Transitions in wear mechanisms of alumina cutting tools

Abstract

Alumina inserts were used to turn medium carbon steel bars in dry, continuous cutting conditions at a cutting speed of 450 m min-1. Three wear mechanisms were identified: plastic deformation-induced necking of asperities; grain spallation; and bulk plastic deformation of alumina. The crater wear land was characterized by spikes nearer the cutting edge and ridges further away because the temperatures and compressive stresses were higher nearer the cutting edge. The crater wear-rate, as measured by the maximum crater depth, was controlled by plastic deformation-induced necking, which was also responsible for the formation of the spikes. The flank wear land was characterized by ridges during the initial stage of machining, formed by the spallation of grains and their subsequent smearing by single glide. As the temperatures and compressive stresses on the flank wear land increased with increasing width of the wear land and machining forces, a transition to a two-zone wear feature (TF) characterized by spikes and ridges occurred, similar to that observed on the rake face. Seizure then followed and a transition in wear rate (TR) was observed in that further increases in the width of the flank wear land occurred by bulk plastic deformation of the tool material at a much reduced rate. The present work shows that there exits a transition zone (from TF to TR) in flank wear, separating the rapid wear mechanism of grain spallation from the slower mechanism of bulk plastic deformation, defined by both machining time and a critical flank wear land width.