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 materia l 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 so ft Al substrate has to be improved if a PVD coating should be applied succe ssfully. 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-carryi ng layer thickness required to protect a soft substrate from plastic deform ation. The method is evaluated, but also used to investigate different load -carrying layers on aluminium. In the experiment, rigid cemented carbide ba lls were used to indent aluminium coated with, compared with PVD coatings, relatively thick (50-150 mu m) foils. The foils were firmly attached to the aluminium substrates using a fusible and strong adhesive. After indentatio n the foil/coating was detached and the amount of plastic deformation on th e substrate surface was determined by measuring the depth of the residual i ndent. The influence of foil thickness, stiffness and hardness on the resis tance to plastic substrate deformation was studied. It was found that the t echnique 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. (C) 2000 Elsevier Science S.A. All rights reserved.