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Solid lubrication is an excellent option to reduce friction and wear in dry sliding conditions, as well as a secondary source of protection in fluid-lubricated systems. Considering the most widely used solid lubricants, carbon-based lubricants are among the most versatile for tribological use. The reasons for this include their availability and their innate ability to adopt various nanostructures. The crystalline ordering of carbon has been identified as the main factor governing its tribological behavior. This work presents and discusses the most significant findings from an ongoing research program aimed at developing carbon-based solid lubricants. In this sense, several carbon-based materials with diverse levels of nanostructural order have been studied: three-dimensional (3D) crystalline graphite (GR); novel carbide-derived two-dimensional (2D) turbostratic carbon (CDC) produced from Fe–SiC (GSF) and B4C–Cr3C2 (GBC) solid-state reactions; NH3 plasma-functionalized multilayer graphene (MLG); in situ fluorinated diamond-like carbon (DLC) (isfDLC); vertically aligned carbon nanotube (CNT) films. Initially, we present a brief description of the processing route for obtaining these materials, followed by their microstructural characterization and a synthesis of the most relevant aspects of tribological evaluation. Dry sliding tests in reciprocating motion were employed with different tribo-pair geometries, specimens and counter-body materials, varied surface topography, and diverse routes to add solid lubricants to the contact (vacuum impregnation of sintered steels, drop-casting, self-lubricating composites and vertically aligned films). The results provide a holistic view of the nature of the tribolayers formed by these materials. Finally, a new micro-Raman (µRaman) analysis technique for quantifying the point and line defects of the carbon present in tribolayers is used to correlate the lubrication and degradation mechanism of carbonaceous solid lubricants with their initial nanostructure and testing conditions. This technique provides new insights into the nature of tribolayers produced by carbon-based solid lubricants.

Abstract Carbon-based solid lubricants are excellent options to reduce friction and wear, especially with the carbon capability to adopt different allotropes forms. On the macroscale, these materials are sheared on the contact along with debris and contaminants to form tribolayers that govern the tribosystem performance. Using a recently developed advanced Raman analysis on the tribolayers, it was possible to quantify the contact-induced defects in the crystalline structure of a wide range of allotropes of carbon-based solid lubricants, from graphite and carbide-derived carbon particles to multi-layer graphene and carbon nanotubes. In addition, these materials were tested under various dry sliding conditions, with different geometries, topographies, and solid-lubricant application strategies. Regardless of the initial tribosystem conditions and allotrope level of atomic ordering, there is a remarkable trend of increasing the point and line defects density until a specific saturation limit in the same order of magnitude for all the materials tested.

Sintered self-lubricating components may present inappropriate mechanical properties despite their excellent tribological properties. In general, alloying elements are used to improve these properties, but with a cost increase. As an alternative, surface enrichment (or surface alloying) with alloying elements may be applied. This study developed and characterized sintered composite materials surface-enriched with nickel or molybdenum. The results showed that the surface enrichment process is useful for increasing the tribomechanical properties. The Mo-enriched samples presented superior tribological results. Compared with the reference samples (not enriched), the Mo-enriched specimens showed an increase of 4,954% in scuffing resistance, lower friction coefficient (0.09), and reduced wear rate (68% and 96% lower for the specimen and the counter body). These results were mainly attributed to microstructural modification.

The dispersion stability of carbon-based solid lubricants/lubricating oils remains a challenge to overcome. Recently, novel processing routes were developed to obtain 2D turbostratic graphite particles via solid-state reactions between B4C and Cr3C2 (GBC) and between SiC and Fe (GSF) that present outstanding tribological properties in a dry scenario, as well as functionalized graphene (GNH). This work investigated the suspension stability of GBC and GSF particles (0.05 wt.%) dispersed in a low-viscosity polyol ester lubricating oil and their tribological performance. Ammonia-functionalized graphene (GNH) particles were also used as a reference. In order to evaluate the dispersion stability, in addition to the classical digital image technique, a much more assertive, reliable, quantitative and rarely reported in the literature technique was used, i.e., the STEPTM (Space and Time-resolved Extinction Profiles) technology. Reciprocating sphere-on-flat tribological tests were carried out, in which before contact, 0.2 μL of pure oil and suspension (POE + 0.05 wt.% of solid lubricant) was applied on a flat surface. The results showed that the GBC particles remained remarkably stable and reduced the sphere wear rate by 61.8%. From the tribosystem point of view, the presence of GBC and GSF reduced the wear rate by 18.4% and 2.2%, respectively, with respect to the pure oil, while the GNH particles increased the wear rate by 4.2%. Furthermore, the wear rate was improved due to the highly disordered carbon tribolayer formation identified on both surfaces.

Abstract This work presents a study on effect of cutting parameters in force, texture and chips morphology obtained during the turning of a self-lubricating sintered composite. The tests were performed varying the cutting speed (vc = 100 and 200 m/min), feed (f = 0.1 and 0.2 mm), depth of cut (ap = 0.5 and 1 mm), as well as the materials manufacturing route in single pressing (SP) and double pressing (DP). The variation in the cutting parameters influenced the components of the machining force. The cutting, feed and passive force were approximately 10%, 10% e 25% higher, respectively, for DP materials. The turning process generated surfaces with values of maximum height (Sz) between 28.3 to 10.8 μm, Kurtosis (Sku) > 3 and Skewness (Ssk) < 0.3. The chip type obtained was segmented and the chip thickness ratio was influenced by the variation of the cutting parameters and the materials manufacturing route.

Sintered iron-based self-lubricant composites in which graphite reservoirs are generated by the dissociation of silicon carbide (SiC) particles feature low friction coefficients and scuffing resistance combined with high mechanical stress. This work evaluated the influence of the sintering temperature and the SiC polytype on the graphite reservoirs morphology, structure, and friction behaviour. The reservoirs were studied by field emission gun-scanning electron microscopy (FEG-SEM), micro-Raman spectroscopy, and transmission electron microscopy (TEM). The friction behaviour was evaluated by cylinder on plane tests (friction coefficient and scuffing resistance). The results showed that the SiC polytype and the sintering temperature modified the graphite structures. The SiC dissociation promoted the formation of different nano graphites, presenting different turbostratic 2D and 3D arrays. The results showed that the presence of the turbostratic graphite was beneficial to the friction behaviour of these composites, which offered better tribological performance than the composite with 3D graphite.

In this study, powder technology was used to obtain Fe-SiC composites in which SiC particles act as precursors to generate a large amount of turbostratic graphite dispersed in the composite matrix. The selection of the alloy composition was studied employing Thermo-Calc® software to obtain the temperature and composition range for the stabilization of the graphite phase in iron with a high yield. The extracted turbostratic 2D graphite particles were dispersed in mineral oil in order to evaluate the potential of these particles as a lubricating oil additive. The structure and morphology of the extracted graphite were examined by Raman spectroscopy and transmission electron microscopy (TEM), indicating the highly disordered nature of turbostratic graphite. Reductions in the friction coefficient and wear rate of a tribological pair were observed when compared to the pure mineral oil and mineral oil with commercial graphite particles added. The misorientation and increase in interplanar distances of turbostratic 2D graphite induce a low degree of interaction between these atomic planes, which contributes to the low-friction coefficient and the lower wear rate obtained for this system.

The objective of this work is to investigate the influence of the crystallographic texture on the corrosion resistance of 16% Cr ferritic stainless steel. Samples of ASTM S43000 ferritic stainless steel, both niobium-stabilised and non-stabilised, were used. The samples were subjected to crystallographic characterisation (EBSD) and analysed using an inverse pole figure (IPF) and a crystalline orientation distribution function (CODF). The samples also underwent anodic potentiodynamic polarisation tests (deaeration by high-purity argon gas) in 3.56% NaCl and 1N H2SO4 solutions, and the surface was examined by SEM after the tests. The results showed a clear influence of the crystallographic texture on the corrosion resistance. The niobium decreases the amount of the preferred orientation and thus the influence of the texture on the corrosion resistance, although the corrosion resistance of the stainless steel is increased as niobium carbides are formed.

The morphology, dimensions and chemical composition of tribolayers strongly depend on the pressures and temperatures acting on the contact. They are formed by reactions between surfaces in contact with each other as well as with the atmosphere, lubricants and possible contaminants. In this paper, the influence of test time (180, 500, 1000 and 2500 h) on the formation and characteristics of tribolayers in pairs of DLC-stainless steel that were tested under an atmosphere of refrigerant gas R134a and without lubricating oil was analyzed. The characterization was performed using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) (morphology and chemical composition) and white light interferometry (thickness). The tribolayer thicknesses ranged from 100 to 500 nm, and they were composed of elements originating from mutual transfers between the tribological pairs, as well as oxides that were more pronounced on the stainless steel surface. The results show that the tribolayers are chemically stable, maintaining the same composition over time, and their thicknesses remained constant after 1000 h of testing.

The present work analyzes the tribological behavior of surface modifications often used in cold rolling mill rolls. Different surface modifications were carried out on samples produced from a fragment of the rolling mill roll: i) texturing; ii) chromium plating; iii) texturing with subsequent hard chrome plating; iv) and hard chrome plating with subsequent texturing. Before the surface modifications the samples were heat treated and ground on both faces. Wear tests were performed using a reciprocating movement of a ball over flat configuration under a load of 9.8 N. It is observed that there is no significant change in the coefficient of friction as a function of surface modification. Surface texturing increases the wear of the counter body, while the hard chromium coating reduces it. The addition of hard chromium coating promotes the formation of a tribolayer on the counter body consisting of chromium and oxygen. On the other hand, for the samples without hard chrome coating, the tribolayer consists of iron and oxygen

Friction and wear control can be achieved primarily by considering the nature of the counter faces, together with the environmental conditions. In most cases, a transfer film is found on the sliding surfaces. Environment plays a crucial role on the kinetics of formation and on the composition of the transfer film, and thus strongly influences friction levels and wear rates. In this paper, the effect of the actual environment (refrigerant) present in hermetic compressors on the tribological behaviour of a Si rich multifunctional DLC coating deposited on 1020 steel is analyzed. Unlubricated reciprocating pin-on- disk tests are performed using a High Pressure Tribometer under different atmospheres (Air, CO2 and R600a). Samples tested in R600a environment present the lowest friction coefficient and the lowest wear rate for both body and counter-body

When the surfaces of two elastic bodies present relative motions under certain amount of contact pressure the mechanical system can be unstable. Experiments conducted on elastic bodies in contact shown that the dynamic system is self-excited by the non-linear behavior of the friction forces. The main objective of this paper is to estimate the friction force using the vibrations signals, measured on a reciprocating wear testing machine, by the proposed non-linear signal analysis formulation. In the proposed formulation the system global output is the sum of two outputs produced by a linear path associated in parallel with a non-linear path. This last path is a non-linear model that represents the friction force. Since the linear path can be identified by traditional signal analysis, the non-linear function can be evaluated by the global input/output relationships. Validation tests are conducted in a tribological system composed by a sphere in contact with and a prismatic body, which has an imposed harmonic motion. The global output force is simultaneously measured by a piezoelectric and by a piezoresistive load cells. The sphere and prismatic body vibrations are measured by a laser Doppler vibrometer and by an accelerometer respectively. All signals are digitalized at the same time base and the data is transferred to a microcomputer. The non-linear signal analysis technique uses this data to identify the friction force.

Durability of surfaces has been reported as the main factor affecting tribological behavior of steam oxidised sintered iron. The presence of surface pores and their negative influence on load bearing capacity, suggest that surface topography might play an important role on the durability of the oxide layer. In this paper, the influence of compaction pressure and powder grade on surface topography, and as a consequence, its effect on the tribological behavior of steam oxidised sintered iron has been analysed. Specimens prepared from atomised iron powders with different sizes were compacted using 4 different pressures, sintered, and then subjected to steam treatment. Tribological characterisation was carried out in a reciprocating sliding wear test. Although the processing parameters affected the surface topography to a considerable extent, the main influence may be attributed to powder grade. A strong influence of surface topography on the durability distance, evaluated in terms of the evolution of contact resistance with total sliding distance, has been highlighted. Surfaces which were smoother and had high load-carrying capacity were always associated with a higher durability distance.

O consumo regular de bebida ácida tem sido associado com o aumento da incidência de erosão dental, o que pode levar a graves conseqüências para a saúde bucal pela perda de tecido mineralizado, resultando em sensibilidade, dor e má aparência. O presente trabalho teve como objetivo estudar a importância do pH da dieta líquida industrializada na etiologia das lesões de erosão dental, bem como avaliar, in vitro, a influência do refrigerante de limão (Soda limonada®), do suco à base de soja sabor laranja (Ades®), da bebida isotônica sabor tangerina (Gatorade®) e do refrigerante de Cola (Coca-cola®) empregadas rotineiramente na alimentação de alunos em idade escolar, em função do tempo de exposição. Para este estudo foram utilizadas cinco coroas de incisivos e molares permanentes humanos livres de trincas e/ou fraturas, extraídos na Clínica de Cirurgia do Centro Universitário do Triangulo (UNITRI). Os espécimes foram divididos aleatoriamente em cinco grupos, sendo o pH das bebidas foi aferido utilizando-se um pH-metro, imediatamente após a abertura de cada frasco de bebida industrializada. Os ciclos de imersão foram realizados sob agitação, durante cinco minutos, três vezes ao dia, com intervalos de quatro horas, durante 30 dias. Todas as coroas de incisivos e molares submetidos ao protocolo foram observadas por microscopia eletrônica de varredura e após análise concluiu-se que as quatro bebidas avaliadas apresentaram potencial erosivo, sendo que o refrigerante à base de cola alterou o esmalte de forma mais intensa seguido pelo refrigerante de limão, bebida isotônica e suco à base de soja.

Abstract This work is a review of previous works, presenting and discussing the most important results obtained by an ongoing research program towards the development of innovative, low-cost, self-lubricating composites with a low friction coefficient and high mechanical strength and wear resistance. Special emphasis is given to uniaxial die pressing of solid lubricant particles mixed with matrix powders and to metal injection moulding associated with in situ generation of solid lubricant particles. Initially, a microstructural model/processing route (powder injection moulding followed by plasma-assisted debinding and sintering) produced a homogeneous dispersion of in situ generated solid lubricant particles. Micrometric nodules of graphite with diameter smaller than 20 μm were formed, constituting a nanostructured stacking of graphite foils with nanometric thickness. Micro Raman analysis indicated that the graphite nodules were composed of turbostratic 2D graphite having highly misaligned graphene planes separated by large interlamellae distance. Large interplanar distance between the graphene foils and misalignment of these foils were confirmed by transmission electron microscopy and were, probably, the origin of the outstandingly low dry friction coefficient (0.04). The effect of sintering temperature, precursor content, metallic matrix composition and surface finish is also reported. Furthermore, the influence of a double-pressing/double-sintering (DPDS) technique on the tribological performance of self-lubricating uniaxially die-pressed hBN + graphite-Fe-Si-C-Mo composite is also investigated. Moreover, the tribological behaviour of die-pressed Fe-Si-C matrix composites containing 5, 7.5 and 10 wt% solid lubricants (hBN and graphite) added during the mixing step is analysed in terms of mechanical properties and wear mechanisms. Finally, the synergy between solid lubricant particles dispersed in a metallic matrix and fluid lubricants in a cooperative mixed lubrication regime is presented.

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 ε 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.

Noncarious cervical lesions (NCCLs) are considered to be of multifactorial origin, normally associated with inadequate brushing. This study assessed the influence in vitro of simulated brushing on NCCL formation. Fifteen human premolars were submitted to brushing in the cementoenamel junction region, using hard-, medium- and soft-bristled brushes associated with a toothpaste of medium abrasiveness under a 200 g load, at a speed of 356 rpm for 100 minutes. The surface topography of the region was analyzed before and after brushing, by means of a laser interferometer, using "cut-off" values of 0.25 and considering roughness values in mm. The initial roughness (mm) results for dentin (D / bristle consistency: 1 - soft, 2 - medium and 3 - hard) were as follows: (D1) 1.25 ± 0.45; (D2) 1.12 ± 0.44; (D3) 1.05 ± 0.41. For enamel (E / bristle consistency: 1 - soft, 2 - medium and 3 - hard), the initial results were: (E1) 1.18 ± 0.35; (E2) 1.32 ± 0.25; (E3) 1.50 ± 0.38. After brushing, the following were the values for dentin: (D1) 2.32 ± 1.99; (D2) 3.30 ± 0.96; (D3) Over 500. For enamel, the values after brushing were: (E1) 1.37 ± 0.31; (E2) 2.15 ± 0.90; (E3) 1.22 ± 0.47. Based on the results of the ANOVA and Tukey statistical analyses (a = .05) it was concluded that soft, medium and hard brushes are not capable of abrading enamel, whereas dentin showed changes in surface roughness by the action of medium- and hard-bristled brushes.