Optimization of mechanical and tribological properties of a dental SiO2–Al2O3–K2O–CaO–P2O5 glass-ceramic

Journal Pre-proof Optimization of mechanical and tribological properties of a dental SiO2–Al2O3–K2O– CaO–P2O5 glass-ceramic Gaoqi Wang, Kun Fu, Shouren Wang, Bingbing Yang PII:

S1751-6161(19)31363-3

DOI:

https://doi.org/10.1016/j.jmbbm.2019.103523

Reference:

JMBBM 103523

To appear in:

Journal of the Mechanical Behavior of Biomedical Materials

Received Date: 18 September 2019 Revised Date:

3 November 2019

Accepted Date: 4 November 2019

Please cite this article as: Wang, G., Fu, K., Wang, S., Yang, B., Optimization of mechanical and tribological properties of a dental SiO2–Al2O3–K2O–CaO–P2O5 glass-ceramic, Journal of the Mechanical Behavior of Biomedical Materials (2019), doi: https://doi.org/10.1016/j.jmbbm.2019.103523. 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 Ltd.

Optimization of mechanical and tribological properties of a dental SiO2-Al2O3-K2O-CaO-P2O5 glass-ceramic Gaoqi Wang 1☆, Kun Fu 1☆, Shouren Wang 1*, Bingbing Yang

1

School of Mechanical Engineering, University of Jinan, Jinan 250022, China

Abstract Dental glass-ceramics with main crystal of fluorapatite are usually employed as veneering porcelain. However, failure of the porcelain happens clinically in the form of fracture or excessive wear. The aims of the study were to evaluate the influence of microstructures of a SiO2-Al2O3-K2O-CaO-P2O5 fluorapatite glass-ceramic on its mechanical properties and tribological behaviors, and to improve the comprehensive performance by adjusting content and sintering process. The glass-ceramics were fabricated by sintering method with different CaO contents and heat treatment regimes. Phase compositions and crystal length of specimens were characterized by X-ray diffractometer and scanning electron microscope. Combined with mechanical properties, friction and wear behaviors in both dry and artificial saliva lubrication conditions were investigated. The results show that different content of CaO and heat treatment temperature could change crystallinity of main fluorapatite crystal. Larger crystallinity improves the mechanical properties, significantly influencing friction and wear behaviors. The specimens with 6.0 wt. % CaO and sintered at 1100 °C have the best comprehensive performance, which show excellent mechanical properties and wear resistance. Keywords: Friction and wear; Glass-ceramics; Mechanical property; Microstructure

☆

These authors contributed equally to this work.

*

Corresponding authors at: School of Mechanical Engineering, University of Jinan, 336 Nanxin-

zhuang West Road, Jinan 250022, China. Tel.: +86 531 82765476; fax: +86 531 87154048. E-mail addresses: [email protected] (Shouren Wang).

1

1. Introduction Glass-ceramics are materials composed of various crystalline phases and glass matrix. Due to their biocompatibility and appearance similar to teeth, glass-ceramics have been widely used in dental restorative (Khvostenko et al., 2016; Khvostenko et al., 2013), including mainly lithium disilicate glass-ceramics, leucite glass-ceramics, fluorapatite glass-ceramics, and feldspathic glass-ceramics (Sailer et al., 2007). Glass-ceramics with a main crystalline phase of fluorapatite (Ca5(PO4)3F, FAp) are commonly employed as veneering material of all-ceramic crown or partial denture. Fluorapatite glass-ceramics have many advantages: 1) they have similar morphology with enamel and have better biocompatibility; 2) they can release trace fluorine to prevent dental caries; 3) they have better acid and alkali corrosion resistance (Ghosh et al., 2015). However, failure happens clinically in forms of fracture or excessive wear due to their mechanical and tribological properties, which also limit their application range (Stijacic et al., 2018). Thus, the reliability of the glass-ceramics needs to be improved and optimized. Strength is an important property of glass-ceramics, which determine the load bearing capacity of the denture (Baino and Verné, 2017; Wang et al., 2016). The commonly-used veneering materials fluorapatite glass-ceramics and feldspathic glass-ceramics usually have strength of about 60-90 MPa, which are almost the lowest values among the dental ceramics (Zhang et al., 2017). Although restorations can be made by monolithic zirconia ceramic (with strength of above 1100 MPa) without veneering in posterior region, veneering porcelain is indispensable in anterior repairation clinically because of their aesthetic properties. Therefore, it is a key factor to improve the strength of the veneering glass-ceramic to reduce the failure probability. In addition, hardness and elastic modulus are also mechanical properties affecting the clinical performance of the ceramic material. Similar hard-

2

ness and elastic modulus with natural enamel is ideal for a glass-ceramic, because significant larger hardness and elastic modulus could cause increasing stresses within the enamel and give rise to damage (Sailer et al., 2015). The wear of ceramic materials is usually the result of small deformation, fracture of the surface, and the physicochemical interaction between the surface and the environment during friction (Baino and Verné, 2017; Sailer et al., 2007), thereby significantly affected by mechanical properties of the material (Borrero-Lopez et al., 2019; Oh et al., 2002). Many literatures have studied the wear of dental ceramic materials. Zhang et al. (Zhang et al., 2018) employed thermally controlled crystallization to change crystal size to improve wear resistance and mechanical properties of lithium disilicate glass ceramics, showing lithium disilicate glass-ceramic with medium-sized crystals of 0.92 (±0.14µm) had the highest wear resistance and mechanical properties. Aboushahba et al. (Aboushahba et al., 2018) reported that zirconia ceramics had lower wear than lithium disilicate ceramics, because zirconia ceramics were able to maintain a smooth contact surface to reduce the friction due to the high hardness. Theocharopoulos et al. (Theocharopoulos et al., 2013a; Theocharopoulos et al., 2013b) showed that leucite reinforced glass-ceramics with small-sized crystals had higher flexure strength and resulted in a reduction in wear. Zhang et al. (Zhang et al., 2018) reported commercial fluorapatite glass-ceramic has lower wear resistance than feldspathic glass-ceramic. Introducing certain oxide additives and changing preparation process are effective methods to achieve better mechanical properties of glass-ceramics. Mollazadeh et al. (Mollazadeh et al., 2013) studied the effects of ZrO2 and BaO on the mechanical property of apatite glass-ceramics, confirming that the highest strength of the glasses occurred in BaO containing glass due to it crystallizing

3

more extensively rather than the others. Wang and Zheng et al. (Wang et al., 2010; Zheng et al., 2008) found that lithium disilicate glass-ceramics with P2O5 of 1 mol.% content achieved better fracture strength. Chen et al. (Chen et al., 2011) optimized crystallization and flexural strength of leucite glass-ceramic by controlling the crystallization parameters. Denry et al. (Denry and Holloway, 2014) found the crystallization temperature and activation energy of crystallization for FAp would increase with the Ca/Al ratio in the SiO2-Al2O3-P2O5-MgO-Na2O-K2O-CaO-CaF2 system. However, limited studies are available regarding the relationship between crystalline structure and mechanical and tribological properties of fluorapatite glass-ceramics. In

our

previous

study

(Fu

et

al.,

2018),

the

crystallization

mechanism

of

SiO2-Al2O3-K2O-CaO-P2O5 system fluorapatite glass-ceramics was studied. Based on that, the influencing mechanism of phase compositions and crystal length on the mechanical and tribological properties were further investigated in the present study. The results indicate that the mechanical properties and wear resistance of the glass-ceramic are improved by optimizing the composition and fabrication process. 2. Materials and methods 2.1 Material preparation The sintering process of the glass-ceramics has been described in detail in the previous study (Fu et al., 2018). Chemical compositions of samples C1-C5 with different compositions are shown in Table 1. Reagent grade powders of SiO2, Al2O3, Na2CO3, K2CO3, CaCO3, CaHPO4, CaF2, and so on were used as starting materials. The calcium oxide content increased from 3.8% (in wt. %) to 6.9%, while other contents were kept constant. The sintering temperatures of samples C1-C5 were all 1100 °C for 30 min. Based on our preliminary experiments the influence of temperature on the crystalliza-

4

tion processes of samples with various compositions may be different, but the main trends were similar. Therefore, 4 groups of samples (marked R1-R4) with the same composition as C1 were fabricated in different heat treatment temperatures from 830 ℃ to 1100 ℃, which are listed in Table 2. After weighing and mixing batches, the starting materials were melted in an electric furnace at 1600 °C for 3 h (SIOMM, SQFL-1700, China), and quenched in distilled water. The obtained frits were milled to glass powers and sieved by a 48 µm mesh sieve. The powers were pressed under 20 MPa to form disk (15 mm×15 mm×2 mm) and bar (35 mm×4 mm×3 mm) shape specimens, and then sintered following the parameters shown in Table 2. Table 1. Chemical compositions of sample C1-C5 (wt. %) (Fu et al., 2018). NO.

SiO2

Al2O3

Na2O

K2O

P2O5

ZrO2

F

TiO2

CeO2

Li2O

B2O3

ZnO

CaO

C1

56.6

14.6

8.6

4.2

4

1.5

0.7

1

0.8

0.2

1

3

3.8

C2

56.6

14.4

8.5

4.1

4

1.5

0.7

1

0.8

0.2

1

3

4.2

C3

55.6

14.3

8.5

4.1

4

1.5

0.7

1

0.8

0.2

1

3

5.3

C4

54.6

14.4

8.6

4.2

4

1.5

0.7

1

0.8

0.2

1

3

6.0

C5

54.1

14.4

8.4

4

4

1.5

0.7

1

0.8

0.2

1

3

6.9

Table 2. Heat treatment parameters of sample R1-R4. Samples

Crystallization temperature (°C)

Crystallization time (min)

Heating rate (°C/min)

R1

830

30

5

R2

930

30

5

R3

1000

30

5

R4

1100

30

5

2.2 Characterization 2.2.1 XRD analysis The crystalline phases were distinguished by X-ray diffractometer (XRD, Siemens, D500, Germany). The powdered glass-ceramics were analyzed at a scanning rate of 2°/min using Cukα radiation (λ=0.15405 µm) at 40 kV and 40 mA in the two theta range 10° to 70°. The step size was

5

0.02°. In addition, peak separation technology and Rietveld refinements were used to obtain the crystallized volume fractions (crystallinity) of fluorapatite by Jade 6.0 within the error range of about ± 2% (Wang et al., 2018). 2.2.2 SEM observation The glass-ceramics were corroded in 4 vol. % HF solution for 60 s to acquire microstructures using field emission scanning electron microscope (SEM, JEOL, JSM-7610F, Japan). In order to evaluate the porosity of the samples, glass-ceramics were ground, polished and observed using SEM at low magnification. 2.2.3 Density test The density was tested by the Archimedes drainage method. All glass-ceramics samples were polished for the density test. Density (ρ, g/cm3) was calculated using the equation as follows (Ge et al., 2017; Han et al., 2018) w ×ρ

ρ= ws -w1 1

(1)

2

where ρ1 is density of distilled water at room temperature (0.9958g/cm3), ws is the dry weight of sample (g), w1 is weight of sample when it is immersed in water but not suspended (g), and w2 is the weight of sample when it is immersed in water and suspended (g). 2.2.4 Microhardness test Vicker’s microhardness (HV) was measured using a microhardness tester (Optical Instrument Corp, HXD-1000TMSC/LCD, China) by loading 500 gf for 15 s on glass-ceramics after grinding and polishing. It was calculated using the equation as follows HV=1.85544×

P

(2c)2

where P is the load (N), and c is impression half-diagonal (µm).

6

(2)

Fig. 1. Schematic diagram of (a) flexural strength test; (b) elastic modulus test.

2.2.5 Flexural strength test The flexural strength ( f , MPa) of the material was tested by four-point bending method with an outer span of 30 mm, and inner span of 15 mm on a micro-computer controlled universal testing machine (MTS Systems Corp, CMT 5305, USA). After sintering, the parallelepiped specimens were grinded with SiC papers (from 500-1500 grid). The final size of samples was 35 mm×4 mm×3 mm. The displacement rate was 0.5 mm/min (shown in Fig. 1 a). The flexural strength was calculated using the equation as follows (Cesar et al., 2006)

σf =

3PL 4bd2



(3)

where P is the fracture load (N), d is the specimen thickness (mm), 3 mm, b is the specimen width (mm), 4 mm, and L is the outer span (mm), 30 mm. 2.2.6 Elastic modulus test The elastic modulus was tested via electronic strain gauge by four-point bending method. Same as the flexural strength test, the sintered parallelepiped specimens were grinded with SiC papers (from 500-1500 grid). Then strain gauge (Donghua Test Corp, BX120-3AA, China) was attached to the center of specimen with size of 35 mm×4 mm×3 mm. Then the specimen was positioned on the

7

universal testing machine and loaded to a maximum loading of 60 N with a displacement rate of 0.1 mm/min. Wires of the strain gauges were connected to a signal collecting system (Cooperative Technology Corp, XL2118C, China) through quarter bridge circuits and modulators (Li et al., 2002). During loading, strain and stress were measured with the signal collecting system (shown in Fig. 1 b). It was calculated using the equation as follows (Bocchini et al., 2010) E=

L(P2-P1) bd2 (ε2-ε1)

×10-3

(4)

where P1 is the initial load (N), P2 is the maximum load (N), ε1 and ε2 are strains under P1 and P2 (dimensionless). 2.2.7 Friction and wear tests Relative motion between enamel was a typical low-speed reciprocating friction. Thus friction and wear tests were conducted on a pin-on-disc reciprocating horizontal tribometer (RTEC, MFT-50, USA). Many studies showed that different individuals had different angles of the cusps, which made the contact stress changed during wear. Therefore, the geometry of cups had a great influence on wear. Wassell (Wassell et al., 1994) reported that steatite produced similar coefficients of friction to enamel (correlation coefficient, r = 0.98). The wear rate of the steatite abrader was slightly greater than that against enamel, but the two abraders were reasonably correlated (r = 0.94). Therefore in this research, steatite ceramics was a suitable substitute for enamel. Steatite (Enstatite, MgSiO3, PDF 73-1937) ceramics were manufactured by Tianbu High Frequency Ceramics Corp (Haimen, China). The shape of steatite abrader was shown in Fig. 2 (a). The length was 18 mm, and the diameter of the ball head was 3.5 mm. Each test glass-ceramics sample was grounded, using abrasive papers, from 200, 500, 600, to 1200 grit, and polished. The tribometer is shown schematically in Fig. 2 (b). 90 min test on each specimen was conducted in artificial saliva lubrication and dry environment re-

8

spectively. Tests were conducted with a normal load of 35 N and reciprocating amplitude of 1.5 mm at room temperature. This sequence was repeated at 2Hz for 90 min for each specimen. The compositions of artificial saliva are shown in Table 3. The depth of worn surface and wear volume were obtained using a white light interferometer (RTEC, UP-WLI, USA); The average specific wear rate WS (mm3/N·m) was defined by (Lhymn, 1987) Ws=

∆V

(5)

FN S

Where ∆V (mm3) is the average wear volume defined by five data, FN (N) is the normal load, S (m) is the sliding distance. Wear morphology was analyzed by SEM. Energy dispersive spectroscopy (EDS, Oxford Instruments, X-max, UK) was carried out to detect the compositions of worn surfaces.

Fig. 2. (a) steatite abrader; (b) schematic illustration of pin-on-disc reciprocating tribometer.

Table 3. Compositions of artificial saliva (Zheng and Zhou, 2007). Compositions

Quantity (g)

NaCl

0.4

KCl

0.4

CaCl2·2H2O

0.795

NaH2PO4·2H2O

0.78

Na2S·9H2O

0.005

Urea

1

Distilled water

1000

9

The density, microhardness, flexural strength, elastic modulus, and friction and wear tests were repeated in five independent assays. The friction coefficients and wear results were analyzed with one-way analysis of variance (ANOVA) followed by Tukey’s HSD test at a significance level of α= 0.05 (SPSS Statistics ver.25, SAS, USA). 3. Results and discussion 3.1 Phase composition and microstructure The microstructures of the glass-ceramics C2 and C4 after being etched are shown in Fig. 3 (a) and (b). Needle-like FAp surrounded by remaining glassy matrix could be observed. As shown in Fig. 4 (a), the main crystalline phase of C1-C5 was FAp (Ca5(PO4)3F, PDF 73-1727), however with CaO content increasing the minor crystalline phase changed from low albite (NaAlSi3O8, PDF 84-0982) into anorthite (Na0.25Ca0.71Al2Si2O8, PDF 78-2330). The mean length of FAp is shown in Fig. 4 (b), which were determined from more than 200 crystals parallel to the observed surface by analyzing SEM micrographs. Obviously, the length did not linearly depend on calcium oxide content. The mean crystal length of sample C4 was 368 nm, which was the largest among all the samples.

Fig. 3. Partial SEM micrographs of the glass-ceramics after being etched. (a) sample C2 sintered at 1100 °C; (b) C4 sintered at 1100 °C; (c) R1 sintered at 830 °; (d) R3 sintered at 930 °.

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Fig. 4. (a) XRD patterns of samples; (b) mean crystal length of C1-C5; (c) mean crystal length of R1-R4.

For R1 and R3 specimens (Fig. 3 c,d), a large number of FAp crystals in glassy matrix were spherical or tiny needle-like. As shown in Fig. 4 (a), for sample R1-R4 crystalline phases were FAp and low albite. With the increase of sintering temperature, peaks attributed to FAp became sharper because the crystals became larger (Fig. 4c). The crystallinity of FAp also raised at a higher sintering temperature as shown in Table 4. Fulcher equation (Diaz-Mora et al., 1998) has an explanation of the dependence of the crystal length upon the temperature log10 n=-A+B/(T-T )

(6)

where n is the viscosity of the glass, A and B are constant, T0 is the initial temperature, and T is the final temperature (°C). Obviousely, increasing the sintering temperature made the viscosity decline, promoting ions to move quickly to crystallize, which improve the crystallinity. Therefore, it can be concluded that the heat treatment temperature has an important influence on the morphology of the crystal and crystallinity of the FAp glass-ceramic.

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Table 4. The crystallinity of glass-ceramcis. Sample

Treatment schedule

Crystllinity (%)

R1

830°C/30 min

9.78

R2

930°C/30 min

12.64

R3

1000°C/30 min

16.37

R4 (C1)

1100°C/30 min

24.64

C2

1100°C/30 min

25.87

C3

1100°C/30 min

23.24

C4

1100°C/30 min

23.93

C5

1100°C/30 min

22.94

3.2 Physical properties Fig. 5 (a) shows densities of the samples, in which C1-C5 fluctuated around 1.68 g/cm3, while R1-R4 increased from 1.642 g/cm3 to 1.684 g/cm3. Due to the sintering method, a lot of pores were produced inside the sample (as shown in Fig. 6). With the increase of sintering temperature, the size of pores was smaller. Thus the higher heat treatment temperature made the sample slightly denser. As shown in Fig. 5 (b-d), the microhardness, flexural strength and elastic modulus of specimens with different CaO content (C1-C5) fluctuated in a certain range, but apparently increased in higher temperatures (R1-R4). Compared with the crystal length in Fig. 4 (b) and (c), the trends in density, microhardness, flexural strength and elastic modulus were relevant to the crystal length.

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Fig. 5. Densities, microhardness, flexural strength and elastic modulus of specimens.

Fig. 6 The pores in the typical samples. (a) R2 sintered at 930°C; (b) R3 sintered at 1000°C; (c) C1 sintered at

1100°C; (d) C5 sintered at 1100°C.

During sintering process, residual stress was generated inside the material due to the different thermal expansion coefficient between glassy phase and FAp crystal (the thermal expansion coeffi-

13

cient of minor crystalline phase was ignored due to very low content). The residual stress of the material could be described as follows (Selsing, 1961) 2∆T(α2 -α1 )

P = 1+v

1 +2×(1-2v2 ) E1 E2



(7)

where ∆T is the cooling range from sintering temperature to room temperature, α1, α2 are thermal expansion coefficients of the glassy phase and FAp crystal, respectively, E1, E2 are elastic modulus of the glassy phase and FAp crystal, respectively, v1, v2 are Poisson’s ratios. According to reference (Baik et al., 2005), α1 is 65×10-7/°C, α2 is 90×10-7/°C. FAp possessed higher elastic modulus than glass substrate, thence E2 was larger than E1. v1 and v2 were small and have little impact on the result, so it is assumed v1 ≈ v2. Therefore, P is positive. The residual stress produced by spherical FAp crystal could be decomposed into radial (σr) and tangential (σt) stress on glassy phase, which could be described as follows r

σr = P(R)3 P r

σt = - 2 (R)3

(8) (9)

where r is the crystal radius, R is the distance from center of crystal to glassy phase subjected stress. σr is positive and σt is negative when P>0. Thus, along radial direction of crystal, the glassy phase was subjected to tensile stress; while along tangential direction, the glassy phase was subjected to compressive stress. In this case, cracks tended to propagate bypass the FAp crystal (Wei and Becher, 1984). Due to high mechanical properties of FAp crystal, cracks were hard to propagate. Thereby FAp crystal could enhance the fracture toughness of the glass-ceramic. According to Eq. (8) and (9), for the same position in the glassy phase (R were equal), the larger the FAp crystal radius was, the larger σ was and the smaller the σt was, making the crack harder to propagate. Meanwhile, the modified rule of mixtures used to predict the strength of short-rod crystal composite material is de-

14

scribed as follows (Fu and Lauke, 1996) σcu=χ1χ2Vfσfu+σmVm (10) where σcu and σfu are the ultimate strength of the glass-ceramics and FAp crystal, and are assumed to be equal for different samples. Vf and Vm are the volume fraction of the FAp and glassy phase, respectively. χ1 and χ2 are the crystal orientation and crystal length factor, as a result χ1χ2 is the FAp crystal efficiency factor for the strength of the glass-ceramics. FAp crystal was randomly distributed in glass-ceramics. So χ1 of different samples should be equal (especially C1-C5). As FAp crystal has higher strength than glass matrix, the larger the values of χ2 (crystal length factor) and Vf (fraction of the FAp) are, the higher the glass-ceramic strength will be. Besides, in the process of crack propagation, larger FAp showed pull-out effect to absorb the fracture energy to enhance the material fracture toughness due to typical needle-like crystal, according to the morphology of fracture surface after strength test (shown in Fig. 7). As pointed out in previous studies (Fischer et al., 2008; Henry and Hill, 2004) larger needle-like crystals interlaced with glassy matrix showed an interconnected network, further enhancing the mechanical properties of the material.

Fig. 7. Micrographs of fracture of the sample C4 after strength test.

The above is indicated that the mechanical properties of the glass-ceramic have positive correlation with crystallinity, which is influenced by crystal length and number. The specimens sintered at 1100℃ (C1-C5) had larger crystallinity than other samples, improving the mechanical properties. C4

15

performed the largest average flexural strength and elastic modulus among all the specimens. 3.3 Friction and wear behavior 3.3.1 Friction coefficients and wear rate The average friction coefficients are summarized in Fig. 8 (a). ANOVA revealed that friction coefficient was significantly affected by friction environment (p < 0.05). In dry friction condition, the coefficients of sample R1-R3 increased gradually. Subsequently, the coefficients of R4 decreased rapidly from about 0.65 of R3 to 0.55. Samples C1-C5 had little differences in the friction coefficients, in which C4 showed the lowest. In artificial saliva lubrication condition, 0.098-0.130 decrease of friction coefficients were observed for samples compared with dry friction conditions. The whole trend in the coefficient of different samples was similar with that in dry condition. Depths of worn surfaces in dry condition are shown in Fig. 8 (b). The scar depths of sample C1-C5 was below 107.3 µm, in which the wear of C4 was the minimum. The scar depth decreased from 141.7 µm of sample R1 to 107.3 µm of sample R4. Despite coefficient of R1-R4 increasing gradually, microhardness of samples was also increased to improve wear resistance of materials. The scar depths in artificial saliva lubrication condition are shown in Fig. 8 (c). They decreased 10.3-28.7 µm compared with these in dry conditions. With the raising heat treatment temperature, the scar depth of sample R4 also significantly decreased. The scar depths of C1-C5 specimens were in the range from 74.4 µm to 83.6 µm, showing little differences. The average specific wear rate of sample was shown in Table 5, which has similar trend as wear depth.

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Fig. 8. (a) Friction coefficients of samples; (b) depths of worn surfaces in dry friction condition; (c) depths in saliva lubrication condition. Values with the same uppercase or lowercase letters are not significantly different at (α= 0.05)

Table 5. The average specific wear rate of samples (×10-3mm3/N·m). R4 R1

R2

R3

C2

C3

C4

C5

(C1) Dry friction

0.56

0.54

0.46

0.43

0.41

0.39

0.33

0.35

Saliva lubrication

0.48

0.45

0.43

0.39

0.37

0.36

0.32

0.32

3.3.2 Wear morphology Figs. 9 (a-d) show the typical wear morphology of R1, R3, C4 and C5 in dry condition. Severe delamination wear debris appeared on the surfaces of R1 and R3. Because of larger mechanical strength, slighter delamination of sample C4 was observed compared with R1 and R3. The shallow pits and surface spall on the surfaces of C4 indicate the occurrence of fatigue wear. The mechanism

17

of wear could be further analyzed by element transfer between steatite and FAp ceramics. Steatite ceramics contains Mg, Si, O and other elements, while FAp ceramics and artificial saliva do not contain Mg element. It meant that the accumulation of Mg on the worn surface could be used to confirm that whether adhesive wear occurred. Fig. 10 (a-d) shows the distribution and content of Mg on the worn surfaces in dry condition. The worn surfaces of R1, R2, and C5 had Mg of 1.8 wt. %, 1.4 wt. %, and 1.3 wt. %, respectively, implying the severe adhesive wear. The content of Mg was much lower on surface of C4, suggesting the lower extent of adhesive wear. Consequently, adhesive wear was the dominate wear mechanism of R1 and R3 specimens. While C4 and C5 specimens mainly showed fatigue wear and accompanied by mild adhesive wear.

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Fig. 9. Typical SEM micrographs of worn surfaces. (a-d) are R1, R3, C4, and C5 in dry condition, respectively; (e-h) are R1, R3, C4, and C5 in saliva lubrication condition, respectively.

The worn morphologies of samples in artificial saliva condition are shown in Fig. 9 (e-h). There were distinct characters where plenty of strong ploughs along the direction of sliding appeared in the wear scars other than delamination in dry condition. For sample R1 and R3, deep ploughs combined with plastic deformation were observed. The surface of C4 and C5 showed slighter ploughs than samples R1 and R3 attributed to their larger strength and hardness. Fig. 10 (e-h) represents the compositions of the worn surfaces in artificial saliva condition. Compared with dry condi-

19

tion, each surface had almost no Mg element, which meant that there was few adhesive wear. The main factors that caused adhesive wear were load and temperature. It is known the heat transfer coefficient of air convection is 3-10 W/(m2·°C), while that of water convection is 200-1000 W/(m2·°C). As a result, the heat generated in the progress of friction could be rapidly transmitted by saliva to prevent from the happening of adhesive wear. Abrasive wear was the main wear mode for all samples in saliva lubrication condition.

Fig. 10. The contents of elements on the worn surfaces of typical samples. (a-d) are R1, R3, C4, and C5 in dry condition, respectively; (e-h) are R1, R3, C4, and C5 in saliva lubrication condition, respectively.

3.4 Relationship between microstructure and properties In dry friction condition, wear mechanism was mainly adhesive wear for samples R1 and R3, which had smaller crystal length and less degree of crystallinity than C4 and C5. It has been mentioned above that the FAp crystal has better mechanical properties than glass substrate, indicating smaller dimension and quantity of FAp crystal lead to worse strength and hardness performance. In the process of friction, tribo-pairs moved to each other to generate the instantaneous high temperature zone to make the glassy phase soften. At this time, the adhesion force between the two surfaces increased. The surface material fell off and adhered to the worn surfaces due to the low surface

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hardness and strength, which resulted in adhesive wear. With the increase of heat treatment temperature, the samples C4 and C5 crystallized better, which greatly increased the quantity and length of FAp crystal and obtained higher mechanical properties. The higher hardness and strength made the material harder to produce microcrack, reducing the probability of delamination (Albashaireh et al., 2010; Zhou and Zheng, 2008). Thus, the adhesive wear between the frictional pairs was significantly decreased, leading to lower friction coefficient and wear rate. Saliva reduced the friction coefficient and wear rate of the samples, and changed the wear mechanism from adhesive and fatigue wear to abrasive wear dominated by the plough effect. The ploughs on surface of samples sintered at low temperature (R1 and R3) were deeper with more plastic deformation (Fig. 9). The improvement of mechanical properties due to high sintering temperature (C4 and C5) made it harder to produce plough and plastic deformation on the glass-ceramic surface, reducing the friction coefficient and wear rate. Sample C4, which contained 6.0 wt. % CaO and was sintered at 1100 °C, had the best comprehensive performance among all the specimens. It not only has large FAp crystal size and shows the best flexural strength among the samples, but also exhibits low friction coefficient and wear rate in both dry and lubrication conditions. It has potential on reducing the fracture possibility and retarding the wear progress of all-ceramic denture, but the clinical performance of the material should be further evaluated by in vivo experiments. 4. Conclusion The influence of microstructures of SiO2-Al2O3-K2O-CaO-P2O5 fluorapatite glass-ceramics on its mechanical properties and tribological behaviors were evaluated, and the comprehensive performance was improved by adjusting content and sintering process. The conclusions were summarized

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as: 1) Content of CaO could change the length of main fluorapatite crystal, but have little influence on the minor crystalline phase. With the increase of heat treatment temperature, crystallinity of fluorapatite increases gradually. 2) Larger crystallinity of fluorapatite improves the mechanical properties of the glass-ceramic. Increasing heat treatment temperature can also decrease the pore size of the glass-ceramic and made it denser. Therefore, adjusting the CaO content and raising the heat treatment temperature are valid ways to improve the mechanical properties. 3) Content of CaO and heat treatment temperature significantly influence friction and wear behavior by changing the material crystal structure and mechanical properties. The specimen which contains 6.0 wt. % CaO and sintered at 1100 °C have the best comprehensive performance, which shows excellent mechanical properties and low friction and wear in both dry and lubrication conditions. Acknowledgment This work was supported by National Natural Science Foundation of China (No. 51872122), Shandong Provincial Natural Science Foundation, China (No. ZR2019QEM006), Project of Shandong Province Higher Educational Science and Technology Program, China (No. J18KZ002), Taishan Scholar Engineering Special Funding, China (2016-2020). Conflict of Interest The authors deny any actual or potential conflict of interest. References Aboushahba, M., Katamish, H., Elagroudy, M., 2018. Evaluation of hardness and wear of surface treated zirconia on enamel wear. An in-vitro study. Future Dental Journal 4, 76-83.

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.