Study On The Deposition, Tribological Properties And Applications Of CVD Diamodn Films Coated Ceramics

Co cemented tungsten carbide (WC-Co) material has been widely used in cutting tools, drawing dies, drills, mills, etc. due to its high hardness, wear resistance, thermal rigid, corrosion resistance, anti-oxidation resistance and other characteristics. But the production of WC-Co consumes large quantities of tungsten, cobalt, titanium, nickel and other important strategic resource. Structural ceramics have high strength, hardness, wear resistance, corrosion resistance, anti-oxidation, small creep characteristics at high temperatures, and wide variety of sources, which makes it become the most promising alternative materials of WC-Co. However, the biggest drawback of structural ceramics is that its poor plastic deformation ability and low toughness make it difficult to form. For structural ceramics, once made, its microstructure is difficult to be improved through the deformation. In particular, its holes, micro-cracks and harmful impurities can not be deformed or eliminated. Chemical vapor deposition (Chemical Vapor Deposition, CVD) diamond film has high hardness, elastic modulus, thermal conductivity, low friction coefficient, good chemical stability, etc. Therefore, it is a protective coating material. Ceramic material, with thermal expansion coefficient close to the one of diamond, can reduce the thermal expansion stress between the CVD diamond film and ceramic substrate, and thus produce good bonding between them. Moreover, CVD diamond film can not only fill surface defects of ceramic material but also increase its surface hardness, lower coefficient of friction and improve wear-corrosion resistance. In this paper, based on ceramic substrates, CVD diamond deposition process, surface topography, tribological properties and practical application will be studied. Main contents of this article are as follows:1. Hight quality CVD diamond films are deposited on ceramic substrates. Conventional micro-diamond (MCD) films, fine-grained diamond (FGD) thin films and composite micro-diamond (PMCD) films are deppsited on silicon nitride substrates,using the hot filament CVD (HFCVD) method. The MCD film has many spikes on the surface and its grain size is about 2 ~ 3μm. Diamond grains of the FGD film, which are very small, get together and form clusters. The FGD film has a rather flat surface. The PMCD film has a more smooth surface than the MCD film and its grain size is 2 ~ 3μm. The Ra values of MCD, FGD and PMCD films are 275, 131 and 210 nm, respectively. The MCD and PMCD diamond films have high purity; while the FGD film, in addition to diamond, also contain a certain amount of graphite.2. Tribological properties of the silicon nitride substrate, MCD film, FGD film and PMCD film are evaluated on a ball-on-plate reciprocate tester, using silicon nitride, WC-Co, ball-bearing steel and copper as counterpart materials. The tribological influence of films'surface morphology, films'quality and counterpart materials is revealed. For sliding against WC-Co, the FGD film presents the best overall performance, namely the lowest coefficient of friction and very good wear resistance. However, for sliding against the silicon nitride substrate, although the FGD film produces the lowest friction coefficient, it suffers very serious wear and tear; while the PMCD film presents low friction coefficient (~ 0.07) , which is very close to the friction coefficient (~ 0.06) of the FGD film, and the surface wear of the PMCD film is almost negligible. For sliding against ball-bearing steel, the PMCD film shows the smallest friction coefficient and no wear or tear is discovered on its surface. There is no clear distinction among the friction coefficients of CVD diamond films against copper.3. The cutting performance of as-deposited diamond coated silicon nitride inserts is evaluated in dry turning glass fiber-reinforced plastics materials. Compared to the MCD film, the lower friction coefficient of the FGD film results in smaller friction force and delays the shedding of the diamond film. In consequence, the MCD and FGD coated inserts present as approximately 2.5 and 5 times lifetime as the uncoated one.4. CVD diamond films are deposited on the inner surface of silicon carbide (SiC) drawing dies using the HFCVD method and as-fabricated CVD diamond film coated SiC drawing dies are used in processing coaxial cables and copper tubes. In the coaxial cable processing, the lifetime of CVD diamond coated drawing dies is as 7~13 times as WC-Co drawing dies. In the copper tubes processing, the lifetime of CVD diamond coated drawing dies is as 10 times as WC-Co drawing dies. In addition to substantial increase in the lifetime of drawing dies, the use of CVD diamond-coated drawing dies can also improve the surface quality of the coaxial cables and copper tubes, increase productivity and save raw materials.