Liu Zhengyi

Laser remelting of plasma sprayed Al2O3 ceramic coatings and subsequent wear resistance

This paper deals with the laser melting of Al2O3 and Al2O3+13wt.%TiO2 ceramic coatings deposited by atmospheric plasma spraying. After laser melting, metastable phase γ-Al2O3 transforms into stable phase α-Al2O3 in Al2O3 coatings, whereas in Al2O3+13wt.%TiO2 coatings besides this phase transformation, TiO2 may react with Al2O3 and transform into TiAl2O5. The melting layer becomes much denser and, hence, its hardness has been greatly developed despite the existence of some cracks. Furthermore, the increase in hardness of the melting layers is almost independent of coating type and generally increases with the laser energy density. However, the coating type has a strong influence on the thickness of the laser melting layers. As to the coatings with the same coating type, the depth of laser melting roughly increases with the laser energy density. Finally, it has been found that the introduction of TiO2 can improve the wear resistance of ceramic coatings, particularly, of the melting layers, although TiO2 reduces the hardness of these ceramic coatings.

Interfacial phenomena in the plasma spraying Al2O3+ 13 wt.% TiO2 ceramic coating

The interface between the bond layer and ceramic layer as well as between the bond layer and the substrate in a duplex coating system consisting of a NiCrAl bond coat layer and an Al2O3+13 wt.% TiO2 ceramic coat layer has been investigated by scanning electron microscopy and transmission electron microscopy. The lamellar bond layer mainly consists of lamellae aligning approximately parallel to the substrate surface, in which the individual lamellae consist of a thin outer amorphous and nanocrystalline film, an intermediate layer of fine columnar and equiaxed grains and an inner microcrystalline region of big columnar grains. The morphology of the interfacial regions of the above two interfaces is shown to be similar and consists of a thickness of 0.7 μm, or far less, of a mixture of amorphous and nanocrystalline phases. However, besides a certain percentage of area of perfect contact, a small amount of area of non-contact has also been detected among the lamellae or at the interfaces.