| Titanium fiber inherits the advantage of high specific strength and good corrosion from titanium alloy,while its porous structure makes them suitable for noise reduction and filtration applications.Micro-arc oxidation（MAO）can fabricate a Ti O2 ceramic coating on the surface of titanium fiber,which can enhance the fiber corrosion and wear resistance.At the same time,the coating can make it with certain photocatalytic performance,which can improve the purification efficiency of titanium fibers.However,MAO on titanium fiber surface can causes local“over-dissolution”on the substrate by concentrated plasma spark discharge,and the coating is induced“over-growth”into the“over-dissolution pits”subsequently.The coating“over-growth”behavior leads crack initiation at the interface during deformation prematurely,resulting in effect on the mechanical property of coated workpiece.Therefore,this paper studied the effect of concentrated plasma discharge under ultimate different voltages（280 V-450 V）on local“over-dissolution”of fiber and“over-growth”of coating different voltages.In addition,analyzed the influence mechanism of different sizes of coating“over-growth”on the crack density and peeled area on the coating surface by bending and tensile test.Moreover,the finite element model of the coating with“over-growth”on the surface of titanium fibers was established,and the mechanical response of“over-dissolution pit”defect to titanium fibers under tensile stress was studied.Furthermore,studied the catalytic efficiency of MAO coating fabricated by different voltages by testing the catalytic degradation of Cr（VI）on the titanium fibers.The results show that the thickness of the titanium fiber surface film increases from 2.1μm to 9.1μm with the increase of the termination voltage,and the pore size on the surface increases from 0.51μm to 1.62μm under the influence of discharge spark.Meanwhile,the hydrophilicity of the titanium fiber surface is enhanced by Ti O2 coating.The wetting angle decreases from 101o to 57o with the increase of voltage,which showing stronger hydrophilicity.In addition,it is found that larger concentrated discharge sparks cause a large size of local“over-dissolution”of the titanium fiber substrate under higher voltage based on the analysis of the cross-section morphology,accompanied by more coating“over-growth”into the substrate,making the interface between the fiber and film layer turn to"zigzag"The residual stress concentration caused by the“over-growth”defect of the coating,which provides a driving force for crack initiation and propagation during deformation.The crack density and peeled area of the coating are positively correlated with the“over-growth”size of coating Bending and tensile test shows that the coating peeled off under the ultimate voltage of420 V and 450 V,whereas the coating cracking and minor peeling under 280 V and 320 V.In addition,the crack propagated from the bottom of“over-dissolution”near interface into the titanium fiber,which reduces the tensile strength compared with the original fiber.The tensile strength and elongation of titanium fibers decreased with increasing size of fiber“over-dissolution pits”.Moreover,part of sub-stable anatase phase Ti O2transformed to thermally stable rutile phase Ti O2 with the applied voltage increased,which decreased the photocatalytic performance of coating,and the catalytic efficiency of 450 V is only 56%of 280 V.In this study,under the applied stress,the cracks initiate at the interface has less effect on coating adhesion and fiber tensile property when the coating fabricated on titanium fiber surface by lower voltages（280 V,320 V）.Meanwhile,the coating fabricated under lower ultimate voltage shows higher catalytic efficiency.This study provides potential application and gives a theoretical basis for the optimize ceramic coating. |
