Abstract
There are many applications including aeroengine design where one would like to operate Ti or its alloys at higher temperatures, but the threat of oxidation or fire remains a longstanding challenge. Here, we have designed a bilayer nitride coating for Ti and its alloys produced by magnetron sputter deposition of a SiAlN coating (1.2 μm thick) along with a Mo interlayer. We have taken advantage of interdiffusion and inter-reaction at the interface during cyclic oxidation at 800 °C to form a layered nitride coating system comprising: a SiAlN top layer, a TiN0.26 and Ti5Si3 mixed phase interlayer, and a Ti-Mo solid solution. The novel TiN0.26 interlayer exhibits adaptive conformability via mechanical twinning, thereby accommodating the thermal mismatch strain between the coating and substrate. This, along with high adhesion, confers excellent thermal cycling life with no cracking, spallation and oxidation of the coating, evident after hundreds of hours of cyclic oxidation (>40 cycles) in air at 800 °C. This work provides a design pathway for a new family of coatings displaying excellent adhesion, adaptive conformability and superior environmental protection for Ti alloys at high temperature.
Highlights
Ti and Ti alloys are widely used in the aeroengine gas turbine and automotive industries as well as for medical implants due to their low density, high specific strength and excellent corrosion resistance [1À8]
The selective area diffraction (SAD) pattern of the SiAlN coating shows a diffraction halo, as shown in Fig. 1a inset, which indicates that this coating is amorphous
The top surface of the as-deposited SiAlN coating was milled by Ar+ ions and the XPS quantification is performed after each etch cycle, as shown in supplementary Figure s1
Summary
Ti and Ti alloys are widely used in the aeroengine gas turbine (e.g. fan blade, compressor, etc.) and automotive industries as well as for medical implants due to their low density, high specific strength and excellent corrosion resistance [1À8]. Increasing the gas turbine inlet temperature can increase engine efficiency and thereby increase fossil fuel efficiency [3]. Ti alloys have inadequate oxidation resistance during high temperature exposure, which restricts their application. In order for Ti alloys to be safely applied at higher temperature, various techniques have been used to improve the oxidation resistance by bulk alloying, surface treatments and coating technologies. Of these, coating methods have been found to be the most effective way to improve the oxidation resistance [9,13À17]
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