Measurement of High Temperature Elastic Moduli of Infrared Transparent Materials

Insight into a material’s high temperature mechanical and microstructural properties can be gained from knowledge of its elastic moduli. For a single-crystal material, the elastic modulus, Cijkl, provides information on the strain (εkl) stress (σij) relationship given by σij = Cijkl•εkl. This relationship shows that, for a given strain, a large elastic modulus will induce a large stress in the material. In regards to mechanical properties, the crystallographic axis with the largest elastic modulus is the most likely for failure to occur. Before any IR transparent material can be considered for application in a high stress environment, such as IR dome applications, the elastic moduli must be known. Laser-based ultrasonics provides a non-contact, non-destructive means of measuring the elastic moduli of IR transparent materials in an elevated temperature environment. In this paper, the application of laser ultrasonics to IR materials characterization is reviewed. Specific calculations for determining the elastic moduli for isotropic and trigonal crystal symmetries, as exhibited by single-crystal sapphire, are presented. Measurements of the elastic moduli as a function of temperature for borosilicate glass and fused silica, elastically isotropic materials, are presented. In addition, a room-temperature ultrasonic measurement of germanium (Ge), an elastically anisotropic material, is shown.