Microstructure, properties and thermal stability of W/B4C multilayer coating synthesized by ion beam sputtering

Microstructure, hardness and toughness as well as the thermal stability of W/B₄C multilayer coating synthesized by ion beam sputtering were investigated. The results show well-defined layered structure and uniform monolayers can be found in the as-deposited multilayer coatings with various period thickness at fixed period ratio (W:B₄C = 1:1.5). The crystallinity of W layer and the roughness of W/B₄C interfaces are changed as the period thickness increasing. It can be concluded that the nano multilayer coatings have superior strengthening and toughening effects. The hardness of as-deposited (W_5.2nm/B₄C_7.6nm)₂₀, (W_10.5nm/B₄C_15.8nm)₁₀ and (W_14.5nm/B₄C_21.5nm)₇ coatings is greatly enhanced and higher than that of pure B₄C coating, and the toughness of W/B₄C multilayer coating is significantly higher than that of pure B₄C coating. Subsequently, the investigations about thermal stability of (W_10.5nm/B₄C_15.8nm)₁₀ multilayer coatings indicate that the layered structural is stable when the heat treatment temperature is lower than 500 °C. With increasing of heat treatment temperature, the oxidation of W layer and the formation of WO₃ compounds were found. The nano layered structure was dramatically destroyed accompanying severe oxidation and slight carbonization when the temperature increase up to 600 °C. The hardness of (W_10.5nm/B₄C_15.8nm)₁₀ multilayer coating increases up to 37 GPa when the coating is heat treated below 500 °C, while it declined sharply when the coating is heat treated at 600 °C due to the a degradation of layered structure. In addition, the toughness of the multilayer coatings monotonously decreases with the increase of heat treatment temperature.