Carbon-Based Nano-composite Films And Their Applications

The rapid development of modern manufacturing requires increasingly high-performance cutting-tools. The cemented carbide has the tendency of replacing high-speed steel in the field of cutting-tools. In recent years, with the development of machining industries towards rapid and continuous processing of complex and small-scale materials, super-fine grain (less than 0.3μm) cemented carbide is increasingly demanded. The strength, toughness and wear resistance of super-fine grain cemented carbide have reached unprecedented levels and the performance of cutting-tools has been largely improved, due principally to grain refinement. However, grain refinement is becoming more and more difficult and coating technologies are in many cases the only ways to further enhance the performance of cemented carbide tools. At present, the comprehensive performance of cemented carbide is increasingly improved and the processing conditions for cutting-tools are more stringent, so traditional hard films such as TiN, TiAIN, and Ti/TiN cannot meet the requirement for super-fine-grain cemented carbide. Therefore, there is an urgent need for new films to match the super-fine grain cemented carbide.Diamond-like carbon (DLC) film has low friction coefficient and can be the first choice for surface modification for super-fine grain cemented carbide. However, the application of DLC film has been restricted by it's low thermal stability. This paper aimed at designing and preparing carbon based nanocomposite films for super-fine grain cemented carbide surface modification. Composite films with hard MeN (Me=Cr,V,Zr) crystalline phase embedded within DLC amorphous matrix are obtained by doping Me and V simultaneously. The Me and N co-doped DLC composite films possess high hardness, low friction coefficient, and high thermal stability as compared with single DLC films. The contents of this thesis are divided into sections as follows:(1) First, several groups of CNX films were prepared on super-fine grain cemented carbide substrate using pulsed bias arc ion plating by controlling the process parameters. For the CNχfilms deposited at different nitrogen flow rates, the hardness and elastic modulus of the film increase and reach a maximum values of 32.1 GPa and 411.8 GPa as the nitrogen content increases to 8.1%. However, a further increase in the nitrogen content will rapidly decrease the hardness and the elastic modulus of the films.(2) Transition metal element Cr was added into CNχfilms to prepare C-N-Cr film. The effect of nitrogen flow rates on the microstrucute and properties of the films were investigated. The nitrogen content increases linearly with the nitrogen flow rate from 5 to 50 Sccem, and then increases slowly with further increase of the nitrogen flow rate from 50 to 100 Sccm. The Cr content first keeps stable when the nitrogen flow rate is not more than 20 Sccm and decreases linearly with further increase of the nitrogen flow rate from 20 to 100 Sccm. The structures vary with composition. CrN peak appears in the film and diamond-like feature is not obvious when nitrogen flow rate is not more than 20 Sccm, above which no peaks appear and diamond-like feature is obvious. The C-N-Cr films have high hardness (>30 GPa) and elastic modulus (>500 GPa) when the nitrogen flow rate is not more than 20 Sccm, above which the hardness and elastic modulus decrease drastically and only have the values of 13.6 GPa and 190.8 GPa when the nitrogen flow rate is 100 Sccm.(3) Transition metal element Zr was added into CNχfilms. First of all, C-N-Zr composite films with different compositions were deposited on super-fine grain cemented carbide substrates at different nitrogen flow rates by pulsed bias arc ion plating. The film surfaces were all uniform, smooth and dense. The C contents are more than 60% and the N content increases and the Zr content decreases with increasing nitrogen flow rate. The Raman spectra indicated that the deposited films were typical diamond-like carbon. XRD results suggested that an ZrN crystalline phase was also present in the films. The hardness and elastic modulus were closely related to the composition and structure of the films and decrease with increasing nitrogen flow rates, principally due to the increase in the sp2 content and the decrease in the ZrN crystalline phase. Then a series of C1-χ-yNχZry composite films were deposited on a cemented carbide substrate by adjusting the nitrogen flow rate and the Zr target arc current simultaneously while fixing the graphite target arc current. The Zr and N contents in the films increase linearly while the C content shows an inverse trend as the Zr target arc current and nitrogen flow rate increase. The hardness first increases and then decreases with increasing Zr and N contents and reaches the highest superhard value of 43.6 GPa atχ=0.19, y=0.28. The variation of Zr and N contents have significant influence on the phase structure, relative concentration of ZrN crystalline phase and DLC amorphous phase, hence has great effect on the properties of the films.(4) V and N co-doped DLC films were deposited on cemented carbide substrates by pulsed bias arc ion plating. The effects of V and N co-doping on the microstructure and properties of the as-deposited films were investigated. It has been found that V target arc current and nitrogen flow rate have significant effects on the composition and microstructure of the films, including the V and N contents, Raman intensity, VN content and grain size, formation of nano-diamond, and hence dominate the hardness and elastic modulus of the films. The Raman spectra, GIXRD and HRTEM results indicated that the deposited films were DLC-VN composite films with VN nanocrystalline phase imbedded within DLC amorphous matrix. The hardness and elastic modulus, which closely related to the composition and microstructure of the films, first increase and then decrease with increasing V and N contents and have the maximum values of 36.8 GPa and 569.7 GPa respectively when the N content is 20.4% and the V content is 21.8%. The nano-diamond also contributes to the high hardness of film. The VN nanocrystallite present in the films could act as a catalyst to promote the formation of nano-diamond, which lead to an increase in the hardness and elastic modulus of the C-N-V film.(5) The super-fine grain cemented carbide coated with C-N-Me films shows lower coefficient of friction. The films investigated in this work were applied in industry. For the copper tube peeling mould fabrication, scratches on the tube surface are often produced when using uncoated cemented carbide mould. The tubes fabricated using moulds coated with CNχfilm showed uniform and bright surface and the service life of the mould increased by three times. The extruding tests were carried out on uncoated and coated super-fine grain cemented carbide dies and the results showed that the service life of the C-N-Zr film coated die increased by at least two times. The drilling number of super-fine grain cemented carbide micro-drill coated by C-N-Zr film was 5005, while that of the uncoated micro-drill is only 1083.