Oxidation of MoSi2-based materials with the addition of W, B and Al

Licentiate thesis, 2009

MoSi2-based composites are intermetallic materials that combine metallic and ceramic properties. Because of their excellent oxidation resistance above 1000°C, they are commonly used in high temperature applications, such as furnace heating elements, where they can be used up to approximately 1800°C. However, there are some problems that limit a broader use of the material, such as accelerated oxidation/‘pesting’ at intermediate temperatures (400°C–700°C). A key issue is therefore improving the mechanical properties of this material and the pesting behaviour while maintaining the high temperature oxidation resistance. The aim of this work was to study the oxidation behaviour of the silica-forming (Mo,W)Si2-based composite with additions of boron and the alumina-forming Mo(Si,Al)2-based composite at temperatures between 300 and 1000°C (in dry oxygen/air). The initial oxidation of the Mo(Si,Al)2-based composite at high temperatures was also investigated. Oxidation kinetics were studied using thermogravimetric analysis (TGA), and the oxidized samples were analysed with a wide range of methods, including SEM/EDX, FIB/BIB, STEM/SEM, XRD and AES. The boron-reinforced (Mo,W)Si2 composite showed rapid oxidation in the intermediate temperature region (about 500–700°C), although pesting was not observed. The vaporization of MoO3 (and to some extent WO3) resulted in porous and poorly protective oxide scales. A dense and protective SiO2/B2O3 scale was established above 700°C. The presence of boron lowered the viscosity compared to a pure silica scale and is believed to be beneficial to the oxidation behaviour. The Mo(Si,Al)2-based composite showed excellent oxidation behaviour in the temperature regime investigated. No accelerated oxidation was observed, and the mass gains were low at all temperatures studied. At 300–700°C, the oxide consisted of a mixture of the oxides of Al, Si and Mo. At 600 and 700°C, the evaporation of MoO3 resulted in a molybdenum depletion of the oxide. At 1000°C, a protective alumina scale formed with a subjacent Mo5(Si,Al)3 region. The initial oxidation of the Mo(Si,Al)2-based composite at 1500 and 1450°C resulted in the formation of a dense and protective Al2O3 scale. The Al supply to the growing alumina scale was mainly provided by the Mo(Si,Al)2 phase in the substrate and resulted in the formation of a Mo5(Si,Al)3 region directly below the scale and an underlying region of Mo(Si1.4,Al0.6). Keywords: (Mo,W)Si2, Mo(Si,Al)2, accelerated oxidation, temperature dependence, protective oxide scales, SEM/EDX, XRD.