José Daniel Biasoli de Mello

Thermal Stability of the MoS2 Phase in Injection Moulded 17-4 PH Stainless Steel

In the present paper, an analysis of the stability of the MoS2 compound in 17-4 PH stainless steel matrix during the sintering of powder injection moulded samples is presented. A feedstock containing 10% vol. of the solid lubricant phase MoS 2 , mixed with 17-4 PH stainless steel powder was prepared and injected. The sintering was carried out at various temperatures ranging from 650°C to 1,300oC. The progress of the dissociation process of MoS 2 as a function of sintering temperature and the formation of new phases were analyzed via X-ray diffraction. The microstructure of the resulting material was analyzed by SEM/EDS. As expected, for temperatures above 650oC, the results confi rm the decomposition of MoS 2 and formation of others sulphides during the sintering cycle. In addition, there occurs dissolution of molybdenum resulting from MoS2 decomposition.

Effects of Different Plasma Nitrided Layers on the Tribological Performance of DLC Coatings

When multifunctional surface engineering processes that combine purpose-oriented phases are applied to soft substrates, a combination of high wear resistance, high load support and low coefficients of friction can be achieved. In this study, the effects of different nitrided layers on the tribological behaviour of a diamond-like carbon (DLC) film deposited on a SAE 1040 steel were investigated. The nitriding was carried out under different temperatures and gas mixtures to create three distinct nitrided layers: two compound layers with predominant e and γ phases and a diffusion layer. All of the surfaces were then coated with DLC deposited via plasma-enhanced chemical vapour deposition (PECVD). The tribological tests indicated that the best performance was achieved for a specific combination of hardness, surface roughness and nitride type. The best load-bearing capacity between the DLC coating and the soft substrate was achieved when the nitrided layer was primarily a diffusion layer.

Metallurgical Aspects of Self-lubricating Composites Containing Graphite and MoS2

The performance of dry self-lubricating bulk materials is directly related to microstructural aspects such as solid lubricant chemical composition and distribution. In this paper, dry powder mixtures were prepared from iron powder and 9-16.5xa0vol.% of solid lubricants (graphite and MoS2), both combined and isolated. The results showed that interactions and reactions occurred during processing, either between the solid lubricants or between the lubricants and the matrix, generating carbides and sulfides. On account of that, the lubricant distribution in the microstructure is greatly altered, and the microhardness, friction coefficient and wear rate are increased. The best results were achieved by adequate powder particle size, solid lubricant content and sintering temperature control. In the composite containing 9%MoS2xa0+xa02.5%C, values of friction coefficient and wear rate lower than 0.08 and 8xa0×xa010−6xa0mm3xa0N−1xa0m−1 were reached.

Tribological Behaviour of Plasma-Functionalized Graphene as Low-Viscosity Oil Additive

The influence of plasma functionalization of multilayer graphene (MG) as an additive for low-viscosity polyolester (POE) oil in terms of dispersion stability and tribological behaviour was investigated. Pure MG and MG functionalized via N2 and NH3 plasma were analysed. The plasma functionalization significantly improved the substrate wettability and the dispersion stability of the nanofluids. The tribological behaviour of the nanofluids was investigated using a reciprocating cylinder on plane configuration. 0.05xa0wt% of the functionalized nanoparticles dramatically increased the scuffing resistance and significantly improved the anti-wear properties of the POE oil (over 60% wear reduction). Optical microscopy, white light interferometry, scanning electron microscopy and micro-Raman spectroscopy were used to identify the wear mechanisms. The functionalization provides a well-dispersed suspension, which contributes to the formation of a continuous and homogeneous anti-wear tribofilm. Once between the sliding surfaces, the MG improves the load-carrying capacity of the oil, avoiding the seizure of the tribolayer.

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

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.