SINTERING BEHAVIOR OF LZS GLASS-CERAMICS

Materials Science Forum Vols. 727-728 (2012) pp 1028-1033 Online available since 2012/Aug/24 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.727-728.1028

SINTERING BEHAVIOR OF LZS GLASS-CERAMICS Jaime Domingos Teixeira1,a, Antonio Pedro Novaes de Oliveira1,2,b Lourival Boehs2,c, Francielly Roussenq Cesconeto1,d, Cristina Siligardi3,e, Manuel Alfredo Pereira4,f 1

Group of Ceramic and Glass Materials (CERMAT) 2 Department of Mechanical Engineering (EMC) Federal University of Santa Catarina (UFSC) Campus Universitário – Trindade, 88040-900 Florianópolis, SC, Brazil 3 Department of Materials and Environmental Engineering (DIMA) University of Modena and Reggio Emilia (UNIMORE) Via Vignolese 905, 41100, Modena, MO, Italy 4 Federal Institute of Santa Catarina (IFSC) Avenida Mauro Ramos, 950 – Centro, Florianópolis, SC, Brazil a

[email protected], [email protected], [email protected], [email protected], [email protected], [email protected]

d

Keywords: ceramics, glass-ceramics, sintering, crystallization.

Abstract. Sintering and crystallization behaviors of a LZS glass-powder were investigated by means of thermal shrinkage, differential thermal analysis, X-ray diffraction as well as density and mechanical properties measurements. The melted glass, 9.56Li2O.22.36ZrO2.68.08SiO2 (wt%) first was cast into water to provide a frit for milling. The milled glass powder (mean particle size 5.0 µm) was then uniaxially pressed at 100 MPa and the obtained samples were isothermally sintered in the 800-950°C temperature range in air for appropriated time intervals (15-120 min). Sintering was found to start at about 640°C and crystallization took place just after completion of sintering and was almost complete at 920°C. The glass powder compacts crystallized into lithium and zircon silicates so that glass-ceramics with relative densities between 84 and 99% were obtained reaching maximum hardness and bending strength values of 8 ± 0.5 GPa and 214 ± 20 MPa, respectively. Introduction Glass-ceramic materials are polycrystalline solids containing residual glassy phase made by controlled crystallization of particular formulated glasses [1,2]. These materials show several important properties, such as low coefficient of thermal expansion, high abrasion and scratch resistances and good chemical and thermal shock resistances. For these reasons glass-ceramic materials have found applications in different fields of society and industry [1-3]. The usual technology for fabrication of glass-ceramics materials consists on preparation of monolithic glass articles by the application of glass technology and subsequently crystallization [4,5]. However, this technology requires great investments and can be justified only for high production volumes [3]. On the other hand, the production of glass-ceramic materials processed from glass powders and consolidate by sintering and crystallization seems to be a valid alternative since is possible to use the same equipments of a ceramic plant for the production of components with very complicated shapes [3,6,7]. The process involves the glass melting and its cooling (parent glass frit); pulverization; forming by ceramic technology and sintering and crystallization. Consequently, sintered glass-ceramics shows, over the crystalline and glassy phases, residual porosity. In the midst of these materials, the Li2O-ZrO2-SiO2 (LZS) system has been widely studied [8-17] since it is technologically important in some areas including glass-ceramics, refractories and glazes [18]. As the properties of the glass-ceramic materials are determined by their chemical composition and resulting microstructure, the details of the sintering behavior and crystals formation in the LZS system are of interest in particular in applications where mechanical properties such as mechanical strength and hardness are important requirements.

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Thus, this article reports results for the behavior between sintering and crystallization of a 9.56 wt% Li2O, 22.36 wt% ZrO2, 68.08 wt% SiO2 glass powder composition formed by uniaxial pressing. This work is a preliminary step aiming to find the optimized heat-treatment conditions to obtain high performance LZS glass-ceramics for a specific application. Experimental Procedures A glass with wt% composition 9.56 Li2O.22.36 ZrO2.68.08 SiO2 was prepared from ZrSiO4, Li2CO3 and SiO2 as raw materials. A batch to produce 2 kg glass was placed in a platinum crucible and melted at 1550°C in 7 h in an electric furnace. The melt was cast into deionized water to provide a frit for milling. Subsequently, the glass frit was dried and dry-crushed in an alumina ball-mill for 1 h, and then sieved to yield a powder of particle size lower than 100 µm. The crushed glass powder was then wet milled in laboratory alumina ball mill so that a powder with an average particle size of 5.0 µm was found by laser scattering analysis (CILAS 1064 L). Chemical composition of the LZS parent glass powder was determined by X-ray fluorescence spectroscopy (Philips, PW 2400) and by atomic absorption (UNICAM, Solar 969) for the lithium determination. The resulting suspension, from the wet milling processes, was also dried to a humidity content of about 6% and then uniaxially pressed by means of a hydraulic press at 100 MPa in a 10 mm diameter steel die. Thermal linear shrinkage (TLS) of compacted samples was measured by using an optical dilatometer (Expert System Solutions, MISURA ODHT) at 10°C.min-1 in air. The crystallization temperature of the glass powders were measured using differential thermal analysis, DTA (Netzsch, STA EP 409) in air at a heating rate of 10°C.min-1 using powdered specimens of about 60 mg in an alumina sample holder with an empty alumina crucible as reference material. Compacted samples were also isothermally sintered in an electric laboratory furnace (Jung, Model TB 213) in air for appropriated time intervals (15-120 min) at T = 800º, 850º, 900º, and 950ºC (±5ºC). After completion of sintering, samples were air-quenched to room temperature. The theoretical density (ρt) of the sintered samples was measured by using a He-picnometry and the apparent density (ρap) was measured by the Archimedes principle in water immersion at 20oC (ASTM C373-72). Taking in account the apparent density and theoretical density measurements, the relative density (ρr) and porosity were calculated (ρr=ρap/ρt). To investigate the crystalline phases formed during heat-treatments, powdered samples were analyzed with a Philips PW 3710 Xray (Cu Kα) powder diffractometer (XRD). Bending strength (σf) of the sintered samples was measured in a test machine (EMIC, Model DL 2000) using very well finished (machined and polished with 1 µm alumina paste) rectangular samples (10 samples) with nominal dimensions of 4 x 8 x 36 mm. In this case, samples were formed by slip casting. In order to determine the hardness of the obtained materials, samples were mounted in epoxy resin and surfaces were ground smooth, and then polished with 1 µm alumina paste. Subsequently, microhardness measurements were performed with a Vickers automatic hardness tester (SHIMADSU, Model HMV) at a load level of 3.0 N. A total time of 15 s was used for each indentation. Each value of hardness is the average of ten measurements with the respective standard deviation. Results and Discussion The parent glass (frit) batch composition and the analyzed composition are given in Table 1. Table 1. Theoretical and analyzed chemical composition of the LZS glass. Glass composition

Constitutive oxides [wt%] Al2O3

CaO

Fe2O3

K2O

Li2O

Na2O

SiO2

TiO2

ZrO2

Theoretical

-

-

-

-

9.56

-

68.08

-

22.36

Analyzed

0.374

0.028

9.44

0.034

67.68

0.011 0.053

0.068 22.31

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As it can be seen from table the produced glass frit shows a chemical composition very closed to the calculated composition since other oxide in minor amount are presents and they came from the used raw materials and milling processes. Even so, these oxides in minor amounts did not affect, apparently, the expected performance of the LZS glass-ceramics. The thermal linear shrinkage curve obtained by dilatometry measurements is shown in Fig. 1.

Linear thermal shrinkage (%)

0

-5

-10

-15

-20

-25 0

200

400

600

800

1000

1200

Temperature (°C)

Fig. 1. Shrinkage-temperature curve of a LZS glass powder compact. From Fig. 1 it can be seen that sintering starts at ~640°C and is completed over a short temperature interval (∆T ≈ 280°C). Crystallization starts soon after the completion of sintering. A slight increase in volume is observed up to ~1080°C when melting of the crystalline phases takes place. Fig. 2 which is the DTA curve of the glass powder shows almost the same effects: an endothermic inflection at ~633°C related to the glass transition; an exothermic peak having maximum at ~920°C related to the crystallization processes; and an endothermic effect having maximum at ~990°C related to the melting of crystalline phases.

Fig. 2. DTA plot of the investigated LZS glass powder composition.


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The effect of sintering time and temperature on the densification is shown in Fig. 3. Fig. 3 shows that the relative density of sintered samples increases as the temperature decreases from 950° to 800°C and it is almost constant as the heating time increases for samples sintered at 850°C.

Fig. 3. Relative density-sintering time curves of the investigated LZS glass composition. For samples sintered at 800 and 950°C the relative density decreases as the heating time increases. Same behavior, as before, can be observed for samples sintered at 900ºC in the 15-60 min time range. However, for samples sintered at 900ºC in the 60-120 min time range the relative density increases. This indicates that most of the densification takes place in a narrow range of heating time (15 min) even if for samples sintered at 900°C the relative density was almost the same reaching a maximum of 94% after 120 min in the sintering temperature. The changes in the relative density with the sintering temperature and time can be related to the crystallization processes. In fact, XRD analyses performed showed that the main crystalline phases formed in the studied sintering temperatures are zircon (ZrSiO4), file JCPDS n°6-266 and lithium dissilicate (Li2Si2O5) files JCPDS n°24-651, 30-767. The relative crystallinity reached a maximum of 80% for samples sintered at temperatures higher than 850°C for 120 min. As the crystalline phases have different specific volumes respect to the residual glassy phase, porosity can be generated also in this case over the glassy phase. Fig. 4 shows microhardness (HV) values for samples sintered in the studied temperature and time intervals.

Fig. 4. Microhardness as a function of temperature and sintering times for the studied LZS glass.

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No significant variations were observed in the microhardness values as the sintering temperature and time changed. However, the magnitudes of the microhardness values are of practical interest and they reached a maximum of 8 GPa for samples sintered at 900°C for 120 min. Even if samples sintered at 800°C for 30 min showed the highest relative density value (99%), crystallinity was not enough since the microhardness value in this case was 6 GPa. The relatively high microhardness value obtained in the sample sintered at 900°C for 120 min can be associated to the zircon crystals which show hardness values between 9 and 10 GPa and bending strength of 320 MPa according to measurements conducted by Shi et.al [19] in high purity hot-pressed zircon ceramics. Considering the best combination of crystallinity/hardness and porosity results samples with appropriated dimensions in a number of 10 were formed and sintered at 900°C for 120 min for bending tests. The average bending strength value obtained was 214 ± 20 MPa that is very interesting in a number of practical applications. In fact, according to the literature [1] glass-ceramic materials show, generally, bending strength values between 70 and 350 MPa and ceramics with high alumina content between 212 and 353 MPa. Summary Sintering and crystallization of a glass with composition 9.56 Li2O.22.36 ZrO2.68.08 SiO2 (wt%) was investigated. Sintering was found to start at about 640°C and crystallization took place just after completion of sintering and was almost complete at 920°C. The glass powder compacts crystallized into lithium and zircon silicates so that glass-ceramics sintered at 900°C for 120 min with relative density of 94% were obtained reaching hardness and bending strength values of 8 ± 0.5 GPa and 214 ± 20 MPa, respectively. Acknowledgements The authors are grateful to Capes and CNPq/Brazil for funding this work. References [1] Z. Strnad: Glass-ceramic materials - Glass science and technology Vol. 8 New York, Elsevier, 1986. [2] W. Höland and G. Beall, Glass-ceramic technology. American Ceramic Society, Westerville, Ohio, 2002. [3] E.M. Rabinovich: Journal of Materials Science Vol. 20 (1985), p. 4259. [4] P.W. Mc Millan: Glass Ceramics. 2nd Edn, edited by Academic Press, New York, 1979. [5] J.H. Simmons et al:Nucleation and Crystallization in Glasses, in: Advances in Ceramics. Edited by American Ceramic Society, Vol. 4 1982. [6] D.M. Miller, US Patent 3926648 (1975). [7] C.I. Helgesson: Science of Ceramics Vol.8 (1976), p. 347. [8] A.P.N. Oliveira., T. Manfredini, C. Leonelli andG.C. Pellacani: Journal of the American Ceramic Society Vol. 79 (4) (1996), p.1092. [9] A.P.N. Oliveira., T. Manfredini and C. Leonelli: Thermochimica Acta Vol. 286 (1996), p. 375. [10] A.P.N. Oliveira and C. Leonelli: Physics and Chemistry of Glasses Vol. 39 (4) (1998), p. 213. [11] A.P.N. Oliveira., T. Manfredini, G.C. Pellacani and C. Leonelli: Journal of the American Ceramic Society Vol. 81 (3) (1998), p. 777. [12] A.P.N. Oliveira, C. Lira, R. Marimbondo, L. Pandini and O.E. Alarcon: Cerâmica Informação Vol. 5 (1999), p. 78. [13] A.P.N. Oliveira, O.E. Alarcon, T. Manfredini, G.C. Pellacani and C. Siligardi: Physics and Chemistry of Glasses Vol. 41 (2) (2000), p. 100. [14] A.P.N. Oliveira and T. Manfredini:. Journal of Materials Science Vol. (36) (2001), p.1. [15] A.P.N. Oliveira, C. Siligardi and T. Manfredini: American Ceramic Society Bulletin Vol. 83 (4) (2004), p. 9401.


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[16] A.P.N. Oliveira, O.R.K. Montedo, G.M. Reitz, F.M. Bertan, D. Hotza and C. Siligardi: American Ceramic Society Bulletin Vol. 83 (8) (2004), p. 9201. [17] A.P.N. Oliveira and T. Manfredini: Ceramic Society Bulletin Vol. 88 (4) (2009), p.28. [18] P. Quintana and A.R. West: J. Br. Ceram. Soc. Vol. (80) (1981), p. 91. [19] Y. Shi, X. Huang and D. Yan: Ceramics International Vol. 23 (1997), p. 457.

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