In vitro fracture strength and thermal shock resistance of metal-ceramic crowns with cast and machined AuTi frameworks

STATEMENT OF PROBLEM: AuTi alloys with 1.6% to 1.7% (wt%) Ti provide sufficient bond strength to veneering ceramics, but the strength of entire metal-ceramic restorations fabricated from these alloys is not known. However, this information is important to assess the clinical performance of such materials. PURPOSE: This in vitro study evaluated the fracture strength and thermal shock resistance of metal-ceramic crowns with AuTi frameworks produced by milling or casting. MATERIAL AND METHODS: Frameworks of the alloy Au-1.7Ti-0.1Ir (wt%) (Esteticor Vision) were produced by milling or casting (test groups). A high-gold alloy (Esteticor Special) was used as the control. The frameworks were veneered with ceramic (VMK 95). Specimens (n=7) were loaded until fracture. Loads at failure (N) were recorded and the mean values statistically evaluated using 1-way analysis of variance and a post hoc Dunnett test (alpha=.05). To assess the crazing resistance of the veneering ceramic, 6 additional crowns of each group were subjected to a thermal shock test. Fractured surfaces were documented by scanning electron microscopy. Coefficients of thermal expansion of the materials used were measured (n=2) to assess the thermal compatibility between alloys and ceramic. RESULTS: The mean fracture strength of the crowns with machined AuTi frameworks (1294 +/- 236 N) was significantly lower (P=.012) than that of the cast AuTi frameworks (1680 +/- 150 N), but statistically not different than the high-gold alloy (1449 +/- 159 N). Bonding failure to the AuTi alloy predominantly occurred at the alloy-oxide interface. For the high-gold alloy, more ceramic residues were observed. In the thermal shock test, crowns with milled AuTi frameworks showed significantly higher thermal shock resistance compared to the other groups. The coefficients of thermal expansion (Esteticor Vision cast: 14.5 microm/m.K; Esteticor Vision milled: 14.3 microm/m.K; Esteticor Special cast: 13.7 microm/m.K) did not correlate with the results of the thermal shock test. CONCLUSION: The in vitro fracture strength of crowns with milled AuTi frameworks is lower than that obtained with cast AuTi frameworks, but comparable to those crowns produced with a high-gold alloy.

Factors affecting thermal shock resistance of polyphase ceramic bodies

Mode of access: Internet.

Thermal shock – and its perfect imperfections

Dissertação de Mestrado em Arte e Ciência do Vidro

Heat shock resistance traits in specimens of the seaweed Gracilaria vermiculophylla originating from native and non-native populations

We compared the responses of native and non-native populations of the seaweed Gracilaria vermiculophylla to heat shock in common garden-type experiments. Specimens from six native populations in East Asia and from eight non-native populations in Europe and on the Mexican Pacific coast were acclimated to two sets of identical conditions before their resistance to heat shock was examined. The experiments were carried out twice - one time in the native range in Qingdao, China and one time in the invaded range in Kiel, Germany - to rule out effects of specific local conditions. In both testing sites the non-native populations survived heat shock significantly better than the native populations, The data underlying this statement are presented in https://doi.pangaea.de/10.1594/PANGAEA.859335. After three hours of heat shock G. vermiculophylla exhibited increased levels of heat shock protein 70 (HSP70) and of a specific isoform of haloperoxidase, suggesting that both enzymes could be required for heat shock stress management. However, the elevated resistance toward heat shock of non-native populations only correlated with an increased constitutive expression of heat shock protein 70 (HSP70). The haloperoxidase isoform was more prominent in native populations, suggesting that not only increased HSP70 expression, but also reduced allocation into haloperoxidase expression after heat shock was selected during the invasion history. The data describing expression of HSP70 and three different isoforms of haloperoxidase are presented in https://doi.pangaea.de/10.1594/PANGAEA.859358.

Sol-Gel Alumina Nano Composites for Functional Applications

The thesis entitled "Sol-Gel Alumina Nano Composites for Functional Applications" investigate sol-gel methods of synthesis of alumina nanocomposites special reference to alumina-aluminium titanate and alumina-lanthanum phosphate composites. The functional properties such as thermal expansion coefficient and thermal shock resistance, machinability of composites as well as thermal protection are highlighted in addition to novel approach in synthesis of composites.A general introduction of alumina matrix composites materials, followed by brief coverage of alumina-aluminium titanate and alumina-lanthanum phosphate composites is highlight of the first chapter. The second chapter deals with the sol-gel synthesis of aluminium titanate and alumina-aluminium titanate composite. The synthesis of machinable substrate, based on alumina and lanthanum phosphate forms the basis of the third chapter. The fourth chapter describes the sol-gel coating of mullite on SiC substrate for the possible gas filtration application.

Development of zirconia-magnesia/zirconia-yttria composite solid electrolytes

Composite solid electrolytes were prepared by thoroughly mixing ZrO2:8 mol% MgO (Z8Mg) and ZrO(2):3 mol% Y(2)O(3) (Z3Y) ceramic powders followed by pressing and sintering at 1500 degrees C/1 h. The properties of the sintered pellets were studied by X-ray diffraction for evaluation of the structural phases by the Rietveld method, by high-temperature dilatometry for analysis of the thermal shrinkage/expansion behavior, and by impedance spectroscopy for determination of the oxide ion conductivity. The x(Z8Mg)+(1-x)(Z3Y) specimens, x= 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0, are partially stabilized (monoclinic, cubic and tetragonal phases) with density >94% of the theoretical density and show thermal shock resistance and electrical conductivity values suitable for high-temperature oxygen gas detection. One-end closed tube samples of the composite solid electrolytes were assembled in Pt/Z8Mg+Z3Y/Cr+Cr(2)O(3)/Pt electrochemical cells for exposure to different levels of oxygen in the 1-850 ppm range. The total electrical conductivity increases for increasing the relative Z3Y content. Addition of Z3Y to Z8Mg (80 wt.%-20 wt.%) suppresses the electronic contribution to the electrical conductivity at 620 degrees C. (c) 2008 Elsevier B.V. All rights reserved.

Fractografia quantitativa e o conceito de tenacidade à fratura para a análise do efeitodo choque térmicos em TiO2

The present study evaluated the use of semicircular bending test (SCB) as an alternative to conventional bending test to examine the effect of thermal shock. Still, studies the behavior of fracture surfaces generated by monofractal behavior analysis, which allows us to evaluate the contributions of the microstructure and the mechanical context in forming reliefs during the crack propagation. The fractal analysis is made from elevation maps obtained by reconstruction method by extension of the focus stacks of digital images acquired in microscope. The specimens used were samples semicircular pressed TiO2 (rutile) with and without heat shock, prepared for testing mode I loading. Were also produced, specimens in the form of bars for Weibull statistical analysis. From the three-point bending test, we found the variation of fracture toughness between the samples after the heat shock and natural condition. The SCB test was feasible for the analysis of thermal shock resistance. The results showed that the value of the fracture toughness decreases as the heat shock treatment

Monitoring the process of densification of 3D and 4D (tridirectional and tetradirectional) carbon/carbon composites

Carbon fiber reinforced carbon composites can be made by iterative liquid impregnation or gas phase carbon deposition routes. In both cases, at the final processing stage the carbon fiber is embedded in carbon matrix which results in unique properties such as low density, high thermal conductivity and thermal shock resistance, low thermal expansion and high modulus, in relation to other refractory materials. In the present study assembled three-directional and four-directional preforms, having 50% volume of pores, were densified by iterative cycles of thermoset resin impregnation followed by pyrolysis under inert atmosphere, until appropriate densities were achieved. The thermoset resin is converted in a carbon matrix during pyrolysis. The iterative manufacturing process of the carbon fiber reinforced carbon composites is evaluated by means of nondestructive techniques based on X-ray computed tomography and electrical resistivity. X-ray computed tomography gives a general mapping view of the filling pores of the preforms which impacts results of the electrical resistivity. After six processing cycles and heat treatments up to 2000?, the final densities of the three-directional and four-directional carbon fiber reinforced carbon composites were 1.16g/cm(3) and an electrical resistivity of approximate to 0.07m. The configuration of preforms, three-directional or four-directional, did not alter the densification profile, in terms of increasing density and reducing porosity during the processing cycles.

Spark Plasma Sintering of Si3N4-based Ceramics : Sintering mechanism-Tailoring microstructure-Evaluationg properties

Spark Plasma Sintering (SPS) is a promising rapid consolidation technique that allows a better understanding and manipulating of sintering kinetics and therefore makes it possible to obtain Si3N4-based ceramics with tailored microstructures, consisting of grains with either equiaxed or elongated morphology. The presence of an extra liquid phase is necessary for forming tough interlocking microstructures in Yb/Y-stabilised α-sialon by HP. The liquid is introduced by a new method, namely by increasing the O/N ratio in the general formula RExSi12-(3x+n)Al3x+nOnN16-n while keeping the cation ratios of RE, Si and Al constant. Monophasic α-sialon ceramics with tailored microstructures, consisting of either fine equiaxed or elongated grains, have been obtained by using SPS, whether or not such an extra liquid phase is involved. The three processes, namely densification, phase transformation and grain growth, which usually occur simultaneously during conventional HP consolidation of Si3N4-based ceramics, have been precisely followed and separately investigated in the SPS process. The enhanced densification is attributed to the non-equilibrium nature of the liquid phase formed during heating. The dominating mechanism during densification is the enhanced grain boundary sliding accompanied by diffusion- and/or reaction-controlled processes. The rapid grain growth is ascribed to a dynamic ripening mechanism based on the formation of a liquid phase that is grossly out of equilibrium, which in turn generates an extra chemical driving force for mass transfer. Monophasic α-sialon ceramics with interlocking microstructures exhibit improved damage tolerance. Y/Yb- stabilised monophasic α-sialon ceramics containing approximately 3 vol% liquid with refined interlocking microstructures have excellent thermal-shock resistance, comparable to the best β-sialon ceramics with 20 vol% additional liquid phase prepared by HP. The obtained sialon ceramics with fine-grained microstructure show formidably improved superplasticity in the presence of an electric field. The compressive strain rate reaches the order of 10-2 s-1 at temperatures above 1500oC, that is, two orders of magnitude higher than that has been realised so far by any other conventional approaches. The high deformation rate recorded in this work opens up possibilities for making ceramic components with complex shapes through super-plastic forming.

Studio del comportamento meccanico di smalti porcellanati per substrati metallici

Composite porcelain enamels are inorganic coatings for metallic components based on a special ceramic-vitreous matrix in which specific additives are randomly dispersed. The ceramic-vitreous matrix is made by a mixture of various raw materials and elements and in particular it is based on boron-silicate glass added with metal oxides(1) of titanium, zinc, tin, zirconia, alumina, ecc. These additions are often used to improve and enhance some important performances such as corrosion(2) and wear resistance, mechanical strength, fracture toughness and also aesthetic functions. The coating process, called enamelling, depends on the nature of the surface, but also on the kind of the used porcelain enamel. For metal sheets coatings two industrial processes are actually used: one based on a wet porcelain enamel and another based on a dry-silicone porcelain enamel. During the firing process, that is performed at about 870°C in the case of a steel substrate, the enamel raw material melts and interacts with the metal substrate so enabling the formation of a continuous varying structure. The interface domain between the substrate and the external layer is made of a complex material system where the ceramic vitreous and the metal constituents are mixed. In particular four main regions can be identified, (i) the pure metal region, (ii) the region where the metal constituents are dominant compared with the ceramic vitreous components, (iii) the region where the ceramic vitreous constituents are dominant compared with the metal ones, and the fourth region (iv) composed by the pure ceramic vitreous material. It has also to be noticed the presence of metallic dendrites that hinder the substrate and the external layer passing through the interphase region. Each region of the final composite structure plays a specific role: the metal substrate has mainly the structural function, the interphase region and the embedded dendrites guarantee the adhesion of the external vitreous layer to the substrate and the external vitreous layer is characterized by an high tribological, corrosion and thermal shock resistance. Such material, due to its internal composition, functionalization and architecture can be considered as a functionally graded composite material. The knowledge of the mechanical, tribological and chemical behavior of such composites is not well established and the research is still in progress. In particular the mechanical performances data about the composite coating are not jet established. In the present work the Residual Stresses, the Young modulus and the First Crack Failure of the composite porcelain enamel coating are studied. Due to the differences of the porcelain composite enamel and steel thermal properties the enamelled steel sheets have residual stresses: compressive residual stress acts on the coating and tensile residual stress acts on the steel sheet. The residual stresses estimation has been performed by measuring the curvature of rectangular one-side coated specimens. The Young modulus and the First Crack Failure (FCF) of the coating have been estimated by four point bending tests (3-7) monitored by means of the Acoustic Emission (AE) technique(5,6). In particular the AE information has been used to identify, during the bending tests, the displacement domain over which no coating failure occurs (Free Failure Zone, FFZ). In the FFZ domain, the Young modulus has been estimated according to ASTM D6272-02. The FCF has been calculated as the ratio between the displacement at the first crack of the coating and the coating thickness on the cracked side. The mechanical performances of the tested coated specimens have also been related and discussed to respective microstructure and surface characteristics by double entry charts.

Nanostructured tungsten as a first wall material for the future nuclear fusion reactors

The lack of materials able to withstand the severe radiation conditions (high thermal loads and atomistic damage) expected in fusion reactors is the actual bottle neck for fusion to become a reality. The main requisite for plasma facing materials (PFM) is to have excellent structural stability since severe cracking or mass loss would hamper their protection role which turns out to be unacceptable. Additional practical requirements for plasma facing materials are among others: (i) high thermal shock resistance, (ii) high thermal conductivity (iii) high melting point (iv) low physical and chemical sputtering, and (v) low tritium retention.

Refratários magnesianos de panela de aciaria: redução da oxidação inicial, formação da fase espinélio MgAl2O4 e resistência ao dano por choque térmico

A campanha dos refratários magnesianos aplicados como revestimento de trabalho de panelas de aciaria depende da soma de diversos fatores como resistência à corrosão, resistência à oxidação do carbono, estabilidade termomecânica, entre outros. A concepção microestrutural do refratário pode influenciar de forma benéfica ou deletéria no desempenho do refratário in situ. Nesta tese de doutorado os refratários magnesianos comerciais de panela de aciaria foram estudados sob três diferentes aspectos: redução da oxidação prematura do carbono, formação da fase espinélio de alumina e magnésio e resistência ao choque térmico e ao dano por choque térmico. Para reduzir a oxidação precoce do carbono foi desenvolvido um coating cerâmico que atua como uma eficiente barreira física, reduzindo o contato do oxigênio da atmosfera de aquecimento com o carbono presente no refratário. Como resultado reduz-se a oxidação prematura do carbono e eleva-se a vida útil do revestimento. A formação da fase espinélio de magnésia e alumina também influencia o desempenho termomecânico destes refratários, principalmente devido ao incremento volumétrico decorrente de sua formação. Nesta tese foram estudados os mecanismos de formação desta fase in situ, demonstrando experimentalmente o caminho preferencial que leva à formação desta fase mineralógica. O comportamento termomecânico dos refratários magnesianos foi determinado em função da resistência ao choque térmico (parâmetros R, R\'\'\') e quanto à resistência ao dano por choque térmico (parâmetro R\'\'\'\' e Rst). Estes parâmetros foram correlacionados com as respectivas características microestruturais destes refratários. Os resultados apresentados por esta tese de doutorado compõe uma importante ferramenta técnica para as indústrias produtoras de aço e de refratários por fornecer subsídio técnico e científico para fundamentar alterações em refratários já existentes e colaborar com o desenvolvimento de novos refratários de engenharia com elevado desempenho e maior vida útil.

The Mechanics of the drawing of polygonal sections from round at elevated temperature

Economic factors such as the rise in cost of raw materials, labour and power, are compelling manufacturers of cold-drawn polygonal sections, to seek new production routes which will enable the expansion in the varieties of metals used and the inclusion of difficult-to-draw materials. One such method generating considerable industrial interest is the drawing of polygonal sections from round at elevated temperature. The technique of drawing mild steel, medium carbon steel and boron steel wire into octagonal, hexagonal and square sections from round at up to 850 deg C and 50% reduction of area in one pass has been established. The main objective was to provide a basic understanding of the process, with particular emphasis being placed on modelling using both experimental and theoretical considerations. Elevated temperature stress-strain data was obtained using a modified torsion testing machine. Data were used in the upper bound solution derived and solved numerically to predict drawing stress strain, strain-rate, temperature and flow stress distribution in the deforming zone for a range of variables. The success of this warm working process will, of course, depend on the use of a satisfactory elevated temperature lubricant, an efficient cooling system, a suitable tool material having good wear and thermal shock resistance and an efficient die profile design which incorporates the principle of least work. The merits and demerits of die materials such as tungsten carbide, chromium carbide, Syalon and Stellite are discussed, principally from the standpoint of minimising drawing force and die wear. Generally, the experimental and theoretical results were in good agreement, the drawing stress could be predicted within close limits and the process proved to be technically feasible. Finite element analysis has been carried out on the various die geometries and die materials, to gain a greater understanding of the behaviour of these dies under the process of elevated temperature drawing, and to establish the temperature distribution and thermal distortion in the deforming zone, thus establishing the optimum die design and die material for the process. It is now possible to predict, for the materials already tested, (i) the optimum drawing temperature range, (ii) the maximum possible reduction of area per pass, (iii) the optimum drawing die profiles and die materials, (iv) the most efficient lubricant in terms of reducing the drawing force and die wear.

Comprehensive process maps for synthesizing high density aluminum oxide-carbon nanotube coatings by plasma spraying for improved mechanical and wear properties

Plasma sprayed aluminum oxide ceramic coating is widely used due to its outstanding wear, corrosion, and thermal shock resistance. But porosity is the integral feature in the plasma sprayed coating which exponentially degrades its properties. In this study, process maps were developed to obtain Al2O3-CNT composite coatings with the highest density (i.e. lowest porosity) and improved mechanical and wear properties. Process map is defined as a set of relationships that correlates large number of plasma processing parameters to the coating properties. Carbon nanotubes (CNTs) were added as reinforcement to Al2O 3 coating to improve the fracture toughness and wear resistance. Two novel powder processing approaches viz spray drying and chemical vapor growth were adopted to disperse CNTs in Al2O3 powder. The degree of CNT dispersion via chemical vapor deposition (CVD) was superior to spray drying but CVD could not synthesize powder in large amount. Hence optimization of plasma processing parameters and process map development was limited to spray dried Al2O3 powder containing 0, 4 and 8 wt. % CNTs. An empirical model using Pareto diagram was developed to link plasma processing parameters with the porosity of coating. Splat morphology as a function of plasma processing parameter was also studied to understand its effect on mechanical properties. Addition of a mere 1.5 wt. % CNTs via CVD technique showed ∼27% and ∼24% increase in the elastic modulus and fracture toughness respectively. Improved toughness was attributed to combined effect of lower porosity and uniform dispersion of CNTs which promoted the toughening by CNT bridging, crack deflection and strong CNT/Al2O3 interface. Al2O 3-8 wt. % CNT coating synthesized using spray dried powder showed 73% improvement in the fracture toughness when porosity reduced from 4.7% to 3.0%. Wear resistance of all coatings at room and elevated temperatures (573 K, 873 K) showed improvement with CNT addition and decreased porosity. Such behavior was due to improved mechanical properties, protective film formation due to tribochemical reaction, and CNT bridging between the splats. Finally, process maps correlating porosity content, CNT content, mechanical properties, and wear properties were developed.