How do I measure the adhesion of thermal sprayed coatings?

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 to determine coating adhesion 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.