In Situ Generated Turbostratic 2D Graphite: A New Way to Obtain High-Performance Self-Lubricating Iron-Based Composites

Abstract

The production of self-lubricating composites containing second phase particles is one of the most promising choices for controlling friction and wear in energy efficient modern systems. Initially, we present a new microstructural model/processing route able to produce a homogeneous dispersion of in situ generated, discrete, solid lubricant particles in the volume of sintered composites. The high mechanical and tribological performances of the composites are a result of the combination of matrix mechanical properties and structural parameters, such as the degree of continuity of the metallic matrix, the nature, the amount, and the lubricant particle size and shape which determine the mean free path between solid lubricant particles and the active area covered by each lubricant particles. This new route was achieved by in situ formation of graphite nodules due to the dissociation of a precursor (SiC particles) mixed with metallic matrix powders during the feedstock preparation. Thermal debinding and sintering were performed in a single thermal cycle using a plasma-assisted debinding and sintering (PADS) process. Nodules of graphite (size ≤20 μm) presenting a nanostructured stacking of graphite foils with thickness of a few nanometers were obtained. Micro-Raman spectroscopy indicated that the graphite nodules are composed of a so-called turbostratic 2D graphite which has highly misaligned graphene planes separated by large interlamellae distance. The large interplanar distance and misalignment among the graphene foils has been confirmed by transmission electron microscopy and is, probably, the origin of the remarkably low dry friction coefficient (0.06). The effects of precursor content (0 to 5 wt% SiC) and of sintering temperature (1100 °C, 1150 °C and 1200 °C) on tribolayer durability and average friction coefficient in the lubricious regime (μ < 0.2) are presented and discussed. In addition, the effect of the metallic matrix composition (Fe-C; Fe-C-Ni; Fe-C-Ni-Mo) is presented. Friction coefficient decreased and durability drastically increased with the amount of graphite formed during sintering, whereas friction coefficient was little affected by sintering temperature. However, the durability of the tribolayer was greatly increased when lower sintering temperatures were used. The addition of alloying elements considerably reduced wear rate and friction of specimens and counter-bodies. Friction coefficient values as low as 0.04 were obtained for the Fe-C-Ni-Mo composites. We also analyzed the effect of precursor content and of sintering temperature on the tribological behavior under constant normal load sliding tests. Again, the presence of graphite nodules significantly reduced the friction coefficients and wear rates, whereas the sintering temperature hardly affected these parameters. The results were compared with those caused by other forms of graphite (nodules in nodular cast iron and powder graphite) and were discussed in terms of the crystalline structure of the analyzed graphite using micro-Raman spectroscopy. Chemical analyses of the wear scars using scanning electron microscopy (SEM – EDX) and Auger electron spectroscopy (AES) showed a tribolayer that was composed predominantly of carbon and oxygen. This tribolayer is removed and restored during sliding and is continuously replenished with graphite. Finally, the strong effect of surface finishing is presented and discussed.