PHASE-TRANSFORMATION IN CHEMICALLY VAPOR-DEPOSITED K-ALUMINA

The kappa --> alpha transformation in kappa-Al2O3 Coatings obtained by chemical vapour deposition (CVD) has been studied by isothermal heat treatments and high speed metal-cutting tests. The phase analyses were carried out using X-ray diffraction and thc morphological variations occurring upon the kappa --> alpha transformation were elucidated using scanning electron microscopy. Based on isothermal transformation curves, a time-temperature-transformation diagram was constructed for CVD kappa-Al2O3. The effects of the kappa --> alpha transformation on adhesion and cutting performance were studied by scratch adhesion and cutting tests respectively. As a result of the volume contraction encountered in the kappa --> alpha transformation, severe cracking of the Al2O3 coating layer occurred and the coating properties deteriorated. It was further confirmed that the kappa --> alpha transformation may occur in kappa-Al2O3 coatings as a result of high speed metal cutting.