4.7 Article

Improving phase stability, hardness, and oxidation resistance of reactively magnetron sputtered (Al,Cr,Nb,Ta,Ti)N thin films by Si-alloying

Journal

SURFACE & COATINGS TECHNOLOGY
Volume 416, Issue -, Pages -

Publisher

ELSEVIER SCIENCE SA
DOI: 10.1016/j.surfcoat.2021.127162

Keywords

High-entropy metal-sublattice nitrides (HESN); High-entropy alloys (HEA); Oxidation; Thermal stability; Magnetron sputtering; PVD

Funding

  1. Austrian COMET Program [872176]
  2. Swedish Research Council VR-RFI [2017-00646_9]
  3. Swedish Foundation for Strategic Research [RIF14-0053]
  4. TU Wien Bibliothek

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The addition of silicon in high-entropy metal-sublattice (Al, Cr, Nb, Ta, Ti)N coatings can enhance the hardness initially but lead to a decrease in hardness at higher silicon contents. Silicon alters the decomposition temperature and oxidation behavior of the coatings, affecting the structure and pore size of the oxide scale formed.
Reactively magnetron sputtered high-entropy metal-sublattice (Al,Cr,Nb,Ta,Ti)N coatings have been alloyed with Si concentrations between x(si) = 6.4 and 15.0 at.%. All coatings are single-phase fcc structured and their hardness initially increases from similar to 32 to 35 GPa with Si-alloying up to x(si) = 9.8 at.%, and then decreases to similar to 24 GPa for higher Si contents. Contrary, the indentation modulus E continuously decreases from similar to 470 to 350 GPa by Si-alloying. Also, the decomposition of the fcc structure during vacuum annealing is shifted from 1000 to 1200 degrees C with the addition of Si. The hardness initially increases during vacuum annealing and reaches a maximum of 37 GPa with T-a = 1000 degrees C at x(si) = 12.0 at.%. During oxidation experiments in ambient air at 850 degrees C for up to 100 h, a 2700 nm single-phase rutile-structured oxide scale forms at the Si-free (Al,Cr,Nb,Ta,TON with a parabolic growth rate. The rate changes to a logarithmic-like behavior with the addition of Si, resulting in only similar to 280 nm oxide scale after 100 h. Also, for the Si-containing coatings, the oxide scale shows only one crystalline rutile structure. The pore size in the oxide scale of the Si-free coating is considerably reduced by Si-addition. The oxides growing at the Si-containing coatings show an opposing Si- and Cr-gradient - with much smaller pores in the Si-rich inner region - which shows a gradual transition to the remaining nitride. Ab initio based calculations confirm that the formation of a single-phase rutile-structured solid solution oxide, (Al,Cr,Nb,Ta,Ti)O-2, is energetically preferred over separate phases above 509 K, due to the higher configurational entropy. Below this temperature the decomposition towards (Al,Ta,Ti)O-2 + (Cr,Nb)O-2 would be favored (when considering just chemical contributions), but kinetically restricted.

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