Robert J. Stearn

The effect of porosity in thermal shock

The effects of porosity on cracking during thermal shock have been studied by directly observing the cracks that formed after quenching heated porous alumina bars into water. The porosity was introduced by adding different volume fractions of fugitive inclusions and the behaviour compared with that obtained by partial sintering of a powder compact. Where fugitive inclusions had been used, there was little effect of either pore size or pore volume fraction over the ranges studied. The extent of cracking was always slightly less than that of a monolithic, dense alumina and gave reasonable agreement with predictions using experimentally measured data. However, cracks grew much further in the partially sintered material. This discrepancy became greater as the temperature change increased, inconsistent with existing analyses. It is suggested that this difference in behaviour arises predominantly because of the greater measured fracture energy of the alumina made using fugitive inclusions compared with that made by partial sintering.

Deposition of TiN/NbN Superlattice Hard Coatings by Ionised Magnetron Sputter Deposition

We have investigated the effect of ion bombardment on the structure and hardness of thin coatings of TiN/NbN multilayered structures and monolithic films of both TiN and NbN. A radio frequency coil was used to generate an additional inductively coupled plasma between the substrate and the target enabling the sample to be bombarded by a high flux of relatively low energy ions under the appropriate conditions. It is shown that the effect of such bombardment in the case of the monolithic films is to reduce the porosity. This gave an increase in the hardness of both the TiN and the NbN films up to a power of 100 W (using a coil with a cross-sectional area of 2 × 10 mm). Further increasing the power density led to a decrease in hardness. TiN/NbN multilayer coatings were made under the optimum deposition conditions for the monolithic materials and gave hardnesses greater than those observed in either TiN or NbN and approximately 50% greater than that predicted by a mixtures rule.

The Growth of Bamboo-Structured Carbon Tubes Using a Copper Catalyst

Abstract : Catalytic decomposition of methane has been used to grow bamboo-structured carbon tubes at temperatures ranging from 1233 K to 1291 K. No tube growth was observed at temperatures less than 1233 K, whilst above 1291 K pyrocarbon was the dominant product. It is shown that the average size of the copper catalyst particles was influenced by the reaction temperature, with the reciprocal of the maximum size of the copper particle decreasing linearly with temperature. This is consistent with the idea that the melting point can be reduced by surface energy effects. Observations show that under the conditions here the catalyst particle penetrates into the carbon fibre and a mechanism is proposed for development of the bamboo structure based upon the energy changes that take place.