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 coefficien ts of thermal expansion of surface coatings of homogeneous and graded compositions. In this method, a number of identical substrate specimen s are coated simultaneously with surface layers of fixed or graded com positions, and specimens with different layer thicknesses are periodic ally removed from the deposition chamber. It is shown that the followi ng results can be obtained from a knowledge of the strain or curvature and thermal history of the coated specimens, in conjunction with simp le four-point bend tests and thermal loading/cycling at different temp eratures: (i) the magnitude of the processing-induced intrinsic stress es through the thickness of the coating, (ii) the in-plane Young's mod ulus, E, as a function of the coating thickness, (iii) the coefficient of thermal expansion, alpha, as a function of the coating thickness, (iv) the variation of E and alpha as a function of temperature at any thickness location within the coating, and (v) the separation of inter nal stresses arising From thermal expansion mismatch between different constituent phases or layers from those arising from the deposition p rocess (so-called ''intrinsic'' or ''quench'' stresses). The thermomec hanical analyses underlying this method are discussed in detail, and i ts 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 homogen eous and graded compositions. (C) 1997 Acta Metallurgica Inc.