- Email: [email protected]
Machinability of a newly developed pre-sintered zirconia block for dental crown applications
Journal Pre-proofs Machinability of a newly developed pre-sintered zirconia block for dental crown applications Noor Faeizah Amat, Andanastuti Muchtar, Hsu Zenn Yew, Muhammad Sufiyan Amril, Rahimi L. Muhamud PII: DOI: Reference:
S0167-577X(19)31628-3 https://doi.org/10.1016/j.matlet.2019.126996 MLBLUE 126996
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 September 2019 21 October 2019 10 November 2019
Please cite this article as: N.F. Amat, A. Muchtar, H.Z. Yew, M.S. Amril, R.L. Muhamud, Machinability of a newly developed pre-sintered zirconia block for dental crown applications, Materials Letters (2019), doi: https://doi.org/ 10.1016/j.matlet.2019.126996
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Machinability of a newly developed pre-sintered zirconia block for dental crown applications Noor Faeizah Amata, Andanastuti Muchtara,* , Hsu Zenn Yewb, Muhammad Sufiyan Amrila and Rahimi L. Muhamudc a
Centre for Materials Engineering and Smart Manufacturing, Faculty of Engineering and Built Environment,
Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b
Centre for Restorative Dentistry, Faculty of Dentistry, Universiti Kebangsaan Malaysia, Jalan Raja Muda
Abdul Aziz, 50300 Kuala Lumpur, Malaysia c
RS Advanced Technology Sdn Bhd, No. 35G, Jalan Mutiara Subang 1, Taman Mutiara Subang 47500 Subang
Jaya, Selangor, Malaysia. *Corresponding author: Tel: +60389118379; Email address: [email protected] Abstract This study aimed to investigate the machinability of zirconia blocks pre-sintered at four different temperatures (900 °C, 1000 °C, 1100 °C, and 1200 °C). Vickers hardness and fracture toughness were measured to obtain the brittleness index (BI) of the zirconia blocks. The shrinkage percentage of the blocks pre-sintered at different temperatures was also determined upon final sintering at 1500 °C. The blocks were then subjected to machining, and the machined surfaces were morphologically analyzed. Although the materials pre-sintered at 900 °C and 1000 °C could be machined, noticeable trace lines and grooves formed on the machined surface due to lack of strength prior to sintering. BI and surface morphology confirmed that the new zirconia block can be milled at a pre-sintering temperature of 1100 °C and can achieve an acceptable shrinkage value for machining upon final sintering. Keywords: Machinability; Bioceramics; Shrinkage; Brittleness; Hardness; Sintering 1.
Introduction
Yttria tetragonal zirconia polycrystals have emerged as promising ceramic materials for dental restoration applications because of their unique mechanical strength, excellent esthetics, and biocompatibility [1]. Zirconia restorations can be fabricated by soft machining a pre-sintered blank/block and then sintering such block to full density or hard machining a fully sintered block through computer-aided design/computer-aided manufacturing (CAD/CAM) [2]. Soft machining is more favorable than hard machining because the former generates lower friction during milling and causes minimal cutting tool wear [1]. By contrast, hard machining allows clinicians
1
to obtain accurate anatomical prosthesis dimensions because it is independent of dimensional shrinkage [3]. However, hard machining demonstrates low machinability because of the high hardness of materials; thus, it may occasionally create cracks, induce tetragonal-to-monoclinic phase transformation, wear milling tools, and consume excessive processing time [4]. Previous studies have shown that the harder a material is, the more difficult machining will be; this observation can be related to fabrication and sintering techniques [5]. A new zirconia block developed in our laboratory and fabricated by combining colloidal treatment and cold isostatic pressing (CIP) exhibits excellent mechanical properties that are appropriate for zirconia restoration [6,7]. However, studies regarding the shrinkage and machinability of this newly developed zirconia have yet to be performed. Therefore, the present work aimed to investigate the machinability and shrinkage of zirconia blocks pre-sintered at four different temperatures (900 °C, 1000 °C, 1100 °C, and 1200 °C) and fully sintered at 1500 °C. The correlation of machinability with mechanical properties and surface morphology at the pre-sintering stage was also examined. 2.
Materials and Methods
Commercially available 3 mol% yttria-stabilized zirconia (Inframat Advanced Materials, USA) with an average particle size of 30 nm was used to fabricate zirconia blocks through colloidal treatment and CIP techniques. Zirconia blocks with a diameter of 40 mm and a thickness of 20 mm were prepared in accordance with the method reported by Amat et al. [8]. The blocks were pre-sintered at 900 °C, 1000 °C, 1100 °C, and 1200 °C with a holding time of 2 h and heating and cooling rates of 3 °C/min in a furnace (VMK-1800, Linn High Therm, Germany). This range was selected on the basis of a previously patented work that indicated that presintering temperature ranging from 850 °C to 1200 °C can be used to prepare pre-sintered zirconia dental blocks [9]. The blocks were further sintered to a final temperature of 1500 °C with heating and cooling rates of 3 °C/min and a holding time of 2 h. These values are in accordance with several studies on the appropriate sintering temperatures for zirconia [10,11]. The dimensions of the zirconia blocks at each sintering step were measured to calculate volume shrinkage percentage upon final sintering. The machinability criteria of the pre-sintered zirconia were determined by calculating the brittleness index (BI), which is the hardness-to-fracture toughness ratio of materials introduced by Boccaccini (1997) in previous studies on the machinability of glass-ceramics [12]. Fundamentally, hardness indicates the ability of a material to resist deformation. Meanwhile, fracture toughness indicates the ability of a material to resist cracking. On the basis of the hardness-to-fracture toughness ratio, the machinability of a material is poor when its BI is high.
2
Fracture toughness and Vickers hardness tests were performed following the method previously reported by Amat et al. (2019) [6]. The zirconia blocks were machined into a crown framework using an open system fiveaxis CAM milling machine (Dentaswiss, Biodenta, Germany). The morphological characteristics of milled zirconia surfaces were observed under a field-emission scanning electron microscope, FESEM (Zeiss Merlin, Germany) to evaluate the effect of pre-sintering temperature on zirconia surface quality upon machining. 3.
Results and Discussion
Table 1 provides the mechanical properties of the zirconia samples. Hardness and fracture toughness increased as pre-sintering temperature rose from 900 °C to 1200 °C. The BI of the samples pre-sintered at 900 °C, 1000 °C, and 1100 °C ranged from 0.33 µm−1/2 to 0.38 µm−1/2. The samples pre-sintered at 1200 °C exhibited the highest BI (2.26 µm−1/2). In terms of hardness, the samples pre-sintered between 900 °C and 1100 °C may be suitable for CAD/CAM milling because the hardness value is consistent with that in a previous study [13], in contrast with that of the sample pre-sintered at 1200 °C. Nevertheless, Boccaccini (1997) [12] stated that appropriate machinability frequently occurs when materials achieve BI values of ≤4.3 µm–1/2. All the samples in this study attained values within this range. Therefore, all the pre-sintered samples in the present work were presumed suitable for machining and subjected to milling. The experiments showed that all the samples can be machined into a crown (Fig. 1). The zirconia surface obtained after milling was characterized via FESEM to confirm the machinability of the samples (Fig. 2). Morphological examination indicated that the samples pre-sintered at 900 °C and 1000 °C exhibit a coarse surface with noticeable pores. The rough surface, including grooves and trace lines, can be attributed to the effect of the burr cutter used in grinding and milling the loose structure of the blocks. Handling extremely soft materials may be difficult because they are fragile and easily crack. The block sample pre-sintered at 1100 °C presented a flat and smooth surface with low porosity. Meanwhile, cracks formed on the milled surfaces of the block pre-sintered at 1200 °C due to the block’s high hardness properties; these cracks may eventually compromise mechanical properties and exacerbate tool wear [4]. The FESEM characterization of zirconia surfaces confirmed that a pre-sintering temperature of 1100 °C is suitable for zirconia blocks with appropriate hardness, enabling handling and milling of these blocks. Fig. 3 presents the volume shrinkage of zirconia samples upon final sintering. The highest shrinkage of 48.0% was obtained at a pre-sintering temperature of 900 °C; however, this value slightly decreased to 42.9% when temperature was increased to 1000 °C. Such remarkable shrinkage was possibly generated because zirconia
3
remained in particle form during this pre-sintering temperature, indicating that the consolidation temperature was not reached. The shrinkage percentage of commercially available pre-sintered zirconia for dental machining ranges from 20% to 25% [3]. In the present study, the sample pre-sintered at 1100 °C yielded a shrinkage percentage within the range of commercial zirconia, and the sample pre-sintered at 1200 °C exhibited the lowest shrinkage (9.0%) among all the samples. A high shrinkage percentage, such as the value exhibited by the samples pre-sintered at 900 °C and 1000 °C, may require considerable enlargement (nearly 50%) to compensate for shrinkage during final sintering. However, this requirement is presumed uneconomical. The samples with low shrinkage percentage at a pre-sintering temperature of 1200 °C are cost-effective because low-dimensional enlargement is necessary prior to final sintering, increasing the number of restoration units that can be milled per block. However, the machining feasibility of blocks at this particular pre-sintering level affects the hardness properties of blocks and their machinability. 4.
Conclusions
The effects of pre-sintering temperature on the machinability and shrinkage of zirconia fabricated through colloidal treatment and CIP techniques were evaluated. Pre-sintering temperatures of 900 °C and 1000 °C produced zirconia blocks with hardness and BI values that were within the allowable machining range. However, trace lines and grooves evidently formed because of the weak block structure. The optimal temperature for machinability was 1100 °C, as indicated by the appropriate hardness and BI values for machining and the excellent machining surfaces of the zirconia block. Acknowledgments This work was supported by Universiti Kebangsaan Malaysia (UKM) via the research sponsorship of DIP-2018025. The authors gratefully acknowledge the Center for Research and Instrumentation Management (CRIM), UKM for excellent testing equipments and Genlab Dental Laboratory Sdn Bhd for providing profesional services of dental CAD/CAM machining.
References [1]
I. Denry, J.R. Kelly, State of the art of zirconia for dental applications, Dent. Mater. 24 (2008) 299–307.
[2]
F. Komine, M.B. Blatz, H. Matsumura, Current status of zirconia-based fixed restorations., J. Oral Sci. 52 (2010) 531–539.
[3]
O.S. Abd El-Ghany, A.H. Sherief, Zirconia based ceramics, some clinical and biological aspects:
4
Review, Futur. Dent. J. 2 (2016) 55–64. [4]
A.R. Alao, R. Stoll, X.F. Song, T. Miyazaki, Y. Hotta, Y. Shibata, L. Yin, Surface quality of yttriastabilized tetragonal zirconia polycrystal in CAD/CAM milling, sintering, polishing and sandblasting processes, J. Mech. Behav. Biomed. Mater. 65 (2017) 102–116.
[5]
J. Fan, T. Lin, F. Hu, Y. Yu, M. Ibrahim, R. Zheng, S. Huang, J. Ma, Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics, Ceram. Int. 43 (2017) 3647–3653.
[6]
N.F. Amat, A. Muchtar, M.S. Amril, M.J. Ghazali, N. Yahaya, Effect of sintering temperature on the aging resistance and mechanical properties of monolithic zirconia, J. Mater. Res. Technol. 8 (2019) 1092–1101.
[7]
M. Aboras, A. Muchtar, C. Husna, N. Yahaya, J.C.W. Mah, Enhancement of the microstructural and mechanical properties of dental zirconia through combined optimized colloidal processing and cold isostatic pressing, Ceram. Int. 45 (2019) 1831–1836.
[8]
N.F. Amat, A. Muchtar, M.S. Amril, M.J. Ghazali, N. Yahaya, Preparation of presintered zirconia blocks for dental restorations through colloidal dispersion and cold isostatic pressing, 44 (2018) 6409– 6416.
[9]
H. Moon, Slip-casting method of fabricating zirconia blanks for milling into dental appliances, United States Pat. US 2009/01 (2009). https://www.google.com/patents/US20090115084 (accessed December 22, 2016).
[10]
C.H. Chin, A. Muchtar, C.H. Azhari, M. Razali, M. Aboras, Influence of sintering temperature on translucency of yttria-stabilized zirconia for dental crown applications, J. Teknol. 78 (2016) 13–18.
[11]
C.H. Chin, A. Muchtar, C. Husna, M. Razali, Influences of the processing method and sintering temperature on the translucency of polycrystalline yttria-stabilized tetragonal zirconia for dental applications, Ceram. Int. 44 (2018) 18641–18649.
[12]
A.R. Boccaccini, Machinability and brittleness of glass-ceramics, J. Mater. Process. Technol. 65 (1997) 302–304.
[13]
A.R. Alao, L. Yin, Nano-scale mechanical properties and behavior of pre-sintered zirconia, J. Mech. Behav. Biomed. Mater. 36 (2014) 21–31.
5
Table captions Table 1 Mechanical properties and BI of zirconia samples at different pre-sintering temperatures Pre-sintering temperature (°C)
Vickers hardness (GPa)
Fracture toughness, KIC (MPa·m1/2)
Brittleness index, BI (µm−1/2)
900
0.9 ± 0.18
2.57 ± 0.21
0.35
1000
1.06 ± 0.26
2.77 ± 0.28
0.38
1100
1.27 ± 0.21
3.89 ± 0.22
0.33
1200
9.55 ± 0.3
4.23 ± 0.35
2.26
Figure captions
Zirconia crown Fig. 1 CAD/CAM milled zirconia crown
6
Magnification 30 K
Magnification 1 K
900 °C
Porosity Porosity
1000 °C
Trace lines and grooves
1100 °C
1200 °C
Cracks
7
Fig. 2 FESEM micrograph of zirconia surface after CAD/CAM milling
Mean value for shrinkage (%)
50
40
30
20
10
0 900
1000
1100
Pre-sintering temperature (°C)
Fig. 3 Shrinkage percentage of zirconia at different pre-sintering temperatures
8
1200
Highlights
Machinability of zirconia blocks is dependent on pre-sintering temperature High pre-sintering temperatures lead to high hardness of the zirconia blocks Pre-sintering at 1100C produces zirconia blocks with appropriate hardness Shrinkage percentage fell within the range of commercial zirconia
9
S0167-577X(19)31628-3 https://doi.org/10.1016/j.matlet.2019.126996 MLBLUE 126996
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 September 2019 21 October 2019 10 November 2019
Please cite this article as: N.F. Amat, A. Muchtar, H.Z. Yew, M.S. Amril, R.L. Muhamud, Machinability of a newly developed pre-sintered zirconia block for dental crown applications, Materials Letters (2019), doi: https://doi.org/ 10.1016/j.matlet.2019.126996
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Machinability of a newly developed pre-sintered zirconia block for dental crown applications Noor Faeizah Amata, Andanastuti Muchtara,* , Hsu Zenn Yewb, Muhammad Sufiyan Amrila and Rahimi L. Muhamudc a
Centre for Materials Engineering and Smart Manufacturing, Faculty of Engineering and Built Environment,
Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b
Centre for Restorative Dentistry, Faculty of Dentistry, Universiti Kebangsaan Malaysia, Jalan Raja Muda
Abdul Aziz, 50300 Kuala Lumpur, Malaysia c
RS Advanced Technology Sdn Bhd, No. 35G, Jalan Mutiara Subang 1, Taman Mutiara Subang 47500 Subang
Jaya, Selangor, Malaysia. *Corresponding author: Tel: +60389118379; Email address: [email protected] Abstract This study aimed to investigate the machinability of zirconia blocks pre-sintered at four different temperatures (900 °C, 1000 °C, 1100 °C, and 1200 °C). Vickers hardness and fracture toughness were measured to obtain the brittleness index (BI) of the zirconia blocks. The shrinkage percentage of the blocks pre-sintered at different temperatures was also determined upon final sintering at 1500 °C. The blocks were then subjected to machining, and the machined surfaces were morphologically analyzed. Although the materials pre-sintered at 900 °C and 1000 °C could be machined, noticeable trace lines and grooves formed on the machined surface due to lack of strength prior to sintering. BI and surface morphology confirmed that the new zirconia block can be milled at a pre-sintering temperature of 1100 °C and can achieve an acceptable shrinkage value for machining upon final sintering. Keywords: Machinability; Bioceramics; Shrinkage; Brittleness; Hardness; Sintering 1.
Introduction
Yttria tetragonal zirconia polycrystals have emerged as promising ceramic materials for dental restoration applications because of their unique mechanical strength, excellent esthetics, and biocompatibility [1]. Zirconia restorations can be fabricated by soft machining a pre-sintered blank/block and then sintering such block to full density or hard machining a fully sintered block through computer-aided design/computer-aided manufacturing (CAD/CAM) [2]. Soft machining is more favorable than hard machining because the former generates lower friction during milling and causes minimal cutting tool wear [1]. By contrast, hard machining allows clinicians
1
to obtain accurate anatomical prosthesis dimensions because it is independent of dimensional shrinkage [3]. However, hard machining demonstrates low machinability because of the high hardness of materials; thus, it may occasionally create cracks, induce tetragonal-to-monoclinic phase transformation, wear milling tools, and consume excessive processing time [4]. Previous studies have shown that the harder a material is, the more difficult machining will be; this observation can be related to fabrication and sintering techniques [5]. A new zirconia block developed in our laboratory and fabricated by combining colloidal treatment and cold isostatic pressing (CIP) exhibits excellent mechanical properties that are appropriate for zirconia restoration [6,7]. However, studies regarding the shrinkage and machinability of this newly developed zirconia have yet to be performed. Therefore, the present work aimed to investigate the machinability and shrinkage of zirconia blocks pre-sintered at four different temperatures (900 °C, 1000 °C, 1100 °C, and 1200 °C) and fully sintered at 1500 °C. The correlation of machinability with mechanical properties and surface morphology at the pre-sintering stage was also examined. 2.
Materials and Methods
Commercially available 3 mol% yttria-stabilized zirconia (Inframat Advanced Materials, USA) with an average particle size of 30 nm was used to fabricate zirconia blocks through colloidal treatment and CIP techniques. Zirconia blocks with a diameter of 40 mm and a thickness of 20 mm were prepared in accordance with the method reported by Amat et al. [8]. The blocks were pre-sintered at 900 °C, 1000 °C, 1100 °C, and 1200 °C with a holding time of 2 h and heating and cooling rates of 3 °C/min in a furnace (VMK-1800, Linn High Therm, Germany). This range was selected on the basis of a previously patented work that indicated that presintering temperature ranging from 850 °C to 1200 °C can be used to prepare pre-sintered zirconia dental blocks [9]. The blocks were further sintered to a final temperature of 1500 °C with heating and cooling rates of 3 °C/min and a holding time of 2 h. These values are in accordance with several studies on the appropriate sintering temperatures for zirconia [10,11]. The dimensions of the zirconia blocks at each sintering step were measured to calculate volume shrinkage percentage upon final sintering. The machinability criteria of the pre-sintered zirconia were determined by calculating the brittleness index (BI), which is the hardness-to-fracture toughness ratio of materials introduced by Boccaccini (1997) in previous studies on the machinability of glass-ceramics [12]. Fundamentally, hardness indicates the ability of a material to resist deformation. Meanwhile, fracture toughness indicates the ability of a material to resist cracking. On the basis of the hardness-to-fracture toughness ratio, the machinability of a material is poor when its BI is high.
2
Fracture toughness and Vickers hardness tests were performed following the method previously reported by Amat et al. (2019) [6]. The zirconia blocks were machined into a crown framework using an open system fiveaxis CAM milling machine (Dentaswiss, Biodenta, Germany). The morphological characteristics of milled zirconia surfaces were observed under a field-emission scanning electron microscope, FESEM (Zeiss Merlin, Germany) to evaluate the effect of pre-sintering temperature on zirconia surface quality upon machining. 3.
Results and Discussion
Table 1 provides the mechanical properties of the zirconia samples. Hardness and fracture toughness increased as pre-sintering temperature rose from 900 °C to 1200 °C. The BI of the samples pre-sintered at 900 °C, 1000 °C, and 1100 °C ranged from 0.33 µm−1/2 to 0.38 µm−1/2. The samples pre-sintered at 1200 °C exhibited the highest BI (2.26 µm−1/2). In terms of hardness, the samples pre-sintered between 900 °C and 1100 °C may be suitable for CAD/CAM milling because the hardness value is consistent with that in a previous study [13], in contrast with that of the sample pre-sintered at 1200 °C. Nevertheless, Boccaccini (1997) [12] stated that appropriate machinability frequently occurs when materials achieve BI values of ≤4.3 µm–1/2. All the samples in this study attained values within this range. Therefore, all the pre-sintered samples in the present work were presumed suitable for machining and subjected to milling. The experiments showed that all the samples can be machined into a crown (Fig. 1). The zirconia surface obtained after milling was characterized via FESEM to confirm the machinability of the samples (Fig. 2). Morphological examination indicated that the samples pre-sintered at 900 °C and 1000 °C exhibit a coarse surface with noticeable pores. The rough surface, including grooves and trace lines, can be attributed to the effect of the burr cutter used in grinding and milling the loose structure of the blocks. Handling extremely soft materials may be difficult because they are fragile and easily crack. The block sample pre-sintered at 1100 °C presented a flat and smooth surface with low porosity. Meanwhile, cracks formed on the milled surfaces of the block pre-sintered at 1200 °C due to the block’s high hardness properties; these cracks may eventually compromise mechanical properties and exacerbate tool wear [4]. The FESEM characterization of zirconia surfaces confirmed that a pre-sintering temperature of 1100 °C is suitable for zirconia blocks with appropriate hardness, enabling handling and milling of these blocks. Fig. 3 presents the volume shrinkage of zirconia samples upon final sintering. The highest shrinkage of 48.0% was obtained at a pre-sintering temperature of 900 °C; however, this value slightly decreased to 42.9% when temperature was increased to 1000 °C. Such remarkable shrinkage was possibly generated because zirconia
3
remained in particle form during this pre-sintering temperature, indicating that the consolidation temperature was not reached. The shrinkage percentage of commercially available pre-sintered zirconia for dental machining ranges from 20% to 25% [3]. In the present study, the sample pre-sintered at 1100 °C yielded a shrinkage percentage within the range of commercial zirconia, and the sample pre-sintered at 1200 °C exhibited the lowest shrinkage (9.0%) among all the samples. A high shrinkage percentage, such as the value exhibited by the samples pre-sintered at 900 °C and 1000 °C, may require considerable enlargement (nearly 50%) to compensate for shrinkage during final sintering. However, this requirement is presumed uneconomical. The samples with low shrinkage percentage at a pre-sintering temperature of 1200 °C are cost-effective because low-dimensional enlargement is necessary prior to final sintering, increasing the number of restoration units that can be milled per block. However, the machining feasibility of blocks at this particular pre-sintering level affects the hardness properties of blocks and their machinability. 4.
Conclusions
The effects of pre-sintering temperature on the machinability and shrinkage of zirconia fabricated through colloidal treatment and CIP techniques were evaluated. Pre-sintering temperatures of 900 °C and 1000 °C produced zirconia blocks with hardness and BI values that were within the allowable machining range. However, trace lines and grooves evidently formed because of the weak block structure. The optimal temperature for machinability was 1100 °C, as indicated by the appropriate hardness and BI values for machining and the excellent machining surfaces of the zirconia block. Acknowledgments This work was supported by Universiti Kebangsaan Malaysia (UKM) via the research sponsorship of DIP-2018025. The authors gratefully acknowledge the Center for Research and Instrumentation Management (CRIM), UKM for excellent testing equipments and Genlab Dental Laboratory Sdn Bhd for providing profesional services of dental CAD/CAM machining.
References [1]
I. Denry, J.R. Kelly, State of the art of zirconia for dental applications, Dent. Mater. 24 (2008) 299–307.
[2]
F. Komine, M.B. Blatz, H. Matsumura, Current status of zirconia-based fixed restorations., J. Oral Sci. 52 (2010) 531–539.
[3]
O.S. Abd El-Ghany, A.H. Sherief, Zirconia based ceramics, some clinical and biological aspects:
4
Review, Futur. Dent. J. 2 (2016) 55–64. [4]
A.R. Alao, R. Stoll, X.F. Song, T. Miyazaki, Y. Hotta, Y. Shibata, L. Yin, Surface quality of yttriastabilized tetragonal zirconia polycrystal in CAD/CAM milling, sintering, polishing and sandblasting processes, J. Mech. Behav. Biomed. Mater. 65 (2017) 102–116.
[5]
J. Fan, T. Lin, F. Hu, Y. Yu, M. Ibrahim, R. Zheng, S. Huang, J. Ma, Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics, Ceram. Int. 43 (2017) 3647–3653.
[6]
N.F. Amat, A. Muchtar, M.S. Amril, M.J. Ghazali, N. Yahaya, Effect of sintering temperature on the aging resistance and mechanical properties of monolithic zirconia, J. Mater. Res. Technol. 8 (2019) 1092–1101.
[7]
M. Aboras, A. Muchtar, C. Husna, N. Yahaya, J.C.W. Mah, Enhancement of the microstructural and mechanical properties of dental zirconia through combined optimized colloidal processing and cold isostatic pressing, Ceram. Int. 45 (2019) 1831–1836.
[8]
N.F. Amat, A. Muchtar, M.S. Amril, M.J. Ghazali, N. Yahaya, Preparation of presintered zirconia blocks for dental restorations through colloidal dispersion and cold isostatic pressing, 44 (2018) 6409– 6416.
[9]
H. Moon, Slip-casting method of fabricating zirconia blanks for milling into dental appliances, United States Pat. US 2009/01 (2009). https://www.google.com/patents/US20090115084 (accessed December 22, 2016).
[10]
C.H. Chin, A. Muchtar, C.H. Azhari, M. Razali, M. Aboras, Influence of sintering temperature on translucency of yttria-stabilized zirconia for dental crown applications, J. Teknol. 78 (2016) 13–18.
[11]
C.H. Chin, A. Muchtar, C. Husna, M. Razali, Influences of the processing method and sintering temperature on the translucency of polycrystalline yttria-stabilized tetragonal zirconia for dental applications, Ceram. Int. 44 (2018) 18641–18649.
[12]
A.R. Boccaccini, Machinability and brittleness of glass-ceramics, J. Mater. Process. Technol. 65 (1997) 302–304.
[13]
A.R. Alao, L. Yin, Nano-scale mechanical properties and behavior of pre-sintered zirconia, J. Mech. Behav. Biomed. Mater. 36 (2014) 21–31.
5
Table captions Table 1 Mechanical properties and BI of zirconia samples at different pre-sintering temperatures Pre-sintering temperature (°C)
Vickers hardness (GPa)
Fracture toughness, KIC (MPa·m1/2)
Brittleness index, BI (µm−1/2)
900
0.9 ± 0.18
2.57 ± 0.21
0.35
1000
1.06 ± 0.26
2.77 ± 0.28
0.38
1100
1.27 ± 0.21
3.89 ± 0.22
0.33
1200
9.55 ± 0.3
4.23 ± 0.35
2.26
Figure captions
Zirconia crown Fig. 1 CAD/CAM milled zirconia crown
6
Magnification 30 K
Magnification 1 K
900 °C
Porosity Porosity
1000 °C
Trace lines and grooves
1100 °C
1200 °C
Cracks
7
Fig. 2 FESEM micrograph of zirconia surface after CAD/CAM milling
Mean value for shrinkage (%)
50
40
30
20
10
0 900
1000
1100
Pre-sintering temperature (°C)
Fig. 3 Shrinkage percentage of zirconia at different pre-sintering temperatures
8
1200
Highlights
Machinability of zirconia blocks is dependent on pre-sintering temperature High pre-sintering temperatures lead to high hardness of the zirconia blocks Pre-sintering at 1100C produces zirconia blocks with appropriate hardness Shrinkage percentage fell within the range of commercial zirconia
9