Microstructural Evolution and Oxidation Behaviour of Spark Plasma Sintered Mn+1AXn Ceramics

Abstract

MAX phases are promising candidates for high-temperature wear, hypersonic and nuclear applications. Understanding the formation of MAX phase microstructures is crucial to their application because they often coexist with phases such as M-A intermetallics, M-X or A-X binary compounds which are intermediate products of the synthesis. The relation between processing and fired microstructures in Ti2AlN ceramics made by spark plasma sintering (SPS) was examined using a range of advanced microscopy techniques. When sintered at 1300 ºC, nearly single-phase Ti2AlN ceramics with elongated (~22×6×6 μm) grains were obtained. After sintering at 1200 ºC and chemical etching, Ti2AlN nanowhiskers (150-200 nm dia., 1-5 μm long) were exposed in pores coexisting with TiAl, TiN and Ti2AlN grains. The nanowhiskers are believed to form by diffusion of TiN into TiAl during SPS and are exposed during the chemical etch. Microstructural studies of Ti3AlC2/W composites prepared by SPS at 1300 ºC revealed “core-shell” microstructures in which a TixW1-x “shell” surrounded a W “core” in a Ti3AlC2 matrix. Above 1200 ºC, W reacted with Ti out-diffused from Ti3AlC2 to form TixW1-x solid solution with crystal structure isotypic with bcc W and non-stoichiometric Ti3-xAlC2. The oxidation mechanism of MAX phases is not fully understood which may limit their applications at high temperature in oxidising atmosphere. To understand the fundamental nature of the oxidation mechanism of MAX phases requires investigation of the microstructural evolution of the oxide scale during oxidation. Microstructural development during high-temperature oxidation of Ti2AlC below 1300 ºC involves gradual formation of an outer discontinuous TiO2 layer and an inner dense and continuous α-Al2O3 layer. At 1400 ºC, a mixed outer layer of TiO2 and Al2TiO5 and a cracked α-Al2O3 inner layer formed. During high-temperature oxidation of dense Ti2AlN ceramics below 1200 ºC layered microstructures containing anatase, rutile and α-Al2O3 formed on the surface. Above 1200 ºC, more complex layered microstructures containing Al2TiO5, rutile, α-Al2O3 and continuous void layers formed. The planar defects formed after Ti2AlC oxidation at 1200 ºC and Ti2AlN oxidation at 1100 ºC for 1h were identified as twins and stacking faults. After heating both Ti2AlC and Ti2AlN to 1400 ºC for 1h and cooling to room temperature, cracks propagate in TiO2 grains. Planar defects and cracks may arise from stress generation in the oxide scale. The thermal stresses formed during cooling may result from thermal expansion mismatch of phases (TiO2, Al2O3 and Al2TiO5) in the oxide scale, the high anisotropy of thermal expansion in Al2TiO5 and thermal expansion mismatch between the oxide scale and Ti2AlC or Ti2AlN substrate. Growth stresses formed during isothermal oxidation treatment may arise from the volume changes associated with oxidation reactions of Ti2AlC or Ti2AlN. An oxidation mechanism for Ti2AlC is proposed, in which the growth of oxide scale is caused by inward diffusion of O2- and outward diffusion of Al3+ and Ti4+. The weakly bound Al leaves the Al atom plane in the layered structure of Ti2AlC, and diffuses outward to form a protective inner α-Al2O3 layer between 1100 and 1300 ºC. However, the α-Al2O3 layer becomes cracked at 1400 ºC, providing channels for rapid ingress of oxygen to the body, leading to heavy oxidation. An oxidation mechanism for Ti2AlN is proposed, which involves initial reaction with atmospheric oxygen to form oxide phases, demixing of the mixed oxide phases, and void formation due to the Kirkendall effect and gaseous NOx release. The oxidation resistance of Ti2AlC (up to 1400 ºC) is better than that of Ti2AlN (up to 1200 ºC). Stress generation and gas formation appear to play important roles in the different oxidation mechanisms of Ti2AlC and Ti2AlN. Microstructural development during high-temperature oxidation of Ti3AlC2/W composites involves α-Al2O3 and rutile formation ≥1000 ºC and Al2TiO5 formation at ~1300 ºC while tungsten oxides may have volatilised above 800 ºC. Likely due to exaggerated, secondary grain growth of TiO2-doped alumina, fine (<1 μm) Al2O3 grains formed dense, anisomorphic laths on Ti3AlC2/5wt%W surfaces ≥1200 ºC and coarsened to large (>5 μm), dense, TiO2- doped Al2O3 clusters on Ti3AlC2/10wt%W surfaces ≥1400 ºC, which were more protective than well-dispersed Al2O3 grains. W may affect the oxidation behaviour of Ti3AlC2/W composites in two ways: a) beneficially by weakening the Ti-Al bond in Ti3AlC2 by attracting Ti to form Ti1-xWx resulting in a higher diffusivity of Al which diffuses outward to form a protective α-Al2O3 layer during high-temperature oxidation; and b) detrimentally by releasing volatile tungsten oxides so generating pores in the oxide scale. However, at high temperature (≥1400 ºC) the former beneficial effects appear to be dominant over the latter detrimental effect.