Effect Of Hydrothermal And High Temperature On Stability And Tribological Property Of Micro-arc Oxidation Coatings On Titanium

Titanium micro-arc oxidation(MAO)coating has been widely used because of its excellent properties such as high hardness,wear resistance and corrosion resistance.However,the stability and tribological property of the MAO coating will face severe challenges when it is used in hydrothermal and high temperature environments.Therefore,different types of titanium MAO coatings are prepared,and then these MAO coatings are placed in liquid-phase hydrothermal environment,vapour-phase hydrothermal environment and high temperature environment respectively.The effects of three environments on the microstructure and composition of titanium MAO coatings are analyzed by SEM,EDS and XRD,the evolution of tribological properties of MAO coatings was investigated by microhardness tester and wear test machine.Under the liquid-phase hydrothermal environment,MAO coating prepared in silicate electrolyte flakes off because amorphous Si O₂ in the coating dissolves in water.Amorphous Ti O₂ in MAO coating prepared in phosphate electrolyte follows the dissolution precipitation mechanism and forms a layer of spherical anatase particles on the surface of the coating,which destroys the microstructure of the dense layer of the coating and consequently leads to a significant decrease in the microhardness and tribological property of the coating.For MAO coating prepared in aluminate electrolyte,after liquid-phase hydrothermal treatment,not only spherical anatase particles but also claviform boehmite particles were formed on the surface,and the microhardness and tribological property of the coating were greatly reduced.Under the vapour-phase hydrothermal environment,the phase and tribological property of the MAO coating prepared in the silicate electrolyte have no visible changes.Anatase spherical particles appeared on the surface of MAO coating prepared in phosphate electrolyte,and the content of anatase increased slightly,but the tribological property of MAO coating did not change obviously.MAO coating prepared in aluminate electrolyte is greatly affected by the formation of spherical anatase particles and claviform boehmite particles on the surface of the coating,and the microhardness and tribological property of the coating are significantly reduced.Under the high temperature environment,when the temperature is 400�C and 500�C,a small amount of microcracks appear on the surface of the MAO coating prepared in silicate electrolyte under the action of thermal stress,and the phase,microhardness and tribological property of the coating are almost unchanged.However,under the action of thermal stress,the microcracks on the surface of the MAO coating prepared in aluminate electrolyte significantly increase and expand,which lead to a slight decrease in the microhardness and tribological property of the coating.When the temperature rises to600�C,the MAO coating prepared in silicate or aluminate electrolyte is cracked and peeled off,which is because the effect of thermal stress is more significant at 600�C.In the process of heating and cooling,thermal stress occurs between different phases of the coating and between the coating and the substrate.