The Crystallization and Fracture Toughness of Transparent Glass-ceramics with Various Al2O3 Additions for Mobile Devices

ZHENG Weihong ,LI Changqin ,YUAN Jian ,TIAN Peijing ,TENG Fei ,WANG Weizheng ,PENG Zhigang,WANG Wei

(1.State Key Laboratory of Silicate Building Materials,Wuhan University of Technology,Wuhan 430070,China;2.Shahe Research Institute of Glass Technology,Shahe 054100,China;3.School of Automotive Engineering,Wuhan University of Technology,Wuhan 430070,China;4.Hubei Key Laboratory of Advanced Technology for Automotive Components,Wuhan University of Technology,Wuhan 430070,China;5.Hubei Collaborative Innovation Center for Automotive Components Technology,Wuhan University of Technology,Wuhan 430070,China)

Abstract: Li2O-Al2O3-SiO2 (LAS) glass-ceramics were prepared by a melting method.Effects of different Al2O3 content on the structure,crystallization,transmittance and fracture toughness of LAS glassceramics were investigated by means of XRD,FESEM and other methods as well.The results showed that the glass transition temperature and crystallization temperature of samples increased as the content of Al2O3 increased from 4.1 wt% to 13.1 wt%,which restrained the precipitation of lithium disilicate crystals.The main crystalline phase of glass-ceramics transformed from lithium disilicate and petalite to silicon dioxide,which reduced the fracture toughness of glass-ceramics.When the Al2O3 content was 7.1 wt%,the specimen had outstanding transmittance and fracture toughness.The transmittance was 90.32%.The fracture toughness was 1.13 MPa • m1/2.Compared with high-alumina glass,the fracture toughness of the glass-ceramic was greatly improved,and it could be used as a new type of protective material for mobile devices.

Key words: transparent glass-ceramics;crystallization;Al2O3;lithium disilicate;petalite

1 Introduction

In recent years,the screen of mobile devices has increased from 4 inches to 6.5 inches,and especially 2.5D and 3D full-screen curved glass had been adopted.However,the fracture toughness of common high-alumina glass is 0.66-0.76 MPa•m.Due to the greater size,curved shape and insufficient fracture toughness of high-alumina glass,it was easy to be broken when impacted.With the development of mobile devices,the currently used high-alumina glass can’t meet the requirements.Therefore,it is necessary to prepare a new type of glass-ceramics to improve the fracture toughness of the mobile devices through the precipitation of crystals.

Generally,glass-ceramics with large crystal size and high crystallinity had high fracture toughness.But at this time,glass-ceramics were opaque or translucent,which made it unable to transmit light and thus cannot be applied to mobile devices.Nevertheless,mobile devices required a relatively high transmittance of glass material.Therefore,it is necessary to prepare a glass-ceramic with high transmittance and high fracture toughness.

Transparent glass-ceramics usually were MgOAlO-SiO(MAS) system,oxyfluoride system and LiO-AlO-SiO(LAS) system.The precipitated crystals of the MAS system transparent glass-ceramics were mostly spherical crystals,such as spinel crystals,the size of crystals was about 50-70 nm.And with the increase of the crystallization temperature,the crystal growth rate was fast,which easily led to devitrification.Hu B

et al

prepared the MgO-AlO-SiO-KO-TiO-ZrOsystem glass-ceramics by the melting method,the crystal phases were MgAlOand AlZnOspinel,the hardness was 874.8 kgf/mm,but the transmittance was only 75%.The crystals precipitated from the oxyfluoride transparent glass-ceramics were mostly nano-granular crystals,such as LaFcrystals,the crystal size was about 5-10 nm,and its mechanical properties were not outstanding.The precipitated crystals of LAS system glassceramics were usually

β

-quartz solid solution and

β

-spodumene.

β

-quartz solid solution crystals were mostly granular nanocrystals with a size of about 20-30 nm,with high optical performance and low mechanical properties.

β

-spodumene crystals were mostly platelike or column-like crystals,with a size of about 0.5-2 µm.The crystal size was relatively large,resulting in devitrification of the glass-ceramics.At present,most of researches on LAS transparent glass-ceramics were concentrated on the

β

-quartz solid solution system.In addition,petalite and lithium disilicate crystals can also be precipitated in LAS glass-ceramics.Lithium disilicate crystals were often plate-like and column-like crystals,which can provide high mechanical properties because the crystals form an interlocking microstructure distributed in the glass.Petalite was often a layered crystal with a refractive index similar to that of the base glass and lithium disilicate.Both crystal sizes can be less than 100 nm,which can increase the transmittance of the glass.LAS glass-ceramics with lithium disilicate and petalite as the main crystal phases will have excellent transmittance,fracture toughness and other properties.It is an excellent protective material for mobile devices.However,the main crystal phases formed in glass-ceramics depend on the composition of parent glass and heat treatment process.The content of AlOhad a prominent effect on the crystallization of glassceramics,especially for MAS and LAS system glassceramics.There are many reports on the effect of AlOcontent on the performance of MAS glass-ceramics and oxyfluoride glass.Ke XF

et al

studied the effect of AlOcontent on the hardness and fracture toughness of MAS glass-ceramics.The results confirmed that with the increase of AlOcontent,the hardness and fracture toughness of the glass increased significantly.Biswas K

et al

studied the effect of gradually adding AlOinstead of SiOon the structure,thermal and mechanical properties of SiO-BaF-KO-GdF-SbOsystem oxyfluoride glass.The results confirmed the structural modification with the gradual addition of AlOin the glass matrix revealing the dominant presence of [AlO]tetrahedral units in 10 mol%AlOsamples unlike in 5 mol% AlOsamples which exhibited the manifestation of [AlO]units.And the mechanical properties of the sample had increased.Fernandes HR

et al

studied the influence of AlOand KO content on the structure,crystallization and properties of LiO-SiOsystem glass-ceramics.The study confirmed that as the content of AlOand KO decreased,more lithium disilicate crystals will be precipitated.These research results show that with the increased AlOcontent,the structure of the glass is changed,which will affect the crystallization of glassceramics and further affect the performance of glassceramics.

At present,few reports systematically studied the effect of AlOcontent on the structure,crystallization and performance of LAS glass-ceramics with petalite and lithium disilicate as the main crystal phases.Therefore,in this paper,LAS glass-ceramics were prepared by the melting method and the influence of AlOcontent on the structure,crystallization and performance of LAS glass-ceramics were studied.Through research to determine an appropriate amount of AlOintroduced,a transparent glass-ceramic with high transmittance and fracture toughness with the main crystal phases of petalite and lithium disilicate was prepared.

2 Experimental

LAS1,LAS2,LAS3 and LAS4 samples in the LAS system were melted with 4.1 wt%,7.1 wt%,10.1 wt% and 13.1 wt% of AlOcontent,respectively.The chemical composition of samples was presented in Table 1.Laboratory reagent-grade LiCO,SiO,AlO,ZrO,SbO,CeO,BO,(NH)HPO,YOpowders were chosen as raw materials.These were placed in a platinum crucible,which was introduced into a preheated furnace at 450 ℃ to allow for removal of HO,COand NHfollowed by melting at 1 580 ℃for 3 h.Upon removal of the crucible from the furnace,melted glass was then poured into a preheated cast iron mould at 500 ℃ for 2 h to relieve internal stress.The glass block obtained by cooling to room temperature was cut into different dimensions samples.

The Raman spectra were recorded on a LABHRev-UV spectrometer using a He-Ne laser(532 nm) of power before the entrance optics.The measurements were performed on bulk samples and made in the range of 200-1 500 cmat room temperature.The glass frits were used for differential scanning calorimetry measurement with DSC-machine(model Netzsch 404 F3).The temperature range was from room temperature to 1 150 ℃ in a dry nitrogen atmosphere and the heating rate amounted to 10 ℃/min.The crystalline phases were determined by X-ray diffraction (XRD) analysis (Rigaku UltimaIV)with Cu-Kα radiation and scanning from 10° to 60°at a scanning speed of 5 °/min.The highly polished surface of the glass-ceramic sample was etched with 5 vol% hydrofluoric acid solution for the 30 s.After ultrasonic cleaning,samples were gold-sputtered and the microstructure was observed using FESEM(JSM-7500F).Transmittance value was expressed by real in-line transmission,which was measured in the 300-1 000 nm range by ultraviolet/visible spectrum spectrometer (Lambda 950).The fracture toughness of the sample was tested with the hardness tester (SCV-50A/T),the maximum applied load was 1 kgf,and the holding time was 10 s.Five measurements were made on each sample and averaged.Indentation fracture toughness was calculated according to the following formula:

where

IFR

is the indentation fracture toughness,MPa•m,

E

is the elastic modulus of the sample,GPa,and

F

is the indentation load,N,

c

and

c

is the crack length in the diagonal direction of the indentation,mm,

d

and

d

is the diagonal length of the indentation,mm.

3 Results and discussion

3.1 Raman spectra analysis

The Raman spectra of the four groups of glasses were shown in Fig.1.It was observed that with the increase of AlOcontent,Raman spectra of each sample had not been significantly changed.However,the shape and area of the peak had been changed,indicating that the glass structure had been changed.Three distinct bands can be found in the spectra at 488,956 and 1 076 cm.

Raman bands at 488 cmbelong to symmetrical valence vibrations of the Si-O-Si group.The most prominent bands at 1 076 and 956 cmbelong to the antisymmetric valence vibrations of the Si-O-Si group.In the glass,Alcan exist in two forms,forming an aluminum oxide tetrahedron [AlO]in a four-coordinate form,and forming an aluminum oxide octahedron[AlO]in a six-coordinate form.With the increase of AlOcontent,the vibration peaks around 956 and 1 076 cmgradually broadened because Alin the glass formed more aluminum oxide tetrahedral [AlO].[AlO]participated in the glass network to connect with[SiO]to fill the network,which resulted in the glass structure becoming more compact.

3.2 DSC analysis of basic glass

The DSC results of the base glass were shown in Fig.2(a) and Fig.2(b) was a partially enlarged view of the DSC curve at 600-800 ℃.

Fig.1 Raman spectra of basic glass with different amounts of Al2O3 content

Table 2 Characteristic temperature of samples with different AlO content

According to Fig.2,the characteristic temperature of the base glass was determined.As Table 2 showed,

T

represented the glass transition temperature,

T

represented the crystallization peak temperature,and Δ

T

represented the crystallizing ability of the glass.Fig.2 and Table 2 showed that as the content of AlOincreased from 4.1 wt% to 13.1 wt %,the

T

,

T

and Δ

T

gradually increased,indicating that the glass crystallization ability decreased.LAS1 and LAS2 had two crystallization peaks.LAS3 and LAS4 had only one crystallization peak.

T

gradually increased from 510.8 to 521.4 ℃,the first crystallization peak temperature (

T

) gradually increased from 687.6 to 768.3 ℃,the second crystallization peak (

T

)gradually weakened,and its temperature increased from 769.5 to 818.5 ℃.In this study,when the crystallization temperature was

T

,the crystal size grew too large,resulting in devitrification of the glass,so the crystallization temperature was chosen as the initial crystallization temperature of

T

.According to the DSC curve,in LAS1,the

T

was 510.8 ℃,and the initial crystallization temperature of

T

was 624.5 ℃.Therefore,the nucleation temperature was 550 ℃ and the crystallization temperature was 625 ℃.

Fig.2 DSC curves of basic glass with different amounts of Al2O3 content

3.3 XRD analysis of glass-ceramics

Fig.3 showed the XRD patterns of glass-ceramics nucleated at 550 ℃ for 6 h and crystallized at different crystallization temperatures for 1 h.XRD patterns of glass-ceramics with different AlOcontent when crystallized at 625 ℃ were shown in Fig.3(a).At this temperature,the samples were all transparent.Petalite and lithium disilicate crystals were precipitated in LAS1 and LAS2 samples with the content AlOincreasing from 4.1% to 7.1%.Petalite and cristobalite crystals were precipitated in the LAS3 sample,but no crystals were precipitated in LAS4.As shown in Fig.3(b),the crystallization temperature increased to 650 ℃,petalite and lithium disilicate crystals were precipitated in the LAS1 sample.While the petalite and lithium disilicate crystals were precipitated,a small amount of lithium metasilicate crystals were also precipitated in the LAS2 sample.Petalite,lithium metasilicate and cristobalite crystals were precipitated in the LAS3 sample,and the sample was devitrified.Quartz crystals were precipitated in the LAS4 sample.It showed that the precipitation of lithium disilicate was reduced with the increase of AlOaddition.

Fig.3 XRD patterns of glasses with different Al2O3 content crystallized at different crystallization temperatures for 1 h:(a) 625 ℃,(b) 650 ℃

3.4 Microstructure

Fig.4 showed the microstructure features of glasses with different AlOcontent when crystallized at 625 and 650 ℃.When crystallized at 625 ℃,the crystals in samples LAS1 and LAS2 (Fig.4(a) and (b))were petalite and lithium disilicate,and the crystal grains were uniform and compact.Lithium disilicate crystals were small cylindrical crystals that form a sheet-like structure in an interlocked state.The petalite crystals were small,the crystal size was 20-30 nm.The crystal distribution of LAS1 was denser than that of LAS2.The crystals in LAS3 (Fig.4(c)) were petalite and cristobalite.The crystal size was about 20-30 nm.No crystals precipitated in LAS4 (Fig.4(d)).

When crystallized at 650 ℃,the crystals in samples LAS1 (Fig.4(e)) were petalite and lithium disilicate,and the crystal grains were uniform and compact.Lithium disilicate crystals were cylindrical crystals that formed a sheet-like structure in an interlocked state.The crystals in LAS2 (Fig.4(f)) were petalite and lithium disilicate,mixed with a small amount of lithium metasilicate.The lithium disilicate crystals were small cylindrical crystals that formed a sheet-like structure in an interlocked state.The black pits corresponded to the lithium metasilicate crystal phase because the lithium metasilicate was soluble in the diluted hydrofluoric acid solution.The crystals in LAS3 (Fig.4(g)) were petalite,lithium metasilicate and cristobalite,and the crystal size was not uniform.The crystal size of cristobalite and lithium metasilicate was larger,ranging from 100 to 200 nm.The lithium metasilicate crystals were staggered between each crystal phase.The crystals in LAS4 (Fig.4(h)) were quartz,with uniform crystal grains,and with crystals under 20 nm,it was granular.

As the content of AlOincreased,it could be found that the precipitation of lithium disilicate crystals in the glass decreased,and the crystal structure changed from sheet-like to granular.

3.5 Transmittance of the glass-ceramic

Since no crystals were precipitated in the LAS4 sample treated at 625 ℃,the LAS3 sample was devitrified at 650 ℃.Therefore,the transmittance of the LAS4 sample treated at 625 ℃ and the LAS3 sample treated at 650 ℃ was not measured.Fig.5 showed that the transmittance of the glass increased with the content of AlOincreased.At 625 ℃,the transmittance increased from 89.74% to 90.37%.At 650 ℃,the transmittance increased from 89.77% to 90.32% and then to 91.44%.

Based on the theoretical equation of Rayleigh-Gans:

where

σ

is the total turbidity,

N

is the particle number density,

V

is the particle volume,

a

is the particle radius,

k

is 2π/

λ

,

n

is the refractive index of the crystal,and Δ

n

is the refractive index difference between the crystal and the host.

Therefore,it can be known that the transmittance of glass-ceramics is mainly determined by the following factors:

First,for a fixed glass-ceramic system,the difference in refractive index was basically fixed.In the system of petalite and lithium disilicate as the main crystalline phase,the refractive index of petalite crystal was 1.51,the refractive index of lithium disilicate crystal was 1.55,and the refractive index of the matrix glass was between 1.47 and 1.50.The refractive index difference Δ

n

between the glass phase and the crystal phase was less than 0.1.Therefore,it met the requirements and could have a higher transmittance.

Fig.4 FESEM images of glasses with different Al2O3 content crystallized at 625 ℃ and 650 ℃ for 1 h:(a)LAS1,625 ℃;(b)LAS2,625 ℃;(c)LAS3,625 ℃;(d)LAS4,625 ℃;(e)LAS1,650 ℃;(f)LAS2,650 ℃;(g)LAS3,650 ℃;(h)LAS4,650 ℃

Fig.5 Visible light transmittance curves of glasses with different Al2O3 content treated at different crystallization temperatures for 1 h:(a)625℃,(b)650 ℃

Second,according to the formula,as the size of the crystal decreases,the absorption and scattering of light by the crystal will decrease,and the transmittance of glass-ceramics will increase significantly.Therefore,the glass precipitates small enough crystals to have a higher transmittance.

According to XRD patterns,microstructure features and transmittance curve,it could be found that the transmittance difference of the four groups of glassceramics was small,and all had high transmittance.The precipitated crystals of LAS1 and LAS2 were mainly petalite and lithium disilicate,the crystal size was about 20-70 nm,the refractive index difference of crystal phase was less than 0.1,and the transmittance was higher than 89%.The increase of AlOcontent was not conducive to the precipitation of lithium disilicate crystals.In LAS2,the precipitation of lithium disilicate crystals decreased,so the transmittance was higher than that of LAS1.The crystals precipitated by LAS3 were petalite and cristobalite.At 625 ℃,the crystal size was less than 30 nm.The refractive index difference Δn between the glass phase and the crystal phase was reduced,and its transmittance was greater than that of LAS1 and LAS2.However,at 650 ℃,the crystal size increased to more than 100 nm,and at the same time,lithium metasilicate crystals were precipitated,resulting in the devitrification of the glass.In LAS4,the quartz crystals were precipitated,the crystal size was less than 20 nm.Therefore,it had the highest transmittance.

3.6 Fracture toughness of glass-ceramic

Fig.6 Fracture toughness curves of glasses with different Al2O3 content after crystallization at different crystallization temperatures for 1 h

As shown in Fig.6,when crystallized at 625℃,with the AlOcontent increased,the fracture toughness of the glass decreased.The fracture toughness decreased from 1.10 to 1.08 and then to 1.05 MPa•m,and the lowest fracture toughness in LAS4 was only 1.02 MPa•m.When crystallized at 650 ℃,the fracture toughness of the glass first decreased,then increased,and finally decreased.The fracture toughness of the LAS3 sample was the largest,and the fracture toughness of the LAS4 sample was the smallest.The fracture toughness of glass decreased from 1.16 to 1.12,then increased to 1.38 MPa•m,and finally decreased to 1.09 MPa•m.In Fig.7,the Vickers indentation photos of the glass showed its fracture toughness.With the increase of AlOcontent,the crack length of the indentations increased.Except for the LAS3 sample crystallized at 650 ℃,the sample had the shortest crack length and the largest fracture toughness,which corresponds to Fig.6.

Fig.7 Optical images showing the Vickers impressions with radial cracks under an indentation load of 1 kgf on the glassceramic samples in different states

Generally,the type of crystals in the glassceramics and the microstructure of the glass-ceramics affect the fracture toughness of glass-ceramics.With the increase of AlOcontent,the crystals precipitated in the glass were changed,so that the fracture toughness of the glass was also changed.It is found from the above spectra that as the content of AlOincreased,the fracture toughness of the glass mainly showed a downward trend.In LAS1,petalite and lithium disilicate crystals were precipitated,with uniform size and dense crystal grains.Lithium disilicate crystals interlocked to form a sheet-like structure.In addition,crack propagation can be effectively prevented,which played an important role in improving mechanical properties,so the fracture toughness of the LAS1 sample was high.In LAS2,petalite and lithium disilicate were precipitated as the main crystalline phases,and a sheet-like structure was formed.However,as the content of AlOincreased,the precipitation of lithium disilicate crystals was inhibited,and the lithium metasilicate crystals were precipitated,the crystal size increased,and the density was reduced,so the fracture toughness of the LAS2 sample had decreased.When crystallized at 625 ℃,the crystals of petalite and cristobalite in LAS3 were precipitated,and no lithium disilicate crystals had been precipitated.Its crystal size was small,and the crystal was spherical,crack propagation was rarely prevented,so the LAS3 sample had lower fracture toughness.However,when crystallized at 650 ℃,the crystal size increased greatly,reaching 150 nm,and the lithium metasilicate crystal was precipitated.The crystals were dense,the glass was devitrified,and the residual glass phase was reduced.Lithium metasilicate crystals were arranged in a staggered arrangement in the crystal phases of petalite and cristobalite to form an interlocking state,which prevented crack propagation more effectively.And the fracture toughness of the LAS3 sample was maximized.Quartz crystals were precipitated in LAS4,its crystal size was small,only 10-20 nm,and it was a spherical crystal,crack propagation was limited to be prevented,so its fracture toughness was the lowest.

4 Conclusions

In this study,LAS system glass-ceramics with different AlOcontent were prepared by the melting method.The structure,crystallization and properties of the glass-ceramics were studied,and the following conclusions were drawn:

With the increase of AlOcontent from 4.1 wt%to 13.1 wt%,the glass transition temperature was increased from 510.8 to 521.4 ℃,crystallization peak temperature (

T

) of the glass was increased from 687.6 to 768.3 ℃,and

T

was increased from 769.5 to 818.5℃,respectively.Therefore,the crystallization ability of glass-ceramic was decreased with the addition of AlOincreased.

With the increase of AlOcontent from 4.1 wt%to 7.1 wt%,petalite and lithium disilicate crystals were precipitated as the main crystalline phases in the glass-ceramics.When the content of AlOincreased from 7.1 wt% to 13.1 wt%,petalite and cristobalite or quartz crystals were precipitated as the main crystalline phase in the glass-ceramics.However,the formation of lithium disilicate was inhibited.When the addition of AlOwas 10.1 wt%,lithium disilicate was not precipitated.

When the content of AlOwas 7.1 wt%,petalite and lithium disilicate were precipitated in the glass when nucleated at 550 ℃ for 6 h and crystallized at 650 ℃ for 1 h.The crystal size was uniform and the crystal grains size was 40-70 nm.Lithium disilicate crystals interlocked to form a sheet-like structure,which resulted in the fracture toughness of glassceramics being 1.13 MPa•m.At the same time,it had a higher transmittance of 90.32%.