Thermal Spray Coatings: Part Two

Arc Spray Process

The arc spray process is the fastest, least complicated, and most inexpensive, and efficient way to produce metal coatings. A wire feeder pushes two wires through the arc spray gun. The heat created by the arc melts the wires. Compressed air propels the molten particles onto the prepared component.

Arc spraying can apply three basic types of coatings:
• Wear-resistant coatings that resistant abrasion, corrosion, erosion, fretting, friction or galling
• Coatings for rebuilding worn areas, salvaging improperly machined parts, or improving the characteristics of finished parts
• Corrosion resistant coatings such as zinc, aluminum, aluminum magnesium, and zinc aluminum.

Flame Spray Process

The flame spray process uses the heat generated by the controlled ignition of a combustible fuel gas to melt the wire. The heat zone created by the gas ignition melts the wire and the compressed air blows the molten particles onto the substrate. The molten metal impinges the substrate and cools to the ambient substrate temperature.

Applications for flame spray process are similar to those of arc spray. Although flame spray is not as efficient as arc spray, it does offer the advantage of being more portable than most arc spray equipment. It can be used in the application of ceramics and fusible materials and is less capital intensive than the arc spray process.

Plasma Arc Spray Process

The plasma arc spray process uses a hot ionized gas (plasma) as the heat source to melt powdered materials. Plasma system provides controllable temperature higher than the melting range of most substances.

In the plasma process a gas or gas mixture passes through an arc created between a coaxially aligned tungsten cathode and an orifice in a copper anode. The gas partially ionizes during the heating process and produces plasma. Injected into the plasma, the powder melts and the high velocity plasma stream propels it onto the substrate. The type of nozzle, arc current, gas mixture ratio and gas flow rate control the heat content, temperature and velocity of the plasma stream. They are operated on direct current from a rectifier-type power supply. A central control unit regulates electric power to the arc, the plasma gas, the flow of cooling water and sequences these elements to allow the process to be initiated reliably and with precision.

The primary plasma-forming gas is either nitrogen or argon. A secondary gas, either hydrogen or helium, may be added to increase the heat content and velocity of the plasma.

Plasma spray is used to form deposits of greater than 50 micrometers of a wide range of industrial materials, including nickel and ferrous alloys, refractory ceramics, such as aluminum oxide and zirconium-based ceramics. For high performance applications, in order to approach theoretical bulk density and extremely high adhesion strength, plasma spray is carried out in a reduced pressure inert gas chamber - vacuum plasma spray (VPS), which operates at pressures of approximately between 50 and 200 mbar, or through the use of a shrouded flame, where, for example, argon or nitrogen excludes oxygen from the vicinity of the flame and the work piece. Shrouding, however, has no known effect on particle velocity, whereas gas and particle velocity are significantly increased within the reduced pressure chamber. This, as for HVOF, yields a higher density deposit. The benefits of reduced pressure and shrouded plasma spraying will increase substantially in the future.

The key features of plasma spraying are the following:
• Deposits metals, ceramics or any combinations of these materials
• Forms microstructures with fine, equiaxed grains and without columnar boundaries
• Produces deposits that do not change in composition with thickness (length of deposition time)
• Can change from depositing a metal to a continuously varying mixture of metals and ceramics (i.e., functionally graded materials)
• High deposition rates (>4 kg/hr)
• Fabricate free-standing forms of virtually any material or any materials combinations
• Processes materials in virtually any environment; e.g., air, reduced pressure inert gas, high pressure.

Vacuum Plasma Spray Process (VPS)

VPS is in large extent displacing EB-PVD (electron beam - physical vapor deposition) for the production of high quality metallic (MCrAlY) coatings. The compositional flexibility afforded by VPS and the high coating rates achieved though liquid droplet transfer versus the limitations of evaporation in EB-PVD caused a major shift to VPS during the 1980's.

As with all thermal spray processes, VPS is limited to line-of-sight. With the VPS process, individual parts are fixtured on a manipulator within a load-locked transfer chamber. The load-lock is pumped down and the parts are preheated to about 900-1000ºC before being transferred to the coating chamber. Prior to initiation of the plasma spray, the part is usually treated through reverse transferred arc sputtering to remove any traces of oxide that may have formed during preheat. The part is then plasma sprayed in a non-transferred mode. The coating distribution is determined by computer controlled gun and part motion. Powder feed rates vary from ~3 to 5 kg/hr depending on the application.

Of further importance is the ability of VPS to process oxygen sensitive materials, such as reactive metals and intermetallic compounds. For example, considerable work has been carried out on the VPS processing of nickel aluminides and molybdenum disilicide, which have potential uses in the aerospace industry. It was demonstrated that the VPS process was capable of producing dense, free-standing forms which showed impressive mechanical properties. The deposits were ultra-fine grained and illustrated the capability of VPS in manufacturing of rapidly solidified intermetallics.

Some studies have been published on the VPS processing of composites based on Ni₃Al and MoSi₂. High density deposits are obtained and some promising toughness increases are found. There is a clear important potential for VPS in the processing of intermetallics as both protective coatings and as free-standing forms.

At the other end of the thermal spray spectrum is the limitation in obtaining thin films. A major shortcoming of traditional thermal spray technology has been the thinness to which a deposit could be formed. Fine-sized feedstock particles would be required for the production of thin film deposits (< 1 micrometer). To force fine particles into such a flame would require increasing the carrier gas pressure, leading to increased flame turbulence, and thus to a disturbed particle trajectory. These problems, as well as others, could be largely solved by using axial injection of the materials feedstock. New axial-feed plasma gun designs can avoid many of the limitations imposed by non-axial plasma devices.

A synthesis of the traditional two-wire electric arc method and the plasma arc is a single-wire plasma spray gun, which uses a single metallic conductor wire to deposit coatings displaying harnesses and wear resistances of high performance plasma (powder) deposits. While mainly recognized by and being evaluated for the automotive industry, there is no question that wear coatings of high temperature materials could be built-up using this and related processes. For example, it would be interesting to evaluate this technique with cored wires (e.g., NiCr/Cr₃C₂).

High Velocity Oxygen Fuel Spray Process

High Velocity Oxygen Fuel (HVOF) systems are relatively new in the thermal spray industry. HVOF systems combine supersonic spray velocities with improved powder particle heating and melting characteristics. These systems produce high quality coatings with excellent density, hardness and bond strength.

Various fuel gases including propylene, hydrogen, acetylene, propane and MAPP can be used. This permits selection of the most economical gas to produce the required coating characteristics. Materials with high melting points, like ceramics or refractory metals such as molybdenum, can be sprayed with some HVOF systems. Industries are finding many new uses for these systems, with the most notable being the application of tungsten carbide-cobalt coatings.

The HVOF process is versatile and offers premium coatings for wear resistance, corrosion protection, thermal and electronic insulation. However, of particular importance is the fact that HVOF hard facings (e.g., WC/Co, Stellite) have harnesses and wear resistances superior to such materials plasma sprayed in air.

Spray parameters can be regulated over a wide range to provide the best temperature and velocity for specific coating applications. High deposition rates (6-7 kg/h for many materials) shorten the spray time on large parts. As-sprayed surface finishes below 5 micrometers are common with HVOF systems, and many coatings can be finished below 0.2 micrometers. The aircraft industry has expressed an interest in the HVOF process.