Studies On The Plasma-Sprayed And Reaction Forming AI₂0₃ Gradient Thermal Resistance Coatings

In this paper the oxidation resistance and the thermal shock resistance properties of the plasma-sprayed A1203 coatings with various bond layer was studied and compared with Ni/Al-Zr02 gradient coating. The reaction-forming mechanism and its correlating factor besides of the above two properties of the reaction-forming gradient Al203 coatings with Al+Ni/Al bond layer were discussed. The results show that as follows: 1. Ni/Al-Al203 gradient coating with Ni/Al bond layer (1) The oxidation resistance properties of the three kinds of Ni/Al-Al203 gradient coating (3-layer 0.3mm, 5-layer 0.3mm, 5-layer 0.1mm) are better than that of the Ni/Al-Zr02 gradient coating because there are lower porosity in Ni/Al- Al203 gradient coatings than that in the Ni/Al-Zr02 gradient coating whereby protects the coatings from oxidation effciently. (2) According to the thermal shock tests it is showen that the destruction process of the NiAl-A1203 gradient coatings includes three steps: the cracks forming, expanding and shelling. The multi-layer gradient coating structure can remarkably raised the thermal shock resistance properties because it can relax the thermal stress and hinder the cracks forming, expanding and shelling. (3) The thermal shock resistance properties of the 3 kinds of Ni/Al-A1203 gradient coatings is slightly interior to that of the Ni/Al-Zr02 gradient coating at 8000C because their stress-relax effect is not very good. In them the thermal resistance properties of the 5-layer 0.1mm gradient coating with 20% mass gradient deference is better than other coatings and the thermal shock resistance properties of the coating is the best at 10000C because it抯 proper coating structure relaxs thermal stress efficiently and it抯 low porosity hinders oxidation of the coating. III (4) Generally speaking9 The thermal resistance properties of the 5-layer 0.1mm A1203 gradient coating with 20% mass gradient deference is the best. 2. NiIA1-A1203 reaction-forming gradient coating with Al+NiIA1 bond layer (1) When the coating was heated above 1000C,there were A1203+Ni/Al transition layer formed between the bond layer and the ceramic coating which leads to NiIA1-At203 gradient coating structure. (2) The higher the heating temperature, the longer the heating time, the higher the content of Al in bond layer, the obvious the reaction-forming gradient Ni/Al- Al203 layer is. (3) The oxide resistance properties of the Ni/Al-A1203 reaction-forming gradient coatings with Al+Ni/Al bond layer are remarkably better than that of the NiIA1- A1203 and NiIA1-Zr02 non-gradient coatings and better than that of the Ni/Al- A1203 and Ni/Al-Zr02 gradient coatings because the reaction-forming transition layer protects the inner coating and substrate from further oxidation by lowering the porosity of the coating and relaxing thermal stress whereby hindering the oxidation caused by cracks. (4) The oxidation resistance properties of the Ni/A1-A1203 reaction-forming gradient coating with 20%Al+80%Ni/Al bond layer is the best when heated longer than 7.5h at 1000C and so does that of the Ni/Al-Al203 reaction- forming gradient coating with 5%A1+95%NiIAl bond layer when heated shorter than 7.5h. The thermal shock resistance properties of...