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. Themost 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 cylindricalspecimen 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 crosssectional 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, failurewill 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.
This test method is described by the ASTM standard C633-01.
An alternative approach allows measurement of coating adhesion on flat plate specimens. The test uses apparatus known as a portable pull-off adhesion tester and follows the method described by the ASTM standard D 4541 - 95. Theadhesion tester uses a micro hydraulic system to apply a tensile load to a cylindrical test piece that is adhesively bonded to the coating surface. The pull force is distributed evenly and dynamically through the test specimen. Theforce is increased while balanced continually between the four hydraulic legs until fracture occurs.
References
| ASTM C633-01 | Standard test method for adhesion or cohesive strength of flame-sprayed coatings |
| ASTM D4541-1995 | Test method for pull-off strength of coatings using portable adhesion-testers |
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