Fatigue fracture mechanism of the bismuth free machining steel with Super hard and self-lubrication coating

Abstract

Influence of diamond like carbon (DLC) and MoS_2 coating on the fatigue reliability of bismuth machining steel was investigated with the aim of the fatigue fracture mechanism analysis. The main results were as follows. (1) In comparison with the virgin material, fatigue strength of bismuth free machining steel which conducted DLC is the 15% lowering. And bismuth free machining steel which conducted micro poral layer and MoS_2 improved 12%. (2) In the DLC material, the initial crack is held to 79.0% of the fatigue life ratio by surface restraining force of the carbide membrane. This crack becomes a trigger, and the rapid fracture has been caused. It is the factor in which a rapid hardness inclination from the surface causes the rapid fracture of this material. (3) In the MoS_2 material, micro poral layer which exists on the surface suppresses the sliding, and the initial crack has been delayed. The compressive residual stress added on the surface suppresses the progress, even if the crack is generated. It is the factor in which the fatigue strength of the MoS_2 material is improved by the control of generation and progress of the crack.