3.2.1 Chemistry and Types of Vitrified Bonds
Vitrified bonds consist of silicates (red and white clay), kaolin (also known as white clay, Al2Si2O5(OH)4), field spar (KAlSi3O8-NaAlSi3O8-CaAl2Si2O8), quartz (also known as silicon oxide, SiO2), and frits, i. e. pre-molten bonding components [BEYE04, HADE66, PADB93, JACK11, p. 92]. The bond is sintered at temperatures above 800 °C and results in a structure where the grits are enclosed by the bond (Fig. 3.10). The bond bridges between grits leave room for pores. Due to the high sintering temperatures, reactions between grits and bond are likely (Sect. 3.2.3 “Performance/Grit Retention”). In contrast to resin and metallic bonds, vitrified bonds have a significant amount of pores.
Vitrified bonds vary in their appearance due to the proportion of ingredients and sintering conditions. The two bond type extremes are melted bonds and sintered bonds [KLOC09, KREB06, TYRO03b]:
• Melted or fusible bonds are glassy bonds with a high amount of glass phase [JACK95]. They result from a high proportion of clay and frits as raw materials. Melted bonds melt totally, flow around the grits and react with the grit surface [HELL94]. The frits enhance the melting properties of the bond resulting in higher grit retention at lower firing temperatures [KREB06].
• Sintered bonds occur as porcelain bond type with a small amount of the glass phase. A high proportion of fieldspar and a low amount of frits leads to this bond type. The grits are only “glued” together by the partially molten bonding. The proportion of bond and the firing temperatures are comparatively high [KREB06]. Nevertheless, this bond type is used for critical grinding operations with low forces and for silicon carbide tools [KREB06]. High glass contents can decompose the silicon carbide grits [KREB06].