Nitriding is a plasma based processing technique that is used to improve the surface properties of components and products in many areas including the aerospace, automotive and biomedical industries to name a few. Active Screen Plasma Nitriding (ASPN) is a relatively new nitriding technique which has potential advantages over the more traditional nitriding techniques such as Direct Current (DC) plasma nitriding where high substrate biases can be problematic. However, there is considerable debate as to the mechanism for nitriding in ASPN. This thesis focuses on investigating the mechanism for nitriding in a commercial ASPN system. Commercial ASPN treatments of nitrideable alloy steels were found to be unsatisfactory unless a sufficient bias was applied. The level of bias required to produce a satisfactory nitriding response, in terms of the cross sectional hardness, was found to depend on the concentration of strong alloy nitride forming elements present in the steel. Although active screen material was found to be transferred to the workload, no evidence was found that this process played a significant role in enhancing the nitriding response. The primary mechanism for nitrogen mass transfer in ASPN was found to be dependent on the active screen/workload separation distance. When this separation is small (less than approximately 10cm for the conditions used in this study) then nitrogen mass transfer in the form of energetic ions or neutrals can occur between the active screen and the workload. This allows samples to be treated without a substrate bias. On the other hand, when the active screen/workload separation distance is large (greater than approximately 10cm) as is normally the case in a commercial environment, this mechanism for nitrogen mass transfer breaks down and a substrate bias is essential. In this latter case, nitrogen ions attracted to the workload using a bias is the primary nitrogen mass transfer mechanism and the role of the active screen is primar ily to uniformly heat the workload.

No presente trabalho foi investigada a nitretação a plasma do aço rápido ASTM M2 utilizando diferentes configurações, envolvendo as técnicas convencional e com tela ativa. Na técnica convencional, o material a ser nitretado atua como o cátodo do sistema, sendo coberto diretamente pela bainha da descarga luminescente. Nos tratamentos conduzidos com a técnica de tela ativa, uma estrutura de tela em formato cilíndrico atua como cátodo e o material a ser nitretado é posicionado no seu interior. Neste novo processo, como não há potencial elétrico externo aplicado aos substratos, a descarga luminescente se desenvolve apenas na estrutura da tela, evitando problemas intrínsecos do método convencional de nitretação a plasma. Todos os tratamentos investigados foram executados com os seguintes parâmetros: Temperatura de 500 °C, mistura gasosa composta 76% de volume de gás nitrogênio e 24% de volume de gás hidrogênio e pressão de 3 milibar. Para os tratamentos com tela ativa, os substratos foram mantidos eletricamente isolados. Os resultados foram comparados observando a diferença de método de nitretação (com e sem o uso de tela ativa) e a influência do tempo de tratamento (variando sua duração em 1, 4 e 8 horas) Todos os experimentos foram realizados utilizando uma fonte de potência de tensão retificada aplicada na tela ou nas amostras, dependendo da configuração. Adicionalmente, para o tempo de nitretação de 4 horas uma fonte de potência com tensão pulsada também foi aplicada. As amostras nitretadas foram caracterizadas por ensaios de rugosidade, ensaios de dureza e microdureza, microscopia ótica, microscopia eletrônica de varredura e difração de raios-X. Ensaios tribológicos para avaliação da resistência ao desgaste das diferentes condições de tratamento foram conduzidos. Dentre os principais resultados foi observada uma clara diferença na profundidade de camada nitretada, que foi sempre mais profunda nos tratamentos convencionais. Apesar de formadas camadas menos profundas, as amostras nitretadas oriundas dos tratamentos com tela ativa demonstraram melhor desempenho nos ensaios tribológicos, resultando em taxas de desgaste até sete vezes inferior do que as amostras do nitretadas convencionalmente, este resultado foi atribuído a não formação da zona de compostos nas amostras nitretadas com tela ativa. / In the present work an investigation on the plasma nitriding of ASTM M2 High-Speed Steel using different configurations was carried out, involving traditional and active screen techniques. In the traditional technique, the material to be nitrided act as the system cathode, being directly covered by the glow discharge sheath. In the active screen treatments, a cylindrical mesh structure (screen) plays the role of the cathode and the material to be nitrided is positioned in its interior. In this new process, as there is no external electric potential applied to the specimens, the glow discharge develops only in the screen structure, avoiding intrinsic problems from the conventional plasma nitriding method. All the studied treatments were carried out with the following parameters: temperature of 500 °C, gas mixture of 76 vol.-% N2 and 24 vol.-% H2 and pressure of 3 millibar. For the active screen treatments, the samples were kept electrically insulated. The results were compared observing the differences in the nitriding method (with and without the use of active screen) and the nitriding time influence (varying its duration in 1, 4 and 8 hours) All experiments were carried using a rectified voltage power supply applied to the screen or to the samples, depending on the configuration. Additionally, for the nitriding time of 4 hours a pulsed voltage power supply was also employed. The nitrided samples were characterized by roughness tests, hardness and microhardness tests, optical microscopy and scanning electron microscopy, X-ray diffraction. Tribological tests to evaluate the wear resistance of the different treatment conditions were also carried out. Among the main results a clear difference in the case layer depth was noticed, which was always deeper in the conventional treatments. Although forming shallower case depths, the active screen nitrided specimens presented better performance in the tribological tests, resulting in wear rates up to seven times lower than the wear rate for conventionally plasma nitrided samples, this result was assigned due to the non-formation of compound layer in the active screen plasma nitrided samples.

No presente trabalho foi investigada a nitretação a plasma do aço rápido ASTM M2 utilizando diferentes configurações, envolvendo as técnicas convencional e com tela ativa. Na técnica convencional, o material a ser nitretado atua como o cátodo do sistema, sendo coberto diretamente pela bainha da descarga luminescente. Nos tratamentos conduzidos com a técnica de tela ativa, uma estrutura de tela em formato cilíndrico atua como cátodo e o material a ser nitretado é posicionado no seu interior. Neste novo processo, como não há potencial elétrico externo aplicado aos substratos, a descarga luminescente se desenvolve apenas na estrutura da tela, evitando problemas intrínsecos do método convencional de nitretação a plasma. Todos os tratamentos investigados foram executados com os seguintes parâmetros: Temperatura de 500 °C, mistura gasosa composta 76% de volume de gás nitrogênio e 24% de volume de gás hidrogênio e pressão de 3 milibar. Para os tratamentos com tela ativa, os substratos foram mantidos eletricamente isolados. Os resultados foram comparados observando a diferença de método de nitretação (com e sem o uso de tela ativa) e a influência do tempo de tratamento (variando sua duração em 1, 4 e 8 horas) Todos os experimentos foram realizados utilizando uma fonte de potência de tensão retificada aplicada na tela ou nas amostras, dependendo da configuração. Adicionalmente, para o tempo de nitretação de 4 horas uma fonte de potência com tensão pulsada também foi aplicada. As amostras nitretadas foram caracterizadas por ensaios de rugosidade, ensaios de dureza e microdureza, microscopia ótica, microscopia eletrônica de varredura e difração de raios-X. Ensaios tribológicos para avaliação da resistência ao desgaste das diferentes condições de tratamento foram conduzidos. Dentre os principais resultados foi observada uma clara diferença na profundidade de camada nitretada, que foi sempre mais profunda nos tratamentos convencionais. Apesar de formadas camadas menos profundas, as amostras nitretadas oriundas dos tratamentos com tela ativa demonstraram melhor desempenho nos ensaios tribológicos, resultando em taxas de desgaste até sete vezes inferior do que as amostras do nitretadas convencionalmente, este resultado foi atribuído a não formação da zona de compostos nas amostras nitretadas com tela ativa. / In the present work an investigation on the plasma nitriding of ASTM M2 High-Speed Steel using different configurations was carried out, involving traditional and active screen techniques. In the traditional technique, the material to be nitrided act as the system cathode, being directly covered by the glow discharge sheath. In the active screen treatments, a cylindrical mesh structure (screen) plays the role of the cathode and the material to be nitrided is positioned in its interior. In this new process, as there is no external electric potential applied to the specimens, the glow discharge develops only in the screen structure, avoiding intrinsic problems from the conventional plasma nitriding method. All the studied treatments were carried out with the following parameters: temperature of 500 °C, gas mixture of 76 vol.-% N2 and 24 vol.-% H2 and pressure of 3 millibar. For the active screen treatments, the samples were kept electrically insulated. The results were compared observing the differences in the nitriding method (with and without the use of active screen) and the nitriding time influence (varying its duration in 1, 4 and 8 hours) All experiments were carried using a rectified voltage power supply applied to the screen or to the samples, depending on the configuration. Additionally, for the nitriding time of 4 hours a pulsed voltage power supply was also employed. The nitrided samples were characterized by roughness tests, hardness and microhardness tests, optical microscopy and scanning electron microscopy, X-ray diffraction. Tribological tests to evaluate the wear resistance of the different treatment conditions were also carried out. Among the main results a clear difference in the case layer depth was noticed, which was always deeper in the conventional treatments. Although forming shallower case depths, the active screen nitrided specimens presented better performance in the tribological tests, resulting in wear rates up to seven times lower than the wear rate for conventionally plasma nitrided samples, this result was assigned due to the non-formation of compound layer in the active screen plasma nitrided samples.

No presente trabalho foi investigada a nitretação a plasma do aço rápido ASTM M2 utilizando diferentes configurações, envolvendo as técnicas convencional e com tela ativa. Na técnica convencional, o material a ser nitretado atua como o cátodo do sistema, sendo coberto diretamente pela bainha da descarga luminescente. Nos tratamentos conduzidos com a técnica de tela ativa, uma estrutura de tela em formato cilíndrico atua como cátodo e o material a ser nitretado é posicionado no seu interior. Neste novo processo, como não há potencial elétrico externo aplicado aos substratos, a descarga luminescente se desenvolve apenas na estrutura da tela, evitando problemas intrínsecos do método convencional de nitretação a plasma. Todos os tratamentos investigados foram executados com os seguintes parâmetros: Temperatura de 500 °C, mistura gasosa composta 76% de volume de gás nitrogênio e 24% de volume de gás hidrogênio e pressão de 3 milibar. Para os tratamentos com tela ativa, os substratos foram mantidos eletricamente isolados. Os resultados foram comparados observando a diferença de método de nitretação (com e sem o uso de tela ativa) e a influência do tempo de tratamento (variando sua duração em 1, 4 e 8 horas) Todos os experimentos foram realizados utilizando uma fonte de potência de tensão retificada aplicada na tela ou nas amostras, dependendo da configuração. Adicionalmente, para o tempo de nitretação de 4 horas uma fonte de potência com tensão pulsada também foi aplicada. As amostras nitretadas foram caracterizadas por ensaios de rugosidade, ensaios de dureza e microdureza, microscopia ótica, microscopia eletrônica de varredura e difração de raios-X. Ensaios tribológicos para avaliação da resistência ao desgaste das diferentes condições de tratamento foram conduzidos. Dentre os principais resultados foi observada uma clara diferença na profundidade de camada nitretada, que foi sempre mais profunda nos tratamentos convencionais. Apesar de formadas camadas menos profundas, as amostras nitretadas oriundas dos tratamentos com tela ativa demonstraram melhor desempenho nos ensaios tribológicos, resultando em taxas de desgaste até sete vezes inferior do que as amostras do nitretadas convencionalmente, este resultado foi atribuído a não formação da zona de compostos nas amostras nitretadas com tela ativa. / In the present work an investigation on the plasma nitriding of ASTM M2 High-Speed Steel using different configurations was carried out, involving traditional and active screen techniques. In the traditional technique, the material to be nitrided act as the system cathode, being directly covered by the glow discharge sheath. In the active screen treatments, a cylindrical mesh structure (screen) plays the role of the cathode and the material to be nitrided is positioned in its interior. In this new process, as there is no external electric potential applied to the specimens, the glow discharge develops only in the screen structure, avoiding intrinsic problems from the conventional plasma nitriding method. All the studied treatments were carried out with the following parameters: temperature of 500 °C, gas mixture of 76 vol.-% N2 and 24 vol.-% H2 and pressure of 3 millibar. For the active screen treatments, the samples were kept electrically insulated. The results were compared observing the differences in the nitriding method (with and without the use of active screen) and the nitriding time influence (varying its duration in 1, 4 and 8 hours) All experiments were carried using a rectified voltage power supply applied to the screen or to the samples, depending on the configuration. Additionally, for the nitriding time of 4 hours a pulsed voltage power supply was also employed. The nitrided samples were characterized by roughness tests, hardness and microhardness tests, optical microscopy and scanning electron microscopy, X-ray diffraction. Tribological tests to evaluate the wear resistance of the different treatment conditions were also carried out. Among the main results a clear difference in the case layer depth was noticed, which was always deeper in the conventional treatments. Although forming shallower case depths, the active screen nitrided specimens presented better performance in the tribological tests, resulting in wear rates up to seven times lower than the wear rate for conventionally plasma nitrided samples, this result was assigned due to the non-formation of compound layer in the active screen plasma nitrided samples.

Low-temperature plasma nitrocarburizing treatments are applied to improve the surface properties of austenitic stainless steels by forming an expanded austenite layer without impairing the excellent corrosion resistance of the steel. Here, low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wall reactor to compare the effects of two active screen materials: (i) a steel active screen with the addition of methane as a gaseous carbon-containing precursor and (ii) an active screen made of carbon-fibre-reinforced carbon (CFC) as a solid carbon precursor. By using both active screen materials, ASPNC treatments at variable plasma conditions were conducted using AISI 316L. Moreover, insight into the plasma-chemical composition of the H2-N2 plasma for both active screen materials was gained by laser absorption spectroscopy (LAS) combined with optical emission spectroscopy (OES). It was found that, in the case of a CFC active screen in a biased condition, the thickness of the nitrogen-expanded austenite layer increased, while the thickness of the carbon-expanded austenite layer decreased compared to the non-biased condition, in which the nitrogen- and carbon-expanded austenite layers had comparable thicknesses. Furthermore, the crucial role of biasing the workload to produce a thick and homogeneous expanded austenite layer by using a steel active screen was validated.

The effect of a controlled oxygen admixture to a plasma nitrocarburizing process using active screen technology and an active screen made of carbon was investigated to control the carburizing potential within the plasma-assisted process. Laser absorption spectroscopy was used to determine the resulting process gas composition at different levels of oxygen admixture using O2 and CO2, respectively, as well as the long-term trends of the concentration of major reaction products over the duration of a material treatment of ARMCO® iron. The short-term studies of the resulting process gas composition, as a function of oxygen addition to the process feed gases N2 and H2, showed that a stepwise increase in oxygen addition led to the formation of oxygen-containing species, such as CO, CO2, and H2O, and to a significant decrease in the concentrations of hydrocarbons and HCN. Despite increased oxygen concentration within the process gas, no oxygen enrichment was observed in the compound layer of ARMCO® iron; however, the diffusion depth of nitrogen and carbon increased significantly. Increasing the local nitrogen concentration changed the stoichiometry of the ε-Fe3(N,C)1+x phase in the compound layer and opens up additional degrees of freedom for improved process control.

Active screen plasma nitrocarburizing (ASPNC) increases the surface hardness and lifetime of austenitic stainless steel without deteriorating its corrosion resistance. Using an active screen made of carbon opens up new technological possibilities that have not been exploited to date. In this study, the effect of screen power variation without bias application on resulting concentrations of process gas species and surface modification of AISI 316L steel was studied. The concentrations of gas species (e.g., HCN, NH3, CH4, C2H2) were measured as functions of the active screen power and the feed gas composition at constant temperature using in situ infrared laser absorption spectroscopy. At constant precursor gas composition, the decrease in active screen power led to a decrease in both the concentrations of the detected molecules and the diffusion depths of nitrogen and carbon. Depending on the gas mixture, a threshold of the active screen power was found above which no changes in the expanded austenite layer thickness were measured. The use of a heating independent of the screen power offers an additional parameter for optimizing the ASPNC process in addition to changes in the feed gas composition and the bias power. In this way, an advanced process control can be established.