Wear behavior of Ti1-xAlxN-based coatings during turning

Abstract: Ti1-xAlxN coatings are extensively used on cutting tools used for metal cutting. In this thesis, the wear behavior of TiN, TiAlN with different Al-content, and TiAlWN and TiAlMoN coatings is investigated after turning, using electron microscopy and X-ray absorption spectroscopy techniques. An in operando study using high-energy X-ray diffraction during turning is also carried out, to understand the strain and phase evolution of TiAlN coatings during machining. The main wear mechanisms when cutting stainless steel 316L with cutting speeds from 60 m/min up to 220 m/min are investigated. The results show a difference in wear behavior with cutting speed. At low cutting speeds, adhesive wear is the main mechanism that generates coating failure, causing fracture of the coating. The higher Al-content coatings (x ≥ 0.53) perform better compared to lower Al-content coatings, likely due to a better fracture resistance. At higher cutting speeds, 220 m/min, crater wear due to abrasive wear and chemical reactions between coating and workpiece material occurs. In this case, the high temperatures achieved during turning results in formation of hexagonal (h)-AlN in Ti0.38Al0.62N coatings, which lowers their wear resistance. For TiAlMoN and TiAlWN, an improved wear behavior is observed compared to pure TiAlN, because it retards spinodal decomposition and the subsequent formation of h-AlN. Investigations of the sliding area using TEM, EDS and XANES spectra from the Ti 1s-edge reveals that there are differences in level of spinodal decomposition, thus differences in temperature, in different regions of the tool. During in operando orthogonal turning of alloy steel, spinodal decomposition was observed to take place after only 10 s of turning for the highest Al-content coatings. Decomposition occurs where the temperature of the rake face is the highest. In summary, the results achieved lead to a better understanding of the interactions between tool and workpiece material and the different wear mechanisms which may expand the application envelope for these coatings.

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