Effect of temperature on the mechanical properties and anisotropy of C-plane sapphire: Insights from indentation tests and MD simulations

Abstract

Sapphire, known for its exceptional properties, is widely used in various applications requiring superior surface quality and precise geometry. However, sapphire's inherent hardness, brittleness and anisotropy pose significant challenges during machining. To address these issues, the impact of temperatures on mechanical properties and anisotropy of C-plane sapphire was studied to enhance its processability. MD simulations were employed to unravel the underlying mechanisms of material microstructure changes at various temperatures. Additionally, Berkovich and Vickers indentation tests were performed at 25 °C and elevated temperatures (up to 1000 °C) to evaluate the anisotropy of brittle fracture and hardness. Results revealed increasing temperature led to a significant reduction in sapphire's hardness, brittle fracture and anisotropy. MD simulations showed dislocation proliferation was the primary driver of hardness reduction below 600 °C, while above this temperature, the abundance of amorphous atoms predominated. The formation of amorphous atoms and face-centered cubic structures disrupted the original crystal lattice, contributing to a reduction in anisotropy. Importantly, the hardness difference between different indenter orientations diminished with increasing temperature, stabilizing at minimal levels beyond 600 °C. This study recommends a temperature of 600 °C for laser-assisted processing or surface modification of sapphire to effectively mitigate its hardness, brittleness, and anisotropy, thereby improving its processability.