Effect of fluorine content on sintering and crystallisation behaviour of CaO–Al2O3–SiO2 glass ceramic system S. Banijamali, B. Eftekhari Yekta*, H. Rezaie and V. K. Marghussian The effect of fluorine content on the sinterability and crystallisation of glass belonging to CaO– Al2O3–SiO2 system was investigated. It was shown that increasing fluorine content promotes sinterability and crystallisation simultaneously. The Avrami exponent and activation energy for crystallisation of the most promising specimen were determined according to the Marotta method. The calculated Avrami exponent confirms a surface crystallisation mechanism. Results in the present paper suggest that with increasing fluorine content, the sinterability, crystallisation and mechanical properties of the system are improved whereas the chemical resistance is degraded. Keywords: Sinterability, Glass ceramics, Fluorine
Introduction Glass ceramics can be produced either by crystallisation of glass articles or by sintering and crystallisation of compacted glassy powders. In the case of sintered glass ceramics, well crystallised and completely dense products are desired.1–3 To obtain this condition, sintering should be performed before complete crystallisation of specimens. Therefore, it seems that by considering factors such as heat treatment programme, glass particle size and its composition, a desirable sintering and crystallisation procedure can be obtained.4–9 In this regard, Rabinovich found that glass powders with heterogeneous crystallisation tendency are more suitable than glasses that are homogeneously nucleated.10 It is assumed that in the latter case the overall rate of crystallisation will be fast and crystalline phases will be developed rapidly. In this case, the viscosity of specimen is increased gradually with crystallisation and so sintering is interrupted.10,11 In the present work, the sintering and crystallisation behaviour of CaO–Al2O3–SiO2 glass system was investigated as a function of CaF2 amounts. Glass ceramics belonging to this system present excellent mechanical and chemical properties and can be obtained from available and low cost raw materials.1,2 It seems that because of their high hardness, abrasion and chemical resistances, these glass ceramics are suitable for being used as tiles in the building industry.9 Although various nucleating agents for induction of bulk crystallisation in CAS glasses have been commentated, these nucleants are not very effective and CAS glasses usually crystallise from surface, which makes this Ceramic Division, Department of Materials Engineering, Iran University of Science and Technology, Tehran, Iran *Corresponding author, email
[email protected]
ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 5 July 2006; accepted 15 August 2006 DOI 10.1179/174367606X152679
glass system desirable for preparation of glass ceramic tiles by sintering route.1 The present work focused on using CaF2 as a nucleating agent and also flux, in a CaO–Al2O3–SiO2 glass system. Although CaF2 is a common nucleating agent, there is not enough information about its role in the kinetics and mechanism of crystallisation of glasses mentioned above. In this way the initial composition was chosen with respect to crystallisation of wollastonite and anorthite.2 Thus various amounts of CaF2 were added to the base composition and its optimum amount was determined in terms of the chemical and mechanical properties.
Experimental The chemical compositions of glasses shown in Table 1 were prepared from commercial grade CaCO3, a-Al2O3, silica and fluorine. The mixtures of raw materials were transferred to a zircon crucible and melted at 1450uC for 1 h in an electric furnace. Then the melts were water quenched and the resulted frit powders were wet milled to the required size (,75 mm). The particle size distributions of glass powders were characterised by a laser particle size analyser (Fritsch analysete 22). The glass powders were completely mixed with carboxy methyl celloluse (through a 1 wt-%CMC solution in water) and then were uniaxually pressed at 30 MPa in a 20 mm diameter cylindrical steel mould. Compacted powders were then sintered in an electric laboratory furnace between 850 and 1050uC, with temperature intervals of 50 K. In this way a constant heating rate of 7 K min21 and a soaking time of 1 h were used and then the furnace was allowed to cool down naturally to room temperature. Linear shrinkage and water absorption (EN 99) were measured to evaluate sintering. The thermal behaviour of the glasses was monitored by differential thermal analysis (Netzsch 404) using a heating rate of 10 K min21 in air. Alumina was used as the reference material.
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Crystalline phases precipitated in the sintered samples were identified using X-ray diffractometers (Jeol JDX-8030, Siemens D500). The bending strength was measured using three point bending method according to the C158 ASTM standard in MTS machine (10/M) at a crosshead speed of 0.6 mm min21. The average value is obtained from the measurement of five samples. The microhardness of the glass ceramics was measured using a Vickers tester (Buehler, Micromete 1) under an indentation load of 100 grf for 30 s. The average value was obtained from 10 indentations. The chemical resistance of the glass ceramics was examined according to a modified EN106 standard, in which chemical resistance was determined as the weight difference of specimens, before and after a chemical leaching. The microstructures of the sintered samples were evaluated by a scanning electron microscope (Cambridge S360, Phillips XL30) after polishing and etching in a 5%HF solution for 15 s.
Results and discussion 1 Differential thermal analysis of samples (,75 mm) at 10 K min21
Figure 1 shows the DTA profiles of glasses containing different amounts of CaF2, at a heating rate of 10 K min21. It can be seen that by increasing fluorine amount, the dilatometric softening point and crystallisation peak temperatures of glasses decrease gradually.
2 X-ray diffraction patterns of sintered samples at crystallisation peak temperature for 1 h
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3 X-ray diffraction patterns of SF9 sintered at different temperatures
It means that the viscosity of glasses decreases gradually with the addition of CaF2. It seems that by increasing fluorine content, non-bridging oxygen atoms are substituted by F2 ions and develop weaker Si–F bonds in the glass network.3,10 It can also be seen that by increasing fluorine content, a second weak exothermic peak appears in SF9 and SF12 samples. X-ray diffraction analysis indicated that this peak is related to the crystallisation of gehlenite (2CaO.Al2O3.SiO2). Furthermore, an endothermic peak can be observed at temperatures above 1050uC that is related to the liquidus and/or the melting temperature of crystalline phases. Increasing fluorine content is accompanied with enlargement of this peak area, which means introduction of more liquid phase to the system at nearly the same temperature. Figure 2 shows the X-ray diffraction patterns of fired compacted glass powders at their main DTA crystallisation peak temperature for 1 h. As it can be seen the same crystalline phases, i.e. wollastonite (CaO.SiO2), calcium aluminium silicate (CaO.Al2O3.SiO2) and anorthite (CaO.Al2O3.2SiO2) were precipitated at this peak temperature. The intensity of this peak increases with CaF2, presumably through the reduction of glass viscosity, which facilitates the mobility and diffusion of the ions. It is worth noting that the present authors could not track any fluorine compound by X-ray diffraction and SEM in the Table 1 Chemical composition of glasses, wt-% Glass
SiO2
Al2O3
CaO
CaF2
SF6 SF9 SF12
33.63 33.63 33.63
42.84 42.84 42.84
23.54 23.54 23.54
6 9 12
a SF6; b SF9; c SF12 4 Sintering behaviour of samples sintered at different temperatures for 1 h
specimens fired between dilatometric softening points, i.e. sintering temperature interval. These observations convince the authors that perhaps fluorine atom has not entered into a crystalline structure and has been remained dissolved in the glass matrix. Phase evaluation of the bodies during sintering indicated that after 1000uC, wollastonite and calcium aluminium silicate start to dissolve in the residual glasses and create the endothermic peaks in the DTA profiles. Figure 3 shows this trend in SF9 sample. Figure 4 shows the sintering behaviour of three samples. It can be seen that increasing the fluorine content can improves both the sinterability and crystallisation of glasses, simultaneously. It should be expected that the sinterability could be degraded with reduction of the crystallisation temperature.10 However, it seems that as the residual glass becomes richer in F2 ions with further crystallisation,12 more addition of CaF2 will facilitate the flow ability of the residual glass and so the sinterability of specimens. Figure 5 depicts the SEM microstructure of different samples sintered at 1000uC for 1 h. As it was expected, a finer grain size microstructure accompanies more addition of CaF2 and lower DTA crystallisation peak temperature. It can also be seen that surface
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6 Determination of crystallisation activation energy for sample SF9 by Marotta method
and finer crystallite size which take places with more CaF2 content. On the contrary, the chemical resistance of samples has been degraded with increasing CaF2, so glass ceramic SF12 possesses the least chemical resistance. This effect can be attributed to a weaker and greater amount of residual glass phase, which comes from its higher modifier F2 ions and also increases the glass–crystalline phase interfaces.13 Therefore, it seems that the glass ceramic SF9 was the most promising specimen in view of mechanical and chemical resistance points. The mechanism of crystallisation and activation energy for crystallisation of SF9 were determined according to the Marotta method14 lna~{E=RTp zconstant lnDT~{nE=RTzconstant
5 Images (SEM) of samples sintered at 1000uC for 1 h
crystallisation of glass particles governs the process as crystalline phases have been started to precipitate at glass particle surfaces. Table 2 shows the results of measurement of mechanical and chemical properties of the glass ceramics. The bending strength and Vickers microhardness of the sintered glass ceramics are controlled by their crystallinity, relative density and microstructure that are influenced by CaF2 amounts. As it can be observed, the mechanical properties of glass ceramics were improved with increasing CaF2 content. This improvement is attributed to more crystallisation, better sinterability,
here DT, Tp, E, n and R are related to deviation of each point of crystallisation peak from baseline, crystallisation peak temperature, activation energy for crystallisation, the Avrami exponent and gas constant respectively. The results have been shown in Figs. 6 and 7. The activation energy for crystallisation and the Avrami exponent which was extracted from the above mentioned method were 625.7 kJ mol21 and 0.8 respectively. The value of the Avrami exponent is close to a unity, which confirms the SEM results. The obtained Avrami exponent value is in agreement with those obtained by other researchers. However, the calculated activation energy for crystallisation is slightly more. It is probably related to the composition differences and the usage of finer powders by them.11
Conclusions The main results of this research are listed as follows.
Table 2 Mechanical and chemical properties of glass ceramics sintered at 1000uC Properties
SF6
SF9
SF12
Bending strength, MPa 107.75 123.15 127.33 Vickers hardness, GPa 5.40 5.64 5.95 Chemical resistance (weight loss), % 2.07 2.20 2.28
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7 Variation of lnDT versus T21 for determination of Avrami exponent
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1. Addition of CaF2 to the base glass was associated with the following results: the dilatometric softening point of glasses decreased with CaF2 and their sinterability increased with it; the endothermic peak (which deduced here as the liquidus temperature of system) became sharper and shifted to lower temperature with CaF2 and there were not any characteristics peaks of CaF2 and its compounds in the XRD patterns and the present authors could not track them down in the specimens fired at the temperatures between the dilatometric softening point – sintering temperature interval by SEM. Therefore, it seems that fluorine not only acts as a nucleating agent but also improves crystallisation by decreasing viscosity. 2. The kind of crystalline phases which precipitated were not influenced by the amount of CaF2 but increased by it. 3. While the sinterability and mechanical properties of glass ceramics were improved with CaF2 enhancement, the chemical resistance was degraded. 4. The Avrami exponent and SEM results indicate that surface crystallisation governs the procedure of precipitation.
Effect of fluorine on sintering and crystallisation of CaO–Al 2 O 3 –SiO 2
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