91av

TO MAKE longer-lasting engine parts for ships, researchers in the US Navy
have developed metal and ceramic composite materials that are soft in some parts
and hard in others. They worked out how to do it by modifying one of the oldest
metal technologies of them all—casting.

Called sedimentation casting, the technique was developed by Steven Fishman
and his colleagues at the Office of Naval Research in Arlington, Virginia. The
researchers were able to make aluminium bronze parts—in this case,
propeller shaft seals—coated with a thin layer which is 75 per cent
tungsten carbide.

The first step is to make an ingot using a uniform molten mix of aluminium
bronze and a small amount of tungsten carbide ceramic particulate. Once the
ingot cools, it is remelted in a mould, and the heavier ceramic particles sink
to the bottom of the mould. “The part still has the good mechanical properties
of bronze, but these extremely hard tungsten carbide particles make that surface
much more wear-resistant,” Fishman told 91av.

“For clutch plates, seals and other applications, it means parts that will
last longer—perhaps even the life of a ship, eventually. And we can also
mix in other types of particles in the same layer, to improve lubricity.” The
new technique is influenced by centrifugal casting, patented by US Navy
researchers in 1992, which casts materials while spinning them to create a
structure of variable composition.

Krishan Chawla, a materials scientist at the University of Alabama at
Birmingham, comments that both these technologies “allow us to make materials
with controlled microstructure, hard only where needed, for example, and to do
it much faster and more economically than with other methods”.