Fracture in self-lubricating inserts: A case study

Abstract

A case study is carried out to reveal the fracture mechanism of self-lubricating inserts. The insert is made up of 98.5 wt% Zirconia Toughened Alumina (ZTA) reinforced with 1.5 wt% CuO. The insert is fabricated through the powder metallurgy route, followed by the hot isostatic pressing (HIP) technique (for densification). After densification, the insert is shaped and sized according to ISO SNUN 120408. The workpiece material used for machining is AISI 4340 steel. A conventional NH-26 lathe is used for machining as well as for creating facture inside the insert. In this investigation, the machining operations are carried out till the failure of inserts does not take place. The fracture analysis suggested a synonymous fracture mechanism that occurs for brittle facture (facture without any deformation). The fracture analysis seen through FESEM images observed that the initiation of crack that happened without any dislocation movement results in chipping, cracking, and flaking on the surface. The crack is initialized due to the generation of high localized stresses. The stresses were developed due to the continuous movement of chips concentrated slightly away from the cutting edges. The developed stresses are relieved by creating small cracks beneath the surface. These cracks are propagated in the radial direction without any dislocation-induced plasticity. The mechanism is well confirmed through the FESEM images of fracture specimens.