The field of nanocomposites has recently attracted considerable attention as researchers strive to enhance composite properties and extend their utility by using nanoscale reinforcements instead of the more conventional particulate-filled composites.[13] While smaller reinforcements have a better reinforcing effect than larger ones, applying the ball-milling technique for composite fabrications must have the following merits:
since ball milling is processed at room temperature, the disadvantages of the liquid metallurgy method for producing undesirable materials can be avoided
moreover, the ball-milling process can produce homogeneous nanocomposite powders instead of the more conventional coarse particulate-filled composites
In 1998, El-Eskandarany[14] employed a high-energy ball mill to fabricate metal matrix, a composite of Al reinforced with SiC with distinct nanocrystalline characteristics. In his experiments, pure elemental powders of Al (99.99%, 10 mm) and SiC (P + a phases) (99.9%, 100 mm) were weighed to give the nominal composition of SiCxAl100-x (x = 2, 5, 7, and 10 vol %) and mixed in a glove box under a purified argon atmosphere, using sapphire mortar and pestle. The initial mixed powders were then charged and sealed in a stainless steel vial (SUS 316, 250 ml in volume) together with fifty stainless steel balls (SUS 316, 10 mm in diameter). The ball-to-powder weight ratio was maintained as 20:1. The ball-milling experiments were performed in a high-energy planetary ball mill at a rotation speed of
3.3 s-1. To avoid the formation of any undesired contaminated phase(s), no lubricant solutions that are mainly hydrocarbon compounds were added to the charge. The ball-milling experiments were stopped periodically (every 1.8 ks) and then resumed when the temperature of the vial decreased to about 300 K. The final product (86 ks of milling) of the mixed powders were then consolidated into compacts in vacuum at 823 K, which is far below the melting point of Al (934 K), with a pressure of 19.6–38.2 MPa for 0.3 ks, using a plasma activated sintering method (PAS). The as-milled and as-consolidated samples were characterized by means of x-ray diffraction (XRD) with CuKα radiation, scanning electron microscopy (SEM) using a 20 kV microscope, transmission electron microscopy (TEM) using a 200 kV microscope, and chemical analyses, using the induction coupled plasma emission and helium carrier fusion-thermal conductivity methods. The gas (oxygen, nitrogen, and hydrogen) and the iron, that was introduced to the samples from the milling tools, contamination contents in the end-product of composite SiCp/Al were determined to be less than 0.07 and 0.2 (at. %), respectively. The density of the consolidated samples was determined by Archimedes’ principle, using water immersion. The hardness of the compacted sample was determined using a Vickers indenter with a load of 10 kg. Moreover, the elastic properties of the bulk samples were investigated by a pulse-echo overlap ultrasonic technique, using an ultrasonic detector.