| Electroplating Cr coating on the gun bore is one of the effective measures to improve the service life of gun barrel.The traditional electroplating Cr coating can't meet the actual needs any longer due to its brittleness,cracks,adhesion,and severe environmental pollution etc.It is essential and urgent to develop a new preparation coating technology to replace the traditional electroplating Cr technology,so as to improve the comprehensive properties of the gun bore.After analyzing the technology on gun barrel life prolongation home and abroad,the thesis prepared three coatings about 25?m,electroplating Cr coating,magnetron sputtering Cr coating and multi arc ion plating Cr coating on the surface of PCrNi3MoVA steel,with the preparation method of the coating and the treatment scheme of the gun bore being considered.The properties of three-kind Cr coatings were analyzed through micro hardness test,friction and wear test,scratch test,thermal shock test,high temperature oxidation test,corrosion test,etc.The results were summarized as followings:The average micro hardness of the three-kind Cr coatings increased significantly compared with that of the PCrNi3MoVA steel which was 4.98GPa.The test results of multi arc ion plating Cr coating increased by 77%;magnetron sputtering Cr coating increased by65%;and the electroplating Cr coating increased by 54%.The friction coefficient of the three-kind Cr coatings decreased significantly compared with that of the PCrNi3MoVA steel which was 0.7-0.85.The test result of multi arc ion plating Cr coating was 0.35-0.45,magnetron sputtering Cr coating's result was 0.5-0.6,and electroplating Cr coating's result was 0.6-0.7.The wear rates of the Cr coatings decreased compared with that of the PCrNi3MoVA steel which was 0.00092mm3/?N·m?.The test result of multi arc ion plating Cr coating was 0.00014mm3/?N·m?;magnetron sputtering Cr coating was 0.00025mm3/?N·m?,electroplating Cr coating was 0.00073mm3/?N·m?.Therefore,it showed that the three-kind Cr coatings has more effective function on anti-friction and anti-wear than the PCrNi3MoVA steel.The wear mechanism of electroplating Cr coating was fatigue wear and adhesive wear.Magnetron sputtering Cr coating was adhesive wear,and multi arc ion plating Cr coating was adhesive wear with slight abrasive wear.The bcc phase were observed in all three-kind Cr coatings,but the grain size of multi arc ion plating was smaller,the microstructure was denser,so the hardness and wear resistance of multi arc ion plating were higher.The critical load for scratch test of three-kind Cr coatings with PCrNi3MoVA steel from high to low was listed as followings:multi arc ion plating Cr coating was 42.6N,magnetron sputtering Cr coating was40.3N,electroplating Cr coating was 26.4N.The thermal shock failure cycles of multi arc ion plating Cr coating was 21;magnetron sputtering Cr coating was 18 cycles;electroplating Cr coating was 5 cycles.The average oxidative weight gain per unit area of the three-kind Cr coatings decreased varyingly compared with that of the PCrNi3MoVA steel which was 21.223mg/cm3.The multi arc ion plating Cr coating decreased by 81.23%;the magnetron sputtering Cr coating decreased by67.44%;and the electroplating Cr coating decreased by 33.06%.Dense oxide scales were not formed on the substrate of the PCrNi3MoVA steel and the surface of electroplating Cr coating.The oxide thickness of magnetron sputtering Cr coating was about 2?m,and that of multi arc ion plating Cr coating was about 1.7?m.The Cr2O3 oxide scale with rhombohedral system can improve the oxidation resistance of gun steel significantly.The main reason for the high temperature performance difference of the three-kind Cr coatings is the growing defects and the preparation defects of the Cr coatings.The CrH with hexagonal crystal decomposes into Cr with bcc phase will produce micro-cracks to release stress in electroplating;The columnar crystal of magnetron sputtering Cr coating was observed;Although the micro-droplet defects were observed below 5?m on the surface of the multi arc ion plating Cr coating,the coating was denser.The ion energy had a significant effect on the surface oxidation kinetics of Cr coatings,and then affected the oxidation resistance of Cr coating under high temperature.The followingfactswereshown:the50eVionenergyreflectedone-dimensional preferential-growth manner of the oxide grains;the 150eV ion energy reflected two-dimensional preferential-growth manner of the oxide grains;However,as the energy of the Cr ions increased to 250eV,the preferential-growth manner of the oxide grains disappeared.With the increase of ion energy,the gas inclusion like Ar was released and the activation energy was decreased,growth rate of oxide scale decreased obviously,and the thickness of oxide scale decreased as well.The corrosion current density of the three-kind Cr coatings decreased compared with that of the PCrNi3MoVA steel which was 50.125?A/cm2.The multi arc ion plating Cr coating dropped by 99.68%;magnetron sputtering Cr coating dropped by 74.87%;and the electroplating Cr coating dropped by 93.69%.The corrosion resistance of the three-kind Cr coatings had been improved significantly,which was mainly due to the formation of the protective film on the surface of the coatings.The residue of gun bore has a certain effect on the corrosion properties of PCrNi3MoVA steel.The results of the factorial test showed the residue of gun bore and the preparation coating technologies had a significant effect on the corrosivity of the gun body,and the interaction effect exited.In the orthogonal test with the four residue of gun barrel,Cu2+,K+,Zn2+,NO3-,the Cu2+?K+?NO3-promoted corrosion.The corrosion promotion of Cu2+and K+increased with the rising of ion concentration,while NO3-promoted corrosion at a low level of concentration.Reduction of Cu2+,K+,Zn2+,NO3-was helpful to improve the corrosion resistance of gun steel.Among the three-kind Cr coatings,the multi arc ion plating Cr coating exhibits the best overall properties,which might be originated from the better adhenrence and more compact microstructure of the coating.The service life of gun bore will be considerably extended as the multi arc ion plating Cr coatings is used. |
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