Distribution of internal strain and fracture strength in various single-crystal diamonds

The internal strain states of three typical types of industrial single-crystal diamonds, i.e., natural type Ia, synthetic type Ib and synthetic type IIa diamonds, were quantitatively evaluated by birefringent retardation (Δn), residual stress and microfracture strength. The variation width of retardation between crystals was Δn = 10 to 200 nm for natural Ia diamonds, Δn = 10 to 100 nm for synthetic Ib diamonds, and Δn = 1 to 10 nm for synthetic IIa diamonds. Natural Ia diamonds with a retardation of 80 nm or more had a mosaic-shaped strain, and diamonds with moderate or less strain exhibited growth striation-like strain. Synthetic Ib diamonds showed no such patterns, but radial patterns were observed. No distinct strain patterns were observed in the synthetic IIa diamonds. The residual stress (GPa) distributions within each crystal were estimated by the Raman peak shift (cm−1). The variation in the residual stress of the natural Ia diamonds was about ±0.1 GPa, while that in the synthetic Ib diamonds was as small as ±0.005 GPa, and almost no variation was observed in the synthetic IIa diamonds. The distribution of microfracture strength of each crystal was evaluated by measuring the crack initiation load (N) with a spherical nano-polycrystalline diamond (NPD) indenter of Φ50 μm. The variation in the crack initiation load within the crystal was also as large as 21 N (from 4 to 25 N) for natural Ia diamond, but as small as 6 N (12–18 N) for synthetic Ib diamond, and even smaller as 3 N (19–22 N) for synthetic IIa diamond. In addition, in synthetic Ib diamond, localized internal strains were observed where dislocation defects derived from the seed crystal existed, and it was confirmed that the residual stress at these locations changed largely and the strength decreased.