Doutorado em Engenharia Mecânica

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Navegando Doutorado em Engenharia Mecânica por Autor "Bozzi, Antonio Cesar"

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    Comportamento em desgaste por erosão-cavitação e em ensaios por esclerometria linear da liga Stellite 250 submetida a tratamentos térmicos e termoquímico de carbonitreção a plasma
    (Universidade Federal do Espírito Santo, 2023-12-04) Oliveira, Daniela Bahiense de; Bozzi, Antonio Cesar; http://lattes.cnpq.br/3017292130810807; https://orcid.org/0000-0002-6173-6316; http://lattes.cnpq.br/6977424481086523; Franco Júnior, Adonias Ribeiro; Silva, Sidney Nicodemos da; Orlando, Marcos Tadeu D Azeredo; https://orcid.org/0000000283876504; http://lattes.cnpq.br/3562894103432242; Milan, Julio Cesar Giubilei
    Cobalt superalloys are widely used due to their ability to maintain their mechanical, physical and chemical properties when used at high temperatures. However, concerns about the increasing demand for cobalt, unstable prices and few suppliers, make it essential to assess the efficiency of the materials applied. In this context, the present work investigated the effect of heat treatments (solution and aging treatments) and thermochemical plasma carbonitriding on the tribological behavior of the Stellite 250 alloy. The solution treatment was carried out at 1200°C for 1,5 hours and aging at 850°C for 4,5 hours. Thermochemical treatments were carried out at 360°C and 380°C for 9 hours. For tribological characterization, scratch tests were carried out with a progressive load (2 to 20N) and cavitation erosions tests were performed during 15 hours of exposure. The cross-sectional area of the scratches was verified by 3D profilometry. The wear mechanisms of the tribological tests were analyzed by scanning electron microscopy (SEM). The result of the microstructural characterization showed that for the cast alloy, the structure is formed by coarse grains, consisting of the α and ε phases. From the heat treatments, the structure remained mixed. Under all carbonitring conditions there was formation of a double layer of S-phase, composed of an N-enriched phase (SN) and another C-enriched layer (SC). The surface hardness values observed under carbonitrided conditions are in the order of 1100-1500 HV0.01. The scratch tests results showed that plasma carbonitriding condition enabled a considerable reduction in the coefficient of friction in relation to that observed for the Stellite 250 cast alloy. The wear mechanisms were tensile cracks in carbonitrided conditions; and in heat-treated conditions: formation of slip bands on the sides and cracks. The solution treated condition and subsequently carbonitrided at 380°C (SC380) showed greater resistance to cavitation erosion wear than the Stellite 250 cast alloy. Wear mechanisms include premature damage with formation and coalescence of pits, crater formation and brittle behavior of the material due to fatigue.
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    Superlubricidade no deslizamento de cerâmicos lubrificados com água
    (Universidade Federal do Espírito Santo, 2019-11-22) Strey, Nathan Fantecelle; Scandian, Cherlio; https://orcid.org/0000-0002-4393-719X; http://lattes.cnpq.br/8466752738430250; https://orcid.org/0000-0002-2568-116X; http://lattes.cnpq.br/3613706957012460 ; Sinatora, Amilton; https://orcid.org/0000-0002-5004-4947; http://lattes.cnpq.br/2988437176012972; Souza, Roberto Martins de; https://orcid.org/; http://lattes.cnpq.br/; Bozzi, Antonio Cesar; https://orcid.org/0000-0003-4857-0216; http://lattes.cnpq.br/3017292130810807; Murray, Henara Lillian Costa
    Superlubricity is the state of a tribosystem characterized by extremely low friction coefficients, with μ < 0.01. The superlubricity of ceramic pairs sliding in water has been reported for several similar and dissimilar tribopairs: Si3N4-Si3N4; SiC-SiC, Al2O3-Al2O3, ZrO2-ZrO2, Si3N4-SiC, Si3N4 Al2O3, Al2O3-SiC, ZrO2-SiC, etc. By hypothesis, some requirements must be satisfied for superlubricity in these tribosystems to occur: low enough mean contact pressure, adequate surface finish, a mild wear regime during running-in where tribochemical mechanisms are dominant, and formation of a soft tribofilm adhered to surfaces. These conditions were reevaluated through different approaches: (i) the polishing process parameters were optimized during the preparation of engineering ceramics used in tribological components and a better surface finish was obtained for SiC surfaces when compared to Al2O3 or ZTA ones; (ii) wear maps were developed for several dissimilar ceramics pair sliding in water which showed that the combination of tribologically compatible ceramic counterparts extends the operating conditions range of ceramic sliding bearings and that the mild wear regime is a necessary but not sufficient condition for the occurrence of superlubricity. Finally, (iii) the construction of Stribeck curves for Si3N4-SiC, ZrO2-SiC, Si3N4-Al2O3 and SiO2-SiO2 pairs showed that superlubricity is associated with mixed or hydrodynamic lubrication regime, depending on operating conditions, and that the ratio between the minimum thickness of the hydrodynamic film and the amplitude of the surface roughness peaks (λ ratio) is the parameter that best explains the friction coefficient of these tribosystems.