@ Komal, you may use a glass cutter to make a straight mark of the required length with it on the alumina sample. After that gently hold the alumina substrate and while applying pressure on the crack line, break it. It is exactly similar to the glass cutting procedure with the glass cutter.

Thankyou J. C. Tarafdar sir for your response. Sir, Alumina is a 9 on the Mohs scale. It cannot be scratched by anything with a lower hardness, so alumina can only be damaged by very few materials, including diamond. But again the issue is precise cutting in millimetre dimensions.