Thermodynamics, Synthesis, Oxidation Properties Of Ti₂AlC And Cr₂AlC Ceramics

MAX is the abbreviation of Mn+1AXn, in which M is the transition metals, A is ⅢA or ⅣA group elements, X is C or N, the coexistence of layered crystal structure and the valence characteristics of M-X covalent bond and M-A metal bond provide such compounds with both metallic and ceramic properties, and make them have a good application prospect in the fields of the anti-oxidation coating and high temperature structural materials.Ti2AlC and Cr2AlC are both typical members of the MAX compounds and have good thermal physical properties. They have excellent oxidation resistance due to their moderate Al content favorable for generating compact oxide layer of Al2O3, and make MAX compounds become a current research hotspot. Nowadays, the application research of such compounds mainly concentrates in the fields of the adhesive layer of thermal barrier coatings and interconnects materials of the solid oxide fuel cell ("SOFC").The synthesis of single-phase compounds is one of critical problems in relevant research. Studies have shown that the existence of second phase will seriously affect the performance of the single-phase compounds. Take Ti2AlC for example, the impurity phases such as TiC, TiAlx, etc. in the Ti2AlC compound will make the high temperature oxidation resistance of the material severely weakened and make the material become brittle.The preparation of single-phase compounds is very difficult, and this is mainly determined by the complex phase equilibrium of Ti-Al-C and Cr-Al-C ternary system. Take Ti-Al-C for example. There are three ternary compounds namely Ti2AlC, Ti3AlC and Ti2AlC3and nine binary compounds, and due to the phase equilibrium relationship between binary compounds such as Ti2AlC and TiC, Ti3Al and TiAl, etc., it is easy to generate impurity phases during the preparation of Ti2AlC. Meanwhile, because the saturation vapor pressure of component Al is much higher than the other components, it will loss severely in the process of synthesis and deviate from the best alloy composition point obtained from phase relationship research and design, so that the product deviates from the designed phase equilibrium area. To fabricate single-phase compounds, we need have clear understanding about the phase equilibrium and the nature of the element volatilization loss during the synthesis process, and strictly control the raw material and the preparation conditionBy using the CALPHAD method, we can predict the phase equilibrium and phase transition of multi-system and calculate the phase composition of specified compositon in the specific process conditions. On the other hand, based on the thermodynamic model of the material, we can calculate the saturation vapor pressure of each component under different conditions and predict volatile degree of each component to determine the ratio of raw materials and process conditions required for the synthesis of single-phase compounds.This paper, taking the Al-C-Cr-Ti-O system as the key point, we using the CALPHAD method to build the thermodynamic database based on the self-consistent principle of the thermodynamics and phase equilibrium behavior. Thermodynamic database contains the element O because of the research of oxidation property of Ti2AlC and Cr2AlC.Through the collection and organization of the literature data regarding sub-binary and ternary system, we have unified the thermodynamics model, conduct thermodynamic optimization of Ti-Al-C and Cr-Al-C system, and obtained some important model parameters in the thermodynamic database. For the shortage of the thermodynamic data about the ternary compounds Ti2AlC, Ti3AlC2, Ti3AlC and Cr2AlC during the thermodynamic optimization, this writer applies the first principles of quantum mechanics combined with the super-cell perturbation method to calculate the Gibbs free energy of end-number compounds within the whole temperature range. Taking Gibbs free energy calculated by the first principles as initial value of thermodynamic optimization makes the optimization results more reasonable.Based on the established Al-C-Cr-Ti-O system thermodynamics database, the writer used the CALPHAD method to predict the synthesis conditions of Ti2AlC and Cr2AlC and successfully prepared a high single-phase Ti2AlC and Cr2AlC compounds using the hot pressing method. For the component volatilization loss during the vacuum preparation, the writer calculated he saturation vapor pressure of each component and their theoretical evaporation rate. We find that the saturation vapor pressure of component Al is much higher than the other component, quantitatively predict the loss degree of component Al through phase diagram calculation and find higher volatile degree will happen in high-temperature synthesis which lead to the deviation from the optimum alloy composition point, so that the product deviates from the designed phase equilibrium region. Meanwhile, verification test is carried out for the volatilization loss of component Al by a vacuum without pressure sintering experiment. By thermodynamic calculation combined with cavuum sintering experiments at different temperatures, the reaction pathways of the synthesis process of Ti2AlC and Cr2AlC was analyzed in detail and the thermodynamic mechanism and element trend in the synthesis process were clearly understood.The writer has studied the phase composition of oxidation products of Ti2AlC and Cr2AlC by using XRD and SEM, etc. For Ti2AlC, we found the mixed oxidation products of TiO2and Al2O3is evenly distributed on the surface of Ti2AlC oxidation layer, and a small amount of bigger TiO2particles is distributed in the interfacial boundaries which states part of Ti diffuses from the interfacial boundaries to outside and grow up in the interfacial boundaries during the oxidation process, and a dense Al2O3layer is distributed under the oxidation surface. For Cr2AlC, we found Al(Cr)2O3solid solution is evenly distributed on the oxidation surface of Cr2AlC and the dense Al2O3layer is under the oxidation surface. The oxidation layer structure of Cr2AlC is different from Ti2AlC, it is that the obvious Cr7C3layer without Al id distributed under the Al2O3layer and the thickness is nearly equal to the Al2O3layer. Besides, there are holes on the junction of Cr7C3layer and Al2O3layer. Through the thermodynamic calculation during the oxidation process, it stated that the stratification of the oxidation products is caused by the oxygen partial pressure gradient in the material internal.By analyzed the isothermal oxidation weigh gain curve, we found Ti2AlC and Cr2AlC material is total reaction control in the initial stage of oxidation among1100～1300℃and meet the parabolic weight gain rule, so we can confirm it is mianly the diffusion control during the oxidation process. The oxidation weight parabolic constants of Ti2AlC and Cr2AlC are smaller than the conventional high-temperature antioxidant alloy of NiCrAlY and both two materials have good isothermal oxidation resistance performance. Moreover, we found it is mainly interfacial boundaries diffusion during the oxidation process for both compounds. By studying the matrial cycle oxidation property, we found that no weigh loss happened during high-temperature cycle oxidation process for Ti2AlC and oxidation product is closely combined with the substrate. However, weigh loss happened during this process for Cr2AlC and oxidation product is peeling.The Cr2AlC coatings more than200um were successfully prepared by High Velocity Oxy-Fuel (HVOF) on GH4169nickel alloy. We measured the microhardness and porosity of the coatings and observed the cross-section morphology of the coatings. It showed that the coating was closely combined with the substrate and the microstructure of the coating was dense. We could obtain a higher density coating with finer powder to spray by studying coating phase composition with EDS analysis and X-ray diffraction. When the Cr2AlC coatings are being oxidized, we observed that the structure of coatings is basically the same to the bulk oxidation products, but the oxidation rate is faster than the Cr2AlC bulk materials due to the inherent porosity of the Cr2AlC coatings making the oxidation accelerated.This paper has established the thermodynamic database of Al-C-Cr-Ti-O system and predicts the conditions for synthetizing the single phase Ti2AlC and Cr2AlC. To solve the element loss in the synthesis, we calculated the saturated vapor pressure, predict the evaporation rate of element, and proposed the measure to prevent component segregation as well. Moreover, we studied the high temperature oxidation resistance during synthetizing Ti2AlC and Cr2AlC compounds and analyzed the stratification of oxidation products. This paper applied the material design methods to successfully synthetize the Ti2AlC and Cr2AlC single phase materials, studied the properties and application fundament of the new materials and made a very useful work for the practical application of such materials.