This study describes a new approach to the quality assessment of thermally sprayed
carbide and ceramic coatings produced by High Velocity Oxy-Fuel (HVOF) and Air
Plasma Spray (APS) processes. The aim of the work was to develop an experimental
methodology based on Acoustic Emission (AE) monitoring of a dead-weight Vickers
indentation to assess the degree of cracking and hence the toughness of the coating. AE
monitoring was also applied to an industrial process as a contribution to the possibility
of quality assessment during the deposition process.
AE data were acquired during indentation tests on samples of coating of nominal
thickness 250-325 μm at a variety of indentation loads ranging from 49 to 490 N.
Measurements were carried out on six different thick-film coatings (as-sprayed HVOFJP5000/
JetKote WC-12%Co, HIPed HVOF-JetKote WC-12%Co, as-sprayed HVOFJP5000
WC-10%Co-4%Cr, conventional powder APS-Metco/9MB Al2O3 and fine
powder HVOF-theta gun Al2O3) and also on soft and hard metallic samples and metals.
The raw AE signals were analysed along with force and displacement history and the
total surface crack length around the indent determined. Also, a selection of the indents
was sectioned in order to make some observations on the sub-surface damage. The
results show characteristic AE time evolutions during indentation for tough metals, hard
metals, and carbide and ceramic coatings. Within each category, AE can be used as a
suitable surrogate for crack length measurement for assessing coating quality.
Finally, a preliminary observation on AE monitoring during HVOF (JP5000) WC-
10%Co-4%Cr thermal spraying was made. It was found that AE is sensitive to
individual particle landings during thermal spraying and therefore can, in principle, be
used to monitor the spray process
