2014 INTERNATIONAL SCIENTIFIC CONFERENCE 21 – 22 November 2014, GABROVO

NANOSTRUCTURED COATINGS FOR CUTTING TOOLS Miroslav Radovanović, Miloš Madić University of Niš, Faculty of Mechanical Engineering, Serbia Abstract Quality of cutting tools is one of the limiting criteria in the metal cutting. In metal cutting process is normal to use modern cutting tools. Coated cutting tools are most widely used. They achieve significantly higher cutting performance. One further development in coated cutting tools is the deposition of nanostructured coatings. Nanostructured coatings are superhard materials for cutting tools. Nanostructured coatings are coatings with dimensions of grain size or individual layers less than 100 nm. They offer improved durability and performance over traditional cutting tool materials. Nanostructured coatings can be categorized as: nanocrystalline, nanomultilayer and nanocomposites. This paper presents characteristics of nanostructured coatings for cutting tools. Keywords: cutting tool, coated cutting tools, nanostructured coatings.

INTRODUCTION Metal cutting has progressed and developed with three elements: workpiece materials, cutting tools and machine tools. New workpiece materials are being developed continuously with development of metalworking industry. These materials have high strength and toughness but generally abrasive and chemically reactive with cutting tool materials. The difficulty of cutting these materials efficiently and the need for improving the performance in metal cutting the more common engineering materials have led to important development in materials of cutting tools. Development of wear resistant cutting tools is imperative. Many types of cutting tool materials are used in today’s metalworking industry. Success in metal cutting depends upon the selection of the proper cutting tool material for a given workpiece material. Choosing a cutting-tool material is not an easy task. Resistance to deformation and wear, resistance to chipping, and resistance to chemical attack by the workpieces material are the principal characteristics of a successful cutting tool material. Cutting tool material must possess: hot hardness, toughness and impact strength, thermal shock resistance, wear resistance, and chemical stability and inertness. It is tendency that cutting tool materials have hot hardness and wear resistance at high temperature with hot toughness. Demand for better performance III-298

and more efficiency is generating lively development of cutting tool materials which have big influence on steadiness of cutting tools. In last time it is evident the application of cutting tools from new materials, which enable the work with high cutting speed. Selecting tool material on the basis of longer tool life is a safe choice A wide range of cutting tool materials is available with a variety of properties, performance capabilities, and cost. Cutting tool materials are classified as:  High speed steels,  Coated high speed steels,  Cemented carbides,  Coated cemented carbides,  Ceramics,  Super hard tool materials (polycrystalline diamond and polycrystalline cubic boron nitride) , and  Coated super hard tool materials. One of the most revolutionary changes in the cutting tool materials has been development of thin film hard coatings. Coatings considerably improve tool life and boost the performance of cutting tools in high productivity, high speed and high feed cutting. Nowadays, 50% of high speed steels, 85% of carbides and 40% of super-hard materials used for cutting tools are coated. There are five primary reasons for using coated tools: to

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increase wear resistance, to increase oxidation resistance, to reduce friction, to increase resistance to metal fatigue, and to increase resistance to thermal shock. Coatings have unique properties, such as lower friction, higher adhesion, higher resistance to wear and cracking, acting as diffusion barrier and higher hot hardness and impact resistance. Coated cutting tools can have tool lives 10 times longer than those of uncoated tools, allowing for high cutting speeds and thus reducing both the time required for machining operation and production cost. This improvement has a major impact on the economics of machining operation in conjunction with continued improvement in the design and construction of modern machine tools and their computer controls. As a result, coated cutting tools now are used in 40 to 80% of all machining operations, particularly in turning, milling and drilling. In turning, the percentage is about 95% and in milling about 60%. Survey has indicated that the use of coated cutting tools is more prevalent in larger companies than in smaller one. Nanostructured coatings are superhard materials that rival diamond in performance. Nanostructured coatings are defined as materials having features, such as grain size or individual layers, with dimensions less than 100 nm. The superior mechanical and physical properties of nanostructured coatings make them ideal for metal cutting. Nanostructured coated cutting tools offer improved durability and performance over conventional tools. COATED CUTTING TOOLS Cutting tools which are most widely used in machining operations for the production of steel and cast-iron workpieces are coated cutting tools. Coated cutting tools achieve significantly higher cutting performance compared with traditional cutting tools. High hardness at elevated temperatures is the main advantage of coated cutting tools, which allows them to be used at significantly higher cutting speeds. There are basically four major groups of coating materials. The first group, the most popular group, is titanium based coating materials as titanium nitride (TiN), titanium carbide (TiC), and titanium carbon nitride

(TiCN). The second group represents ceramic type coatings as aluminum oxide (Al2O3). The third group includes super-hard coatings, such as chemical vapor deposition diamond. The fourth group includes solid lubricant coating. Coated cutting tools can classified as  Coated inserts for cutting tools,  Coated solid cutting tools. Coatings are applied by four basic types of techniques 11:  Chemical vapor deposition(CVD),  Physical vapor deposition(PVD),  Thermal diffusion (TD), and  Dynamic compound deposition (DCD). Chemical vapor deposition (CVD) involves a chemical reaction between a gaseous phase (e.g. titanium and nitrogen) and the surface of a substrate heated to approximately 1000◦C. CVD processes are chemical reactions that take place in gaseous phase under rough vacuum conditions (103–105 Pa) and with the addition of thermal or radiant energy, forming technically useful solids (hard materials) as well as volatile products. Because CVD coating is a gaseous process, all the surfaces of the substrate may be uniformly coated. CVD coatings have been commercially available for about 30 years, and the fact that more than half of the inserts sold are CVD coated testifies to the effectiveness of these coatings. CVD coatings usually are deposited in multi-layer composition. A TiC–TiN multi-layer, for instance, provides the lubricity of TiN and the abrasion resistance of TiC. Coating thickness is in the range of 5–10 μm. Chemical vapour deposition (CVD) methods are:  High temperature CVD (HTCVD)  Medium temperature CVD (MTCVD)  Plasma enhanced CVD (PECVD)  Plasma activated CVD (PACVD)  Electron cyclotron resonance CVD (ECRCVD) The early 1980s saw the succession of the CVD process by the PVD process in various process variants (vacuum evaporation, ArcPVD, sputtering). The initial benefit of these methods was the possibility of coating high speed steels (HSS) tools with complex geometries. In the meantime, these processes

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are also being used for cemented carbides. The move to PVD coating technology in the production of cutting tools has resulted in edges that are much sharper than can be achieved with CVD. Physical vapor deposition (PVD) is process in which the metal component of the coating is produced from solid, in a high vacuum environment. The generation of the metal atoms is accomplished by evaporation or ion bombardment methods, at temperatures of approximately 500C. Physical vapour deposition (PVD) coatings are ceramic materials usually applied in 1-15 µm thicknesses on tools made of high speed steels and cemented carbides.[4,13] Physical vapour deposition (PVD) methods are:  Thermal evaporation (TE) - Pulsed laser deposition (PLD) - Electron beam deposition (EBPVD)  Sputter deposition - Magnetron sputtering - Ion beam sputtering  Arc vapour deposition - Vacuum arc deposition - Filtered arc deposition  Ion implantation - Ion beam deposition Thermal diffusion (TD) is applied in a molten borax bath, with the addition of vanadium, at approximately 1000C. The resultant vanadium carbide coating has very good results in numerous applications 11. Dynamic compound deposition (DCD) coating process (developed by Richter Precision Co.) is a proprietary low-temperature coating process that synthesized dry-film lubricant and wear-resistant coating components 11. NANOSTRUCTURED COATINGS Nanotechnology also involves sputtering, i.e. deposition, by PVD. It is being used today in a wide variety of industries. Such coating technology is being investigated for cutting tools because unusual combinations of layers III-300

can be built up. The key concept is to deliberately mix alternating hard and tough layers. Crack that might be initiated in a hard, brittle layer is arrested when it meets the tough layer thereby increasing the overall fracture toughness of the whole ensemble. Almost any refractory hard material can be used as the hard nanolayer; almost any compatible metal can be used as the tough nanolayer. Whereas each layer of the multiple layer “sandwich” may only be a few nanometers thick, the total thickness can be in the range of 2-5 µm. The number of potential combinations of materials that can be used for nanostructured coatings is unlimited. 10 Generally, the nanostructured coatings can be divided into two main categories: a) Multifunctional nanostructured coatings b) Superhard nanocomposites coatings Multifunctional nanostructured coatings offer high hardness (20 to 35 GPa) and excellent wear resistance combined with other important properties, such as high toughness, impact resistance, and dry lubrication. Superhard nanocomposites are ceramic based coatings e.g. nc-MeN/a-Si3N4 (Me = Ti, V), nc-TiN/a-BN/a-TiB2, nc-(Ti1-xAlx)N/aSi3N4. These coatings offer superhardness (H ≥ 50 GPa), high resistance to brittle fracture, high thermal stability and oxidation resistance; In terms of their nanostructure, coatings can be categorized as:  Nanocrystalline coatings,  Nanomultilayer coatings (nanocoatings),  Nanocomposite coatings. Nanocrystalline coatings display a lower wear rate. This improvement in wear resistance is attributed to the high hardness and toughness of nanostructured materials, and the change of fracture and material removal mechanism due to ultrafine particle size. The major challenge for synthesizing nanocrystalline coatings is to retain the particle size of the powders or inhibit their growth at high temperatures. Various methods such as reconstituting nano particles into micro particle “balls” combined with plasma spray have been developed to solve this problem. In Fig. 1 is shown nanocrystalline coating. 13

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Fig. 1. Nanocrystalline coating

Fig. 2. Nanomultilayer coating

Using PACVD coating technology, now it is capable of depositing nanomultilayer coatings. Nano-multilayer coating is the conventional structure for the so-called nanocoatings. These are multilayer coatings with individual layers whose thickness is given in nanometres. Nanoscale multilayer coatings, which consist of alternating layers of materials, improve the performance of single-layer nanostructured coatings. Lowering the thickness of the coating increases hardness, toughness and wear resistance. One further development in coated cutting tools is the deposition of nanolayers or coatings with superlattice structures. These are coatings consisting of a large number of extraordinarily thin layers, the thickness of which amounts to only a few nanometres. In contrast to conventional, usually columnar layers, coating materials deposited as nanolayers are characterized by their significantly higher hardness. This is based on the fact that the hardness of a material increases significantly below a certain layer thickness. This increase in hardness is explained by the phenomenon that diminishing layer thicknesses have altered crystal lattices and thus Young’s modulus as well. For example, AlN has a hexagonal lattice structure at layer thicknesses > 10 nm and a cubic structure when the layer thickness is < 10 nm. AlN is, with a hardness of about 1200 HV0.05, relatively soft in comparison to hard materials like TiC, however it has a hardness of over 3000 HV0.05 when the coating thickness is below 10 nm. In Fig. 2 is shown nanomultilayer coating. 6,7

Types of materials, bonding characteristics, and crystal structures differentiate multilayer coatings. Nanomultilayer coatings produce super hardness and super modulus effects. Optimum mechanical properties can be achieved by varying the number of layers. In Fig. 3 is shown nanomultilayer coating TiN30x(TiCN-TiN), with coating thickness of 5.2 µm, and single layer thickness of approximately 80 nm. 6

Fig. 3. Coating TiN-30x(TiCN-TiN)

In Fig. 4 is shown nanomultilayer coating TiN-18x(Al2O3-TiN), with coating thickness of 2 µm, and single layer thickness of approximately 50 nm. 6

Fig. 4. Coating TiN-18x(Al2O3-TiN)

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In Fig. 5 is shown nanomultilayer coating 1000x(TiN-AlN), with coating thickness of 9 µm, single layer thickness of approximately 9 nm, and coating hardness of 3200 HV0.05. 6

Fig. 5. Coating 1000x(TiN-AlN)

Another way to improve a coating’s properties is to produce so-called nanocomposites. In Fig. 6 is shown nanocomposite coating 6.

Fig. 6. Nanocomposite coating

Nanocomposite coatings are commercially available since 2003. These are noncrystalline

isotropic multiphase systems, in which two mutually insoluble phases are deposited during the coating process. In nanocomposite coatings, different materials are combined to achieve new properties that cannot be obtained from a single material. Nanocomposites represent a new class of materials, consisting in two or more phases coexisting in a very low volume, crystals having dimensions of 3-10 nm. In the case of nanocristallyne materials the number of atoms in a crystal grain is comparable, or even less, than the number of atoms that are in the grain limits. In such conditions the formation of dislocations is inhibited by the grain limits, and mechanical deformation takes place by the mechanism of slipping at the grain limits, not by dislocation movement, which is the mechanism of deformation in conventional materials. This leads to a signifycant increasing of hardness of nanocristallyne materials and to the development of superhard materials. In general, nanocomposite coatings comprise at least two phases: nanocrystalline phase and matrix phase. The matrix can be either nanocrystalline or amorphous. These types of coatings are reported to have improved mechanical and thermal properties. By depositing different kinds of materials, the components (Ti, Cr, Al, and Si) are not mixed, and two phases are created. The nanocrystalline TiAlN or AlCrN grains become embedded in an amorphous Si3N4 matrix. The design and synthesis of multilayer nanocomposite coatings significantly improved wear life of the coatings.

Table 1. Performances of nanoctructured coatings Coating

Colour

nACo=nc-AlTiN/a-Si3N4 Violet-blue nACRo=nc-AlCrN/a-Si3N4 Blue-grey nATCRo=nc-AlTiCrN/a-Si3N4 Blue-grey 3 nACo =TiN+AlTiN+nACo Violet-blue 3 nACRo =CrN+AlTiCrN+nACRo Blue-grey nATCRo3=CrTiN+AlTiN+AlTiCrN/SiN Blue-grey nACoX3=TiN+AlTiN+nACo+AlCrO(N) Black 3 TiXCo =TiN+nACo+TiSiN Copper 3 AlCrN =CrN+Al/CrN Multi/Nanolayer+AlCrN Black AlTiCrN3=Cr(Ti)N+Al/CrN Multi/Nanolyer+AlTiCrN Blue-grey AlTiCrN3+=CrN+Al/Ti/CrN Multi/Nanolyer+CrCN Blue-grey III-302

Nano Thickness Friction Max. usage hardness (m) coefficient temperature (GPa) (C) 45 1-4 0.45 1200 40 1-7 0.35 1100 42 1-4 0.40 1150 45/34 1-5 0.45 1200/900 40/34 1-5 0.35 1100/900 42/34 1-5 0.40 1150/900 40/30 4-18 0.40 1200 40/70 1-5 0.55 1200 32/35 1-7 0.40 900 34/32 1-10 0.60 900 32/34 1-7 0.50 900

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The wear mechanisms in nanocomposite coatings are different. The nanocomposites can be deposited as monolayer or multilayer coating systems. The boundary layers/grain boundaries in the nanolayer/nanocomposite layers are energy dissipating barriers to cracks. Cracking and the speed of crack growth are thereby reduced. Nanocomposite layers are thus characterized not only by extremely high hot hardness and high temperature wear resistance but also by favorable toughness attributes. Performances of nanoctructured coatings are shown in Table 1 [6]. Nanocomposite coating nc-AlTiN/a-Si3N4 have extremely high nanohardness and extremely high heat and oxidation resistance. Nanocrystaline grains are embedded into an amorphous matrix. Nanocomposite coating ncAlCrN/a-Si3N4 have high nanohardness, high heat resistance and high coating thickness possible. Fig. 7 shows the tool life comparison when drilling 42CrMoV, hardness of HRC 30-32, with coated solid carbide drills, diameter of 8 mm, successive cutting, drilling depth 24 mm, cutting speed 150 m/min, feed 0.15 mm/rev, emulsion 8%. [15]

Fig. 7. Tool life of coated solid carbide drills

CONCLUSION Acceptable performance requires a cuttingtool with good hot hardness and abrasion resistance, and high chemical inertness. The best cutting tool is not necessarily the most expensive. This is the one that has been carefully chosen to get the job done quickly, efficiently and economically. There is a wide range of materials available for cutting tools. Nanostructured coated cutting tools with a very good wear resistance can make important functional tool properties in such specific applications like high speed cutting, high feed cutting, hard machining, dry machining, etc. Now nanostructured coatings for cutting tool

applications are used commercially. Thus the technology is expected to grow in the future. ACKNOWLEDGEMENT The paper is part of the project III41017 Virtual Human Osteoarticular System and its Application in Preclinical and Clinical Practice, sponsored by Ministry of Education, Science and Technological Development of the Republic of Serbia for period 2011-2014. REFERENCES [1] Malshe A., Jiang W., Dhamdhere A., Nanostructured coatings for machining and wearresistant applications, JOM, September 2002, pp. 28-30. [2] Duane D., Cutting tool coating production, Production Machining, September 2011, pp. 38-42. [3] Vereshchaka A. A., Vereshchaka A. S., Mgaloblishvili O., Morgan M., Batako A., Nano-scale multilayered-composite coatings for the cutting tools, Int. J. Adv. Manuf. Technol., Published online: 14 February, 2014. [4] Coating catch-up, www.machinery.co.uk, March 2007. [5] Kopač J., Advanced tool materials for highspeed machining, 12th International Scientific Conference Achievements in Mechanical & Materials Engineering - AMME, 2003, pp. 1119-1128. [6] Platit compendium 2014 (www.platit.com) [7] Cselle T., Coddet O., Galamand C., Holubar P., Jilek M., Jilek J., Luemkemann A., Morstein M., Triplecoatings - new generation of PVDcoatings for cutting tools, Journal of Machine Manufacturing, Vol. XLIX, Issue E1, 2009, pp. 19-25. [8] Radovanović M., Dašić P., Ječmenica R., Advanced cutting tool materials, 2nd International Conference Research and development in mechanical industry - RaDMI, 2002, pp. 346-351. [9] Klocke F., Manufacturing processes 1, Cutting, Springer, 2011. [10] Trent E., Wright P., Metal cutting, Butterworth-Heinemann, 2000. [11] Astakhov V., Tribology of metal cutting, Elsevier, 2006. [12] Pogrebnjak A., Beresnev V., Hard nanocomposite coatings, their structure and properties (http://dx.doi.org/10.5772/50567 [13] Davim P., Machining of hard materials, Springer, 2011. [14] www.diamondproductsolutions.nl [15] Golombek K., Dobrzanski L., Hard and wear resistance coatings for cutting tools, Journal of Achievements in Materials and Manufacturing Engineering, 2007, Vol. 24, No. 2, pp. 107-110 [16] The most important criteria of coating users = The most important reasons for in-house coating, Werkzeug Technik, 25 February 2014, Nr. 138 (www.werkzeg-technik.com)

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