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Nano-crystalline filtered arc deposited (FAD) TiAlN PVD coatings for high-speed machining applications
Surface and Coatings Technology 177 – 178 (2004) 800–811
Nano-crystalline filtered arc deposited (FAD) TiAlN PVD coatings for high-speed machining applications G.S. Fox-Rabinovicha,*, G.C. Weatherlya, A.I. Dodonovb, A.I. Kovalevc, L.S. Shusterd, S.C. Veldhuisa, G.K. Dosbaevaa, D.L. Wainsteinc, M.S. Migranovd a
McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S4L7 Vacuum Ion Technology Company (VIT), 206, IEC VNIIETO, Pochtovaya Street, Istra-2, Moscow region 143500, Russia c Surface Phenomena Researches Group (SPRG), 2nd Baumanskaya str., 9y23,CNIICHERMET, Moscow 107005, Russia d Ufa Avia Institute (UAI), Ufa 450083, Russia
b
Abstract The main advantage of the filtered arc deposition (FAD) technique is a significant grain refinement that leads to the formation of nano-crystalline (grain size approx. 60–80 nm) PVD coatings. This technique improves the wear resistance of FAD TiAlN coatings under high-speed machining conditions when cutting tool oxidation wear is dominant. A study of the surface structure characteristics of the FAD TiAlN coatings using SEM, EDS, TEM, AES, SIMS and EELFS was performed. The microhardness of the coatings were measured. The microstructure of the chips was analyzed. The compression of the chips as well as shear angle was measured. The friction parameter on the rake surface of the tools was determined in-situ. It was shown that the major cause of the high wear resistance of the FAD coatings during high-speed machining was the formation of the thin protective oxide films on the cutting tool surface. The grain size refinement of the coating promotes the formation of protective alumina films. These films are formed at the surface of nano-crystalline FAD TiAlN coating during high-speed machining. They mainly consist of protective alumina, whereas the films that are forming on the surface of TiAlN commercial coatings with coarser grains consist only of non-protective titania. The formation of the protective alumina films, composed of complicated amorphous– crystalline structures, significantly improves the friction and wear performance of coated tools. The tendency of the work piece material to adhere is reduced and considerably more heat is dissipated via chip removal. This is a major cause of the enhanced wear resistance of the FAD coating technique. Tribo-oxidation during cutting is a typical process of the self-organization of TiAlN coatings that results in a quasi-stabilization of cutting tool wear. The self-organization and friction control for coated cutting tools is considered based on a non-equilibrium thermodynamic approach. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Filtered arc deposited TiAlN PVD coatings; Nano-crystallline structure; Self-organization; High-speed machining
1. Introduction Recent improvements in the life of cutting tools have been achieved by the development of titanium aluminium nitride (Ti,Al)N coatings. Films such as TiAlN w1,2x display a unique combination of properties, viz. a high hardness at elevated temperature together with thermal and chemical stability, as well as low thermal conductivity. An extremely important advantage of (Ti,Al)N coatings is their high oxidation stability w3x, which is *Corresponding author. Tel.: q1-905-525-9140x24980; fax: q1905-521-2773. E-mail address: [email protected] (G.S. Fox-Rabinovich).
due to the formation of relatively stable surface oxide films w4,5x. Furthermore, improvement of the wear resistance of TiAlN coatings during cutting could be achieved through grain size refinement down to the nanoscale (grain size less than 100 nm) level w6x. This could be done by alloying of TiAlN coatings w7x, as well as through advanced deposition techniques (such as FAD) w8–11x. Due to the high plasma ionization rate in the chamber of the coater unit and the relatively low deposition rate, the current temperature at the crystallization front of FAD coatings is low. The coating deposition conditions are close to nano-scale heating w12x, when the kinetic energy of bombarding ions is transferred into very small areas of actively growing thin coating layers, which are
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.05.004
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2. Experimental Two types of TiAlN coatings were studied: 1. Commercial monolayered TiAlN PVD coating. 2. Advanced FAD TiAlN PVD coating.
Fig. 1. Scheme of the filtered arc-evaporation system.
then very quickly conveyed into the samples vicinity. Thus, the cooling rate, during a strongly non-equilibrium FAD process, is very high. This leads to the formation of nano-crystalline films w12x. This results in the formation of nano-crystalline coatings and could be very beneficial for wear resistant coatings especially well suited for high-speed machining applications. Significant improvements of the TiAlN PVD coatings for high-speed machining applications, under conditions when the oxidation wear prevails, could be achieved based on a better understanding of the mechanism of the tribo-oxidation of the coating during cutting. The generation of a dynamically stable and continuous film of alumina on the surface with high tribological properties is the objective for the new coating development. The goal of this work is to study the friction and wear behavior of the cutting tools with TiAlN PVD coatings and determine the impact of the FAD coatings nanocrystalline structure on the cutting tools performance under high-speed machining conditions.
The advanced coatings under study were deposited using a FAD PVD unit of NNV-6.6-I1 type. These types of units have up to three removable targets, also referred to as FAD modules. A sketch of the FAD module is shown in Fig. 1. Parameters of the PVD coating deposition process are shown in Table 1. Temperatures on the surface of the sample were measured using an optical pyrometer. Heating the substrate up to the deposition temperature was performed by Argon ions during FAD at the substrate biasing voltage of 1 kV. After obtaining the required substrate temperature, the substrate biasing voltage value was reduced and nitrogen was fed into the chamber for deposition of the TiAlN films. The system was shown to affect the physical-chemistry of the plasma-chemical processes significantly when depositing refractory compounds. This is due to an increase in the ionization rates of both metals and reactive gases w9,10x. The system included a hollow steel tube approximately 300 mm in diameter, which was a quarter segment of a torus (Fig. 1). The tube was installed on the body of a standard unit of the arcevaporation PVD instead of on a conventional arcevaporator. An induction coil was placed inside the tube to create the magnetic field needed to control the plasma path. Regulation of the plasma flow in the FAD System is based on the principles of plasma-optics. The influence of the magnetic field upon the plasma flow allows the radial electric field to appear; as a result, regulation of the plasma path inside the internal volume of the FAD system can be achieved. Whereas only electrically charged particles are focused in the FAD system, uncharged particles (so called ‘droplet’ phase) are not affected by the magnetic and electric fields. The uncharged particles flying from the cathode surface parallel to the cathode axis do not reach the substrate surface and are thus deposited on the inside of the tube. However, the ions follow the bend and are focused on the substrate surface. The value of the continuous current running through the magnetic coil affects the distribution of the ion flow density. The body of the FAD system is
Table 1 Parameters of the FAD TiAlN PVD coating deposition 噛
Target
Arc current, A
Nitrogen pressure, Torr
Time of deposition, min
Coil current, A
Separator current, A
Separator voltage, V
Temperature of deposition, 8C
1
TiAl alloy (cast monolithic)
200
;5=10y3
30
20
20
16
340–370
802
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biased with regard to the PVD unit’s body. This is required if the substrate biasing is not enough to achieve a suitable value for the deposition rate. The FAD system allows the substrate to be heated up to the deposition temperature by use of Argon ions, which are rarely achieved when applying conventional arc evaporation PVD technology. This was shown to occur due to the high ionization rate of the gases obtained when using the FAD system with elevated energy w9x. The filtering efficiency of the FAD technology for ‘droplet’ phase deposition prevention is well known w11x. But there is another advantage of the FAD technique that could be very important for hard coatings. High plasma ionization rate during FAD results in the formation of nanocrystalline coatings structure. As it is mentioned above this could be very beneficial for wear resistant coatings. Cast monolithic TiAl targets were used. These targets were made by GfE Company (Germany) using their arc melting technique in argon with AlyTi ratio 1.0. The coatings were deposited on Sandvik H1P cemented carbide substrate. The chemical composition of the cemented carbides is the following: WC-85.5; TiC-7.5; TaC-1.0; Co-6.0 wt.%. Surface morphology of the coatings was studied by means of SEM, EDS and AFM. The microstructure of the coatings was investigated by transmission electron microscopy (TEM) using JEOL JEM-2010F microscope with an acceleration voltage of 200 kV. Coatings for TEM observations were deposited on a cemented carbide substrate with a thickness 2.5–3 mm. TEM samples were prepared using a focused ion beam (FIB) technique on a JEOL JFIM-2100 system. The samples were thinned to approximately 0.1 mm by Ga ions with the acceleration voltage and current of the Ga ion source of 30 kV and 2.0 mA, respectively. The chemical composition of secondary phases emerging on the tool surface during cutting was studied by means of a secondary ion mass spectroscopy (SIMS). This was carried out with the aid of an ESCALAB MK2 (VG) electron spectrometer equipped with an SQ300 ion analyzer of quadruple type and AG-61 scanning ion gun, which allows the flow of argon primary ions with energy up to 5 keV to be focused on a spot up to 200 mm in diameter on the surface of a sample. The ion etching speed was in the order of 0.2-mL miny1; the analysis was carried out in the static mode. We then studied the average chemical composition of the wear zone of the coating. To do this the argon primary ion beam was directed at the target spot on the surface of the sample. In some cases an ion beam was moved along the direction chosen. Atomic structure of the films forming on the tool surface during cutting was studied by means of EELFS with the aid of an ESCALAB MK2 (VG) electron spectrometer. The friction surface was studied in the zones free of workpiece material sticking. High magni-
fication (2000=) was applied. Primary electron energy was Eps1000 eV. The fine structure of the spectrum energy loss was recorded close to the line of elastically scattered electrons in the range 250 eV. Analysis conditions were chosen in such a way as to ensure the best energy resolution with a good signalynoise ratio. The fundamentals of the method’s details are presented elsewhere w13x. Wear of coatings was studied while turning 1040 steels. The ccutting test conditions are as following: material to be machined—Steel 1040; hardness HB 200; parameters of cutting; speed, 250–450 m miny1; depth, 0.5 mm; feed, 0.11 mm ry1. Wear of tetragonal indexable inserts with TiAlN coatings was studied. It is known that as the flank wear exceeds 0.3 mm, the cutting tool loses serviceability w14x. Eight cutting tests using two tetragonal inserts were performed for each kind of coating. The scatter of the tool life measurements was approximately 10%. The friction parameter at the rake face during cutting was also determined. Cutting forces were measured in-situ with a force dynamometer w15x. The metallography analysis of the chips formed during cutting was made using an SEM. The chip compression ratio and chip shear plane angle were determined using standard methods w15x. The microhardness of TiAlN coatings was measured using Shimadzu nanoindentation tester using Vickers type indentor. The load applied was 200 mN. The thickness of the coating was measured using a ballcratering device with accuracy of approximately 0.1 mm. 3. Results The coating’s characterization data are shown in Table 2. The results presented in Table 2 show that the two coatings under analysis are close to a stoichiometric composition and the AlyTi ratio in regular TiAlN coating is lower (0.88) when compared to FAD (1.0). The major feature of the filtered coatings is the ultrafine grain size. The grain sizes are approximately 60– 80 nm instead of 100–120 nm for commercial TiAlN coatings (Fig. 2). There is almost no significant difference in the microhardness of the coatings under study. Oxidation stability of the coatings is also different. Weight gain of FAD coating was found to be lower than the commercially supplied one (Table 2). Wear resistance of the coatings critically depend on the cutting test conditions used. At moderate cutting speeds (250 mymin) commercial TiAlN coating tool life is superior to FAD (Fig. 3a). But the wear resistance of this coating drastically decreases at cutting speeds above 350 mymin. During high speed machining oxidation tool wear dominates w16,17x and the oxidation stability of the coating determines the cutting tool life. That is why FAD coatings,
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Table 2 TiAlN PVD coatings characterisation data TiAlN PVD
Chemical composition, at. %
Microstructure
Properties
Coatings
Chemical formula (quantitative AES data)
AlyTi ratio
Grain sizes nm
Thickness mm
Uncoated carbide tool Commercial Filtered
–
–
Ti0.25Al0.22N0.53 Ti0.22Al0.22N0.56
0.88 1.0
100–120 60–80
with improved oxidation resistance (Table 2), have much better tool life at high cutting speeds in the range of 450 mymin (Fig. 3a). However, the 1.5 time improvement of the oxidation resistance of the FAD coatings, on its own, cannot explain the tool life increase of four
3.0 2.8
Microhardness GPa
Weight gain mgycm2 oxidation in air at 900 8C during 1 h
15
54.94
33 35
15.75 10.69
times when compared to the commercial coating (Fig. 3a). Additional investigations of the cutting toolyworkpiece interface have been done to further our understanding of this phenomenon.
Fig. 2. TEM images of the cross-section of FAD (a–b) and commercial (c–d) TiAN coatings.
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Fig. 3. Tool life of cutting inserts with commercial and FAD TiAlN coatings (a) vs. speed of cutting (b) length of cut for under high speed machining conditions (cutting speed 450 mymin).
Fig. 4a,b present the SEM images of the worn rake surface of cutting inserts with TiAlN coatings. Formation of the relatively large adhesion zones of the workpiece material on the tool surface takes place in the case of commercial TiAlN coatings (Fig. 4a). The wear behavior of the filtered coatings is different (Fig. 4b). The dotty-like sticking of workpiece materials takes place and numerous small sticking zones with dimension of 10–100 mm are formed. The sticking zones are generating on the asperities that are formed as a result of tool surface grinding. An intensive tribo-oxidation of the cutting tool surface takes place during high-speed machining. Fig. 5 shows the Auger electron spectra for worn tools with (a) commercial and (b) FAD coatings. The oxidation of the rake surface is obvious due to the high amount of oxygen present in both spectra. Intensive iron peaks correspond to the sticking zones of the workpiece material. The line of Ti is significantly diffused (Fig.
5b) for this zone; this is due to the oxidation process. An increased amount of alumina is observed on the spectrum of FAD coatings that is displayed as a shift in the aluminium line to the zone of a lower energy (60 eV). At the same time the intensity of metallic Al LMM line near the 68 eV level decreases. Fig. 6a,b presents a series of positive secondary ion spectra for both commercial and FAD TiAlN coatings. Fig. 6c,d shows a spectra of negative secondary ions for the same coatings. On both positive SIMS spectra the intensity of the TiO line is high; this is due to intensive tribo-oxidation that forms rutile-like films. But some amount of alumina forms only on the surface of FAD coatings (Fig. 6c). This effect can be observed at the spectrum of negative secondary ions (Fig. 6c). Formation of alumina films on the cutting tool surface drastically changes the heat dissipation associated with chip removal. This can be illustrated through the SEM images of the cross-sections of the chips as shown in
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Fig. 4. SEM images of cutting inserts rake surface for (a) the commercial and (b) FAD TiAlN coatings after cutting at 450 mymin; length of cut is 2280 and 8900 m correspondingly (see Fig. 3b).
Fig. 7. In general chips consist of three different zones (Fig. 7) w18x. It is known that dynamic re-crystallization of the chip contact area takes place during cutting (zone 3) w6,14x. More heat flux goes into the chips causing more intensive re-crystallization to occur. This is exhibited in the chip grain coarsening within the contact zone (Fig. 7). Fig. 7a,b show re-crystallization of the chip contact zone for cutting tools with commercial TiAlN coatings, whereas Fig. 7c,d present similar images for tools with FAD coatings. We can see that more intensive
re-crystallization of the contact zone takes place for FAD coatings. When chips slide along the rake face of the cutting tool the curved flow lines are formed due to friction. An extended deformation zone (zone 2) can be observed close to the contact area of the chip (zone 3). Zone 1 is located at the chip outer surface w18x. The chips that are formed with TiAlN commercial coatings show a deformation zone 2, that zone was found to be thicker than the corresponding one with the FAD TiAlN coat-
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Fig. 5. Auger electron spectra of cutting inserts rake surface (wear zone) for (a) the commercial and (b) FAD TiAlN coatings after cutting at 450 mymin; length of cut is 2280 and 8900 m correspondingly (see Fig. 3b).
ings. At the same time the toolyworkpiece interfaces are different. For the case of the commercial coating a severe seizure was observed in the cutting toolychip interface. This corresponds to the morphology of the rake face with the large sticking zones, as presented in Fig. 4. The interface of the chips formed using the FAD
coating technique is very smooth and corresponds to Fig. 4b with small, point-like sticking zones. These data exhibit the reduction of friction for the tool with FAD coatings. The chip characteristics (compression ratio and share angle) as well as the friction parameter on the rake surface of the coated inserts measured during cutting (Table 3) also exhibit a significant improvement in the tribological characteristics for the tools with the FAD TiAlN coatings. Atomic structure of the films that are generating on the surface of the wear zone during tribo-oxidation of the TiAl coatings was compared to the oxide layer that is formed during oxidation of the binary TiAl alloy under equilibrium conditions (isothermal oxidation test, see Tables 4 and 5). Fig. 8 depicts the Fourier transforms as a result of the mathematical proceeding of the fine structure of electron spectra close to the line of elastically scattered electrons. The peaks position at the Fourier transforms corresponds to the interatomic distances for the nearest coordination spheres. The data interpretation was based on the analysis of known crystal characteristics of the oxides forming on the surface. The interpretation of the data obtained is shown in Tables 4 and 5. The positions of the main peaks on the Fourier transforms are in good agreement with the interatomic distances associated with Al2O3 and TiO2 lattices (Fig. 8). The peaks intensity of the Fourier transforms is connected to the amount of inter-atomic bonds of the type for nearest atomic neighbours. The peak’s intensity at the Fourier transform is higher for a bigger coordination number at the sphere of the definite radius and for a higher ordered crystal structure. Major peak positions in Fig. 8a,b are similar due to the formation of titanium and aluminium oxide films in both cases. However, experimental distances in Fig. 8b (Table 5) are closer to the theoretical data when compared to the similar distances in Fig. 8a (Table 4). This means that equilibrium oxides are forming on the surface of binary TiAl alloy during the process of oxidation in air. The oxide films that are forming during tribo-oxidation have atomic structure with higher levels of defects. In this case, the actual interatomic distances differ from the equilibrium theoretical data (Table 4). The peaks inten˚ decrease, this means that sity at the distances above 4 A the degree of order is reduced and amorphization of the oxide film atomic structure is taking place. However, the peaks at the Fourier transforms could be observed ˚ as well as long (above 5–8 for close (below 4–5 A) ˚ interatomic distances. Based on this data we can A) conclude that the surface oxide film has a complicated amorphous–crystalline structure. 4. Discussion Two major improvements in the characteristics of coating surfaces can be attributed to the filtered tech-
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Fig. 6. SIMS spectra of cutting inserts rake surface (wear zone) for the commercial and FAD TiAlN coatings after cutting at 450 mymin. Length of cut is 2280 and 8900 m correspondingly (see Fig. 3b). Positive secondary ion spectra (a–b). Negative secondary ion spectra (c–d).
nique. The first, and most widely known one is a full or partial filtering of the ‘droplet phase’ w9,10,19x. This results in a better surface finish, which has the effect of lowering the adherence of workpiece material to the tool surface. It also improves the friction and wear behavior of the filtered coatings. This is very important for low speed machining conditions, in the domain of build-up formation w14,20x. But for high-speed machining conditions when oxidation wear dominates w16,17x the ability of the coatings to form the protective surface films during friction becomes more important. This ability is improved for TiAlN filtered coatings due to their nano-crystalline structure. Nano-crystallinity of the coating helps to form an alumina protective layer w12x on the surface due to acceleration of aluminium diffusivity during oxidation. A coating with a finer grain structure has more grain boundaries; thus more diffusion paths are available for outward diffusion of Al and inward diffusion of oxygen. This promotes the formation of protective aluminum oxide films w21x and results in an increase in the oxidation resistance of the surface (Table 2) and thus significantly improves tool life during high-speed machining (Fig. 3). From this we can assume that tribo-oxidation is a very important and beneficial process for high speed
machining conditions. Tribo-oxidation of the cutting tools is very far from an equilibrium state in this case. This highlights the peculiarities of this process as compared to regular isothermal oxidation. Tribo-oxidation of the TiAlN coating results in structural adaptation of the surface layers to severe conditions of high-speed machining. Adaptation is a beneficial process based on the self-organizing phenomenon w16,22x that results in cutting tool life improvements. The coatings that have this ability could be described as adaptive and FAD TiAlN coatings exhibit these adaptive characteristics under high-speed machining conditions. The metalbased oxygen-containing compounds that are forming during cutting can act as a shield that protects the tool surface. Based on the data presented in Figs. 3, 5 and 6 we can conclude that the oxide films that are forming on the tool surface with TiAlN coatings are a mixture of alumina and rutile but only alumina layer is protective w23x. Friction control in this case promotes the formation of protective alumina films. As soon as a significant portion of alumina films starts to form, some beneficial phenomena occur at the cutting toolyworkpiece material interface. During high-speed machining, alumina films formed at the surface are limiting the interaction of the underlying coating with the workpiece material. The
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Fig. 7. SEM images of the chips cross-sections for (a–b) commercial and (c–d) FAD TiAlN coatings after cutting at 450 mymin; initial stage of cutting Zone 3-chip contact area; Zone 2-extended deformation zone; Zone 1-chip outer surface.
data presented at Fig. 8 shows that there are two types of the protective aluminium-based oxygen containing films formed at the surface during cutting: amorphouslike and crystalline. Previous detailed studies of protective film formation during cutting at low and moderate cutting speeds show that there is only one type of protective film formed on the surface as a result of the
self-organizing phenomena. These films have amorphous-like structure with high plasticity and improved lubricity w16,24x. During high speed machining more complicated phenomena take place. These are low intensity peaks that are found at remote atomic distances on the Fourier transforms, which are presented in Fig. 8. From this we can assume that the majority of the films
Table 3 TiAlN PVD coatings frictional characteristics TiAlN PVD coatings
Commercial Filtered
Friction characteristics Chip parameters Chip compression ratio
Chip shear angle, 8
1.35 1.19
40.58 44.49
Friction parameter on the cutter rake surface 0.986 0.857
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Table 4 Interpretation of Fourier-transforms for FAD (TiAl)N coating at the wear zone of cutting tool Phase
Bonds
Distances ˚ (Theoretical, A)
Phase
Phase composition
Distances ˚ (Experimental, A)
噛 of line
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
Al–O Al–O O–O O–O O–O Al–Al O–O Al–Al 2=O–O 2=O–O
1.89 1.93 2.57 2.64 3.62 4.25 4.60 5.12 2.64=2s5.28 3.62=2s7.24
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
Al2O3
1.61 1.85 2.45 2.55 3.46 4.25 4.60 5.12 5.30 7.30
1 2 3 4 5 6 7 8 9 10
TiO2 TiO2 TiO2 TiO2
O–O Ti–O Ti–O O–O
3.55 4.37 5.26 6.46
TiO2 TiO2 TiO2 TiO2
TiO2
3.51 4.40 5.15 6.30
11 12 13 14
that are forming during tribo-oxidation under high-speed machining conditions are also amorphous-like. Only a small amount of crystalline films are forming as a result of the thermal activation process w25x. Additionally, crystalline alumina films improve the wear behavior because they have a low thermal conductivity w26x that prevents the intensive heat generating during cutting from conducting into the cutting tool surface. Therefore, a significant part of the heat leaves with the chips (Fig. 7). At the same time alumina as a chemically stable
material prevents intensive interaction at the workpiecey tool interface during cutting and depresses the adherence of the machined material (Fig. 4) to the cutting tool surface. It is worth noting that the adaptation process during high-speed machining exhibits the formation of alumina films with amorphous-like structure in a most clear manner. Spots of work piece material sticking at the toolychip interface could not be observed and a perfect smooth contact zone was formed (Fig. 7b,c and d). This is a result of the chip flow improvement w18x
Fig. 8. Fourier transforms of EELFS close to the line of elastic scattered electrons for the films forming on the surface of: (a) TiAlN FAD coatings after cutting at 450 mymin; (b) binary TiAl alloy after isothermal oxidation in air.
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Table 5 Interpretation of Fourier-transformant of the oxidized surface for the binary TiAl alloy 噛 of line
Phase
Bonds
Distances ˚ (theoretical, A)
Phase
Phase composition
Distances ˚ (experimental, A)
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
Al–O Al–O O–O O–O O–O Al–Al O–O
1.89 1.93 2.57 2.64 3.62 4.25 4.60
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
Al2O3
1.89 1.93 2.57 2.64 3.46 4.25 4.60
1 2 3 4 5 6 7
TiO2 TiO2 TiO2 TiO2 TiO2
Ti–O Ti–O O–O O–O O–O
1.91 1.97 2.46 3.55 4.37
TiO2 TiO2 TiO2 TiO2 TiO2
TiO2
1.97 2.01 2.44 3.54 4.49
8 9 10 11 12
most probably due to the increased lubricity of the amorphous-like alumina films. Ultimately it leads to the significant improvement of frictional characteristics during cutting (Table 5). The friction parameter measured in-situ on the rake surface for FAD coatings is lower (0.857) compared to regular coating (0.986). The characteristics of the chips also exhibit the same trend: chip compression ratio is lower and chip shear angle is higher for FAD coating compared to commercial TiAlN coatings (Table 3). As a result of the favourable change in the friction conditions the wear rate of the process was found to stabilize (Fig. 3b) and thus significantly increase the cutting tool life. The entire diversity of the processes that take place during friction can be divided into two groups: quasiequilibrium steady-state processes, which are encountered during steady state friction and wear, and non-equilibrium non-steady states, which are associated with surface damage processes w27x. A rapid increase in wear rate testifies unsteady wear conditions and surface damage. This is typical for commercial TiAlN coatings under high-speed machining conditions (Fig. 3b). Friction control in this context implies the existence of a stable tribosystem, which resists any instability leading to intensive wear and surface damage w25,27x. The transition from a thermodynamically non-equilibrium condition to a more stable, quasi-equilibrium condition is connected to the accelerated formation of a beneficial surface structure formed as a result of the self-organizing process. From the point of view of self-organization both natural and synthetic processes can be considered during friction. Therefore, it is necessary to try and control (or modify) the synthetic processes to encourage the evolution of those natural processes that lead to a minimum wear rate. The data presented at Fig. 3b prove that FAD TiAlN coating application is one of the effective ways for achieving friction control under severe service conditions of high-speed machining.
As it was mentioned above, the TiAlN family of coatings has a great potential for high-speed machining applications due to its favourable combination of thermal, mechanical properties, its oxidation resistance w16,17x and as outlined in this study due to the adaptability of these coatings. Based on the studies performed, we can outline some general trends associated with the future development of TiAlN coatings for high-speed machining applications. To improve the wear resistance and adaptability of these coatings a formation of the both types of protective alumina films (crystalline triboceramics and amorphous-like) should be promoted. To achieve this goal, the formation of continuous protective alumina films at the surface during high-speed machining should be ensured. However, the formation of the protective amorphous-like alumina films with low friction at high temperatures should also be improved. This could be done by the optimization of the coating’s chemical composition as well as by the transformation of its structure to the nano-scale level in particular due to the application of FAD techniques. 5. Conclusion The main advantage of the FAD technique for highspeed machining applications is a significant grains refinement of TiAlN PVD coatings that leads to the formation of a nano-scaled (grains size approx. 60–80 nm) surface layer. It was shown that the major cause of high wear resistance of the FAD coatings during highspeed machining is the generation of thin protective oxide films on the cutting tool’s surface as a result of the self-organizing phenomena. The FAD TiAlN coating grain size refinement, down to the nano-scale level, promotes the formation of protective aluminum oxide films. Oxide layers that form at the surface of nanocrystalline FAD TiAlN coating during high-speed cutting mainly consist of alumina-like protective thin films.
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Whereas the films formed on the surface of TiAlN commercial coatings with more coarse grains consist only of non-protective titania. The films that are forming on the cutting tool surface as a result of the adaptation process have complex amorphous-crystalline structures. The complex structure of the film has a beneficial impact on the wear behavior of the cutting tools. Adaptive FAD TiAlN PVD coatings with nano-crystalline structure exhibit promising combinations of properties ideally suited for high-speed machining applications. Acknowledgments The authors wish to thank S. Koprich for SEM studies, F. Pearson from McMaster University for TEM sample preparation and for TEM investigations. This research was funded by the Natural Sciences and Engineering Research Council of Canada. References w1x H.K. Tonshoff, ¨ B. Karpuschewski, A. Mohlfeld, H. Seegers, Surf. Coat. Technol. 116–119 (1999) 524. w2x S.K. Wu, H.C. Lin, P.L. Liu, Surf. Coat. Technol. 124 (2000) 97. w3x D.-Y. Wang, Y.-W. Li, W.-Y. Ho, Surf. Coat. Technol. 114 (1999) 109. w4x W.-D.M. Muntz, J.Vac.Sci.Technol. A 4 (6) (1986) 2717. w5x J.H. Woo, J.K. Lee, S.R. Lee, D.B. Lee, Oxid. Met. 53 (5–6) (2000) 529. w6x J. Musil, R. Daniel, Surf. Coat. Technol. 166 (2003) 243. w7x Y. Tanaka, N. Ichmiya, Y. Onishi, Y. Yamada, Surf. Coat. Technol. 146–147 (2001) 215–221. w8x V.I. Gorokhovsky, R. Bhattacharya, D.G. Bhat, Surf. Coat. Technol. 140 (2) (2001) 82.
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w9x G. Barinov, A. Dodonov, V. Ogay, Ion Plating Technology, Machinostroenie, Moscow, 1987, p. 34. w10x P. Martin, R. Netterfield, A. Bendavid, T. Kinder, Annual Technical Conference Proceeding of the Society for Vacuum Coaters, Society of Vacuum Coaters, Boston, 1993, p. 375. w11x I. Konyashin, G.S. Fox-Rabinovich, J. Mater. Sci. 32 (1997) 6029–6036. w12x J. Musil, Surf. Coat. Technol. 125 (2000) 322. w13x D.L. Wainstein, A.I. Kovalev, Surf. Interface Anal. 34 (2002) 230. w14x E.M. Trent, P.K. Wright, Metal Cutting, 4 ed, Butterworth, 2000, p. 211. w15x M.C. Shaw, Metal Cutting Principles, Oxford University Press, New York, 1996, p. 594. w16x G.S. Fox-Rabinovich, A.I. Kovalev, G.C. Weatherly, in: G. Totten, L. Xie, K. Funatani (Eds.), Modeling and Simulation for Materials Selection and Mechanical Design, 1, Marcel Dekker, New York, 2002, p. 125, Chapter 3. w17x K Holmberg, A Matthews, Coating Tribology: Principles, Techniques, and Application in Surface Engineering, Elsevier Science B.V, Amsterdam, The Netherlands, 1994, p. 257. w18x B. Denkena, H.K. Tonshoff, T. Friemuth, C. Gey, C. Podolsky ICMCTF 2002, Book of Proceeding of (in press). w19x S.G. Harris, E.D. Doyle, A.C. Vlasveld, P.G. Dolder, Surf. Coat. Technol. 146–147 (2001) 305. w20x G.S. Fox-Rabinovich, Scientific principles of material choice for wear-resistant cutting tools and dies from the point of view of surface’s structure optimization. DSc Thesis, All-Russian Railway Transport Research Institute, Moscow, Russia, 1993. w21x B.A. Pint, J. Leibowitz, J.K DeVan, Oxid. Met. 51 (1999) 45. w22x I. Pigogine, From Being to Becoming, W.H. Freeman and Company, San Francisco, 1980, pp. 84–87. w23x H. Schulz, J. Dorr, I.J. Rass, M. Schulze, T. Leyenecker, G. Erkens, Surf. Coat. Technol. 146–147 (2001) 480. w24x A.I. Kovalev, D.L. Wainstein, V.P. Mishina, G.S. Fox-Rabinovich, J. Electron Spectrosc. Relat. Phenom. 105 (1999) 63. w25x B.I. Kostetsky, J. Friction Wear VI (2) (1985) 201. w26x H. Schulz, J. Dorr, I.J. Rass, M. Schulze, T. Leyenecker, G. Erkens, Surf. Coat. Technol. 146–147 (2001) 480. w27x V.S. Ivanova, N.A. Bushe, I.S. Gershman, J. Friction Wear 18 (1) (1997) 74.
Nano-crystalline filtered arc deposited (FAD) TiAlN PVD coatings for high-speed machining applications G.S. Fox-Rabinovicha,*, G.C. Weatherlya, A.I. Dodonovb, A.I. Kovalevc, L.S. Shusterd, S.C. Veldhuisa, G.K. Dosbaevaa, D.L. Wainsteinc, M.S. Migranovd a
McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S4L7 Vacuum Ion Technology Company (VIT), 206, IEC VNIIETO, Pochtovaya Street, Istra-2, Moscow region 143500, Russia c Surface Phenomena Researches Group (SPRG), 2nd Baumanskaya str., 9y23,CNIICHERMET, Moscow 107005, Russia d Ufa Avia Institute (UAI), Ufa 450083, Russia
b
Abstract The main advantage of the filtered arc deposition (FAD) technique is a significant grain refinement that leads to the formation of nano-crystalline (grain size approx. 60–80 nm) PVD coatings. This technique improves the wear resistance of FAD TiAlN coatings under high-speed machining conditions when cutting tool oxidation wear is dominant. A study of the surface structure characteristics of the FAD TiAlN coatings using SEM, EDS, TEM, AES, SIMS and EELFS was performed. The microhardness of the coatings were measured. The microstructure of the chips was analyzed. The compression of the chips as well as shear angle was measured. The friction parameter on the rake surface of the tools was determined in-situ. It was shown that the major cause of the high wear resistance of the FAD coatings during high-speed machining was the formation of the thin protective oxide films on the cutting tool surface. The grain size refinement of the coating promotes the formation of protective alumina films. These films are formed at the surface of nano-crystalline FAD TiAlN coating during high-speed machining. They mainly consist of protective alumina, whereas the films that are forming on the surface of TiAlN commercial coatings with coarser grains consist only of non-protective titania. The formation of the protective alumina films, composed of complicated amorphous– crystalline structures, significantly improves the friction and wear performance of coated tools. The tendency of the work piece material to adhere is reduced and considerably more heat is dissipated via chip removal. This is a major cause of the enhanced wear resistance of the FAD coating technique. Tribo-oxidation during cutting is a typical process of the self-organization of TiAlN coatings that results in a quasi-stabilization of cutting tool wear. The self-organization and friction control for coated cutting tools is considered based on a non-equilibrium thermodynamic approach. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Filtered arc deposited TiAlN PVD coatings; Nano-crystallline structure; Self-organization; High-speed machining
1. Introduction Recent improvements in the life of cutting tools have been achieved by the development of titanium aluminium nitride (Ti,Al)N coatings. Films such as TiAlN w1,2x display a unique combination of properties, viz. a high hardness at elevated temperature together with thermal and chemical stability, as well as low thermal conductivity. An extremely important advantage of (Ti,Al)N coatings is their high oxidation stability w3x, which is *Corresponding author. Tel.: q1-905-525-9140x24980; fax: q1905-521-2773. E-mail address: [email protected] (G.S. Fox-Rabinovich).
due to the formation of relatively stable surface oxide films w4,5x. Furthermore, improvement of the wear resistance of TiAlN coatings during cutting could be achieved through grain size refinement down to the nanoscale (grain size less than 100 nm) level w6x. This could be done by alloying of TiAlN coatings w7x, as well as through advanced deposition techniques (such as FAD) w8–11x. Due to the high plasma ionization rate in the chamber of the coater unit and the relatively low deposition rate, the current temperature at the crystallization front of FAD coatings is low. The coating deposition conditions are close to nano-scale heating w12x, when the kinetic energy of bombarding ions is transferred into very small areas of actively growing thin coating layers, which are
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.05.004
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2. Experimental Two types of TiAlN coatings were studied: 1. Commercial monolayered TiAlN PVD coating. 2. Advanced FAD TiAlN PVD coating.
Fig. 1. Scheme of the filtered arc-evaporation system.
then very quickly conveyed into the samples vicinity. Thus, the cooling rate, during a strongly non-equilibrium FAD process, is very high. This leads to the formation of nano-crystalline films w12x. This results in the formation of nano-crystalline coatings and could be very beneficial for wear resistant coatings especially well suited for high-speed machining applications. Significant improvements of the TiAlN PVD coatings for high-speed machining applications, under conditions when the oxidation wear prevails, could be achieved based on a better understanding of the mechanism of the tribo-oxidation of the coating during cutting. The generation of a dynamically stable and continuous film of alumina on the surface with high tribological properties is the objective for the new coating development. The goal of this work is to study the friction and wear behavior of the cutting tools with TiAlN PVD coatings and determine the impact of the FAD coatings nanocrystalline structure on the cutting tools performance under high-speed machining conditions.
The advanced coatings under study were deposited using a FAD PVD unit of NNV-6.6-I1 type. These types of units have up to three removable targets, also referred to as FAD modules. A sketch of the FAD module is shown in Fig. 1. Parameters of the PVD coating deposition process are shown in Table 1. Temperatures on the surface of the sample were measured using an optical pyrometer. Heating the substrate up to the deposition temperature was performed by Argon ions during FAD at the substrate biasing voltage of 1 kV. After obtaining the required substrate temperature, the substrate biasing voltage value was reduced and nitrogen was fed into the chamber for deposition of the TiAlN films. The system was shown to affect the physical-chemistry of the plasma-chemical processes significantly when depositing refractory compounds. This is due to an increase in the ionization rates of both metals and reactive gases w9,10x. The system included a hollow steel tube approximately 300 mm in diameter, which was a quarter segment of a torus (Fig. 1). The tube was installed on the body of a standard unit of the arcevaporation PVD instead of on a conventional arcevaporator. An induction coil was placed inside the tube to create the magnetic field needed to control the plasma path. Regulation of the plasma flow in the FAD System is based on the principles of plasma-optics. The influence of the magnetic field upon the plasma flow allows the radial electric field to appear; as a result, regulation of the plasma path inside the internal volume of the FAD system can be achieved. Whereas only electrically charged particles are focused in the FAD system, uncharged particles (so called ‘droplet’ phase) are not affected by the magnetic and electric fields. The uncharged particles flying from the cathode surface parallel to the cathode axis do not reach the substrate surface and are thus deposited on the inside of the tube. However, the ions follow the bend and are focused on the substrate surface. The value of the continuous current running through the magnetic coil affects the distribution of the ion flow density. The body of the FAD system is
Table 1 Parameters of the FAD TiAlN PVD coating deposition 噛
Target
Arc current, A
Nitrogen pressure, Torr
Time of deposition, min
Coil current, A
Separator current, A
Separator voltage, V
Temperature of deposition, 8C
1
TiAl alloy (cast monolithic)
200
;5=10y3
30
20
20
16
340–370
802
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biased with regard to the PVD unit’s body. This is required if the substrate biasing is not enough to achieve a suitable value for the deposition rate. The FAD system allows the substrate to be heated up to the deposition temperature by use of Argon ions, which are rarely achieved when applying conventional arc evaporation PVD technology. This was shown to occur due to the high ionization rate of the gases obtained when using the FAD system with elevated energy w9x. The filtering efficiency of the FAD technology for ‘droplet’ phase deposition prevention is well known w11x. But there is another advantage of the FAD technique that could be very important for hard coatings. High plasma ionization rate during FAD results in the formation of nanocrystalline coatings structure. As it is mentioned above this could be very beneficial for wear resistant coatings. Cast monolithic TiAl targets were used. These targets were made by GfE Company (Germany) using their arc melting technique in argon with AlyTi ratio 1.0. The coatings were deposited on Sandvik H1P cemented carbide substrate. The chemical composition of the cemented carbides is the following: WC-85.5; TiC-7.5; TaC-1.0; Co-6.0 wt.%. Surface morphology of the coatings was studied by means of SEM, EDS and AFM. The microstructure of the coatings was investigated by transmission electron microscopy (TEM) using JEOL JEM-2010F microscope with an acceleration voltage of 200 kV. Coatings for TEM observations were deposited on a cemented carbide substrate with a thickness 2.5–3 mm. TEM samples were prepared using a focused ion beam (FIB) technique on a JEOL JFIM-2100 system. The samples were thinned to approximately 0.1 mm by Ga ions with the acceleration voltage and current of the Ga ion source of 30 kV and 2.0 mA, respectively. The chemical composition of secondary phases emerging on the tool surface during cutting was studied by means of a secondary ion mass spectroscopy (SIMS). This was carried out with the aid of an ESCALAB MK2 (VG) electron spectrometer equipped with an SQ300 ion analyzer of quadruple type and AG-61 scanning ion gun, which allows the flow of argon primary ions with energy up to 5 keV to be focused on a spot up to 200 mm in diameter on the surface of a sample. The ion etching speed was in the order of 0.2-mL miny1; the analysis was carried out in the static mode. We then studied the average chemical composition of the wear zone of the coating. To do this the argon primary ion beam was directed at the target spot on the surface of the sample. In some cases an ion beam was moved along the direction chosen. Atomic structure of the films forming on the tool surface during cutting was studied by means of EELFS with the aid of an ESCALAB MK2 (VG) electron spectrometer. The friction surface was studied in the zones free of workpiece material sticking. High magni-
fication (2000=) was applied. Primary electron energy was Eps1000 eV. The fine structure of the spectrum energy loss was recorded close to the line of elastically scattered electrons in the range 250 eV. Analysis conditions were chosen in such a way as to ensure the best energy resolution with a good signalynoise ratio. The fundamentals of the method’s details are presented elsewhere w13x. Wear of coatings was studied while turning 1040 steels. The ccutting test conditions are as following: material to be machined—Steel 1040; hardness HB 200; parameters of cutting; speed, 250–450 m miny1; depth, 0.5 mm; feed, 0.11 mm ry1. Wear of tetragonal indexable inserts with TiAlN coatings was studied. It is known that as the flank wear exceeds 0.3 mm, the cutting tool loses serviceability w14x. Eight cutting tests using two tetragonal inserts were performed for each kind of coating. The scatter of the tool life measurements was approximately 10%. The friction parameter at the rake face during cutting was also determined. Cutting forces were measured in-situ with a force dynamometer w15x. The metallography analysis of the chips formed during cutting was made using an SEM. The chip compression ratio and chip shear plane angle were determined using standard methods w15x. The microhardness of TiAlN coatings was measured using Shimadzu nanoindentation tester using Vickers type indentor. The load applied was 200 mN. The thickness of the coating was measured using a ballcratering device with accuracy of approximately 0.1 mm. 3. Results The coating’s characterization data are shown in Table 2. The results presented in Table 2 show that the two coatings under analysis are close to a stoichiometric composition and the AlyTi ratio in regular TiAlN coating is lower (0.88) when compared to FAD (1.0). The major feature of the filtered coatings is the ultrafine grain size. The grain sizes are approximately 60– 80 nm instead of 100–120 nm for commercial TiAlN coatings (Fig. 2). There is almost no significant difference in the microhardness of the coatings under study. Oxidation stability of the coatings is also different. Weight gain of FAD coating was found to be lower than the commercially supplied one (Table 2). Wear resistance of the coatings critically depend on the cutting test conditions used. At moderate cutting speeds (250 mymin) commercial TiAlN coating tool life is superior to FAD (Fig. 3a). But the wear resistance of this coating drastically decreases at cutting speeds above 350 mymin. During high speed machining oxidation tool wear dominates w16,17x and the oxidation stability of the coating determines the cutting tool life. That is why FAD coatings,
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Table 2 TiAlN PVD coatings characterisation data TiAlN PVD
Chemical composition, at. %
Microstructure
Properties
Coatings
Chemical formula (quantitative AES data)
AlyTi ratio
Grain sizes nm
Thickness mm
Uncoated carbide tool Commercial Filtered
–
–
Ti0.25Al0.22N0.53 Ti0.22Al0.22N0.56
0.88 1.0
100–120 60–80
with improved oxidation resistance (Table 2), have much better tool life at high cutting speeds in the range of 450 mymin (Fig. 3a). However, the 1.5 time improvement of the oxidation resistance of the FAD coatings, on its own, cannot explain the tool life increase of four
3.0 2.8
Microhardness GPa
Weight gain mgycm2 oxidation in air at 900 8C during 1 h
15
54.94
33 35
15.75 10.69
times when compared to the commercial coating (Fig. 3a). Additional investigations of the cutting toolyworkpiece interface have been done to further our understanding of this phenomenon.
Fig. 2. TEM images of the cross-section of FAD (a–b) and commercial (c–d) TiAN coatings.
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Fig. 3. Tool life of cutting inserts with commercial and FAD TiAlN coatings (a) vs. speed of cutting (b) length of cut for under high speed machining conditions (cutting speed 450 mymin).
Fig. 4a,b present the SEM images of the worn rake surface of cutting inserts with TiAlN coatings. Formation of the relatively large adhesion zones of the workpiece material on the tool surface takes place in the case of commercial TiAlN coatings (Fig. 4a). The wear behavior of the filtered coatings is different (Fig. 4b). The dotty-like sticking of workpiece materials takes place and numerous small sticking zones with dimension of 10–100 mm are formed. The sticking zones are generating on the asperities that are formed as a result of tool surface grinding. An intensive tribo-oxidation of the cutting tool surface takes place during high-speed machining. Fig. 5 shows the Auger electron spectra for worn tools with (a) commercial and (b) FAD coatings. The oxidation of the rake surface is obvious due to the high amount of oxygen present in both spectra. Intensive iron peaks correspond to the sticking zones of the workpiece material. The line of Ti is significantly diffused (Fig.
5b) for this zone; this is due to the oxidation process. An increased amount of alumina is observed on the spectrum of FAD coatings that is displayed as a shift in the aluminium line to the zone of a lower energy (60 eV). At the same time the intensity of metallic Al LMM line near the 68 eV level decreases. Fig. 6a,b presents a series of positive secondary ion spectra for both commercial and FAD TiAlN coatings. Fig. 6c,d shows a spectra of negative secondary ions for the same coatings. On both positive SIMS spectra the intensity of the TiO line is high; this is due to intensive tribo-oxidation that forms rutile-like films. But some amount of alumina forms only on the surface of FAD coatings (Fig. 6c). This effect can be observed at the spectrum of negative secondary ions (Fig. 6c). Formation of alumina films on the cutting tool surface drastically changes the heat dissipation associated with chip removal. This can be illustrated through the SEM images of the cross-sections of the chips as shown in
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Fig. 4. SEM images of cutting inserts rake surface for (a) the commercial and (b) FAD TiAlN coatings after cutting at 450 mymin; length of cut is 2280 and 8900 m correspondingly (see Fig. 3b).
Fig. 7. In general chips consist of three different zones (Fig. 7) w18x. It is known that dynamic re-crystallization of the chip contact area takes place during cutting (zone 3) w6,14x. More heat flux goes into the chips causing more intensive re-crystallization to occur. This is exhibited in the chip grain coarsening within the contact zone (Fig. 7). Fig. 7a,b show re-crystallization of the chip contact zone for cutting tools with commercial TiAlN coatings, whereas Fig. 7c,d present similar images for tools with FAD coatings. We can see that more intensive
re-crystallization of the contact zone takes place for FAD coatings. When chips slide along the rake face of the cutting tool the curved flow lines are formed due to friction. An extended deformation zone (zone 2) can be observed close to the contact area of the chip (zone 3). Zone 1 is located at the chip outer surface w18x. The chips that are formed with TiAlN commercial coatings show a deformation zone 2, that zone was found to be thicker than the corresponding one with the FAD TiAlN coat-
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Fig. 5. Auger electron spectra of cutting inserts rake surface (wear zone) for (a) the commercial and (b) FAD TiAlN coatings after cutting at 450 mymin; length of cut is 2280 and 8900 m correspondingly (see Fig. 3b).
ings. At the same time the toolyworkpiece interfaces are different. For the case of the commercial coating a severe seizure was observed in the cutting toolychip interface. This corresponds to the morphology of the rake face with the large sticking zones, as presented in Fig. 4. The interface of the chips formed using the FAD
coating technique is very smooth and corresponds to Fig. 4b with small, point-like sticking zones. These data exhibit the reduction of friction for the tool with FAD coatings. The chip characteristics (compression ratio and share angle) as well as the friction parameter on the rake surface of the coated inserts measured during cutting (Table 3) also exhibit a significant improvement in the tribological characteristics for the tools with the FAD TiAlN coatings. Atomic structure of the films that are generating on the surface of the wear zone during tribo-oxidation of the TiAl coatings was compared to the oxide layer that is formed during oxidation of the binary TiAl alloy under equilibrium conditions (isothermal oxidation test, see Tables 4 and 5). Fig. 8 depicts the Fourier transforms as a result of the mathematical proceeding of the fine structure of electron spectra close to the line of elastically scattered electrons. The peaks position at the Fourier transforms corresponds to the interatomic distances for the nearest coordination spheres. The data interpretation was based on the analysis of known crystal characteristics of the oxides forming on the surface. The interpretation of the data obtained is shown in Tables 4 and 5. The positions of the main peaks on the Fourier transforms are in good agreement with the interatomic distances associated with Al2O3 and TiO2 lattices (Fig. 8). The peaks intensity of the Fourier transforms is connected to the amount of inter-atomic bonds of the type for nearest atomic neighbours. The peak’s intensity at the Fourier transform is higher for a bigger coordination number at the sphere of the definite radius and for a higher ordered crystal structure. Major peak positions in Fig. 8a,b are similar due to the formation of titanium and aluminium oxide films in both cases. However, experimental distances in Fig. 8b (Table 5) are closer to the theoretical data when compared to the similar distances in Fig. 8a (Table 4). This means that equilibrium oxides are forming on the surface of binary TiAl alloy during the process of oxidation in air. The oxide films that are forming during tribo-oxidation have atomic structure with higher levels of defects. In this case, the actual interatomic distances differ from the equilibrium theoretical data (Table 4). The peaks inten˚ decrease, this means that sity at the distances above 4 A the degree of order is reduced and amorphization of the oxide film atomic structure is taking place. However, the peaks at the Fourier transforms could be observed ˚ as well as long (above 5–8 for close (below 4–5 A) ˚ interatomic distances. Based on this data we can A) conclude that the surface oxide film has a complicated amorphous–crystalline structure. 4. Discussion Two major improvements in the characteristics of coating surfaces can be attributed to the filtered tech-
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Fig. 6. SIMS spectra of cutting inserts rake surface (wear zone) for the commercial and FAD TiAlN coatings after cutting at 450 mymin. Length of cut is 2280 and 8900 m correspondingly (see Fig. 3b). Positive secondary ion spectra (a–b). Negative secondary ion spectra (c–d).
nique. The first, and most widely known one is a full or partial filtering of the ‘droplet phase’ w9,10,19x. This results in a better surface finish, which has the effect of lowering the adherence of workpiece material to the tool surface. It also improves the friction and wear behavior of the filtered coatings. This is very important for low speed machining conditions, in the domain of build-up formation w14,20x. But for high-speed machining conditions when oxidation wear dominates w16,17x the ability of the coatings to form the protective surface films during friction becomes more important. This ability is improved for TiAlN filtered coatings due to their nano-crystalline structure. Nano-crystallinity of the coating helps to form an alumina protective layer w12x on the surface due to acceleration of aluminium diffusivity during oxidation. A coating with a finer grain structure has more grain boundaries; thus more diffusion paths are available for outward diffusion of Al and inward diffusion of oxygen. This promotes the formation of protective aluminum oxide films w21x and results in an increase in the oxidation resistance of the surface (Table 2) and thus significantly improves tool life during high-speed machining (Fig. 3). From this we can assume that tribo-oxidation is a very important and beneficial process for high speed
machining conditions. Tribo-oxidation of the cutting tools is very far from an equilibrium state in this case. This highlights the peculiarities of this process as compared to regular isothermal oxidation. Tribo-oxidation of the TiAlN coating results in structural adaptation of the surface layers to severe conditions of high-speed machining. Adaptation is a beneficial process based on the self-organizing phenomenon w16,22x that results in cutting tool life improvements. The coatings that have this ability could be described as adaptive and FAD TiAlN coatings exhibit these adaptive characteristics under high-speed machining conditions. The metalbased oxygen-containing compounds that are forming during cutting can act as a shield that protects the tool surface. Based on the data presented in Figs. 3, 5 and 6 we can conclude that the oxide films that are forming on the tool surface with TiAlN coatings are a mixture of alumina and rutile but only alumina layer is protective w23x. Friction control in this case promotes the formation of protective alumina films. As soon as a significant portion of alumina films starts to form, some beneficial phenomena occur at the cutting toolyworkpiece material interface. During high-speed machining, alumina films formed at the surface are limiting the interaction of the underlying coating with the workpiece material. The
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Fig. 7. SEM images of the chips cross-sections for (a–b) commercial and (c–d) FAD TiAlN coatings after cutting at 450 mymin; initial stage of cutting Zone 3-chip contact area; Zone 2-extended deformation zone; Zone 1-chip outer surface.
data presented at Fig. 8 shows that there are two types of the protective aluminium-based oxygen containing films formed at the surface during cutting: amorphouslike and crystalline. Previous detailed studies of protective film formation during cutting at low and moderate cutting speeds show that there is only one type of protective film formed on the surface as a result of the
self-organizing phenomena. These films have amorphous-like structure with high plasticity and improved lubricity w16,24x. During high speed machining more complicated phenomena take place. These are low intensity peaks that are found at remote atomic distances on the Fourier transforms, which are presented in Fig. 8. From this we can assume that the majority of the films
Table 3 TiAlN PVD coatings frictional characteristics TiAlN PVD coatings
Commercial Filtered
Friction characteristics Chip parameters Chip compression ratio
Chip shear angle, 8
1.35 1.19
40.58 44.49
Friction parameter on the cutter rake surface 0.986 0.857
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Table 4 Interpretation of Fourier-transforms for FAD (TiAl)N coating at the wear zone of cutting tool Phase
Bonds
Distances ˚ (Theoretical, A)
Phase
Phase composition
Distances ˚ (Experimental, A)
噛 of line
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
Al–O Al–O O–O O–O O–O Al–Al O–O Al–Al 2=O–O 2=O–O
1.89 1.93 2.57 2.64 3.62 4.25 4.60 5.12 2.64=2s5.28 3.62=2s7.24
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
Al2O3
1.61 1.85 2.45 2.55 3.46 4.25 4.60 5.12 5.30 7.30
1 2 3 4 5 6 7 8 9 10
TiO2 TiO2 TiO2 TiO2
O–O Ti–O Ti–O O–O
3.55 4.37 5.26 6.46
TiO2 TiO2 TiO2 TiO2
TiO2
3.51 4.40 5.15 6.30
11 12 13 14
that are forming during tribo-oxidation under high-speed machining conditions are also amorphous-like. Only a small amount of crystalline films are forming as a result of the thermal activation process w25x. Additionally, crystalline alumina films improve the wear behavior because they have a low thermal conductivity w26x that prevents the intensive heat generating during cutting from conducting into the cutting tool surface. Therefore, a significant part of the heat leaves with the chips (Fig. 7). At the same time alumina as a chemically stable
material prevents intensive interaction at the workpiecey tool interface during cutting and depresses the adherence of the machined material (Fig. 4) to the cutting tool surface. It is worth noting that the adaptation process during high-speed machining exhibits the formation of alumina films with amorphous-like structure in a most clear manner. Spots of work piece material sticking at the toolychip interface could not be observed and a perfect smooth contact zone was formed (Fig. 7b,c and d). This is a result of the chip flow improvement w18x
Fig. 8. Fourier transforms of EELFS close to the line of elastic scattered electrons for the films forming on the surface of: (a) TiAlN FAD coatings after cutting at 450 mymin; (b) binary TiAl alloy after isothermal oxidation in air.
G.S. Fox-Rabinovich et al. / Surface and Coatings Technology 177 – 178 (2004) 800–811
810
Table 5 Interpretation of Fourier-transformant of the oxidized surface for the binary TiAl alloy 噛 of line
Phase
Bonds
Distances ˚ (theoretical, A)
Phase
Phase composition
Distances ˚ (experimental, A)
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
Al–O Al–O O–O O–O O–O Al–Al O–O
1.89 1.93 2.57 2.64 3.62 4.25 4.60
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
Al2O3
1.89 1.93 2.57 2.64 3.46 4.25 4.60
1 2 3 4 5 6 7
TiO2 TiO2 TiO2 TiO2 TiO2
Ti–O Ti–O O–O O–O O–O
1.91 1.97 2.46 3.55 4.37
TiO2 TiO2 TiO2 TiO2 TiO2
TiO2
1.97 2.01 2.44 3.54 4.49
8 9 10 11 12
most probably due to the increased lubricity of the amorphous-like alumina films. Ultimately it leads to the significant improvement of frictional characteristics during cutting (Table 5). The friction parameter measured in-situ on the rake surface for FAD coatings is lower (0.857) compared to regular coating (0.986). The characteristics of the chips also exhibit the same trend: chip compression ratio is lower and chip shear angle is higher for FAD coating compared to commercial TiAlN coatings (Table 3). As a result of the favourable change in the friction conditions the wear rate of the process was found to stabilize (Fig. 3b) and thus significantly increase the cutting tool life. The entire diversity of the processes that take place during friction can be divided into two groups: quasiequilibrium steady-state processes, which are encountered during steady state friction and wear, and non-equilibrium non-steady states, which are associated with surface damage processes w27x. A rapid increase in wear rate testifies unsteady wear conditions and surface damage. This is typical for commercial TiAlN coatings under high-speed machining conditions (Fig. 3b). Friction control in this context implies the existence of a stable tribosystem, which resists any instability leading to intensive wear and surface damage w25,27x. The transition from a thermodynamically non-equilibrium condition to a more stable, quasi-equilibrium condition is connected to the accelerated formation of a beneficial surface structure formed as a result of the self-organizing process. From the point of view of self-organization both natural and synthetic processes can be considered during friction. Therefore, it is necessary to try and control (or modify) the synthetic processes to encourage the evolution of those natural processes that lead to a minimum wear rate. The data presented at Fig. 3b prove that FAD TiAlN coating application is one of the effective ways for achieving friction control under severe service conditions of high-speed machining.
As it was mentioned above, the TiAlN family of coatings has a great potential for high-speed machining applications due to its favourable combination of thermal, mechanical properties, its oxidation resistance w16,17x and as outlined in this study due to the adaptability of these coatings. Based on the studies performed, we can outline some general trends associated with the future development of TiAlN coatings for high-speed machining applications. To improve the wear resistance and adaptability of these coatings a formation of the both types of protective alumina films (crystalline triboceramics and amorphous-like) should be promoted. To achieve this goal, the formation of continuous protective alumina films at the surface during high-speed machining should be ensured. However, the formation of the protective amorphous-like alumina films with low friction at high temperatures should also be improved. This could be done by the optimization of the coating’s chemical composition as well as by the transformation of its structure to the nano-scale level in particular due to the application of FAD techniques. 5. Conclusion The main advantage of the FAD technique for highspeed machining applications is a significant grains refinement of TiAlN PVD coatings that leads to the formation of a nano-scaled (grains size approx. 60–80 nm) surface layer. It was shown that the major cause of high wear resistance of the FAD coatings during highspeed machining is the generation of thin protective oxide films on the cutting tool’s surface as a result of the self-organizing phenomena. The FAD TiAlN coating grain size refinement, down to the nano-scale level, promotes the formation of protective aluminum oxide films. Oxide layers that form at the surface of nanocrystalline FAD TiAlN coating during high-speed cutting mainly consist of alumina-like protective thin films.
G.S. Fox-Rabinovich et al. / Surface and Coatings Technology 177 – 178 (2004) 800–811
Whereas the films formed on the surface of TiAlN commercial coatings with more coarse grains consist only of non-protective titania. The films that are forming on the cutting tool surface as a result of the adaptation process have complex amorphous-crystalline structures. The complex structure of the film has a beneficial impact on the wear behavior of the cutting tools. Adaptive FAD TiAlN PVD coatings with nano-crystalline structure exhibit promising combinations of properties ideally suited for high-speed machining applications. Acknowledgments The authors wish to thank S. Koprich for SEM studies, F. Pearson from McMaster University for TEM sample preparation and for TEM investigations. This research was funded by the Natural Sciences and Engineering Research Council of Canada. References w1x H.K. Tonshoff, ¨ B. Karpuschewski, A. Mohlfeld, H. Seegers, Surf. Coat. Technol. 116–119 (1999) 524. w2x S.K. Wu, H.C. Lin, P.L. Liu, Surf. Coat. Technol. 124 (2000) 97. w3x D.-Y. Wang, Y.-W. Li, W.-Y. Ho, Surf. Coat. Technol. 114 (1999) 109. w4x W.-D.M. Muntz, J.Vac.Sci.Technol. A 4 (6) (1986) 2717. w5x J.H. Woo, J.K. Lee, S.R. Lee, D.B. Lee, Oxid. Met. 53 (5–6) (2000) 529. w6x J. Musil, R. Daniel, Surf. Coat. Technol. 166 (2003) 243. w7x Y. Tanaka, N. Ichmiya, Y. Onishi, Y. Yamada, Surf. Coat. Technol. 146–147 (2001) 215–221. w8x V.I. Gorokhovsky, R. Bhattacharya, D.G. Bhat, Surf. Coat. Technol. 140 (2) (2001) 82.
811
w9x G. Barinov, A. Dodonov, V. Ogay, Ion Plating Technology, Machinostroenie, Moscow, 1987, p. 34. w10x P. Martin, R. Netterfield, A. Bendavid, T. Kinder, Annual Technical Conference Proceeding of the Society for Vacuum Coaters, Society of Vacuum Coaters, Boston, 1993, p. 375. w11x I. Konyashin, G.S. Fox-Rabinovich, J. Mater. Sci. 32 (1997) 6029–6036. w12x J. Musil, Surf. Coat. Technol. 125 (2000) 322. w13x D.L. Wainstein, A.I. Kovalev, Surf. Interface Anal. 34 (2002) 230. w14x E.M. Trent, P.K. Wright, Metal Cutting, 4 ed, Butterworth, 2000, p. 211. w15x M.C. Shaw, Metal Cutting Principles, Oxford University Press, New York, 1996, p. 594. w16x G.S. Fox-Rabinovich, A.I. Kovalev, G.C. Weatherly, in: G. Totten, L. Xie, K. Funatani (Eds.), Modeling and Simulation for Materials Selection and Mechanical Design, 1, Marcel Dekker, New York, 2002, p. 125, Chapter 3. w17x K Holmberg, A Matthews, Coating Tribology: Principles, Techniques, and Application in Surface Engineering, Elsevier Science B.V, Amsterdam, The Netherlands, 1994, p. 257. w18x B. Denkena, H.K. Tonshoff, T. Friemuth, C. Gey, C. Podolsky ICMCTF 2002, Book of Proceeding of (in press). w19x S.G. Harris, E.D. Doyle, A.C. Vlasveld, P.G. Dolder, Surf. Coat. Technol. 146–147 (2001) 305. w20x G.S. Fox-Rabinovich, Scientific principles of material choice for wear-resistant cutting tools and dies from the point of view of surface’s structure optimization. DSc Thesis, All-Russian Railway Transport Research Institute, Moscow, Russia, 1993. w21x B.A. Pint, J. Leibowitz, J.K DeVan, Oxid. Met. 51 (1999) 45. w22x I. Pigogine, From Being to Becoming, W.H. Freeman and Company, San Francisco, 1980, pp. 84–87. w23x H. Schulz, J. Dorr, I.J. Rass, M. Schulze, T. Leyenecker, G. Erkens, Surf. Coat. Technol. 146–147 (2001) 480. w24x A.I. Kovalev, D.L. Wainstein, V.P. Mishina, G.S. Fox-Rabinovich, J. Electron Spectrosc. Relat. Phenom. 105 (1999) 63. w25x B.I. Kostetsky, J. Friction Wear VI (2) (1985) 201. w26x H. Schulz, J. Dorr, I.J. Rass, M. Schulze, T. Leyenecker, G. Erkens, Surf. Coat. Technol. 146–147 (2001) 480. w27x V.S. Ivanova, N.A. Bushe, I.S. Gershman, J. Friction Wear 18 (1) (1997) 74.