Low-temperature Sintering, Microstructures And Properties Of Silicon Carbide Ceramics

SiC ceramics have been widely applied in many industrial areas such as mechanical seals and wear-and corrosion-resistant parts, due to excellent properties e.g. high hardness and fracture strength, good abrasive and corrosion resistance as well as relatively low coefficient of thermal expansion. A low-temperature sintering technology of SiC ceramics without a protective atmosphere was developed in this work. In terms of optimization and design of main key technical parameters such as SiC particle size and gradation, binder content, forming pressure, type and content of sintering additive, sintering temperature and holding time, high-performance SiC ceramics were fabricated at a relatively temperature. And, microstructures and properties of the SiC ceramics were investigated.BaO-Al2O3-Y2O3 ternary system is the most effective liquid sintering additive, compared with the CaO-Al2O3-Y2O3 and BaO-Al2O3-CaO systems. During sintering in air, partial oxidization of SiC powder takes place to form SiO2, which reacts with the sintering additive to produce a liquid-phase melt. The melt has a suitable viscosity and a high wettability with the SiC particles. So, mass transportation and densification of the SiC ceramics was accelerated via effects of the capillary force and the liquid-phase mass transformation. After sintering, the melt in the SiC ceramics mainly transforms into BAS and other secondary phases such as Y2Si2O7, CaAl2Si2Og and etc. It can effectively avoid brittle glass phase existed in the ceramics and hence improves microstructures and properties of the SiC ceramics. In the SiC ceramics, SiC particles combine with BAS tightly, it is mainly of cleavage fracture with an even fracture surface. However, the fracture surface of the secondary phases is rather rough and then crack propagation path is then extended. Fracture toughness of the SiC ceramics is therefore improved.On the conditions of the forming pressure of 400 MPa, the binder content of 5 wt% and the weight ratio of 3:1 for the 3.5 um and 0.5μm SiC powders, the SiC preforms have the highest green density (2.00 g/cm3). In terms of the unique powder bed sintering technique, the SiC ceramics sintered at 1550℃ for 30 min through adding 30 wt% BaO-Al203-Y2O3 sintering additives have the highest properties, i.e. density of 3.14 g/cm3, bending strength of 520 MPa and fracture toughness of 6.7 MPa·m1/2, respectively.