ADVANCED CERAMIC MATERIALS FOR DENTAL APPLICATIONS SINTERED BY MICROWAVE HEATING

[EN] Zirconia has become a widely utilized structural ceramic material with important applications in dentistry due to its superb mechanical properties, biocompatibility, aesthetic characteristics and durability. Zirconia needs to be stabilized in the t-phase to obtain improved mechanical properties such as hardness and fracture toughness. Fully dense yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) materials are normally consolidated through the energy-intensive processing of powders at very high temperatures (>1000 °C). Innovative non-conventional approaches are being developed to reduce time and energy consumption and, consequently, environmental impact in ceramic powder processing. Microwave sintering is one such approach aimed at fully-densifying ceramics by using a different heating mechanism based on the material's inherent dielectric properties. The main purpose of this work is to obtain highly dense Y-TZP dental materials from commercial and lab-prepared sources via microwave sintering with mechanical and microstructural qualities that are similar or even improved with respect to their conventionally sintered counterparts. Therefore, its effect on important aspects related to dental applications has been studied. First, Y-TZP ceramics have been characterized upon sintering to determine whether the resulting properties meet the minimum mechanical requirements for structural dental applications. Second, the influence of microwave sintering on hydrothermal degradation, a spontaneous ageing phenomenon that affects zirconia materials in wet conditions, has been investigated. And third, the behavior under fretting wear conditions of microwave and conventionally sintered materials has been assessed to evaluate their durability and performance. The main conclusions indicate that microwave sintering allows proper consolidation of dental Y-TZP materials resulting in a finer microstructure due to shorter processing time and mechanical properties comparable, and in some cases enhanced, to those obtained in conventional sintered materials at lower dwell temperatures. Additionally, a higher resistance to hydrothermal degradation has been determined for microwave sintered materials due to a finer grain size and lower sintering temperatures that reduce the presence of cubic phase, which is responsible for destabilizing neighboring tetragonal phase grains. Finally, a similar wear rate has been obtained between microwave and conventional sintering of zirconia materials under fretting wear conditions. In addition, humidity can reduce the wear volume loss due to the lubricative effect of water and wear of degraded materials might increase the resistance due to the formation of a protective debris layer. In general, microwave sintering can be an interesting alternative for obtaining fully-densified Y-TZP dental materials providing certain advantages over conventional methods. Nonetheless, more studies are still necessary to have a better understanding of the advantages and disadvantages of microwave sintering of zirconia ceramics.[ES] La circona es un material ampliamente utilizado como cerámica estructural con aplicaciones en el ámbito dental debido a sus propiedades mecánicas, biocompatibilidad, características estéticas y durabilidad. Para poder aprovechar las altas propiedades mecánicas de la circona, es necesario estabilizarla en su fase tetragonal. Los materiales de circona policristalina estabilizada con itria (Y-TZP) se consolidan normalmente a través de polvos mediante procesos energéticamente intensivos a altas temperaturas (>1000 °C). Actualmente, se están desarrollando técnicas basadas en métodos no convencionales para reducir el tiempo y el consumo energético en el procesado de polvos cerámicos. La sinterización por microondas tiene por objetivo la densificación completa mediante la utilización de mecanismos de calentamiento basados en las propiedades dieléctricas del material. El objetivo principal es la obtención de materiales dentales de Y-TZP altamente densos mediante la sinterización por microondas con propiedades mecánicas y microestructurales similares, o incluso por encima de las obtenidas por el método convencional. Para ello, se estudian aspectos relevantes al ámbito dental. En primer lugar, los materiales son caracterizados con el fin de determinar si las propiedades finales cumplen con los requisitos mecánicos para aplicaciones dentales. Además, se ha investigado la influencia de la sinterización por microondas en la degradación hidrotérmica, un fenómeno espontáneo de envejecimiento que afecta a los materiales de circona en condiciones de humedad. Finalmente, se ha evaluado el comportamiento en condiciones de desgaste fretting de los materiales sinterizados para determinar su durabilidad. Las conclusiones principales indican que la sinterización por microondas permite la consolidación adecuada de estos materiales, resultando en una microestructura más fina debido a los tiempos más cortos de procesado y en propiedades mecánicas comparables a las de materiales obtenidos mediante el método convencional, incluso a temperaturas más bajas. Una mayor resistencia a la degradación hidrotérmica se ha determinado en materiales sinterizados por microondas. Al emplear temperaturas más bajas se reduce la presencia de fase cúbica, la cual es responsable por la desestabilización de granos adyacentes de fase tetragonal. Tasas de desgaste similares han sido observadas entre materiales sinterizados por microondas y convencionalmente bajo condiciones de desgaste fretting. Adicionalmente, la humedad puede reducir sustancialmente la pérdida de volumen de desgaste debido al efecto lubricante del agua y los materiales degradados pueden aumentar la resistencia a este tipo de desgaste como consecuencia de la formación de una capa protectora de material que se desprende más fácil. En general, la sinterización por microondas es una alternativa interesante para obtener materiales dentales de Y-TZP altamente densos con ciertas ventajas sobre los métodos convencionales pero deben considerarse también las desventajas de esta técnica.[CA] La circona és un material àmpliament utilitzat com a ceràmica estructural amb aplicacions en l'àmbit dental a causa de les seues propietats mecàniques, biocompatibilidad, característiques estètiques i durabilitat. Per a poder aprofitar les altes propietats mecàniques de la circona, és necessari estabilitzar-la en la seua fase tetragonal. Els materials de circona policristalina estabilitzada amb itria (Y-TZP) es consoliden normalment mitjançant processos energèticament intensius a altes temperatures (>1000 °C). Actualment, s'estan desenvolupant tècniques basades en mètodes no convencionals per a reduir el temps i el consum energètic en el processament de la pols ceràmicas. La sinterització per microones té per objectiu la densificació completa mitjançant la utilització de mecanismes d'escalfament basats en les propietats dielèctriques del material. L'objectiu principal d'aquesta tesi és l'obtenció de materials dentals de Y-TZP altament densos mitjançant la sinterització per microones amb propietats mecàniques i microestructurals superiors a les obtingudes per mètodes convencionals. En primer lloc, els materials seràn caracteritzats per a determinar si les propietats finals compleixen amb els requisits mecànics per a aplicacions dentals. En segon lloc, s'investigarà la influència de la sinterització per microones en la degradació hidrotèrmica, un fenomen espontani d'envelliment que afecta als materials de circona en condicions d'humitat. I en tercer lloc, s'avaluarà el comportament en condicions de desgast fretting dels materials sinteritzats per a determinar la seua durabilitat. Les conclusions principals indiquen que la sinterització per microones permet la consolidació adequada i millorada de materials de Y-TZP, amb una microestructura més fina a causa dels temps més curts de processament i propietats mecàniques comparables a les de materials obtinguts mitjançant el mètode convencional, fins i tot a temperatures més baixes. Un factor positiu ha sigut la major resistència a la degradació hidrotèrmica en materials sinteritzats per microones. A més, al emprar temperatures més baixes es redueix la presència de fase cúbica, la qual és la responsable de la desestabilització de grans adjacents de fase tetragonal. Finalment, sota condicions de desgast fretting, s'han observat taxes de desgast similars entre materials sinteritzats per microones i via convencional. Addicionalment, en condicions de 100% d'humitat es pot reduir substancialment la pèrdua de volum de desgast a causa de l'efecte lubrificant de l'aigua i materials degradats, els quals poden augmentar la resistència a aquest tipus de desgast com a conseqüència de la formació d'una capa protectora de material que es desprèn amb més facilitat. En general, la sinterització per microones és una alternativa molt interessant per a obtindre materials dentals Y-TZP òptims i amb certes avantatges sobre els mètodes convencionals, però han de considerar-se també algunes desavantatges d'aquesta tècnica.Presenda Barrera, Á. (2016). ADVANCED CERAMIC MATERIALS FOR DENTAL APPLICATIONS SINTERED BY MICROWAVE HEATING [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/68510TESI

Effect of annealing ambient conditions on crack formation mechanisms of bulk Bi-2212 ceramic systems

This study paves way to examine the influence of different annealing conditions (temperature range of 830-850°C and duration intervals 24-48 h) on the fundamental mechanical performance and characteristic quantities of polycrystalline Bi2.1Sr2.0Ca1.1Cu2.0Oy (Bi-2212) superconducting ceramics by means of Vickers microindentation hardness tests at the various indentation test loads (0.245 N≤F≤2.940 N) and some available theoretical approaches. The annealing ambient plays an important role on the operable slip systems and crystal quality. The bulk Bi-2212 superconducting compound prepared at 840 °C and 24 h is found to be the least sensitive to the applied test load due to less structural problems, voids, cracks and stress raisers in the crystal system. Conversely, the excess annealing ambient complicates remarkably the control of crack growth size and velocity. Thus, relatively lower load can lead to the formation of crack and acceleration of crack rate up to the critical size and terminal velocity. The samples exhibit the typical indentation size effect (ISE) behavior as a result of predominant character of elastic recovery mechanism. As for the theoretical examination in the saturation limit regions, the indentation-induced cracking (IIC) model wins the comparison as it provides the most accurate results to the experimental findings

Wear Behavior Characterization For The Screening Of Magnesium-Based Alloys

This research is focused on the development of a systematic approach to evaluate the selection of materials for Mg-based alloys under wear conditions for biomedical applications. A pilot study was carried out in order to establish an accurate and reliable wear testing technique for magnesium and its alloys. This pilot study was conducted on aluminum (Al) and pure Mg, and showed that aluminum has a lower wear rate compared to Mg. The technique displayed good repeatability and high precision. For the main study, an ERC Mg-based alloy was to be compared with pure Mg. The same technique, when applied to pure Mg from a different vendor, produced up to 90% scatter in the data. Microstructure was studied to see if it had any correlation with the scatter. It was discovered that Mg ingot from the second vendor had outsized grains that contributed to the disproportional scatter in the wear data. Increasing the stroke length during wear testing was required so that the wear data would be averaged over multiple grains and reduces the variation in computed wear rates

Evaluating the Wear of Polycrystalline Diamond Compact Drill Bit Cutters using Indentation and Scratch Tests

Abstract–Polycrystalline diamond compact (PDC) drill bits are widely used in oil and gas drilling. The wear of PDC cutters is a major problem during drilling. It leads to severe time losses which affect the overall drilling operation cost. Therefore, it is essential to evaluate the wear tendency for these cutters using predictive approaches. The present research is focused on studying the wear mechanisms of PDC cutters and the effect of their mechanical properties on the extent of wear. The volume of wear for the PDC cutters was determined experimentally using micro- and nano-scratch tests by implementing an approach based on the geometry of the removed material after micro- and nano-scratch tests. The experimental wear results were compared to the predictions from current models in the literature.Various wear models are evaluated for micro- and nano-scratch tests on both layers of the PDC samples. The study shows that the wear of the PDC cutters can be predicted from the material mechanical properties, applied load, sliding distance, and hardness of the PDC cutters. The study could be extended for the evaluation of wear intensity of PDC cutters from various manufactures without using the previous techniques of abrasion testing

Development of ceramic reinforced iron aluminide based composite coatings for wear resistant applications

Les composés intermétalliques Fe₃Al et leurs revêtements composites sont des matériaux structuraux potentiels pour des applications tribologiques. Parmi les composites, ceux obtenus par broyage mécanique à haute énergie possèdent plusieurs avantages, en particulier une fabrication rentable. Le broyage à billes à haute énergie permet également une large gamme de fraction volumique des particules de renforcement. Dans cette recherche, Nous avons préparé des revêtements composites à matrice d'aluminiure de fer, basés sur la composition chimique de Fe₃Al avec des particules de renforcement de TiC et de TiB₂ en utilisant un broyeur à billes à haute énergie et déposé par la technique HVOF (High Velocity Oxy Fuel). L'effet des paramètres de traitement tels que la durée du broyage et le traitement thermique subséquent sur les la matière première destinés à la projection par HVOF a été étudié. Les paramètres de traitement ont joué des rôles importants sur la poudre composite et par la suite sur la microstructure, les propriétés mécaniques et tribologiques des revêtements. Le but de la première phase expérimentale de ce travail était d'étudier l'effet des particules de TiC in situ sur la microstructure, le comportement mécanique et tribologique des revêtements de Fe₃Al déposés par HVOF. Dans cette étape, des poudres composites Fe₃Al / TiC avec différentes quantités de carbure de titane ont été produites par broyage à haute énergie. Un mélange de Fe₃Al-Ti-C a été broyé pendant 6 h suivi d'un traitement thermique à 1000 °C pendant 2 h sous vide poussé. Des revêtements composites d'aluminure de fer renforcés au TiC in situ ont été préparés pour améliorer la dureté Vickers et la résistance à l'usure des intermétalliques de Fe₃Al. Les revêtements composites consistent principalement en une phase de TiC uniformément dispersée dans des lamelles de la matrice de Fe₃Al. Les revêtements composites ont montré une dureté Vickers croissante avec l’augmentation de la quantité de TiC, allant jusqu'à 70 % en moles de TiC. La résistance à l'usure par glissement à sec des revêtements a été augmentée avec l'addition de particules de TiC formées in situ. Les revêtements composites de Fe₃Al déposés par HVOF avec des renforts en TiC de 50 % et 70 % en moles présentaient une excellente résistance à l'usure par glissement. Le mécanisme d'usure dominant de ces revêtements était l'abrasion et l'oxydation. Dans une autre étape de ce travail, des poudres composites de Fe₃Al-TiB₂ avec deux quantités différentes de borure ont été produites par le dépôt par high Velocity Oxy Fuel (HVOF) sur un substrat en acier. Les revêtements composites consistaient principalement en une phase de TiB₂ pré-synthétisée et uniformément dispersée dans des lamelles de la matrice de Fe₃Al. Il a été montré qu'en augmentant la fraction volumique du TiB₂, la dureté Vickers et la résistance à l'usure par glissement des revêtements contre le contre-corps en alumine (6,33 mm de diamètre) étaient augmentées. L'augmentation de la résistance à l'usure était censée être liée à l'amélioration de la dureté, qui à son tour est due à la présence de particules de TiB₂ dans la matrice Fe₃Al. Le taux d'usure de glissement des revêtements a augmenté pour atteindre un maximum lorsque la vitesse de glissement augmente, puis il a diminué avec l'augmentation supplémentaire de la vitesse de glissement. Les analyses chimiques des surfaces usées ont montré que des vitesses de glissement plus élevées entraînent une oxydation plus élevée de la surface, probablement en raison de la température locale plus élevée. Une telle couche d'oxyde semble agir comme une barrière entre deux corps coulissants, diminuant ainsi le taux d'usure.Fe₃Al intermetallic compounds and their composite coatings are potential structural materials for tribological applications. High-energy ball milled powders possess several advantages, especially cost-effective fabrication and lower cost of reinforcement. High-energy ball mill also allows for a wide range of reinforcement volume fraction. In this research, Iron Aluminide matrix composite coatings based on Fe₃Al chemical composition with TiC and TiB₂ particles were prepared using a high-energy ball mill and deposited via the High Velocity Oxy Fuel (HVOF) technique. The effect of processing parameters such as ball milling duration and subsequent heat treatment soaking time and temperature on the phases of products as a feed stock for the HVOF gun was studied. The processing parameters played important roles on the microstructure, mechanical and tribological properties of the coatings. The aim of the first experimental stage of this work was to study the effect of in-situ TiC particles on microstructure, mechanical and tribological behavior of HVOF deposited Fe₃Al coatings. In this stage Fe₃Al/TiC composite powders with different carbide quantities were produced via high-energy ball milling of Fe₃Al-Ti-C system for 6 h followed by heat treatment at 1000 °C for 2 h under high vacuum. In-situ TiC-reinforced iron aluminide composite coatings were prepared to improve the Vickers hardness and wear resistance of Fe₃Al intermetallics. The composite coatings mainly consist of a TiC phase uniformly dispersed within lamellae of the Fe₃Al matrix. The composite coatings showed increasing Vickers hardness with increasing TiC content up to 70 mol% TiC. The dry sliding wear resistance of coatings was increased with the addition of in-situ formed TiC particles. HVOF deposited Fe₃Al composite coatings with 50 and 70 mol% TiC reinforcements exhibited excellent sliding wear resistance. The dominant wear mechanism in those coatings was abrasion and oxidation. In another stage of this work Fe₃Al-TiB2 composite powders with two different boride quantities were produced by the high Velocity Oxy Fuel (HVOF) spray deposition on a steel substrate. The composite coatings mainly consisted of a TiB₂ phase uniformly dispersed within lamellae of the Fe₃Al matrix. It was shown that by increasing the volume fraction of TiB₂ both the Vickers hardness and sliding wear resistance of the coatings against alumina counterbody (6.33 mm in diameter) were increased. The increase of wear resistance was believed to be related to the hardness enhancement, which, in turn, is due to the presence of TiB₂ particles within the Fe3Al matrix. The sliding wear rate of the coatings increased to reach a maximum as the sliding speed increases, and then it decreased with further increase of the sliding speed. The chemical analyses of the worn surfaces showed that higher sliding speeds result in higher oxidation of the surface, most likely due to the higher local temperature. Such an oxide layer seems to act as a barrier between two sliding bodies, thus decreasing the wear rate

Development of a transparent indenter measurement system and indentation analysis for material mechanical property evaluation

Since Tabor showed the application of the spherical indentation approach to obtain material post-yielding true stress-strain curves, the indentation technique has been investigated to determine mechanical properties besides hardness measurement. With the development of various thin film materials, depth-sensing indentation is often applied to evaluate mechanical properties of thin film materials. However, indentation testing is a complicated mechanical process and analytical solutions are difficult to obtain. As a result, much of the understanding of the indentation process has been acquired through experiments and finite element simulations. Thus accurate measurement of indentation parameters is critical in the determination of surface mechanical properties using indentation method.;In this research, a Transparent Indenter Measurement (TIM) method was developed for material inspection and mechanical property measurement. The TIM method is capable of on-site, in-situ, (i) mechanical property measurement (hardness, Young\u27s modulus, yield strength and post-yielding stress strain data) of material samples/machined parts and (ii) qualitative inspection of material surface conditions. The residual surface deformation after spherical indentations was first investigated on IN783 superalloy samples using phase-shifting moire and Twyman-Green interferometry. The elastic-plastic boundary was identified based on the characteristic of the out-of-plane deformation fringe patterns. Then using the measured in-plane deformation, yield strength of the tested material was obtained. Using the TIM system, real-time in-situ measurement of indentation-induced out-of-plane deformation and contact radius are directly measured during an indentation process. Coupled with elastic recovery theories and 2D finite element analyses, a procedure was developed to determine the material stress-strain curve. It is also demonstrated that the TIM method is suitable for debonding inspection of thin film materials

Nanoindentation characterization on local plastic response of Ti-6Al-4V under high-load spherical indentation

After high-load spherical indentation treatment, the variations of hardness on the plastic zone of Ti-6Al-4V were investigated via nanoindentation method. The hardness within the center of plastic zone was measured by nanoindenter, and the magnitude decreased gradually along the depth, which were caused by the different extent of plastic deformation under the residual imprint. The microstructure of indentation were observed using scanning electron microscope (SEM) before and after surface etching, and the results showed that the microhardness revealed the average hardness of α and β phases of Ti-6Al-4V. The maximum hardness reached 6.438 GPa in the depth of 132 μm. In addition, the two and three dimensional contour profiles of residual imprint introduced by high-load spherical indentation were measured by the white-light interferometer and the shape of residual imprint was obtained. All results were discussed in detail

Effective Boronizing Process for Age Hardened Inconel 718

Boronizing or boriding is a technique to mitigate wear damage in industrial valves made of age-hardenable Inconel® 718 which is a nickel-based superalloy. Boriding involves immersing the part in a patented boron-based compound and heating over 800 oC. Boriding combined with aging has a detrimental thermal effect and was the subject of this investigation. The effects on hardness, wear and grain size, of boriding and aging separately, subsequently, and simultaneously, were investigated to observe the microstructure and mechanical properties. The results show that boriding has negligible effect on the grain size and the hardness of the substrate. Besides, a boride coating can improve the surface hardness by a factor of five and reduce the wear damage by over 75%. As the wear performance strongly depends on the coating thickness, we found that initial aging and subsequent boriding delivers the thickest layer and is the best method for boriding of Inconel® 718

Dry-sliding wear behavior of 3Y-TZP/Al2O3-NbC nanocomposites produced by conventional sintering and spark plasma sintering

[EN] This work presents the initial results of the dry-sliding wear behavior of 3 mol% yttria-stabilized zirconia reinforced with 5 vol% alumina-niobium carbide (3Y-TZP/5 vol% Al2O3-NbC) nanocomposites sintered by conventional sintering and spark plasma sintering methods in the temperature range of 1350-1450 degrees C. The reinforcement of 3Y-TZP matrix with hard nanoparticles aimed to improve wear strength of the composites. Wear tests were performed by the ball-on-disc method using alumina (Al2O3) and tungsten carbide with 6 wt% cobalt cermet (WC-6%Co) balls as counter-materials, a load of 15 N, a sliding distance of 2000 m, and a sliding speed of 0.1 m/s. Wear behavior was evaluated in terms of wear rate and FE-SEM micrograph analysis of the wear tracks. The nanocomposite sintered at 1450 degrees C by conventional sintering exhibited the least wear when tested with the WC-6%Co ball. Generally, the wear mechanism showed evidence of severe wear regime with both counter-materials.The authors acknowledge the Brazilian institutions CAPES-PVE (grant number 23038.009604/2013-12), FAPESP (grant number 2015/07319-8), Fundação Araucária (grant number 810/2014), European Union/Erasmus Mundus for doctorate mobility (grant number EB15DM1542), and the Spanish Ministry of Economy and Competitiveness (RYC-2016-20915).Salem, R.; Gutiérrez-González, C.; Borrell Tomás, MA.; Salvador Moya, MD.; Chinelatto, AL.; Chinelatto, AS.; Pallone, E. (2019). Dry-sliding wear behavior of 3Y-TZP/Al2O3-NbC nanocomposites produced by conventional sintering and spark plasma sintering. International Journal of Applied Ceramic Technology. 16(3):1265-1273. https://doi.org/10.1111/ijac.13151S12651273163Liu, H., Zhao, W., Ji, Y., Cui, J., Chu, Y., & Rao, P. (2017). Determination of fracture toughness of zirconia ceramics with different yttria concentrations by SEVNB method. 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