Influence of Cutting Surface Condition and Composition on High-Temperature Oxidation Behavior of Ti2 Al C MAX Phase Ceramics
Authors: Yamaguchi Naoya , Okada Manabu , Kuo Yen-Ling , Nanko Makoto 
Published: December 2024

Ti2AlC, one of the MAX phase ceramics, has metal-like properties (electrical conductivity, high fracture toughness and good machinability) and ceramic-like properties (low density, high-temperature oxidation resistance and high bending strength). Ti2AlC ceramics can be cut with cemented carbide tools and the mechanical strength does not decrease significantly after cutting. Ti2AlC ceramics have potential as high-strength machinable ceramics for use in high temperature components. However, the oxidation resistance of Ti2AlC ceramics decreases after cutting because the cutting damages inhibit the formation of protective Al2O3 scale. There are few discussions on the effect of cutting surface conditions on oxidation resistance of Ti2AlC ceramics. The continued growth of the protective Al2O3 scale requires a sufficient Al supply during high-temperature oxidation. The oxidation resistance of Ti2AlC ceramics is reduced when a non-protective TiO2 scale forms on the surface. As one of the common methods to prevent the growth of the TiO2 scale is Nb addition in Ti-based alloys and compounds. The oxidation resistance of as-machined Ti2AlC ceramics may be improved by increasing the amount of Al and Nb additions. This study reports the influences of cutting surface conditions and composition on oxidation behavior of Ti2AlC ceramics.

Commercial Ti, Al, C and Nb powders were mixed in Ti:Al:C:Nb molar ratios of 2:1.2:0.9:0 and 1.9:1.2:0.9:0.1 with the corresponding samples receiving the designations Al1 and Al1.2Nb. The powder mixture was annealed for 12 h in a vacuum at 1300°C. The synthesized powder was consolidated over 15 min using a pulsed electric current sintering technique at 1300°C, in a vacuum, and under a uniaxial pressure of 30 MPa. The phases present in the sintered samples were also identified by XRD. Milling tests were performed on a 4 x 4 mm sample surface using a 2 mm diameter end-mill mounted on a plane milling machine. Dry cutting was performed with a spindle speed, feed rate, and cut depth of 4000 rpm, 46 mm/min, and 100 μm. Threading tests were cut into an JIS-M6 bolt shape with a thickness of 3 mm with a lathe and diamond saw. The surface roughness of each sample was measured using a surface texture measuring instrument. The samples were heated in an electric furnace at 1200°C for 4 h in laboratory air. The surface and cross-sections of the oxidized samples were observed via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX).

The main peaks of all sintered samples in these XRD patterns were identified as originating from Ti2AlC and minor peaks originating from TiAl3 (Al-rich phase). The average arithmetic mean roughness (R
a) values of the end-milled and threaded surfaces were 0.8 and 4.5 μm. The SEM and EDX results show that a continuous Al2O3 scale formed on the end-milled surface of Al1.2 and Al1.2Nb after the oxidation test. The results of SEM and EDX investigations indicate that a non-protective TiO2 scale had formed on the threaded surface of Al1 and a continuous Al2O3 scale with a thickness of less than 4 μm was formed on the threaded surface of Al1.2Nb after the oxidation test. The Al-rich phase provided the Al necessary for the formation of a continuous Al2O3 scale on the end-milled surface. The oxidation resistance of Ti2AlC ceramics was greatly reduced in the case of screw-like shapes with high R
a values and many corners on the cutting surface. The addition of Nb improves the oxidation resistance of Ti2AlC ceramics under such cutting surface conditions. The cause of non-protective oxidation was the destruction of the Al2O3 scale because of the growth of TiO2 scale. The TiO2 scale tends to form at corners where continuous Al2O3 scale formation is difficult. One of the factors that improves the oxidation resistance of Ti2AlC ceramics with screw shapes is the suppression of TiO2 growth by Nb addition. The higher Al concentration and the addition of Nb remarkably increase the oxidation resistance of the Ti2AlC ceramics after cutting.