Chrome Plating Alternative
Thermal Spray
Submitted by
William Kalivas
Summer 2003
Emerging Technologies (TECH
3023)
Rogers University
Table of Contents
Introduction
History
The
Process
Thermal
Spray Processes
Arc Spray
Combustion
Plasma
High Velocity Oxygen Fuel
Detonation Gun
High-Velocity
Air-Fuel
Why Replace Chrome?
Aircraft Systems
Landing Gear
Propeller Hubs
Industrial Application
Optical Mirror Surface
Consistency
Productivity
Oil and Natural Gas Tooling
Resources
Introduction
For over 60 years, thermal spray
has achieved outstanding technological and commercial progress in the
aerospace, gas turbine, engineering and biomedical Industries.
The applications of the little known
Thermal Spray industry are endless. Every manufactured product in memory either
comes in contact, or is impacted by the process. Applications range from
aerospace, to farm implements, to down hole tooling, to the pulp and paper
industry. There is a tremendous economic benefit from Thermal Spray. In addition
to the economic gains, there are ecological pluses associated with Thermal
Spray too. Unlike Chrome and similar plating technologies, Thermal Spray
“waste” is easily collected, the scrap is sold for profit, (the metals can be
reclaimed) and there are no superfund clean up sites to contend with. In
addition, the Thermal Spray process allows designers to use less expensive,
lighter and more easily processed base materials. A coating under 0.010” thick
can easily add corrosion, wear and thermal protection to any base metal.
Early
History
The metal spraying industry has its beginnings early in the 20th century when Dr. M.U. Schoop of Zurich, Switzerland, developed the first process for spraying metal and, subsequently, the first equipment to spray metal in wire form. The early commercial applications for the "Schoop Process" or "metallizing" took place in Germany, and later in France. Schoop subsequently sold his rights to a German firm known as Metallizator. It was this firm that made and sold spray units in Europe, England and the United States beginning in the early 1920"s. Among the early U.S. companies to adopt the technology were Metal Coatings Company and Metal-weld of Philadelphia and Metallizing Company of Los Angeles. Early applications included the coating of railroad tank cars, U.S. Navy ship tanks, coal barges and the spraying of the emergency gates for the Panama Canal.
Applications for industrial plants accelerated during the "Great Depression," and during this decade the greatest push for what was then known as "flame spraying" occurred. Four entrepreneurs – Larry Kunkler, Rea Axline, Charles Boyden, Sr. and Charles Stipp from the Metallizing Company of America – were largely responsible for pushing metallizing into the American industrial scene. In 1932, Rea Axline (a subsequent founder of Metco) exhibited his company’s "Three-in-one Metallizing Unit" at a meeting of the Galvanizer’s Institute at the Hotel Statler in St. Louis.
Sixteen
years later, the American Metallizing Contractors Association, the predecessor
of the International Thermal Spray
Association (ITSA), was founded in the same hotel. The meeting was arranged by
Walter B. Meyer of St. Louis Metallizing Company and William H Fatka of Metallizing,
Inc. of Chicago. Soon they were publishing a newsletter, AMCA News, to share new thermal spray technology information and
identify new market opportunities for members.
With
the advent of World War II, the American thermal spray industry went into high
gear with the members of the association playing a key role in providing the
"metallizing" desperately needed for replacement parts for industrial
equipment. Walter Meyer and Tom Lufkin of Tranter Manufacturing Company worked
with the Army in the China-Burma-India theater; Knowles Smith of Dix
Engineering Company worked with the Navy. By the end of the war,
"metallizing" was firmly established as a major industrial process.
Applications included large elevated water tanks, tuna fishing boats, chemical
industry tanks and tank cars, capacitor castings and pipe.
In
response to an increasingly sophisticated market, ITSA drew up industry
specifications for the application of corrosion-resistant coatings and spelled
out the methods of inspection. These specifications were distributed to
engineering firms, designers, and educational institutions throughout the world
and resulted in increased business opportunities for the entire metallizing
industry. The advent of fusible alloys, flame spraying of ceramics and plasma
spraying were soon to follow.
In
1976, the association co-sponsored, with the American Welding Society (AWS),
the first International National Thermal Spray Conference held in the United
States. The event took place in September 1976 in Miami Beach, Florida.
Accounts described the event (which drew 515 people from 28 countries) "as
the most successful international conference to date." Eight ITSA members
presented technical papers at the event, receiving international recognition
for their "contributions to the world body of technical knowledge."
This event paved the way for ITSA sponsorship of the National Thermal Spray
Conference.
ITSA
members were also important contributing authors and researchers for the AWS
manual, Thermal Spraying – Practice and
Theory Application. Published in 1985, this was the first definitive work
on thermal spray produced in the United States.
Today,
as an organization, ITSA is working to raise the level of awareness of general
industry and government on the advanced capabilities of thermal spray
technology and the vitally important problems it can solve. Thermal spray
applications have moved from a traditional base in aviation to encompass
ground-based turbines, automotive, biomedical electronics, highway
infrastructure and virtually every industry. According to 1991 figures, the
thermal spray market is expected to reach over $2 billion the beginning of year
2000.
The
Process
The thermal spray coating process takes the energy from the likes of inflammable gas, ionized gas, explosive gas, or electric energy. The thermal spray powder is heated to a point sufficient (not necessarily to melting) that the feed stock will deform on impact. It is propelled by the energy source gas stream at a
Target, that the feed stock literally explodes onto the target, cools and forms a mechanical bond on the surface.
The establishment of the coating on the surface will greatly improve the
character of abrasion resistance, erosion resistance, heat resistance, chemical
resistance regarding the part and the product. It can also give the character
of the electric conductivity, insulation, and thermal conduction control.
Schematic Breakdown of the different
Thermal Spray Processes
The Thermal Spray Processes
Arc Spraying - Electric Arc
Wire Thermal Spray Process
Schematic Diagram of the Arc Spraying Process
In the Arc Spraying
Process a pair of electrically conductive wires are melted by means of an
electric arc. The molten material is atomized by compressed air and propelled
towards the substrate surface. The impacting molten particles on the substrate
rapidly solidify to form a coating. This arc spraying process carried out
correctly is called a "cold process" (relative to the substrate
material being coated) as the substrate temperature can be kept low during
processing avoiding damage, metallurgical changes and distortion to the
substrate material.
Arc Gun
Arc spray coatings are
normally denser and stronger than their equivalent combustion spray coatings.
Low running costs, high spray rates and efficiency make it a good tool for
spraying large areas and high production rates. Disadvantages of the arc
spraying process are that only electrically conductive wires can be sprayed and
if substrate preheating is required, a separate heating source is needed.
Combustion Powder Thermal
Spray Process - Flame Spraying
Schematic Diagram of Combustion Powder Thermal Spraying Process
This flame spraying
process is basically the spraying of molten material onto a surface to provide
a coating. Material in powder form is melted in a flame (oxy-acetylene or
hydrogen most common) to form a fine spray. When the spray contacts the
prepared surface of a substrate material, the fine molten droplets rapidly
solidify forming a coating. This flame spraying process carried out correctly
is called a "cold process" (relative to the substrate material being
coated) as the substrate temperature can be kept low during processing avoiding
damage, metallurgical changes and distortion to the substrate material.
The main advantage of this
flame spraying process over the similar Combustion wire spray process is that a
much wider range of materials can be easily processed into powder form giving a
larger choice of coatings. The flame spraying process is only limited by
materials with higher melting temperatures than the flame can provide or if the
material decomposes on heating.
Combustion Powder Gun
Powder Thermal Spray Process Plasma Spraying
Schematic
Diagram of the Plasma Spraying Process
The Plasma Spraying
Process is basically the spraying of molten or heat softened material onto a
surface to provide a coating. Material in the form of powder is injected into a
very high temperature plasma flame, where it is rapidly heated and accelerated
to a high velocity. The hot material impacts on the substrate surface and
rapidly cools forming a coating. This process carried out correctly is called a
"cold process" (relative to the substrate material being coated) as
the substrate temperature can be kept low during processing avoiding damage,
metallurgical changes and distortion to the substrate material.
The plasma gun comprises a
copper anode and tungsten cathode, both of which are water cooled. Plasma gas
(argon, nitrogen, hydrogen, helium) flows around the cathode and through the
anode which is shaped as a constricting nozzle. The plasma is initiated by a
high voltage discharge which causes localized ionization and a conductive path
for a DC arc to form between cathode and anode. The resistance heating from the
arc causes the gas to reach extreme temperatures, dissociate and ionize to form
a plasma. The plasma exits the anode nozzle as a free or neutral plasma flame
(plasma which does not carry electric current) which is quite different to the
Plasma Transferred Arc coating process where the arc extends to the surface to
be coated. When the plasma is stabilized ready for spraying the electric arc
extends down the nozzle, instead of shorting out to the nearest edge of the
anode nozzle. This stretching of the arc is due to a thermal pinch effect. Cold
gas around the surface of the water cooled anode nozzle being electrically
non-conductive constricts the plasma arc, raising its temperature and velocity.
Powder is fed into the plasma flame most commonly via an external powder port
mounted near the anode nozzle exit. The powder is so rapidly heated and
accelerated that spray distances can be in the order of 25 to 150 mm.
Plasma Spray Gun in Use
The plasma spraying
process is most commonly used in normal atmospheric conditions and referred as
APS. Some plasma spraying is conducted in protective environments using vacuum
chambers normally back filled with a protective gas at low pressure, this is
referred as VPS or LPPS.
Plasma spraying has the
advantage that it can spray very high melting point materials such as refractory
metals like tungsten and ceramics like zirconia unlike combustion processes.
Plasma sprayed coatings are generally much denser, stronger and cleaner than
the other thermal spraying processes with the exception of HVOF and detonation
processes. Plasma spray coatings probably account for the widest range of
thermal spray coatings and applications and makes this process the most
versatile. Disadvantages of the plasma spraying process are relative high cost
and complexity of process.
High Velocity Oxygen Fuel Thermal Spray 
Schematic Diagram of the HVOF Process
The HVOF (High Velocity
Oxygen Fuel) Thermal Spray Process is basically the same as the combustion
powder spray process (LVOF) except that this process has been developed to
produce extremely high spray velocity. There are a number of HVOF guns which
use different methods to achieve high velocity spraying. One method is
basically a high pressure water cooled combustion chamber and long nozzle. Fuel
(kerosene, acetylene, propylene and hydrogen) and oxygen are fed into the
chamber, combustion produces a hot high pressure flame which is forced down a
nozzle increasing its velocity. Powder may be fed axially into the combustion
chamber under high pressure or fed through the side of laval type nozzle where
the pressure is lower. Another method uses a simpler system of a high pressure
combustion nozzle and air cap. Fuel gas (propane, propylene or hydrogen) and
oxygen are supplied at high pressure, combustion occurs outside the nozzle but
within an air cap supplied with compressed air. The compressed air pinches and
accelerates the flame and acts as a coolant for the gun. Powder is fed at high
pressure axially from the center of the nozzle.
High Velocity Oxygen Fuel Spray Gun in Use
The Coatings are very dense,
strong and show low residual tensile stress or in some cases compressive
stress, which enable very much thicker coatings to be applied than previously
possible with the other processes.
The very high kinetic
energy of particles striking the substrate surface do not require the particles
to be fully molten to form high quality coatings. This is certainly an
advantage for the carbide cermet type coatings and is where this process really
excels.
HVOF coatings are used in
applications requiring the highest density and strength not found in most other
thermal spray processes. New applications, previously not suitable for thermal
spray coatings are becoming viable.
Detonation Spray Process
Schematic Diagram of the Detonation Thermal Spray Process
The Detonation gun
basically consists of a long water cooled barrel with inlet valves for gases
and powder. Oxygen and fuel (acetylene most common) is fed into the barrel
along with a charge of powder. A spark is used to ignite the gas mixture and
the resulting detonation heats and accelerates the powder to supersonic
velocity down the barrel. A pulse of nitrogen is used to purge the barrel after
each detonation. This process is repeated many times a second. The high kinetic
energy of the hot powder particles on impact with the substrate result in a
build up of a very dense and strong coating.
Activated Combustion High-Velocity Air-Fuel Spraying
Emerging Technology
Activated
Combustion High-Velocity Air-Fuel Spraying (AC-HVAF) is a new industrial
thermal spray technology, which incorporates the best features of both HVOF and
Cold Spray processes. We have done what traditional HVAF has failed to deliver.
Activated Combustion HVAF is a material deposition process in which coatings
are applied by exposing a substrate to a high-velocity (600-800 m/s) jet of
10-45 um particles accelerated and heated by supersonic jet of low-temperature
'air-fuel gas' combustion products.
Like HVOF
Activated Combustion HVAF process utilizes high velocity heated particles. But spraying at temperatures
(1000-15000F)lower than the particles melting point, this new
process avoids negative effects caused by HVOF high temperatures, (1500-22000F)such
as oxidation and deterioration of spray material, thus producing superb
coatings of metals and carbides for a wide range of industrial needs.
Activated Combustion High-Velocity
Air-Fuel process was first demonstrated in early 1990s by Dr. Viacheslav
Baranovski and colleagues at the Institute of Durability and Longevity of
Machines (Belorussian Academy of Science). At that time, they demonstrated a
material deposition process based on the use of 20- to 50-um solid metal
particles introduced into a large supersonic gas stream formed by
low-temperature products of air-propane combustion. This research continued in
the U.S. in early 1999.
Why Replace Chrome?
Chrome plating provides a
very effective surface treatment for the reduction of wear, and in some cases
corrosion. It has been in use for more than 70 years, and has proved to be a
relatively cheap, effective solution. Over the past ten years HVOF coating
technology has developed the point where it is widely available commercially.
Equipment and powders are available from a number of suppliers, and the process
is offered by aerospace-qualified job shops across North America, as well as
elsewhere in the world. In general, it is found that the performance of HVOF
coatings is far superior to chrome in wear, fatigue, and impact resistance, and
is at least equal in corrosion resistance. Furthermore, the HVOF deposition
process is faster than chrome (typically an hour or two versus 24 hours) and
HVOF does not cause hydrogen embrittlement (which also eliminates embrittlement
relief heat treatments). The consequent reduction of in-shop time makes many
landing gear components significantly cheaper to coat by HVOF than by chrome
plating. Even where HVOF is a more expensive process, its improved performance
lowers the expected overhaul frequency, reducing the lifetime cost of
ownership. On the basis of overall cost and performance, therefore, it makes
sense to replace hard chrome with HVOF coatings.
The environmental problem
with hard chrome is not with chrome itself, but with the severe environmental
problems associated with the plating process. Chrome itself is essentially
inert, and is in fact safely used in everyday items such as stainless steel
cutlery, and even inside the body in implanted prosthetic hips and knees. The
primary problem lies with the hard chrome plating process, which uses a chromic
acid solution, which the plating process releases into the air in the form of a
fine mist. This mist contains chromium ions in the hexavalent (Cr6+) state,
which has long been known to be carcinogenic and to cause a host of other
medical problems, including perforated nasal passages and skin rashes.
As a result, under the
authority of the Clean Air Act, the EPA has recently promulgated new and more
stringent rules to limit hexavalent chrome releases to the environment. These
rules have increased the cost and complexity of hard chrome plating in many
shops. Added to these rules, OSHA is expected soon to issue new regulations
that will very seriously curtail the permitted levels of hexavalent chrome to
which plating shop workers can be exposed. The limits, which are currently set
at 100 micrograms per cubic meter, will almost certainly need to be reduced -
perhaps to as low as 0.5 micrograms per cubic meter to reach the desired level
of risk. The cost of reaching these levels will be very high - the National
Defense Center for Environmental Excellence estimates $22 million for the first
year and $46 million for each subsequent year, for the Navy alone.
While the cost of chrome
plating is rising to meet stricter standards, technologies developed over the
last ten years have proved to be cleaner, more effective, and sometimes cheaper
than chrome plating. Their superior performance is predicted to lead to
significant reductions in life-cycle cost of ownership of military systems,
while improving military readiness. As a result DoD has recognized the value of
reducing its reliance on chrome plating.
Aircraft Systems Being Validated
Chrome Replacements on Landing Gear
The major use of hard chrome in aircraft is on landing
gear. A landing gear is a large hydraulic unit up to 6 feet long and 8" or
so in diameter that runs inside the outer cylinder, which is attached to
underside of the aircraft - essentially it is a giant version of the shock
absorber in a car. It is a high pressure gas-over-fluid system that is
subject to a great deal of stress and side-loading, both on landing and during
taxiing across airports with a loaded aircraft. For carrier-based
aircraft the stresses are much worse, not just because landings are
harder. During takeoff tremendous stresses are put on the landing gear by
the catapult, since the catapult attaches to the nose landing gear, pulling the
whole fully loaded aircraft along the flight deck to launch speed in a few
seconds. Any coating must withstand the flexure this creates on the
landing gear.
A typical landing gear
inner cylinder, or piston, is shown in the figure. The inner cylinder is
chrome plated, as are the journals where the wheels attach. Typically the
chrome is about 0.003" (75 microns) thick. Periodically (every 10,000
flight hours for most commercial jets, but more often for military aircraft)
the landing gear must be overhauled. On overhaul the chrome is frequently
found to be worn, chipped, or otherwise damaged, and it must be replaced by
stripping off the old chrome and plating on new.
HVOF Sprayed C –17
Landing Gear Post
A joint team of US
Department of Defense repair depots, laboratories, and US and Canadian
manufacturers of aircraft and landing gear are producing all the data necessary
to replace chrome plate with HVOF WC-Co and WC-CoCr - both clean thermal spray
coatings that produce no hexavalent chrome during deposition, grinding, or
stripping. In order to ensure that all of the data needed is obtained,
all the stakeholders (manufacturers, Air Force single item manager for landing
gear, NAVAIR, and the depot maintenance engineers) have agreed to a Joint Test
Protocol, which defines the testing to be done and the pass/fail criteria
(which in general are defined as performance equal to or better than hard
chrome). Tests include fatigue, wear, corrosion, hydrogen embrittlement,
impact, and proper definition of the materials and deposition methods to ensure
reproducibility.
Chrome Replacement on Propeller Hubs
Although most military and passenger aircraft today
are jet powered with gas turbine engines, there is still substantial use of
propeller aircraft such as the C-130 Hercules transport, the E-2, and the P-3
Orion anti-submarine aircraft. Wear in
the propeller hub region in these types of aircraft is a significant source of
chrome usage for NADEP Cherry Point in North Carolina and Warner Robbins ALC in
Georgia.
A sub-team of the HCAT has
been formed to produce all the data necessary to replace chrome plate with HVOF
WC-Co and/or HVOF Tribaloy 800.
In order to ensure that all
of the data needed is obtained, all the stakeholders (manufacturers, Air Force
single item manager for landing gear, NAVAIR, and the depot maintenance
engineers) have agreed to a Joint Test Protocol, which defines the testing to
be done and the pass/fail criteria (which in general are defined as performance
equal to or better than hard chrome). Tests include fatigue, wear,
corrosion, toxicity leaching, and proper definition of the materials and
deposition methods to ensure reproducibility.
Industrial Applications
HVOF and
Detonation spray technologies for application of tungsten carbide and chrome
carbide based coatings have proved to be cleaner and more effective than chrome
plating. Their superior performance is predicted to lead to significant reductions
in life-cycle cost of ownership in industry. However, several technical
problems are holding back a wide application of these technologies:
1.Remaining porosity in sprayed coatings
limits possibility to superfinish coating surface to better than Ra 1.0
microinch (0.025 micron). It is generally not a problem for hard chrome
plating.
2.Consistency of coating quality is poor due
to quick deterioration of spray gun hardware.
3.Low productivity of equipment results in
high coating cost.
Development of
Activated Combustion HVAF technology is expected to become a “break-through” in
thermal spray applications for hard chrome replacement since this novel
technology overcomes above-mentioned problems.
Optical Mirror Surface
Note:
Surface finish is considered of “mirror quality” if Ra is better than 4
microinch. “Optical mirror” surface must exhibit Ra better than 0.5 microinch.
It is a
general knowledge that HVOF tungsten carbide based coatings can not be
superfinished to better than 1.0 microinch Ra. WC-20Cr-7Ni coatings, sprayed by
Super-Detonation process, have been reported superfinished to 0.5-0.7
microinch. However, this is rather brittle but not hard coating compared to
WC-based alternatives (WC-CoCr, WC-Co, etc.).
On the contrary, all WC-based and Cr3C2-based coatings, sprayed by AC-HVAF
process, are routinely superfinished to optical mirror range. This is
considered to be due to superior density of AC-HVAF coatings and absence of
brittle phases usually formed due to oxidation and thermal deterioration of
carbides.
Consistency of Coating Quality
Low temperature of air-fuel gas
combustion is a primary reason for low heating of the Intelli-Jet gun parts,
providing their long-term performance. Since sprayed carbides are heated
substantially lower than their melting point and due to large diameter of the
gun nozzle, spray particles never deposit on the nozzle wall. Thus, nozzle
clogging does not limit its lifetime. As a result, the hardware performance in
the AC-HVAF process is significantly better than in HVOF processes, providing
an order of magnitude longer lifetime of limiting parts (nozzles, injectors,
etc.). In addition, due to production rate, the AC-HVAF process needs 5-10
times shorter time for application of a coating compare to HVOF.
These factors provide a good basis for significant improvement in coating
quality consistency for AC-HVAF process.
Productivity as a Key Factor in Cost Efficiency
While consuming about the same
amount of spray powder as HVOF processes, the AC-HVAF technology provides 5-10
time faster application rate. This way, application cost is reduced many folds
compare to HVOF and Detonation spraying. Insignificant cost of compressed air
versus oxygen is another obvious factor of cost reduction. For large
applications, the AC-HVAF cost efficiency becomes a breakthrough factor in
using thermal spray for hard chrome replacement.
Oil and Gas Tooling
This
application has long been dominated by chrome and Nickel plating to protect
tooling. As the conditions under which extracting petrochemicals become more
and more extreme, currently, wells are acidified to brake up pockets of oils so
that they can be more easily recovered. The abrasive nature of sand, rock and
mud also takes its toll on plating. Chrome and Nickel plate simply cannot hold
up under these conditions. The determining factor in fully taking advantage of
Thermal Spray HVOF to replace plating in this application is cost. The costs of
traditional HVOF are prohibitive in all but the most extreme cases. It is
likely that the success of the lower cost HVAF systems will expand the market
in the oil and natural gas tooling industries.
Resources
Ace Thermal
Spray Service
Gordon
England
http://www.gordonengland.co.uk/
International
Thermal Spray Association
Hard Chrome
Alternative Team
Southwest
United Industries
Halliburton - Duncan OK
Stork
Cellramic
http://www.cellramic.storkgroup.com/