Preparation And Properties Of The V-Al-(C,N) Nanocomposite Coating

Manufacturing is one of important pillar industries for the development of the country,and traditional hard coating of transition metal carbides and nitrides have been widely applied as protective coating in drilling,cutting and anti-wear components.However,their limited hardness,poor toughness,oxidation resistance and thermal instability at elevated temperatures cannot meet the demand in high-speed manufacturing processes and tribologically stressed components application.Nanocomposite coatings composed of crystallite and amorphous phases have recently received wide attention owing to fundamental research and industrial applications by virtue of their excellent properties such as high hardness and toughness,good wear resistance and oxidation resistance.V-based coatings has a lower friction coefficient over a wide temperature range（293–993K）as self-lubrication during sliding,which can meet green cutting demand.In this thesis,the preparation and properties of V-Al-C and V-Al-C-N nanocomposite coatings are focused,the coating performance are discussed in terms of the structure evolution.Firstly,V-Al-C and V-Al-C-N coatings were prepared by direct current magnetron sputtering,and the effect of N doping on their chemical composition,microstructure and mechanical properties of V-Al-C coatings were studied.The results showed that the V-Al-C coating displayed columnar structure with coarse grain extending through the coating thickness.When the nitrogen was incorporated,the coating structure evolved into nanocomposite structure composed of nanocrystallite and amorphous carbon.The hardness increased from（14±0.48）GPa to（24.5±0.8）GPa and the toughness was significantly improved（H/E>0.1）,and the coating adhesion to substrate was improved from 42 N to 53 N.The effect of N doping on friction behavior of the coatings under different contact conditions（air,distilled water and artificial seawater）were studied.In air condition,the friction coefficient decreased from 0.70 to 0.42,owning to the synergy interaction between V₂O₅ and amorphous carbon during sliding.The friction coefficients of the both coatings in distilled water and artificial seawater were lower than those in air due to the boundary lubrication forming lubricative film by absorbed water.The friction coefficient in seawater was lower than those in distilled water,resulting from the formation of Mg（OH）₂ and CaCO₃ during sliding.However,the wear rates of the both coatings in artificial seawater were larger than in distilled water,which demonstrated a synergism between corrosion and wear in artificial water.The V-Al-C coating was all worn out under different contact condition owing to severe abrasive wear.However,the V-Al-C-N coating showed better wear resistance,with a wear rate of 3.0×10-16 m3/Nm in air and 1.4×10-15 m3/Nm in artificial water,respectively.Finally,V-Al-C coatings with various carbon contents have been prepared by direct current reactive magnetron using V2 AlC target under different CH₄ flow rate.This work was focused on effect of CH₄ flow rate on composition,morphology and microstructure of the nanocomposite coatings.The results showed that the primary V-Al-C coating displayed columnar structure with coarse grain.When CH₄ was introduced,the columnar became fibrous and dense.The coating evolved into nanocomposite structure composed of nanocrystalline and amorphous carbon further increasing CH₄ flow rate to 10 sccm.The coating with 5 sccm CH₄ flow rate displayed the maximum hardness of 28.74 GPa and the lowest wear rate of 2.8×10-16 m3/Nm,meanwhile the toughness was significantly improved（H/E>0.1）.The primary V-Al-C coating was worn out owing to the severe abrasive wear,while all coatings with nanocomposite structure exhibited lower friction coefficient and wear rate than the primary coating,which was attributed to the synergistic lubricant effect derived from both amorphous carbon and V₂O₅ Magnéli phases formed during friction stage.