
Legg, Keith; Alternatives for Hard Chrome Plating in Aerospace Industry
66
Legg, Keith; Alternatives for Hard Chrome Plating in Aerospace Industry
67
Bolelli, Giovanni; Corrosion Resistance of HVOF-sprayed Coatings for Hard Chrome Replacement
38
oxides on non-passivating materials leading to excessive oxidation. The Corrodkote test
is an industry standard to test the corrosion prevention abilities of various materials and
surface treatments by subjecting a material or coating to a mixture of acids in a
controlled high temperature, high relative humidity for extended periods of time. It can
replicate common corrosive environments in a faster, controlled, empirical setting.
In the HCl test, the WC-based HVOF performed superior to its hard chrome
counterpart because the WC-based coatings tested do not passivate and form the outer
protective oxide layer therefore the Cl had nothing to attack. The HVOF coatings tested
also have more noble corrosion protection ability in an electrolytic environment and will
act as a cathode and build material leading to its protection. However, corrosion and
deterioration was evident on the WC-based HVOF spray for the same reason as the
above.
One common failure mechanism of the HVOF coating was due to corrosion forming
at the crevices within the coating. The resultant of this failure is corrosion that travels
within the coating and parallel to the surface of the substrate. This shows that it is very
unlikely for the corroding agent to reach the substrate.68 Figure 10 shows the separation
of an HVOF coating parallel to surface of the substrate.
These crevices are the result of poor melting of the powder during the spray
operation and due to oxide inclusions within the coating. These are process parameters
that can be controlled during process development and should be controlled through the
actual specification of the HVOF process.
68
Bolelli, Giovanni; Corrosion Resistance of HVOF-sprayed Coatings for Hard Chrome Replacement
39
Figure 10: SEM micrograph of WC-17Co coating cross-section after the polarization test in 0.1 N
HCl showing an extensive crevice corrosion phenomenon.69
As a result of these tests performed, WC-based HVOF coatings can be employed in
many settings where corrosion and oxidation are a problem and where hard chrome
plating has been used.
3.3.4
STRESS AND THERMAL EFFECTS
The main stress and thermal effect concern with thermal spray coatings is largely due to
the cooling during the spraying process. As mentioned during the process discussion of
WC based HVOF coatings, the residual stress that remains during the cooling is also
called intrinsic or quenching stresses.70 Spray particles will reach speeds over 4400 ft/s
and temperatures exceeding 3000°C.71 Residual stresses easily form within coating and
substrate, caused by both mechanical impact of molten spray and temperature, possibly
69
Bolelli, Giovanni; Corrosion Resistance of HVOF-sprayed Coatings for Hard Chrome Replacement
70
Bemporad, F.M.; Modellling, Production, and Characterisation of Duplex Coatings(HVOF and PVD) on
Ti-6Al-4V Substrate for Specific Mechanical Applications
71
Toparly, Mustafa; Thermal Stress Analysis of HVOF Sprayed WC-Co/NiAl Multilayer Coatings on
Stainless Steel Substrate Using Finite Element Methods
40
leading to cracking on cooling of the spray due to these high quenching stresses
exceeding the limits of the coating.
Thermal stresses also exist in the coating due to normal thermal contractions during
designed operation of the coating. Testing and operation of HVOF coatings, such as
WC-Co, WC-Co-Cr, NiCrBSi-WC have proven effective up to 500°C (Modern Surface
Technology, Bach).
During coating selection, the operating temperatures of the
substrate naturally should be considered so the thermal interaction and any possible
expansion/contraction between the HVOF coating and the substrate should be accounted
for.
However, the common employment of the HVOF coating in the oil wetted
environments will limit the temperature the HVOF coating sees due to the temperature
control nature of the oil.
3.4 ENVIRONMENTAL HEALTH & SAFETY ISSUES AND RISKS
Thermal sprays, in general, are largely safe to both workers and the environment because
of the controlled nature of the process and due to its insolubility in water. Simple,
inexpensive measures can easily be taken and are standard within several thermal spray
industry shops. However, risks remain when handling thermal spray powder, equipment
and sprayed parts.
Primary health risks of tungsten-carbide are associated with inhalation of the
powders and dermal contact with the powders which even in the High Velocity Oxygen
Fuel process are unlikely because of the following:
•
Powder is handled in a controlled nature.
•
The HVOF jet exhaust is performed in a localized, controlled booth where
access is limited to workers.
•
All thermal spray booths, HVOF included, have ventilated exhaust to
remove any gaseous tungsten-carbide that may exit the local spray area. 72
72
http://www.cdc.gov/niosh/topics/noise/
41
3.4.1
NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND
HEALTH
The National Institute for Occupational Safety and Health (NIOSH) is a United States
government organization that falls under the direction of the Center for Disease Control
(CDC) which is a part of the Department of Health and Human Services. NIOSH has
provided precautions and exposure limits for both tungsten-carbide and cemented
tungsten-carbide materials.
NIOSH conducted a comprehensive study spanning a few decades investigating
health problems amongst workers primarily exposed to metal cutting using tungstencarbide tools. A significant, yet still relatively small, percentage of workers were noted
to have obtained respiratory illnesses including, but not limited to, upper respiratory tract
irritation, coughing, exertional dyspnea (i.e. shortness of breath) and weight loss.
Exposure to tungsten-carbide and cemented tungsten-carbide materials including cobalt
were a common thread amongst those workers and labeled as the most likely cause of
their illness. However, in several of the workers analyzed, respiratory distress subsided
within a year of being removed exposed work environments and the thermal spray
industry was not included in the results.
A small carcinogenic study was performed to try and understand cancer effect on
humans of tungsten-carbide metals. Two instances of deaths caused by lung based
cancers were statistically inconclusive due to other factors affecting the workers that no
conclusion can be drawn to tungsten-carbide. However, a reference is made in the
document to cobalt being a cancer driver in other animal testing but no human
connections have been made.
It is also of note that none of these tests were conducted in a thermal spray specific
shop but were commonly performed amongst workers primarily involved with metal
cutting using cemented tungsten-carbide with cobalt binders or metal foundries, both of
which have many other metals in gaseous and vapor forms within the air at sub micron
levels.
These various test results led to NIOSH publishing exposure standards,
precautionary measures, employee education and recommended corrective action
procedures for WC based materials in 1977 titled “Criteria for a Recommended
42
Standard: Occupational Exposure to Tungsten and Cemented Tungsten Carbide”. This
document is still referenced by the CDC and other government agencies.
The report provides an eight part system of recommendations that have become the
basis for proper worker and environmental protection for areas where tungsten-carbide
based materials may pose a risk:
1. Environmental (workplace air) – set requirements for soluble, insoluble and
cemented tungsten-carbide based work areas
2. Medical – worker pre-evaluation, respirator education and evaluation,
treatment and education of exposed workers
3. Labeling & Posting – health warnings and requirements and locations of
containers and work areas.
4. Personal Protective Equipment – respiratory standards and requirements, eye
and skin protection
5. Informing Employees of Tungsten Hazards – warning, education and
exposure information, MSDS sheet use
6. Work Practices – engineering controls, spillage and leakage controls,
handling and general practices
7. Sanitation – dining requirements, hand washing, hazardous/toxic waste
disposal
8. Environmental Monitoring & Recordkeeping – air quality and personnel
monitoring, corrective action (if necessary), recordkeeping.
3.4.2
PROCESS RISKS
3.4.2.1 Sound
HVOF process volumes can reach noise levels of up to 120dBA.73 Tests performed by
the Centre for Human Performance & Health in Ontario, Canada have created a decibel
level and list 90-95dBA the level at which a sustained exposure may result in hearing
loss74.
The OSHA Daily Permissible Noise Level Exposure lists 115dBA as the
73
Bach, Friedrich-Wilhelm; Modern Surface Technology
74
http://www.gcaudio.com/resources/howtos/loudness.html
43
maximum noise level exposure which no worker should be submitted to for periods
longer than 0.25 hours per day per OSHA Standards 29 CFR 1926.52.
The Criteria for Recommended Standard: Occupational Noise Exposure published
by NIOSH (Publication No. 98-126) provides a system of guidelines for determining
damaging noise levels, measuring standards and protective measures and requirements.
In this document it states the reduction in noise is the best solution for worker safety but
lists a system of hearing protection measures to combat unsafe noise in the work place.75
Also, as discussed previously, HVOF is conducted in a booth which aside from
protecting the workpiece and spray environment from outside, unwanted factors, also
protects bystanders from the excessive noise.
3.4.2.1.1 Temperature
During the spray process, temperatures of the HVOF jet flame can exceed 3000°C.76 As
a result, the work piece will see very high temperatures but will not see the same
temperature due to certain process parameters, specifically nozzle stand-off distance and
dwell time or nozzle translation speed. Regardless, time should pass before contact is
made with the part. Risks associated with this are burning and possibly damage to
coating and the part.
75
http://www.cdc.gov/niosh/topics/noise/
76
Bach, Friedrich-Wilhelm; Modern Surface Technology
44
4. CONCLUSIONS
This document presented an overview of the hard chromium electroplating process and
the environmental dangers associated with it.
It has shown the use of hexavalent
chromium within the process as a grave danger to the entire environment. For many
years, hard chromium plating has overcome the environmental risks discussed in this
report because it has effectively protected parts subjected to high wear rates and where
corrosion and oxidation may pose a risk. A coating that can achieve the same wear,
oxidation and corrosion benefits, but without the environmental concerns, as the hard
chrome plating would be ideal. Tungsten carbide high velocity oxygen fuel thermal
spray coatings have established themselves as a viable option because they have attained
these qualities with preventable and minimal health and safety concerns to just the
workers involved, not a general public.
45
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http://www.hazmat-alternatives.com/Documents/Briefings&Presentations/TS&SEAHimley_UK-4-20-05.pdf
www.modernhc.com/capabilities.html
http://www.electro-coatings.com/wear-resistant/hard-chrome.php
http://www.nhml.com/resources/1999/7/1/hard-chromium-plating
www.finishing.com/faqs/chrome.html
www.wmrc.uiuc.edu/manuals/finishing.htm
http://www.youtube.com/watch?v=mJ3KhaIATlA
http://www.campuserc.org/resources/EHSguide/EPCRA/Pages/default.aspx
http://www.gcaudio.com/resources/howtos/loudness.html
http://www.osha.gov/pls/oshaweb/searchresults.category?p_text=chromium&p_title=&p
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http://www.cdc.gov/niosh/topics/noise/
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48
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