TWI Industrial Member Report Summary 1029/2013
By M A Riley
The strength of the bond between a thermal spray coating and substrate is critical for many coating applications, and its measurement is often required for process and material quality assurance. Several standards exist in this regard, as follows:
- ASTM C633-01 (Reapproved 2008) - Standard test method for adhesion or cohesion strength of thermal spray coatings.
- BS EN 582:1994 - Thermal spraying. Determination of tensile adhesive strength.
- ASTM D4541-09 - Standard test method for pull-off strength of coatings using portable adhesion testers.
- ASTM F1044-05 - Standard test method for shear testing of calcium phosphate coatings and metallic coatings.
These tests involve bonding the coating surface to another surface to create a test specimen assembly which is then subjected to tensile or shear loading. Due to the high strengths of some thermal spray coatings, it is not unusual for test assemblies to fail in the glue rather than at the coating bond line. TWI has previously used high strength film adhesives eg FM1000 for this purpose, but there is a need for easy-to-use, lower cost glue with a long shelf life with a similarly high bond strength. There is also a requirement for glues which cure at low temperatures, preferably ambient, to avoid thermal damage to thermally sensitive substrates and coatings. These issues are addressed in the first part of this work, and two suitable glues are identified for use in the above tests.
The tensile test methods are widely used whereas shear tests are far less commonplace.
Despite the potential for shear failure in many applications where a coating operates in relative motion to an opposing surface including orthopaedic implants, shafts, journals and bearings there is little published data for thermal spray coatings. This work has generated shear strength data for a selection of industrially representative thermal spray coatings and metallic substrates, using the adhesive selected from the first phase of the study and tested in accordance with ASTM F1044.
Cohesive strength of a coating is also important, especially in applications where coatings are applied for dimensional restoration or to spray form a component. It is also relevant where failure might occur at particle boundaries or within the spray particles which make up the coating eg particle pull out or spallation as a result of wear. This study has evaluated the "Tubular Coating Tensile" (TCT) test method developed for cold spray coatings (Schmidt, 2008) for measurement of the cohesion strength of a range of industrially relevant thermal spray coatings.
- Identify alternative high strength glues for use in thermal spray coating adhesion and cohesion tests, namely ASTM C633 (tensile test); ASTM D4541-09 (portable adhesion test), and ASTM F1044 (shear test).
- Measure the adhesion and cohesion of a range of thermal spray coatings in tension in accordance with ASTM C633 using alternative glue.
- Assess the suitability of the ASTM F1044 shear strength test (originally developed for medical coatings) for measuring the shear strength of a range of thermal spray coatings.
- Assess the suitability of the 'Tubular Coating Tensile' (TCT) test (developed for cold spray coatings) for measuring the cohesive strength of thermal sprayed coatings, and generate TCT test data for of a selection of industrially representative high velocity oxy-fuel, twin wire arc and flame spray coatings.