Development of stress on quartz grain in illite ceramics during cooling stage of firing

Abstract

If the volume fraction of quartz in traditional ceramics is higher than 10% then the use of Selsing’s formula is questionable. A model of two concentric spheres, in which the inner sphere is a quartz grain and its spherical cladding is glassy phase, is proposed. The development of the stress on the grain surface during cooling stage of the firing and its influence on the microcracking is described. The tensile tangential stress which is developed between the glass transformation temperature and β → α transition of quartz can be a source of the first microcracking. After this transition, the tangential stress becomes compressive. The radial stress on the grain surface, which is compressive before the β → α transition of quartz, turns into tensile after this transition as a steep change of 40–80 MPa (for 10–50% of quartz content). These changes are in a narrow temperature interval around the β → α transition of quartz passing through zero value and no cracks are expected. This is confirmed with acoustic emission (AE) and short recovery of Young’s modulus. When the β → α transition of quartz is finished and the temperature decreases, the creation of the cracks continues. The radial tensile stress on the grain and in its close vicinity reaches 100–180 MPa, consequently, circumferential cracks can be formed. This is indirectly confirmed with a decrease of Young’s modulus and weak AE activity.