Annealing of sputter-deposited nanocrystalline Cr-Ta coatings in a low-oxygen-containing atmosphere

As a protective hard coating on glassmolding dies, Cr-Ta coatings were fabricated on binderless tungsten carbide substrates with a Ti interlayer by RF magnetron sputtering. The nanocrystalline Cr-Ta coatings were deposited at 550 degrees C, which revealed one nanocrystalline phase for the Ta-rich coating and two nanocrystalline phases for the Cr-rich coating. Annealing treatment was conducted at 600 degrees C in a 12 ppm O-2-N-2 atmosphere to evaluate the coating performance in a realistic glassmolding environment. Both Auger electron spectroscopy and X-ray photoelectron spectroscopy depth profiles verified the outward diffusion of Cr, which formed a protective coating for the Cr-rich coatings. A scale of Cr2O3 and a Cr-depleted transition zone near the surface were identified by conducting a transmission electron microscopy investigation on the annealed Cr0.71Ta0.29 coating. The Cr-rich coating absorbed a smaller amount of oxygen, exhibited greater hardness, and maintained nanoscale surface roughness after annealing in the glass molding atmosphere, thus making it an appropriate protective coating for the die material. (C) 2012 Elsevier B.V. All rights reserved.