Microstructural evolution and 1500 °C oxidation resistance of Mo(Al,Si)2 fabricated via an innovative two-step SHS-SPS technique

• An innovative two-step method of SHS and SPS was designed to synthesize Al-alloyed MoSi 2 ceramics. • Mo(Si,Al) 2 powders were in-situ synthesized by SHS, and they were used to produce dense alloyed ceramics via the SPS. • Al-Si-O composite oxide layer formed on Mo(Si,Al) 2 ceramics, enhancing high-temperature oxidation resistance. • Trace Al addition into MoSi 2 enhanced its high-temperature oxidation resistance while excessive Al reduced it. An innovative two-step approach of self-propagating high-temperature synthesis (SHS) and spark plasma sintering (SPS) was developed to rapidly fabricate MoSi 2 and Mo(Al,Si) 2 ceramics for high-temperature anti-oxidation applications. The SHS process predominantly promoted the synthesis of high-purity and high-yield MoSi 2 and Mo(Si,Al) 2 phases in the alloyed powders. Subsequently, dense and crack-free MoSi 2 and Mo(Al,Si) 2 ceramics were produced using SPS. 1500 °C oxidation tests of the ceramics (100 h) revealed the formation of a protective SiO 2 oxide layer on the surface of MoSi 2 ceramics, while an Al-Si-O composite glassy oxide layer formed on Mo(Si,Al) 2 ceramics, which exhibited better thermal stability and lower oxygen permeability compared to the single SiO 2 oxide layer. However, an excessive Al content (>0.05 at.%) compromised the oxidation resistance due to the emergence of a Si-depleted Mo 5 (Si,Al) 3 layer with inferior oxidation resistance, which was caused by the high-temperature diffusion of Si. Therefore, via this novel two-step SHS-SPS technique compact and crack-free Mo(Si,Al) 2 ceramics can be rapidly synthesized at high temperatures. When trace amount of Al was added (0.05 at.%), Mo(Si 0.95 Al 0.05 ) 2 showed optimum high-temperature oxidation resistance.