Erlangen, Nürnberg, Univ., Diss., 2002. / Dateien im PDF-Format. Computerdatei im Fernzugriff.

Erlangen, Nürnberg, University, Diss., 2002. / Dateien im PDF-Format.

Metal-ceramic (Cermets) materials that combine properties of both: high hardness, high wear resistance, and high working temperatures of ceramics and the ductility, toughness, and heat conductance of metals. Cold gas dynamic spraying, or simply cold spray, is a solid state thermal spray process that has been in development for the last 25 years. In the cold spray process, ductile materials are accelerated in a supersonic flow. These particles impact a substrate and adhere by plastic deformation. The continuous accumulation of these particles covers the substrate and creates a dense coating. The cold spray process is beginning to become a popular method to consolidate some select cermet materials into coatings. This technique can be advantageous when an erosion and wear resistant coating is required. During the deposition of these coatings, researchers have shown that the ceramic particles have a dramatic influence on the deposition behavior by causing an increase in deposition efficiency and coating adhesion. These effects have been presented in several experiments but have yet to be thoroughly explained. The goal of this investigation is to increase the knowledge, on a fundamental level, with regards to the deposition behavior of metal-ceramic blending and cermet powders. Ultimately, the focus is to prove the feasibility of these coatings for the requirements needed in the engineering industry. The first part of the investigation is a fundamental study on the deposition behavior of metal-ceramic blends with different compositions. Three theories that aim to explain the increase in deposition efficiency were proposed in the literature and further investigated in this study. One proposed mechanism for the increase in deposition efficiency was established by probability analysis to be too unlikely to contribute to the increment in deposition efficiency. The other two proposed mechanism, the presence of asperities caused by ceramic particles, and the oxide removal produced by the impact of ceramic particles, shown to play a major role in increasing the deposition efficiency. The effect of the ceramic particle morphology on the deposition behavior of metal-ceramic blending was studied in the second part of the investigation. This study greatly complements the previous one adding more depth to the investigation and confirming results. The increment in deposition efficiency normally seen with the addition of small amounts of angular alumina was not seen when spherical alumina was added instead. The creation of asperities during deposition was explored for the two morphologies and was determined that spherical alumina does not produce the same asperities at the surface. In addition, the coating sprayed with spherical alumina showed very little ceramic retention compared with the ones sprayed with angular alumina. These results have a direct impact on the mechanical properties of the coatings. Wear resistance for coatings sprayed with spherical alumina showed no improvement compared with pure aluminum coating due to the low ceramic content. Hardness was lower in coatings sprayed with spherical alumina for the same feedstock powder composition but was harder when the final coating composition was considered. Adhesion strength significantly increases with the addition of ceramic content in the feedstock powder; this increase was greater for coating sprayed with spherical alumina. The third part of the investigation focuses on understanding the mechanism of deposition for cermet particles with various morphologies. Six commercially available CrC-NiCr powders were studied, varying in morphology and metal/ceramic ratio. Spherical powders led to the erosion of the substrate and no coating was formed. Porous agglomerated and sintered powder lead to severely cracked coatings. For dense agglomerated and sintered powders, the outcome of powder depended on the initial metal/ceramic ratio, powders with 25%wt.NiCr led to erosion while 35%wt.NiCr powders led to a dense coating. Finally blended ceramic metal powders also lead to a successful coatings. All coatings obtained had lower ceramic content than the initial feedstock powder. Interrupted deposition tests, FEA analysis, and SEM observation were used to draw conclusions on the deposition behavior and explain the results. Finally, the last part of this investigation aims to apply the knowledge learned to an applied engineering problem. The problem that is targeted is the replacement of chrome plating for the aerospace industry. A commercially available cermet powder CrC-NiCr (65/35) was proposed as a replacement of chromium plating as well as a restoration for this coating and its alternatives (electroless nickel-plating, and WC-Co-Cr HVOF). The coatings and restoration were analyzed by SEM and tested by strip rupture rest, neutral salt spray fog, and fluid immersion testing. The adhesion strength, porosity, and hardness of the cold spray coating was also tested. The deposition and restoration of coatings were successful; a hard and dense coating was obtained with good adhesion strength. The process of restoration chromium-plating and its alternatives was also developed with a clean interface was achieved in each case. Coatings and restoration passed strip to rupture rest as well as fluid immersion test in two selected industry fluids. Neutral salt spray fog test revealed that the cold spray coating and repairs may have a path that allows the solution to penetrate the substrate and start the corrosion process. This behavior was found in a few select spots and should be further investigated. Overall, the coating proved to have potential as an alternative of chromium-plating or to restore damaged hard coatings.

This master thesis includes a literature study and experimental work to understandthe conditions where a binder phase layer, Co-capping, could be produced orinhibited, for three different cermet grades in order to suggest changes in thesintering processes and two production units. The effect of C activity and sinteringatmosphere, e.g. flow rate, pressure were investigated. The results show that the Co-capping occurs on the cooling stage, when the binder phase, Co, solidifies. Co-capping could be inhibited by using a high C activity and high pressure (50 bar). Itwas further found that Co-capping could be evaporated using low pressure, i.e. vacuum, which has not been discussed in earlier studies on Co-capping. Evaporation was also found to have a relation with the solidification temperature of the grades,since grades with higher solidification temperature get Co-capping at a highertemperature, which consequently will be exposed to higher temperatures.

A obtenção do Cermet de níquel-zircônia via moagem de alta energia (Mechanical Alloying MA) foi estudado visando a preparação de anodos de células a combustível de óxido sólido (SOFC). O níquel metálico foi adicionado em três concentrações: 30, 40 e 50% em volume. As operações de moagem foram conduzidas em moinho vibratório de alta energia do tipo SPEX. Estudou-se a influência do tempo de moagem, a eficiência de aditivos para controle do processo, tipo e geometria dos potes de moagem. A influência destas variáveis foram avaliadas através de análises de tamanho de partículas, determinação de área superficial e morfologia do material resultante. O uso de pote de teflon resultou em contaminação por carbono. Por outro lado, o uso de pote de aço aumenta a contaminação por impurezas metálicas. As diversas geometrias projetadas para os potes mostraram que potes com maiores raios de concordância (R.15) apresentaram melhor rendimento. Após a conformação e sinterização a 1300°C em atmosfera de argônio, as amostras apresentaram valores de densidade entre 60 a 80% da densidade teórica. As microestruturas observadas por microscopia eletrônica de varredura revelaram uma boa homogeneidade na distribuição de fases do Cermet. A técnica de moagem de alta energia apresentou-se como boa opção na fabricação de Cermet Ni-ZrO2. / The ZrO2 and metallic Ni Cermet obtained by Mechanical Alloying MA is studied in the present work with the objective to prepare solid oxide fuel cells anodes (SOFC). Metallic Ni is added under three different concentrations: 30, 40 and 50% volume. The millings were conducted in SPEX vibratory mill where the influence of milling time, process control additives efficiency, type and geometry of milling vessels were studied. The study of the influence of these variables was made under particle size analysis, surface area determination and resulting material morphology. The use of teflon vessel causes contamination by carbon. On the other side, steel vessel increases the contamination by metallic impurities. The several geometries projected and analyzed for the vessels showed that vessels with larger bottom radius (R.15) showed the best results. After conformation and sintering at 1300°C in argon atmosphere the samples reached densities between 60 and 80% of the theoretical density. Microstructures observed by scanning electron microscopy reveal good homogeneity in the Cermet phases distribution. The mechanical alloying technique was considered a good option to obtain Ni- ZrO2 Cermet.

A obtenção do Cermet de níquel-zircônia via moagem de alta energia (Mechanical Alloying MA) foi estudado visando a preparação de anodos de células a combustível de óxido sólido (SOFC). O níquel metálico foi adicionado em três concentrações: 30, 40 e 50% em volume. As operações de moagem foram conduzidas em moinho vibratório de alta energia do tipo SPEX. Estudou-se a influência do tempo de moagem, a eficiência de aditivos para controle do processo, tipo e geometria dos potes de moagem. A influência destas variáveis foram avaliadas através de análises de tamanho de partículas, determinação de área superficial e morfologia do material resultante. O uso de pote de teflon resultou em contaminação por carbono. Por outro lado, o uso de pote de aço aumenta a contaminação por impurezas metálicas. As diversas geometrias projetadas para os potes mostraram que potes com maiores raios de concordância (R.15) apresentaram melhor rendimento. Após a conformação e sinterização a 1300°C em atmosfera de argônio, as amostras apresentaram valores de densidade entre 60 a 80% da densidade teórica. As microestruturas observadas por microscopia eletrônica de varredura revelaram uma boa homogeneidade na distribuição de fases do Cermet. A técnica de moagem de alta energia apresentou-se como boa opção na fabricação de Cermet Ni-ZrO2. / The ZrO2 and metallic Ni Cermet obtained by Mechanical Alloying MA is studied in the present work with the objective to prepare solid oxide fuel cells anodes (SOFC). Metallic Ni is added under three different concentrations: 30, 40 and 50% volume. The millings were conducted in SPEX vibratory mill where the influence of milling time, process control additives efficiency, type and geometry of milling vessels were studied. The study of the influence of these variables was made under particle size analysis, surface area determination and resulting material morphology. The use of teflon vessel causes contamination by carbon. On the other side, steel vessel increases the contamination by metallic impurities. The several geometries projected and analyzed for the vessels showed that vessels with larger bottom radius (R.15) showed the best results. After conformation and sintering at 1300°C in argon atmosphere the samples reached densities between 60 and 80% of the theoretical density. Microstructures observed by scanning electron microscopy reveal good homogeneity in the Cermet phases distribution. The mechanical alloying technique was considered a good option to obtain Ni- ZrO2 Cermet.

No description available.

This thesis focuses on comparison of cermet cutting materials, which are used to manufacture reamer cutting edges. It contains review of current market reamer tools and suppliers of cermet semi-finished products. Cermet cutting tips were used to machine parts from carbon steel and analysis was made to compare wearing of the tools from different suppliers. This test was conducted on turning machine in working conditions typical for reaming. During this test there were taken notes on the wear of the tools and surface roughness of the part. The output of this is work is data processing, which indicates durability of each tested cermet.

A series of cermet powders of composition NixCo(1-x)O-YSZ were synthesized for testing as cermet anode materials for SOFCs. The Co is found by powder XRD to become incorporated into the crystal lattice of the NiO, thus forming a true alloyed material. SEM and EDS results show two types of particles upon sintering to 1380oC: small, amorphous particles of YSZ and large, crystalline particles of nickel. The electrochemical oxidation of hydrogen on a cermet anode composed of Ni0.7Co0.3O-YSZ was investigated using a series of many button cells. Through EIS data, cyclic voltammetry data, the exchange current densities for these button cells were determined. Although a relatively large variation was found (expected to be due to microstructural variation) the average values for both methods of measurement is in good agreement in hydrogen. Following reduction in pure hydrogen, the fuel was changed to a mixture with high concentration of H2S. It was found that a concentration of 10 % H2S/H2 produced a sudden change in anode microstructure and resulted in loss of exchange current density. Lowering the amount of H2S in the initial fuel feed, which allowed for a more gradual microstructural change, allowed the cell to eventually function at concentrations in excess of 10 % H2S/H2. It was determined by OCV values in various concentrations of H2S/H2 that hydrogen is the predominant fuel of choice, even if H2S is available. Following electrochemical testing, slow cooling in a 10 % H2S/H2 mixture following produced metal sulfide spheres, as determined by SEM and EDS. Investigation in hydrocarbon, alcohol and biodiesel fuels was then undertaken to test the fuel variability of the given cermet anode material. Methane containing 10 % H2S was found to have increased exchange current density relative to poisoned hydrogen. Ethane and biodiesel experienced no increase in exchange current density, but a lengthening of the functional lifetime of the cell was observed, indicating reduced carbon poisoning. Methanol is a promising oxygen-containing SOFC fuel since it produced exchange current density values larger than hydrogen, and showed no evidence of coke formation by post-mortem SEM. Since oxygen-containing fuels are known to decompose in the gas phase at typical SOFC operating temperatures, the performance in a mixture of various CO/H2 fuels was then investigated. The Ni0.7Co0.3O-YSZ cermet anode gave higher exchange current density values for low ratio of CO/H2 fuels in the range 20/80 and 30/70 compared to pure H2. This is the first example of a Ni-based anode providing higher performance with a CO/H2 mixed fuel than for a pure H2 fuel. Finally, continuous running of a cell with fuel ratio 25/75 CO/H2 for 7 days produced exchange current density values, which were observed to increase significantly above the values for pure H2 during days 1-4 followed by deterioration below the value for hydrogen on subsequent days.

A series of cermet powders of composition NixCo(1-x)O-YSZ were synthesized for testing as cermet anode materials for SOFCs. The Co is found by powder XRD to become incorporated into the crystal lattice of the NiO, thus forming a true alloyed material. SEM and EDS results show two types of particles upon sintering to 1380oC: small, amorphous particles of YSZ and large, crystalline particles of nickel. The electrochemical oxidation of hydrogen on a cermet anode composed of Ni0.7Co0.3O-YSZ was investigated using a series of many button cells. Through EIS data, cyclic voltammetry data, the exchange current densities for these button cells were determined. Although a relatively large variation was found (expected to be due to microstructural variation) the average values for both methods of measurement is in good agreement in hydrogen. Following reduction in pure hydrogen, the fuel was changed to a mixture with high concentration of H2S. It was found that a concentration of 10 % H2S/H2 produced a sudden change in anode microstructure and resulted in loss of exchange current density. Lowering the amount of H2S in the initial fuel feed, which allowed for a more gradual microstructural change, allowed the cell to eventually function at concentrations in excess of 10 % H2S/H2. It was determined by OCV values in various concentrations of H2S/H2 that hydrogen is the predominant fuel of choice, even if H2S is available. Following electrochemical testing, slow cooling in a 10 % H2S/H2 mixture following produced metal sulfide spheres, as determined by SEM and EDS. Investigation in hydrocarbon, alcohol and biodiesel fuels was then undertaken to test the fuel variability of the given cermet anode material. Methane containing 10 % H2S was found to have increased exchange current density relative to poisoned hydrogen. Ethane and biodiesel experienced no increase in exchange current density, but a lengthening of the functional lifetime of the cell was observed, indicating reduced carbon poisoning. Methanol is a promising oxygen-containing SOFC fuel since it produced exchange current density values larger than hydrogen, and showed no evidence of coke formation by post-mortem SEM. Since oxygen-containing fuels are known to decompose in the gas phase at typical SOFC operating temperatures, the performance in a mixture of various CO/H2 fuels was then investigated. The Ni0.7Co0.3O-YSZ cermet anode gave higher exchange current density values for low ratio of CO/H2 fuels in the range 20/80 and 30/70 compared to pure H2. This is the first example of a Ni-based anode providing higher performance with a CO/H2 mixed fuel than for a pure H2 fuel. Finally, continuous running of a cell with fuel ratio 25/75 CO/H2 for 7 days produced exchange current density values, which were observed to increase significantly above the values for pure H2 during days 1-4 followed by deterioration below the value for hydrogen on subsequent days.