Theoretical Estimation of the Strength of Thin-film Coatings

Abstract

The theoretical strength of a perfect crystal lattice, which corresponds to the simultaneous breaking of intermolecular bonds, is very high – only ten times less than the Young’s modulus. Strength of real solids is several orders of magnitude smaller. This is linked with the existence of lattice defects. In the work of the various types of defects are considered only crack. Although really brittle materials is very small. Question of cracks in brittle solids is of great practical importance because many plastic materials (metals) are destroyed “brittle” manner. Problem of brittle fracture paid much attention. Trying to explain the discrepancy between the actual and theoretical values of strength in the presence of cracks was made by Griffith in his theory of brittle fracture of amorphous materials. He suggested that the real materials have a large number of small cracks, which can act as stress concentrators, increasing their value to the theoretical strength. The process reduces the gap while increasing the length of cracks until complete separation of the sample into two parts. In this paper we propose a simplified model of the phenomenon of destruction of elastic material, which allows to establish a connection between the theoretical tensile strength and the actual breaking stress. The model is based on the idea that each representative elementary particle continuum elastic medium incorporates the micro-crack. So that the material, as in Griffith’s theory, there is a whole network of micro-cracks. Fracturing process occurs due to the merger with the apparent micro-cracks (assuming no cracks) voltage equal to the actual tensile strength, and tensile material reaching peaks between neighboring cracks voltage theoretical tensile strength. Evaluation carried out calculations of the magnitude of micro-cracks and damage some materials. Installed in the connection between the theoretical tensile strength and the real destructive voltage calculation results correspond to known concepts and reference data.