The use of Raman spectroscopy to study fatigue type processes in polycrystalline diamond (PCD)

Abstract

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 tungsten carbide substrate. The 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. Because the PCD is virtually a two phase material comprising 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. The fact that the tool-bits fail after repeated use gives rise to the possibility of fatigue-type processes in diamond. X-ray diffraction and Raman spectroscopy techniques were used to investigate residual stresses in leached and unleached polycrystalline diamond disc samples. The 514:5 nm line of an Ar+ ion laser was used as an excitation source with an 1800 grooves=mm grating in the single spectrograph mode of a Jobin-Yvon T64000 Raman spectrometer. Raman stress measurements at room temperature were carried out using a 36 point mapping array in area close to the size of the PCD samples. The mapping points provided histograms of the magnitude and nature of these small individually stressed regions showing a general compressive stress for the lower numbers of fatigue cycles which deteriorates to a high proportion of tensile regions. The data are also illustrated by 2-D surface maps as an alternative mode of presentation again confirming the change form surface stresses being dominantly compressive to dominantly tensile with exposure to the higher numbers of fatigue cycles. A systematic investigation and evaluation of the average in-plane residual stress fields on a fatigue loaded leached and unleached PCD disc sample was also conducted using the the Bruker D8 Discover machine equipped with a graphite monochromator and a Cu X-ray tube of wavelength 1:544 A operated at a voltage of 40 kV and a current of 40 mA. To determine the full stress tensor, samples were measured at 6 different azimuth angles = 0o; 180o; 90o; 270o;45o, and 225o. Whilst the Raman technique showed a progress shift of the residual stresses state on the surface of the samples, from a compress stress state to an average tensile stress state with increasing number of loading cycles, the X-ray diffraction method recorded compressive stresses for all the measurements even at the highest number of loading cycles. Whilst the results are seemingly in disagreement the non destructive nature of the the two techniques employ in the study has been shown. Whilst the two methods are considered to be surface techniques there exists fundamental differences in way they determine the residual stress, which can be used to explain the divergent set of results for the two methods. Scanning electron microscope (SEM) images showing evidence of possible localized damage at large numbers of cycles for some of the unleached samples, were obtained using a JEOL JSM7500F field emission scanning electron microscope operating at 15 keV in a vacuum of 5:1 10��4 Pa. Using the same equipment elementary analysis was done for the unleached samples and the observations indicated that the samples had similar chemical compositions.