Relationship between the microstructure and nanoindentation hardness of thermally evaporated and magnetron-sputtered electrochromic tungsten oxide films

Tungsten oxide (WOx) films were fabricated by (i) reactive thermal evaporation (RTE) at room temperature with oxygen ambient pressure PO(2)as a parameter, and (ii) reactive magnetron sputtering (RMS) with substrate temperature Tsas a parameter. The film structure revealed by x-ray photoelectron spectroscopy, x-ray diffraction, density measurements, infrared absorption, and atomic force microscopy was correlated with the nanoindentation hardness H. The RTE WOxfilms deposited at high PO(2)were amorphous and porous, while H depended appreciably on normalized penetration depth hD(indentation depth/film thickness) due to the closing of the pores at the point of indentation. Decrease in PO(2)from 10 to 2 × 10-3retort led to smaller porosity, weaker hDdependence of H, and higher average H (measured at hD≈ 0.2 to 0.3, for example). The RMS WOxfilm deposited at room temperature was amorphous and denser than all RTE films. The rise in substrate temperature Tsfirst densified the film structure (up to 110 °C) and then induced crystallization with larger grain size for Ts≥ 300 °C. Correspondingly, the hDdependence of H became weaker. In particular, H of the RMS sample deposited at 110 °C showed a peak at hDslightly above 1 owing to pileup at the contact point of indentation. For higher Ts, pileup occurred at shallower hDand the average H (measured at hD≈ 0.2 to 0.3, for example) rose, accompanied by the increase of grain size.