Abstract
Objectives
Fine-grained, high strength, translucent leucite dental glass-ceramics are synthesized via controlled crystallization of finely milled glass powders. The objectives of this study were to utilize high speed planetary milling of an aluminosilicate glass for controlled surface crystallization of nano-scale leucite glass-ceramics and to test the biaxial flexural strength.
Methods
An aluminosilicate glass was synthesized, attritor or planetary milled and heat-treated. Glasses and glass-ceramics were characterized using particle size analysis, X-ray diffraction and scanning electron microscopy. Experimental (fine and nanoscale) and commercial (Ceramco-3, IPS Empress Esthetic) leucite glass-ceramics were tested using the biaxial flexural strength (BFS) test. Gaussian and Weibull statistics were applied.
Results
Experimental planetary milled glass-ceramics showed an increased leucite crystal number and nano-scale median crystal sizes (0.048–0.055 μm 2 ) as a result of glass particle size reduction and heat treatments. Experimental materials had significantly ( p < 0.05) higher mean BFS and characteristic strength values than the commercial materials. Attritor milled and planetary milled (2 h) materials showed no significant ( p > 0.05) strength difference. All other groups’ mean BFS and characteristic strengths were found to be significantly different ( p < 0.05) to each other. The mean (SD) MPa strengths measured were: Attritor milled: 252.4 (38.7), Planetary milled: 225.4 (41.8) [4 h milling] 255.0 (35.0) [2 h milling], Ceramco-3: 75.7 (6.8) and IPS Empress: 165.5 (30.6).
Significance
Planetary milling enabled synthesis of nano-scale leucite glass-ceramics with high flexural strength. These materials may help to reduce problems associated with brittle fracture of all-ceramic restorations and give reduced enamel wear.
1
Introduction
Leucite glass-ceramics are utilized in restorative dentistry to replace tooth substance lost or removed due to disease, trauma and/or for esthetic reasons. When used for all-ceramic restorations, brittle fracture and wear of the opposing teeth are the main disadvantages . Nano-technology is increasingly utilized to improve the properties of restorative dental biomaterials including: strength, wear and translucency . Nano-technology has found numerous applications in the field of resin based composites , surface nano-composite coatings , glass ionomers and high-performance bio-ceramics . Nanotechnology using sol–gel (bottom up) processing has been used to produce nano-scale leucite crystallites (80 nm) for dispersion in a glassy matrix. These nano-leucite composites were however only able to achieve flexural strengths of mean (SD): 109 (10.7) MPa , in the range expected for current leucite glass-ceramics (120–140 MPa) . More recently, hydrothermally derived super-fine leucite (115 nm crystallites) glass-ceramics have also been produced, but no strength data is yet available . A combination of conventional top–down glass processing (milling) and crystallization heat treatments should also be considered to yield improved properties in leucite glass-ceramics. Chen et al. developed fine-grained, translucent leucite glass-ceramics with minimal matrix micro cracking and the highest mean (SD) biaxial flexural strength (253.8 (53.3) MPa) reported to date for dental leucite glass-ceramics. This was achieved using a combination of conventional top–down glass powder size reduction using attritor milling, together with controlled crystallization processing. Attritor mills can be very efficient in terms of energy transfer, thus producing powder sizes that are in the micrometer range . Chen et al. established an exponential correlation between glass powder particle size (attritor milled) and the leucite crystal size achieved in the glass-ceramic. The surface crystallization mechanism prevalent in these milled aluminosilicate glasses can therefore be utilized to achieve very fine leucite crystallites in the glassy matrix , since a leucite crystal can grow readily to a larger size than the glass particle from which it originates. Another high-energy design is that of a planetary ball mill where nanometre range particle sizes have been achieved that could facilitate the surface crystallization of nano-scale leucite glass-ceramics. Targeted synthesis and careful processing of leucite glass-ceramic formulations have previously led to high strength and reliability optimization , with a reduced leucite crystal size associated with smaller amounts of enamel wear . Further crystal size reduction to the nano-scale level may be possible in this system and would be advantageous to maintain high strength and give enhanced tooth wear properties.
The aim of this study was to utilize high speed planetary milling of an aluminosilicate glass for controlled surface crystallization of high strength nano-scale leucite glass-ceramics. The hypotheses were that; (a) high speed planetary milling would produce enhanced glass surface nucleation and growth of nano-scale leucite crystallites, (b); there would be an increase in biaxial flexural strength associated with these materials.
2
Materials and methods
2.1
Glass and glass-ceramic synthesis
A glass with the following mol% composition was batched: 72.6% SiO 2 , 10.7% Al 2 O 3 , 7.9% K 2 O, 2.1% CaO, 0.3% TiO 2 , 4.7% Na 2 O, 1.1% Li 2 O, and 0.5% MgO. The glass batch was heated in an electric kiln (Fredrickson Kiln Co., Alfred, NY, USA) at 10 °C/min to 1316 °C (7 h hold). The frit was then cooled, ground and sieved through a 125 μm nylon sieve (glass A). Samples of glass A were then wet-ground in a high-speed planetary mill (Pulverisette P7, Fritsch, Idar-Oberstein, Germany) at 1000 rpm using 1 mm YTZ grinding media (LOT:52600590100, Tosoh, Tokyo, Japan) for up to 4 h. Samples were retrieved at 2, 6, 10, 16, 30 (PM0.5A), 46, 60 (PM1A), 120 (PM2A) and 240 min (PM4A). The samples were freeze dried in a freeze drier (Virtis Advantage, Virtis, Gardiner, New York, USA). The glass particle size (PSA) was then measured using a laser diffraction analyser (LS13320, Beckman Coulter, High Wycombe, UK). The powders were dispersed via ultra-sonication using the on stage ultrasonic probe for 45 min, in an aqueous solution of sodium hexametaphosphate and anhydrous sodium carbonate mixed in the ratio of 5:0.7 wt , before taking measurements.
Samples of glass A were also wet-ground in an Attritor Mill (Model 1-S Lab Attritor, Union Process, Akron, OH, USA) at 400 rpm with 5 mm yttria-stabilized zirconia (YTZ) grinding media (Tosoh Inc., Grove City, OH, USA) for 120 min and with 2 mm YTZ media for another 120 min (M1A). Glass powders (M1A, PM0.5A, PM1A, PM2A, PM4A) sequentially received controlled crystallization heat-treatments in a furnace (RHF 1600, Carbolite, Hope Valley, UK) from 23 °C at a ramp of 10 °C/min to 610 °C (1 h hold) then to 1050 °C (1 h hold) followed by air quenching and regrinding.
2.2
X-ray diffraction analysis
X-ray diffraction (XRD) was carried out on the glass and glass-ceramic powders (X’-Pert Pro X-ray diffractometer, PANalytical, Almelo, The Netherlands). Flat plate θ / θ geometry and Ni-filtered Cu-Kα radiation ( λ 1 = 0.1540598 nm and λ 2 = 0.15444260 nm) were used. Data were continuously collected with an X’Celerator solid state multistrip detector from 5° to 120° (2 θ range) with a step size of 0.0334°and a step time of 200.03 s. Calibration was carried out using NIST standard reference material 660a (lanthanum hexaboride). The structural model of tetragonal leucite (ICDD: 00-038-1423) was used for phase identification.
2.3
Specimen fabrication
Experimental (PM2A, PM4A, M1A) and commercial (Ceramco 3 dentin, Batch No 02111576, Dentsply, Ceramco, Burlington, USA) glass-ceramic powders were used to fabricate 30 disk specimens per group. Specimens were fabricated by moistening 1.1 g of powder with 0.4 ml of modeling liquid (CH B: 24066, Vita Zahnfabrik, Bad Sackingen, Germany). The experimental group specimens were transferred to a pre-heated (538 °C) porcelain furnace (Multimatt MCII, Dentsply, Konstanz, Germany) and sintered under vacuum at a rate of 38 °C/min to 1040 °C with 2 min hold. A 3 min cooling cycle (Multimatt MCII) was applied at the end of the firing. Specimens were next wet-ground with P1000 grade silicon carbide paper and then received one stain and one overglaze simulated firings as per Chen et al. , with a 3 min cooling cycle as previously.
Ceramco 3 specimens were sintered using one dentin firing and one simulated glaze firing cycle as per manufacturer’s instructions, and were then wet wet-ground with P1000 grade silicon carbide paper. Ceramco-3 (Dentsply, Ceramco, New Jersey, USA) and IPS Empress Esthetic glass-ceramics (Ivoclar-Vivadent, Schaan, Liechtenstein) were fabricated as commercial comparison materials. Heat extruded IPS Empress Esthetic glass-ceramic (Lot: H22624, ETC2, Ivoclar-Vivadent, Liechtenstein) used in a previous study also served as a commercial reference material. The tensile test surface of the IPS Empress Esthetic (EE) specimens was left as sandblasted (50 μm glass beads at 1.5 × 10 5 Pa pressure) and the opposite test surface (under loading indenter) was wet ground with P1000 grade silicon carbide grinding paper. Specimens were then fired in a porcelain furnace (Multimatt MCII, Dentsply) according to the manufacturers’ recommended firing cycles (2 simulated stain and 2 simulated glaze firings).
2.4
Biaxial flexural strength testing
Disk specimens (14 mm × 2 mm) were centrally loaded using the ball-on-ring test using a 10 mm diameter knife-edge support, a 4 mm diameter spherical indenter and a crosshead speed of 1 mm/min. The biaxial flexural strength (BFS) was calculated by using :
σ max = p h 2 ( 1 + v ) [ 0.485 × ln a h + 0.52 ] + 0.48
where σ max is maximum tensile stress, P is recorded load at fracture, h is specimen thickness, a is radius of the knife-edge support ring and v the Poisson’s ratio (set as 0.25, ISO-6872). Parametric (one-way-ANOVA, Tukey’s tests, Sigma Stat ver. 2.03, SPSS Inc., Chicago, USA) and Weibull statistics (WinSMITH™ Weibull & Visual 2.0M, Fulton Findings™, Torrance, CA, USA) were used to analyse the strength data. Ranked (ascending) BFS test values were plotted against the double logarithm of 1/(1–median rank). Median rank regression methods were then applied to fit a straight line through the data points. The two parameter Weibull distribution function was used ( P f = 1 − exp [−( σ / σ 0 ) m ], P f is Probability of failure, σ is strength at a given P f value, σ 0 is Characteristic strength and m is Weibull modulus). Significant differences between Weibull m and characteristic strength values were detected based on the lack of overlap of the 95% double sided confidence intervals.
2.5
Secondary electron imaging
Polished and etched (0.1% HF, 60 s) and fractured specimens were gold coated and viewed in a scanning electron microscope (FEI Inspect F., Europe NanoPort, Eindhoven, The Netherlands) in the secondary electron imaging (SEI) mode. Micrographs (×15,000, area = 342 μm 2 ) were used to quantify crystal size, number, and area fraction using image analysis software (Sigma Scan Pro 5.0, Systat Software, Inc., Chicago, IL, USA). Median crystal size data were analysed for significant differences between test groups ( p < 0.001, Dunn’s multiple comparisons test, Sigma stat, version 2.03, SPSS Inc.).
2
Materials and methods
2.1
Glass and glass-ceramic synthesis
A glass with the following mol% composition was batched: 72.6% SiO 2 , 10.7% Al 2 O 3 , 7.9% K 2 O, 2.1% CaO, 0.3% TiO 2 , 4.7% Na 2 O, 1.1% Li 2 O, and 0.5% MgO. The glass batch was heated in an electric kiln (Fredrickson Kiln Co., Alfred, NY, USA) at 10 °C/min to 1316 °C (7 h hold). The frit was then cooled, ground and sieved through a 125 μm nylon sieve (glass A). Samples of glass A were then wet-ground in a high-speed planetary mill (Pulverisette P7, Fritsch, Idar-Oberstein, Germany) at 1000 rpm using 1 mm YTZ grinding media (LOT:52600590100, Tosoh, Tokyo, Japan) for up to 4 h. Samples were retrieved at 2, 6, 10, 16, 30 (PM0.5A), 46, 60 (PM1A), 120 (PM2A) and 240 min (PM4A). The samples were freeze dried in a freeze drier (Virtis Advantage, Virtis, Gardiner, New York, USA). The glass particle size (PSA) was then measured using a laser diffraction analyser (LS13320, Beckman Coulter, High Wycombe, UK). The powders were dispersed via ultra-sonication using the on stage ultrasonic probe for 45 min, in an aqueous solution of sodium hexametaphosphate and anhydrous sodium carbonate mixed in the ratio of 5:0.7 wt , before taking measurements.
Samples of glass A were also wet-ground in an Attritor Mill (Model 1-S Lab Attritor, Union Process, Akron, OH, USA) at 400 rpm with 5 mm yttria-stabilized zirconia (YTZ) grinding media (Tosoh Inc., Grove City, OH, USA) for 120 min and with 2 mm YTZ media for another 120 min (M1A). Glass powders (M1A, PM0.5A, PM1A, PM2A, PM4A) sequentially received controlled crystallization heat-treatments in a furnace (RHF 1600, Carbolite, Hope Valley, UK) from 23 °C at a ramp of 10 °C/min to 610 °C (1 h hold) then to 1050 °C (1 h hold) followed by air quenching and regrinding.
2.2
X-ray diffraction analysis
X-ray diffraction (XRD) was carried out on the glass and glass-ceramic powders (X’-Pert Pro X-ray diffractometer, PANalytical, Almelo, The Netherlands). Flat plate θ / θ geometry and Ni-filtered Cu-Kα radiation ( λ 1 = 0.1540598 nm and λ 2 = 0.15444260 nm) were used. Data were continuously collected with an X’Celerator solid state multistrip detector from 5° to 120° (2 θ range) with a step size of 0.0334°and a step time of 200.03 s. Calibration was carried out using NIST standard reference material 660a (lanthanum hexaboride). The structural model of tetragonal leucite (ICDD: 00-038-1423) was used for phase identification.
2.3
Specimen fabrication
Experimental (PM2A, PM4A, M1A) and commercial (Ceramco 3 dentin, Batch No 02111576, Dentsply, Ceramco, Burlington, USA) glass-ceramic powders were used to fabricate 30 disk specimens per group. Specimens were fabricated by moistening 1.1 g of powder with 0.4 ml of modeling liquid (CH B: 24066, Vita Zahnfabrik, Bad Sackingen, Germany). The experimental group specimens were transferred to a pre-heated (538 °C) porcelain furnace (Multimatt MCII, Dentsply, Konstanz, Germany) and sintered under vacuum at a rate of 38 °C/min to 1040 °C with 2 min hold. A 3 min cooling cycle (Multimatt MCII) was applied at the end of the firing. Specimens were next wet-ground with P1000 grade silicon carbide paper and then received one stain and one overglaze simulated firings as per Chen et al. , with a 3 min cooling cycle as previously.
Ceramco 3 specimens were sintered using one dentin firing and one simulated glaze firing cycle as per manufacturer’s instructions, and were then wet wet-ground with P1000 grade silicon carbide paper. Ceramco-3 (Dentsply, Ceramco, New Jersey, USA) and IPS Empress Esthetic glass-ceramics (Ivoclar-Vivadent, Schaan, Liechtenstein) were fabricated as commercial comparison materials. Heat extruded IPS Empress Esthetic glass-ceramic (Lot: H22624, ETC2, Ivoclar-Vivadent, Liechtenstein) used in a previous study also served as a commercial reference material. The tensile test surface of the IPS Empress Esthetic (EE) specimens was left as sandblasted (50 μm glass beads at 1.5 × 10 5 Pa pressure) and the opposite test surface (under loading indenter) was wet ground with P1000 grade silicon carbide grinding paper. Specimens were then fired in a porcelain furnace (Multimatt MCII, Dentsply) according to the manufacturers’ recommended firing cycles (2 simulated stain and 2 simulated glaze firings).
2.4
Biaxial flexural strength testing
Disk specimens (14 mm × 2 mm) were centrally loaded using the ball-on-ring test using a 10 mm diameter knife-edge support, a 4 mm diameter spherical indenter and a crosshead speed of 1 mm/min. The biaxial flexural strength (BFS) was calculated by using :
σ max = p h 2 ( 1 + v ) [ 0.485 × ln a h + 0.52 ] + 0.48
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