Study On Mechanical Properties Of Typical Steel Nitriding Diffusion And TC4 Surface Coating Strengthening

As a typical steel and light alloy requiring surface strengthening in practical applications,such as 316L stainless steel,M50 die steel,W6 high speed steel and TC4titanium alloy,ion nitriding and coating strengthening are commonly used as surface strengthening means.The surface ion nitriding of 316L,M50 and W6 steels was processed by hot wire enhanced plasma ion nitriding and magnetron sputtering.By analyzing the differences of chemical composition,crystal structure and mechanical properties,the diffusion mechanism of nitrogen in typical steel was studied,and the factors affecting the diffusion of nitrogen were clarified and the diffusion mechanism was revealed.Ti/TiN and Ti（N）/TiN multilayer coatings were deposited on the surface of TC4 titanium alloy by Ti composite ion nitriding and Ti（N）/TiN multilayer coatings with different thickness modulation ratios.The mechanical properties of modified TC4titanium alloy surface multilayer coatings were studied by the thickness modulation of TC4 matrix surface strengthened multilayer coatings and the design of ion nitride of soft titanium layer.The correlation between soft/hard layer thickness and coating deformation was clarified,and the strengthening and failure mechanisms were revealed.The results are helpful to establish the design principles of ion nitriding and coating strengthening,and will provide theoretical basis for industrial production practice.（1）For the low temperature ion nitrization on the surface of 316L,M50 and W6steels,N atoms preferally diffused into octahedral gaps.The number and size of octahedral gaps in steels with body-centered cubic structure are larger than those in face-centered cubic structure steels,so the thickness of nitrizing layers in M50 and W6steels is larger than that in 316L steels.The substituted solid solution element Cr in316L steel has greater affinity for N,which hinders the diffusion of N atom.The high carbon content accelerates the alloying of elements.It is calculated that the diffusion activation energy of N atom in 316L steel is 76.02 k J.mol^-1 at 450℃,which is much higher than that in M50 steel and W6 steel,which is 23.37 k J.mol^-1 and 23.16 k J.mol^-1.（2）For Ti composite ion nitride deposited on the surface of TC4 titanium alloy,nano-hardness and modulus of Ti composite nitride on the surface of TC4 titanium alloy increased,and the wear rate decreased from 10^-3mm³·N^-1·m^-1 to 10^-4mm³·N^-1·m^-1.With the increase of target current,the deposition rate is accelerated,the coating is more loose,which is conducive to the diffusion of N atoms and the formation of Ti₂N phase,and the mechanical properties of the coating are improved.（3）On the surface of TC4,Ti/TiN and Ti（N）/TiN multilayer coatings and TiN monolayer with different thickness modulation ratios were deposited:Ti and Ti（N）as soft bonding layers inhibited the growth of TiN columnar crystals and refined the size of TiN columnar crystals.When subjected to normal stress and shear stress,the thickness modulation is relatively high,that is,the plastic deformation of the thick soft Ti and Ti（N）layer is serious,which leads to the hard TiN layer cracking in the multi-layer structure.When the thickness modulation is low,the plastic deformation of the soft Ti and Ti（N）layer absorbs some external work,and the interfacial stress is reduced,which effectively prevents the crack propagation in the hard TiN layer and improves the strength and toughness of the coating.Due to the matching of hardness and modulus,the Ti（N）layer of ion nitriding has stronger strengthening and toughening effect on the multilayer structure than that of pure metal Ti layer.