Sintering Process

In the sintering process, the bond components melt and flow around the abrasive grits. Bonds with a large amount of clay melt at higher temperatures; bonds with more frit content melt at lower temperatures [MALK08, p. 27]. The mechanisms of melting, wetting, resolidification, and forming of bonding bridges between grits are highly complex [BOTS05, MOSE80].

The bond mixture is formed into non-uniform glass of complex composition including several ceramic bond minerals that form during the sintering process [JACK11, p. 83, 85]. Jackson studied the vitrification behavior of sintered and fusible bonds for conventional grinding wheels [JACK95]. Bot-Schulz analyzed the chemical reactions and sintering mechanisms for varying bonding composition for white corundum and Sol-Gel-corundum [BOTS05]. If the bond contains MgO, mullite (3Al2O32SiO2), cordierite (2MgO2Al2O3-5SiO2), and spinel

(MgO Al2O3) are formed during devitrification [JACK11, p. 83 ff]. If the bond contains CaO, anorthite (CaOAl2O32SiO2) and mullite (3Al2O32SiO2) can form [JACK11, p. 83 ff].

One problem in manufacturing of vitrified bonded tools is the shrinkage after melting and sintering. A higher amount of abrasives and secondary abrasives reduces the shrinkage, but might have a negative effect on the grinding tool per­formance. Pore builders that burn leave voids, which sometimes collapse during the sintering shrinkage and counter the purpose. New approaches to reduce sintering shrinkage are to use fired clay (“grog”) or crushed firebricks in sizes between 30-200 pm. Cerium oxide, andalusite, sillimanite, willemite, or a combination thereof serves the same purpose. [HUZI12]

Several chemical processes happen during sintering [BOTS05, 25 f.]:

• 20-600 °C: Free and bound water dissolves. The temporary binders burn out. Quartz (SiO2) changes volume at 573 °C. Gas pressure and volumetric shrinkage lead to high internal stresses in the tool.

• 600-900 °C: Carbonates, organic material, chemically combined water in the clay, and other volatile materials drive out.

• Furthermore, chemical reactions between grits and bond are possible.

The sintering temperature must not harm the abrasive grits by inducing unwanted oxidization or other chemical reactions. The bondings for CBN and diamond have to fuse together under lower forming pressures and vitrification temperatures than for conventional grits [JACK07]. Because CBN is thermally more stable than diamond, CBN grits can be used with a wider range of vitrified bonds [MALK08, p. 26]. Special coatings protect CBN grits from chemical reac­tions above 800 °C with the water and alkali in most frits [MALK08, p. 27]. For diamond wheels, a non-oxidizing atmosphere is applied to enable a sintering temperature higher than the diamond oxidation temperature of around 700 °C.

Different furnace types are used depending on application and batch size. Continuous furnaces, also called tunnel furnaces or traveling kiln, are often used for the mass production of conventional grinding wheels and can be 70 m long. Periodic hood-type furnaces are used for smaller batch sizes. In general, about 80 % of the inner furnace volume can be used for producing products. Additonal accessories enable special gas atmospheres in the furnace or pressure control. Furnaces are heated by electrical energy, with gas or oil [KLOC09]. Gas furnaces are usually cheaper but they cannot be used for all processes.

The temperature distribution is not constant along the furnace cross-section. Temperature fluctuations inside the furnace should not exceed ±10 °C. A smaller range enhances process capability. Tools properties can depend on their position on the kiln car [RAMM74].

The temperature curve has to be controlled precisely. The rate of heating, peak temperature, and isothermal soaking time define the grinding wheel properties [JACK95, p. 82]. The temperature profile consists of a heating, soaking and cooling phase. The total time can be 100 h [TYRO03]. For example, corundum wheels in a tunnel furnace are heated up to 1260 °C over 1 to 2 days, held for about 12 h at the maximum temperature, then slowly cooled down [MALK08, p. 27]. The cooling process has to be carefully controlled to avoid thermal stresses or cracking of the wheels [MALK08, p. 27]. The cooling process can take weeks for very large wheels [MALK08, p. 27].

Updated: 24.03.2016 — 11:54