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

Reactively magnetron sputtered high-entropy metal-sublattice (Al,Cr,Nb,Ta,Ti)N coatings have been alloyed with Si concentrations between xSi = 6.4 and 15.0 at.%. All coatings are single-phase fcc structured and their hardness initially increases from ~32 to 35 GPa with Si-alloying up to xSi = 9.8 at.%, and then decreases to ~24 GPa for higher Si contents. Contrary, the indentation modulus E continuously decreases from ~470 to 350 GPa by Si-alloying. Also, the decomposition of the fcc structure during vacuum annealing is shifted from 1000 to 1200 °C with the addition of Si. The hardness initially increases during vacuum annealing and reaches a maximum of 37 GPa with Ta = 1000 °C at xSi = 12.0 at.%. During oxidation experiments in ambient air at 850 °C for up to 100 h, a 2700 nm single-phase rutile-structured oxide scale forms at the Si-free (Al,Cr,Nb,Ta,Ti)N with a parabolic growth rate. The rate changes to a logarithmic-like behavior with the addition of Si, resulting in only ~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)O2, is energetically preferred over separate phases above 509 K, due to the higher configurational entropy. Below this temperature the decomposition towards (Al,Ta,Ti)O2 + (Cr,Nb)O2 would be favored (when considering just chemical contributions), but kinetically restricted.