The Wear and Friction Characters of Polycrystalline Diamond Under Wetting Conditions

Polycrystalline diamond compacts (PDCs) are the main cutting unit of drill bits and are a major factor in determining the drilling efficiency and service life of drill bits. Drill bit failure is caused by the severe abrasive wear it undergoes during the drilling process. The drill bit failure can prolong the drilling period, which can result in borehole instability and cause collapse in the material. A solution that can address this issue is developing an appropriate drilling method that can expel the dust in a manner that will not increase the abrasive wear on the drill bit. Here, an Amsler friction and wear-testing machines was used to investigate the friction and wear characteristics of PDC and to study the effects of the dust expelled during drilling on the wear performance of drill bits under dry air and wetting conditions. The microstructures of the worn surfaces were examined by a scanning electron microscope (SEM) and metalloscope. In addition, the chemical compositions of the PDCs' surfaces were analyzed using X-ray diffraction (XRD) after the wear and friction tests. The results demonstrate that the friction coefficients and wear rate obtained in dry air were higher than those under wetting conditions. As expected, these values are mainly ascribed to the absence of the absorber layer and lubrication under dry air. Furthermore, under wetting conditions a number of cracks were observed on the PDC surface after testing at 700 °C, which was mainly caused by two factors: The different thermal expansion coefficients between the diamond and Cobalt phase; and the residual stress generated inside the PDC under wetting conditions.