TEM investigation of wear mechanisms during metal machining

Abstract

An enhanced technique for the preparation of cross-sectional thin foils of CVD coated cemented carbide inserts after metal cutting for studies by analytical transmission electron microscopy is presented. The technique is described in detail and involved the following main steps: (i) controlled CVD deposition, (ii) cutting test, (iii) deposition of a protective PVD film atop the wear crater and (iv) preparation of the cross-sectional thin foils of the chip-tool interface. The technique ensured electron transparency to be obtained on the crater wear land and the whole chip-tool contact length area could be studied in cross-section by transmission electron spectroscopy (TEM). The use of CVD multilayer technique together with PVD technique contributed to the further facts: the positions of the thin foils within the coating layer could be controlled in detail and the coating-chip interface could be studied. The coating investigated was a κ-Al 2O 3-TiC multilayer which was produced by chemical vapour deposition (CVD). The κ-Al 2O 3→ α-Al 2O 3 phase transformation in the metastable κ-Al 2O 3 coating was confirmed to occur locally during metal cutting on the rake face. The transformed α-Al 2O 3 exhibited a relatively high degree of plasticity during metal cutting. Plastic deformation and ductile fracture of α-Al 2O 3 formed as a result of the phase transformation of κ-Al 2O 3 were identified as being the main mechanisms leading to crater wear. The wear mechanisms of Al 2O 3 coating in general and the microstructural changes occurring in the κ-Al 2O 3 and transformed α-Al 2O 3 regions during metal cutting are described in detail.