Thermophysical properties of ceramic substrates with modified surfaces.

M Rohde, S Schreck

Forschungszentrum Karlsruhe GmbH, Institute for Materials Research I, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein, Germany

Laser induced surface melting and alloying are important techniques to improve the properties of a material without affecting the bulk since only the surface area is involved in the modification process. In ceramic components these methods can be used to optimise the fracture toughness, friction and wear [1] but also the thermal and electrical conductivity [2].

Within this study we have examined the changes in the thermophysical properties of selected ceramic substrates due to the laser supported modification process. Two types of substrates have been included, namely pure Al₂O₃ with a porosity of 4-6 % and Al₂O₃ reinforced with 10-wt.% ZrO₂. As a modifying material during the laser process hard metal powder like TiN, WC and W has been applied to produce a metal-ceramic composite with a metal concentration of about 50 wt.%. Standard measurement techniques like the Laser Flash method, DSC and push-rod dilatometry have been used to measure the thermal diffusivity, the heat capacity and the thermal expansion of the ceramics before and after the laser processing. These properties have been evaluated within a temperature range from room temperature up to 1300 °C.

Our results show that the heat capacity data follow approximately the rule of mixtures in the modified region. The WC-modified ceramics revealed a high sensitivity against small amounts of oxygen at high temperatures, which was present in the Ar-purge gas of the DSC. At high oxygen loading this leads to a complete WC to WO_x transformation with a strong shift of the melting point.

The thermal expansion data of the both substrate materials used are very similar over the measured temperature range with the exception of a volume change due to the monoclinic-tetragonal phase transition of the ZrO₂-phase in the reinforced ceramic at about 900 °C.

Depending on the hard metal powder introduced into the ceramic surface a more or less strong enhancement of the thermal diffusivity could be observed over the whole temperature range. The improvement of the thermal diffusivity exhibits the highest value at room temperature and decreases with increasing temperature. These data will be discussed within the framework of theoretical models for the heat transport in inhomogeneous materials.

1. K.-H. Zum Gahr, J. Schneider: Surface modification of ceramics for improved tribological properties, Ceram. Int. 26 (2000) 363 – 370

2. S. Rüdiger, H. Gruhn, R. Heidinger, M. Rohde, J. Schneider, K.-H. Zum Gahr: Laser induced surface modification of cordierite, Surface Engineering EUROMAT 99-Vol. 11 (ed. H.Dimigen) (1999) 510 – 515