The stability behaviour of hard material coatings on alloyed steel substrates

Abstract

The stability behaviour of hard material coatings made by CVD on different alloyed and carbon steel substrates depends on the properties of both substrate and coating. SEM in‐situ investigations on the tension loaded surface of bended samples may be used to observe crack nucleation and growth in brittle hard material coatings such as TiC, TiCN on low alloyed and carbon steels. From the crack distance distributions some important parameters of mechanical stability such as the threshold value of fracture σth, the Weibull‐parameter β and the ratio of the strength distribution function F and a characteristic length D may be estimated. The threshold value for fracture in the coatings σth decreases with increasing coating thickness for all steel‐coating combinations. The threshold value also decreases if a heat treatment is made on the steel‐coating system. This is due to the relaxation of the internal stress in the coating. The mean strength of the TiC‐coatings decreases and the strength variance (measured by the Weibull‐parameter β) increases with increasing coating thickness and therefore the mechanical stability of coatings decreases with growing thickness. The ratio of the strength distribution function F and the characteristic distance between defects D as a measure of crack density depends on the deformation behaviour of the steel as well as on the strength and the defect structure of the coating.

Mechanical Properties of TiN Coatings

“…This behavior can be understood by taking into account that the stored elastic energy in the coating is proportional to both its thickness and the square of the acting stress. [12] The higher the thickness, the higher the stored elastic energy. The acting compressive stresses and the stored elastic energy are the ¹driving forcesª for buckling, cracking at the places of highest curvature and at least for complete delamination, where the stored elastic energy is mainly transformed into the energy of the created free surfaces of the substrate and the coating.…”

“…Therefore, the adhesive failure is controlled by the ratio, R, of elastic strain energy to surface energy. [16,12] (4) where d L = layer thickness, E L = Young's modulus of the layer, c = mean surface energy. Now this complex stress state caused by the moving stylus is superimposed on the residual stress and/or the external bending stress.…”