| In the present paper, material design on the background of inertial instrument application is carried out with the aim of acquiring good machinability,high special elastic modulus, high damping properties and good dimension stability, which is on the basis of SiC/Gr/Al composites. For this purpose, flaky graphite (volume fraction of 3%~7%, and particle size of 1μm~70μm) were additioned. Effects of graphite particles on microstructure, mechanical properties, machinability and dimension stability were studied by using scanning electron microscope (SEM), transmission electron microscope(TEM), optical microscope , friction-wear tester, atomic forces microscope (AFM), cutting force tester, thermal conductivity tester, tensile testing machine, Brinell hardness, X-ray diffraction and internal friction apparatus. And their influencing factors were also discussed.Graphite with solid lubrication is selected according to chemical stability and lubrication, and wear-resistence is explained by solid lubrication theory. The appropriate upper limit of the addition of graphite particles should be 7.2% on the based of analysis H-S model and the mixing rule. Effect of graphite particles on thermal expansion was analysised according to theory model, and influencing factors to dimension stability was analysised. Those factors was summarized, the design principle of SiC/Gr/Al was established.TEM observations indicate SiC-Al and Gr-Al interfaces are clean and free from interfacial reaction products, which are attributed to fabrication process. Meanwhile, there is no interface reactivity in Gr-Al interfaces, which is due to high degree of graphization. The bonding strength between graphite and matrix alloy is weak, and the lamellar is sliding easily. Micro-stress is relaxed in the process of fabrication and heat treatment, The addition of graphite particles decreases the dislocation density in the composite. Moreover, the dislocation in the casting microstructure generates the sub-boundary, and the density of precipitate phase dependent on the dislocation nucleation also reducesThe coefficient of thermal expansion (CTE) and daming properties are tested, and the results show that CTE is between 11.5~13×10-6/℃and Q-1 for SiC/Gr/Al composite is 2~6 times higher than that of the aluminum matrix. These meet the requirements of thermal expansion matching the corresponding materials and high damping in precision instrument.The increment of graphite volume fraction, the decrement of particle spacing, destroyed continuity of matrix, result in the reducing of tensile strength of SiC/Gr/Al composites。The increment of interfacial area and reduction of partilcle spacing result in the increasing amount of crack source and destroy the continuity of aluminum matrix, which is the main reason for decrease of mechanical properties.Tribological conditions of tool/ composite in the process of cutting are analsized by wear test. Results show that the tribo-layer formed during wear cause the decreasing of friction coefficient and wear loss. Further analysis to the mechanism of the increase in wear resistance, the worn surface of SiC/Gr/Al composite covered by a continuous or thin tribo-layer can also improve wear resistance.Graphite particles improve that machinability from tool wear, cutting force and surface roughness. It is due to the decreasing of hardness and strength of composites, which has decreased the coefficient of chip distortion and promoting crack propagation of chip, and solid lubrication to tool. With increasing Vf of graphite particles, tool wear increase. Particle size of graphite has little effect on the tool wear, but has very remarble effect on cutting force. The characteristic of thrust force (Ft) higher than cutting force (Fc) is not change by the addition of graphite particles. The SiC+Al matrix failed in ductile manner while graphite particle fail in a brittle manner. The elastic modulus was estimated by H-S models and the estimated value is 93.5% of the tested values.The investigation of dimension stability show that: the addition of graphite is beneficial to improving dimensional stability Sufficient aging precipitation, stable dislocation structure and lower macro residual stress ensure dimension stability by appropriate thermal cycling procedure (A+TCC(0)and A+TCC(-70)) when compared with aging procedure. Besides thermal cycling procedure, relaxation of micro stress by graphite particles further improved dimension stability of SiC/Gr/Al. From analysis both micro-deformation resistance and dimension stability under no load, A+TCC (0) is the optimum procedure for improving the dimension stability.In the paper, we design SiC/Gr/Al composites according to the requirement of instrument materials, and find proper heat treatment process. For the 40%SiC/5%Gr(6μm)/Al composites, the tool life maching the composite was prolonged by 40%, the elastic modulus are maintained 150GPa, variable dimentional stability is lower than 0.61×10-6, which meet the requirements of instrument properties. |
