Thermally induced structural evolution and age-hardening of polycrystalline V1-xMoxN (x approximate to 0.4) thin films

Rocksalt-structure (B1) (V,Mo)N alloys are inherently hard and tough ceramics. However, the mechanical properties and thermal stability of (V,Mo)N solid solutions at temperatures greater than or similar to 700 degrees C of relevance for practical applications have not been previously investigated. In this work, we synthesize single-phase B1 polycrystalline V0.57Mo0.43N0.95 coatings to investigate the effects induced by temperature on the nanostructural evolution and hardness (H) of the material. Nanoindentation measurements show that the as-deposited film (H = 23 +/- 3 GPa) becomes approximate to 30% harder (up to 31 +/- 2 GPa) upon annealing at 730 C. Experimental characterization and analyses, based on dispersive X-ray spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), reveal that the age-hardening effect originates from decomposition of the solid solution into coherent strained cubic VN-rich/MoN-rich domains. The experimental results are complemented by the composition/temperature (V,Mo)N phase diagram - constructed upon ab initio molecular dynamics free-energies - which indicates that the separation observed in the solid solutions is of spinodal nature. Films annealed at temperatures exceeding 850 degrees C undergo structural coarsening, with formation of hexagonal MoxNy and cubic VN phases, which cause a decrease in hardness to approximate to 22 GPa. Our present findings indicate that (V,Mo)N coatings may offer outstanding mechanical performances during operation at elevated temperatures.Funding Agencies|Slovak Research and Development AgencySlovak Research and Development Agency [APVV-17-0320]; Scientific Grant Agency [VEGA 1/0381/19]; Operational Program Research and Development [ITMS 26210120010]; Swedish Research CouncilSwedish Research Council [2016-07213]; VINN Excellence Center Functional Nanoscale Materials (FunMat-2) [2016-05156]; Olle Engkvist Foundation; Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linkoping University (Faculty Grant SFO-MatLiU) [2009-00971]