Effect of Humidity on Wear of TiN Coatings: Role of Capillary Condensation

Coated ball-on-disk wear configuration was used to study the effect of relative humidity, water vapor pressure, and water on wear of TiN coatings in the temperature range of room temperature to 100 °C. Two kinds of experiments were designed: one at constant temperature with varying humidity and the second at constant water vapor pressure with varying temperature. Temperature variation experiments were also conducted in water. The trends in wear volume after a fixed sliding distance were analyzed. At constant temperature, the wear volumes increased with humidity/water vapor pressure. However, at a constant vapor pressure, wear volumes remained roughly invariant with temperature until a critical temperature below which they rose sharply. In contrast, the wear rate increased with temperature for tests in liquid water. Considering calculations based on Kelvin’s equations and further characterization of the wear surfaces using profilometry, XPS and FIB techniques, it has been strongly suggested that the anomalous temperature dependence of wear in humid air might be due to capillary condensation occurring at the contacting asperities. The wear of TiN showed two regimes in the influence of humidity. Where the humidity was below a threshold value of ~ 50 pct relative humidity (RH), there was negligible condensation and wear was low. Above 50 pct RH, the wear rate increased due to the availability of liquid water at asperities. The reason for the difference in the wear rate in vapor and liquid was hypothesized, based on XPS data, to depend on the formation of soft hydroxides in the presence of water, in contrast to the harder barrier oxide formed in the presence of oxygen, the dominant gas species in the vapor phase.