Cohesion of Dissimilar Splats in Hybrid Plasma-Sprayed Coatings: A Case Study for Al2O3-TiO2

Hybrid plasma spraying combines deposition of coarse dry powders and liquids, i.e., suspensions or solutions, thus expanding range of microstructures and functional properties achievable by thermal spraying. One of the crucial questions is how the miniature liquid-originated splats bond to the coarse splats deposited from powders and how their interaction influences the overall coating integrity. In this paper, cohesion of hybrid coatings was studied on model coatings deposited from Al2O3 powder and TiO2 suspension. Spraying distance and deposition temperature were varied as these two process variables influence the bonding between the splats in both conventional and liquid plasma spraying and the same may be expected for hybrid plasma spraying. Coatings without additional miniature splats were also prepared for comparison. Cohesion of the coatings was studied by four independent mechanical tests using different loading modes, namely tensile adhesion/cohesion test, cavitation test, conventional hardness test and instrumented indentation test. Results showed that interfaces between the dissimilar splats are not equivalent and that the coating strength greatly depends not only on the presence of the miniature TiO2 phase but also on the loading mode and involved interaction volume. When compared to the reference conventional coating deposited from Al2O3 powder, hybrid coatings were found to be less durable in adhesion/cohesion test but more durable in cavitation test and comparable in both hardness tests. Miniature TiO2 splats tended to be more strongly bonded to the above-laying Al2O3 splats than to the below-laying ones as large Al2O3 droplets possess greater ability to partially remelt previously deposited TiO2 than vice versa and even to form Al2TiO5 compound. Importance of splats deposition sequence and their remelting ability were confirmed also by the simplified analytical model.