Some Considerations for the Routine Testing of Thermal Sprayed Coatings

Revealing the true properties is of utmost importance for the optimized use of thermal sprayed coatings and for the realization of the ultimate spray booth. The necessity of the true properties for a successful correlation as well as development of process windows for on-line control cannot is emphasized. Only the true properties can be expected to correlate to the spray parameters or the particle properties in the flame. What is actually meant when stating the following? - The coating has 5% porosity - Tensile bond is 50MPa - Hardness is 1000HV - Coating thickness is 100 nm Different loopholes during the steps of metallographic preparation, hardness and tensile bond strength testing are highlighted and discussed. The microstructure of a thermally sprayed coating typically consists of a multiphase matrix (often a mix between hard, soft and amorphous), pores, oxides, delaminations, cracks, grit residues and unmelted particles. Due to this complexity in the structure there are a number of possible errors that often can be made in metallographic laboratories. Several examples and explanations are given. It is shown that improper handling in the materials laboratory may lead to smearing and pullouts etc. This will ruin the results and effectively hide the true microstructure. Furthermore the use of different epoxies and curing methods effectively varies the tensile bond strength test results by more than 300% for various coatings. By using different preparation routines the micro hardness can be varied by more than HV_0.3 100.