Fabrication and Tribological Properties of High Strength Silicified Graphite Composites

Silicified graphite composites, as a type of ceramic-based solid lubrication material, possess the superior properties of silicon carbide ceramics, such as high strength and high hardness, along with high-temperature strength, and self-lubricating performance of graphite materials. It has extensive application prospects in aerospace, chemical engineering, and nuclear energy. The reactive melting infiltration (RMI) method features near-net shape forming, a short processing cycle, and low cost, as an efficient approach for fabricating silicified graphite composites. Nevertheless, the infiltration reaction of liquid silicon into the porous carbon preform is extremely rapid after driven by reactive wetting and capillary forces, resulting in a sharp consumption of carbon. Hence, controlling the graphite content emerges as a crucial issue for the development of silicified graphite composites with superior lubrication properties. In this paper, a carbon source was fabricated through carbon source modification to prepare composite graphite spheres and coat on the surface of the composite graphite spheres with nano-carbon black. Subsequently, silicified graphite composites were prepared via compression molding and reaction melt infiltration. The carbon content of the composites was regulated via altering the particle size of the composite graphite spheres. The influence of the carbon content on the mechanical and tribological properties of the silicified graphite composites was discussed, and the film formation and lubrication mechanisms of the silicified graphite composites with a high carbon content under reciprocating friction conditions were analyzed.

Composite graphite spheres were prepared by mixing spherical graphite with liquid phenolic resin. First, the liquid phenolic resin and spherical graphite were weighed and mixed according to a mass ratio of 8:10, and mechanically stirred at 45 ℃ for 20–30 min. It was cured at 80 ℃ for 2 h and 150 ℃ for 10 h to obtain the cured product in a drying oven. The cured product was carbonized in a tubular furnace with nitrogen atmosphere. The temperature increased to 1000 ℃ at 1 ℃·min^-1, and the heat kept for 2 h. Finally, the bulk materials were crushed and screened, and five kinds of composite graphite particles with different particle sizes were obtained. The average particle sizes of the selected composite graphite spheres were 203 μm, 153 μm, 135 μm, 105 μm and 75 μm, respectively.

The composite graphite particles and nano carbon black were evenly dispersed in alcohol, and the mixed particles were obtained under magnetic stirring in a water bath at a constant temperature of 70 ℃ until dry. Subsequently, prefabricated 5% PVA solution was added to the dry mixed particles as a binder, and after being mixed evenly, the porous carbon preform was formed at 10 MPa for 30 s. The porous carbon preforms reacted with liquid silicon at 1550 ℃ for 30 min to obtain the silicified graphite composites. According to the size of the composite graphite spheres from large to small, the silicified graphite composites were labeled as samples GS-NCB-1, GS-NCB-2, GS-NCB-3, GS-NCB-4 and GS-NCB-5, respectively.

The introduction of nano-sized carbon black facilitates to the protection of carbon phase. The carbon content of the composites fabricated with the composite graphite particles coated by nano-sized carbon black obviously increases, compared with the composites without nano-sized carbon black. This is because that nano-sized carbon black with a higher reactivity forms a continuous and dense silicon carbide layer on the surface of the composite graphite powders, which effectively inhibits the reaction between the composite graphite powders and silicon to form β-SiC. In addition, the increase of particle size of the composite graphite particles has a positive effect on the carbon content of the silicified graphite composites. The carbon content of silicified graphite composites increases from 20.08% to 43.03%, the mechanical properties gradually decreases from 151 MPa to 96 MPa and the friction coefficient and wear rate reduce (the minimum: 0.11, 5×10^–7 mm³∙N^–1∙m^–1, 15 N) as the particle size of composite graphite powders increases, indicating superior self-lubricating properties. The results show that the improvement in the lubricating properties of silicified graphite composites is achieved due to the formation of lubricating films. The dense and continuous lubrication film avoids a direct contact with the abrasive material and reduces the coefficient of friction and wear rate. The decrease of the mechanical properties of the silicified graphite composites is due to the increase of the soft carbon content.

Nano-sized carbon black layer on the surface of the composite graphite particle first contacted with liquid silicon to form a silicon carbide layer during the reactive melting process, preventing the further infiltration and reaction of liquid silicon to the composite graphite particle, playing a protective role in the composite graphite particle, and leading to the high carbon content of the silicified graphite composites.

The increase in the particle size of the composite graphite particle could improve the carbon content of the silicified graphite composites because the thickness of the silicon carbide layer after the reaction of the composite graphite particle of different particle sizes was the same, and the increase in the particle size could reduce the specific surface area of the porous carbon preform, thereby reducing the reaction contact area with liquid silicon. The silicon carbide content reduced and the carbon content increased.

The improvement of lubrication performance of silicified graphite composites was achieved due to the formation of surface lubrication film. The dense and continuous lubrication film could avoid a direct contact between grinding materials, effectively reducing the coefficient of friction and wear rate.