A peel adhesion test for thermal spray coatings
Creators: name: Sexsmith, Michael
Publisher: name: University of British Columbia
Year: 2009

description: In order for the thermal spray industry to progress, informative and reliable coating evaluation techniques are needed. The measurement of adhesion is an important problem and existing tests have severe limitations. A peel adhesion test has been adapted to thermal spray coatings from the adhesives industry which should address this problem. In a peel test, a thin metal foil is coated by plasma spraying. The foil is then peeled off of the coating at a constant speed. The force required for separation is monitored as a function of crack position'. The force is than converted into a peel strength which is equivalent to the energy required for separation. The test allows the detection of variations in adhesion within a single sample. Interface defects were detected and correlated with features of the peel test curve. The measured parameter of energy per unit area of fracture, was found to be representative of interfacial fracture toughness. The peel test was used to determine interface toughnesses in the range of 10 to 60 J/m2 for ceramic coatings, 150 to 250 J/m2 for cermet coatings and 160 to 300 J/m2 for metal coatings. A literature review of interfacial fracture mechanics revealed the complexity of determining the interface stress intensities so an energy balance approach is used to determine the energy consumed in separating the coating from the substrate. The balance accounts for the energies associated with adherand deformation, friction, applied loads, and residual stress. Both a numerical and experimental method of determining the work of deformation in the substrate were developed. The numerical method allowed the calculation of the work of deformation based on an arbitrary elastoplastic stress-strain curve for the foil material. The experimental method measured the energy of deformation by imitating the peeling process without an interface. Both methods were in agreement for the adherand materials tested. The deformation work was found to be a significant fraction of the total energy. Deformation energies, when normalized to the foil area, were found in the range 30 J/m2 to 400 J/m2 depending on the material. Friction was also measured experimentally so that all of energies consumed in the test could be determined independently. It was found to add about 10 to 15% to the measure loads. The adhesion of a range of different thermal spray coatings (TSCs) was measured. The adhesion of the spray pattern profile was studied as well as the adhesion of uniform coatings. The results were compared with the ASTM standard for TSC adhesion measurement, the expected range of interface toughnesses and the Vickers hardness of the coating. The comparisons indicate that the measurement is self consistent and produces results comparable to other toughness measurements. A comparison between the ASTM standard test and the peel test was not possible due the fact that the ASTM test is unable to test coatings stronger that 70 MPa. Most of the coatings studied were much stronger than this limit. Comparison with hardness showed that harder and more brittle coatings adhered with lower energies than softer coatings. This is consistent with our understanding of the fracture of materials. It was found that the adhesion of a uniform coating was comparable to the adhesion of the central region of the spray pattern. This indicates that the poor adhesion of the periphery particles does not significantly affect the adhesion of a coating. The need to do further work in understanding the effects of residual stress on bonding was identified. The residual stress in a coating can alter the type of loading which the interface experiences and can reduce the adhesion of a coating to very low values as shown. A complete mechanical description of the peeling process should be developed which includes the effects of residual stress. In order to allow the relationship between the peel test and the service life of a coating, performance comparisons should be made between the tested coatings. The peel test was found to be a simple inexpensive method for reliably determining the adhesion of a thermal spray coating.