Thermal stability and mechanical properties of TiAlN-based multilayer and monolithic coatings
Sammanfattning: This thesis explores the thermal stability, microstructure, mechanical properties and cutting performance of multilayer and monolithic cubic TiAlN hard coatings. The aim is to increase the understanding of how the coatings’ microstructure and properties are affected by a layered structure when exposed to high temperatures.The coatings were deposited on cemented carbide substrates, using a full scale industrial reactive cathodic arc evaporation system at Seco Tools AB. The thermal stability was investigated by differential scanning calorimetry and the microstructure was characterized with analytical transmission electron microscopy, x-ray diffractometry and atom probe tomography. The mechanical properties and cutting performance were studied by nanoindentation and metal machining, respectively.The decomposition of cubic TiAlN transpire in two steps, first by an isostructural decomposition to cubic AlN- and cubic TiN-rich domains, which is followed by a phase transformation of cubic AlN to hexagonal AlN. In this work I show that the isostructural decomposition occurs in two stages, namely: Spinodal decomposition (initial stage) and coarsening (latter stage). During the initial stage, the phase separation proceeds with a constant size of the AlN- and TiN-rich domains, with a measured wavelength of ~2.8 nm. The time needed for the initial stage depends on the temperature as well as the composition. Following the spinodal decomposition, the AlN- and TiN-rich domains coarsen. The coarsening process is kinetically limited by diffusion and is not dependent on the composition.If the cubic TiAlN is grown as a multilayer coating, with TiN as the alternating layer type, the decomposition behavior will be different. The isostructural spinodal decomposition in the multilayers starts at a lower temperature compared to the monolithic TiAlN, while the subsequent transformation from cubic AlN to hexagonal AlN is delayed to higher temperatures. The TiN-layers confine the coarsening of the hexagonal AlN resulting in smaller domains. Mechanical testing reveals that, despite the 60 vol. % of the softer TiN, the asdeposited multilayers show a similar or slightly higher hardness than the monolithic Ti0.34Al0.66N. In addition, the multilayers show a more pronounced age hardening compared to the monoliths.For short annealing times (<1 min) at 850 °C a layer rich in AlN followed by areas rich in TiN is observed parallel to the TiAlN/TiN interfaces in the multilayer stack. This microstructural feature indicates the presence of surface directed spinodal decomposition in the multilayer coatings. The lack of a layered structure further into the TiAlN-layer is due to the growth induced elemental fluctuations, which trigger an earlier onset of the coarsening. The coherency stresses generated across the multilayer interfaces also influence the decomposition. However, in this case the surface directed spinodal decomposition is the dominating mechanism for the altered thermal stability.Finally, during metal machining of AISI-316L stainless steel the Ti0.34Al0.66N/TiN multilayers, regardless of period, show an improved crater wear resistance compared to a Ti0.34Al0.66N monolith. The multilayer structure and the local coherency across the multilayer interfaces, seen in the as-deposited state, is present also after the metal machining. It is further revealed that the Ti0.34Al0.66N layer decomposes to AlN- and TiN-rich domains during the cutting operation.
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