Research On Preparation And Properties By Pressureless Infiltration Process Of SiC/Al Composites With Double Size Distribution

SiC/Al composites reinforced with high volume fraction of SiC powders have excellent mechanical and thermal properties. Nowadays, seeking for low cost manufacture processes for SiC/Al composites have attracted considerable attention from material researchers. In comparison with other preparation methods, pressureless infiltration process is the most economical one because it is a near-net-shape process with relatively less requirement for equipments. In this paper, a concise pressureless infiltration process for the making of SiC/Al composites was investigated. And in order to obtain SiC/Al composites reinforced with high volume fraction, double size distribution SiC powers with different size ratios were used to prepare the SiC preforms by self-oxidation bonding at low temperature, and Ferric Nitrate crystal was used for the first time in prepare the SiC preforms as pore-forming agents. Liquid aluminum alloy spontaneous infiltration into the SiC preforms which were putted in the crucible furnace filled with nitrogen gas. SiC/Al composites with different SiC particle sizes and different SiC volume fractions were successfully achieved by pressureless infiltrating process. The composition, micro-structure and micro-morphology of SiC/Al composites were analyzed by optical microscope (OM), scan electron microscope (SEM), energy dispersion spectrometer (EDS) and X-Ray diffraction (XRD). The effects of SiC particle size and SiC volume content on the mechanical and thermal properties of the composites were also examined.SiC powders were heated and oxidized synchronously,and Ferric Nitrate crystals were decompounded at 1000℃. A layer of SiO₂ was formed on the surface of SiC, the hardened of Sodium Silicate between SiC powders, and Fe₂O₃ was formed by the thermal decomposition of Ferric Nitrate, which bonded SiC powders together to form a ceramic skeleton, the sites occupied by Ferric Nitrate turned into pores and Fe₂O₃ when Ferric Nitrate decompounded in high temperature. Three dimensional co-continuous net-works of pores distributed uniform in the preform which was formed through the combination of original intervals among SiC powders and pores formed by Ferric Nitrate decompounded. The oxidation of SiC powders and the decomposition of Ferric Nitrate led to a 2².5% linear expansion to form a SiC perform, and this expansion increased slowly with the increase of Ferric Nitrate content. SiC preforms with different volume of porosity were obtained by modulating Ferric Nitrate content. SiC preforms with 35⁴6% porosity were obtained using 85μm +28μm SiC powders and 46μm +14μm SiC powders. The compressive strength of SiC preform increased with the increase of Ferric Nitrate content, and the compressive strength reached 180 MPa when the content of Ferric Nitrate was 10% in weight.With the aid of magnesium, Liquid aluminum alloy reacted with SiO₂ film on the surface of SiC powders to form MgO and MgAl₂O₄, which gave out heat and raised the temperature on the infiltration front to promote the wettability for SiC-Al system, and the pressureless infiltration of liquid aluminum into the intervals among SiC powders. Compact SiC/Al composites were fabricated by pressureless infiltration of liquid aluminum alloy into porous SiC preforms at 900℃in nitrogen gas. Preforms had no change in shape and dimension after infiltration, thus near-net-shape composites were easily achieved.Brittle rupture was mostly observed in SiC/Al composites. Cracks spreaded along the interface between SiC and Al matrix, traversed across the sintering necks among SiC particles. When Cracks spreaded through the sample, tough metal matrix was teared away with some observations of plastic deformation. The strength of Aluminum alloy was improved remarkably as high volume SiC particles added in. The flexure strength of the composites were all over 300MPa, the highest one reached 381MPa. The flexure strength did not increased monotonously with the increasing Ferric Nitrate content but first increased and then reduced slowly, and the flexure strength of SiC/Al composites based on coarse powders was slightly higher than that based on fine powders. In fact, the flexure strength of SiC/Al composites has little change with different Ferric Nitrate content.As the porosity of SiC preform changed from 35% to 46%, the thermal conductivity (TC) of SiC(85μm+28μm)/Al-Mg-Si composites varied from 116 to 131 W·m^-1·K^-1, and the TC of SiC(46μm+14μm)/Al-Mg-Si composites varied from 114 to 132 W·m^-1·K^-1. In the case of same filler volume content, the TC of SiC/Al composites based on coarse powders was slightly higher than that based on fine powders.