Diffraction analysis of nonuniform stresses in surface layers: Application to cracked TiN coatings chemically vapor depo
- PDF / 631,455 Bytes
- 18 Pages / 612 x 792 pts (letter) Page_size
- 48 Downloads / 173 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Diffraction analysis of nonuniform stresses in surface layers: Application to cracked TiN coatings chemically vapor deposited on Mo W. G. Sloof, B. J. Kooi, R. Delhez, Th.H. de Keijser, and E. J. Mittemeijer Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands (Received 1 August 1994; accepted 13 October 1995)
Variations of residual stresses in layers on substrates can occur in directions parallel and perpendicular to the surface as a result of compositional inhomogeneity and/or porosity or cracks. Diffraction methods to evaluate such stress variations are presented. Comparison of the experimental value for the stress with a calculated value of the “diffraction-averaged stress,” on the basis of a model for the local stresses, proved to be a useful method of stress analysis. It is shown that a direct evaluation of occurring stress-depth profiles is less practical. The method of stress analysis proposed, is applied to chemically vapor deposited TiN coatings on Mo substrates. In these coatings a large tensile stress parallel to the surface develops during cooling from the deposition temperature, due to difference in thermal shrink between coating and substrate. As a result of the cooling-induced stress, cracking of the coating occurs. The mesh width of the crack pattern allows determination of the fracture-surface energy and the fracture toughness of the coating material. Conceiving the cracked coatings as assemblies of freestanding columns, and assuming full elastic accommodation of the thermal mismatch at the column/substrate interface, the stress variations in the coating are calculated. On this basis the diffraction-averaged stress and the depth profile of the laterally averaged stress can be predicted accurately for the cracked TiN layers.
I. INTRODUCTION
The presence of residual stresses in materials can largely govern their properties.1–5 This in particular holds for thin films and coatings. Comprehensive knowledge is not available on the development and relaxation of stresses in layer/substrate assemblies. The case normally considered involves a homogeneous layer subjected to a laterally constant and depth independent stress.6,7 However, the occurrence of compositional inhomogeneity, porosity, and cracks induces residual stress variations in directions parallel and perpendicular to the surface of the layer. The treatment for the (x-ray) diffraction analysis of a composition-depth profile is presented in Refs. 8–10. The present paper deals with stress variations in a thin layer. In particular, the attention will be focused on the case of a layer exhibiting loss of internal coherency by the presence of cracks; i.e., the noncoherent layer is composed of entities that more or less are disconnected with respect to each other, but are strictly (coherently) bonded to the joint substrate at the layer/substrate interface, where the stress-inducing misfit is imposed. The stress distributions in the layer will b
Data Loading...
