Spherical indentation of polycrystalline cubic boron nitride (PcBN): Contact damage evolution with increasing load and microstructural effects

The extremely severe conditions at which cutting tools are exposed in hard machining operations require tool materials with outstanding mechanical properties. Excellent combinations of high hardness and fracture toughness are aimed to enhance wear and contact damage resistance; and thus, reliability of the tool. Here, spherical indentation is used to induce controlled damage, under monotonic loads, and to assess the resulting damage scenario of polycrystalline cubic boron nitride (PcBN). In doing so, two grades with different content of cBN particles as well as chemical nature of the binder are investigated. Systematic and detailed inspection of residual imprints, by means of optical and scanning electron microscopy, is conducted to document critical loads for damage emergence and evolution through different stages, as a function of increasing load and microstructural assemblage. It is found that H-PcBN grade, with higher content of cBN particles and a metallic binder, exhibits higher resistance to crack nucleation as well as a more gradual transition between distinct damage scenarios. This enhanced response is linked to the concomitant increase of hardness and fracture toughness observed for that grade, as compared to the L-PcBN one, consisting of a significantly lower amount of superhard particles bound by a binder of ceramic nature. In-depth inspection of interaction between fissures and microstructure indicates the existence of four different crack paths for both PcBN grades - transgranular through cBN particles, across the binder phase, and intergranular along either cBN/cBN or cBN/binder interfaces. Relative frequency of these crack paths as well as operative toughening mechanisms define the effective crack growth resistance of each PcBN grade. In this regard, a higher fracture toughness is assessed for H-PcBN, as a result of crack bridging by metallic ductile ligaments together with the effective barrier-like role played by substructural features within cBN particles. Meanwhile, L-PcBN grade shows a rather low crack growth resistance, in direct correlation with the intrinsic brittle nature of the contiguous ceramic binder through which crack extension predominantly takes place. These relative differences in resistance to crack nucleation and growth become key for understanding the enhanced contact damage behaviour determined for the grade with higher content of cBN particles in this study.