| 316L austenitic stainless steel has good plasticity,corrosion resistance and weldability.Plasma nitriding and magnetron sputtering technologies can improve the surface tribological properties of 316 L austenitic stainless steel,accompanied by the ceretaining its original excellent properties,thus becoming the two most commonly techniques for surface modification of stainless steels.However,the surface properties of materials are becoming more and more demanding with the rapid development of economy and society.Any single modification technology can no longer meet the requirements of some special workpieces for high fatigue strength,low friction wear and high load carrying capacity,especially the stainless steel parts widely used in the marine environment.The in-situ composited treatments of nitriding and coating were realized onto the surface of 316 L stainless steel using a plasma-enhanced magnetron sputtering system,which can form a gradient distribution sub-layer,which consist of metastable supersaturated solid solution with high nitrogen content,combined with a harder and more wearable outer-layer,thereby achieving a good transition between the hard film and the soft substrate,significantly improving the film-based bonding force and prolonging the service life of the workpiece.In this paper,the solid solution and precipitation mechanism of nitrogen in austenitic stainless steel and the thermal stability of expanded austenitic phase were studied,accompanied by the characterization of phase structure and tribological properties of CrN films combined with nitriding and(Cr,Al)N multilayer films.The results show that the nitrogen atom preferentially dissolves in the tetrahedral interstitial site and then occupies the octahedral interstitial siteduringthe nitriding.As the nitriding time increased,the supersaturated nitrogen atom occupied the tetrahedral interstitial site preferentially precipitatd and formed the compounds with Fe and Cr,and the precipitation time decreased as the substrate current density increased.The diffusion coefficient of nitrogen atoms increased linearly with the substrate current density,up to 5.37×10-10 cm~2/s.The expanded austenite γN phase decomposed to form α-Fe and CrN during laser heat treatment.The precipitation of the CrN phase contributed to the improvement of the wear resistance of the nitrided layer,but the demage of the corrosionresistance.After CrN in-situ denpostited on the nitridedaustenite,the increased surface roughness caused by nitriding resulted in the orientation of CrN changing to(111).As the nitriding time increased,the thickness of the nitrided layer increased,and the wear resistance of the composited layer increased.However,the increase of nitrogen content in the nitrided layer leaded to the precipitation of nitride,which caused the adhesive force between the CrN film and the substrate decreasing from 40.8 N to 19.3 N.The addition Cr/CrN~x intermediate layer can inhibit nitride precipitation by absorbing out-diffused nitrogen atoms,increasing the adhesive force to 25.7 N,reducing surface roughness,decreasing friction coefficient,and enhancing wear resistance.The gradient transition of the CrNx layer played more effective role in the tribological properties.Compared with the single-layer film,the multi-layer film can effectively inhibit the growth of columnar crystals,thereby reducing the surface roughness,and changing the preferred orientation of the(Cr,Al)N film from(111)to(200).The nanohardness of the Cr/CrN/(Cr,Al)N film increased to 26.09 GPa,the modulus decreased to 286.8 GPa,and the adhesive forece strengthened.Similarly,nitriding contributed to improve the wear resistance and adehesive strength between the multilayer film and the substrate. |
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