Effect of AlF₃·3H₂O Content on Properties of Silicon Carbide Whiskers Self-Reinforced Porous Ceramics

Silicon carbide porous ceramics are widely used in many fields because of their corrosion resistance, good thermal shock resistance, large specific surface area and superior mechanical properties. Silicon carbide porous ceramics are mainly prepared by different methods (i.e., pore-forming agent method, organic foam impregnation method, freeze–drying method, and foaming method). Among these, the foaming method with simple operation and controllable density and shape of the products is a common approach to prepare silicon carbide porous ceramics with a high porosity. However, silicon carbide porous ceramics still have the disadvantage of low mechanical properties. The In-situ generation of SiC whiskers can greatly improve the mechanical properties of the materials. Therefore, in this paper, silicon carbide whiskers self-reinforced porous ceramics were prepared by foaming method and reactive sintering with silicon carbide powder of 70% (in mass fraction) as a raw material, 30% silicon powder and carbon black as a self-bonding phase, AlF₃·3H₂O as an additive. In addition, the effect of AlF₃·3H₂O content on the phase composition, microstructure and mechanical properties of silicon carbide whiskers self-reinforced porous ceramics was also investigated.

In this work, silicon carbide powders were used as raw materials. Silicon powders and carbon black were used as self-bonding phase materials. AlF₃·3H₂O was selected as an additive. Triethanolamine dodecyl sulfate was used as a blowing agent. The ground powders were slowly added to a solution containing 0.5% Isobam 104 and 0.15% sodium carboxymethylcellulose, which was vigorously stirred in a mechanical stirrer to produce a slurry containing 50% (in volume fraction) of ground powders. Subsequently, 1.0% (in volume fraction) triethanolamine dodecyl sulphate was added to the obtained slurry and then vigorously stirred to prepare a homogeneous wet foam, which was rapidly poured into moulds with the dimensions of 160 mm × 80 mm × 40 mm and dried at ambient temperature for 48 h. Afterwards, the demoulded foam green bodies were dried at 40–100 ℃. Finally, the dried foam green bodies were fired in argon atmosphere furnace at 1600 ℃ for 2 h to obtain porous ceramics.

, The average large spherical pore size and annular pore size in as-prepared silicon carbide whiskers self-reinforced porous ceramics both firstly decrease and then increase with increasing the content of AlF₃·3H₂O from 0 to 4%. These results are due to the minimum surface free energy of the slurry with 2% AlF₃·3H₂O, leading to the stable bubbles existing in the slurry. Adding an appropriate amount of AlF₃·3H₂O can promote the formation of silicon carbide whiskers and improve the strength of as-prepared silicon carbide whiskers self-reinforced porous ceramics. At AlF₃·3H₂O content of 2%, the flexural and compressive strengths of porous ceramic samples with a porosity of 76.9% are 2.6 MPa and 3.7 MPa, respectively, which are significantly higher than those of the porous ceramic samples prepared without AlF₃·3H₂O (i.e., 1.1 MPa and 1.4 MPa). The enhanced mechanical properties can be related to the microstructure of the porous samples. The more uniform pore distribution and smallest average spherical pores and annular pores in porous sample with AlF₃·3H₂O addition of 2% are formed, resulting in improved mechanical properties of porous sample. Also, a large number of silicon carbide whiskers with the largest length are generated, which act as a reinforcing phase for further improving the mechanical properties of porous samples.

Adding an appropriate amount of AlF₃·3H₂O could promote the formation of silicon carbide whiskers and improve the strength of porous ceramic specimens. At AlF₃·3H₂O content of 2%, the flexural and compressive strengths of as-prepared silicon carbide whisker self-reinforced porous ceramic samples with a porosity of 76.9% were 2.6 MPa and 3.7 MPa, respectively. The length and the diameter of SiC_w whiskers generated in the samples were 4 μm and 62 nm, respectively, and their growth was controlled due to the gas-solid reaction mechanism.