High-temperature, high-pressure (HTHP) hard-rock drilling frequently causes chamfer spalling of polycrystalline diamond compact (PDC) cutters, leading to ~20% loss in the rate of penetration (ROP) and large torque oscillations. We propose a surface-gradient chamfer comprising a thin SiC interlayer (t_SiC ≈ 0.7 μm) and a nanocrystalline diamond topcoat (t_D ≈ 5 μm, d_D ~100 nm), combined with shallow cobalt leaching (L_deCo ≈ 100 μm). The structure was verified by microscopy/spectroscopy and evaluated by scratch adhesion, SEVNB toughness, instrumented impact, thermal shock, 400 °C pin-on-disc wear, and bench-scale granite drilling with vibration/torque monitoring. A coupled thermo-mechanical finite-element model, calibrated with Raman stress maps and thermal measurements, was used to interpret failure trends. Relative to untreated cutters, the gradient design reduced peak tensile residual stress by ~45% and lowered high-temperature wear volume by ~40%. In the present impact dataset (limited cutters per condition), the observed spall incidence at 1.0 J decreased from 2/3 (baseline) to 1/5 (gradient-treated). Short bench drilling runs suggested improved signal separability between healthy and pre-spall states (ROC-AUC ≈ 0.85 vs. ~0.65 for baseline, evaluated using a leave-one-cutter-out protocol); these drilling results should be interpreted as trend-level evidence given the limited number of cutters. These gains arise from mitigated thermal mismatch and residual stresses at the chamfer.