Failure modes of an alumina-based machining ceramic

Attempts have been made to reproduce "comb" cracks in disposable cuboid alumina tool tips as a preliminary to an investigation into catastrophic failure during machining.
During lathe tests the tips withstood rates of machining several times greater than the normally accepted "comb" crack thresholds.
Chemical analysis revealed the anomalous presence of calcia in the current tips which was thought to be responsible for the presence of a liquid phase during sintering.
Examination by electron microscopy of the broken tool tips
revealed a predominance of intergranular fracture and two distinct modes of transgranular cleavage, one of which was influenced by pore concentrations in geometric patterns.
Moduli values were some 40% lower than previously used batches of tips, which permitted the absorption of greater strain induced by either mechanical or thermal sources.
No damage was evident in the tool tips after the first cut up to a feed rate of 0.03125 in/rev but catastrophic failure always occurred at the instance of re-engagement at feed rates of 0.028 in/rev or greater.
The difference in chemical analysis, porosity and modulus would appear to be responsible for the absence of "comb" cracking and for the phenomenon of catastrophic failure.
A test rig was designed for operation on an induction heater to
simulate the condition of abrupt disengagement during machining. Failure thresholds were established for a variety of induction coils under closely controlled conditions of heatining and loading.

Thesis submitted to the Council for National Academic Awards
in partial fulfilment of the requirements for the Master of Philosophy degree.