Coating on high-entropy alloy survives jet engine temperatures
Stable, oxidation-resistant, nano-grain-sized coating layers on a high-entropy alloy have reportedly been developed.
The layers are made using a sequential, two-step, boron (B)–silicon (Si) pack cementation process.
The technology, developed at Hanbat National University, South Korea, could have implications for defence and aerospace in components exposed to high-temperature flames, such as in fighter jets and missiles.
As metallic materials with higher operating temperatures are developed, the speed of aircraft can be enhanced and fuel consumption reduced, so research on high-temperature materials directly links to aircraft performance.
Nickel (Ni)-based alloys with ceramic coatings have been the primary materials used for such high-temperature applications for decades. However, the scientists explain how this is limited because of the intrinsic softening of Ni-based alloys. Consequently, their operating temperature cannot exceed ~1,100°C.
The paper, Superior oxidation behaviors of stable nano-grain-sized coating layers produced via sequential two-step pack cementation coatings by B and Si of TiTaNbMoZr high-entropy alloys, says that the new coatings enable high-entropy alloys to be used at significantly higher temperatures.
‘Sequential pack cementation, in which each deposition step is independently controlled, has been reported to offer advantages such as more precise regulation of coating composition, simplified processing and improved oxidation resistance,’ explains the paper. ‘However, studies on sequential pack cementation using Si and B have not been well documented.’
The paper continues, ‘In this study, we systematically investigate the microstructural evolution of the titanium-zirconium-niobium-tantalum-molybdenum (TiZrNbTaMo) alloy subjected to Si-pack cementation and sequential B-Si-pack cementation treatments to enhance its oxidation resistance through surface modification.
‘In addition, the oxidation behaviours of the as-cast alloy, the Si-pack cementation-coated alloy, and the BSi-pack cementation-coated alloy are compared.
‘This study highlights that merely applying Si-pack cementation coatings is not sufficient to enhance the oxidation resistance of high-entropy alloys. Instead, it emphasises the importance of employing composition-dependent coating strategies based on the specific characteristics of each alloy system.’
They find that the as-cast, untreated alloy experiences extreme oxidation at 1,300°C, while the Si-pack cementation-coated high-entropy alloy also shows crack formation. This is due to the oxidation of Zr-rich metal silicides (XSi2) to zirconium dioxide (ZrO₂), compromising coating integrity.
On the other hand, the B-Si-pack cementation-coated TiTaNbMoZr alloy is reported to develop a structurally stable surface layer, comprising, XB2, XSi2 and X5SiB2. It demonstrates oxidation resistance even at very high temperatures.
The as-cast alloy and Si-pack cementation-coated alloy also demonstrate high mass gains after oxidation at 1,300°C for 10h, while their B-Si-pack cementation-coated counterpart is said to exhibit significantly lower mass gain under the same conditions. Furthermore, the parabolic rate constant is highlighted as being ‘quite small’ after the protective oxide layer forms.
The coating layer ‘maintain[s] its nanostructure while effectively protecting the substrate even at very high temperatures’, find the researchers.
They claim this is the first time this has been established. The refractory high-entropy alloy used exhibits a Young’s modulus of 153GPa, a Vickers microhardness of 4.9GPa, a compressive yield strength of 1,390MPa and an apparent plastic strain to failure of ~6%, reads the paper.
‘Overall, our results confirm the potential of high-entropy alloys for use in high-temperature environments and emphasise the critical role of selecting suitable coating strategies tailored to the alloy composition,’ concludes Professor Joonsik Park at the University.
