Testing and Characterization Methods to Describe the Mechanical Properties of An HVOF Coating Sprayed with Ultrafine Powder

HVOF spraying with ultrafine (-10 +2 µm) and spherodized powder materials dramatically improves coating quality concerning spraying accuracy, surface roughness, density, constant hardness and a homogeneous phase distribution of the applied material. Those performance characteristics are forming the baseline of high quality coatings. Furthermore a close control of coating thickness with a accuracy of +- 10 µm and a very low, as-sprayed surface roughness with Ra < 1.2 µm dramatically reduces grinding and finishing costs of the coated part.

But developing a High-Performance-Coating for Applications in Aerospace, Landing Gear, Oil & Gas and other demanding Industries means even more:
it’s all about achieving high residual compressive stresses, bond-strength and ductility enhancement.

To develop coatings with performance characteristics like this it is essential to have different characterization and testing methods. The Almen-Test is well known to characterize the amount of residual compressive stresses in a coating. A modified Bend-Test provides the necessary feedback to develop coatings which do not spell-off in case of resulting tensile stresses. To characterize the bond-strength of thus coatings in a reproducible and reliable way, a modified Shear-Test was developed. Furthermore this test allows analyzing the behavior of the coating, when it is exposed to massive shear-forces. It is possible to show an interrelationship between the crack formation in the coating structure and the before chosen spray and powder-spherodization parameters. The delivered information enable a systematic procedure to increase the ductility, by interpreting the crack behavior of the coating.

It is shown that the test results are measurable and reproducible facts which deliver the necessary feedback to develop High-Performance-Coatings, with enhanced residual compressive stresses and substantial improved bond-strength and overall ductility in comparison to state of the art HVOF coatings.