Isothermal phase transformations of a dental porcelain.

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.     

Effects of local cooling rate and processing variables on leucite in dental porcelain

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.     

Mechanical properties and phase transformation of super-elastic Ni-Ti alloy wires. Part 1: Relation between super-elasticity and phase transformation

The relationship between mechanical properties and thermal behavior of super-elastic Ni-Ti alloy orthodontic wires was investigated in order to use them effectively in clinics. Three types of super-elastic Ni-Ti alloy orthodontic wires, SENTALLOY-BLUE(SB), YELLOW(SY) and RED(SR), were used. Two parameters were introduced from the load-deflection curves in three point bending test to evaluate their mechanical properties. One was the super-elastic index (SEI) for super-elasticity, and the other was the E-load for the load in the super-elastic region. Their thermal behavior in the course of the transformation analyzed with DSC, and compared with the mechanical properties. The specimens heat-treated at 500 degrees C for 30 min were also examined. The differences in mechanical properties of the super-elastic Ni-Ti alloy orthodontic wires used in this study were thought to be more affected by the height of the thermic peak and the form of the DSC curves than the transformation temperature. The higher the thermic peak, the higher tended to be the SEI and the lower E-load. To the contrary, the lower thermic peak, the higher was the E-load and the lower was the SEI, and the continuous force could not be achieved. The remaining work-hardened structure was thought to influence these differences.     

Flexural strength optimisation of a leucite reinforced glass ceramic.

OBJECTIVES: The aims of the study were to process a ceramic material with a fine leucite particle size using hot pressing techniques, to increase the flexural strength, reliability and ease of use. METHODS: A starting glass composition of wt%; 64.2% SiO(2), 16.1% Al(2)O(3), 10.9% K(2)O, 4.3% Na(2)O, 1.7% CaO, 0.5% LiO and 0.4% TiO(2) was used to produce a leucite reinforced ceramic material. Twenty-one porcelain discs were produced by sintering the ceramic frit (group 1) and sixty-three discs by heat pressing the frit (groups 2, 3 and 4). Twenty-one Empress 1 ceramic discs were also heat pressed (group 5). Disc specimens were tested using the biaxial flexure test at a crosshead speed of 0.15mm/min and the data analysed using the Scheffé F multiple comparison test and Weibull statistics. Specimens were characterised using X-ray diffraction (XRD), secondary electron imaging and energy dispersive X-ray analysis where applicable. RESULTS: The heat pressed groups (2, 3 and 4) had higher mean biaxial flexural strengths and characteristic strength values than groups 1 and 5 (p<0.05). XRD revealed the presence of tetragonal leucite in all test groups. Fine leucite crystals, tabular platelets and minimal matrix microcracking were found in the microstructure of test groups (1-4) with a more uniform leucite distribution in the heat pressed specimen groups (2, 3 and 4), which were associated with a significant increase in the biaxial flexural strength and reliability. SIGNIFICANCE: Optimisation of the microstructure by producing a fine microstructure and controlling the distribution via the correct pressing parameters may be extremely advantageous in these systems.     

Stress analysis of porcelain laminate veneers. (1)

The stress distributions in the porcelain laminates under various kinds loading are analysed numberically in order to make clean the reason of their exfoliation or fracture. The two-dimensional finite element method is used to determine the principal stresses developed in the porcelain laminated and the teeth substance of a restored maxillary central incisor. The thicknesses of enamel preparations are assumed to be 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm and 0.8 mm in the analysis. As a result of this analysis, the following points are made clear: 1) The stress concentration are observed at the vicinity of a loading point and in the cervical regions, independently of the loading conditions on its location and direction. 2) In case of the vertical loading, the location of loading point has no effect on the overall stress magnitude and distribution. 3) In the case when the load is applied in the 45 degrees-direction to the vertical axis, the overall stress level is increased, as the location of loading point is far from the supporting regions. 4) In the case when the load is applied normal to the vertical axis at the incisal edge, the high stress is obtained in comparison with other loading conditions.     

Mechanical properties and phase transformation of super-elastic Ni-Ti alloy wires. Part 2: Changes of properties through heat treatment

Three types of super-elastic Ni-Ti alloy orthodontic wires, SENTALLOY-BLUE (SB), YELLOW (SY) and RED (SR), on the change of properties were studied especially the reversible change of the load level, using three point bending test and DSC. The heat treatment at 440 degrees C caused the largest reduction in mechanical properties of SB and SY, and the 400 degrees C treatment did that of SR. The load level of SR was less changeable than those of SB and SY by the difference of heat treatment temperature. Mechanical properties of SY were almost reversible by the alternate heat treatment at 440 degrees C and 510 degrees C. Then the 440 degrees C-heat treatment increased the transformation temperature, and the 510 degrees C-heat treatment decreased them. The variation of the mechanical properties completely corresponded to the change in transformation temperature. The variation appears to result from some internal structure changes. Changes in mechanical properties are observed by heat treatment at 360, 400 and 480 degrees C, but the reversibilities were not so clear. The change of the properties of super-elastic wires are influenced by the kind of wire and the heat treatment condition. The reversible change in the mechanical properties seems to have many advantages over super-elastic Ni-Ti alloy orthodontic wires.     

Creep and phase transformation in dental amalgam.

Creep measurements were performed on different types of amalgams as a function of heat treatment after specimen preparation before testing. The types of amalgams included lathecut, spherical, nonpreamalgamated amalgams, and an amalgam containing Ag-Cu particles. Heat treatment significantly reduced the creep of all alloy types studied. Creep reduction was associated with the phase and transformation gamma1 leads to beta1 (Ag-Hg), which was detected by metallographic and microprobe analysis.     

The effect of polishing porcelain laminates on induced I-Bar wear.

Partial-coverage porcelain laminate restorations may successfully be used to create undercuts for removable partial denture abutment teeth. If recontouring of such restorations becomes necessary after placement, surface polishing would be necessary prior to removable partial denture fabrication. In this study the wear of luted porcelain laminates, which received a polished surface treatment and contacting I-bar clasps, was quantified over a simulated 2-year period when subjected to removable partial denture placement and removal. Scanning electron photomicrographs and computer imaging were used for wear measurement. These data were compared to previously recorded data on wear of I-bar tips against human enamel and glazed, luted, porcelain-laminate restorations under the same experimental conditions. No measurable wear was observed on any of the laminate specimens. I-bar clasps tested against the glazed laminate restorations exhibited the greatest mean wear, and those tested against the polished laminates exhibited the least mean wear.     

Adherence of a nickel-chromium alloy and porcelain.

1. Surface oxide favors adherence. The results of this study do not support the hypothesis that chromium oxide has a deleterious effect on adherence as measured by the torsion test. 2. Gritblasting the surface before porcelain application improves adherence. Smooth uncoated specimens where oxide was removed before enameling developed insufficient amounts of oxide for bonding. 3. The particular coating agent used on this study indirectly promotes adherence, since additional oxide forms during its application. Also, it may be directly involved by its influence on the residual stress distribution in the interface porcelain. 4. The more extensive the reaction zone at the interface, the further away the fracture surface is from the interface.     

Fatigue and fracture properties of yttria partially stabilized zirconia crown systems.

OBJECTIVES: The purpose of this study was to test in vitro the fatigue and fracture properties of the Zirconia Everest core material after being veneered with a sintered or a heat-pressed veneer material. METHODS: Fifty zirconium copings were made using Kavo Everest ZS-blanks and the CAD/CAM technology. These were divided equally into two groups. Group one was veneered by a heat-pressed material (IPS e.max ZirPress) and group two was veneered by a sintered material (IPS e.max Ceram). All the crown shapes were subjected to 50,000 cycles of cyclic loading in water between 20 and 200 N, at a rate of 1 Hz and then loaded dynamically at a crosshead speed of 1 mm/min, until failure. Specimens of the core and the veneering materials were characterized using X-ray diffraction (XRD) and secondary electron imaging (SEI). RESULTS: There was no statistically significant difference in the mean failure load (N+/-S.D.) between group 1: 2135.6+/-330.1 and group 2: 2189.9.1+/-317.6 (p>0.05). No difference was found in the Weibull modulus and characteristic failure load values between the two groups (p>0.05). The specimens displayed mainly cohesive failure within the veneering material. XRD of the zirconium powder before sintering revealed tetragonal and monoclinic phases while after sintering only tetragonal zirconia could be identified. SIGNIFICANCE: There was no difference in the fatigue properties of the Zirconia Everest core material following sintering or heat pressing of the veneering material (p>0.05).     

核饰瓷厚度和基底颜色对铸瓷贴面修复四环素牙光学性能的影响

目的 分析不同瓷层厚度和变色基底对IPS e.max铸瓷贴面修复四环素牙的色彩学性能和半透性的影响。方法 用热压铸造方法制作IPS e.max LT核瓷,在其表面堆塑体瓷,制作饰/核瓷厚度分别为0.25 mm/0.25 mm、0.50 mm/0.25 mm、0.25 mm/0.50 mm、0.50 mm/0.50 mm、0.25 mm/0.75 mm的铸瓷试件。利用分光光度计测量试件在模拟四环素牙背景及黑白背景下的L^*、a^*、b^*值,计算试件置于模拟四环素牙背景上与背景本身色差ΔE₀₀1,及其与试件在白背景上色差ΔE₀₀2和半透性参数(TP值)。采用SPSS 17.0软件包对数据进行统计学分析。结果 ΔE₀₀1随着核饰瓷厚度增加而增加,但饰瓷厚度为0.50 mm时,核瓷从0.25 mm增加到0.50 mm,ΔE₀₀1间无统计学差异(P>0.01)。除浅灰背景外,其余背景下ΔE₀₀2随核饰瓷厚度增加而减小,但饰瓷厚度0.25 mm时,核瓷0.50 mm与0.75 mm组间无统计学差异(P>0.01)。不同四环素牙背景下铸瓷试件的ΔE₀₀1和ΔE₀₀2差异均有统计学意义(P<0.01)。随核饰瓷厚度增加,试件TP值均降低。不同核饰瓷厚度组TP值均有统计学差异(P<0.01)。结论 核饰瓷厚度和四环素牙颜色显著影响IPS e.max LT铸瓷贴面的遮色性能和色彩匹配性;IPS e.max LT铸瓷贴面修复偏黄四环素牙时色彩性能最优,修复浅灰四环素牙时色彩匹配性最差。核饰瓷厚度显著影响IPS e.max LT铸瓷贴面的半透性。     

The thermomechanical properties of gutta-percha. Part V. Volume changes in bulk gutta-percha as a function of temperature and its relationship to molecular phase transformation

A variety of gutta-percha materials was subjected to dilatometric analysis to measure volume changes which take place with heating and cooling. The volume changes were found to be related directly to the molecular transformation kinetics of the polymer material and to the temperature ranges within which they take place. If the gutta-percha in the apical segment is not elevated above 45 degrees C, molecular transformation is avoided and the ultimate volume changes which accompany temperature cycling are small, predictable, and controllable.     

Influence of phase transformation on the torsional and bending properties of nickel-titanium rotary endodontic instruments

AIM: To investigate the relationship between the functional properties and the phase transformation of nickel-titanium endodontic instruments. METHODOLOGY: Five types of rotary nickel-titanium endodontic instruments with a 0.30 mm diameter tip (EndoWave, HERO 642, K3, ProFile.06, and ProTaper) were selected to investigate torsional and bending properties, and phase transformation behaviour. A torsional test was performed according to ISO publication 3630-1, and maximum torque and angular deflection at fracture were measured. Bending load of the instruments was measured in a cantilever-bending test at 37 degrees C with the maximum deflection of 4.0 mm. A stainless steel K-file was used for reference. Phase transformation behaviour was measured by differential scanning calorimetry (DSC). From the DSC curve, transformation temperatures were calculated. Data were analysed by anova and the Tukey-Kramer's test. RESULTS: The maximum torsional torque values of HERO, K3 and ProTaper were significantly higher (P < 0.05) than those of EndoWave, ProFile and K-file. The K-files had the lowest torque value. Angular deflection at fracture was significantly higher (P < 0.05) for K-files than that for any nickel-titanium instrument. The bending load values of HERO and K3 were significantly higher (P < 0.05) than those of EndoWave, ProFile, ProTaper and K-file. The K-files had the lowest load value, although residual deflection remained. The transformation temperatures of HERO and K3 were significantly lower (P < 0.05) than those of EndoWave, ProFile and ProTaper. CONCLUSIONS: The functional properties of nickel-titanium endodontic instruments, especially their flexible bending load level, were closely related to the transformation behaviour of the alloys.     

Thermal induced deflection of a porcelain–zirconia bilayer: Influence of cooling rate

Objective

To determine the thermal expansion of a porcelain (VM9) and tetragonal zirconia (Y-TZP) as well as the deflection upon re-heating and cooling of a bilayer fabricated from these two materials after slow and rapid cooling during initial fabrication.

Restoration of maxillary anterior teeth using porcelain jacket crowns and porcelain veneers.

Combining different types of restorations when restoring maxillary anterior teeth poses a difficult esthetic problem. This article describes a method of restoring maxillary anterior teeth using a combination of porcelain jacket crowns and indirect porcelain laminate veneers.