C.W. Fairhurst

Calibration of FTIR conversion analysis of contemporary dental resin composites

Composite restorative material has undergone gradual change in composition since its introduction in the 1960s. Early commercial resins were mixtures of BIS-GMA and TEGDMA. Today, these mixtures are still present, but urethane dimethacrylates and large oligomeric structures of BIS-GMA-urethanes exist. Because of these changes in composition, the past methods of calculating monomer conversion by means of infrared spectroscopy may need modification. This research investigates different methods used to formulate calibration curves for determination of monomer conversion by infrared spectroscopy of contemporary commercial composites containing aromatic structures. Conversion calibration procedures using various baseline methods with BIS-GMA, Bisphenol-A/TEGDMA, and a hydrogenated bonding resin were established. Both peak and area infrared absorptions were determined. One particular baseline method proved the best fit to the Beer-Lambert law. Bisphenol-A was found unsuitable as an infrared calibration model for resin composites. The BIS-GMA/TEGDMA calibration model closely simulates conversion values obtained when a hydrogenated commercial resin model was used.

Materials Science Comparative Evaluation of Ceramic-metal Bond Tests Using Finite Element Stress Analysis

Eleven porcelain-fused-to-metal bond tests were analyzed for interfacial shear stress distribution using finite-element stress analysis. Stress concentration effects are significant in ten of the 11 tests. A high probability of tensile failure within porcelain or the interfacial region was found in eight of the 11 tests analyzed.Eleven porcelain-fused-to-metal bond tests were analyzed for interfacial shear stress distribution using finite-element stress analysis. Stress concentration effects are significant in ten of the 11 tests. A high probability of tensile failure within porcelain or the interfacial region was found in eight of the 11 tests analyzed.

Fatigue of Dental Ceramics in a Simulated Oral Environment

Fatigue in ceramics refers to the subcritical growth of cracks, aided by the combined influence of water and stress. The dynamic fatigue (constant stressing rate) method was used to obtain subcritical crack growth parameters for three dental ceramics: a feldspathic porcelain, an aluminous porcelain, and a fine-grain, polycrystalline core material. The constant stressing rate experiments were carried out at 37°C for all three ceramics in distilled water, and, for the feldspathic porcelain, in artificial saliva as well. Considerable differences were found in the value of the crack growth exponent (n) among the three ceramics. The feldspathic porcelain was lowest in n-value, while the fine-grain ceramic had the highest n-value. No differences were found for the feldspathic porcelain with respect to n measured in water and in the artificial saliva. Lifetime prediction curves in 37°C water, constructed from the n-values and inert strengths, showed that fatigue failure within five years is a good possibility for feldspathic porcelain specimens at stress levels which can reasonably be anticipated to occur in the oral environment. Little likelihood of failure was perceived for the fine-grain ceramic. The aluminous porcelain was intermediate between these two materials with respect to failure probability.

Oxide Adherence and Porcelain Bonding to Titanium and Ti-6A1-4V Alloy

The bonding of an experimental low-fusing porcelain to titanium and Ti-6AI-4V was evaluated by an x-ray spectrometric technique that measures the area that remains covered with porcelain following a controlled deformation of the metallic substrate. Oxide adherence strength values for titanium and Ti- 6AI-4V oxidized at 750° and 1000°C were measured in tension with use of high-strength adhesives. The effect of further oxidation that would occur during porcelain firing was evaluated via simulated porcelain firings without actual porcelain application. Interface cross-sections of the titanium-porcelain and Ti-6AI-4V-porcelain bonds were examined in a scanning electron microscope (SEM). The porcelain was found to delaminate completely from the metal substrate, leaving less than 1% of the surface covered with porcelain. The oxide adherence of the specimens oxidized at 750°C was good, but those oxidized at 1000°C exhibited significantly lower oxide adherence (p = 0.001). The simulated porcelain-firing oxidation treatments also produced a significant decrease in oxide adherence (p = 0.004). The 750°C oxidation treatments produced oxide films too thin to be visualized in the SEM, whereas the 1000°C oxidation treatments produced oxide films approximately 1 µm thick. The lower oxide adherence of the 1-~,m-thick oxide films is consistent with reports in the titanium literature of oxide delamination when the oxide film reaches 1 µm in thickness.

Fracture toughness of commercial dental porcelains

Abstract Ceramic materials suffer from a number of limitations which restrict their use in dental restorations. Chief among these shortcomings is their extreme brittleness which is manifested as a low ability to absorb elastic strain energy during structural failure. This property is embodied in the material parameter, fracture toughness or K IC . The fracture toughness values of two types of dental porcelains, the feldspathic and aluminous porcelains, were determined by the indentation technique. The aluminous porcelains were significantly tougher than the feldspathic porcelains. These differences in K IC were attributed to differences in the nature of crack-microstructure interaction occurring in the two types of porcelain.

The effect of the leucite transformation on dental porcelain expansion

Abstract Nearly all commercial dental porcelains designed for fusing to metal rely upon the crystalline phase leucite to supply the needed high thermal expansion. Repeated firings and certain heat treatments are known to alter dental porcelain thermal expansion behavior, and such changes in expansion could induce cracking, checking, or spalling of the porcelain. Because of the likelihood of involvement of leucite in these expansion changes, this study sought to determine the role of leucite thermal expansion, and particularly the displacive (martensitic) transformation of leucite, in the expansion behavior of the leucite-containing Component No. 1 of the Weinstein patent. The thermal expansion of this frit was modelled as a weighted average of the leucite expansion (including the discontinuous volume change that accompanies the transformation) as measured by hot-state x-ray diffraction and the expansion of the glass matrix calculated from its composition. Close agreement was found between the predicted and measured expansion for Component No. 1. The non-linearity of the α curve for Component No. 1 is explained by the discontinuous volume change during the leucite transformation.

The effect of glaze on porcelain strength

The self-glazing technique provides an esthetic and hygienic surface for crowns and fixed partial dentures that use porcelain veneers. A study of the biaxial flexure strengths of polished vs. glazed specimens is needed to verify that current laboratory methods are appropriate for planned fatigue studies. Four groups of 50 porcelain disk specimens each were subjected to the following polishing and firing procedures: group one was fired, glazed-no hold, and polished; group two was fired, polished, and glazed-no hold; group three was fired, polished and glazed-1 min. hold; group four was fired, polished, and not glazed. The piston-on-three-ball method was used for testing biaxial flexure strengths. Significantly lower differences in biaxial flexure strengths were noted when group two values were compared with values from groups one, three and four. The results show that the Weibull distribution is an appropriate model for our studies. Differences in glaze thickness among the groups were noted in SEM examination; however, bulk (interior) microcrack density differences were absent. The specimens that were fired, polished to a 1 micron surface finish, and not glazed (group four) were significantly higher in flexure strength than groups one and three at the p less than 0.001 level. The hypothesis that glazing of porcelain surfaces improves the biaxial flexure strength of test specimens was rejected.

High-temperature Behavior of a Pd-Ag Alloy for Porcelain

The mechanism of formation of nodular material on the surface of a Pd-Ag-based alloy for porcelain during pre-porcelainization heat treatment was investigated using scanning electron microscopy, x-ray diffraction, quantitative metallography, and Auger electron spectroscopy. The nodules were found to form by a Nabarro-Herring creep mechanism driven by the internal oxidation of tin and indium. Implications of this process with regard to porcelain bonding and discoloration are discussed.

Adherence Controlling Elements in Ceramic-Metal Systems. II. Nonprecious Alloys

Nickel-chromium alloy-ceramic couples used in this study exhibited Cr, Ni-Cr, and Ti-Cr interactions with bonding agent-ceramic complexes. Bonding agents may broaden or suppress the width of interaction zones. Metal oxides such as NiO, Cr2O 3, or more complex forms such as TiO · Cr2O3 and NiCr2O4 may play a prominent role in controlling ceramic-metal adherence.

Clinical progress of sealed and unsealed caries. Part I: Depth changes and bacterial counts

1. A method for measuring depth changes in carious lesions has been developed. 2. A method for estimating bacterial viability in the study lesions has been developed. 3. Sealed cavities showed little or no change in depth over a 1-year period. 4. Open lesions showed a significant increase in depth as compared to sealed lesions. 5. Elimination or marked decrease in viable organisms under the sealant occurred.