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| Home > Publications database > Microstructural evolution of TiAlN hard coatingsat elevated pressures and temperatures |
| Dissertation / PhD Thesis | DESY-2014-03100 |
2014
Linköping University - Tryck
Linköping
ISBN: 978-91-7519-372-4
Abstract: A typical hard coating on metal cutting inserts used in for example turning,milling or drilling operations is TiAlN. At elevated temperatures, TiAlNexhibits a well characterized spinodal decomposition into coherent cubic TiNand AlN rich domains, which is followed by a transformation from cubic tohexagonal AlN. Using in-situ synchrotron x-ray radiation, the kinetics of thesecond transformation was investigated in this thesis and the strongtemperature dependence on the transformation rate indicated a diffusionbased nucleation and growth mechanism. The results gave additionalinformation regarding activation energy of the transformation and the criticalwavelength of the cubic domains at the onset of hexagonal AlN. Afternucleation and growth, the hexagonal domains showed a striking resemblancewith the preexisting cubic AlN microstructure.During metal cutting, the tool protecting coating is subjected totemperatures of ~900 ºC and pressure levels in the GPa range. The results inthis thesis have shown a twofold effect of the pressure on the decompositionsteps. Firstly, the spinodal decomposition was promoted by the appliedpressure during metal cutting which was shown by comparisons with annealedsamples at similar temperatures. Secondly, the detrimental transformationfrom cubic to hexagonal AlN was shown to be suppressed at elevatedhydrostatic pressures. A theoretical pressure/temperature phase diagram,validated with experimental results, also showed suppression of hexagonalAlN by an increased temperature at elevated pressures.The spinodal decomposition during annealing and metal cutting was inthis work also shown to be strongly affected by the elastic anisotropy ofTiAlN, where the phase separation was aligned along the elastically softer<100> directions in the crystal. The presence of the anisotropicmicrostructure enhanced the mechanical properties compared to the isotropiccase, mainly due to a shorter distance between the c-AlN and c-TiN domainsin the anisotropic case. Further improvement of the metal cutting behaviorwas realized by depositing individual layers with an alternating bias. Theindividual bias layers exhibited microstructural differences with differentresidual stress states. The results of the metal cutting tests showed anenhanced wear resistance in terms of both crater and flank wear compared tocoatings deposited with a fixed bias.
Keyword(s): Dissertation
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