Vitrified bonds are formed by mixtures from the natural silicates red and white clay, kaolin and feldspar, as well as quartz and, as an additive, frits [HADE66, PADB93].
Frits are glassy, previously melted and pulverised organic and inorganic mixes which serve as fluxing agents and give the vitrified bond certain properties. Among other things, they help lower the firing temperature of the bond mixture. For grinding wheels, these mixtures are composed mostly of boron silicates or glass containing magnesium. Especially with frits containing boron, the propensity of borax, as in the hard soldering of metals, to form molten masses with metal oxides at low temperatures (these masses then congealing into glassy state after cooling) is evident [HADE66].
The usual components and their volumetric portions determine the strength of the bond bridges. All components are prepared, after checking the raw material, by crushing, milling, sifting and air sifting. Mixture proportions are varied in accordance with the desired target properties. The bonds must be attuned to the respective machining conditions and the abrasive grit material in use. In view of this, we distinguish according to the proportion of the glass phase in the bond mixture
• fused bonds with a high amount of glass phase,
• bonds with a medium amount of glass phase and
• sintered bonds with a small amount of glass phase.
In fused bonds, grits bonding occurs by means of a glass phase with low viscosity. In sintered bonds, adhesive force is a result of rearrangement or solid reactions [PADB93]. The sintering and melting points of the bond mixture are essential, characteristic values for every bond. They determine the firing temperature and the process behaviour of the abrasives.
Furthermore, the bond should be made to conform to the abrasive’s properties. Certain sintered corundum materials, for example, are attacked by alkaline silicates. Besides a diffusion of sodium and silicon in the interior of the grit, crystal growth transpires when exposed to higher temperatures. This interferes with the self-sharpening properties of the abrasive grits. Thus, in the case of sintered corundum, low-alkaline fused bonds with the lowest possible melting point are used.
Should high requirements be placed on the grinding component’s profile accuracy and dressing capability of the wheel, the hardness and strength of the bond can be enhanced by increasing the amount of glass phase. Thereby, next to an improvement in the bonding of the individual grit, a facilitation of the dressing process is achieved [PADB93].
In essence, the properties of ceramic bonds can be summarised as follows:
• brittle and thus comparatively sensitive to impact,
• high elasticity modulus,
• temperature-resistant, yet sensitive to temperature change,
• chemically resistant to oil and water.