Dissertations / Theses on the topic 'CRC32'

The master's thesis deals with encoding payment data invoices into QR codes in accordance with the by square standard. The SAP system along with a process of encoding data into QR codes and by square PAY standard is introduced in my thesis. In compliance with the standard function module is designed for the SAP system which is in the ABAB language, using Smartforms form allowing the QR codes to generate the invoice. The module is then subjected to testing. The results are evaluated in conclusion of the master's thesis and a description of potential further development is assessed as a final step.

O presente estudo está dividido em duas partes. A primeira parte está relacionada à preparação de pós de Cr3C2-25(Ni20Cr) nanoestruturados através do processo de moagem de alta energia, bem como à caracterização dos pós moídos e no estado como recebido. A análise dos dados obtidos nesta etapa do trabalho foi feita utilizando-se uma abordagem essencialmente teórica. A segunda parte deste estudo refere-se à produção e caracterização de revestimentos preparados com os pós de Cr3C2-25(Ni20Cr) nanoestruturados e como recebido. O comportamento destes revestimentos sob erosão-oxidação em alta temperatura foi comparado com base em uma abordagem de caráter mais tecnológico. O tamanho médio de cristalito do pó de Cr3C2-25(Ni20Cr) decresceu rapidamente de 145 nm para 50 nm nos estágios iniciais de moagem e, posteriormente, com o aumento do tempo de moagem, decresceu mais lentamente até atingir um estado estacionário para um tamanho de cristalito em torno de 10 nm. Este estado estacionário corresponde ao início do processo de recuperação dinâmica. A máxima deformação da rede cristalina (δ = 1,17%) foi observada para pós moídos por 16 horas, caracterizando um tamanho crítico de cristalito da ordem de 28 nm. Por outro lado, o parâmetro de rede atingiu um mínimo para pós moídos por 16 horas. Após atingir o tamanho crítico de cristalito, a densidade de discordâncias praticamente não mais varia (estado estacionário) e toda deformação plástica posteriormente introduzida no material é acomodada através de eventos que ocorrem nos contornos de grão, particularmente por meio do processo designado deslizamento de contorno de grão (grain boundary sliding). A energia de deformação armazenada na rede cristalina dos pós de Cr3C2-25(Ni20Cr) moídos com diferentes tempos de moagem foi determinada por meio de medidas da variação de entalpia. Estes resultados indicaram que a máxima variação de entalpia (ΔH = 722 mcal) também ocorreu para pós moídos por 16 horas. Analogamente, a máxima variação do calor específico (ΔCp = 0,278 cal/gK) ocorreu para pós moídos por 16 horas. As seguintes propriedades mecânicas dos revestimentos de Cr3C2-25(Ni20Cr), preparados utilizando-se o processo HVOF de aspersão térmica, foram determinadas: microdureza Vickers, módulo de Young e tenacidade à fratura. As propriedades dos revestimentos preparados com os pós nanoestruturados e como recebido foram comparadas. A dureza e o módulo de Young dos revestimentos preparados com os pós nanoestruturados foram aproximadamente 26% maiores que aqueles preparados com os pós como recebido. A tenacidade à fratura dos revestimentos nanoestruturados foi aproximadamente 36% maior do que o verificado para os revestimentos produzidos com pós no estado como recebido. A resistência à erosão-oxidação do revestimento produzido com o pó nanoestruturado foi em torno de 52% maior do que a do revestimento preparado com o pó no estado como recebido, a 800ºC. Ambos os revestimentos mostraram um aumento da taxa de erosão-oxidação para temperaturas acima de 450ºC.
This study is divided in two parts. The first part is about the preparation of nanostructured Cr3C2-25(Ni20Cr) powders by high energy milling followed by characterization of the milled and the as received powder. Analyses of some of the data obtained were done using a theoretical approach. The second part of this study is about the preparation and characterization of coatings prepared with the nanostructured as well as the as received Cr3C2-25(Ni20Cr) powders. The high temperature erosion-oxidation (E-O) behavior of the coatings prepared with the two types of powders has been compared based on a technological approach. The average crystallite size of the Cr3C2-25(Ni20Cr) powder decreased rapidly from 145 nm to 50 nm in the initial stages of milling and thereafter decreased slowly to a steady state value of around 10 nm with further increase in milling time. This steady state corresponds to the beginning of a dynamic recovery process. The maximum lattice strain (δ = 1,17%) was observed in powders milled for 16 hours, and this powders critical crystallite size was 28 nm. In contrast, the lattice parameter attained a minimum for powders milled for 16 hours. Upon reaching the critical crystallite size, the dislocation density attained a steady state regime and all plastic deformation introduced in the material there after was in the form of events occurring at the grain boundaries, due mainly to grain boundary sliding. The deformation energy stored in the crystal lattice of the Cr3C2-25(Ni20Cr) powders milled for different times was determined from enthalpy variation measurements. These results indicated that the maximum enthalpy variation (ΔH = 722 mcal) also occurred for powders milled for 16 hours. In a similar manner, the maximum specific heat variation (ΔCp = 0,278 cal/gK) occurred for powders milled for 16 hours. The following mechanical properties of Cr3C2-25(Ni20Cr) coatings prepared using the HVOF thermal spray process were determined: Vickers micro-hardness, the Young Modulus and the fracture toughness. The properties of the coatings prepared with the nanostructured and the as received powders were compared. The hardness and Young Modulus of the coatings prepared with nanostructured powders were approximately 26% higher than that of the coatings prepared with as received powders. The fracture toughness of the nanostructured coating was 36% higher. The erosion-oxidation resistance of the coating produced with the nanostructured powder was around 52% higher than that of the coating prepared with the as received powders at 800 ºC. The E-O wastage of both types of coatings increased with temperature beyond 450 ºC.

This thesis investigates the mechanisms of erosion-corrosion of Cr3C2-NiCr thermal spray coatings under high temperature, high erodent velocity, turbine conditions. Erosion-corrosion is a generalised wear phenomenon where the combined effect of each degradation mechanism generates more extensive mass loss than the sum of each mechanism acting independently. Previous research has highlighted several theoretical mechanisms under this generalized process, ranging from the erosion induced breakdown of oxide scales in corrosive environments, through to the development of oxide layers in highly erosive environments. Prior to this current work experimental simulation of these mechanisms has focused on bulk alloy materials with well characterised oxidation responses, under conditions of low temperature, low erodent impact velocity and high erodent flux, conditions which are readily generated within laboratory scale rigs and which tend towards the low impact energy conditions encountered within fluidised bed combustors. Few works have addressed erosion-corrosion under simulated turbine conditions of high temperature, high erodent impact velocity and low erodent flux. While comparative trials have been run under such conditions on a purely mass loss basis, little has been presented regarding the microstructural analysis of such degradation, particularly for materials that rely on the industrially relevant, slow growing oxide scales Cr2O3 and Al2O3. Thermally sprayed Cr3C2-NiCr coatings are routinely applied to combat wear at high temperature due to the high wear resistance imparted by the hard carbide particles and the high temperature oxidation resistant nature of the Cr2O3 oxide formed over both phases. However, most published work characterising the erosion-corrosion response of these coatings has been conducted on a comparative basis by contrasting coatings of various composition ratios, deposited by various techniques, with the response of well characterised bulk materials. Little has been presented on the microstructural mechanism of erosioncorrosion of Cr3C2-NiCr coatings, a point highlighted by the limited understanding of the oxidation mechanism of the Cr3C2 phase, the oxidation mechanisms of the combined composite Cr3C2-NiCr and the influence of the coating splat structure on the oxidation response. While the erosion response of thermal spray coatings and bulk cermets is more widely understood, most works have been conducted under milder conditions than used in the current work. In addition previous works have been conducted primarily on assprayed coatings with few works taking into account the effect of heat treatment induced changes in the coating composition and microstructure that occur with extended in-service exposure at elevated temperature. In addressing the short comings in the current state of knowledge, the aim of this work was to characterise the mechanism of erosion-corrosion of high velocity sprayed Cr3C2-NiCr thermal spray coatings under turbine conditions, incorporating the effect of variation in the composition and carbide distribution with inflight degradation, variations in starting powder morphology, heat treatment, erosion conditions and exposure temperature. 75 Cr3C2-25(Ni20Cr) coatings were deposited by Aerospray HVAF, GMA Microjet HVOF, Stellite Jet Kote HVOF and TAFA JP-5000 HVOF spraying under optimised conditions using agglomerated/sintered and blended powders. The prealloyed powder based coatings, characterised in terms of microhardness, porosity content and phase degradation, were found to exceed the average values of coating quality presented in the literature. The blended powder based coating of this work was comparable with the coating attributes presented in the literature for plasma and HVOF coatings based on this powder morphology. Based on these results the coatings were considered representative of those sprayed industrially and therefore the responses of the samples in this work to oxidation and erosion were considered indicative of the response of industrially applied coatings of this composition in service. Heat treatment trials were conducted on the Aerospray HVAF and Microjet HVOF coatings at 900ºC in air and argon for up to 60 days to simulate the compositional and microstructural development of these coatings under elevated temperature conditions in service. In the prealloyed powder based coatings, rapid carbide precipitation occurred within the first two days in both coatings to reach the steady state composition of 75-80vol%. Minimal in-flight carbide dissolution in the HVAF coating led to preferential carbide precipitation on the retained carbide grains. In the Microjet HVOF coatings, which suffered extensive inflight carbide dissolution, carbide precipitation occurred as fine precipitates in the carbide-free zones, forming large sponge-like agglomerates. With extended exposure Ostwald ripening led to coarsening of the individual carbide grain size and widespread agglomeration of the carbide grains into an extensive three dimensional network after 30 days exposure, with minimal development out to 60 days. Compositionally, heat treatment led to a dramatic reduction in the supersaturated matrix phase Cr content, with the steady state Cr composition of the Microjet HVOF coating exceeding that of the Aerospray HVAF coating based on XRD analysis. Cr3C2 was the only carbide detected with heat treatment. Heat treatment of the blended powder based coating led to sintering of the single phase splats. Diffusion of the carbide elements into the matrix phase splats occurred, allowing fingers and nodules of the carbide to develop into this phase as well as increasing the matrix phase Cr concentration. Oxidation of Cr3C2 by hot stage XRD analysis at 600ºC, combined with TGA analysis at 600-850ºC, supported the mechanism of stepwise decarburisation prior to Cr2O3 formation, presented as one possible mechanism in the literature. Oxidation of the Cr3C2-NiCr coatings over the range 700-850ºC was dependent on the starting powder morphology and the extent of dissolved carbide in the matrix phase. Oxidation of the as-sprayed prealloyed powder based coatings was dictated by the matrix phase, the high Cr concentration resulting from carbide dissolution leading to rapid growth of the Cr2O3 phase over the oxidising carbide grains. Growth stresses induced by such overgrowth lead to the formation of interfacial voids over the carbide grains at high temperature. Heat treatment reduced the matrix phase Cr concentration, resulting in the coating phases oxidising independently with a reduced magnitude of lateral matrix based scale growth over the carbide phase. In the blended powder based coating, bulbous Ni oxides dominated the scale topography. With extended exposure a continuous Cr2O3 scale formed below the faster growing Ni oxides, which enabled lateral growth of the scale over the carbide based Cr2O3 scales. Following heat treatment the matrix phase Cr concentration increased, minimising the development of Ni oxides on this phase. Erosion studies were carried out in a custom built high temperature erosion apparatus. Ambient temperature trials were conducted using Al2O3 erodent at velocities of 150m/s. The as-sprayed prealloyed powder based coatings exhibited a brittle impact response, which was accentuated in the Microjet HVOF coating by the increased extent of in-flight carbide dissolution and the splat structure which made this sample more susceptible to brittle erosion mechanisms. Heat treatment of these coatings led to sintering of the splats and a more ductile impact response due to the increased ductility of the matrix phase. The as-sprayed blended powder based coatings exhibited a range of impact responses from brittle erosion of the carbide through to ductile erosion of the matrix based splats. Mass loss was accentuated by the poor intersplat adhesive strength. Heat treatment led to sintering of the splats, resulting in a more microstructural based erosion response. The two prealloyed powder based coatings generated similar erosion rates under the aggressive conditions, distinctly more erosion resistant than the blended powder based coatings. Heat treatment improved the erosion resistance of all the coatings, however, the duration of heat treatment had a negligible effect on the magnitude of erosive mass loss. Erosion at 800ºC, with an impact velocity of 235m/s, lead to significantly deeper erodent penetration into the coating than noted at ambient temperature. The significant increase in the matrix phase ductility at elevated temperature minimized the impact of carbide dissolution on the matrix impact response in the prealloyed powder based coatings. The primary effect of carbide dissolution was to reduce the carbide concentration, allowing deformation of the matrix to dictate the coating response. Carbide development with heat treatment significantly reduced the ability of the matrix to deform in this manner. The increased matrix phase ductility in the blended powder based coating reduced the concentration of impact energy on the splat boundaries, leading to a more microstructural based erosion response. Heat treatment had a negligible effect on the coating response, given the reduced significance of the splat boundary adhesion. Erosion at 700ºC generated similar erosion responses in the prealloyed powder based coatings to those noted at 800ºC, the lower matrix phase ductility reflected in the more brittle response evident as brittle cracking and fracture. The effect of carbide development with heat treatment was not as dramatic as at 800ºC due to the reduced matrix phase ductility at this temperature. Erosion of the blended powder based coating at 700ºC generated the same spectrum of erosion response as noted at 800ºC in both the as-sprayed and heat treated states, with the variation in matrix phase ductility with temperature overshadowed by the heterogeneous coating impact response resulting from the heterogeneous phase distribution. The steady state erosion rate at 700ºC was comparable across all of the coatings in both the as-sprayed and heat treated conditions. At 800ºC, heat treatment had a negative impact on the prealloyed powder based coatings, but no definitive effect on the blended powder based coatings. The Microjet HVOF coatings were more erosion resistant than the Aerospray HVAF coatings under these conditions. These results pointed to a reduction in the significance of the coating splat structure on the magnitude of erosion, in favour of a more microstructural based response at high temperature. In both the ambient and elevated temperature trials the coating microhardness values proved to be a poor indicator in predicting the magnitude or relative ranking of the erosion response of the different coatings under these aggressive erosion conditions. Erosion-corrosion under turbine conditions of high temperature, high erodent velocity and low erodent flux, was simulated by oxidizing samples at 900ºC and subjecting them to one second of erosion every 48 hours over a period of 60 days. The degradation testing was assessed in accelerated testing in additional trials by polishing the oxide scale formed at 900ºC from the sample surface every 48 hours over the same time period. Under these conditions the coatings formed thick oxide scales that penetrated into the coating. Preferential internal oxidation of the Cr3C2 phase occurred in the coating, consuming the grains in the near surface zone through the formation of Cr2O3 to a depth dependent upon the test temperature. Oxygen ingress occurred along the carbide-matrix interface and was accentuated in the regions of impact damage surrounding the erodent indentations. Internal oxidation of the carbide phase sealed off the pockets of matrix phase which were eventually consumed by oxidation once they were no longer able to maintain a protective Cr2O3 oxide. The extent of internal attack was consistent in the 20, 40 and 60 day samples, suggesting that the internal oxidation front proceeded at a constant rate in front of the erosion front into the coating. The prealloyed powder based coatings were more resistant to such internal degradation relative to the blended powder based coatings, with an internally oxidised zone of 6ìm relative to the 10ìm thick internally oxidized band in the blended powder based coating. While each erodent impact event may be classified as oxidation affected erosion, the low erodent flux effectively led to a long-term response more accurately described as erosion affected oxidation.

The Seamless Stressometer® roll (Seamless STR) is used to measure the flatness of aluminum and steel strip when there is an extreme demand on the surface finish. To protect the roll and strip, the roll is coated with two layers deposited by high velocity oxygen fueled spraying (HVOF), Cr-Ni(Si,B) closest to the roll and WC-Co on top. This solution has several disadvantages; high cost and complicated logistics, corrosion sensitivity and high residual stresses creates the need for two coatings which in turn complicates the process. Cobalt is, in addition, sensitive to low pH coolants and environmentally unfriendly. These problems have given rise to the idea of switching both the method and material of the coating. In the first part of this work, high velocity air fueled spraying (HVAF) was evaluated as an alternative deposition method. Three materials, Cr3C2-NiCr, WC-Co and WC-CrC-Ni were deposited on steel coupons with varying chamber pressure, powder feed rate and distance from the nozzle, in order to evaluate if HVAF can be a valid technique for use in this application and to optimize the spraying recipe. The objectives were to get a sufficiently high thickness per sweep, to be able to make the depositions in a manageable number of sweeps, and to get low porosity, since the coatings need to be dense to be hard and possible to polish smooth. The tests showed that all three materials can be sprayed with the high settings on the parameters to obtain coatings that exceeded the set limits of the objectives. In the second part of this work, the recipe obtained from the first part was used to deposit samples for further analysis. The coatings were compared regarding cost, hardness, friction, wear and pick-up properties to evaluate if a switch in material from WC-Co was possible. The coatings showed both similarities and differences. The friction was very similar for the three materials. Cr3C2-NiCr was substantially cheaper than the other two, had lower hardness and higher porosity, but still probably acceptable values, and was satisfactory regarding wear and pick-up. WC-Co and WC-CrC-Ni were very similar to each other regarding cost, hardness and porosity but WC-Co was the best regarding wear and pick-up, where WC-CrC-Ni was the worst. The only clear advantage of WC-CrC-Ni over WC-Co is the lack of cobalt. Taking everything into consideration, including the fact that the wear and pick-up tests in this work was quite exaggerated, Cr3C2-NiCr is an interesting option for this application due to its low cost and acceptable test results, WC-Co had the best results but is expensive and contains cobalt and WC-CrC-Ni had as good results as WC-Co except for the wear and pick-up tests and does not contain cobalt.

I pappersmaskiner används kräppblad för att kräppa av pappret från en stor roterande cylinder, en så kallad yankee-cylinder. Dessa blad utstår stora nötningar och beläggs därför med antingen en kromoxidbeläggning eller en hårdmetallsbeläggning för att öka bladets nötningsbeständighet. Bladen beläggs med termisk sprutning, kromoxiden via plasmasprutning och hårdmetallen via "High-Velocity-Oxy-Fuel"-sprutning (HVOF). I detta arbete har slitagetester utförts på belagda kräppblad genom att använda en "slider-on-flat-surface" (SOFS) tribometer för att ta reda på vilken av dessa beläggningar som är bäst lämpad för att användas på kräppblad. Vid användning av SOFS belastas kräppbladen med en normalkraft och glider mot en platta till önskad total glidsträcka är uppnådd. För att kunna genomföra slitagetesterna i SOFSen behövdes en fixtur konstrueras och tillverkas och kräppblad behövde kapas till önskad geometri. Det skapades tre koncept för fixturen och med en beslutsmatris gjordes valet vilken av dem som skulle tillverkas. Den tillverkade fixturen håller kräppbladen stabila i både sidled och i glidriktningen och håller kräppbladen i en vinkel likt i verkligheten. Slitagetesterna utfördes vid tre olika glidsträckor med konstant normallast. Efter slitagetesterna analyserades bladen i stereomikroskop och svepelektronmikroskop för att identifiera nötningsmekanismer. De dominerande nötningsmekanismerna var abrasiv och adhesiv nötning. På grund av porositet i beläggningarna ökades antalet sprickor och mer och mer fragment lossnade vid ökande glidsträcka under slitagetesterna. Det visade sig efter mätningar av bortnött bredd på bladet att det hårdmetallbelagda kräppbladet hade bäst nötningsbeständighet. Detta på grund av dess hårda tungstenkarbider som minskade den abrasiva nötningen och spricktillväxten i beläggningen och dess högre duktilitet jämfört med kromoxiden.
Creping blades are used in paper machines for creping tissue paper from a rotating cylinder called a yankee cylinder. These blades experience severe wear and are therefore coated by either chromium oxide or a hard metal coating to make the blades more wear resistant. The coatings are sprayed on to the blade by thermal spraying processes, the chromium oxide coating with plasma spraying and the hard metal coating with "High-Velocity-Oxy-Fuel" (HVOF) spraying. In this thesis wear testing of the coated creping blade using a "slider-on-flat-surface" (SOFS) tribometer has been done to find out which of these coatings that is best suited for coating on the creping blade. When using the SOFS, the creping blade is subjected to a chosen normal force and is scraped against a counterface for a set total distance. In order to execute the wear tests in SOFS, construction and manufacturing of a holder for the creping blades was needed and the creping blades had to be cut to correct geometry. Three concepts were created for the holder and the decicion about which one to manufacture was made with a decision matrix. The new holder holds the creping blades stable in side movements and in the sliding direction. It also keeps the blade at a set angle to the counterface as in the real application. The wear tests consisted of three sliding distances with constant normal load. The blades were analyzed in a stereo microscope and in a scanning electron microscope to identify the wear mechanisms when the wear tests were completed. The major wear mechanisms were abrasive and adhesive wear. Because of porosity in the coatings, the number of cracks increased and fragments from the blades came loose when increasing the sliding distance during the wear tests. It showed that by measuring the eroded width on the blades that the hard metal coated blade had the best wear resistance because of the hard tungsten carbides that lowered the abrasive wear and the crack growth in the coating and because of its higher ductility in comparison to chromium oxide.

Dans les procédés de projection thermique du type HVOF (High Velocity Oxygen Fuel), les propriétés du revêtement sont sensibles aux caractéristiques des particules en vol qui dépendent elles-mêmes des paramètres opératoires. En raison de la complexité des réactions chimiques et des processus thermodynamiques qui interviennent dans la formation du dépôt, l’obtention d’un modèle multi-physique complet et l'accès à une description quantitative du procédé HVOF représentent encore actuellement un défi scientifique majeur. L'étude réalisée a eu pour but de développer une méthodologie robuste reposant sur la méthodologie des réseaux de neurones artificiels (ANN), en vue de répondre à ce besoin dans le cas des revêtements NiCr-Cr3C2 élaborés par HVOF en mettant en oeuvre des paramètres opératoires variables.En premier lieu, 40 séries d’expériences de projection HVOF ont été conduites et les propriétés du revêtement obtenus ont été déterminées et analysées afin de créer les modèles ANN visés. La relation entre les paramètres opératoires, les caractéristiques des particules en vol et les propriétés du revêtement a été étudiée dans une approche initiale "intuitive", réalisée sur la base de considérations physico-chimiques classiques, ce qui a fourni une compréhension préliminaire du processus HVOF et des revêtements. Toutefois, bien que l’effet des paramètres du procédé sur ces derniers puisse être décrit qualitativement, il est impossible d’établir descorrélations quantitatives..Aussi deux modèles ANN ont été développés et mis en œuvre pour prédire les performances du revêtement (en termes de micro-dureté, porosité et taux d’usure) et traduire l’influence des paramètres opératoires (distance de projection, débit d’oxygène et débit de carburant) en faisant intervenir les variables intermédiaires (température et vitesse de particules en vol). Une procédure détaillée de création de ces deux modèles ANN est présentée, qui intègre les phénomènes physiques régissant le processus HVOF. Une série d’expériences supplémentaires a également été menées pour valider la fiabilité et estimer la précision de ces modèles ANN. Les résultats montrent que les modèles implicites ainsi développés ont des qualités prédictives satisfaisantes. La clarification des relations entre les conditions de projection, les comportements des particules en vol et les performances du revêtement est un élément clef pour assurer un contrôle optimal des revêtements déposés par HVOF. De plus, une analyse de la « valeur d’impact moyenne » (MIV) a été réalisée pour évaluer quantitativement l’influence relative de chaque entrée sur les données de sortie en vue d’affiner la prédiction.Enfin, les modèles ANN mis en forme ont été programmées et intégrés dans le système de contrôle de projection HVOF tel qu'il est mis en œuvre au laboratoire, pour parvenir un système de contrôle intelligent. À l’aide de ce système, la température et la vitesse des particules en vol peuvent être calculées à partir des paramètres opératoires et les propriétés de revêtement spécifiques peuvent être estimées. Un modèle ANN inversé a également été développé, qui prédit les paramètres opératoires optimaux à appliquer en fonction du niveau de micro-dureté visé pour le revêtement.Ce travail contribue donc à la mise au point d’un système de contrôle intelligent applicable aux procédés de projection HVOF et la démarche suivie peut en outre être étendue à d’autres techniques de projection thermique.Ainsi, sur la base d'un nombre substantiel de données, ce travail a non seulement produit une analyse de la relation entre les paramètres opératoires du procédé, le comportement des particules en vol et les propriétés du revêtement, mais a également fourni une méthode de prédiction applicable aux revêtements déposés par HVOF, au travers d'un modèle ANN optimisé. Enfin, un logiciel prototype concrétisant un tel système de contrôle intelligent adapté au procédé de projection HVOF a également été élaboré
In the high velocity oxygen fuel (HVOF) spray process, the coating properties are sensitive to the characteristics of in-flight particles, which are mainly determined by the process parameters. Due to the complex chemical and thermodynamic reactions during the deposition procedure, obtaining a comprehensive multi-physical model or analytical analysis of the HVOF process is still a challenging issue. This study proposes to develop a robust methodology via artificial neural networks (ANN) to solve this problem for the HVOF sprayed NiCr-Cr3C2 coatings under different operating parameters.First, 40 sets of HVOF spray experiments were conducted and the coating properties were tested for analysis and to build up the data set for ANN models. The relationship among the process parameters, behaviors of in-flight particles, and coating properties were investigated from an initial view, which provided a preliminary understanding of the HVOF process and sprayed coatings. Even though the effect of process parameters on the behaviors of in-flight particles and thus on the coating’ properties can be roughly summarized, it is impossible to build up direct connections among them.Second, two ANN models were developed and implemented to predict coating’s performances (in terms of microhardness, porosity and wear rate) and to analyze the influence of operating parameters (stand-off distance, oxygen flow rate, and fuel flow rate) while considering the intermediate variables (temperature and velocity of in-flight particles). A detailed procedure for creating these two ANN models is presented, which encodes the implicitly physical phenomena governing the HVOF process. A set of additional experiments was also conducted to validate the reliability and accuracy of the ANN models. The results show that the developed implicit models can satisfy the prediction requirements. Clarifying the interrelationships between the spraying conditions, behaviors of in-flight particles, and the final coating performances will provide better control of the HVOF sprayed coatings. Additionally, mean impact value (MIV) analysis was conducted to quantitatively explore the relative significance of each input on outputs for improving the effectiveness of the predictions.Lastly, the well-trained ANN models were programmed and integrated into the homemade HVOF spray control system to realize an intelligent control system. With this system, the temperature and velocity of in-flight particles can be calculated by entering process parameters, and thereafter obtaining specific coating properties. A reverse ANN model was also integrated, which calculates process parameters based on the microhardness of the coating to guide the selection of the best parameters. This integration provides a preliminary idea for the construction of an intelligent control system for HVOF spray process and can be promoted to other thermal spray technologies.Overall, based on a large data set, this work not only intuitively analyzed the relationship among process parameters, behaviors of in-flight particles, and coating’s properties, but also provided a prediction method for the HVOF spray process and HVOF sprayed coatings via the optimized and well-trained ANN model. In addition, a prototype to realize an intelligent control system for HVOF spray process has also been suggested

Agglomerated and sintered 25Ni(Cr)-75Cr3C2 powder was deposited onto 304 stainless steel substrates by HVOF thermal spraying. Post spraying thermal treatments were carried out on as-sprayed samples at different temperatures (700°C and 800°C) in air (oxidising). Samples were heat treated from 1hour up to 16 days and furnace cooled. The powder, coatings and substrate were characterised by a variety of techniques including optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness testing and thermogravimetric analysis (TGA). The results of agglomerated and sintered powder coating microstructural characterisation show that the as-sprayed coating comprised the carbide phase Cr3C2, a small amount of amorphous phase and a metallic phase which was Ni-rich. The spray process also led to the formation a small amount of Cr2O3 in the coatings. In the heat treated coatings, the carbides Cr7C3 and Cr23C6 were found at the coating substrate interface, the amorphous structure phase was eliminated. With heat treatment, matrix phase supersaturation was reduced, while widespread carbide nucleation and growth generated an expansive carbide skeletal network especially near the coating/substrate interface. An initial softening of the coatings occurred but subsequently the hardness increased again after 2 days was a function of carbide development. The oxidation kinetics of the 25Ni(Cr)-75Cr3C2 powder and coatings during elevated temperature oxidation in air at 650°C to 1000°C have also been studied. The oxidation behaviour was found to be governed by a parabolic rate law up to 800°C. The activation energy of the coating oxidation reaction, Q, was found to be 164 kJ/mol. The mechanism of high temperature oxidation of 25Ni (Cr)-75Cr3C2 coating on top surface has been discussed. Inter-diffusion between the coating and the 304 stainless steel substrate has been investigated. A distribution of the precipitate phase Cr23C6 within the 304 steel was found from etched substrate samples. The decrease of the microhardness measured with distance away from the interface also revealed that diffusion occurred between coating and substrate during the annealing period. Carbon diffusion distances at different annealing conditions in this work have been measured according to the appearance of the Cr23C6 precipitates and variation in microhardness away from the interface. A simple mathematical model has been used to calculate the theoretical carbon diffusion distance in substrate.

There have been enormous advances in technologies for thermal spray over the past few decades. One such application is the replacement of electroplated hard chrome for aerospace and automotive industries. Hard chrome electroplating has been a valuable surface treatment for parts in corrosion and wear applications due to its high hardness, ability to passivate, as well as its low coefficient of friction. In the past two decades, there have been concerns due to limitations in hard chrome's performance as well as environmental effects due to hexavalent chromium produced during processing. High velocity oxy-fuel (HVOF) spray processing has been developed to produce exceptional coating quality due to the very dense microstructures formed with limited porosity.

Cr₃C₂-NiCr has been shown to be a viable replacement to electroplated hard chrome when deposited by HVOF spray techniques. In order to produce optimized coatings with this technology, a process mapping methodology was implemented to understand the relationship between process variables. The variations of oxygen to fuel ratios as well as total volume flows of gases were examined to interrelate process variables with the particle state, stress evolution during deposition, and properties of the coatings. The performance of the coatings in aqueous corrosion and sliding wear environments were correlated back to the properties of the coatings. It has been demonstrated that monitoring the in-flight particles and evolution of stress can be directly correlated to the properties of the coating and in-directly to the coating performance.

Cr₃C₂-NiCr property and performance was also compared to hard chrome and WC-CoCr coatings. It was shown that the optimized coatings of Cr₃C₂-NiCr outperformed hard chrome in both aqueous corrosion as well as sliding wear environments. Cr₃C₂-NiCr was outperformed by WC-CoCr in sliding wear applications due to its lower hardness, but outperformed WC-CoCr in aqueous corrosion due to the denser microstructures that were produced.

Este trabalho investigou o fenômeno da degradação de revestimentos obtidos por aspersão térmica pelas técnicas de HVOF e plasma spray, de um sistema metal cerâmico à base de NiCr (NiCr-Cr3C2), submetido à erosão, desde a temperatura ambiente até 800°C, relacionando a resistência à erosão à sua microestrutura. Os revestimentos erodidos tiveram o sistema NiCr-Cr3C2 com percentual das fases variável. No caso dos revestimentos obtidos por HVOF investigou-se o comportamento em revestimentos com 0, 35, 70 e 75% de fase Cr3C2 na matriz metálica, NiCr. Já no caso dos revestimentos obtidos por plasma spray avaliou-se o comportamento em revestimentos com 0, 35, 70 e 100% de fase Cr3C2 na matriz metálica, NiCr. Para os ensaios de erosão foi construído um equipamento capaz de variar as condições experimentais como velocidade, fluxo, ângulo de incidência do erodente, além da temperatura de ensaio. Os revestimentos foram obtidos a partir dos melhores parâmetros de deposição avaliados em pré-testes, visando-se controlar a porosidade das amostras, conforme a limitação de cada técnica empregada. A superfície das amostras foi levemente lixada antes de serem erodidas para a retirada de lamelas não bem aderidas, visando-se obter um padrão semelhante de rugosidade em cada grupo. As amostras foram submetidas a um fluxo controladonde partículas erodentes de alumina eletrofundida, em ângulos de incidência de 30, 45, 60 e 90º, a uma velocidade mantida em torno de 50 m/s. As temperaturas estudadas foram de 25, 200, 400, 600 e 800ºC. O material foi caracterizado quanto a sua microestrutura (microscopia ótica e eletrônica de varredura, porosidade, rugosidade, fases presentes e sua morfologia), também quanto a propriedades mecânicas como (micro- e nano-) dureza e módulo de elasticidade. O desgaste foi medido pela perda de volume do corpo erodido. Os resultados indicaram que a relação entre a porosidade, temperatura e quantidade de fase Cr3C2 foi determinante no desgaste erosivo dos revestimentos. O aumento de temperatura proporcionou um acréscimo na incrustação de partículas erodentes, na oxidação e na plasticidade dos revestimentos, esta última sendo verificada pelo decréscimo da dureza do material. Além disso, o fenômeno erosão-oxidação fez-se presente acima de 400°C O acréscimo de fase Cr3C2 a matriz metálica proporcionou uma diminuição da incrustação de partículas erodentes, tendo em vista a diminuição da plasticidade dos revestimentos, além do aumento da oxidação e porosidade dos revestimentos. A porosidade, de fato, influenciou decisivamente na resistência à erosão, dividindo claramente os revestimentos em dois grupos, no intervalo de porosidades superior (revestimentos obtidos por plasma spray) e inferior (revestimentos obtidos por HVOF) a 4%. Nos revestimentos de maior porosidade mesmo o acréscimo de uma fase mais dura não proporcionou um aumento da resistência à erosão, pois a porosidade implicou em uma menor área de contato entre as lamelas, facilitando seu arrancamento do revestimento sob ataque erosivo. Já para os revestimentos de menor porosidade o incremento de Cr3C2 proporcionou um aumento da resistência ao desgaste para todo o intervalo de temperatura investigado, devido ao aumento da dureza. Quanto aos mecanismos que proporcionaram o desgaste nos revestimentos observou-se uma preponderância de desgaste dúctil (corte – 30º e platelet – 90º) para os revestimentos metálicos NiCr0,5% e NiCr9%. Com o acréscimo de fase Cr3C2 há uma diminuição da característica dúctil de desgaste, mais acentuada nos revestimentos obtidos por plasma spray. E a partir do acréscimo de 70% de Cr3C2 observou-se alguns mecanismos de desgaste frágeis, como a formação de trincas, e pits. O acréscimo de temperatura proporcionou o aumento da plasticidade dos revestimentos, a diminuição de formação de platelets e no caso do desgaste eminentemente frágil (para o revestimento CrC28%) o arredondamento das bordas desgastadas.
In this work, the degradation behavior of thermal-sprayed NiCr-Cr3C2-based metal-ceramic coatings under erosion at temperatures ranging from room temperature to 800°C was investigated in order to establish correlations between the erosion resistance and the microstructure. In order to obtain coatings with different microstructures, the coatings were produced employing two different techniques - HVOF and plasma spray -; and different amounts of the Cr3C2 ceramic phase in the metallic NiCr matrix: 0, 35, 70, 75% (only by HVOF) and 100% (only by plasma spray). For the erosion tests, an equipment capable of using different conditions of speed, flux and angle of incidence of the erodent at different temperatures was developed. The samples were subjected to a controlled flux of fused alumina erodent particles, with angles of incidence of 30, 45, 60 e 90º, at a speed of about 50m/s. Temperatures of 25, 200, 400, 600 e 800ºC were employed in the tests. The material was characterized regarding its microstructure (optical microscopy, scanning electron microscopy, porosity, rugosity, phase composition and phase morphology) and its mechanical properties (micro- and nano-hardness and Young modulus). The wear was evaluated by measuring the volume loss of the eroded bodies. The results indicated that porosity, temperature and amount of Cr3C2 had a critical role in the erosive wear of the coatings. Higher temperatures increased the incrustation of erodent particles, the oxidation (which was observed above 400°C and was also influenced by porosity), and the plasticity of the coatings. Increasing the Cr3C2 content in the metallic matrix has lowered the incrustation by making the coatings less plastic, and has also increased the oxidation and porosity. In fact, the porosity had a crucial role in the erosion resistance, in such way that it is possible to divide the obtained coatings in two groups: those with porosity level above 4% (obtained by plasma spray) and those with porosity below that level (obtained by HVOF). In the high porosity group, even increasing the level of hard phase was not enough to make the coatings more resistant, because the porosity has lowered the contact area between the lamellas, easing its pull-out by erosive attack. For the low-porosity coatings, on the other hand, increasing the Cr3C2 content made the coatings more wear-resistant in the whole temperature range under investigation. As for the wear mechanisms involved, a preponderance of ductile wear (cutting – 30º and platelet – 90º) was observed for the NiCr0,5% and NiCr9% coatings. With increasing Cr3C2 content, a decrease in the ductile wear characteristic was observed, more prominently in the plasma sprayed coatings. From the 70% Cr3C2 content on, fragile wear mechanisms were observed, as cracks and pits formation. Increasing temperature made the coatings more plastic, reducing the platelet formation and, for the eminently fragile mechanism observed for the CrC28% coating, making the edges of the worn areas less sharp.

O desgaste é um dos fatores que causam a maior parte das falhas e redução na vida útil de ferramentas, além de resultar em produtos com condições superficiais inadequadas. A crescente demanda da indústria por menores custos, maior produtividade e melhor qualidade estão entre as justificativas para que se busquem maneiras de incrementar o desempenho dessas ferramentas. Neste contexto, o trabalho tem como objetivo avaliar e caracterizar superfícies projetadas para resistência ao desgaste utilizando o processo de aspersão térmica oxicombustível de alta velocidade (HVOF). Fez-se um estudo comparativo de caracterização e comportamento tribológico de um aço ferramenta com deposição por aspersão térmica de dois diferentes revestimentos. O estudo abrange o aço ABNT H13, utilizando o tratamento superficial de aspersão térmica HVOF com revestimentos de Cr3C2-25(NiCr) e WC-10Co-4Cr. A avaliação da superfície e do comportamento tribológico envolveu perfis de microdureza, análise de fases por difração de raios X, metalografias, obtenção da composição química, caracterização da rugosidade, ensaio de abrasão e adesão, pino-disco e ensaio do anel. O objetivo de conseguir aumentar a resistência ao desgaste utilizando um aço ferramenta com deposição via processo de HVOF, bem como o de caracterizar este processo foi alcançado. A utilização do aço ABNT H13 com deposição via aspersão térmica HVOF e revestimentos à base de carboneto de cromo e tungstênio demonstrou-se recomendável para proteção de substratos, fornecendo alta dureza e boa resistência ao desgaste. O revestimento de WC-10Co-4Cr obteve o melhor desempenho geral, apresentando uma microdureza 192% maior frente ao aço sem revestimento, uma redução de 315% do coeficiente de atrito, além de uma adesão maior ao substrato quando comparado com o revestimento Cr3C2-25(NiCr).
Wear is one of the factors that cause most of the failures and reduction of lifespan for tools, it also results in products with no quality in the surface conditions. The increasing demand of the industry for lower costs, higher productivity and better quality are among the justifications for finding ways to increase the performance of these tools. In this context, the objective of this work is to evaluate and characterize surfaces designed for wear resistance using the high velocity oxyfuel (HVOF) thermal spray process. A comparative study of characterization and tribological behavior of a tool steel with thermal spray deposition was performed for two different coatings. The study covers the ABNT H13 steel, using the surface treatment of HVOF thermal spray with coatings of Cr3C2-25(NiCr) and WC-10Co-4Cr. The surface and the tribological behavior evaluation involved microhardness profiles, phase analysis by x-ray diffraction, metallography, chemical composition, roughness characterization, abrasion and adhesion test, pin-on-disk and ring test. The objective of increasing the wear resistance using a tool steel with deposition through HVOF process, as well as to characterize this process was achieved. The use of ABNT H13 steel with chrome and tungsten carbide coatings deposited by thermal spray HVOF has proved to be recommended for substrates protection, providing high hardness and good wear resistance. The WC-10Co-4Crcoating had the best overall performance, presenting a microhardness 192% higher than the steel with no coating, a reduction of 315% of the coefficient of friction, and a higher adhesion to the substrate compared to the Cr3C2-25(NiCr) coating.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
O aço AISI 4340 tem sido utilizado largamente devido as suas propriedades físicas, químicas, mecânicas e resistência à fadiga, atendendo ao rigoroso e crescente aumento das exigências para aplicações na engenharia. Revestimentos superficiais são aplicados objetivando proteção contra corrosão e desgaste, por exemplo, a eletrodeposição de Cromo, pois apresenta resistência à corrosão, alta dureza, resistência ao desgaste abrasivo e adesivo, soldabilidade e baixo coeficiente de atrito. Por outro lado, microtrincas derivadas da camada de Cromo diminuem a resistência à fadiga, com isso o componente é submetido ao tratamento superficial para deformação plástica, shot peening, criando tensões residuais compressivas na superfície, que retardam ou eliminam a propagação de trincas, tendendo a aumentar sua vida em fadiga. Entretanto, durante o processo de deposição do Cromo duro é liberada uma substância cancerígena, o Cromo hexavalente. Por essa razão, estuda-se a substituição do Cromo por outros revestimentos que apresentem propriedades mecânicas similares, mas que não tenham restrições ambientais. O processo HVOF (High-Velocity-Oxigen-Fuel) para deposição de revestimentos alternativos, como carbeto de tungstênio, é considerado uma técnica promissora, pois fornece alta dureza e boa resistência ao desgaste e uma menor redução da resistência à fadiga do substrato quando comparado à redução fornecida pelo revestimento de Cromo duro aplicado ao aço AISI 4340. Portanto, o objetivo deste estudo é avaliar o comportamento mecânico do aço AISI 4340 com alta resistência mecânica (50 HRC), revestido com WC-CrC-Ni, WC-10Ni, Ni-20Cr, Ni-Cr-B-Si- Fe, e Cr3C2-NiCr, depositados através do processo HVOF; revestimentos alternativos à camada de Cromo duro. Os dados de fadiga axial são representados por curvas σ x N...
AISI 4340 steel is being used in the aeronautical industry because its good physical, chemical, mechanical and fatigue properties. Chromium coatings are used in applications to guarantee protection against wear and corrosion, combined with chemical resistance and good lubricity. The reduction in the fatigue strength of base material and since this technology presents detrimental environmental and health effects, resulted in the search on coatings viewed as being capable of replacing hard chrome plating. Thermally sprayed HVOF coatings are being considered to replace galvanic chromium deposits in industrial applications with comparable performance for wear and corrosion resistance. With respect to fatigue life, the HVOF technique induces tensile residual stress on the interface. In this case, the initiation and propagation phases of fatigue process are accelerated because the coating; on the other hand, compressive residual stresses close to the surface increases fatigue life. The technique to improve the coated materials fatigue strength is the shot peening process, which induces compressive residual stress in the surface, which delay nucleation and propagation process In the present research is verified the influence of WC-CrC-Ni; WC-10Ni; Ni-20Cr; Ni-Cr-B-Si-Fe; Cr3C2-NiCr HVOF process, coated high strength AISI 4340 with and without shot peening on the axial fatigue strength, in comparison with EHC. Corrosion resistance is also conducted by salt spray tests. S-N curves are obtained in axial fatigue tests. In order to study the influence of residual stresses on fatigue life, the stress field is measured by X-ray tensometry. Scanning electron microscopy and optical metallographic are used to investigate the fatigue source appearance. Experimental data show lower axial fatigue resistance for HVOF coated specimens in comparison to base material...(Complete abstract click electronic access below)

Resumo: O aço AISI 4340 tem sido utilizado largamente devido as suas propriedades físicas, químicas, mecânicas e resistência à fadiga, atendendo ao rigoroso e crescente aumento das exigências para aplicações na engenharia. Revestimentos superficiais são aplicados objetivando proteção contra corrosão e desgaste, por exemplo, a eletrodeposição de Cromo, pois apresenta resistência à corrosão, alta dureza, resistência ao desgaste abrasivo e adesivo, soldabilidade e baixo coeficiente de atrito. Por outro lado, microtrincas derivadas da camada de Cromo diminuem a resistência à fadiga, com isso o componente é submetido ao tratamento superficial para deformação plástica, shot peening, criando tensões residuais compressivas na superfície, que retardam ou eliminam a propagação de trincas, tendendo a aumentar sua vida em fadiga. Entretanto, durante o processo de deposição do Cromo duro é liberada uma substância cancerígena, o Cromo hexavalente. Por essa razão, estuda-se a substituição do Cromo por outros revestimentos que apresentem propriedades mecânicas similares, mas que não tenham restrições ambientais. O processo HVOF (High-Velocity-Oxigen-Fuel) para deposição de revestimentos alternativos, como carbeto de tungstênio, é considerado uma técnica promissora, pois fornece alta dureza e boa resistência ao desgaste e uma menor redução da resistência à fadiga do substrato quando comparado à redução fornecida pelo revestimento de Cromo duro aplicado ao aço AISI 4340. Portanto, o objetivo deste estudo é avaliar o comportamento mecânico do aço AISI 4340 com alta resistência mecânica (50 HRC), revestido com WC-CrC-Ni, WC-10Ni, Ni-20Cr, Ni-Cr-B-Si- Fe, e Cr3C2-NiCr, depositados através do processo HVOF; revestimentos alternativos à camada de Cromo duro. Os dados de fadiga axial são representados por curvas σ x N ...(Resumo completo, clicar acesso eletrônico abaixo)
Abstract: AISI 4340 steel is being used in the aeronautical industry because its good physical, chemical, mechanical and fatigue properties. Chromium coatings are used in applications to guarantee protection against wear and corrosion, combined with chemical resistance and good lubricity. The reduction in the fatigue strength of base material and since this technology presents detrimental environmental and health effects, resulted in the search on coatings viewed as being capable of replacing hard chrome plating. Thermally sprayed HVOF coatings are being considered to replace galvanic chromium deposits in industrial applications with comparable performance for wear and corrosion resistance. With respect to fatigue life, the HVOF technique induces tensile residual stress on the interface. In this case, the initiation and propagation phases of fatigue process are accelerated because the coating; on the other hand, compressive residual stresses close to the surface increases fatigue life. The technique to improve the coated materials fatigue strength is the shot peening process, which induces compressive residual stress in the surface, which delay nucleation and propagation process In the present research is verified the influence of WC-CrC-Ni; WC-10Ni; Ni-20Cr; Ni-Cr-B-Si-Fe; Cr3C2-NiCr HVOF process, coated high strength AISI 4340 with and without shot peening on the axial fatigue strength, in comparison with EHC. Corrosion resistance is also conducted by salt spray tests. S-N curves are obtained in axial fatigue tests. In order to study the influence of residual stresses on fatigue life, the stress field is measured by X-ray tensometry. Scanning electron microscopy and optical metallographic are used to investigate the fatigue source appearance. Experimental data show lower axial fatigue resistance for HVOF coated specimens in comparison to base material...(Complete abstract click electronic access below)
Orientador: Herman Jacobus Cornelis Voorwald
Coorientador: Maria Odila Hilário Cioffi
Banca: Marcelo dos Santos Pereira
Banca: Carlos de Moura Neto
Banca: Carlos Antonio Reis Pereira Baptista
Banca: Lindolfo Araujo Moreira Filho
Doutor

碩士
國立成功大學
材料科學(工程)學系
84
Alumina is one of the most popular and utilized structural ceramics. However, the intrinsic brittleness of alumina makes itsreliability limited and prevents wider usage. Alumina can be strengthened by incorporating Cr3C2 particles through hot- pressingand pressureless sintering process. Fully dense compacts with Cr3C2 particulate reinforced alumina matrix composites have excellent mechanical properties. This investigation concentrates on the effect of sintering atmosphere,interfacial properties and injection molding of Al2 O3/Cr3C2 composites. Experimental results show that the relative density and bendingstrength of Al2O3/Cr3C2 sintered in vacuum was better than in argon.By sintering in vacuum, flexural strength and toughness of 490MPa and 6.0MPa·m1/2 were obtained. The interface of Al2O3/Cr3C2 sintered in vacuum have no obvious reactions. The ceramic injection molding of Al2O3/Cr3C2 composites ceramics is possible and provides assembly process suitable for mass production. In the present work, The effect of Cr3C2 (2mm) additions on rheology of ceramic injection molding blends were investigated. Fluidity is strongly influenced by powder loading, temperature and Cr3C2 contents. The addition of Cr3C2 apparently reduced the viscosity of ceramic injection molding blends. Cold isostatic press before sintering considerably enhanced therelative density, hardness and toughness of sintered samples. By controlling the amount of Cr3C2, flexural strength and toughness of Al2O3/Cr3C2 of 480MPa and 5.0MPa.m1/2 were obtained. Cr3C2 apparently inhabited the grain growth of alumina.

碩士
國立成功大學
材料科學(工程)學系
86
Alumina is one of the most popular commercialized structural ceramics. However, the intrinsic brittleness of alumina makes its reliability and usage limited. It was previously reported that the alumina can be strengthedby incorporating Cr3C2 particles. The Al2O3 incorporated with 10vol.% Cr3C2 were used in this experiment. The injection moldimg technique is a good way for making complex-shaped components and was chosen for making test specimens in this study. The result of both dynamic and static fatigue tests indicate the slow crackgrowth parameters of alumina were enhanced by adding Cr3C2. Both Indentation Strength in Bending (ISB) and Dummy-ISB were used toinvestigate R-curve behaviors in alumina and Cr3C2/Al2O3 composite. The R-curve behavior was detected in Cr3C2/Al2O3 composite which was attributedto the crack tip shielding caused by crack deflection and crack bridging.

博士
國立成功大學
材料科學及工程學系碩博士班
91
This study focused on investigating the influence of heat and environmental effects on mechanical properties of Cr3C2/Al2O3 composite. 　　The crack resistance (R-curve) of heat treated Cr3C2/Al2O3 composite was studied using the indentation-strength-in-bending technique. The oxidation layer containing alumina and alumina/chromia solid solution was in a compression of �{179 MPa and �{195 MPa as evaluated by X-ray analysis and indentation method, respectively. The results were slightly different from the thermal mismatch calculation (�{139 MPa). The relatively compressive, low porous oxidation layer caused significant increase in crack resistance. Crack healing could contribute also partially to crack resistance. 　　The ultrasonic technique is proposed for assessment of thermal shocked damage of Cr3C2/Al2O3 composites. This includes narrowband tone burst and broadband pulse measurements from different frequency. The results reveal the acoustic properties strongly depend on the quenching temperatures. The shock-induced surface residual stresses decreased with increasing quenching temperatures. 　　The wear resistance and fracture mechanisms of Cr3C2/Al2O3 composite were investigated with a pin-on-disk test configuration against a silicon carbide sphere at room temperature in unlubricated sliding. The results suggest that the microcracking induced from residual stresses could play an important role in predicting the wear behavior. Although the addition of Cr3C2 to Al2O3 matrix improved its crack growth resistance, it enhanced the wear resistance. The mechanisms of material removal was related to the failure of the particle-matrix interface, resulting in particle pullout, wear debris and chipping of the matrix. A microabrasive mechanism and microplastic deformation were also observed. The worn-induced surface residual stress (�{142 MPa) was similar to that induced by #270 diamond wheel grinding (�{158 MPa). 　　Concerning the effects of erosive particle size and impact angle on the wear rate and surface residual stress, the impact damage behavior of injection moulded Cr3C2/Al2O3 composite was investigated. The experiments were performed using a gas blast erosion rig with SiC and Al2O3 erodents. The results suggest that the larger erodent particles are more effective in displacing and damaging the surface compared to the smaller particles, and lead to higher wear rate and surface residual stress. The variation in surface residual stress with kinetic energy is consistent with the erosion rate and post-erosion strength. This study has been made of erodent size, liquid pressure and impact angle on wet erosion wear, surface residual stress and impact damage behavior of injection moulded Cr3C2/Al2O3 composites. Spontaneous microcracking induced from sintering process, due to thermal expansion mismatch between alumina and chromium carbide, played a key role in erosion mechanism. Deformed microstructures included basal twins and dislocations were observed when composite erdoded by SiC #80 grit at a water pressure 500 psi. This observation suggests the basal twins and the partial dislocation have the lowest critical resolved stress to activation.

碩士
國立成功大學
材料科學(工程)研究所
83
Alumina is one of the most developed and utilized structural ceramics because of its low cost, excellent physical and chemical properties. This investigation concentrate on the toughening of alumina through a pressureless sintering process. There are various phases with chromium carbide, such as Cr3C2, Cr7C3 and Cr23C6. Experimental results in this study proved that the phase stability of this material is quite sensitive to the temperature and sintering environment. By controlling the phase stability, the amount and particle size of Cr3C2, flexural strength and toughness of 500 MPa and 5.9 MPa ‧m1/2 were obtained. Crack deflection and microcracking were the two major toughening mechanisms. The addition of Cr3C2 apparently played an important role in the grain growth of alumina. Colescence of Cr3C2 was observed along the grain boundaries of alumina as the addition of Cr3C2 reached a critical amount. Transgranular was observed in alumina grains while cracks were obviously deflected from Cr3C2.

碩士
國立成功大學
材料科學及工程學系碩博士班
98
Metal-organic chemical vapor deposition (MOCVD) and spouted bed were employed to prepare the nanoscaled particles deposited on alumina. The amorphous Cr2O3 deposited on the Al2O3 ceramic powder by means of pyrolysis of Cr(CO)6 at 300℃. The composition of decomposed Cr(CO)6 includes Cr2O3, CrC1-x, and C. Cr2O3 reacts with carbon to transform into chromium carbide. The Cr3C2/Al2O3 nanocomposites were fabricated using a spark plasma sintering (SPS) technique at 1350℃, and their densification behavior and mechanical properties were investigated. Using spark plasma sintering, the Cr2O3/Al2O3 nanocomposites were formed at 1200 ℃. But the successful densification of Cr3C2/Al2O3 nanocomposites was obtained at a temperature of 1350 ℃. Cr3C2/Al2O3 nanocomposites were found to possess microstructures of fine Cr3C2 particles dispersed within the Al2O3 matrix grains and/or at the grain boundaries. These nanocomposites exhibit an average fracture toughness of 4.8 MPa m1/2, hardness of 25.6 GPa, and flexural strength of 780 MPa compared with 3.7 MPa m1/2, 21.8 GPa, and 625 MPa for the pure Al2O3 compacts, respectively. The fracture mode changes from intergranular fracture of monolithic Al2O3 to transgranular fracture of nanocomposites, and step-wise fracture surface is also observed. From nanoindentation tests, the Young's modulus of Cr3C2/Al2O3 nanocomposites is found to be higher than pure alumina. Addition of chromium carbide can be delayed fracture of sintered. When the sintered starts fracture and plastic deformation, the elastic modulus begin to decrease and plastic work / total work ratio was the largest ratio.

碩士
國立臺灣科技大學
機械工程系
104
In this study, sintered Ni60-WC-Cr3C2 composites with different carbide concentrations were processed at different temperatures. There are three parts in this study. In the first part, the specimens were analyzed by scanning electron microscope (SEM) to examine the densification behavior, and XRD and EDXA to examine the constituting phases. In the second part, Optimization of the hardness was achieved by plotting the broken-line graph of carbide concentration, temperature and hardness. In the last part, the optimized composite in the second part were tested by corrosion test, wherein a 70% acid was used. It was found the optimized composition and temperature is 30.0 vol% carbide sintered at 1150°C. The microstructural ananlysis indicted hardness is 899.9 kgf/〖mm〗^2, corrosion rate is 7.0 mg/〖cm〗^2．hr, porosity is 1.65%.

碩士
國立臺北科技大學
製造科技研究所
97
The study is using high velocity oxy-fuel spraying to achieve NiCrBSi self fluxing alloy coating. Moreover, WC or Cr3C2 is added into the self fluxing alloy coating. The coatings were re-melted at 950°C, 1000°C and 1050°C respectively. The relation between different conducting parameters and the coating’s surface formation, microstructure, hardness and porosity is systematically analyzed. At last, the mechanical properties and wear characteristics of the coating are in comparison. The results show the NiCrBSi coating which is treated by re-melting at 950°C results in the presence hardening phases of Ni3B and Cr7C3 and approach of 0% porosity. Besides, the NiCrBSi coating which treated by 1050°C causes the coating itself collapsed, that decrease the hardness of the coating. The NiCrBSi coating with the addition of Cr3C2 can effectively enhance the wear resistance. After the coating was re-melted, it will occurs micro cracks resulting from the phenomenon of decarburization, resulting in decreased hardness of the coating. The NiCrBSi coating with the addition of WC is stacked up by the WC and NiCrBSi. After re-melting, the phase of WC does not change and the porosity is less than 1%. The bonding between the alloy coating and substrates shifts from mechanical bonding to metallurgical bonding. Furthermore, the average hardness of the coating is above 700Hv and the excellent wear resistance at either room temperature or high temperature.

碩士
國立臺北科技大學
材料科學與工程研究所
105
Vanadis 4 steel is a powder metallurgy tool steel which has high levels of vanadium, chromium, and molybdenum elements in the matrix. Generally speaking, Vanadis 4 tool steels manufactured by powder metallurgy have been widely used due to their high strength, wear resistance and ductility. In addition, metal matrix composites (MMCs) is are new materials with different types of ceramics as reinforcement phase dispersed in the metal matrix. Vanadium carbide (VC) and chromium carbides (Cr3C2) have excellent hardness and high melting temperatures, effectively inhibiting grain growth and thermal stability with Fe-based alloys; therefore, they are great options for reinforcing tool steels. According to the above statements, this study utilizes the VC and Cr3C2 carbides to improve the mechanical properties of the Vanadis 4 tool steels. In this study, different ratios of VC (1, 3 and 5 wt%) and Cr3C2 (1, 3 and 5 wt%) powders were added to Vanadis 4 steel powders and then mixed by ball milling in order to find the optimal vacuum sintering parameters of Vanadis 4 tool steels. After compressing and forming, all the specimens were sintered from 1225°C to 1275°C by vacuum sintering for 1 h. Furthermore, a series of heat-treatments (quenching followed by tempering) was performed, by which the optimal samples were heated to 1020°C and maintained at that temperature for 100 min for quenching, with 0.5 MPa of N2 for quenching media, and then subjected to sub-zero treatment (-150°C for 60 min). Meanwhile, the tempering temperature was held at 480°C for 180 min and repeated twice. Finally, various material characterization techniques were used to evaluate the specimens’ properties, including porosity, hardness and transverse rupture strength (TRS). Corrosion tests were also conducted; the microstructures were analyzed using XRD, OM, SEM and EDS techniques. The experimental results showed that good mechanical properties were obtained by the addition of 3 wt% VC powders sintered at 1250°C for 1 h. Meanwhile, the apparent porosity was 0.27%, and TRS and hardness reached 1953.3 MPa and 82.1 HRA, respectively. On the other hand, this study also revealed that the apparent porosity reached to 0.22%, and that a TRS value of 1821.2 MPa and a hardness value of 82.4 HRA were obtained by the addition of 5 wt% Cr3C2 powders, which were sintered at 1250°C for 1 h. Furthermore, when the optimal sintered specimens of Vandis 4 tool steel were obtained by adding VC and Cr3C2 powders (3 wt% VC and 5 wt% Cr3C2 sintered at 1250°C for 1 h, respectively), followed by heat-treatments and sub-zero treatments, the TRS obviously increased to 2066.3 and 2085.0 MPa, respectively. However, there is no significant difference in the hardness value.