Study of HPHT single crystal diamond as precision cutting tool material

High pressure high temperature (HPHT) synthetic diamond is rapidly replacing natural diamond in precision machining industry due to its consistency in quality and cost. It is believed by some manufacturers that HPHT diamond cutting tool is virtually free of impurities, inclusions and cracks. This paper is to analyze defects inside HPHT diamond crystal by density test, optical microscopy, SEM, Raman, and XRD. Results show that HPHT diamond is relatively clean, with less chemical elements inside in comparison with natural diamond. However, there are cracks, pit holes, and bubbles in/on the crystal, some tracing amount of cementite (Fe3C), γ-Fe, and nickel based carbides also exist in the HPHT diamond crystal, causing less accurate cutting tool edge and surface integrity. Residual stresses in HPHT diamond are compressive but high in value, may lead to cutting tool brittleness and micro-chipping, which is a common reason for diamond tool failing in precision machining.

► This research reveals that HPHT diamond has lower density (3.396–3.452 g/cm3) than its natural peers (3.52 g/cm3) in our test.
► Its residual stress is compressive and is in the range of 4.52–4.59 GPa, which could be desirable as a cutting tool material.
► SEM study shows that HPHT diamond is 96.15% of carbon and 3.85% of oxygen by weight, or 97.08% carbon and 2.92% oxygen by atom.
► Both Raman and XRD tests further confirm the samples are high quality diamond with no graphite inclusions.
► Surface analysis with optical and SEM microscopes reveal numerous pit holes on HPHT SCD surface and multiple bubbles are visible inside the crystal, which are signs of defects.