Keywords

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Synonyms

Tools with thin hard protective surface films; Tools with thin hard protective surface layers

Definition

Coatings deposited on cutting tools are ceramic layers of few micrometers in thickness which exhibit high mechanical strength and hardness, chemical inertness, and low thermal conductivity. As such, a significant increase in performance is realized over uncoated tools.

Extended Definition

Coated tools have compound material structure, consisting of the substrate covered with a hard, antifriction, chemically inert, and thermal isolating layer, up to several micrometers thick. In this way, coated tools compared to uncoated ones offer better protection against mechanical and thermal loads, diminish friction and interactions between tool and chip, and improve wear resistance in a wide cutting temperature range. Coatings follow the topomorphy of the tool substrate surface. Depending on the deposition process, the film thickness may vary on the flank and rake faces of the tool. In addition to the inherent properties of the film, coating adhesion is also pivotal for the cutting performance. An electron micrograph of a cross section through a coated cemented carbide insert is presented in Fig. 1.

Fig. 1
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Electron micrograph of a cross section through a coated carbide insert

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Theory and Application

History

Coatings produced by chemical vapor deposition (CVD) were already commercialized for carbide inserts in the 1960s. Physical vapor deposition (PVD) was developed almost 20 years later, and today both CVD and PVD are sharing the coating market of cutting tools.

TiN coatings were first applied industrially on cutting tools. The next generation of coatings was composed of chromium nitride (CrN) and titanium carbonitride (TiCN). The evolution of TiAlN, by adding aluminum to the TiN base composition, provided not only a higher hardness but also a remarkable improvement of high-temperature strength and inertness. The high hot hardness and oxidation resistance up to ca. 900 °C contributed to remarkable improvement in machining productivity. The next evolution of TiAlN coatings is usually known as AlTiN coatings, for their higher Al content, implying a better thermal resistance. The addition of silicon in the composition enabled further increases in cutting temperature (Flink et al. 2009). The hardness of AlCrN coating is similar to that of TiAlN, but what makes this coating outstanding is its high adhesion with the substrate material, due to Cr content and its high oxidation resistance, up to 1200 °C. AlCrN-based coating has been successfully applied in hobbing, drilling, and milling where both high temperature and oxidation resistance of the coating are required (Endrino and Derflinger 2005; Bouzakis et al. 2011a). From a materials perspective, alloying TiAlN coatings with different elements provides a large number of further possibilities: for example, TiAlCrN, TiAlCrSiN, and TiAlCrYSiN compositions are reported (Bohlmark et al. 2011) and even more the addition of dopants like Zr, V, B, or O (Bouzakis et al. 2008a; López de Lacalle et al. 2010).

Alumina is uniquely suited for metal cutting tools due to its chemical inertness and high hot hardness at the temperatures typically reached in these applications (Quinto 1988). In addition to sintered alumina-based ceramics, Al2O3 is also important as a coating material. Alumina-coated cemented carbide tools are used, for example, in turning and milling steel and cast iron. The Al2O3 coatings are typically manufactured by CVD. CVD has been used for about 30 years for industrial deposition of wear-resistant coatings and still dominates the market of Al2O3 coatings on cemented carbide tools. Crystalline alumina PVD coatings offer high potential for an application in cutting operations. Beneficial of these types of coatings are high chemical inertness, high hot hardness, and high oxidation resistance (Bouzakis et al. 2002). One promising candidate is γ-Al2O3, which can be deposited at lower temperatures and is more fine-grained than α-Al2O3. At high temperatures, γ-Al2O3 transforms into α- Al2O3, which could limit the application temperature (Erkens 2007; Bobzin et al. 2010).

CVD diamond thin films offer the hardness and wear resistance of diamond but on geometrically complex tools such as drills and end mills. The manufacturing chain of CVD diamond-coated cemented carbide tools commences with the identification of a suitable substrate as well as the substrate pretreatment to remove cobalt from the surface layer. This is necessary to prevent a catalytic reaction of cobalt with diamond and to provide a mechanical bond between substrate and diamond film. These manufacturing steps are followed by cleaning and diamond seeding measures before CVD diamond deposition is carried out (Uhlmann and Koenig 2009; Haubner and Kalss 2010). Residual stresses develop in a diamond film mainly due to epitaxial crystal differences and thermal expansion coefficients mismatch of the diamond coating and its cemented carbide substrate. The residual stresses usually enhance the diamond coating adhesion since they contribute to roughness peaks locking in the coating-substrate interface. However, they may overstress the substrate material in its interface region, thus deteriorating the coating adhesion (Skordaris et al. 2016). For selected applications, the extreme properties of diamond can be exploited.

Theory

Introduction

PVD covers a broad family of vacuum coating processes in which the metal comprising the film material is physically removed from a source or “target” by evaporation or sputtering. Then, they are transported in a vacuum or partial vacuum by the energy of the vapor particles and condensed as a film on the surfaces of appropriately positioned parts in the vacuum chamber. It is most common to deposit ceramic films, and these are formed by introducing a reactive gas (nitrogen, oxygen, or simple hydrocarbons) containing the desired chemical elements, which once reacted with the target materials form the required coating composition. PVD coatings can be deposited at temperatures lying in the range of 450–550 °C, which allows the film deposition on high-speed steel tools. Most of the PVD processes are known by various phrases or acronyms, and they are typically named for the physical vapor target, for example, diode or triode sputtering, planar or cylindrical magnetron sputtering, direct current (DC) or radio frequency (RF) sputtering, electron beam evaporation, activated reactive evaporation, and ARC evaporation (DC or alternate current (AC)) (Erkens 2007; Bobzin et al. 2009).

CVD, unlike to PVD vacuum processes, is a heat-activated process based on the reaction of gaseous chemical compounds within a reaction chamber containing the parts to be coated. It is possible to control the coating composition, crystal or lattice structure, and thickness by adjusting the reactor pressure, temperature, and/or reactant composition. Primary reactive vapors can be either metal halides or metal carbonyls, as well as hydrides and organometallic compounds. Typical deposition temperatures range from 800 to 1200 °C. The ability to provide uniformly thick coatings with refined grain is also influenced by the deposition temperature. Fewer CVD reactions are available for use at temperatures below 800 °C than above (moderate temperatures, MT-CVD). However, the temperature required for a given reaction can be lowered by exposing the substrate to an electrical plasma in the gas phase during deposition, referred to as plasma-assisted CVD (PA-CVD) (Shimada et al. 2010). Metal-organic CVD (MO-CVD) has been reported for strengthening Al2O3-based ceramic tools.

The ability to control thicknesses on the edges, when PVD is employed, guarantees a sharp-coated cutting edge. High intrinsic hardness and compressive stresses, inhibiting the crack growth in tool material, are among the beneficial properties of PVD films (Klocke and Krieg 1999). The possibility to produce thick layers by CVD at increased deposition rates renders the CVD-coated tools suitable for high material removal operations, whereas the PVD ones are selected in medium-finish and finish operations. PVD films can be produced without any chemical interaction with the substrate. CVD coatings easily interact with the substrates, occasionally producing brittle carbides at the interfaces. The ease of decoating and resharpening of PVD-coated tools opened a large industrial market highly sensitive to cost-reducing opportunities. Both coating processes contributed to significant enhancement of high-speed cutting (HSC) and high-performance cutting (HPC) (Toenshoff 2011).

Figure 2a illustrates a variety of coated cutting tools, from inserts to solid tools and hobs which can be industrially produced. Figure 2b exhibits characteristic fixtures for attaching cutting tools in the deposition chamber. Where the plasma flux during PVD is quasi-parallel to the insert rake (see Fig. 2c), a thicker coating is formed on the flank and vice versa. As a result, cutting inserts are coated with slightly variable film thickness on the rake and flank, depending on the incidence directions of the plasma flux. HPPMS technology contributed to the elimination of this phenomenon (Bobzin et al. 2009).

Fig. 2
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(a) Typical coated cutting tools produced, (b) characteristic fixtures for attaching the cutting tools in the deposition chamber, and (c) tool orientation against ion flux during PVD

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The potential to increase tool life via the employment of coated tools is demonstrated in Fig. 3. In this figure a characteristic example in milling cast iron with coated and uncoated cemented carbide inserts is displayed. When an appropriate PVD film for this workpiece material is applied, an impressive number of cuts (tool life) can be attained compared to the uncoated tool. The used coating consists of a crystalline Al2O3 layer over a (Ti46Al54)N film. The substrate is a cemented carbide insert appropriate as coated or uncoated for milling cast iron. The coated tool managed to cut 650 × 103 cuts up to a flank wear of 0.2 mm (Bouzakis et al. 2002). The uncoated insert achieved only 50 × 103 cuts up to the same flank wear width. This superior performance of coated tools has resulted in their wide application in cutting processes, rendering the employment of uncoated ones as an exception.

Fig. 3
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Flank wear development of uncoated and coated tools in milling cast iron

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Coated Tools Technical and Thermal Loads During Cutting

The elevated cutting performance of coated tools can be also explained by the mechanical and thermal loads acting on the cutting edge during the material removal. In an example of milling hardened steel, the maximum equivalent stress in the coating determined by finite element method (FEM) calculations reaches 5.6 GPa on the cutting-edge roundness close to the flank, remaining below the film yield stress of 5.9 GPa (see Fig. 4) (Bouzakis et al. 2008b). Additionally, the substrate is less stressed (max. equivalent stress ≈ 3 GPa) compared to the uncoated tool (4.9 GPa). In the uncoated tool, the stress of 4.9 GPa exceeds its yield strength of ca. 3.2 GPa, thus leading to cutting-edge micro-breakages and accelerating the wear growth. The maximum temperature in the coated insert amounts to ca. 266 °C, at a tool–chip contact time of 4.8 ms. In the case of an uncoated tool, a comparatively higher amount of the total cutting energy is conducted into the tool, leading to a maximum temperature of up to 652 °C, thus affecting its cutting performance. In interrupted cutting, depending on the tool–workpiece contact time, the maximum tool temperature is commonly lower than the corresponding steady-state temperature of continuous material removal processes.

Fig. 4
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Decrease of mechanical and thermal loads of cemented carbide tools by the application of PVD coatings

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Wear Development on Coated Tools

The wear mechanisms of coated tools in cutting vary from application to application, and the ones dominating in steel milling are displayed in Fig. 5 (Bouzakis et al. 2013). Mechanical overstressing as well as the exceeding of the fatigue strength during material removal leads to microchipping of the coating mainly at the transient cutting-edge region from the flank to the tool rake (region I). The development of this wear phenomenon increases the width of the flank wear land at low cutting velocities, without any significant wear on the tool rake, and causes tool failure. Moreover, depending on the temperature developed and coating composition, oxidation and diffusion mechanisms develop at higher cutting velocities, mainly on the tool rake face (region II). High cutting temperatures may also lead to coating decomposition (Alling et al. 2009). Due to these mechanisms, a deterioration of the coating’s mechanical properties occurs, which accelerates its abrasive wear. Furthermore, the film adhesion quality significantly affects coating wear, since inadequate interlocking of the coating with the substrate increases the developing stresses (Bouzakis et al. 2011b). These mechanisms appear in the cutting wedge region I and lead to film fracture and rapid tool wear. Finally, adhesive micro-welding leading to micro-peeling can occur at low cutting speeds, in part, as a result of common elements between workpiece and coating materials.

Fig. 5
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Developed wear mechanisms in coated cutting tools

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The wear mechanisms are significantly affected by the applied cutting speed. As it is shown at the upper part of Fig. 6, at the cutting speed of 200 m/min, coating fatigue fracture at the rounded transient region of the flank to the cutting edge develops, restricting the tool life. At elevated cutting speeds, in the present case over 300 m/min, besides the aforementioned mechanically overstressed flank region, tribo-oxidation along with increased abrasion develops. These mechanisms appear on the rake face close to the cutting edge and are predominant in limiting tool performance. The electron micrographs of the tool rake face shown at the bottom of the figure exhibit the aforementioned coating wear mechanisms. On one hand, in the frame of these investigations, the width of flank wear land VB up to a value of 0.2 mm is evenly distributed along the cutting edge. On the other hand, a coating failure appears at the indicated wedge locations 1 and 2, but the wear extent and the number of the achieved cuts vary, depending on the cutting speed.

Fig. 6
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Coating failure at a flank wear of 0.2 mm, investigated by electron micrographs, at various cutting speeds

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The film adhesion crucially affects the wear development of a diamond-coated tool. The diamond film adhesion can be assessed employing methodologies described in Skordaris et al. (2016). Characteristic SEM micrographs exhibiting the wear evolution on the NCD-coated tools possessing improved adhesion or insufficient adhesion after 1.6 × 106 or 5 × 104 cuts, respectively, in milling AA7075 T6, are shown in Fig. 7. Coating detachment in a restricted region of the tool rake also develops even in cases of well-adherent diamond-coated tools, when the shear strength of the coating interface is exceeded, among others, due to film thickness decrease on the cutting-edge roundness because of wear (Skordaris et al. 2016).

Fig. 7
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Characteristic SEM micrographs of worn NCD-coated tools with different adhesion qualities after various numbers of cuts

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Coated Tools’ Material and Functional Properties Determination

The cutting performance of coated tools can be significantly improved by tailoring the coating properties to application-specific requirements. For achieving this target, a thorough understanding of the coated tools wear mechanisms is required. Since CVD and PVD thin films are very hard and brittle materials, properties such as fatigue, toughness, residual stresses, and adhesion along with tribological and dimensional ones play a pivotal role in cutting with coated tools. To quantify these parameters, experimental–analytical test procedures have been developed. These provide information concerning material and functional properties of the film and its substrate as well as the actual coated tool geometry. In Fig. 8 methods for determining material, dimensional, and functional data of coated tools are displayed. Combinations of these procedures jointly with FEM-supported computations contribute to the explanation of the cutting tool films’ failure mechanisms, thus restricting the experimental cost for optimizing cutting conditions. Characteristic examples will be introduced in the following sections.

Fig. 8
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Characteristic methods for determining coating material, dimensional, and functional properties

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Several test procedures are applied for determining material, dimensional, and functional properties of coated tools. Moreover, experimental in combination with FEM-supported techniques are used for assessing the performance of coated tools. Some of these procedures are significant for cutting tools and will be briefly described:

  • Strength properties and hardness at various temperatures: The determination of the coating strength properties and hardness is conducted by nanoindentations at ambient and elevated temperatures. With this technique, in situ measurements are conducted in a wide range of temperatures, enabling an accurate estimation of coating properties. Based on FEM simulations of the indentation procedure, experimental results may be evaluated and coating stress–strain curves as well as hardness at various temperatures are determined (Bouzakis et al. 2005a).

  • Fatigue strength at various temperatures and impact force signals: Coated tools’ film failures commonly develop during cutting, due to fatigue problems. The films’ fatigue properties at ambient and elevated temperatures are determined by perpendicular impact tests, employing appropriate devices (Knotek et al. 1992; Bouzakis et al. 2001, 2010a, 2012). In these experiments, the impact force signal (force and duration) is pivotal for coating failure. Moreover, the possibility to calculate the developed cutting temperature field allows the correlation between the coating impact resistance and the tool wear at various cutting speeds and cutting-edge entry impact durations (Bouzakis et al. 2007, 2013).

  • Coating adhesion: The film adhesion can be qualitatively assessed by Rockwell and scratch test methods. These test procedures do not always yield reliable results, due to limitations of the test procedures (Bouzakis et al. 2011a). The evolution of the inclined impact test renders possible the accurate and quantitative coating adhesion evaluation (Bouzakis et al. 2010b).

Enhancement of Coating Adhesion

Prior to coating deposition, cemented carbide cutting tool substrates are mechanically treated via various methods for improving the film adhesion. As a side effect of these treatments, different surface topographies are generated (Toenshoff et al. 1997).

Typical mechanical pretreatments of cemented carbide tools are presented in Fig. 9a. The applied processes are grinding (G) or grinding with subsequent polishing (P) for achieving a medium or a low roughness, respectively. Furthermore, microblasting (mb) is conducted in all the examined cases. After polishing and microblasting, the exposed WC carbides are better embedded in the binding material, compared to ground and microblasted substrates. Thus the effective film adhesion is enhanced, and a cutting performance increase is realized. Milling tests validate these models (see Fig. 9b). The coated inserts with ground and microblasted substrates reach a tool life of approximately 55 × 103 cuts, at a flank wear width of 0.2 mm. Moreover, the results exhibit a further wear resistance growth, by polishing and subsequent substrate microblasting. Inserts with polished or ground substrates managed to cut ca. 28 × 103 and 35 × 103 times, respectively, up to the same flank wear width. The wear resistance improvement is evident, when coated inserts with polished and microblasted substrates are used (Bouzakis et al. 2005b).

Fig. 9
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Effect of substrate’s microblasting on (a) its microstructure and (b) coated tools’ cutting performance

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Further ways for improving the coating adhesion are via deposition of thin adhesive interlayers between substrate and coating (Bouzakis et al. 2010b) and through surface nitriding, prior to the PVD film deposition (Erkens et al. 2011).

Improvement of Coated Tools Cutting Performance

Microblasting on PVD-coated tool surfaces may be an efficient method for improving the cutting performance. Through microblasting on coated cemented carbide inserts, it is possible to enhance film strength properties and thus the tool cutting performance. On one hand, microblasting induces residual compressive stresses into the film structure, resulting to coating hardness increase (Bouzakis et al. 2009). On the other hand, the film becomes more brittle (Bouzakis et al. 2011b). As blasting materials, aluminum oxide (Al2O3) with sharp-edged grains or ZrO2 with smooth surfaces is commonly used. The blasting grains are transferred by compressed air (dry) or combination of compressed air with water (wet) and can cause coating’s material deformations (strengthening effect), as well as material removal (abrasive effect). This is qualitatively demonstrated in Fig. 10. The residual compressive stresses, which are simultaneously induced into the film structure, lead to an increase in coating hardness and strength properties. Microblasting parameters such as pressure, time, abrasive grains’ size, and quality affect the coated tool’s cutting performance. According to experimental and computational results, by microblasting pressure augmentation, an enlargement of the plastically deformed film region into its depth takes place. Although an increased microblasting pressure is beneficial for enhancing the coating hardness, this can cause the substrate revelation as well as increased film brittleness, and in this way, a deterioration of the tool life may occur, as it is shown in the diagram at the bottom of Fig. 10 (Klocke et al. 2009; Bouzakis et al. 2011b). According to these results, microblasted tools at a pressure of 0.2 MPa exhibited the best cutting performance, reaching a tool life of approximately 130,000 cuts. A slight tool life reduction at 120,000 cuts up to the same flank wear of 0.2 mm was encountered at a pressure of 0.3 MPa. Hence, over this critical microblasting pressure, which depends on the microblasting conditions, the higher microblasting pressure increases the film brittleness. In this way, the coated tool wear resistance is also deteriorated. At the higher microblasting pressure of 0.4 MPa, the cutting performance is additionally restricted by substrate revelation effects.

Fig. 10
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Effect of microblasting on coated tools’ properties and cutting performance

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Reconditioning of PVD-Coated Tools of Complex Geometry

In contrast to simple cutting inserts, solid tools such as twist drills, milling cutters, gear hobs, gear-shaping wheel cutters, and broaching tools have to be reconditioned after achieving the wear limit, due to their elevated cost compared to one-use cutting inserts. The potential of reconditioning worn-coated cemented carbide and high-speed steel tools through sequential electrochemical coating removal, tooth rake regrinding, microblasting, and PVD recoating has a wide industrial importance. However, the effect of all these procedures on the substrate mechanical properties, on the edge sharpness, and on the cutting performance, especially of cemented carbide tools, must be given due consideration. The mechanical properties of the cemented carbide substrate may deteriorate after the first film deposition (Denkena and Breidenstein 2010) and may further degrade, albeit to a lesser extent in further recoating steps. As such, a deterioration in the cementing of carbide grains by the cobalt binder may arise leading to cutting-edge chipping, which in turn reduces cutting performance and process reliability. The appropriate control of procedures such as macro- and microblasting enhances the cutting performance after tool reconditioning and improves the productivity when using cemented carbide tools (Bouzakis et al. 2008b; Klocke et al. 2009).

Key Applications

The application of coatings is currently a “must” on the vast majority of cutting tools. This is particularly true where high-speed and high-performance cutting is required or in the machining of difficult-to-cut materials. For the majority of industrial applications, the use of coated tools is widespread and absolutely necessary for realizing satisfactory productivity and tool life.

High-Performance Cutting (HPC) and High-Speed Cutting (HSC)

The objective of high-performance cutting is the shortening of machining times by applying increased cutting speeds and/or feed rates with due consideration of tool wear and the surface quality of the generated workpiece. High-performance cutting applications place high demands on the properties of PVD coatings in terms of wear resistance, thermal stability, and oxidation resistance and hardness at elevated temperatures. Al-, Ti-, and Si-containing coatings such as AlTiN, AlCrN, and TiSiN on cemented carbide tools generally provide high performance in metal cutting applications.

Machining of Difficult-to-Cut Materials

Difficult-to-cut materials are characterized by low machinability, which prescribes the application of convenient tool and coating materials as well as machining conditions, for guaranteeing a satisfactory tool life. Characteristic materials with low machinability are aerospace alloys, e.g., titanium or nickel alloys. Aerospace alloys possess high strength, work hardening, and dynamic shear strength at ambient and elevated temperatures and are characterized by low thermal diffusivity and chemical reactivity with tool materials, associated with increased tool cutting loads and temperatures, as well as with extreme abrasion. For such demands, despite the trend to use uncoated tools, the application of single and multilayer TiAlSiN and CrAlN coatings containing alternatively Zr or Y dopants on cemented carbide tools can provide a superior cutting performance (Bouzakis et al. 2008a; Klocke et al. 2010).

Cross-References