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 sub-mum alumina coatings with a thickness of 5-10 mum on steel substrates. Indentations with maximum loads between 20 and 200 mN were performed on single crystalline (12 10) surfaces of sapphire and with sub-mum sintered Al2O3 samples. The indentation size effect of the hardness can be obtained without a variation of the maximum load directly from loading curves if the Young's modulus of the material is known and if the shape of the penetration curve is analyzed carefully. With the small loads used, the effects of loading rate and of creep in the sintered materials are small compared with the scatter caused by the microstructure of even the most homogeneous and dense sub-mum polycrystals. In the single crystalline surfaces, however, extended creep complicates the determination of the hardness. Similar influences of testing load (indentation size effect) and grain size are observed as known from conventional measurements. At small testing loads of 25-50 mN, the Vickers hardness 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. (C) 2001 Elsevier Science Ltd. All rights reserved.