Influence of dovetail microstructures on adhesive tensile strength and morphology of thermally sprayed metal coatings

Abstract

An approach for a reduction of weight and fuel consumption of passenger cars consists in a replacement of grey cast iron by aluminium alloys as base material of combustion engine blocks. Regarding the insufficient chemical resistance of these alloys to the fuels with its additives, material compounds with thermally sprayed coatings are used as cylinder linings. For machining and in service a high connection strength between the substrate and the coating is demanded. This can be gained by a predefined microstructuring of the substrate surface to attain a local form fit with the coating. But, the microstructured surface also influences the local morphology of the layer. Another possibility to improve the connection strength is the application of a primer between the substrate and the coating.For basic investigations, spiral dovetail microstructures with different pitches (200 µm – 500 µm) are machined by face turning using tools with CVD diamond tippings. The substrates consist of the aluminium alloy EN AW-5754 and their microstructured surfaces are coated with a nickel-aluminium layer. The coating with a thickness of about 200 µm is produced by high-velocity arc spraying. The material is usually used as a primer between the substrate and an iron-based coating. The substrate microstructures and the morphology of the coatings are analysed by SEM and 3D laser scanning microscopy. To determine the adhesive tensile strength of the coating tensile adhesion strength tests are performed.The results show that a higher number of structure elements per length increase the tensile adhesion strength, the hardness and the oxide proportion of the coating. Furthermore, tensile adhesion strength tests reveal failure mechanisms of the compound of substrate and coating depending on the substrate microstructure.