X-ray diffraction topography was used to investigate the relationship between sub-surface damage, near-surface microstructure, and fracture strength in a series of sapphire modulus of rupture (MOR) bars which had been fabricated to proof test fabrication processes. The strength of the bars was determined by failure in four point bending. The tensile surface of the bars was examined using optical microscopy and non-contacting surface profilometry. Both show that the bars have good surface finish, with typical RMS roughness of 0.7 nm. No correlation was found between the surface finish and fracture strength. Although the bars appeared to be indistinguishable, topographs taken prior to fracture testing revealed that they are of two distinct types. Type 1 has an oriented microstructure consisting of a pattern of linear features running the length of the bars. Type 2 was typical of well-polished sapphire, containing individual dislocations and occasional damage from handling. We attribute Type 1 microstructure to fabrication damage that was not removed by subsequent processing and/or polishing. Fracture strength data showed that the Type 1 (damaged) bars had strengths less than 70 % of the bars without damage. Topography is sensitive to surface damage which can be correlated to fracture strength. Neither low magnification optical microscopy nor conventional surface finish statistics could be correlated to strength.
Proceedings Title: Materials Research Society Symposium
Conference Dates: December 1, 1999
Conference Location: Warrendale, PA
Conference Title: Materials Research Society
Pub Type: Conferences
sapphire, surface damage, surface finish, x-ray topography