Antônio César Bozzi

O aço inoxidável superaustenítico ASTM A 744 Gr. CN3MN é aplicado na fabricação de equipamentos que trabalham em ambientes sob corrosão severa com solicitação mecânica. Nesse trabalho investigou-se a influência do tratamento térmico de solubilização na microestrutura e nas propriedades desse tipo de material. Foram realizados tratamentos térmicos de solubilização na faixa de temperaturas entre 1100 e 1250°C. Ensaios de impacto (Charpy) em temperatura ambiente e a -46°C foram realizados nas amostras tratadas termicamente. As análises microestruturais foram feitas por meio de microscopia eletrônica de varredura, eletrônica de transmissão e difração de raios X. Concluiu-se que, para maximizar a resistência ao impacto, a solubilização deve ser feita a 1200°C, pois tal medida produz a menor fração volumétrica de precipitados. As amostras solubilizadas a 1200 e 1240°C apresentaram fase sigma (s) e carboneto M6C.

PURPOSE To evaluate in situ the effect of different types of toothpaste on enamel surface topography, microhardness and surface roughness (Ra) after bleaching with 7.5% hydrogen peroxide (H2O2). METHODS This was a single-center, triple-blind, randomized, two-period, crossover in situ study in 15 subjects with 60 bovine enamel blocks (9.0×9.0×3.0 mm) fixed to acrylic palatal devices which were divided into six groups (n=10) : G1: control: placebo toothpaste (PT) and placebo bleaching (PB)G2: conventional toothpaste (CT) and PB; G3: whitening toothpaste (WT) and PB; G4: PT and H2O2; G5: CT and H2O2; and G6: WT and H2O2. After 14 days of whitening and brushing regime, the mineral volumetric loss of the enamel surface was evaluated according to qualitative scores, Ra was measured using 3D surface topography while Vickers microhardness (VH) (50g/10 seconds) comparing the test area with the control area and using scanning electron microscopy imaging. RESULTS G1 and G2 showed no visible vo...

A avaliação do comportamento em erosão-cavitação (EC) da liga Co-30Cr-19Fe quando nitretada em baixas temperaturas foi realizada através de ensaios de cavitação vibratória. Visando caracterizar os mecanismos de desgaste foi utilizada a microscopia óptica e a microscopia eletrônica de varredura para análise do progresso do mesmo. As amostras foram submetidas a um tratamento térmico de solubilização, realizado a 1200°C por 12 horas e subsequente têmpera em água. Posteriormente, foram conduzidas ao tratamento termoquímico de nitretação em baixas temperaturas: 380°C, 400°C e 420°C, respectivamente. A mistura gasosa foi de 75% de H2 e 25% de N2, o tempo de tratamento de 20 horas e pressão de trabalho de 2,5 Torr para todas as condições de nitretação. As amostras foram caracterizadas antes e após cada tratamento realizado por meio de microscopia ótica, medidas de microdureza e difração de raios X. Os ensaios de cavitação vibratória, para todas as condições trabalhadas, foram realizados no equipamento ultrassônico da marca Telsonic DG-2000. O ensaio foi interrompido periodicamente a fim de avaliar a evolução da perda de massa das amostras e de registrar a evolução do mecanismo de desgate por microscopia eletrônica de varredura. Antes do tratamento termoquímico foi observado uma microestrutura de matriz cujas fases são cobalto α (CFC) e ε (HC), com poucos carbonetos e precipitados. E após o tratamento de solubilização houve dissolução parcial desses poucos carbonetos e precipitados que existiam. A fase S foi obtida em todas as condições. A espessura e a dureza das camadas aumentaram de acordo com o acréscimo da temperatura de nitretação. Foi constatado que a condição que apresentou maior resistência ao desgaste por erosão cavitacional foi a N380 a partir da análise das curvas de perda de massa acumulada com o tempo de exposição e da avaliação dos mecanismos de desgaste. No entanto, o tratamento termoquímico não foi efetivo para melhorar a resistência ao desgaste por EC das condições nitretadas a 400°C e 420°C em relação a condição solubilizada. De forma geral o mecanismo de desgaste observado para todas as condições nitretadas foi ausência de deformação plástica significativa e consequentemente ausência de tempo de incubação, com provável propagação de trincas por fadiga e desprendimento frágil do material. Palavras chave: Liga Co-30Cr-19Fe; Nitretação a plasma; Austenita expandida; Erosão cavitacional

Abstract Superalloys are a potential material class for selection in some tribological systems, particularly those subjected to wear. When subject to cavitation erosion, the cobalt-based alloys are often applied. Surface treatments have been employed to improve the tribological resistance without loss of corrosion performance for several class of materials. In this context, the thermochemical treatment of low temperature plasma carbonitriding was carried out in a superalloy of the Co-Cr-Fe system in order to improve cavitation erosion resistance. The carbonitriding treatments were performed at temperature of 380°C over a period of 3, 9 and 15 hours. The gas mixture was composed of 72.5% H2, 25% N2 and 2.5% CH4. After the thermochemical treatment, phases formation was identified by X-ray diffraction and the glow discharge optical emission spectroscopy was used for mass concentration depth profiling of carbon and nitrogen. Cavitation erosion resistance of specimens were evaluated by a vibratory test. Mass losses were periodically measured as a function of exposure time during test. Specimens were evaluated in terms of mean depth of erosion, incubation time and mass loss. In addition, the cavitation erosion mechanism was investigated by scanning electron microscopy observation of damaged surfaces. S-phase formation was observed for all treatments. Results showed that low temperature plasma carbonitriding and the formation of expanded phase are effective to increase cavitation erosion resistance of the studied cobalt-based alloy when treatment was carried out during 3 and 9 hours.

Abstract The tribological behavior of a cast cobalt-chromium alloy, during microabrasion testing, was investigated. The alloy's microstructure contains coarse grains of fcc cobalt rich dendrites, interdendritic segregations and almost no precipitates. The tests were done using a 0.1 g/cm 3 Al 2 O 3 /distilled water slurry, 0.3 N applied load and 20 rpm rotational speed. Two-body abrasion (grooving wear) was the dominating wear micromechanism. A severely deformed nanocrystalline layer containing random oriented ∼20 nm grains was observed, immediately below the worn surface. Finite element modeling of a single Al 2 O 3 particle scratching the Co-Cr alloy surface showed that severe plastic deformation occurs in a region 1–2 μm beneath the worn surface, giving rise to the observed nanocrystalline layer.

Abstract Cobalt alloys are used in several industrial applications due to their good mechanical properties at high temperatures. During manufacturing of 13% Cr super martensitic stainless steel seamless tubes by Mannesmann process, the hot rolling guides made of cast Co30Cr19Fe alloy are currently used, but present severe wear, limiting their service life. Seeking an alternative, three cobalt-based coatings (Stellite 1, 6 and 12) were selected. These coatings were evaluated and compared with the alloy currently used by sliding wear tests performed at room temperature and at 500 °C in a tribometer PLINT TE67 with pin-on-disc configuration, without lubrication and varying the normal load. The pin was made of 13% Cr super martensitic stainless steel. The discs were manufactured with cast Co30Cr19Fe alloy and some of them were coated by laser cladding process with Stellite alloys, that is, a total of four materials. The worn volume of the wear surface was analyzed by 3D profilometry. The micromechanisms were observed with a stereoscopic and scanning electron microscope. To assess the elemental composition of the wear track, analyses were performed using energy dispersive spectroscopy (EDS). The results indicate that the samples coated with Stellite presented better performance than the cast Co30Cr19Fe alloy. Among the coatings, the Stellite 1 showed the best wear resistance, both at room temperature and at 500 °C. The Co30Cr19Fe, Stellite 6 and 12 alloys, presented predominantly plastic removal mechanisms. Whereas Stellite 1 presented micro-cutting and oxidative wear.

The effect of the nitrogen potential on the micro-abrasive wear resistance of AISI D2 plasma nitrided tool steel was evaluated. Plasma nitriding experiments were conducted in a pulsed plasma reactor using different nitrogen potentials. The results indicate that a nitrided layer, with finely dispersed nitrides, can be formed when shorter nitriding times and higher nitrogen potentials were used. Although the thickness of this layer is small, its wear resistance is higher than that observed at longer nitriding times. Increasing the nitriding time resulted in the formation of thicker layers, albeit with grain boundary nitrides, which are deleterious to wear resistance. For untreated material, three-body abrasive wear rolling was the dominant mechanism, and for nitrided material, the mechanism transition was not clearly observed in spite of its better wear resistance.

Ultra-low carbon (ULC) steels exhibit low yield strength and excellent formability. Plasma Assisted Physical Vapor Deposition (PAPVD) is a potential coating method for enhancing the strength at the surface of these steels [1]. However, when deposited onto low strength alloys PAPVD coatings may undergo premature failure if the substrate plastically deforms under heavy load. Extra load support is usually required for hard coatings to perform satisfactorily. Combined treatments involving plasma nitriding and PAPVD coating have been used to improve the load-bearing capacity of hard films [2]. This work describes the characterization and micro-abrasive wear behavior of Ti-stabilized ULC steels after surface modification by D.C Triode Plasma Nitriding (DC-TPN) and sequential coating with CrAlN by Electron Beam Plasma Assisted Physical Vapor Deposition (EB-PAPVD). Ti-ULC steel, plasma nitrided Ti-ULC steel and Ti-ULC duplex system were characterized by SEM, EDS, XRD analyses, micro-hardnes...

In this study, the three-body abrasion resistance of AISI 1020 annealed steel samples coated with WC–12%Co coatings produced by the Jet Kote® thermal spray process was evaluated. The tests were performed using SiO2, Al2O3 and SiC as abrasives and five levels of stress ranging from 0.10 to 0.60 MPa. By measuring the weight loss, the wear rate was evaluated after reaching the steady state of wear. Characterisation of the abrasives was performed using an image analyser and scanning electron microscopy. The microstructure and wear mechanisms of the coatings were characterised by scanning electron microscopy. The relationship between stress and wear rate was found to be dependant on abrasive type; this is due to different degrees of fragmentation of the abrasive. The effect of stress on the wear rate depended on the nature of the abrasive due to the fragmentation process of the abrasives. The results indicated that abrasion resistance and predominant wear mechanisms were strongly depende...