Search

The interface fatigue strength of coated tools with nano-crystalline diamond (NCD) coatings affects their milling performance. Depending on the developed cutting loads and temperatures, the repetitive impacts on the tool deteriorate progressively the coating-substrate interface strength, thus influencing the effective NCD film adhesion. Aiming at a comprehensive investigation of this effect, NCD coated cemented carbide inserts with improved or insufficient adhesion were manufactured. Hereupon, a parameter characterizing the coatings' interface adhesion strength, the shear failure stress (SFLS) was estimated by inclined impact tests and FEM calculations at ambient and elevated temperatures. This metric is significantly affected, among others, by the compressive residual stresses developed in the NCD film during cooling after its deposition and the coating operational temperature. A correlation between the determined SFLS at the investigated adhesion levels and the FEM calculated maximum shear stress in the coating interface during milling AA 7075T6 contributed to the explanation of the wear evolution on the employed NCD coated tools. Prime novelty statement The submitted paper aims at investigating the effect of the interface fatigue strength of NCD coated hardmetal inserts on their cutting performance in milling. The fatigue strength of the NCD coating-substrate interface is a dominant mechanical property for the NCD coated tool life. In the submitted work, NCD coated cemented carbide inserts with different adhesion qualities were manufactured. The interface fatigue strength at temperatures up to 400 °C was quantified using a critical shear failure stress (SFLS). This was attained by conducting appropriate FEM supported evaluation of the inclined impact test results at ambient and elevated temperatures. A correlation between the determined SFLS at various adhesion levels and the FEM calculated maximum shear stress in the film interface during milling AA 7075T6 contributed to the explanation of the wear evolution on the employed NCD coated tools. This is a new and original contribution to the field of testing NCD coatings and their effective application in cutting. The submitted paper is not being considered for publication elsewhere.

In the described investigations, nano-impact tests were conducted on PVD films with graded mechanical properties. The properties gradation was induced by micro-blasting at different pressures for constant duration. Micro-blasting improves the film mechanical properties due to the induced residual compressive stresses into the coating structure. The film mechanical properties as well as the ratio of the film yield to rupture stress (SY/SM) grow up to a certain micro-blasting pressure. The augmentation of S Y/SM ratio increases simultaneously the film brittleness. For analyzing the effect of the SY/SMratio on the film brittleness, a multi-layer 3D-FEM model simulating the nano-impact test was developed employing the LS-DYNA software package. The FEM calculated results converge sufficiently with the attained experimental ones. Based on these results, the film's failure initiation and evolution can be predicted depending on the SY/SMratio of the individual film layers.

The knowledge of coated tool wear mechanisms in milling is pivotal for explaining the film failure and selecting the appropriate cutting strategy and conditions. In this paper, tool wear experiments were carried out in milling of four different steels using coated cemented carbide inserts. The variable stress, strain and strain rate fields developed in the tool during cutting affect the film-substrate deformations and in this way the resulting coatings loads and its fatigue failure. For investigating the influence of cyclic impact loads magnitude and duration on the films' fatigue of coated specimens, an impact tester was employed which facilitates the modulation of the force signal. The attained tool life up to the films' fatigue failure was associated to a critical force for the film fatigue endurance and to the cutting edge entry impact duration. These factors converge sufficiently to the tool life in all examined milling kinematics and workpiece material cases.

The knowledge of coated tool wear mechanisms in milling is central for explaining the film failure and selecting the appropriate cutting strategy and conditions. The described tool wear investigations in up and down milling were carried out at various cutting conditions, by coated cemented carbide inserts fixed on tool holders of different diameters. The variable stress, strain and strain rate fields developed in the tool during cutting affect the film-substrate deformation and in this way the resulting coating's loads and its fatigue failure. The effective tool life, up to a certain flank wear width was experimentally determined and correlated to the cutting edge entry impact duration (CEEID) into the workpiece material. Using a commercial CAD/CAM system, the geometry of the chip and of the contact between the cutting edge and workpiece was calculated in various milling cases and the corresponding tool wear evolution predicted. The latter converges with cutting edge en try impact duration and with the associated film fatigue critical force, affected by the coating strain rate.

The grain size of coated cemented carbide tools may significantly affect the tool life by influencing either the coating adhesion to the substrate or the coating growth during deposition. In the present study three variously grained cemented carbides were coated with two different PVD films. Various pre- and post-treatments were applied, while their effect on the films' mechanical properties and adhesion was assessed by nanoindentations and inclined impact tests. The coatings deposited on ultra-fine-grained substrates exhibited the highest mechanical properties overall. The research revealed elevated adhesive properties for the fine-grained substrate compared to the normal and ultra-fine-grained one. The film adhesion of normal and fine-grained substrate can be further enhanced through micro-blasting. Post-treatments of the coatings were beneficial in all substrate cases, as brushing of the film surface removed droplets while at the same time inducing surface residual s tresses at the coating resulting in a hardness increase.

Coated tools constitute the majority of the tools applied in material removal processes, rendering the employment of uncoated ones as an exception. A broad growing market of coated cutting tools has been developed. Moreover, numerous material- and manufacturing-engineers have joint their expertise, aiming at developing coatings meeting the needs for processing the most difficult-to-cut materials at the most extreme cutting conditions. The emerging of new workpiece, tool and film materials, the evolution of sophisticated coatings' characterization methods and the continuous need for higher productivity rates, maintain vivid the industrial and scientific interest for further advancing this field.

Nanoindentation is usually applied on thin films at ambient temperatures for hardness determination. Recently, instruments for conducting nanoindentation at elevated temperatures have been developed facilitating measurements up to 700 oC. Both indenter and specimen, if necessary, are heated in an inert atmosphere to avoid film oxidations. In the described investigations, nanoindentations were conducted on cemented carbides and high speed steel specimens, coated with various films, up to 400 oC. The obtained results were subjected to statistical analysis to estimate their reliability. Moreover, the results were evaluated by appropriate FEM (Finite Element Method) algorithms for determining the coatings’ elasticity modulus, yield and rupture stress as well as hardness at various temperatures. The results reveal a non-linear temperature dependence of the coating properties.

The cutting performance of coated tools can be significantly improved by tailoring the compound film-interface-substrate properties to specific application requirements. In the paper, examples highlight such innovative procedures, facilitating the adaption of cutting conditions to the film properties. The fatigue strength of PVD-films at high strains and strain rates, an important parameter for milling operations, is evaluated by repetitive impacts of various magnitudes and durations. Considering the attained results, the tool wear evolution is described dependent on the cutting speed and the cutting edge entry impact duration. Characteristic application examples are presented in the case of NC milling of a hardened steel.

The cutting performance of coated tools can be significantly improved by tailoring the coatings properties to application of specific requirements. For achieving this target, optimised PVD processes as well as appropriate substrate pre- and posttreatments of the coated surfaces have to be employed. To check the effectiveness of all these methods, innovative quality inspection tests should be applied. In the recent 3 years, various innovative methods for characterising the coatings properties towards explaining their performance in cutting applications have been developed. Characteristic related methods are presented in this paper and applications results are provided for demonstrating the effectiveness of these methods. In this context, nano-impact test is an effective tool for assessing the coatings strength and their brittleness as well. With the aid of its FEM simulation, a theoretical explanation of the nano-impact test results, concerning the film fracture initiati on and evolution, is attained. For investigating the effect of strain rate on the surface response and film fatigue fracture of coated specimens subjected to cyclic impact loads, an impact tester was developed, facilitating the impact force modulation concerning its signal pattern. The surface response depends, among others, on the substrate and coating strain, strain rate properties. Repetitive impact loads with variable duration are exercised on coated cutting edge in interrupted machining operations, depending on the applied kinematics and cutting conditions, as for example in milling. The coated tool life under certain cutting conditions was correlated to the film strain rate-dependent fatigue behaviour. In this way, the coated inserts cutting performance at various conditions was explained, considering the impact test results with modulated repetitive force. Finally, inclined impact test is a further procedure, which is analytically described appropriately by (FEM- based techniques, allowing the determination of films adhesion, in cases where the Rockwell indentation tests fail to deliver reliable results.

The die and mold industry registers nowadays a significant growth due to current developments among others in the sectors of energy and aerospace. The manufacturing procedure of dies and molds primarily includes milling operations, which are commonly linked to complicated chip geometry and contact conditions between tool and workpiece. Therefore, the optimisation of the cutting conditions and the description of the tool wear is a great challenge for production engineers. In the described experiments, cement-coated carbide inserts fixed on milling cutters with various diameters were applied in down and up milling for monitoring the wear behaviour at various cutting edge entry conditions. The corresponding developed strain rates cause different film-substrate deformations and resulting loads. These phenomena were investigated with the aid of a new impact tester with adjustable impact force characteristics. The effective tool life up to a certain flank wear land width vers us the cutting edge entry duration was explained and analytically described.

The determination of the temperature dependent interface fatigue strength of Nano-Crystalline Diamond (NCD) coatings facilitates a thorough understanding of the NCD coated cutting tools wear mechanisms. In the present paper, the fatigue strength of the interface region between a NCD film and its hardmetal substrate was investigated by inclined impact tests at various temperatures. Depending upon the impact load and the applied temperature, after a certain number of impacts, damages in the film-substrate interface develop, resulting in coating detachment and lifting. These effects were attributed among others to the release of highly compressive residual stresses in the NCD coating structure. The attained inclined impact testâs results contributed to the explanation of the wear-evolution of NCD-coated tools with diverse film-substrate adhesion qualities. The related milling experiments using as work material AA 7075 T6 verified the dominant effect of the film adhesion on the NCD coated tool life.

The eternal goal increasing the productivity in gear manufacturing can be obtained, among others, by efficient process planning systems. Gear manufacturing is associated with complicated generating kinematics, chip formation and tool wear mechanisms. In the present paper the number of cuts per coated tool tooth in a generating position, up to the first substrate revelation and up to a flank wear width of 0.15 mm were predicted using appropriate coating Woehler-like diagrams; hereupon the developed maximum equivalent stress in the critical cutting edge region leading to film fatigue failure and subsequent flank wear was considered. The required data for these calculations were determined experimentally by 3-axis milling investigations. This method facilitates the simulation of gear hobbing chips in a less time-consuming way, compared to procedures with gear cutting kinematics. An application example for predicting the tool wear by the developed methodologies is presented.