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TiN coated tool sliding contact
Tribology International 97 (2016) 337–348
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Effect of Si and Cr additions to carbon steel on material transfer in a steel/TiN coated tool sliding contact T. Aiso a,n, U. Wiklund a, M. Kubota b, S. Jacobson a a b
Tribomaterials group, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Steel Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 1-8 Fuso-Cho, Amagasaki, Hyogo, 660-0891, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2015 Received in revised form 20 January 2016 Accepted 21 January 2016 Available online 29 January 2016
A crossed cylinders sliding test, simulating the contact between the chip and the tool in machining, is used to evaluate material transfer and friction characteristics of a TiN coating against specifically designed model steels. These include one base reference, only alloyed with C (Base steel) and two alloyed also with 1 mass% Si or Cr. When sliding against the Base steel, an Fe–O layer is formed on the coating. Against the Si and Cr alloyed steels, Fe–Si–O and Fe–Cr–O layers are formed. In these oxides, Si and Cr are enriched, i.e. preferentially transferred from the steels. Compared to the Base steel, the friction coefficient is significantly lower against the Si alloyed steel and higher against the Cr alloyed steel. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Transfer Coating Sliding
1. Introduction Material transfer often occurs between work materials and cutting tools, and the amount and the mode of it depend on machining conditions, tools and work material types. It is wellknown that transfer layers on the cutting edge sometimes trigger the formation of built-up edges and deteriorate the quality of the machined surface [1]. This indicates that transfer should be minimized to avoid such effects. On the other hand, it is also suggested that transfer layers on the rake face of the tool prevent the wear of the tool [2–4]. To promote the formation of transfer layers with a beneficial impact on machining performance and tool life, it is of great importance to understand and control the material transfer. Recent research in this area reflects a strong industrial interest; see for example Ref. [5–7]. In dry hobbing of case hardening steels, it was reported by Yoshimura et al. [8] that transfer layers formed on the rake face reduced the crater wear. Gerth et al. [9] reported on transfer layers on the rake faces of hob teeth, consisting of oxides of Fe, Cr, Mn and Si. All these elements except oxygen were constituents of the steel work material. They concluded that the roles of the oxide layer were probably wear protection of the tool and friction reduction between the chip and the tool. They also suggested that the oxide layer composition, and thus its properties, was n
Corresponding author. E-mail address: [email protected] (T. Aiso).
http://dx.doi.org/10.1016/j.triboint.2016.01.032 0301-679X/& 2016 Elsevier Ltd. All rights reserved.
influenced by the composition of steel work material. Therefore, understanding the effects of different alloy elements is valuable for several reasons. For example, to the component manufacturers such knowledge will tell how the element influences the machinability of the steel and hint at how to tune the machining parameters. To the steel manufacturers it can provide means for tuning their products for optimized machinability in terms of low friction and high tool wear protection. Since machining is a complex process, several simplifying sliding tests have been used to simulate the contact between the chip and the rake face [10–13]. In recent years, Gerth et al. [14] used a crossed cylinders sliding test. This excludes the chip forming in the primary shear zone and focuses on simulating only the contact between the chip and the rake face. Very similar oxide layers were formed in the sliding test as in actual milling. It was concluded that the test is capable of reproducing the work material transfer occurring in milling, and thus it is a powerful tool for studying the influence of oxide layers on friction and wear. This work focuses on the effect of Si and Cr, two commonly used alloy elements, on material transfer and friction behavior during sliding. To isolate and study the influence of the individual alloy elements, model steels designed specifically for this purpose were necessary. Binary and ternary alloys were prepared to have a similar microstructure and hardness, with the only significant difference being the chemical composition, i.e. the addition of 1 mass% of either Si or Cr. The steels were slid against a TiN coated high speed steel (HSS) cylinder, representing a commonly used tool surface.
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56 mm right before starting the test, to ensure a clean surface was achieved. The turning was carried out with coated carbide tools under dry condition, with a cutting speed of 250 m/min, a feed of 0.061 mm/rev and a cutting depth of 0.1 mm. This resulted in similar surface roughness; about Ra 0.8, 0.6 and 0.5 μm for the Base steel, the Si alloyed steel and the Cr alloyed steel, respectively. The microstructure and the Vickers hardness of the three cylinders are shown in Fig. 1 and Table 1, respectively. All steels mainly contained a fine pearlite microstructure, although the Base steel possibly included some amount of tempered martensite, and the Si and Cr alloyed steels contained small amounts of ferrite. The hardness of all steels was around 200 HV.
2. Material and methods 2.1. Materials 2.1.1. Tool material Cylinders with a diameter of 5 mm and a length of 20 mm were produced from a powder metallurgy HSS ASP2023 with a hardness of about 850 HV. The cylinders were polished and coated with PVD TiN. After coating deposition, the cylinders were polished and the final roughness, measured with white light interference profilometry, was Ra 0.083 70.009 μm. The coating thickness was about 3.6 μm. The coating hardness, measured using nanoindentation with an indent depth of 200 nm, was 25 72 GPa. 2.1.2. Work material Three types of steels, nominally containing 0.55 mass% C, were used as the work materials. The chemical compositions of the steels are shown in Table 1. In the Base steel, C was the only alloy element. The Si and Cr alloyed steels also contained nominally 1.0 mass% of Si or Cr, respectively. The steels were melted in a laboratory vacuum melting furnace, cast into ingots 180 kg in weight, hot forged into round bars with a diameter of 70 mm, heated to 1523 K for 2 h and cooled in air to room temperature as a homogenization treatment to reduce the segregation of the alloy elements, and then austenitized at 1203 K for 30 min and cooled in air for the purpose of normalizing. Subsequently, each steel underwent different heat treatments to minimize differences in microstructure and hardness. The heat treatment cycles are shown in Table 1. The austenitizing and annealing temperatures were changed to control the austenite grain size and the hardness, respectively. The work materials were austenitized at 1123 or 1223 K for 30 min, and oil-quenched or cooled in air to room temperature, and then annealed at 763, 873 or 903 K for 300 min. Here, the oil quench was used for accelerated cooling to obtain sufficient hardness for the Base steel. Cylinders for the sliding experiments, 57 mm in diameter and 500 mm in length, were cut out from the heat-treated round steel bars. The cylinders were turned to a diameter of approximately
2.2. Sliding contact rig and analysis The crossed cylinders sliding test rig was the same as used in some previous studies [14–16]. The small TiN coated HSS cylinder represented the rake face of a cutting tool and the large rotating work material cylinder represented the chip. The normal load, sliding speed, the total sliding distance and sliding time were 75 N, 100 m/min, 3.0 m and 1.8 s respectively, see Table 2. All tests were repeated three times, and performed at room temperature in air. Shorter sliding times of 0.025, 0.05, 0.1, 0.3, 0.6, 1.2 s were also used once in order to study how the material transfer developed with sliding time. Based on a previous study in the same rig [15], concluding that the same type of oxide layer formed in both intermittent and continuous sliding, all tests in this work were performed in continuous sliding. After the tests, the tool cylinder surfaces were studied using optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Auger electron spectroscopy (AES). Cross-sections of transfer layers were prepared using a focused ion beam (FIB) technique. Before preparing the crosssection, the platinum layer was deposited on top to protect the surface. The acceleration voltages for SEM imaging and EDS analysis were either 3, 10 or 15 kV.
Table 1 Chemical composition (mass%), hardness (HV) and heat treatment of the work materials.
Base Si Cr
C
Si
Cr
Mn
S
Hardness
Heat treatment
0.57 0.56 0.56
0.005 1.001 0.002
0.001 o 0.001 1.007
0.001 o 0.001 o 0.001
0.001 0.001 0.001
202 199 194
1123 K/30 min – Oil quench-763 K/300 min 1223 K/30 min – Air cooling-873 K/300 min 1123 K/30 min – Air cooling-903 K/300 min
Base
Si
0.2 mm
Cr
0.2 mm
0.2 mm
Fig. 1. Microstructure of the steel work materials. All steels mainly contained a fine pearlite microstructure. Small amounts of ferrite included in the Si and Cr alloyed steels are indicated by arrows.
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3. Results 3.1. Friction characteristics The three work material steels exhibited pronounced differences in friction behavior, see Fig. 2. Compared with the Base steel, the use of the Si and Cr alloyed steels resulted in a lower and a higher friction coefficient, respectively. The friction coefficient against the Base steel and the Si alloyed steel rapidly stabilized at approximately 0.9 and 0.65, respectively. On the other hand, the friction coefficient against the Cr alloyed steel changed drastically during the test and it also showed larger fluctuations than the Base steel and the Si alloyed steel. The friction coefficient against the Cr alloyed steel initially rose for about 0.15 s to a value of about 0.9, kept increasing slightly to reach about 1.05 after some 0.9 s, decreased gradually until about 1.5 s and then stabilized at about 0.9. The change in fluctuation at about 1.5 s corresponds to cracking of the coating and a subsequent change of material transfer, as described below. 3.2. Surface appearance of tool cylinders and transfer layer composition Even as quickly as 0.025 s, different types of transfer layers had formed in zone I on all TiN coated cylinders, i.e. in the inner part of the contact mark as indicated in Fig. 3. The transfer layer in this region became light gray after sliding against the Base steel, black against the Si alloyed steel and it got a silver color against the Cr alloyed steel. When sliding against the Base steel, the area of zone I grew larger during the first 0.1 s and then remained almost constant in size. Against the Si alloyed steel, zone I expanded over the entire test duration, i.e. 1.8 s. When sliding against the Cr alloyed steel, the area of zone I expanded until 0.3 s and then remained almost constant until 1.2 s. At 1.8 s, a large contact mark Table 2 Sliding test conditions. Test parameter Sliding speed Normal load Sliding distance Test duration Lubrication Atmosphere Temperature
100 m/min 75 N Up to 3.0 m Up to 1.8 s None (Dry) In air 21 °C (RT)
Cr
Base
Si
Fig. 2. Representative friction curves of the Base steel, the Si alloyed steel and the Cr alloyed steel.
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and a transfer lump were observed, which is consistent with the change in friction fluctuation at about 1.5 s in Fig. 2. Against all steels, also another transfer layer was formed in a narrow region outside zone I, here indicated zone II. The TiN coated cylinder slid against the Base steel for 0.025 s illustrates that zone I is positioned in the center of contact, where the contact pressure and temperature were the highest, see Fig. 4. The entire elliptical contact mark is made visible using a low acceleration voltage in the SEM, here 3 kV. The EDS element maps in Fig. 5 show that zone I contained Fe and O after sliding against the Base steel, Fe, Si and O against the Si alloyed steel and Fe, Cr and O against the Cr alloyed steel. For all steels, zone II contained Fe and O only. To represent sliding against the Cr alloyed steel, the contact mark was analyzed after 0.6 s, i.e. at a time when the friction coefficient is at its stable high level. Material transfer in zone I is further described in the following section. 3.3. Material transfer behavior 3.3.1. Transfer from the Base steel The gray transfer layer from the Base steel was found to be Fe– O while the bright transfer layer was almost metallic steel, see Fig. 6. On the TiN coated cylinder slid only 0.025 s, it appears as if the transfer layer has grown by the accumulation of thin layers. Local metallic steel transfer, as shown in the white dashed rectangle in Fig. 6a, was initiated at the front edges of the Fe–O transfer. After 1.8 s the transfer layer has expanded towards the front and it has become more homogeneous, although some metallic steel transfer can still be found. A FIB cross-section of the TiN coated cylinder slid against the Base steel for 1.8 s revealed that the transfer layer was homogeneous and fully covering the coating surface, see Fig. 7. In the analyzed region it was approximately 1.2 μm thick. 3.3.2. Transfer from the Si alloyed steel The layer transferred from the Si alloyed steel developed in a more complex way, see Fig. 8a–d. Zone I could be divided into two regions; one rough and one smooth. The rough region was positioned in the central part of zone I. The smooth region surrounded the rough region at the sides and in front of it. Magnified images of three areas A, B and C are shown in Fig. 8c and d. A is positioned in the rough region, B in the border zone between the rough and smooth regions, and C in the smooth region. It was confirmed from EDS spectra, see Fig. 8e and f, that the dark transfer layer was Fe– Si–O while the bright transfer was almost metallic steel from the work material. The Si is strongly enriched within the dark transfer layer, cf. Fig. 8e and g. In area A after 0.025 s, the pieces of Fe–Si–O and metallic steel transfer were small and non-coherent, see Fig. 8c. The size of the transferred pieces increased with sliding time, see Fig. 8d. In area B, both smooth Fe–Si–O transfer and metallic steel transfer were observed after 0.025 s, see Fig. 8c. As on the TiN coated cylinder slid against the Base steel, the steel transfer was positioned at the front edges of the oxidized transfer, this time on the Fe–Si–O, see the white dashed rectangle. After 0.3 s, the Fe–Si–O layer had grown to include more and larger pieces of steel transfer, resulting in a mixed and irregular surface, see Fig. 8d. In area C, a smooth Fe–Si–O layer was seen after 0.025 s, see Fig. 8c. After 0.3 s, this layer had grown, apparently by accumulation of thin layers, and contained a small amount of metallic steel transfer, see Fig. 8d. While the smooth region expands by growth of the smooth Fe–Si–O layer, a concurrent process of Fe–Si–O formation and metallic steel transfer occurs in the center and rear areas, resulting in simultaneous expansion of the rough region into the previously smooth region.
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Fig. 3. Optical microscope images of the contact marks on TiN coated cylinder after sliding against (a) the Base steel, (b) the Si alloyed steel and (c) the Cr alloyed steel obtained in separate tests with different sliding times. Images from tests for 0.025, 0.6 and 1.8 s are chosen to illustrate the change in transfer. The sliding direction of the work material is from right to left in the plane of paper. The two different modes of transfer layers are indicated by arrows showing zone I and II. The apparent full contact area, which is referred to in Fig. 16, is defined as the area surrounded by the white dashed line.
Sliding direction of work material Edge of entire contact mark
Transfer layer (zone I) 100 μm Fig. 4. An SEM image of the entire contact mark on the TiN coated cylinder after sliding against the Base steel for 0.025 s. The electron beam acceleration voltage was 3 kV.
After 1.8 s, the front area of the transfer layer from the Si alloyed steel exhibits smooth Fe–Si–O hills with minute grooves, see Fig. 9. These hills actually surround most of the front edge, see e.g. the position of the white arrow in Fig. 9a. In the center, Fig. 9b, the transfer layer contained both higher and lower parts. The higher parts displayed very faint grooves along the sliding direction, as shown by the black arrows in Fig. 9c, while no such grooves were visible in the lower parts. This indicates that most of the rough region is not in contact with the sliding counter surface. The transfer layer in the center area was rough, around a micrometer thick, consisting of a mixture of Fe–Si–O (dark) and metallic steel (bright), see Fig. 10a. In the front area the transfer layer consisted mainly of Fe–Si–O, containing small amounts of steel pieces apparently distributed in layers, see Fig. 10b. In this region, the transfer layer was fully covering the coating and reached a thickness of approximately 6.4 μm.
3.3.3. Transfer from the Cr alloyed steel The transfer from the Cr alloyed steel showed two modes after 0.025 s, see Fig. 11a. AES analyses revealed that the dark transfer layer was Fe–Cr–O while the bright was metallic steel, see Fig. 12. The steel transfer tended to adhere on the front edges of the Fe–Cr– O transfer, as shown in the white dashed rectangle in Fig. 11a. After 0.6 s, the transfer area was broader and the steel was transferred in larger pieces, see Fig. 11b. As shown in Fig. 12, in the dark area Fe– Cr–O covered the coating to a thickness of about 150 nm. In the bright area, the same kind of oxide, of approximately the same thickness, covered the coating and on top of that an approximately 250 nm thick metallic steel was added. The observations of the layer transferred from the Cr alloyed steel after 0.6 s were further strengthened by the cross-section study shown in Fig. 13. An extensive Fe–Cr–O layer was indeed covering the TiN coating and steel adhered on top of that. Within the entire analyzed cross section, the Fe–Cr–O layer was rather thin, while the steel transfer was much thicker. After sliding for 1.8 s against the Cr alloyed steel, cracks had formed in the coating and the steel was transferred along the cracks, see Fig. 14. Despite the cracks, the substrate was not exposed, as proven by the absence of EDS signal from W, which is abundant in the substrate.
4. Discussion The experimental results clearly showed that alloy element additions of 1 mass% had a significant effect on both the material transfer mechanism and the friction coefficient. The contact situations against the three steels are schematically shown in Fig. 15. Please note that the vertical scale is extremely exaggerated to visualize the contact more clearly. The transfer layers from (a) the Base steel and (c) the Cr alloyed steel show only small topography and consequently almost the full zone I areas help to
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Fig. 5. EDS maps of the contact marks on the TiN coated cylinders after sliding against (a) the Base steel, (b) the Si alloyed steel and (c) the Cr alloyed steel. The electron beam acceleration voltage was 10 kV.
support the normal load. Contrastingly, in order to explain the contact situation of the Si alloyed steel, the contributions from both the rough and smooth regions have to be considered. The large topography of the transfer layer from the Si alloyed steel prevents the load from being supported by the full area. In fact, it was shown in Fig. 9 that the rough region included both higher and lower parts, and that the higher parts but not the lower parts contributed to the contact. This means the transfer layer supports the load in the smooth region plus the higher parts of the rough region as shown in Fig. 15. In a previous study [14] it was concluded that the friction coefficient increased with an increase in contact area covered with the iron oxide in zone II. However, in this study, it is shown that the transfer layer in zone I, rather than that in zone II, has the largest influence on the friction. This is because zone I now covers a substantial fraction of the contact mark. In Fig. 16, the size of the apparent full contact areas as defined in Fig. 3, and the size of zone I are plotted. Please note that since the values in Fig. 16 were obtained from separate tests with different test durations, there is
some variability in the curves. Except for the very early stage and when the Cr alloyed steel caused coating cracking after 1.8 s, the apparent full contact areas are almost stable and have about the same level, see Fig. 16a. This is reasonable since the size of the contact area is decided mainly by the normal load and the hardness of the work material, and the hardness of all tested steels is almost the same. On the other hand, the sizes of zone I were different between all steels, see Fig. 16b. When sliding against the Base steel, the change in size of zone I was small, while against the Cr alloyed steel, the size increased for 0.3 s and then became stable until it increased at 1.8 s. These tendencies for the Base steel and the Cr alloyed steel seem to match the friction coefficients in Fig. 2. However, when sliding against the Si alloyed steel, zone I continuously grew with increasing sliding time. This tendency differs from that of the friction coefficient. This means the area of zone I shown in Fig. 16b is over-estimated. The real contact size of the rough region for the Si alloyed steel is difficult to measure. Instead, we can assume that the size of the contact area is not very different from that formed against the
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Gray transfer
SEM images after 0.025 s
Bright transfer
Sliding direction of work material
50 μm
2 μm
SEM images after 1.8 s
50 μm
2 μm
EDS spectra Gray transfer
Bright transfer
Fig. 6. SEM images and EDS spectra of the transfer layer on the TiN coated cylinder after sliding against the Base steel for (a) 0.025 s and (b) 1.8 s. Overview images are shown to the left and the areas in the black rectangles are magnified in the images to the right. A white dashed rectangle indicates an area with steel transfer. Gray and bright transfers are indicated by black arrows. (c) EDS spectra from the gray and bright transfer area. The electron beam acceleration voltage was 3 kV.
Sliding direction of work material
Pt protective layer Transfer layer
TiN
High speed steel 1 μm Fig. 7. An SEM image of a FIB cross-section of the transfer layer in the contact mark on the TiN coated cylinder after sliding against the Base steel for 1.8 s. The electron beam acceleration voltage was 10 kV.
Base steel. That would mean the lower friction against the Si alloyed steel, compared with that against the Base steel, is due to easier shearing of the interfacial region between the Fe–Si–O and the work material at the elevated temperature. Umino et al. [17] reported that the Fe–Si–O produced in the interface of tool and chip in milling gives a lubricating effect. They suggested that this is because the Fe–Si–O, which has a low eutectic temperature, softens at elevated temperature. Actually, the eutectic temperature of Fe–Si–O is about 190 K lower than the melting point of Fe–O; about 1453 K for Fe–Si–O, about 1643 K for Fe–O [18]. This fact supports the hypothesis of easier shearing of an interfacial region including Fe–Si–O in the present work. The friction coefficient against the Si alloyed steel was almost constant throughout the test, c.f. Fig. 2. This suggests that the real contact area was almost constant even though the zone I area and the topography changed.
T. Aiso et al. / Tribology International 97 (2016) 337–348
Overview SEM images 0.025 s
343
0.3 s
Sliding direction of work material A
B
Rough
C
A
C B
Rough
Smooth
Smooth
50 μm
50 μm
Magnified SEM images 0.025 s
Bright transfer 4 μm
4 μm
4 μm
4 μm
Dark transfer
4 μm
0.3 s
EDS spectra Dark transfer
Bright transfer
4 μm
Work material
Fig. 8. SEM images of the transfer layers after (a) (c) 0.025 s and (b) (d) 0.3 s of sliding against the Si alloyed steel. Overview images are shown in (a) and (b), and the areas A, B and C in the white rectangles are magnified in (c) and (d). EDS spectra obtained from (e) the dark transfer, (f) bright transfer and (g) the bulk work material. Rough and smooth regions are indicated by white arrows in (a) and (b). Dark and bright transfers are indicated by white arrows in (c). The electron beam acceleration voltage was 3 kV.
In contrast, the friction coefficient increased against the Cr alloyed steel. The high level seems to be consistent with the large area of the transfer layer. Crack generation of TiN coated HSS has
been explained to be caused by the high tensile stresses generated in the coating during sliding [11]. It is also reported that this cracking is aggravated by an increased sliding speed because
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50 μm
4 μm
4 μm
1 μm
Fig. 9. SEM images of the contact mark on the TiN coated cylinder after sliding against the Si alloyed steel for 1.8 s, tilted 65°. Overview images are shown in (a) and the areas in the black rectangles are magnified in (b)–(d). The white arrow in (a) indicates the smooth Fe–Si–O hills. The black arrows in (c) indicate fine grooves. The electron beam acceleration voltage was 10 kV.
Front area
Sliding direction of work material
Pt protective layer Center area
Sliding direction of work material
Transfer layer Transfer layer Pt protective layer
TiN
TiN
1 μm
High speed steel
1 μm
High speed steel
Fig. 10. SEM images of FIB cross-sections in the transfer layer on the TiN coated cylinder after sliding against the Si alloyed steel for 1.8 s. The cross-sections were made in (a) the lower part of the center area and (b) the front area in the transfer layer. The electron beam acceleration voltage was 10 kV.
friction heat softens the substrate, which then deforms to a degree that leads to coating fracture [11,16,19]. In this test, only the Cr alloyed steel leads to friction heating and tensile stresses sufficient to result in coating cracks. The wide fluctuation of the friction curve of the Cr alloyed steel was actually the result of an induced vibration. Analysis of the friction curve showed a wavelength of about 1 mm (i.e. a period of about 0.6 ms) and the track on the work material also showed cyclic changes in appearance with a wavelength of about 1 mm. This wavelength is not linked to the much smaller length scales of the microstructure. It is interesting at this point that only the Cr alloyed steel induced this vibration while the exact reason for that is not yet known. Frictional vibrations can generate various forms of errors in the friction measurement, as reported e.g. in [20]. The
friction presented for the Cr alloyed steel can possibly be influenced by such errors. The oxides formed in the center of the contact differ from the native oxides that are present on the surface of the work materials. Even though the very initial oxide transferred to the TiN surface may simply be the native oxide scraped off the work material, the growing oxide rapidly changes in character to something else. In a previous study [15], the oxide formation in the center of the contact was concluded to be very different from the native oxide on the work material. It was suggested to rely on reactions occurring in a condition with high temperature, high pressure and low oxygen supply. Furthermore, in another study [16], involving low sliding speed, almost no such oxide was formed. Together, this suggests that friction heating allows new reactions that are needed for this
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0.025 s
345
Dark transfer
Bright transfer
Sliding direction of work material
50 μm
2 μm
0.6 s
50 μm
2 μm
Dark transfer area
Bright transfer area
100
100
Fe
Ti + N
Atomic concentration (at%)
Atomic concentration (at%)
Fig. 11. SEM images of the transfer layers on the TiN coated cylinder after sliding against the Cr alloyed steel for (a) 0.025 s and (b) 0.6 s. Overview images are shown to the left and the areas in the black rectangles are magnified in the images to the right. Dark and bright transfers are indicated by black arrows. The electron beam acceleration voltage was 3 kV.
80
O
60
40
Cr 20
0
Fe
0
100
200
300
400
500
Sputter depth (nm)
Ti + N
80
60
O
40
Cr
20
0
0
100
200
300
400
500
Sputter depth (nm)
Fig. 12. AES depth profiles of (a) the dark transfer layer and (b) the bright transfer layer formed against the Cr alloyed steel after 0.025 s. The depth was calculated using the sputtering rate of approximately 17.1 nm/min in Ta2O5 as a reference.
new oxide to form. Against the Si alloyed steel, the oxide growth is more complex. The smooth region follows the description above, but the rough region is influenced by fragments fractured from the smooth front region. In the rough region the growth of the oxide is likely a combined result of the new reactions and agglomerated fragments, producing the high topography. It is interesting to note that Si and Cr are extremely enriched in the oxide layers, as shown in Figs. 8e and 12a. This is reasonable, because these alloy elements are known to oxidize more easily than Fe [21]. Although most atoms that become oxidized are still Fe because of its dominance over Si and Cr in the work material, the alloy elements will become enriched in the oxides. As the
oxide grows, more or less metallic steel is concurrently transferred from the work material to the oxide, depending on the type of oxide and the steel composition. Material transfer is reported to be influenced by the topography and adhesion tendency [22]. Considering Figs. 6a, 8c and 11a, the topography is believed to give a strong contribution to the metallic steel transfer in the present work. It was concluded in the previous studies [14,15] that these sliding experiments reproduced the material transfer in milling. The type and hardness of the tool material, the hardness of the work material and sliding contact parameters used in the present work are very similar to those in the previous studies. Thus, we
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Sliding direction of work material
a
Transfer layer
b Pt protective layer
TiN
Ti
Fe
Cr High speed steel 1 μm Fig. 13. (a) An SEM image and (b) EDS maps of a FIB cross-section in the transfer layer on the TiN coated cylinder after sliding against the Cr alloyed steel for 0.6 s. The EDS maps were obtained from the black rectangle in (a). The electron beam acceleration voltage was 10 kV.
Crack
Sliding direction of work material
Steel 100 μm
Fe
50 μm
O
W
Fig. 14. Backscattered electron images of (a) the entire material transfer region on the TiN coated cylinder after sliding against the Cr alloyed steel for 1.8 s and (b) a magnification of the area marked with the white rectangle in (a). EDS maps of the magnified area are shown in (c). Cracks of the coating and the steel transfer are indicated by black arrows. The electron beam acceleration voltage was 15 kV.
believe that also the tendencies of material transfer and friction behavior in the present work are relevant for real milling.
5. Conclusions The influence of Si and Cr additions to a carbon steel on material transfer and friction characteristics during sliding between the steels and TiN coated HSS was investigated. Three especially designed model steels were produced to allow studies of the individual roles of the alloy elements; Fe–0.55C (Base steel), Fe–0.55C–1Si (Si alloyed steel) and Fe–0.55C–1Cr (Cr alloyed steel) in mass %. The main conclusions are as follows.
● In the center of the contact areas on the TiN coated cylinder, different types of transfer layers were formed depending on the steel composition; ○ Base steel: A homogeneous Fe–O with a small amount of metallic steel was formed. ○ Si alloyed steel: A mixed layer composed of Fe–Si–O and metallic steel transfer. The oxide contained a large amount of Si. ○ Cr alloyed steel: An Fe–Cr–O was formed on the TiN coating, and metallic steel locally adhered on the top of the Fe–Cr–O. The oxide contained a large amount of Cr. ● Compared to the Base steel, the friction coefficient was lower against the Si alloyed steel, mainly explained by easier shearing of the interfacial region between the Fe–Si–O and the work material at elevated temperature.
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Base
347
Direction of rotation
Work material Deformation of work material
Transfer layer (Fe-O)
TiN
Normal load
Work material
Si
Direction of rotation
Deformation of work material Transfer layer
Rough region (Fe-Si-O & metallic steel)
TiN
Smooth region (Fe-Si-O)
Normal load
Cr
Work material
Direction of rotation
Deformation of work material
Transfer layer (Fe-Cr-O & metallic steel)
TiN
Normal load
Size of apparent full contact areas
Size of zone I
0.24
0.16 Coating cracking
Coating cracking
0.20 Base 0.16
Si Cr
0.12
0.08 0.0
0.5
1.0
1.5
2.0
Area of transfer (mm2)
Apparent full contact area (mm 2)
Fig. 15. Cross-sectional illustrations of the contact situation along the central lines of contact, when the material transfer and the friction coefficient have become relatively stable, i.e. at 1.8 s for the Base steel and the Si alloyed steel, and at 0.6 s for the Cr alloyed steel.
0.12 Base 0.08
Si Cr
0.04
0.00 0.0
0.5
1.0
1.5
2.0
Sliding time (s)
Sliding time (s)
Fig. 16. (a) Apparent full contact areas, (b) areas of zone I, measured using optical microscope images.
● Compared to the Base steel, the friction coefficient was higher against the Cr alloyed steel, high enough to cause coating cracking.
Acknowledgments Dr. Ulf Bexell, Dalarna University, is acknowledged for assistance with the AES analysis. Financial support from Nippon Steel & Sumitomo Metal Corporation is acknowledged.
References [1] Trent EM, Wright PK. Metal cutting. 4th ed. Oxford: Butterworth-Heineman; 2000. [2] Ohgo K, Nakajima K, Awano T. Layer formation on the rake face of a cutting tool and its effect on tool life. Wear 1976;40:85–92. [3] Yamane Y, Usuki H, Yan B, Narutaki N. The formation of a protective oxide layer in machining resulphurized free-cutting steels and cast irons. Wear 1990;139:195–208. [4] Qi HS, Mills B. On the formation mechanism of adherent layers on a cutting tool. Wear 1996;198:192–6. [5] Kümmel J, Gibmeier J, Müller E, Schneider R, Schulze V, Wanner A. Detailed analysis of microstructure of intentionally formed built-up edges for improving wear behaviour in dry metal cutting process of steel. Wear 2014;311:21–30.
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[6] Kümmel J, Braun D, Gibmeier J, Schneider J, Greiner C, Schulze V, Wanner A. Study on micro texturing of uncoated cemented carbide cutting tools for wear improvement and built-up edge stabilisation. J Mater Process Technol 2015;215:62–70. [7] Atlati S, Haddag B, Nouari M, Moufki A. Effect of the local friction and contact nature on the Built-Up Edge formation process in machining ductile metals. Tribol Int 2015;90:217–27. [8] Yoshimura H, Miyazaki K, Hatashita Y, Moriwaki T, Shibasaka T. Study on dry cutting in hobbing. J Jpn Soc Abras Technol 2007;51:106–11. [9] Gerth J, Larsson M, Wiklund U, Riddar F, Hogmark S. On the wear of PVDcoated HSS hobs in dry gear cutting. Wear 2009;266:444–52. [10] Olsson M, Söderberg S, Jacobson S, Hogmark S. Simulation of cutting tool wear by a modified pin-on-disc test. Int J Mach Tools Manuf 1989;29:377–90. [11] Hedenqvist P, Olsson M, Söderberg S. Influence of TiN coating on wear of high speed steel tools as studied by new laboratory wear test. Surf Eng 1989;5:141–50. [12] Grzesik W, Zalisz Z, Nieslony P. Friction and wear testing of multilayer coatings on carbide substrates for dry machining applications. Surf Coat Technol 2002;155:37–45. [13] Zemzemi F, Rech J, Ben Salem W, Dogui A, Kapsa P. Development of a friction model for the tool–chip–workpiece interfaces during dry machining of AISI4142 steel with TiN coated carbide cutting tools. Int J Mach Mach Mater 2007;2:361–77. [14] Gerth J, Heinrichs J, Nyberg H, Larsson M, Wiklund U. Evaluation of an intermittent sliding test for reproducing work material transfer in milling operations. Tribol Int 2012;52:153–60.
[15] Heinrichs J, Gerth J, Bexell U, Larsson M, Wiklund U. Influence from surface roughness on steel transfer to PVD tool coatings in continuous and intermittent sliding contacts. Tribol Int 2012;56:9–18. [16] Heinrichs J, Gerth J, Thersleff T, Bexell U, Larsson M, Wiklund U. Influence of sliding speed on modes of material transfer as steel slides against PVD tool coatings. Tribol Int 2013;58:55–64. [17] Umino M, Sera T, Okada Y, Murakami D, Murakami R, Tubakino H. Effect of silicon content and lubricant on the machinability of hot working tool steel. Tetsu-to-Hagané 2003;89:601–8. [18] Levin EM, Robbins CR, McMurdie HF. Phase diagrams for ceramists, volume I. USA: The American Ceramic Society; 1964. [19] Hedenqvist P, Olsson M. Sliding wear testing of coated cutting tool materials. Tribol Int 1991;24:143–50. [20] Kado N, Tadokoro C, Nakano K. Measurement error of kinetic friction coefficient generated by frictional vibration. Trans Jpn Soc Mech Eng Ser C 2013;79:2635–43. [21] Handbook of Iron and Steel. 4th ed. (CD-ROM). The Iron and Steel Institute of Japan; 2002. [22] Heinrichs J, Olsson M, Jacobson S. Mechanisms of material transfer studied in situ in the SEM: explanations to the success of DLC coated tools in aluminium forming. Wear 2012;292–293:49–60.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of Si and Cr additions to carbon steel on material transfer in a steel/TiN coated tool sliding contact T. Aiso a,n, U. Wiklund a, M. Kubota b, S. Jacobson a a b
Tribomaterials group, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Steel Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 1-8 Fuso-Cho, Amagasaki, Hyogo, 660-0891, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2015 Received in revised form 20 January 2016 Accepted 21 January 2016 Available online 29 January 2016
A crossed cylinders sliding test, simulating the contact between the chip and the tool in machining, is used to evaluate material transfer and friction characteristics of a TiN coating against specifically designed model steels. These include one base reference, only alloyed with C (Base steel) and two alloyed also with 1 mass% Si or Cr. When sliding against the Base steel, an Fe–O layer is formed on the coating. Against the Si and Cr alloyed steels, Fe–Si–O and Fe–Cr–O layers are formed. In these oxides, Si and Cr are enriched, i.e. preferentially transferred from the steels. Compared to the Base steel, the friction coefficient is significantly lower against the Si alloyed steel and higher against the Cr alloyed steel. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Transfer Coating Sliding
1. Introduction Material transfer often occurs between work materials and cutting tools, and the amount and the mode of it depend on machining conditions, tools and work material types. It is wellknown that transfer layers on the cutting edge sometimes trigger the formation of built-up edges and deteriorate the quality of the machined surface [1]. This indicates that transfer should be minimized to avoid such effects. On the other hand, it is also suggested that transfer layers on the rake face of the tool prevent the wear of the tool [2–4]. To promote the formation of transfer layers with a beneficial impact on machining performance and tool life, it is of great importance to understand and control the material transfer. Recent research in this area reflects a strong industrial interest; see for example Ref. [5–7]. In dry hobbing of case hardening steels, it was reported by Yoshimura et al. [8] that transfer layers formed on the rake face reduced the crater wear. Gerth et al. [9] reported on transfer layers on the rake faces of hob teeth, consisting of oxides of Fe, Cr, Mn and Si. All these elements except oxygen were constituents of the steel work material. They concluded that the roles of the oxide layer were probably wear protection of the tool and friction reduction between the chip and the tool. They also suggested that the oxide layer composition, and thus its properties, was n
Corresponding author. E-mail address: [email protected] (T. Aiso).
http://dx.doi.org/10.1016/j.triboint.2016.01.032 0301-679X/& 2016 Elsevier Ltd. All rights reserved.
influenced by the composition of steel work material. Therefore, understanding the effects of different alloy elements is valuable for several reasons. For example, to the component manufacturers such knowledge will tell how the element influences the machinability of the steel and hint at how to tune the machining parameters. To the steel manufacturers it can provide means for tuning their products for optimized machinability in terms of low friction and high tool wear protection. Since machining is a complex process, several simplifying sliding tests have been used to simulate the contact between the chip and the rake face [10–13]. In recent years, Gerth et al. [14] used a crossed cylinders sliding test. This excludes the chip forming in the primary shear zone and focuses on simulating only the contact between the chip and the rake face. Very similar oxide layers were formed in the sliding test as in actual milling. It was concluded that the test is capable of reproducing the work material transfer occurring in milling, and thus it is a powerful tool for studying the influence of oxide layers on friction and wear. This work focuses on the effect of Si and Cr, two commonly used alloy elements, on material transfer and friction behavior during sliding. To isolate and study the influence of the individual alloy elements, model steels designed specifically for this purpose were necessary. Binary and ternary alloys were prepared to have a similar microstructure and hardness, with the only significant difference being the chemical composition, i.e. the addition of 1 mass% of either Si or Cr. The steels were slid against a TiN coated high speed steel (HSS) cylinder, representing a commonly used tool surface.
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56 mm right before starting the test, to ensure a clean surface was achieved. The turning was carried out with coated carbide tools under dry condition, with a cutting speed of 250 m/min, a feed of 0.061 mm/rev and a cutting depth of 0.1 mm. This resulted in similar surface roughness; about Ra 0.8, 0.6 and 0.5 μm for the Base steel, the Si alloyed steel and the Cr alloyed steel, respectively. The microstructure and the Vickers hardness of the three cylinders are shown in Fig. 1 and Table 1, respectively. All steels mainly contained a fine pearlite microstructure, although the Base steel possibly included some amount of tempered martensite, and the Si and Cr alloyed steels contained small amounts of ferrite. The hardness of all steels was around 200 HV.
2. Material and methods 2.1. Materials 2.1.1. Tool material Cylinders with a diameter of 5 mm and a length of 20 mm were produced from a powder metallurgy HSS ASP2023 with a hardness of about 850 HV. The cylinders were polished and coated with PVD TiN. After coating deposition, the cylinders were polished and the final roughness, measured with white light interference profilometry, was Ra 0.083 70.009 μm. The coating thickness was about 3.6 μm. The coating hardness, measured using nanoindentation with an indent depth of 200 nm, was 25 72 GPa. 2.1.2. Work material Three types of steels, nominally containing 0.55 mass% C, were used as the work materials. The chemical compositions of the steels are shown in Table 1. In the Base steel, C was the only alloy element. The Si and Cr alloyed steels also contained nominally 1.0 mass% of Si or Cr, respectively. The steels were melted in a laboratory vacuum melting furnace, cast into ingots 180 kg in weight, hot forged into round bars with a diameter of 70 mm, heated to 1523 K for 2 h and cooled in air to room temperature as a homogenization treatment to reduce the segregation of the alloy elements, and then austenitized at 1203 K for 30 min and cooled in air for the purpose of normalizing. Subsequently, each steel underwent different heat treatments to minimize differences in microstructure and hardness. The heat treatment cycles are shown in Table 1. The austenitizing and annealing temperatures were changed to control the austenite grain size and the hardness, respectively. The work materials were austenitized at 1123 or 1223 K for 30 min, and oil-quenched or cooled in air to room temperature, and then annealed at 763, 873 or 903 K for 300 min. Here, the oil quench was used for accelerated cooling to obtain sufficient hardness for the Base steel. Cylinders for the sliding experiments, 57 mm in diameter and 500 mm in length, were cut out from the heat-treated round steel bars. The cylinders were turned to a diameter of approximately
2.2. Sliding contact rig and analysis The crossed cylinders sliding test rig was the same as used in some previous studies [14–16]. The small TiN coated HSS cylinder represented the rake face of a cutting tool and the large rotating work material cylinder represented the chip. The normal load, sliding speed, the total sliding distance and sliding time were 75 N, 100 m/min, 3.0 m and 1.8 s respectively, see Table 2. All tests were repeated three times, and performed at room temperature in air. Shorter sliding times of 0.025, 0.05, 0.1, 0.3, 0.6, 1.2 s were also used once in order to study how the material transfer developed with sliding time. Based on a previous study in the same rig [15], concluding that the same type of oxide layer formed in both intermittent and continuous sliding, all tests in this work were performed in continuous sliding. After the tests, the tool cylinder surfaces were studied using optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Auger electron spectroscopy (AES). Cross-sections of transfer layers were prepared using a focused ion beam (FIB) technique. Before preparing the crosssection, the platinum layer was deposited on top to protect the surface. The acceleration voltages for SEM imaging and EDS analysis were either 3, 10 or 15 kV.
Table 1 Chemical composition (mass%), hardness (HV) and heat treatment of the work materials.
Base Si Cr
C
Si
Cr
Mn
S
Hardness
Heat treatment
0.57 0.56 0.56
0.005 1.001 0.002
0.001 o 0.001 1.007
0.001 o 0.001 o 0.001
0.001 0.001 0.001
202 199 194
1123 K/30 min – Oil quench-763 K/300 min 1223 K/30 min – Air cooling-873 K/300 min 1123 K/30 min – Air cooling-903 K/300 min
Base
Si
0.2 mm
Cr
0.2 mm
0.2 mm
Fig. 1. Microstructure of the steel work materials. All steels mainly contained a fine pearlite microstructure. Small amounts of ferrite included in the Si and Cr alloyed steels are indicated by arrows.
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3. Results 3.1. Friction characteristics The three work material steels exhibited pronounced differences in friction behavior, see Fig. 2. Compared with the Base steel, the use of the Si and Cr alloyed steels resulted in a lower and a higher friction coefficient, respectively. The friction coefficient against the Base steel and the Si alloyed steel rapidly stabilized at approximately 0.9 and 0.65, respectively. On the other hand, the friction coefficient against the Cr alloyed steel changed drastically during the test and it also showed larger fluctuations than the Base steel and the Si alloyed steel. The friction coefficient against the Cr alloyed steel initially rose for about 0.15 s to a value of about 0.9, kept increasing slightly to reach about 1.05 after some 0.9 s, decreased gradually until about 1.5 s and then stabilized at about 0.9. The change in fluctuation at about 1.5 s corresponds to cracking of the coating and a subsequent change of material transfer, as described below. 3.2. Surface appearance of tool cylinders and transfer layer composition Even as quickly as 0.025 s, different types of transfer layers had formed in zone I on all TiN coated cylinders, i.e. in the inner part of the contact mark as indicated in Fig. 3. The transfer layer in this region became light gray after sliding against the Base steel, black against the Si alloyed steel and it got a silver color against the Cr alloyed steel. When sliding against the Base steel, the area of zone I grew larger during the first 0.1 s and then remained almost constant in size. Against the Si alloyed steel, zone I expanded over the entire test duration, i.e. 1.8 s. When sliding against the Cr alloyed steel, the area of zone I expanded until 0.3 s and then remained almost constant until 1.2 s. At 1.8 s, a large contact mark Table 2 Sliding test conditions. Test parameter Sliding speed Normal load Sliding distance Test duration Lubrication Atmosphere Temperature
100 m/min 75 N Up to 3.0 m Up to 1.8 s None (Dry) In air 21 °C (RT)
Cr
Base
Si
Fig. 2. Representative friction curves of the Base steel, the Si alloyed steel and the Cr alloyed steel.
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and a transfer lump were observed, which is consistent with the change in friction fluctuation at about 1.5 s in Fig. 2. Against all steels, also another transfer layer was formed in a narrow region outside zone I, here indicated zone II. The TiN coated cylinder slid against the Base steel for 0.025 s illustrates that zone I is positioned in the center of contact, where the contact pressure and temperature were the highest, see Fig. 4. The entire elliptical contact mark is made visible using a low acceleration voltage in the SEM, here 3 kV. The EDS element maps in Fig. 5 show that zone I contained Fe and O after sliding against the Base steel, Fe, Si and O against the Si alloyed steel and Fe, Cr and O against the Cr alloyed steel. For all steels, zone II contained Fe and O only. To represent sliding against the Cr alloyed steel, the contact mark was analyzed after 0.6 s, i.e. at a time when the friction coefficient is at its stable high level. Material transfer in zone I is further described in the following section. 3.3. Material transfer behavior 3.3.1. Transfer from the Base steel The gray transfer layer from the Base steel was found to be Fe– O while the bright transfer layer was almost metallic steel, see Fig. 6. On the TiN coated cylinder slid only 0.025 s, it appears as if the transfer layer has grown by the accumulation of thin layers. Local metallic steel transfer, as shown in the white dashed rectangle in Fig. 6a, was initiated at the front edges of the Fe–O transfer. After 1.8 s the transfer layer has expanded towards the front and it has become more homogeneous, although some metallic steel transfer can still be found. A FIB cross-section of the TiN coated cylinder slid against the Base steel for 1.8 s revealed that the transfer layer was homogeneous and fully covering the coating surface, see Fig. 7. In the analyzed region it was approximately 1.2 μm thick. 3.3.2. Transfer from the Si alloyed steel The layer transferred from the Si alloyed steel developed in a more complex way, see Fig. 8a–d. Zone I could be divided into two regions; one rough and one smooth. The rough region was positioned in the central part of zone I. The smooth region surrounded the rough region at the sides and in front of it. Magnified images of three areas A, B and C are shown in Fig. 8c and d. A is positioned in the rough region, B in the border zone between the rough and smooth regions, and C in the smooth region. It was confirmed from EDS spectra, see Fig. 8e and f, that the dark transfer layer was Fe– Si–O while the bright transfer was almost metallic steel from the work material. The Si is strongly enriched within the dark transfer layer, cf. Fig. 8e and g. In area A after 0.025 s, the pieces of Fe–Si–O and metallic steel transfer were small and non-coherent, see Fig. 8c. The size of the transferred pieces increased with sliding time, see Fig. 8d. In area B, both smooth Fe–Si–O transfer and metallic steel transfer were observed after 0.025 s, see Fig. 8c. As on the TiN coated cylinder slid against the Base steel, the steel transfer was positioned at the front edges of the oxidized transfer, this time on the Fe–Si–O, see the white dashed rectangle. After 0.3 s, the Fe–Si–O layer had grown to include more and larger pieces of steel transfer, resulting in a mixed and irregular surface, see Fig. 8d. In area C, a smooth Fe–Si–O layer was seen after 0.025 s, see Fig. 8c. After 0.3 s, this layer had grown, apparently by accumulation of thin layers, and contained a small amount of metallic steel transfer, see Fig. 8d. While the smooth region expands by growth of the smooth Fe–Si–O layer, a concurrent process of Fe–Si–O formation and metallic steel transfer occurs in the center and rear areas, resulting in simultaneous expansion of the rough region into the previously smooth region.
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Fig. 3. Optical microscope images of the contact marks on TiN coated cylinder after sliding against (a) the Base steel, (b) the Si alloyed steel and (c) the Cr alloyed steel obtained in separate tests with different sliding times. Images from tests for 0.025, 0.6 and 1.8 s are chosen to illustrate the change in transfer. The sliding direction of the work material is from right to left in the plane of paper. The two different modes of transfer layers are indicated by arrows showing zone I and II. The apparent full contact area, which is referred to in Fig. 16, is defined as the area surrounded by the white dashed line.
Sliding direction of work material Edge of entire contact mark
Transfer layer (zone I) 100 μm Fig. 4. An SEM image of the entire contact mark on the TiN coated cylinder after sliding against the Base steel for 0.025 s. The electron beam acceleration voltage was 3 kV.
After 1.8 s, the front area of the transfer layer from the Si alloyed steel exhibits smooth Fe–Si–O hills with minute grooves, see Fig. 9. These hills actually surround most of the front edge, see e.g. the position of the white arrow in Fig. 9a. In the center, Fig. 9b, the transfer layer contained both higher and lower parts. The higher parts displayed very faint grooves along the sliding direction, as shown by the black arrows in Fig. 9c, while no such grooves were visible in the lower parts. This indicates that most of the rough region is not in contact with the sliding counter surface. The transfer layer in the center area was rough, around a micrometer thick, consisting of a mixture of Fe–Si–O (dark) and metallic steel (bright), see Fig. 10a. In the front area the transfer layer consisted mainly of Fe–Si–O, containing small amounts of steel pieces apparently distributed in layers, see Fig. 10b. In this region, the transfer layer was fully covering the coating and reached a thickness of approximately 6.4 μm.
3.3.3. Transfer from the Cr alloyed steel The transfer from the Cr alloyed steel showed two modes after 0.025 s, see Fig. 11a. AES analyses revealed that the dark transfer layer was Fe–Cr–O while the bright was metallic steel, see Fig. 12. The steel transfer tended to adhere on the front edges of the Fe–Cr– O transfer, as shown in the white dashed rectangle in Fig. 11a. After 0.6 s, the transfer area was broader and the steel was transferred in larger pieces, see Fig. 11b. As shown in Fig. 12, in the dark area Fe– Cr–O covered the coating to a thickness of about 150 nm. In the bright area, the same kind of oxide, of approximately the same thickness, covered the coating and on top of that an approximately 250 nm thick metallic steel was added. The observations of the layer transferred from the Cr alloyed steel after 0.6 s were further strengthened by the cross-section study shown in Fig. 13. An extensive Fe–Cr–O layer was indeed covering the TiN coating and steel adhered on top of that. Within the entire analyzed cross section, the Fe–Cr–O layer was rather thin, while the steel transfer was much thicker. After sliding for 1.8 s against the Cr alloyed steel, cracks had formed in the coating and the steel was transferred along the cracks, see Fig. 14. Despite the cracks, the substrate was not exposed, as proven by the absence of EDS signal from W, which is abundant in the substrate.
4. Discussion The experimental results clearly showed that alloy element additions of 1 mass% had a significant effect on both the material transfer mechanism and the friction coefficient. The contact situations against the three steels are schematically shown in Fig. 15. Please note that the vertical scale is extremely exaggerated to visualize the contact more clearly. The transfer layers from (a) the Base steel and (c) the Cr alloyed steel show only small topography and consequently almost the full zone I areas help to
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Fig. 5. EDS maps of the contact marks on the TiN coated cylinders after sliding against (a) the Base steel, (b) the Si alloyed steel and (c) the Cr alloyed steel. The electron beam acceleration voltage was 10 kV.
support the normal load. Contrastingly, in order to explain the contact situation of the Si alloyed steel, the contributions from both the rough and smooth regions have to be considered. The large topography of the transfer layer from the Si alloyed steel prevents the load from being supported by the full area. In fact, it was shown in Fig. 9 that the rough region included both higher and lower parts, and that the higher parts but not the lower parts contributed to the contact. This means the transfer layer supports the load in the smooth region plus the higher parts of the rough region as shown in Fig. 15. In a previous study [14] it was concluded that the friction coefficient increased with an increase in contact area covered with the iron oxide in zone II. However, in this study, it is shown that the transfer layer in zone I, rather than that in zone II, has the largest influence on the friction. This is because zone I now covers a substantial fraction of the contact mark. In Fig. 16, the size of the apparent full contact areas as defined in Fig. 3, and the size of zone I are plotted. Please note that since the values in Fig. 16 were obtained from separate tests with different test durations, there is
some variability in the curves. Except for the very early stage and when the Cr alloyed steel caused coating cracking after 1.8 s, the apparent full contact areas are almost stable and have about the same level, see Fig. 16a. This is reasonable since the size of the contact area is decided mainly by the normal load and the hardness of the work material, and the hardness of all tested steels is almost the same. On the other hand, the sizes of zone I were different between all steels, see Fig. 16b. When sliding against the Base steel, the change in size of zone I was small, while against the Cr alloyed steel, the size increased for 0.3 s and then became stable until it increased at 1.8 s. These tendencies for the Base steel and the Cr alloyed steel seem to match the friction coefficients in Fig. 2. However, when sliding against the Si alloyed steel, zone I continuously grew with increasing sliding time. This tendency differs from that of the friction coefficient. This means the area of zone I shown in Fig. 16b is over-estimated. The real contact size of the rough region for the Si alloyed steel is difficult to measure. Instead, we can assume that the size of the contact area is not very different from that formed against the
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Gray transfer
SEM images after 0.025 s
Bright transfer
Sliding direction of work material
50 μm
2 μm
SEM images after 1.8 s
50 μm
2 μm
EDS spectra Gray transfer
Bright transfer
Fig. 6. SEM images and EDS spectra of the transfer layer on the TiN coated cylinder after sliding against the Base steel for (a) 0.025 s and (b) 1.8 s. Overview images are shown to the left and the areas in the black rectangles are magnified in the images to the right. A white dashed rectangle indicates an area with steel transfer. Gray and bright transfers are indicated by black arrows. (c) EDS spectra from the gray and bright transfer area. The electron beam acceleration voltage was 3 kV.
Sliding direction of work material
Pt protective layer Transfer layer
TiN
High speed steel 1 μm Fig. 7. An SEM image of a FIB cross-section of the transfer layer in the contact mark on the TiN coated cylinder after sliding against the Base steel for 1.8 s. The electron beam acceleration voltage was 10 kV.
Base steel. That would mean the lower friction against the Si alloyed steel, compared with that against the Base steel, is due to easier shearing of the interfacial region between the Fe–Si–O and the work material at the elevated temperature. Umino et al. [17] reported that the Fe–Si–O produced in the interface of tool and chip in milling gives a lubricating effect. They suggested that this is because the Fe–Si–O, which has a low eutectic temperature, softens at elevated temperature. Actually, the eutectic temperature of Fe–Si–O is about 190 K lower than the melting point of Fe–O; about 1453 K for Fe–Si–O, about 1643 K for Fe–O [18]. This fact supports the hypothesis of easier shearing of an interfacial region including Fe–Si–O in the present work. The friction coefficient against the Si alloyed steel was almost constant throughout the test, c.f. Fig. 2. This suggests that the real contact area was almost constant even though the zone I area and the topography changed.
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Overview SEM images 0.025 s
343
0.3 s
Sliding direction of work material A
B
Rough
C
A
C B
Rough
Smooth
Smooth
50 μm
50 μm
Magnified SEM images 0.025 s
Bright transfer 4 μm
4 μm
4 μm
4 μm
Dark transfer
4 μm
0.3 s
EDS spectra Dark transfer
Bright transfer
4 μm
Work material
Fig. 8. SEM images of the transfer layers after (a) (c) 0.025 s and (b) (d) 0.3 s of sliding against the Si alloyed steel. Overview images are shown in (a) and (b), and the areas A, B and C in the white rectangles are magnified in (c) and (d). EDS spectra obtained from (e) the dark transfer, (f) bright transfer and (g) the bulk work material. Rough and smooth regions are indicated by white arrows in (a) and (b). Dark and bright transfers are indicated by white arrows in (c). The electron beam acceleration voltage was 3 kV.
In contrast, the friction coefficient increased against the Cr alloyed steel. The high level seems to be consistent with the large area of the transfer layer. Crack generation of TiN coated HSS has
been explained to be caused by the high tensile stresses generated in the coating during sliding [11]. It is also reported that this cracking is aggravated by an increased sliding speed because
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50 μm
4 μm
4 μm
1 μm
Fig. 9. SEM images of the contact mark on the TiN coated cylinder after sliding against the Si alloyed steel for 1.8 s, tilted 65°. Overview images are shown in (a) and the areas in the black rectangles are magnified in (b)–(d). The white arrow in (a) indicates the smooth Fe–Si–O hills. The black arrows in (c) indicate fine grooves. The electron beam acceleration voltage was 10 kV.
Front area
Sliding direction of work material
Pt protective layer Center area
Sliding direction of work material
Transfer layer Transfer layer Pt protective layer
TiN
TiN
1 μm
High speed steel
1 μm
High speed steel
Fig. 10. SEM images of FIB cross-sections in the transfer layer on the TiN coated cylinder after sliding against the Si alloyed steel for 1.8 s. The cross-sections were made in (a) the lower part of the center area and (b) the front area in the transfer layer. The electron beam acceleration voltage was 10 kV.
friction heat softens the substrate, which then deforms to a degree that leads to coating fracture [11,16,19]. In this test, only the Cr alloyed steel leads to friction heating and tensile stresses sufficient to result in coating cracks. The wide fluctuation of the friction curve of the Cr alloyed steel was actually the result of an induced vibration. Analysis of the friction curve showed a wavelength of about 1 mm (i.e. a period of about 0.6 ms) and the track on the work material also showed cyclic changes in appearance with a wavelength of about 1 mm. This wavelength is not linked to the much smaller length scales of the microstructure. It is interesting at this point that only the Cr alloyed steel induced this vibration while the exact reason for that is not yet known. Frictional vibrations can generate various forms of errors in the friction measurement, as reported e.g. in [20]. The
friction presented for the Cr alloyed steel can possibly be influenced by such errors. The oxides formed in the center of the contact differ from the native oxides that are present on the surface of the work materials. Even though the very initial oxide transferred to the TiN surface may simply be the native oxide scraped off the work material, the growing oxide rapidly changes in character to something else. In a previous study [15], the oxide formation in the center of the contact was concluded to be very different from the native oxide on the work material. It was suggested to rely on reactions occurring in a condition with high temperature, high pressure and low oxygen supply. Furthermore, in another study [16], involving low sliding speed, almost no such oxide was formed. Together, this suggests that friction heating allows new reactions that are needed for this
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0.025 s
345
Dark transfer
Bright transfer
Sliding direction of work material
50 μm
2 μm
0.6 s
50 μm
2 μm
Dark transfer area
Bright transfer area
100
100
Fe
Ti + N
Atomic concentration (at%)
Atomic concentration (at%)
Fig. 11. SEM images of the transfer layers on the TiN coated cylinder after sliding against the Cr alloyed steel for (a) 0.025 s and (b) 0.6 s. Overview images are shown to the left and the areas in the black rectangles are magnified in the images to the right. Dark and bright transfers are indicated by black arrows. The electron beam acceleration voltage was 3 kV.
80
O
60
40
Cr 20
0
Fe
0
100
200
300
400
500
Sputter depth (nm)
Ti + N
80
60
O
40
Cr
20
0
0
100
200
300
400
500
Sputter depth (nm)
Fig. 12. AES depth profiles of (a) the dark transfer layer and (b) the bright transfer layer formed against the Cr alloyed steel after 0.025 s. The depth was calculated using the sputtering rate of approximately 17.1 nm/min in Ta2O5 as a reference.
new oxide to form. Against the Si alloyed steel, the oxide growth is more complex. The smooth region follows the description above, but the rough region is influenced by fragments fractured from the smooth front region. In the rough region the growth of the oxide is likely a combined result of the new reactions and agglomerated fragments, producing the high topography. It is interesting to note that Si and Cr are extremely enriched in the oxide layers, as shown in Figs. 8e and 12a. This is reasonable, because these alloy elements are known to oxidize more easily than Fe [21]. Although most atoms that become oxidized are still Fe because of its dominance over Si and Cr in the work material, the alloy elements will become enriched in the oxides. As the
oxide grows, more or less metallic steel is concurrently transferred from the work material to the oxide, depending on the type of oxide and the steel composition. Material transfer is reported to be influenced by the topography and adhesion tendency [22]. Considering Figs. 6a, 8c and 11a, the topography is believed to give a strong contribution to the metallic steel transfer in the present work. It was concluded in the previous studies [14,15] that these sliding experiments reproduced the material transfer in milling. The type and hardness of the tool material, the hardness of the work material and sliding contact parameters used in the present work are very similar to those in the previous studies. Thus, we
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Sliding direction of work material
a
Transfer layer
b Pt protective layer
TiN
Ti
Fe
Cr High speed steel 1 μm Fig. 13. (a) An SEM image and (b) EDS maps of a FIB cross-section in the transfer layer on the TiN coated cylinder after sliding against the Cr alloyed steel for 0.6 s. The EDS maps were obtained from the black rectangle in (a). The electron beam acceleration voltage was 10 kV.
Crack
Sliding direction of work material
Steel 100 μm
Fe
50 μm
O
W
Fig. 14. Backscattered electron images of (a) the entire material transfer region on the TiN coated cylinder after sliding against the Cr alloyed steel for 1.8 s and (b) a magnification of the area marked with the white rectangle in (a). EDS maps of the magnified area are shown in (c). Cracks of the coating and the steel transfer are indicated by black arrows. The electron beam acceleration voltage was 15 kV.
believe that also the tendencies of material transfer and friction behavior in the present work are relevant for real milling.
5. Conclusions The influence of Si and Cr additions to a carbon steel on material transfer and friction characteristics during sliding between the steels and TiN coated HSS was investigated. Three especially designed model steels were produced to allow studies of the individual roles of the alloy elements; Fe–0.55C (Base steel), Fe–0.55C–1Si (Si alloyed steel) and Fe–0.55C–1Cr (Cr alloyed steel) in mass %. The main conclusions are as follows.
● In the center of the contact areas on the TiN coated cylinder, different types of transfer layers were formed depending on the steel composition; ○ Base steel: A homogeneous Fe–O with a small amount of metallic steel was formed. ○ Si alloyed steel: A mixed layer composed of Fe–Si–O and metallic steel transfer. The oxide contained a large amount of Si. ○ Cr alloyed steel: An Fe–Cr–O was formed on the TiN coating, and metallic steel locally adhered on the top of the Fe–Cr–O. The oxide contained a large amount of Cr. ● Compared to the Base steel, the friction coefficient was lower against the Si alloyed steel, mainly explained by easier shearing of the interfacial region between the Fe–Si–O and the work material at elevated temperature.
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Base
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Direction of rotation
Work material Deformation of work material
Transfer layer (Fe-O)
TiN
Normal load
Work material
Si
Direction of rotation
Deformation of work material Transfer layer
Rough region (Fe-Si-O & metallic steel)
TiN
Smooth region (Fe-Si-O)
Normal load
Cr
Work material
Direction of rotation
Deformation of work material
Transfer layer (Fe-Cr-O & metallic steel)
TiN
Normal load
Size of apparent full contact areas
Size of zone I
0.24
0.16 Coating cracking
Coating cracking
0.20 Base 0.16
Si Cr
0.12
0.08 0.0
0.5
1.0
1.5
2.0
Area of transfer (mm2)
Apparent full contact area (mm 2)
Fig. 15. Cross-sectional illustrations of the contact situation along the central lines of contact, when the material transfer and the friction coefficient have become relatively stable, i.e. at 1.8 s for the Base steel and the Si alloyed steel, and at 0.6 s for the Cr alloyed steel.
0.12 Base 0.08
Si Cr
0.04
0.00 0.0
0.5
1.0
1.5
2.0
Sliding time (s)
Sliding time (s)
Fig. 16. (a) Apparent full contact areas, (b) areas of zone I, measured using optical microscope images.
● Compared to the Base steel, the friction coefficient was higher against the Cr alloyed steel, high enough to cause coating cracking.
Acknowledgments Dr. Ulf Bexell, Dalarna University, is acknowledged for assistance with the AES analysis. Financial support from Nippon Steel & Sumitomo Metal Corporation is acknowledged.
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