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Wear characteristics of TiC, Ti(C,N), TiN and Al2O3 coatings in the turning of conventional and Ca-treated steels
IntemaUonalJournalof
REFRACTORYMETALS & HARDMATERIALS ELSEVIER
International Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
Wear characteristics of TiC, Ti(C,N), TiN and A1203 coatings in the turning of conventional and Ca-treated steels S. Ruppi a*, B. H6grelius a, M. Huhtiranta b "Seco ToolsAB, SE-737 82 Fagersta, Sweden bImatra Steel Oy Ab, FIN-55100 Imatm, Fmland
Recewed 9 March 1998; accepted 14 July 1998
Abstract
The wear characteristics of single layers of TIC, Ti(C,N), TiN and A1203 were investigated during turning of conventional and Ca-treated quenched and tempered Al-killed steels. The experimental coatings were deposited using chemical vapour deposition (CVD) or moderate temperature CVD (MTCVD) on cemented carbide substrates of a single composition and the coatings were of simdar thicknesses (7_+ 1/~m). The wear mechamsms and layer formation were studied using scannmg electron microscopy, optical microscopy and X-ray diffraction. Inclusion modification appeared to be an effective means of enhancing machmability and all experimental coatings exhibited about 20% better performance as a result of Ca-treatment. In particular, the crater wear of the experimental coatings - - excluding A1203 - - was clearly reduced. Comparative cutting tests revealed important differences between the coating materials Wear mechanisms of the experimental coatings are discussed in detail. © 1998 Elsevier Science Ltd. All rights reserved. Keywords Wear mechanisms, Coating materials; Conventionalsteel; Ca-treated steel, Metal cutting
1. Introduction
The machinability of Al-killed steels can be enhanced by Ca-treatment, which modifies the hard alumina inclusions into less abrasive aluminates. When machining Ca-treated steels the cutting speed, for example, can be substantially increased. It is generally assumed that the coating materials do not manifest the same properties in Ca-treated steels as compared with conventional steels. Even though these differences are most probably reflected in the cutting performance of the coated tools, there are only a few mvestigations [1,2] concerning the wear behaviour of coating materials in Ca-treated steels. The earlier investigations concerning wear mechanisms of coated tools in Ca-treated steels [1,2] and conventional steels [3,4] have been carried out using commercial, usually multilayered, coatings. The aim of this work was to address this situation and to elucidate the wear properties of single layers of TiC, TiN, Ti(C,N) and Al203 in turning of both conventional and Ca-treated steels. Based on these observations, the *Corresponding author
wear processes could be understood in more detail and multilayer coatings may then be optimized.
2. Experimental 2.1. Coatings
The experimental coatings of A1203, TiC and TiN were deposited using conventional chemical vapour deposition (CVD) (deposition temperatures ~ 1000°C). The Ti(C,N) coatings were deposited using both conventional CVD and moderate temperature CVD (MTCVD, deposition temperature ~700°C). The experimental coatings and processes are presented in Table 1 in more detail and cross-sectional scanning electron microscopy (SEM) images of the coating layers can be seen in Fig. l a - e . The coating thicknesses were 7_+1#m and all the coatings were deposited on the cemented carbide substrates of a single composition containing 94 wt% WC and 6 wt% Co. All the experimental coatings were deposited under non-decarburizing conditions and the q-phase forma-
0263-4368/98/$-- see front matter © 1998 Elsevier ScienceLtd All rights reserved PII: S0263-4368(98)00039-0
S Ruppt et al/Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
354 Table 1 Experimental coatings Coating
Process
Deposition temperature (°C)
Precursors in H2 carrier gas
Thickness (/zm)
Lattice parameter (nm)/ alumina phase (c,/tc)
AlzOs TI(C,N) TIN T1C Ti(C,N)
CVD MTCVD CVD CVD CVD
~ 1000 ~ 700 ~ 1000 ~ 1000 ~ 1000
CO2+AIC13 CH3CN N2+T1CL CHa+TiC14 CHa+Nz+TiC14
7_+ 1 7_ l 7+ 1 7+ 1 7+ 1
50% e+50% x 0 4285 0.4236 0.4320 0 4287
tion was consequently avoided [5]. The MTCVD Ti(C,N) coatings were composed of relatively large columnar crystals. The CVD Ti(C,N) coatings were composed of fine-grained ( < 1/~m) equiaxial crystals. The lattice parameters and consequently the carbon contents of the Ti(C,N) coatings were almost equal (Table 1). The pronounced columnar growth, typically exhibited by CVD TiN coatings, was suppressed by renucleation [6]. The A1203 coatings were composed of a mixture of ~-Al203 and ~:-A1203 [7]. In the case of A1203, a thin ( < 1/~m) layer of Ti(C,N) was deposited first on the cemented carbide substrates to ensure good adhesion (Fig. la). 2.2. Workptece matertals
Two charges of 42CrMo4 steel according to EN 10083 with and without Ca-treatment were produced for the turning tests. The Ca-treatment was used to modify the hard Al203 inclusions of Al-killed steels into less abrasive aluminates. The inclusions in Ca-treated steels were composed of rounded aluminates surrounded by a soft envelope of CaS and MnS, Fig. 2. Both experimental steels were quenched and tempered to about the same hardness (Table 2) and the chemical compositions were equal with the exception of the Ca-content (Table 3). The steel bar size used was ~90 x 350 mm which was peeled to 86 mm. 2.3. Cutting tests
All the cutting tests were performed dry under the cutting conditions shown in Table 4. The turning tests were carried out under such conditions that plastic deformation of the substrate was minimized and, consequently, wear properties of the coatings could be elucidated. The edges of the experimental inserts were turned for 2, 5, 9 and 15 rain. After cutting, the inserts were investigated using X-ray diffraction (XRD) and SEM before and after removing the adherent workpiece material with a boiling HC1 solution. Crater wear, flank wear and notch wear were measured using optical microscopy (OM) and SEM.
The experimental coatings were also subjected to additional tool life tests which were performed according to ISO 3685. The tool life tests were carried out for all the experimental coatings in Ca-treated steel, and for AlzO3 and TiN in conventional steel. The cutting forces were measured using a Kistler threecomponent dynamometer, type 9441.
3. Results
3.1. Wear evaluation
The crater depths (KT) and flank wear areas (VB) measured using OM after cutting for 2, 5, 9 and 15 min (13 rain for A1203) in conventional and Ca-treated steel are given in Table 5. The SEM images of the representative cutting edges are shown in Figs 3-8. The inserts were characterized with respect to flank wear, crater wear and notch wear. The rankings of the coatings with respect to these properties based on OM and SEM observations are summarized in Table 6.
3.1.1. Flank wear The best coatings with respect to flank wear were the Ti(C,N) coatings deposited by CVD and MTCVD, followed by the CVD TiC coatings in both workpiece materials. Even TiN coatings exhibited relatively good flank wear resistance, the width of the flank wear area being only slightly greater than for the Ti(C,N) coatings. The SEM examinations revealed, however, that the TiN coatings became worn through on the flank face after turning for 9 min in both conventional and Ca-treated steels (arrowed, Figs 5c and 6c). This was not the case for the TiC or the Ti(C,N) coatings. The least flank wear resistance was offered by A1203. The A1203 coatings became worn through on the flank face after turning for only 2 rain (Figs 3a and 4a). Once the A1203 coating became worn through, the substrate beneath was worn very rapidly resulting in strongly increasing flank wear which was clearly reflected in the VB values measured using OM. This observation for Al~O3 could be made in both workpiece materials.
S Ruppt et al/Internanonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
(a)
(b)
(c)
(d)
(e) Fig 1. SEM cross-secnonal views of the experimental coatings. (a) A1203, (b) MTCVD TI(C,N); (c) T1N, (d) TaC; (e) CVD TffC,N)
355
S Ruppz et al./lnternatzonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
356
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Fig 2. Schematic presentation of Inclusion control using Ca-treatment [8] The A1203 inclusions and elongated sulphide inclusions present in conventional steel (e) are modified into less abrasive, rounded, alummates encased by softer sulphides typically found m Ca-treated steels (d)
3.1.2. Crater wear
According to the solution wear model [9], A1203 should be the most wear-resistant material, at least in conventional steel. A1203 was, however, the worst coating material with respect to crater wear in both conventional and Ca-treated steels (Figs 3a-8a). The A1203 coatings exhibited typically a massive crater wear and became worn through in less than 5 rain. The MTCVD Ti(C,N) and the CVD Ti(C,N) coatings were the most wear-resistant coatings with respect to crater wear in both workpiece materials (Figs 3b,e-8b,e). In conventional steel the CVD Ti(C,N) appeared to be the best coating (Fig. 7e), and in Ca-treated steel the MTCVD Ti(C,N) coating offered most crater wear protection (Fig. 8e). The TiN and TiC coatings exhibited higher rates of crater wear than the Ti(C,N) coatings in both workpiece materials (see Figs 3c-8c, 3d-Sd, respectavely). Crater wear of all the experimental coatings - except for A1203 - - was clearly reduced as a result of Ca-treatment. For example, the TiN coatings became Table 2 Hardness of workplece materials Distance from surface (mm)
Hardness HB-2 5/187 5 kg Conventional
Ca-treated
5 10 15 20 25
264 268 267 264 257
277 277 275 272 267
Average hardness
264
273
worn through after 9 min of turning (Fig. 5c) at 200 m mm 1, while m Ca-treated steel the TiN coatings exhibited the same crater depth after turning at 240 m rain -1 for 15 rain (Fig. 8c). 3.1.3. Notch wear
The TiN and A 1 2 0 3 coatings were superior to the other coatings with respect to notch wear. The TiC and the Ti(C,N) coatmgs showed clear tendencies for notching (indicated in Figs 3-8). These observations are in accordance with earlier investigations [8, 9]. Table 3 Chemical compositions (wt%) of the workplece materials
C $1 Mn P S Cr N1 Mo V Tl Cu A1 Pb As Sn B Nb Co Ca Sb
Conventional steel
Ca-treated steel
0.42 0.24 0 72 0.018 0.029 1 04 0 12 0 17 0.005 0.005 0.13 0 015 0 001 0 008 0.008 0.000 0.004 0.013 0.0010 0.001
0.44 0.29 0.68 0.019 0.029 1.04 0.25 0.18 0.005 0.004 0 20 0.012 0.003 0.009 0.010 0.000 0.005 0.012 0.0044 0.001
S Ruppt et aL/InternanonalJoumal of Refractory Metals & Hard Materials 16 (1998) 353-368
3.1.4. Tool life tests The results of the tool life tests are presented in Fig. 9 showing the cutting speeds corresponding to the tool life of 15 min (v15). The results are in good accordance with SEM observations. The measured v15 values were, in general, 20% higher in Ca-treated steels. This was due to enhanced crater wear resistance and the tool life was - - excluding A1203 - - limited by flank wear. When turning conventional steel, crater wear became more important. It is emphasized that the A1203 coatings were outperformed by about 20% by all other experimental coatings in both workpiece materials. 3.2. Tool wear mechamsms
The chip-tool contact areas were examined by SEM and XRD before and after the adherent workpiece material was dissolved with HC1. X-ray diffraction confirmed the formation of an adherent layer when cutting Ca-treated steel. A layer that was composed of Ca, S, Mn and AI was observed on all the experimental Table 4 Cutting condmons and tool geometries
Cutting speed (m rain -1) Feed (mm rev -1) Depth of cut (mm) Cutting time (rain) Coohng Tool geometry Edge radius (/~m) Rake angle (deg) Clearance angle (deg)
Conventional 42CrMo4
Ca-treated 42CrMo4
200 03 2.5 2. 5, 9, 15 no SNUN 120408 40 - 6 - 6
240 0.3 2.5 2, 5, 9, 15 no SNUN 120408 40 - 6 - 6
357
coatings when turning Ca-treated steel. No layer formation was observed as far as conventional steels were concerned. General views of the chip-tool contact areas after HC1 treatment are shown in Figs 10 and 11 for conventional and Ca-treated steel, respectively. The chipA1203 contact area is shown after cutting for 2 min. For all the other coatings the chip-tool contact areas are shown after cutting for 9 rain. The contact lengths were 670 and 640/tin for A1203 coatings in Ca-treated and conventional steel, respectively. For the other coating materials the contact lengths were of the order of 550 and 530/~m in conventional and Ca-treated steels, respectively. These very small variations in the contact lengths cannot explain either the observed differences in the cutting performances of the experimental coatings or the differences in the machmability of the workpiece materials. It is also clear from Figs 10 and 11 that the contact area exhibited three different zones. These zones are referred to here as sticking (A), mixed internal shear and sliding (B) and pure sliding (C). As can be seen from the SEM micrographs the crater wear occurred typically in zone C in both workpiece materials and was very pronounced on A1203 coated inserts (Figs 10a-lla). When cutting Ca-treated steels, an adherent inclusion layer was formed in the outer part of zone B (indicated in Figs 11b-d), where an undeformed coating surface could be detected. This could clearly be seen on the cutting edges where the chip flow direction (CFD) was not parallel with the grinding marks as is the case, for example, in Fig. llc. The inclusion layer was, however, either not formed in zone C, or had been viscous due to a higher temperature in this part of the contact area, allowing abrasive components of steel to affect the coating surface. The resulting ridge
Table 5 Flank wear areas and crater wear depths for the experimental coatings after cutting of conventional and Ca-treated steels Coating
Flank wear (VJmm) A1203 TI(C,N)-MTCVD T1N TiC TI(C,N)-CVD
Conventional steel cutting time
Ca-treated steel cutting Ume
2 mm
5 mm
9 mln
15 mln
2 mm
5 mm
9 mm
15 mln
0.06 0.07 0.1 0 09 0 05
0 11 0 07 0 13 0.09 0.07
0 21 0 09 0 14 0.09 0 09
> 0.39 0.11 0.17 0.11 0.09
0 009 0 08 0.1 0 07 0.08
0 15 0 09 0 13 0 07 0.09
0.33 0.12 0.16 0 11 0 12
> 0 60 0 16 0 23 0.22 0 18
3 5 0 0 5
8 6 6 8 6
15 7 9 10 7
> 195 18 23 20 18
0 0 0 0 0
24 0 0 0 0
75 2 6 1 2
> 165 3 16 3 3
Crater weat (Kr/pm)
A1203 TI(C,N)-MTCVD T1N T1C Ta(C,N)-CVD
358
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S Ruppt et al /International Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
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S Ruppl et al/InternattonalJoumal of Refractory Metals & Hard Matenals 16 (1998) 353-368
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(e) Fig 6. SEM images of the cutting edges after turning Ca-treated steel for 9 mln (a) A1203. (b) MTCVD Ta(C,N); (c) TIN, (d) T1C, (e) CVD Ti(C,N).
362
S Ruppl et al./IntemaUonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
(a)
(b)
(c)
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(~) Fig 7. SEM images of the cutting edges after turning conventional steel for 15 mm: (a) AlzO3 (13 mm), (b) MTCVD TI(C,N), (c) TiN, (d) TiC, (e) CVD Ti(C,N).
S. Ruppt et al./Intematlonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
(a)
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(o) Fig 8. SEM images of the cutting edges after turning Ca-treated steel for 15 mm (a) A1203 (13 mm), (b) MTCVD TI(C,N), (c) TIN, (d) T1C, (e) CVD Ti(C,N).
S Ruppt et al /Internattonal Journal of Refiactory Metals & Hard Materials 16 (1998) 353-368
364
Table 6 Relatwe wear rates and rankmgs of the expenrnental coating materials
Flank wear rate Crater wear rate Notch wear rate
Conventmnal steel
Ca-treated steel
A1203 >>TIN> TIC-~ MTCVD TI(C,N) > CVD TI(C,N) A1203 >>TIN >TIC-~ MTCVD TI(C,N) > CVD TI(C,N) TIC -~MTCVD TI(C,N) - CVD TI(C,N) >>TIN ~-A1203
A1203 >>T1N ~ TIC> MTCVD Ti(C,N)---CVD TI(C,N) A1203 >>T1N ~ TIC > MTCVD Ti(C,N)-~ CVD Ti(C,N) T1C ~ MTCVD TI(C,N) -~CVD Ti(C,N) >>TiN -- A1203
formation in zone C was clearly observed on all coatings and especially o n AI2O3 coatings. The ridge formation and crater wear were clearly reduced on all coatings, except for A1203, as a result of Ca-treatment (Figs 11b-e). This was obviously due to a more favourable inclusion structure exhibited by Ca-treated steel. It is emphasized that the alumina layers behaved in a similar way in both conventional and Ca-treated steels. No evidence for a chemical reaction between the inclusion layer and the A1203 coating could be seen when turning Ca-treated steels as suggested earlier [2]. Crater wear rates of A1203 coatings were not increased as a result of Ca-treatment even though the cutting speed was higher. The wear mechanism was clearly plastic deformation and ductile fracture of the coating asperities (zone B) in combination with abrasion due
to hard inclusions (in zone C). The observed wear characteristics and the behaviour of A1203 relative to TiC, Ti(C,N) and TiN are in accordance with an earlier study where these coating materials were investigated in a variety of conventional steels and cast irons [10].
3. 3. Cutting forces The main cutting force components (axial, radial and tangential) were measured when cutting conventional and Ca-treated steel using AlzO3 and TiN coated inserts. The results are given in Table 7. The observed differences in the cutting forces between the experimental coatings were very small as observed earlier [10]. It is also clear that the cutting forces were very similar for both workpiece materials.
250
200
150
E g
==Ca treated steel, 273 HB [3 Conv. treated steel, 264 HB 100
50
Depth of cut 2.5 m m . , .. Feed '0.4 mm/rev ' . ': , : Separate chip breaker insert 4 mm distance from ~Utting edge Cutting fluid not used i : .' ' Steels 42CrMo4, Q+T, bar size o 90 x 350 turn ipeeledt0 0"86 mm
TiC
Ti(C,N) CVD
Ti(C,N) MTCVD
TiN
AI203
Fig 9 Tool hfe test results. Cutting speeds corresponding to the tool hves of 15 mln (v~5) are shown for the experimental coatings
S Ruppt et aL/InternattonalJoumal of Refractory Metals & Hald Materials 16 (1998) 353-368
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(b)
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365
(e) Fig. 10. SEM images of the chip-tool contact area. Sticking zone (A), mtxed shear and sliding (B) and pure sliding (C) are indicated (a) A1203 after 2 min; (h) MTCVD Ti(C,N) after 9 min, (c) T1N after 9 mm, (d) TIC after 9 min; (e) CVD Ti(C,N) after 9 mm Note that all the micrographs have been obtained after the adherent workplece material was removed by using a boiling HC1 solution
S Ruppz et al /Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
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(e) Fig 11 SEM images of the chip-tool contact areas after cuttmg Ca-treated steel. (a) AleO3 after 2 min Pronounced abraswe wear m zone C can be seen U n d e f o r m e d coating surface is indicated by [] (b) M T C V D Ti(C,N) after 9 m m U n d e f o r m e d coating surface is mdicated by [] (c) T1N after 9 m m Chip flow direction (CFD) and grlndmg marks are arrowed The undeformed coating surface (marked with []) is clearly seen when the CFD and grinding marks are not parallel (d) TiC after 9 mm. The undeformed coating surface is marked by [] (e) CVD TI(C,N) after 9 m m U n d e f o r m e d coating surface between zones C and B is now more difficult to distinguish when the CFD and grinding marks are parallel. Note that the mlcrographs have been obtained after removing the adherent inclusion layer using a bolhng HC1 solution.
367
S. Ruppt et al /Intemanonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
Table 7 Axial (FA), radial (FrO and tangential (FT) components of the main cutting forces for TIN and A1203 coated inserts when turning conventional and Ca-treated steels Workplece material
Cutting force (N) FA
Conventional steel Ca-treated steel
FR
FT
TIN
A1203
TIN
A1203
TIN
A1203
768 755
768 762
609 618
613 614
1692 1719
1648 1712
4. Conclusions Tool wear mechanisms and layer formation have been studied during turning of conventional and Ca-treated Al-killed quenched and tempered steels using cemented carbide inserts coated by single layers of TiC, TiN and Ti(C,N) and Al203. The following conclusions could be drawn: • The crater wear of the coated inserts was most probably governed by plastic deformation/ductile fracture in combination with abrasion due to hard inclusions present in both conventional and Ca-treated Al-killed steels. The chemical effects may be neglected on the rake face under these experimental conditions. • Ca-treatment of Al-killed steels, which modifies the hard A1203 inclusions into less abrasive aluminates encased by a softer sulphide envelope, is an effective means of enhancing machinability. All experimental coatings showed a better performance m Ca-treated steels than in conventional steel. The v15 values were about 20% higher for Ca-treated steels. • The A1203 coatings behaved in the similar way in both conventional and Ca-treated steels exhibiting about 20% lower v15 values in both workpiece materials than the other coating materials. No evidence for enhanced dissolution wear of A1203 into adherent inclusion layers could be found when cutting Ca-treated steel. • The adherent inclusion layer was formed on all experimental coatings when turning Ca-treated steels. However, the inclusion layer was absent in that part of the contact length where sliding and abrasion occurred. In spite of this, the wear in the sliding zone was dearly reduced on all coatings - except for A1203 - - due to less abrasive inclusions in Ca-treated steel. Increased machinability of Ca-treated steels is consequently not solely due to a 'wear isolating' inclusion layer. This layer can be
seen partly as a consequence of successful inclusion control. • The crater wear rates of TiC, Ti(C,N) and TIN coatings were clearly reduced as a result of Ca-treatment. The best coatings in this respect were the Ti(C,N) coatings. With respect to notch wear the A1203 and TiN outperformed the Ti(C,N) and TiC coatings in both workpiece materials. • The better machinability of Ca-treated steel relative to that of conventional steel cannot be explained by the very small differences in the cutting forces and in the chip-tool contact lengths, which were observed when cutting these workpiece materials.
Acknowledgements We thank Ms I. Karlsson, Seco Tools AB, for excellent word processing of the manuscript.
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S Ruppz et al /International Journal of Refiactory Metals & Hard Materials 16 (1998) 353-368
[7] Vuorlnen S, Karlsson L Phase transformation in CVD kappa alumina. Thin Solid Films 1992;214'132-41. [8] Vamola RV. Holappa LEK, Karvonen PHJ. Modern steelmaking technology for special steels Modern Steelmaklng Technology 1995;53'453-65.
[9] Kramer BM, Suh NP Tool wear by solution: a quantitative understanding Eng Ind 1980,102 303-9. [10] Ruppl S. Hogrehus B Wear mechanisms of different CVD and MTCVD coating materials m machining of steel and cast Iron In press
REFRACTORYMETALS & HARDMATERIALS ELSEVIER
International Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
Wear characteristics of TiC, Ti(C,N), TiN and A1203 coatings in the turning of conventional and Ca-treated steels S. Ruppi a*, B. H6grelius a, M. Huhtiranta b "Seco ToolsAB, SE-737 82 Fagersta, Sweden bImatra Steel Oy Ab, FIN-55100 Imatm, Fmland
Recewed 9 March 1998; accepted 14 July 1998
Abstract
The wear characteristics of single layers of TIC, Ti(C,N), TiN and A1203 were investigated during turning of conventional and Ca-treated quenched and tempered Al-killed steels. The experimental coatings were deposited using chemical vapour deposition (CVD) or moderate temperature CVD (MTCVD) on cemented carbide substrates of a single composition and the coatings were of simdar thicknesses (7_+ 1/~m). The wear mechamsms and layer formation were studied using scannmg electron microscopy, optical microscopy and X-ray diffraction. Inclusion modification appeared to be an effective means of enhancing machmability and all experimental coatings exhibited about 20% better performance as a result of Ca-treatment. In particular, the crater wear of the experimental coatings - - excluding A1203 - - was clearly reduced. Comparative cutting tests revealed important differences between the coating materials Wear mechanisms of the experimental coatings are discussed in detail. © 1998 Elsevier Science Ltd. All rights reserved. Keywords Wear mechanisms, Coating materials; Conventionalsteel; Ca-treated steel, Metal cutting
1. Introduction
The machinability of Al-killed steels can be enhanced by Ca-treatment, which modifies the hard alumina inclusions into less abrasive aluminates. When machining Ca-treated steels the cutting speed, for example, can be substantially increased. It is generally assumed that the coating materials do not manifest the same properties in Ca-treated steels as compared with conventional steels. Even though these differences are most probably reflected in the cutting performance of the coated tools, there are only a few mvestigations [1,2] concerning the wear behaviour of coating materials in Ca-treated steels. The earlier investigations concerning wear mechanisms of coated tools in Ca-treated steels [1,2] and conventional steels [3,4] have been carried out using commercial, usually multilayered, coatings. The aim of this work was to address this situation and to elucidate the wear properties of single layers of TiC, TiN, Ti(C,N) and Al203 in turning of both conventional and Ca-treated steels. Based on these observations, the *Corresponding author
wear processes could be understood in more detail and multilayer coatings may then be optimized.
2. Experimental 2.1. Coatings
The experimental coatings of A1203, TiC and TiN were deposited using conventional chemical vapour deposition (CVD) (deposition temperatures ~ 1000°C). The Ti(C,N) coatings were deposited using both conventional CVD and moderate temperature CVD (MTCVD, deposition temperature ~700°C). The experimental coatings and processes are presented in Table 1 in more detail and cross-sectional scanning electron microscopy (SEM) images of the coating layers can be seen in Fig. l a - e . The coating thicknesses were 7_+1#m and all the coatings were deposited on the cemented carbide substrates of a single composition containing 94 wt% WC and 6 wt% Co. All the experimental coatings were deposited under non-decarburizing conditions and the q-phase forma-
0263-4368/98/$-- see front matter © 1998 Elsevier ScienceLtd All rights reserved PII: S0263-4368(98)00039-0
S Ruppt et al/Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
354 Table 1 Experimental coatings Coating
Process
Deposition temperature (°C)
Precursors in H2 carrier gas
Thickness (/zm)
Lattice parameter (nm)/ alumina phase (c,/tc)
AlzOs TI(C,N) TIN T1C Ti(C,N)
CVD MTCVD CVD CVD CVD
~ 1000 ~ 700 ~ 1000 ~ 1000 ~ 1000
CO2+AIC13 CH3CN N2+T1CL CHa+TiC14 CHa+Nz+TiC14
7_+ 1 7_ l 7+ 1 7+ 1 7+ 1
50% e+50% x 0 4285 0.4236 0.4320 0 4287
tion was consequently avoided [5]. The MTCVD Ti(C,N) coatings were composed of relatively large columnar crystals. The CVD Ti(C,N) coatings were composed of fine-grained ( < 1/~m) equiaxial crystals. The lattice parameters and consequently the carbon contents of the Ti(C,N) coatings were almost equal (Table 1). The pronounced columnar growth, typically exhibited by CVD TiN coatings, was suppressed by renucleation [6]. The A1203 coatings were composed of a mixture of ~-Al203 and ~:-A1203 [7]. In the case of A1203, a thin ( < 1/~m) layer of Ti(C,N) was deposited first on the cemented carbide substrates to ensure good adhesion (Fig. la). 2.2. Workptece matertals
Two charges of 42CrMo4 steel according to EN 10083 with and without Ca-treatment were produced for the turning tests. The Ca-treatment was used to modify the hard Al203 inclusions of Al-killed steels into less abrasive aluminates. The inclusions in Ca-treated steels were composed of rounded aluminates surrounded by a soft envelope of CaS and MnS, Fig. 2. Both experimental steels were quenched and tempered to about the same hardness (Table 2) and the chemical compositions were equal with the exception of the Ca-content (Table 3). The steel bar size used was ~90 x 350 mm which was peeled to 86 mm. 2.3. Cutting tests
All the cutting tests were performed dry under the cutting conditions shown in Table 4. The turning tests were carried out under such conditions that plastic deformation of the substrate was minimized and, consequently, wear properties of the coatings could be elucidated. The edges of the experimental inserts were turned for 2, 5, 9 and 15 rain. After cutting, the inserts were investigated using X-ray diffraction (XRD) and SEM before and after removing the adherent workpiece material with a boiling HC1 solution. Crater wear, flank wear and notch wear were measured using optical microscopy (OM) and SEM.
The experimental coatings were also subjected to additional tool life tests which were performed according to ISO 3685. The tool life tests were carried out for all the experimental coatings in Ca-treated steel, and for AlzO3 and TiN in conventional steel. The cutting forces were measured using a Kistler threecomponent dynamometer, type 9441.
3. Results
3.1. Wear evaluation
The crater depths (KT) and flank wear areas (VB) measured using OM after cutting for 2, 5, 9 and 15 min (13 rain for A1203) in conventional and Ca-treated steel are given in Table 5. The SEM images of the representative cutting edges are shown in Figs 3-8. The inserts were characterized with respect to flank wear, crater wear and notch wear. The rankings of the coatings with respect to these properties based on OM and SEM observations are summarized in Table 6.
3.1.1. Flank wear The best coatings with respect to flank wear were the Ti(C,N) coatings deposited by CVD and MTCVD, followed by the CVD TiC coatings in both workpiece materials. Even TiN coatings exhibited relatively good flank wear resistance, the width of the flank wear area being only slightly greater than for the Ti(C,N) coatings. The SEM examinations revealed, however, that the TiN coatings became worn through on the flank face after turning for 9 min in both conventional and Ca-treated steels (arrowed, Figs 5c and 6c). This was not the case for the TiC or the Ti(C,N) coatings. The least flank wear resistance was offered by A1203. The A1203 coatings became worn through on the flank face after turning for only 2 rain (Figs 3a and 4a). Once the A1203 coating became worn through, the substrate beneath was worn very rapidly resulting in strongly increasing flank wear which was clearly reflected in the VB values measured using OM. This observation for Al~O3 could be made in both workpiece materials.
S Ruppt et al/Internanonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
(a)
(b)
(c)
(d)
(e) Fig 1. SEM cross-secnonal views of the experimental coatings. (a) A1203, (b) MTCVD TI(C,N); (c) T1N, (d) TaC; (e) CVD TffC,N)
355
S Ruppz et al./lnternatzonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
356
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Fig 2. Schematic presentation of Inclusion control using Ca-treatment [8] The A1203 inclusions and elongated sulphide inclusions present in conventional steel (e) are modified into less abrasive, rounded, alummates encased by softer sulphides typically found m Ca-treated steels (d)
3.1.2. Crater wear
According to the solution wear model [9], A1203 should be the most wear-resistant material, at least in conventional steel. A1203 was, however, the worst coating material with respect to crater wear in both conventional and Ca-treated steels (Figs 3a-8a). The A1203 coatings exhibited typically a massive crater wear and became worn through in less than 5 rain. The MTCVD Ti(C,N) and the CVD Ti(C,N) coatings were the most wear-resistant coatings with respect to crater wear in both workpiece materials (Figs 3b,e-8b,e). In conventional steel the CVD Ti(C,N) appeared to be the best coating (Fig. 7e), and in Ca-treated steel the MTCVD Ti(C,N) coating offered most crater wear protection (Fig. 8e). The TiN and TiC coatings exhibited higher rates of crater wear than the Ti(C,N) coatings in both workpiece materials (see Figs 3c-8c, 3d-Sd, respectavely). Crater wear of all the experimental coatings - except for A1203 - - was clearly reduced as a result of Ca-treatment. For example, the TiN coatings became Table 2 Hardness of workplece materials Distance from surface (mm)
Hardness HB-2 5/187 5 kg Conventional
Ca-treated
5 10 15 20 25
264 268 267 264 257
277 277 275 272 267
Average hardness
264
273
worn through after 9 min of turning (Fig. 5c) at 200 m mm 1, while m Ca-treated steel the TiN coatings exhibited the same crater depth after turning at 240 m rain -1 for 15 rain (Fig. 8c). 3.1.3. Notch wear
The TiN and A 1 2 0 3 coatings were superior to the other coatings with respect to notch wear. The TiC and the Ti(C,N) coatmgs showed clear tendencies for notching (indicated in Figs 3-8). These observations are in accordance with earlier investigations [8, 9]. Table 3 Chemical compositions (wt%) of the workplece materials
C $1 Mn P S Cr N1 Mo V Tl Cu A1 Pb As Sn B Nb Co Ca Sb
Conventional steel
Ca-treated steel
0.42 0.24 0 72 0.018 0.029 1 04 0 12 0 17 0.005 0.005 0.13 0 015 0 001 0 008 0.008 0.000 0.004 0.013 0.0010 0.001
0.44 0.29 0.68 0.019 0.029 1.04 0.25 0.18 0.005 0.004 0 20 0.012 0.003 0.009 0.010 0.000 0.005 0.012 0.0044 0.001
S Ruppt et aL/InternanonalJoumal of Refractory Metals & Hard Materials 16 (1998) 353-368
3.1.4. Tool life tests The results of the tool life tests are presented in Fig. 9 showing the cutting speeds corresponding to the tool life of 15 min (v15). The results are in good accordance with SEM observations. The measured v15 values were, in general, 20% higher in Ca-treated steels. This was due to enhanced crater wear resistance and the tool life was - - excluding A1203 - - limited by flank wear. When turning conventional steel, crater wear became more important. It is emphasized that the A1203 coatings were outperformed by about 20% by all other experimental coatings in both workpiece materials. 3.2. Tool wear mechamsms
The chip-tool contact areas were examined by SEM and XRD before and after the adherent workpiece material was dissolved with HC1. X-ray diffraction confirmed the formation of an adherent layer when cutting Ca-treated steel. A layer that was composed of Ca, S, Mn and AI was observed on all the experimental Table 4 Cutting condmons and tool geometries
Cutting speed (m rain -1) Feed (mm rev -1) Depth of cut (mm) Cutting time (rain) Coohng Tool geometry Edge radius (/~m) Rake angle (deg) Clearance angle (deg)
Conventional 42CrMo4
Ca-treated 42CrMo4
200 03 2.5 2. 5, 9, 15 no SNUN 120408 40 - 6 - 6
240 0.3 2.5 2, 5, 9, 15 no SNUN 120408 40 - 6 - 6
357
coatings when turning Ca-treated steel. No layer formation was observed as far as conventional steels were concerned. General views of the chip-tool contact areas after HC1 treatment are shown in Figs 10 and 11 for conventional and Ca-treated steel, respectively. The chipA1203 contact area is shown after cutting for 2 min. For all the other coatings the chip-tool contact areas are shown after cutting for 9 rain. The contact lengths were 670 and 640/tin for A1203 coatings in Ca-treated and conventional steel, respectively. For the other coating materials the contact lengths were of the order of 550 and 530/~m in conventional and Ca-treated steels, respectively. These very small variations in the contact lengths cannot explain either the observed differences in the cutting performances of the experimental coatings or the differences in the machmability of the workpiece materials. It is also clear from Figs 10 and 11 that the contact area exhibited three different zones. These zones are referred to here as sticking (A), mixed internal shear and sliding (B) and pure sliding (C). As can be seen from the SEM micrographs the crater wear occurred typically in zone C in both workpiece materials and was very pronounced on A1203 coated inserts (Figs 10a-lla). When cutting Ca-treated steels, an adherent inclusion layer was formed in the outer part of zone B (indicated in Figs 11b-d), where an undeformed coating surface could be detected. This could clearly be seen on the cutting edges where the chip flow direction (CFD) was not parallel with the grinding marks as is the case, for example, in Fig. llc. The inclusion layer was, however, either not formed in zone C, or had been viscous due to a higher temperature in this part of the contact area, allowing abrasive components of steel to affect the coating surface. The resulting ridge
Table 5 Flank wear areas and crater wear depths for the experimental coatings after cutting of conventional and Ca-treated steels Coating
Flank wear (VJmm) A1203 TI(C,N)-MTCVD T1N TiC TI(C,N)-CVD
Conventional steel cutting time
Ca-treated steel cutting Ume
2 mm
5 mm
9 mln
15 mln
2 mm
5 mm
9 mm
15 mln
0.06 0.07 0.1 0 09 0 05
0 11 0 07 0 13 0.09 0.07
0 21 0 09 0 14 0.09 0 09
> 0.39 0.11 0.17 0.11 0.09
0 009 0 08 0.1 0 07 0.08
0 15 0 09 0 13 0 07 0.09
0.33 0.12 0.16 0 11 0 12
> 0 60 0 16 0 23 0.22 0 18
3 5 0 0 5
8 6 6 8 6
15 7 9 10 7
> 195 18 23 20 18
0 0 0 0 0
24 0 0 0 0
75 2 6 1 2
> 165 3 16 3 3
Crater weat (Kr/pm)
A1203 TI(C,N)-MTCVD T1N T1C Ta(C,N)-CVD
358
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360
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(a)
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(e) Fig 5 SEM images of the cutting edges after turning conventional steel for 9 mm (a) A1203, (b) MTCVD TffC,N), (c) T1N; (d) T1C, (e) CVD Ti(C,N)
S Ruppl et al/InternattonalJoumal of Refractory Metals & Hard Matenals 16 (1998) 353-368
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(e) Fig 6. SEM images of the cutting edges after turning Ca-treated steel for 9 mln (a) A1203. (b) MTCVD Ta(C,N); (c) TIN, (d) T1C, (e) CVD Ti(C,N).
362
S Ruppl et al./IntemaUonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
(a)
(b)
(c)
(d)
(~) Fig 7. SEM images of the cutting edges after turning conventional steel for 15 mm: (a) AlzO3 (13 mm), (b) MTCVD TI(C,N), (c) TiN, (d) TiC, (e) CVD Ti(C,N).
S. Ruppt et al./Intematlonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
(a)
(b)
(~)
(d)
363
(o) Fig 8. SEM images of the cutting edges after turning Ca-treated steel for 15 mm (a) A1203 (13 mm), (b) MTCVD TI(C,N), (c) TIN, (d) T1C, (e) CVD Ti(C,N).
S Ruppt et al /Internattonal Journal of Refiactory Metals & Hard Materials 16 (1998) 353-368
364
Table 6 Relatwe wear rates and rankmgs of the expenrnental coating materials
Flank wear rate Crater wear rate Notch wear rate
Conventmnal steel
Ca-treated steel
A1203 >>TIN> TIC-~ MTCVD TI(C,N) > CVD TI(C,N) A1203 >>TIN >TIC-~ MTCVD TI(C,N) > CVD TI(C,N) TIC -~MTCVD TI(C,N) - CVD TI(C,N) >>TIN ~-A1203
A1203 >>T1N ~ TIC> MTCVD Ti(C,N)---CVD TI(C,N) A1203 >>T1N ~ TIC > MTCVD Ti(C,N)-~ CVD Ti(C,N) T1C ~ MTCVD TI(C,N) -~CVD Ti(C,N) >>TiN -- A1203
formation in zone C was clearly observed on all coatings and especially o n AI2O3 coatings. The ridge formation and crater wear were clearly reduced on all coatings, except for A1203, as a result of Ca-treatment (Figs 11b-e). This was obviously due to a more favourable inclusion structure exhibited by Ca-treated steel. It is emphasized that the alumina layers behaved in a similar way in both conventional and Ca-treated steels. No evidence for a chemical reaction between the inclusion layer and the A1203 coating could be seen when turning Ca-treated steels as suggested earlier [2]. Crater wear rates of A1203 coatings were not increased as a result of Ca-treatment even though the cutting speed was higher. The wear mechanism was clearly plastic deformation and ductile fracture of the coating asperities (zone B) in combination with abrasion due
to hard inclusions (in zone C). The observed wear characteristics and the behaviour of A1203 relative to TiC, Ti(C,N) and TiN are in accordance with an earlier study where these coating materials were investigated in a variety of conventional steels and cast irons [10].
3. 3. Cutting forces The main cutting force components (axial, radial and tangential) were measured when cutting conventional and Ca-treated steel using AlzO3 and TiN coated inserts. The results are given in Table 7. The observed differences in the cutting forces between the experimental coatings were very small as observed earlier [10]. It is also clear that the cutting forces were very similar for both workpiece materials.
250
200
150
E g
==Ca treated steel, 273 HB [3 Conv. treated steel, 264 HB 100
50
Depth of cut 2.5 m m . , .. Feed '0.4 mm/rev ' . ': , : Separate chip breaker insert 4 mm distance from ~Utting edge Cutting fluid not used i : .' ' Steels 42CrMo4, Q+T, bar size o 90 x 350 turn ipeeledt0 0"86 mm
TiC
Ti(C,N) CVD
Ti(C,N) MTCVD
TiN
AI203
Fig 9 Tool hfe test results. Cutting speeds corresponding to the tool hves of 15 mln (v~5) are shown for the experimental coatings
S Ruppt et aL/InternattonalJoumal of Refractory Metals & Hald Materials 16 (1998) 353-368
(a)
(b)
(c)
(d)
365
(e) Fig. 10. SEM images of the chip-tool contact area. Sticking zone (A), mtxed shear and sliding (B) and pure sliding (C) are indicated (a) A1203 after 2 min; (h) MTCVD Ti(C,N) after 9 min, (c) T1N after 9 mm, (d) TIC after 9 min; (e) CVD Ti(C,N) after 9 mm Note that all the micrographs have been obtained after the adherent workplece material was removed by using a boiling HC1 solution
S Ruppz et al /Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
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(e) Fig 11 SEM images of the chip-tool contact areas after cuttmg Ca-treated steel. (a) AleO3 after 2 min Pronounced abraswe wear m zone C can be seen U n d e f o r m e d coating surface is indicated by [] (b) M T C V D Ti(C,N) after 9 m m U n d e f o r m e d coating surface is mdicated by [] (c) T1N after 9 m m Chip flow direction (CFD) and grlndmg marks are arrowed The undeformed coating surface (marked with []) is clearly seen when the CFD and grinding marks are not parallel (d) TiC after 9 mm. The undeformed coating surface is marked by [] (e) CVD TI(C,N) after 9 m m U n d e f o r m e d coating surface between zones C and B is now more difficult to distinguish when the CFD and grinding marks are parallel. Note that the mlcrographs have been obtained after removing the adherent inclusion layer using a bolhng HC1 solution.
367
S. Ruppt et al /Intemanonal Journal of Refractory Metals & Hard Materials 16 (1998) 353-368
Table 7 Axial (FA), radial (FrO and tangential (FT) components of the main cutting forces for TIN and A1203 coated inserts when turning conventional and Ca-treated steels Workplece material
Cutting force (N) FA
Conventional steel Ca-treated steel
FR
FT
TIN
A1203
TIN
A1203
TIN
A1203
768 755
768 762
609 618
613 614
1692 1719
1648 1712
4. Conclusions Tool wear mechanisms and layer formation have been studied during turning of conventional and Ca-treated Al-killed quenched and tempered steels using cemented carbide inserts coated by single layers of TiC, TiN and Ti(C,N) and Al203. The following conclusions could be drawn: • The crater wear of the coated inserts was most probably governed by plastic deformation/ductile fracture in combination with abrasion due to hard inclusions present in both conventional and Ca-treated Al-killed steels. The chemical effects may be neglected on the rake face under these experimental conditions. • Ca-treatment of Al-killed steels, which modifies the hard A1203 inclusions into less abrasive aluminates encased by a softer sulphide envelope, is an effective means of enhancing machinability. All experimental coatings showed a better performance m Ca-treated steels than in conventional steel. The v15 values were about 20% higher for Ca-treated steels. • The A1203 coatings behaved in the similar way in both conventional and Ca-treated steels exhibiting about 20% lower v15 values in both workpiece materials than the other coating materials. No evidence for enhanced dissolution wear of A1203 into adherent inclusion layers could be found when cutting Ca-treated steel. • The adherent inclusion layer was formed on all experimental coatings when turning Ca-treated steels. However, the inclusion layer was absent in that part of the contact length where sliding and abrasion occurred. In spite of this, the wear in the sliding zone was dearly reduced on all coatings - except for A1203 - - due to less abrasive inclusions in Ca-treated steel. Increased machinability of Ca-treated steels is consequently not solely due to a 'wear isolating' inclusion layer. This layer can be
seen partly as a consequence of successful inclusion control. • The crater wear rates of TiC, Ti(C,N) and TIN coatings were clearly reduced as a result of Ca-treatment. The best coatings in this respect were the Ti(C,N) coatings. With respect to notch wear the A1203 and TiN outperformed the Ti(C,N) and TiC coatings in both workpiece materials. • The better machinability of Ca-treated steel relative to that of conventional steel cannot be explained by the very small differences in the cutting forces and in the chip-tool contact lengths, which were observed when cutting these workpiece materials.
Acknowledgements We thank Ms I. Karlsson, Seco Tools AB, for excellent word processing of the manuscript.
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S Ruppz et al /International Journal of Refiactory Metals & Hard Materials 16 (1998) 353-368
[7] Vuorlnen S, Karlsson L Phase transformation in CVD kappa alumina. Thin Solid Films 1992;214'132-41. [8] Vamola RV. Holappa LEK, Karvonen PHJ. Modern steelmaking technology for special steels Modern Steelmaklng Technology 1995;53'453-65.
[9] Kramer BM, Suh NP Tool wear by solution: a quantitative understanding Eng Ind 1980,102 303-9. [10] Ruppl S. Hogrehus B Wear mechanisms of different CVD and MTCVD coating materials m machining of steel and cast Iron In press