Commercial nanosized alumina and titania particles were selected as raw materials to prepare the blended slurry with composition of A1203-13%TiO2 （mass fraction）, which were reconstituted into micrometer-sized granules by spray drying, subsequently sintering at different temperatures to form nanostructured feedstock for thermal spraying, and then A1203-13%TiO2 nanocoatings were deposited by plasma spraying. The evolution of morphology, microstructure, and phase transformation of the agglomerated powder and as-sprayed coatings were characterized by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The results show that A1203 retains the same a phase as the raw material during sintering, while TiO2 changes from anatase to futile. During plasma spraying, some a-A1203 phases solidify to form metastable y-A1203, and the volume fraction of a-A1203 decreases as CPSP increases. However, peaks of the TiO2 phase are not observed from the as-sprayed coatings except for the coatings sprayed at the lower CPSP. As the CPSP increases, nanostructured TiO2 is dissolved easily in y-A1203 or z-A1203＇TiO2 phase. After heat treatment, y-A1203 in the coatings transforms to a-A1203, and rutile is precipitated.
Al_(2)O_(3)-Y_(2)O_(3) composite powder with TiO_(2) additive was plasma sprayed to prepare Al_(2)O_(3)-Y_(2)O_(3) composite coatings.The micro structure and properties evolution of the Al_(2)O_(3)-Y_(2)O_(3) coatings during high temperature and thermal shock resistance were investigated.The results show that the micro structure of the Al_(2)O_(3)-Y_(2)O_(3)-TiO_(2) coating is more uniform than that of the Al_(2)O_(3)-Y_(2)O_(3) coating.Meanwhile,amorphous phase is formed in the two coatings.The Al_(2)O_(3)-Y_(2)O_(3)(-TiO_(2)) coatings were heat treated for 2 h at temperatures of 800,1000 and 1200℃,respectively.It is found that the microstructure and properties of the two coatings have no obvious change at 800℃.Some of the amorphous phase is crystallized at1000℃,and meanwhile Y_(2)O_(3) and Al_(2)O_(3) react to form YAG phase and YAM phase.At 1200℃,all of the amorphous phases are crystallized.After heat treatment,the micro hardness of the two coatings is increased.The thermal shock resistance of the Al_(2)O_(3)-Y_(2)O_(3) system coatings can be improved by using TC4 titanium alloy as substrate and with NiCrAlY bonding layer.Moreover,the Al_(2)O_(3)-Y_(2)O_(3)-TiO_(2) coating exhibits better thermal shock resistance due to the addition of TiO_(2).
Al_(2)O_(3)-ZrO_(2)-TiO_(2)coatings were successfully prepared by plasma spraying Al_(2)O_(3)-ZrO_(2)composite powders with and without TiO_(2)addition.The effects of TiO_(2)on the phase composition,microstructure and properties of the Al_(2)O_(3)-ZrO_(2)coating were studied.The results show that the Al_(2)O_(3)-ZrO_(2)-TiO_(2)composite powder was composed of t-ZrO_(2),a-Al_(2)O_(3),m-ZrO_(2)and rutile,while the Al_(2)O_(3)-ZrO_(2)-TiO_(2)composite coating consisted of t-ZrO_(2),a-Al_(2)O_(3)and c-Al_(2)O_(3).The diffraction peaks of TiO_(2)could not be detected in the Al_(2)O_(3)-ZrO_(2)-TiO_(2)coating even up to10 wt%TiO_(2)addition.The reason may be that TiO_(2)was dissolved in the amorphous phase or formed solid solution with c-Al_(2)O_(3)phase in the coating during cooling.Compared with the Al_(2)O_(3)-ZrO_(2)coating,the as-prepared Al_(2)O_(3)-ZrO_(2)-TiO_(2)coating had denser microstructure,less microcracks and more amorphous phases.The density of the Al_(2)O_(3)-ZrO_(2)-TiO_(2)coating increased with the increase of TiO_(2)content.The Al_(2)O_(3)-ZrO_(2)-10 wt%TiO_(2)coating had the most uniform and dense microstructure,possessed higher toughness,adhesive strength and wear resistance compared with the Al_(2)O_(3)-ZrO_(2)coating,which was due to its lower porosity and more uniform microstructure.
Two kinds of different structured alumina-titania composite powders were used to prepare alumina matrix ceramics by microwave sintering. One was powder mixture of alumina and titania at a micron-submicron level, in which fused-and-crushed alumina particles （micrometers） was clad with submicron-sized titania. The other was powder mixture of alumina and titania at nanometer-nanometer level, in which nano-sized alumina and nano-sized titania particles were homogeneously mixed by ball-milling and spray dried to prepare spherical alumina-titania composite powders. The effect of the microstructure of composite powders on microstructure and properties of microwave sintered alumina matrix ceramics were investigated. Nano-sized composite （NC） powder showed enhanced sintering behavior compared with micro-sized composite （MC） powders. The as- prepared NC ceramic had much denser, finer and more homogenous microstructure than MC ceramic. The mechanical properties of NC ceramic were significantly higher than that of MC ceramic, e.g. the flexural strength, Vickers hardness and fracture toughness of NC ceramic were 85.3%, 130.3% and 25.7% higher than that of MC ceramic, respectively. The improved mechanical properties of NC ceramic compared with that of MC ceramic were attributed to the enhanced densification and the finer and more homogeneous microstructure through the use of the nanostructured composite powders.
