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The Al 2 O 3 -Y 2 O 3 system composite coatings were prepared by plasma spraying Al 2 O 3 -Y 2 O 3 composite powders with or without TiO 2 addition. The effects of TiO 2 on the microstructure and properties of the Al 2 O 3 -Y 2 O 3 coatings were investigated. The results show that the composition of the composite powders had great influence on their microstructure evolution. The Al 2 O 3 -Y 2 O 3 coatings were composed mainly of α-Al 2 O 3 and c-Y 2 O 3 . There were amorphous phase, Y 3 Al 5 O 12 (YAG) phase and Y 4 Al 2 O 9 (YAM) phase formed in the Al 2 O 3 -Y 2 O 3 coating. The content of Y 3 Al 5 O 12 in the Al 2 O 3 -Y 2 O 3 -TiO 2 coating increased with addition of 2 wt% TiO 2 , and the content of Y 4 Al 2 O 9 and amorphous phases decreased correspondingly. There was no Y 3 Al 5 O 12 formed in the Al 2 O 3 -Y 2 O 3 -TiO 2 coatings with further increasing of TiO 2 , while the content of amorphous phase increased. The microhardness of the Al 2 O 3 -Y 2 O 3 -TiO 2 coating was improved with the addition of 2 wt% TiO 2 , which was attributed to its lower porosity, more uniform microstructure, more YAG phase, and less amorphous phase. The microhardness of the Al 2 O 3 -Y 2 O 3 -TiO 2 coatings decreased significantly with further increasing of TiO 2 due to the significant decrease of YAG phase and the increase of the amorphous phase. The Al 2 O 3 -Y 2 O 3 -TiO 2 coating with addition of 2 wt% TiO 2 possessed higher toughness, adhesive strength and wear resistance compared with the Al 2 O 3 -Y 2 O 3 coating, which was due to its lower porosity, smaller and spherical pores, and more YAG phase. [ABSTRACT FROM AUTHOR]