Preparation And Oxidation Resistance Of Ni-P/Al₂O₃Coating On Niobium Alloy

Niobium and niobium alloys are the most promising candidate as structural materials used in aerospace engines, weapons propeller, nuclear reactors, submersible and gas turbines, due to high melting point, low density, good machining properties and excellent high temperature mechanical properties. However, a key problem is poor oxidation resistance from the formation of NbO, NbO2and Nb2O5at high temperature, which strongly restricts their applications widely. In order to enhance oxidation resistance, high temperature protective coating is always suggested to be prepared on the Niobium alloys.In this paper, Al2O3coating was prepared on the Niobium alloy by Sol-gel method, and electroless plating was sandwiched into the Al2O3coating and Niobium alloy substrate to improve the interfacial strength. The chemical composition, phases composition and microstructure of the two coatings (Al2O3coating and Ni-P/Al2O3coating) before and after thermal exposure were analyzed by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The following conclusions can be drawn:1、 A12O3precursor solution was prepared by aluminum isopropoxide, deionized water and HNO3. Al2O3solution was stable and transparent, and became Al(OH)3after three step reaction, and eventually turned into boehmite AlOOH solution. Al2O3was formed after being coated and vacuum heat treatment.2、 Al2O3coating was prepared by Sol-gel method on niobium alloy surface. The coating was compact, covered over niobium alloy. There were three primary intermediate phases A12O3, AlNi3and AlPO4as well as three secondary oxide phases NbO, NbO2and Nb2O5after high-temperature oxidation. Oxidation mass gain curves showed that Al2O3coating could effectively improve the oxidation resistance of Nb-based alloys. With prolonging oxidation time, the coating was thickened. A lot of cracks could be seen from the coating after oxidation, leading to the decline of protection ability.3、 Electroless plating Ni-P intermediate layer was prepared with94.81wt%Ni and5.19wt%P, which exhibits amorphous structure. The microstructure could improve the ratio of surface area of the coating attaching to substrate. Therefore, Electroless plating layer could enhance the interfacial adhesion of the Al2O3coating to the niobium substrate.4^Compared to the single Al2O3coatings, Ni-P/Al2O3coating exhibited more excellent oxidation resistance. There are not cracks found in the interlayer after high temperature oxidation. Calculations indicated internal stress was about-3.7125GPa as△T was900℃in the Al2O3coating. So the Ni-P intermediate layer could effectively reduce the internal stress and improve the interfacial bonding between the coating and the substrate.