Posted by Alexei Goncharov (195.96.79.2) 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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