Thermally enhanced mechanical properties of arc evaporated Ti 0.34 Al 0.66 N / TiN multilayer coatingsIn contrast to the monolithic c-Ti 1Àx Al x N, the isostructural spinodal decomposition to c-AlN and c-TiN in c-Ti 1Àx Al x N/TiN multilayers has almost the same onset temperature for the compositions x ¼ 0.50 and 0.66. Differential scanning calorimetry also shows that the decomposition initiates at a lower temperature compared to the monoliths with the same Al-content. Z-contrast scanning transmission electron microscopy imaging reveals a decomposed structure of the multilayers at temperatures where the monoliths remain in solid solution. In the multilayers, the decomposition is initiated at the internal interfaces. The formation of an AlN-rich layer followed by a TiN-rich area parallel to the interface in the decomposed Ti 0.34 Al 0.66 N/TiN coating, as observed in atom probe tomography, is consistent with surface directed spinodal decomposition. Phase field simulations predict this behavior both in terms of microstructure evolution and kinetics. Here, we note that surface directed spinodal decomposition is affected by the as-deposited elemental fluctuations, coherency stresses, and alloy composition. V C 2013 American Institute of Physics. [http://dx.

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Structure and mechanical properties of nanoscale multilayers of ZrN/Zr 0.63 Al 0.37 N grown by reactive magnetron sputtering on MgO (0 0 1) substrates at a temperature of 700°C are investigated as a function of the Zr 0.63 Al 0.37 N layer thickness. The Zr 0.63 Al 0.37 N undergoes in situ chemical segregation into ZrN-rich and AlN-rich domains. The AlN-rich domains undergo transition from cubic to wurtzite crystal structure as a function of Zr 0.63 Al 0.37 N layer thickness. Such structural transformation allows systematic variation of hardness as well as fracture resistance of the films. A maximum fracture resistance is achieved for 2 nm thick Zr 0.63 Al 0.37 N layers where the AlN-rich domains are epitaxially stabilized in the metastable cubic phase. The metastable cubic-AlN phase undergoes stress-induced transformation to wurtzite-AlN when subjected to indentation, which results in the enhanced fracture resistance. A maximum hardness of 34 GPa is obtained for 10 nm thick Zr 0.63 Al 0.37 N layers where the wurtzite-AlN and cubic-ZrN rich domains form semi-coherent interfaces.

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Reactive cathodic arc deposition was used to grow substoichiometric solid solution cubic c-(Ti 1-x Al x)N y thin films. The films were removed from the substrate and then heated in an argon environment to 1400 °C. Via solid state reactions, formation of MAX phase Ti 4 AlN 3 was obtained. Additional phases such as Ti 2 AlN, c-TiN, w-AlN, Al 5 Ti 2 and Al 3 Ti were also present during the solid state reaction. Ti 4 AlN 3 formation was observed in samples with an Al metal fraction x < 0.63 and a nitrogen content 0.4 < y < 0.6. Regardless of the initial composition, formation of Ti 4 AlN 3 started in Ti 2 AlN crystal plates in the temperature range between 1200-1400 °C. Accompanying the onset of Ti 4 AlN 3 was the presence of an intermediate structure identified as Ti 6 Al 2 N 4 , consisting of alternating layers of intergrown Ti 2 AlN and Ti 4 AlN 3 phases with a half-unit-cell stacking. We suggest that the formation of Ti 4 AlN 3 occurred via intercalation of aluminum and nitrogen along the basal plane accompanied by a simultaneous detwinning process. In addition we propose that this formation mechanism can be used to obtain MAX phases of high n order.

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