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, D3,2, 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, f(mc), 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 microm. The porcelains were partitioned according to whether their mean leucite particle diameters, D3,2, fell above or below Dc, and their values of f(mc) were analyzed by a permutation test with random re-sampling. The porcelains with mean leucite particle diameters below Dc had a significantly lower fraction of cracked particles compared with the porcelains with mean leucite particle diameters above Dc (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 microm.

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 degrees C. The lattice parameters a, b, c, and beta 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, alpha, 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 beta = 116.02 degrees +/- 0.01 degree (mean +/- 95% confidence interval). The linear thermal expansion coefficient, a, over the range from room temperature to 700 degrees C was determined to be 4.1 x 10(-6) K(-1) +/- 0.6 x 10(6) 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 degrees to 700 degrees C is 28 x 10(-6) K(-1)), the conversion of leucite to sanidine during porcelain heat treatments would produce a detrimental lowering of the porcelain thermal expansion.

Elimination, via high-rate laser dilatometry, of structural relaxation during thermal expansion measurement of dental porcelains

OBJECTIVES

Thermal expansion measurement of glassy materials is complicated by thermal history effects. Excess volume--trapped in quenched dental porcelains after firing--collapses via structural relaxation on first slow heating during conventional dilatometry, making the thermal expansion coefficient (alpha) obtained on first heating unreliable. The purpose of this study was to determine whether porcelain thermal expansion measurement at high thermal rates could minimize the influence of thermal history.

METHODS

Eight thermal expansion specimens each of six body porcelains and the Component No. 1 (leucite-containing) frit prepared according to the patent by Weinstein et al. (US Patent No. 3,052,982) were subjected to three heat-cool conventional dilatometry runs at 3 degrees C/min, while eight thermal expansion specimens of each porcelain were reserved as untreated controls. Eight hollow, cylindrical specimens of the same brands were subjected to three heat-cool laser dilatometer thermal expansion runs at 600 degrees C/min, while eight cylindrical specimens of each porcelain were reserved as untreated controls. Thermal expansion data (25-500 degrees C) of all specimens were subjected to repeated measures analysis of variance.

RESULTS

The alpha obtained on first slow heating was significantly lower than values for succeeding slow heat and cool runs in all porcelains (P < 0.001). High-rate alpha obtained on first heating was not significantly different from values of succeeding heat and cool runs in all porcelains (P > 0.05).

SIGNIFICANCE

Conventional dilatometer measurements demonstrated occurrence of structural relaxation, as evidenced by the significant difference in the first heating and subsequent runs. High-rate laser dilatometry eliminated structural relaxation, thereby providing a thermal expansion measurement that is free of interference from thermal history effects.

Cyclic fatigue of a model feldspathic porcelain

OBJECTIVES

This study was conducted to evaluate the fatigue parameters of a model porcelain based on the Weinstein patent using cyclic fatigue and to compare the parametric values obtained from cyclic fatigue tests with those from dynamic fatigue tests previously reported by Fairhurst et al. (1993).

METHODS

Cyclical biaxial flexure of 1 mm thick and 12 mm diameter disks was performed at 37 degrees C in distilled water at a frequency of 4 Hz with constant stressing rates between a minimum and maximum stress. Three groups of samples (50, 40, 40) were tested with a maximum stress of 51, 47, and 43 MPa, respectively. The crack growth exponent, n, and the scaling constant, sigma fo, were derived from the regression constants obtained from a linear regression of the logarithm of the median time to failure with the logarithm of the maximum stress.

RESULTS

No significant differences were found between the cyclic fatigue parameters, n and sigma fo, derived from the median time to failure and those obtained from dynamic fatigue data.

SIGNIFICANCE

Within the limits of error in this determination, the median cyclic fatigue life can be estimated by the use of fatigue parameters obtained from dynamic fatigue testing.

Isothermal anneal effect on leucite content in dental porcelains

Porcelain thermal expansion behavior is a function of leucite content, and leucite changes can produce porcelain-metal incompatibilities. The purpose of the present study was to determine whether isothermal anneal treatments (such as are encountered during "post-soldering" operations) could result in alterations of leucite content of dental porcelain. Six commercial dental porcelains and the "Component No. 1" frit of the Weinstein et al. patent (1962) were studied. Twenty-eight coupon specimens of each porcelain were fired and divided randomly into seven groups of four specimens each. One group served as a control, while the other six groups were subjected to isothermal anneal treatments--three groups at 500 degrees C and three groups at 750 degrees C, for 4, 8, and 16 min, respectively. Leucite volume fraction was measured via quantitative x-ray diffraction. From an equation for the growth of particles in competition for diffusing material, an expression was derived for the potential simultaneous processes of leucite crystallization and reaction of leucite+glass to form sanidine or other compounds. This exponential growth/decay curve was fit through the experimental data for each porcelain by minimization of the sum of the squares of the residuals through iteration. For each porcelain, a statistically significant correlation was obtained between leucite volume fraction and isothermal anneal duration at 750 degrees C (p < 0.01), whereas at 500 degrees C, only CII exhibited a statistically significant correlation (p < 0.01). The control (non-annealed) leucite volume fractions for the commercial porcelains ranged from (mean +/- SD) 0.155 +/- 0.002 for WIL to 0.442 +/- 0.003 for Component No. 1. The 750 degrees C isothermal anneal increased the leucite volume fraction by between 6.1% and 21.3%.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic fatigue of feldspathic porcelain

Several studies (Sherrill and O'Brien, 1974; Southan and Jørgensen, 1974; Jones, 1983) have shown that stress corrosion fatigue occurs in dental porcelains. Morena et al. (1986) reported on an assessment of slow crack growth parameters for dental ceramics. The purpose of the study reported here was to evaluate the fatigue parameters of a model experimental porcelain using dynamic fatigue testing. This test procedure makes use of several constant stressing rates to perform strength tests. Dynamic stress testing was first described by Evans (1974) and later defined as a distinct test modality by Ritter (1978). From such data, the fatigue parameters can be calculated. These fatigue parameters, n and sigma f0, are, respectively, the crack growth exponent from the crack velocity expression and a materials constant which is dependent on the test environment and the inert (moisture-free) strength. The model porcelain was made from 60% component 1 and 40% component 3 according to the Weinstein patent (Weinstein, et al., 1962). The biaxial flexure strength of 300 specimens 1 mm thick was tested in 37 degrees C water by testing 50 samples at each of 6 constant stressing rates: 100, 10, 1, 0.1, 0.01, and 0.001 MPa/s. One hundred specimens were tested in a moisture-free environment at 100 MPa/s using a servo-mechanical testing machine. A commercial porcelain (Jelenko Gingival-Lot # 2012, Jelenko Dental Health Products, Armonk, NY, USA) was chosen as a reference material. One hundred twenty specimens were tested using the same procedures as those used for the model porcelain; however, only 20 samples were tested for 5 stressing rate groups and an inert group.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of thermal history on porcelain expansion behavior

Porcelain-fused-to-metal restorations are fixed several hundred degrees above the glass-transition temperature and cooled rapidly through the glass-transition temperature range. Thermal expansion data from room temperature to above the glass-transition temperature range are important for the thermal expansion of the porcelain to be matched to the alloy. The effect of heating rate during measurement of thermal expansion was determined for NBS SRM 710 glass and four commercial opaque and body porcelain products. Thermal expansion data were obtained at heating rates of from 3 to 30 degrees C/min after the porcelain was cooled at the same rate. By use of the Moynihan equation (where Tg systematically increases in temperature with an increase in cooling/heating rate), the glass-transition temperatures (Tg) derived from these data were shown to be related to the heating rate.

A rapid heating and cooling rate dilatometer for measuring thermal expansion in dental porcelain

Herein we describe a dilatometer that consists of a low-mass infrared furnace for rapid heating or cooling, an optical pyrometer, and a laser interferometer. The dilatometer facilitates observations of thermal expansion at rates comparable with those in dental laboratory practice over the temperature range necessary for comparison of thermal expansion of dental porcelain and alloy. Examples of thermal expansion data obtained at a 600 degrees C/min heating rate on NIST SRM 710 glass and dental porcelain are reported. To a limited extent, thermal expansion data above the glass-transition temperature range of dental porcelain were obtained. A shift of the glass-transition temperature range to higher temperatures was observed for both materials, compared with data obtained at 20 degrees C/min.

Glass transition temperatures of dental porcelains at high heating rates

The glass transition temperature (Tg) of a dental porcelain is a factor in determining the magnitude of residual stresses introduced in a dental porcelain during cooling of a porcelain-fused-to-metal prosthesis. Tg is known to vary with changes in heating or cooling rate. However, available commercial instrumentation does not permit Tg to be obtained at the very high cooling rates commensurate with actual dental laboratory practice. Tg values are reported here for a number of commercial dental porcelains and other materials. These data were obtained by the bending beam technique, employing a special low thermal-mass furnace to permit rapid heating and cooling rates. Measurements were made at rates as high as 600 degrees C/min. Coefficients of determination (r2) for 1/Tg vs. ln (heating rate) were excellent. This relation is consistent with previously reported low rate Tg data obtained by a different technique.

Interactive effect of stress and temperature on creep of PFM alloys

The creep rates of six alloys for porcelain-fused-to-metal (PFM) restorations were determined as a function of flexural stress and temperature. Although two Pd-Cu alloys demonstrated excellent resistance to creep at low-stress, high-temperature conditions, they were especially susceptible to deformation at high stresses and temperatures near the glass transition temperature of dental porcelains. In comparison, a Ni-Cr alloy and a Pd-Co alloy demonstrated superior creep resistance at high-stress, low-temperature conditions. This indicates a relatively low potential for deformation due to stresses which may result from a thermal contraction differential between these two alloys and incompatible dental porcelains.

Thermal shock resistance of porcelain discs

Thermal shock testing of opaque porcelain-body porcelain discs and body porcelain discs was performed on three porcelain products. Significant differences exist between the mean failure temperature differentials, delta T, for the composite opaque-body porcelain discs. The residual tensile stress and the transient tensile stress distribution are more severe for one of the three porcelains tested.

Slots vs pins: a comparison of retention under simulated chewing stresses

The specimens of composite resin retained by pins exhibited significantly lower cycle life than did all other groups. At low loads, the pins and slots are comparable (when composite resin is used), but there is a dramatic decrease in the life of restorations retained with pins as the load is increased. At the 30-micrometer end point, the composite restorations retained with pins failed at a mean of 250 cycles, whereas the composite restorations retained with a slot failed at 250,000 cycles - a 1000X increase in cycle life. In general, cycle life of composite resin retained with a slot is comparable to or greater than that of amalgam retained with a slot or pins. Whether the life difference between the pin- and slot- retained amalgam restoration is clinically significant is not known; however, the baseline for a clinical study of the two types of retention for definitive amalgam restorations has been completed and will be reported in the future.

Correlation of thermal shock resistance with thermal compatibility dat for porcelain-metal systems

On the basis of this study the following conclusions may be made: (1) A strong linear relationship exists between the thermal shock resistance and compatibility index values of the systems studied. (2) Compatibility index analysis and thermal shock testing are promising approaches for evaluation of porcelain-metal compatibility. (3) Further refinements of the model are necessary to account for the effects of stress relaxation, thermal diffusivity, thermal history, elastic modulus, porcelain tensile strength, and geometry on the predicted stress state of porcelain-metal systems.

Thermal expansion of dental alloys and porcelains

The effect of thermal coefficient of expansion (alpha) mismatch on porcelain-metal bonding is frequently referred to in the dental literature. Thermally induced stresses may develop at metal-porcelain system interfaces due to differences in the coefficients of thermal expansion of the porcelains, metals, and metal oxides. The objective of this research is to characterize alloy and porcelain expansion behavior as a first step in developing a more specific definition of thermal compatibility. It is clear from comparisons of porcelain data and alloy data that the porcelain has expansion characteristics which are quite different from those of the alloys. The overall differences in values between these alloys and porcelains constitute a mismatch. First run dilatometric heating measurements for porcelain yield large differences between delta L/L and alpha values as compared to cooling measurements. For a comparison of alloy and porcelain expansion characteristics, data should be obtained at several temperatures up to the glass transition temperature (Tg) of the porcelain.