Microstructure and mechanical properties of

MgO–Al2O3–SiO2–TiO2 glass–ceramics

Hua Shao*, Kaiming Liang, Feng Zhou, Guoliang Wang, Anming Hu

Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China

Received 9 April 2004; received in revised form 2 November 2004; accepted 24 November 2004

Abstract

Crystallization sequences of MgO–Al2O3–SiO2–TiO2 system glass were investigated by means of DTA,

XRD, SEM and EDS. The mechanical properties of samples, including density, microhardness and elastic

modulus, were characterized as well. The relationship between crystalline phases, heat treatment methods and

mechanical properties is discussed. The results have shown that: the glass first underwent extensive phase

separation into titanium-rich droplets in a silica-rich matrix, then magnesium aluminotitanate (MAT) initially

precipitated in the droplet phase. With the crystallization temperature increased, b-quartzss, sapphirine, a-quartzss,

a-cordierite and cristobalite made their appearance successively. When the glass was heated at 1100 8C for 2 h, the

maximum elastic modulus of 137 GPa (accompanied by a microhardness of 8.5 GPa and a density of 2.924 g/cm3)

was obtained.

# 2004 Elsevier Ltd. All rights reserved.

1. Introduction

Cordierite (2MgO–2Al2O3–5SiO2) and cordierite-based glass–ceramics have been extensively studied

in various applications including ceramic matrix composites (CMC) [1,2] and substrates for high speed

computers [3,4] mainly due to their beneficial properties, i.e. low thermal expansion coefficient, low

dielectric constant and high chemical durability [5–8]. Generally, the properties of glass–ceramics are

determined by the main crystalline phases and the microstructures depending on composition of the

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Materials Research Bulletin 40 (2005) 499–506

* Corresponding author. Tel.: +86 106 277 3392; fax: +86 106 277 3392.

E-mail address: sh00@mails.tsinghua.edu.cn (H. Shao).

0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2004.11.005

parent glass as well and the addition of nucleating agents [9]. So, it is very important to design the

composition of the glasses and control the crystallization of the glass to achieve a homogenous

microstructure. Although the relationship between phase separation, nucleation and crystallization in

this kind of glass ceramics had been widely investigated [10,11], the microstructure-property relationship

during crystallization appear rarely reported.

The aim of the present work is to investigate the crystallization sequences of the MgO–Al2O3–SiO2

system glass containing TiO2 as nucleating agent, and deal with the relationship between crystalline

phases, heat treatment methods and mechanical properties of this glass–ceramics.

2. Experimental procedure

2.1. Preparation of the glass–ceramics

The material investigated was produced by mixtures of the following reagent oxides: MgO

(10–20 wt.%), Al2O3 (15–35 wt.%), SiO2 (20–40 wt.%), TiO2 (5–10 wt.%). Glass batches were ball-

milled for 24 h, and thereafter melted in a platinum crucible at 1500–1600 8C for 3 h. The melts were

poured onto a steel plate, annealed for 1 h at 600 8C, and then cooled to ambient temperature in the

furnace.

2.2. DTA analysis

The resulting glass was crushed and sieved through a 200 mesh to produce glass powder suitable for

DTA analysis employing a Dupont DTA with the temperature range of 20–1200 8C. The glass powder

with the weight of 50 mg was contained in a platinum crucible and the reference material was a-Al2O3

powders. The samples were heated in air from ambient temperature to 1200 8C at the heating rate of

10 K/min.

2.3. X-ray powder diffraction

The amount and types of crystalline phases existing in a sample after heat treatment were determined

by X-ray powder diffraction (D/max-RB) using Cu Ka radiation, working voltage 40 kV, working

current 25 mA, scanning speed of 48 min�1.

2.4. Scanning electron microscope

The bulk specimens after heat-treatment were surface polished with diamond paste to a 0.5 mm finish

and etched in 1% hydrofluoric acid for 30 s (20 8C). Then the microstructural studies on these specimens

were done by scanning electron microscope (6301 F).

2.5. Determination of density, microhardness Hv, and elastic modulus

Density of samples were determined by Archimedes’ immersion method, involving boiling in water

for 2 h and a further soaking of 24 h at ambient temperature.

H. Shao et al. / Materials Research Bulletin 40 (2005) 499–506500

Vickers microhardness measurements of heat treated glass–ceramic samples were made with a Vickers

microhardness tester (Shimadazu HMV 2000). Specimens were prepared using conventional metallo-

graphic techniques and a load of 500 g were used to indent their surfaces. In order to obtain reliable

statistical data, at least 10 measurements were made on each sample.

Modulus of elasticity was measured by the pulse-echo method with an ultrasonic tester (USIP 12).

3. Results and discussion

3.1. Differential thermal analysis

The differential thermal analysis (DTA) record of sample was shown in Fig. 1. As we can see

from Fig. 1, the glass transition temperature (Tg) of the curve was evidently about 780 8C, two

apparently exothermal peaks (Tp1, Tp2) were observed which associate the formation of various

crystalline phases.

3.2. XRD analysis

The X-ray diffraction patterns of samples isothermally treated at various temperatures are shown in

Fig. 2. The crystallization sequences of the system glass identified by XRD are summarized in Table 1. It

was found that the sample is fully amorphous after heat-treatment at 780 8C for 2 h. The magnesium

aluminotitanate (Mg2Al6Ti7O25, MAT) appears at 900 8C although a significant amount of glassy phase

remained. According to Fig. 2b, at 950 8C, besides the reflections of MAT, reflections belonging to a

small amount of b-quartzss could be detected. As we can see from Fig. 2c, heat treatments over 1100 8Cfor 2 h resulted in the formation of a-cordierite and transformation of b-quartzss to a-quartzss, and a trace

of reflection of sapphirine was detected as well. After heat treatment at 1200 8C for 2 h, the reflections of

b-quartzss and sapphirine disappeared, a-cordierite became the primary phase and the reflections of a-

quartzss, cristobalite and aluminum titanate (Ai2TiO5, AT) could be detected, and the glassy phase

minimized.

H. Shao et al. / Materials Research Bulletin 40 (2005) 499–506 501

Fig. 1. DTA curve of the glass sample at the heating rate of 10 K/min.

3.3. Microscopic examinations

The extensive phase separation after crystallization at 780 8C for 2 h is shown in Fig. 3a. No

crystalline phase made its appearance confirmed by the result of XRD as shown in Fig. 2a. The results

of EDS shown in Fig. 4 give evidence for the droplets (A area) rich in titanium, while leading the

matrix (B area) rich in silica. SEM micrograph of sample heated at 950 8C for 2 h as shown in Fig. 4b.

The initial MAT made its appearance from the titanium-rich zone heated at 950 8C for 2 h which was

confirmed by the result of XRD as shown in Fig. 2b. The morphology of the samples heated at

1200 8C for 2 h shown in Fig. 3c. The average crystal size is 1 mm and the maximal crystal size is

about 3 mm.

3.4. Mechanical test

The curves of the relationship between of density, Vickers hardness, elastic modulus and crystal-

lization temperatures are shown in Fig. 5. The values of density, r, and elastic modulus, E, of the parent

H. Shao et al. / Materials Research Bulletin 40 (2005) 499–506502

Fig. 2. XRD patterns of samples at different heat treatment conditions. (a) 780 for 2 h, (b) 950 8C for 2 h, (c) 1100 8C for 2 h, (d)

1200 8C for 2 h.

Table 1

Crystalline phases of samples at various heat treatment conditions

Treatment condition Crystalline phases

780 8C, 2 h Glass

900 8C, 2 h MAT, glass

950 8C, 2 h MAT, b-quartzss, glass

1000 8C, 2 h b-Quartzss, a-quartzss, sapphirine, glass

1100 8C, 2 h a-Cordierite, a-quartzss, sapphirine,

1200 8C, 2 h a-Cordierite, a-quartzss, cristobalite, aluminum titanate

H. Shao et al. / Materials Research Bulletin 40 (2005) 499–506 503

Fig. 3. SEM micrographs of samples at different heat treatment conditions. (a) 780 8C for 2 h, (b) 950 8C for 2 h, (c) 1200 8Cfor 2 h.

glass without thermal treatment are 2.619 g/cm3 and 98 GPa, respectively. After heat treatment at 900 8Cfor 2 h, the elastic modulus is slightly larger (107 GPa) while the density remains constant. The Vickers

hardness also slightly increases from 6.1 GPa for the glass to 6.9 GPa after thermal treatment at 950 8Cbecause the crystal content is larger with the crystallization temperature increased. Subsequent crystal-

lization at 1000 8C for 2 h leads to a notable increase of density, Vickers hardness and elastic modulus to

2.723 g/cm3, 7.2 and 121 GPa, respectively because a-quartzss precipitated, which has higher density and

hardness (r = 2.65 g/cm3, Moh’s hardness number is 7) than b-quartzss (r = 2.53 g/cm3, Moh’s hardness

number is 6.5–7) [12,13]. Increasing the crystallization temperature to 1100 8C for 2 h, a-cordierite

appeared and b-quartzss transformed to a-quartzss, which results in a further improvement of the

mechanical properties. Here, a density of 2.924 g/cm3 and an elastic modulus as high as 137 GPa is

obtained and the Vickers hardness amounts to 8.5 GPa. Tempering at 1200 8C for 2 h, however, results in

a decrease of Vickers hardness, elastic modulus and density because the cristobalite, aluminum titanate

which have low density and elastic modulus appeared which confirmed by the results of XRD shown in

Fig. 2d [14].

H. Shao et al. / Materials Research Bulletin 40 (2005) 499–506504

Fig. 4. EDS results of (a) A area and (b) B area in Fig. 3a.

4. Conclusion

1. The glass investigated in this paper first underwent extensive phase separation into titanium-rich

droplets in a silica-rich matrix, then magnesium aluminotitanate (MAT) initially precipitated in the

droplet phase. With the crystallization temperature increased, b-quartzss, sapphirine, a-quartzss, a-

cordierite, cristobalite made their appearance successively.

2. Mechanical properties of this material mainly depend on heat treatment conditions. When the glass is

heated at 1100 8C for 2 h, the maximum elastic modulus of 137 GPa combined with a hardness of

8.5 GPa and a density of 2.924 g/cm3 were obtained.

H. Shao et al. / Materials Research Bulletin 40 (2005) 499–506 505

Fig. 5. The curves of the relationships between density, Vickers hardness, elastic modulus and crystallization temperatures.

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