Microstructure and age hardening of C276 alloy coatings

C276 alloy layers were deposited on 4140 steel substrates by plasma enhanced magnetron sputtering. The as-deposited 12-μm-thick coatings exhibit a face centered cubic nanocrystalline structure with a grain size of 37-46 nm and a uniform composition that replicates the C276 alloy deposition source. The as-deposited coating hardness H = 6.2 ± 0.3 GPa is four times larger than the known bulk H = 1.5-1.9 GPa. This is attributed to the small grain size and associated Hall-Petch hardening. Annealing in air for 1 h at 600 °C, 700 °C, and 800 °C causes negligible changes in grain size but results in changes in microstructure and texture and a significant increase in H to 7.3-8.5 GPa. Compositional maps by Auger electron and energy-dispersive spectroscopies as well as a secondary x-ray diffraction peak suggest that this increase is due to segregation and precipitation of Cr- and Mo-rich regions.