Or just extremely durable stuff. That was the deal apparently in the Empire: the machines still existed, but nobody knew how to fix them, so when they broke down the inhabitants were "back to coal and oil".

So Asimov came _that close_ to inventing steampunk. Personally I'd be interested in reading a story about these coal-powered starships.

Other space barbarians:

In a series of John Brunner stories (collected in a single volume by DAW sometime, but I can't remember the title... just the ugly yellow spine of the book) automated starships ply the spacelanes long after the collapse of the culture that had produced them.

Then there's Anderson's Galactic Empire where the "barbarians" are peoples who've been sold technology by irresponsible businessmen, Nicholas Van Rijn, and only use it to kill rob and oppress, which is very different from establishing an empire.