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.

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Utilising a series of mechanically over‐excavated cavities along borehole is a novel technique for enhancing the permeability of soft coal seams and, consequently, gas drainage. The evolution of cracks induced by a wide range of pressure‐relief around an over‐excavated hole is intrinsically complex. In this study, the mechanical behaviour and crack evolution of the specimens containing an over‐excavated hole under uniaxial compression loading were studied by experimental and 3D numerical simulation. The results indicated that the peak strength and elastic modulus of the specimens gradually decrease with increasing cavity diameter and length, which is also verified by the numerical simulation. The inclusion of cylindrical cavities in over‐excavated holes results in reduced crack initiation stress and a greater degradation of peak stress and elastic modulus, despite having an equivalent volume to the ellipsoidal cavity. This is likely attributed to the difference in stress concentration between the cylindrical and ellipsoidal cavities. The crack propagation process can be classified into four stages based on the acoustic emission (AE) event counts, initial crack compaction, stable crack propagation, unstable crack propagation and post‐peak failure stage. The two AE indices, rise angle and average frequency value, demonstrated that the failure is dominated by tensile crack and gradually transformed to shear crack. Stress redistribution is essential in the initiation and propagation of cracks. Tensile stress concentration leads to cracks forming at the top and bottom of the hole, which propagate in the direction of loading. Compressive stress concentration results in shear cracks forming at the left and right sides of the hole, which propagate diagonally. The failure pattern of the specimen is ultimately determined by a combination of tensile and mixed crack propagation. The experimental and numerical results contribute to a deeper understanding of the crack evolution mechanism of coal seams with over‐excavated holes.