Rock-breaking mechanism of diamond drill bits

The rock‑breaking action of a diamond drill bit is accomplished by the diamond grains. To understand how the bit breaks rock, one must first examine the rock‑breaking behavior of a single diamond grain. In hard formations, under drilling pressure, a single diamond grain subjects the rock to extremely high stress—on the order of 4,200–5,700 MPa, with some reports indicating values as high as 6,300 MPa—inducing a lithological transition that transforms the rock from brittle to ductile. As the diamond grain penetrates the formation, it cuts and fractures the rock under torque; the cutting depth is essentially equal to the penetration depth of the diamond grain. This process resembles plowing, which is why it is referred to as the “plow‑type” cutting action of a diamond drill bit. In certain highly brittle rocks—such as sandstone or limestone—the diamond grains on the bit, subjected simultaneously to drilling pressure and torque, fracture a volume of rock far exceeding the combined volume of the diamond grain’s penetration and rotation. At lower pressures, only small grooves are formed along the direction of the diamond’s motion; increasing the pressure causes deeper fractures in the rock both beneath and on either side of these grooves, extending beyond the cross‑sectional dimensions of the diamond grain itself. The rock‑breaking efficiency of a diamond drill bit depends not only on the lithology and external factors influencing it—such as pressure, temperature, and the properties of formation fluids—but also, crucially, on the magnitude of the drilling pressure. Like roller‑cone bits, diamond bits employ three primary mechanisms during rock breaking: surface crushing, fatigue fracturing, and volumetric fragmentation. Only when the diamond grains achieve sufficient specific pressure to penetrate the formation and induce volumetric fragmentation can optimal rock‑breaking performance be attained.