| With the rapid development of high-tech industries such as aeronautics,astronautics and nuclear power etc.,the advanced tool coating materials and technology are required urgently and rigorously for machining high-strength Al,Ni,Ti alloys and others in high speed and large power.On these harsh conditions,the cutting temperature often rises to 700°C,the blade and workpiece are inevitably subjected to high temperature,varying temperature and oxidation environments,consequently the traditional coatings can't maintain a low friction coefficient and wear rate,which seriously restricts the improvement of metal processing quality and the development of high performance cutting machine.Therefore,the developing of excellent coatings self-lubricating in wide temperature range has significant theoretical and practical values for accelerating the advancement of high-performance cutting tool and machine of our country.This dissertation developed VN base self-lubrication composite coatings based on multi arc ion plating technology,and studied the yield rules of VN base composite coating/substrate system in different coating thicknesses and elasticity moduli by numerical method,then explored the friction and wear characteristics of VCN and VN/Ag coatings in wide temperature range,furthermore revealed the influence of working conditions on tribology characteristics of VCN coating.The plane strain model of VN base composite coatings in micro contact with a rigid plane were established,and the critical yield features of VN base coating/substrate system were researched under normal compression.It turned out that the plastic inception of super thick coatings was located in a certain depth below contact center,and kept a monotone increasing relation vs.Poisson's ratio.The critical contact parameters were derived for the super thick VN base composite coatings on the onset of plasticity.For a thinner coating,the yield rule of VN base coating/substrate system was studied in different coating thicknesses and elasticity moduli,the results showed that the plastic zone or brittle crack could be produced in coating,substrate or at their interface with the variation of parameters,moreover,the yield resistance can be reduced by increasing the difference of elasticity moduli between the VN base composite coatings and substrate.The VN and VCN coatings were prepared by evaporating V targets in N2 and C2H2 atmosphere,the microstructure studies indicated that C element formed sp2C-C and sp3C-C hybrid bonds in VCN coating,which changed the morphology,crystal orientation and grain sizes of VCN coating,and increased the hardness and elasticity modulus of VCN coating,but decreased the binding force between the coating and substrate.The tribology characteristics of VN and VCN coatings were studied under the temperature of 25°C,300°C,500°C and 700°C,it was found that the hybrid bonds reduced the friction and wear of VCN coating in 25°C~500°C range,but lost efficacy at 700°C.The fluctuations in N2 discharge influencing the thickness of VCN coating were considered,it was found that the flow rate of 350 sccm satisfied the gas consumption in coating synthesis,when the substrate bias was-50 V,C2H2 flow rate was 60 sccm and target current was 60 A,furthermore,the coating thickness was little changed with the flow enhancement of N2 discharge.A L4(23)orthogonal experiment was designed to study the influence mechanisms of load,temperature and frequency on the tribology behaviors of VCN coating,the favourable working conditions were obtained for VCN coating to get excellent tribology characteristics.The self-lubricating VN/Ag composite coating was prepared by evaporating V and Ag targets in pure N2 gas,it was found that Ag element formed nanocrystals in VN lattice,which increased the surface roughness,and changed the crystal orientation of composite coating.In addition,the VN/Ag coating got a higher bonding force,but a lower hardness and elasticity modulus in compare with VN coating.The friction and wear characteristics of VN/Ag coating were researched in temperature range of 25°C~700°C,demonstrating the Ag nanoparticles improved the tribological properties of composite coating under room temperature and 300°C,nevertheless the oxidation products such as V2O5,Ag3VO4 and Ag VO3 and so forth became the major lubrication substance under 500°C and 700°C.The further investigation was carried out on the structure,mechanics properties and friction wear characteristics in wide temperature range of VN/Ag multilayer coatings with heterogenous and transition interfaces,which showed that the Ag content of transition interface was 48 wt.% lower than that of heterogenous interface,but the corresponding V content grew by 71 wt.%.Moreover,a higher load-carrying capacity and failure resistance were demonstrated for the VN/Ag multilayer coating with transition interface.Tribology investigations showed the coating with transition interface displayed a lower friction coefficient at room temperature in compare with the coating with heterogenous interface,however both coatings exhibited a rising friction at the temperature of 300°C on account of Ag shortage,afterwards,Ag particles accelerated the migration and diffusion rates towards the coating surface under 500°C and 700°C,hence reducing the friction coefficient due to the formation of vanadate and metavanadate which have a low shear strength. |
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