## 2 Materials and Methods  
Spherical 6061 Al powders with the diameter range of 16 \(\upmu\)m to 86 \(\upmu\)m, gas-atomized by JW-25 vacuum metal atomization furnace, were used as the matrix. To reduce the agglomeration of graphite, NAGF powders were used that were prepared by pre-coating commercial irregular graphite flakes (diameter range of 20 \(\upmu\)m-70 \(\upmu\)m and average thickness of about 3 \(\upmu\)m), with nano-Al powders (average diameter of 80 nm, purity of 99%, purchased from Jiaozuo Ban-Lv Nano. Eng. Co. Ltd.). The detailed process of pre-coating is described in our earlier works [13].  
The 6061 Al and NAGF powders with graphite content of 0 vol.%, 5 vol.%, 10 vol.% and 15 vol.% were respectively mixed in QM-3A mill for 30 min at a rotation rate of 1400 r/min. The blended powders were put into a mold, assembled by pyrophyllite bulk embedded with a dolomite tube, a NaCl tube and a graphite tube. Then, the mold was dried and put into a 6 MK-3 WY-60 cubic hinge presser, in which the UHP sintering was performed for 40 min at a pressure of 5.2 GPa and temperature of 450 \({}^{\circ}\)C, 550 \({}^{\circ}\)C, 650 \({}^{\circ}\)C and 750 \({}^{\circ}\)C, respectively. To analyze the improvement of the sintering pressure on the properties, G/6061 Al bearing 0 vol.%, 5 vol.% and 10 vol.% graphite were also HP-sintered at 35 MPa and temperature of 520 \({}^{\circ}\)C. Both of the two sintering processes were performed in avacuum atmosphere lower than \(1.3\times 10^{-3}\) Pa.  
The relative density was the ratio of practical density vs theoretical density. The practical density was measured by a wax-coated method, based on the principle of Archimedes. The theoretical density \(\rho_{\rm T}\) (g/cm\({}^{3}\)) was calculated according to Formula (1):  
\[\rho_{\rm T}=\mathrm{V}_{Al}\rho_{Al}+\mathrm{V}_{\rm G}\rho_{\rm G} \tag{1}\]  
where \(\rho_{Al}\) and \(\rho_{\rm G}\) are theoretical densities of 6061 Al and graphite (g/cm\({}^{3}\)); \(\mathrm{V}_{Al}\) and \(\mathrm{V}_{\rm G}\) are volume fractions of 6061 Al and graphite (cm\({}^{3}\)). The HRA hardness of the as-pressed sample was measured by Electric Rockwell hardness tester (500 MRA) with a diamond cone indenter, at a load of 60 kgf and dwell time of 10 s. In order to evaluate the tensile property, the sintered composite was machined to specimens with a gauge section size of 2.5 mm-long, 1.4 mm-wide and 1 mm-thick. The specimens were tested by Instron 5848 tensile tester at a tensile rate of 0.15 mm/min. A dry sliding test was carried out on MMW-1 vertical universal friction and a wear testing machine, under the load of 20 N and rotation speed of 240 r/min for about 900 s (distance of about 135,000 mm). The samples for the test were machined to 12.7 mm length and 4.8 mm diameter. The sample was weighed before and after the dry sliding test and the weight loss was obtained. The wear rate of the composite was evaluated by the ratio of the weight lost to the dry sliding distance.  
The morphology of the powder, microstructure of the composite, fracture surfaces of tensile samples and worn surfaces of wear samples were observed by OLYMPUS GX51 optical microscope, field emission scanning electron microscopy (SEM; FEI Quanta 250 FEG; ZEISS Sigma 300) and transmission electron microscope (High Resolution TEM, JEM-2100). The distribution of Al, C and O was investigated by means of energy dispersive spectroscopy (EDS). Phase analysis of the composite was carried out by X-ray diffractometer (Smart-lab, Rigaku, Japan), at 40 kV and 200 mA and using CuK\(\alpha\) radiation (\(\lambda\) = 0.15406 nm), within 2\(\theta\) = 20\({}^{\circ}\)-85\({}^{\circ}\), at scanning speed of 10\({}^{\circ}\)/min.