ONLINE EXCLUSIVE: Improving Surfaces with HVOF-Sprayed Coatings
Many different processes can be used to produce ceramic coatings over bulk materials. For thin-film coatings, physical vapor deposition and chemical vapor deposition methods are often used. For thick-film coatings, thermal spray techniques such as air plasma, wire arc or flame spraying are typically the application methods of choice.
High-velocity oxygen-fuel (HVOF) deposition is the latest commercial thermal spray development and produces the highest quality coatings of the thermal spray processes. Ceramic oxide coatings applied through the HVOF process are currently used to provide wear and corrosion resistance, as well as thermal protection in heat-sensitive applications. Applications are also being found in the semiconductor equipment, electrical and electronic, and sensor industries. As more is learned about this efficient, high-performance coating method, additional applications are likely to emerge.
Hypersonic CombustionHVOF spraying is a hypersonic combustion flame process through which ceramic materials can be built up on bulk materials to develop engineered coatings. The coatings are formed by particles that are heated through convective heat transfer from a hot flame and accelerated to high velocities by the drag forces associated with exhaust gases. One of the key advantages to thermal spraying with HVOF is that the coatings are produced without overheating the substrate.
Originally, the only way to produce hypersonic coatings was with a detonation gun, which was a proprietary device developed by Union Carbide Corp. several decades ago for in-house coating services. In the detonation gun process, the coatings are formed by explosion-initiated shockwaves. The gun essentially consists of a water-cooled barrel several feet long and about 1 in. in diameter with associated valves for gases and powder injection. A mixture of oxygen and fuel gases and a charge of powder are fed into the barrel, where a spark is used to ignite the mixture to produce a detonation wave. The barrel heats and accelerates the powder charge onto the component to be coated. Typical particle velocities are reported to reach 730 meters per second (m/s).1 The process is intermittent in operation (8-10 times per second) and must be repeated many times for a substantial coating layer to be deposited.
In recent years, the thermal spray field has seen the introduction of commercially available hypersonic flame spray guns (see Figure 1). Unlike earlier technologies, the HVOF method is a continuous process that enables thick coatings to be formed very quickly.
The fundamental idea behind HVOF technology is the formation of an internal combustion jet exiting through a relatively long water-cooled nozzle. Oxygen, along with gaseous and/or liquid fuel (hydrogen, propylene, MAPP, natural gas, etc.), is fed through inlet ports into a closed combustion chamber, where combustion of the reactants occurs. The hot, high-velocity gas passes through a nozzle feed port where material is injected.2 The combustion product is then accelerated through a water-cooled constricting nozzle, forming the hypersonic jet.
The exact behavior of an HVOF spray system depends on the gun hardware configuration, processing parameters, gas pressures and flows, cooling water effectiveness, powder characteristics, and the powder feed injection method. However, HVOF processes typically generate gas velocities of 4900-6500 ft per second (fps)-about 4-5 times the speed of sound-and powder particle velocities (the speed at which particles of coating material travel during their flight from the spray gun to the part being coated) of 1800-2600 fps.
The particle velocity is a crucial factor in producing the coating's microstructural and physical properties. Higher particle velocities generally yield lower-porosity coatings with higher hardness and greater tensile adhesion strength. With HVOF technology, the particle velocity is several times higher than with any other thermal spray process.
When using propane (C3H6) or hydrogen (H2)-gases that are known to produce dense, low-porosity coatings-the flame temperatures of the HVOF process are typically 4800-5200 degrees F. By using acetylene, the highest-flame-temperature fuel gas commercially available, the process can be successfully used to spray high-melting-point oxide ceramics, including chromium oxide.
Properties of HVOF Sprayed CeramicsHVOF coatings are formed by the impingement and solidification of ceramic particles as they are deposited in successive layers. Each layer consists of several lamella (thin layers) deposited on top of each other. The solidification of these lamellae depends on the particle size, velocity, temperature, substrate surface conditions and physical properties of the impinging ceramic material.
A variety of microstructure and mechanical tests are used to determine coating properties. The coating microstructure is typically evaluated using optical microscopy and scanning electron microscopy. Mechanical properties such as tensile adhesion strength and coating hardness are evaluated using ASTM C633-69 and ASTM E-384 guidelines, respectively.
Test results on two different ceramic coatings produced using an HVOF system* are provided in the following paragraphs.3 The spray conditions for the HVOF system used in these tests are provided in Table 1.
*Jet Kote™, manufactured by Stellite Coatings, Goshen, Ind.
MicrostructureFigure 2 is an SEM image of the HVOF sprayed sintered alumina-40% titania alloy, and Figure 3 is an SEM image of the coating produced from a composite material. Both images show a very dense, well-adhered deposit.
Hardness and Tensile AdhesionThe hardness of the coatings was evaluated using Rockwell 15N indenter and diamond pyramid hardness (DPH) testing equipment with a 300-gram load. Hardness was also compared to conventional APS deposits. The hardness data on both the APS and HVOF sprayed coatings are shown in Table 3. All of the coatings showed similar hardness on the order of RN 89-91; however, the DPH readings of the HVOF sprayed coatings were slightly lower than those of the APS coatings, possibly due to greater melting in the APS process.
Pin-on-Disc WearPin-on-disc wear was also performed on both the HVOF and APS alumina-40% titania coatings. The load produces stress-relieving subsurface microcracks that eventually lead to spalling of the material. Initially, the ceramic materials showed virtually no wear, but wear steadily increased as the spalling occurred. In all cases, the HVOF sprayed coatings outperformed the APS coatings.3
Other Test ResultsJ. Beczkowiak4 and others have successfully sprayed chromia, alumina, alumina-titania alloys and zirconia with HVOF and detonation gun systems using acetylene to provide a higher energy flame to the higher melting point oxides. The coating hardnesses produced with HVOF using acetylene are shown in Table 5.
A number of other tests on ceramic oxides using a variety of commercially available HVOF systems have also been performed, all with positive results. In general, the coating hardness is the highest with chromia and alumina coatings, with the latter showing only mild wear in unlubricated pin-on-disk testing. (Alumina-titania coatings are not as robust due to their high titania content.) In most cases, the coating hardness is comparable to monolithic ceramics.
The superior characteristics of HVOF sprayed oxide ceramic coatings make them candidates for a variety of functional applications that require high wear and corrosion resistance (Al2O3, Cr2O3), thermal shock (Al2TiO5), dielectric strength (Al2O3) and other high-performance properties.