The thermal stability of nanocomposite W-Si-N coatings, which had been sputter deposited at increased silicon target currents, was studied by annealing at 800 or 900 degrees C in vacuum by using x-ray diffraction, x-ray photoelectron spectroscopy, electron probe microanalysis, scanning electron microscopy, atomic force microscopy, and, microhardness testing. The crystalline W2N coatings were decomposed to W with the loss of interstitial N atoms and the presence of microcracks at their surfaces after annealing at 900 degrees C. The coating consisting of nanocrystalline (nc-) W2N imbedded in amorphous (a-) Si3N4 matrices and the amorphous coatings underwent partial recrystallization, the loss of interstitial N atoms, and morphological changes upon annealing. The nC-W2N/a-Si3N4 coating had the least N in grain boundaries and the lowest atomic O/N ratio, and it had no failure after annealing at 900 degrees C, showing high thermal stability. The hardness of the coatings was decreased, but the nc-W2N/a-Si3N4 coating maintained the highest hardness (46.6 +/- 3.7 GPa) after annealing at 900 degrees C. (c) 2006 American Vacuum Society.