The hardness, modulus, internal stress, fracture toughness and cyclic fatigue properties of polycrystalline chemical vapor deposited (CVD) diamond are being investigated on thick (~100 to 300 mm) free-standing films. Specifically, the fracture toughness, Kc, of diamond was determined using indentation methods and for the first time by the tensile testing of pre-notched fracture-mechanics type compact-tension samples. Measured Kc values were found to be ~ 5.5 MPa-m1/2 by either method and to be apparently independent of grain size and shape. Toughness of CVD diamond film can be enhanced by promoting intergranular fracture in order to induce grain bridging. This is achieved by increasing the soot content in the grain boundaries. Three samples with different graphitic content were studied. The toughness went up from 5.5 MPa-m1/2 to 8.2 MPa-m1/2 after a small increase in the non-diamond carbon content but came down to 5.9 MPa-m1/2 after further addition of such phases. Preliminary micro-Raman measurements indicate higher graphitic content at the grain boundary and the fracture mechanism is observed to be predominantly intergranular. Corresponding experiments to examine susceptibility to cyclic fatigue are currently being performed using indentation-precracked cantilever beams cycled in three-point bend.
Diamond disk-shaped compact-tension DC(T) specimen used to measure fracture toughness Kc values.
Backscattered SEM image of the radial cracks used to measure indentation fracture toughness Kc values.
Prof. Robert Ritchie
“Fracture of Synthetic Diamond,” M. D. Drory, R. H. Dauskardt, A. Kant, and R. O. Ritchie, Journal of Applied Physics, vol. 78, no. 5, September 1995, pp. 3083-3088.
“Fracture Toughness and Subcritical Crack Growth of CVD Diamond,” A. Kant, M. D. Drory, and R. O. Ritchie, Mechanical Behavior of Diamond and Other Forms of Carbon, MRS Symposium Proceedings, vol. 383.