- Email: [email protected]
Characteristics features of wear in tools made of high-speed steels with surface engineered coatings
WEAR ELSEVIER
Weartgs (1996) 28o-286
Characteristic features of wear in tools made of high-speed steels with surface engineered coatings II: Study of surface engineered high-speed steel cutting tools by AES, SIMS and EELFAS methods G.S. Fox-Rabinovich *, A.I. Kovalev, S.N. Manasyev RIMET Corap., P.O. Box 167, Moscow 125239, Russia
ReceivedS August1995;accepted3 April1996
Abstract Changes in chemicalcompositionand fine (at atomiclevel) structurecharacteristicsof surfacesduringthe life time of cuttingtools were investigatedby using AES, SIMS and EELFAS methods.High-speedsteels M2 and TI5 with coatingsincorporatingion nitriding and (Ti, Cr) nitrideCAPDP (cathode arc plasmadepositionprocess) wereused. It was found that, under the wear conditionsstudied (such as cuttingof mediumcarbon steels), some componentsof the protectivehard coating dissociated,formingquasi-oxideamorphousfilms.This showsthe paralleldevelopingprocessesof: 1. Dissociationof chemicalcompoundin coatingmaterial (Ti,Cr)N and intensivediffusion(absorption) of nitrogeninto chip. 2. Creat!onon tool surfaceof stableamorphousprotectingquasi-oxideTi-Ofilms.The diseoverexlchangesin chemicaland phasecomposition of contact surfacesof the "tool-workpiece" tribopair improveservicecharacteristicsof the cutting tool and promote its adaptationto service conditions (named "self-organization"). The adaptationobservedis expressedmore on steel TI5 with high heat resistanceand leads to a significantincreaseof tool life. Keywords: Cuttingtools;Steel;Metallurgicalcoatings
1, Introduction Cutting tools fall into the range of highly loaded tribosysterns. That is why the damage to contact surfaces and, consequently, very low wear resistance during operation are inevitable [1 ]. Nevertheless, thanks to recent ad',,ances in tribology [2,3], one can discern some scientifically substantiated trends that enable one to move closer to adaptable fundamentally new self-organized tool tribosystems. There are a wide variety of methods for achieving such tribosystems [3-6]. So modification of surface properties during the service period secured by specially organized composition and structure of surface layers is a way to prolong the workability of the tool. To solve this problem one needs to study complex physical and chemical phenomena on the toot surface. These phenomena improve service characteristics of extremely stressed microvolumes of the tool during operation and promote its adaptation to service conditions (named "selforganization"). The self-organization is a spontaneous * Correspondingauthor. 0043.1648/961515.00© 1996ElsevierScienceS.A.Allrightsreserved PIi S0043-1648 (96) 07204-3
formationof stable and ordered phases that protect the surface against damage from friction [ 3]. The present paper considers both technological and material science aspects of the problem solution that are concerned with tool surface modification. The approaches under consideration are based on the self-organization of surfaces under friction [3]. The phenomenon of self-organization constitutes a physical basis for possible friction control [2-4]. The latter provides both for improvementofsuffacetribotechnical properties and for significant changes in mechanisms of contact interaction between the tool and the workpiece. Solution of this task would make it possible to pass into a completely differentlevel of wear resistance and machining productivity. Taking all this into account, one may see the purpose of this paper as investigation of self-organization in the cutting tools modified by coatings'technology. The change in chemical and phase composition on the contact surfaces of the "tool-workpieee" tribopair was studied. Tools based on high-speed steels with surface engineered coatings (ion nitriding and TiN nitride coating applied by the CAPDP-
(7.$. Fox.Rabinovichet al. / Wear 198 (1996) 280-286
cathode arc plasmadeposition process-method) were usedat increased cutting speeds in the vicinity of 100 m rain- !. As the researchc;arriedout shows, the use of such coatings prorides for stable cutting with a wide range of tool service [6-9]. This effect is ensured by localizingexternal thermo. mechanical loads in surface layers of a high-melting compound [8]. All these lay the grounds for the assumptionthat the phenomenonof tool surface self-organizationis observed in the process of friction. The experimental data obtained confirmthis assumption. In the present paper, all the physicaland chemicalphenomena on the interfacebetweenbuild-upor chip and hard coating in a wear hollow were investigated,For this purpose surface sensitive analytical techniques were used. At present, a number of experimental techniquesare competitive for the study of .~urfaces.There is an impressivearray of methods available for the analyses of the structure and composition of the outermost layers of a surface. Auger electron spectroscopy (AF_,S),secondary ion mass spectroscopy (SIMS), electron energy loss fine Auger structure (EELFAS) methods have a high in-depth resolution at the surface, nearly several interatomicdistances,high lateral resolution (AES 30nm, SIMS 0.2 p.m, EELFAS atomic diameter) [ 101. The information on these analytical techniques is very good. It, SIMS, cluster ions and molecule fragments can be used in trace analysis of the surface chemical environment. To obtain meaningful information on the origin of interatomic interaction and its bond length a technique is needed with a precision of 0.01-0.02 ~. F.ELFAS provides this kind of accuracyand can be used for discovering microcrystalline,amorphous,orderedand disorderedsystems [11,12]. These techniques have irrefutablepreferencesunder TEM and XRD when thin layers on interfaces are investigated.In the present work AES, SIMS and EELFAS were used for investigation of chemical and phase composition thin layers on interfacesbetween hard coating in the wear hollow of the tool and build-up or chip.
2, Experimental procedure Chemical and phase composition of "tool-workpiece" contact surfaces were studied by means of Auger-spectros. copy (AES) and secondary ion mass-spectroscopy(SIMS). For this we employed a ESCALB MK2 (VG) spectrometer equipped with LEG 200 electron gun, AG 6 ion gun and SQ 300 quadrupole spectrometer. The scanning Auger-spectroscopywas used to analyze the composition of the surface of the hollow on the face cutting tool edge at different stages of its development.Besides, this type of spectroscopy was employed for the analysis of chip contact surface. In each of these cases several areas 15 X 15 p,m in size were chosen on the surface underanalysis. Television scan rates were used for the beam of primary electrons 2000 it in diameter. Auger-spectrawere recorded
281
at energy E=2500eV in the mode CRR (constants retard ratio) modulationamplitudeof 2 eV and at speed 2,1 ©Vs- i. After cleaningby standardtechniques (argon bombardment) the samples were investigatedon vacuum 10-s Pa. Phase composition of the sur2"aceof the wear hollow was analyzed with the aid of scanning mass-spectroscopyof secondary ions. For this purpose an argon ions beam 0.5 IJ.min diameter was scanned at television scan rates at a 5.0 keV accelerationvoltage and 2X 10-s Pa press~::e,wi:]l the area 15 × 15 Itm in size being analyzed. In the selected mode, ion etching speed was sufficiently low and did not exceed 0.5 monolayer per minute, The analysis was made in the mode approximating static conditions. Investigationof amorphyzationand the nearest atomicsurrounding on the surface of wear hollow of hard coating cutting tool was carded out by means of the i~ELFS method. The investigationwas cardedout on an ESCALBMK2 (VG) electron spectrometer equipped with the hemispherical capacitor type energy analyzer. The extended fine structure of electrons'spectrawas analyzedin the range of 250 eV close to the lines of elastic scattering and KLL.iines of nitrogen, carbon and oxygen. Electronspectra were recorded atthe rate of 0.4 eV s- ~. This was done in the integral form in the CADmode (constant analyzer energy) at 10-fold signal accumulation. All physical and technical conditions were chosen in such a way as to ensure the best energy resolutionat a good signalnoise ratio. The energy of primary electrons was 1500eV. The EELFAS method is based on the analysis of extended fine structure spectra of electrons with energy losses at their reflection. Recently this method has been used for precision investigations of atomic structure in thin surface layers. By analogy with the EXAFS (extended X-ray-absorption fine structure) spectroscopy [ 1! ], electron spectra contain information about the structure of the nearest atomic surrounding (the beam interaction area) on the surface. The necessary experimental equipment for such investigations is considerably simpler than that in the EXAFS method: any Augerspectrometercan be used, The main difficultyencounteredin obtaining the electron spectra fine structure is obtaining a high energy resolution, precision accumulation of a weak signal and availabilityof software for mathematicalprocessing of spectra and noise filtering. The theory that describes an EXAFS-Iike structure on the electron spectrum is still underdevelopmen:[ 11]. The methodsof mathematicalspecIra processing adopted in the EXAFS-spectroscopy [ 12], however,can be used to analyze the fine structureon electron spectra. These enable one to determine lengths of atomic bonds in the nearest atomic surrounding, with these results being in good agreement with crystallographicdam [ 13]. It is possible to give a physical picture of the phenomena observed. In electron spectra close to the lines determinedby elastic scattering, and emission of photo- and Auger-clueIrons, there is always a background gradually decreasing in intensity on the low kinetic energy side. This background is determinedby the detectionoftbe flowof eleclronssubjected
282
G.S.Fox.Sabinovichet al. I Wear198(1996)280-286
to nonelastic scattering in substance. The greatest contribution to inelastic scatt.ering is made by the electron-electron interaction [ 13]. In contrast to the photoeffect initiated by X-ray or ultraviolet radiation with the energy of a quantum being completely absorbed, the electron excitation of atom levels causes the emission of electrons which feature a continuous spectrum of kinetic energy. On passing the thinnest surface layers, a flow of such electrons undergoes inelastic and elastic scattering as well. All these processes, taken together, considerably enhance the emission of electrons that have a continuous spectrum of kinetic energy when the sample is electron irradiated. In this case the object to be analyzed seems to possess a source of variable energy electrons that is due to the excitation of multiple emission centers. Since electrons possess a wave nature, in the process of their scattering in the nearest atomic surrounding and under certain conditions in the crystal lattice, there is a possibility of the phenomena of resonance and antiresonanoe occurring. These are connected with the enhancement or attenuation of electron wave function amplitude in a given point of space and, in particular, in vacuum and in a detector. These conditions are determined both by the electron wavelength (or energy) and by the distance from the emission center to the scattering center (or interatomic distance). As a result, for electrons with different kinetic energy the electron emission spectrum features a modulation in detected signal intensity. Emission of electrons and their nonelastic scattering on the nearest atomic surrounding is of a probabilistic nature. Due to this, the amplitude modulation of signal intensity becomes diffused over the energy scale and creates an extended fine structure of background close to the lines of elastic scattering, absorption edge and Auger-lines. The extent of this fine structure achieves 250-300 eV. Depending on the wave vector of electron K the oscillation on such a spectrum has the following equation [ 12]: - ~ KR~ x(K)=~'/
K(E-I) ~_[0.263(E- Eo) (eV) ] Ij2
(3)
A theoretical basis of such an analysis is analogous to that presented in [ 12-15]. AES, SIMS and EELFAS spectra were obtained on the surface of the cutting tool face edge hollow for different service times using square indexable inserts.
3. Results and discussion In this work we investigated the change in chemical and phase compositions of contact surfaces, The "tool-workpiece" tribosystem was studied on the surface of the hollow of cutting tool face edge, and on the chip contact surface. The study of indicated contact elements of the tribosystem under analysis permitted more complete and objective information about processes occurring at the contact surfaces to be obtained. Chemical composition analysis on the surface of the cutting tool face edge hollow shows that, from initial stages of cutting in the running-in zone, there is a reduction of nitrogen concentration and increasing of oxygen concentration. These effects are clearly presented by a series of Auger-spectra (Fig. 1) obtained from the surface of the wear hollow at different durations of cutting. After considerable cutting duration, a significant increase in Auger-lines intensity for iron and carbon is observed due to adhesion of machined steel
I o
I
I c
. . . . . . .{_q~,3t)j , ,-,,e-2~z.,~e-zR~/A~g) slntg/t/cj (1)
To determine atomic distances in the nearest coordination spheres (averaged and partial functions of radial distribution), the Fourier analysis of this amplitude modulation is applied. Calculation data are in good agreement with crystallographic data. A small difference is due only to the phase shift, To determine the coordination number by the peak intensity on Fourier transform we need the amplitude analysis whose principals are given in [12,13]. In the present work we used the Fourier transform of the type: Kmt~ F(R) = ( - 1) [ x(K)W(K)I~' exp( - 2/f~R) dk
(2)
L~,, 800 700 t~O 500 4~, ~U
I c E,eV
d
ICon
The K-wave vector is represented as:
Fig. 1. Mger-s~¢tnl from tuning tool surface: (a) TiN coating before service; (b) 30 s; (¢) 180 s; (d) 300 s; (e) 2100 s cutting ti~s,
G.5. Fox-Rabinovichet al. I Wear I98 (1996) 280--286
283
W.~O~.m= etrb. unl% L'ID
/
:: .
]
°'" ~ - ~ "
s
:(_.:.
/
,o k Mlln
&m.U.
70
~
~ II,m.U,
m
f,s
Ilamt ,l',m.U.
Fig.3. SIMSspec~aof TiNandquasi.oxidesTi-O filmsvs servicedine of
c,~.
tool madeofM2(P6M5) withcoatings:(a) cuttingequaito 15 s; (b) cut'ling equalto90 s;(~)cuttingequal~o 120 s,
40&S. :lO-
found on mass-spectra it is assumed that the discovered TiO clusters were emitted from non-stoichiometric quasi-oxide Ti-O films. Titanium oxide [22-24] has high resistance at
20.
16In.
cutting [91 and can effectivelyfulfilla protectivefunction t
i
t
i
60 100 1~0 ~
~0 :~0 'l~ 4~0;111~|1tB022011Ti4 l"
c, a~ •
o
1050
c SO 100 1~ 200 ~
300 3~.040Q 4,~0210021~OTtS
Fig. 2, Dependence of wear value and chemical composition of the "tootworkpiece" tdbopair elements on cutting time: (a) change of wear value
on the facecuttingtool edge (areaof wearhollow);(b) changein AES elementcompositionofthewearhollowsurface;(c) changein AESelement compositionof the chipcuttingzone;curves4, tool madeof M2(P6MS) steelwithcoating;curve5, toolmadeof TIS(PI2M3(IY2KS)powdersteel withcoating. on the surface of cutting tool, and at the final stage of cutting by the destruction of the coating. There is a good correlation between the chany~ein wear resistance of the cutting tool and the element composition of the coating (Fig. 2(a, b)). It is possible that, during the transition from the running-in stage to that of steady-state wear, the depiction of the coating surface of niUogen and its enrichment by oxygen is connected with the dissociation of titanium nitride, adsorption of oxygen from the environment, and development of titanium oxide. Th~se data are in accordance with results of other investigations [21 ]. These data are confirmed upon analyzing the coating phase composition. In fact, during the transition frora the runningin stage to that of normal wear (Fig. 2(a), :.,ryes 4), the development of increased Tie cluster quantity (Fig, 3(a)(c)), was discovered with the aid of secondary ion massspectroscopy (SIMS). Because tint Tie2 (mass 80) was not
[251. Using the insights of modem triboiogy [3], one can state that quasi-oxide Ti-O fixed on the surface constitutes thin films. These layers form at cutting and are known to be corn. pounds of the surface material with the oxygen introduced into the cutting zone from the environment. Oxide rich phases act as a shield that protects the surface [3,4]. That is why the emergence of increased Ti-.O quantity at transition from the running-in stage to that of normal wear (Fig. 3(b), (c)) testifies to the self-organization of "tool-workpiece" tribosystem. As a result, the wear intensity is reduced and this process enters its steady-state stage (Fig. 2(a), curve 4). Further on, the quantity of oxide component remains practically unchanged as the cutting time grows (Fig. 3(b), (c)). It is typical that the adhesion of medium carbon steel to the workpi~cc increases in the running-in period, then stabilizes and does not change within the stage of normal wear. This is evidenced by the constant content of iron traces on the surface of the hollow (Fig. 2(b)). Additional information about the processes in operation on contact surfaces of the coating is suggested by the results of chemical composition analysis on the chip contact zone surface. The latter arc obtained by means of ABS. These data correspond to the results of element composition analysis for the adjoining cutting tool surface, It was found that as the cutting time grows the concentration of nitrogen and oxygen in this surface zone changes according to a nonlinear function. At initial operation stages, for example at the running-in stage (up to 30 s of cutting, Fig. 2(a), curve 4), there is an intense oxidation of chip surface (Fig. 2(c) ), In this case the oxygen content is maximum, whereas the nitrogen concentration is minimal. Upon completion of the running-in stage (60 s of cutting) the picture is changed: the nitrogen concentrations on the surface increase while the oxygen concentration drops. This agrees with the onset of the normal wear stage (90120 s of cutting, Fig. 2(a), (c), curves 4). Further on, due to the stabilization of processes occurring during the steady-
284
G.S. Fox-Rabinorichet aLI Weur198(1996)280-286
state friction stage, the chemical composition of the chip surface remains constant up to the catastrophic wear. Decreases of nitrogen content in the coating and nitrogen absorption on the contact surface of chip are tied together and to individual stages of tool wear. The more stable the cutting process is, the more intensive is the indicated effect. Thus, the tool made from composite material M2(P6MS) with a surface engineered coating has a higher degree of wear at cutting and a lower friction process stability than one made from T15.PI2M3tlr2K5 (Fig. 2(a), curve 4,5). As is seen, the nitrogen concentration in chips at cutting with the T15-steel tool is higher than that at cutting with the M2-base tool (Fig. 2(c)). Amorphization and fine structural changes in the nearest atomic surrounding on the contact surface were investigated by means of EELFS method when analyzing the extended fine structure of spectra of electrons with energy losses. Fig. 4(a) presents an EELFS-picture obtained close to the line of elastically scattered electrons for a TiN-coated sample. The spectrum is given as a function of K-wave vector after background subtraction and smoothing.
t A/ ] I k ,
_ _ . . _ v _
v a~b. 3o unlls 2o -
~r
Co)
C"O~ti~
to-
o
i i
i i 1 I
i ~
, I
"'--
RIAI olb. 200.
(c)
Fig. 4. (a, b) structuralfunctionX(K) and FouriertransformF(R) from
I~LFS spectraTi,CrN-coatingon nitridcdTI5(PI2M3gr2K.5)stcei; (c) FouriertransformfromEELFAS-spcctrain regionof KLL-nilrogcnlia¢on the sane sample.
Fig. 4(b) depicts the Fourier transform which contains averaged information about the coating structure after the mathematical processing of the spectrum given in Fig. 4(a). This function represents several diffuse peaks in the range of interatomic distances of 3 A and 4-5 A. To identify this averaged picture, the partial functions were determined. To do this, we analyzed EELFS on spectra close to KLL-lines of oxygen, nitrogen and carbon, The Fourier transform characterizing the individual nearest surrounding of nitrogen atoms in TiN-coating is represented in Fig. 4(e). The data are obtained after processing the fine structure close to KLL-tine of nitrogen in the energy range 390-280 eV. The sole pronounced peak corresponds to the length of bonds of 1,95 A and specifies a radius of the first coordination sphere surrounding the nitrogen atoms. The measured value R~ = 1.95 .~ is close to the theoretical value of Ti-N-bond length received on the basis of Ti and N ion radii (RTi+RN=IA5+O.61=2.06A). These data have good accordance with crystallographic parameters for TiN and earlier measurements by the EELFS method [ 13]. Partial functions like these were obtained after analyzing the fine structure close to other characteristic Auger-lines of the spectrum. These allowed the nature of peaks on the integral picture, gained when investigating the spectrum close to the lines of hack scattering, to he identified more precisely. Values of the first coordination sphere radii in the surrounding O-, Ti-, C- and N-atoms representthe bonds between atoms of various elements in the TiN-coating. The nearest atomic surrounding in the coatin,,, is characterized by interatomie bonds Ti-N, Ti-O, Ti-Ti 1_hepeak at R, = 1.26 A is due to the presence of C and O adsorbed over the coating surface (Re+Re=0.51 +0.73= 1.24 A). The peak at larger interatomic distances of the order of 4-5 A is indicative of a more remote order in the arrangement of atoms and crystalline structure of TiN.coating. These crystaUostruetural data correspond to others obtained for TiN massive samples by EELFS [ I ~]. The analysis of fine structure on electron spectra obthined from the surface of the wear hollow at different stages ( see Fig, 5 (a)(c)) made it possible to demonstrate changes in the structure of the thinnest surface layers of the coating at its interaction with chips. When the cutting lasts 30 s (i.e. at the tool running-in stage), the me1 surface features titanium nitride with crystalline structure. This is attested by the position of the peak at R2 = 2.0 A,, as weU as by the peak of average intensit~ at more remote interatomie distances of the order of 4-5 A (see Fig. 5(a)). The broad peak at 4-5 A.has an individual shape and intensity that are distinguished from nons~ctural oscillations in interval 6--8 ~,. This peak characterizes long range older in the TiN coating. When the cutting lasts 180 s, titanium oxide develops in the coating, and the degree of remote order in the crystal lattice is reduced. This is shown by the appearance of the peak at Re = 2.20 )[ (R.ri + Re = 1.45 + 0,'/3 = 2,18 A,). Besides, there is a peak intensity at more remote interatomic distance,s 4-5 A decrease to intensity cf nonstructural oscil-
G.$. Fox.Rabino~ichel aLI Wear 198 (1996) 280.-286
C,.O lo
~°VVI
: \ , . J
i
: '°1/# "1~ I
i t i
l i
-3o-t
0
v
I
t
2
3
4
-
~
-
e
#
"
0
orb. so" units 40-
9~1
30. 20,
TbO
lb. ~'
o, -10'
-20' ,~o, .40,
I I I i i
i i i
I i
2
1
3
4
1 i i I i 6
6
7
8
llp,] aib. 80-
C-Fo
40-2o.
I
units ~'3°"
~' " - ~
:IJiW
.....-ill" .30
(c)
iI ~
-
it/
~
"
I 2
3
4
!i
8
?
8
Fig. 5. Fourier transform f r o m P P L F $ a~nlysis in wear hollow, T i , C r N -
coatingonni~dedTI5 steel:(a) 30 s: (b) 180s; (c) 2100s curing, lations. This established fact indicates decreaseof long range crystallography order Tie or amorphization of coating (see Fig. 5(b) ). At the onset of catastrophic deterioration (Fig. 4(c)) ef the coating, at a cutting duration of the order of 2100 s, the coating becomes destroyed and the steel surface is exposed. This is evidenced by the change in the form of Fourier transform. Its first peak is located at distance R, -- 1.7 A, the second at distance R2 = 2.65 ,~. These peaks correspond to the bonds C-Fe and Fc-Fe (Rc+Rr~=0.51 + 1.26= !.77/~,; RF¢÷ Rr~= 1.26 + 1.26 -- 2.25 ~,). Such interatomic bonds, as well as a more remote order revealed at distances dose to 4-.5 A, a,'e typical of BCC.lattice of martensitic matrix in the substrate. The analysis carried out made it possible to evaluate the change in the structure of the coating applied to high-speed steels of different heat resistances. On cutting at the stage of steady-state wear, there is a transformation of titanium nitride into oxide-likelyTie films in the coating. This process develops to a greater degree in a coating applied to steel of higher heat resistance.
285
The observnd process of self-organization is no doubt useful and above all controllable, Tribotechnological mutes to such control are quite clear: 1. One can use chemical compounds as PVD-coatings that make it possible to considerably enhance the development of protective oxide-likely films in the process of speed cutting. Such chemical compounds can be, for instance, carbo-nitrides of high-melting melals [ 16-18]. The use of carbo-nitrides increases the beat-barrier effect and raises the temperature gradient on the surface [6--8], This is contributing to the enhancementof self.organization. 2. As a tool substrat¢ one can use specially heat*resistant materials that not only encourage the ccnfincment of external effects to the coating layer, but possess selforganization properties at friction as well, For example, we may employ HSS-based powder alloys with a sU'engthening phase of TiC or TiN-type or other similar materials [18-20]. 3. One can determine metalworking parameters of highspeed steel cutting tools [2] through discovery of the optimal relationship between the maximally attainable cutting speed and ,*hepossibility of preserving the process stability. With these ideas realized, the creation of self-organizing cuuing materials with a modified (hardened) surface may become possible. In our opinion, such materials may well prove to Ix: competitive against existing materials (hard alloys) and replace them under specified cutting of 50I00 m rain- '.
4. Conclusions Changes in chemical composition and fine (at atomic level) structure characteristics of the surface during the life time of cutting tools were investigated by using AES, SIMS and EELFAS methods. High-speed steels M2 and TI5 with comings incorporating the ion nilliding and the (Ti,Cr) niu'ioe UAPDP (cathode arc plasma deposition process) coatings were used. It was found that under the wear conditions studied (such as cutting of medium carbon steels), some components of the protective hard coating dissociate, forming quasi-oxide amorphous films. It shows in parallel developing processes of: 1, dissociation of the chemical compound in the coating material (Ti,Cr)N and intensive diffusion (absorption) of nitrogen into the chip; 2. creation on the tool surface of stable amorphous protecting quasi-oxide Ti.-O films. Discovering changes in chemical and phase composition of contract surfaces of "toolworkpiecc" tribopair improves service characieristics of the cutting tool and promotes its adaptation ~o service conditions (named "self-organization"). The adaptation to be observed is expressed more on steel TI5 with high heat resistance and leads to significantincreaseoftoollife.
286
G.S. Fox-Rabinovich etal. I Wear 198 (1996) 280-286
The study of self-organization in tools with a hardened surface layer may well lay the basis for tribotechnicalcontrol of friction and become an initial stage in developing selforganizing composite materials with a radically new level of wear resistance,
Appendix A. Nomenclature Wx 10 -6
C T I mass
x(K) F(R) K
R(E) ON(E)IOE R~ N7
Aj(K, qr) ¢(K) 0.2
value of wear on the face surface (m ~) concentration of elements (at.%)
time of cutting process (s) intensity of signal (arbitraryunits) atom masses (atomic mass units) structural function (arbitraryunits) Fourier transformer (arbitrary units) wave vector (/I,-1) length of between atomic bonds (,~) intensityof Auger signals (arbitrary units) distance between centers of emission and cattering (it) coordination number amplitude of back-scattering phase shift of an electron at its scattering
A(K) K.~. Kma~ K"
factor correcting for heat oscillation of atoms in the lattice depth of electron exit (A.) onset of spectrum processing range (A.- ,) end of spectrum processing range (A.- ~) weighting factor (1
W(K) E Eo
window function energy of electrons (eV) position of the characteristic line (eV)
References [ ! ] K. Trent, Metal Cutting, London, 1980. [2] L.1. Bershadski, Boris Ivanovich Kostetsky and general conception in tribology, in Friction and Wear, Nanka i techica, Minsk, 1993, Vel. 14, No. 1, pp. 6--19 (in Russian). [ 3] B.I. Kostet~ki,Evaluation ofstrdetural and phase state and mechanisms of material structuring under external friction condition, in Friction and Wear, Nauka i technika, Minsk, 1993, Vol. d, pp. 773.-784 (in Russian).
[4] M.B. Gordon, Tim role of ~ysical and chemical processes in metal cutting, theory of friction, coolant and nmchineabiliW of metals, Reports ofserainar, 1981, Chelx~ari, pp. 3-11. [5] G.S. Fox-Rabioovieh,Stmctureofcomplex coating, Wear, 160 (1993) 67-76. [6] N.P. Sub, New theories ofwearand their implications fortool material, Wear, 62 (1980) !-20. [7] G.S. Fox-Rabinovich, Combined coating for cutting Iools. Cutting, wedge contact behavior combined coating role under adhesive wear, in Friction and Wear, Nauka i technica, Minsk, 1994, Vol. 15, pp. 636-645 (in Russian). [8] G.S. Fox-Rabinovich, A.I. Kava]or and D.L. Wainstein, Investigation of structuring mechanismin complex Ti-N-based coating during work of cutting tool using EELFAS and AES methods, J. 5pectwsc. feint. Phenom., 1995. [9] B.M. Kramer and P.R. Judd, Composition design of wear coatings, J. Vac. $ci. TechnoL, A3 (6) (1985) 2439-2444. [ 10] G.P. Hoffmann,F.E. Huggins,LJ. Cuddy, F.W. Lytlo and R.B.Gregor, Scripta Met., 18 (1984) 719-724. [ ! 1] D.P. Woodruff, Su~ $ci., 189-190 (1987) 64-68. [ 12] E.A. Stern and D.E. Sayers, Phys. Re~., liB (12) (1975) 4836-4845. [ 13] M.A. Sherwood,in D. Briggs and M.E Sench (eds.),PracticalS~ace Analysis by Auger and X-ray Photoelectron Spectroscopy, 1983. [14] J. Denier, E. Chainet, M. De Crescenzi and C. Nogue~ Su~ $ci., 189-I90 (1987) 590-604. [ 15] A.l. Kava]or and G.V. Sched~edinski,Modern methods of metals and alloys surfaces stody, Metallurgia, 1981 (in Russian). [ 16] S.B. Sant and K.S. Gill, Growth and characterization of cathode arc evaporated CrN, (TiAl)N, and (TiZx)N films, Su~ Coat. Technol., 16 (1995) 152-157. [ 17] F, ~ d o , R.M. Rodrigues and M.C. Polo, Microstructure analysis of CAPDP TiCN hard coatings, Surf.. Coat. Tecl~wl., 45 ( 10-1 I) (1994) 997-1001. [ 18] W. Konig and D. Kammermeiner, Fanktionelles Verhalten van TiN, TiCN and (TiAI)N beschichteten H$S Werk~engen, Gc;~,'~llsch~gflit Tribologie, Essan, September, I ~ . [ 19] R. Oskatson and P. Van Holst, Sandvick coronite--a new composed material for end mills, in Reports of 12th Int. Plansee Seminar, 1989, Vol. 2, pp. 531-539. [20] N. Uchida and H. Naknmura, Influence of chemical composition of matrix powders on some properties of TiN dispersed and carbide enriched HSS, in Rcpom of l2th int. Plansee Seminar, 1989, Vol. 2, pp. 541-555. [21] L. Singer, S. Fayenble and P.D. Edwi, Friction and wear behavior of TiN in air: the chemistry of tranfer films and debris formation, Wear, 149 (1991) 375-394. [22] M.N. Gardos, The effect of anion vacancies on the tribological properties of futile, TriOoloTrans., 32 ( 4 ) (1988) 427-436. [23] M.N. Gardos, The tribooxidativebehavior of rotile-forwdngsubstrates, in L.E. Pope, L.L. Fehrenbacher and W.O. Wirier (eds.), New Materials Approach to Tribology, Theory and Applications, MRS Syrup. Proc., 'Col. I40, Materials Research Society, Pittsburgh, PA, 1989, pp. 325-338. [24] M.N. G~dos, The effect of Magneli phases on the tribologifal properties of polycrystalline faille, in M. Kozma (od.), Eurotrib'93, Proc. 6th Int. Conf. on Tribology, Budapest, Vol. 3, International Tribalogy Council, Budapest, 1993, pp. 201-206. [25] H. So, The mechanism of oxidational wear, Wear, I84 (1995) 161167.
Weartgs (1996) 28o-286
Characteristic features of wear in tools made of high-speed steels with surface engineered coatings II: Study of surface engineered high-speed steel cutting tools by AES, SIMS and EELFAS methods G.S. Fox-Rabinovich *, A.I. Kovalev, S.N. Manasyev RIMET Corap., P.O. Box 167, Moscow 125239, Russia
ReceivedS August1995;accepted3 April1996
Abstract Changes in chemicalcompositionand fine (at atomiclevel) structurecharacteristicsof surfacesduringthe life time of cuttingtools were investigatedby using AES, SIMS and EELFAS methods.High-speedsteels M2 and TI5 with coatingsincorporatingion nitriding and (Ti, Cr) nitrideCAPDP (cathode arc plasmadepositionprocess) wereused. It was found that, under the wear conditionsstudied (such as cuttingof mediumcarbon steels), some componentsof the protectivehard coating dissociated,formingquasi-oxideamorphousfilms.This showsthe paralleldevelopingprocessesof: 1. Dissociationof chemicalcompoundin coatingmaterial (Ti,Cr)N and intensivediffusion(absorption) of nitrogeninto chip. 2. Creat!onon tool surfaceof stableamorphousprotectingquasi-oxideTi-Ofilms.The diseoverexlchangesin chemicaland phasecomposition of contact surfacesof the "tool-workpiece" tribopair improveservicecharacteristicsof the cutting tool and promote its adaptationto service conditions (named "self-organization"). The adaptationobservedis expressedmore on steel TI5 with high heat resistanceand leads to a significantincreaseof tool life. Keywords: Cuttingtools;Steel;Metallurgicalcoatings
1, Introduction Cutting tools fall into the range of highly loaded tribosysterns. That is why the damage to contact surfaces and, consequently, very low wear resistance during operation are inevitable [1 ]. Nevertheless, thanks to recent ad',,ances in tribology [2,3], one can discern some scientifically substantiated trends that enable one to move closer to adaptable fundamentally new self-organized tool tribosystems. There are a wide variety of methods for achieving such tribosystems [3-6]. So modification of surface properties during the service period secured by specially organized composition and structure of surface layers is a way to prolong the workability of the tool. To solve this problem one needs to study complex physical and chemical phenomena on the toot surface. These phenomena improve service characteristics of extremely stressed microvolumes of the tool during operation and promote its adaptation to service conditions (named "selforganization"). The self-organization is a spontaneous * Correspondingauthor. 0043.1648/961515.00© 1996ElsevierScienceS.A.Allrightsreserved PIi S0043-1648 (96) 07204-3
formationof stable and ordered phases that protect the surface against damage from friction [ 3]. The present paper considers both technological and material science aspects of the problem solution that are concerned with tool surface modification. The approaches under consideration are based on the self-organization of surfaces under friction [3]. The phenomenon of self-organization constitutes a physical basis for possible friction control [2-4]. The latter provides both for improvementofsuffacetribotechnical properties and for significant changes in mechanisms of contact interaction between the tool and the workpiece. Solution of this task would make it possible to pass into a completely differentlevel of wear resistance and machining productivity. Taking all this into account, one may see the purpose of this paper as investigation of self-organization in the cutting tools modified by coatings'technology. The change in chemical and phase composition on the contact surfaces of the "tool-workpieee" tribopair was studied. Tools based on high-speed steels with surface engineered coatings (ion nitriding and TiN nitride coating applied by the CAPDP-
(7.$. Fox.Rabinovichet al. / Wear 198 (1996) 280-286
cathode arc plasmadeposition process-method) were usedat increased cutting speeds in the vicinity of 100 m rain- !. As the researchc;arriedout shows, the use of such coatings prorides for stable cutting with a wide range of tool service [6-9]. This effect is ensured by localizingexternal thermo. mechanical loads in surface layers of a high-melting compound [8]. All these lay the grounds for the assumptionthat the phenomenonof tool surface self-organizationis observed in the process of friction. The experimental data obtained confirmthis assumption. In the present paper, all the physicaland chemicalphenomena on the interfacebetweenbuild-upor chip and hard coating in a wear hollow were investigated,For this purpose surface sensitive analytical techniques were used. At present, a number of experimental techniquesare competitive for the study of .~urfaces.There is an impressivearray of methods available for the analyses of the structure and composition of the outermost layers of a surface. Auger electron spectroscopy (AF_,S),secondary ion mass spectroscopy (SIMS), electron energy loss fine Auger structure (EELFAS) methods have a high in-depth resolution at the surface, nearly several interatomicdistances,high lateral resolution (AES 30nm, SIMS 0.2 p.m, EELFAS atomic diameter) [ 101. The information on these analytical techniques is very good. It, SIMS, cluster ions and molecule fragments can be used in trace analysis of the surface chemical environment. To obtain meaningful information on the origin of interatomic interaction and its bond length a technique is needed with a precision of 0.01-0.02 ~. F.ELFAS provides this kind of accuracyand can be used for discovering microcrystalline,amorphous,orderedand disorderedsystems [11,12]. These techniques have irrefutablepreferencesunder TEM and XRD when thin layers on interfaces are investigated.In the present work AES, SIMS and EELFAS were used for investigation of chemical and phase composition thin layers on interfacesbetween hard coating in the wear hollow of the tool and build-up or chip.
2, Experimental procedure Chemical and phase composition of "tool-workpiece" contact surfaces were studied by means of Auger-spectros. copy (AES) and secondary ion mass-spectroscopy(SIMS). For this we employed a ESCALB MK2 (VG) spectrometer equipped with LEG 200 electron gun, AG 6 ion gun and SQ 300 quadrupole spectrometer. The scanning Auger-spectroscopywas used to analyze the composition of the surface of the hollow on the face cutting tool edge at different stages of its development.Besides, this type of spectroscopy was employed for the analysis of chip contact surface. In each of these cases several areas 15 X 15 p,m in size were chosen on the surface underanalysis. Television scan rates were used for the beam of primary electrons 2000 it in diameter. Auger-spectrawere recorded
281
at energy E=2500eV in the mode CRR (constants retard ratio) modulationamplitudeof 2 eV and at speed 2,1 ©Vs- i. After cleaningby standardtechniques (argon bombardment) the samples were investigatedon vacuum 10-s Pa. Phase composition of the sur2"aceof the wear hollow was analyzed with the aid of scanning mass-spectroscopyof secondary ions. For this purpose an argon ions beam 0.5 IJ.min diameter was scanned at television scan rates at a 5.0 keV accelerationvoltage and 2X 10-s Pa press~::e,wi:]l the area 15 × 15 Itm in size being analyzed. In the selected mode, ion etching speed was sufficiently low and did not exceed 0.5 monolayer per minute, The analysis was made in the mode approximating static conditions. Investigationof amorphyzationand the nearest atomicsurrounding on the surface of wear hollow of hard coating cutting tool was carded out by means of the i~ELFS method. The investigationwas cardedout on an ESCALBMK2 (VG) electron spectrometer equipped with the hemispherical capacitor type energy analyzer. The extended fine structure of electrons'spectrawas analyzedin the range of 250 eV close to the lines of elastic scattering and KLL.iines of nitrogen, carbon and oxygen. Electronspectra were recorded atthe rate of 0.4 eV s- ~. This was done in the integral form in the CADmode (constant analyzer energy) at 10-fold signal accumulation. All physical and technical conditions were chosen in such a way as to ensure the best energy resolutionat a good signalnoise ratio. The energy of primary electrons was 1500eV. The EELFAS method is based on the analysis of extended fine structure spectra of electrons with energy losses at their reflection. Recently this method has been used for precision investigations of atomic structure in thin surface layers. By analogy with the EXAFS (extended X-ray-absorption fine structure) spectroscopy [ 1! ], electron spectra contain information about the structure of the nearest atomic surrounding (the beam interaction area) on the surface. The necessary experimental equipment for such investigations is considerably simpler than that in the EXAFS method: any Augerspectrometercan be used, The main difficultyencounteredin obtaining the electron spectra fine structure is obtaining a high energy resolution, precision accumulation of a weak signal and availabilityof software for mathematicalprocessing of spectra and noise filtering. The theory that describes an EXAFS-Iike structure on the electron spectrum is still underdevelopmen:[ 11]. The methodsof mathematicalspecIra processing adopted in the EXAFS-spectroscopy [ 12], however,can be used to analyze the fine structureon electron spectra. These enable one to determine lengths of atomic bonds in the nearest atomic surrounding, with these results being in good agreement with crystallographicdam [ 13]. It is possible to give a physical picture of the phenomena observed. In electron spectra close to the lines determinedby elastic scattering, and emission of photo- and Auger-clueIrons, there is always a background gradually decreasing in intensity on the low kinetic energy side. This background is determinedby the detectionoftbe flowof eleclronssubjected
282
G.S.Fox.Sabinovichet al. I Wear198(1996)280-286
to nonelastic scattering in substance. The greatest contribution to inelastic scatt.ering is made by the electron-electron interaction [ 13]. In contrast to the photoeffect initiated by X-ray or ultraviolet radiation with the energy of a quantum being completely absorbed, the electron excitation of atom levels causes the emission of electrons which feature a continuous spectrum of kinetic energy. On passing the thinnest surface layers, a flow of such electrons undergoes inelastic and elastic scattering as well. All these processes, taken together, considerably enhance the emission of electrons that have a continuous spectrum of kinetic energy when the sample is electron irradiated. In this case the object to be analyzed seems to possess a source of variable energy electrons that is due to the excitation of multiple emission centers. Since electrons possess a wave nature, in the process of their scattering in the nearest atomic surrounding and under certain conditions in the crystal lattice, there is a possibility of the phenomena of resonance and antiresonanoe occurring. These are connected with the enhancement or attenuation of electron wave function amplitude in a given point of space and, in particular, in vacuum and in a detector. These conditions are determined both by the electron wavelength (or energy) and by the distance from the emission center to the scattering center (or interatomic distance). As a result, for electrons with different kinetic energy the electron emission spectrum features a modulation in detected signal intensity. Emission of electrons and their nonelastic scattering on the nearest atomic surrounding is of a probabilistic nature. Due to this, the amplitude modulation of signal intensity becomes diffused over the energy scale and creates an extended fine structure of background close to the lines of elastic scattering, absorption edge and Auger-lines. The extent of this fine structure achieves 250-300 eV. Depending on the wave vector of electron K the oscillation on such a spectrum has the following equation [ 12]: - ~ KR~ x(K)=~'/
K(E-I) ~_[0.263(E- Eo) (eV) ] Ij2
(3)
A theoretical basis of such an analysis is analogous to that presented in [ 12-15]. AES, SIMS and EELFAS spectra were obtained on the surface of the cutting tool face edge hollow for different service times using square indexable inserts.
3. Results and discussion In this work we investigated the change in chemical and phase compositions of contact surfaces, The "tool-workpiece" tribosystem was studied on the surface of the hollow of cutting tool face edge, and on the chip contact surface. The study of indicated contact elements of the tribosystem under analysis permitted more complete and objective information about processes occurring at the contact surfaces to be obtained. Chemical composition analysis on the surface of the cutting tool face edge hollow shows that, from initial stages of cutting in the running-in zone, there is a reduction of nitrogen concentration and increasing of oxygen concentration. These effects are clearly presented by a series of Auger-spectra (Fig. 1) obtained from the surface of the wear hollow at different durations of cutting. After considerable cutting duration, a significant increase in Auger-lines intensity for iron and carbon is observed due to adhesion of machined steel
I o
I
I c
. . . . . . .{_q~,3t)j , ,-,,e-2~z.,~e-zR~/A~g) slntg/t/cj (1)
To determine atomic distances in the nearest coordination spheres (averaged and partial functions of radial distribution), the Fourier analysis of this amplitude modulation is applied. Calculation data are in good agreement with crystallographic data. A small difference is due only to the phase shift, To determine the coordination number by the peak intensity on Fourier transform we need the amplitude analysis whose principals are given in [12,13]. In the present work we used the Fourier transform of the type: Kmt~ F(R) = ( - 1) [ x(K)W(K)I~' exp( - 2/f~R) dk
(2)
L~,, 800 700 t~O 500 4~, ~U
I c E,eV
d
ICon
The K-wave vector is represented as:
Fig. 1. Mger-s~¢tnl from tuning tool surface: (a) TiN coating before service; (b) 30 s; (¢) 180 s; (d) 300 s; (e) 2100 s cutting ti~s,
G.5. Fox-Rabinovichet al. I Wear I98 (1996) 280--286
283
W.~O~.m= etrb. unl% L'ID
/
:: .
]
°'" ~ - ~ "
s
:(_.:.
/
,o k Mlln
&m.U.
70
~
~ II,m.U,
m
f,s
Ilamt ,l',m.U.
Fig.3. SIMSspec~aof TiNandquasi.oxidesTi-O filmsvs servicedine of
c,~.
tool madeofM2(P6M5) withcoatings:(a) cuttingequaito 15 s; (b) cut'ling equalto90 s;(~)cuttingequal~o 120 s,
40&S. :lO-
found on mass-spectra it is assumed that the discovered TiO clusters were emitted from non-stoichiometric quasi-oxide Ti-O films. Titanium oxide [22-24] has high resistance at
20.
16In.
cutting [91 and can effectivelyfulfilla protectivefunction t
i
t
i
60 100 1~0 ~
~0 :~0 'l~ 4~0;111~|1tB022011Ti4 l"
c, a~ •
o
1050
c SO 100 1~ 200 ~
300 3~.040Q 4,~0210021~OTtS
Fig. 2, Dependence of wear value and chemical composition of the "tootworkpiece" tdbopair elements on cutting time: (a) change of wear value
on the facecuttingtool edge (areaof wearhollow);(b) changein AES elementcompositionofthewearhollowsurface;(c) changein AESelement compositionof the chipcuttingzone;curves4, tool madeof M2(P6MS) steelwithcoating;curve5, toolmadeof TIS(PI2M3(IY2KS)powdersteel withcoating. on the surface of cutting tool, and at the final stage of cutting by the destruction of the coating. There is a good correlation between the chany~ein wear resistance of the cutting tool and the element composition of the coating (Fig. 2(a, b)). It is possible that, during the transition from the running-in stage to that of steady-state wear, the depiction of the coating surface of niUogen and its enrichment by oxygen is connected with the dissociation of titanium nitride, adsorption of oxygen from the environment, and development of titanium oxide. Th~se data are in accordance with results of other investigations [21 ]. These data are confirmed upon analyzing the coating phase composition. In fact, during the transition frora the runningin stage to that of normal wear (Fig. 2(a), :.,ryes 4), the development of increased Tie cluster quantity (Fig, 3(a)(c)), was discovered with the aid of secondary ion massspectroscopy (SIMS). Because tint Tie2 (mass 80) was not
[251. Using the insights of modem triboiogy [3], one can state that quasi-oxide Ti-O fixed on the surface constitutes thin films. These layers form at cutting and are known to be corn. pounds of the surface material with the oxygen introduced into the cutting zone from the environment. Oxide rich phases act as a shield that protects the surface [3,4]. That is why the emergence of increased Ti-.O quantity at transition from the running-in stage to that of normal wear (Fig. 3(b), (c)) testifies to the self-organization of "tool-workpiece" tribosystem. As a result, the wear intensity is reduced and this process enters its steady-state stage (Fig. 2(a), curve 4). Further on, the quantity of oxide component remains practically unchanged as the cutting time grows (Fig. 3(b), (c)). It is typical that the adhesion of medium carbon steel to the workpi~cc increases in the running-in period, then stabilizes and does not change within the stage of normal wear. This is evidenced by the constant content of iron traces on the surface of the hollow (Fig. 2(b)). Additional information about the processes in operation on contact surfaces of the coating is suggested by the results of chemical composition analysis on the chip contact zone surface. The latter arc obtained by means of ABS. These data correspond to the results of element composition analysis for the adjoining cutting tool surface, It was found that as the cutting time grows the concentration of nitrogen and oxygen in this surface zone changes according to a nonlinear function. At initial operation stages, for example at the running-in stage (up to 30 s of cutting, Fig. 2(a), curve 4), there is an intense oxidation of chip surface (Fig. 2(c) ), In this case the oxygen content is maximum, whereas the nitrogen concentration is minimal. Upon completion of the running-in stage (60 s of cutting) the picture is changed: the nitrogen concentrations on the surface increase while the oxygen concentration drops. This agrees with the onset of the normal wear stage (90120 s of cutting, Fig. 2(a), (c), curves 4). Further on, due to the stabilization of processes occurring during the steady-
284
G.S. Fox-Rabinorichet aLI Weur198(1996)280-286
state friction stage, the chemical composition of the chip surface remains constant up to the catastrophic wear. Decreases of nitrogen content in the coating and nitrogen absorption on the contact surface of chip are tied together and to individual stages of tool wear. The more stable the cutting process is, the more intensive is the indicated effect. Thus, the tool made from composite material M2(P6MS) with a surface engineered coating has a higher degree of wear at cutting and a lower friction process stability than one made from T15.PI2M3tlr2K5 (Fig. 2(a), curve 4,5). As is seen, the nitrogen concentration in chips at cutting with the T15-steel tool is higher than that at cutting with the M2-base tool (Fig. 2(c)). Amorphization and fine structural changes in the nearest atomic surrounding on the contact surface were investigated by means of EELFS method when analyzing the extended fine structure of spectra of electrons with energy losses. Fig. 4(a) presents an EELFS-picture obtained close to the line of elastically scattered electrons for a TiN-coated sample. The spectrum is given as a function of K-wave vector after background subtraction and smoothing.
t A/ ] I k ,
_ _ . . _ v _
v a~b. 3o unlls 2o -
~r
Co)
C"O~ti~
to-
o
i i
i i 1 I
i ~
, I
"'--
RIAI olb. 200.
(c)
Fig. 4. (a, b) structuralfunctionX(K) and FouriertransformF(R) from
I~LFS spectraTi,CrN-coatingon nitridcdTI5(PI2M3gr2K.5)stcei; (c) FouriertransformfromEELFAS-spcctrain regionof KLL-nilrogcnlia¢on the sane sample.
Fig. 4(b) depicts the Fourier transform which contains averaged information about the coating structure after the mathematical processing of the spectrum given in Fig. 4(a). This function represents several diffuse peaks in the range of interatomic distances of 3 A and 4-5 A. To identify this averaged picture, the partial functions were determined. To do this, we analyzed EELFS on spectra close to KLL-lines of oxygen, nitrogen and carbon, The Fourier transform characterizing the individual nearest surrounding of nitrogen atoms in TiN-coating is represented in Fig. 4(e). The data are obtained after processing the fine structure close to KLL-tine of nitrogen in the energy range 390-280 eV. The sole pronounced peak corresponds to the length of bonds of 1,95 A and specifies a radius of the first coordination sphere surrounding the nitrogen atoms. The measured value R~ = 1.95 .~ is close to the theoretical value of Ti-N-bond length received on the basis of Ti and N ion radii (RTi+RN=IA5+O.61=2.06A). These data have good accordance with crystallographic parameters for TiN and earlier measurements by the EELFS method [ 13]. Partial functions like these were obtained after analyzing the fine structure close to other characteristic Auger-lines of the spectrum. These allowed the nature of peaks on the integral picture, gained when investigating the spectrum close to the lines of hack scattering, to he identified more precisely. Values of the first coordination sphere radii in the surrounding O-, Ti-, C- and N-atoms representthe bonds between atoms of various elements in the TiN-coating. The nearest atomic surrounding in the coatin,,, is characterized by interatomie bonds Ti-N, Ti-O, Ti-Ti 1_hepeak at R, = 1.26 A is due to the presence of C and O adsorbed over the coating surface (Re+Re=0.51 +0.73= 1.24 A). The peak at larger interatomic distances of the order of 4-5 A is indicative of a more remote order in the arrangement of atoms and crystalline structure of TiN.coating. These crystaUostruetural data correspond to others obtained for TiN massive samples by EELFS [ I ~]. The analysis of fine structure on electron spectra obthined from the surface of the wear hollow at different stages ( see Fig, 5 (a)(c)) made it possible to demonstrate changes in the structure of the thinnest surface layers of the coating at its interaction with chips. When the cutting lasts 30 s (i.e. at the tool running-in stage), the me1 surface features titanium nitride with crystalline structure. This is attested by the position of the peak at R2 = 2.0 A,, as weU as by the peak of average intensit~ at more remote interatomie distances of the order of 4-5 A (see Fig. 5(a)). The broad peak at 4-5 A.has an individual shape and intensity that are distinguished from nons~ctural oscillations in interval 6--8 ~,. This peak characterizes long range older in the TiN coating. When the cutting lasts 180 s, titanium oxide develops in the coating, and the degree of remote order in the crystal lattice is reduced. This is shown by the appearance of the peak at Re = 2.20 )[ (R.ri + Re = 1.45 + 0,'/3 = 2,18 A,). Besides, there is a peak intensity at more remote interatomic distance,s 4-5 A decrease to intensity cf nonstructural oscil-
G.$. Fox.Rabino~ichel aLI Wear 198 (1996) 280.-286
C,.O lo
~°VVI
: \ , . J
i
: '°1/# "1~ I
i t i
l i
-3o-t
0
v
I
t
2
3
4
-
~
-
e
#
"
0
orb. so" units 40-
9~1
30. 20,
TbO
lb. ~'
o, -10'
-20' ,~o, .40,
I I I i i
i i i
I i
2
1
3
4
1 i i I i 6
6
7
8
llp,] aib. 80-
C-Fo
40-2o.
I
units ~'3°"
~' " - ~
:IJiW
.....-ill" .30
(c)
iI ~
-
it/
~
"
I 2
3
4
!i
8
?
8
Fig. 5. Fourier transform f r o m P P L F $ a~nlysis in wear hollow, T i , C r N -
coatingonni~dedTI5 steel:(a) 30 s: (b) 180s; (c) 2100s curing, lations. This established fact indicates decreaseof long range crystallography order Tie or amorphization of coating (see Fig. 5(b) ). At the onset of catastrophic deterioration (Fig. 4(c)) ef the coating, at a cutting duration of the order of 2100 s, the coating becomes destroyed and the steel surface is exposed. This is evidenced by the change in the form of Fourier transform. Its first peak is located at distance R, -- 1.7 A, the second at distance R2 = 2.65 ,~. These peaks correspond to the bonds C-Fe and Fc-Fe (Rc+Rr~=0.51 + 1.26= !.77/~,; RF¢÷ Rr~= 1.26 + 1.26 -- 2.25 ~,). Such interatomic bonds, as well as a more remote order revealed at distances dose to 4-.5 A, a,'e typical of BCC.lattice of martensitic matrix in the substrate. The analysis carried out made it possible to evaluate the change in the structure of the coating applied to high-speed steels of different heat resistances. On cutting at the stage of steady-state wear, there is a transformation of titanium nitride into oxide-likelyTie films in the coating. This process develops to a greater degree in a coating applied to steel of higher heat resistance.
285
The observnd process of self-organization is no doubt useful and above all controllable, Tribotechnological mutes to such control are quite clear: 1. One can use chemical compounds as PVD-coatings that make it possible to considerably enhance the development of protective oxide-likely films in the process of speed cutting. Such chemical compounds can be, for instance, carbo-nitrides of high-melting melals [ 16-18]. The use of carbo-nitrides increases the beat-barrier effect and raises the temperature gradient on the surface [6--8], This is contributing to the enhancementof self.organization. 2. As a tool substrat¢ one can use specially heat*resistant materials that not only encourage the ccnfincment of external effects to the coating layer, but possess selforganization properties at friction as well, For example, we may employ HSS-based powder alloys with a sU'engthening phase of TiC or TiN-type or other similar materials [18-20]. 3. One can determine metalworking parameters of highspeed steel cutting tools [2] through discovery of the optimal relationship between the maximally attainable cutting speed and ,*hepossibility of preserving the process stability. With these ideas realized, the creation of self-organizing cuuing materials with a modified (hardened) surface may become possible. In our opinion, such materials may well prove to Ix: competitive against existing materials (hard alloys) and replace them under specified cutting of 50I00 m rain- '.
4. Conclusions Changes in chemical composition and fine (at atomic level) structure characteristics of the surface during the life time of cutting tools were investigated by using AES, SIMS and EELFAS methods. High-speed steels M2 and TI5 with comings incorporating the ion nilliding and the (Ti,Cr) niu'ioe UAPDP (cathode arc plasma deposition process) coatings were used. It was found that under the wear conditions studied (such as cutting of medium carbon steels), some components of the protective hard coating dissociate, forming quasi-oxide amorphous films. It shows in parallel developing processes of: 1, dissociation of the chemical compound in the coating material (Ti,Cr)N and intensive diffusion (absorption) of nitrogen into the chip; 2. creation on the tool surface of stable amorphous protecting quasi-oxide Ti.-O films. Discovering changes in chemical and phase composition of contract surfaces of "toolworkpiecc" tribopair improves service characieristics of the cutting tool and promotes its adaptation ~o service conditions (named "self-organization"). The adaptation to be observed is expressed more on steel TI5 with high heat resistance and leads to significantincreaseoftoollife.
286
G.S. Fox-Rabinovich etal. I Wear 198 (1996) 280-286
The study of self-organization in tools with a hardened surface layer may well lay the basis for tribotechnicalcontrol of friction and become an initial stage in developing selforganizing composite materials with a radically new level of wear resistance,
Appendix A. Nomenclature Wx 10 -6
C T I mass
x(K) F(R) K
R(E) ON(E)IOE R~ N7
Aj(K, qr) ¢(K) 0.2
value of wear on the face surface (m ~) concentration of elements (at.%)
time of cutting process (s) intensity of signal (arbitraryunits) atom masses (atomic mass units) structural function (arbitraryunits) Fourier transformer (arbitrary units) wave vector (/I,-1) length of between atomic bonds (,~) intensityof Auger signals (arbitrary units) distance between centers of emission and cattering (it) coordination number amplitude of back-scattering phase shift of an electron at its scattering
A(K) K.~. Kma~ K"
factor correcting for heat oscillation of atoms in the lattice depth of electron exit (A.) onset of spectrum processing range (A.- ,) end of spectrum processing range (A.- ~) weighting factor (1
W(K) E Eo
window function energy of electrons (eV) position of the characteristic line (eV)
References [ ! ] K. Trent, Metal Cutting, London, 1980. [2] L.1. Bershadski, Boris Ivanovich Kostetsky and general conception in tribology, in Friction and Wear, Nanka i techica, Minsk, 1993, Vel. 14, No. 1, pp. 6--19 (in Russian). [ 3] B.I. Kostet~ki,Evaluation ofstrdetural and phase state and mechanisms of material structuring under external friction condition, in Friction and Wear, Nauka i technika, Minsk, 1993, Vol. d, pp. 773.-784 (in Russian).
[4] M.B. Gordon, Tim role of ~ysical and chemical processes in metal cutting, theory of friction, coolant and nmchineabiliW of metals, Reports ofserainar, 1981, Chelx~ari, pp. 3-11. [5] G.S. Fox-Rabioovieh,Stmctureofcomplex coating, Wear, 160 (1993) 67-76. [6] N.P. Sub, New theories ofwearand their implications fortool material, Wear, 62 (1980) !-20. [7] G.S. Fox-Rabinovich, Combined coating for cutting Iools. Cutting, wedge contact behavior combined coating role under adhesive wear, in Friction and Wear, Nauka i technica, Minsk, 1994, Vol. 15, pp. 636-645 (in Russian). [8] G.S. Fox-Rabinovich, A.I. Kava]or and D.L. Wainstein, Investigation of structuring mechanismin complex Ti-N-based coating during work of cutting tool using EELFAS and AES methods, J. 5pectwsc. feint. Phenom., 1995. [9] B.M. Kramer and P.R. Judd, Composition design of wear coatings, J. Vac. $ci. TechnoL, A3 (6) (1985) 2439-2444. [ 10] G.P. Hoffmann,F.E. Huggins,LJ. Cuddy, F.W. Lytlo and R.B.Gregor, Scripta Met., 18 (1984) 719-724. [ ! 1] D.P. Woodruff, Su~ $ci., 189-190 (1987) 64-68. [ 12] E.A. Stern and D.E. Sayers, Phys. Re~., liB (12) (1975) 4836-4845. [ 13] M.A. Sherwood,in D. Briggs and M.E Sench (eds.),PracticalS~ace Analysis by Auger and X-ray Photoelectron Spectroscopy, 1983. [14] J. Denier, E. Chainet, M. De Crescenzi and C. Nogue~ Su~ $ci., 189-I90 (1987) 590-604. [ 15] A.l. Kava]or and G.V. Sched~edinski,Modern methods of metals and alloys surfaces stody, Metallurgia, 1981 (in Russian). [ 16] S.B. Sant and K.S. Gill, Growth and characterization of cathode arc evaporated CrN, (TiAl)N, and (TiZx)N films, Su~ Coat. Technol., 16 (1995) 152-157. [ 17] F, ~ d o , R.M. Rodrigues and M.C. Polo, Microstructure analysis of CAPDP TiCN hard coatings, Surf.. Coat. Tecl~wl., 45 ( 10-1 I) (1994) 997-1001. [ 18] W. Konig and D. Kammermeiner, Fanktionelles Verhalten van TiN, TiCN and (TiAI)N beschichteten H$S Werk~engen, Gc;~,'~llsch~gflit Tribologie, Essan, September, I ~ . [ 19] R. Oskatson and P. Van Holst, Sandvick coronite--a new composed material for end mills, in Reports of 12th Int. Plansee Seminar, 1989, Vol. 2, pp. 531-539. [20] N. Uchida and H. Naknmura, Influence of chemical composition of matrix powders on some properties of TiN dispersed and carbide enriched HSS, in Rcpom of l2th int. Plansee Seminar, 1989, Vol. 2, pp. 541-555. [21] L. Singer, S. Fayenble and P.D. Edwi, Friction and wear behavior of TiN in air: the chemistry of tranfer films and debris formation, Wear, 149 (1991) 375-394. [22] M.N. Gardos, The effect of anion vacancies on the tribological properties of futile, TriOoloTrans., 32 ( 4 ) (1988) 427-436. [23] M.N. Gardos, The tribooxidativebehavior of rotile-forwdngsubstrates, in L.E. Pope, L.L. Fehrenbacher and W.O. Wirier (eds.), New Materials Approach to Tribology, Theory and Applications, MRS Syrup. Proc., 'Col. I40, Materials Research Society, Pittsburgh, PA, 1989, pp. 325-338. [24] M.N. G~dos, The effect of Magneli phases on the tribologifal properties of polycrystalline faille, in M. Kozma (od.), Eurotrib'93, Proc. 6th Int. Conf. on Tribology, Budapest, Vol. 3, International Tribalogy Council, Budapest, 1993, pp. 201-206. [25] H. So, The mechanism of oxidational wear, Wear, I84 (1995) 161167.