【作者】 洪雨；

【导师】 吴玉程；

【作者基本信息】 合肥工业大学 ， 材料学， 2009， 硕士

【摘要】 高增强体含量SiC/Al复合材料具有高的力学性能和好的热学性能,作为一种新材料具有很大的发展潜力,而探求低成本的制造工艺是该材料当前研究的热点。与诸多制造工艺相比,SiC预成形坯无压渗透工艺具有近净成形能力强、设备投入少等优点,因而成为一种极具经济竞争力的制备技术。本文首先研究了SiC粉体氧化条件下的液态铝合金无压渗透现象及其机理。而为了获得高增强体含量的SiC/Al复合材料,作者采用双尺寸颗粒的SiC粉体进行配比后低温自氧化烧结,并首次采用Fe（NO₃）₃·9H₂O作为造孔剂制备SiC预成形坯,在氮气气氛中将自制液态铝合金无压渗入SiC预成形坯内,成功制备出不同增强体粒度及含量的SiC/Al复合材料。应用金相显微镜（OM）、扫描电子显微镜（SEM）、X射线衍射（XRD）、能谱分析（EDS）等测试手段对复合材料进行成分、结构与形貌分析,并研究SiC粒度配比和造孔剂的添加量对复合材料力学和热学性能的影响。在1000℃低温烧结时,SiC自身发生氧化反应,Fe（NO₃）₃因受热分解而去除;SiC粉体由其自身缓慢氧化产生的SiO₂膜、水玻璃的硬化作用形成硅酸凝胶以及Fe（NO₃）₃·9H₂O的分解形成的Fe₂O₃的共同作用而聚合在一起,形成SiC陶瓷骨架。SiC粉体间的本征孔隙和Fe（NO₃）₃去除后留下的孔隙一起构成一个三维相互连通的、开放的孔隙网络;Fe（NO₃）₃去除在SiC预成型坯中留下的孔隙分布均匀,没有偏聚现象。SiC预成型坯在烧结过程中产生2².5%左右的线膨胀,膨胀量随Fe（NO₃）₃含量的增加而增大,通过调整Fe（NO₃）₃含量,能获得不同孔隙率的SiC预成形坯,对于85μm+28μm和46μm+14μm粒度的SiC配比的双尺寸SiC预成型坯,均能获得35⁴6%的孔隙率;预成型坯的抗压强度随着造孔剂含量的增加而增大,当硝酸铁含量在10%时,其抗压强度高达180MPa。在助渗剂镁的作用下,液态铝合金与SiC表面的SiO₂膜生成MgO和MgAl₂O₄,且液态铝合金中的基体Al与Fe（NO₃）₃热分解后形成的Fe₂O₃发生铝热反应,放出反应热,使熔渗前沿温度升高,有效改善了SiC与Al液之间的润湿性,诱发Al液自发向SiC粉体间隙中渗透。在900℃的氮气气氛中,液态铝合金能无压渗入SiC预成形坯内颗粒间的孔隙,获得接近全致密的SiC/Al复合材料。液态铝无压渗入后,SiC预成形坯无任何形状和尺寸的变化,易于实现近净成形。SiC/Al复合材料的断裂方式以脆性断裂为主。其裂纹走向主要沿着SiC-Al分界面、SiC与SiC间较脆弱的烧结颈扩展。延性的金属在裂纹扩展过程中产生少量的塑性变形后被撕裂。铝基体中引入高体积分数的SiC增强体后强度显著提高,抗弯强度均在300MPa以上,最高达381MPa。其抗弯强度并不随着造孔剂Fe（NO₃）₃添加量的增加单调增加,而表现出先增后减的趋势。且粗颗粒配比预成型坯渗透得到的复合材料强度略高于细颗粒配比。但总体上,对高增强体含量SiC/Al复合材料,其强度随增强体含量改变而波动的幅度不大。当SiC预成型坯中的孔隙率从35%增加到46%时,85μm+28μm双尺寸SiC颗粒增强Al-Mg-Si复合材料的导热系数从116W·m^-1·K^-1增至131W·m^-1·K^-1,而46μm+14μm双尺寸SiC颗粒增强Al-Mg-Si复合材料的导热系数从114W·m^-1·K^-1增至132 W·m^-1·K^-1。总体来说在相同体积含量下粗颗粒SiC增强铝基复合材料的导热率比细颗粒略高。更多还原

【Abstract】 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.更多还原