The influence of TiO₂ content on the properties of glass ceramics: Crystallization, microstructure and hardness

Abstract

The effects of compositional variation, crystallization behavior, crystalline phases and microstructure formed in the SiO₂3Al₂O₃3CaO (SAC) glass system using various amounts of TiO₂ as nucleating agent were investigated by Differential Thermal Analysis (DTA), X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDAX) and Fourier transform infrared spectroscopy (FTIR) techniques. The crystallization kinetics and mechanical properties of SAC glass ceramics were studied using crystallization peak temperature (T_p) of three different glasses as obtained from DTA, the activation energy (E) and Avrami exponent (n) were also determined. The crystallization peak temperature (T_p) and activation energy (E) were found to increase with the increase in TiO₂ content. The major crystalline phases were anorthite and wollastonite along with gehlenite and titanite as the minor crystalline phases present in the glass ceramic system. The studies showed that the three dimensional crystalline structure and the microhardness increased with the increase of TiO₂ content in the glass ceramics system.

Introduction

In the glass-ceramics system the nucleating agents such as CaF₂, TiO₂ were used in order to induce bulk crystallization of the phases and to reduce the crystallization peak temperature [1], [2], [3]. Several researchers highlighted the effects of TiO₂ as a nucleating agent in mica-based glass ceramics and observed that the addition of small amount of titania had a remarkable effect on the crystallization processes [4], [5], [6], [7]. The nature of the crystallinity and distribution of the crystalline phase(s) formed during crystallization were dependent on the type and amount of nucleating agent used [3]. Researchers had revealed that TiO₂ could be used as a good nucleating agent in many silicate systems [8], [9]. Barry et al. [10] observed that the addition of TiO₂ had acted as a surface active agent and increased the nucleation rate in the LiO₂3Al₂O₃3SiO₂ (LAS) glass system. Zdaniewski [11] reported that the presence of TiO₂ in glass-ceramics system decreased the viscosity of the base glasses at high temperature, which favored the nucleation and crystal growth. The decrease in surface tension and increase in the rate of nucleation in LAS glass system by using 11.5 wt% TiO₂ as nucleating agent were reported in literature [12], [13]. TiO₂, ZrO₂, and mixtures of them were used successfully to make strong spodumene glass ceramics in the LAS glass system [14], [15]. Lee and Hsu [16] investigated the effects of TiO₂ in the calcium phosphate glass ceramic for biological applications. Bach and Krause [17] and Hu et al. [18] reported the use of TiO₂ as nucleating agent for making low-thermal-expansion LAS glass ceramics with a very wide range of commercial applications, like manufacturing mirrors, telescopes, cooker tops, etc. In general the sintering and crystallization parameters were strongly dependent on the composition of base glass with respect to the added nucleating agents [2].

The aim of the present work is to investigate the possible role of TiO₂ in the SiO₂3Al₂O₃3CaO glass ceramic system with respect to its nucleation, crystallization behavior, activation energy, phase separation, microstructure, microhardness and density.