Experimental And Simulation Study On Fracture Toughness Of WC-Ni60A Wear Resistant Coating

The soil contacting parts of agricultural machinery work in the soil environment,and fatigue wear,corrosion and fracture become the main failure forms.Generally,researchers will improve its wear resistance by coating a coating with relatively high hardness.However,the wear mechanism indicates that toughness will also affect the wear behavior of abrasive particles.So far,no one has studied the toughness of wear-resistant coatings.In this paper,Ni60A wear-resistant coatings containing different mass fractions（15%,25%,35%）of WC were prepared by plasma surfacing and plasma spraying.The differences of microstructure,mechanical properties and wear-resistant properties of different coatings were obtained through structure and performance characterization.Combined with three-point bending experiment and finite element simulation,the effects of WC content,preparation process and coating thickness on WC-Ni60A were explored.（1）The surface of WC-Ni60A surfacing layer is composed of a large number ofγ-（Fe,Ni）composition,in which dendrite deposit Cr₇C₃is distributed.With the increase of WC content,WC particles and W crystals gradually appear in the surfacing layer.A small number of pits and holes can be seen on the surface of the spraying layer,which are all over the patterned and strip Cr₇C₃ and W₂C areas.Compared with the surfacing layer,there are more unmelted WC particles in the spraying layer.Due to the strengthening of solid and solution of WC and Cr₇C₃,the coating hardness increases when WC proportion rises.increase of The coating hardness with 35%WC content is the highest,but it is not different from that with 25%WC content.Moreover,due to the large number of unmelted WC particles on the surface of 35%coating,the coating hardness distribution is uneven.In the same proportion,the hardness of WC surfacing layer is slightly higher than that of spraying layer.Under the action of high temperature,the coarse crystal of WC is transformed into fine crystal W₂C,and the microstructure and properties are improved.The wear resistance shows a changing trend with time.In the early stage of wear,35%WC surfacing layer shows good wear resistance with high hardness.In the middle and late stage of wear,35%wear amount gradually increases,while 15%and 25%coating wear amount shows a downward trend.In the middle and late stage of wear,with the falling off of WC particles,WC particles form WC fragments under the action of tangential force,cutting and ploughing the coating surface,resulting in coating surface damage.Under the action of friction heat,Ni base will appear softening effect to avoid severe wear.（2）Among the three components,the WC nickel base coating with 25%mass fraction has strong ability to resist crack propagation under the multiple effects of hardness and micro morphology.With the increase of thickness,the ability of the coating to resist crack propagation increases.The plane strain fracture toughness of 25%WC surfacing layer with thickness of 1mm can reach 69.697 MPa√8).The surfacing layer and spraying layer show different fracture micro morphology.Sufficient energy can be obtained to penetrate into the matrix before the surfacing layer breaks,and the crack shows deep crack penetrating into the matrix.Under the action of tangential force,the spray coating crack extends to the interface joint to form the interface crack.With the increase of bending deflection of the sample,the deep crack is connected with the interface crack to form the peeling of the coating.The fracture of 15%WC+Ni60A surfacing layer presents typical fan-shaped dissociation pattern;The fracture of 25%WC+Ni60A surfacing layer shows quasi dissociation fracture combined with micropore fracture and small river dissociation;The fracture section crack of 35%WC+Ni60A surfacing layer propagates along the hole inclusion and forms an annular river pattern around the defect.The fracture morphology also belongs to the category of dissociative fracture.（3）XFEM and CZM fracture models are used to simulate the different fracture phenomena of 25%surfacing layer and spraying layer with the best comprehensive performance,obtain the stress nephogram,and visually analyze the fracture process.In the process of establishing the model,the extended finite element model requires the enrichment definition of the fracture zone to give the damage model at the crack initiation,and the cohesive force model gives the cohesive element to the prefabricated expansion path to obtain the interactive damage properties.The results show that the cracks in the surfacing layer propagate vertically along the longitudinal direction.When the load is3000N,the localization phenomenon appears in the system.When the load is 4700N,the cracks show the trend of in-depth propagation.When the loading force of the spraying layer is 500N,the stress concentration phenomenon appears in the coating.When the load reaches 4940N,the crack begins to expand.Finally,the virtual K_ⅠC of 25%surfacing layer is 72.6 MPa√8),and the virtual K_ⅠC of 25%spraying layer is 5.8 MPa√8).the simulation effect is good.Comprehensive analysis shows that 25%WC+Ni60A surfacing layer has the best performance in terms of microstructure and mechanical properties.It can be used for the preparation of wear-resistant coating for soil contacting parts of agricultural machinery.