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Elasticity is the ability of a coating material to resist changes in its volume or shape under mechanical stress due to increase in internal energy. The main characteristic of this property is the elasticity modulus (Young's modulus), which describes the resistance of a coating material to deformation. Numerically it represents the ratio of applied stress increment to the ensuing increment of elastic deformation. In other words, elasticity is a measure of the stiffness of the coating material.Resistance to elastic deformation is not only used as a measure of strength and hardness of material. This value is also required to calculate residual stresses, one of the main characteristics of a coating. Other common elastic properties, such as shear modulus and the Poisson coefficient, are rarely determined for coatings.Below the point corresponding to the limit of proportionality, the deformation in medium-carbon and high-carbon steels is proportional to the applied stress, and the slope of the linear part of the bending curve characterizes the bending elasticity modulus. Further loading of bulk metal specimens leads to deviation from Hooke's law (i.e. the metal flows). The behavior of metal coatings of similar chemical composition is quite different (Fig. 2.l, curve 2). (J 8 Fig. 2.1. Bending stress (a)-deformation (b) diagram for several types of coatings. 1 nonmetallic inorganic brittle coating; 2 high-strength metal coating; 3 metal coating of medium and low-strength L. I. Tushinsky et al., Coated Metal

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