Silicon nitride ball bearing ceramic flaw sizing

Recent (week-ago) office visit. Used to make the five-microns vs. half-inch flaw-size point about why ceramics fail differently from metals.

If you're talking about a ceramic — I had some guys in the office from a company last week, and they're trying to make better ball bearings for jet engines. They want to make them out of silicon nitride because it's one of the toughest ceramics. The size flaws they've got to be looking for are five microns, because five microns will be a critical flaw size in fracture mechanics for a ceramic. For a metal, maybe half an inch, two inches is a critical flaw size. You can have some pretty crappy welds in a metal and it still is not going to fail. Although if you have a crappy steel and crappy welds, it will. That's helped put my kids through college, okay.