Al₂O₃ coating for densification of SiC ceramics and sintering kinetics

Introduction: Silicon carbide (SiC) has attractive physical and chemical properties [1]. Hence, dense SiC ceramics are widely used industrially [2]. The materials are normally produced by sintering techniques requiring extreme conditions such as hot pressing [3], hot-isostatic pressing [4], and spark plasma sintering [5]. The rigorous sintering conditions raise the process complexity and materials cost. Lowering the sintering temperature for densification while not compromising the materials properties is highly desirable albeit challenging. Al₂O₃ is a sintering additive for atmospheric pressure sintering of SiC. Al₂O₃ and SiC are generally mixed by traditional milling techniques but the differences in size, shape, and density results in poor dispersion of Al₂O₃ in SiC. Moreover, micro-segregation of the second phase when Al₂O₃ agglomerates undermines the performance of SiC products. Hence, it is preferable to mix Al₂O₃ with SiC on the nanoscale. 
Materials and Methods: Briefly, 25 g of pristine superfine SiC powders were mixed with a certain amount of aluminum isopropoxide [Al(OCH(CH₃)₂)₃] in 500 mL of anhydrous ethanol at room temperature under stirring. After full impregnation, the samples were dried using a rotary evaporator. The product was aged in wet air for 12 h and subsequently calcined at 450 °C for 4 h. A sintering temperature of 1950 °C, heating rate of 10 °C/min, soaking time of 60 min, and cooling rate of 3 °C/min were selected for ceramic preparation. The sintering kinetics was investigated under isothermal conditions. When the sample reached the desired temperature (within 10 s), an optical lens system started to record the linear dimension change of the sample. 
Results and Discussion: The uniform Al₂O₃ coating promotes densification of SiC via enhanced liquid mass transfer while inhibiting grain growth and recombination during sintering (Figure 1). The effects of the Al₂O₃ content on the ceramic structure and mechanical properties are investigated. Coating a single layer of Al₂O₃ nano-particles on SiC particles produce the best interfacial activity, sintering density, as well as mechanical properties. The SiC sample with 7.5wt% Al₂O₃ has the best properties such as a relative density of 97.98%, flexural strength of 434.40 MPa, fracture toughness of 5.23 MPa·m^1/2, and Vickers hardness of 28.21 GPa. The sintering kinetics follows the Singh model with an apparent activation energy of 574.64 kJ/mol and the sintering rate is determined by both the interface reaction and diffusion (Figure 2). 
Conclusions: High-quality SiC ceramic samples are prepared by atmospheric pressure sintering with the aid of Al₂O₃ doping. The raw SiC granules are coated evenly with a layer of Al₂O₃ nanoparticles by a facile impregnation method. Coating a single layer of Al₂O₃ nano-particles on SiC particles yield the best interfacial activity, sintering density, and mechanical properties. The results provide theoretical and experimental guidance for future materials design and pave the way for commercial batch production.