Fabrication And Structure Control Of SiC Ceramic Foams And Related Reinforced Al Matrix Composistes

SiC ceramic foams are one of the most suitable candidates for the filtration of molten metals due to their excellent properties such as high temperature resistance, corrosion resistance, low thermal conductivity, low thermal expansion coefficient and the unique structures of extremely interconnected cell. SiC ceramic foam reinforced Al matrix composites have wide application prospects in aerospace, electronics, automobile and advanced weapon system based on the high relative strength, relative modulus, good thermal and electric conductivity and size stability. According to the problems that the strength of SiC ceramic foams is relatively low, the cell structure is difficult to control and the related composite interface is not well bonded, in this paper SiC ceramic foams and related reinforced Al matrix composites are fabricated, and the ways of structure controlling and the methods of strengthening are mainly focused on.First, by polymer templates replica method and pressureless sintering technique, SiC ceramic foams with light weight, high strength and controllable structure are fabricated. By using sintering aids of Al2O3, Y2O3 or MgO, Al2O3 and SiO2, SiC slurry and ceramic foams are prepared. The cell structure and microstructure is adjusted, and the relationship between compressive strength and the structure is established. The results suggest that the viscosity of SiC slurry appears shear thinning behavior which is good for the polymeric sponge immersing process. The SiC ceramic foam preforms with plump cell strut are prepared using non-water based SiC slurry of 50% solid content, after the sponge is burnt out at 600℃. The cell is uniformly distributed and well interconnected. The porosity is 77～97%, the cell size is 1.08-2.20mm which decrease with the increasing PPI values of and the coating time. The cell strut diameter is 260-690μm, rising with the decreasing PPI values and the increasing coating time. The average linear contraction of SiC ceramic foams is 15% before and after sintering at 1500℃. According to the microstructure, the SiC ceramic foams with no sintering aids contain a lot of unavoidable pores. Those with Al2O3 and Y2O3 as sintering aids achieve fully dense strut, the liquid phase of which is Y3Al5O12 after sintering at 1700℃. And the SiC particle of those with MgO, Al2O3 and SiO2 as sintering aids is tied by glass phase, which are Mg2Al4Si5O18 and MgSiO3 after sintering at 1300℃and then decompose during high temperatures. The compressive strength of SiC ceramic foams increases with the increasing relative density. With the same relative density, the bigger the PPI values (or the smaller the cell sizes) of polymer templates are chosen, the higher the compressive strength presents. The scatters of the compressive strength distribute in the much higher numerical range with the same relative density of 0.2, which is contributed to the corresponding increase in microstructure densification degree.And then, the preparation and structure formation mechanisms of SiC ceramic foam reinforced Al matrix composites are investigated. The SiC ceramic foams are pressurelessly infiltrated by molten Al under high temperature. The phase compositions, microstructure and micro hardness are characterized. The results suggest that the main phases are Al and SiC, while the rest Si and MgAl2O4 phases are formed by infiltration reaction. The macroscopic contour of SiC reinforcement in the composites is clear and the interface of Al and SiC is well bonded. The infiltration of Al into the hollow strut of SiC ceramic results in the mutual restriction, which is beneficial to the entire strength of the composites. The hardness of Al is 700MPa, that of the SiC side near the interface is lOGPa, and that inside the SiC strut slightly increases to 11GPa. The bringing of Mg into the molten Al with high temperature improves the wettability of Al and SiC. The liquid Si phase formed by reaction enters into the molten Al and separates out when cooling down, which will further boost the mechanical strength of Al matrix composites.