Microstructure And Tribological And Corrosive Properties Of Cr-Si-C-N Coatings In Water Environment

CrSiCN and CrSiC coatings with different silicon contents were deposited on Si(100)wafer and316 L stainless steels using unbalanced magnetron sputtering via adjusting trimethylsilane(Si(CH3)3H,TMS)flows.The influences of Si element content on the composition,microstructure,mechanical and tribological properties of CrSiCN and CrSiC coatings in water lubrication and their electrochemical property in seawater was investigated systematically,and their corresponding wear mechanisms in water environment and the corrosion resistance in seawater were discussed:1.CrSiC coatings with different silicon contents were prepared using unbalanced magnetron sputtering via adjusting TMS flows.The content of Si increased from 2.0 at.% to 7.4 at.% with an increase in the TMS flows.The hardness fluctuated slightly around 13 GPa due to the amorphous structure and a higher volume fraction of sp3C-C.The friction coefficient of CrSiC as sliding against SiC balls was 0.2~0.3 due to the lubrication effect of silica gel and graphitic carbon produced from the tribochemical reaction.As sliding against Si3N4 and SUS440C balls,the friction coefficient was similar and fluctuated around 0.4.However,the friction coefficient increased from 0.47 to 0.72 as sliding against Al2O3 balls due to poor fracture toughness and fatigue failure of coatings,because it was difficult to form a hydrated reaction film for Al2O3 balls.The friction coefficient of CrSiC coatings sliding against four kinds of balls could be arranged as follows: ?SiC<?SUS440C<?Si3N4<?Al2O3.2.The contents of Si and C elements increased from 0 at.% to 9.8 at.% and from 8.7 at.% to 33.4at.%,respectively when the TMS flows increased from 0 sccm to 30 sccm.The highest hardness of21.3 GPa was obtained due to the combination of solution strengthening and nanocrystalline composite structure of nc-Cr(C,N)/(a-Si3N4,a-SiC)at a Si content of 2.1 at.%.And then the hardness rapidly decreased to 13 GPa as superfluous volume fraction of amorphous phase(a-Si3N4/a-C).The lower friction coefficient and lower wear rate were obtained as mating with ceramics balls at a lower Si content.However,with an increase in Si content,the wear resistance of coatings was weakened due to their low hardness and poor resistance to crack.As sliding against SUS440 C balls,the iron oxides adhered on the wear track and acted as the grinding medium,and therefore the friction coefficient increased to 0.68~1.02.When the Si content increased to 5.4~9.8 at.%,the friction coefficient decreased to 0.40~0.53 due to the lubricating effect of tribolayer.The friction and wear behavior of CrSiCN/SUS440 C tribopairs was influenced by the hardness of coatings and anti-oxidative ability of steel balls in water.Furthermore,the formation of hydrated silica gel layer on the friction interfaceplayed an important role to improve the viscosity and capacity of the lubricant film,which effectively reduced friction coefficient and wear rates.But the mechanical wear dominated at a high Si content,so as to the friction coefficient and wear rate all increased.3.The friction behavior of CrSiCN/SiC tribopairs was investigated in water environment(deionizer water and seawater)at different normal loads(2~12 N)and sliding speeds(0.1~0.5 m/s).The lubrication mechanism of CrSiCN/SiC tribopairs in water was mainly boundary lubrication according to Stribeck curves.At a low normal load(<6 N),the low friction coefficient was obtained due to the lubrication effect of silica gel and graphite carbon derived from tribochemical reaction,and the wear tracks were ultra smooth and without peeling off.As compared in deionized water,the wear and corrosion made the interaction in corrosive solution.When the tribotests were carried out at higher loads,there were some spallings and fatigue failures on the wear tracks.The friction coefficient(0.027~0.119)in seawater was lower than that in deionized water(0.018~0.223)due to the lubrication effect of calcium magnesium salt precipitation and high viscosity of seawater.In addition,with an increase of the load(6N~12 N),the local spalling and fatigue failure appeared on the wear tracks in deionized water and seawater.The wear-mechanism maps for the CrSiCN/SiC tribopairs were developed as follows:(I)Mechanical wear for high load and low velocity;(II)Tribochemical wear +local mechanical wear for low load and velocity;(III)Partial mechanical wear + Partial tribochemical wear for high load and velocity;(IV)Tribochemical wear for low load and high velocity.4.The friction behavior of Cr-Si-C-N coatings sliding against SiC balls was investigated in seawater.The lowest friction coefficient of 0.12 was obtained for CrSiCN-30 coatings.That indicated the amorphous nitride phase was helpful to improve the tribological properties.As compared with deionizer water,the lower wear rates were obtained under lubrication of seawater,and the ploughing effect was alleviated,that result was attributed to the high viscosity of seawater,the lubrication effect of the calcium magnesium salt precipitation and well corrosion resistance of coatings.Furthermore,there were obvious corrosion marks on the wear track of the coatings due to the corrosion of the seawater.5.The electrochemical properties of Cr-Si-C-N coatings were investigated in seawater.In the case of CrSiC coatings,the pore resistance of CrSiC coatings increased by two orders of magnitude with an increase of Si content,whilste the corrosion current density and the polarization resistance decreased accordingly.As compared with CrCN coating,the moduli of impedance of CrSiCN coatings were higher than that of CrCN coatings over a low frequency range.Furthermore,the pore resistance of CrSiCN coatings was higher than that of CrCN coating by one or two orders ofmagnitude.In addition,the CrSiCN coatings demonstrated a significantly lower porosity than that of Cr CN coating.Those results indicated that the formation of a-Si3N4/a-SiC/a-C was useful to reduce grain boundary defects.The CrSiCN coatings showed better corrosion resistance than that of CrCN coating due to higher polarization resistance.As compared with three kinds of coatings,the CrSiC coatings showed better corrosion resistance due to dense amorphous structure accompanied with higher polarization resistanceand low corrosion current.