Investigating the strengthening of quartz ceramics by hydrothermal treatment

Abstract

Increasing application of quartz ceramics is accompanied by an increase in the dimensions of the articles, in the complexity of their shapes, a constant increase in demands placed on dimensional accuracy, and also the need to reduce energy expenditure. As a result of all this, extensive use can be found for precision technology in strengthening the components by hydrothermal treatment of quartz-ceramic castings in steam autoclaves [1]. Making additions of alkali-metal oxides to the castings and processing in saturated steam causes them to be strengthened; in the author's opinion this is due to the transfer of the main substance to sites of intergrain contacts, thereby increasing their area and healing the microdefects such as Griffith's cracks [1]. The bending strength of quartz ceramics subjected to hydrothermal processing in an autoclave reaches 75-80 N/mm 2, which is higher than that of fired ceramics, but the autoclaved materials possesses reduced heat resistance and thermal-shock resistance. The reduction in these properties is due to the formation of fusible hydrosilicates of alkali-metal oxides [2]. However, in our opinion, the healing of the microdefects, at least on the level of Griffith's cracks, is not the main factor responsible for the high strength of the material, even if we take into consideration that there are significantly large pores and other macrodefects. Moreover, work has been done on the strengthening of quartz glass by the hydrothermal method [3]. Following hydrothermal treatment of quartz glass it was established that an increase in the bending strength of the specimens to 200 N/ram is due to the formation of silanol complexes, conferring microplasticity and relaxation of stresses under loading [3]. Since the chemical composition of quartz ceramic and quartz glass should not be very different, it can be assumed that the nature of the high strength of these materials after hydrothermal strengthening is identical. Having understood the true nature of the strengthening mechanism and the factors influencing it, and also having eliminated the negative effects of oxides of alkali metals, it might subsequently be possible to find the conditions for obtaining a material with even better strength properties which would enable us to extend the use of quartz ceramics. The present article deals with a study of the strengthening of quartz ceramics by hydrothermal treatment and methods of eliminating the factors that reduce the heat resistance and thermal-shock resistance. Investigations were carried out on cast quartz ceramics with a porosity of 9.5-13 % and a specific surface on the solid phase of 3-10 m2/g, prepared with the well-known method [4]. Hydrothermal treatment was completed in laboratory autoclaves of 1 liter and 5 liter capacity. The working medium in the autoclaves consisted of a mixture of steam and ammonia, with an ammonia concentration of 15-20% ; temperature 110-250~ and pressure 0.15-4.6 MPa. The choice of the steam-ammonia media is based on the fact that ammonium hydroxide, without forming stable compounds with silica, increases its solubility and rate of solution [5], and hence the rate of mass transfer. This provides Specimens of quartz ceramic with a bending strength of 40-80 N/mm 2 after 8-16 h hydrothermal treatment. We first investigated the chemical composition of the specimens to be treated, the dynamics of the changes with increase in test temperature, and also the relationship between the chemical composition and physical properties of the material. After treatment at 105~ for 2 h, the specimens were studied using IR-spectroscopy and diffusion reflection; they were subjected to thermogravimetric study and also mechanical testing. It was found that in specimens that had been heated to 105 ~ (Fig. 1) the IR-spectra contain bands that are typical of valence oscillations in OH-groups at 3650-3690 cm-1 and deformation and valence oscillations of adsorbed water at 1650 and 3450 cm-I; also present are various bands corresponding to amorphous silica (1100, 800, and 4700 cm-1). At 300~ there is a reduction in the intensity of the valence oscillations for the OH-groups