Depth sensing hardness in sapphire and in sintered sub-µm alumina

Single crystals and polycrystalline compacts with different porosity were investigated by a recording indentation technique to obtain calibration data for the evaluation of the state of microstructural arrangement in sintered submicron alumina coatings with a thickness of 5 - 10 µm on steel substrates. Indentations with maximum loads between 20-200 mN were performed on (1210) surfaces of sapphire and with sub-µm sintered Al2O3. Similar influences of testing load (indentation size effect) and grain size are observed as in conventional measurements. The indentation size effect of the hardness can be obtained without a variation of the maximum load directly from loading curves if Young's modulus is known and if the shape of the penetration curve is analyzed carefully. With the small loads used, effects of loading rate and creep are small in the sintered materials. In the single crystals, however, extended creep complicates the determination of the hardness. At small testing loa ds of 25-50 mN, the Vickers hardness of sub-µm polycrystals rises to 25-30 GPa depending on the microstructure. The effect of the porosity on the hardness exhibits the same correlation as observed in conventional measurements of compact ceramics.