Optical properties and mechanical stress in cubic boron nitride and diamond
Date
2013-03-19
Authors
Erasmus, Rudolph Marthinus
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Abstract
Raman and photoluminescence (PL) spectroscopy techniques were used to investigate the
mechanical properties of diamond and cubic boron nitride (cBN) by optical means. Both these
materials have extreme mechanical properties that make them the material of choice for many
industrial applications, ranging from cutting and grinding to wire-drawing dies. The results
obtained on single crystal diamond, polycrystalline diamond (PCD) and cBN are summarised
below.
Micro-Raman and photoluminescence (PL) spectroscopy were used to map the threedimensional
(3D) stress and deformation distributions surrounding a plastic impression made in a
synthetic, type Ib single crystal diamond. Using data from the Raman peak position, a 3D map of
the stress contours surrounding the impression was generated, while the Raman width data
yielded a map of the plastic deformation volume. The stress contours compare favorably with the
resolved shear stress contours calculated for diamond. PL intensity maps of the zero phonon line
(ZPL) associated with the [N-V]– defect centre at 1.945eV provide images of the extent of
vacancy formation and movement during the impression process. Data concerning the position
and width of the ZPL correspond well with the Raman results.
Polycrystalline diamond (PCD) tools commonly consist of a PCD layer sintered onto a
cobalt-tungsten carbide (Co-WC) substrate. These tools are used in diverse applications and both
the magnitude and distribution of the stresses in the PCD layer affect tool behavior. These
stresses in sample drillbits were investigated by means of micro-Raman spectroscopy. Cyclic
annealing of a sample drillbit to 600 oC shows that the tool properties are retained after 5 cycles,
while similar cycling to 800 oC resulted in a permanent decrease of the average surface
compressive stress. This implies a reduction in the drillbit’s ability to resist crack formation and
propagation and is thus a degradation of the tool properties.
The method of Raman mapping of stress and deformation in diamond was also applied to
single crystals of cBN. Indentations on cubic boron nitride (cBN) crystals and polycrystalline
cubic boron nitride (PcBN) composites were mapped and the shifts of the cBN Raman lines from
their unstressed positions used to quantify the residual stresses in the boron nitride due to the
deformation brought about by the indentation. These were found to be of the order of 1 GPa.
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These measurements illustrated for the first time the use of Raman spectroscopy to study residual
stresses in boron nitride.
Defects in cBN were studied using photoluminescence spectroscopy at low temperature
(< 10 K) of two types of cBN irradiated at ambient temperature with 1.9 MeV electrons. All the
samples were small (<1 mm diameter) single crystals of cBN. Three defect centres (with narrow
lines at 2.28 eV, 2.15 eV and 1.98 eV) were introduced in both the amber-coloured and blackbrown
coloured samples by the irradiation. The amber coloured sample also showed a defect
centre (at 1.65 eV) that is present before and after irradiation. Line shape analysis of the zero
phonon lines of all three irradiation-induced centres showed that the lines are predominantly
Gaussian in character, suggesting that linetype defects such as dislocations are a prevalent
characteristic of these crystals.
Raman spectroscopy of cBN single crystals as a function of temperature was performed
over a wide temperature range from 4 K to 1373 K. The low temperature measurements extended
the data previously reported in literature, as this data ranged from room temperature upwards. It
was concluded from the shift of Raman peak position with temperature that both linear expansion
and anharmonic effects were required to adequately account for the observed data. This is in
agreement with previously published findings. Both 3- and 4-phonon processes were required to
account for the observed linewidths as a function of temperature, again in agreement with
literature.
The results presented here and in the associated journal publications illustrate clearly
how optical spectroscopy techniques can serve as non-destructive characterisation tools for the
mechanical properties of ultra-hard materials.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, in fulfilment of the requirements for the degree of Doctor of
Philosophy