Ž . Wear 225–229 1999 273–284 Tool life and wear mechanism of uncoated and coated milling inserts Jie Gu a, ) , Gary Barber b , Simon Tung c , Ren-Jyh Gu b a GM Powertrain, USA b Oakland UniÕersity, Rochester, MI, USA c GM Research and DeÕelopment Center, USA Abstract A systematic study was conducted for face milling inserts cutting 4140 preheat treated steel. The flank wear of uncoated C5 carbide insert, as well as TiN, TiAlN, and ZrN coated inserts was evaluated and ranked. Tool life was expressed as the function of cutting speed and feed. This information is useful for production optimization. Wear mechanisms of attrition, abrasion, mechanical fatigue, and thermal fracture were identified and were represented by wear maps. The TiN and TiAlN coatings provided significant improvement in tool life. The ZrN coated inserts performed about as well as the uncoated C5 carbide inserts. q 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Tool; Wear; Milling inserts 1. Introduction Cutting tool wear is the result of load, friction, and high temperature between the cutting edge and the workpiece. Several wear mechanisms can occur during metal cutting: adhesive wear, abrasive wear, diffusion wear, oxidation w x wear, and fatigue wear 1,2 . Adhesive wear, also known as attrition wear, occurs mainly at low temperature. This mechanism often leads to Ž . the formation of a build up edge BUE . It is a dynamic process, with successive layers from the chip being welded and becoming part of the cutting edge. The BUE can be sheared off but it will start to form again. When it reaches an unstable size, it breaks away in small pieces or fracture. When higher cutting temperatures are reached, the condi- tions for this phenomenon are largely removed. Abrasive wear is mainly caused by the hard particles of the work- piece material. The ability of the cutting edge to resist abrasive wear is related to its hardness. Diffusion wear is more affected by chemical factors during the cutting process. The chemical properties of the tool material and affinity of the tool material to the work- piece material will determine the development of the diffu- sion wear mechanism. The metallurgical relationship be- tween the materials will determine the amount of wear. ) Corresponding author Fatigue wear is often a thermo-mechanical combination. Temperature fluctuations and the loading and unloading of cutting forces can lead to cracking and breaking of cutting edges. Intermittent cutting action leads to repetitive heating and cooling as well as shock from cutting edge engage- ment. Pure mechanical fatigue can also occur when the cutting forces are too high for the mechanical strength of the cutting edge. Many tool life equations have been proposed to predict w x flank wear 3–8 . While these equations provide insight to wear trends, they are valid only in a limited range where the appropriate constants of the equation have been deter- mined. A useful way of presenting the flank wear is using the dimensionless wear rate. Maps representing the flank wear rates consist of an abscissa of cutting speed and an w x ordinate of feed rate 9–16 . The tribological properties of a single tool material never satisfy all performance requirements. Coated tools can produce high wear resistance on the surface with high toughness in the substrate material. Properly applied coat- ings increase the surface hardness of cutting tools at high cutting temperatures, thus minimizing abrasive wear. The coating provides a chemical barrier to decrease diffusion or reaction between the tools and the workpiece, thus reducing tool wear. Most of the heat generated during machining goes into the chips, and the tool substrate stays cooler than with uncoated tools. The high lubricity of most 0043-1648r99r$ - see front matter q 1999 Published by Elsevier Science S.A. All rights reserved. Ž . PII: S0043-1648 99 00074-5