Determination of processing-induced stresses and properties of layered and graded coatings: Experimental method and results for plasma-sprayed Ni-Al₂O₃

An experimental method is proposed which enables the determination of processing-induced intrinsic stresses, elastic modulus, and coefficients of thermal expansion of surface coatings of homogeneous and graded compositions. In this method, a number of identical substrate specimens are coated simultaneously with surface layers of fixed or graded compositions, and specimens with different layer thicknesses are periodically removed from the deposition chamber. It is shown that the following results can be obtained from a knowledge of the strain or curvature and thermal history of the coated specimens, in conjunction with simple four-point bend tests and thermal loading/cycling at different temperatures: (i) the magnitude of the processing-induced intrinsic stresses through the thickness of the coating, (ii) the in-plane Young's modulus, E, as a function of the coating thickness, (iii) the coefficient of thermal expansion, α, as a function of the coating thickness, (iv) the variation of E and α as a function of temperature at any thickness location within the coating, and (v) the separation of internal stresses arising from thermal expansion mismatch between different constituent phases or layers from those arising from the deposition process (so-called "intrinsic" or "quench" stresses). The thermomechanical analyses underlying this method are discussed in detail, and its significance and limitations are addressed. The proposed method is used to determine the evolution of processing-induced stresses during the successive build-up of plasma-sprayed Ni-Al₂O₃ coatings of homogeneous and graded compositions.