Large solubility of silicon in an incongruent nitride: The case of reactively magnetron co-sputtered W-Si-N coatings

Here we examine the equilibrium solubility of silicon in a nitride matrix with a relatively low bond ionicity (i.e., the incongruent W2N). By reactively magnetron co-sputtering under a kinetically unconstrained growth condition, we systematically increased the amount of silicon in W-Si-N coatings, and investigated in detail the composition, chemical bonding, phases, and microstructure through a combination of energy-dispersive x-ray spectrometry, x-ray photoelectron spectroscopy, x-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. Up to a 12 at.% Si content was dissolved in the W2N matrix; the well-crystallized NaCl-structured W-Si-N solid solution exhibited an uninterrupted fibrous growth structure through the whole thickness (similar to 4 mu m). Beyond this critical solubility, the coatings turned into nanocomposites in which some silicon nitride was segregated as a second phase and interrupted the vertical growth of nanocolumns. The large solubility was rationalized in terms of the electrochemical affinity between silicon nitride and the incongruent W2N. This finding highlights the importance of thermodynamic aspects in developing novel nanocomposite structures.