This study investigates the internal oxidation of Nb-Ru multilayer coatings with a cyclically gradient chemical concentration distribution along the growth direction after annealing in a 1% O-2-Ar atmosphere at 600 degrees C for 1 h. X-ray photoelectron spectroscopy and transmission electron microscopy were used to analyze the oxidation behavior of Nb-Ru coatings. The results show that Nb oxidized preferentially in a Nb-Ru coating and partial Nb atoms oxidized to Nb5+, Nb4+, and Nb2+, but Ru remained in a metallic state. After annealing, the microstructure evolved to a laminated structure consisting of alternative crystalline Nb2O5 and Ru layers. (C) 2013 Elsevier B. V. All rights reserved.