The ASTM C633 test method is used to determine the adhesion or cohesion strength of a thermal spray by subjecting it to tension perpendicular to the surface.
The substrate fixture that spray coating is applied to is most commonly a cylinder made of the same metal that would be used as the substrate of the coating in its actual use. If a substrate is not specified for your application, then the ASTM C633 standard specifies to use SAE 1018 or 1020 steel. A similar, but uncoated, fixture is then glued to the surface of the coating with a suitable adhesive bonding agent. The adhesive bonding agent should not be the "weakest link", but should be at least as strong as the minimum required adhesion or cohesion strength of the coating.
To ensure the direction of force application is perpendicular to the coated surfaces, ASTM C633 specifies to use a self-aligning load fixture to attach these substrate cylinders to the test frame. The assembly is pulled at a constant speed between 0.030 in/min and 0.050 in/min until failure. The maximum force is then normalized over the cross-sectional area to give adhesion or cohesion strength of the thermal spray. If failure is entirely at the coating-substrate interface, then the result is referred to as the adhesion strength. If failure occurs within the coating, the result is referred to as the cohesion strength.
The challenges of testing to this standard are:
- Data rate used to capture the maximum peak
- Fixtures in accordance with the standard
- Data rate used to capture the maximum peak – Instron’s 5900 series frames enable data capture of up to 2.5 kHz to ensure that all fast changing test events are captured. Too low of a bandwidth may “dull” the system and lead to a missed peak, resulting in adhesion or cohesion strength values.
- Fixtures in accordance with the standard – Instron’s fixtures for testing in accordance with ASTM C633 provide a self-aligning end which natively adapts to the load cell and base adapters present on eletromechanical testing systems. In addition to this, we find it extremely useful to incorporate an axial slack coupling into the design of the loading fixture as it greatly improves setup time. Different size blocks can be made to test the full dimensional range of specimens allowable by the standard.
It is important to review ASTM C633-2011 in order to fully understand the test setup, procedure, and results requirements.