Abstract: Three free-standing diamond films with 125 mm in diameter and thickness exceeded 1 mm were deposited using DC arc plasma jet and microwave plasma chemical vapor deposition. In order to reduce the influence of sample size on the results of fracture toughness, diamond film samples were cut into square cross-sections. Morphology and phase were characterized by SEM, XRD, Raman, CT, et al. Influences of defects and grain size on fracture toughness in three directions of growth surface, nucleation surface and edge surface were studied. The results indicate that during the growth process, many defects (including pores) are introduced into the film. Especially, on the side near the growth surface in thicker films, the size of pores reaches the micrometer level, so that affects the fracture toughness under different directional loads. Due to the smallest grain size, all diamond free-standing films have the maximum fracture toughness at growth surface, which are 7.8, 8.3, and 9.2 MPa·m^1/2, respectively. For thinner samples, the fracture toughness of edge surface cracks is between growth surface and nucleation surface. This is consistent with the relationship between grain size, indicating that fracture toughness is influenced by grain size. However, when the thickness exceeds 0.8 mm, the number of pores near the growth side will increase, resulting in the smallest fracture toughness (5.4 MPa·m^1/2) of diamond films with thicker side. This study provides guidance for selecting the direction of load application.

Key words: diamond thick film, fracture toughness, CVD, grinding, defect, grain size