Phase transformation in kappa- and gamma-Al2O3 coatings on cutting tool inserts

• The phase transformation in as-deposited, metastable kappa- and gamma-alumina coatings on cutting inserts has been studied by a combination of X-ray diffraction, scanning electron microscopy and cathodoluminescence. In the case of kappa-alumina, mechanical forces during metal cutting lower the transformation temperature of the metastable phases to the thermodynamically stable alpha-alumina phase from 1050 degrees C to 930 degrees C. This is the reason why that coating has a similar performance during metal cutting when compared to the stable ce-alumina phase. The transformation temperature in gamma-alumina coated cutting tools is found to be as low as 950-975 degrees C. Cathodoluminescence (CL) has been demonstrated as a possible method for differentiation between the various alumina phases. Specifically, both kappa- and gamma-alumina films have revealed a strong room temperature CL with different peak energies depending on the phase. CL of the metastable alumina coatings annealed at the transformation temperatures corresponds to the stable alpha-alumina phase.

Nyckelord

Alumina

Phase transformation

Coatings

X-ray diffraction

Cathodoluminescence

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