Crystallization and properties of some CaO-Al₂O₃-SiO₂ system glass-ceramics with Y₂O₃ addition

ZHENG Wei-hong (郑伟宏), CHENG Jin-shu (程金树), QUAN Jian (全 健)

LOU Xian-chun (楼贤春), LIU Jian (刘  健)

Key Laboratory of Silicate Materials Science and Engineering, Wuhan University of Technology，

Ministry of Education, Wuhan 430070, China

Received 10 April 2006; accepted 25 April 2006

Key words:

CaO-Al2O3-SiO2; glass-ceramics; Y2O3 doped; crystallization;

Calcium aluminum silicate (CAS) glass-ceramics have excellent wear-resistant and chemical durability, and have achieved great industrial and economic importance [1-3]. However, there are still a series of problems at present. Firstly, the high sintering temperature and long sintering time are not energy-saving. Secondly, the poor mechanical properties led to the thickness of products over 10 cm, which made the cost expensive. Therefore reducing sintering temperature and improving mechanical properties are urgent to be solved.

         It is reported that rare-earth oxide can be added into ceramics or glass-ceramics as flux to promoted crystallization. For example, HU and LIANG[4] prepared LAS glass ceramics by adding CeO₂ as flux and the results showed that addition of 5%(mass fraction, the same below if not mentioned) CeO₂ serving as a flux could promote crystallization. The glass transition temperature and the crystallization peak temperature decreased. The transformations of glass to β-quartz and of β-quartz to β-spodumene were also accelerated. YAO and QIU [5] investigated the effect of three kinds of rare-earth oxide (Y₂O₃, La₂O₃ and Sm₂O₃) on the sintering and mechanical properties of alumina ceramics. The results show that rare-earth oxide improves the sintering process and increases the relative densities of alumina ceramics, the flexural strength and fracture toughness reach over 430 MPa and 5.15 MPa?m^1/2, respectively.

In this paper, we prepared CAS glass-ceramics doped with Y₂O₃ and investigated the effects of Y₂O₃ addition on the crystallization and mechanical properties of glass-ceramics.

2 Experimental

The initial materials were analytical grade reagents SiO₂, Al₂O₃, CaCO₃, BaCO₃, ZnO, Na₂CO₃, B₂O₃ and Y₂O₃. The detailed compositions of these glasses are given in Table 1[2,3,6-8]. The calculated proportions were thoroughly mixed and melted in Al₂O₃ crucible in an electrical furnace at about 1 480 ℃ (depending on glass composition) for 2 h. The melts were quenched into glass particles whose diameter were 0.5-6 mm, and were then dried up.

The glass particles were paved into the fireproof

Table 1  Chemical composition of glass-ceramics with different Y₂O₃ (mass fraction, %)

molds (7 cm×5 cm×1.5 cm) and heat-treated in a silicon-molybdenum furnace. CAS glass-ceramic is a top grade decorative material, which requires excellently flat and bright surface. The flatness and brightness have a direct relation with treated temperature and holding time. In this paper, adopted different sintering temperatures but the same holding time to obtain the same optimal flatness and brightness for the glass-ceramics. The heating temperature was raised from room temperature to optimal sintering temperature and held for 1 h, with a heating rate of 6 ℃/min. The resultant samples were annealed at about 600 ℃ for 30 min.

The crystalline phase of specimen was identified by X-ray diffraction analysis (XRD, D/Max-Radiffractomer, Rigaku, Japan) with Cu K_α radiation. The specimens for microstructural observation were cut from the crystallized glass body. The surface morphologies of specimens, polished and chemically etched in HF acid (3%) at room temperature for 30 s, were examined by scanning electron microscopy (SEM, JSM-5610LV). Density of samples was determined by Archimedes’ immersion method at room temperature. The bending strength was measured by an MTS-810 Ceramic Test System in air with an internal spanning of 30 mm and a crosshead speed of 0.5 mm/min at room temperature. In order to obtain reliable statistical data, at least 10 measurements were made on each sample.

3 Theoretical analysis

Glass-ceramics may be considered composite materials made up of different crystalline phases (ci) and a residual glass (g). The volume of a glass-ceramics, V_gc, is practically an additive function of the volumes of the vitreous and crystal phases present[9].

                             (1)

As a consequence, the density of the glass-ceramic can be related to the percentage and densities of the different phases by the formula as

                           (2)

where  ρ_gc is the density of the glass-ceramic, ρ_g is the density of the residual glass phase and x_ci and ρ_ci are the mass percent and the density of the crystal phase i, respectively.

         In this glass-ceramic, only wollastonite was precipitated. So Eqn.(2) can be described as

                              (3)

where  ρ_w is the density of wollastonite. Under real conditions, only a portion of the parent glass is transformed to a crystal phase. It is assumed that the volume of the parent glass, V_g, consists of two parts: V_gt with the same composition as the crystal phase which is to be formed and V_gr with the same composition as the parent glass, V_g.

So the percentage crystal phase (x) can be obtained through

                        (4)

And the volume fraction of crystalline phase (V_f) can be determined by

                         (5)

Fig.1 shows the XRD patterns of samples thermally treated at different temperatures. The main reflection at 2θ=30.033? might be attributed to β-CaSiO₃ (a=10.10, b=11.05, c=7.305, α=99.53, β=100.56, γ=83.44). Additional reflections attributable to β-CaSiO₃ also occur at 23.230?, 25.362?, 26.923? and 28.938?. These reflections were accorded with No.42-0547 JCPDS card. The main phases of four samples were all wollastonite despite of different Y₂O₃ contents.

Fig. 1 XRD patterns of samples thermally treated at different temperatures

Fig.2 (a) shows the SEM micrograph of CASY-0 with crystals having size of about 6 ?m. The micrograph of CASY-2 is shown in Fig.2(b). The crystals are significantly larger than those precipitated in CASY-0, which indicates that the addition of Y₂O₃ by 2 % can enhance crystal growth. As shown in Fig.2(c), the microstructure changes further in CASY-3.25 with Y₂O₃ 3.25 % addition. The size of crystals becomes larger.

ionally, needle-like shaped wollastonite oriente and interlock, with equal size, can be discerned. It can be seen from Fig.2(d) that the crystals continues to grow and the oriented and interlocked microstructure changes.

4.2 Mechanical properties

Fig.3 shows the bending strength of the samples investigated. The values show that the rupture strength increases from 84 MPa to 87.85 MPa with 2% Y₂O₃. Increasing the addition of Y₂O₃ to 3.25% results in a further improvement of mechanical properties. Here, the rupture strength of 110 MPa is obtained. The addition of Y₂O₃ over 3.25 %, however, results in a decrease of mechanical properties. Fig.4 shows that the density increases from 2.5 to 2.89 g/cm³ when the addition of Y₂O₃ increases from 0 to 4.5%.

5 Discussion

The volume fraction of crystal (V_f) can be figured out by Eqn.5. The values of ρ_gt and ρ_w are available in by MARZURIN[10], which was 2.87 and 2.92 g/cm³, respectively. The densities of the glasses (ρ_g) were measured by experiments. The values of V_f and ρ_g are shown in Fig.5.

Fig.5 shows that the addition of Y₂O₃ increasing from 0 to 4.5 % leads to the volume fraction of crystalline phase rising from 37% to 57%. According to YAO and QIU[5], the distribution of rare earth ion atgrain boundary is non-equilibrium thermodynamics. The content of rare earth at grain boundary is higher than that in crystalline phase. Therefore, Y³⁺ ions cluster together around the surface of glassy grain. Additionally,

Fig.2 SEM micrographs of samples: (a)—CASY-0; (b)—CASY-2; (c)—CASY-3.25; (d)—CASY-3.5

Fig.3  Relationship between rupture strength and Y₂O₃ content

Fig. 4  Relationship between density and Y₂O₃ content

Fig.5 Relationship between ρ_g and V_f and addition of Y₂O₃

Y³⁺ ions are high field intensity cations and can destroy the network of glass, which results in the decrease of viscosity. The surface of glassy grain could form liquid phase at a lower temperature and the crystallization is accelerated.

The oriented and interlocking wollastonite crystals

were precipitated with the addition of Y₂O₃ by 3.25%. The high content of crystalline phase and interlocking structure cause cracks to divert or deflect away from the initial direction. Crack deflection becomes more effective in limiting the flaw sizes within the materials, which increases the absorption of energy released upon crack propagation. Such energy-absorbing fracture propagation must increase the effective surface energy of fracture. When the absorption of energy was great enough, crack could be blunted and its fracture mode appeared to be mainly intergranular, thus strength increased [11]. While the flake diameter and aspect ratio exceed the critical values, as the addition of Y₂O₃ was over 3.5 %, the bending strength decreased because the orientated and interlocked microstructure was destroyed.

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(Edited by LONG Huai-zhong)

Corresponding author: ZHENG Wei-hong; Tel: +86-27-62624330; Fax:+86-27-87860801; E-mail: guiguiss@126.com