1999_Klocke-F._Krieg-T._CIRP-Ann-Manuf-Technol

8/6/2019 1999_Klocke-F._Krieg-T._CIRP-Ann-Manuf-Technol

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Keynote Papers

Coated Tools for Me tal Cutt ing - Features an d ApplicationsF Klocke (11, T. KriegLaboratory for Machine Tools and Production Engineering Dept. of Machining TechnologyRWTH Aachen, Aachen, Germany

AbstractDemands on products and production processes are the driving factors behind developments in today's cut-ting technologies. Innovations such as the application of advanced work material concepts, together withneeds for non-pollutant machining processes, increased flexibility and improved cost-effectiveness triggerthe applicationof high performance processes, imposing higher stresses on tools. This often reveals inade-quate wear resistance n conventional tool materials. Coating technology is one means of achieving a crucialenhancement in tool performance. However, there is such a huge variety of available coating materials,coating structures and coating processes that careful selection of a suitable coating system is essential. Us-ing accessible know-how concerning coated cutting tools and their behaviour in a wide range of differentmachining tasks, the paper shows methods to test, evaluate and influence the properties of tool coatings.Applying this know-how may contribute to improving the systematic selection and development of coatingsfor specialised cutting operations.Keywords: Machining, Tool Coating, Tribology

0 AcknowledgementsThe authors would like to acknowledge al l who havecontributed to this paper with suggestions, discussionsand documents of their work. Special thanks are given to:Akiyama K., Altan, T., Bouzakis K.-D., Brinksmeier E.,Dautzenberg J.H., Jawahir I.S.. Leopold J., Moriwaki T..Ostafiev V., Schulz H . , Seytoyama M., Tonshoff H.-K.,Uhlmann E.. Weinert K., WerlheimR..Yamada Y.1 IntroductionSome important keywords describing the environment ofcurrent cutting processes are High Speed Machining,near net shape technology, hard machining, hard-to-machine materials, environmentally compatible proc-esses and precision machining. These technologies arebeing developed and implemented in response to de-mands on actual products in terms of productivity, flexi-bility, accuracy and environmental compatibility (m1).

Figure 1 Driving factors - current cutting processes.Just as the demands imposed on products change ma-chining processes, current cutting processes in their turn

affect, i.e. very often intensify, demands on the wearresistance of coated cutting tools.An effective, systematic approach is essential for thesuccessful implementation and sophistication of modemcutting processes. There is accordingly a current need todevelop predictive models for the various parameterswhich govern machining performance, like cutting forces,tool wear, chip formation, surface integrity and part accu-racy. The properties of tool materials need to be includedas an integral part of such attempts at predictive model-ling [I,1. In addition, it is necessary to formulate theeffective mechanisms by which process modificationsaffect tool stresses and the ways in which coating prop-erties influence wear behaviour under a given set ofprocess parameters. The following sections are intendedto contribute to a basic understandingof these interrela-tionships.2 Propertiesof coatingsIn order to select or develop a suitable tool coating, it isnecessary to identify the primary wear mechanisms in-herent in the specific machining task. The ability of acoating to reduce wear sufficiently is the criterion forchoosing it.Fiaure2 shows that there are two major ways in which acoating may influence tool wear. On the one hand, thefive wear mechanisms defined in DIN 50320 can be influ-enced directly by increasing wear resistance. These wearmechanisms may firstly be classified into the three sur-face effects of adhesion, abrasion and tribo-oxidation.Diffusion is a mechanism which begins at the tool face,but which also influences the properties of the bulk mate-rial and can therefore also be regarded as a volume ef-fect. Finally, fatigue is a typical volume effect that leads tolosses of tool material due to fractures which follow theformation of cracks.On the other hand, tool coatings can help to vary contactconditions by altering friction, heat generation or heat

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flow. These are indirect means of influencing wear bydecreasing wear attack.FrictionHea t generationHeat flow

Altered contact

Altered wear7 esistanceComplex stressesl Surface effecrs: AbrasionAdhesionTribo-oxidation combined wearDiffusion phenomena

ScoringPlastic deformationVolume effects: Crack initiation

source: WZLFigure 2: Influence of coatings on wear mechanismsand contact conditions.2.1 Coating structuresBecause the structure of the tool coating determines bothits wear resistance and the tribological conditions in thecontact zones, it is essential to adapt the coating struc-ture to the demands of a specific machining task. Themain influences on the structure are:+ the choice of coating material+ layer growth during the coating process and+ the structural design of the single layers to form amulti-layer.2.1.1 Philosophy behind choice of materialsBasically, there are four major groups of hard coatingmaterials on the market. The most popular is the group oftitanium-based coating materials such as TIN, Tic andTi(C,N). The metallic phase is often supplemented byother metals like A1 or Cr, whose role is to improve prop-erties like hardness, oxidation resistance etc.. A verysuccessful example of such coatings is (Ti,AI)N. Thesecond group represents ceramic coatings like AO Inthe last few years, two urther groups have been added tothe list of available coatings for cutting tools. These arethe super-hard coatings, like CVD-diamond, and the solidlubricant coatings (hard coatings with a very low coeffi-cient of friction), such as amorphous metal-carbon, Me-C:H.Additionally, recent years have seen the introduction ofsoft coatings, which are deposited on top of a hard coat-ing to reduce friction and wear, especially in its firststages. Examples are MoS, or pure graphite. WClC pro-viding a somewhat higher hardness can also be countedamong this type of coatings due to a self polishing effectunder tribological oadings.The Ti-based coatings earn their popularity from the factthat they combine coverage of a broad "medium range" ofmechanical and thermal properties with an adequate rateof deposition during the coating process and good bond-ing to the usual tool substrates. Ceramic coatings exhibitgood resistance to abrasive wear and possess high ther-mal stability. Except for AO this group of coatings hasnot yet reached a very high level of application for metalcutting tools (31.This may be due to their brittleness andpoor bonding to the tool substrate. Moreover, at leastsome PVD-processes are not suitable for depositingceramic coatings.Since diffusion is a very strong wear mechanism in metalcutting, the choice of a coating material must follow a

basic guideline. The enthalpy of formation of the chosencoating material must be as negative as possible, in orderto shift the temperatures at which diffusion occurs to-wards high values [4]. For example, if steel is used as awork material, it is important that the tool material has amuch more negative enthalpy as compared to any pos-sible combination of iron with one of the elements of thetool material. If the enthalpies of different materials arecompared [5, 61, it can be inferred that most of the poten-tial carbide coating materials like TIC, HfC, rC etc. aremore suitable for steel cutting than WC. This likewiseapplies to most of the nitrides except CrN, up to a tem-perature of some 1500C. The oxides are also very sta-ble and are suitable as tool materials.Transformation in response to tribological stresses is ananticipated and desired property of certain metallic coat-ing materials. A frequently discussed effect is the possi-ble formation of AIO, and TiO, on a (Ti,AI)N coating athigh temperatures. This transformation could help toprovide protection against tribo-oxidation for coated re-gions which are temporarily or continuously exposed tohigh temperatures and air [7] .Some special effects like the influence of different coatingmaterials on the wetting of tool surfaces by cooling lubri-cants (influence of coating polarity and topography) arestill being researched (8) .2.1.2 Influence of the coating processThe morphology of a coating depends mainly on thecoating process applied. The relevant processes for thecoating of tools may roughly be differentiated into CVDand PVD processes. CVD and PVD processes may befurther classified into sub-types, each with its effects oncoating structures and on the tribological properties of thecoated tools.The main characteristic of a CVD-process is the highsubstrate temperature needed to deposit a coating. Hightemperatures during the coating process promote an-nealing processes in HSS substrates and also affect thetoughness and the transverse rupture strength (TRS) ofcemented carbide substrate materials, due to the forma-tion of a brittle q-phase (Co,W,C,) [9, 101. Using a stan-dard CVD process at about 1100C can reduce strengthby 30 percent. The problem can be alleviated by usingthe moderate temperature process (MT-CVD) at 850Ccoating temperature. A further advantage of the moderatetemperature processes is that stresses decrease andtoughness is improved significantly due to the lower ma-terial expansion at 850 "C. Co-enrichment at the toolfaces has been identified as another means of improvingthe TRS of CVD coated tools.The PVD process, which is usually performed at 200 "Cto 500 'C, has virtually no impact on the transverse rup-ture strength of the coated material [7. lo]. In PVD-processes, the materials needed to form the coatingmaterial (e.g. metals) are evaporated and subsequentlycondense on the tool substrate. Further components ofthe coating material can be added by using a reactivegas. The method used to evaporate the coating materialis an important feature of a specific PVD-process. It canbe induced by heating, by an electron beam or by sput-tering with a process gas (often Argon) accelerated to thetarget. Differentiation into three types of PVD-processes(evaporation, sputtering and ion plating (IP)) is in line withthe energy imparted to the evaporated particles[ill. ighionisation levels, as realised in ion plating, can be ex-ploited to improve important properties of tool coatings,like hardness, coating-substrate bonding, structure aswell as chemical and thermal stability. A negative biasvoltage is therefore used to accelerate the particle streamfrom the target to the substrate. Popular IP-processesareArc-lon-Plating (AIP), Magnetron-lon-Sputtering (MSIP),

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_I____(_

High-lonisation-Sputtering (H.I.S.TM) and Low-Voltage-Arc-Discharge (Bakers) [7].In Fiaure 3, SEB-analyses of the cross-sectional mor-phology of different (Ti,AI)N coatings illustra te the inter-relationships between coating structure, aluminium con-tent and ionisation [12]. MSIP-coatings with an AI/Ti-ratioof 0.6 show a coarse columnar structure with pinholes(Figure 3-A). The diameter of the grains varies from 0.3pm up to 0.6 pm. A higher aluminium content leads to adense, fine, columnar structure (Figure 3-8). The endfaces of these columns are flat rather than spherical, asshown in Figure 3-A. Further increasing the aluminumcontent decreases the grain diameter, but also producesa less dense columnar film structure (Figure 3-C).

Graded layerouce:cememFigure 3: Influence of PVD-process and Al-content onthe structure of (Ti,AI)N-Coating.The H.I.S.TM process (Figure 3-0, -E, -F ) leads to en-hanced ionisation as compared to MSIP-technology.Irrespective of the aluminium content, all cross-sectionsare dense and non-columnar. The surface topographiesare smooth. With increasing ionisation, more argon andmetal ions reach the surface. The higher the energy ofthe arriving ions, the denser are the obtained films [13].MSIP- and H.I.S.lM coatings reache their greatest hard-ness at an AITTi-ratio = 1 O. The microhardness of H.I.S.lM-coatin gs was found to improve irrespective of theAVTi-ratio. The microhardness of the coatings correlatesclosely with the structure of the films. The denser theobtained structure, the greater will be the microhardnessof the films. The decrease in microhardness as alumin-ium content rises can be explained by the higher contentof the softer and hexagonal AIN-phase (= 1000HVO.l)2.1.3 Philosophy behind the structuring of mono-

and multilayer coatingsFiaure 4 gives an overview of currently available mono-and multilayer structrures. There are three main drivingfactors behind the application of multi-layered coatings inthe field of conventional hard coatings:

[141*

Some coating materials provide good bonding to thesubstrate, so that they are often used as an interfaciallayer between the substrate and the actual hardcoating. An example is Tic in a typical CVD TiC-AI,03-TiN coating.Some multi-layers are designed to improve mechani-cal properties of the complete coating, like hardnessand toughness. Since some of the Ti-based coatingshave high residual stresses, nano-layer structures oreven superlattice structures are used to improvetoughness. As a result, a greater coating thicknesscan be realised without adverse effects on bonding.One example is the use of numerous alternating lay-

ers of TiN and (Ti,AI)N to provide a coating whichcombines all the advantages of (Ti.AI)N with goodbonding and high toughness. A large number of in-terfaces between the single layers are also thought toprovide a barrier against crack propagation [7].3. Multi-layer design can also b e aimed at realising acombination of functions provided by different coating

materials. Multi-layers with different functional inter-mediate layers are applied for this reason. Functionsmay include the high thermal stability offered by anintermediate layer, high hardness provided by the toplayer or even reduction of the friction coefficient by asoft top layer or a solid lubricant layer.If graded layers are interpreted as multi-layer systemsthey can be designed for two different purposes, to pro-vide a smooth and graded transition either to a goodcoating-substrate bonding or to special properties on thecoating surface.

Mono-layer (hard thin film)t = 0.5 ... 50 pmTypical multi-layer withfunctional intermediate ayerst = 0.5 ... 10 pmMultilayer (nano-structure)I j t = few atomic cells ... 100 nm

Super-hard coatings(CVD-DP I BN )Hard and soft compounds(MoS,, WC/C, graphite etc.)

I SHard film + solid lubricant film 8(a-Me-C:H) 1Source:WZL, LMMFigure 4: Commonly used layer structures.A frequently cited property of coatings, especially in con-nection with function al intermediate layers, is the thermalinsulation of the substrate. FEM-analysis of the influenceof 15pm thick coatings with thermal properties of (Ti,AI)N( can be used to show that there is no significanteffect on either the temperature fieldlmaximum tempera-ture or on a delay in the increase of heat at the substratesurface (A) and at a depth of 0.3 mm (B). A low heattransmission coefficient likewise has no effect on thetemperature in the substrate. Rather, there is highertemperature loading of the coating itself. These state-ments have been deduced from the fact that three calcu-lations for different thermal properties produced almostidentical results for the temperature curves in point A andpoint 8. In addition lower thermal conductivity of thecoating ma terial as we ll as a low heat transmission coef-ficient in the coating-substrate interface leads to highermaximum temperatures in the coating.Since the model is based on the assumption of a con-stant heat flux into the coated body, any measurablethermal relief of the substrates in metal cutting withcoated tools has to be traced back to altered contactconditions which lead to lower heat generation. De-

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creased heat transfer between chip and tool as a result ofthe coating is also conceivable. influences of grinding, water peening and micro blastingon the bonding of a PVD-(Ti,AI)N coating to the ce-mented carbide substrate were investigated by means ofa spherical indent test ( Evaluation of the pat-tern of the chipped coating material around the sphericalindent shows that both microblasting and water peeningcan decisively improve the bonding between coating andsubstrate. It is important to use a blasting material withlow grain size for microblasting. Low pressure seems tohave an additional positive effect.

Figure5: FEM-analysis of thermal isolation-effects.2.2 InteractionsMany interactions take place between the coating andsubstrate materials as well as between the coating proc-ess and the coated tool, decisively determining its wearresistance and performance. These interactions begin ona very low scale, with the dependence of the latticestructure of single layers in a superlattice on the layerthickness. (A) and (B) show the X-ray diffractionpatterns of TiNlAlN superlattices with h = 30 nm and h =2.5 nm periods respectively. For the superlattice with 30nm period, the diffraction patterns of TIN (NaCI-type) andWurtzite-type AIN are identified. However, the diffractionof Wurtzite-type AIN is not identified for a period of 2.5nm (A). Only one diffraction pattern exists. It is equivalentto face centered cubic structures and its lines lie betweenthe positions of TIN and NaCI-type AIN. The results sug-gest that AIN in the superlattice with h = 2.5 nm trans-forms into a cubic structure (NaCI-type) and that TIN andAIN distort each other. An increase of hardness HK with adecrease of the period of the superlattice was identified nthe same study. At the period of 2.5 nm, hardnessreached a maximum value of approximately 4000 HK,which is 1.6 times that of a TIN single layer film. The HKvalue for a period of 13 nm is only about 2700 MPa [15].

A A A Wurtziie AIN

30 40 50 602 Q [deslIII

I i iNI AIN (NaCI type)

I I AIN (Wurtzite type)I 1 I

I I I I I

Source: SumitornoFigure 6: X-ray diffraction patterns of TiNlAIN superlat-tice. Periods: (A) h = 2.5 nm, (B) h = 30 nm.Besides the properties of the coating itself, interactionsbetween coating and substrate, especially the bonding,are important for the tribological behaviour of a coatedtool.Different methods for pre-treatment of the substrate canbe applied to improve coating-substrate adhesion. The

Figure 7: Influence of different substrate pre-treatmentson coatinglsubstrate adherence.Diffusion of Co and W from the substrate into the coatingis decisive for bonding between the substrate and CVDcoatings on cemented carbides. Cutting tests at a highfeed rate showed that a good adhesion is obtained whena suitable amount of these elements diffuses into thecoating. EDX analyses show (Eiaure 83 greater diffusionof these elements if a layer such as Tic or Ti(C,N) withgranular crystals is present in the coatingkubstrate in-terface [16].

Columnar TiCN on1: substrate

0.0 1.0 2.0 3.0Distance fromsubstrate surface l pmSource: MitsubishiFigure8: Effect on an interfacial layer on (W+Co) diffu-sion into the CVD-TiCN coating.As with all other technical systems, residual stresses inthe surface and subsurface zones of coated tools deter-mine their resistance to mechanical stresses, especiallydiscontinuous loads. CVD coatings exhibit tensilestresses, PVD coatings, compressive stresses (The stress characteristics of the PVD coating, in combi-nation with the usually small layer thickness (2-5 pm),provides good cutting edge strength, fracture toughnessand bending strength. Lower stresses are normally in-duced in CVD-AI,O, coatings as compared to CVD-Ti(C,N) and -TiN layers [ lo].

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CVD Coating (5-10) pm PVD Coating (2-6) pm

Source: lscarFigure9: General stress distribution.In addition to the influence of the coating process onresidual stresses in the coating and substrate material,the grinding process affects stress values in the substrate( U r e 1Q). Polishing removes the deformed andsmeared subsurface of ground WC-based carbides andat the same time almost completely relieves the residualstresses.

tJ J 0-200-600

2 -1000? MPa

2mmmUm-.-

insert SNGN 120412 rangeSubstrate: Micro blasting: Water peening: X-ray diffractionHW-P30/40 AI,O,, p=6 bar p,=lOOO bar measurement'

s=30mm t=20 s radiation (CuK,)s=45 mmSource: IFWFigure 10: Substrate pretreatment- residual stresses.Different stresses can be induced in carbides by rnicro-blasting the substrate surface before coating. Coarse

blasting material causes strong local plastic deformationsand leads to higher, more homogeneous compressivestresses as compared to the ground state. Fine grainedblasting material reduces the plastic deformation of sub-surface layers. The abrasive effect of microblasting withfine grained materials is also greater, leading to a stresslevel which is similar to that in the ground state, but muchmore constant stress [I1.Water peening removes the Co-binder of WC-carbides.The compressive stresses at the surface of the substratealso increase. The process characteristics of waterpeening always induce homogenous stress distributionsat the surface.2.3 Methods fortesting and evaluating coatingsIt is not intended for this paper to focus on the individualmethods in detail, because they are mostly standardevaluation methods and widely known. A reference toeach of the more unconventional methods is thereforeincluded. Some methods are illustrated n m r e 1.It is more important to cluster the methods in three sub-groups:a) The measurement of coating properties such as+ chemical composition by EDX, ESMA, Auger, Simsetc.+ residual stresses by X-ray diffraction [I81 or by me-chanical methods [ I91+ topography by mechanical or optical methods as wellas Atomic Force Microscopy+ morphology and growth by SEM analyses of fractures(s. Figure 3)+ plastic hardness by microhardnessHV or+ nano-hardness and elasticity by universal hardnessHU and nano-indentation (Figure 11 ) [DIN 50359,201+ thermal conductivity by the thermal wave method [21]b) The measurement of tribological properties such as+ Coating-substrate adhesion by scratch test or Rock-well indent (s.Figure7, Figure 11 )+ resistance to abrasion by the spherical calotte test[22] or the pin on disk method [23]

S o m : LMMFigure 11: Determinationof mechanical coating properties1251.

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resistance to tribooxidation by abrasion testing onthermally pre-loaded coatingsadhesion resistance by measurement of materialtransfer between sliding partnersresistance to diffusion by ESMA analysis of clampedand heated tool material - work material specimensfriction between sliding partners on a tribometer withrealistic pressingsresistance to fatigue by means of an impact test [24]Evaluation by modellingSimulation of impact test, generation of Smith-Mlbhlerdiagrams for determination of fatigue resistance ofcoatings [24]

PI

Suitable combinations of these individual methods haveto be found in order to determine the wear resistance of aspecific coating to the primary wear mechanisms that areactive in the real tribosystem, i.e. the cutting process. Itis, therefore, often necessary to identify modifications ofcoating properties and wear resistance during or after athermal, chemical or mechanical stress. Unfortunately.there is still no complete description of the ways in whichcoating structures affect coating properties and in whichspecific coating properties are related to resistanceagainst specific wear mechanisms.

Source: LMMFigure 12: FEM evaluation of the impact test. determina-A rather new aspect of coating characterisation is theapplication of computational methods. FEM analysis ofthermal or mechanical stresses often provides qualitative

tion of SmithMlOhler diagrams.

statements concerning the influence of coatings on con-tact conditions (s.Figure 5), or even quantitative results.The latter are possible if mechanical test methods can besupplemented by computational evaluation strategies.The impact test is used to determine the fatigue behav-iour of coatings (Fiaure 17, top left). In the impact test, aplane coating-substrate compound is exposed to contactpressure by impacting its surface with a cemented car-bide ball. Graphs plotting the contact stress which leadsto coating fatigue fracture versus the correspondingnumber of impacts can be obtained in this way (Figure12, top right). FEM simulation of the impact test trans-forms critical impact loads into critical stress values asso-ciated with specific and distinct failure modes. Coatingfatigue behaviour can thus be expressed in the form of aSmith diagram of the critical stress components for cohe-sive failure mode, i.e. the von Mises stresses that ensuretheir persistence (Figure 12,bottom) [24].3 Solutions and performanceof coated toolsThe combination of basic coating features decisivelyaffects the suitability of a tool for a certain technologicalapplication. In consequence, a coating has to be selectedor designed with the aim of adapting its performance todemands arising from the chosen technology, work mate-rial or operation. The dominant wear mechanisms occur-ring in the cutting operation have to be determined and allcoating parameters, like material, structure, coating proc-ess and substrate pretreatment, have to be adapted to itsystematically.3.1 Technology drivenThe following sections of the paper will present a numberof results showing how modern tool coatings perform incurrent machining processes. In this context, technologi-cal requirements have to be regarded as the driving forcebehind coating developments.3.1.1 High speed cuttingThe application of coated cemented carbides for highspeed cutting operations on steel materials is still verymuch restricted by the fact that tool temperature in-creases as the cutting speed rises. Here, we often findpolycrystalline boron nitride BN or ceramic as the toolmaterial. However, some high speed operations likemilling of steel materials or machining of light-weightalloys with coated carbide grades are common applica-tions for coated carbides tools.An example for the performance of coated tools at highcutting speeds is shown in Fiaure 13. (Al, Ti)N reducesthe rise in flank wear when milling hardened steel at acutting speed of 600 m/min, as compared to uncoatedand TiNcoated tools. Three properties of the coatinghave been related to this improved wear resistance:higher hardness (2720HVvs. 1930 HV for TIN), improvedoxidation temperature (840' C vs. 620' C for TiN) andbetter bonding of the coating to the substrate [26].The use of Al-containing coating materials is reported tobe advantageous, especially for machining operationswhich combine high cutting temperatures with high me-chanical stresses on the tool material. This is usuallyexplained by two effects. The formation of a thin A1,03layer on tool faces which come into contact with O2 ro-tects the coating from tribo-oxidation. This is important forinterrupted cutting operations as well as for reducingnotch wear at the minor cutting edge of coated carbidetools. The second reason for the good wear resistance of(Ti,AI)N coatings is their comparatively high hardness at

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elevated temperatures (Fiaure 14). This provides goodresistance to abrasive wear in high speed cutting [27].

0.07E 0.06E- .05@ 0.043 0.03

0.01L

h

5 0.02

Cutting speed (dmin)Outside diameterof endmills 1OmmWork material (hardness) X40CrMoV5-1 (52HRC)Cutting speed 100-600tn/rninFeed rate O.lOmm/toothDepthof cut 4 = 1Omm

width of cut a = 0.5mmRemarks Side milling, air blow

Cutting direction Downcut millingCutting ength 50msovce:Kobela,Figure 13: Influence of tool coatings on wear in highspeed milling of steel.

of 8 urn P V n - m3000

> 2500Iu) 2000u)t 1500Ea 1000tI500

00 200 400 600 800 'C 1200

TemperatureSource:QuintoFigure 14:Hot hardness of some PVD-coatings[27].3.1.2 Environmentally compatible technologiesIt is desirable to eliminate the use of cooling lubricantscompletely, or at least partially, in order to reduce theenvironmental impact of cutting fluids. The use of envi-ronmentally friendly fluids, i.e. biodegradable coolinglubricants, is aimed at in cases where a dry operationcannot be conducted. Either option will, however, lead toa loss of the tribological functions of the cooling lubricantas compared to conventional fluids. The absence ofcooling, lubricating or chip transport functions inducesgreater stresses on the tools. Coating technology fre-quently offers a means of compensating for this deterio-ration in contact parameters.A very detailed overview of the current state of the art indry cutting technology was given in a ClRP keynote pa-per in 1997 [28]. Treatment here will therefore be re-stricted to describing the influence of tool coatings in avery critical though representative drilling operation onAISi9Cu3 (aaure 15).

source:WZLFigure 15: Positive influence of low friction layers on drydrilling of AISi9Cu with carbide drills.These tests showed that a complete dry drilling operationwith uncoated tools is not possible, due to the tendencyof chips to stick to the chip flutes. This results in the endof tool life after only 6 holes. But it was possible to detectsignificant differences between the performance of asingle hard coating (PVD Ti(C,N)) and two different com-binations of a hard coating with a solid lubricant layer. Inall tests, the stop criterion or tool life criterion was eitherclogging of the chip spaces or tool breakage. While theTi(C, N) coating achieved no significant improvements ascompared to the uncoated tool, the TiAICN+MoS, combi-nation made it possible to machine as many as 87 holes.Improvements were even more noteworthy with theTiAICN+Me-C:H coating (104 to 130 drilled holes). Al-though aluminium material had stuck to the margins andin the chip spaces at the end of the test cycles for all thecoatings, the tool life test demonstrates that coatings witha low friction coefficient have an anti-adhesive effect.Nevertheless, minimal quantity lubrication (MQL) is rec-ommended under production conditions. Using MQLreduces adhesion and significantly improves chip trans-port. This is to some extent true even for the convention-ally Ti(C,N) coated tool, which, in short tests, producedresults comparable to those for both the multilayers interms of tool wear and surface roughness [29].Another means of reducing the environmental impact ofcooling-lubricants (CL) is the application of biodegrad-able, non-water miscible fluids. Synthetic esters are oneof the first commercially available examples. Applyingthese esters instead of emulsion in drilling operations onaustenitic steel with conventionally coated tools shortenstool lives. This may be related to two main effects: 1. thetool temperature rises, due to a reduced cooling functionof the fluid and 2. the CL-flow through the tool falls, dueto the higher viscosity of the fluid. Such altered contactconditions increase adhesive material transfer in the chipflutes and hence the resistance to chip flow. This resultsin chipping of the cutting edge comers, eventually fol-lowed by breakage of the drilling tools.Only the application of a hard and soft compound coatingmaterial consisting of (Ti,AI)N and WC/C could compen-sate for the increased wear attack (Fiaure 18). SEM-analyses showed that WC/C flattens the tool surface.WC/C remains only in the roughness valleys, e.g. thegrooves left by the grinding process. This minimisesadhesive material transfer as well as the resistance to

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chip flow. The result is a significant reduction of cuttingedge corner chipping or tool breakages.

Number of holes nTool material: HC-K20F Material: X5CrNil8-10

vc= 42 dmin.M.08mm, d=6 mm, I=18 mmPocket hole, inner CL-supply

swce:WZLFigure 16:Wear in drilling of austenitic steel with syn-3.2 Work material drivenThe work material becomes the driving factor for coatingselection and coating development, if the machinedpart-the product-needs to possess special propertieswhich mean that the workpiece becomes more difficult tomachine.

thetic ester - influence of different coatings.

3.2.1 Light weight alloysIn recent years, developments in the automotive industryhave brought a rapid increase in the use of light compos-ite materials based on At. Examples of these new materi-als are hypereutectic AI-Si alloys with a Si content be-tween 17 and 25 wt.-% for tribological applications. Thematerials are produced by spray deposition and hotforming. As with metal matrix composites (MMC), themain problem in machining hypereutectic AI-Si alloys isabrasive tool wear caused by hard Si crystals. A study ofthe wear resistance of differently coated carbide toolsand a DP tool ( u r e 12) revealed that, because of itshigh hardness up to 5000 HV, the DP tool suffers thelowest amount of flank wear. Compared to uncoatedcemented carbide, the CVD-diamond coating and the(Ti,AI)N coated tools achieve lower wear rates. In con-trast to the other tool materials, the diamond coating issubject to progressively increasing tool wear. This iscaused by chipping at the rake and clearance face. Theunprotected substrate consequently comes into contactwith the Si phases of the workpiece material. By com-parison with CVD-diamond, the hardness of the (Ti,AI)Ncoating amounts only to 2450 HV. Its resistance to initialabrasive wear attack by abrasive Si crystals is lower andhigher tool wear is therefore observable. Although thehardnesses of TiN and (Ti,AI)N are quite similar, the wearrate at the flank of the TIN coated tool is much higher.This can be explained by a high chemical affinity of TIN toaluminium. The coating is dissolved by the mechanical,thermal and tribochemical stresses in the contact zones[4] and cannot protect the tool against wear [30].As in the case of the hypereutectic AI-Si alloys, the mainwear mechanism in machining of GAI,O,+SiC particlereinforced magnesium alloys is abrasion. m u r e 18shows that the abrasive wear attack in drilling of such aworkpiece material is so intense that there is no im-provement of the wear resistanceof the uncoated carbidetool by application of a (Ti,AI)N coating. Only the high

hardness of the CVD-diamond coating produces ade-quate enhancement of the wear resistance. The highdegree of abrasive wear may be related to the hardnessof the Sic phases in the workpiece material. When thesame Mg-matrix reinforced with softer &AI,O, shortfibers was drilled, the (Ti,AI)N coating provided wearprotection comparable to that of the CVD-diamond coat-ing [31].0.07

5 mm3% 0.03G 0.01m

0 200 400 s 800Cutting-time6

Cutting speed : v = 400 m h i n Tool geometry:Feed rate : = 0.1 mmDepthofcut :a p = 1 mmCutting time : t = 750 ssource:SFFigure 17: Tool wear in turning of spray depositedAISi25X.

0.6 I I I I I Imm>2p 0.2xC((I

LL0 200 mm 500

Drilling length 4Reinforcement :5 vol.%Tool diameter :d = 6 mmDrill hole length : = 20 mmCutting speed :vFeed rate : = 0.25 mmlrev

&4l2O3+i5 vol.% Sic= 100 mlmin

Source:SFFigure 18:Tool wear in drilling of reinforced Mg.Comprehensive tests have shown that the suitability ofthe cutting tool material depends mainly on the hardnessof the reinforcements. But the relation between the grainsize of the cutting materials and the size of the rein-forcements is also important, because these factors influ-ence the dominant wear mechanism [32, 33, 34, 51, i.e.the dominant subtypes of abrasive wear: microplough-ing. microcutting or microcracking/fatigue1361.Microploughing occurs when hard abrasives interact onthe surface of a material causing high levels of plasticdeformation. In general this mechanism is not dominantconcerning the tool wear when machining MMC, becausethe cutting tool materials have a high hardness andtherefore a high resistance against plastic deformation.During microcutting the reinforcement removes material

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from the cutting edge in form of microchips. This mecha-nism is dominant if the reinforcement is harder than thecutting tool material and the dimension of the reinforce-ment is higher than the grain size of the hard phases ofthe cutting tool material.Microcracking and fatigue occur when cracks are inducedinto the cutting tool material by high dynamic loadings.These mechanisms are dominant, if the hardness andlorgrain size of the cutting tool material is higher as com-pared to the reinforcement particles.3.2.2 Difficult to machine materialsTitanium alloys play an important part amongst the mate-rials for components subjected to high thermal and me-chanical stresses. The good suitability of such materialsfor certain products is, however, associated with a severedecline in machinability. A characteristic feature of tita-nium alloys is their tendency to form built up edges (BUE)as well as to stick on the tool flank, especially if HSStools are used. This leads to higher friction on the toolflank and also higher temperatures in the contact zone.Cohesive failure of the tool substrate is related to thewelding of chips on the tool surface. The use of coatingslike TiN or Ti(C,N) even intensifies this effect.Baure 19 shows results obtained for TiA16V4 milling,using different coatings at a cutting speed of 70 m/min.The studies showed that the coated tools have a greatersusceptibility to BUE formation, owing to the chemicalaffinity between the coating and workpiece materials. Anadditional anti-stick coating based on MoS, can diminishthe interactions between coating and workpiece materialas compared to the other coatings, but achieves no im-provement as compared to the uncoated substrate. Fur-ther investigations have shown, that negative experiencewith coated HSS tools coincides with results obtainedusing coated carbide tools [37, 381.

mo1 r - / A l l

HSS HSS HSS HSS

TiA16V4Ippl;D,=~OIIUTIz = 4v, = 70 rnlrninf, = 0,08mrng = 5 m m% = 2 m mdry machiningdow ncut millingfailure of tool

uncoated TIN-- TiCN- TiCN+coated coated MoSicoatedsouw: IWTFigure 19: Coatings increase tool wear in milling ofLonger tool lives or higher cutting speeds are possible inexternal cylindrical turning operations on a high-strengthaluminium bronze if CVD-diamond coatings on cementedcarbides (K10) or nonsxide ceramics are employed( m r e 24). The highest cutting speeds are obtained inturning operations using coated ceramics. This is duepartly to geometrical modification in form of a chamferedcutting edge and partly to closer harmonisation of therespective coefficients of thermal expansion a of thediamond layer and the ceramic substrate. The a-ratio is1/4 for the diamond layedceramic combination, but 1/7for the diamond layerkemented carbide composite [39].This may contribute to better coating-substrate adhesion

TiA16V4.

during the cutting process.

10min

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the substrate. This is also a typical application for PVDcoatings. Here, currently thin film technology is openingup new fields of application for CVD coatings as alreadymentioned. However, there still seem to be certain re-strictions concerning the application of CVD coatinas oncarbide drills and endmills [27]. -

0 Milling of hypereutecticAlsovc%:wzLFigure21 :Evaluation-scheme for demands on coatingproperties (operation and material driven).

4 Trends and Future DevelopmentsOwing to the great suitability of hard thin films for pro-longing tool lives [41, 2 et al.], between40 % and 80 %of all machining operations are now conducted withcoated tools, depending on the process concerned, whilevirtually all .first-choice" grades for turning, drilling andmilling processes are coated [43, 41. The coating ofcomplex special tools is also becoming more economi-cally viable. A dominant role is played by CVD as op-posed to PVD coatings, usually with multilayer structures.A recently published market survey revealed that largercompanies in particular are turning to coated tools (65%)while smaller companies only have 35% of their toolscoated. The use of CVD is growing, especially among thelarger companies, with a market share of some 65 %.With respect to tool materials it was found that conven-tional TIN and Tic coatings still represent about two-thirds of the total coatings deposited [40].Among thecoating systems, titanium-based ternary or quaternaryhard thin films and coatings with oxidic interlayers aregaining importance. The technological improvementsmentioned above widen the field of possible applicationsfor both CVD and PVD coatings, promising substantialoverlaps between their basic application profiles in thefuture. However, the inputs to this paper reveal a strongscientific focus on characterisation and development ofPVD coatings.Recapitulating the contents of the previous sections, it isevident that the main factors for future coating develop-ments are material driven, so far as the machining ofnon-ferrous materials is concerned. Due to their high

resistance to abrasive and adhesive wear, there seem tobe good prospects for CVD-diamond coatings in this field.In the case of steel materials, the main factors are tech-nological, i.e. the demands for ecological machining andhigh cutting speeds will encourage the development oftool coatings. These demands often entail substitutionand compensation for the lost functions of the coolinglubricant. Research and development work on the actualsurface of the tools and coatings will therefore be neces-sary. Some concepts have already been launched on themarket, including hard and soft compound layers likeMoS, and WC/C. or solid lubricant coatings such as Me-C:H.Since CVD-diamond coatings are very successful inmachining of non-ferrous materials, it could be very at-tractive to have a suitable superabrasive coating for themachining of steel, too. CBN in therory could be such acoating material. However, unlike diamond, which is asingle element, cBN is a compound, which makes thegrowth process more complex for a number of reasonsincluding the problems of stoichometry and purity of thecubic phase. Despite a significant level of research effortover many years, virtually all cBN layers are actuallymulti-phase microcrystalline material. Even in this form itmay be possible to develop coated tool application, al-though to date this has not been clearly demonstrated[45,46].5 References

van Luttervelt, C.A., Childs, T.H.C., Jawahir, I.S.,Klocke, F., and Venuvinod, P.K., 1998, "PresentSituation and Future Trends in Modelling of Ma-chining Operations," Keynote Paper, Annals of theJawahir I.S., Balaji A. K., 1999,The effects of ToolCoatings on Machining Performance, not publishedYetLoffler F., Barimani A., 1990,PVD-Schichten fur dentribologischen Einsatz, Ingenieur-Werkstoffe 12. 2,Taminiau D.A., Dautzenberg J.H., 1999, How toUnderstand Friction and Wear with Classical Phys-ics, unpublished manuscriptKubaschewski O., Alcock C.B., 1979,Metallurgicalthermochemistry, Pergamon PressBarin I., Knacke0.. 1973,Thermochemical proper-ties of inorganic substances, Springer VerlagDerfinger V. et al., 1999, Balinit SchichtendieLasung fur moderne Zerspanungsaufgaben, 3.VSM-Seminar "neue Entwicklungen in derspanenden Bearbeitung, January 28, Regensdorf-CHLugscheider E., Bobzin K., 1999.unpublished prog-ress report, collaborate research center SFB 442,RWTH-AachenQuinto D. T., Santhanam A. T. Jindal P. C.. 1989,Int. J. Refrac. Hard Materials 8,2,95-I01Wertheim R., 1998,Development and Applicationsof Coated Cutting Tool Carbides, Rewrite of an OralPresentation held at CIRP General Assembly inAthensHaefer R.A.. 1987,Oberflachen- und Dunnschicht-technologie, Teil I, Beschichtungen von Ober-When, Springer-Verlag, Berlin

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[12] Tdnshoff H.-K. et al., 1997,Wear Mechanisms of(Ti,.x, AIJN Coatings in Dry Drilling , Surface andCoatings Technology. 94-95, pp. 603-609[I31 R. Messier, A. P. Giri, R. A. Roy, Vac. Sci. Techn.[I41 H. Satoh, Vac. Sci. Techn. A, July/Aug. 1992[15] Seytoyama M., et al., 1996, Formation of Cubic-AINin TiN/AIN Superlattice, Surface and CoatingsTechnology, 86-87, pp. 225-230[16] Akiyama K. et al., 1997, A Study of the AdhesionBetween CVD Layers and a Cemented CarbideSubstrate by AEM Analysis, Surface and CoatingsTechnology, 94-95, pp. 328-332s[17] Tdnshoff H.-K.. Mohlfeld A.. 1998, Increasing Inter-face Strength by Mechanical Substrate Treatment ,Production Engineering,V/1, pp. 603-609[I81 Chollet L; Laufenburger A; Biselli C, 1988, Relationbetween residual stresses and adhesion of hardcoatings, Second Int. Conf. on Residual StressesICRS2, Societe Fraincaise de Metallurgie, Nancy, F,[19] Vijgen R.O.E., Dautzenberg J.H., 1995, Mechanicalmeasurement of the residual stress in thin PVDfilms, Thin Solid Films 270, pp. 264-269[20] Neumaier P., 1989, Hartebestimmung anBeschichtungen von Metalloberflachen, Metallober-flache, 2[21] Lugscheider E.. Geiler H.D., Lake M., ZimmermannH., 1996. Investigation of thermophysical propertiesof AIP coated cutting tools for dry machining, Sur-face and Coatings Techn. 86-87. pp. 803-808[22] Klocke F. et al, 1998, Improved Cutting Processeswith Adapted Coating Systems, Annals of the CIRP,[23] Czichos H.. Habig K.-H., 1992, Tribologie - Hand-buch: Reibung und VerschleiR; Systemanalyse,Pruftechnik Werkstoffe und Konstruktionselemente.Vieweg Wiesbaden[24] Bouzakis K.-D., et al., 1998, Experimental and FEMAnalysis of the Fatigue Behaviour of PVD-Coatingson HSS Substrate in Milling, Annals of the CIRP.[25] Bouzakis K.-D., Vidakis, N., 1999, Advanced Physi-cally Vapour Deposited Coatings - State of the Art,Innovations and Future Trends, Tribology in Indus-try, too be published[26] Yamada Y., 1996, High Speed Cutting Performanceof (Al, Ti)N Coated Endmills, Proc. 3rd Int. Conf.On Progress of Cutting and Grinding Nov. 19-22.[27] Quinto, J. et al., 1999, PVD coatings for turning,Cutting tool engineereing, 02, pp. 42-52[28] Klocke F.. EisenbUtter G., 1997, Dry cutting, Annalsof the CIRP, 4612[29] Fleischer W. et al.. 1999, PVD coatings for DryCutting of AI-alloy and Bronze, Proc. of SVC Conf.Boston, pp. 80-88[30] Weinert K. et al.. 1998, Machining of Spray Depos-

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aluminium alloy, ASME Int. Conf. On Wear of Mate-rials, Vancouver Canada 14.4.-18.4., pp. 539-544[34] Tomac N., Tanessen K., 1992, Machinability ofParticulate Aluminium Matrix composites, Annals ofthe CIRP, 4111, pp. 55-58[35] Weinert K., 1993. A Consideration of Tool wearMechanism when Machining Metal Matrix Compos-ites MMC, Annals of the CIRP, 4211, pp. 95-98[36] Zum Gahr K.-H., 1987, Microstructure and wear ofmaterials, Tribology series 10. Elsevier, Amsterdam[37] Brinksmeier E., Berger U., Janssen R., 1997, HighSpeed Machining of TiA16V4 for Aircraft Applica-tions, Proc. 1st French and German Conf. On High

Speed Machining, June 17-18, Metz, pp. 295-306[38] Brinksmeier E.. Berger U., Janssen R., 1998, Ad-vanced Sensoric and Machining System for Manu-facturing and Repair of Jet Engine Components,Proc. Of 31st CIRP Int. Seminar on Manuf. Syst.,26.-28.5, Berkley[39] Uhlmann E., Brucher M., Lachmund U., 1998. Ein-satzverhalten diamantbeschichteter Werkzeuge beider Trockenbearbeitung hochfester und abrasiverNE-Legierungen, Werkstofhvoche. Symposium 5,Fertigungstechnik, Milnchen[40] Weiner M.. 1999, Coatings move forward, cuttingtool engineering, February, pp. 22-29[41] Pulker, H. K. et al.. 1989, Wear and Corrosion re-sistant Coatings by CVD and PVD, Ehningen beiBoblingen: expert-Verlag[42] Venkatesh, V. C., Ye, T. C., Quinto, D. T., Hoy, D.E. P., 1991, Performance Studies of Uncoated,CVD-Coated and PVD-Coated Carbides in Turningand Milling, Annals of the CIRP, 40/1, 545-550[43] N.N., 1991, Die Schneide verdient das Geld.Schneidstoffe - lndikatoren vielschichtiger interna-tionaler Entwicklungen, Fertigung, Landsberg, 2Sonderheft Werkzeuge, pp. 12-15[44] N.N.. 1994, AB Sandvik Coromant: Modem MetalCutting[45] Clark I.E., Sen P.K., 1998, Advances in the Devel-opment of Ultrahard Cutting Tool Materials, Ind.Diamond Rev., 2, pp. 40-44[46] Stock H.R., 1997, Stand der cBN-ProzeRentwicklung (BAD), Proc. of MaTech-Statusseminar: Superharte Schichten far tribolo-gische Anwendungen, June 4-5, Braunschweig

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1999_Klocke-F._Krieg-T._CIRP-Ann-Manuf-Technol