Effect of the crystallisation time and metal oxide pigments on translucency and the mechanical and physical properties of mica glass-ceramics

Journal of Non-Crystalline Solids 528 (2020) 119730

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Effect of the crystallisation time and metal oxide pigments on translucency and the mechanical and physical properties of mica glass-ceramics

T

Thapanee Srichumponga, Sukanda Angkulpipata, Sahadsaya Prasertwonga, Noparat Thongpunb, Chayada Teanchaib, Paolo Veronesic, Kallaya Suputtamongkolb, Cristina Leonellic, Greg Henessa, ⁎ Duangrudee Chaysuwana, a

Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand Department of Prosthodontics, Faculty of Dentistry, Mahidol University, Bangkok 10400, Thailand c Department of Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia, 41125, Modena, Italy b

A R T I C LE I N FO

A B S T R A C T

Keywords: Chemical solubility Crystallisation Dental materials Metal oxide Mica glass-ceramics Translucency

Mica glass-ceramics tend to be opaque making them less desirable, from an aesthetic point of view, for dental materials. This research focuses on the development of a mica glass-ceramic for use as a restorative dental material with improved translucency. A ceria-based pigment and a zircon-based pigment consisting of Pr-ZrSiO4 and Fe-ZrSiO4 were added to glass-ceramic and the effect on translucency, phase development and morphology, strength, thermal expansion and chemical solubility were studied. The addition had no effect on the level of crystallinity obtained, the morphology of the crystals formed or the strength. The use of the pigments allowed for the control of the translucency and values of the translucency parameter similar to natural teeth were produced. The characteristic strength values and chemical solubility obtained to make these materials acceptable as dental ceramics type II class 2, for single-unit anterior or posterior prostheses adhesively cemented, according to ISO 6872.

1. Introduction Glass-ceramic is an advanced bioceramics produced through a controlled in situ glass crystallisation process within the bulk amorphous glass during two-stage heat treatment [1,2]. Two important factors in the production of glass-ceramics influence the size and number of crystals: 1) nucleating agents and 2) the temperature and time used in the heat treatment process [3,4]. If the production process can control these factors, it can control the properties of glass-ceramics [5–8]. Normally, mica-based glass-ceramics are interesting materials for dental applications due to their good aesthetic properties, high strength, low thermal expansion and machinability. These properties are imparted to the material by the crystallisation of fluorphlogopite (K2Mg6Al2Si6O20F4) in the glass matrix [9]. The microstructure of the mica-based phase consists of randomly oriented, small interlocking plate-like crystals [10]. These plate-like crystals are easily cleaved when subjected to shearing stresses and provide the glass-ceramic with good machinability [11–13]. These crystals also act as a reinforcement resulting in a flexural strength approximately twice that of leucite glassceramic. The cations in mica glass-ceramics are commonly potassium

⁎

cation but they can be other alkali or alkaline earth cations such as Na+, Ca2+ Sr2+, Ba2+, Rb+ and Cs+ [9]. Introduction of Sr2+ to the interlayer ions of the mica structure could modify the hardness of this material. It matches with human enamel while growing some fluorapatite (Ca5(PO4)3F) within the calcium mica. The fluorapatite does increase its flexural strength and fracture toughness [11]. Translucency and colour are important aesthetic factors for materials selection.However, fluorphlogopite or mica is opaque making it unsuitable as a dental restorative material. The translucency of glassceramics can be controlled by heat treatment. In addition, the aesthetics can be adjusted by the addition of metal oxides such as ceria (CeO2), iron oxide (Fe2O3) or zircon-based pigments [14], for example, Yuan et al. [15] used zircon-based tricolour pigments consisting of praseodymium zircon yellow (Pr-ZrSiO4), ferrous zircon red (Fe-ZrSiO4) and vanadium zircon blue (V-ZrSiO4) to produce tooth-like colours. In addition, CeO2 can be used as a flux and a nucleating agent [16,17]. Zircon-based pigments are widely used in the ceramic industry because of their high colour intensity and excellent colour stability. It is thus possible that these glass-ceramics can deliver the correct colouration along with the hardness and machinability needed for these dental

Corresponding author. E-mail address: [email protected] (D. Chaysuwan).

https://doi.org/10.1016/j.jnoncrysol.2019.119730 Received 8 August 2019; Received in revised form 2 October 2019; Accepted 4 October 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.

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2.3. Coefficient of thermal expansion (CTE)

applications. The aims of this study were to investigate the influence of crystallisation time and the addition of CeO2, Pr-ZrSiO4 and Fe-ZrSiO4 pigments on the thermal expansion, biaxial flexural strength, chemical solubility and translucency of a mica-based glass-ceramic and its ability to be used as a restorative dental material. Any changes are described through observation of phase formation, crystalline structures and morphology.

The CTE measurements were carried out using a dilatometer (DIL 402 Expedis Classic, NETZSCH, Selb, Germany). The specimens of our glass and glass-ceramics were prepared as bar 5 mm × 5 mm × 50 mm with the smooth tip. Each sample had flat ends, perpendicular to the long axis. The specimen was heated from 25° to 800 °C with heating rate of 5 °C/min. The glass transition temperature (Tg), the softening temperature (Ts) and the CTE (α) were determined. Data was manipulated using Pyris Manager software.

2. Experimental procedures 2.1. Preparation of glass

2.4. X-Ray diffraction (XRD) The glass ceramic was prepared from composition in weight percent SiO2 37%, MgO 15.9%, SrCO3 13.6%, Al2O3 11.9%, MgF2 11.5%, CaCO3 6.5%, P2O5 3% and CaF2 0.6% as per a previous study [18]. All chemicals were mixed in a ball mill for 2 h and then melted in an alumina crucible for 1 h at 1450 °C in a high temperature furnace (Nabertherm, HT 16/18, Lilienthal, Germany) The melt was then quenched into water to form glass frit which was dried then crushed in a planetary mill (PULVERISETTE 6 classic line, Germany) and sieved through a 325 mesh (45 µm) sieve. The pigments were added to the fine glass frits and mixed by ball mill for 2 h. The pigments are CeO2, a yellow substance, and a zircon-based pigment (ZrSiO4) consisting of PrZrSiO4 (yellow shade) and Fe-ZrSiO4 (deep brownish red shade) mixed in a ratio of 10: 3. The compositions and heat treatment schedules of the glass-ceramic mixes are in Table 1 and compare with the original condition (GCF) as the nucleation time 180 min and crystallisation time 180 min [11] to confirm the complete crystallisation. The percent addition of pigment was determined from previous work, the ceria-based being more intense and requiring less quantity [18]. All compositions were remelted at 1450 °C and poured into a carbon mould, annealed at 582 °C in another furnace (8,2/1100 LHM01, SNOL, Utena, Lithuania) for 90 min to relieve residual stresses and then cooled to room temperature in the furnace to obtain 15.6 mm diameter glass rod [19].

After heat treatment, the glass-ceramics were characterised by X-ray diffraction using Cu Kα (Philips, PW3040/00, Eindhoven, Netherlands). The specimens in each group were scanned from 5° to 85° with a step size of 0.01°, count time per step of 0.5 s at 30 kV and 40 mA. 2.5. Scanning electron microscopy (SEM) The microstructures were characterized using SEM (FEI QUANTA 450, Oregon, USA) with an accelerating voltage of 10 kV. The specimens were progressively wet ground from 600 to 2500 grit with silicon carbide abrasive papers and then polished with 0.1 μm alumina powder slurries. They were then sonicated for 10 min in distilled water, etched in 10% HF solution for 10–15 s to reveal microstructures and then gold sputtered. 2.6. Translucency parameter (TP) The translucency of a material can be described using the translucency parameter. It is a method that allows relating visual perception to translucency. It is defined as the colour difference of a material of a given thickness over white and black backgrounds [8,21–23]. The larger the TP the more translucent the material. The specimens were cut to 1.0 ± 0.05 mm thick discs from the glass-ceramic rod with a watercooled low-speed diamond. All specimens were wet ground to 1200-grit silicon with silicon carbide paper and ultrasonically cleaned in distilled water for 10 min prior to measurement. The translucency was measured using a Spectro colorimeter (XE, Hunter Lab Ultra Scan, Virginia, USA) with D65 as the standard daylight illuminant and a 10° standard observer [24,25]. To ensure the consistency, 15 specimens in each group were measured. The translucency parameter for each sample was calculated using Eq. (1):

2.2. Thermal analysis Differential thermal analysis (DTA) was used to determine the optimum nucleation and crystallisation temperatures for the heat treatment of the glass. Samples of each of the mixes were heated in air at a rate of 10 °C/min from room temperature to 1200 °C in a differential thermal analyser (DTA7, Perkin Elmer, Boston, USA) with α-Al2O3 powder used as the reference material. The optimum nucleation temperature was determined following Marotta's regime [20]. This required the determination of Tp2* -Tp2. Tp2 is the second crystallisation peak and Tp2* is found by soaking the glass at Tg − 30 °C, Tg − 15 °C, Tg, Tg + +15 °C and Tg + +30 °C for 1 h. Peak shifts are now observed when the DTA is run again. The new peak position for the second crystallisation peak is Tp2*. The optimum nucleation temperature is determined by plotting Tp2* -Tp2 vs these soaking temperatures. The temperature at which Tp2* -Tp2 is a maximum gives the optimum nucleation temperature whereas the crystallisation temperature is taken as 20 °C above Tp2.

TP = [(Lb* − Lw* )2 + (ab* − a w*)2 + (bb* − bw*)2]1/2

where L* represents the lightness/ darkness of a colour, a* is a measure of redness (positive) or greenness (negative) and b* is a measure of yellowness (positive) or blueness (negative). The subscripts “b” and “w” refer to colour coordinates over the black and white backgrounds, respectively. 2.7. Biaxial flexural strength

Table 1 Compositions and heat treatment schedules of the glass-ceramics. Groups of glassceramics

Glass frit (wt%)

Pigments (wt%) CeO2 ZrSiO4

Nucleation Temp (°C) Time (min)

Crystallisation Temp (°C) Time (min)

GCF [11] GC GC_Ce5 GC_Ce35 GC_Zr12.5 GC_Zr30

100 100 99 99 96 96

– – 1 1 – –

643 642 642 642 642 642

892 897 897 897 897 897

– – – – 4 4

180 10 10 10 10 10

(1)

The biaxial flexural strength test (piston on three balls) was employed according to ISO 6872:2015 Dentistry - Ceramic materials using a universal testing machine (LF Plus, Lloyd Instruments, Florida, USA) at a crosshead speed of 0.5 mm/min until failure occurred. More than 15 polished disks with a nominal diameter of 15.6 mm and a thickness of 2.0 mm for each condition were tested. The load at fracture was recorded and the biaxial flexural strength of each specimen was calculated by an equation according to ISO 6872: 2015. The strength distribution was analysed by Weibull distribution according to ISO 6872: 2015, using the following Eq. (2):

180 10 5 35 12.5 30

2

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1 ⎞ lnln ⎛⎜ ⎟ = m lnσc − m lnσ0 − Pf ⎠ 1 ⎝

3.2. Coefficient of thermal expansion (2)

The coefficients of thermal expansion (CTE), for each of the glassceramics, over temperature range of 50–500 °C are presented in Table 2.

where Pf is the probability of failure, σc is the strength, σ0 is the characteristic strength (at 63% failure probability) and m is the Weibull modulus. The Weibull modulus indicates the distribution of strength of the samples. For ceramic materials with a low distribution of strength the value should have a value greater than10 [26]. The failure probability was calculated assuming the following relation: Pf = (i − 0.5/ N) where Pf is the failure probability of the ith specimens (ranked in order of ascending strength) and N is the total number of specimens tested.

3.3. X-ray diffraction XRD patterns of the glass and mica-based glass-ceramics particles are presented in Fig. 3. The XRD result of the glass shows a broad peak at around 20–35° 2θ indicating that it is an amorphous material. Phases identified for the glass-ceramic are phlogopite-Ca mica (CaMg6Al2Si6O20F4, JCPDS: 025–0155), anorthite (CaAl2 (SiO4)2, JCPDS: 00–002–0523), fuorapatite (Ca5(PO4)3F, JCPDS: 00–060–0667), stishovite (SiO2, JCPDS: 00–015–0026), magnesium strontium phosphate (SrMg2P2O8, JCPDS: 00–052–1590), calcium magnesium silicate (CaMg3(SiO4)3, 00–002–0455) and magnesium aluminium silicate (MgAlSi1.5O6, 01–087–0038). The broad amorphous peak is still present and confirms that an amorphous phase still remains.

2.8. Chemical solubility The chemical solubility measurements were performed in accordance with ISO 6872:2015 with the total surface area exposed to the test solution around 33.6 cm2. The surfaces of the specimens were polished with a series of silicon carbide abrasive papers from 350 to 2500 grit. They were washed, dried at 150 ± 5 °C for 4 h, then weighed. They were then placed in 4% acetic acid solution (analytical grade) for 16 h at 80 °C. The samples were rinsed and dried at 150 ± 5 °C for 4 h and reweighed to calculate the chemical solubility, represented by the weight loss in terms of micrograms per square centimetre.

3.4. Microstructural features Representative SEM photos of etched surfaces are shown in Fig. 4. For this mica-based glass-ceramics the plate-like, interlocked crystal structure is clearly observed surrounded by a glass matrix. This structure makes these materials easy to machine by conventional metallic tools or dental CAD/CAM milling machines. Table 4 shows the level of crystallinity achieved for these specimens, the percent crystallinity of mica-based glass-ceramics was determined from SEM photography using the ImageJ/Fiji software [30] by area. As might be expected, the shorter crystallisation times of the GC_Ce5 and GC_Zr12.5 specimens result in a lower level of crystallinity.

3. Results and discussion 3.1. Differential thermal analysis Fig. 1 shows the DTA thermogram of as-cast glass. The glass transition (Tg) is given by the endothermic peak shoulder at approximately 632 °C. The two exothermic peaks at 774 °C and 877 °C are the first and second crystallisation peaks, Tp1 and Tp2, respectively. The several endothermic peaks above 1000 °C represent the decomposition, softening and melting points of the different crystalline phases. The broad shape of the melting endothermic peak suggests that the glass-ceramic is composed of many phases. As stated, before Tp2 is 877 °C, this makes the crystallisation temperature as 897 °C. The plot of Tp2* -Tp2 vs soak temperature is shown in Fig. 2. From this graph, it can be seen that the optimum nucleation temperature is 642 °C. These temperatures, 897 °C and 642 °C, were used in the fabrication of glass-ceramics in this research.

3.5. Translucency parameter (TP) Table 3 gives measured values of the translucency parameter. A visual depiction of these values is given in Fig. 5. One-way ANOVA revealed a significant difference in TP, amongst some of the materials (p<0.05). When the material is completely opaque the TP is about zero. As the TP increases the translucency of the material increases. 3.6. Biaxial flexural strength The biaxial flexural strengths are shown in Table 4. The strengths

Fig. 1. DTA curve of the as-received glass. 3

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Fig. 2. Tp2*-Tp2 plotted against soaking temperature for optimum nucleation temperature determination.

strength of GC_Zr12.5 was the lowest. The Weibull analysis was applied to the data and is shown in Table 4 and Fig. 6.

Table 2 Coefficient of thermal expansion of resultant mica-based glass-ceramics. Materials

CTE (x10−6/°C) at 50–500 °C

Glass GC GC_Ce5 GC_Ce35 GC_Zr12.5 GC_Zr30 IPS e.max Ceram IPS Empress2 (layering ceramic) VITA VM 11

7.45 ± 0.53 9.30 ± 0.65 10.82 ± 0.61 10.77 ± 0.48 9.32 ± 0.49 9.97 ± 0.54 9.5 [27] 9.7 [28] 11.2–11.6 [29]

3.7. Chemical solubility Table 5 shows the results of the chemical solubility tests. The dentistry-ceramic materials standard ISO 6872:2015 type II class2 states that partially or fully covered substructure ceramic for single-unit anterior or posterior (class 2a) must have a chemical solubility less than 2000 µg/cm2 and for a monolithic ceramic for single-unit anterior or posterior (class 2b) that is in direct contact with the oral environment, should be less than 100 µg/cm2. 4. Discussion

with the same superscript indicate they are not significantly different (p > 0.05). The results from one-way ANOVA analysis of variance and Tukey's HSD test showed that biaxial flexural strength of GC_Zr30 was higher than those of GC_Ce35, GC and GC_Ce5 while the biaxial flexural

This work investigated the effect of two different pigments on a range of properties; thermal expansion, translucency, biaxial strength and chemical solubility. In general, the change in properties can be

Fig. 3. XRD patterns of as-received glass and glass-ceramics crystallized at 897 °C. 4

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Fig. 4. SEM micrographs of the polished and etched surfaces of the mica-based glass-ceramics (a) GC (b) GC_Ce5 (c) GC_Ce35 (d) GC_Zr12.5 and (e) GC_Zr30, 1000X.

attributed to the effect, or in some cases the lack of effect, on the phase development and crystal development. Only a small effect on thermal expansion was found on the addition of pigments. The parent glass has the lowest thermal expansion value with crystallisation increasing the expansion coefficient (Table 2), as to be expected. The GC_Ce35 exhibited a CTE close to GC_Ce5 and GC_Zr30 close to that of GC_Zr12.5 showing that pigment type affects the thermal expansion of the glass-ceramic. In addition, the average thermal expansion of GC_Zr12.5 is close to GC. These results come as no surprise as the CTE of CeO2 is higher (11 × 10−6 °C−1) when compared to ZrO2 (≈9 × 10−6 °C−1), similar to that of the GC. For dental applications, the CTE of each material is an important factor because it must be matched with porcelain glazes applied as veneers. There are

Table 3 Translucency parameter of glass and resultant glass-ceramics at 1.0 mm thickness. Materials

Translucency parameter

Glass GC GC_Ce5 GC_Ce35 GC_Zr12.50 GC_Zr30

94.20 ± 0.63 1.13 ± 0.22 17.80 ± 1.06 10.77 ± 0.48 * 14.08 ± 0.56 10.82 ± 0.40 *

Remark: * not significantly different (p > 0.05).

5

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Fig. 5. The specimens (a) Glass (b) GC (c) GC_Ce5, (d) GC_Ce35, (e) GC_Zr12.5 and (f) GC_Zr30. Table 4 Biaxial flexural strength and Weibull modulus of the glass-ceramics in this study. Materials

GCF [11] GC GC_Ce5 GC_Ce35 GC_Zr12.5 GC_Zr30 ISO 6872:2015 Type II Class2 [11]

Biaxial flexural strength (MPa)

Weibull modulus (m)

% Crystallinity

139.4 106.6 ± 105.2 ± 115.2 ± 101.5 ± 118.9 ± ≥100

17.8 21.6 12.4 19.8 15.3 21.9

– 75 66 74 69 74

5.4A 9.5A 6.6B 7.5A 6.1B

Table 5 Chemical solubility and percentage of crystallinity values of glass-ceramics.

Characteristic Strength (MPa)

141 106 112 118 105 120

Materials

Chemical solubility (μg/cm2)

%Crystallinity

GC GC_Ce5 GC_Ce35 GC_Zr12.5 GC_Zr30

332.3 688.4 157.5 418.0 269.9

75 66 74 69 74

glass-ceramics show the absence of magnesium strontium phosphate peaks. Why this is, it is not apparent at the moment and is being investigated. An obvious difference in the patterns is the peak areas. The lowest peak areas occur in the GC_Zr12.5, followed by the GC_Ce5 with last two glass ceramics having the highest, and similar peak heights. The peak heights can depend on many things, but the quantity of the crystalline phases and size of the crystals are two factors. It is seen in Fig. 3 that, in general, the areas of the main peaks increase with the level of crystallinity. The broad amorphous peak also decreases in size with the increase in peak areas. Comparison of the peak areas for the ceria-based pigments and zircon-based pigments shows clearly that the former promotes nucleation. For example, the glass-ceramic with the ceria-based pigment and crystallisation time of 5 min show larger peaks than the glass-ceramic with the zircon-based pigment and 12.5 min crystallisation time. Ceria will enhance the crystallisation of a glass-ceramic by decreasing the activation energy of glass for crystallisation [31], decreasing the viscosity of the melted glass and crystallisation time and temperature [31–33]. The effect

Remark: strengths with the same superscript indicate they are not significantly different (p > 0.05).

several commercial porcelain veneers that can be matched to our glassceramics (Table 2) such as VITA VM11, IPS e.max Ceram and IPS Empress 2 with CTE values between 6.9 and 11.6 × 10−6/°C at 50–500 °C. The phases formed are typical for this glass system. The broad amorphous peak is still present after heat treatment, suggesting an amorphous phase still exists. This is supported by the scanning electron micrographs in Fig. 4, showing crystals surrounded by an amorphous phase. The ceria-based pigments did not affect the crystalline phases formed with both XRD patterns showing the same diffraction peaks as that of the glass-ceramic with no additives. The zircon-based pigmented

Fig. 6. Weibull analysis of biaxial flexural strength of mica-based glass-ceramics. 6

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glass-ceramic [15]. The effect on colour can be seen in Fig. 5. However, it is encouraging that CeO2 and Pr-ZrSiO4/FeZrSiO4 could be used for shading of the mica-based glass-ceramics. Modification of the amount and ratio of pigments added should be further investigated to obtain a wide variety of yellow red colour and translucency suitable for the dental application [39]. By controlling the nucleation and crystallisation times, the translucency of CeO2 and Pr-ZrSiO4/Fe-ZrSiO4 added mica-based glass-ceramic could be modified to a satisfactory level which was appropriate for dental applications. The crystalline phase of Ca-mica in these glass-ceramics is important as its presence provides extremely good machinability [40,41]. Since the interlayer cations are weakly bound, cleavage occurs easily on the application of shear stresses, which is especially useful in the CAD/CAM fabrication of dental restorations [11,18]. Comparison of the biaxial strengths with the per cent crystallinity (Table 4) shows that the strength is a function of the amount of glassy phase present. The strength of the glass ceramic (from a previous study) shows that this trend extends to higher crystallinity levels [18]. Where there was no significant difference in% crystallinity, there was no significant difference in biaxial strength. The representative SEM pictures of fracture surfaces after biaxial fractural strength of all glass-ceramics used in this study are shown in Fig. 7. The plate-like mica structures surrounded by the glass matrix are clearly observed. Because of randomly oriented mica crystals, the fracture surface provides a tortuous path for the cracks. The fracture surface appears to be mainly in the glass phase, though fractured crystals can be observed. As the strength increases, and the level of crystallinity, the fracture surface becomes rougher (more tortuous). The Weibull modulus for all materials is greater than 10 and closer to 20 for a couple of materials. This shows that within each group of materials there is a low variation in strength making them reasonably reliable for ceramics. A value greater than 10 is considered good for engineering ceramics. The two materials with the highest Weibull modulus, GC_Ce35 and GC_Zr30, also have the highest crystal content. One would expect, the less glassy phase, the less variation in strength. The characteristic strength (strength with a minimum probability of failure of 63%) is also shown in Table 4. It shows the same trend as the mean strengths. Observation of the graph shows that there is no significant difference between the two lower percent crystalline materials nor between the two higher percent crystalline materials as was the case for biaxial strength. The characteristic strengths (Table 4) make these materials acceptable as dental ceramics type II class 2, partially or fully covered substructure ceramic for single-unit anterior or posterior prostheses adhesively cemented, according to ISO 6872. Consequently, the Ca-mica allows the easy production of many restorative dental parts such as single unit, multi-unit, bridges, crowns, jackets, inlays, onlays, veneers etc. Another important phase found in the present research is fluorapatite. It is well known that it possesses biocompatibility and promotes bioactivity [42,43]. The effect of crystallinity was also found to be important for chemical solubility. Chemical solubility is an important property for dental restorations as a high solubility or low resistance to erosion will strictly limit the effective lifetime of the restorations. Additionally, this chemical degradation directly affects the strength of the glass-ceramic. The glassy phase in glass-ceramics is generally more soluble than the crystalline phases formed on heat treatment [4]. It was found that, in general, the chemical solubility decreased with increasing crystallinity. This is particularly so within each of the two systems and is in line with previous findings [44]. However, the difference observed between the two systems at the higher crystallinity level is not as clear cut and cannot be explained simply by the level of crystallinity. In Table 5, it was found that at the same percentage of crystallinity (74%), the Zr-based pigment presented less chemical solubility than Ce-based. Observation of Fig. 3 shows there is no evidence of the formation of the magnesium strontium phosphate in the glass-

seems to be more prominent at lower crystallisation times, as the crystallisation time increases the difference, as might be expected, in the level of crystallisation approaches similar values, as shown with similar peak areas for both types of glass-ceramics at 30 and 35 min. However, the little difference in appearance of Fig. 4b and Fig. 4d as far as the level of crystallinity is concerned. This is supported by the values of crystallinity in Table 4, where the values for GC, GC_Ce5 and GC_Zr12.5 are not significantly different, despite significantly different crystallisation times. The ceria-based pigment, GC_Ce5, does show larger crystals than that of the GC_Zr12.5 material, despite the latter's longer crystallisation time, which may explain the peak areas discussed earlier. This will influence the intensity of the crystalline peaks. For example, the GC_Ce5 shows less glassy phase and larger crystals than that of the GC_Zr12.5 material, despite the longer crystallisation time, as predicted with the XRD patterns. The type of pigment added did not affect the type of crystals with no apparent differences among the crystals grown, crystallisation times considered, for all four types of glass-ceramics. Observation of Fig. 4 shows little difference in morphology for the longer crystallisation time specimens. Crystals have grown and coalesced to produce microstructures very similar with the same level of crystallinity. This supports the discussion on the XRD patterns. The addition of the pigments has increased the translucency (Table 3). The glass-ceramic without any pigmentation is very opaque with a TP of nearly zero (TP = 1.13) while a maximum for TP of 17.8 was found for the GC_Ce5 material. The translucency of human teeth has been measured at a TP of 15–19 based on 1 mm thick human teeth [34,35], suggesting the samples with the longest crystallisation times may not be suitable for dental applications, on the basis of translucency. The translucency can be controlled by the nucleation and crystallisation of the parent glass. The translucency is a function of the amount of light scattering dependent on the crystalline phases present, crystal size and refractive index [36]. For these materials we have seen the crystalline phases of all samples are similar, thus the refractive index and crystal size are the main factors of translucency. Clearly, the used pigments do have an effect on the TP. The pigments do increase the TP, from 1 for the non-pigmented glass-ceramic to over 10. This is a much larger increase than the decrease in crystallinity might suggest (from 82% to around 70%). The increase in TP for the pigmented glassceramics over a crystallinity change from 74% to 66% is only a change in TP of 7. However, with the pigments added, the crystallinity effect seems to take over. Comparison of Tables 3 and 4 show that as the crystallinity increases the TP decreases. This is to be expected [31]. As the crystal size, and the amount of crystals increases an increase in light scattering would be produced and a subsequent reduction in transmittance. In addition, in term of translucency of glass-ceramic, it derives from the formation of crystals among the glassy phase matrix. The dimension data of crystals were measured by ImageJ/Fiji software on SEM photography approximately in length and width as 20–50 µm and 4–8 µm, respectively, and aspect ratios as 4.0–5.5. The CeO2 and zirconia-based ceramic pigments can also be used for shade adjusting of mica-based glass-ceramics without any significant effect on the number of mica crystal structure. CeO2 was generally used to enhance the crystallisation of a glass-ceramic by decreasing the viscosity of the melted glass and crystallisation time and temperature [31–33]. Pr-ZrSiO4 and Fe-ZrSiO4 have the colour characteristics of yellow and coral red pigmentation. In addition, zircon-based pigments including blue vanadium-ZrSiO4 pigment are widely used in ceramic glazes because of their chemical stability and excellent thermal resistance at high temperature production processes [15,37,38]. These pigments are often used in a triaxial system that provides a wide range of colour. Tooth colour is in the range of yellow red, therefore, PrZrSiO4 and FeZrSiO4 have been determined to be useful to colour the mica-based glass-ceramics. These zircon-based pigments are able to produce tooth-like colours without significant effects on physical or mechanical properties when they are mixed with lithium disilicate 7

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Fig. 7. SEM micrographs of the fracture surface of the mica-based glass-ceramics (a) GC (b) GC_Ce5 (c) GC_Ce35 (d) GC_Zr12.5 and (e) GC_Zr30, 2500X.

magnesium silicate, and magnesium aluminium silicate. The addition of the pigments did not affect the phase development except for the zircon-based pigment which seems to suppress the development of the magnesium strontium phosphate. The pigments had no effect on crystal development. As would be expected, the level of crystallinity and the morphology of the crystals formed were entirely controlled by heat treatment. The pigments had some effect on the thermal expansion coefficient with the ceria-based pigment having the larger effect. The CTE was either similar to or about 10% higher than the glass-ceramic without pigment. In all cases, the pigmented glass-ceramics had a CTE that matches many commercial veneers and thus would be suitable form a thermal expansion perspective for dental use. Biaxial flexural strength was measured on all samples and was found to depend on the level of crystallinity regardless of pigment type and this corelated with the fractography of the broken specimens with the fracture path predominantly in the glassy phase or at the glass-crystal interface. The

ceramics with the zircon-based pigment whereas the ceria-pigment materials do. Both magnesium and strontium apatites are considered to have low chemical solubility, their presence in the GC_Ce35 and their absence in the GC_Zr30 material may explain the higher solubility in the latter [3]. The results for all the mica-based glass-ceramics specimens tested are below the 2000 µg/cm2 limit set by the ISO standard for covered substructure ceramics, but above the 100 µg.cm2 limit set for a monolithic ceramic.

5. Conclusion Glass ceramics based on the SiO2, MgO, SrCO3, Al2O3, MgF2, CaCO3, P2O5 and CaF2 system were prepared with CeO2, and a zircon-based pigment consisting of Pr-ZrSiO4 and Fe-ZrSiO4 additions. The phases formed on heat treatment are phlogopite-Ca mica, anorthite, fluorapatite, stishovite, magnesium strontium phosphate, calcium 8

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increase in crystallinity provided a more tortuous path for the fracture process. The values of characteristic strength make these materials acceptable as dental ceramics type II class 2, partially or fully covered substructure ceramic for single-unit anterior or posterior prostheses adhesively cemented, according to ISO 6872. The pigments had some effect on the chemical solubility, though the crystallinity had the major effect. The glassy phase is more soluble than the crystalline phases and thus an increase in crystallinity reduces the chemical solubility. For the higher concentration pigment materials with the same crystallinity the suppression of the more chemically insoluble magnesium strontium phosphate by the zircon-based pigment resulted in a higher solubility. All the specimens met the requirements of ISO 6872:2015 for a covered substructure dental ceramic. The pigments are mainly added to control the aesthetics of the dental material and they did affect the translucency of the glass-ceramics, increasing the translucency. They had more of an effect than the crystallinity. Values of the translucency parameter similar to that of natural teeth were achieved.

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