Effect of carbide additives on the structure and behaviour of polycrystalline diamond

Abstract

It is widely known that the addition of vanadium carbide to tungsten carbide significantly enhances the performance of the tungsten carbide tool by improving the hardness as well as the abrasion resistance of the material. This thesis describes the addition of different concentrations of vanadium carbide to polycrystalline diamond (PCD), and investigates the microstructure and behaviour of the PCD. It was observed that a (V,W)Cx mixed carbide phase was formed in the PCD together with residual vanadium carbide in solution with the cobalt binder. The (V,W)Cx carbide deposited predominantly at the WCCo/ diamond interface. The 1wt% vanadium carbide enhanced PCD showed the presence of tungsten carbide evenly distributed within the PCD, whereas the 3wt% vanadium carbide enhanced PCD and the 5wt% vanadium carbide enhanced PCD revealed the absence of tungsten carbide in the sintered PCD. Both TEM and SEM analysis confirmed the presence of the (V,W)Cx mixed carbide phase within the PCD. The sandstone milling test revealed the vanadium carbide enhanced PCD to be more thermally stable with approximately 40% improvement in performance, whilst both the Paarl Granite Turning test and Vertical borer tests showed the vanadium carbide enhanced PCD to be more wear resistant than the standard cobalt based PCD. Heat treatment experiments were undertaken to determine the thermal stability of the (V,W)Cx mixed carbide phase. Using hot stage XRD, the (V,W)Cx mixed carbide phase was found to be stable up to a temperature of 1100 °C In addition, the vanadium carbide enhanced PCD showed a reduced amount of graphite being formed during hot stage XRD analysis. This suggests that the vanadium carbide enhanced PCD would be more thermally stable during tool use as compared to the conventional PCD. Other carbide additives such as molybdenum carbide, chromium carbide and titanium carbide were also investigated. The Cr3C2 did not seem to react with the WC in the PCD to form a mixed carbide. The addition of titanium carbide to the vanadium carbide enhanced PCD led to the formation of a proposed mixed (Ti,V,W)Cx phase. Vertical borer test results showed that the VC-TiC enhanced PCD exhibited superior durability. The addition of vanadium carbide and chromium carbide to fine grained PCD was also investigated in terms of abnormal grain growth (AGG). It was observed that the grain size of the AGG in the standard PCD ranged from 100-400 _m with an AGG region of 250-300 _m, the vanadium carbide doped PCD showed an AGG size of 100-200 _m with an AGG region of 250-300 _m and the chromium carbide doped PCD showed an AGG size of 100-250 _m with an AGG region of 650-700 _m. The exaggerated AGG present in the chromium carbide doped PCD was likely due to the increase in the carbon activity in the PCD. From this body of work, two patents were filed. A patent was filed by Kaveshini Naidoo et al. with the publication number: US2010285335A1 that described the performance improvement of the vanadium carbide enhanced PCD. An additional patent was filed by Kaveshini Naidoo et al. with the publication number: US2015151410A1 that described the performance improvement of the VC-TiC enhanced PCD.