Study Of The Microstructure And Properties Of Plasma-Sprayed Nanostructured Al₂O₃-13wt.% TiO₂ Coating

This paper deals with the preparation of thermally sprayable powders by spray-drying and sintering using nanoparticles Al₂O₃ and TiO₂ as start materials, and the deposition of nanostructured Al₂O₃-13wt.%TiO₂ coating by plasma spraying the as-prepared powders. The influences of the process parameters on the structural morphologies and properties of the spray-dried and sintered powders were investigated. The structure of the as-sprayed nanostructured coating was characterized using XRD, SEM and TEM, and the mechanical properties and wear resistance were measured. The mechanism of the formation of the coating microstructure and the mechanisms of the toughening and wear behavior of the nanostructured coating were also discussed.It was shown that the spray-dried powders obtained under the conditions of 2.5% binder, 30% solid contend and 300～350℃drying temperature exhibited spherical and higher bulk density and flowability. After sintering treatment at 1100℃, the size of nanoparticles in the spray-dried powders was not obviously changed and the feed rate of the powders measured in the actual powder feeding system was as about two times as that of conventional powders.It was revealed that the nanostructured coating was composed two distinct regions: the fully melted region (FM) resulted from the solidification of liquid phase and the partially melted region (PM) consisting of mainly unmelted and/or semi-melted nanoparticles. The partially melted region in the nanostructured coating presented the morphologies of spherical, strip-shaped and of the particle-disperse distribution type structure, respectively. At a low spray parameter CPSP, the partially melted region was most spherical, and it developed gradually to continual and interrupted strip-shaped as CPSP increased. The TEM analysis showed that in the fully melted region of the nanostructured coating there were equiaxed grains and irregular grains with the grain size ranging from several ten to hundreds nanometers. In the partially melted region, semi-meltedα-Al₂O₃ particles in the size of smaller than 100 nm were observed distributed in a dispersed state in the Ti-rich matrix. It was identified that the phases presented in the nanostructured coating wereα-Al₂O₃,γ-Al₂O₃ and rutileTiO₂, while the conventional coating consisted essentially ofγ- Al₂O₃,rutile TiO₂ and Al2TiO5.The microhardness and fracture toughness of the nanostructured Al₂O₃-13wt.%TiO₂ coating were about 40% higher than and tow times those of the conventional coating respectively, as was determined by the special microstructure of the coating. Under the condition of 500, 700, 900 and 1000°C heating and water quenching, the nanostructured coating demonstrated all higher anti-thermal shock property than the conventional coating. The thermal shock failure mode of the nanostructured coating was the formation of cracks on surface and extending, leading to local break-off. For the conventional coating, however, cracks originated principally from and extending along intrer-splats, resulting in integral exfoliation of the coating.The nanostructured coating showed smaller friction coefficient than the conventional coating, and the difference grew up with increasing load applied. In the condition of 20-100 N load and un-lubrication, the wear resistance of the nanostructured coating was more than one time that of the conventional coating. The greater the wear load and the longer the wear time, the better the wear resistance of the nanostructured coating with respect to that of the conventional coating. The wear mechanism of the two coatings was mainly fracture wear, with certain abrasive wear. The wear of the conventional coating was produced by fracture and peeling-off of parts of a splat, which was caused by inter-splat cracks and surface cracks extending through the whole section of a splat. While the fracture and peeling-off of the nanostructured coating happened in the layered composite splats and progressed layer by layer in a splat. Accordingly, the spallation pits of the conventional coating were deep and showed smooth facets. But the he spallation pits of the nanostructured coating were shallow and the fractured surface exhibited rough and tortuous.