Y. W. Bao

Preparation of TiC free Ti3SiC2 with improved oxidation resistance by substitution of Si with Al

Abstract Ti3SiC2 combines many of the merits of both metals and ceramics and has potential in diverse high temperature applications. However, TiC always exists as an impurity phase, which is deleterious to the high-temperature oxidation resistance of Ti3SiC2. Although many attempts have been made, it is difficult to eliminate TiC from Ti3SiC2 due to the close structural relations between the two compounds. In this paper we describe our innovative work to prepare TiC free Ti3SiC2 by the substitution of a small amount of Si with Al. The substitution of Si with Al resulted in the formation of aTi3Si1, AlxC2 solid solution. The mechanical properties of the Al doped Ti3SiC2 are close to those of Ti3SiC2, but the oxidation resistance is significantly improved due to the formation of a protective Al2O3 layer during high- temperature oxidation.

Oxidation of Zr 2 (Al(Si)) 4 C 5 and Zr 3 (Al(Si)) 4 C 6 in air

The oxidation behavior of Zr(2)[Al(Si)](4)C(5) and Zr(3)[Al(Si)](4)C(6) in air has been investigated. The oxidation kinetics of bulk Zr(2)[Al(Si)](4)C(5) and Zr(3)[Al(Si)](4)C(6) at 900-1300 degrees C generally follow a parabolic law at a very short initial stage and then a linear law for a long period with the activation energy of 237.9 and 226.8 kJ/mol, respectively. The oxide scales have a duplex structure, consisting of mainly an outer porous layer of ZrO(2), Al(2)O(3), and aluminosilicate/mullite, and a thin inner compact layer of these oxides plus remaining carbon. The oxidation resistance of Zr(2)[Al(Si)](4)C(5) and Zr(3)[Al(Si)](4)C(6) has been improved compared with Zr(2)Al(3)C(4), and is much better than Zr(3)[Al(Si)](4)C(6) to larger fraction of protective oxidation products, Al(2)O(3) and aluminosilicate/mullite.

Synthesis and oxidation of Zr3Al3C5 powders

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 °C and 1200 °C, 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.

Isothermal oxidation of bulk Zr 2 Al 3 C 4 at 500 to 1000 °C in air

The isothermal oxidation behavior of Zr 2 Al 3 C 4 in the temperature range of 500 to 1000 °C for 20 h in air has been investigated. The oxidation kinetics follow a parabolic law at 600 to 800 °C and a linear law at higher temperatures. The activation energy is determined to be 167.4 and 201.2 kJ/mol at parabolic and linear stages, respectively. The oxide scales have a monolayer structure, which is a mixture of ZrO 2 and Al 2 O 3 . As indicated by x-ray diffraction and Raman spectra, the scales formed at 500 to 700 °C are amorphous, and at higher temperatures are α-Al 2 O 3 and t-ZrO 2 nanocrystallites. The nonselective oxidation of Zr 2 Al 3 C 4 can be attributed to the strong coupling between Al 3 C 2 units and ZrC blocks in its structure, and the close oxygen affinity of Zr and Al.

Elastic and thermal properties of Zr2Al3C4: Experimental investigations and ab initio calculations

This article presents the results of combined experimental and theoretical studies of elastic and thermal properties of Zr(2)Al(3)C(4) carbide. The full set of second order elastic constants, bulk modulus, shear modulus, and Youngs modulus of Zr(2)Al(3)C(4) were calculated and compared with those of Zr(3)Al(3)C(5) and ZrC. The experimentally measured Youngs modulus and shear modulus are in good agreement with theoretical ones. The calculated Debye temperature from elastic constants of Zr(2)Al(3)C(4) is 830 K, which is slightly higher than that of Zr(3)Al(3)C(5), and exhibits pronounced enhancement in comparison with that of ZrC. The highest Debye temperature of Zr(2)Al(3)C(4) is related with its highest specific stiffness, i.e., the stiffness-to-weight ratio. The heat capacity and thermal conductivity of Zr(2)Al(3)C(4) were measured by means of the flash method. The thermal conductivity of Zr(2)Al(3)C(4) decreases with increasing temperature, for instance the values at room temperature and 1600 K are 15.5 and 10.1 W/m K, respectively. The investigations provide information on elastic and thermal properties of Zr(2)Al(3)C(4) with promising high temperature applications. (c) 2007 American Institute of Physics.

Improving the high-temperature oxidation resistance of Zr2Al3C4 by silicon pack cementation

Silicon pack cementation has been applied to improve the oxidation resistance of Zr2Al3C4. The Si pack coating is mainly composed of an inner layer of ZrSi2 and SiC and an outer layer of Al2O3 at 1200 degrees C. The growth kinetics of silicide coating at 1000-1200 degrees C obey a parabolic law with an activation energy of 110.3 +/- 16.7 kJ/mol, which is controlled by inward diffusion of Si and outward diffusion of Al. Compared with Zr2Al3C4, the oxidation resistance of siliconized Zr2Al3C4 is greatly improved due to the formation of protective oxidation products, aluminosilicate glass, mullite, and ZrSiO4.

A simple method for measuring tensile and shear bond strength of ceramic-ceramic and metal-ceramic joining

Abstract.nA simple method for measuring the ceramic-ceramic and metal-ceramic bond strength was presented, by which uniaxial tensile stress normal to the interface or shear stress in the interface can be produced using uniaxial compression load on a cross-bonded sample. Both tensile and shear bond strength were obtained by this testing technique for Ti3SiC2–TiO2 and Ti3SiC2–Al2O3 composite as well as for glued steel samples, respectively. The novel method provided a solution for determining bond strength in solid (especially brittle) materials, and it is also demonstrated as a useful method for evaluating the tensile and shear strength of various glues.