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FAQ: What quality assurance measures can be applied to thermally sprayed coatings?

   

Frequently Asked Questions

NDT methods for assessing general coating quality have limited success with sprayed deposits.

Current NDT techniques do not give reliable measures of coating features such as cracking, porosity, oxide levels, coating disbond or adhesion. Crack detection by dye penetrant techniques often fails to detect isolated defects against the general presence of porosity. Radiography, eddy current and ultrasonic inspections are of little use because of the heterogeneous nature of sprayed coatings and the small dimensions of features and defects.

Therefore, greater emphasis must be given to destructive techniques such as adhesion tests and metallography. Destructive testing on a small percentage of components is acceptable where large numbers of similar articles are being processed. However, many thermal spraying jobs involve coating of small numbers of components, and often in reclamation work, only one part may be repaired. In these circumstances, it is necessary to coat a test coupon. It is essential to ensure that the coating on the test coupon is similar to that on the component. This requires not only consistency in spraying conditions but also an appreciation of the effects of, for example, differences in component section thickness and cooling rate on deposit structure and properties.

Metallographic examination is widely used to assess deposit quality, but care must be taken in preparation of the section and in interpretation of the results to avoid misleading conclusions. At its best, such examination can show up defects such as cracking, inclusions and un-melted particles, give an indication of porosity and highlight features at the interface.

Hardness is commonly measured on coating cross sections, but its usefulness is limited. Tests of the Brinell or Vickers type give a general indication of coating quality, but the presence of porosity will lead to an apparently low hardness compared with that of similar solid material.

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The adhesion of a thermal sprayed coating is often an important property to determine, particularly as the coatings are not fused to the substrate and coating adhesion is very dependent on surface preparation prior to coating. The most common approach is to measure the tensile load needed to detach the coating from the substrate. The test usually consists of depositing the thermal sprayed coating on to the face of a cylindrical specimen and then bonding the face of a similar uncoated cylinder to the coating surface using a high strength epoxy resin. The tensile strength of the coating results from dividing maximum load applied at failure by the cross sectional area. The bond strength is given if failure occurs at the coating substrate interface. The cohesive strength of the coating is given if the failure occurs entirely within the coating. Often, for the better coatings, failure will occur in the epoxy adhesive. This gives an upper bond strength as measured by this method of about 80MPa, i.e. the tensile strength of the adhesive.

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