Microcrack Blunting and Etching Elimination of Surface Microscopic Defects (Based on SEM Morphological Characterization): During the high-temperature and high-pressure (HTHP) synthesis, purification, and sieving processes of synthetic diamond, numerous tiny growth steps, microcracks, sharp edges, and superficial impurities inevitably remain on the crystal surface. When subjected to compressive loads, the tips of these surface microcracks are highly prone to severe stress concentration, acting as crack initiation sites that lead to premature brittle fracture of the grits. By comparing the SEM images before and after treatment , it can be observed that molten potassium nitrate at 560∘C∼600∘C exerts a preferential oxidation and mild etching effect on high-energy defect sites, microcrack tips, and surface impurities on the diamond surface. This gentle chemical polishing and etching action effectively removes surface adherents, blunts the originally sharp tips of surface microcracks (increasing the radius of curvature), or even directly eliminates superficial microcracks. In accordance with the Griffith fracture theory, this significantly reduces the surface stress concentration factor and markedly enhances the diamond's ability to resist external compressive loads.

Crystal Thermal Annealing and Internal Stress Relaxation (Based on Birefringence Pattern Characterization): During the rapid growth and subsequent cooling and pressure-unloading processes of synthetic diamond, substantial internal stress remains within the crystal due to differences in the thermal expansion coefficients of internal inclusions as well as lattice distortion. By comparing the birefringence patterns of the original diamond and the chemically treated diamond , the study found a distinct improvement in the birefringence extinction pattern of the treated diamond. Holding the diamond grits in a uniform molten salt temperature field at 560∘C∼600∘C for 30 min∼60 min not only induces chemical etching on the surface, but also provides an effective and uniform thermal annealing treatment for the diamond crystal. This promotes the partial recovery of lattice distortion, effectively reduces and homogenizes the residual thermal stress inside the diamond, and consequently further enhances the compressive strength of the grits in terms of macroscopic mechanical performance.