Studies On Fracture Toughness And Fracture Mechnism Of Micro-nano Structure TaC Coating

With the increasing of the strength of surface ceramic reinforcement layer,the toughness decreases sharply.The reciprocal relationship between the strength and the toughness restricts further development and application of surface ceramic material.Therefore,it is important for the coating industry to improve the molding or preparation process of the materials to increase the toughness of the surface ceramics and to find a scientific,accurate and reliable means to evaluate the fracture toughness of the surface ceramics.In this paper,the microstructure,phase constitution and distribution,grain size and crystal orientation of the micro-nano structure TaC ceramic layer prepared by in-situ reaction method combined with pretreatment and heat treatment were investigated by scanning electron microscopy(SEM),energy dispersive spectrum(EDS)and electron backscatter diffraction(EBSD)technique.The "true" hardness of the TaC ceramic layer was analyzed by the Meyer rule,the PSR model and the MPSR model.The mechanical properties which include nano-hardness,elastic modulus and fracture toughness were studied by Vickers indentation method and Nano indentation technique.Combined with indentation morphology and fracture morhology,the toughening mechanism and fracture mechanism were discussed.The main conclusions are as follows:(1)At the beginning of the heat treatment,the "Package" microstructure which named initial reaction zone was observed at the interface between the substrate and the Ta plate.With the increasing holding time,the initial reaction zone was fused and a uniform TaC ceramic layer was formed,the graphite phase in the matrix gradually decreased.The TaC ceramic layer obtained at 1135℃ for 45 min was well metallurgically bonded to the substrate,with a thickness of about 230 μm,the structure was dense,uniform and free from diffusion.The surface and cross-section of TaC ceramic layer showed different microstructure characteristics.(2)The phase constitution of TaC ceramic layer cross-section was TaC phase and Fe phase of fcc structure.Fe phase was mainly distributed at the grain boundary of TaC ceramic layer.The average grain size of TaC was 311nm,and there existed preferred orientation inside the grains;The TaC ceramic layer surface was more dense and the average grain size of TaC was 193nm,the ratio of small angle grain boundary is more than that of cross-section,and the grain orientation was almost random distribution.(3)TaC ceramic layer had obvious standard indentation size effect(ISE),and the classical Meyer empirical equation could describe the relationship between the indentation dimension of TaC ceramic layer and the applied load.The relational expressions of the PSR model and the MPSR model could be used to quantify the indentation size effect of the Vickers hardness of TaC ceramic layer.(4)The accurate evaluation of the KIC value of the TaC ceramic layer strongly depended on the judgment of the crack system and the choice of the KIC formula.It could be determined that the cracking mode was radial crack when the load was less than 10N,according to the curve method,polishing method and indentation profile method.The radial crack formula proposed by Niihara was selected to calculate KIC value of the TaC ceramic layer according to the Chicot theory and Palmqvist crack experience method.(5)There was a certain relationship between the fracture toughness(or fracture surface energy)and the microstructure of TaC ceramic layer.The fracture toughness of TaC ceramic layer cross-section was higher than that of surface,and the toughening mechanism was microstructure toughening(elastic modulus,grain size,plastic phase and grain boundary),crack deflection and crack bridge.