Abstract

Predominantly single-phase Hf₂[Al(Si)]₄C₅ ceramic has been fabricated by an in situ reaction/hot-pressing method using Hf, Al, Si and graphite as starting materials. Hf₂[Al(Si)]₄C₅ shows comparable mechanical properties to Zr₂[Al(Si)]₄C₅, and lower hardness and stiffness but higher strength and toughness than HfC. The stiffness decreases slowly with temperature and at 1600^oC it remains 83% of that at ambient temperature. Compared to Zr₂[Al(Si)]₄C₅ and HfC, however, Hf₂[Al(Si)]₄C₅ exhibits a relatively higher coefficientof thermal expansion, an intermediate specific heat capacity and a lower thermal conductivity.

Abstract

The oxidation behaviors of a Ti₃AlC₂/20(vol%)TiB₂ composite, synthesized by means of in situ reactions of Ti, Al, graphite, and B₄C powder mixtures under hot pressing, have been investigated at 1000–1400^oC in air. The Ti₃AlC₂/20TiB₂ composite followed the logarithmic oxidation law, and had a lower oxidation rate than the matrix Ti₃AlC₂. During the oxidation at 1000–1400^oC, a continuous Al₂O₃ formed on the composite, meanwhile other different oxides formed on the top surface of the Al₂O₃ layer, depending on the oxidation temperature: discontinuous α-TiO₂ at below 1200^oC, mixture of Al₂TiO₅ and TiO₂ at 1300^oC, and continuous Al₂TiO₅ at 1400^oC. The selective oxidation of Al occurred at the oxide/substrate interface through inward diffusion of oxygen, and TiO₂ overgrew the Al₂O₃ layer to form coarse grains through outward diffusion of titanium ions. When the temperature was above 1280^oC, Al₂TiO₅ appeared due to a eutectic reaction between TiO₂ and Al₂O₃. The incorporation of TiB₂ promoted the generation of great numbers of voids at the Al₂O₃/substrate interface and in the Al₂O₃ inner layer, which is proposed as being the main reason for the Ti₃AlC₂/20TiB₂ composite to follow a logarithmic oxidation law.

Abstract

Hydrothermal oxidation of bulk Ti₃SiC₂ in continuous water flow was studied at 500–700 ^oC under a hydrostatic pressure of 35 MPa. The oxidation was weak at 500–600 ^oC and accelerated at 700 ^oC due to the formation of cracks in oxides. The kinetics obeyed a linear time-law. Due to the high solubility of silica in hydrothermal water, the resulting oxide layers only consisted of titanium oxides and carbon. Besides general oxidation, two special modes are very likely present in current experiments: (1) preferential hydrothermal oxidation of lattice planes perpendicular to the c-axis inducing cleavage of grains and (2) uneven hydrothermal oxidation related to the occurrence of TiC and SiC impurity inclusions. Nonetheless the resistance against hydrothermal oxidation is remarkably high up to 700 ^oC.

Abstract

HfAl₄C₄, a new ternary aluminum carbide, was discovered and its crystal structure was determined by a combination of X-ray diffraction, transmission electron microscopy, and first principles calculations. The crystal structure is trigonal belonging to the P 3m1 space group. The refined lattice constants are a=0.3308 nm, c=2.190 nm. First-principles method was used to calculate the theoretical second-order elastic constants, bulk modulus, shear modulus, and the Young’s modulus of HfAl₄C₄.  It shows that HfAl₄C₄ has relatively high elastic stiffness.

Abstract

Magnetron sputtering deposition Cu and subsequent annealing in the temperature range of 900–1100 ^◦C for 30–60 min were conducted with the motivation to modify the surface hardness of Ti₃SiC₂. Owing to the formation of TiC following the reaction Ti₃SiC₂ + 3Cu→3TiC_0.67 +Cu₃Si, the surface hardness was enhanced from 5.08 GPa to a maximum 9.65 GPa. In addition, the surface hardness was dependent on the relative amount of TiC, which was related to Cu film thickness, heat treatment temperatures and durations of annealing. Furthermore, after annealing at 1000 ^◦C for 30 min the Cu-coated Ti₃SiC₂ has lower wear rate and lower COF at the running-in stage compared with Ti₃SiC₂ substrate. The reaction was triggered by the inward diffusion of Cu along the grain boundaries and defects of Ti₃SiC₂. At low temperature and short annealing time, i.e. 900 or 1000 ^◦C for 30 min, Cu diffused inward Ti₃SiC₂ and accumulated at the trigonal junctions first. At higher temperature of 1100 ^◦C or prolonging the annealing time to 60 min, considerable amount of Cu diffused to Ti₃SiC₂ and filled up the grain boundaries leaving a mesh structure.

Abstract

The microstructure, mechanical and thermal properties, as well as oxidation behavior, of in situ hot-pressed Zr₂[Al(Si)]₄C₅–30 vol.% SiC composite have been characterized. The microstructure is composed of elongated Zr₂[Al(Si)]₄C₅ grains and embedded SiC particles. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young’s modulus of 386 GPa), strength (bending strength of 353MPa), and toughness (fracture toughness of 6.62MPam1/2) compared to a monolithic Zr₂[Al(Si)]₄C₅ ceramic. Stiffness is maintained up to 1600 ^◦C (323 GPa) due to clean grain boundaries with no glassy phase. The composite also exhibits higher specific heat capacity and thermal conductivity as well as better
oxidation resistance compared to Zr₂[Al(Si)]₄C₅.

Abstract

Ti₂AlC and Ti3AlC2 are the most light-weight and oxidation resistant layered ternary carbides belonging to
the MAX phases. This review highlights recent achievements on the processing, microstructure, physical,
mechanical and chemical properties of these two machinable and electrically conductive carbides. Ti₂AlC
and Ti₃AlC₂ display superior properties such as fracture toughness, electrical and thermal conductivities, and oxidation resistance over their binary counterpart. This paper provides a comprehensive overview of the processing-microstructure-property correlations of these two carbides. Potential fields of applications for Ti₂AlC and Ti₃AlC₂ are surveyed. In addition, we point out methods for further improving their properties in
some specific applications through appropriate structural design and modification.

Abstract

Ti₃SiC₂ shows a unique combination of the properties of both metals and ceramics. However, its stiffness and strength lose rapidly above 1050 ^oC, which is the main obstacle for the high-temperature application of this material. To improve the high-temperature mechanical properties of Ti₃SiC₂, Zr, Hf, or Nb were used as dopants in Ti₃(SiAl)C₂. At room temperature, the Zr-, Hf-, or Nb-doped Ti₃(SiAl)C₂ ceramics have comparable stiffness, hardness, strength, and fracture toughness with those of Ti₃(SiAl)C₂. At high temperatures, however, a significant improvement in stiffness and strength has been achieved for (Ti_{1- x}T_x)₃(SiAl)C₂ (T=Zr, Hf, or Nb). (Ti_1-xT_x)₃(SiAl)C₂ can retain high degrees of stiffness and strength up to 1200 ^oC,
which is 150 ^oC higher than those for Ti₃(SiAl)C₂.

Abstract

Cr₂AlC coating was deposited at 370 and 500 °C by D.C. magnetron sputtering from an as-synthesized bulk
Cr₂AlC target. The phase composition and preferential orientation of the coating were investigated using XRD, and the microstructure of the coating was characterized by TEM. Results indicated that Cr₂AlC coating
with a strong (110) preferential orientation could be obtained. The coating microstructure was clearly affected by the deposition temperature. At 370 °C, the deposited coating possessed a triple-layered structure with an α-(Cr, Al)₂O₃ inner layer, an amorphous intermediate layer and a crystalline Cr₂AlC outer layer.  However, the coating deposited at 500 °C had a single-layered structure consisting of crystalline Cr₂AlC layer.  The growth mechanism of the Cr₂AlC coating at different deposition temperatures is discussed.

Abstract

Ti₃Si(Al)C₂ films were electrophoretically deposited at 3 V on indium-tin-oxide (ITO) conductive glass from Ti₃Si(Al)C₂ aqueous suspension with 1 vol% solid loading at pH 9 in the absence of any dispersant. The surface morphology, cross section microstructure, and preferred orientation of the films were investigated by scanning electron microscopy and X-ray diffraction.  The as-deposited Ti₃Si(Al)C₂ films exhibited (00l) preferred orientation and the thickness can be controlled by the deposition–drying–deposition method. These results demonstrate that electrophoretic deposition is a simple and feasible method to prepare MAX-phases green films at room temperature.

Abstract

In order to achieve better understanding of the structural/property relationships of La₂T₂O₇ (T = Ge, Ti, Sn, Zr, Hf) pyrochlore, first-principles calculations were conducted to investigate the bonding characteristics, elastic stiffness, structural stability and minimum thermal conductivity. The results show that the relatively weak La–O bonds play a predominant role in determining the structural stability, mechanical and thermal properties of these compounds. In addition, the elastic and thermal properties are influenced when the T atom changes from Ge to Hf. When the bonding strength is enhanced by applying hydrostatic pressure, apart from c₁₁, c₁₂, and B, which normally increase at high pressures, it is found that the shear elastic moduli, c₄₄ and G, which relate to the shear deformation resistance, abnormally remain almost constant. The underlying mechanism may help to explain the damage tolerance of pyrochlore compounds. After comprehensive consideration of the elastic anisotropy, a modified David Clarke-type equation is used to calculate the minimum thermal conductivity of the studied pyrochlore materials, which display an extraordinary low thermal conductivity.