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Applying microwave technology to sintering dental zirconia
Applying microwave technology to sintering dental zirconia Abdulredha A. Almazdi, DDS, MS,a Hasan M. Khajah, DDS, MS,b Edward A. Monaco Jr, DDS,c and Hyeongil Kim, DDS, MSd Al-Amiri Dental Center, Kuwait City, Kuwait; School of Dental Medicine, State University of New York at Buffalo, Buffalo, NY Statement of problem. When sintering zirconia, conventional processing may not provide uniform heating and consumes more energy than an alternative method using microwave energy. Purpose. The purpose of this study was to compare the surface quality, mechanical and physical properties, and dimensional stability obtained by sintering yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) in a conventional furnace versus a microwave furnace. Material and methods. Twenty bars of Y-TZP were prepared from Zircad blocks. Ten specimens were used for sintering in a conventional furnace. The remaining 10 specimens were sintered in a microwave furnace. The sintering temperature used for both techniques was 1500˚C. The flexural strength of all specimens was measured with the 3-point bend test with a universal testing machine with a cross head speed of 1.0 mm/min. Density was measured by applying the Archimedes method, and specimen length, width, and thickness were measured with a digital micrometer. The phase composition and average grain size of these ceramics were examined by using X-ray diffraction, and microstructure characteristics were studied with scanning electron microscopy. Data obtained were analyzed by using independent t tests (α=.05). Results. No significant difference between conventional and microwave sintering for either flexural strength, t18=0.49 (P=.63) or density, t18=0.07 (P=.95) was found. Specimens in both groups exhibited a uniform firing shrinkage of approximately 24.6% in all dimensions. The surface of selected specimens examined with a scanning electron microscope showed no visible difference in grain shape or porosity size between the 2 sintering methods. Conclusions. Under the conditions of this study, it appears that either microwave or conventional zirconia sintering may be used for processing zirconia for dental use. However, microwave energy provides uniformity of heating, allowing the use of higher heating rates, which can increase productivity and save energy. (J Prosthet Dent 2012;108:304-309)
Clinical Implications
Based on the study results, a microwave furnace, which creates a more uniform heat distribution and uses less energy, may be used to sinter dental zirconia. Because of its esthetic qualities, durability, and biocompatibility, dental zirconia has become a popular alternative to metals as a dental restorative material. However, the fabrication
of ceramic crowns in a conventional sintering oven is a time-consuming and expensive procedure, requiring the heating of a large oven to a high temperature and then slow cooling to
prevent cracking. In a standard conventional furnace, heat is applied to the external surface of the restoration and reaches the core by thermal conduction, producing high tempera-
Senior Registrar, Fixed Prosthodontics, Al-Amiri Dental Center. Senior Specialist, Fixed Prosthodontics, Al-Amiri Dental Center. c Assistant Professor and Director, Postgraduate Prosthodontics, Department of Restorative Dentistry, School of Dental Medicine, State University of New York at Buffalo. d Assistant Professor, Postgraduate Prosthodontics, Department of Restorative Dentistry, School of Dental Medicine, State University of New York at Buffalo. a
b
The Journal of Prosthetic Dentistry
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November 2012 ture gradients and stresses within the material. It is not a uniform heating process. Dental zirconia can also be sintered in a microwave furnace, in which the material is rapidly heated both internally and externally.1 In addition to the heat generated within the material, susceptors (materials that absorb electromagnetic energy and convert it to heat) made of silicon carbide or molybdenum silicate are placed around the ceramics to heat the specimen externally by thermal conduction. The presence of susceptors modifies the electromagnetic field distribution and its mean intensity, introduces heat sources external to the specimens, and may shield, to some extent, the zirconia specimens from the heat field.2 As a result of this internal and external volumetric heating, the thermal gradient and flow of heat in the ceramic body is evenly applied. This results in more uniformly and rapidly heated specimens with less thermal stress than conventional methods. Research has shown that many ceramics do not absorb microwaves well at room temperature.3 This creates the need to use susceptors that absorb microwaves at room temperature and act as heating elements. The possibility of the microwave processing of ceramics was first described in the 1950s and the processing of industrial ceramics in the 1960s.4 Microwave sintering has many attractive features, including rapid volumetric heating and low cost. Higher production rates and lower energy requirements make this process commercially attractive. Estimates of energy saving are 25% to 95%.5 In the 1980s, investigations demonstrated the benefit of microwave sintering. By using 28 GHz microwave radiation, alumina can be sintered at 300°C to 400°C below the temperature required for sintering in a conventional oven.6,7 Recently, a custom microwave oven (ThermWave, TW 1.3; EPL Ceramic Materials LLC, Youngstown, NY) was fabricated in cooperation with Alfred University (Alfred, NY)
Almazdi et al
that enables the temperature to rise to 1600°C. A water jacket with a circulating water supply is provided to cool the system. A small heating chamber made of insulating material (fibrous alumina) encloses 3 silicon carbide microwave susceptors (Thermcepts; Research Microwave Systems, Troy, NY). Thermcepts are used to raise the temperature of microwave-processed material for both microwave-absorbing specimens and nonabsorbing or lowabsorbing specimens. The thermcepts can be used to heat materials through temperature zones at which the material does not readily absorb microwave energy at 2.54 MHz. The temperature inside the heating chamber is recorded with a pyrometer/thermocouple inserted in the top of the oven, and the temperature is controlled inside the heating chamber with a temperature sensor.8 Zirconia is known as a polymorphic material that presents in 3 forms: the monoclinic phase, which is stable at room temperature up to 1170°C; the tetragonal phase, which is stable at temperatures above 1170°C up to 2370°C; and the cubic phase, which is the form that the material changes into above 2370°C.9 Compressive stress is initiated because of volumetric expansion in zirconia, and crack propagation resistance occurs; this is known as transformation toughening.10 Compared to other ceramic materials, yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is characterized by good physical properties such as high fracture toughness and relatively low elastic modulus as well as some degree of translucency.11-13 In dentistry, zirconia ceramics have been used for posts and cores, frameworks for fixed partial dental prostheses, orthodontic brackets, implant abutments, and implants.14-16 Radiographically, zirconia substructures appear opaque, which enhances the evaluation of marginal integrity and early detection of recurrent caries.17 Although some zirconia systems have demonstrated marginal discrepancies, these have been shown to be of
no significance in many of the systems used currently.18 Y-TZP has become the core material of choice for many categories of ceramic restorations because of its toughness and esthetic qualities. Computer-aided design and computer-aided manufacturing (CAD/CAM) technology has solved the difficulties of processing this hard material by milling it when it is partially sintered or presintered. A Y-TZP substructure can be veneered with pressable or layering ceramics with excellent esthetic results.19 This could be done either with airborne-particle abrasion or liner-applied treatments, with the latter having a stronger in vitro shear bond strength.20 In this study, the flexural strength and density of zirconia processed in a conventional or microwave sintering furnace were compared. The null hypothesis was that there are no significant differences in flexural strength and density between the sintering techniques.
MATERIAL AND METHODS Twenty (25 × 4.5 × 2.5 mm) bars were milled from partially sintered 3% Y-TZP (e.max Zircad B40 Blocks (lot# L17952)); Ivoclar Vivadent, Schaan, Liechtenstein) with an electrical high precision saw (Isomet 1000 Precision Saw; Buehler Ltd, Lake Bluff, Ill) with a diamond wafering blade (4”, high concentration Cat #M412H, 4” × .012” × 1/2” Blade; Metlab Corp, Niagara Falls, NY) under water irrigation. The bars were assigned to 1 of 2 groups (n=10 per group) in either a conventional or microwave furnace. The bars sintered in a conventional furnace were sintered at 1500°C for 8 hours in a high-temperature sintering furnace for zirconia (Sintramat; Ivoclar Vivadent, Buffalo, NY). All 10 bars were sintered simultaneously according to the manufacturer’s recommendations (Fig. 1). A microwave oven (ThermWave, TW 1.3; EPL Ceramic Materials LLC) (Fig. 2) was custom designed in col-
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1 Conventional sintering machine used in study.
2 Microwave furnace used in study.
3 Three SiC susceptors placed at far ends of box arranged around specimens.
4 Zirconia bar mounted for 3-point bend test.
5 Archimedes method used to measure theoretical density under water. laboration with EPL ceramic materials as previously described. A controller box was attached to the microwave to precisely control the temperature inside the chamber. The bars were sintered at 1500°C for 30 minutes. The prepared zirconia bars (2 specimens per firing cycle) were placed on the floor of an insulation box fabri-
cated from low-density rigid alumina insulation board (Eco25B; Zircar Ceramics Inc, Florida, NY). Three 25 gram SiC susceptors (Research Microwave Systems, Troy, NY) were placed at the far ends of the box so that 1 was on either side of the specimens (Fig. 3). The top was placed on the insulation box, and the box was
The Journal of Prosthetic Dentistry
positioned inside the microwave oven on a base made of the same insulation board to reduce the amount of heat absorbed by the turntable of the microwave. The temperature was set to 1500°C. After sintering, the box was then removed with thermal insulated gloves, and after approximately 20 minutes of cooling, the specimens
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6 SEM of conventional oven specimen (×5) showing slightly larger porosity than microwave oven specimen.
7 SEM of microwave oven specimen (×5) showing well condensed grains with minimum porosity.
8 Conventional EDS showing presence of Zr and O elements on surface of specimen.
9 Microwave EDS showing presence of Zr and O elements on surface of specimen. were carefully removed from the box. Each bar was carefully hand polished on the side subjected to tensile force with a MiniMet electrical polishing machine (Buehler, Lake Bluff, Ill) with 200 mm silicon carbide waterproof paper, grit sizes 320, 400, and 600 (Met Lab Corporation, Niagara Falls, Canada). Flexural strength testing followed final polishing. Each bar was oriented
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on a holding device with the modified surface subjected to tensile force (facing opposite to the indenter). The span between the 2 points supporting each bar was 20 mm. The load was applied at the center of the bar with a blunt rounded chisel-shaped indenter, measuring 20 mm in length and 1 mm in width at the tip (Fig. 4). Following ISO guidelines (ISO stan-
dards, 2006), flexural strength was determined in MPa units after loading each bar in a 3-point configuration in a universal testing machine (Instron series 4204 Load Frame; Instron, Norwood, Mass) with a 5 kN load cell at a crosshead speed of 1.00 mm/min. Bulk density measurements were made according to the American Society for Testing and Materials (ASTM)
308 standards. Postsintered weight was measured as a dry weight (D). Specimens were placed in deionized water, boiled for 2 hours, and left undisturbed for 12 hours. Saturated weight (S) was measured by using the Archimedes method (Fig. 5). Care was taken to keep each specimen just below the surface of the water. The specimens were removed, dried thoroughly, and wet weight (W) was recorded. Volume (V), bulk density (B), and percentage density (PD) were recorded by using the following formula: V=W–S B = D/ V PD = B/ Theoretical density × 100 A theoretical density of 6.06 was used to calculate percentage density. The length, width, and thickness of the sintered bars were measured with a digital micrometer (Adjustable Micrometer 0-300 mm; Digital Micrometers Ltd, Sheffield, UK) 3 measurements per variable were made for each bar. Volumetric shrinkage was calculated from the measurement differences between the presintered and the postsintered specimens. The specimens were thermally etched by heating the furnace and holding for 20 minutes at 1420°C and were then slowly cooled to room temperature. They were sputter coated with gold palladium for 3 minutes (300 angstrom thick coating) and examined under a scanning electron microscope (SU-70 Field Emission SEM, 20.Kev Acceleration Voltage; Hitachi, Tokyo, Japan). This was done by using the fine-slow mode to provide better image resolutions, provide more frame integration, and average out any noise. Surface elemental analysis was obtained by energy dispersive X-ray spectroscopy (INCA silicon drift EDS Detector; Oxford Instruments, Concord, Mass). Independent t tests were used to test differences in density and flexural strength between sintering methods. All tests were conducted at an alpha level of .05.
Volume 108 Issue 5 RESULTS The volumetric shrinkage in both techniques yielded the same results, with approximately 24.6% volume shrinkage in the specimens used. The mean flexural strength values (standard deviation) in MPa were 1080.08 (79.37) for the conventional zirconia furnace and 1108.33 (162.55) for the microwave furnace. The mean percentage density values (standard deviation) were 99.9 (0.22) for the conventional zirconia furnace and 99.9 (0.16) for the microwave furnace. The results indicated no significant difference between the conventional and microwave sintering furnace for either flexural strength, t18=0.49, P=.63 or density, t18=0.07, P=.95. Effect sizes in standard deviation units were 0.023 for density and 0.35 for flexural strength. Effect sizes of 0.2 or less are considered small.21 They may also be interpreted in terms of percentage overlap between the 2 groups. For an effect size of 0.023, there is over 93% overlap; for an effect size of 0.35, the overlap is approximately 75%.21 Post hoc power analysis revealed that a minimum sample size to show significant difference between the 2 methods was 431 specimens per group for flexural strength and 3440 specimens per group for density. Thus, it appears that there is no real difference in flexural strength or density between the sintering methods.
DISCUSSION The results of this study do not support rejection of the null hypothesis that there are no significant differences in flexural strength and density between the sintering techniques. Zirconia shrinks by approximately 25%, achieving a density of greater than 99%22 and a flexural strength greater than 1100 MPa during the sintering process at 1500°C.23 In this study, specimens in both groups exhibited a uniform firing shrinkage of approximately 24.6% in all dimensions,
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which is similar to the manufacturer’s specifications. The surface of selected specimens examined under a scanning electron microscope showed no visible difference in grain shape and size or porosity (Figs. 6, 7) between the 2 sintering methods. However, the average porosity size of the microwave-sintered specimens appeared smaller than the conventionally sintered specimens. In general, the microwave-sintered specimens appeared to have more uniform grain size distribution and to be more closely packed than conventionally sintered specimens. This might be related to the fact that microwave sintering provides more uniform specimens and suggests confirmation of the volumetric heating phenomenon. Using a microwave furnace limits the number of specimens to 2 per sintering cycle. This could be enhanced by developing microwave furnaces that could hold more than 2 specimens per sintering cycle. The surface elemental analysis revealed the presence of Zr and O on the surface of specimens for both sintering methods (Figs. 8, 9). Among the limitations of this study is the lack of marginal fit assessment between conventional sintering techniques and that of microwave furnace sintering technique. Another limitation is the need to study other physical properties of the zirconia copings produced using both methods. Future research could involve scanning actual dies from prepared teeth and comparing the physical properties of zirconia copings as well as measure the marginal fit with both techniques.
CONCLUSION Results of this study suggest that either a microwave or conventional zirconia furnace satisfies the general clinical requirements of manufacturing zirconia for dental use. Microwave energy provides a greater uniformity of heating, allowing the use of higher heating rates to increase productivity and save energy.
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November 2012 REFERENCES 1. Sutton W. Microwave processing of ceramic materials. Am Ceram Soc Bull 1989;68:376-86. 2. Iskander M. Computer modeling and numerical simulation of microwave heating systems. M R S Bulletin 1993;18:30-6. 3. Committee on Materials for High-Temperature Semiconductor Devices, Commission on Engineering and Technical Systems, National Research Council. Materials for high-temperature semiconductor devices. (Pub. NMAB-474) Washington DC: National Academy Press; 1994. 4. Tinga W R, Voss WA. Microwave power engineering. New York: Academic Press; 1968. p. 189-99. 5. Sheppard L. Manufacturing ceramics with microwaves: the potential for economical production. American Ceramic Society Bulletin 1988;67:1656-61. 6. Janney MA, Kimrey HD. Microwave sintering of alumina at 28 GHz. In: Ceramic powder science II. Messing GL, Fuller ER, Hausner H, eds. Westerville: American Ceramic Society; 1988. p. 919-24. 7. Janney M, Kimrey H. Microstructure evolution in microwave sintered alumina. In: Ceramic transaction, sintering of advanced ceramics. Handwerker C, Blendell J, Kaysser W, eds. Westerville: American Ceramic Society; 1990. p. 382-6. 8. Thermal pod and thermal box user’s guide. Troy, New York: Research microwave systems; 1998. p. 5-11.
9. Piconi C, Maccauro G. Zirconia as ceramic biomaterial. Biomaterials 1999;20:1-25. 10.Garvie R, Hannink R, Pascoe R. Ceramic steel? Nature 1975;258:703-4. 11.Evans A. Perspective on the development of high toughness ceramics. J Am Ceram Soc 1990;65:187-206. 12.Baldissara P, Llukacej A, Ciocca L, Valandro FL, Scotti R. Translucency of zirconia copings made with different CAD/CAM systems. J Prosthet Dent 2010;104:6-12. 13.Spyropoulou PE, Giroux EC, Razzoog ME, Duff RE. Translucency of shaded zirconia core material. J Prosthet Dent 2011;105:304-7. 14.Keith O, Kusy R, Whitley J. Zirconia brackets: an evaluation of morphology and coefficients of friction. Am J Orthod Dentofacial Orthop 1994;106:605-14. 15.Prestipino V, Ingber A. Esthetic high strength implant abutments. Part I. J Esthet Dent 1993; 5:29-36. 16.Akagawa Y, Ichikawa Y, Nikai H. Interface histology of unloaded and early loaded partially stabilized zirconia endosseous implant in initial bone healing. J Prosthet Dent 1993;69:599-604. 17.Raigrodski A. Contemporary all-ceramic fixed partial dentures: a review. Dent Clin North Am 2004;48:531-44. 18.Baig MR, Tan KB, Nicholls JI. Evaluation of the marginal fit of a zirconia ceramic computer-aided machined (CAM) crown system. J Prosthet Dent 2010;104:216-27
19.Kugel G, Perry RD, Aboushala A. Restoring anterior maxillary dentition using alumina-and zirconia-based CAD/CAM restorations. Compend Contin Educ Dent 2003;24:569-79. 20.Kim HJ, Lim HP, Park YJ, Vang MS. Effect of zirconia surface treatments on the shear bond strength of veneering ceramic. J Prosthet Dent 2011;105:315-22. 21.Cohen J. Statistical power analysis for the behavioral sciences. 2nd ed. New York: Academic Press; 1977. p. 22-40. 22.Denry I, Kelly J. State of the art of zirconia for dental applications. Dent Mater 2008; 24:299-307. 23.Pittayachawan P, McDonald A, Petrie A, Knowles JC. The biaxial flexural strength and fatigue property of lava Y-TZP dental ceramic. Dent Mater 2007;23:1018-29. Corresponding author: Dr Hasan M. Khajah Al-Amiri Dental Center P.O. Box 899 Qurtubah KUWAIT E-mail: [email protected] Acknowledgments The authors thank Dr Elaine Davis for providing statistical analysis. Copyright © 2012 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Biological aging of implant surfaces and their restoration with ultraviolet light treatment: a novel understanding of osseointegration. Att W, Ogawa T. Int J Oral Maxillofac Implants. 2012 Jul;27:753-61. Abstract The topographic and physicochemical features of implant surfaces influence the process of osseointegration. The biologic properties of implant surfaces have been considered to remain stable over time, ie, the capability of osseointegration of implant surfaces presumably does not change over time after manufacturing. However, recent reports have demonstrated that titanium surfaces undergo a progressive change in their biologic characteristics over time, resulting in a significant decrease in osseointegration capability. In comparison to newly prepared titanium surfaces, 4-week-old titanium surfaces (ie, stored for 4 weeks after processing) required more than twice as much healing time to achieve a similar strength of osseointegration. The bone implant contact percentage for the 4-week-old surfaces was less than 60%, as opposed to more than 90% for the new surfaces. In vitro, the 4-week-old surfaces showed only 20% to 50% of the levels of recruitment, attachment, settlement, and proliferation of osteogenic cells versus new surfaces. On the other hand, a series of recent papers reported the generation of highly cell-attractive and osteoconductive titanium surfaces by ultraviolet (UV) light treatment. The phenomenon, defined as photo functionalization, caused a fourfold acceleration in the process of osseointegration and resulted in nearly 100% bone-implant contact. Remarkably enhanced behavior and response of osteogenic cells around UV-treated surfaces exceeded the levels observed for the newly prepared surfaces. These studies indicated that UV treatment reverses the time-dependent biologic degradation of titanium and even enhances the surface beyond its innate potential. The present paper summarizes the findings about the aging-like time-dependent biologic degradation of titanium surfaces as well as about the discovery of UV photo functionalization as a solution for this phenomenon. It also provides a novel understanding of osseointegration and calls for immediate attention to a new avenue of exploration in the science and therapeutics of implant dentistry.
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Clinical Implications
Based on the study results, a microwave furnace, which creates a more uniform heat distribution and uses less energy, may be used to sinter dental zirconia. Because of its esthetic qualities, durability, and biocompatibility, dental zirconia has become a popular alternative to metals as a dental restorative material. However, the fabrication
of ceramic crowns in a conventional sintering oven is a time-consuming and expensive procedure, requiring the heating of a large oven to a high temperature and then slow cooling to
prevent cracking. In a standard conventional furnace, heat is applied to the external surface of the restoration and reaches the core by thermal conduction, producing high tempera-
Senior Registrar, Fixed Prosthodontics, Al-Amiri Dental Center. Senior Specialist, Fixed Prosthodontics, Al-Amiri Dental Center. c Assistant Professor and Director, Postgraduate Prosthodontics, Department of Restorative Dentistry, School of Dental Medicine, State University of New York at Buffalo. d Assistant Professor, Postgraduate Prosthodontics, Department of Restorative Dentistry, School of Dental Medicine, State University of New York at Buffalo. a
b
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November 2012 ture gradients and stresses within the material. It is not a uniform heating process. Dental zirconia can also be sintered in a microwave furnace, in which the material is rapidly heated both internally and externally.1 In addition to the heat generated within the material, susceptors (materials that absorb electromagnetic energy and convert it to heat) made of silicon carbide or molybdenum silicate are placed around the ceramics to heat the specimen externally by thermal conduction. The presence of susceptors modifies the electromagnetic field distribution and its mean intensity, introduces heat sources external to the specimens, and may shield, to some extent, the zirconia specimens from the heat field.2 As a result of this internal and external volumetric heating, the thermal gradient and flow of heat in the ceramic body is evenly applied. This results in more uniformly and rapidly heated specimens with less thermal stress than conventional methods. Research has shown that many ceramics do not absorb microwaves well at room temperature.3 This creates the need to use susceptors that absorb microwaves at room temperature and act as heating elements. The possibility of the microwave processing of ceramics was first described in the 1950s and the processing of industrial ceramics in the 1960s.4 Microwave sintering has many attractive features, including rapid volumetric heating and low cost. Higher production rates and lower energy requirements make this process commercially attractive. Estimates of energy saving are 25% to 95%.5 In the 1980s, investigations demonstrated the benefit of microwave sintering. By using 28 GHz microwave radiation, alumina can be sintered at 300°C to 400°C below the temperature required for sintering in a conventional oven.6,7 Recently, a custom microwave oven (ThermWave, TW 1.3; EPL Ceramic Materials LLC, Youngstown, NY) was fabricated in cooperation with Alfred University (Alfred, NY)
Almazdi et al
that enables the temperature to rise to 1600°C. A water jacket with a circulating water supply is provided to cool the system. A small heating chamber made of insulating material (fibrous alumina) encloses 3 silicon carbide microwave susceptors (Thermcepts; Research Microwave Systems, Troy, NY). Thermcepts are used to raise the temperature of microwave-processed material for both microwave-absorbing specimens and nonabsorbing or lowabsorbing specimens. The thermcepts can be used to heat materials through temperature zones at which the material does not readily absorb microwave energy at 2.54 MHz. The temperature inside the heating chamber is recorded with a pyrometer/thermocouple inserted in the top of the oven, and the temperature is controlled inside the heating chamber with a temperature sensor.8 Zirconia is known as a polymorphic material that presents in 3 forms: the monoclinic phase, which is stable at room temperature up to 1170°C; the tetragonal phase, which is stable at temperatures above 1170°C up to 2370°C; and the cubic phase, which is the form that the material changes into above 2370°C.9 Compressive stress is initiated because of volumetric expansion in zirconia, and crack propagation resistance occurs; this is known as transformation toughening.10 Compared to other ceramic materials, yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is characterized by good physical properties such as high fracture toughness and relatively low elastic modulus as well as some degree of translucency.11-13 In dentistry, zirconia ceramics have been used for posts and cores, frameworks for fixed partial dental prostheses, orthodontic brackets, implant abutments, and implants.14-16 Radiographically, zirconia substructures appear opaque, which enhances the evaluation of marginal integrity and early detection of recurrent caries.17 Although some zirconia systems have demonstrated marginal discrepancies, these have been shown to be of
no significance in many of the systems used currently.18 Y-TZP has become the core material of choice for many categories of ceramic restorations because of its toughness and esthetic qualities. Computer-aided design and computer-aided manufacturing (CAD/CAM) technology has solved the difficulties of processing this hard material by milling it when it is partially sintered or presintered. A Y-TZP substructure can be veneered with pressable or layering ceramics with excellent esthetic results.19 This could be done either with airborne-particle abrasion or liner-applied treatments, with the latter having a stronger in vitro shear bond strength.20 In this study, the flexural strength and density of zirconia processed in a conventional or microwave sintering furnace were compared. The null hypothesis was that there are no significant differences in flexural strength and density between the sintering techniques.
MATERIAL AND METHODS Twenty (25 × 4.5 × 2.5 mm) bars were milled from partially sintered 3% Y-TZP (e.max Zircad B40 Blocks (lot# L17952)); Ivoclar Vivadent, Schaan, Liechtenstein) with an electrical high precision saw (Isomet 1000 Precision Saw; Buehler Ltd, Lake Bluff, Ill) with a diamond wafering blade (4”, high concentration Cat #M412H, 4” × .012” × 1/2” Blade; Metlab Corp, Niagara Falls, NY) under water irrigation. The bars were assigned to 1 of 2 groups (n=10 per group) in either a conventional or microwave furnace. The bars sintered in a conventional furnace were sintered at 1500°C for 8 hours in a high-temperature sintering furnace for zirconia (Sintramat; Ivoclar Vivadent, Buffalo, NY). All 10 bars were sintered simultaneously according to the manufacturer’s recommendations (Fig. 1). A microwave oven (ThermWave, TW 1.3; EPL Ceramic Materials LLC) (Fig. 2) was custom designed in col-
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Volume 108 Issue 5
1 Conventional sintering machine used in study.
2 Microwave furnace used in study.
3 Three SiC susceptors placed at far ends of box arranged around specimens.
4 Zirconia bar mounted for 3-point bend test.
5 Archimedes method used to measure theoretical density under water. laboration with EPL ceramic materials as previously described. A controller box was attached to the microwave to precisely control the temperature inside the chamber. The bars were sintered at 1500°C for 30 minutes. The prepared zirconia bars (2 specimens per firing cycle) were placed on the floor of an insulation box fabri-
cated from low-density rigid alumina insulation board (Eco25B; Zircar Ceramics Inc, Florida, NY). Three 25 gram SiC susceptors (Research Microwave Systems, Troy, NY) were placed at the far ends of the box so that 1 was on either side of the specimens (Fig. 3). The top was placed on the insulation box, and the box was
The Journal of Prosthetic Dentistry
positioned inside the microwave oven on a base made of the same insulation board to reduce the amount of heat absorbed by the turntable of the microwave. The temperature was set to 1500°C. After sintering, the box was then removed with thermal insulated gloves, and after approximately 20 minutes of cooling, the specimens
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6 SEM of conventional oven specimen (×5) showing slightly larger porosity than microwave oven specimen.
7 SEM of microwave oven specimen (×5) showing well condensed grains with minimum porosity.
8 Conventional EDS showing presence of Zr and O elements on surface of specimen.
9 Microwave EDS showing presence of Zr and O elements on surface of specimen. were carefully removed from the box. Each bar was carefully hand polished on the side subjected to tensile force with a MiniMet electrical polishing machine (Buehler, Lake Bluff, Ill) with 200 mm silicon carbide waterproof paper, grit sizes 320, 400, and 600 (Met Lab Corporation, Niagara Falls, Canada). Flexural strength testing followed final polishing. Each bar was oriented
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on a holding device with the modified surface subjected to tensile force (facing opposite to the indenter). The span between the 2 points supporting each bar was 20 mm. The load was applied at the center of the bar with a blunt rounded chisel-shaped indenter, measuring 20 mm in length and 1 mm in width at the tip (Fig. 4). Following ISO guidelines (ISO stan-
dards, 2006), flexural strength was determined in MPa units after loading each bar in a 3-point configuration in a universal testing machine (Instron series 4204 Load Frame; Instron, Norwood, Mass) with a 5 kN load cell at a crosshead speed of 1.00 mm/min. Bulk density measurements were made according to the American Society for Testing and Materials (ASTM)
308 standards. Postsintered weight was measured as a dry weight (D). Specimens were placed in deionized water, boiled for 2 hours, and left undisturbed for 12 hours. Saturated weight (S) was measured by using the Archimedes method (Fig. 5). Care was taken to keep each specimen just below the surface of the water. The specimens were removed, dried thoroughly, and wet weight (W) was recorded. Volume (V), bulk density (B), and percentage density (PD) were recorded by using the following formula: V=W–S B = D/ V PD = B/ Theoretical density × 100 A theoretical density of 6.06 was used to calculate percentage density. The length, width, and thickness of the sintered bars were measured with a digital micrometer (Adjustable Micrometer 0-300 mm; Digital Micrometers Ltd, Sheffield, UK) 3 measurements per variable were made for each bar. Volumetric shrinkage was calculated from the measurement differences between the presintered and the postsintered specimens. The specimens were thermally etched by heating the furnace and holding for 20 minutes at 1420°C and were then slowly cooled to room temperature. They were sputter coated with gold palladium for 3 minutes (300 angstrom thick coating) and examined under a scanning electron microscope (SU-70 Field Emission SEM, 20.Kev Acceleration Voltage; Hitachi, Tokyo, Japan). This was done by using the fine-slow mode to provide better image resolutions, provide more frame integration, and average out any noise. Surface elemental analysis was obtained by energy dispersive X-ray spectroscopy (INCA silicon drift EDS Detector; Oxford Instruments, Concord, Mass). Independent t tests were used to test differences in density and flexural strength between sintering methods. All tests were conducted at an alpha level of .05.
Volume 108 Issue 5 RESULTS The volumetric shrinkage in both techniques yielded the same results, with approximately 24.6% volume shrinkage in the specimens used. The mean flexural strength values (standard deviation) in MPa were 1080.08 (79.37) for the conventional zirconia furnace and 1108.33 (162.55) for the microwave furnace. The mean percentage density values (standard deviation) were 99.9 (0.22) for the conventional zirconia furnace and 99.9 (0.16) for the microwave furnace. The results indicated no significant difference between the conventional and microwave sintering furnace for either flexural strength, t18=0.49, P=.63 or density, t18=0.07, P=.95. Effect sizes in standard deviation units were 0.023 for density and 0.35 for flexural strength. Effect sizes of 0.2 or less are considered small.21 They may also be interpreted in terms of percentage overlap between the 2 groups. For an effect size of 0.023, there is over 93% overlap; for an effect size of 0.35, the overlap is approximately 75%.21 Post hoc power analysis revealed that a minimum sample size to show significant difference between the 2 methods was 431 specimens per group for flexural strength and 3440 specimens per group for density. Thus, it appears that there is no real difference in flexural strength or density between the sintering methods.
DISCUSSION The results of this study do not support rejection of the null hypothesis that there are no significant differences in flexural strength and density between the sintering techniques. Zirconia shrinks by approximately 25%, achieving a density of greater than 99%22 and a flexural strength greater than 1100 MPa during the sintering process at 1500°C.23 In this study, specimens in both groups exhibited a uniform firing shrinkage of approximately 24.6% in all dimensions,
The Journal of Prosthetic Dentistry
which is similar to the manufacturer’s specifications. The surface of selected specimens examined under a scanning electron microscope showed no visible difference in grain shape and size or porosity (Figs. 6, 7) between the 2 sintering methods. However, the average porosity size of the microwave-sintered specimens appeared smaller than the conventionally sintered specimens. In general, the microwave-sintered specimens appeared to have more uniform grain size distribution and to be more closely packed than conventionally sintered specimens. This might be related to the fact that microwave sintering provides more uniform specimens and suggests confirmation of the volumetric heating phenomenon. Using a microwave furnace limits the number of specimens to 2 per sintering cycle. This could be enhanced by developing microwave furnaces that could hold more than 2 specimens per sintering cycle. The surface elemental analysis revealed the presence of Zr and O on the surface of specimens for both sintering methods (Figs. 8, 9). Among the limitations of this study is the lack of marginal fit assessment between conventional sintering techniques and that of microwave furnace sintering technique. Another limitation is the need to study other physical properties of the zirconia copings produced using both methods. Future research could involve scanning actual dies from prepared teeth and comparing the physical properties of zirconia copings as well as measure the marginal fit with both techniques.
CONCLUSION Results of this study suggest that either a microwave or conventional zirconia furnace satisfies the general clinical requirements of manufacturing zirconia for dental use. Microwave energy provides a greater uniformity of heating, allowing the use of higher heating rates to increase productivity and save energy.
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9. Piconi C, Maccauro G. Zirconia as ceramic biomaterial. Biomaterials 1999;20:1-25. 10.Garvie R, Hannink R, Pascoe R. Ceramic steel? Nature 1975;258:703-4. 11.Evans A. Perspective on the development of high toughness ceramics. J Am Ceram Soc 1990;65:187-206. 12.Baldissara P, Llukacej A, Ciocca L, Valandro FL, Scotti R. Translucency of zirconia copings made with different CAD/CAM systems. J Prosthet Dent 2010;104:6-12. 13.Spyropoulou PE, Giroux EC, Razzoog ME, Duff RE. Translucency of shaded zirconia core material. J Prosthet Dent 2011;105:304-7. 14.Keith O, Kusy R, Whitley J. Zirconia brackets: an evaluation of morphology and coefficients of friction. Am J Orthod Dentofacial Orthop 1994;106:605-14. 15.Prestipino V, Ingber A. Esthetic high strength implant abutments. Part I. J Esthet Dent 1993; 5:29-36. 16.Akagawa Y, Ichikawa Y, Nikai H. Interface histology of unloaded and early loaded partially stabilized zirconia endosseous implant in initial bone healing. J Prosthet Dent 1993;69:599-604. 17.Raigrodski A. Contemporary all-ceramic fixed partial dentures: a review. Dent Clin North Am 2004;48:531-44. 18.Baig MR, Tan KB, Nicholls JI. Evaluation of the marginal fit of a zirconia ceramic computer-aided machined (CAM) crown system. J Prosthet Dent 2010;104:216-27
19.Kugel G, Perry RD, Aboushala A. Restoring anterior maxillary dentition using alumina-and zirconia-based CAD/CAM restorations. Compend Contin Educ Dent 2003;24:569-79. 20.Kim HJ, Lim HP, Park YJ, Vang MS. Effect of zirconia surface treatments on the shear bond strength of veneering ceramic. J Prosthet Dent 2011;105:315-22. 21.Cohen J. Statistical power analysis for the behavioral sciences. 2nd ed. New York: Academic Press; 1977. p. 22-40. 22.Denry I, Kelly J. State of the art of zirconia for dental applications. Dent Mater 2008; 24:299-307. 23.Pittayachawan P, McDonald A, Petrie A, Knowles JC. The biaxial flexural strength and fatigue property of lava Y-TZP dental ceramic. Dent Mater 2007;23:1018-29. Corresponding author: Dr Hasan M. Khajah Al-Amiri Dental Center P.O. Box 899 Qurtubah KUWAIT E-mail: [email protected] Acknowledgments The authors thank Dr Elaine Davis for providing statistical analysis. Copyright © 2012 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Biological aging of implant surfaces and their restoration with ultraviolet light treatment: a novel understanding of osseointegration. Att W, Ogawa T. Int J Oral Maxillofac Implants. 2012 Jul;27:753-61. Abstract The topographic and physicochemical features of implant surfaces influence the process of osseointegration. The biologic properties of implant surfaces have been considered to remain stable over time, ie, the capability of osseointegration of implant surfaces presumably does not change over time after manufacturing. However, recent reports have demonstrated that titanium surfaces undergo a progressive change in their biologic characteristics over time, resulting in a significant decrease in osseointegration capability. In comparison to newly prepared titanium surfaces, 4-week-old titanium surfaces (ie, stored for 4 weeks after processing) required more than twice as much healing time to achieve a similar strength of osseointegration. The bone implant contact percentage for the 4-week-old surfaces was less than 60%, as opposed to more than 90% for the new surfaces. In vitro, the 4-week-old surfaces showed only 20% to 50% of the levels of recruitment, attachment, settlement, and proliferation of osteogenic cells versus new surfaces. On the other hand, a series of recent papers reported the generation of highly cell-attractive and osteoconductive titanium surfaces by ultraviolet (UV) light treatment. The phenomenon, defined as photo functionalization, caused a fourfold acceleration in the process of osseointegration and resulted in nearly 100% bone-implant contact. Remarkably enhanced behavior and response of osteogenic cells around UV-treated surfaces exceeded the levels observed for the newly prepared surfaces. These studies indicated that UV treatment reverses the time-dependent biologic degradation of titanium and even enhances the surface beyond its innate potential. The present paper summarizes the findings about the aging-like time-dependent biologic degradation of titanium surfaces as well as about the discovery of UV photo functionalization as a solution for this phenomenon. It also provides a novel understanding of osseointegration and calls for immediate attention to a new avenue of exploration in the science and therapeutics of implant dentistry.
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