Maximising the Interfacial Fracture Toughness of Thin Coatings and Substrate through Optimisation of Defined Parameters.

Abstract

The influence of three parameters i.e. interfacial roughness, coating thickness and the size of impurity at the interface on interfacial fracture toughness has been investigated within the framework of two approaches i.e. thermodynamics and fracture mechanics. Mathematical relationship for both the approaches have been designed independently and then fused to form a governing law for evaluating the interfacial toughness. Simulation techniques founded on the experimental studies, have been developed during this research in order to find the optimised values of three parameters. These optimised values act as critical values (boundary point) between coating fail-safe and coating fail conditions and can be used to avoid coating failure due to loss of interfacial toughness. The experimental design considering three parameters has been divided in to the three setups. Each setup is used to analyse the effect of one variable parameter on interfacial toughness while keeping the other two parameters constant; (i) Setup 1: Constant coating thickness and constant impurity size with variable roughness (ii) Setup 2: Constant roughness and constant impurity size with variable coating thickness (iii) Setup 3: Constant coating thickness and constant roughness with variable impurity size. Three samples for each setup were prepared considering the requirements of constant and variable parameters for each setup. ASTM-B117 salt spray (fog) test methodology was deployed for conducting experiments. Conditioned samples were exposed to simulated environment in the test chamber for up to 150 hrs. Experimental observations were recorded every 24hrs. The effect of ASTM-B117 test environment on interfacial toughness of coated samples under each setup has been studied using 3D- surface interferometry, scanning electron microscopy (SEM) and micro-indentation techniques. These experiments showed excellent, quantitative agreement with the simulation trends predicted by the theoretical model.