US Patent 7160825 Machinable ceramic

A high-strength machinable ceramic capable of high-precision fine machining has a coefficient of thermal expansion close to that of silicon and preferably a uniform blackish color which facilitates image processing of machined parts. The ceramic comprises a main constituent and a sintering aid. The main constituent comprises 30–59.95 mass % of boron nitride, 40–69.95 mass % of zirconia, optionally up to 20 mass % of silicon nitride and 0.05–2.5 mass % (calculated as an element) of at least one coloring additive, which is selected from C, Si, elements of Groups IIIA–IVB in the fourth period, elements of Groups IVA–VB in the fifth period, elements of Groups IVA–VIB in the sixth period of the long form periodic table, and compounds of these elements.