An experimental method for evaluation of the load-carrying capacity of coated aluminium: the influence of coating stiffness, hardness and thickness

If a brittle ceramic PVD coating is applied to a soft and compliant material such as aluminium, it might crack when the component is taken into use, i.e. when it is loaded. This means that the load-carrying capacity of the soft Al substrate has to be improved if a PVD coating should be applied successfully. One solution is to introduce a relatively thick intermediate load-carrying layer between the soft substrate and the thin ceramic PVD coating. This paper presents a new experimental test method to find the load-carrying layer thickness required to protect a soft substrate from plastic deformation. The method is evaluated, but also used to investigate different load-carrying layers on aluminium. In the experiment, rigid cemented carbide balls were used to indent aluminium coated with, compared with PVD coatings, relatively thick (50–150 μm) foils. The foils were firmly attached to the aluminium substrates using a fusible and strong adhesive. After indentation the foil/coating was detached and the amount of plastic deformation on the substrate surface was determined by measuring the depth of the residual indent. The influence of foil thickness, stiffness and hardness on the resistance to plastic substrate deformation was studied. It was found that the technique of gluing foils on Al well simulates the load-carrying properties of a traditionally deposited layer. The indentation experiments also showed that the load-carrying capacity of the coated aluminium increases with the thickness of the foil and the elastic modulus of the foil. No significant influence of the foil/coating hardness on the load-carrying capacity could be seen.