Structure And Tribological Properties Of CrCN Based Coatings In Different Environment

CrMoCN coatings with various Mo contents were deposited on TC4 disks and Si?100?wafers by using closed-filed magnetron sputtering system.Meanwhile,three CrCN-based kinds of CrSiCN,CrBCN and CrSiBCN coatings were prepared on 316L disks and Si?100?wafers.The crystallite phases and chemical bonding structures of all coatings were characterized using X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?,respectively.The surface and cross-sectional profiles of coatings were observed and shot using scanning electron microscopy?SEM?.The mechanical properties such as hardness and elastic modulus of coatings were measured by a nano-indenter with a diamond indenter.Besides,the adhesion bond of coatings to the substrates was determined by an acoustic emission scratch tester.The tribological properties of coatings in air and seawater conditions were tested by a ball-to-disk tribo-tester.After the end of friction test,the wear marks on coatings and ball marks were taken using a metallographic optical microscope and the cross-section curves of wear scars were observed and analyzed by the non-contact white light interference three-dimensional profilometer.In order to clarify the wear mechanism of coatings under different environments of air and seawater lubrication,scanning electron microscopy?SEM?and energy dispersive spectroscopy?EDS?were used to further observe the wear scar of coatings.The corrosion resistances of CrMoCN coatings in seawater environment were tested by standard three-electrode electrochemical workstation?CHI660E?.The experimental test indicators included open circuit voltage?OCP?,AC impedance spectrum?EIS?and dynamic potential polarization curve.1)CrMoCN coatings containing different Mo contents?1.4%⁹.8%?were prepared using magnetron sputtering technique by changing the sputtering currents of Mo target?0.5 A³.0 A?.After the structures were characterized,it was found that Cr,Mo and N elements mainly existed in the form of?Cr,Mo?N solid solution crystals,while C element mainly existed as amorphous a-C?a-CN_x?.As the Mo contents in the coatings increased,the surface morphology became more compact.After the mechanical properties of coatings were tested,it was found that the hardness and elastic modulus values increased first and then decreased.CrMoCN4 coating with 6.4%Mo content showed the highest hardness and elastic modulus,which were 26.8 GPa and 327.8 GPa,respectively;and the highest?H/E?and?H³/E²?,which were 0.082 and 0.179,respectively.In the air,after sliding against Al₂O₃ balls,CrMoCN6 coating with the highest Mo content of 9.8%showed the lowest friction coefficient and the lowest wear rate,of which the values were 0.46 and 1.33×10^-7 mm³/?N·m?,respectively.This was mainly because CrMoCN coatings were susceptible to tribochemical reaction with moisture in the air to form layered lubricant MoO₃ with low shear strength.An increase in the Mo content further promoted the formation of MoO₃ friction lubricating layer.2)In the seawater environment,it was found that the friction coefficients and wear rates of CrMoCN coatings decreased overall with the increase in the Mo content,and CrMoCN6 coating with the highest Mo content showed the best tribological properties,while the wear rates of SiC balls were also reduced.This was mainly because CrMoCN coatings underwent a tribochemical reaction with water during seawater lubrication to form MoO₃ which could improve the lubricating effect.At the same time,due to the hydration reaction of SiC material in the water environment,a gel-like SiO₂·2H₂O with a certain lubricating effect was formed.The degree of wear of both was alleviated due to the synergistic lubrication and wear-reducing effect of two friction layers.According to the observation and the elemental analysis of wear scar,it was concluded that the wear mechanisms of CrMoCN/SiC tribopairs were abrasive?mechanical?wear accompanied by tribochemical wear.In the seawater environment,it was found that after sliding against Al₂O₃ balls,the friction coefficients of CrMoCN coatings and the wear rates of Al₂O₃ balls decreased overall with the increase in the Mo content of coating,but the wear rates of CrMoCN coatings themselves were rising.This was mainly due to the fact that the MoO₃ lubricating thin layer formed under water lubrication conditions effectively reduced the friction coefficient and protected the Al₂O₃ balls,but the lubricating effect of coating enhanced the loss to material itself at the same time.Therefore,the wear rates of CrMoCN coatings increased.According to the observation and the elemental analysis of wear scar,it was concluded that the wear mechanisms of CrMoCN/Al₂O₃ tribopairs were abrasive?mechanical?wear and tribochemical wear.After the CrMoCN coatings were tested using a standard three-electrode electrochemical workstation,it was found that CrMoCN2 coating with the Mo content of 2.9%exhibited the lowest corrosion current density value of 4.675×10^-8 A/cm² and the highest polarization resistance value of 8.67×10⁵??cm².The data indicated that the corrosion resistances of CrMoCN coatings were affected by Mo contents.Due to the increase in the density of the surface of CrMoCN coatings within a certain range of Mo contents,the reduction of defects such as fine columnar pores could effectively inhibit the penetration of corrosive medium in the seawater into the substrate,thereby improving the corrosion resistance of coating material.3)Three CrCN-based kinds of coatings were prepared by closed unbalanced magnetron sputtering system,namely CrSiCN,CrBCN and CrSiBCN coatings respectively.After structural characterization,it was found that Cr element in the coatings mainly existed in the form of CrN?200?and Cr?210?crystals,and there were a small amount of CrN?111??220?and Cr?200??310?crystals at the same time.Among them,Si,B and C elements were mainly present in amorphous a-SiN_x,Si-C-N,a-BN_x and a-C?a-CN_x?.After the mechanical properties of three kinds of coatings were tested,it was found that CrSiBCN coating exhibited the highest hardness of 27.1 GPa.Meanwhile,it also showed the highest?H/E?and?H³/E²?values,which were 0.097 and 0.255,respectively.While three kinds of coatings slid against WC+6%Co,ZrO₂ and Si₃N₄ materials in seawater,CrSiBCN coating showed the lowest friction coefficient,and the values were 0.27,0.40 and 0.29,respectively.This indicated that the synergistic lubrication effect of colloidal SiO₂·2H₂O and layered H₃BO₃ formed by the hydration reaction of CrSiBCN coating in seawater would be the most beneficial to the tribological performance improvement of coatings.In addition,it was found that CrSiCN coatings presented the highest wear rates when three kinds of coatings slid against YG6 balls in the seawater,which was caused by greater friction coefficient between the tribopairs.When three kinds of coatings slid against ZrO₂ balls,the wear rates of CrBCN and CrSiBCN coatings were higher.It could be attributed to the H₃BO₃ lubricating layer with layered structure was formed by the hydration reaction of coating itself under seawater lubrication conditions.The friction coefficient was effectively reduced but the loss of coating material itself was enhanced.Therefore,the wear rates of CrBCN and CrSiBCN coatings containing boron element was higher.When three kinds of coatings slid against Si₃N₄ balls,the wear rates of CrBCN and CrSiBCN coatings were lower,which could be attributed to their superior mechanical properties,including higher hardness,?H/E?and?H³/E²?values.Based on the observation and the elemental analysis of wear scar,it was concluded that the wear mechanisms included abrasive?mechanical?wear and tribochemical wear when three kinds of coatings slid with different materials in seawater.