X-ray computed micro tomography (CT) is an alternative technique to the classical methods such as mercury intrusion (MIP) and gas pycnometry (HP) to obtain the porosity, pore-size distribution, and density of porous materials. Besides the advantage of being a nondestructive method, it gives not only bulk properties, but also spatially resolved information. In the present work, uniaxially pressed porous alumina performs activated by titanium were analyzed with both the classical techniques and CT. The benefits and disadvantages of the applied measurement techniques were pointed out and discussed. With the generated data, development was proposed for an infiltration model under ideal conditions for the production of metal matrix composites (MMC) by pressureless melt infiltration of porous ceramic preforms. Therefore, the reliability of the results, received from different investigation techniques, was proved statistically and stereologically.

Common activation mechanisms, which are responsible for infiltration in other non‐wetting systems, play only a secondary role in the Ti activated Al2O3/X3CrNiMo13‐4 system for pressureless alloy infiltration into porous ceramic preforms. Experiments with a model pore consisting of two juxtaposed parallel alumina plates with lithographically imprinted arrays of Ti‐dots show that transport through the vapour phase and subsequent condensation of the steel components onto the activator are the key steps of the activation mechanism. The mechanism was additionally confirmed by infiltration experiments with porous alumina preforms consisting of alternating layers with and without Ti parallel to the infiltration direction. Only layers with activator particles were infiltrated.

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Metal matrix composites (MMC) represent a class of materials of broad technological and commercial significance designed for applications where the contrasting material properties of both metals and ceramics are needed. [1] MMC combine the high strength and wear resistance of ceramics with the ductility as well as the thermal and electrical conductivity of metals. The ceramic phase within the metal matrix guarantees a failure-tolerant behavior and low thermal expansion. [2] Many fabrication routes have been described in previous studies, [3] the most important ones are pressureless and pressure assisted metal melt infiltration techniques as well as powder metallurgical methods. The choice of a suitable manufacturing route is conditioned by cost factors, form flexibility and production temperature, [3,4] For certain applications in the food, pharmaceutical and automotive industry, the required properties are best met by composites made of alumina (Al 2 O 3 ) and high-melting Fe-based alloys (>1400 8C) such as X3CrNi13-4. [5][6][7][8][9] Pressureless infiltration would be the manufacturing route of choice for many of these applications, but the alumina-steel system lacks an important prerequisite, that makes infiltration without pressure possible: Oxide ceramics in general, due to their ionic character, show bad wettability by metal melts. The equilibrium contact angle u Y , which depends on the surface tensions of the phases involved, is given by Young's equation [10][11][12] (Eq. 1):and is in general well over 908 for such systems. [13][14][15][16] It is therefore not surprising that steel melts do not wet alumina surfaces.Pressure assisted infiltration and previous activation of the metal and/or the ceramic preform by incorporating reactive metals like titanium (Ti) are the two methods at disposal to circumvent the wettability problem. [2,7] Moreover, it was shown that a pressure drop within preforms, caused by the reaction of reactive metals such as e.g. Ni, Ti or Al and thus generation of a vacuum atmosphere is favorable for successful infiltration. [17] Applying pressure to the melt allows overcoming the negative capillary forces, but the pressurizing systems are technically demanding. The addition of a reactive element such as Ti enhances the infiltration performance either by lowering the surface tension of the metal melt and/or by the reactive modification of the ceramic surface. [18] It is well known that the addition of Ti to brazing solders for joining ceramic components lowers the contact angle of the metal melt with alumina due to the formation of a reactive layer. [19,20] Addition of titanium to steel melts lowers the surface tension, although there is a large spread in the data which are due to the presence or absence of surface active species in the gas phase. The formation of a TiO x reactive layer, and also modification of fluid flow and active metal adsorption are additional mechanisms that have been postulated to enhance wettability. [21,22] The activator may be prealloyed into the metal or dis...