An advanced investigation of polycrystalline diamond compact's degradation mechanism during granite turning

The fundamental degradation mechanism of Polycrystalline Diamond Compact (PDC) cutters during granite turning were investigated at various cutting forces and cutting temperature by changing the turning depths and using cooling water or not. The microstructures and compositions of the worn PDC surfaces were characterized by Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS) and Raman Spectroscope. In this experiment, PDC turning granite tests under dry and wet conditions with four different cutting depths were conducted.Three different failure modes of PCD were observed. Under wet turning, when the turning depths were 0.2 mm and 0.4 mm, the main failure was the abrasion of diamond due to the cobalt at grain boundaries catalyzing sp3 to sp2 transformation. When the turning depth reaches 0.6 mm, both the graphitization on surface and the cobalt thermal expansion subsurface occurred, which result in the flaky pits on diamond surface. Under dry turning, the temperature of the PDC surface increases gradually with increasing depth of cut. Cobalt at surface and beneath undergoes significant thermal expansion and the introduced thermal stresses break the diamond to diamond (D-D) bondings. In summary, there are two main mechanisms of PDC degradation, the inverse catalysis of cobalt and the in -situ thermal expansion of cobalt were found. For wet turning test, the PDC cutting temperature grows relatively slow, cobalt at grain boundaries on the the diamond surface is firstly abraded. At the same time, due to the thermal expansion at high temperatures cobalt could be partially squeezed out of the grain boundaries, accelerating the transformation of diamond to graphite and leading to the scratching of diamond grains by sharp granite. For dry turning test, the PDC cutting temperature dramatically increases, cobalt at grain boundaries in -situ thermal expanses and cause the huge thermal stresses, resulting in diamond transgrainular cracking and spalling out.