High-temperature mechanical characterization of single-crystal sapphire using laser-based ultrasonics

Of current interest in the missile community is the high-temperature mechanical behavior of single-crystal sapphire. For endo-atmospheric infrared (IR) transparent windows, single crystal sapphire is the material of choice(1). However, sapphire has been found to undergo a significant change in the mechanical properties, leading to a potential fracture of the window at the high temperatures encountered during typical flight conditions. A critical figure of merit used in considering a material's usefulness as a dome material is its thermal shock resistance. A material's thermal shock resistance, R, is given by(2): R = S(1-v)k/alpha E where S is the mechanical strength, v is Poisson's ratio, k is the thermal conductivity, a is the thermal expansion coefficient, and E is the elastic modulus. Thus, an important aspect in understanding the material's survivability as it undergoes aero-heating is the study of the elastic moduli as a function of temperature. Single-crystal sapphire displays trigonal symmetry, requiring the measurement of six independent elastic moduli. Laser-based ultrasonics (LBU) offers a non-contact, non-destructive method of measuring the elastic moduli of a material at temperature(3). The Johns Hopkins Applied Physics Laboratory has successfully adapted LBU to the measurement of the mechanical properties of IR transparent materials. Currently, focus is being placed upon studying the behavior of the elastic moduli of single crystal sapphire as a function of temperature.