The wettability and adherence of metal-ceramic systems were investigated, using a precise sessile-drop method. The surface tension and wettability of silicon, iron, and nickel in contact with various refractpry materials were evaluated at elevated temperatures. Measurements were carried out in vacuo, in hydrogen, and in helium. Generally, nonwetting contact angles were observed for iron and nickel with oxides, whereas silicon was found to wet the oxides partially. In all cases the wettability was greater for the metalgraphite and beryllium carbide systems than for the metal-oxide systems. The nature of the atmosphere was found to have a significant effect on the wettability and surface tension of the metal. Wettability results are interpreted on the basis of surface tension and by the consideration of free-energy changes.

The microstructure of tungsten carbide and titanium carbide base cermets has been evaluated on the basis of wettability of the carbide phase. Incomplete wetting was found to result in a coalescence of the carbide phase leading to an apparent increase in the carbide particle size. Improvement in the wettability of the nickeltitanium carbide systems by the addition of molybdenum was found to promote the dispersion of the carbide phase and to lead to improved hardness and impact resistance.

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315volume fraction of pores according to the Loeb equation. Depending on the extent of the pores, the conductivities of the other specimensfall into the region bonded by the dashed and solid lines in Fig. 1. These results demonstrate the importance of the shape and distribution of pores in their effect on conductivity. It is therefore important that the pore structure be ascertained before any of the relations between porosity and conductivity are employed. Unless the pore structures correspond to the model on which the relations are based, errors in the calculated conductivities may result. ( 5 ) Effect of Grain BoundariesThe thermal conductivity measurements on specimens of differing crystal sizes (Figs. 4 to 8) show that identical values were obtained when the effect of porosity and impurities was taken into account. These results support the conclusions reached in the theoretical analysis and indicate that for the range of grain sizes, materials, and temperatures considered, the effect of grain boundaries on thermal conductivity is negligible. Further confirmation has been obtained by the identical values between the aggregates and single crystals at temperatures below 300°C. Finally, the temperature dependence of the polycrystalline aggregates also indicates that in dense pure materials, no other resistance except the lattice resistivity is operative.(1956).

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