"Instrumented Strain-Gage Measurement Of Functional Coating Adhesion Strength"

ASTM C633 has been an industry standard for determining thermal spray coating adhesion for nearly 40 years. The test, however, has several drawbacks that can greatly affect the results including epoxy curing procedure, specimen alignment, and coating surface preparation. The epoxies used cannot withstand stresses greater than 12,000psi, producing data that suggests coatings cannot function beyond the epoxy threshold and data that is in an ‘unknown functional space’ due to loading in the normal direction. Published data shows coatings functioning beyond C633 limits, yet there is no standardized test to show true functional stress limitations. To fill this gap, a 4-point bend test method with an instrumented strain-gage has been used to measure true coating adhesion strengths. A strain-gage is applied to a prepared coating surface on a bend bar, and then loaded under tension or compression. The MTS test system load data is used to calculate the stress at the coating/substrate interface by beam theory equations, allowing for stress-strain curves to be generated. This data can indicate microscopic coating behavior (cracking, de-bonding) as a result of test sensitivity and can ultimately be used as design criteria rather than attempting to interpret C633 data as sufficient for an application.