Properties of Al2O3 coatings by High Velocity Suspension Flame Spraying (HVSFS): Effects of injection systems and torch design

Through the High Velocity Suspension Flame Spraying (HVSFS) process, fine (sub-micrometric) oxide ceramic particles can be sprayed at high velocity to achieve denser and more finely structured layers, compared to those obtained by conventional thermal spraying. This paper explores the effect of some key HVSFS hardware components on the properties of Al2O3 coatings. Two feedstock powders with different particle size distributions, dispersed in isopropanol medium, were processed by setting up the HVSFS torch with combustion chambers of different lengths and with two distinct injection systems: a water-cooled mechanical injector, delivering a con- tinuous stream of suspension, or a gas-atomiser injector. The use of the atomising injection system and of longer combustion chambers, promoting better fragmentation of the liquid stream and better homogenisation with the gas flow, can suppress the formation of defects between coating layers, resulting in remarkably high tensile adhesive/cohesive strength values, up to ≈70 MPa. All coat- ings are under compressive residual stress, the magnitude of which varies between −150 MPa and −300 MPa depending on the maximum surface temperature attained during spraying, and have a microhardness of 1000–1200 HV0.1. Quite mild wear rates of ≈1 ∗ 10−6 mm3/(Nm) are obtained by ball-on-disk tests performed under sliding conditions severe enough to cause complete failure of reference HVOF-sprayed Al2O3. Wear mech- anisms involve the formation and fatigue delamination of a smooth surface tribofilm. Detailed analyses of both coatings and single splats also reveal that extremely fast quenching of very small mol- ten droplets caused the formation of ≈20 wt.% glassy Al2O3. Glass transition at 527 °C and three crystallisation reactions at about 800, 1000 and 1180 °C were revealed by differential thermal analysis (DTA).