Fabrication And Properties Of ?-Si₃N₄ Whickers Reinforced Al Matrix Composites

As the representative of light structural metal materials,Al and its alloys are widely used in various fields.However,the disadvantages of low strength,poor wear resistance and poor high temperature stability make it difficult for Al matrix materials to meet the requirements of high-speed,high-load and high-reliability of equipment.Nowadays,Al matrix composites?AMCs?,which have higher specific strength,specific modulus,wear resistance and thermal deformation resistance,can overcome the disadvantages of poor comprehensive performances of traditional metal materials by introducing high performance reinforcements into Al matrix.?-Si₃N₄ whiskers??-Si₃N₄w?have excellent mechanical properties,thermal properties and wear resistance.Compared with the ceramics particles,the special shape of whisker makes them more difficult to peel off from the matrix,so the reinforcing effect is better.Compared with the long fibers,the preparation process of whiskers reinforced AMCs is much simpler and the cost is effectively reduced.Therefore,?-Si₃N₄w is an ideal reinforcing phase for composites.In this paper,the preparation of?-Si₃N₄ reinforced AMCs by hot-pressing sintering process was carried out,and the effects of sintering parameters,whisker content,whisker surface modification,thermal processing and other factors on the properties of the composites were systematically studied.?-Si₃N₄w could significantly enhance the mechanical properties of Al matrix,but it reduced the density of the composites.The increase in sintering temperature was helpful to improve the densification of the material,so as to further enhance the whisker's strengthening effect.The Vickers hardness,ultimate tensile strength?UTS?,bending strength and ultimate compressive strength?UCS?of the composites with 20vol.%whisker content can reach up to 94.6,312 MPa,443 MPa and 339 MPa,respectively.Compared with the pure Al samples prepared under the same conditions,the properties of the composites were improved by 85%,102%,119%and 117%,respectively.The strengthening mechanisms of composites mainly included grain refinement,dislocation strengthening and stress transfer of whisker.In addition,excessive whiskers content would lead to a rapid decline in the mechanical properties of composites.The presence of the amorphous SiO₂ layer on the?-Si₃N₄w's surface after pre-oxidation could effectively improve the wettability and interfacial bonding strength between the whisker and the pure Al matrix,as well as promote the density of the material,thus enhancing the reinforcement effect of?-Si₃N₄w.When the whisker pre-oxidation time was 1 hour,the Vickers hardness and UTS of the composites could be improved by a maximum of 9.7 and 24 MPa,respectively.Prolonged oxidation time could reduce the mechanical properties of the whisker,which would easily lead to whisker fracture during the preparation and processing.As a result,the enhancement effect of whiskers was reduced.Ag-coated?-Si₃N₄w could not only effectively improve the wettability between the whisker and the pure Al matrix,but also promote the densification of composites.The Ag coating on the surface of?-Si₃N₄w could optimize the interphase structure and effectively lower the dislocation density and thermal resistance near the interface,thus improving the ductility,toughness and thermal conductivity of composites.The maximum values of the elongation at fracture and the fracture toughness of Ag-coated?-Si₃N₄w reinforced AMCs were 15.5%and 23.9MPa·m^1/2,respectively,which were 23%and 18.9%higher than those of the initial?-Si₃N₄w reinforced AMCs.Moreover,the maximum room temperature thermal conductivity of composites reached up to 198.6 W/m·K,which was close to the theoretical value and 21%higher than that of the initial?-Si₃N₄w reinforced AMCs?Hot extrusion could not only realize the orientation of whiskers in the composites,but also make the Al matrix show obvious<111>_Al texture,thereby leading to the anisotropy of the mechanical properties of composites.There was a semi-coherent interface in the hot-extruded composites with an orientation relationship of?12?11????111?_Al.Hot extrusion significantly improved the mechanical properties of the composites.When the whisker content was 15 vol.%,the UTS,yield strength?YS?and elongation at fracture of composites reached up to 289 MPa,170 MPa and 12.3%,respectively.Compared with the as-fabricated composites,the strength and ductility were improved by 33%,36%and 101%,respectively.Besides,when the whisker content was 5 vol.%,the measured YS of composites was almost consistent with the theoretical value.The addition of?-Si₃N₄w could obviously improve the wear resistance of composites.Due to the effect of work hardening,the higher the load was,the stronger the reinforcing effect of?-Si₃N₄w was.With the increase of?-Si₃N₄w content,the wear mechanism of composites changed from severe delamination wear and abrasive wear to slight abrasive wear.?-Si₃N₄w could play a certain self-lubricating role,and a more stable mechanically mixed layer?MML?was formed on the surface with the increase of the?-Si₃N₄w content,thus reducing the friction coefficient of the composite.In addition,the higher rotating speed would promote the formation of the dense oxide layers on the worn surface which were also helpful to reduce the friction coefficient of composites.