A research group led by Prof. WANG Aiying at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS), successfully fabricated dense Cr₂AlC MAX phase coatings, and revealed that the oxidation kinetics of Cr₂AlC MAX phase coatings follow a two-stage parabolic relationship at 900-1100 °C. The study was published in Corrosion Science (Corros. Sci.). 

As one member of the novel family of ternary layered compounds called MAX phases, Cr₂AlC was first identified in the 1980s. Benefiting from its high bulk modulus, high thermal-shock resistance, high electrical conductivity, and strong resistance to damage, it has the potential to be a protective coating material in accident tolerant fuel (ATF) systems and other harsh environments. However, Cr₂AlC coatings prepared by the traditional physical vapor deposition (PVD) techniques show relatively poor oxidation resistance, due to the columnar structure and growth defects. 

Through an easy combined arc/sputtering deposition method followed by annealing, researchers at NIMTE prepared dense Cr₂AlC MAX phase coatings (purity: 91 wt%) on Hastelloy substrates. The developed coatings were free of columnar structures, including an inner layer and an outer Cr₂AlC layer. Therefore, the coatings were able to remain intact even under flowing air for 40 h at 900-1100 °C.  

Moreover, researchers delved into the oxidation mechanism of Cr₂AlC coatings at relatively long-term and high temperatures exceeding 1000 °C, which has not been fully examined in previous studies.  

The oxidation behavior investigation of the Cr₂AlC coatings at the 900-1100 °C temperature range showed that oxidation kinetics followed a parabolic relationship at each temperature. The parabolic relationship consisted of two stages and the second stage exhibited a slightly lower rate of oxidation. Furthermore, a dense α-Al₂O₃ layer, exhibiting coherent epitaxial growth with the oxidized coatings and the substrates, was formed during the oxidation at 1100 °C, which provided strong adherence for the oxidized coatings even after 60 cycles of thermal shock tests. 

This study illuminated the oxidation mechanisms of Cr₂AlC MAX phase coatings at 900-1100 °C. The novel fabrication method improves the oxidation resistant properties of Cr₂AlC MAX phase coatings, thus can promote its applications in high-temperature or other harsh environments. 

This work was financially supported by the National Science and Technology Major Project (NO. 2017-VII-0012-0108), National Natural Science Foundation of China (No. 51875555, 51901238), and Zhejiang Provincial Natural Science Foundation (No. LQ19E010002).