片層石墨尺寸對鋁基復合材料的力學性能和熱導率的影響

片層石墨尺寸對鋁基復合材料的力學性能和熱導率的影響Effect of graphite flakes size on the strength and thermal conductivity of graphite flakes/Al composites

采用粉末冶金法制備了名義尺寸150、300、500 μm的片層石墨增強鋁基（50vol.%Gf/Al）復合材料，得到密度均接近理論密度的致密復合材料坯錠。片層石墨與鋁合金基體結合緊密，界面處無裂紋、孔洞等缺陷。片層石墨的xy方向基本與坯錠的xy方向平行，但受粉末冶金工藝的影響，較小片層石墨的xy方向與坯錠的xy方向略有偏差，隨著片層石墨的尺寸增大，偏差逐漸減少。復合材料的強度隨著片層石墨尺寸增加逐漸降低。150 μm片層石墨復合材料的彎曲強度為82 MPa，當片層石墨尺寸增至500 μm時，強度降低至39 MPa。片層石墨強度較低，裂紋容易沿片層石墨的層間擴展，隨著片層石墨尺寸增大，這一現象更加明顯，容易在斷口中觀察到片層石墨剝離的現象。復合材料xy方向的熱導率隨片層石墨尺寸增大而增加，最高可達604 W/m ? K，與尺寸較小的片層石墨相比提高63%。300、500 μm片層石墨復合材料的界面換熱系數略低于理論值，但150 μm片層石墨復合材料的界面換熱系數明顯小于理論值。除了片層石墨的尺寸，其形狀、分布和內部缺陷等對復合材料的熱導率也有一定的影響。

Graphite flakes reinforced Al matrix composites (Gf/Al) with low density, good machining property and high thermal conductivity are considered an excellent heat sink materials used in electronic industry. When the composites are manufactured by liquid method such as liquid infiltration, it is easy to achieve a high thermal conductivity composite. However, the Al4C3 phase would be formed in the composite, which will decrease the corrosion properties of the composites. The powder metallurgy technique could avoid the formation of the Al4C3 phase. In this study, three seized graphite flakes (150, 300, 500 μm) were used to investigate the effect of the graphite flake size on the strength and thermal conductivity of Gf/Al alloy composites. The 50% Gf/Al alloy (volume fraction) composites were fabricated by the powder metallurgy technique. The density of all the three Gf/Al alloy composites were similar to the theoretical density. The graphite flakes had a well bonding with Al matrix without cracks and pores. The xy plane of the graphite flakes were almost parallel to the xy plane of the composites. However, for the small graphite flakes, their xy plane was not well parallel to the xy plane of the composite due to the powder metallurgy process. For the large graphite flakes, they exhibited a good orientation in the xy plane. The strength of the Gf/Al alloy composites decreased with the increase of the graphite flake size. For the 150 μm graphite flake, the bending strength of the Gf/Al alloy composite was 82 MPa. However, for the 500 μm graphite flake, the bending strength of the composite decreased to 39 MPa. Due to the low strength between the layers of the graphite flake, the cracks were prone to expand in the graphite flake. As the size of the graphite flake increased, this phenomenon became more obviously. It is easy to observe that the graphite flakes peeled off on the fracture surfaces. When the size of the graphite flake increased from 150 μm to 500 μm, the thermal conductivity increased by 63%. The highest thermal conductivity was 604 W/m?K. The interfacial thermal conductance (hc) of the composites were calculated by the Maxwell-Garnett type effective medium approximation model. The hc of 300 and 500 μm graphite flake Gf/Al alloy composites were similar to the theoretical value (calculated by the acoustic mismatch model). However, the hc of the 150 μm graphite flake Gf/Al alloy composite was lower than that of the theoretical value. Besides the size of the graphite flakes, the shape, distribution and defect of the graphite flakes also influenced the thermal conductivity of the composites.

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