Additives are employed to modify the properties of alumina as described below. Examples of additives include chromium oxide, titanium oxide, zirconium oxide, and vanadium oxide.
5.4.3 Pink Alumina
The addition of chromium oxide produces pink alumina. White alumina is alloyed with <0.5% chrome oxide to give the distinctive pink hue of pink alumina. The resulting grain is slightly harder than white alumina, while addition of a small amount of TiO2 increases its toughness. The resultant product is a medium-sized grain available in elongated, or blocky, but sharp, shapes.
5.4.4 Ruby Alumina
Ruby alumina has a higher chrome oxide content of 3% and is more friable than pink alumina. The grains are blocky, sharp-edged, and extremely cool cutting making them popular for tool room and dry grind application on steels (e. g., ice skate sharpening). Vanadium oxide has also been used as an additive giving a distinctive green hue.
5.4.5 Zirconia-Alumina
Zirconia is added to alumina to refine the grain structure and produce a tough abrasive. At least three different zirconia-alumina compositions are used in grinding wheels:
• 75% Alox, 25% ZrO2
• 60% Alox, 40% ZrO2
• 65% Alox, 30%ZrO2, 5%TiO2
Manufacture usually includes rapid solidification to enhance the nature of the grain structure. The resulting abrasives are fine grain, extremely tough, and give excellent life in medium to heavy stock removal applications such as billet grinding in foundries.