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Researchers Develop a Coating Technique by High Speed Particles with Velocity of 1,000m/s

Published on 2012-08-31. Author : SpecialChem

A research group in the NIMS High Temperature Materials Unit, in joint work with Kagoshima University and 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.

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, and Dr. Makoto Watanabe, Senior Researcher, and Dr. Hiroshi Araki, Chief Engineer, of the same unit, in joint research with Associate Professor Hiroshi Katanoda of Kagoshima University and 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.

These research results were announced at the 95th Spring National Meeting of Japan Thermal Spray Society held at the Rijyo Kaikan in Hiroshima, Japan.

About National Institute for Material Sciences

NIMS is a research institution specializing in materials research in metals, organic or inorganic materials. The world is made up of various "substances" and in these "materials" the basis of our everyday lives can be found. Materials fall into two major categories such as organic/polymeric materials and inorganic materials, the latter in turn being divided into metals and ceramics. From the Stone Ages - by way of the Industrial Revolution - up to today, the advance in materials has contributed to the development of humankind and now it is being focused upon as offering a solution for global problems. NIMS specializes in carrying out research concerning these materials. Their materials research is managed with the next generation within perspective, in line with their theme, "Materials research for creating tomorrow".

Source: National Institute for Material Sciences


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