Physicochemical aspects of formation of the adhesion bond of detonation coatings. II. Features of failure of the coating-base composite

The problem of studying adhesion, that is, the capacity to resist separation of different materials classically has two aspects of investigation [i], formation of the adhesion contact and failure of the adhesion bond formed because it is only possible to quantitatively evaluate the strength of cohesion by breaking this bond. In this work investigation of the adhesion bond of detonation coatings follows this tradition. The process of formation of detonation coatings has been investigated in [2, 3]. It has been shown that it reflects the specifics of local pulsed deformation of the contacting surfaces. Coatings obtained in one or two cycles (detonations) possess porosity and abut the base poorly. The bond of a thin coating with the base is weak. In order to hold such a coating on the base in preparation of polished specimens special protective measures are necessary (application of an adhesion or an electrolytic layer). The surface relief of the base changes and the number of microprojections per unit of length decreases in comparison with that obtained in abrasive-structural preparation. With an increase in coating thickness its porosity practically disappears, it tightly abuts the base, and the cohesion strength increases. Changes also occur in the base. The depressions at the boundary of contact acquire an oval form, the grains of material under the depressions recrystallize, and the deformation of them, as in explosive welding, reaches 100%. The changes in the structure of the base and coating during forming of it are the result of the effect of hot pressing by the flow of particles. The fact that close to the interface the changes stop after 7-10 detonations indicates damping of the pulse from the impact load by the particles, the pressing action of which extends to a depth of 20-40 ~m. The correlation between the increase in adhesion strength and plastic deformation of the base in the zone of contact indicates an influence on strength of the stressed condition of the contacting materials. The residual stresses in the layers of the base adjoining the coating are compressive after etching off of the sprayed material [4] and their level is 80-100 MPa higher than the yield strength of the metal. Attempts to reduce the level of stresses such as by ultrasonics or a short thermal pulse lead to peeling off of the coating or to loss of strength. The contribution of chemical interaction of the material being sprayed with the base to cohesion strength was studied on specially prepared oblique specimens. Many investigations of the transition zone of coatings of aluminum oxide and of tungsten and chromium carbides with a nickel binder do not make it possible to draw a conclusion about any significant role of chemical reactions in formation of strong cohesion of the coating with the base. It was not possible to observe phases containing iron and nickel with a size of not less than 1 #m, which requires local x-ray spectral analysis. It is not possible for a chemical reaction to occur to full degree under conditions of pulsed spraying. Even if after the first detonation such interaction occurred in thin layers, in subsequent spraying the contact, of the coating with the base was disturbed during reforming of the interface. Therefore the main contribution to the strength of the bond of the coating and the base is made by the stressed and strained condition in the zone of contact. In connection with the anomalously high rates of mass exchange in detonation spraying (an opinion widely prevalent in the literature) it must be noted that the question of the possibility of occurrence of diffusion processes and chemical reactions under conditions of pulsed loading has been discussed for many years.