Investigation of the fracture toughness of radio frequency magnetron sputtered Al–Cu–Fe films via white-beam synchrotron radiography/topography

Abstract

A novel white-beam synchrotron radiography/topography substrate curvature technique has been used to study stress development in situ during annealing of Al–Cu–Fe quasicrystalline and approximant coatings, as well as to image their failure modes in real time. Single crystal Si and sapphire substrates were coated with a 2.55 µm precursor coating by RF sputtering from an Al65Cu23Fe12 powder composite target and subsequently annealed at 585°C while stress and imaging data were acquired. After the initial ramp to the annealing temperature, a stress plateau was reached for coatings on both Si and sapphire substrates, although the magnitude of the stress plateau was different in each case. A tensile stress developed in the coatings during cooling due to differential thermal expansion between the coating and substrate, allowing for calculation of both the coefficient of thermal expansion and elastic modulus of the film. During cooling, the films exhibited different stress evolution above and below 470°C, a temperature of interest in Al–Cu–Fe quasicrystal and approximant phase development. The Al–Cu–Fe coating on the Si substrate fractured at approximately 954 MPa, while the coating on the sapphire substrate fractured at approximately 431 MPa. From these values the fracture toughness was calculated to be 1.9 MPa m1/2 and 0.76 MPa m1/2 for the coatings on Si and sapphire, respectively.