Development of coating technique by high speed particles with velocity of 1,000m/s
Coating (formation of a film on a material) is an extremely important technology for modern industry, as it dramatically improves the heat resistance, corrosion resistance, and wear resistance of materials, essentially creating new materials with performance that did not exist in the past. In this work, a research group headed by Dr. Seiji Kuroda, Unit Director of the High Temperature Materials Unit, National Institute for Materials Science (NIMS), a team led by Dr. Naoyuki Ohno, Director of the Engineering Department of Plasma Giken Co., Ltd., improved the warm spray method, which is a NIMS original coating process, and increased the velocity of the sprayed particles projected on the substrate material to 1,000m/s by achieving a combustion pressure 4 times higher than that in the conventional process. The improved process enables formation of high quality titanium alloy coating films, which had been difficult with the conventional technique.
When solid metal particles impact on a substrate material at high velocity, the particles are deformed to a flat shape, create depressions in the substrate, and are repelled. However, if the impact velocity exceeds a certain value, large shear plastic deformation (shear instability) is generated locally at the interface between the two, oxides and other substances on the surface are removed, and bonding occurs between the particles and the substrate. This technology was discovered in Russia in the 1980s, and a process called cold spraying, which employs this phenomenon, is now attracting attention. NIMS discovered that densification of the film can be promoted by further heating the particles to an appropriate temperature below their melting point, and named this process warm spraying in 2006. NIMS is continuing joint research and development of this technology with Kagoshima University and Plasma Giken Co., Ltd.
In the present research, Associate Prof. Katanoda, who is an expert on compressible gas-dynamics, created the basic design of the device with the aim of achieving a higher particle velocity. Plasma Giken was responsible for the actual design and manufacture of the device, and NIMS carried out the verification experiments. When the average velocity of the sprayed particles was measured by particle image velocimetry, it was found that the velocity of titanium particles with a diameter of 30μm achieved 1,000m/s under appropriate conditions.
The developed process was applied to a Ti-6Al-4V alloy, which is a type of high strength titanium alloy and is widely used in aircraft engines and similar applications. Under the optimum conditions, it was possible to obtain an alloy film with porosity of less than 1vol% and an oxygen content of 0.25 mass% (content in the raw material powder: 0.15 mass%). These results exceed the results of film forming using helium, which is an expensive working medium, in the cold spray process.