Abstract

Passivation behavior, corrosion kinetics and film formation mechanism of Ti₃AlC₂ in 1M NaOH and 1M H₂SO₄ solutions were investigated by linear potential scan, electrochemical impedance spectroscopy, cyclic voltammetry, SEM and XPS. The corrosion resistance mainly depended on the formation of passivating films on Ti₃AlC₂ in 1M NaOH and 1M H₂SO₄, which led to different corrosion processes. Ti₃AlC₂ displayed good
corrosion resistance in NaOH due to the formation of dense and protective Ti oxides as the passivating film. However, it exhibited poor corrosion resistance in H₂SO₄ which attributed to the formation of permeable Ti sub-oxides as the pseudo-passivating film.

Abstract

Reticulated porous Ti₃AlC₂ ceramic, a member of the MAX phase family (M_n+1AX_n phases, where M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen), was prepared from the highly dispersed aqueous suspension by a replica template method. Through a cathodic electrogeneration method, nanocrystalline catalytic CeO₂ coatings were deposited on the conductive porous Ti₃AlC₂ supports. By adjusting the pH value and cathodic deposition current, coatings exhibiting nanocellar, nanosheets-like, or bubble-free morphologies can be obtained. This work expects to introduce a novel practically feasible material system and a catalytic coating preparation technique for gas exhaust catalyst devices.

Abstract

A new-layered compound, (Ti_0.5Nb_0.5)₅AlC₄, which belongs to MAX phases was discovered. The new-layered carbide was synthesized by reactive hot pressing of Ti, Nb, Al, and C powders.  The crystal structure was determined by the combination of X-ray diffraction and transmission electron microscopy analyses, which consists of a 5(Ti, Nb)–C bond chain linked by an Al layer. The lattice parameters are a=3.100 A ˚ , c=28.89 A ˚ , with Ti(Nb)1 at (0, 0, 0), Ti(Nb)2 at (1/3, 2/3, 0.0877), Ti(Nb)3 at (2/3, 1/3, 0.1723), Al at (0, 0, 0.25), C1 at (2/3, 1/3, 0.0463), and C2 at (0, 0, 0.1566).

Abstract

The oxidation behavior of Hf–Al–C ceramics containing 37.5 wt% Hf₃Al₃C₅, 30.5 wt%Hf₂Al₄C₅, and 32.0 wt% Hf₃Al₄C₆ has been investigated at 9001–1300 ^oC in air. The oxidation kinetics approximately follows a linear law with the activation energy of 194±12 kJ/mol. The oxidation resistance of Hf–Al–C ceramics at high temperature is superior to HfC. The oxide scale is porous and composed of well-mixed t-HfO₂ and Al₂O₃ as well as residual free carbon. The stabilization mechanisms of t-HfO₂ and free carbon have been discussed. The simultaneous oxidation of Hf and Al in Hf–Al–C ceramics can be attributed to their close oxygen affinity as well as the strong coupling between Hf–C blocks and Al–C units in the crystal structures.

Abstract

TaC–TaSi₂ composites were fabricated at 1700 ^oC by an in situ reaction/hot pressing method using Ta, Si, and graphite as initial materials. TaSi₂ content was 0–100 vol%. The microstructure and mechanical properties of the composites were investigated.  It was found that the relative densities of composites were above 97.5% when the volume content of TaSi₂ was above 10%. The TaC/10 vol% TaSi₂ composite presented the highest flexural strength of 376 MPa. When the TaSi₂ content was 30–50 vol%, the composites showed the highest fracture toughness of about 4.3MP am1/2. In addition, the composites could retain high Young’s modulus up to at least 1525 ^oC.

5Abstract

Pressureless sintering of titanium aluminum carbide (Ti₃AlC₂) is difficult due to its easy decomposability at high temperatures, thus decomposition must be avoided during sintering. In this work, pressureless sintering was performed in different embedded powders and Al₄C₃ was found to be effective to inhibit Ti₃AlC₂ from decomposition due to the offering of Al rich ambience. High-density Ti₃AlC₂ was obtained by pressureless sintering in Al₄C₃ powder bed without additives. The good sinterability is due to the special crystal structure of Ti₃AlC₂ and the easy diffusion of Al atoms. The mechanical properties of pressureless sintered Ti₃AlC₂ are comparable to those of the hot-pressed ones.

Abstract

A novel method to fabricate titanium aluminum carbides/ alumina-laminated composites with weak interfaces was developed through heat treatment titanium aluminum carbides at 1300 ^oC and low oxygen partial pressure. The laminated composites consist of 0.5-mm-thick layers of titanium aluminum carbides joined together by the in situ formed Al₂O₃ interlayers.  The interfaces are free of cracks or delaminations. These laminated materials exhibit better damage tolerance compared with the monolithic counterparts. Crack deflection along the Al₂O₃ interlayers was the main mechanism for the noncatastrophic failure. The fracture toughness was increased from 4.9 MPa .m^1/2 for Ti₂AlC to 7.3 MPa .m^1/2 for Ti₂AlC/Al₂O₃ composite and from 6.5 MPa .m^1/2 for Ti₃AlC₂ to 10.4 MPa .m^1/2 for Ti₃AlC₂/Al₂O₃ composite, and the work of fracture required to break samples was increased from 153 J/m² for Ti₂AlC to 676 J/m² for Ti₂AlC/Al₂O₃ composite and from 300 J/m2 for Ti₃AlC₂ to 1317 J/m² for Ti₃AlC₂/Al₂O₃ composite.