Shape, Morphology

Corundum appears as polycrystalline material and is broken down to the desired grit sizes. The size of the single crystals within the larger abrasive grits depends on the manufacturing route. Pink corundum appears as blocky grits with sharp edges and an average crystal size of 750 pm [WASH12b]. Zirconium corundum can have much smaller crystal sizes down to 10-12 pm [WASH12b]. Sol-gel corundum is known for the very fine crystals below 500 nm, enabling a very distinctive fracture behavior [ENGE02, p. 7].

2.1.1.1 Toughness, Breaking Behavior, Friability, Hardness

The different corundum types have a broad bandwidth of hardness and friability from 1300 to 2300 HK and a friability index between 5 and 75 (Fig. 2.3). Contents of Cr2O3 and TiO2 increase the toughness, whereas Na2O contents are disadvan — tasgeous, because Na2O induces the softer P-11Al2O3-Na2O [TYRO03b].

Corundum is considered to be the softest of the four main abrasives for grinding tools with a Knoop hardness of 2100-2400 kgf/mm2 for the purer molten corundum types, down to around 1600 kgf/mm2 or less for zirconium corundum (Fig. 2.3) [KLOC09, p. 27]. The compressive strength of Al2O3 is around 569 N/mm2 [MERB03, p. 6].

Table 2.1 Chemical composition of typical molten corundum for grinding applications [TYRO03b, WASH12b] Ak03 (%) Na20 (%) СьО-, (%) TiOo (%) SiOo (%) Fe203 (%) Others (%) Hardness (HK) Brown corundum 96.1 2.70 0.67 0.11 2090 White corundum 99.8 0.2 2000-2160 Pink corundum 99.5 0.2 0.25-0.3 0.05 0.03 2160 Ruby corundum 97.4 0.2-0.26 2.1-2 0.01 0.08 2150 Mono-crystalline corundum 99.3 <0.09 0.3-0.6 Zirconium corundum (40 %) 59-60 <0.03 0.15-0.3 <0.35 <0.15 39—40 % ZrOo 0.6-0.8 % Y0O3 1600-1700 Zirconium corundum (25 %) 75 0.08 0.1 0.3 0.3 23 % ZrOo 1450

to

friable

2 Mono crystalline Al2O3

3 Regular Al2O3

4 Micro crystalline Al2O3

5 Zirconium Al2O3

1000

0 10 20 30 40 50 60 70 80

Friability

Polycrystalline corundum experiences increasing toughness and hardness when the crystal grain sizes get smaller [KLOC03, KREL95]. Presumably, the yield stresses from dislocations lead to micro cracks at the grain boundaries, which increase the toughness [EVAS90].