| Combining high strength and ductility of matrix with high hardness and abradability of coating, coated cutting tools are widely used for high rate and accuracy automatic process. Due to their great potential and broad prospect in high-speed cutting field, coated cutting tools have great economic benefits. In this paper, TiN-TiCN-Al2O3-TiN multi-layer coated carbide tools were studied. Deposition thermodynamics, fluid mechanics, deposition dynamics of coating, the microstructure of multi-layer coating, diffusion of elements in multi-layer coating, residual stress distribution, interface cohesion and oxidation resistant were investigated in details. The conclusions were as followings:1)TiN-TiCN-Al2O3-TiN multi-layer composite coated carbide tools were investigated, relation between Gibbs free energy and temperature of sub-coating as well as fluid mechanics characteristic parameters of tray, such as:diffusion length, Peclet, Thiele length, Thiele modulus, Reynolds, Damkohler and Knudsen were calculated, deposition thermodynamics of MT-TiCN and Al2O3coatings was discussed. The results showed that the deposition of MT-TiCN coating was controlled by surface reaction kinetics in860℃-890℃, activation energy was calculated to be254KJ/mol. The deposition of Al2O3coating was mainly controlled by formation of water-vapor. A guide for design of processing parameters was supplied by calculating thermodynamics, fluid mechanics and dynamics.2) TiN/MT-TiCN/Al2O3/TiN multilayer composite coating were designed and prepared, and the microstructure of every sub-coating in multilayer system were studied in detail by SEM, TEM, EDS and SIMS. The results indicated that the growth pattern of TiN transition layer in WC grain was primarily epitaxial growth, whereas that in Co grain was primarily nucleation growth. MT-TiCN coating which the content of C and N was gradient distributed were designed and prepared, and the microstructure of MT-TiCN coating in different growth stage was obviously different. TiCO coating was designed and prepared inside MT-TiCN coating and Al2O3coating. Al2O3was nucleated renewedly, and the growth pattern of Al2O3coating was cylindrical polycrystalline growth.3) The diffusion behavior of elements in matrix/multilayer interface was discussed. The results showed that Co in matrix diffused along grain boundary inside the coating, whereas the diffusion pattern of W was grain boundary diffusion and volume diffusion, and the diffusion distance of W inside the coating was larger than other elements. The diffusion behavior of Ti and Al in TiCO/Al2O3interface indicated that the diffusion distances of Ti and Al in TiCO/Al2O3interface were both short, and they even did not diffuse.4) The residual stress distribution, fracture energy, critical thickness of crack in the coating, critical crack stress and critical temperature of TiN/MT-TiCN/Al2O3/TiN multilayer composite coating were calculated. The residual stress was measured by XRD analysis. The results demonstrated that small difference of thermal expansion rate between matrix and coating can form relatively large thermal stress. It is the primary methods that the improvement of critical thickness, critical crack stress and critical temperature of crack in the coating can be improved by enhancing the ductility of the matrix and fracture energy of the coating. The relaxing of residual stress was studied when the crack of coating happened. The results pointed out that sand blasting and shot peening can effectively increase the crack density, and release the residual stress of the coating.5) The bond strength of mutil-layer coating was investigated by scratch test, and the morphology of sample scratched was also analyzed. The results indicated that good combination between mutil-layer composite coating and cemented carbide matrix was formed. The influence of component of matrix and combination modes of coating on the bond strength was discussed. The bond strength was affected obviously by particle size of WC, contents of Co, addition of cubic carbide and the formation of cobalt-rich layer. The bond strength decreased clearly with the increasing of coating thickness and the formation of Al2O3coating. The strength increased in indentation test caused by the substitution of TiC in TiN/TiC/Al2O3/TiN multilayer with different matrix. There are various kinds of fracture form exist in multi-layer composite coating, and fracture was not only formed between coating and matrix but also exist in each sub-coating.6) The oxidation resistance of four kinds of multilayer coating sample was studied under the condition of600℃-950℃and30min-120min. Compared with the oxidation of cemented carbide, the activation energy of coating sample is lower and was controlled by diffusion dynamics more easily. Hence, coating sample exhibiting much better oxidation resistance compared with cemented carbide matrix. Both internal and external of multilayer coating without Al2O3were oxidized, and furthermore, the oxidation of Al2O3sub-coating was processed along with the heating crack in surface. Dense and integrated Al2O3sub-coating can protect the multilayer coating and matrix effectively, which also improved the oxidation resistance obviously.
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