Microwave-liquid nitrogen synergy on granite fracture surfaces induced by Brazilian splitting

The efficient fracturing of deep hard rock remains a critical challenge in resource exploration. To enhance the upper limit of mechanical rock-breaking capacity for polycrystalline diamond compact bits, microwave heating and liquid nitrogen cooling (LNC) are receiving increasing attention due to their superior auxiliary rock-breaking effectiveness. This study investigates the synergistic effects of microwave-LNC on the mechanical properties, fracture morphology, and energy evolution of granite. Brazilian splitting tests are conducted on granite specimens subjected to varying microwave durations (0–16 min) and thermal cycles (1–20 cycles). Three-dimensional scanning and fractal analysis are employed to quantify the roughness and anisotropy of fracture surfaces. The results reveal that microwave-LNC pretreatment significantly reduces tensile strength (up to 79.5% after 15 cycles) and alters fracture morphology. The fracture surfaces exhibit an inverse proportionality between surface undulation (maximum peak to valley height Z and root mean square slope Z2) and geometric complexity (fracture dimension D), while demonstrating anisotropic characteristics. The anisotropy of the fracture surface has larger roughness at 15°–45° and 105°–165°. Energy analysis highlights strong correlations between dissipated energy and roughness parameters, particularly along the loading direction. Microstructural analysis identifies five distinct thermal damage stages, governed by intergranular crack propagation, mineral expansion anisotropy, and crack tip blunting.