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Microstructural study of microwave sintered zirconia for dental applications
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 1255–1261 www.elsevier.com/locate/ceramint
Microstructural study of microwave sintered zirconia for dental applications C. Monacoa, F. Preteb, C. Leonellic, L. Espositoa, A. Tuccia,n a
Department of Biomedical Science and Neuromotor Sciences, University of Bologna, Italy b Centro Ceramico Bologna, Italy c Department of Materials and Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia, Italy Received 19 September 2013; received in revised form 16 July 2014; accepted 25 August 2014 Available online 19 September 2014
Abstract Conventional sintering techniques for zirconia-based materials, which are commonly used in dental reconstruction, may not provide uniform heating, with the consequent generation of microstructural flaws in the final component. A sintering system using microwave heating may represent a viable alternative. The purpose of this study was to compare the dimensional variation and physical and microstructural characteristics of commercial zirconia (Y-TZP), used as a dental restoration material, sintered in conventional and microwave furnaces. A physicalmineralogical-microstructural characterisation was carried out to evaluate the level of densification and the presence of flaws in the sintered specimens. Use of the microwave systems allowed the length of the sintering cycle to be reduced to a few minutes, compared with the several hours necessary with a ‘traditional’ heating system. Additionally, the maximum temperature used to reach the required density decreased from 1450–1480 1C with the electric furnace to 1200 1C in the microwave furnace. An important clinical implication is that the reduced sintering time could allow the introduction of zirconia in chair-side treatments, if used as a monolithic material. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Microwave sintering; Y-TZP; Dental ceramic; Microstructure
1. Introduction A well-known review on zirconia for dental applications [1] noted that in the last 20 years, the diffusion of metal-free restorations in dental practice has increased considerably due to the growing demand for highly aesthetic and natural-appearing components. Bioceramics [2] are particularly suitable for the use in prosthodontics as possible metal substitutes because of their wear resistance, superior mechanical properties, high biocompatibility, and excellent aesthetic appearance. Particular attention has been paid to “yttria-tetragonal zirconia polycrystalline ceramics” (Y-TZPs), which have been used as framework materials for dental crowns and fixed partial dentures (FDPs), because their aesthetic appearance is similar to that of n Correspondence to: Department of Biomedical and Neuromotor Sciences, Division of Prosthodontics and Maxillo-facial Rehabilitation via S. Vitale, 59 40125 Bologna, Italy. Tel.: þ 39 0512 088 186; fax: þ39 0512 25208. E-mail address: [email protected] (A. Tucci).
http://dx.doi.org/10.1016/j.ceramint.2014.09.055 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
natural teeth and their mechanical characteristics are good: indeed, the highest reported for any dental ceramic [3]. Both the chemistry and processing of these materials allow obtaining a fully dense polycrystalline zirconia, in a tetragonal phase, with a homogeneous distribution of submicron zirconia grains, giving a translucent aspect, which meets the requirements for natural teethlooking restorations [2]. Furthermore, compared with other ceramics, 3 mol% yttria-stabilised tetragonal zirconia polycrystals (3Y-TZP) are characterised by high fracture toughness and flexural strength, caused by a stress-induced phase transformation, from a tetragonal (t) phase to a monoclinic (m) phase, which increases its crack-propagation resistance [4]. The combination of these particular mechanical and physical characteristics has allowed Y-TZP to become the core material of choice for many categories of dental ceramic restorations. Additionally, CAD/CAM system technology has solved the difficulties of processing this hard material by working it when partially sintered or presintered [5]. In the usual practice of dental laboratories, pre-sintered Y-TZP blocks are rapidly
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CAD/CAM-converted into restoration components that require a final firing step both to reach a higher density and to eliminate any stress induced by the strong surface working actions [6]. This final sintering is currently performed in large, electrically heated ovens to a high temperature, followed by slow cooling to prevent cracking, which is a time-consuming and expensive procedure. The thermal cycles typically indicated by the manufacturers are characterised by maximum temperatures of 1350–1550 1C, at which the ceramic components remain for almost 60 min. Due to the slow cooling step, the total sintering cycle is generally 6–10 h. Many ceramic powders can be sintered using microwaves at lower temperatures and for shorter times than with conventional electrical heating sources, allowing finer and more uniform microstructures to be obtained. Microwave irradiation, also known as dielectric heating, as applied to the sintering of advanced ceramic components, has become an important topic of scientific research [7–10]. In conventional sintering, heat is transferred from the radiant elements of the furnace to the surface of the ceramic component, reaching the core of the component through conduction mechanisms. In microwave sintering, the heat is produced as a consequence of an interaction between the ceramic sample and the electromagnetic waves and involves the whole sample volume; in this way, the heating is more rapid and uniform [11]. The extent of the energy transfer from the electromagnetic field to the matter depends strongly on the dielectric properties of the material, the temperature, and the radiation frequency [12–15]. The microwave sintering of yttria-doped zirconia has been investigated extensively using several types of microwave furnace. Due to its poor coupling with microwaves below 400 1C and moderate coupling at higher temperatures, it is necessary to use susceptors that absorb microwaves at room temperature and act as heating elements to increase the initial temperature of the material to the critical value at which it starts to absorb more effectively. For this purpose, several studies have reported the use of silicon carbide susceptors, because they absorb microwave energy and subsequently transfer it, in the form of heat, to the material, via conduction. This approach, often referred to in the literature as ‘hybrid microwave heating’, uses hybrid heating devices such as multi-mode cavities with SiC rods [16] or SiC powder [17] acting as a susceptor, or microwave/ conventional hybrid furnaces [18]. When samples are heated in an electric furnace or a microwave furnace, two methods can be used to control the temperature: 1) intermittent powering of the magnetron at a fixed power output (on/off control method or time-control method), or 2) continuous powering of the magnetron with a variable power output (powercontrol method). The first method involves the use of the magnetron at its highest output power as typically programmed in domestic ovens, while the second is commonly used in industrial processes, where continuous adjustments of output power are necessary to follow the desired heating profile. It has been pointed out that there is no difference between the two methods in terms of grain growth or sample densification level, but the power-control method gives more precise control of temperature versus the on/off control method [19].
The multiplicity of experimental procedures reported makes obtaining a clear view of the behaviour of zirconia powder during microwave sintering difficult, despite the useful review in [20]. Regarding the sintering of nano Y-TZP, hybrid conventional-microwave heating sintering allowed obtaining 499% TD dense ceramics with an average grain size of less than 100 nm [18], but also near-theoretical density values for 3Y-TZP using a multi-mode microwave sintering furnace at 2.45 GHz [21]. Microwave sintering has been also proposed to overcome the problem of grain growth associated with conventional heating in nanocrystalline 3Y-TZP, by obtaining more homogenous microstructures with higher mechanical characteristics [22]. Further improvements in the physical and mechanical properties of Y2O3-ZrO2 ceramics have been achieved through the use of nanopowders and the use of microwave sintering [23]. The results of these studies underlined that, compared with conventional sintering, the use of microwaves provides several advantages, such as rapid and volumetric heating, lower heating temperatures, enhancements in densification, grain growth limitations, and cost savings. The aim of this work was to assess the possibility of using hybrid microwave sintering with a 3Y-TZP pre-sintered material for dental applications. Two microwave methods, multi- and single-mode, were used for the tests, both of which were fed with continuous variable power to better control the process temperature. The results indicated that the density of the microwave-fired samples depended strongly on the firing changes, which, once optimised, allowed the generation of highly dense zirconia samples with a firing time of only a few minutes. 2. Material and methods A 3% Y-TZP pre-sintered commercial material (Biotech Srl, Milano, Italy, one of the most utilised in Italian Dentistry and in some countries of Europe), suitable for shaping using CADCAM technology, was used for the sintering tests. From the commercial supplies, provided in the form of cylinders, rectangular specimens, of about 20 10 14 mm3, were cut with an high-precision electric saw (Isomet 1000 Precision Saw, Buehler Ltd., Düsseldorf, Germany) and subjected to three heating treatments: conventional, multi-mode, and singlemode microwave sintering. Conventional sintering was conducted in an electric furnace, using the following sintering cycle: 12 1C/min up to 300 1C, 5 1C/min up to the maximum temperature, holding time 60 min, with natural cooling. After several preliminary trials, two maximum temperatures were used: 1450 and 1480 1C. These sintering cycles required 10 h at either maximum temperature. Microwave sintering was performed with a commercial CEM-MAS 7000 multi-mode applicator (CEM Corporation, Matthews, NC) at 2.45 GHz (950 W, nominal power) and on a TE10 n single-mode applicator (0.5–3 kW output power), connected to a 2.45–GHz TM030 microwave generator (Alter Power System, Long Beach CA). The multi-mode applicator
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can generate a lower field density than the single-mode applicator, which is why we compared the two systems. Additionally, a single-mode process can be designed or adjusted according to the load to ensure that the sample is in the region of high microwave intensity. Because microwave sintering causes very rapid temperature increases in tested samples, the resulting thermal shock could destroy the zirconia specimens. For this reason, several trials using different arrangements using SiC susceptors or a refractory crucible were conducted to determine the optimum operating conditions, allowing us to obtain sintered specimens with no cracks. In this study, the following sintering conditions were used. For the multi-mode applicator, a fibre insulating housing was placed inside the microwave chamber. Temperature measurements were conducted using a k-type thermocouple inserted into the multi-mode cavity and placed in direct contact with the specimen. Each sample was located inside a cordierite crucible full of alumina powder. The general scheme of the experimental set-up is shown in Fig. 1. For the tests conducted with the single-mode applicator, each sample was located inside a SiC crucible filled with alumina powder. The crucible was positioned inside a refractory support (Fig. 2a) and then placed in the microwave chamber (Fig. 2b). Temperature was detected in contact with the sample using an optical fibre and transformed to a temperature signal. Manual adjustment to keep the sample in the maximum electric field intensity was performed throughout the heating cycle by means of a shorting plunger, which was positioned at the end of the single-mode applicator. The relative density and apparent porosity of each sample were measured before and after conventional or microwave sintering following European Standard EN 623-2 (1993) [24]. The degree of shrinkage after firing was calculated, and the thickness of each rectangular specimen was measured using a digital micrometer before and after sintering. To avoid errors due to possible distortion of the specimens, the resulting values are the means of three measurements made at different parts of the bars. The phase fraction amounts of zirconia in the pre-sintered and sintered conditions were evaluated by X-ray diffraction
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Fig. 2. Sample arrangement used in the microwave single mode applicator; (a) SiC crucible filled with alumina powder inserted into a refractory support and (b) refractory support positioned inside the single mode cavity.
(XRD) using a diffractometer (Philips PW 3830; Koninklijke; the Netherlands), with Cu Kα radiation (0.021 step-scan, 10 s per step). Zirconia diffraction peaks were deconvoluted using a Lorentz function to obtain the integral breadth. The monoclinic phase fraction of the zirconia was calculated following reference [25]. The microstructures of the zirconia specimens before and after the sintering step were determined by analysing gold sputtercoated fresh-fractured and sintered surfaces using a scanning electron microscope (EVO 40; Carl Zeiss Microscopy GmbH, Oberkochen, Germany) equipped with an EDS system (Inca Energy 250; Oxford Analytical Instruments, Uedem, Germany). 3. Results
Fig. 1. Sample arrangement inside the microwave multi mode applicator.
The results of the physical-mineralogical characterisation of the tested material in the as-received, pre-sintered condition are shown in Table 1. Although it had a rather compact microstructure with a density consistent with other commercial materials of the same class, SEM analysis of the bulk of the samples showed some microcracks, usually connected with
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fragments of agglomerates originating from the starting powder, which were not completely destroyed by the processing operations (Fig. 3). The zirconia grains with an equiaxed geometry were rather fine, with a diameter of 80 nm, confirming that the presintering heating cycle did not allow grain growth. The crystalline phases present were tetragonal (96 wt%) and monoclinic (4 wt%) phase; after the different sintering cycles, only the tetragonal phase was detected. In Table 2, the density, apparent porosity, and shrinkage values of the samples sintered under the different conditions are reported. Samples B1 and B2, sintered with the two conventional cycles in the electrical furnace, reached high density values, 499% TD, and similar shrinkage values. Use of the lower temperature, 1450 1C, provided a good level of compactness, even if some small pre-existing flaws remained present, as evidenced by the SEM analysis (Fig. 4a). With an increase in the maximum sintering temperature to 1480 1C almost total elimination of the flaws was observed, although the higher temperature caused an increase in grain size dimensions (Fig. 4b). Samples B3 and B4 were microwave-sintered in the system CEM-MAS 7000 multi-mode applicator using two cycle lengths, both reaching a maximum temperature of 1200 1C. The shortest, a total length of 6 min, caused only partial sintering of the specimens; the apparent porosity was rather high, 5.20%, with a density that reached 92.1% TD. Increasing the time to 25 min improved the final shrinkage, but did not increase the density meaningfully, which reached only 95.6%TD with an apparent porosity of 1.03%. Microstructural analysis showed the presence
Table 2 Physical characteristics of the tested samples, after the different sintering conditions. Sintering Thermal Density Apparent temperature cycle length (g/cm3) porosity (vol%) (1C) (min)
Shrinkage (%)
Conventional sintering B1 B2
1450 1480
600 600
6.06 6.06
0.01 0.01
21.77 21.70
Multi mode microwaves sintering B3 B4
1200 1200
6 25
5.60 5.82
5.20 1.03
18.90 21.52
Single mode microwaves sintering B5 B6 B7
1100 1200 1200
4 4 6
5.05 5.98 6.01
14.52 0.80 0.01
16.71 21.49 21.32
Sample
Table 1 Physical-mineralogical characteristics of the pre-sintered material. Density (g/cm3)
Porosity (%)
t phase (wt%)
m phase (wt%)
Grain size (nm)
3.04
49.62
96
4
80
Fig. 3. SEM-SEI micrograph of the fracture surface of the tested material, in the pre-sintered condition. An evident microcrak, sorrounding rests of agglomerates of the starting powder, is arrowed.
Fig. 4. SEM-SEI micrograph of the fracture surface (a) of sample B1, conventional sintering 1450 1C/1 h, microflaws are arrowed and (b) of the fracture surface of sample B2, conventional sintering 1480 1C/1 h.
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of very small pores distributed rather homogeneously among the submicron zirconia grains (Fig. 5). They were likely responsible for the low density value. Samples B5 and B6, microwave-sintered with the TE10n single-mode applicator, reached maximum temperatures of 1100 and 1200 1C, respectively, with sintering cycle lengths of 4 and 6 min, respectively. At 1100 1C, samples B5 did not reach an acceptable level of density or shrinkage. Its microstructure was characterised by a rather homogeneous distribution of small pores; however, microcracks present in the presintered samples and agglomerates of microporosity linked to the presence of fragments of the original powder agglomerates are recognisable (Fig. 6). This sintering condition allows a rapid increase in the density of the zirconia, in the areas where the particles are closely packed, but is unable to eliminate the larger flaws, which would likely require a higher temperature and/or longer time. Use of an identical same sintering time (4 min) but a temperature of 1200 1C in sample B6 increased the density
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meaningfully, although a certain amount of open porosity remained; the apparent porosity was 0.80%. From the microstructural analysis, together with very small isolated pores some open porosity was evident (Fig. 7), although no appreciable growth of zirconia grains was visible. With the use of a longer time, 6 min, also at 1200 1C, the zirconia sample B7 reached the maximum density value for microwave sintering, which comparable with those reached using the longer-duration conventional heating treatments.
4. Discussion
Fig. 5. SEM-SEI micrograph of the fracture surface of sample B4, multimode microwaves sintered, maximum temperature 1200 1C, total length of the thermal cycle 25 min.
The results of this study underline that it is possible to reach a high level of densification and shrinkage using single-mode microwave sintering, such as with the conditions used with samples B7, 6 min at 1200 1C. While the open porosity of the specimens was minimal, the shrinkage was slightly lower than that of the conventionally heated samples B1 and B2. This can be better understood by examining the microstructural features. From Fig. 8a, a cross-section of B7, it is evident that at the border near the external surface the material is perfectly compact, with no pores, while in the bulk some small, closely packed pores, which are responsible for the reduced shrinkage, are evident (Fig. 8b). Reduced grain growth was observed for all microwavesintered, in comparison with conventionally sintered, samples even when the lower temperature (1450 1C) was used. This aspect is of particular importance for ceramic materials, because the presence of a coarse grain size distribution or a few grains characterised by exaggerated grain growth can be deleterious to the mechanical characteristics of a structural component. Under these conditions, microwave sintering yields specimens with a more uniform microstructure, confirming the volumetric heating phenomenon. The elemental EDX microanalysis performed on the external surfaces of the differently microwave-sintered specimens identified no extraneous element; only zirconium, oxygen, and yttrium were
Fig. 6. SEM-SEI micrograph of the fracture surface of sample B5, single mode microwaves sintered, maximum temperature 1100 1C, total length of the thermal cycle 4 min. A microcrack is arrowed, an agglomerates of pores is circled.
Fig. 7. SEM-SEI micrograph of the fracture surface of sample B6, single mode microwaves sintered, maximum temperature 1200 1C, total length of the thermal cycle 4 min.
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– The less drastic sintering conditions, in terms of reduced temperature and thermal cycle length, resulted in limited grain growth, which may improve the mechanical characteristics of the sintered zirconia components. – The dielectric heating method has advantages in terms of energy efficiency, process simplicity, and savings in terms of equipment maintenance and operator costs.
References
Fig. 8. SEM-SEI micrograph of the fracture surface of sample B7, single mode microwaves sintered, maximum temperature 1200 1C, total length of the thermal cycle 6 min. (a)The external surface is at the topbright of the micrograph and (b) magnification of the inner part of a).
detected. This indicates that no contamination from the alumina powder used to eliminate thermal shock phenomena occurred. The results of the present study suggest the interesting possibility of greater use of microwave sintering, although several improvements are necessary to provide larger furnace chambers and more user-friendly temperature control systems. Further work regarding these issues is in progress.
5. Conclusions A microwave sintering study of zirconia material was performed, using two different systems to avoid thermal shock cracks in the specimens. The results of the present work allow the following conclusions to be drawn: – By optimising the microwave sintering conditions, it was possible to obtain dense specimens, greatly reducing both the maximum sintering temperature and the total thermal cycle length, from several hundreds of minutes at 1450 1C to a few minutes at 1200 1C.
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 1255–1261 www.elsevier.com/locate/ceramint
Microstructural study of microwave sintered zirconia for dental applications C. Monacoa, F. Preteb, C. Leonellic, L. Espositoa, A. Tuccia,n a
Department of Biomedical Science and Neuromotor Sciences, University of Bologna, Italy b Centro Ceramico Bologna, Italy c Department of Materials and Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia, Italy Received 19 September 2013; received in revised form 16 July 2014; accepted 25 August 2014 Available online 19 September 2014
Abstract Conventional sintering techniques for zirconia-based materials, which are commonly used in dental reconstruction, may not provide uniform heating, with the consequent generation of microstructural flaws in the final component. A sintering system using microwave heating may represent a viable alternative. The purpose of this study was to compare the dimensional variation and physical and microstructural characteristics of commercial zirconia (Y-TZP), used as a dental restoration material, sintered in conventional and microwave furnaces. A physicalmineralogical-microstructural characterisation was carried out to evaluate the level of densification and the presence of flaws in the sintered specimens. Use of the microwave systems allowed the length of the sintering cycle to be reduced to a few minutes, compared with the several hours necessary with a ‘traditional’ heating system. Additionally, the maximum temperature used to reach the required density decreased from 1450–1480 1C with the electric furnace to 1200 1C in the microwave furnace. An important clinical implication is that the reduced sintering time could allow the introduction of zirconia in chair-side treatments, if used as a monolithic material. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Microwave sintering; Y-TZP; Dental ceramic; Microstructure
1. Introduction A well-known review on zirconia for dental applications [1] noted that in the last 20 years, the diffusion of metal-free restorations in dental practice has increased considerably due to the growing demand for highly aesthetic and natural-appearing components. Bioceramics [2] are particularly suitable for the use in prosthodontics as possible metal substitutes because of their wear resistance, superior mechanical properties, high biocompatibility, and excellent aesthetic appearance. Particular attention has been paid to “yttria-tetragonal zirconia polycrystalline ceramics” (Y-TZPs), which have been used as framework materials for dental crowns and fixed partial dentures (FDPs), because their aesthetic appearance is similar to that of n Correspondence to: Department of Biomedical and Neuromotor Sciences, Division of Prosthodontics and Maxillo-facial Rehabilitation via S. Vitale, 59 40125 Bologna, Italy. Tel.: þ 39 0512 088 186; fax: þ39 0512 25208. E-mail address: [email protected] (A. Tucci).
http://dx.doi.org/10.1016/j.ceramint.2014.09.055 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
natural teeth and their mechanical characteristics are good: indeed, the highest reported for any dental ceramic [3]. Both the chemistry and processing of these materials allow obtaining a fully dense polycrystalline zirconia, in a tetragonal phase, with a homogeneous distribution of submicron zirconia grains, giving a translucent aspect, which meets the requirements for natural teethlooking restorations [2]. Furthermore, compared with other ceramics, 3 mol% yttria-stabilised tetragonal zirconia polycrystals (3Y-TZP) are characterised by high fracture toughness and flexural strength, caused by a stress-induced phase transformation, from a tetragonal (t) phase to a monoclinic (m) phase, which increases its crack-propagation resistance [4]. The combination of these particular mechanical and physical characteristics has allowed Y-TZP to become the core material of choice for many categories of dental ceramic restorations. Additionally, CAD/CAM system technology has solved the difficulties of processing this hard material by working it when partially sintered or presintered [5]. In the usual practice of dental laboratories, pre-sintered Y-TZP blocks are rapidly
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CAD/CAM-converted into restoration components that require a final firing step both to reach a higher density and to eliminate any stress induced by the strong surface working actions [6]. This final sintering is currently performed in large, electrically heated ovens to a high temperature, followed by slow cooling to prevent cracking, which is a time-consuming and expensive procedure. The thermal cycles typically indicated by the manufacturers are characterised by maximum temperatures of 1350–1550 1C, at which the ceramic components remain for almost 60 min. Due to the slow cooling step, the total sintering cycle is generally 6–10 h. Many ceramic powders can be sintered using microwaves at lower temperatures and for shorter times than with conventional electrical heating sources, allowing finer and more uniform microstructures to be obtained. Microwave irradiation, also known as dielectric heating, as applied to the sintering of advanced ceramic components, has become an important topic of scientific research [7–10]. In conventional sintering, heat is transferred from the radiant elements of the furnace to the surface of the ceramic component, reaching the core of the component through conduction mechanisms. In microwave sintering, the heat is produced as a consequence of an interaction between the ceramic sample and the electromagnetic waves and involves the whole sample volume; in this way, the heating is more rapid and uniform [11]. The extent of the energy transfer from the electromagnetic field to the matter depends strongly on the dielectric properties of the material, the temperature, and the radiation frequency [12–15]. The microwave sintering of yttria-doped zirconia has been investigated extensively using several types of microwave furnace. Due to its poor coupling with microwaves below 400 1C and moderate coupling at higher temperatures, it is necessary to use susceptors that absorb microwaves at room temperature and act as heating elements to increase the initial temperature of the material to the critical value at which it starts to absorb more effectively. For this purpose, several studies have reported the use of silicon carbide susceptors, because they absorb microwave energy and subsequently transfer it, in the form of heat, to the material, via conduction. This approach, often referred to in the literature as ‘hybrid microwave heating’, uses hybrid heating devices such as multi-mode cavities with SiC rods [16] or SiC powder [17] acting as a susceptor, or microwave/ conventional hybrid furnaces [18]. When samples are heated in an electric furnace or a microwave furnace, two methods can be used to control the temperature: 1) intermittent powering of the magnetron at a fixed power output (on/off control method or time-control method), or 2) continuous powering of the magnetron with a variable power output (powercontrol method). The first method involves the use of the magnetron at its highest output power as typically programmed in domestic ovens, while the second is commonly used in industrial processes, where continuous adjustments of output power are necessary to follow the desired heating profile. It has been pointed out that there is no difference between the two methods in terms of grain growth or sample densification level, but the power-control method gives more precise control of temperature versus the on/off control method [19].
The multiplicity of experimental procedures reported makes obtaining a clear view of the behaviour of zirconia powder during microwave sintering difficult, despite the useful review in [20]. Regarding the sintering of nano Y-TZP, hybrid conventional-microwave heating sintering allowed obtaining 499% TD dense ceramics with an average grain size of less than 100 nm [18], but also near-theoretical density values for 3Y-TZP using a multi-mode microwave sintering furnace at 2.45 GHz [21]. Microwave sintering has been also proposed to overcome the problem of grain growth associated with conventional heating in nanocrystalline 3Y-TZP, by obtaining more homogenous microstructures with higher mechanical characteristics [22]. Further improvements in the physical and mechanical properties of Y2O3-ZrO2 ceramics have been achieved through the use of nanopowders and the use of microwave sintering [23]. The results of these studies underlined that, compared with conventional sintering, the use of microwaves provides several advantages, such as rapid and volumetric heating, lower heating temperatures, enhancements in densification, grain growth limitations, and cost savings. The aim of this work was to assess the possibility of using hybrid microwave sintering with a 3Y-TZP pre-sintered material for dental applications. Two microwave methods, multi- and single-mode, were used for the tests, both of which were fed with continuous variable power to better control the process temperature. The results indicated that the density of the microwave-fired samples depended strongly on the firing changes, which, once optimised, allowed the generation of highly dense zirconia samples with a firing time of only a few minutes. 2. Material and methods A 3% Y-TZP pre-sintered commercial material (Biotech Srl, Milano, Italy, one of the most utilised in Italian Dentistry and in some countries of Europe), suitable for shaping using CADCAM technology, was used for the sintering tests. From the commercial supplies, provided in the form of cylinders, rectangular specimens, of about 20 10 14 mm3, were cut with an high-precision electric saw (Isomet 1000 Precision Saw, Buehler Ltd., Düsseldorf, Germany) and subjected to three heating treatments: conventional, multi-mode, and singlemode microwave sintering. Conventional sintering was conducted in an electric furnace, using the following sintering cycle: 12 1C/min up to 300 1C, 5 1C/min up to the maximum temperature, holding time 60 min, with natural cooling. After several preliminary trials, two maximum temperatures were used: 1450 and 1480 1C. These sintering cycles required 10 h at either maximum temperature. Microwave sintering was performed with a commercial CEM-MAS 7000 multi-mode applicator (CEM Corporation, Matthews, NC) at 2.45 GHz (950 W, nominal power) and on a TE10 n single-mode applicator (0.5–3 kW output power), connected to a 2.45–GHz TM030 microwave generator (Alter Power System, Long Beach CA). The multi-mode applicator
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can generate a lower field density than the single-mode applicator, which is why we compared the two systems. Additionally, a single-mode process can be designed or adjusted according to the load to ensure that the sample is in the region of high microwave intensity. Because microwave sintering causes very rapid temperature increases in tested samples, the resulting thermal shock could destroy the zirconia specimens. For this reason, several trials using different arrangements using SiC susceptors or a refractory crucible were conducted to determine the optimum operating conditions, allowing us to obtain sintered specimens with no cracks. In this study, the following sintering conditions were used. For the multi-mode applicator, a fibre insulating housing was placed inside the microwave chamber. Temperature measurements were conducted using a k-type thermocouple inserted into the multi-mode cavity and placed in direct contact with the specimen. Each sample was located inside a cordierite crucible full of alumina powder. The general scheme of the experimental set-up is shown in Fig. 1. For the tests conducted with the single-mode applicator, each sample was located inside a SiC crucible filled with alumina powder. The crucible was positioned inside a refractory support (Fig. 2a) and then placed in the microwave chamber (Fig. 2b). Temperature was detected in contact with the sample using an optical fibre and transformed to a temperature signal. Manual adjustment to keep the sample in the maximum electric field intensity was performed throughout the heating cycle by means of a shorting plunger, which was positioned at the end of the single-mode applicator. The relative density and apparent porosity of each sample were measured before and after conventional or microwave sintering following European Standard EN 623-2 (1993) [24]. The degree of shrinkage after firing was calculated, and the thickness of each rectangular specimen was measured using a digital micrometer before and after sintering. To avoid errors due to possible distortion of the specimens, the resulting values are the means of three measurements made at different parts of the bars. The phase fraction amounts of zirconia in the pre-sintered and sintered conditions were evaluated by X-ray diffraction
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Fig. 2. Sample arrangement used in the microwave single mode applicator; (a) SiC crucible filled with alumina powder inserted into a refractory support and (b) refractory support positioned inside the single mode cavity.
(XRD) using a diffractometer (Philips PW 3830; Koninklijke; the Netherlands), with Cu Kα radiation (0.021 step-scan, 10 s per step). Zirconia diffraction peaks were deconvoluted using a Lorentz function to obtain the integral breadth. The monoclinic phase fraction of the zirconia was calculated following reference [25]. The microstructures of the zirconia specimens before and after the sintering step were determined by analysing gold sputtercoated fresh-fractured and sintered surfaces using a scanning electron microscope (EVO 40; Carl Zeiss Microscopy GmbH, Oberkochen, Germany) equipped with an EDS system (Inca Energy 250; Oxford Analytical Instruments, Uedem, Germany). 3. Results
Fig. 1. Sample arrangement inside the microwave multi mode applicator.
The results of the physical-mineralogical characterisation of the tested material in the as-received, pre-sintered condition are shown in Table 1. Although it had a rather compact microstructure with a density consistent with other commercial materials of the same class, SEM analysis of the bulk of the samples showed some microcracks, usually connected with
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fragments of agglomerates originating from the starting powder, which were not completely destroyed by the processing operations (Fig. 3). The zirconia grains with an equiaxed geometry were rather fine, with a diameter of 80 nm, confirming that the presintering heating cycle did not allow grain growth. The crystalline phases present were tetragonal (96 wt%) and monoclinic (4 wt%) phase; after the different sintering cycles, only the tetragonal phase was detected. In Table 2, the density, apparent porosity, and shrinkage values of the samples sintered under the different conditions are reported. Samples B1 and B2, sintered with the two conventional cycles in the electrical furnace, reached high density values, 499% TD, and similar shrinkage values. Use of the lower temperature, 1450 1C, provided a good level of compactness, even if some small pre-existing flaws remained present, as evidenced by the SEM analysis (Fig. 4a). With an increase in the maximum sintering temperature to 1480 1C almost total elimination of the flaws was observed, although the higher temperature caused an increase in grain size dimensions (Fig. 4b). Samples B3 and B4 were microwave-sintered in the system CEM-MAS 7000 multi-mode applicator using two cycle lengths, both reaching a maximum temperature of 1200 1C. The shortest, a total length of 6 min, caused only partial sintering of the specimens; the apparent porosity was rather high, 5.20%, with a density that reached 92.1% TD. Increasing the time to 25 min improved the final shrinkage, but did not increase the density meaningfully, which reached only 95.6%TD with an apparent porosity of 1.03%. Microstructural analysis showed the presence
Table 2 Physical characteristics of the tested samples, after the different sintering conditions. Sintering Thermal Density Apparent temperature cycle length (g/cm3) porosity (vol%) (1C) (min)
Shrinkage (%)
Conventional sintering B1 B2
1450 1480
600 600
6.06 6.06
0.01 0.01
21.77 21.70
Multi mode microwaves sintering B3 B4
1200 1200
6 25
5.60 5.82
5.20 1.03
18.90 21.52
Single mode microwaves sintering B5 B6 B7
1100 1200 1200
4 4 6
5.05 5.98 6.01
14.52 0.80 0.01
16.71 21.49 21.32
Sample
Table 1 Physical-mineralogical characteristics of the pre-sintered material. Density (g/cm3)
Porosity (%)
t phase (wt%)
m phase (wt%)
Grain size (nm)
3.04
49.62
96
4
80
Fig. 3. SEM-SEI micrograph of the fracture surface of the tested material, in the pre-sintered condition. An evident microcrak, sorrounding rests of agglomerates of the starting powder, is arrowed.
Fig. 4. SEM-SEI micrograph of the fracture surface (a) of sample B1, conventional sintering 1450 1C/1 h, microflaws are arrowed and (b) of the fracture surface of sample B2, conventional sintering 1480 1C/1 h.
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of very small pores distributed rather homogeneously among the submicron zirconia grains (Fig. 5). They were likely responsible for the low density value. Samples B5 and B6, microwave-sintered with the TE10n single-mode applicator, reached maximum temperatures of 1100 and 1200 1C, respectively, with sintering cycle lengths of 4 and 6 min, respectively. At 1100 1C, samples B5 did not reach an acceptable level of density or shrinkage. Its microstructure was characterised by a rather homogeneous distribution of small pores; however, microcracks present in the presintered samples and agglomerates of microporosity linked to the presence of fragments of the original powder agglomerates are recognisable (Fig. 6). This sintering condition allows a rapid increase in the density of the zirconia, in the areas where the particles are closely packed, but is unable to eliminate the larger flaws, which would likely require a higher temperature and/or longer time. Use of an identical same sintering time (4 min) but a temperature of 1200 1C in sample B6 increased the density
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meaningfully, although a certain amount of open porosity remained; the apparent porosity was 0.80%. From the microstructural analysis, together with very small isolated pores some open porosity was evident (Fig. 7), although no appreciable growth of zirconia grains was visible. With the use of a longer time, 6 min, also at 1200 1C, the zirconia sample B7 reached the maximum density value for microwave sintering, which comparable with those reached using the longer-duration conventional heating treatments.
4. Discussion
Fig. 5. SEM-SEI micrograph of the fracture surface of sample B4, multimode microwaves sintered, maximum temperature 1200 1C, total length of the thermal cycle 25 min.
The results of this study underline that it is possible to reach a high level of densification and shrinkage using single-mode microwave sintering, such as with the conditions used with samples B7, 6 min at 1200 1C. While the open porosity of the specimens was minimal, the shrinkage was slightly lower than that of the conventionally heated samples B1 and B2. This can be better understood by examining the microstructural features. From Fig. 8a, a cross-section of B7, it is evident that at the border near the external surface the material is perfectly compact, with no pores, while in the bulk some small, closely packed pores, which are responsible for the reduced shrinkage, are evident (Fig. 8b). Reduced grain growth was observed for all microwavesintered, in comparison with conventionally sintered, samples even when the lower temperature (1450 1C) was used. This aspect is of particular importance for ceramic materials, because the presence of a coarse grain size distribution or a few grains characterised by exaggerated grain growth can be deleterious to the mechanical characteristics of a structural component. Under these conditions, microwave sintering yields specimens with a more uniform microstructure, confirming the volumetric heating phenomenon. The elemental EDX microanalysis performed on the external surfaces of the differently microwave-sintered specimens identified no extraneous element; only zirconium, oxygen, and yttrium were
Fig. 6. SEM-SEI micrograph of the fracture surface of sample B5, single mode microwaves sintered, maximum temperature 1100 1C, total length of the thermal cycle 4 min. A microcrack is arrowed, an agglomerates of pores is circled.
Fig. 7. SEM-SEI micrograph of the fracture surface of sample B6, single mode microwaves sintered, maximum temperature 1200 1C, total length of the thermal cycle 4 min.
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– The less drastic sintering conditions, in terms of reduced temperature and thermal cycle length, resulted in limited grain growth, which may improve the mechanical characteristics of the sintered zirconia components. – The dielectric heating method has advantages in terms of energy efficiency, process simplicity, and savings in terms of equipment maintenance and operator costs.
References
Fig. 8. SEM-SEI micrograph of the fracture surface of sample B7, single mode microwaves sintered, maximum temperature 1200 1C, total length of the thermal cycle 6 min. (a)The external surface is at the topbright of the micrograph and (b) magnification of the inner part of a).
detected. This indicates that no contamination from the alumina powder used to eliminate thermal shock phenomena occurred. The results of the present study suggest the interesting possibility of greater use of microwave sintering, although several improvements are necessary to provide larger furnace chambers and more user-friendly temperature control systems. Further work regarding these issues is in progress.
5. Conclusions A microwave sintering study of zirconia material was performed, using two different systems to avoid thermal shock cracks in the specimens. The results of the present work allow the following conclusions to be drawn: – By optimising the microwave sintering conditions, it was possible to obtain dense specimens, greatly reducing both the maximum sintering temperature and the total thermal cycle length, from several hundreds of minutes at 1450 1C to a few minutes at 1200 1C.
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