New approaches to plasma enhanced sputtering of advanced hard coatings

Surface & Coatings Technology 201 (2007) 4806 – 4812 www.elsevier.com/locate/surfcoat

New approaches to plasma enhanced sputtering of advanced hard coatings Georg Erkens ⁎ CemeCon AG, Adenauerstrasse 20B1, D-52146, Würselen, Germany Available online 30 August 2006

Abstract PVD coatings and cutting tool materials experienced a rapid development in the past due to the increased demands of modern, innovative HPC (High Performance Cutting) production processes. The development of new materials progressed so rapidly that it is partly not known, how these materials can be economically processed with respect to measures of reducing costs and thus increasing productivity. In order to meet the different requirements a range of developments have been brought forward in the area of coating technology. Al2O3 was and still is an important component of conventional coatings. Especially the different crystalline modifications synthesized by CVD have gained considerable economic significance. Distinct chemical resistance, higher hot hardness and less interaction with almost all production relevant materials than most hard coatings are the main success factors of Al2O3 films. Within the first step the plasma enhanced PVD technology for the deposition of ternary coatings with higher aluminum content was developed. These films continuously generate a protective crystalline alumina layer at elevated temperatures. In addition to RF- and DC-sputter sources, pulsed plasma sources are gaining increased attention in sputter technology. This interest is driven by the significantly enhanced ionization of pulsed plasmas as compared to conventional DC and RF techniques. Using highly ionized plasmas the High Ionization Sputtering (H.I.S.) and the High Ionization Pulsing (H.I.P.) processes have been developed to deposit a novel grade of high performance hard coatings, the supernitrides. The present paper provides a survey of latest approaches to plasma enhanced sputtering and related coating systems, with regard to the demands of high performance applications. © 2006 Published by Elsevier B.V. PACS: 68.37.Hk; 81.07.Bc; 81.15.Cd Keywords: PVD coatings; High ionization sputtering (H.I.S.); Pulsed DC; Supernitride; HPC

1. Introduction An increase in innovative cutting applications such as high speed cutting (HSC), hard machining, dry machining or high performance cutting (HPC) applications is expected globally more and more within the following years. In view of productivity the machine tool manufacturers as well as the tool manufacturers prepare themselves best to meet the future demands. Advanced hard coatings as part of the overall strategic planning in tooling will play a significant role to ensure the increase in productivity. In view of the ever increasing demands on the cutting tools the tool manufacturers have to consider that the cutting tool of the future has to be treated as a system. The know-how and competence of different professional disciplines have to be combined to find specific system solutions. The tool ⁎ Tel.: +49 2405 4470 460; fax: +49 2405 4470 299. E-mail address: [email protected]. 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.07.022

material, the macro and micro geometry, the edge and surface preparation together with the different coatings have to be adapted best to the applications. Cutting parameters have to be considered and adapted as well. Just to coat a tool will not give necessarily a better performance if the cutting parameters have to be increased under more severe conditions. The tool and thus the coating have to be adapted best to the application. Novel coating generations combine the outstanding mechanical properties of e.g. typical group IVA nitrides with the superior chemical inertness of oxides. New innovative plasma enhanced process technologies like the High Ionization Sputtering (H.I.S) and the High Ionization Pulsing (H.I.P.) make it possible to synthesize oxidic- and nitridic coatings with outstanding mechanical properties and a high chemical resistance. Plasma enhanced sputtering technologies give the chance to transfer a gas species dominated plasma to a material species dominated plasma. This plasma enhanced sputtering is a request to synthesize e.g. Supernitride (SN) coatings. Supernitrides (SN) are

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distinguished by a dense, nano-structured morphology and an appropriate surface finish and show superior adhesion, thermal stability, oxidation and chemical resistance as well as high hot hardness. Smooth e.g. micro-finished surfaces are gaining more and more importance in several cutting operations where chip flow and chip removal are most important. The properties of these coatings make them a number one choice in innovative cutting applications with high mechanical and thermal load like dry hard milling, hard turning and other high performance cutting applications. Also functional components have to be adapted best to their applications. Usually a material is coated, if it meets particular requirements as far as manufacturing, stability, stiffness and costs are concerned but the actual surface cannot tolerate the occurring loads and the hard wear. Hydraulic cylinders, bearings and engine parts like pistons or diesel injection parts are just examples for components that are currently coated. Carbon based tribological coatings to reduce friction and wear are essential for these parts. Modern diesel engines would not work without coatings on specific parts because the substrate material could not sustain the very high pressure of several thousand bars. Several different tribological coatings like CrN, (Cr,Al)N, (Al,Cr)N, a-C:H, a-C:H:W (WC/C) or different DLC coatings are currently first choice. The most advanced DLC coatings are currently deposited with pulsed plasmas. Here the High Ionization Pulsing (H.I.P.) is the key technology. Plasma enhanced sputtering in general can also be combined with PACVD (Plasma Assisted CVD) in one unit and one process to produce laminated and graded films. The maximum temperature can be kept below 160 °C, which is essential for most component materials not to run the risk to lose hardness. Complex oxidation resistant coatings, multilayer coatings with e.g. integrated lubricating layers, nano-structured and nanolaminated coatings and nano-composites are recent developments in the area of PVD coating technology [1–3]. Coatings, tool materials and tool geometries received a distinct push in innovation due to the increasing requirements of modern production processes. Besides the deposition technique a quality improvement could also be achieved through the optimization of the preparation for surface modifications and through specific rounding of edges according to the application and e.g. the cleaning in high and midfrequency stimulated plasma [4,5]. For manufacturers and end users, the variety of superior and modern coating materials, coating systems and combinations of coatings is of interest in order to create distinguishing features for the market by configuring individual coatings for their top products. Success factors for this motivation are besides technical performance capability and reliability, the optical distinction. 2. High performance coating materials The basis for selecting of coating materials is the so-called Hägg's phases. These are compounds of transition metals with metalloids that are hard, chemically resistant but electrically conductive and therefore having a metallic character. At the beginning of the developments in the field of tool coatings,

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CVD coatings like TiN and TiCN had gained considerable economical importance. In addition Al2O3 was and still is an important component of conventional coatings. Especially the crystalline high temperature modifications have gained considerable economic significance for turning operations, where high thermal stability is demanded. All developments in the area of coating processes have been always closely related to efforts of reducing the coating temperatures. At the beginning of the 1980's in Germany developments were initiated, that allowed to synthesize a compound also at temperatures of approximately 500 °C e.g. not in thermodynamic balance (metastable, relatively long-lived conditions), that is conductive, however, it also shows similar insulation and oxidation behavior like Al2O3 [6,7]. This material, the so-called Titanium–Aluminum– Nitride ((Ti,Al)N), is playing the central role for coatings of high performance tools in the meantime. The hardness of this material can be increased up to 36 GPa and higher. The oxidation resistance lies within 850–1100 °C depending on the Ti/Al ratio and the microstructure. Due to the thermal insulation effect, this material is still today the basis of all coating systems which are used for modern production processes like dry machining, high speed cutting as well as hard machining. At the moment, the development of this material focuses on the deposition of very dense, nano-structured and nano-composite coatings with a smooth surface in variable coating thickness. Among other things, this is possible with the H.I.S.- and H.I.P. PVD process, which allows to ionize an increased amount of the sputtered material. To achieve further improvements especially in terms of increased oxidation resistance and thermal stability investigations are on the way to add other elements like Cr, B, Si, Y, Hf, V etc. to (Ti,Al)N based coatings. The ever increasing demand on the performance of modern cutting tools results in the necessity to improve further existing coating systems, the related technologies and to search for new materials and material combinations. The following sections will present the development in plasma enhanced sputtering technology, the development of the supernitrides and their potential for innovative cutting processes. 2.1. Materials for wear protection In order to select or develop a suitable tool coating, it is necessary to identify the primary wear mechanisms inherent in the specific machining task. The ability of a coating to reduce wear sufficiently is the primary criterion for choosing it. In addition to the direct influence on the wear mechanism adhesion, abrasion, tribo-oxidation, diffusion and fatigue of a wear resistant coating can influence the wear indirectly by affecting the contact conditions by altering friction, heat generation or heat flow [8]. Coatings deposited with the PVD process are characterized through the formation of compressive stresses. Within certain limits and depending on the exact application this stress condition is wanted for cutting processes. From expert analysis it is known that the compressive stress value should be preferably in the range of appr. −1 to −2.5 GPa depending on the cutting application. Most suitable coating systems for interrupted cutting applications

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require generally compressive stress values at the upper end of that range. Coatings for drilling and turning operations request values rather close to the lower end. Most advanced plasma enhanced process techniques provide the opportunity to adapt these values best to the demands. By doing that, properties like e.g. the coating thicknesses can be varied within a wide range. In contrast to PVD coatings, CVD coatings develop tensile stresses, leading to crack formation within these coatings. The trend in CVD is to transfer the tensile stresses after deposition to compressive stresses by mechanical post-treatment through blasting. Most common commercially available hard coatings for tools are TiN, TiC, Ti(C,N), (Ti,Al)N, CrN, Al2O3 as well as a combination of these materials. Besides these materials especially coating materials with a low friction coefficient like a:C, a:C–H, a:C–H–Me or other DLC's, MoS2, WS2 and a combination of these films with the above wear resistant coatings are successfully used as wear protective coatings. Based on those coating systems a further improvement can be achieved e.g. by adding other elements. In this context two competing approaches to the optimization of hardness as well as wear resistance of multicomponent hard coatings can be distinguished. The superior coating properties of the partly metastable mixed phases like (Ti, Al)N, (Cr,Al)N or Ti(C,N) can be explained by miscible hardening in comparison to the pseudobinary bonding systems. On the other hand coating properties can significantly be influenced by deposition conditions and design. Multilayered structured coatings can combine properties provided by different coating materials. The synthesis of nano-crystalline, isotropic single or multiphase systems like (Ti,Al)N based supernitrides or TiN–Si3N4 is a recent development to optimize the properties of innovative coatings [9]. (Ti,Al)N based films deposited with the plasma enhanced sputtering technique protect the substrate material against excessive heat, oxidation and tribochemical wear due to the thermal insulating effect. Oxides based on alumina yield the necessary oxidation resistance. Due to the high thermodynamic stability of these oxides, the tribochemical wear attack is also significantly reduced. Particularly (Ti,Al)N coatings that have been deposited with the H.I.S. process technology, are characterized through the fact that an α-Al2O3 layer is formed on the surface during operation. If one takes a look at the XRD spectrum of H.I.S.-(Ti,Al)N coated samples annealed in air and shown in Fig. 1, peaks are detected (b012N, b024N, b116N, b300N) that are characteristic for Al2O3 coatings usually deposited with the CVD process technology. This effect is enforced through the nanostructured formation of this coating (cubic (Ti,Al)N in a Al rich bonding phase), that seems to enhance the diffusion of aluminum to the surface at elevated temperatures. This effect is essential especially for cutting operations under dry conditions. The described effects make coatings like plasma enhanced sputtered (Ti,Al)N ideally suitable for a wide range of different applications and materials especially in the area of high speed cutting, dry machining and hard cutting [10]. This is true for both, continuous as well as for interrupted cutting operations. For medium and low cutting parameters the performance potential of these high oxidation resistant coatings cannot fully be exploited. In this parameter field, multilayer coatings based on (Ti,Al)N with

Fig. 1. XRD plot of a H.I.S.-(Ti,Al)N probe after annealing for 10 h at 850 °C in air.

a variable aluminum content offer the possibility to increase the performance. 2.2. High Ionization Sputtering (H.I.S.) technology The known PVD (physical vapor deposition) coating processes like low-voltage arc evaporation, ARC evaporation and cathode sputtering are applied to modify the surface characteristics of tools and components providing all types of functionality from increased wear resistance to decorative qualities. Common to all processes is their line-of-sight characteristic. In addition, there are significant differences and restrictions created by the method of transferring the target material into the vapor phase. With the low-voltage arc and ARC processes the target material is melted before evaporation. A large share of the evaporated material is ionized. This leads to dense, compact, however, not always intact structures grown or condensed on the substrate surface. In the case of low-voltage arc evaporation, where the target material is melted and evaporated by means of an electron beam, it is usually only possible to homogeneously deposit binary or ternary systems like TiN or Ti(C,N) or, in general, alloys with single metallic elements and multiple gas constituents. With the ARC process, where the target material is locally melted and evaporated by means of an arc, droplets are driven out from the melt during the evaporation process. These are defined as “droplets” or more recently as macros or macroparticles, and these are embedded into the coating. With the cathode sputtering, the target material is transferred directly from the solid state into the vapor phase. There are virtually no restrictions of the target material or composition. Based on state-of-the-art, the H.I.S. (High Ionization Sputtering) process was developed further so that material of the target cathode can be sputtered for reactive deposition of hard material coatings onto blasted and ground surfaces with a dense, compact fracture structure. As a result, a “dimpled surface” appeared due to an increase in the concentration of material ions. Fig. 2 illustrates the morphology and the “dimpled” characteristic of a (Ti,Al)N

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Fig. 2. Surface topography (left) and fracture (right) of a (Ti,Al)N based supernitride layer.

coating (Ti/Al = 1) deposited by means of high ionization sputtering. With classic MSIP (magnetron sputter ion plating) processes the plasma mainly consists of evaporated neutral particles (ionization degree approximately 5%) and other ionized gas species (working gas, reactive gas). The H.I.S.-module enhances the potential gradient in the vacuum chamber so that the degree of ionization of the sputtered metal atoms is increased to more than 50%. The control circuit plays a crucial role in the effect on the electron density distribution within the chamber. Since the potential of the substrate is set more negative than the potential of the anode, the result is that on the one hand the trajectory of the electrons mainly proceeds from the target cathode to the anode for maintaining the plasma, and simultaneously from the target cathode to the substrate for the reinforcement of the ionization of the metal atoms sputtered from the target cathode. The amplified ionization is a direct result of the coinciding transport paths of the metal atoms from the target cathode to the substrate and the electron trajectory. Through the high degree of ionization of the metal atoms sputtered from the target the H.I.S. process is able to deposit “dimpled surface” coatings on the substrate even if the surfaces of the substrate are rough, for example due to a blasting treatment. The higher the energy of the arriving ions is, the denser the obtained films will become [11]. Furthermore, the potential gradient within the chamber also has the consequence that the potential of the plasma generated between target cathode and anode in the immediate area of the substrate is generally more positive than the substrate potential. This improves deposition by the fact that ions from the plasma in this area have a higher probability of being transported into the direction of the substrate resulting in film growth. At the same time, however, the condition must be avoided where the electrons reach the chamber wall eliminating their effectiveness to enhance the ionization of the metal atoms. For this reason, normally the substrate has the same potential as the target cathode or it is positively biased against the target cathode, so that electrons contributing to the ionization of the target material will be pulled into the direction of the substrate. If the potential is set in any way that the positive electrical potential between the anode and the chamber wall is smaller than the positive electrical

potential between the anode and the substrate and the substrate is arranged next to the target cathode, the optimization of the electron density distribution within the chamber in front of the target cathode is achieved, effectively and efficiently. Due to the negative bias voltage between the substrate and the anode and the proximity of the substrate to the target cathode the share of electron trajectories proceeding to the substrate is further increased and with that the ionization of the metal atoms from the target cathode is improved. Since there is no liquid phase developed in the sputtering process, the coating formation is homogeneous and free of droplets or macro-particles. With ternary and other complex materials, coatings can be deposited with homogeneous composition based on the composition of the sputtered target material. Extremely good values for coating adhesion can be reached through a superimposed DC, mid-frequency (50–100 kHz) or radio frequency (13.65 MHz) ion etching device in combination with the high ionization sputtering process. By repressing the occasionally appearing “re-sputtering” effects, cutting edges can be coated gently with a medium high ionic current. The so-called “target poisoning” can be avoided through the application of a suitable working gas mixture. This is essential because the chemisorption of reactive gas at the target surface often forms a compound with a lower sputter rate than the target material. The described High Ion Sputtering technology is the basis to deposit most modern high performance coatings. 2.3. Supernitrides The supernitrides represent a novel class of coating systems that combine the outstanding wear properties of hard coatings with the chemical stability of typical oxides. Supernitrides show improved hot hardness, high thermal and oxidation stability and high chemical resistance. They combine the outstanding mechanical properties of modern, typical group IVA nitrides with the superior chemical inertness of, e.g. oxides. They are deposited as nano-structured and nano-laminated films or as nano-composites. On definition supernitrides as a novel generation of coating systems can only be synthesized by High Ionization Sputtering and Pulsing (H.I.S., H.I.P.). The plasma enhanced sputtered

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supernitride films are characterized by a possible high content of oxide forming elements like Al, Si, Cr, Y, B and others or a useful combination of these elements. For a (Ti,Al)N coating with an Al content of approximately 65 mol% AlN in the (Ti,Al)N the deposition in the range of the spinodal decomposition will lead to the precipitation of a second phase (AlN) on the grain boundaries and thus generates a nano-composite structure. To interrupt the film growth of the main phase and force the composition for instance of a growing (Ti,Al)N based supernitride film temporarily to the range of the spinodal decomposition the following parameters can be varied for a user-defined process: change of the Al content in the vapor, change of bias voltage and current, change of the gas composition in the atmosphere, and change of the deposition temperature. These variations during the deposition lead to the generation of a second phase on the grain boundaries of the main phase. Nano-structured materials in general are characterized by special properties in terms of the related process technology and the final product [12]. By manipulating atoms, molecules and molecule clusters nano-structured materials are changed in the way that they are not characterized by a homogeneous volume as traditionally but predominantly marked by interfaces. At these interfaces different physical and chemical rules are valid. The properties of nano-structured materials are not anymore material specific but dominated by the structure itself [13–15]. Coatings and the related properties can be tailored specific to the demands. Nano-composites are latest developments in the field of advanced coatings. Nano-crystallites (3 to 10 nm) incorporated into an amorphous matrix delimit the dislocation mobility, turn around occurring cracks and limit the crack propagation. A high ratio of grain boundaries causes a macro-ductility due to shearing strain of the grain boundaries across nano-pores and crack meshes that open up along the grains. This results in coatings with a high toughness [16]. Besides the comparable hardness of (Ti,Al)N based supernitrides with the high hardness of the well known, commercially available coatings with Al/Ti ratios of about 1 these films generate a protective crystalline Al2O3 film much faster. Microprobe analysis of oxidized (Ti,Al)N based supernitride coatings proved that with increasing AlN content a dense oxide layer is generated much faster. No rutile is generated that usually affects bulk diffusion. The main advantage of these films is the fact that they generate these protective alumina films permanently during use. The supernitrides could be nano-structured, nano-laminated or deposited as nano-composite and represent a novel generation of hard coating systems with higher hot hardness, higher oxidation and chemical resistance [17], higher thermal stability and outstanding mechanical properties for various applications especially in the field of High Performance Cutting (HPC) applications [18]. Fig. 3 shows a micrograph of a supernitride film deposited on cemented carbide. 3. Future oriented H.I.P.™ technology With pulsed plasmas a new set of process parameters appear including pulse duration, duty cycle and pulse amplitude. Compared to conventional DC a significant distinction between

Fig. 3. Fracture of a (Ti,Al)N based supernitride with a nano-crystalline structure and increased Al content representing one variant of the novel generation of coating system.

average and pulse parameters can be adjusted, which allows for extreme plasma parameters during the pulses. Pulsed sputter deposition increases significantly the ionization rate. The deposition rate can be 2–5 times higher for the deposition of oxidic films than with conventional DC technology [19]. The deposition processes may be operated at momentary high power rates, leading to more energetic and ionized particles in the plasma. These extreme plasma parameters for a very short period of time lead to highly improved film properties even during low temperature deposition [20–22]. Plasma enhanced sputtering in general but by means of bipolar pulsed DC in particular gives the chance to transfer a gas species dominated plasma to a material species dominated plasma. Pulsed DC sputtering processes and mid-frequency plasmas (50– 350 kHz) as well as ultra dense plasmas generated by high power pulsing (N 3000 W/cm2) are gaining increasing attention in a wide range of different applications. Innovative PVD coatings deposited with these technologies led to a significant reduction of wear and friction and to a tremendous increase in performance of modern cutting tools and highly loaded components for instance engine parts, diesel injection parts or bearing components. Advanced tribological coatings like gradient a-C:H:Me films deposited with the pulsed DC technology open up a wide field of future applications for functional components. Fig. 4 shows the structure of such a-C:H:Me film. After an enthusiastic start in the 90's, there have not been a lot of publications on further new developments of PVD alumina for the last 5 years. In 1995, Reschke et al. [23] reported on the MAD (Magnetron Activated Deposition) process to deposit reactively alumina layers. They combined a high rate evaporation with bipolar pulsed magnetron discharges using different power supplies (sin wave and square wave pulsing). They claimed that it is necessary to select the appropriate generators and match them to the plasma conditions. Thick alumina layers with a dense structure and a smooth surface were deposited. About the coating properties no information was given but they reported that there is still

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much work to do, to make the related technology available for industrial application. In 1996 Fitzke et al. [24] reported on crystalline Al2O3 layers with hardness values of 22 GPa produced by bipolar pulsed magnetron sputtering of an aluminum target in an Ar/O2 atmosphere. They used a DMC (Dual Magnetron Sputtering) system with a bipolar pulsed square wave power supply in the MF range. They highlighted that at a temperature of 550 °C textured γ-alumina could be deposited. Increasing temperature to 650 °C a mixture of 50% γ-phase and 50% α-phase alumina could be found. This switched to pure α at a temperature of 750 °C. They also talked about the combination of e.g. (Ti,Al)N with these Alumina films to coat cutting tools like cemented carbide inserts or HSS tools in large scale. They found that an intermediate layer of (Ti,Al)N led to the best adhesion of the crystalline alumina films. According to the above reviewed approaches the novel High Ionization Pulsing (H.I.P.) process technology provides the capability to deposit reactively conductive and insulating coatings in virtually any stoichiometry. Furthermore through the realization of extremely dense plasmas it is also possible to deposit crystalline high temperature modifications. This became possible through the application of bipolar pulsed plasmas and a new arrangement in the vacuum chamber, that increases the potential difference between the pulse electrodes and thus the ion energy and which guides the dense plasmas specifically to the substrate. Highest plasma ionization, optimal performance for insulating coatings, nano-composites, perfect surfaces and optimized deposition rates are the results. The ion flux can be varied and the coatings can be designed specifically for the different applications. Al2O3 is of interest for wear and corrosion protection [25] or as diffusion barrier [26] and for applications in microelectronics [27,28]. The thermodynamically stable crystalline α-modification is often favored for applications with high thermal and mechanical load, but crystalline γ-Al2O3 has proven to be a suitable alternative to the α-modification in certain cases [29]. With the High Ionization Pulsing technology fine grained and dense, untextured crystalline Al2O3 films could be reactively deposited in an Ar/O2 atmosphere on cemented carbide substrates and on HSS tools as well. The hardness of these alumina films was in the range of 21–23 GPa. For cutting tests these films were

Fig. 4. Graded a-C:H:CrN coating on 100Cr6, max. dep. temp. 150 °C, pulsed sputtering 4 × 2 kW, 50% duty cycle, 50 kHz, MF bias (type superDLC).

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Fig. 5. (Al,Ti)N based supernitride/H.I.P. crystalline alumina.

applied onto (Al,Ti)N coated inserts (Fig. 5). As Fitzke [24] already reported the alumina films showed very good adhesion to (Ti,Al)N interlayers. Scratch tests resulted in critical loads of more than 70 N. 1.5 μm thick untextured Al2O3 films were deposited onto 3 μm (Al,Ti)N coated inserts for cutting tests. The (Al,Ti)N was orientated in the b111N direction. Due to the higher residual compressive stresses this orientation is appropriate for interrupted cutting applications. Pulsed plasmas also provide the opportunity to coat nonconductive substrate materials with high performance coatings. Ceramics made from Si3N4, mixed oxides or c-BN are PVD coated for better evaluation of wear propagation or to protect the binder against oxidation. High ionization pulsing makes it possible to coat such substrates in large scale. 4. Conclusions Modern application technologies have created a significant advance in innovation for tool materials and coatings. The improved systems lead to tremendous increases of performance, not only for dry cutting but also for hard cutting and high speed cutting. Today almost all modern high performance coatings are still founded on (Ti,Al)N as base material. Systems made of pure (Ti,Al)N films with an elevated aluminum oxide content or crystalline diamond as a coating yield an optimal performance according to the area of application. This cutting performance could be even further increased for dry and wet machining through an a-C or a-C:H based solid state lubricant coating on top of the hard coating, e.g. for drilling, milling and turning processes, for hobbing processes and for very ductile materials. Wear resistant lubricious coatings based on (Ti,Al)N with metal boridic content could gain significant importance within the next years. This will also be valid for adding other elements like Si, V, Cr, Hf, Yt, etc. Due to the deeper understanding of the total processes in the future, the high performance coatings presented in this paper will find even wider applications, particularly through the application of theoretically and experimentally obtained results of mesomechanical investigations of the cutting process. The supernitrides are nano-structured and represent a novel generation of hard coatings with higher hot hardness, higher chemical and

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thermal resistance and outstanding mechanical properties for various applications especially in the field of High Performance Cutting (HPC) applications. New process technologies like High Ionization Pulsing (H.I.P.) will lead to novel coating systems. Plasma enhanced reactive sputtering by means of pulsed DC will provide the opportunity to apply e.g. non-conductive crystalline films like ZrO2, (Cr,Al)2O3, Al2O3, composites from the Si–O–N system, or all in combination onto high performance tools and functional components. Tool and component manufacturers as well as end users will find a wide range of coating materials, coating systems, pre- and post-treatments to create coatings for their high performance products to meet the demands of modern high performance applications. References [1] [2] [3] [4] [5] [6]

I. Wadsworth, et al., Surf. Coat. Technol. 94–95 (1997) 315. F. Vaz, et al., Surf. Coat. Technol. 108–109 (1998) 236. M. Diserens, et al., Surf. Coat. Technol. 108–109 (1998) 241. K.-D. Bouzakis, et al., Surf. Coat. Technol. 163–164 (2003) 625. K.-D. Bouzakis, et al., Surf. Coat. Technol. 200 (2005) 128. O. Knotek, et al., J. Vac. Sci. Technol., A, Vac. Surf. Films 5 (4) (Jul/Aug 1987) 2173. [7] T. Leyendecker, Über neuartige Schneidwerkzeugbeschichtungen auf Titan- und Aluminiumbasis, Dissertation RWTH Aachen, 1985, 11–20.

[8] F. Klocke, T. Krieg, CIRP Ann. 42/2 (1999) 515. [9] R. Cremer, Verkürzung der Entwicklungszeiten multikomponentiger funktionaler Werkstoffe mittels kombinatorischer Ansätze, Habilitationsschrift, RWTH Aachen, 2002, 9–10. [10] K.D. Bouzakis, et al., Surf. Coat. Technol. 120–121 (1999) 34. [11] R. Messier, et al., Vac. Sci. Technol. A2 (2) (1984) 500. [12] Magazine Metalloberfläche Jahrg. 56 (2002) 11–12, 34–35. [13] Magazine Metalloberfläche Jahrg. 58 (2004) 12, 21–26. [14] J. Musil, J. Vlcek, Surf. Coat. Technol. 142–144 (2001) 557. [15] S. Veprek, J. Vac. Sci. Technol. A17 (1999) 2401. [16] Magazine Metalloberfläche Jahrg. 56 (2002) 11–12, 42. [17] K. Weinert, Proceedings: Int. Conf. High Performance Cutting (HPC), ISBN: 3-926690-01-1, 2004, p. 163. [18] G. Erkens, et al., Surf. Coat. Technol. 177–178 (2004) 727. [19] G. Bräuer, R. Fellenberg, Proceedings: Werkstoffe und Werkstofftechnische Anwendungen, Bd. 16, ISBN: 3-00-011951-5, 2003, p. 206. [20] M. Witthaut, et al., Fresenius' J. Anal. Chem. 361 (1998) 639. [21] J.M. Schneider, et al., Appl. Phys. Lett. 75 (5) (1999) 612. [22] A. Anders, Surf. Coat. Technol. 93 (2–3) (1997) 158. [23] J. Reschke, et al., Surf. Coat. Technol. 76–77 (1995) 763. [24] F. Fitzke, et al., Suf. Coat. Technol. 86–87 (1996) 657. [25] J.M. Schneider, et al., J. Vac. Sci. Technol. A, 15(3), 1084. [26] R. Cremer, et al., Surf. Coat. Technol. 108–109 (1998) 48. [27] C.S. Bathia, et al., J. Vac. Sci. Technol. A, 7(3), 1298. [28] R. Cremer, et al., Proceedings of the 7th European Conference on Applications of Surface and Interface Analysis, John Wiley and Sons, Ltd., Chichester, 1997, p. 927. [29] Ch. Täschner, et al., Surf. Coat. Technol. 108–109 (1998) 257.