Micro-Raman and X-ray diffraction stress analysis of residual stresses in fatigue loaded leached polycrystalline diamond discs

Polycrystalline diamond (PCD) cylindrical tool-bits used in oil well drilling are susceptible to fracture due to the hostile environment of randomly occurring high impact loads. These tool-bits generally comprise of a PCD layer sintered onto a Co-cemented tungsten carbide substrate. The cobalt metallic phase primarily aids the formation of the diamond to diamond bonds, however during application the same cobalt expands much quicker than the diamond, breaking the very same bonds it helped to form in the first place, leading to premature failure of the tool bits. As the PCD is virtually a two-phase material comprised of cobalt and diamond, substantial volumes of the metallic phase can be removed through a leaching process without compromising the cohesiveness of the diamond matrix, with reported improved wear resistance and thermal stability. X-ray diffraction and Raman spectroscopy techniques were used to investigate residual stresses in leached polycrystalline diamond disc samples. A systematic investigation and evaluation of the average in-plane residual stress fields using the Raman technique showed a progressive shift of the residual stress state from a compressive stress state to an average tensile stress state as a function of increasing number of loading cycles. In contrast the X-ray diffraction method recorded compressive stresses for all the measurements even at the highest number of loading cycles. The apparent disagreement between the two sets of results were satisfactorily explained by considering the probing beam size and sampling depth for the two different but complementary techniques.