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The oxidation process of Ti2AlC ceramics in dry synthetic air at 1200° C was monitored with thermogravimetry. The microstructural evolution of the oxide scale with time was characterised by X-ray diffractometry, scanning electron microscopy, X-ray microanalysis and electron back scattering diffraction. The oxide scale is comprised of a continuous α-Al2O3 inner layer with isolated coarse TiO2 particles on top. At the early oxidation stage (roughly less than 0.5 h), α-Al2O3 and TiO2 are the main reaction products, while at longer reaction times the oxidation only leads to the formation of α-Al2O3. The α-Al2O3 grains in the oxide scale grow in size upon high-temperature oxidation with the grain size being uniform throughout the thickness of the scale. As diffusion of oxygen along the grain boundaries dominates the oxide scale growth, the change in grain size affects the oxide scale growth kinetics. A simple oxide scale growth model, that takes into account this change in fast diffusion paths, describes the experimentally observed oxide scale growth kinetics perfectly. [ABSTRACT FROM AUTHOR]

Abstract: The linear and mass ablation rates of Ti2AlC ceramics under an oxyacetylene flame at a temperature up to 3000°C were examined by measuring the dimensions and weight change of the ablated samples. The linear ablation rate was decreased from 0.14μms−1 for the first 30s of the ablation to 0.08μms−1 after 180s. Ti2AlC ceramics gained small amounts of weight upon ablation, which is attributed to the formation of oxidation products on the ablated surface. The ablation surface exhibits a two-layer structure: an oxide outer layer, consisting mainly of α-Al2O3 and TiO2 and some Al2TiO5, and a porous sub-surface layer containing Ti2Al1−x C and TiC x O y . With increasing ablation time, the content of TiO2 and Al2TiO5 in the outer layer increased, and more pores developed in the sub-surface layer. The thermal oxidation of Ti2AlC under the flame and scouring of the viscous oxidation products by high-speed flow of gas torch are the main ablation mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: The nucleation and growth of oxide scale at the early stages of oxidation of Ti3AlC2 ceramics was studied via oxidizing at 1100°C in air for short times (≤900s). The nucleation of nanosized Al2O3 particles mainly occurs at the ledges of the fractured lamellar Ti3AlC2 grains as well as on the {0001} basal surfaces. The Al2O3 nuclei mainly grow along these ledges to form oxide strings, and then spread on the terraces and the {0001} basal surfaces. An oxide layer consisting predominantly of nanosized α-Al2O3 forms after oxidizing for 180s. The formation of lenticular hexagonal pores in Ti3AlC2 grains is attributed to the faster consumption of Ti, Al and C atoms along direction than along 〈0001〉 direction. With further oxidation, rutile-TiO2 particles form on top of the α-Al2O3 layer, and grow to form a rutile-TiO2 layer. Further oxidation leads to the formation of pores underneath the primary α-Al2O3 layer. In this porous layer both Al2O3 and TiO2 were present with a preference for Al2O3 to stay adjacent to the inward moving interface of Ti3AlC2 substrate. [Copyright &y& Elsevier]

Abstract: In situ microcantilever bending tests were carried out to evaluate the healing efficiency of pre-notched Ti2AlC ceramic after annealing at 1200°C for 1.5h. Microcantilevers of different orientations were fabricated with focused ion beam method at different locations, i.e. in a singular Ti2AlC grain, at a grain boundary or at the Ti2AlC–Al2O3 interface after healing. Ti2AlC microcantilever shows an anisotropic bending strength (ranging between 9.6GPa and 4.6GPa depending on the precise crystallographic orientations) that is closely related to the different atomic bonds in the layered structure. After healing, the Ti2AlC–Al2O3 microcantilevers exhibit almost the same strength of about 5.2GPa, i.e. slightly higher than the cleavage strength (4.6GPa) of the initial Ti2AlC microcantilevers. It suggests that the orientation of the matrix grain has no significant influence on the strength of healed microcantilevers. Furthermore, it turns out that the strength of the microcantilever with a healed grain boundary is at least twice the strength of the initial Ti2AlC cantilever with a grain boundary. It is concluded that the oxidation dominated self-healing mechanism of Ti2AlC ceramics can result in a perfect recovery of mechanical performance. The paper shows that the in situ microcantilever bending test provides a quantitative method for the evaluation of the strength of self-healing ceramics. [Copyright &y& Elsevier]

A recent paper on “microstructural evolution during high-temperature oxidation of Ti2AlC ceramics” (Cui et al., 2011 ) claims that the oxide product is solely TiO2 (rutile) when Ti2AlC is oxidized at 900°C for 1h. This comment presents experimental evidence and thermodynamic arguments confirming the formation of a primary Al2O3 layer instead of pure TiO2 scale under the same oxidation condition. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a detailed microstructural analysis of the crystallographic relationships between α-Al2O3 oxide scale and Ti2AlC parent material, and examines the atomic diffusion and formation of oxide scale on Ti2AlC during the initial oxidation stage at 1200°C. It is shown that the α-Al2O3 oxide scale can be either a continuous or a discontinuous capping layer on Ti2AlC. A Ti-rich intermediate layer that consists mostly of TiC interrupts the continuity of the α-Al2O3 layer. The channels for inward diffusion of O and outward diffusion of Ti and Al run not only along grain boundaries of α-Al2O3, but also through the Ti-rich intermediate layer. The outward diffusion of Al atoms is either parallel to the (001) basal plane or parallel to prism planes of Ti2AlC. Crystallographic orientation relationships between α-Al2O3 oxide scale and Ti2AlC were observed: // , // and // , // . [Copyright &y& Elsevier]

The microstructure of oxide scales formed on Ti2AlC surface after oxidation at 1200°C is investigated. The results reveal an inhomogeneous morphology of the oxide scales, i.e. islands of coarse rutile grains pile up on fine-grained Al2O3 layer. Local cross-sectional observations show that the rutile islands grow on top of cavities in the parent material. A model is proposed to describe the resulting microstructure after oxidation. These results are relevant in optimizing the self-healing conditions of Ti2AlC at high temperature. [ABSTRACT FROM AUTHOR]

Abstract: Mechanically activated hot-pressing technology was used to synthesize a fine-crystalline Cr2AlC ceramic at relatively low temperatures. A mixture of Cr, Al and C powders with a molar ratio of 2:1.2:1 was mechanically alloyed for 3h, and then subjected to hot pressing at 30MPa and different temperatures for 1h in Ar atmosphere. The results show that a dense Cr2AlC ceramic with a grain size of about 2μm can be synthesized at a relatively low temperature of 1100°C. The synthesized fine-grained Cr2AlC has a high density of 99%, which is higher than the 95% density for the coarse-grained Cr2AlC (grain size of about 35μm) as synthesized by hot pressing unmilled Cr, Al and C. The flexural strength, fracture toughness and Vickers hardness of the fine-grained Cr2AlC were determined and compared with the values for the coarse-grained Cr2AlC. [ABSTRACT FROM AUTHOR]