ASTM C633 determines the bonding strength of thermal spray coatings. The test is performed by applying a thermal spray coating to one face of a substrate fixture, bonding this coating to the face of a loading fixture, then applying a tensile force normal to the plane of the coating. Thermal spray coating materials include ceramics and metals. In some case a coating is formed of different spray materials. The substrate is generally a metal but may be a ceramic.
ASTM C633 is mainly a quality control test but may be used to qualify thermal spray equipment. It is useful for comparing adhesion or cohesion strengths of coatings of similar types of thermal spray materials. The results of the test should not be used to make absolute calculations relating to how much stress a part can withstand during use due to the shape differences between the part and the ASTM C633 test fixture.
In order to measure the bond strength, the thermal spray coating is applied to the substrate which is in the shape of a cylinder. The substrate with coating is then glued to another cylinder of equal diameter. The resulting assembly including a self aligning mechanism that ensures normal forces only are applied to the thermal spray coating when mounted in a testing machine.
Prior to conducting ASTM C633, it is important to read the entire specification in the relevant ASTM publication.
Apply a tensile force to the fixture at a fixed crosshead rate.
The maximum force is recorded during the test and the bond strength is determined by dividing the maximum force by the cross-sectional area of the cylinder. The entire ASTM C633 specification should be reviewed prior to performing the test to ensure correct substrate surface preparation, application of coatings and test speeds.