Critical depth of cut in scribing experiments (with increasing depth of cut) is defined as the depth as which material removal transitions from ductile to brittle, with appearance of first crack. So to answer your question, ductile regime ends at the criticial depth of cut, and brittle regime begins.
One needs to take a critical second look at the "ductile regime" grinding of brittle materials.
Material properties are characterized at strain rates of 10^-1 to 1. Much oif the machining the strain rates are of the order of 10^5 and for grinding it is even higher. (Strain rate = Velocity / chip thickness). Under these conditions material properties are not well known. Hence to use analysis based on low strain properties of materials may be a mistake.
Then there is a matter of micro mini crack generation, compression and flow of the secondary glass phase in most ceramics. Researchers have to take a close look at this combination of phenomena to see if these are the factors which might be mistakenly attributed as "Ductile flow". The later requires dislocations and their movement, which may not be present in ceramics at any strain rate?
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