Synthesis, sintering, and effect of surface roughness on oxidation of submicron Ti₂AlC ceramics

Abstract

Submicron Ti2AlC MAX phase powder was synthesized by molten salt shielded synthesis (MS3) using a Ti:Al:C molar ratio of 2:1:0.9 at a process temperature of 1000°C for 5 hours. The synthesized powder presented a mean particle size of ~0.9 µm and a purity of 91 wt. % Ti2AlC, containing 6 wt. % Ti3AlC2. The Ti2AlC powder was sintered by pressureless sintering, achieving a maximal relative density of 90%, hence field‐assisted sintering technology/spark plasma sintering was used to enhance densification. The fine‐grained microstructure was preserved, and phase purity of Ti2AlC was unaltered in the latter case, with a relative density of 98.5%. Oxidation was performed at 1200°C for 50 hours in static air of dense monolithic Ti2AlC with different surface finish, (polished, ground and sandblasted) which resulted in the formation of an approx. 8 µm thin aluminum oxide (Al2O3) layer decorated with titanium dioxide (rutile, TiO2) colonies. Surface quality had no influence on Al2O3 scale thickness, but the amount and size of TiO2 crystals increased with surface roughness. A phenomenon of rumpling of the thermally grown oxide (TGO) was observed and a model to estimate the extent of deformation is proposed.

Processing of MAX phases: From synthesis to applications

“…However, sintering of pre‐reacted MAX phase powders by SPS often leads to a better control of the microstructure. For example, Badie et al have recently synthesized submicron Ti 2 AlC powders by MS3, followed by densification by SPS at 1200°C for 10 minutes to produce dense Ti 2 AlC with an average grain size below 1 µm, which is the finest microstructure reported for a dense MAX phase to date 195 …”

Multifunctional performance of Ti2AlC MAX phase/2D braided alumina fiber laminates

“…Finally, the third step is the densification of the laminate, which was carried out in a FAST/SPS in order to fully densify the samples, control the phase purity, and limit the grain growth, as reported in a previous work. 22 F I G U R E 1 Processing, design, and angles of the twodimensional alumina fiber braids After sintering, the monolithic material and the Ti 2 AlC/Al 2 O 3f laminate present high density, specifically 99.9% of the theoretical density, and no cracks, flaws, or delamination were detected in the laminate (Figure 2A). This is caused by different reasons, where one of the most determinant is the coefficient of the thermal expansion (CTE) match between the alumina Nextel610 fibers (7.9 x 10 −6 K −1 ) and Ti 2 AlC (8.2 x 10 −6 K −1 ), which strongly reduces the thermal stresses.…”

Injection Molding and Near-Complete Densification of Monolithic and Al2O3 Fiber-Reinforced Ti2AlC MAX Phase Composites

“…Ti 2 AlC powder, synthesized via the method of molten salt-shielded synthesis, described elsewhere [24], was mixed with a binder system that was optimized in a previous study [34], to produce feedstocks with 50 vol.% solid content. The binder was composed of 60 vol.% paraffin wax (Paraffin 65, Sigma-Aldrich CHEMIE GmbH, Taufkirchen, Germany), 35 vol.% of a polyethylene copolymer (Hostalen GA 7260 G, Basell Polyolefine GmbH, Frankfurt am Main, Germany), and 5 vol.% stearic acid (Merck KGaA, Darmstadt, Germany).…”

“…The Ti 2 AlC powder used in this work had an irregular agglomerate shape [24]. After passing through a 500-mesh sieve, the following particle size values were obtained: D 10 = 7.5 µm, D 50 = 13.6 µm, and D 90 = 22.9 µm.…”

“…Furthermore, the densification of MAX phases-and, more specifically, Ti 2 AlC-is known to be challenging through conventional pressureless sintering at a high temperature of 1300 • C [24,25], and even up to 1400 • C [24,26]. Sintering additives can only contribute to a limited extent to increase relative density [26] and higher sintering temperatures are undesired as they promote the decomposition of the MAX phase [27].…”