Andreas Krell

The Influence of shaping method on the grain size dependence of strength in dense submicrometre alumina

Abstract The grain size dependence of the strength of pressureless sintered aluminas is investigated with specimens fabricated by uniaxial pressing, cold isostatic pressing, pressure filtration, gel casting, and combinations thereof. The strength depends on the flaw population and the observed grain size effect is different with different shaping approaches: with reduced grain sizes the diversity of measured strength averages broadens including increasingly high values for some of the shaping procedures only. Submicrometre grain sizes are, however, not an indispensable prerequisite for a high strength: grain sizes of 1–2 μm are sufficient to achieve 800–900 MPa by pressureless sintering, but present microstructures with grain sizes

Nanoindentation hardness of submicrometer alumina ceramics

Abstract Nanoindentations at loads of 20–200 mN were performed on submicrometer sintered alumina ceramics of different residual porosity. The indentation size effect of the hardness can be obtained without variation of the maximum load directly from loading curves, if the Youngs modulus of the material is known and if the shape of the penetration curve is analyzed carefully. Similar influences of testing load (indentation size effect), grain size and porosity are observed as known from conventional measurements. At small testing loads of 25–50 mN, the Vickers hardness of dense sub-μm corundum ceramics rises to 25–30 GPa depending on the microstructure.

Improved hardness and hierarchic influences on wear in submicron sintered alumina

Abstract Pressureless sintering is used to produce submicron alumina bodies with a hardness that equals the level of advanced hot-isostatically pressed Al 2 O 3 /TiC composites with 35 vol.% carbide. Wear can be reduced by almost one order of magnitude by reducing the grain size from 4 to 0.4 μm, but the grain size effect on wear strongly depends on the environment. Wear is governed by the pull-out behaviour of grains with a secondary influence of the hardness. Therefore, the hierarchic order of the parameters which affect wear is dominated by the micromechanical stability of the grain boundaries.

Grain size dependent residual microstresses in submicron A12O3 and ZrO2

Abstract X-ray measurements of lattice spacings are used to determine residual microstresses present in sintered alumina and in tetragonal zirconia polycrystals due to thermal expansion anisotropy (TEA). In A1 2 O 3 with grain sizes larger than 1 μm the microstresses are 30–100 MPa, in submicrometer samples the grain size influence becomes small and residual stresses range between 20 and 30 MPa. For the grain sizes between 0.3 and 9 μm there is no indication of a change in the high-temperature relaxation mechanism. In ZrO 2 with grain sizes of 0.5–1 μm the residual stresses are similar as observed in A1 2 O 3 (20–60 MPa), they decrease further at grain sizes 0.2–0.4 μm. The results are independent of technological approaches like powder processing or sol/gel used to produce the sintered bodies.

A new look at grain size and load effects in the hardness of ceramics

A simple model describes the load effect (size effect) in the hardness, assuming an increasing microplastic deformability, when for the further extension of the plastic zone growth and multiplication of pre-existing elements of plasticity are more effective than the generation of new dislocations or twins in the virgin material around the indentation site. The model explains experiments with sintered alumina which indicate a reduced load effect in increasingly fine-grained microstructures due to a grain size effect that is more pronounced at higher testing loads (larger indents) than in the microhardness range. A large difference between the hardness of plastically deformed volumes in single crystals and in polycrystalline microstructures consisting of grains with the same size, respectively, reveals a substantial contribution of the grain boundaries to inelastic deformation at the indentation site even at room temperature and even for coarser microstructures.

Nanocorundum—advanced synthesis and processing

Abstract Inorganic and organic approaches for the synthesis of nanocorundum powders and products are described on a scale of 50 g batches. The median particle size of the volume distribution of redispersed powders is about 50 nm after calcination at T 21 GPa at a testing load of 10 kg. Porous sintered corundum bodies with an average pore size between 15 and 60 nm and a porosity of about 40–55% have been produced within the framework of the described synthesis by both powder or sol/gel technologies for the use as chemically highly stable filtration membranes or as catalyst supports. Other applications are amorphous or crystalline nanoporous products with average pore sizes of about 1 nm.

Subcritical crack growth in Al2O3 with submicron grain size

Abstract Gelcast sintered α-Al 2 O 3 (corundum) ceramics were developed with a sub-μm grain size at densities >99%. Highly perfect samples with a minimum of flaws were prepared by an approach that maintains the high purity of the raw powder >99.99% Al 2 O 3 throughout processing. As a consequence, all grain boundaries are free of even thinnest amorphous interface films, amorphous triple junctions are ⩽150 nm, and their frequency is low. Subcritical crack growth was investigated by an approach recording growth rates as low as 10 −13 m/s. The outstanding purity of grain boundaries gives rise to a resistance against subcritical crack growth which is similar or even below that of coarser conventional alumina ceramics. No significant promotion of subcritical crack growth by water was observed for the new gelcast high-purity ceramics with grain sizes K I0 below which no crack growth would occur. The results suggest that in sintered alumina ceramics with a given purity of grain boundaries the subcritical crack-growth mechanism of stress corrosion is independent of the grain size. With their high mechanical reliability, these corundum grades are promising candidates for the use in new prostheses for joints with a high load bearing capability and with small calliper sizes.

On grain boundary strength in sintered A12O3

Abstract Scanning and transmission electron microscopy and the C-Pt replica technique have been used to investigate grain boundary structures in sintered polycrystalline A12O3 with varying concentrations of MgO dopant. A method is proposed for evaluating the fracture toughness, Kβb c, and the strength of grain boundaries from the percentage of intergranular fracture as observed along the path of a propagating macrocrack. The resulting Kβb c is found to be between 0·1 and 0·4 of the value characterizing the fracture toughness of the A12O3 lattice, the observed grain boundary strengths being 400–1300 MPa. A comparison of various alumina structures of different grain boundary strengths but similar microcrack densities and fracture properties indicates that for microscopic fracture processes the distribution of flaw sites is more important than the grain boundary strength.

TiC-strengthened Al2O3 by powder tailoring and doping procedures

High-carbide Al2O3/TiC composites have been fabricated without hot-pressing by different processing techniques. Transient liquid phase sintering with additions of TiH2 turned out to be the most promising of the doping procedures. The highest strength, (> 800 MPa), however, was measured when omitting all additives and using a tailored alumina raw material with narrow grain size distribution. The influence of the alumina grain size distribution on sintering and strength is discussed as being associated with the amount of grains and aggregates larger than 1 μm and with the mutual homogenization of oxide and carbide components. In contrast, the reduction of the median powder grain size plays a minor role only.

On crack-propagation-related phenomena in Al2O3 + ZrO2 and Al2O3 sintered in air and hydrogen

Abstract Experiments on Al2 + ZrO2 ceramics reveal higher Young′s moduli and fracture toughness after sintering in air then in hydrogen. Since X-ray measurements showed a higher monoclinic phase content in the air-sintered material, Young′s moduli of these ceramics indicate the existence of higher residual stresses associated with a lower degree of spontaneous transformation-induced microcracking. This effect corresponds to a lower microcrack density observed on fracture surfaces of air-sintered single-phase alumina compared with the same Al2O3 sintered in H2. These results are in agreement with the estimated grain boundary strengths of the examined ceramic materials.