Failure Process And Mechanism Of TC4/Electroplated Ni Coating By Gradual Curvature Method

Titanium alloys are important structural materials which are widely used aerospace,biomedical,chemical and petrochemical industries due to their high strength to weight ratio,good corrosion resistance and biocompatibility.However,the poor wear resistance of titanium alloy limits its further application.Therefore,Nickel,copper,chromium and other metal layers are often electroplated on the surface of titanium alloy to improve the wear resistance of titanium alloy.The coating is mechanic ally bonded to the substrate and its bonding performance is insufficient.Heat treatment can effectively improve the adhesion between the coating and the substrate by forming Intermetallic compounds at the interface.At present,the diffusion heat treatment after plating has been used to strengthen the adhesion between the coating and the substrate the Ni coating TC4,In these studies the grid method and frustrate method have been used to the adhesion of the coating.nevertheless,these kind of qualitative testing methods are not accurate enough.meanwhile,the failure process of the coating cannot be analyzed.Therefore,the mechanism of the effect of heat treatment on the bonding performance of the coating and the failure mechanism of the coating are still unclearFirst of all,independently inventing a gradual curvature method to observe the failure process of coating.The principle of this method is to bend the metal material sample into a gradual curvature shape.With the gradual increase of curvature,the bending stress of the material will gradually increase,and the material Experiences the process of crack initiation,propagation and failure.The curvature is used as the evaluation index of the coating bonding performance when the coating fails.The gradual curvature curve is solved by MATLAB software,and the structure of the device is designed,which is composed of forming unit,guiding unit,locking unit and blocking unit.ABAQUS software is used to simulate the gradual curvature method.The simulation results show that the bending stress increases gradually along the length of the sample.At the same time,the effect of vacuum heat treatment on the microstructure of TC4/Ni coating interface was studied.The results are as follows:(1)the interface of TC4/Ni plating changed from mechanical bonding to metallurgical bonding,and three kinds of Ni-Ti intermetallic compounds,Ni₃Ti,NiTi and Ti₂Ni,were formed at the interface.(2)With the increase of heat treatment time,the thickness of NiTi and Ti₂Ni layer increases,and Ni₃Ti layer first increases and then decreases.When diffusion is carried out at 850?for 9h,Ni₃Ti sublayer disappears,so NiTi shape memory alloy layer is formed on TC4 surface.(3)In the early stage of diffusion,the growth of Ni₃Ti,NiTi and Ti₂Ni can meet the parabolic relationship.With the increase of diffusion time,the coating cannot provide enough Ni atoms in the long-term diffusion.The results show that Ni₃Ti,NiTi and don't satisfy the parabolic law,while Ti₂Ni satisfies the parabolic law.Finally,the effect of heat treatment time on TC4/Ni coating interface failure process was analyzed by gradual curvature method:(1)With the increase of heat treatment time,the cracks initiate in Ni₃Ti sublayer.With the increase of curvature,the bending strain of each sublayer increases,and Ti₂Ni phase also initiates.The crack propagation of Ni₃Ti and Ti₂Ni internal cracks is hindered by NiTi phase.When the curvature increases to a certain value,the NiTi sublayer finally breaks.Therefore,the toughness of each sublayer:NiTi>Ti2Ni>Ni3Ti.(2)With the increase of heat treatment time,the bonding property of the coating first increases and then decreases.the bonding property between the coating and TC4 substrate is the weakest Without heat treatment,and the corresponding curvature of the coating failing is 0.096mm^-1.When the diffusion time increases to 120min,the corresponding curvature of the coating failing is 0.432mm^-1,and the bonding property of the coating is the strongest.(3)With the increase of heat treatment time,the failure mode of the coating changes.the failure mode of the coating is Ni/TC4 interface crack Without heat treatment.When the heat treatment time is 60 min,the failure mode of Ni coating is Ni/Ni₃Ti interface crack.When the heat treatment time is 120 min,the failure mode of Ni coating is coating crack.When the heat treatment time is 180 min,the failure mode of Ni coating is Ti₂Ni/TC4interface crack.