Grain growth is closely controlled in a sulphide matrix. The alumina is separated out by acid leaching without crushing. The grain shape is nodular, which aids bond retention, while the elimination of crushing reduces mechanical defects from processing.
5.4.6 Postfusion Processing Methods
As mentioned above, the type of particle reduction method can greatly affect the resulting grain shape. Impact crushers like hammer mills will create a blocky shape, while roll crushers will cause more splintering. It is further possible using electrostatic forces to separate sharp shapes from blocky grains to provide grades of the same composition but very different cutting action.
5.4.7 Postfusion Heat Treatment
The performance of an abrasive can also be altered by heat treatment, particularly for brown alumina. The grit is heated to 1,100°C to 1,300°C, depending on grit size, in order to anneal cracks and flaws created by the crushing process. This can enhance toughness by 25 to 40%.
5.4.8 Postfusion Coatings
Finally, several coating processes exist to improve bonding of the grains in the grinding wheel. Red iron oxide is applied at high temperature to increase surface area for better bonding in resin cutoff wheels. Silane is applied for some resin bond wheel applications to repel coolant infiltration between bond and abrasive grit and thus protect the resin bond.