New design concept for stableα-silicon nitride based on the initial oxidation evolution at the atomic and molecular levels

As the dominated composition of Si_(3)N_(4)ceramics,α-silicon nitride(α-Si_(3)N_(4))can satisfy the strength and fracture toughness demand in the applications.However,α-Si_(3)N_(4)is oxygen-sensitive at high temperatures,which limits its high-temperature performance.To improve the oxidation resistance ofα-Si_(3)N_(4)ceramics,it is necessary to shed light on the oxidation mechanism.Herein,the initial oxidation ofα-Si_(3)N_(4)was systematically studied at the atomic and molecular levels.The density functional theory(DFT)calculation denotes that the(001)surface ofα-Si_(3)N_(4)has the best stability at both room temperature and high temperature.Besides,the oxidation process of theα-Si_(3)N_(4)(001)surface consists of O adsorption and N desorption,and the consequent formation of nitrogen-vacancy(VN)is the key step for further oxidation.Moreover,the molecular dynamics(MD)simulation indicates that the oxidation rate ofα-Si_(3)N_(4)(100)surface is slower than that ofα-Si_(3)N_(4)(001)surface due to the lower N concentration at the outermost layer.Therefore,the oxidation resistance ofα-Si_(3)N_(4)can be improved by regulating the(100)surface as the dominant exposure surface.In addition,reducing the concentration of N on the final exposed surface ofα-Si_(3)N_(4)by mean of constructing the homojunction of the Si-terminal(100)surface and other N-containing surfaces(such as(001)surface)should be also a feasible approach.