The Joint Research Group of the Metal-based Functional Materials Technology Group of the Institute for Structural and Engineering Materials (ISEM) at the National Institute of Advanced Industrial Science and Technology (AIST), the Institute of Fluid Science Tohoku University and Japan Basic Material Co., Ltd. (Sendai) has developed a mirror-polished CVD diamond slider with a low friction coefficient close to air-floating conditions. A coating technique has been established for CVD diamond film onto machinable titanium silicon carbide. This technology affords a much improved degree of freedom in selecting the shape of the substrate.
Most sliding surfaces use a combination of a lubricant oil and a soft bearing alloy, an oil retaining bearing or rotary bearing. In contrast, solid lubricants without the use of lubricant oils and bearings have the advantage of noise-free operation and of being suitable for use in vacuum as well as high- and extremely low-temperature conditions. Their use has a promising potential for sliding surface applications in the 21st century. The current solid lubricants include Teflon, graphite, ceramics, and diamond-like carbon (DLC).
The newly developed sliding surface can achieve a lower friction coefficient than Teflon. This is the result of newly developed techniques for controlling both the CVD diamond coating and polishing conditions in order to create a surface on which mirror polished parts coexist with microscopically uneven parts. From experience with single-crystal diamond, it is well known that diamond has a very low friction coefficient of only around 0.05. However, the polishing of a CVD diamond has certain drawbacks: (1) It is a laborious procedure; (2) poor sliding performance when two mirror surfaces come in close contact with each other as they will stick together as a result of the vacuum and inter-atomic forces. These are some of the reasons why a CVD diamond has not been applied to sliding surfaces.
The Joint Research Group has been successful in achieving outstanding sliding properties by controlling the particle growth of the CVD diamond and achieving polishing conditions permitting the coexistence of mirror-finished areas with microscopically uneven parts.
The superior sliding performance is attributed to the mixed lubricant effect resulting from a combination of the solid diamond lubricant and the fluid lubricant brought about by the intervening air. The joint research results have shown that it is also possible to use a machinable titanium silicon carbide as the substrate for diamond deposition. Until the present, fabrication of CVD diamond has only been possible on flat substrate surfaces or for small cutting tools. With the new technology, however, it will be possible to use this process for coating any shape of sliding surfaces. In the past, the suitable materials as substrates for CVD diamond were limited to brittle silicon or very hard silicon carbide and silicon nitride, or tungsten carbide that are heavy and difficult to machine.
The newly developed diamond sliding surface essentially presupposes the use of a diamond-to-diamond combination. At low contact pressures, however, it is also possible for the diamond surface to mate with a metal surface, since the diamond film will provide a smooth air-floating like sliding performance also with machined surfaces such as stainless steel.