Research On Preparation By Pressureless Infiltration Process And Properties Of SiC/Al Composites With High Filler Content

SiC/Al composites reinforced with high volume fraction of low cost SiC powder have excellent properties of high specific strength, specific elastic modulus, improved wear resistance, low thermal expansion and high thermal conductivity. Those properties can be tailored at all, which makes the composites have tremendous potential for development in both structure components and functional ones. 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 routine 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. Based on the study of phenomenon and mechanism on the spontaneous infiltration of liquid aluminum alloy into the intervals among oxidized SiC particles which were sealed in the quartz tube filled with nitrogen gas, SiC/Al composites with different SiC particle sizes and different SiC volume fractions were successfully achieved by spontaneous infiltrating routine. SiC preforms were prepared by self-oxidation bonding at low temperature. The composition, micro-structure and micro-morphology of SiC/Al composites were analyzed by optical microscope (OM), scan electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), 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.With the aid of magnesium, Liquid aluminum alloy reacted with SiO₂ film on the surface of SiC powder 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 spontaneous infiltration of liquid aluminum into the intervals among SiC powder. There was a gestation period before the infiltration started when using sheet, magnesium as infiltrating agent and the penetration process was controlled by the reaction of liquid aluminum with SiC, which led to a slow speed of infiltration. The penetration distance increased parabolically with time. As sheet magnesium was substituted by powder one, there was nearly no gestation period and the infiltration process followed well in a parabola relationship with time, and a faster penetration speed was observed than that in using sheet magnesium case.SiC powder and graphite powder were mixed and oxidized synchronously at 1100℃. A layer of SiO₂ was formed on the surface of SiC, which bonded SiC powders together to form a ceramic skeleton and the sites occupied by graphite powders turned into pores when graphite powders burned out. Three dimensional co-continuous net-work of pores was formed through the combination of original intervals among SiC powders and pores formed by burned out graphite particles. The oxidation of SiC-graphite mixture led to a 3～5% linear expansion to form a SiC perform, and this expansion increased slowly with the increase of graphite content. SiC preforms with different volume of porosity were obtained by modulating graphite content. SiC preforms with 38～61% porosity were obtained using W28 SiC powder and SiC preforms with 47～63% porosity were obtained using W7 SiC powder.Compact SiC/Al composites were fabricated by Spontaneous 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. A oxide interface phase with thickness of 0.3～0.5μm was formed between SiC and aluminum alloy. The oxide layer could prevent effectively the contact and reaction between SiC particles and liquid aluminum alloy. Crystallographic orientation relationships exist between the interface phase and the SiC filler or interface phase and the metal matrix, both 4F type and 8F type SiC formed semi-coherent interfaces with MgAl₂O₄. Tensile stress derived from coherent mismatch induced dislocations and stacking faults in the MgAl₂O₄ crystal lattice near the interface. MgAl₂O₄ and aluminum alloy formed a coherent interface with good lattice match, resulting in no such crystal lattice faults along the interface.Brittle rupture was mostly observed in SiC/Al composites. Cracks spreader 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 did not increase monotonously with the increasing SiC content; overmuch SiC volume fraction would depress the strength of the composites. The strength of composites based on ZL101 was higher than that based on industrial pure aluminum. The elastic modulus of SiC/Al composites increased monotonously with the increasing SiC content and the elastic modulus of composites based on ZL101 was higher than that based on industrial pure aluminum.When temperature raised from room temperature to 200℃, the structure of SiC/Al composites was very stable and its dimension increased smoothly. The linear thermal expansion coefficient (CTE) of Aluminum alloy was depressed effectively as high volume SiC particles were introduced in. The CTE of the composites increased with increasing temperature and decreased with the increasing SiC volume content. Metal matrix with a lower CTE for led to a lower CTE of the composites. As SiC volume fraction changed from 39% to 62%, the thermal conductivity (TC) of SiC(W28)/ZL101 composites varied from 146 to 118 W·m^-1·K^-1. As for W7 SiC, the TC varied from 136 to 118 W·m^-1·K^-1 when the filler content changed from 37% to 53%. In the case of same filler volume content, the composites with finer particles usually led to a lower TC.