Challenges in brazing large synthetic diamond grit by Ni-based filler alloy

Large size D711 synthetic single crystal diamonds are vacuum brazed with medium carbon steel using a Ni-Cr-Fe-B-Si alloy and inherent difficulties are critically investigated. Although, the alloy shows appreciable wetting on diamond at the brazing temperature of 1050 °C, micro-cracks develops and pronounces at the bond level in all as-brazed samples. The crack is not arrested even by using cooling rate, as slow as 5 °C/min. The large difference in Young's modulus and thermal expansion coefficient of the substrate, alloy and grit results in unfavorable tensile residual strain on diamond at bond level, causing detrimental stress. Formation of a partially coherent chromium iron carbide phase is identified which seems to facilitate the micro-crack formation under residual stress. The residual stress distribution at the interface region is mapped through finite element analyses. No noticeable difference in the strength of joint is recorded on reducing the cooling rate, up to 5 °C/min. Failure pattern of grits suggests a predominant bond-level breakage of diamond grits, indicating prevalence of residual stresses at bond level. The (110) crystallographic plane of diamond suffers significant graphitization at higher temperatures, even under a very high vacuum environment.