Metallic and ceramic residues usually accompany the diamond/CBN feedstock materials. During size reduction or milling stage of the micronizing process, intrinsic impurities (metallic and graphitic) trapped within the crystals, are released and introduced as extrinsic impurities to the resulting micron powder. Metallic impurities are also introduced as a result of mechanical wearing of the milling equipment.
Silicon oxide (SiO2), silicon carbide (SiC) and aluminium oxide (Al2O3) are the most common ceramic contaminants, while iron, nickel and chromium account for the most common metallic contaminants. Ceramic and metallic impurities are removed from the micron powder during cleaning and purification stages. If present in feedstock material, ceramic residues are difficult to remove through regular cleaning processes; which is why micron powders available on the market are sometimes contaminated with materials such as Al2O3, SiO2 and SiC.
Extrinsic or Bulk Impurities
The presence of extrinsic impurities (non-diamond or non-CBN materials) in superabrasive powders is known to alter its performance. Therefore, superabrasive powders consisting of 100% diamond/CBN material are required, regardless of the application.
Intrinsic or Trapped Impurities
The catalytic high pressure-high temperature synthesis of diamond and CBN involves a solvent-catalyst that assists the phase transformation (graphite to diamond or hexagonal BN to cubic BN). Hence, all diamond and CBN materials synthesized by the HP-HT catalytic process contain impurities that are trapped in the diamond/CBN crystal during the crystal growth process. The most common impurities in diamond crystals are Fe, Co, Ni, and Mn, while most common impurities in CBN crystals are Li, Ca, Mn, Al, Si, and Fe.
To a large extent, the intrinsic impurities as internal crystal defects are responsible for the mechanical, thermal and optical properties of the crystal. The level of intrinsic impurities has a significant impact on mechanical strength and thermal stability.
Technique and Equipment
At Engis we use inductively coupled plasma–optical emission spectroscopy (ICP-OES) as quantitative chemical analysis, which can successfully measure the level of extrinsic and intrinsic impurities.
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