Research On The Preparation And Properties Of NiAl/Al₂O₃ Ceramic Gradient Coatings

In order to release stress concentration of thermal spraying coatings and improve its adhesion strength and shock resistance, the preparation technics and mechanical character of NLAl/Al2O3 ceramic gradient coating were developed in this paper. In the light of different powder mixture, two preparation methods were designed that delivering powder dividually or synchronously after mixing, and the better was choosen through analyzing it's feasibility and the processing parameters were confirmed. The gradual distribution of composition of NiAl/Al2O3 ceramic gradient coating on Q235 steel improves the adhesion strength and shock resistance.The organization and structure of gradient coating were analyzed by metalloscope and XRD. The results show that the gradient coating is laminal structure, from basal body to the surface of coating and along the direction of thickness, the content of Al2O3 is increased and the content of NiAl alloy product is decreased, the composition takes on gradient distribution and it does not exist distinct componential saltation and macroscopic interface among the different compositions, the gradient coatings showed the characteristic that structure is not uniform macroscopically but consecutive microscopically. The gradient coating is multi-phase composite that including α-Al2O4,.γ-Al2O3, δ-Al2O3, Ni, NiO, TiO2 and Ni-Al intermetallic compound, the organization is obvious different along the direction of deposition and its perpendicular direction, the composition shows aeloltropism.Through bonding tensile test, boiling test, Vickers microhardness tester and heatcycling test, the adhesion strength, micro-hardness and thermal-shock resistance were tested and the variation regularity of performance were analyed. The results show that comparing with double layer coating, the gradient coating has the higher adhesion strength, the adhesion strength of pure NiAl layer is highest, following increasing of Al2O3, adhesion strength is descending, it gives priority to mechanical bonding and have both metallurgical bonding between the basal body and coatings; The pore of NiAl layer is least and A12O3 layer is most, when the content of Al2O3 less than 50%, following increasing of Al2O3, porosity is heightened, and when the content of Al2O3 is 60% and 80%, porosity is reduced; Following increasing of Al2O3 hard phase, microhardness of coating is increased and it is the most when the content of Al2O3 is about 80%, after this it begin to decrease; gradient coating has the better thermal-shock resistance, it's thermal-shock resistance is triplication comparing with double layer coating.