TWI Industrial Member Report Summary 554/1996
By F J Blunt
Background
Sprayed coatings are variously used to delay corrosion or wear of components in service and one of the most fundamental requirements is good adhesion to the substrate. Alumina has several excellent properties which can be exploited when it is applied as a surface coating. It is resistant to many corrosive liquids, is a good electrical insulator, and has good resistance to abrasive wear.
For a coating such as alumina, it is generally believed that the adhesion of the coating is strongly related to mechanical keying between the sprayed particles and the substrate surface, and hence grit blasting of the substrate surface is routine preparation for spraying. To date no systematic study has been carried out to find out whether there is an optimum roughness, or whether other parameters, such as fuel gas or interpass temperature and time have a significant effect on the adhesion of High Velocity Oxyfuel (HVOF) sprayed alumina coatings.
The most common test of adhesion is the ASTM C633-79 tensile test. This test is adequate for many types of coating, including ceramics and most metals. However, it is relatively expensive, slow and it requires a special test geometry. An alternative more rapid test is available commercially (P.A.T. tester), and this was chosen for comparison. This test is in principle very similar, but the geometry of the test allows it to be carried out directly on sprayed components provided there is a sufficient flat area on which to place the jig.
Objectives
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To identify the best method of preparing of steel surfaces prior to spraying of alumina
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To determine whether the particle velocity, fuel gas or interpass time had a significant effect on adhesion
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To determine whether the P.A.T. test method for coating adhesion gave comparable results to the ASTM C633 pull-off test