Enhanced mechanical properties of ZrO2-Al2O3 dental ceramic composites by altering Al2O3 form

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Enhanced mechanical properties of ZrO2 -Al2 O3 dental ceramic composites by altering Al2 O3 form Ji-Young Seo a , Daniel Oh b , Dae-Joon Kim c , Kwang-Mahn Kim a , Jae-Sung Kwon a,∗ a

Department and Research Institute of Dental Biomaterials and Bioengineering, Yonsei University College of Dentistry, Seoul, Republic of Korea b Department of Orthopedic Surgery, Columbia University, New York, NY, USA c Acucera Co., Inc., 313, Naechon-Myeon, Pocheon-si, Gyeonggi-do, Republic of Korea

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. This study evaluates the difference in physical and mechanical properties of ZrO2

Accepted 14 January 2020

ceramics, commonly used in dental applications, altered by three different forms of Al2 O3

Available online xxx

content; microparticles (m), nanoparticles (n), and microfiber (f). Methods. Three different types of ZrO2 –Al2 O3 composites were formed using microparticle

Keywords:

(m), nanoparticle (n), or microfibre (f) forms of Al2 O3 . The physical and mechanical proper-

Zirconia

ties such as sintering shrinkage, relative density, Vickers hardness, fracture toughness, and

ZrO2 –Al2 O3 composite

biaxial strength were evaluated. A Weibull analysis was performed to assess the strength

Fibre

reliability of the specimens. All data were calculated using the t-test and ANOVA.

Mechanical properties

Results. The sintering shrinkage and relative density of all ceramic composite groups were decreased with the addition of Al2 O3 . The mechanical properties of ZrO2 –Al2 O3 (f) composite were higher than that of ZrO2 –Al2 O3 (m) composite and ZrO2 –Al2 O3 (n) composite. The maximum hardness, fracture toughness, and biaxial flexural strength were observed for 10 vol% of Al2 O3 fibre. When the content of Al2 O3 fibre in the matrix was increased above 20 vol%, agglomeration occurred and resulted in a decrease of hardness and toughness. The Weibull modulus value of the ZrO2 –Al2 O3 (f) composite was the lowest compared to that of other groups. However, characteristic strength (␴0 ) of ZrO2 –Al2 O3 (f) the highest value. Significance. The current study demonstrated that the addition of right amount of Al2 O3 microfibre into the ZrO2 matrix enhanced the mechanical properties of ZrO2 -Al2 O3 (f) composite, which would be favourable for dental applications. © 2020 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.

1.

Introduction

Zirconia (ZrO2 ) has been increasingly used as the dental ceramicdue to its high strength and fracture toughness as well as its colour, which is similar to that of a natural tooth [1–4].

∗

Especially, the development of high-toughness ceramics has led to an increased use of ZrO2 stabilized with Y2 O3 (Y-TZP) in clinical applications [5,6]. Although the addition of Y2 O3 retained the tetragonal phase of the ZrO2 at room temperatures, when a crack initiates on the surface of Y-TZP, the stress concentration at the top of the crack causes the tetragonal crystal to transform into a monoclinic crystal, with associated volumetric expansion [7]. In addition, the fracture toughness of monolithic ceramics is generally very poor and limits their

Corresponding author. E-mail address: [email protected] (J.-S. Kwon). https://doi.org/10.1016/j.dental.2020.01.014 0109-5641/© 2020 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.

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applications. Hence, to minimize undesired events of surface cracking or change in the surface microstructure during clinical usage, the hardness and fracture toughness of tetragonal ZrO2 still needs to be improved [8]. To increase the mechanical properties of ZrO2 , Al2 O3 is recommended as a reinforcement agent for a ZrO2 composite [9–11]. Reinforcement materials for ceramic composites can be chosen in forms of particle-, fibre- or whisker-type. A recent trend in the research for ceramic composites is to incorporate strong ceramic fibre into ceramic matrices to form a fibrereinforced ceramic matrix composite. These composites are being extensively studied to enhance their mechanical properties, leading to the interest of researchers in the development of fibre-reinforced ceramic composite systems [12–15]. Such studies were concentrated on the effect of adding Al2 O3 particles to ZrO2 –Al2 O3 composites. There has been a limited number of research on how different forms of Al2 O3 such as microparticles, nanoparticles, and microfibres would influence the ZrO2 –Al2 O3 composites. Hence, in this study, the alteration of physical and mechanical properties of ZrO2 ceramics according to the addition of different forms of Al2 O3 were investigated in association with the possible use of the ZrO2 –Al2 O3 composites in dentistry. Three different forms of Al2 O3 were considered, which included microparticle (Al2 O3 (m)), nanoparticle (Al2 O3 (n)) and microfibre (Al2 O3 (f)).

2.3.

Shrinkage and relative density

To investigate shrinkage behaviour of each specimen, sintering shrinkage of all specimens was calculated by measuring diameter and thickness before and after sintering. Eighteen specimens were made for each group. The bulk density of each of the sintered samples (n = 5 per group) was measured in water using the Archimedes method. The theoretical density was determined using the rule of mixtures. The relative density was calculated as the ratio of the bulk density of the sintered specimens to the theoretical density.

2.4.

Vickers hardness and fracture toughness

The Vickers hardness and fracture toughness values of the ZrO2 and ZrO2 –Al2 O3 composite specimens (n = 10 per group) were measured using the indentation method. A maximum load of 9.8 N (1 kg) was applied to the specimen surface with a dwell time of 15 s (Microhardness tester, Matsuzawa, Inc., MXD-CX3E, Japan). Three readings were taken for each ten specimens from each group, and the mean Vickers hardness value was recorded. Hardness and fracture toughness of the ZrO2 and ZrO2 -Al2 O3 composite were calculated from the following Eqs. (1) and (2) [16]; Hv = 1.854 ×

2.

Materials and methods

2.1.

Materials

For all test groups, 3 mol % yttria-stabilized tetragonal zirconia powder (KZ-3YF, Kyoritsu, Japan) was used. In terms of microparticle, nanoparticle and microfibre forms of Al2 O3 , micro sized Al2 O3 particles (ALM-43, Sumitomo. Chemical Co., Japan), nanosized Al2 O3 particles (Aeroxide Alu C, Degussa Evonik, Germany), and Al2 O3 fibre (B97N3, Denka Alsen, Denka Co., Japan) were used, respectively.

2.2.

Preparation of ZrO2 –Al2 O3 composite

The ZrO2 and ZrO2 –Al2 O3 composite (90:10 vol %) was prepared using yttria-stabilized tetragonal zirconia powder and their different types of alumina: Al2 O3 (m), Al2 O3 (n) and Al2 O3 (f). Additionally, ZrO2 -based ceramic composites with varying Al2 O3 fibre content (5, 15 and 20 vol%) were prepared. The mixed powders were ball-milled for 24 h in ethanol. The slurries were dried at 80 ◦ C for 24 h and sieved through a 212 ␮m mesh. Disk shape samples (13.00 ± 0.05 mm in diameter, 1.75 ± 0.05 mm in thickness) were prepared from the powders obtained under uniaxial pressure of 1 ton in a steel mould for 2 min. The pressed specimens were sintered in an electric furnace (Super Kanthal Furnace, Lindberg. Blue M, USA) at 1450 ◦ C for 2 h at a heating and cooling rate of 10 ◦ C/min. After sintering, the specimens were cooled to room temperature.

P d2

(1)

where p is the indentation load (N), d is an average length of the two diagonals of the indentation (mm), and Hv is the Vickers hardness (GPa) [17–19]; KIC = 0.0726 ×

P

(2)

3

c2

where P is the applied load for indentation (N), c is the 1/2 of crack length (m) and KIC is the fracture toughness (MPa m1/2 )

2.5.

Biaxial flexural strength and Weibull analysis

The biaxial flexural strength test of eleven specimens per test group was measured by using the piston-on-three ball technique in a universal testing machine at a 1.0 mm/min crosshead speed. The fracture load for each specimen was measured, and the biaxial flexural strength was calculated for measured value using the Eq. (3) [20–23];

S=

P − 0.2387(X − Y) d2

(3)

where s is the maximum centre tensile stress (MPa), P is the load at fracture (N) and d is the thickness at the specimen disk centre (mm). X and Y were calculated using following two Eqs. (4) and (5);

X = (1 + ) ln

  2 2

3

+



(1 − ) 2

   2 2

3

(4)

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Fig. 1 – Shrinkage in diameter and thickness (A) and relative density (B) of the sintered specimens at 1450 ◦ C. Different letters indicate a significant difference at p < 0.05 for each diameter (lower case), thickness (upper case) and relative density (lower case).

  2  1

Y = (1 + ) ln

3

+ (1 − )

  2 1

3

(5)

where v is the Poisson ratio, ␥1 is the radius of the support circle (mm), ␥2 is the radius of the loaded area (mm), and ␥3 is the radius of the specimen (mm). In the present study, v was considered to be 0.25 [20,22,24]. To assess the reliability strength of ceramic specimens, Weibull analysis was performed using the flexural strength values. The Weibull modulus was calculated by following Eq. (6) [4,25–28];



Pf () = 1 − exp −(

 m ) 0

 (6)

where Pf (␴) is the probability of failure for a given flexural strength, ␴ is the fracture strength, ␴0 is the characteristic strength at the fracture probability of 63.2%, and m is the Weibull modulus.

2.6.

Grain size and microstructure

Polished and thermally etched microstructure surface samples were visualized by a field emission scanning electron microscope (FE-SEM, JSM 7800 F, JEOL, Japan). The average grain sizes were measured from the microstructures using the linear intercept method. Additionally, the microstructures of a fracture surface of specimen with varying Al2 O3 fibre contents were visualized by FE-SEM.

2.7.

Statistical analysis

Statistical analysis before and after sintering shrinkage between diameter and thickness were carried out using Student’s t-test (p = 0.05). The statistical analysis of the results from shrinkage, relative density, hardness, fracture toughness and biaxial flexural strength measurements were carried out using one-way ANOVA (p = 0.05).

3.

Results

3.1. Shrinkage and relative density of ZrO2 –Al2 O3 composites The comparative shrinkage of the diameter and thickness of the sintered specimens is presented in Fig. 1A There was a general decrease in shrinkage as the Al2 O3 was added. The shrinkages of both the diameter and the thickness of the ZrO2 specimens were significantly higher than shrinkage of both the diameter and the thickness of all of the ZrO2 /Al2 O3 composite specimen groups (p < 0.05). There were no significant differences in thickness shrinkage between different types of Al2 O3 (microparticle; Al2 O3 (m), nanoparticle; Al2 O3 (n) and microfibre; Al2 O3 (f) (p > 0.05). In terms of the diameter, the shrinkage was lowest with Al2 O3 (m), followed by Al2 O3 (f) and Al2 O3 (n) (p < 0.05). The variations of relative density with different types of Al2 O3 (microparticle; Al2 O3 (m), nanoparticle; Al2 O3 (n) and microfibre; Al2 O3 (f) are shown in Fig. 1B. The relative densities of all sintered specimen groups were over 98 % upon sintering at 1450 ◦ C. Additionally, the relative density decreased with addition of Al2 O3 (p < 0.05) but the relative density was not significantly different among composite groups (p > 0.05).

3.2. Vickers hardness and fracture toughness of ZrO2 –Al2 O3 composites The hardness and fracture toughness of ZrO2 and ZrO2 –Al2 O3 composites containing 10 vol % of the Al2 O3 particles/fibre are shown in Fig. 2 The hardness and fracture toughness of all composite groups were higher than ZrO2 (p < 0.05). The hardness values of the ZrO2 –Al2 O3 (m), ZrO2 –Al2 O3 (n) and ZrO2 –Al2 O3 (f) composite specimens were higher than the value for the ZrO2 by approximately 4.7 %, 4.7 % and 8.6%, respectively (Fig. 2A). No significant difference was found between ZrO2 –Al2 O3 (m) composite and ZrO2 –Al2 O3 (n) composite for hardness (p > 0.05). The hardness of ZrO2 –Al2 O3 (f) composites were higher than those

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Fig. 2 – The Vickers hardness (A) and fracture toughness (B) of ZrO2 and ZrO2 –Al2 O3 composite specimens. Different letters indicate significant difference at p < 0.05.

strength values of ZrO2 –Al2 O3 (m) composite and ZrO2 –Al2 O3 (n) composite. The summary of mean biaxial flexural strengths and results from the Weibull characteristic biaxial strength and modulus is seen in Table 1. The Weibull plots are shown in Fig. 4 The Weibull plots that perform from the biaxial flexural strength results showed that the Weibull modulus (m) value of the ZrO2 –Al2 O3 (f) composite was the lowest compared to other groups, but characteristic strength (␴0 ) had the highest value. The m values of the ZrO2, ZrO2 –Al2 O3 (m) composite and the ZrO2 –Al2 O3 (n) composite were in a similar range.

3.4. Grain size and microstructure of ZrO2 –Al2 O3 composites

Fig. 3 – The biaxial flexure strength of ZrO2 and ZrO2 –Al2 O3 composite specimens. Different letters indicate significant difference at p < 0.05.

of ZrO2 –Al2 O3 (m) composite and ZrO2 –Al2 O3 (n) composites (p < 0.05). The fracture toughness values of the ZrO2 –Al2 O3 (m), ZrO2 –Al2 O3 (n) and ZrO2 –Al2 O3 (f) composite specimens were higher than that of the ZrO2 by approximately 4.2%, 6.7 % and 9.2%, respectively (Fig. 2B). There were no significant differences between ZrO2 -Al2 O3 (m) composite and ZrO2 –Al2 O3 (n) composite, and between ZrO2 –Al2 O3 (n) composite and ZrO2 –Al2 O3 (f) composite for fracture toughness (p > 0.05).

3.3. Biaxial flexural strength and Weibull analysis of ZrO2 –Al2 O3 composites Fig. 3 shows the biaxial flexural strength of ZrO2 and ZrO2 –Al2 O3 composites containing 10 vol % of the Al2 O3 particles/fibres. The biaxial flexural strength for sintered ceramic generally increased with addition of 10 vol % Al2 O3 . However, the one-way analysis of variance showed no statistically significant difference among all groups (p > 0.05). Nevertheless, in terms of the average values, the biaxial strength values for ZrO2 –Al2 O3 (f) composites were higher than the biaxial

Fig. 5 presents SEM images of the polished and thermally etched ZrO2 and ZrO2 –Al2 O3 composites with the addition of 10 vol % Al2 O3 . ZrO2 (the brighter phase) and Al2 O3 (the darker phase) were clearly evident on the image. The mean grain size appeared to be slightly decreased as the Al2 O3 was added to the ZrO2 ceramic. After measurement, pure ZrO2 had a slightly higher mean grain size of 0.32 ␮m compared to grain sizes of ZrO2 –Al2 O3 (m), ZrO2 –Al2 O3 (n) and ZrO2 –Al2 O3 (f), which were 0.28 ␮m, 0.27 ␮m and 0.28 ␮m, respectively.

3.5. Vickers hardness, fracture toughness and fracture surface morphologies on Al2 O3 fibre content The hardness and fracture toughness of sintered ZrO2 and ZrO2 –Al2 O3 (f) composite specimens with additive contents of 0−20 vol % are shown in Fig. 6A and B. The sintered specimens showed relatively high hardness and fracture toughness until the volume content of the Al2 O3 fibre was increased to 15 vol %. When the concentration of Al2 O3 fibre increased above 20 vol %, the mechanical properties were decreased, showing that the critical value of Al2 O3 fibre corresponds to approximately 15 vol %. The hardness and fracture toughness of the ZrO2 -Al2 O3 (f) composite were (maximum value) 11.85 ± 0.29 GPa and 7.63 ± 0.12 MPa m1/2 , respectively, when 10 vol % Al2 O3 fibre was added. Fig. 6C represents photographs of a sintered ZrO2 and ZrO2 –Al2 O3 (f) composite specimen with varying Al2 O3 fibre contents. At 20 vol % of Al2 O3 fibre reinforced ZrO2 , not only low densification but also phenomena related to inho-

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Table 1 – The mean biaxial flexural strength (MPa) and Weibull analytical results of ZrO2 and ZrO2 -Al2 O3 composite specimens. Group

Mean strength (MPa)

Characteristicstrength (␴0 ) (MPa)

Weibull modulus (m)

R2 - value

ZrO2 ZrO2 –Al2 O3 (m) ZrO2 –Al2 O3 (n) ZrO2 –Al2 O3 (f)

1037.2 ± 122 1178.3 ± 110.6 1185 ± 114.5 1191.8 ± 258.4

1089 1228 1236 1306

10.07 12.02 11.70 4.70

0.92 0.93 0.94 0.95

ZrO2 –Al2 O3 (m), Al2 O3 in microparticle form; ZrO2 –Al2 O3 (n), Al2 O3 in nanoparticle form; ZrO2 –Al2 O3 (f), Al2 O3 in microfiber form.

Fig. 4 – Weibull-distribution of pure ZrO2 (A), ZrO2 –Al2 O3 (m) (B), ZrO2 –Al2 O3 (n) (C) and ZrO2 –Al2 O3 (f) (D).

mogeneous warping after sintering were observed because of imperfect sintering, and all specimens (1.35 ± 0.05 mm in thickness) showed similar colour and translucency. Fig. 7 shows the fracture surface morphologies of ZrO2 and 5, 10, 15 and 20 vol % Al2 O3 fibre reinforced ZrO2 composite ceramics. From these micrographs, isolated pores (white dotted circles) were evident in the case of ZrO2 -Al2 O3 (f) composite ceramics (5, 10 and 15 vol %). Also, in the composite with a relatively higher content of Al2 O3 fibre (over 20 vol%) (Fig. 7E), agglomerations were observed (yellow arrows).

4.

Discussion

In dentistry, zirconia has been used for prosthetic infrastructures and implants, including crowns, abutments and bridges [29,30]. Previous studies have attempted to improve mechanical strength such as fracture toughness or damage tolerance by addition of alumina in ceria-stabilized TZP with alumina [6] or machined zirconia [31], as well as development of fibrereinforced ceramic composite systems [12–15].

Here, we examined various important features of zirconia that would be relevant to application in dentistry, focused on three different forms of Al2 O3 : ZrO2 –Al2 O3 (m), ZrO2 –Al2 O3 (n) and ZrO2 –Al2 O3 (f). First, we considered sintering shrinkage and relative density of specimens (Fig. 1). Improving shrinkage accuracy for ZrO2 has been an important challenge for the reliability of the restorations as the changes of dimensional and density result from the densification during sintering [32]. The shrinkage and relative density generally decreased with addition of Al2 O3 , indicating that the addition of Al2 O3 inhibits the densification of the ZrO2 matrix. The shrinkage measurements of the specimen thickness and diameter showed that the diameter shrinkage was higher than that of the thickness shrinkage in all groups. Sintering shrinkage of powder forms is influenced by various factors and by the composition. For example, for heavy metal particles have previously indicated that force will act towards the direction of gravity and influence the thickness shrinkage more than the diameter shrinkage [33,34]. The common process methods of fabricating ZrO2 in dentistry would involve the CAD-CAM (computer-aided design

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Fig. 5 – Microstructures of pure ZrO2 (A), ZrO2 –Al2 O3 (m) (B), ZrO2 –Al2 O3 (n) (C) and ZrO2 –Al2 O3 (f) (D), observed by scanning electron microscope (scale bar is 200 nm).

Fig. 6 – Mechanical properties of ZrO2 –Al2 O3 composite as a function of volume fraction of filler fibre to either Vickers hardness (A) or fracture toughness (B). Photographs of sintered on content of ZrO2 /Al2 O3 fibre composite (C).

and computer-aided manufacturing) process in two different methods. One method is by milling the fully sintered ZrO2 block using CAD-CAM. Another method is milling the presintered ZrO2 block using CAD-CAM, followed by allowing final sintering for improved mechanical properties [35]. The limitation of this study, however, is that such forms have not yet been investigated here. Further study regarding the shrinkage of the second sintering stage using pre-sintered block may provide additional information.

Nevertheless, this study clearly demonstrated the effect of Al2 O3 on mechanical properties of the ZrO2 –Al2 O3 composite (Fig. 2 and Fig. 3). The influences of Al2 O3 additions on mechanical properties are like the results of previous work [36–41]. Santos et al. reported that the hardness of ZrO2 –Al2 O3 composite increased linearly with the addition of Al2 O3 particles [36], and Kim et al. observed the improvement of flexural strength and fracture toughness as the Al2 O3 particle content increased up to 20 vol % [37]. Kirsten et al., who analyzed the

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Fig. 7 – Microstructures of fracture surface of pure ZrO2 (A) and ZrO2 –Al2 O3 (f) composite in different amounts; 5 (B), 10 (C), 15 (D) and 20 (E) vol% Al2 O3 fibre-reinforced ZrO2 composite. White circles are residual pores and yellow arrows are agglomerated Al2 O3 fibres. Scale bar is 200 nm (A, B, C and D) and 10 ␮m (E).

subcritical crack growth behavior of dispersion oxide ceramics, have revealed that substantial amount of Al2 O3 in a ZrO2 matrix produces a significant reduction of subcritical crack growth [38]. Therefore, homogeneously dispersed Al2 O3 created a hindrance in the crack propagation path and absorbed the crack propagation energy. Thus, the gradual increase of hardness and fracture toughness with the addition of Al2 O3 is attributed to the toughening mechanisms of crack bridging and phase transformation. The reliability of ceramics is an important factor for their use in the field of dental prosthesis. In general, a higher Weibull modulus value represents a narrow dispersion of the fracture strength, and thus more homogeneous flaw distribution and higher reliability [42,43]. For this study, the reliability of ZrO2 -Al2 O3 composite ceramics has been considered in terms of Weibull statistical analysis as suggest in ISO 6872 (Fig. 4). The Weibull analysis of ZrO2 and ZrO2 –Al2 O3 composite ceramics showed the high Weibull coefficient in ZrO2 ,

ZrO2 –Al2 O3 (m) composite and in ZrO2 –Al2 O3 (n) composite, indicating that the material shows high reliability. In contrast, ZrO2 –Al2 O3 (f) composite showed the lowest Weibull modulus. Therefore, these results show that in all composite groups, the strength reliability of ZrO2 –Al2 O3 (m) and ZrO2 –Al2 O3 (n) is higher than that of ZrO2 –Al2 O3 (f). Ideally, ZrO2 –Al2 O3 composites for dental application shall have both high level of mechanical strength but also structural reliability [42,43]. Despite the greater mechanical strength by ZrO2 –Al2 O3 (f) composite in terms of surface hardness, fracture toughness and biaxial flexural strength, the use of fibre-form of Al2 O3 has limitation in term of such structural reliability. The lower Weibull modulus may be due to the greater level of flaws and defects in the material as evident in Fig. 7 with higher vol% of fibre-form Al2 O3 . Also, the surface roughness of ceramic has been reported to significantly affect Weibull distribution [44]. Hence, the lowest Weibull modulus of ZrO2 –Al2 O3 (f) composite may also due to the sur-

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faces of the material that was not uniformly polished. Further study searching for optimal volumetric percentage of fibreform ZrO2 , that would enhance mechanical properties while maintaining structural reliability would be warranted in the future. Grain size influences the strength because the grain boundary is known to interact with dislocations. Generally, for most materials, an increase in strength has been linked with decreasing grain size according to the Hall-Petch equation. The presence of properly dispersed second-phase particles can produce a pinning effect and inhibit the grain growth of the matrix, thus contributing to the mechanical performance of the ceramic composite. In this study, the addition of Al2 O3 slightly restrained the ZrO2 grain growth (Fig. 5). Therefore, the dispersed Al2 O3 particles and fibre can act as grain boundary pinning agents in the ZrO2 matrix. The mechanical failure in ceramic matrix composites happens when a great number of cracks are present. The way to improve the mechanical property in ZrO2 -Al2 O3 composite ceramic composites could be the addition of Al2 O3 fibres. However, it is very important to obtain fibres without agglomerates to attain high densification during sintering. According to Fig. 6A and B the hardness and fracture toughness for ZrO2 –Al2 O3 composite increase until the volume content of the Al2 O3 fibre was increased to 15 vol%. The study confirmed the previous findings by Abdullah et al., who have reported stronger mechanical properties with increasing Al2 O3 whisker concentration up to 10 wt % [45]. Residual pores caused by the agglomeration of Al2 O3 fibre become a good pathway for crack propagation. Therefore, when the volume content of Al2 O3 increases to 20 vol %, the hardness and toughness decrease. According to previous reports, agglomeration of the whiskers arises due to stress caused by incompatibility between matrix and whiskers. These stresses occur with increasing whisker concentration [45,46]. Hence, within the limitations of this study and consideration of these findings, it has been evident that the mechanical properties for the application in dental restoration have been improved by adding appropriate amounts of the fibre form Al2 O3 into ZrO2 ceramics. With possible future studies, it is expected that these ZrO2 -Al2 O3 composites can be good candidates for dental restorations in the future.

5.

Conclusions

The ZrO2 –Al2 O3 composites containing different forms of Al2 O3 (microparticle, nanoparticle and microfibre) were developed to evaluate the effect of such differences on the physical and mechanical properties of ZrO2 ceramics. Compared with the ZrO2 , the mechanical properties of ZrO2 –Al2 O3 composites with fibre-form of Al2 O3 (ZrO2 –Al2 O3 (f)) were improved, even though densification was decreased. Additionally, the mechanical properties of ZrO2 and the ZrO2 –Al2 O3 (f) composite with varying Al2 O3 fibre content was studied and the addition of Al2 O3 fibre up to 10 vol % was found to improve the mechanical properties of ZrO2 –Al2 O3 composites. Still, the Weibull modulus (m) value of ZrO2 –Al2 O3 (f) composite was the lowest compared to other groups which would be a limitation of this material. This has been related to the content

of Al2 O3 fibre in the matrix which the agglomeration occurred as the contents increased above 20 vol %, and resulted in a decrease in hardness and toughness. This study speculated that the addition of right amount of Al2 O3 microfibre into the ZrO2 matrix enhanced the mechanical properties of the ZrO2 -Al2 O3 (f) composite for dental applications.

Acknowledgements This study was supported by the Yonsei University College of Dentistry (6-2019-0021).

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Please cite this article in press as: Seo J-Y, et al. Enhanced mechanical properties of ZrO2 -Al2 O3 dental ceramic composites by altering Al2 O3 form. Dent Mater (2020), https://doi.org/10.1016/j.dental.2020.01.014