Synthesis and oxidation behaviour of Ta-Al-C MAX phases

MAX-phase materials, a class of ternary carbides and nitrides that combine favorable properties of metals (conductivity and toughness) and ceramics (high temperature strength) have been shown to autonomously heal micro-cracks by high temperature oxidation. The investigation of Ta and Al containing MAX phases (Ta2AlC & Ta4AlC3) could render self-healing materials at lower temperatures. Mixed (Ta2AlC and Ta4AlC3) and pure dense bulk Ta2AlC MAX phase was synthesized by spark plasma sintering (SPS) of elemental powders. The oxidation reactions of Ta-Al-C MAX phase powders were studied with differential thermal analysis (DTA). The oxidation kinetics of dense bulk Ta-Al-C MAX phase was studied by thermal gravimetric analysis (TGA). The microstructure and composition of the MAX phase materials and its oxides was characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray microanalysis with electron probe microanalysis (EPMA), and X-ray photoelectron spectroscopy (XPS). Linear oxidation kinetics, inward oxide growth and formation of mainly porous Ta-oxides were observed for isothermal oxidation of dense bulk Ta2AlC and mixed MAX phase. Based on the results obtained in this work, the oxides scales formed in the 600 – 800 °C temperature range are believed to consist of various metastable and/or amorphous oxides. Full oxidation of the MAX phase above 900 °C results in the formation of Ta2O5 and ternary TaAlO4. It was found that TaAl-C MAX phases cannot self-heal microcracks by formation of a protective oxide scale at elevated temperatures.