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Fracture strength of optical quality and black polycrystalline CVD diamonds

: Ralchenko, V.G.; Pleuler, E.; Lu, F.X.; Sovyk, D.N.; Bolshakov, A.P.; Guo, S.B.; Tang, W.Z.; Gontar, I.V.; Khomich, A.A.; Zavedeev, E.V.; Konov, V.I.


Diamond and Related Materials 23 (2012), S.172-177
ISSN: 0925-9635
Fraunhofer IAF ()

Three-point method has been used to measure bending strength f 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 f 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 m, 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 mod e 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.