A comparison of different adhesion test methods for thermal sprayed coatings

Following testing methods have been compared and their response assessed to parameter changes in the process of plasma spraying and substrate pretreatment: Pull-off-test, hardness-indentation-test, shear test, and fracture mechanics by 4-point-bend test and double- torsion-test. Different effects on the adhesion properties like mechanical locking, physical bonding forces, residual stress enter the test results in a different way. Thus the results prove, that the shear test is revealing the effect of mechanical locking of coating material with the roughened substrate surface better, while the interfacial hardness indentation test is more sensitive to the physical bonding forces between coating and substrate. Between those two methods the pull-off test takes an intermediate position. Both the 4-point-bend test and the double torsion test arrangement don't give an unimbigous decision, whether the cohesive or the adhesive strength has been measured, because the fraction course run near the interface in the coating material