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The effects of experimental lithium-containing ion exchange pastes on dental porcelain were compared with a commercial ion strengthening paste. Maximum biaxial flexure strength using the experimental pastes was obtained by firing lithium paste containing 200 g per L lithium sulphate dihydrate for 45 minutes at 750 degrees C onto the surface of the porcelain. However, the commercial paste produced the greatest strengthening overall.
The aim of this study was to investigate the correlation between fracture toughness (K(Ic)) and flexural strength (FS) in dental porcelains. Porcelains with different leucite contents and clinical indications were used (A, B, C, D, and E). K(Ic) was determined by surface crack in flexure method (SCF) and FS was determined by four-point-bending test. Microstructural characterization was also carried out. The leucite contents of porcelains A, B, C, D, and E were, respectively, 22, 22, 6, 15, and 0%. Materials with higher leucite content (A and B) presented significantly higher K(Ic) values compared to materials with lower leucite content (C and E). The Weibull moduli (m) of porcelains A and B were statistically higher than those of the other three materials. Regarding characteristic strength (sigma(0)), porcelains D and E showed similar values and statistically higher than those of the other materials which were statistically different from each other. According to the regression analysis, sigma(0) increased with the increase of K(Ic) until approximately 0.75 MPa m(1/2). After that, the increase in K(Ic) was accompanied by a decrease in sigma(0). However, the Weibull modulus increased with the increase in K(Ic), especially for values greater than 0.80 MPa m(1/2).
PURPOSE: This research determined whether there is a measurable effect of local geometry factors on leucite content of dental porcelain in fixed partial dentures (FPD). MATERIALS AND METHODS: Four-unit FPD frameworks (n = 36) were fabricated using a nickel-chromium alloy (Rexillium III). Body porcelain (Crystar, shade A2) was applied in one increment and subjected to two simulated body firings, followed by a simulated glaze firing to achieve a thickness of 1.5 to 2.0 mm. The completed FPD specimens were randomly assigned to three groups of 12 specimens each: (1) simulated post-soldering, (2) multiple firing, and (3) control. The FPDs from each test group were sectioned into individual units: canine retainer, premolar pontic, molar pontic, and molar retainer. The porcelain was removed from each unit, and the leucite content was measured via quantitative x-ray diffraction. RESULTS: Porcelain cracking indicated that the soldering simulation had successfully reproduced conditions in the dental laboratory that result in porcelain cracking during soldering. The leucite content was not significantly different between the retainer and pontic units for either the soldering simulation or control FPDs, although the canine retainer units did have a slightly lower leucite content than the pooled values of the other units. Comparison of the pooled data for the three groups indicated statistically significant differences among the leucite contents. CONCLUSION: Compared to the control, the simulated post-soldering procedure produced a significant increase in leucite, and the multiple firing group exhibited a significant decrease in leucite. Increases in leucite weight fraction during post-soldering operations-and the larger thermally induced stresses that accompany these leucite increases-are responsible for the cracking that occurs.
OBJECTIVE: This study was to investigate the influence of the bending test configurations and the crosshead displacement speeds on the fracture toughness (K(Ic)) of dental porcelains obtained with the indentation strength in bending (ISB) method. METHODS: The strength of the dental veneering porcelains Duceram and Sintagon Zx, Vickers' indented at a load of 2 kg was measured at a crosshead speed of 0.5mm/min with three test configurations, which were 3-point, 4-point, and biaxial bending. Two more groups of Sintagon Zx were tested the same way, but at speeds of 0.1, and 0.05 mm/min, respectively. Both porcelains, the three crosshead speeds, and the three test configurations were compared statistically. RESULTS: Duceram had a higher toughness than Sintagon Zx with all three test configurations and there was no significant difference between three test configurations with either porcelain. Within the crosshead speed groups of Sintagon Zx, a significant difference was found only in the 0.5mm/min group between the 3-point, and 4-point configurations. Within the configuration groups, significant differences were found between all speeds with the 3-point configuration and only between the highest and lowest speed with the 4-point and the biaxial tests. CONCLUSION: The crosshead displacement speed can cause statistically different results of fracture toughness obtained with the ISB method.
OBJECTIVE: Mechanical condensation of powder-method dental porcelains can only achieve a limited effect--the majority of consolidation and porosity elimination is achieved by sintering. However, there is a surprising lack of information on the process in the literature, and the effects of the two basic conditions of sintering, time and temperature, are very poorly described despite theoretical expectations. The present study was to investigate the effects of these conditions on porosity and to consider its relationship to the recognizable firing stages of low and high biscuit, and other aspects of appearance, with a view to schedule recommendations. METHODS: The variation of appearance, pore form and percentage porosity with sintering time and temperature was studied for five dental dentine porcelains, two aluminous: Alpha (Vita Zahnfabrik, Bad Säckingen, Germany) and Vitadur-N (Vita); and three feldspathic: Omega (Vita), VMK68 (Vita) and Carmen (Esprident, Ispringen, Germany). Disc specimens were sintered for 0-1000 min over 750-1100 degrees C in a systematic search pattern to establish limits of acceptable appearance (high biscuit to not quite slumped). Porosity was measured using an image analyser on specimens fired for sintering times of 24 and 30 s; 1, 3, 6 and 30 min; 1, 5, 10, 15 and 20 h; with sintering temperatures from 750 to 950 degrees C for Carmen and 800-1050 degrees C for the others, all with 50 degrees C increments. Measurements were made on a ground and polished surface (1 microm diamond), five fields on each of five specimens per condition. The percentage porosity, pore count, median pore area and pore size distribution were analyzed. RESULTS: The boundaries of the acceptable appearance areas in maps of sintering temperature vs. sintering time were clearly delineated; analysis showed that they may be related to the activation energy of the diffusive processes occurring during sintering. Minimum porosity was obtained at high temperature and short time, close to but not consistently coincident with the manufacturers' recommendations. There is also a conductivity-related minimum sintering time at high temperatures. The theoretical reduction on prolonging sintering at high temperature was not a general result, and porosity increased markedly in the feldspathic porcelains, particularly VMK-68. SIGNIFICANCE: The reduction of porosity of dentine porcelain is much more sensitive to temperature than to time. It is possible systematically to identify optimum conditions using the present approach. Detailed study of the effects of formulation on the sintering process may be made by reference to the activation energy.
A computer simulation using a visco-elastic stress analysis was conducted to clarify the effect of the heating rate on deformation temperature of dental porcelain during firing. In this simulation, the following temperature-dependent factors were incorporated: elastic modulus, viscosity, and coefficient of thermal expansion. The cooling/heating rate dependencies of both the glass-transition temperature and the temperature distribution in the slab were also included. Thermal expansion curves of porcelain with an applied load at various heating rates were computed. Effects of the applied stress and the heating rate on the deformation temperature of porcelain were revealed. The results suggest that the temperature where the incompatibility stress develops in the porcelain-fused-to-metal strips during cooling can be estimated closely from the deformation point of the heating curve of the porcelain with an applied stress of about 1.2 approximately 3.1 MPa. A method for measuring temperature dependence of viscosity as represented by an Arrhenius equation is proposed.
UNLABELLED: This study compared the surface roughness of 4 dental porcelains with different microstructures (d.Sign-D, Finesse-F, Noritake-N and Symbio-S) using varied surface treatments. The porcelain surfaces were submitted to the following surface treatments: 1) g (glazing only); 2) rg (polishing with a rubber wheel before glazing); 3) 2g (reglazing); 4) r (rubber wheels); 5) rp (rubber wheels + diamond paste); 6) d (sandpaper discs) and 7) dp (sandpaper discs + diamond paste). Treatments 3 through 7 were performed after breaking the glaze layer with a diamond bur. Surface roughness (Ra, in m) was determined using a profilometer (n=10). Visual inspection was made using the scanning electron microscope. Microstructural characterization was also performed (hardness, leucite content and particle size). Reglazed specimens presented significantly rougher surfaces compared to glazed specimens. The use of a polishing paste after the sandpaper discs or after the rubber wheel resulted in a reduction of the Ra value for all materials (except for the dp group of porcelain S). Rubber or discs followed by diamond paste were the best surface treatments for porcelains D (D-rp: 0.21 +/- 0.06 microm and D-dp: 0.22 +/- 0.05 microm) and F (F-rp and F-dp: 0.17 +/- 0.03 microm). For porcelains N and S, both reglazing (2g) and the use of rubber or sandpa- per discs followed by diamond paste (groups rp and dp) resulted in similar roughness (N-2g: 0.22 +/- 0.03 microm; N-rp: 0.22 +/- 0.04 microm; N-dp: 0.20 +/- 0.04 microm, S-2g: 0.22 +/- 0.04 microm; S-rp: 0.19 +/- 0.04 microm; S-dp: 0.23 +/- 0.04 microm). CONCLUSION: The best choice of surface treatment for leucite-based porcelains depended on the material considered. Porcelains with lower leucite content (F and S) tended to present lower roughness compared to those with higher leucite content after being polished with rubbers or discs followed by diamond pastes.
Recently, the demand for a more natural and harmonious color of porcelain-fused-to-metal restoration is on the increase due to the recent rise in esthetic values among the patients. However, the usual methodology of shade taking, color transmission, and color evaluation is not so precise. We have thus tried to resolve this problem with the aid of a computer color matching system for the purpose of reproducing the natural and harmonious color of porcelain-fused-to-metal restoration. To obtain the colorimetric and optical fundamental data of dental porcelains, we analyzed its absorption and scattering coefficients by using a spectrophotometer. It was concluded that there was a high positive correlation between the absorption and scattering coefficient to porcelain thickness in both the dentin and enamel porcelain layers. As the wavelength increased, the absorption coefficient tended to decrease and the scattering coefficient tended to increase. From this experiment, it became possible to determine the absorption and scattering coefficient of the dentin and enamel porcelain layers of any shade and thickness.
The flexural strength, number (n), size, and form of pores in seven different dental porcelains were examined. The flexural strength was approximately 100 MPa for the core materials for metal bonding, 116 MPa for traditional alumina porcelains (NBK 1000, Vitadur N), 150 MPa for two new high-alumina porcelains (Vita Hi Ceram, Cerestore), and 240 MPa for a castable glass (Dicor). The mean pore diameter was approximately 10 microns in all porcelains except Dicor, but with a large and varying range. Dicor showed a mean pore size of 1 micron. The pore density varied from 36.5/mm2 in one of the porcelains for metal bonding to 4367/mm2 in Cerestore. The mean cross-sectional area occupied by pores was below 10% for most porcelains except Vitadur N and Cerestore, showing 17.5% and 32.5%, respectively. No correlation was found between flexural strength and frequency or cross-sectional area of pores of the various products.
OBJECTIVES: The purpose of this investigation was to determine if the change in the leucite weight fraction during an isothermal heat treatment could be estimated by observing the deformation of PFM strips in a high-heating-rate, computer-controlled bending beam viscometer (BBV). METHODS: Specimens of a commercial body porcelain were fired according to the manufacturer's instructions-50 disk specimens for quantitative X-ray diffraction (XRD) and 100 bimaterial strip specimens for BBV. The XRD specimens were annealed at temperatures between 650 and 1000 degrees C, and leucite weight fraction was measured using an alumina internal standard. The BBV specimens were annealed in the BBV using time-temperature schedules designed to elucidate the leucite crystallization behavior between 700 and 1000 degrees C. Timoshenko's equation for a bimaterial thermostat was used to estimate the change in the thermal expansion of the porcelain near room temperature. Changes in leucite weight fraction were determined from these thermal expansion changes. RESULTS: The means and SDs were compared to values obtained by quantitative XRD. Good agreement was obtained between values of leucite weight fraction derived from beam deformation and those determined by quantitative XRD (p> or =0.45). The anneal sequence showed that the increase in leucite weight fraction at 800 or 900 degrees C is reversible by an anneal at 1000 degrees C. SIGNIFICANCE: The BBV technique yields comparable results to quantitative XRD and provides the opportunity to efficiently monitor porcelain leucite changes nondestructively over multiple heat treatments. This technique could prove useful for testing firing schedules designed to stabilize the leucite content in dental porcelain.
Luminescence spectra of several non-radioactive luminophors that may be suitable for dental porcelain have been obtained. It was found that porcelain composition and maturation temperature have large effects on excitation and emission spectra. Most of the luminophors had excitation maxima lower in wavelength than have been observed for natural teeth.
STATEMENT OF PROBLEM: New processing techniques have facilitated the use of zirconia core materials in all-ceramic dental prostheses. Zirconia has many potential advantages compared to existing core materials; however, its performance when layered with porcelain has not been evaluated. PURPOSE: This study investigated the strength of a wide variety of layered zirconia and porcelain beams to determine whether the inclusion of zirconia cores results in improved strength. MATERIAL AND METHODS: Eight types of layered or simple zirconia and porcelain beams (n = 10), approximately fixed partial denture-size, were made of a tetragonal polycrystalline zirconium dioxide partially stabilized with yttria core (Lava System Frame) and a feldspathic dental porcelain (Lava Ceram veneer ceramic). Elastic moduli of the materials were measured using an acoustic method. Maximum force and modulus of rupture were determined using 3-point flexural testing and a universal testing machine. Descriptive statistical methods were used. RESULTS: Beams with porcelain tensile surfaces recorded mean tensile strengths or moduli of rupture from 77 to 85 MPa, whereas beams with zirconia tensile surfaces recorded moduli of rupture almost an order of magnitude higher, 636 to 786 MPa. The elastic moduli of the porcelain and zirconia materials were 71 and 224 GPa, respectively. Crack propagation following initial tensile cracking often involved the porcelain-zirconia interface, as well as bulk porcelain and zirconia. CONCLUSION: The layered zirconia-porcelain system tested recorded substantially higher moduli of rupture than have been previously reported for other layered all-ceramic systems.
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