Abstract

Layered ternary ceramics represent a new class of solids that combine the merits of both metals and ceramics. These unique properties are strongly related to their layered crystal structures and microstructures. The combination of atomic-resolution Z-contrast scanning and transmission electron microscopy, selected area electron diffraction, convergent beam electron diffraction represents a powerful method to link microstructures of materials to macroscopic properties, allowing layered ternary ceramics to be investigated in an unprecedented detail. Microstructural information obtained using transmission electron microscopy is useful in understanding the formation mechanism, layered stacking characteristics, and defect structures for layered ternary ceramics down to atomic-scale level; and thus provides insight into understanding the “Processing-Structure-Property” relationship of layered ternary ceramics. Transmission electron microscopic characterizations of layered ternary ceramics in Ti-Si-C, Ti-Al-C, Cr-Al-C, Zr-Al-C, Ta-Al-C and Ti-Al-N systems are reviewed.

Abstract

Putting a soft ceramic in a sandwich of hard ceramics will produce composites combining the merits of both soft ceramics and hard ceramics. To strengthen soft ceramics, two analytical relationships among the bending strength, the residual stresses, and the ratio of the coating thickness to the substrate thickness, R, in sandwich beam samples with strong interfaces were established based on the three-point bending model. When the temperature drop and the material properties of the coating and substrate are fixed, the strength enhancement due to the residual stress can be predicted. Furthermore, an optimum ratio R₀ was derived using a stress equilibrium principle, which makes the designed component having the highest strength. These predictions were confirmed by using a bending test on the hard–soft–hard sandwich samples of Al₂O₃/Ti₃SiC₂/Al₂O₃. The measured maximum strength was 14.5%higher than that of Ti₃SiC₂ when R was 0.10, which was close to the calculated optimum ratio R₀ (0.087).

Abstract

The resistance of Ti₃Si(Al)C₂-based materials to strength-impairing contact damage was investigated using the Hertzian indentation method.  Microstructural analysis indicated that for the three types of testing materials the contact damage was governed by multiple grain slip, crushed grains, and intergranular shear failure. No cone cracking or other macro-cracks were visible on or beneath the contact damage surfaces. Bending tests on the specimens containing single-cycle contact damage revealed that the resistance of Ti₃Si(Al)C₂ to strength degradation was significantly improved by incorporating SiC particles into the matrix. The mechanism of the improvement is ascribed to the increased shear resistance and the fact that the hard SiC particles inhibit the downward extent of the contact damage through restricting the slip and deformation of the Ti₃Si(Al)C₂ grains.

Abstract

The microstructures of bulk Zr₂Al₃C₄ and Zr₃Al₃C₅ ceramics have been investigated using transmission electron microscopy and scanning transmission electron microscopy. These two carbides were determined to have a point group 6/mmm and a space group P63/mmc using selected-area electron diffraction and convergent beam electron diffraction. The atomic-scale microstructures of Zr₂Al₃C₄ and Zr₃Al₃C₅ were investigated through high-resolution imaging and Z-contrast imaging. Furthermore, intergrowth between Zr₂Al₃C₄ and Zr₃Al₃C₅ was identified. Stacking faults in Zr₃Al₃C₅ were found to result from the insertion of an additional Zr–C layer. Cubic ZrC was occasionally identified to be incorporated in elongated Zr₃Al₃C₅ grains. In addition, Al may induce a twinned ZrC structure and lead to the formation of ternary zirconium aluminum carbides.

Abstract

Predominantly single phase Zr₃Al₃C₅ powders were synthesized in an Ar atmosphere using Zr-Al intermetallics and graphite as starting materials. The reaction path of Zr₃Al₃C₅ synthesis was discussed based on differential scanning calorimetry and X-ray diffraction results. Lattice parameters of Zr₃Al₃C₅ determined using the Rietveld method are a = 3.347 Å and c = 27.642 Å. In addition, the oxidation of Zr₃Al₃C₅ powders was tested by using thermogravimetry-differential scanning calorimetry. The starting and complete oxidation temperatures are 400 ^oC and 1200 ^oC, respectively. These temperatures are much higher than those for ZrC, suggesting that Zr₃Al₃C₅ has better oxidation resistance than ZrC. On the other hand, the oxidation degree of Zr₃Al₃C₅, defined for the complete carbide-oxide transformation, overshot 100% during oxidation. This overshooting is attributed to the formation of amorphous carbon. The phase evolution during the oxidation of Zr₃Al₃C₅ was also investigated.

Abstract

Elastic stiffness and electronic structure of La₂Zr₂O₇ were calculated by means of the first-principles pseudopotential total energy method. The equation of state (EOS), elastic parameters (including the full set of second-order elastic coefficients, bulk modulus and Young’s modulus) and elastic anisotropy were reported. Furthermore, pressure dependence of crystal structure, electronic structure, and bond strengths were investigated. It is found that, although the La₂Zr₂O₇ lattice is stable at high pressures, its electronic structure and atomic bonding are definitely disturbed by the applied pressure. The crystal structure of La₂Zr₂O₇ approaches that of the fluoritetype lattice at high pressures. The strengths of different interatomic bonds in La₂Zr₂O₇ are examined by considering bond-length contractions at various pressures. In addition, the results based on quantum-mechanical-scale calculation clarify the nature of low thermal conductivity of La₂Zr₂O₇ at elevated temperatures.

Abstract

In order to modify surface properties of Ti₃SiC₂, boronizing was carried out through powder pack cementation in the 1100–1400 ^oC temperature range. After boronizing treatment, one mixture layer, composed of TiB₂ and β-SiC, forms on the surface of Ti₃SiC₂. The growth of the coating is processed by inward diffusion of boron and obeys a linear rule. The boronizing increases the hardness of Ti₃SiC₂ from 3.7 GPa to a maximal 9.3 GPa and also significantly improves its wear resistance.

Abstract

This paper describes a new method to synthesize AlN nanowires by the nitridation of Ti₃Si_0.9Al_0.1C₂ solid solution. Single-crystalline AlN nanowires with the hexagonal wurtzite structure can be easily prepared using this method. In particular, the resulting AlN nanowires display a new growth orientation of <101¯1> besides <1000> and <0001>.  This work indicates that M_N+1AX_N compounds are promising raw reactants to synthesize one-dimensional (1D) nanostructures of nitrides and oxides.

Abstract

An in situ reactive hot-pressing process using zirconium (zirconium hydride), aluminum, and graphite as staring materials and Si and Y₂O₃ as additives was used to synthesize bulk Zr₃Al₃C₅ ceramics. This method demonstrates the advantages of easy synthesis, lower sintering temperature, high purity and density, and improved mechanical properties of synthesized Zr₃Al₃C₅. Its electrical and thermal properties were measured. Compared with ZrC, Zr₃Al₃C₅ has a relatively low hardness (Vickers hardness of 12.5 GPa), comparable stiffness (Young’s modulus of 374 GPa), but superior strength (flexural strength of 488 GPa) and toughness (fracture toughness of 4.68 MPa .m^1/2). In addition, the stiffness decreases slowly with increasing temperature and at 1600 ^oC remains 78% of that at ambient temperature, indicating that Zr₃Al₃C₅ is a potential high-temperature structural ceramic.

Abstract

Fully dense and single-phase Ti₂AlN ceramic was successfully synthesized by a hot-pressing method using Ti, TiN and Al as starting materials. The present method offers the advantages of short processing time, low applied pressure and high product purity.  The formation mechanism, atomic-scale microstructures and typical mechanical properties for Ti₂AlN are presented.

Abstract

Weibull modulus of bending strength of nanolayer-grained ceramic Ti₃SiC₂ was estimated with over 50 specimens, using the least square method, the moment method and the maximum likelihood technique, respectively. The result demonstrated that the m-value of this layered ceramic ranged from 25 to 29, which is much higher than that of traditional brittle ceramics. The reason of high Weibull modulus was due to high damage tolerance of this material. Under stress, delamination and kinking of grains and shear slipping at interfaces give this material high capacity of local energy dissipation and easy local stress relaxation, leading to the excellent damage tolerance of Ti₃SiC₂. The effect of amounts of specimens on the reliability of the estimated m-values was also investigated.  It was confirmed that the stability of the estimated m-value increased with increasing numbers of specimens. The parameter obtained using the maximum likelihood technique showed the highest reliability than other methods. The ranges of failure probability were determined using the Weibull estimates calculated from the maximum likelihood technique.

Abstract

A simple approach named relative method is developed for determining the elastic modulus and strength of hard coatings. Analytical relationship among the moduli of the film, the substrate, and the film/substrate system was derived based on bending model, from which the elastic modulus of the coating can be determined uniquely via the measured moduli of the samples before and after coating. Furthermore, the relationship between the strength of the films and the bending strength of the coated sample is derived, thus both the modulus and the strength of coating can be evaluated via traditional tests on coated samples. Mathematic expressions of those calculations were derived, respectively for rectangular beam samples with
three types of coating configurations: single face coating, sandwich coating and around coating. Experimental results using various brittle coatings demonstrated the validity and convenience of this method.