Adhesive-Free Bond Strength Test Method for Cold Spray Coatings
By Ben Robinson
Adhesion strength is a commonly measured parameter as part of a robust quality control system for thermally sprayed coatings (e.g. standards from NORSOK, NACE, and ASTM are widely used for measuring the adhesion strength of coatings for mitigating corrosion such as aluminium and zinc). This is of particular relevance in the use of thermal spray techniques for the repair of high value components, which is a growing market.
In the typically used tensile adhesion tests a stud is attached to the coating surface using an adhesive and the bonded assembly is subjected to a controlled strain rate tensile test until the coating fails, which can be at the interface with the substrate, within the coating itself, in the adhesive or a combination thereof. However, recent advances in thermal and cold spray technology have resulted in some coating bond strengths exceeding 70MPa, the maximum strength of available adhesives. This includes high velocity oxy-fuel spray (HVOF) deposited tungsten carbide based coatings and many coatings deposited by the cold spray process. It is desirable to measure the adhesion values of thermal and cold spray coatings with bond strengths in excess of 70MPa in order to validate their performance for load bearing applications. Therefore, a bond strength test that does not require the use of an adhesive needs to be developed and validated, so that the true bond strengths of HVOF and cold spray deposits (which can be in excess of 150 MPa) can be determined. This report describes such a test.
An adhesive-free adhesion test was developed that allowed assessment of coatings with bond strengths beyond 90MPa.
- Validation of the adhesive-free adhesion test design was achieved by testing of cold sprayed 316L, HVOF WC-CoCr and cold sprayed Ti-6Al-4V coatings.
- Finite element modelling of the validated adhesive-free adhesion test design showed inconsistent stress distribution over the test area during loading due to the creation of a notch with the separation of the pin and collar. Modelling therefore showed that while the adhesive-free adhesion test allows measurements of coatings with significantly greater bond strength, this test underestimates the true adhesion strength which must be considered when reporting results. The level of underestimation will depend on coating stiffness.
Loading of adhesive-free adhesion test specimen
Photograph of specimen in a tensile testing machine