Mechanical properties of and cracks and wrinkles in vacuum-deposited MgF 2, carbon and baron coatings

Abstract

The mechanical properties of coatings of MgF 2, carbon and boron were investigated. The internal stress in the MgF 2 coatings on glass substrates, measured by the cantilever method, was tensile and was about 4×10 9 dyn cm −2, while those of carbon and boron were compressive and were about 7×10 9 dyn cm −2 and 2×10 9 dyn cm −2 respectively. Young's moduli of MgF 2 and carbon were determined by the vibrating reed method to be 15×10 11 dyn cm −2 and 2×10 11 dyn cm −2 respectively. Cracks were observed in sufficiently thick MgF 2 coatings on glass substrates and the adhesion energy was estimated to be 270 erg cm −2 from the measured values of the internal stress, Young's modulus and crack initiation thickness. Wrinkles were generated under certain conditions in carbon and boron coatings. It was found that exposure of the coatings to the atmosphere was essential to the initiation of the wrinkles.