Thesis (Ph.D.)--McMaster University, 2006. / Supervisor: Patrick S. Nicholson. Includes bibliographical references (leaves 146-169).

Thesis (Ph.D.)--University of Wollongong, 2008. / Typescript. Includes bibliographical references.

Thesis (M.S.)--University of Michigan, 2001. / Includes bibliographical references.

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Indiana University-Purdue University Indianapolis (IUPUI), Indiana University School of Dentistry / Background: With the emerging of digital dentistry, IPS e.max® CAD lithium disilicate (LD) glass-ceramic material has become one of the most popular esthetic restorative materials in digital assisted dental esthetic restoration. The mechanical and optical properties of this material have been investigated in several studies. However, there is a lack of information and consensus regarding the optical properties of IPS e.max® LD glass-ceramic materials. Objectives: 1) To investigate the optical properties as translucency parameters (TP), Contrast ratio (CR), light transmissions (Lt) and color changes (CC) between high-translucent (HT) and low-translucent (LT) IPS e.max® CAD LD glass-ceramic materials with different crystalline phases and thickness in different firing stages. 2) To investigate the optical properties as TP, CR, Lt and CC of each translucent (HT and LT) IPS e.max® CAD LD glass-ceramic materials with different crystalline phases and thickness in different firing stages. 3) To determine the mathematical relationships of thicknesses of IPS e.max® CAD LD glass-ceramics materials with TP and Lt. Materials and methods: The total of 120 of shade A2 IPS max CAD samples (HT and LT) were prepared into square shape (15.25 mm X 15.25 mm) and were divided into two main groups according to the material translucency (HT and LT) (n=60). Each main group was further divided into 5 sub-groups according to the thickness (1.00, 1.25, 1.5, 1.75, and 2.00 mm) (n=12). Each thickness group was assigned into three groups based on different crystallization (firing) temperatures (750, 820 °C in single stage heating schedule with 1 second and 10 second holding times, respectively, and 840 °C with two-stage heating schedule (RECOM) (820°C, 840 °C with 10 second and 7 min holding time, respectively) as recommended by manufacturer (n=4). CIEL*a*b*, TP, CR, and Lt were measured and calculated for all samples. Statistical analysis: The effects of the test results were evaluated using 3-way ANOVA with factors for Translucency (HT and LT), Firing Temperature (750, 850, and RECOM) and Thickness (1, 1.25, 1.5, 1.75, and 2), as well as all two-way and three-way interactions among the factors. Pair-wise comparisons were made using Least Significant Differences to control the overall significance level at 5%. Results: The mean irradiance and TP for both HT and LT decrease as the thickness of the samples increases from 1 to 2mm with significant difference between the thickness groups within each material translucency groups (HT and LT) and between both HT and LT. The coefficients of absorption (c) of the two materials were calculated. The effective incidence irradiance when material thickness approaches zero (Ie) was also calculated. There is an unexpected spectral peak shift as the thickness of the samples increases. There is no statistically significant difference in Ie at 750˚C and 820 ˚C between the HT and LT. However, there is a statistically significant higher Ie in HT at the recommended firing temperature as expected. Coefficients of translucency parameter (p) of the materials in various firing temperature were defined and the TP of the material as the thickness approaches zero were calculated (TP0). The TP of the materials is directly correlated to the mean irradiance passing through the samples. There is no statistically significant difference in the TP0 and Ie of the HT and LT groups at the recommended firing temperature. Conclusion: In this project we developed modified Beer-Lambert law to describe the parameters governing the effect of thickness on light transmission in dental ceramic material. We also applied the same equation to describe the translucency parameter. The parameters defined in these equations allow us to compare the optical property of dental ceramic material independent of the thickness of the samples.

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The main objective of the thesis is to study the thermal properties of Si-based polymer derived ceramics (PDCs) at elevated temperatures and to classify the main factors affecting the thermal transport through these ceramics. The polymer derived ceramics with the chemistry Si- O-C-N were prepared starting from commercial polycarbosilane, polysiloxane, and polysilazane precursors. These precursors are cross-linked at room temperature to obtain the preceramic, followed by controlled pyrolysis (at different temperatures ranging from 1200 oC to 1800 oC in argon, nitrogen or carbon-di-oxide atmospheres), to get the final ceramic. The first part of the thesis discusses on development and studies of dense polymer derived thin disks having a basic chemistry, Si-C, Si-O- C, and, Si-C-N-O, developed via a casting technique followed by specific pyrolysis cycles. Having a thickness in the range of 100 μm- 300 μm, these ceramic disks were studied to be nanocrystalline/amorphous at least up to a temperature of 1400 oC and were found to have a significant amount of Cfree phase existing in them along with the intended chemistry. The high-temperature thermal properties were primarily investigated on ceramics prepared at a pyrolysis temperature of 1200 oC (ceramic still in nanocrystalline/amorphous glassy phase). The disks were found to have very low expansion coefficients (CTE) measured up to ~900 oC and the thermal diffusivity (k) and thermal conductivity (l) of these disks were also measured. An attempt to understand the influence of the different phases in a SiOC ceramic (mainly the Cfree phase, studied by enriching the carbon percentages using DVB) in determining the final thermal properties was also conducted. The influence of carbon enrichment on the mechanical properties of these disks is also studied as a sub-part of this work. The second part of the work deals with testing the possibility to use these ceramics for high-temperature insulation applications. ‘Reticulated’ ceramic foams of relatively same chemistries as that of the disks were prepared by a template replica approach, using polyurethane (PU) foams (more open-celled to more closed-celled types of PU foams were used in the study) as the template. Porous structures having densities ranging from as low as 0.02 g.cm-3 to 0.56 g.cm-3 and with a porosity ~ 80 % to ~99% were prepared and tested. The developed foams showed excellent thermal stability up to a temperature of 1400 oC and possessed very low thermal expansion. The thermal conductivity measured on them at RT gave values in the range 0.03 W.m-1.K-1- 0.25 W.m-1.K-1. A Gibson-Ashby modeling approach to explain the thermal conductivity of the porous ceramics was also attempted. The developed foams were also found to be mechanically rigid. In a nutshell, the thesis work studies the thermal properties of Si-O-C- N ceramics in detail and probes into the possibility to develop these class of Si-O-C-N ceramics into promising high-temperature insulation material.

An analytical study was conducted of the effect of cracks on creep of polycrystalline, brittle structural ceramics. Two independent mechanisms of contribution of cracks were defined. The mechanism of elastic creep by crack growth represents the rate of increase in strain, with time, resulting from the time-dependent decrease in elastic moduli of the material, due ·to crack growth. The mechanism of crack-enhanced creep provides a measure of the increase in creep rate over that in an identical but crack-free material, due to the local stress field associated with the cracks and the resultant transfer of stress to the adjacent, crack-free material. Creep rates due to these mechanisms were quantified for simple crack geometries. It was shown that the contribution of cracks can result in an idealized 4-stage creep curve for a brittle, polycrystalline ceramic, in contrast to the conventional 3-stage creep curve for metals. The four stages consist of a primary or crack incubation period, a secondary sigmoidal region resulting from growth of microcracks along grain boundary facets, a tertiary or crack-enhanced stage associated with arrested microcracks, and a quarternary stage comprising crack linkage and coalescence. It was demonstrated that the formation and growth of cracks during creep can result in apparent power-law creep, positive grain size dependence of the creep rate, and grain size-dependent creep activation energy. It can also account for observations of decreasing creep rate with increasing time in constant load creep tests, anomalous stress relaxation behavior in structural ceramics, significantly higher creep rates in tension tests than in compression tests, and discrepancy between diffusion coefficients inferred from creep studies and measured in diffusivity experiments. A simple model was presented for the effect of cracks on creep rate in bending, based on the time-rate of change of curvature of a bend specimen. Analysis of the effect of cracks on creep was extended to a general state of multiaxial stress, through matrix formulation of stress, creep rate, and creep compliance tensors. Derivation of components of the creep compliance tensor from analogs in elasticity was demonstrated for crack-enhanced creep, for uniaxial and uniform triaxial tension, for simple crack geometries. It was demonstrated that materials containing cracks can exhibit a finite rate of creep under hydrostatic tension, in contrast to a corresponding creep rate of zero in crack-free materials. Recommendations are made for analysis and interpretation of experimental creep data for structural ceramics. / Ph. D. / incomplete_metadata

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