Thermal compound for Alumina Ceramic Insulator
I'm not sure what construction exactly you are thinking of, but the purpose of thermal compound is to fill the gaps between the device being heat sunk (often metal, but sometimes plastic or other material) and the heatsink (usually metal). Because the surfaces are both rough on a microscopic scale the heat sink compound improves the thermal conductivity between the two surfaces so there will be less temperature drop when power is flowing across the interface.
Different thermal compounds have different performance levels. The zinc oxide type is very common (the white paste) and is inexpensive.

There is probably some extreme with both surfaces at mirror-like polish (and either compliant or very flat) where a thin layer of thermal compound no longer improves the conductivity, but it's a function of the surface state not the materials involved. In a hybrid circuit with an alumina substrate the metalization is in intimate contact with, and bonded to, the alumina so you would simply solder the part down.