Abstract
Objective
The aim of this study was to evaluate the effect of TiO 2 concentration on the properties of apatite-mullite glass-ceramics namely strength and the chemical solubility to comply with the ISO standard recommendations for dental ceramics (BS EN ISO 6872-2008) .
Methods
Ten novel glass-ceramic materials were produced based on the general formula (4.5SiO 2 -3Al 2 O 3 -1.5P 2 O 5 -3CaO-CaF 2 – x TiO 2 ) where x varied from 0.5 to 5 wt%. Glass with no TiO 2 added (HG1T0.0) was used as a reference. Discs of 12 mm diameter and 1.6 mm (±0.2 mm) thickness were prepared for both biaxial flexural strength (BFS) and chemical solubility testing, in accordance with the BS EN ISO 6872-2008 for dental ceramics. All produced materials were investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Energy dispersive X-ray analysis (EDS) was also carried out on some samples to identify the element composition of samples.
Results
Increasing the concentration of TiO 2 from 0.5 wt% to 2 wt% significantly ( P < 0.05) increased the chemical solubility of the material. With the material containing 2.5 wt% of TiO 2 , the solubility significantly reduced ( P < 0.05) and resulted in a solubility value of 228.3 μm/cm 2 and BFS value of 197.9 MPa. Increasing the TiO 2 concentration more than 2.5 wt%, led to a significant ( P < 0.05) increase in solubility and a reduction in BFS.
Conclusions
TiO 2 is an effective agent for improving the durability and the mechanical properties of an apatite-mullite glass-ceramic only up to 2.5 wt% concentration.
1
Introduction
Dental glass-ceramics are materials composed of one or more glass and crystal phases and have been developed with improved properties since 1980s . In spite of advancing technology, strength and durability there are still limitations with these materials and they have yet to equal the fracture resistance and durability of gold or porcelain-fused-to-metal crowns. Thus, there still needs to be an improvement in reliability if these ceramics are to be fully accepted as metal free dental replacement materials.
These materials have to be durable in the oral environment, exhibit high strength, wear resistance and have the appearance of natural tooth structure .
Currently there are many glass-ceramic materials on the dental market. These glass-ceramics are classified as mouldable, sintered and machinable glass-ceramics.
Glass-ceramics also could be classified according to their glass phase as follows:
- 1.
Dental ceramic containing glass-infiltrated ceramic such as In-ceram Alumina, In-ceram Spinel and Inceram Zirconia. This group of dental ceramics are suitable for the manufacture of anterior and posterior single crowns.
- 2.
Dental ceramic containing a high strength ceramic such as Cercon, Procera, DCS and Lava. These can be used as cores for anterior and posterior single crowns, and as bridge frameworks.
- 3.
Dental ceramic containing glass ceramic such as leucite, fluorapatite or lithium disilicate . These ceramics are used as veneers, cores and body ceramics.
The solubility of ceramic systems is of vital importance if the materials are to be exposed to the oral environment. Due to their good castability via the lost-wax casting technique and their ability to be processed by dental CAD–CAM systems , a number of different fluorapatite-mullite glass-ceramics are currently being investigated for dental and medical applications. The materials are glass ionomer derivatives that benefit from good chemical solubility and strength for use as dental core ceramics . Fluorapatite containing glass-ceramics have attracted attention because of their compatibility with the natural apatite of the human bone and teeth . These are bioactive compounds with an apatite-like structure, in which fluorine groups have substituted OH groups.
The hardness of fluorapatite-based glass-ceramics are similar to that of tooth enamel and that makes them new candidates for the substitution of hydroxyapatite in restorative dentistry .
In most cases, bulk crystallization is the dominant mechanism of crystallization of needle-like fluorapatite in glass specimens. However, they have not yet met the ISO standard for body dental ceramics (BS EN ISO 6872:2008) with regard to solubility .
The properties of glass-ceramics can be modified in a predictable way by controlling the chemical compositions , in particular by the addition of nucleating agents, such as TiO 2 to the glass batch composition. Although, apatite-mullite glass-ceramics have the ability to self-nucleate and to undergo nucleation only slightly above the glass transition temperature this is also indicative of a nucleation mechanism involving prior amorphous phase separation, which is also seen in ionomer glasses , TiO 2 is used as a nucleating agent to promote bulk nucleation of nano-phases that work as local heterogeneities for the crystallization of the desired phases to try and enhance formation of more nuclei, which would grow to crystals and reduce the chemical solubility and improve the strength of apatite-mullite glass-ceramics to comply with the ISO standard for body dental ceramics (BS EN ISO 6872:2008) . The nucleating agent starts the heterogeneous nucleation, which is the basic for glass ceramic production in combination with other components in the glass .
At the atomic scale, the amorphous-to-crystal transformation around nucleating agents are important to understand the formation of glass-ceramics, but are still not understood clearly and little is known about the role and the interaction between the glassy matrix and the nucleating agent.
Nucleating agents are sometimes described as a catalyst of the internal crystallization over volume crystallization.
In 1960, Stookey described the use of TiO 2 to reduce the crystallization temperature in some types of glass-ceramic systems and he pointed out that TiO 2 was an effective agent in different glass compositions in amounts of 2–20 wt%. However, a study by Maurer in 1962 on Mgo-Al 2 O 3 -SiO 2 glass compositions showed that TiO 2 did not act as a nucleating agent in Na 2 OAl 2 O 3 -SiO 2 glasses. Moreover, a study by Fokin and Zanotto in 1999 , found that using TiO 2 below 7–8 mol%, does not promote enough volume crystallization of the matrix and only surface crystallization occurred. A study by Chung-lun et al. in 2002 , showed that TiO 2 reduced the crystallization temperature lower than 900 °C for anorthite based glass-ceramics. In lithium-alumino-silicate glass (Li 2 O-Al 2 O 3 -SiO 2 ) known as LAS, titanium dioxide is commonly used as a nucleating agent, which precipitates nucleate in the glass matrix when heated for 1.5 h at 780 °C. When the temperature is raised to 950 °C, the transformation of the glass into a crystalline (96–98%) phase occurs with little change in shape. Many parameters influence crystal development, namely the temperature, the time of treatment and compositions. Even in low concentrations, TiO 2 can strongly influence the physical and mechanical properties of glasses .
A previous study has revealed that TiO 2 is also a good nucleation agent in many silicate systems. However, the role of TiO 2 as a nucleation agent in apatite-mullite glass-ceramics is still not fully understand due to the fact that the chemical composition includes P 2 O 5 and F to promote nucleation and this has not been reported in the open literature. Studies by Fathi et al., confirmed that an apatite-mullite glass-ceramic composition that contained of 0.5 wt% of CaF 2 and 0.5 wt% of TiO 2 had good chemical and mechanical properties but would only be suitable for use in dentistry as a core material, but not as a body ceramic in accordance with the ISO Standard (BS EN ISO 6872; 2008) for dental ceramics .
In an attempt to overcome these problems, the authors aim to investigate the influence of the addition of different amounts of TiO 2 on the properties of apatite-mullite glass-ceramics and to see whether this type of nucleating agent has an effect on developing the crystalline microstructure and thus improve the chemical and the mechanical properties of this glass-ceramic system to comply with the ISO standard (6872:2008) for body ceramic materials.
2
Materials and methods
2.1
Glass production
Materials were produced to assess the effect of TiO 2 concentration, with the aim of improving the crystallization process, the durability and the strength of the material to comply with the ISO standard recommendations for dental ceramics (BS EN ISO 6872; 2008) . Ten novel glass-ceramic compositions have been developed based on the general formula, (4.5SiO 2 -3Al 2 O 3 -1.5P 2 O 5 -3CaO-CaF 2 – x TiO 2 ) where x was varied from 0 to 5. Glass with no TiO 2 added (HG1T0.0) was used as a reference. The glass batch compositions are shown in Table 1 .
| Glasses | SiO 2 | Al 2 O 3 | P 2 O 5 | CaO | CaF 2 | TiO 2 |
|---|---|---|---|---|---|---|
| HGF1Ti0.0 | 34.62 | 23.08 | 11.54 | 23.08 | 7.69 | 0.00 |
| HGF1Ti0.5 | 34.43 | 22.95 | 11.48 | 22.95 | 7.65 | 0.53 |
| HGF1Ti1 | 34.25 | 22.83 | 11.42 | 22.83 | 7.61 | 1.07 |
| HGF1Ti1.5 | 34.09 | 22.73 | 11.36 | 22.73 | 7.58 | 1.51 |
| HGF1Ti2 | 33.91 | 22.61 | 11.30 | 22.61 | 7.54 | 2.03 |
| HGF1Ti2.5 | 33.73 | 22.49 | 11.24 | 22.49 | 7.50 | 2.51 |
| HGF1Ti3 | 33.58 | 22.39 | 11.19 | 22.39 | 7.46 | 2.98 |
| HGF1Ti3.5 | 33.41 | 22.27 | 11.14 | 22.27 | 7.42 | 3.48 |
| HGF1Ti4 | 33.21 | 22.14 | 11.07 | 22.14 | 7.38 | 4.05 |
| HGF1Ti4.5 | 33.06 | 22.04 | 11.02 | 22.04 | 7.35 | 4.48 |
| HGF1Ti5 | 32.85 | 21.90 | 10.95 | 22.90 | 7.30 | 5.01 |
Standard laboratory reagent (SLR, Fisher scientific and ACROS organics, UK) grade chemicals: Al(OH) 3 (Al 2 O 3 ), TiO 2 , CaHPO 4 to give (P 2 O 5 and CaO) and CaF 2 along with Loch Aline sand (99.8% SiO 2 ) were used to produce the glass batches. The individual components were weighed and mixed together for 10 min using a mixer machine. The batches were transferred to a covered alumina crucibles and then heated in an electric furnace, at a heating rate of 10 °C/min to a temperature of 1050 °C for 1 h and then left to heating up to reach the temperature of 1450 °C and then held for 2 h. The molten glass was quenched rapidly into cool water (room temperature) to obtain a glass frit. To increase homogeneity of the glass, the glass frit was re-melted for 2 h at 1450 °C after drying for 4 h at 150 °C. The molten glass was then poured into a metal mould to produce a glass block, that was transferred immediately to a pre-heated oven and annealed for 1 h at 650 °C. The furnace then was turned off and samples were left in the furnace to cool down to room temperature.
2.2
Characterization
Glasses were analyzed using differential thermal analysis (DTA) and X-ray diffraction (XRD). DTA (Perkin Elmer, SII, Pyris Diamond TG/DTA, UK) was used to study the phase evolution in the glass, and to determine the heat treatments for the production of glass-ceramics.
The glasses and resultant glass-ceramics were analyzed using X-ray powder diffraction (XRD) (Philips PW1050, Almelo, Holland) to determine the existence and type of crystalline phases. Cu radiation and an accelerating voltage of 50 kV between 2 θ values of 10–70° were utilized using a step-scanning technique with a fixed step of 0.02 degree and a time of 2°/min. The data were analyzed using STOE WinXPOW search and match software (version 2.0, STOE and Cie GmbH, Hilpertstrasse 10, D 64295, Darmstadt).
2.3
Heat treatments
Two stage heat treatments were used to control the microstructure for material property characterization, namely, chemical solubility and the biaxial flexural strength (BFS).
All samples were heat treated for 1 h at 20 °C above the T g temperature obtained from the DTA data at a heating rate of 5 °C/min and then the temperature increased to the maximum temperature of 1200 °C at a heating rate of 5 °C/min and then held for 5 h. The furnace was turned off and samples were then left in the furnace to cool down to room temperature.
2.4
Scanning electron microscopy (SEM) analysis and energy dispersive X-ray (EDS) analysis
SEM was carried out on all produced glasses, using a high-resolution scanning electron microscope (Philips SEM501) , to examine the microstructure. The samples were prepared by sequential grinding with P600 and P1200 grit silicon carbide papers, and then sequential polishing with 6 μm and 1 μm diamond paste, to give an optical finish. Samples were etched using hydrofluoric acid of 40% concentration. Samples were then washed using distilled water before carrying the SEM or the EDS testing. SEM on a fractured surface was also carried out.
All samples were washed using distilled water. Then the specimens were mounted on aluminium stubs using double-sided carbon adhesive discs. Silver conductive paint was used to ensure good contact between the specimen, disc and stub. The samples and stubs were then coated in gold (for SEM) using Sputter S1508 gold coater for 1 min at 50 mA, to prevent charging and for energy dispersive X-ray (EDS) analysis, the samples and stubs were carbon coated. The samples could then be examined either using a scanning electronic microscope (Philips SEM501) or energy dispersive X-ray analysis (EDS).
2.5
Sample preparation for property characterization
Discs of 12 mm (±0.2 mm) diameter and 1.6 mm (±0.2 mm) thickness were produced for both BFS ( n = 30 each material) and chemical solubility testing ( n = 33 each material) for all ten produced glasses by core drilling the glass blocks to form glass rods using the core drilling machine (Pillar Drill, Sealey Power Products, Suffolk, UK) and then a low speed saw with diamond blade (Buhler, Germany) was used to cut the rods into glass discs of required size according to the ISO standard (BS EN ISO 6872: 2008) for dental ceramics .
The discs produced were tested in their glassy form and as glass-ceramics that had been produced by heat-treatments mentioned in Section 2.3 , corresponding to the glass transition temperature ( T g ), and the crystal growth temperatures for fluorapatite and mullite.
2.6
Chemical solubility and BFS testing
Eleven discs of each material giving 30 cm 2 of exposed surface area were used for the solubility testing. The test was repeated three times, each time the test was carried out on different discs for each material ( n = 33) and tested in accordance with International Standard ISO (6872:2008) , as follows. The discs were ultrasonically cleaned in distilled water for 15 min then dried for 4 h at 150 °C then weighed. The discs were placed into a flask with 100 ml of 4% acetic acid solution and heated on a hot plate at 80 °C for 16 h. The discs were then washed in distilled water again and dried at 150 °C for 4 h. The discs were reweighed, and the solubility calculated as 1 g/cm 2 .
For BFS testing, 30 discs of each material were polished on a rotating polishing wheel using 6 μm followed by 1 μm diamond paste and ultrasonically cleaned in distilled water for 15 min and then were dried in accordance with International Standard ISO (6872:2008) . The test was carried out on a tensile testing machine (Lloyd 2000 R, Hampshire, UK). The mean BFS values for each material studied were then determined using Eq. (1) .
σ max = P h 2 0.606 + log e a h + 1.13
Where r max is the maximum BFS, P is the load to fracture, a is the radius of the knife-edge support and h is the sample thickness.
The results of both the chemical and BFS testing were analyzed using a two-way analysis of variance (ANOVA), at the 95% confidence level ( P \ 0.05) (Minitab, Release 13, Minitab, USA) and the Newman–Keuls multiple comparison summaries was used to indicate if any combinations differed significantly. t tests were also undertaken to compare individual results at the level of P = 0.05.
2
Materials and methods
2.1
Glass production
Materials were produced to assess the effect of TiO 2 concentration, with the aim of improving the crystallization process, the durability and the strength of the material to comply with the ISO standard recommendations for dental ceramics (BS EN ISO 6872; 2008) . Ten novel glass-ceramic compositions have been developed based on the general formula, (4.5SiO 2 -3Al 2 O 3 -1.5P 2 O 5 -3CaO-CaF 2 – x TiO 2 ) where x was varied from 0 to 5. Glass with no TiO 2 added (HG1T0.0) was used as a reference. The glass batch compositions are shown in Table 1 .
| Glasses | SiO 2 | Al 2 O 3 | P 2 O 5 | CaO | CaF 2 | TiO 2 |
|---|---|---|---|---|---|---|
| HGF1Ti0.0 | 34.62 | 23.08 | 11.54 | 23.08 | 7.69 | 0.00 |
| HGF1Ti0.5 | 34.43 | 22.95 | 11.48 | 22.95 | 7.65 | 0.53 |
| HGF1Ti1 | 34.25 | 22.83 | 11.42 | 22.83 | 7.61 | 1.07 |
| HGF1Ti1.5 | 34.09 | 22.73 | 11.36 | 22.73 | 7.58 | 1.51 |
| HGF1Ti2 | 33.91 | 22.61 | 11.30 | 22.61 | 7.54 | 2.03 |
| HGF1Ti2.5 | 33.73 | 22.49 | 11.24 | 22.49 | 7.50 | 2.51 |
| HGF1Ti3 | 33.58 | 22.39 | 11.19 | 22.39 | 7.46 | 2.98 |
| HGF1Ti3.5 | 33.41 | 22.27 | 11.14 | 22.27 | 7.42 | 3.48 |
| HGF1Ti4 | 33.21 | 22.14 | 11.07 | 22.14 | 7.38 | 4.05 |
| HGF1Ti4.5 | 33.06 | 22.04 | 11.02 | 22.04 | 7.35 | 4.48 |
| HGF1Ti5 | 32.85 | 21.90 | 10.95 | 22.90 | 7.30 | 5.01 |
Standard laboratory reagent (SLR, Fisher scientific and ACROS organics, UK) grade chemicals: Al(OH) 3 (Al 2 O 3 ), TiO 2 , CaHPO 4 to give (P 2 O 5 and CaO) and CaF 2 along with Loch Aline sand (99.8% SiO 2 ) were used to produce the glass batches. The individual components were weighed and mixed together for 10 min using a mixer machine. The batches were transferred to a covered alumina crucibles and then heated in an electric furnace, at a heating rate of 10 °C/min to a temperature of 1050 °C for 1 h and then left to heating up to reach the temperature of 1450 °C and then held for 2 h. The molten glass was quenched rapidly into cool water (room temperature) to obtain a glass frit. To increase homogeneity of the glass, the glass frit was re-melted for 2 h at 1450 °C after drying for 4 h at 150 °C. The molten glass was then poured into a metal mould to produce a glass block, that was transferred immediately to a pre-heated oven and annealed for 1 h at 650 °C. The furnace then was turned off and samples were left in the furnace to cool down to room temperature.
2.2
Characterization
Glasses were analyzed using differential thermal analysis (DTA) and X-ray diffraction (XRD). DTA (Perkin Elmer, SII, Pyris Diamond TG/DTA, UK) was used to study the phase evolution in the glass, and to determine the heat treatments for the production of glass-ceramics.
The glasses and resultant glass-ceramics were analyzed using X-ray powder diffraction (XRD) (Philips PW1050, Almelo, Holland) to determine the existence and type of crystalline phases. Cu radiation and an accelerating voltage of 50 kV between 2 θ values of 10–70° were utilized using a step-scanning technique with a fixed step of 0.02 degree and a time of 2°/min. The data were analyzed using STOE WinXPOW search and match software (version 2.0, STOE and Cie GmbH, Hilpertstrasse 10, D 64295, Darmstadt).
2.3
Heat treatments
Two stage heat treatments were used to control the microstructure for material property characterization, namely, chemical solubility and the biaxial flexural strength (BFS).
All samples were heat treated for 1 h at 20 °C above the T g temperature obtained from the DTA data at a heating rate of 5 °C/min and then the temperature increased to the maximum temperature of 1200 °C at a heating rate of 5 °C/min and then held for 5 h. The furnace was turned off and samples were then left in the furnace to cool down to room temperature.
2.4
Scanning electron microscopy (SEM) analysis and energy dispersive X-ray (EDS) analysis
SEM was carried out on all produced glasses, using a high-resolution scanning electron microscope (Philips SEM501) , to examine the microstructure. The samples were prepared by sequential grinding with P600 and P1200 grit silicon carbide papers, and then sequential polishing with 6 μm and 1 μm diamond paste, to give an optical finish. Samples were etched using hydrofluoric acid of 40% concentration. Samples were then washed using distilled water before carrying the SEM or the EDS testing. SEM on a fractured surface was also carried out.
All samples were washed using distilled water. Then the specimens were mounted on aluminium stubs using double-sided carbon adhesive discs. Silver conductive paint was used to ensure good contact between the specimen, disc and stub. The samples and stubs were then coated in gold (for SEM) using Sputter S1508 gold coater for 1 min at 50 mA, to prevent charging and for energy dispersive X-ray (EDS) analysis, the samples and stubs were carbon coated. The samples could then be examined either using a scanning electronic microscope (Philips SEM501) or energy dispersive X-ray analysis (EDS).
2.5
Sample preparation for property characterization
Discs of 12 mm (±0.2 mm) diameter and 1.6 mm (±0.2 mm) thickness were produced for both BFS ( n = 30 each material) and chemical solubility testing ( n = 33 each material) for all ten produced glasses by core drilling the glass blocks to form glass rods using the core drilling machine (Pillar Drill, Sealey Power Products, Suffolk, UK) and then a low speed saw with diamond blade (Buhler, Germany) was used to cut the rods into glass discs of required size according to the ISO standard (BS EN ISO 6872: 2008) for dental ceramics .
The discs produced were tested in their glassy form and as glass-ceramics that had been produced by heat-treatments mentioned in Section 2.3 , corresponding to the glass transition temperature ( T g ), and the crystal growth temperatures for fluorapatite and mullite.
2.6
Chemical solubility and BFS testing
Eleven discs of each material giving 30 cm 2 of exposed surface area were used for the solubility testing. The test was repeated three times, each time the test was carried out on different discs for each material ( n = 33) and tested in accordance with International Standard ISO (6872:2008) , as follows. The discs were ultrasonically cleaned in distilled water for 15 min then dried for 4 h at 150 °C then weighed. The discs were placed into a flask with 100 ml of 4% acetic acid solution and heated on a hot plate at 80 °C for 16 h. The discs were then washed in distilled water again and dried at 150 °C for 4 h. The discs were reweighed, and the solubility calculated as 1 g/cm 2 .
For BFS testing, 30 discs of each material were polished on a rotating polishing wheel using 6 μm followed by 1 μm diamond paste and ultrasonically cleaned in distilled water for 15 min and then were dried in accordance with International Standard ISO (6872:2008) . The test was carried out on a tensile testing machine (Lloyd 2000 R, Hampshire, UK). The mean BFS values for each material studied were then determined using Eq. (1) .
σ max = P h 2 0.606 + log e a h + 1.13
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