Anisotropic Brittle-Ductile Transition of Monocrystalline Sapphire during Orthogonal Cutting and Nanoindentation Experiments
Author(s): Philipp Maas, Yuta Mizumoto, Yasuhiro Kakinuma, and Sangkee Min
Publication: Nanotechnology and Precision Engineering
Citation: Nanotechnology and Precision Engineering, Vol. 1, No. 3, pp.157-171, 2018.
Single-crystal sapphire is utilized as a high-performance engineering material, especially in extreme and harsh environments. However, due to its extreme hardness and brittleness, the machinability of sapphire is still a challenge. By means of nanoindentation and plunge-cut experiments, the anisotropic brittle-ductile transition of the prismatic M-plane and rhombohedral R-plane is examined by analyzing crack morphologies and the critical depth-of-cut (CDC). The experimental results of the nanoindentation tests are correlated to the plunge-cut experiment. Both the prism plane and the rhombohedral crystal plane exhibit a two-fold symmetry of ductility with various crack patterns along the machined grooves. The direction-dependent plasticity of the hexagonal sapphire crystal is mainly connected to a twinning process accompanied by slip dislocation.