Mismatch in thermal expansion properties between veneering ceramic and metallic or high-strength ceramic cores can induce residual stresses and initiate cracks when combined with functional stresses. Knowledge of the stress distribution within the veneering ceramic is a key factor for understanding and predicting chipping failures, which are well-known problems with Yttria-tetragonal-zirconia-polycrystal based fixed partial dentures. The objectives of this study are to develop a method for measuring the stress profile in veneering ceramics and to compare ceramic-fused-to-metal compounds to veneered Yttria-tetragonal-zirconia-polycrystal ceramic.
The hole-drilling method, often used for engineering measurements, was adapted for use with veneering ceramic. Because of the high sensitivity needed in comparison with industrial applications, a high sensitivity electrical measurement chain was developed.
All samples exhibited the same type of stress vs. depth profile, starting with compressive at the ceramic surface, decreasing with depth and becoming tensile at 0.5–1.0 mm from the surface, and then becoming slightly compressive again. The zirconia samples exhibited a stress depth profile of larger magnitude.
The hole drilling method was shown be a practical tool for measuring residual stresses in veneering ceramics.
Yttria-tetragonal-zirconia-polycrystal (Y-TZP) was introduced as a framework material for dental crowns and fixed partial dentures (FPDs) a decade ago because of its esthetic and biocompatibility superiority over traditional metal materials. Y-TZP has high strength and fracture toughness linked to its phase transformation potential, and compares well with other ceramic framework materials such as glass ceramics containing leucite or lithium disilicate crystals, or glass-infiltrated ceramics containing spinel, alumina or crystalline alumina/zirconia. Nevertheless, during the last 5–6 years, clinical reports for Y-TZP based crowns and FPDs have indicated a high rate of short-term failures linked to veneering ceramic fracture (chipping), suggesting that the veneering ceramic is a weak point in the restoration . The mechanism of the chipping is complex and is not well understood. Veneer chipping is reported more often than with ceramic-fused-to-metal structures (PFMs) , and is impairing confidence in the use of zirconia-based prostheses.
Mismatches in the thermal expansion properties of the veneer and framework, and temperature gradients occurring during the cooling and solidification period of the firing process, induce residual stresses within the structure . The presence of these residual stresses within the veneer greatly influences the strength and fracture characteristics of veneered dental restorations, and they are likely a major influence on the observed chipping. Therefore, knowledge of residual stresses in components and their formation during manufacture is of great importance when designing and manufacturing composite components .
Until now, stress profiles in dental prostheses have been studied only through mathematical models. One of the challenges encountered by models is to account for the viscoelastic behavior of the ceramic in the glass transition range, where thermo-physical properties such as the coefficient of thermal expansion, glass transition temperature and viscosity have strong temperature dependence and influence on residual stresses . Asaoka and Tesk have developed analytical models that incorporate variations of the coefficient of thermal expansion to study the influence of cooling rate and thermal expansion coefficient mismatch on residual stress profiles. Recently, DeHoff et al. have investigated the impact of thermal expansion coefficient mismatch on residual stresses in glass-ceramic-based, three-unit, posterior FPDs by using viscoelastic finite element stress analysis and a three-dimensional model. Subsequently, Swain highlighted the critical importance of the cooling rate, thickness and thermal expansion coefficient on residual stresses profile in bilayered structures composed of glass ceramics, alumina and zirconia substrates . In other work, Arman et al. introduced a 3D finite element analysis of PFMs crowns, incorporating the mechanical and thermal properties at various temperatures including the equivalent heat transfer coefficients and the instantaneous elastic moduli.
The “locked-in” character of residual stresses makes them challenging to measure because there are no external loads to manipulate to reveal the internal stresses. Various measurement techniques have been developed for industrial applications, but have not been widely used for dental applications. A major class of residual stress measurement methods used in industry is based on the removal of some stressed material and the measurement of the resulting deformations in the adjacent material . The deformations are measured on the surface, typically using strain gages, from which the residual stresses can be calculated. Fig. 1 illustrates this process for the hole-drilling method, a residual stress measurement method widely used in industry. First, a strain gage is attached to the specimen surface, as schematically shown in Fig. 1 (a). Then a small hole is drilled adjacent to the strain gage. This releases the residual stresses within the hole, and causes the material around the hole to deform slightly, as shown in exaggerated form in Fig. 1 (b). The strain gage measures this deformation, from which it is possible to calculate the size of the original residual stress. In practice, instead of a single strain gage, a specially designed strain gage rosette consisting of three or six strain gages is used. Fig. 2 shows a six-element strain gage rosette that was used in this study. The drilled hole is at the center.
A major objective here is to transfer and adapt an effective industrial method for measuring residual stresses to dental use, and to demonstrate the method for measurement of residual stresses in veneer-metal (VM) and veneer-zirconia (VZr) structures. The hole-drilling method is chosen because of its flexibility and convenience of use, its demonstrated reliability in industrial applications, and the existence of a standardized test procedure . An additional objective is to compare the stress profile in PFM structures to Y-TZP based structures to gain a better understanding of chipping problems encountered with Y-TZP based crowns and FPDs in clinical practice.
Materials and methods
Bilayered disc samples composed of veneering ceramic sintered either on Y-TZP framework (VZr, 9 samples), or on dental CoCr alloy framework (VM, 27 samples) were manufactured following standard dental laboratory procedures and manufacturer’s recommendations. CoCr core discs (Duceralloy C, DeguDent GmbH, Hanau, Germany), 20 mm diameter, were cast and ground sequentially with 80-grit, 180-grit and 500-grit silicon carbide discs (Struers LabPol polishing machine, Copenhagen, Denmark) to a thickness of 1.00 ± 0.02 mm. The surface to be veneered was sandblasted at 4 bars with 125 μm alumina particles.
Y-TZP core discs were made by initially cutting square slices out of a pre-sintered Y-TZP cylinder (Acerma, Wissembourg, France), with a diamond wheel mounted on an IsoMet saw (Buehler Ltd., Lake Bluff, IL, USA). These slices were rounded by polishing, and sintered at 1530 °C for 120 min with heating rate 10 °C/min, and heating time 149 min (Zircomat furnace, Vita Zahnfabrik, Bad Säckingen, Germany). The sintered Y-TZP discs were ground and dimensioned in the same way than CoCr, but not sandblasted.
CoCr and Y-TZP discs were veneered respectively with Vita VM 13 and Vita VM9 feldspar veneering ceramic (shade 3M2) (Vita Zahnfabrik, Bad Säckingen, Germany). A Vita Vacumat 4000 Premium furnace (Vita Zahnfabrik, Bad Säckingen, Germany) was used for all firing procedures, as summarized in Table 1 . All samples were baked on the same ceramic mesh-tray.
|Starting T (°C)||Pre-drying t (min), closing t||Heating rate (°C/min)||Heating t (min)||Firing T (°C)||Holding t (min)||Vacuum holding t (min)|
|Alloy core oxidation||600||3||75||4||900||2||4|
|Vita VM13 Opaque||600||2||75||4||900||1||4|
|Vita VM13 Dentine||600||8||50||6||900||6||6|
|Y-TZP core regeneration firing||500||–||100||5||1000||15||–|
|Vita VM9 Effect bonder||500||6||75||6||950||1||6|
|Vita VM9 Dentine||500||6||75||6||910||4||6|
The sandblasted surfaces of the VM samples were oxidized before ceramic layering according to the manufacturer’s guidelines. Vita VM 13 Opaque ceramic powder mixed with Vita VM opaque fluid was applied to the substructure with a brush, and fired to enhance the bond to the alloy surface. Three layers of dentin ceramic were fired successively. This layering technique promotes adhesion between opaque and dentin ceramic and reproduces the dental laboratory procedure. Samples were ground sequentially with 80-grit, 180-grit and 500-grit silicon carbide discs to the thickness of 3.0 ± 0.02 mm to create a 2 mm thick ceramic layer on a 1 mm thick framework.
For the VZr samples preparation, the Y-TZP discs were exposed to a “regeneration firing”, which is a final thermal treatment of the core to reverse any phase transitions in the zirconia due to the grinding procedures. A thin coat of Vita VM 9 Effect Bonder was applied and fired on the surface to be veneered. Then, Vita VM 9 Base Dentine was progressively layered on the effect bonder and samples were dimensioned in the same way than VM samples.
After final polishing, all specimens were exposed to a last firing procedure (see Table 1 ) to restore the residual stress profile through the veneering ceramic thickness. The samples were tempered by opening the furnace door, as performed in dental laboratories, and removed from the mesh-tray at 200 °C.
Strain gage rosette installation
A specialized three-element Type A strain gage rosette (EA-06-062RE-120, Vishay, Malvern, PA, USA) or a six-element Type C rosette (N2K-06-030RR-350/DP, Vishay, Malvern, PA, USA) was installed on the center of the veneering ceramic surface ( Fig. 1 ). To promote the strain gage bond, the ceramic surface was prepared by etching with 10% hydrofluoric acid for 1 min, and was then cleaned for 5 min in an ultrasonic bath containing 90% alcohol. The strain gage rosette was installed with M-Bond 200 Adhesive (Vishay, Malvern, PA, USA), following the manufacturer’s instructions. The adhesive was allowed to cure overnight to ensure complete curing. The installation was monitored using an optical microscope.
Electrical measurement chain
The strains expected from the strain gages are very small and cannot be measured with sufficient accuracy using conventional industrial equipment. A specialized data acquisition system was therefore built where each strain gage was connected in a Wheatstone bridge circuit with 3 control gages (identical gages attached to an undisturbed sample). All gages and control rosettes were exposed to identical constant temperature conditions. Finally, the very low voltage measurements were performed with specific custom-built electronic equipment ( Fig. 3 ) comprising a precision DC and AC current source 6221 (Keithley Instruments, Inc., Cleveland, OH, USA) and 3 Nanovoltmeters 2182A (Keithley Instruments, Inc., Cleveland, OH, USA). Filtered measurements were recorded on a computer using NI LabView software (National Instruments, Austin, TX, USA).