J.R. Mackert

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.

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.

Factors Affecting Estimation of Dental Amalgam Mercury Exposure from Measurements of Mercury Vapor Levels in Intra-oral and Expired Air

Several investigators have attempted to assess the extent of mercury exposure from dental amalgam restorations through the use of mercury vapor measurements of intra-oral or expired air. The sampling times and flow rates of the mercury vapor analyzers used in intra-oral measurements are not comparable with physiological inhalation times or flow rates of air through the mouth during inhalation. Analysis of the assumptions and measurement techniques employed in previous attempts to estimate mercury exposure from amalgam restorations shows that adequate compensation had not been made for these measurement parameters. Calculation of the mercury vaporization rates responsible for the mercury vapor concentration values previously reported enabled the daily dose of mercury to be estimated for subjects with various numbers of amalgam restorations. The corrected estimates for daily dose of mercury from amalgam restorations are a factor of sixteen lower than those previously reported.

Comparison of in vivo vs. in vitro Bonding of Composite Resin to the Dentin of Canine Teeth

Dogs were utilized in a study to compare the bond strengths of dentin bonding agents made to dentin in vivo and then again in vitro in the same teeth 30 min, one day, one week, and one month post-extraction. No statistically significant differences were observed between bonds made in vivo and those made in vitro at any time period. Contamination of the dentin surfaces with blood or saliva lowered the bond strengths, but these could be restored to control values by re-surfacing of the dentin with a bur.

The Relationship Between Oxide Adherence and Porcelain-Metal Bonding:

The lack of a reliable bond test has hindered the elucidation of the mechanism for porcelain-metal bonding in dental systems, because a test capable of detecting differences among porcelain-metal bonds of various qualities is required before the reasons for these differences may be ascertained. A method was developed in the present study whereby specimens of alloys with differing physical properties may be deformed to a constant strain to yield a fracture surface suitable for measurement of the area fraction of retained porcelain by an x-ray spectrometric technique described previously. The method proved sufficiently discriminating that significant differences could be found in 48 of the possible 66 comparisons among alloys and treatments. Linear regression analysis revealed a strong correlation (r 2 = 0.947) between the area fractions of retained porcelain measured in the present study and the oxide adherence strength values measured previously. This strong correlation, when considered in light of the literature evidence for the presence of an oxide layer at the porcelain-metal interface, provides compelling support for the oxide layer theory of porcelain-metal bonding in dental alloy systems.

Evidence of a Critical Leucite Particle Size for Microcracking in Dental Porcelains

The leucite particles in dental porcelains are often partially encircled by microcracks that are the result of the thermal expansion mismatch between leucite and the surrounding glass matrix. Although the magnitude of the stress at the particle-matrix interface is independent of the particle size (Selsing, 1961), Davidge and Green (1968) showed experimentally that there is a critical particle size below which microcracking is absent. The critical particle size is explained by a Griffith-type energy balance criterion: Below the critical size, the stress magnitude may be sufficient to cause cracking, but there is insufficient strain energy for the creation of the new surfaces of the microcrack. The purpose of the present study was to determine whether the mean leucite particle size of a dental porcelain influences the degree of microcracking in the porcelain. Microcrack density, leucite particle surface area per unit volume, and leucite mean volume-surface diameter, D 3,2, were determined by quantitative stereology on 10 specimens each of 6 dental porcelains and Component No. 1 of the Weinstein et al. patent (US Patent 3,052,982, 1962). The fraction of leucite particles with microcracks around them, fmc, was estimated for each porcelain from the microcrack density and the leucite surface area. Using the equations of Selsing (1961) and Davidge and Green (1968), we calculated the critical particle diameter, Dc , for leucite to be 4 μm. The porcelains were partitioned according to whether their mean leucite particle diameters, D 3,2, fell above or below D c, and their values of fmc were analyzed by a permutation test with random re-sampling. The porcelains with mean leucite particle diameters below D c had a significantly lower fraction of cracked particles compared with the porcelains with mean leucite particle diameters above D c (p < 0.05). This study provides evidence that microcracking in dental porcelain can be minimized by a reduction of the mean leucite particle diameter to less than 4 μm.

Microcracks in Dental Porcelain and Their Behavior during Multiple Firing

Dental porcelains rely on the high-thermal-expansion mineral leucite to elevate their bulk thermal expansion to levels compatible with dental PFM alloys. The microcracks that form around these leucite particles when cooled during porcelain manufacture are a potential source of change in bulk porcelain thermal expansion during fabrication of porcelain-fused-to-metal crowns and bridges. The purpose of the present study was to determine whether multiple firings of commercial dental porcelains could produce changes in microcrack density. Specimens of six commercial porcelains and the Component No. 1 of the Weinstein patent were fabricated and subjected to 1, 2, 4, 8, and 16 firings. The microcrack densities were determined by quantitative stereology, whereby intersections of microcracks were counted with a test grid. The microcrack data were subjected to linear regression analysis and analysis of variance. The microcrack densities of four of the six porcelains and the Component No. 1 frit were not significantly affected by the number of firings (p > 0.05). One porcelain exhibited a weak but highly significant positive correlation between microcrack density and multiple firings ( r 2 = 0.24, p = 0.0003), while the remaining porcelain exhibited a weak but statistically significant negative correlation between microcrack density and multiple firings (r 2 = 0.15, p = 0.006). The results of this study indicate that even for porcelains that exhibit a measurable change in microcrack density as a function of multiple firings, the magnitude of the increase or decrease in microcrack density after several firings is sufficiently small to cause only negligible shifts in porcelain bulk thermal expansion.

Measurement of Oxide Adherence to PFM Alloys

A method has been reported for evaluating adherence of an oxide to its substrate metal to a maximum value of about 40 MPa. Oxidized alloy plates were cemented between two aluminum cylinders with a high-strength cyanoacrylate cement and loaded in tension until failure occurred either at the oxide/metal interface, within the oxide layer, or in the cement itself. Significant differences were found among the oxide adherence values obtained from different PFM alloys. The oxides formed on five of the alloys exhibited adherence strengths in excess of the published value for cohesive strength of dental opaque porcelain, indicating that they possess sufficient adherence to act as the transition zone between the porcelain and the alloy. In addition, a correspondence was found between the quality of porcelain bond for a given alloy and its oxide adherence strength. These results remove the principal objection to the oxide-layer theory of porcelain bonding in dental alloy systems and emphasize the importance of oxide adherence in the establishment of a bond. It is therefore suggested that future work devoted to porcelain-metal bonding should seek to elucidate the mechanism of oxide adherence to PFM alloys and explore the development of new alloys which form adherent oxides.

Isothermal Anneal Effect on Microcrack Density around Leucite Particles in Dental Porcelain

Because of the large differential in thermal expansion coefficient between leucite and the surrounding glass matrix, microcracks form around the leucite crystallites during the manufacture of dental porcelain frits. These microcracks decouple leucite from the surrounding glass matrix and affect the bulk thermal expansion of the porcelain frit (Binns, 1983). The purpose of this study was to determine if the microcrack density in a dental porcelain decreased as a result of isothermal heat treatment. Ten specimens of a commercial dental porcelain that had previously exhibited an increase in thermal expansion as a function of isothermal heat treatment were prepared and divided into two groups. The experimental group was heated to 750°C and held for 16 minutes at that temperature. The control group received no anneal. The mean microcrack densities were determined by quantitative stereology to be 575 cm2/cm 3 ± 75 cm2/cm3 (mean ± SEM) for the control group (no anneal) and 231 cm2/cm3 ± 25 cm2/cm3 for the experimental group (16-minute anneal at 750C). The specimens annealed at 750°C had a significantly lower microcrack density (p < 0.001) than those that received no anneal. A model was developed to estimate the effect of microcracking on thermal expansion of the porcelain, and a 6% increase in the coefficient of thermal expansion of the porcelain was predicted from this model as a result of this decrease in microcrack density.

High-temperature X-ray Diffraction Measurement of Sanidine Thermal Expansion

Dental porcelains that are designed to be fused to PFM (porcelain-fused-to-metal) alloys are formulated by their manufacturers to be closely matched in thermal expansion to these alloys. The high thermal expansion of the mineral leucite has been exploited to regulate porcelain expansion. Leucite, however, has been observed to convert to the sanidine polymorph of feldspar during certain heat treatments within the normal firing range of dental porcelain. The effects of this conversion on porcelain thermal expansion and porcelain-metal thermal compatibility have been uncertain, due to the paucity of published data on the thermal expansion of sanidine. The purpose of this study was to measure the thermal expansion of sanidine by high-temperature x-ray diffraction over the temperature range in which thermal mismatch stresses can develop in porcelain-fused-to-metal restorations, i.e., from room temperature to 700°C. The lattice parameters a, b, c, and /3 were determined from the d-spacings and hkl values of multiple reflections by means of a least-squares iteration. The dependence of each lattice parameter on temperature was determined via analysis of variance, and the coefficient of thermal expansion, a, was obtained from this analysis. The lattice parameters of sanidine at room temperature were determined to be: a = 0.8524 ± 0.0015 nm, b = 1.3020 ± 0.0004 nm, c = 0.7165 ± 0.0002 nm, and β = 116.02° ± 0.01° (mean ± 95% confidence interval). The linear thermal expansion coefficient, α, over the range from room temperature to 700°C was determined to be 4.1 X 10-6 K-1 ± 0.6X 106 K-1 (mean ± 95% confidence interval). Because the coefficient of thermal expansion for sanidine is substantially lower than that of leucite (the effective linear thermal coefficient of thermal expansion of leucite over the range of 25° to 700°C is 28×10-6 K-1), the conversion of leucite to sanidine during porcelain heat treatments would produce a detrimental lowering of the porcelain thermal expansion.