Stress distributions and material response in thermal spraying of metallic and ceramic deposits
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I.
INTRODUCTION
T H E R M A L spraying is increasing in technological importance, and a major effort is under way to understand in detail the effects of processing conditions on the structure and properties of the product. Prominent among the phenomena dictating these properties is the generation of stored residual stresses as a result of differential thermal contraction. Spray forming involves the deposited material cooling from its melting temperature while in intimate contact with the substrate. In general, the substrate itself will be heated and the assembly will then cool down to room temperature after spraying is completed. A degree of differential thermal contraction is therefore inevitable, and in practice, very large stresses are readily generated--which are often sufficient to cause spallation, distortion, or the generation of defects such as cracks. For the case of surface protective coatings in particular, these phenomena are undesirable, but there may also be scope for beneficial effects, such as the promotion of compressive stresses in a thin, well-bonded coating, so as to improve its durability. Conventional practice to combat the effects of deleterious stress buildup includes the use of intermediate bond coats and the avoidance of fully sound or thick deposits. Although the details are often unclear, such measures probably function in many cases by simply increasing the system compliance: this reduces stress levels, but it may impair the strength or efficiency of the coating material. As a variety of different stress states can be produced under different conditions, there is probably scope for control and optimization, provided the mechanisms of stress buildup and response are understood in detail. However, it is now evident that the interplay of effects which determines the final stress state is highly complex. There have been several experimental studies exploring the generation of stresses in sprayed coatings, tl-5] For example, Zhuang and Gu m have used X-ray diffraction peak shifts, with progressive mechanical removal of maS.C. GILL, Graduate Student, and T.W. CLYNE, Lecturer, are with the Department of Materials Science and Metallurgy, Cambridge University, Cambridge CB2 3QZ, United Kingdom. Manuscript submitted May 22, 1989. METALLURGICAL TRANSACTIONS B
terial, to examine the through-thickness stress distribution in ZrO2/MgO coatings on steel. This technique is, however, subject to large errors for this application (particularly in view of concem about changes induced by the mechanical removal). A more versatile approach is to measure the curvature induced in a relatively thin substrate/deposit couple or in either of these individually after chemical removal of the other. For example, Hobbs and Reiter t2] noted the curvatures in ZrO2 coatings (with and without a Ni-Cr-A1 bond coat) after chemical removal of the (relatively thick) aluminum substrates. They deduced from their observations that these coatings were in net compression, although their data actually give information only on the
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