Modificação química de nanoestruturas híbridas (POSS) para aplicação como lubrificantes

Polyhedral oligomeric silsesquioxanes (POSS) are hybrid structures type RSiO15n, with n organic groups R. These molecules can be easily functionalized by simply changing the chemical constitution of the organic groups. In this work, chemical modification of POSS-NH2 was performed by amidation reaction with butyric acid at elevated temperature, 160°C. The formation of the amide group is evinced by the appearance of NH angular deformation band at 1540 cm-1 in the FTIR spectra. Approximately 40% of the amino groups reacted, according to titration results. The formation of the amide groups resulted in a shift of the glass transition temperature (Tg) from -36.9°C to -25.6°C for the modified-POSS sample. Both POSS-NH2 and modified-POSS samples exhibited similar thermal degradation pattern. Analysis of the pairs distribution function (PDF) has determined that the hybrid nanoparticles are separated by a periodic distance of approximately 1.32 nm. POSS-NH2 and modified-POSS exhibit newtonian behavior, which will range from 10-1 s-1 and 1000 s-1. The viscosity decreased with increasing temperature, a typical behavior of liquid lubricants. http://dx.doi.org/10.18226/23185279.v2iss1p19Poliedros oligoméricos silsesquioxanos (POSS) são estruturas híbridas tipo RSiO1,5n  com n grupos orgânicos R. Estas moléculas podem ser facilmente funcionalizadas, simplesmente alterando a constituição química dos grupos orgânicos. Nesse trabalho, a modificação química de POSS-NH2 foi feita por meio de reações de amidação com ácido butírico em elevada temperatura, 160°C. A formação do novo grupo amida é evidenciada pelo do surgimento da banda de deformação angular do NH em 1540 cm-1 nos espectros de FTIR. Aproximadamente 40% dos grupos aminos reagiram, conforme resultados de titulação. A formação dos grupos amidas resultou no deslocamento da temperatura de transição vítrea (Tg) de -36,9°C para -25,6°C para o POSS-modificado. Ambas as amostras, de POSS-NH2 e POSS-modificado, exibem padrão de degradação térmica semelhante. A análise da função de distribuição de pares (PDF) determinou que as nanopartículas híbridas são separadas por uma distância periódica de aproximadamente 1,32 nm. O POSS-NH2 e o POSS-modificado apresentam comportamento newtoniano, na faixa que vai de 10-1 s-1 até 1000 s-1. A viscosidade diminuiu com aumento da temperatura, um comportamento típico de lubrificantes líquidos. http://dx.doi.org/10.18226/23185279.v2iss1p19

Avaliação de métodos para estimativas de propriedades mecânicas de materiais por indentação instrumentada: uma revisão

Neste trabalho é apresentado uma revisão de um conjunto de sistemáticas para a estimativa de propriedades elasto-plásticas de materiais metálicos e cerâmicos pelo método de indentação instrumentada. As sistemáticas analisadas contemplam a metodologia de Oliver e Pharr original e com uma modificação da função de profundidade de contato de maneira a torná-la mais próxima das medições experimentais além de diferentes métodos analíticos baseados na energia de deformação (Stilwell e Tabor; Tuck Giannakopoulos e Suresh e Malzbender). Os resultados obtidos para o módulo de elasticidade e a dureza resultantes da aplicação das sistemáticas propostas foram comparados com os valores obtidos da literatura referentes ao aço inoxidável austenítico 316L, alumina e boreto de ferro e cromo. De modo geral os resultados encontrados para a dureza e módulo de elasticidade calculados utilizando as diferentes sistemáticas apresentaram boa coerência com os valores da literatura, entretanto algumas metodologias necessitaram de aplicações de fatores de correção para a caracterização correta das propriedades das amostras.Palavras-chave: Indentação Instrumentada, Oliver e Pharr, Métodos de Energia, Propriedades Mecânicas.

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Uma das razões do grande esforço dedicado ao estudo do desgaste por deslizamento é devido a sua complexidade, especialmente no que se refere os mecanismos envolvidos. No desgaste por deslizamento estão presentes mecanismos de adesão, fadiga,triboquímico e abrasão. Adicionalmente, podem ocorrer transições na taxa de desgaste devidas ao efeito da carga, velocidade de deslizamento ou da transformação de fases, entre outros fatores. Nos aços inoxidáveis austeníticos ocorre atransformação martensítica por deformação plástica durante o desgaste por deslizamento. O esclarecimento da influência desta transformação é ainda um ponto de atenção para os pesquisadores. Com o objetivo de estudar o comportamento do desgastedos aços inoxidáveis austeníticos, assim como a influência da transformação martensítica induzida por deformação plástica foram realizados ensaios de desgaste por deslizamento com variações na carga normal e na distância. Os aços AISI 304 e AISI316 foram os materiais de estudo e o sistema de ensaio utilizado foi do tipo pino-sobre-disco. Foram caracterizadas as superfícies desgastadas e os resíduos de desgaste mediante microscopia óptica, microscopia eletrônica de varredura e difraçãode raiso-X. Para a caracterização das regiões sub-superficiais foram realizadas medidas de microdureza. No desgaste dos aços inoxidáveis austeníticos a microestrutura e a dureza da sub-superfície são afetadas pela presença da fasemartensitainduzida por deformação plástica. A dureza das regiões sub-superficiais influenciam na magnitude do desgaste. O material sofrerá um desgaste maior, se uma região sub-superficial, mais próxima à superfície desgastada, endurecida fortemente devido à deformação plástica e/ou à presença da martensita, não pode ser suportada pelas regiões menos encruadas correspondentes ao material basea.One of the reasons for the high efforts dedicated to study the sliding wear is its complexity, due to the multiple mechanisms involved. Mechanisms such as adhesion, abrasion, tribochemical effects and fatigue are present in sliding wear. Also transitions in the wear rate due to effects like loading, sliding speed or phase transformation, among other factors can occur. In austenitic stainless steels strain-induced martensitic transformation during sliding wear occurs. The explanation of the influence of that transformation is still a point of attention for researches. To study the austenitic stainless steels wear behavior as well as the influence of strain-induced martensitic transformation, sliding wear tests were performed with various normal loads and sliding distances. The materials studied were stainless steels AISI 304 and AISI 316. The wear tests were performed on pin-on-disk equipment. The wear surfaces and wear debris were characterized by means of optical microscopy, scanning eléctron microscopy and X-ray diffraction. For the characterization of the sub-superficial áreas microhardness measurements were performed. In the sliding wear of austenitic stainless steels the microstructure and sub-surface hardness are affected by the presence of strain-induced martensite. The hardness of sub-superficial areas influences the wear magnitude. The material will suffer a larger wear if the less hardened base region cannot support a strongly hardened sub-superficial area due to plastic deformation and/or martensite presence

The Effect of Counterpart Material on the Sliding Wear of TiAlN Coatings Deposited by Reactive Cathodic Pulverization

This work aims to study the effect of the counterpart materials (100Cr6, Al2O3 and WC-Co) on the tribological properties of TiAlN thin films deposited on AISI H13 steel substrate by reactive magnetron co-sputtering. The structural characterization of the TiAlN films, performed by X-ray diffraction, showed (220) textured fcc crystalline structure. The values of hardness and elastic modulus obtained by nanoindentation were 27 GPa and 420 GPa, respectively, which resulted in films with a relatively high resistance to plastic deformation. Ball-on-disk sliding tests were performed using normal loads of 1 N and 3 N, and 0.10 m/s of tangential velocity. The wear coefficient of the films was determined by measuring the worn area using profilometry every 1000 cycles. The mechanical properties and the chemical stability of the counterpart material, debris formation and the contact stress influences the friction and the wear behavior of the studied tribosystems. Increasing the hardness of the counterpart decreases the coefficient of friction (COF) due to lower counterpart material transference and tribofilm formation, which is able to support the contact pressure. High shear stress concentration at the coating/substrate interface was reported for higher load promoting failure of the film-substrate system for all tribopair

The Effect of Counterpart Material on the Sliding Wear of TiAlN Coatings Deposited by Reactive Cathodic Pulverization

This work aims to study the effect of the counterpart materials (100Cr6, Al2O3 and WC-Co) on the tribological properties of TiAlN thin films deposited on AISI H13 steel substrate by reactive magnetron co-sputtering. The structural characterization of the TiAlN films, performed by X-ray diffraction, showed (220) textured fcc crystalline structure. The values of hardness and elastic modulus obtained by nanoindentation were 27 GPa and 420 GPa, respectively, which resulted in films with a relatively high resistance to plastic deformation. Ball-on-disk sliding tests were performed using normal loads of 1 N and 3 N, and 0.10 m/s of tangential velocity. The wear coefficient of the films was determined by measuring the worn area using profilometry every 1000 cycles. The mechanical properties and the chemical stability of the counterpart material, debris formation and the contact stress influences the friction and the wear behavior of the studied tribosystems. Increasing the hardness of the counterpart decreases the coefficient of friction (COF) due to lower counterpart material transference and tribofilm formation, which is able to support the contact pressure. High shear stress concentration at the coating/substrate interface was reported for higher load promoting failure of the film-substrate system for all tribopair

Using the ratio: maximum load over unload stiffness squared, Pm/Su², on the evaluation of machine stiffness and area function of blunt indenters on depth-sensing indentation equipment

Depth sensing indentation study was conducted in a Fischerscope H100V machine, equipped with a Vickers indenter with a tip roundness of approximately 1330 nm. Tests were carried out on soda-lime glass, fused silica, sapphire, aluminum (1100 alloy), high alloyed steel, titanium and copper. The widely used iterative method of Oliver and Pharr was unsuccessful in the attempts to analyze machine compliance and indenter area function. Therefore, an alternative procedure was adopted. The alternative procedure is based on the ratio between maximum load and unload stiffness squared, Pm/Su². It was found that this procedure, which is not iterative, gives good results. A careful study of the Pm/Su² ratio, lead us to conclude that the Fischercope machine has a low compliance which depends on the sample mounting. This low compliance in conjunction with the recent discovery of the dependence of beta factor on the tip roundness/maximum depth ratio, which appears in the relation between contact stiffness and contact area, explains why the iterative method does not converge. However, variations in beta and machine compliance produces deviation on the hardness and elastic modulus lower that 6% with respect to expected values for the materials and the machine studied in this work