Influence of the microstructure on the contact damage behavior of polycrystalline cubic boron nitride

Abstract

In this work the influence of microstructure on the contact damage of polycrystalline cubic boron nitride (Pc-BN) is studied. Pc-BN is a superhard composite material constituted by c-BN particles and a binder matrix of either metallic or ceramic nature. Pc-BN are extensively used as cutting tools particularly for machining difficult-to-cut materials. As for many composite materials, a large array of microstructural parameters can vary from one specimen to another, resulting in quite different mechanical properties of the material. Within this context, the main aim of this project is to document and analyze the contact damage behavior of three microstructurally distinct Pc-BN materials and to establish correlations between it and the main microstructural features describing the assemblage exhibited by these superhard composites. First, the three studied Pc-BN grades were analyzed on a microstructural scale by means of optical and scanning electron microscopy, in order to assess all the relevant microstructural parameters. A roughness analysis of the samples surface was also performed, followed by a polishing process to decrease the surface topography. A subsequent roughness analysis concluded that the polished surfaces exhibited a satisfying finish for reliable mechanical testing. Contact damage was then induced in a controlled manner by means of spherical indentation techniques. Applied loads, ranging between 300N and 3000N, were then performed on the samples. The emergence and evolution of the resulting damage was evaluated as a function of the applied load, and the crack-microstructure interaction was investigated. It was found that the Pc-BN grade with the highest c-BN content was the most resistant to emergence of damage, followed by the low- and medium- content grades. Regarding damage evolution, it was rather gradual and smooth for the PcBN with medium content of particles, different from more abrupt changes observed in the high- and low- content materials. Such responses may be indirectly related to the effective hardness and toughness of the studied materials, respectively. Crack path was found to be both transgranular and intergranular in the sample with high content of cBN particles, where it was exclusively intergranular in the two grades. Such a behavior is rationalized on the basis of relative differences in average c-BN grain size, chemical nature of the binder and binder mean free path.