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Adhesion

Reason for use:

The adhesion of sprayed coatings to substrates is a matter of great concern to Engineers.

Coatings which become detached during machining must be re-applied whereas those which fail during service will not only cause failure of the coated part but may seriously damage other components and could lead to injuries.

However, provided that the basic rules for using sprayed metal coatings are applied and that materials are correctly sprayed on to properly prepared surfaces, bond failures are rare.

Many techniques have been used to assess the adhesion of coatings. The most commonly employed involve pulling in tension a known area of coating from a suitably prepared substrate. In order to do so, it is necessary to attach a pulling device to the coating with a suitable adhesive.

This method has the advantage of giving a load failure and, knowing the area under test, a failure of bond strength can be calculated. Unfortunately, the test is generally restricted to test pieces which bear little resemblance to engineering components.

Although the test is simple, it is subject to many variables: the strength and curing state of the adhesive; the degree of penetration into porous sprayed deposit (depending on porosity, coating thickness, adhesive viscosity, etc.,) axiality may not be achieved during testing (which may give rise to sheer and peel stresses as well as tensile stresses at the interface).

Complete detachment of the deposit rarely occurs and the fracture is a mixture of bond and cohesive failure. These factors combine to produce considerable scatter and test results and quoted bond strengths should be treated with considerable caution. If adhesion is critical, it is strongly recommended that a practical evaluation of a sprayed component be made before specifying a particular sprayed deposit.

The adhesion test may then be used as a quality control tool rather than a design aid.

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