Insight into the Mechanisms of Nitride Films with Excellent Hardness and Lubricating Performance: A Review.

Transition metal nitride (TMN) films with excellent hardness and lubricating performance are versatile low dimension materials, which are widely used in various fields including industries, transportation, aerospace, and so on. This paper introduces one film design strategy and provides a review of the mechanisms for strengthening and lubricating nitride films. The design strategy refers to two aspects which determine the structures, the performance, the components, and the chemical constitutions of nitride films The strengthening mechanisms of nitride films are then illuminated in detail, including the solid solution effect, the grain size effect, the secondary phase effect, the stress or stress field effect, the template effect, and the valence electron concentration effect. Five lubricating mechanisms are next summarized, including the easy-shear nature, the tribo-chemical reactions, the lubricious fluorides, the textured contact surface, and the synergistic effect. This paper aims to give a comprehensive introduction for understanding the mechanisms of strengthening and lubrication of nitride films for students and researchers, as well as to understand the current research progress in nitride films for exploring research gaps.

Investigation into fresh air delivery performance with vortex ring.

An air supply mode based on vortex ring structures was proposed to deliver fresh air for good indoor air quality. This study focused on the influence of air supply parameters, including formation time (T*), supply air velocity (U0), and supply air temperature difference (ΔT), on the fresh air delivery performance of an air vortex ring using numerical simulations. The cross-sectional average mass fraction of fresh air (Ca) was proposed to evaluate the fresh air delivery performance of the air vortex ring supply. The results showed that the convective entrainment of the vortex ring resulted from the combined effect of the induced velocity generated by the rotational motion of the vortex core and the negative pressure zone. When the formation time T* < 3.5, Ca increases with an increase in the formation time but decreases with an increase in the formation time for T* > 3.5. The variation in Ca is insignificant for U0 < 3 m/s or U0 > 3 m/s but decreases with an increase in the supply air temperature difference (ΔT). Thus, the optimal air supply parameters for air vortex ring supply are identified as T* = 3.5, U0 = 3 m/s, ΔT = 0 °C. Compared with the traditional circular and square free air jets, within 25Ds (Ds: the diameter of air supply inlet) downstream from the air inlet, the fresh air delivery performance of the air vortex ring supply is increased by a maximum of nearly 200 %, with an average improvement of approximately 120 %.