Avaliação tribológica de DLC em regime de lubrificação mista

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 2015.A lubrificação fluida é um ramo da tribologia bastante desenvolvido devido às inúmeras aplicações nos mais diferentes segmentos industriais. O regime de lubrificação limite é responsável por significativa parcela de perdas por atrito e desgaste em sistemas lubrificados. Por esse motivo, os óleos lubrificantes possuem aditivos em suas formulações para que os mesmos reajam com as superfícies em contato e movimento relativo para gerar tribofilmes antidesgaste na região do contato. Os aditivos possuem em sua composição química elementos como fósforo e enxofre que em grandes concentrações podem trazer danos à saúde humana e ao meio ambiente. Consequentemente, legislações cada vez mais severas pressionam para redução, ou até mesmo eliminação desses aditivos nos lubrificantes. Os DLC são revestimentos a base de carbono amorfo que possuem além de alta dureza e estabilidade eletroquímica, propriedades autolubrificantes. Portanto, os DLC possuem grande potencial para redução dos aditivos nos óleos, principalmente em regimes de lubrificação limite. Diversos estudos já foram conduzidos nesse sentido, contudo, a grande maioria deles utiliza óleos e aditivos da indústria automotiva de elevada viscosidade. É exatamente nesse ponto que o presente trabalho exerce grande avanço, pois avaliou o desempenho tribológico do DLC (a-C:H) em um cenário típico da indústria de compressores herméticos para refrigeração, ou seja, com lubrificante de baixa viscosidade (4,2mm²/s a 40°C). Para atingir os objetivos foram realizados testes com a configuração cilindro-plano (contato em linha), onde um cilindro desliza (movimento alternativo) sem rolar sobre uma superfície plana durante tempo e força normal pré-determinados. Os testes foram realizados a seco e repetidos com a presença do óleo. Foram analisadas as marcas de desgaste via microscopia óptica e eletrônica, interferometria óptica de luz branca, espectroscopia Raman e espectroscopia de energia dispersiva. Os resultados mostraram que o comportamento tribológico dos pares testados a seco são governados por tribocamadas, que por sua vez são formadas por partículas de desgaste oxidadas sujeitas a ação do contato. Além disso, foi encontrado carbono na forma de grafite nessas tribocamadas. A presença do óleo reduz o coeficiente de atrito dos pares testados e o aditivo BTP reage com as superfícies metálicas para formar tribocamadas antidesgaste. Mecanismos dúcteis atuam no desgaste do ferro fundido enquanto que no DLC ocorre degradação por mecanismo frágil. A sinergia entre tribocamadas provenientes da reação do aditivo com superfícies metálicas e a possível grafitização do DLC governam o desempenho tribológico dos pares envolvendo DLC e ferro fundido cinzento.Abstract : Liquid lubrication is a well-established technological branch due to several applications in different industrial segments. Boundary regime lubrication is responsible for a significant amount of frictional losses and wear in lubricated systems. For this reason, lubricating oils are usually formulated with different types of additives in order to form protective antiwear tribofilms on the real contact regions. Additives are composed of elements like phosphorus and sulfur that might be hazardous to human health and the environment when added in high concentrations. Therefore, severe regulations demand the reduction or even removal of such additives from lubricants. Diamond-like carbons (DLC) are a family of coatings containing high amounts of amorphous carbon that possess high hardness, chemical stability and self-lubricating properties. For this reason, DLCs have great potential in reducing the usage of additives in oils, especially in boundary lubrication regimes. Many studies have been conducted in this matter. However, these are mainly focused on automotive industry additives and oils with high viscosity. The main contribution of the present study is exactly on this point, since the hydrogenated DLC (a-C:H) in a typical hermetic compressors for refrigeration scenario was evaluated, with low viscosity oils (~4.2 mm²/s). The pair chosen was a cylinder vs plane, where the cylinder slides in a reciprocating movement without rolling on a plane surface during a constant time and normal load pre-determined. Dry and lubricated tests were conducted with the same configurations. Wear scars were analyzed using optical and electronic microscopy, white light interferometry, Raman spectroscopy and EDS. The results show that the tribological behavior of tested pairs under dry conditions are governed by tribolayers, generated by oxidized debris submitted to contact conditions. Furthermore, graphitic carbon was found in these tribolayers. Under lubricated conditions, the coefficient of friction (COF) is reduced and the BTP additive reacts with metallic surfaces in order to form antiwear tribolayers. Ductile mechanisms act in wear of cast iron while DLC wear occurs due fragile mechanisms. The synergy between tribolayers originated from reactions of the additive and metallic surfaces and DLC graphitization during friction determine the tribological behavior of pairs involving DLC and gray cast iron

Genesis and stability of tribolayers in solid lubrication: case of pair DLC-stainless steel

AbstractThe 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 2500h) 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 500nm, 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 1000h of testing

Desempenho tribológio de compósitos autolubrificantes em regime de lubrificação mista

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 2020.Atualmente, uma das grandes tendências mundiais se concentra no desenvolvimento de lubrificantes mais amigáveis ao meio ambiente e a saúde humana, principalmente através da redução ou até mesmo eliminação dos aditivos modificadores de atrito e antidesgaste contendo fósforo (P) e enxofre (S). Esses aditivos são compostos químicos que interagem com as superfícies sob lubrificação limite ou mista para gerar tribocamadas na área real de contato com baixa resistência ao cisalhamento, reduzindo o atrito e desgaste. Uma forma de fazer a função dos aditivos é utilizar materiais com propriedades autolubrificantes, ou seja, aqueles que ao sofrerem ação do contato tribológico forneçam condições para formação de tribocamadas com baixa tensão de cisalhamento. Neste sentido, no presente trabalho foram utilizados compósitos autolubrificantes produzidos por moldagem de pós por injeção (PIM do inglês Powder injection molding), formados por uma matriz metálica e nódulos de grafite 2D turbostrático homogeneamente dispersos no volume. Esses nódulos ou reservatórios de lubrificante sólido foram gerados in situ pela dissociação do carbeto de silício (SiC) durante a etapa de sinterização do processo PIM. Além disso, foram testadas duas formas alotrópicas do SiC, o hexagonal e o cúbico, e as temperaturas de sinterização de 1100 ,1150 e 1200°C. Para avaliação tribológica, testes do tipo movimento alternado com geometria cilindro sobre plano foram realizados a seco e sob lubrificação fluida. O óleo empregado é de baixa viscosidade (4,2 cSt a 40°C) contendo o aditivo antidesgaste BTP. Detalhes das microestruturas dos materiais estudados foram analisados por microscopia óptica (MO), enquanto que a dureza das matrizes metálicas e a topografias foram determinadas por microdureza Vickers e interferometria óptica de luz branca (WLI do inglês white light interferometry) respectivamente. Os reservatórios de lubrificante sólido foram estudados através de análises por microscopia eletrônica de varredura (MEV), microscopia eletrônica de transmissão (MET), espectroscopia de energia dispersiva de raio-X (EDS do inglês energy dispersive X-ray spectroscopy) e espectroscopia Raman (ER). As marcas de desgaste dos testes tribológicos foram avaliadas por MEV, EDS, WLI, ER e espectroscopia de elétrons Auger (AES do inglês Auger electron spectroscopy). As diferentes estruturas cristalinas do SiC e as temperaturas de sinterização não modificaram significativamente o lubrificante sólido gerado in situ, exceto para 1200°C. Testados a seco, os compósitos autolubrificantes apresentaram menor coeficiente de atrito e maior durabilidade em relação a referência, adicionalmente, a taxa de desgaste do contracorpo (Cilindro em aço AISI 52100 nitretado) é três ordens de grandeza menor. Sob lubrificação fluida, os mesmos comportamentos foram observados. Em ambos os casos (seco e lubrificado) os bons resultados tribológicos estão associados a tribocamadas formadas por óxidos de ferro, elementos dos materiais que formam par tribológico e grafite altamente desordenado.Abstract: Nowadays, one of the major global trends in tribology is focused on developing more environmentally friendly and human health lubricants, mainly by the reduction or even elimination of friction modifiers and anti-wear additives containing phosphorus (P) and sulfur (S). These additives are chemical compounds that interact with surfaces under boundary or mixed lubrication regime to generate tribolayers with low shear resistance in the real contact area, reducing friction and wear. One way to realize additives functions is to apply materials with self-lubricating properties, such as those that during tribocontact are able to provide conditions to generation protective tribolayers. In this sense, the present work tested self-lubricating composites produced by powder injection molding (PIM), which is formed by a metallic matrix and 2D turbostratic graphite nodules dispersed through the volume. These solid lubricant nodules or reservoirs were generated in situ by dissociating silicon carbide (SiC) during the sintering step of the PIM process. In addition, hexagonal and cubic SiC polytypes and the 1100°C, 1150 and 1200°C sintering temperatures were used. Cylinder-on-flat reciprocating tests were performed for tribological evaluation under dry and fluid lubricated conditions. The low viscosity (4.2 cSt at 40 °C) oil used contain the BTP antiwear additive. Microstructure details of materials were analyzed by optical microscopy (OM), whereas the matrix hardness and topography were determined by Vickers microhardness, and white light interferometry (WLI), respectively. The solid lubricant reservoirs were analyzed in the scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray energy dispersive spectroscopy (EDS), and Raman spectroscopy (RE). Wear marks from tribological tests also were analyzed by SEM, EDS, WLI, RE and Auger-electron spectroscopy (AES). Distinct SiC structures and the sintering temperatures do not induce significant differences in the solid lubricant generated, except 1200°C. The composites tested in dry condition possesses a lower coefficient of friction and higher scuffing resistance than reference. Additionally, the wear rate of counterbody (nitrided AISI 52100 cylinder) is three orders of magnitude lower, similar to lubricated condition results. In both cases (dry and lubricated), the good tribological effects are associated with tribolayers composed of iron oxide, elements from materials of surfaces, and high disordered graphite

Stability and Tribological Performance of Nanostructured 2D Turbostratic Graphite and Functionalised Graphene as Low-Viscosity Oil Additives

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

Tribologically induced nanostructural evolution of carbon materials: A new perspective

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