The interfacial toughness of two types of Al 2 O 3 -13wt%TiO 2 plasma-sprayed ceramic coatings on steel substrates-"conventional" and "nano"-has been measured using the Rockwell indentation method. The interfacial toughness of the "conventional" coating and the "nano" coating is found to be 22 and 45 J.m Οͺ2 , respectively. The "conventional" coating, which was prepared using a fused feedstock powder available commercially, has a microstructure consisting primarily of fully-molten (FM) and solidified "splats". The feedstock powder for the "nano" coating comprised reconstituted agglomerates of nanocrystalline Al 2 O 3 and TiO 2 powders. The microstructure of the "nano" coating, as characterized using scanning and transmission electron microscopy techniques, consists of regions of FM "splats" interspersed with partially-molten (PM) rounded microstructural features. The substructure in these PM features (20-50 Β΅m diameter) consists of a-Al 2 O 3 grains (0.5-1 Β΅m) surrounded by a TiO 2 -rich amorphous phase. The FM/steel interfaces in both the "conventional" and the "nano" coatings are found to be cracked (before mechanical testing), whereas the PM/steel interfaces in the "nano" coating are found to be adherent. It is believed that the unique bimodal microstructure, together with the presence of the TiO 2 -rich amorphous phase at the PM/steel interface, is responsible for the significantly improved interfacial toughness of the "nano" coating. The key differences in the failure modes in the two types coatings are also discussed, with reference to a simple model.