Japanese researchers improve coating technique
31 Aug 2012
A research group at the National Institute for Materials Science (NIMS) in Japan have improved a coating technique called warm spraying.
The team 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 similar processes.
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 in a flat shape, create depressions in the substrate, and are repelled.
However, NIMS research found that if the impact velocity exceeds a certain value, large shear plastic deformation is generated locally at the interface between the two, oxides and other substances on the surface are removed.
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.
As part of the current study, 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.
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 30?m achieved 1,000m/s under appropriate conditions.
These results exceed the results of film forming using helium, which is an expensive working medium, in the cold spray process.
