Fracture strength of optical quality and black polycrystalline CVD diamonds

Three-point method has been used to measure bending strength σf of optical quality and opaque polycrystalline diamond films with thickness in the broad range of h = 0.06–1.0 mm grown by microwave plasma CVD. The free-standing films were characterized with microRaman spectroscopy, SEM, and optical profilography. For transparent samples the value of σf is found to approach 2200 MPa for thinnest sample when the substrate side is under tensile stress, reducing with film thickness to 600 MPa at h ≈ 1000 μm, while for substrate side under the tension exhibits the strength a factor of two lower. The material tested shows transcrystallite fracture and the strength increase with grain size reduction. Also evaluated are Young modulus E = 1072 ± 153 GPa, and the Weibull moduli m = 6.4 and m = 4.5 for the growth and substrate sides under tension, respectively. In contrast, the (100) textured black diamond films with pronounced columnar structure demonstrate intergranular fracture mode due to relatively weak (with non-diamond carbon component) grain boundaries, lower fracture surface roughness, and the two times lower strength compared to the “white” diamond.

► The bending strength of polycrystalline diamond films of two grades, of optical quality with thickness in the broad range of 0.06–1.0 mm, and opaque ones, both grown by microwave plasma CVD, has been measured a three-point method. The virgin and fractured free-standing films were characterized with microRaman spectroscopy, SEM, and optical profilography.
► While the strength of transparent material turns out to be pretty high (up to 2200 MPa for the thinnest 60 μm samples), the < 100 > fiber texture black diamond films, due to weak defected grain boundaries, show integranular fracture and two times lower strength compared to the “white” diamond of comparable thickness.
► This result underlines the impact of grain boundary structure on mechanical properties of polycrystalline diamond.