Cutting performance optimization of PVD coated inserts in milling, considering the cutting conditions, the tool geometry and the coating material properties K.–D. Bouzakis1, N. Michailidis1, K. Efstathiou1, G. Erkens2 1. Laboratory for Machine Tools and Manufacturing Engineering, Mechanical Engineering Department, Aristoteles University of Thessaloniki, 54006, Greece Tel. 0030 31 996079, 0030 31 996021, Fax. 0030 31 996059,
[email protected] 2. CemeCon GmbH, Adenauerstr. 20B1, D-52146 Würselen, Germany ABSTRACT. The evolution of the Physical Vapour Deposition (PVD) method as a thin film production technique enabled the broad diffusion of thin hard coatings also in manufacturing technology. Coated tools may reach a cutting accomplishment of ten - up to one hundred times greater than the corresponding of the same uncoated ones under the same cutting conditions. However, despite this performance of modern coated systems, thin films experience a variety of failure mechanisms during their operation, which is strongly dependent on the manufacturing case and the cutting conditions. This wide application of thin hard PVD coatings under severe dynamic stress states on cutting tools, leads to the need of the precise knowledge of coating mechanical properties such as the coating fatigue and static stress limits. In the present paper the coating impact test is applied to determine the fatigue behavior of coating-substrate compounds in a form of generally applicable diagrams. The failure mechanisms of coatings are also examined and interpreted in milling, offering an overview of the operational limits of coated tools. In order to utilize the superior characteristics of coatings towards improving the cutting performance, it is highly recommended to optimize the cutting insert wedge radius, as well as the feedrate and the cutting speed. Herewith a premature coating failure and a consequent rapid wear development can be prevented. KEYWORDS: Milling, PVD coatings fatigue, Coating stress-strain curve, Cutting wedge radius, Feedrate
1. Introduction The increasing manufacturing demands, being supported by the improved capabilities of modern machine tools, require the persistent evolution of superior materials for cutting tools. The milling performance of cemented carbide tools, was impressively improved by the development of advanced coating systems [KOE 92], [KLO 98], [KLO 99], [BOU 98a]. The progress of the Physical Vapor Deposition (PVD) as a thin film production technique is the reason for the further broad diffusion of such coatings [TOE 99]. Nowadays, advanced and complicated techniques are incorporated in film production systems, leading in this way to the development of an extended variety of different coating types, so that soft, hard and superhard coatings with superior properties can be produced.
2. Characterisation of the fatigue properties of PVD coatings by means of the impact test The coating impact test is introduced as a convenient experimental method to find out the fatigue strength of hard coatings exposed in alternate impact loads. The contact load that leads to coating fatigue fracture can be recorded in fatigue like diagrams versus the corresponding number of impacts. The upper left part of figure 1 illustrates the CAD solid model of the impact tester, whose application leads to a typical experimentally derived diagram of the impact load versus the number of successive impacts that are required for coating failure (Fimp-N diagram). Gradual or abrupt coating removal and consequent exposure of the substrate material designate the coating fracture. The failure mode can be either cohesive or adhesive. For well adherent modern coatings, the major fatigue danger is the cohesive one, i.e. intrinsic coherence release and microchipping. However, the film failure type and extent, can be easily obtained by SEM investigations and EDX spectroscopy. For each specific experimental branch, there is a critical impact load that for a high number, equal to 106 of successive impacts, the coating does not fail. It is reasonable to claim that the coating stresses that are associated to this contact load are the critical ones that ensure its continuous endurance. The FEM modelling procedure, is based on the axisymetric solid model of the semi-infinite layered half space. The quasi-static simulation of the test has been performed considering two load steps. The first load step, the so-called loading stage, represents the indentation phase towards the coated surface. The superficial nodes, which fulfil the equation of the semi-elliptical pressure distribution within the SEM measured contact circle, are subjected to proportional surface pressure vectors. The elliptical pressure distribution at the measured contact region is a satisfactory approach, as it is confirmed by the comparison between the FEM determined cross section shape of the imprints and the measured ones. During the second load step, the so-called relaxation stage, the pressure distribution is removed, leading to an elastic recovery.
Figure 1.
The Impact Test, its FEM simulation and typical Smith and Woehler diagrams, obtained by its application.
Due to the plastic deformation that develops during the loading stage, the contact area does not fully recover to its initial plane shape, forming herewith a permanent concave imprint. The hardening rule of each specific substrate can be determined by conventional uniaxial tests, or in case of hard materials with special indentation tests
[BOU 99d], [BOU 99e]. The equivalent stress distribution for the loading and the
relaxation stages are also illustrated for the contact load that ensures continuous endurance. By means of the developed FEM simulation of the impact test, the transformation of critical impact loads to critical stress values, associated with specific and distinct failure modes (adhesive, cohesive) is achieved. The coatings fatigue behaviour can be expressed through a Smith diagram of the critical stress components for cohesive failure mode i.e. the von Mises stresses that ensure their continuous endurance as shown in the upper right part of figure 1. The stress limit for coatings is assumed to be their yield stress, since they are considered to be brittle materials. In all examined cases, coating material constitutive law is described through a monolinear approximation. Due to the lack of permanent deformation in most of applications, such as in cutting operations, bearings etc., the coating deforms purely elastically during the cutting phase and fully recovers to the initial position by the end of the contact with the workpiece. For this reason, the fatigue limit for coatings that ensures continuous endurance, derives from the region A-B of the Smith diagram that has a zero minimum stress. By means of this procedure, critical fatigue stress values for coatings, which were examined experimentally and analytically in milling process and are presented in this paper, have derived and are inserted in figure 2. The mono-layers CrN and TINALOX® coatings have a constant elasticity modulus [BOU 96], [CEM 97], [BOU 97a]. The first coating is a relatively soft one and it provides a refined friction coefficient. On the other hand, TINALOX® belongs to a new generation of super hard coatings with dense structure offering significant wear resistance and relatively low stiffness. Furthermore SUPERTIN®, is a structural multilayer coating, consisting of two different Ti- derived nitrides, and corresponding to the alternation of its composition, displays a variation in the elasticity modulus. The SUPERTIN® coating is made of geometrically and mechanically discrete layers with distinct interfaces between them. Finally, TiAlN, as well as TINALOX®, belongs to the ternary (Ti1-xAlx)N coating family having a superior behaviour in comparison to the conventional TiN at elevated temperatures, owning to the self-formation of a thin protective Al2O3 superficial layer, that may prevent coatings from further and deeper oxidation [BOU 99a], [BOU 99b]. According to current research results, the coatings constitutive law can be adequately determined by means of the nanoindentation test and its FEM simulation [BOU 00].
3. Potential wear mechanisms and cutting performance optimization techniques In former investigations, cemented carbide (HM) inserts in interrupted cutting processes indicated a cohesive fatigue failure of the coatings on the transient filleted flank cutting edge region, as the dominant wear mechanism [BOU 98b], [BOU 98c]. The corresponding milling experiments were conducted with improved cutting wedge geometry at relatively low cutting temperatures (
