Residual strain stress coating model

Posted by Alexei Goncharov ( on 14:08:32 21/04/06

Dear Gordon and Colleagues
Even qualitative understanding of the picture of residual stress in coatings could give a chance of adequate choice of the conditions of spraying. In my opinion it s useful to apply the elastic material (rubber, for instance,) expanded and glued to the rigid surface as an analog of coating with strain stress adhered to the substrate. Obviously if rubber O-ring is put on a cylinder both stresses shearing and normal (radial) exist. Due to radial stress the ring is pressed to the cylinder. It means that the respective coating will press the substrate after relaxing of heat given by spraying process. Existing risk of cracks parallel to the axis of cylinder can be minimized by preheating of the substrate. Preheating gives preexpanding of a substrate and increase a chemical reaction rate in adhesion zone. It s worth to remark here that expansion must not be too much as otherwise the shrinkage of the substrate could exceed that of coating and detaching occurs. If we deal with plane substrate and the rubber brick is glued to the surface in strain state the body of the rubber will undergo only shearing (parallel to the substrate) stress everywhere but lateral edges. The free side of the rubber brick and the body of it will slightly short the dimensions in contrast of glued side. The cross section of the brick will transform from a rectangle to a trapeze. The mentioned lateral sides of trapeze will be strained by as shearing as normal stress. The last is directed from the surface of the substrate acting against the glue adhesion. The more degree of shorten dimensions, the more the length of lateral side, the more adhesion needs to keep the rubber edges. In the case of spraying coatings said means, for instance, that cooling of the surface of coating after the end of spraying process increases the risk of detaching of edges of the coating. I will be grateful for any comments and remarks
Best regards
Alexei Goncharov

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