Development of FexN thin films with microcompression analysis

This thesis explores the growth routes of metal nitrides based materials of binary and ternary nitrides. It is believe that this iron nitrides based materials exhibit interesting properties in term of electronic optical, thermal and magnetic properties. However, most experimental and analytical efforts in this research field have been carried out in the form of bulk samples and there was no research of these nitrides in thin films. Consider the unique characteristics of iron nitrides system, the binary and ternary nitrides of both FexN and FexWy-1N thin films were grown using electron beam physical vapor deposition method under metal rich conditions. There exist significant challenges for the development of both binary and ternary nitrides thin films. It required to remain stable during the growth in order to incorporate nitrogen in the system to obtain Fe-N system due to the nature of nitrogen of having strong triple bond. The first aspect of this thesis investigate the development of these nitrides by using four different methods. Results from XRD and XPS reveal that there were formation of iron oxides on the thin films and further XPS data shows the percentage of nitrogen is lower than expected. The highest content of iron was then selected to proceed with second phase of the project with the aim of developing iron nitrides thin films with thicker films under higher growth temperature. The formation of iron nitrides were found in the thin films with higher growth temperature. The XPS analysis that shows the presence of either Fe2+ or Fe3+ species in the sample and further investigation by TEM reveals the film hexagonal crystal structure which corresponds to Fe3N. An attempt of growing ternary nitrides were carried out with different iron to tungsten ratio. However, in this preliminary study, we only managed to obtain W-N composition with a limited amount of iron presence in the sample. It was found that either the iron has been oxidised or some iron oxides or tungsten oxides were formed during the synthesis Microcompression experiment was carried out on the thicker iron nitrides thin films in order to calculate their Young’s modulus. It was found that the value is still lower than the bulk samples which may be corresponds to misalignment between the pillar and the system during the experiments. However, it was observed that there was no deformation such as crack occurred on the thin films during this test. Further study shall be carried out to have a better understanding of these iron nitrides based thin films.Open Acces

Response of a DIN 18MnCrSiMo6-4 continuous cooling bainitic steel to plasma nitriding with a nitrogen rich gas composition

The use of continuous cooling bainitic steels can provide a more energy efficient manufacturing route. However, for their use in mechanical components like gears, it is necessary to improve their surface properties without impacting the core properties to guarantee reliable mechanical performance. The effect of temperature and time on the plasma nitriding response of a DIN 18MnCrSiMo6-4 steel was investigated. The plasma nitriding was performed for 3, 6 and 9 hours, at 400, 450, 500 and 550 °C, using a gas mixture composed of 76 vol.% nitrogen and 24 vol.% hydrogen. Samples were characterized before and after plasma nitriding concerning the microstructure, hardness and microhardness, fracture toughness, phase composition and residual stress states. Based on the results presented, layer growth constants (k) for different temperatures was determined. Moreover, it could be found that 500 °C gave the best results investigated here, as higher temperature took to core and surface hardness decrease. The nitrided samples with thicker compound layers presented a fracture behavior dominated by the formation of Palmqvist cracks. X-ray phase analysis indicated the formation of biphasic compound layer on the surface of all nitrided samples. The diffusion zone presented compressive residual stresses with highest values near the surface

From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase

Austenitic stainless steels are employed in many industrial fields, due to their excellent corrosion resistance, easy formability and weldability. However, their low hardness, poor tribological properties and the possibility of localized corrosion in specific environments may limit their use. Conventional thermochemical surface treatments, such as nitriding or carburizing, are able to enhance surface hardness, but at the expense of corrosion resistance, owing to the formation of chromium-containing precipitates. An effective alternative is the so called low temperature treatments, which are performed with nitrogen- and/or carbon-containing media at temperatures, at which chromium mobility is low and the formation of precipitates is hindered. As a consequence, interstitial atoms are retained in solid solution in austenite, and a metastable supersaturated phase forms, named expanded austenite or S phase. Since the first studies, dating 1980s, the S phase has demonstrated to have high hardness and good corrosion resistance, but also other interesting properties and an elusive structure. In this review the main studies on the formation and characteristics of S phase are summarized and the results of the more recent research are also discussed. Together with mechanical, fatigue, tribological and corrosion resistance properties of this phase, electric and magnetic properties, wettability and biocompatibility are overviewed

Surface Engineering of C/N/O Functionalized Materials

This book discusses the latest developments in the surface engineering of C/N/O functionalized materials, including both experimental and theoretical studies on heat treatment and surface engineering of metals, ceramics, and polymers

Microstructure and wear properties of a low carbon bainitic steel on plasma nitriding at different N2-H2 gas mixtures

For the first time, the influence of gas mixture on first damage resistance of a plasma nitrided DIN 18MnCrSiMo6-4 bainitic steel was investigated. Samples were nitrided at 500 °C with three different N2 -H2 gas mixtures, containing 5, 24, and 76 vol.% N2 . Samples were characterized concerning the resulting roughness, microstructure, compound layer’s phase composition, residual stresses in the diffusion zone, and surface hardness. Tribological ball-on-flat tests were carried out in reciprocal mode using zirconia as ball material for friction coefficient and the compound layer resistance until the first damage. The test results were evaluated statistically by analysis of variance (ANOVA). As the amount of nitrogen in the gas mixture decreases, the ε-Fe2-3(C)N content in the compound layer decreases. A γ’-Fe4 N monophasic compound layer was achieved at 5 vol.% N2 gas mixture. The diffusion zone as expected presented compressive residual stresses with the highest values near the surface. In the tribological tests, better results were obtained for 5 and 24 vol.% N2 in the gas mixture as higher amounts of γ’-Fe4 N were formed. The 76 vol.% N2 gas mixture led to a brittle behavior, due to the biphasic compound layer (γ’-Fe4 N and ε-Fe2-3(C)N) with a predominant content of ε-Fe2-3(C)N

Fe8Nx Thin Films and Nanoparticles: from Intrinsic Properties Towards Magnetic Applications

Iron nitride Fe8Nx could potentially provide an environmentally friendly and resource-efficient functional magnetic material in the areas of permanent magnets, magnetic recording as well as biomedical applications. Despite the amount of research within the last decades, questions remain on whether or not the intrinsic magnetic properties are sufficient and if they can, by sustainable means, be engineered into the useful extrinsic properties. Another key issue is the phase stability in different environments which needs a thorough investigation. In this thesis, the Fe8Nx material synthesis, an analysis of structure and the corresponding magnetic properties, particularly in thin films and nanoparticles, are presented. The focus lies first on the fabrication of buffer-free, phase-pure α'-Fe8Nx and α''-Fe16N2 samples in order to converge towards an unambiguous interpretation of the observed physical phenomena. The main aim of this work is to study the magnetic properties, the thermal stability and consequently feasibility for the proposed applications, by performing advanced synthesis and in-depth characterization of high-quality α'-Fe8Nx and α''-Fe16N2 samples. α'-Fe8Nx thin films are deposited in the full range of 0 ≤ x ≤ 1. The nitrogen incorporation leads to a gradually induced tetragonal unit cell expansion of the compounds which is accompanied by an increase in the magnetic moment, reaching 2.50 ± 0.09μB per Fe atom at 10 K. The origin of the increased magnetic moment is solely the lattice expansion. The uniaxial anisotropy constant increases with c/a ratio (or resp. nitrogen content) reaching a value of 0.54MJm3 for c/a ≈1.1. The interstitial N atoms play a decisive role in stabilizing the enhanced perpendicular magnetocrystalline anisotropy. These findings can be generalized to other nitrogen containing interstitial Fe alloys. The second major activity is the development of a novel route with a high-pressure hydrogen reduction step for the synthesis of α''-Fe16N2 nanoparticles. With this route, phase-pure α''-Fe16N2 nanoparticles are successfully synthesized and characterized. The Ms(0) for α''-Fe16N2 nanoparticles is found to be 215Am2kg-1 and coercivity μ0Hc = 0.22T. Fe-O shells form around the particles when exposed to atmosphere which leads to a reduced magnetization. Overall the Fe8Nx alloys are shown to possess semi-hard magnetic properties as well as relatively poor phase stability, which has direct consequences on applications, such as bulk permanent magnets, nanocomposites and magnetic nanoparticle hyperthermia

Dry sliding behavior od filled PDC coatings applied onto surface modified sintered steel

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 2016.A transformação direta de polímeros precursores em cerâmicas covalentes, via termólise no estado sólido, foi proposta há mais de 30 anos e desde então permitiu avanços tecnológicos significativos na ciência e tecnologia cerâmica. Entre eles, a redução do coeficiente de atrito (µ) de substratos metálicos e cerâmicos com o uso de revestimentos amorfos de carbonitreto de silício (SiCN), produzidos por meio de cerâmicas derivadas de polímeros (PDC). Porém, através de rotas de processamento tecnologicamente sofisticadas, enquanto neste trabalho a viabilidade de processamento desses revestimentos sobre um aço baixa liga sinterizado, por pulverização de suspensões e pirólise em forno, foi explorada. Para compensar a retração do polímero precursor, evitar a formação de trincas e produzir revestimentos auto-lubrificantes para avaliações tribológicas de deslizamento alternado a seco, cargas inertes foram adicionadas a solução do poli(organo)silazano, especificamente nitreto de silício (Si3N4, 30 ou 65% em volume) e nitreto de boro hexagonal (h-BN, 9% em volume). O Si3N4 em pó foi utilizado para reforçar a matriz (SiCN) e reduzir o desgaste, pela formação de uma rede agregada pela fase do ligante precursor e, h-BN para gerar um efeito lubrificante na interface reduzindo o atrito, o desgaste e o aquecimento por atrito. A fim de acomodar as diferenças de propriedades, originadas pela natureza das ligações químicas do substrato metálico e revestimento cerâmico, a tecnologia de plasma de corrente contínua (DC) foi utilizada para modificar as superfícies do aço sinterizado. A nitretação ou o enriquecimento de superfície com Mo seguido de nitretação, tinham o intuito de gerar uma camada de compostos na superfície do substrato com uma zona de difusão gradiente abaixo, de modo a aumentar o suporte de carga e evitar a fratura do revestimento pelo carregamento do contra corpo. A nitretação resultou em um aumento na dureza da superfície (600HV) do aço por precipitação de nitretos de ferro metaestáveis, enquanto o enriquecimento com Mo resultou em carbetos (Fe3Mo3C) dispersos na superfície do substrato. Os últimos, durante a nitretação evoluem para uma camada descontínua de carbonitreto (Fe3Mo3(C,N)), que contribuíram para o aumento da dureza na superfície (800HV). Foi possível produzir revestimentos PDC em amostras sinterizadas com espessura de até 10µm e com adequado preenchimento dos poros na superfície, independente dos teores de carga avaliada (39 ou 74 vol.%). Além disso, a combinação de 65% em volume de Si3N4 e 9% em volume de h-BN resultou em revestimentos cerâmicos homogêneos, sem macro trincas ou falhas coesivas após o processamento sobre o aço sinterizado, independente das modificações de superfície. Conforme projetado para o estudo, após o tratamento térmico de pirólise os precipitados Fe3Mo3(C,N) não foram totalmente decompostos como os nitretos de ferro, apesar da ausência do endurecimento de superfície. O controle da microestrutura e composição química finais são vantagens inerentes aos processos de manufatura utilizados e foram explorados para projetar a microestrutura final dos compósitos. Porém, apesar da utilização de um teor aumentado de cargas passivas ter possibilitado a obtenção de revestimentos cerâmicos sem trincas sobre substratos sinterizados, os compósitos não apresentaram uma resposta tribológica satisfatória, quando avaliados nas condições testadas. No regime permanente, o coeficiente de atrito médio (0,4 a 0,5) dos compósitos permaneceu acima do regime de lubricidade (µAbstract : The directly transformation of organosilicon polymers into covalent ceramics, via solid-state thermolysis, was proposed over 30 years ago, and since then enabled significant technological breakthroughs in ceramic science and technology. Among them, the friction coefficient (µ) reduction of metallic and ceramic substrates with use of amorphous silicon carbonitride (SiCN) coatings, produced by the polymer derived ceramic route (PDC). Nevertheless, technologically sophisticated processing steps are used to produce them, while in this work, the possibility to process such coatings onto sintered low alloy steel samples by simple spraying of poly(organo)silazane suspensions and oven pyrolysis was explored. In order to compensate the precursor shrinkage, avoid crack formation and produce hard and self-lubricating coatings for reciprocating dry sliding evaluations, inert fillers namely silicon nitride (30 or 65 vol.%) and hexagonal-boron nitride (9 vol.%) were added to the polymer precursor. Powdered Si3N4 was used to strengthen the matrix and reduce wear, by formation of a network aggregated by the precursor binder phase, and h-BN to generate a lubricating effect on the interface reducing friction, wear and friction heating. To accommodate dissimilarities of properties, originated from different chemical bonds of the metallic substrate and ceramic coating, the direct current (DC) plasma technology was utilized to modify the surface of the sintered steel. Nitriding or surface enrichment with Mo followed by nitriding was expected to generate a compound layer at the surface of the substrate with a gradient diffusion region below, in order to increase the load support and avoid coating fracture from the counterface load. Nitriding led to an increase in surface hardness of steel (600HV) due to precipitation of metastable iron nitrides, while surface enrichment resulted in dispersed carbides (Fe3Mo3C) onto the substrate surface. The latter evolved to a discontinuous carbonitride layer (Fe3Mo3(C,N)) during nitriding, which increased the surface hardness (800HV). It was possible to produce PDC coatings with thickness below 10µm and with proper filling into substrate surface pores, regardless of total filler amount evaluated (39 or 74 vol.%). Moreover, the combination of 65 vol.% of Si3N4 and 9vol% of h-BN resulted in homogeneous ceramic coatings, free of macro cracks and cohesive failure after processing onto sintered substrates, independent of surface modification. As desired for evaluating, after pyrolysis the Fe3Mo3(C,N) precipitates were not totally decomposed as the iron nitrides, despite the surface hardening absence. The high control of final microstructure and chemical composition are competitive advantages inherent to cited manufacturing processes, explored to project the final microstructures of the steel, interfaces and ceramic coatings involved. However; despite the use of an increased amount of filler (74 vol.%) allowed the successful obtainment of homogeneous and crack-free ceramic coatings, the composites did not presented a satisfactory tribological response when evaluated under tested conditions. The average steady state friction coefficient of composites (0.4 to 0.5) were above the lubricious regime (µ<0.2), regardless of coating composition or surface modification. The maximum contact pressure (0.53GPa) caused failure of coatings, which were in sequence deformed, comminuted and oxidized by substrate and counter-body during sliding, revealing a tribo-chemical wear character. The major contribution to the friction coefficient behavior was from the substrate but controlled by the formed third-body, composed of oxygen and chemical elements from substrate, coatings and counter-body. The third-body presence, indirectly indicated by the contact electrical resistance, increased the wear rates of composites (1.5 to 4.2x10-4.mm3.N-1.m-1) and counter-bodies (7.0 to 8.7x10-5.mm3.N-1.m-1) in relation to the sintered steel tested under same conditions (1 and 0.1x10-4.mm3.N-1.m-1 respectively). The wear rates of composites were further increased with the raise of Si3N4 amount in the coating composition, and with the presence of hard ternary carbonitrides at the substrate surface

The Use of Nitriding to Enhance Wear Resistance of Cast Irons and 4140 Steel

This research is focused on using nitriding to enhance the wear resistance of austempered ductile iron (ADI), ductile iron (DI), and gray iron (GI), and 4140 steel. Three gas nitriding processes, namely Gas nitriding + nitrogen cooled down to 800°F (Blue) , Gas nitriding + cooled down to 300°F (Gray) , and Gas nitriding + oil quenched (Oil) were used for the cast irons. Three salt bath nitriding processes, namely Isonite, QP (Quench, Polish) and QPQ (Quench, Polish, Quench) were used for the 4140 steel. This study was carried out through optical metallography, roughness measurements, microhardness, and SEM. The ball-on-disc wear tests were conducted under lubricated conditions. It was found that COF for all materials in all nitrided conditions was small (\u3c 0.045). The best wear performance was seen for ADI processed using the Gray and Oil gas nitriding processes. For the 4140 steel, The surface microhardness of the ISONITE specimen was around 1400HV. QP and QPQ processes produce a surface microhardness of 2000-2200HV, which suggests that they may show improved wear behaviour compared to ISONITE- treated steels