Deformation and Fracture Behaviours of a YAG Single Crystal Characterized Using Nanoindentation Method

In this work, nanoindentation tests were carried out to understand the deformation and fracture behaviours of yttrium aluminium garnet (YAG) single crystal. The elastic modulus, hardness and fracture toughness of the YAG crystal were 267.8 +/- 1.3 GPa, 24.66 +/- 0.37 GPa and 1.82 +/- 0.23 MPa m(1/2) , respectively. A pop-in was observed on the nanoindentation loading curve at approximately 1 mN load, associated with the occurrence of the elastic-plastic transition. When the indentation load was in the range of 1-6 mN, YAG deformed both elastically and plastically. Evident slip band along ( -110) was observed beneath the indent impression. Dislocations, stacking faults, lattice bending and kinking were found in the plastically deformed subsurface. Radial cracks formed when the indentation load exceeded 6 mN, with the half-penny cracks initiated below the surface during unloading. The knowledge of the fracture toughness of YAG and the insights into its deformation behaviour under quasi-static contact attained from this study would greatly benefit the process optimization of ultra-precision machining of YAG single crystal.