Fracture of synthetic diamond

The fracture behavior of synthetic diamond has been investigated using indentation methods and by the tensile testing of pre-notched fracture-mechanics type samples. Specifically, the fracture toughness of free-standing diamond plates, grown by chemically-vapor deposited (CVD) methods, was measured using Vickers indentations and by the use of disk-shaped compact-tension specimens; the latter method provides an evaluation of the through-thickness fracture properties, whereas the indentation method was performed on the nucleation surface of the sample. Measured fracture toughness (Kc) values were found to be approximately 5–6 MPa√m by both methods, indicating that the fracture resistance of CVD diamond does not vary appreciably with grain size (within the certainty of the testing procedures). Complications, however, arose with the fracture-mechanics testing regarding crack initiation from a relatively blunt notch; further work is needed to develop pre-cracking methods to permit more reliable fracture toughness testing of diamond.