T. Dougherty, Y. Xu
Keywords: aluminum, powder metal, silicon nitride, fiber, microstructure
Summary:There is an increasing demand for new light-weighting materials to improve fuel-efficiency in the automobile and aerospace industries. This has led to an increase in the use of aluminum over traditional metals, such as steel, to increase both efficiency and range. Powder metallurgy is a key enabling technology for the mass production of parts for these industries. Aluminum is known for having excellent specific strength, stiffness and thermal properties, but also is cost-effective for mass production. It suffers, however, from poor wear resistance which lowers its usefulness in applications involving critical moving parts. The inclusion of ceramic reinforcements to produce metal matrix composites (MMCs) can improve both the specific mechanical properties and improve the tribological properties of aluminium. Silicon nitride is a ceramic material with high strength, stiffness and low coefficient of thermal expansion. It also has high toughness for a ceramic material which combines to give it extremely high thermal shock resistance. Fibrous/1D materials provide enhanced material properties in composites (e.g. carbon/glass/polyaramid fibers, carbon nanotubes). Fibrous materials can also enhance the properties of MMCs through dispersion-strengthening and grain boundary-strengthening. The silicon nitride fibers used in this study are sub-micron-diameter, single-crystal fibers with a length 5-24 µm. This is smaller than metal powders allowing the ceramic to coat the metal powder prior to consolidation rather than forming agglomerations that lead to pores and poor fatigue properties. In this study, we have prepared a composite powder using from 2XXX aluminium pre-mix powder and low vol% silicon nitride fibre. This powder was uniaxially pressed and sintered under nitrogen atmosphere. Hardness, tensile properties and elastic modulus data is reported. The mechanical properties are significantly improved even at low loadings. The microstructural analysis clearly shows silicon nitride present in the sintered grain boundaries using EDS to characterise the reinforcement.