Preparation,Microstructure And Properties Of Porous SiC Ceramics

With some appealing properties such as low density,excellent mechanical strength,good oxidation resistance,low thermal expansion coefficient,low thermal conductivity,good wear resistance,good corrosion resistance,high thermal shock resistance and good chemical stability,porous SiC ceramics have attracted the long standing research interest due to their potential application as filters,catalytic supports,high temperature insulation materials,thermoelectric energy conversion,and reinforcement of composites.The purpose of this project is,therefore,to fabricate porous SiC ceramics via polymer sponge technique and novel foaming-freeze-gelcasting method.These two kinds of SiC porous ceramics are applied to molten metal filter and reinforcement of compositers,respectively.Relevant sintering behaviors,mechanical poroperties,porous structures forming mechanism and controlling were also investigated.The present study might open up new approaches to the development of porous SiC ceramics with improved mechanical properties,will promote these applications in various fields.Porous SiC ceramics were fabricated via polymer sponge technique.The influence of pH value,dispersing agents and solid loading on rheology behavior of slurry was investigated.The results show that the Zeta potential of SiC slurry decreases with incease of pH value,and reaches to minimum value at pH = 11.Zeta potential and viscosity of SiC sulrry reaches to minimum when the amount of A-15 dispersant is 0.5%.The viscosity of SiC slurry increases when the solid loading increases.The optimal solid loading of SiC slurry is 73.4%(mass percent)in our experiments.In order to obtain SiC reticulated porous ceramics(SiC RPCs)with optimized mechanical performance,the influence of sintering process,sintering aids,grain composition and recoating technique were investigated in details.It was found that,the mechanical properties of SiC RPCs could be improved via increasing sintering temperature appropriately.The optimum sintering temperature is 1300 ?,and the resultant maximum flexural and compressive strength of SiC RPCs(10 PP1)are 1.04 MPa and 1.12 MPa,respectively.The residual strength after cyclic quenching from 1100 ? to 20 ? for 10 times is 0.42 MPa.Sintering aids can fill into the interspaces between SiC and other ceramic particles and thus can promote the formation of liquid phase to bind ceramic particles.The optimal content of colloidal silica and silica fume are found to be 5%and 17%(mass percent)respectively and the resultant strut density,porosity,flexural strength and compressive strength are 1.73 g/cm3,31.36%,1.08 MPa and 1.18 MPa,respecti,vely.Via grain composition,small particle size SiC powders could fill into the interspaces between the big particles,resulting in improved density and mechanical properties of SiC RPCs.When grain composition is 6.4:2:1.6,strut density,porosity,flexural strength and compressive strength of SiC RPCs are 1.81 g/cm3,29.88%,1.15 MPa and 1.31 MPa,respectively.After recoating,the strength of struts increases with the increasing of recoating slurry viscosity.When the viscosity of recoating slurry is 1.2 Pa·s,the flexural strength and compressive strength of recoated SiC RPCs are 1.61 MPa and 2.10 MPa,respectively.The three-dimensional interconnected hierarchically macroporous SiC(HMS)materials with trimodal macropores were fabricated by a modified freeze-gelcasting method.The pore forming mechanism and porous structure controlling were investigated.The results show that the HMS with uniformly 3D interconnected macroporous structure is consisted of three modes macopores on different scales.The 3D interconnected 1st modal macropores with size range of 0.6 to 1.5 mm distribute homogeneously in the matrix.The pore walls of the 1st modal macropores contain the 2nd modal unidirectional macropores with pore size of?10 microns,while the 3rd modal macropores with pore size of-2 microns exists between the SiC grains.The formation mechanism of the 1st,2nd and 3rd modal macropores is proposed to be due to the H2O2 foaming process,unidirectional freezing and presence of organic porogens(polyacrylamide(PAM)and polyvinyl alcohol(PVA)),respectively.By varying the foaming temperature and time,mass ratios of PVA solution:SiC slurry,concerntration of PVA solution,amounts of AES,the macroporous structures can also be manipulated.The porosity,flexural strength and compressive strength of HMS can be controlled with the range of 70-80%,2.16-3.04 MPa and 2.61-3.54 MPa,respectively.The oxidation behaviors and the corrosion behaviors of HMS were also investigated in the present study.The oxidation experiments showed that HMS exhibited excellent oxidation resistance.The oxidation behavior follows with a parabolicrate law.The oxidation mass gain of HMS after oxidation at 1200 ? for 40 h is varied between 0.6 mg cm-2 and 0.7 mg cm-2.The higher porosity,the lower oxidation activation energy.Moreover,the obtained HMS via modified freeze-gelcasting method also shows good corrosion resistance.The strength loss is less than 5%after immersion corrosion test.The HMS/Al composites were fabricated by gas pressure infiltration method.The microstructure and properties were investigated.The results show that the interface bonding between Al and HMS is strong.The properties of HMS/Al composites depend on the pore size of HMS skeleton.When the average pore size is 1.0 mm,the brinell hardness,compressive strength,wear rate and friction coefficient are 123.4 HBS,364.14 MPa,7.4×10-5 mm·m-1 and 0.53,respectively.The properties of composites all increase after heat treatment.This HMS shows good application value and foreground as reinforcement of composites.