Peter Foy, Matt Napolitano and Dale Moody: Plasma Powders & Systems, Inc.
Thermal spray is a process in which metals, ceramics and plastics, originating as either fine powders or wires, are sprayed onto a surface after being heated to their melting point by an electric arc, plasma or combustion flame. Once applied, the resultant “skin” can be ground, or polished and burnished, then sealed with a patina finish or left au naturel.
For the last 80 years, both artists and architects have discovered that thermal spray provides an exciting new medium for creative expression. Its use enables artisans to economically create durable shapes and finishes, offering unlimited opportunities for discovery and inspiration. In sculpture, there is quick, minimum setup time and no wait for the casting house to do the job.
For the conservator, thermal spray serves as a preservative of art and architecture from the ravages of time and decay.
Examples of Thermal Spray in Art & Architecture and Conservation
Constantin Brancusi’s 98-foot sculpture, The Endless Column, located in Târgu Jiu, Romania, is part of a grouping built as a memorial to those who died defending the town during World War I. It is made up of an elongated module at ground level, followed by fifteen identical, rhomboid-shaped modules and a half-module at the top. .
As original art, all of these elements were made in cast iron with a thermally sprayed brass coating. After coating, they were threaded down a central steel core. The exterior coating was brass. Bronze would have had better survivability under the environmental conditions (acid rain due to local factory operations) that existed at the site. Over the years, the brass deteriorated and allowed the core to rust. In the mid-‘60s and mid-‘70s, ineffective repairs were made to the coating.
In 1996, the World Monuments Fund placed the column on its World Monuments Watch List of 100 endangered sites. Repairs were delayed due to the unique size of the sculpture and politics. Finally, in 2000, the column was disassembled, the coating was thermal sprayed and the column reassembled. Anyone who thinks that restoration is a simple process will want to consider a twenty page critical discussion of the procedure by Art Watch(1).
During the ‘70s and ‘80s, the artist Jeff, gave up wood sculpture, watercolors and acrylics for a new medium – hot metal sprayed onto canvas with a gun like those used by engineers to coat the underbelly of the Apollo spacecraft. Heated metal erupted from the barrel under high pressure and splashed onto the canvas. The artist wore heat-protected coveralls, a hood and an air-breathing mask.
Jeff called his technique “metallizing” canvas and his first successful result was zinc on canvas stretched over a curving six-by-six foot frame, sanded and polished to a rich gloss. He created a texture on the canvas with thick textured acrylic so the metal could bite onto the surface and hold, not slide or fall off.
In 1994, artist Siena Porta stated that “Metallizing offers creative options and a cost-saving alternative to bronze casting” (2). Siena Porta is a sculptor whose work was the subject of a video called “Me and the Mirror”. She has covered materials as divergent as silk, cement, polystyrene, fired clay, acrylic, Mylar, wood and cast resin.
Robinson Iron in Alexander City, Alabama, specializes in fountains, gates and other large structures. In 2001, Robinson was contracted to restore the 56-foot high sculpture of Vulcan by Guiseppe Moretti, which overlooks Birmingham, Alabama, to its 1904 configuration by reinforcing its interior and re-coating and reassembling the statue on the restored 1938 pedestal. In addition, Robinson patternmakers created a new spear point and hammer. Zinc was thermal sprayed to both interior and exterior surfaces of the statue to prevent corrosion which was followed by a primer and a polyurethane coating.
The 2004 sculpture, “Bronze Seed”, by Kathryn Lipke-Vigessa and executed by Bauer Art Metal, is one piece of an environmental garden/sculpture installation titled “Life Cycle”. It is a large bronze seed that has broken apart into three sections, a tree to be planted in the midst of its remnants.
Large blocks of white Styrofoam were shaped into three different seed segments. Stainless steel skeletons were made for each of the three shapes, copper mesh was shaped to the models and the front half was thermal sprayed with zinc to solidify the shape.
The front and back were assembled and sprayed together on the skeleton. The final shape was touched up and thermal sprayed with silicon bronze. It was then highlighted by polishing, enhancing the final appearance of the seed pods with a chemical patina.
Ms. Lipke-Vigesaa chose the thermal spray system because she wanted a totally unique effect and also, due to budget constraints, because of its affordability.
Bridging the worlds of art and architecture, Eric Bauer of Bauer Fabrication & Art Metal, has integrated the thermal spray process into his work, developing fabricating techniques using thermal spray.
Awarded a U.S. patent, the “Bauer Process”, is the building of three-dimensional objects by spraying metal on wire mesh. Almost any metal, from aluminum to zinc, can be melted and sprayed with the process. Since the resultant coating is 100% metal, it retains the basic characteristics of that particular metal, such as base color, durability, corrosion resistance or a pleasing weathered look.
Patina is a chemical finish applied to the cleaned metal surface to give it a special color or appearance. The resultant surface is then sealed to protect the finish and create a moisture-proof barrier that will last indefinitely and is impervious to almost all solvents and liquids. Bauer states that “there’s almost no limit on what you can do with this process.”
In the area of architecture, thermal spray is being used for both beauty and functionality. One of the largest examples is the Burj Al Arab Hotel in Dubai. When the hotel was being built, vital structural components were thermal sprayed to protect against corrosion. In total, 100,000 square feet of steelwork was arc thermal sprayed over an intermittent six-month period.
The elements sprayed included a heli-deck framework, the roof mounted mast and six diagonal support braces, each weighing in excess of 200 tons (3). An epoxy sealer and three-coat paint system were then applied for aesthetics and finished with a topcoat of 50 microns of polyurethane. The aluminum coating guarantees protection against corrosion in the harsh coastal environment in which the Burj Al Arab Hotel stands.
Examples of Thermal Spray on a Variety of Materials
- Copper coating on plaster sprayed with a WireJet 96 Combustion Wire Gun
2. Silicon bronze wire (yellow) and copper and zinc (bottom portion) on concrete sprayed with a WireJet 96 Combustion Wire Gun
- Plywood with aluminum bronze, zinc and copper coating on plywood sprayed with a WireJet 96 Combustion Wire Gun. Note the contours are still visible through the coating
Where to Start as an Artist
Delving into art using thermal spray is easy. Artists can develop their own form and work with a local spray shop to do the coating or invest in setting up their own shop. Combustion wire systems require an investment of less than $8000, are easy to use and are very versatile as to coating materials. Even some plastic “wire” can be used or cord wire can be applied for special effects.
(1) A Restoration Without End? Brancusi’s Column in Târgu-Jiu, Romania, ArtWatch International, Inc., February 14, 2003
(2) Spray Metal: An Artist’s Prospective” International Sculpture Center journal “Maquette”, May/June 1994
Hard Chrome Alternative for Overhaul Operations
Bill Sibree & Peter Foy, Plasma Powders & Systems, Inc.
Traditionally, chrome plating has been a process often contracted to specialty shops using an electroplating process carried out under strict OSHA controls. Today, many overhaul shops have been able to bring chrome equivalent repair work in house using High Velocity Oxy/Fuel (HVOF) coating technology. The new method replaces a slow process with a coating development of approximately one mil per hour which depends on environmentally unfriendly chrome plating operations using toxic chemicals.
The Benefits of HVOF Coatings versus Hard Chrome Plating
The opportunity to bring chrome plating operations in house is a result of work done twenty years ago by the Hard Chrome Alternatives Team (HCAT) (1). HCAT was an industry and government consortium with members from the United States Department of Defense and United States and Canadian manufacturers. Setting a goal to develop technology to replace hazardous chrome plating operations with HVOF coatings of Tungsten Carbide-Cobalt (WC-Co) and Tungsten-Carbide Cobalt/Chrome (WC-CoCr), the task force adopted a Joint Test Protocol to determine the necessary testing and criteria for success. (Note: chrome in WC-CoCr is not hexavalent chrome which is a carcinogenic associated with chrome plating). Tests included corrosion, wear, fatigue, hydrogen embrittlement and impact resistance. Materials and methods were also prescribed to insure reproducibility. The coalition succeeded in establishing a fully defined and proven procedure that could be adopted by overhaul shops.
Advantages of HVOF coatings over hard chrome plating as documented by HCAT include: (2)
- A hard corrosion resistant surface capable of being applied in-house
- A coating with increased abrasion resistance
- Extended seal life for hydraulic components
- Equal or superior corrosion resistance to chrome plating
- Minimum impact on the fatigue performance of the substrate
- Reduced impact on the environment
- Reduced biological and health and safety concerns
- Simplified masking
Airline repair depots now use HVOF for landing gear overhaul (3). Boeing is using HVOF coatings for 767 and 777 landing gear and has developed repair specifications based on the HVOF process (4). HVOF is also being used for hydraulic rams in heavy construction machinery and drill components for oil and gas exploration. HVOF is being used as a hard chrome replacement in hydraulic and cylinder piston rods (5) diesel engines, turbines, hand or foot pumps, construction equipment, manufacturing machinery and civil engineering. It is also servicing paper mill rolls. The paper and pulp industry relies on HVOF coatings for wear resistance, corrosion resistance, low coefficient of friction, cleanability and traction.
What is needed?
Any overhaul operation that incorporates gas welding in their operations is three quarters of the way there for adding HVOF capability. Gas and air requirements are very similar as is safety training and awareness. Often the same handling equipment used for HVOF operations is the same as used for welding. As with welding, HVOF operations require control of fumes and radiation using proper exhaust systems and shielding. The main expense is the HVOF gun and controller.
What are the limits?
The HVOF process is a line-of-sight process and is not able to coat around corners. The second issue is speed. Thickness per pass depends on the gun and material but generally ranges from 0.0002” per pass up to 0.015” per pass. The coating speed in surface feet per minute can be high. This is no problem when coating a shaft mounted in a lathe but may require a robot when coating large flat areas.
How to proceed?
The starting point is to work with a supplier to jointly review requirements and develop a plan that considers the cost of present plating operations and estimates the cost of incorporating the HVOF process. At the same time, other thermal spray processes for different repair needs can be evaluated.
(1) Report “Replacement of Chromium Electroplating Using HVOF Thermal Spray Coatings” B. D. Sartwell, P. M. Natishan, I. L. Singer Naval Research Laboratory, Washington, DC, K. O. Legg, Rowan Catalyst, Inc., Libertyville, IL, J. D. Schell, GE Aircraft Engines, Cincinnati, OH, J. P. Sauer, Metcut Research Inc., Cincinnati, OH
(2) “Choosing a Hard Chrome Alternative” Keith Legg Rowan Technology Group
(3) Report “Status of HCAT/JG-PP Program on Replacement of Hard Chrome Plating With HVOF Thermal Spray Coatings on Landing Gear” Bruce D. Sartwell, Naval Research Laboratory, Washington, DC, Keith Legg, Rowan Technology Group, Chicago, IL, Philip E. Bretz, Metcut Research, Inc., Cincinnati, OH. Paper published in Proceedings of AESF Aerospace Plating and Metal Finishing Forum, March 2000, pp. 131-139
(4) HVOF Technology Insertion @ Delta Air Lines, Inc., Presentation by: Jay Randolph, Lead / Principal Engineer- Component Engineering & Nick Cortese Senior Engineer – Repair Process Engineering Presented at HCAT meeting, November 18/19, 2003, Kennedy Space Center
(5) Report “HCAT PROJECT- FUNCTIONAL ROD/SEAL TESTING AND QUALIFICATION OF HVOF COATINGS ON NAVY ACTUATORS”, Jeff Gribble & James Magno, NAS Patuxent River, MD, January 24, 2006
Hard Chrome Plating Sealing Systems Solution Industry Breakthrough: The HVAF/HVOF Process
New combination process provides alternative to chrome and other coatings.
When seals and seal systems are mentioned, elastomers and compliant materials come to mind. Seldom is thought given to the backbone of the sealing system, the sealing surface. Although many changes have been made with elastomeric members, alterations are also happening with the sealing surface regarding materials and application methods. One notable change is benefiting the hard chrome industry, including repair and rework shops servicing pulp and paper mill rolls, heavy equipment, oil and gas drilling machinery, landing gear and hydraulic cylinders and pistons.
Chrome and Alternatives
Hard chrome plating on rods has presented a particular problem as typical hard chrome plating includes the production of hexavalent chromium compounds. The National Institute for Occupational Safety and Health (NIOSH) classifies all such compounds as occupational carcinogens. In 1996, the U. S. government formed the Hard Chrome Alternate Team (HCAT), with a goal of eliminating these carcinogens from the workplace. A key driver in this program was The Strategic Environmental Research and Development Program (SERDP), DoD’s environmental science and technology program in partnership with DOE and EPA. The HCAT team included representatives from industry and the US and Canadian governments and was dissolved in 2007 once its objective was achieved, a rarity for government programs.
The HCAT project centered on the “line-of-sight” thermal spray process with a focus on the HVOF system that was the highest kinetic energy process available at that time. Thermal spray covers a group of coating processes where finely divided metallic or nonmetallic materials are deposited in a molten or semi-molten condition to form a coating. The coating stock may be in the form of powder, ceramic rod, wire or molten materials. Many different thermal spray processes are in use: wire arc, wire flame, powder flame, plasma, high velocity oxy/fuel (HVOF) and high velocity air/fuel (HVAF).
A significant advantage of thermal spray coatings is the variety of available coating materials. Materials used in specific applications are chrome carbide, tungsten carbide nickel chrome, tungsten carbide cobalt chrome, stainless steels and metals. The variety of coating materials allows tailoring the process to specific applications in various operating environments where electrolytic hard chrome would not perform well, or in fact, may not be considered at all.
HVOF is used throughout the aircraft industry and in military depots where aircraft are repaired. New aircraft landing gear are designed with an HVOF coating on the landing gear cylinder. Many other major components also use the same technology, including aircraft hydraulic actuators, engines, helicopter dynamic components and propeller hubs. Besides eliminating the problematic carcinogen, hexavalent chrome, from past processes used in repair and rework settings, HVOF coatings last longer and demonstrate less wear and corrosion. Note: though chrome is included in some of these coatings, hexavalent chrome is not produced in the process.
In HVOF guns, a mixture of oxygen and gaseous or liquid fuel is fed into a chamber where ignition and continuous combustion occur. The resultant hot gas exits through a converging–diverging nozzle. The jet velocity is supersonic at the exit of the barrel. A powder feed stock is injected into the gas stream and accelerates up to 800 m/s. The stream of hot gas and powder is directed towards the surface to be coated. The powder partially melts in the stream and deposits upon the substrate with a resulting coating of low porosity and high bond strength.
In HVAF guns, spray powders are heated and accelerated by combustion products of hot compressed air and fuel gas. Fuel gas can be propane, LPG with high content of butane, propylene, MAPP or natural gas.
A mixture of air and fuel gas flows into the combustion chamber through hundreds of orifices of a ceramic insert. Initial ignition of the mixture by a spark plug results in heating of the insert above the auto-ignition temperature of the mixture. The hot insert continuously ignites gases, providing stable combustion in a wide range of air:fuel ratios and gas pressures. Air cools the gun, and this preheated air is used for combustion.
HVAF-applied coatings protect parts, vessels and structures against abrasion, erosion and corrosion and benefit surface restoration, conservation and preservation. HVAF is also used for spraying the internal diameters of pipes in preparation for other processes through the application of a thin layer surface and coating stripping.
Combining the Two Processes
HVAF and HVOF have been merged into a single system (HVAF/HVOF) that offers the highest spray rates in the industry in either mode. The HVAF/HVOF thermal spray system uses air as the primary oxidizer and oxygen to elevate the temperature, thereby optimizing the process with breakthrough results. The addition of oxygen provides all existing HVOF owners and new prospects a reliable and less expensive coating with a high quality. Existing HVOF specifications can be achieved and exceeded using the convertible system. Users can achieve a 0.002-in. thick corrosion-proof coating that holds up under high pressure and can be polished to an RA 4 finish without grinding.
In comparison to the HVOF process, the combined process provides a significant increase in the Vickers Hardness of WC-10Co-4 Cr coatings. Weight loss during silt erosion testing is less than 1/3 of the weight loss of the same coating applied with the traditional HVOF process. Furthermore, the cost per kilogram deposited can be as much as half that of using the HVOF process.
In summary, the HVAF/HVOF system produces very economical, high quality coatings with easy to use, reliable equipment.
- HVOF mode delivers coatings of metals that meet HVOF specs with high spray rates and high DE.
- HVOF mode delivers coatings of hard metals that are extremely dense.
- Cost per kg deposited can be as much as half the cost of using the HVOF process.
- HVAF mode delivers higher quality than state-of-the-art HVOF with much higher spray rates and high DE.
- The equipment produces porous free chrome replacement coatings as thin as 0.002-in. thick. A 0.4 RA on an 86-10-4 coating with polishing can be achieved.