The aim of this work was to study the influence of varying nitrogen potential during plasma nitriding of stainless steel ISO 5832-1. The control of nitrogen potential was achieved by pulsing the nitrogen flow for different times (01/19, 02/18, and 05/15), where the numbers represent the time (in minutes) of nitrogen flow on/off, thus creating an intermittent flow of nitrogen during the treatment. For all tested conditions - continuous and pulsed flow of nitrogen - the treatment temperature was kept at 425ºC during 2 and 8 hours. Specimens were characterized by means of X-ray diffraction, scanning electron microscopy, optical microscopy, energy-dispersive X-ray spectroscopy, and nanohardness. Results showed that the layer thickness increases with the increase of total treatment time, and decreased for shorter times of pulsed nitrogen flow. Smaller expansion of the austenite phase, as well as less precipitation of chromium nitrides, were also observed for shorter times of pulsed nitrogen flow. Hence, the use of intermittent nitrogen flow appears to be an efficient approach in order to control the nitrogen concentration within the layer, reducing its brittleness.

Abstract ISO 5832-1 stainless steel specimens were plasma nitrided at different nitrogen potentials. The main goal was to obtain the S-phase with different nitrogen concentrations and free of chromium-based precipitates. The control of nitrogen potential was made by pulsing the gas at predetermined times: 10/10, 05/15, 02/18, and 01/19, where the numbers represent the time in minutes that the nitrogen flow was kept on/off, respectively. For all pulsing conditions, the nitriding was carried out at 450 °C for 2 hours. After nitriding, specimens were characterized by means of optical (OM) and electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and microhardness. Results show that the nitrided layer thickness decreases with decreasing times of nitrogen pulse, and that lower times of nitrogen flow lead to lower precipitation of chromium nitrides. It can be thus concluded that the use of intermittent nitrogen flow is an alternative to control the nitrided layer in terms of thickness, hardness, and the amount of nitrogen present in phase ɣN (S-phase).

RESUMO Corpos de prova de aço inoxidável austenítico ISO 5832-1(similar ao AISI 316L, para aplicações biomédicas) apresentando camada superficial enrriquecida em nitrogênio (fase S), obtidos pelos processos de nitretação por plasma em baixa temperatura e SHTPN (Solution Heat Treatment after Plasma Nitriding -Tratamento de solubilização após nitretação por plasma), além de amostras não tratadas, foram testadas em ensaio de desgaste microabrasivo do tipo esfera livre. Os resultados mostraram que as superfícies nitretadas por plasma em baixa temperatura apresentam o menor coeficiente de desgaste para menores tempos de ensaio. No entanto, devido à pequena espessura de camada nitretada, com o aumento do tempo de ensaio as amostras processadas via SHTPN passam a apresentar desempenho superior, indicando que, dependendo das solicitações de desgaste, um ou outro processo apresenta o melhor desempenho.

ABSTRACT Samples of the ISO 5832-1 austenitic stainless steel (similar to AISI 316L for biomedical applications) with nitrogen-rich surface layer (S-phase), obtained by plasma nitriding at low temperature and SHTPN (Solution Heat Treatment after Plasma Nitriding) processes, as well as non-treated samples, were tested to evaluate its micro-abrasive wear behavior. The micro-abrasive wear test were performed using the free-ball configuration. The results pointed out that the nitrided surfaces present lower wear coeficient for short measuring time. Nevertheless, since nitrided layers are relatively thin, for prolongated wear tests the SHTPN samples presented lower wear cofiecient, indicating that depending on the apllication one of the processes will present the best behaviour.

Specimens of ISO 5832-1stainless steel were submitted to low temperature plasma nitriding treatment and Solution Heat Treatment after Plasma Nitriding (SHTPN) process aiming to obtain a S-phase, chromium precipitates-free, surface layer. The following techniques were used for analysis: scanning electron microscopy (SEM), optical microscopy (OM), microhardness, X-ray diffraction (XRD) and wavelength dispersive spectroscopy (WDS) microanalysis. Corrosion resistance was evaluated by means of potentiodynamic anodic polarization and open-circuit measurements of corrosion potential. Results indicated that the S-phase layer with N concentrations of about 0.9 wt% and 2 µm thick was formed during low temperature plasma nitriding, while layers with concentratios of about 0.45 wt% of N and up to 200 µm thick resulted from the SHTPN process. Results proved the increase in localized corrosion resistance caused by nitrogen in solid solution for both processes, as well as the deleterious effect caused by the precipitation of chromium nitride.

Corpos de prova de aço inoxidável austenítico ISO 5832-1 apresentando camada superficial rica em nitrogênio (fase S), obtidas pelos processos de nitretação por plasma em baixa temperatura e SHTPN (Solution Heat Treatment after Plasma Nitriding), caracterizadas em trabalho anterior, além de amostras na condição de fornecimento, ou seja, solubilizadas, e amostras nitretadas por plasma em 750 ºC por 3 horas foram testadas quanto à resistência a corrosão. A resistência a corrosão foi avaliada com base no método de polarização potenciodinâmica e obtenção de potencial de circuito aberto (Ecorr vs tempo) em uma solução de 0,5 M NaCl. Os resultados comprovaram o ganho na resistência a corrosão localizada proveniente do nitrogênio em solução sólida, tanto para nitretação em baixa temperatura quanto no processo SHTPN, bem como o efeito deletério proveniente da precipitação do nitreto de cromo.

Samples of austenitic stainless steel ISO 5832-1 showing nitrogen-rich surface layer (S-phase), obtained by plasma nitriding at low temperature and SHTPN (Solution Heat Treatment after Plasma Nitriding) processes, characterized in previous work, and samples in starting microstructure, in other words, solubilized and plasma samples nitrided at 750° C for 3 hours, were tested for corrosion resistance. The corrosion resistance was evaluated based on the potentiodynamic anodic polarization test and open circuit measurements of corrosion potential (OCP) using 0.5 M NaCl. The results confirmed the gain in localized corrosion resistance from the nitrogen in solid solution in both low-temperature nitriding and SHTPN process, as well as from the deleterious effect of precipitation of chromium nitride.

Corpos de prova de aço inoxidável ISO 5832-1 foram tratados por dois diferentes processos, nitretação por plasma em baixa temperatura e SHTPN, com objetivo de obter camada superficial rica em nitrogênio (fase S ou γN) e livre de precipitados. O tempo de nitretação para ambos os processos foi o mesmo (3 h), de modo a possibilitar um comparativo dos resultados obtidos. Utilizaram-se as seguintes técnicas de análise: microscopia eletrônica de varredura (MEV), microscopia ótica (MO), microdureza, difração de raios-X (DRX) e espectroscopia por dispersão de comprimento de onda. Os resultados indicaram que a nitretação em baixa temperatura possibilita a formação de camada de fase S com concentrações da ordem de 0,90 % N (peso) com espessuras de pouco mais de 2 µm, enquanto o SHTPN gera camadas com 0,45 % N (peso) e espessuras de até 200 µm.

Samples of stainless steel ISO 5832-1 were nitrided by two different processes, plasma nitriding at low temperature and SHTPN, in order to obtain a nitrogen rich surface layer (S-phase or γN) free from precipitates. The nitriding time for both processes was the same (3 h), to enable a comparison of the results obtained. Treated samples were characterized by means of scanning electron microscopy (SEM), optical microscopy, microhardness, X-ray diffraction (XRD) and wavelength dispersive spectrometry (WDS). The results indicate that low-temperature nitriding enables the formation of S-phase layer with nitrogen concentrations about 0.90 wt.% with a thickness of 2.15 μm, while SHTPN generates layers with 0.45 wt.% and thicknesses up to 200 µm.

Corpos de prova de aço inoxidável ISSO 5832-1 foram nitretados em uma atmosfera de 90% vol. N2 + 10% vol. H2, a uma temperatura de 870ºC. Os tempos de processamento estudados foram 1, 3 e 5 h. Utilizaram-se as seguintes técnicas de análise: difração de raios-x (DRX), microscopia eletrônica de varredura (MEV) e microdureza Vickers, além da análise da variação de massa. Os resultados indicaram um crescimento difusivo da camada composta. O aumento do tempo promoveu uma maior incorporação de nitrogênio no material, porém, devido à maior difusão em volume e a consequente formação de camadas mais espessas, fases mais pobres nesse elemento foram identificadas. Verificou-se, ainda, uma diminuição da dureza da camada nitretada com o aumento do tempo de tratamento. O ganho de massa, bem como o crescimento da camada nitretada, foi crescente com o tempo de tratamento. Dentre os tempos estudados, o que mais se adequou ao posterior tratamento de "SHTPN" foi o de 3h, principalmente devido ao tamanho de grão, associado à incorporação de nitrogênio obtida.

Samples of stainless steel ISO 5832-1 were nitrided in a gas mixture of 90% N2 + 10% H2 at a temperature of 870ºC. The treatment times were 1, 3 and 5 hours. Treated samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and Vickers microhardness, as well as analyze of the mass variation. The results indicated the diffusion growth of the compound layer. The increased time promoted a greater uptake of nitrogen in the material, however, due to higher bulk diffusion and the consequent formation of thicker layers, phases poor in this element were identified. There was also a decrease in hardness of the nitrided layer with increasing treatment time. The gain in mass, as well as growth of the nitrided layer, increased with treatment time. For the subsequent SHTPN treatment the nitriding time of 3 h was the most suitable, mainly due to grain size, associated with the obtained nitrogen incorporation.

Tem-se observado na área odontológica a utilização de dentes naturais para a realização de pesquisas diversas (in vivo, in situ e/ou in vitro ). O comportamento mecânico de dentes, sujeitos aos esforços mastigatórios, geralmente é utilizado como variável de resposta. Durante uma condição de maloclusão, a componente mais relevante para a integridade da estrutura dental é a não-axial (de flexão). Esta, quando atua em um dente em função na cavidade bucal, gera uma situação semelhante à de uma viga em equilíbrio suportando uma carga concentrada na extremidade livre. Com base nessa configuração, foi desenvolvida uma metodologia para o estudo do comportamento de dentes sujeitos a esforços dessa natureza. A metodologia leva em consideração aspectos particulares do dente analisado, tais como área de secção transversal, momento de inércia, etc. Na determinação da área da secção transversal, descreveu-se a utilização de um analisador de imagens. Para a determinação das demais variáveis, implementou-se um programa em linguagem de programação C++ utilizando o compilador Borland C++ Builder, versão 1.0, para ambiente Windows (Borland International, EUA). Os resultados mostraram que, com o emprego da metodologia desenvolvida, alcançam-se resultados mais precisos, sendo sua utilização adequada a pesquisas que utilizam o comportamento à flexão de dentes extraídos como variável de resposta.

The use of natural teeth for performing in vivo , in situ and/or in vitro studies has been widely observed in the dental field. The mechanical behavior of teeth subjected to masticatory efforts is commonly used as a variable of response. In conditions of malocclusion, the most relevant component accounting for the integrity of the dental structure is non-axial (flexural). This force, while acting on a tooth in function in the oral cavity, generates a similar situation as that of a beam in balance supporting a load concentrated on its free extremity. Based on this configuration, a methodology has been developed to investigate the behavior of teeth subjected to this kind of efforts, taking into consideration specific aspects of the tested teeth, such as the transversal section area and the moment of inertia. For determination of the transversal section area an image analysis program was used. For assessment of the other variables, a C++ language program was implemented utilizing the Borland Compiler C++ Builder, version 1.0, for Windows (Borland International, USA). The applicability of the developed methodology is illustrated in this article using bovine teeth. Our findings indicate that more accurate data can be obtained with the application of this methodology and that it is suitable for studies testing the flexural strength of extracted teeth, as a variable of response.