Influence of tungsten coating on microstructure and thermal damage of brazed diamonds

Abstract

Degradation of diamond strength resulting from thermal damage in vacuum brazing process constrains the application of Ni-based brazed diamond tools. To reduce the thermal damage to brazed diamonds, the effectiveness of coating diamond with tungsten was investigated by scanning electron microscopy, Raman spectroscopy and friability test. Tungsten coating was deposited on diamond surfaces by vacuum diffusion method. The experimental results show that tungsten coating, as a buffering layer, plays a stress-relaxing role and brings about fewer cracks in the Ni-based matrix near diamond grains. Graphitization of W-coated diamond after brazing is much slighter than that of uncoated diamond and Ti-coated diamond, according to the morphology and Raman spectra. The isolation of diamond from nickel catalyst by tungsten coating inhibits the graphitization of diamond, which can further explain the increase in the toughness index of brazed diamonds.

A Review of the Diamond Retention Capacity of Metal Bond Matrices

“…The analysis results of XRD or EDS can be used to show the presence of carbides on the diamond surfaces, which could be evidence of the formation of a chemical bond between the matrix and diamonds. For instance, Figure 7 indicates the existence of a tungsten layer, and a chemical combination may occur [35]. Raman spectroscopy can be employed to study diamond graphitization ( Figure 8 illustrates the presence of graphite at the surface of brazed diamond [35]) and thermal residual stresses [78][79][80][81] which are related to diamond retention ability.…”

“…The coatings also fill the micro pores and cracks in the diamond surfaces, and in doing so, can bridge surface defects and enhance the strength of the diamond grits [34]. Figure 1 shows the interfacial morphology around tungsten-coated diamond is more desirable, which exhibits fewer cracks [35]. The theory of diamond surface metallization is based on the formation of a carbide layer due to the intense interfacial chemical reaction between diamond and strong carbide-forming elements under the appropriate process conditions.…”

“…For instance, Figure 7 indicates the existence of a tungsten layer, and a chemical combination may occur [35]. Raman spectroscopy can be employed to study diamond graphitization ( Figure 8 illustrates the presence of graphite at the surface of brazed diamond [35]) and thermal residual stresses [78][79][80][81] which are related to diamond retention ability. The residual stresses σ biaxial can be determined from the shift in the wavenumber w of the Raman-Stokes-peak of a sintered diamond compared to the wavenumber w 0 of the Raman-Stokes-peak of an unsintered and stress-free diamond according to Equation 1[81]…”

“…Backscattered SEM image (a) and corresponding EDS (b) between Ni-Cu alloy and W-coated diamond[35], Taylor & Francis, 2018.…”

“…Representative Raman spectra of diamonds tested at two depths (H = 0 µm and H = 10 µm)[35], Taylor & Francis, 2018.…”