ASTM C633 has been an industry standard for determining thermal spray coating adhesion and cohesion strengths for nearly 40 years. The test, however, has several drawbacks that can greatly affect the results. The epoxies used cannot withstand stresses greater than 15,000psi, producing data that may suggest coatings cannot function beyond the epoxy threshold under uniaxial tensile loading, resulting in data that can only be used for general quality control or acceptance testing. Previously published data shows coatings functioning beyond C633 limits, yet there is no standardized test to show true functional stress limitations. A four-point bend test method with an instrumented strain-gage has been used to show coating adhesion well beyond the yield point of the steel substrates and beyond the C633 limits for three materials and thermal spray processes: electric arc sprayed aluminum bronze, plasma sprayed alumina, and HVOF WC/Co/Cr. A strain-gage is applied to a prepared coating surface on a bend bar and loaded under tension or compression. The MTS universal load frame force data is used to calculate the stress at the coating/substrate interface by beam theory equations, allowing for stress and strain vs displacement curves to be generated and directly compared against C633 data for coating adhesion strengths. The resulting data can indicate microscopic coating behavior (cracking, de-bonding) as a result of the test sensitivity and can ultimately be used as design data for the practicing engineer.

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