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Transfer of titanium in sliding contacts—New discoveries and insights revealed by in situ studies in the SEM
Wear 315 (2014) 87–94
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Transfer of titanium in sliding contacts—New discoveries and insights revealed by in situ studies in the SEM$ Jannica Heinrichs a,n, Mikael Olsson a,b, Istvan Zoltan Jenei c, Staffan Jacobson a a
Tribomaterials Group, The Ångströ m Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Materials Science, Dalarna University, SE-791 88 Falun, Sweden c Instrumentation Physics, Stockholm University, SE-106 91 Stockholm, Sweden b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 December 2013 Received in revised form 4 April 2014 Accepted 5 April 2014 Available online 16 April 2014
Titanium and its alloys generally display poor tribological properties in sliding contacts due to their high chemical activity and strong adhesion to the counter surface. The strong adhesion causes a high tendency to transfer and ultimately galling or build-up edge formation, resulting in severe surface damage. As a result, forming and machining of titanium and its alloys are generally associated with significant problems such as high friction, rapid tool wear and poor surface finish of the formed/ machined surface. In the present study, in situ tests in a scanning electron microscope have been performed to increase the understanding of the mechanisms controlling the initial transfer of titanium (Grade 2) in sliding contact with tool surfaces. Tool materials included cover cold work tool steel, cemented carbide, CVD deposited Al2O3 and PVD deposited DLC. In these tests, a relatively sharp tip, representing the titanium work material, slides against a flat surface, representing the tool. The contact conditions result in plastic deformation of the work material against the tool surface, thereby simulating forming or machining. The limited and well-defined contact, along with the possibility to study the sliding in the SEM, makes it possible to correlate local surface variations to transfer of work material and frictional response. Posttest characterization of the contact surfaces was performed by high-resolution SEM, TEM, EDS and EELS. The initial friction was low and stable against all tested materials, but then gradually escalated against all surfaces except the DLC. The friction escalation was associated to increasing levels of transfer, while the DLC stayed virtually free from transfer. From these very initial sliding tests DLC is a promising tool coating in forming and machining of titanium. & 2014 Elsevier B.V. All rights reserved.
Keywords: Surface topography Coatings Titanium Material transfer Galling Friction
1. Introduction Titanium and its alloys have good mechanical properties, desirable corrosion resistance and low density, and are widely used in e.g. aerospace and chemical process industry. However, in sliding contacts they show poor tribological properties, due to high chemical activity and associated strong adhesion, leading to adhesive transfer to the counter surface. This causes problems in forming and machining, ultimately leading to galling and built-up edge formation, respectively. These processes are associated with high friction and wear, resulting in poor surface finish of the formed or machined titanium surface. The strong tendency to adhesive transfer of titanium and associated issues has been researched by several groups for many years [1–7]. To reduce the transfer tendency various counter
☆ n
This paper was presented at the 2013 World Tribology Congress. Corresponding author. Tel.: þ 46184717236. E-mail address: [email protected] (J. Heinrichs).
http://dx.doi.org/10.1016/j.wear.2014.04.006 0043-1648/& 2014 Elsevier B.V. All rights reserved.
surfaces and surface treatments have been proposed and evaluated. Some ceramic counter surfaces, including alumina and silicon nitride, have been tested with poor results [1–3]. This has been attributed to tribochemical reactions, and for alumina it is due to the formation of titanium aluminides, along with various titanium oxides [1]. Steel counter surfaces have proven to work better than alumina, both when it comes to friction and wear, however still causing adhesive transfer to the same [2]. A carbon based counter surface, polytetrafluoroetylene (PTFE), has on the other hand proven very effective in preventing adhesive transfer [2]. Initial transfer of PTFE to the titanium surface causes a low friction PTFEPTFE sliding contact. Investigations where the titanium surface has been coated to prevent metal to metal contact during sliding has also been reported in the literature. Thermal oxidation or anodization, to form a thick TiO2-oxide, protects the surface efficiently from transfer, however with a relatively high friction [4,5]. Combining the anodizing with addition of PTFE or MoS2 combines surface protection of the oxide with an easily shared layer, which lowers the friction [5]. However, although adhesive transfer of titanium has
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been a thoroughly researched topic, the initiation of transfer is still to be investigated. The objective of the present work is to increase the understanding of the mechanisms behind the initial transfer of titanium in sliding contact with different tool surfaces during forming, of interest for the manufacturing industry. The system is simplified by studying a contact with commercially pure titanium, without any significant amounts of alloying elements. Neither is coating or surface treatment of the titanium taken into account, due to the extensive plastic deformation and surface expansion often associated with forming processes, and like any solid lubrication of the titanium, they would also have to be removed in a later stage during production and is thereby undesired. Four different counter surfaces are studied; cold work tool steel and cemented carbide, representing commonly used tool materials, an alumina coating, representing a traditional coating for cutting inserts, and a DLC coating, representing a coating that previously have proven successful in galling prevention with aluminium and stainless steel [8,9]. The contact is studied in detail by performing well controlled in situ tests in a scanning electron microscope (SEM). The contact is limited to a single titanium asperity (Ø≈100 mm) in sliding contact with a tool material under a load sufficient to cause plastic deformation of the titanium. Transfer particles can be directly observed and correlated to surface features of the tool material and the friction response in the system. Although being a radically simplified test, it mimics central parts of the contact situation in forming, and gives unique insights into the very initial friction and wear mechanisms. This method has recently proven successful in studying the phenomena of initial material transfer of aluminium and stainless steel [8–11].
2. Experimental 2.1. Materials Titanium Grade 2, i.e. commercially pure titanium (499% Ti), with a hardness of 145 HV, was selected to represent the work material in the present study. Cylinders (Ø2.9 mm, length 15– 20 mm) were manufactured by turning, followed by shaping a tip in one end. The final preparation of the tip was grinding with 1000 followed by 4000 grit SiC grinding paper. The investigated tool materials include nitrogen alloyed powder metallurgical cold work tool steel (Uddeholms Vancron 40, wt%: 1.1 C, 1.8 N, 0.5 Si, 8.5 V, 4.5 Cr, 0.4 Mn, 3.2 Mo, 3.7 W, bal. Fe), cemented carbide (94% WC, 6% Co), chemical vapour deposited alumina (Al2O3) and physical vapour deposited diamond-like carbon (DLC), the latter two coatings deposited on cemented carbide and tool steel substrates, respectively. All tool materials were carefully polished, to be able to study differences in tool material chemistry rather than the effect of micro-scale surface roughness. The final polishing step comprised mechanical polishing with 1 μm diamond paste, resulting in mirror-like surface finish with a surface roughness of Rz o 50 nm. The surface appearance after the final polishing was imaged using atomic force microscopy (AFM; PSIA XE150), see Fig. 1. All four samples showed small surface irregularities, in sizes up to single nanometres. Further, the alumina surface showed the presence of nano scratches and thermally induced cracks, and the tool steel displayed somewhat larger protrusions (10–15 nm), deriving from carbonitride particles present in the microstructure.
Fig. 1. AFM topography images showing the surface features over an area of 20 μm 20 μm of the polished tool material flats. (a) Cemented carbide, (b) alumina, (c) tool steel and (d) DLC.
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Before testing, all surfaces were cleaned in an ultrasonic bath with acetone, followed by ethanol, for 3 min, respectively, then rinsed in ethanol and dried in compressed air.
2.2. Method In the sliding tests, a sharp tip with a small flat end surface (ØE 50–100 mm), representing the work material, slides against a flat surface, representing the tool, see Fig. 2. The contact conditions result in plastic deformation of the work material against the harder tool surface already during the initial static loading, thereby simulating forming or machining. The sliding tests were performed in situ in an SEM (Leo 440). The limited and welldefined contact (ØE100 mm after the first static loading), along with the possibility to study the sliding in the SEM, makes it possible to correlate local surface variations to events of work material transfer and friction fluctuations [10]. The titanium tip was tested against each tool material flat, in three parallel tracks, with 1, 5 and 10 passages in the same track, while the friction force and normal load were recorded. All tests were run unlubricated, with a load of 3 N, sliding speed of 2.5 mm/ min and a sliding distance of 2.5–2.8 mm for each passage. Since the testing was performed in an SEM, room temperature and low pressure (1 10 5 Torr) conditions prevailed.
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3. Results 3.1. Friction behaviour The friction curves recorded for the sliding between the titanium tip and the four flat samples are shown in Fig. 3 (each showing 10 repeated passages along a single track). For the cemented carbide the friction coefficient was low during the first passage, slightly above 0.2. However, it increased to about 0.75 already during the second passage. The friction increased even further for each passage, reaching 1.0 during the 10 passages. The trend was similar against the alumina. However, the friction increase between successive passages was more gradual, and the final friction coefficient was just above 0.7. Also against the tool steel the friction increased for each passage. However, the increase was more moderate, reaching a friction coefficient of about 0.4 during the 10th passage. The DLC coated tool steel showed a completely different behaviour. During the first passage the friction was as low, or even lower, as for the other materials, but here it remained low during all 10 passages. No tendency to friction increase could be observed, even in the magnified scale, in this very initial sliding test. However, when a scratch was intentionally introduced in the DLC coating across the tip sliding direction, the friction increased steeply when passing this scratch, see Fig. 4. The friction then remained at a high level for some distance during the subsequent sliding against the polished surface. The high friction region extended for longer and longer sliding distances with each passage.
2.3. Post test characterisation 3.2. Work material transfer The direct observation in the SEM during testing was supplemented by higher resolution studies of the surfaces using FEGSEM (Zeiss Ultra 55, Zeiss 1550, Zeiss Merlin) and cross-section studies using focused ion beam (FIB; FEI Strata DB235) together with transmission electron microscopy (TEM; JEOL JEM-2100F). The SEM micrographs were taken using 3 kV (to achieve a shallow information depth) and no tilt, unless otherwise stated. The elemental composition of the transfer films was analyzed in the TEM using energy dispersive X-ray spectroscopy (EDS). To analyze the oxygen content in the transferred material and along the interface electron energy-loss spectroscopy (EELS), which is better suited for analysis of light elements, was utilized in scanning TEM (STEM) mode. To make the interpretation of the result easier, spectrum imaging (SI) was used together with STEM–EELS.
Material transferred from the titanium tip was observed on all tool material surfaces, except for the polished DLC surface. The cemented carbide showed extensive material transfer already after the first passage, see Fig. 5. Large transfer particles (several micrometres long) were observed. After 5 passages substantially more material had transferred, resulting in a large share of the surface being covered by titanium. The alumina surface showed only occasional transfer particles of some micrometers after a single passage, see Fig. 6. However, after 5 passages the transfer was quite extensive. The transfer events are mainly associated to the small nano-scratches in the polished surface. The tool steel surface was almost bare after the first passage of the Ti tip, see Fig. 7. After 10 passages transfer particles were easily observed, primarily on top of or in front of the slightly protruding carbonitride particles, see Fig. 7b. The transferred titanium particles were considerably smaller than those found on both cemented carbide and alumina. When studying the fully polished DLC surface in the SEM, the track was barely visible even after 10 passages, and no transfer could be observed. However, when testing the DLC surface with a small scratch introduced, titanium was immediately transferred to the scratch, as well as to the polished surface following the scratch, as shown in Figs. 8 and 9a. The transfer extended further and further from the scratch with each passage. After 10 repeated passages, transferred material was found along almost the full length of the track. The transfer was now extensive on the polished surface, see Fig. 9b, resembling the transfer to the polished cemented carbide surface shown in Fig. 5. In contrast, no material transfer was observed before passing the scratch. 3.3. Morphology and composition of the transfer films
Fig. 2. The test equipment, including the titanium tip and the tool material flat, to be operated in the SEM.
To further investigate the morphology and composition of the transferred titanium layers, cemented carbide was chosen for more
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Fig. 3. Friction recordings for10 passages along single tracks on the four materials. Each curve represents one passage of the titanium tip. (a) Cemented carbide, (b) alumina, (c) tool steel and (d) DLC, with a magnified scale inserted. (e) The mean and maximum friction coefficient during each individual passage, for all combinations.
Fig. 4. (a) Friction recordings for 10 passages in the same track for intentionally scratched DLC. The tip reaches the scratch edge, exemplified in (b) after about 0.6 mm sliding.
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Fig. 5. Appearance of the cemented carbide surface partly covered by transferred titanium (darker) after (a) 1 passage and (b) 5 passages of the Ti tip sliding from left to right. (Note that the magnification is the same in both SEM micrographs).
Fig. 6. Alumina coating surface after sliding contact against Ti (from left to right). (a) After 1 passage, showing occasional transfer particles (brighter), and (b) 5 passages. (SEM).
Fig. 7. SEM images showing the tool steel surface (a) practically unaffected after 1 passage and (b) with considerable amount of transfer particles after 10 passages of the Ti tip (sliding from left to right).
detailed studies in the TEM. Electron transparent cross section samples were prepared using FIB. They were prepared from the contact tracks exposed to 1 and 5 passages, oriented along the sliding direction and positioned according to Fig. 10. The material transferred after one passage formed micrometer sized smooth patches on top of the otherwise bare cemented carbide, see Fig. 11. After five passages the transferred material formed larger, smooth patches, however showing some steps, as the one observed
in Fig. 11b. No such steps could be observed after one passage. In these samples, both types of transfer layers showed a maximum thickness of slightly above 100 nm. The interface between the transferred material and the cemented carbide was very dense and showed no pores or cracks, as exemplified in Fig. 12. No indications of titanium diffusing into the Co or WC phases or formation of new phases could be found with EDS analysis in STEM, Fig. 13.
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Interestingly, the degree of oxidation of the transferred titanium was found to be extremely low, as shown in Fig. 14.
4. Discussion The initial friction in unlubricated sliding contact between titanium and four carefully polished tool surfaces was investigated in situ in an SEM. Interestingly, the transfer from the titanium to the tool surface was found to be the primary factor in determining the friction level. On the first passage of the Ti tip, i.e. against the virgin polished tool surface, the friction was low and stable against all tested materials (tool steel, cemented carbide, alumina and DLC). Substantial transfer occurred to the cemented carbide surface, and this contact was also associated with the highest initial friction level (μ E0.25). Against the other three surfaces, no or very limited transfer was found after the first passage and the friction was exceptionally low for unlubricated contact involving Ti (μ E0.11–0.15). On subsequent passages the friction level gradually escalated against all surfaces except against the DLC. Correspondingly, the DLC stayed virtually free from transfer, while the friction escalation on the others was associated with increasing levels of transfer, and the subsequent increasingly rougher surfaces. When testing against the scratched DLC surface, the rough scratch in itself caused local transfer of titanium (as expected). More interestingly, it also caused transfer to the smooth polished surface after passing the scratch. This indicates that the removal
of a layer from the titanium surface “activates” the contact. The mechanical disruption of the sliding surface probably breaks the titanium oxide film and thereby exposes the reactive metallic titanium, making it more prone to adhere to the mating tool surface. When activated in this way, there will be transfer also to the smooth DLC surface, with a corresponding friction increase. A very similar behaviour of aluminium on DLC was reported in Ref. [8], which also elaborates on the different mechanisms of transfer. Consequently, the difference in friction characteristics between the different tool surfaces is to a large extent controlled by the fine-scale micro topography of the polished surfaces. While the DLC surface shows a smooth and defect free surface, the tool steel, cemented carbide and alumina surfaces all show local surface irregularities due to small surface steps in connection to boundaries between the different phases (i.e. carbides, carbonitrides and martensitic matrix in the tool steel, and WC hard phase and Co binder phase in the cemented carbide) or surface defects (pits and cracks in the CVD alumina). Besides a smoother topography after polishing, the DLC surface is believed to show a low (chemical) adhesion tendency towards titanium and titanium oxide, but that is still to be confirmed. Cemented carbide was studied in more detail because of its very intriguing friction and transfer behaviour. The initial friction coefficient was the highest recorded against the polished tool material surfaces. However, the really exceptional part was the extreme friction increase between the first and second passage. During the first passage the titanium tip meets the virgin, polished cemented carbide surface and a lot of material becomes adhered (naturally involving intense local shearing of titanium), however the corresponding friction is still relatively low. During the next passage, when the tip experiences sliding over the cemented carbide surface partly covered by transferred titanium, the friction level triples and the transfer escalates. This may have three possible explanations;
the titanium adheres stronger to the already transferred titanium,
the titanium tip is scratched by the transferred material, adding a larger ploughing term to the friction, or
the titanium tip changes in character.
Fig. 8. Surface appearance of the polished and intentionally scratched DLC surface after 1, 5 and 10 passages of the titanium tip (sliding from left to right). Note the total absence from transfer in front of the scratch. (SEM).
The TEM studies neither confirm nor contradict these explanations, however some important characteristics are revealed. It is possible that the adhesion between the titanium tip and the transferred titanium is stronger than between titanium and cemented carbide. However, the transferred film did not seem to
Fig. 9. The DLC surface, with an intentional scratch, after sliding contact with the Ti tip, sliding from left to right. (a) After 1 passage, about 50 mm after passing the scratch. (b) After 10 passages, about 120 mm after passing the scratch. (SEM).
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Fig. 10. Position of the electron transparent cross-sections prepared from the cemented carbide sample after (a) 1 passage (12 kV) and (b) 5 passages (3 kV, tilt 52 degrees). Ti tip sliding from left to right. (SEM).
Fig. 11. Cross-sections of the transfer film on cemented carbide after (a) 1 passage and (b) 5 passages (TEM).
Fig. 12. High magnification TEM image of the dense interface between transferred work material and cemented carbide after 1 passage of the Ti tip.
grow thicker from the first to the fifth passage, implying that titanium is not primarily added on top of already transferred material, but rather to the cemented carbide surface. The absence of pores and cracks in that interface also implies strong adhesion between the two. The increase in roughness cannot be disregarded. The polished surface showed occasional single nanometer sized roughness, while the transferred particles protrude up to 100 nm from the surface and do act to scratch the tip, thereby increasing the mechanical interaction between the surfaces and thus also increasing the friction. The titanium tip might also change in character, becoming
rougher from the scratching, work harden in the contact, growing a thicker oxide, etc. However, although titanium is known to oxidize rapidly, no significant amount of oxide was incorporated into the transferred material. Since the material being transferred from the tip is mainly metallic, the tip itself cannot be significantly oxidized. Conclusively, the amount of transfer and the topographical character of the surfaces vary between the materials and with the number of passages. As a very simplified rule, we can note that the more transfer, the higher the friction. At this stage, we cannot say how much of this increase is due to an increased ploughing term (rougher surfaces) and how much is due to an increased adhesive term, i.e. due to the change of contacting materials. However, based on the low initial friction levels, we can draw the important conclusion that it is not high friction that leads to transfer, but transfer that leads to high friction!
5. Conclusions The very initial transfer and friction behaviour of titanium onto unlubricated, well-polished flat tool material surfaces has been studied. The initial friction was found to be low and stable against all tested materials (tool steel, cemented carbide, alumina and DLC). On subsequent passages the friction level gradually escalated against all surfaces except the DLC.
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Fig. 13. STEM–EDS analysis of cross-sections of cemented carbide after 1 passage and 5 passages, respectively.
Fig. 14. STEM–EELS spectrum imaging acquired from the interface region between transferred titanium and cemented carbide surface after 1 passage.
The DLC stayed virtually free from transfer, while the friction escalation on the others was associated to increasing levels of transfer, and the subsequent increasingly rougher surfaces. A scratch across the DLC surface caused local transfer and also transfer to the smooth polished surface after the scratch. Based on these very initial tests with a well-defined and limited contact, a well-polished DLC coating seems promising in preventing titanium transfer. It is not high friction that leads to transfer, but transfer that leads to high friction.
References [1] H. Dong, T. Bell, Tribological behaviour of alumina sliding against Ti6Al4V in unlubricated contact, Wear 225–229 (1999) 874–884. [2] J. Qu, P.J. Blau, T.R. Watkins, O.B. Cavin, N.S. Kulkarni, Friction and wear of titanium alloys sliding against metal, polymer, and ceramic counterfaces, Wear 258 (2005) 1348–1356.
[3] N. Chelliah, S.V. Kailas, Synergy between tribo-oxidation and strain rate response on governing the dry sliding wear behaviour of titanium, Wear 266 (2009) 704–712. [4] U. Wiklund, I.M. Hutchings, Investigation of surface treatments for galling protection of titanium alloys, Wear 250–251 (2001) 1034–1041. [5] K.G. Budinski, Tribological properties of titanium alloys, Wear 151 (1991) 203–217. [6] A. Molinari, G. Straffelini, B. Tesi, T. Bacci, Dry sliding wear mechanisms of the Ti6Al4V alloy, Wear 208 (1997) 105–112. [7] G. Straffelini, A. Molinari, Dry sliding wear of Ti–6Al–4V alloy as influenced by the counterface and sliding conditions, Wear 236 (1999) 328–338. [8] J. Heinrichs, M. Olsson, S. Jacobson, Mechanisms of material transfer studied in situ in the SEM: explanations to the success of DLC coated tools in aluminium forming, Wear 292–293 (2012) 49–60. [9] J. Heinrichs, M. Olsson, S. Jacobson, Initiation of galling in forming of ductile metals—differences between aluminium and austenitic stainless steel studied in situ in the SEM, Tribol. Lett. 50 (2013) 431–438. [10] J. Heinrichs, M. Olsson, S. Jacobson, New understanding of the initiation of material transfer and transfer layer build-up in metal forming—in situ studies in the SEM, Wear 292–293 (2012) 61–73. [11] J. Heinrichs, M. Olsson, S. Jacobson, Influence of tool steel microstructure on initial material transfer in metal forming—in situ studies in the SEM, Wear 302 (2013) 1249–1256.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Transfer of titanium in sliding contacts—New discoveries and insights revealed by in situ studies in the SEM$ Jannica Heinrichs a,n, Mikael Olsson a,b, Istvan Zoltan Jenei c, Staffan Jacobson a a
Tribomaterials Group, The Ångströ m Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Materials Science, Dalarna University, SE-791 88 Falun, Sweden c Instrumentation Physics, Stockholm University, SE-106 91 Stockholm, Sweden b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 December 2013 Received in revised form 4 April 2014 Accepted 5 April 2014 Available online 16 April 2014
Titanium and its alloys generally display poor tribological properties in sliding contacts due to their high chemical activity and strong adhesion to the counter surface. The strong adhesion causes a high tendency to transfer and ultimately galling or build-up edge formation, resulting in severe surface damage. As a result, forming and machining of titanium and its alloys are generally associated with significant problems such as high friction, rapid tool wear and poor surface finish of the formed/ machined surface. In the present study, in situ tests in a scanning electron microscope have been performed to increase the understanding of the mechanisms controlling the initial transfer of titanium (Grade 2) in sliding contact with tool surfaces. Tool materials included cover cold work tool steel, cemented carbide, CVD deposited Al2O3 and PVD deposited DLC. In these tests, a relatively sharp tip, representing the titanium work material, slides against a flat surface, representing the tool. The contact conditions result in plastic deformation of the work material against the tool surface, thereby simulating forming or machining. The limited and well-defined contact, along with the possibility to study the sliding in the SEM, makes it possible to correlate local surface variations to transfer of work material and frictional response. Posttest characterization of the contact surfaces was performed by high-resolution SEM, TEM, EDS and EELS. The initial friction was low and stable against all tested materials, but then gradually escalated against all surfaces except the DLC. The friction escalation was associated to increasing levels of transfer, while the DLC stayed virtually free from transfer. From these very initial sliding tests DLC is a promising tool coating in forming and machining of titanium. & 2014 Elsevier B.V. All rights reserved.
Keywords: Surface topography Coatings Titanium Material transfer Galling Friction
1. Introduction Titanium and its alloys have good mechanical properties, desirable corrosion resistance and low density, and are widely used in e.g. aerospace and chemical process industry. However, in sliding contacts they show poor tribological properties, due to high chemical activity and associated strong adhesion, leading to adhesive transfer to the counter surface. This causes problems in forming and machining, ultimately leading to galling and built-up edge formation, respectively. These processes are associated with high friction and wear, resulting in poor surface finish of the formed or machined titanium surface. The strong tendency to adhesive transfer of titanium and associated issues has been researched by several groups for many years [1–7]. To reduce the transfer tendency various counter
☆ n
This paper was presented at the 2013 World Tribology Congress. Corresponding author. Tel.: þ 46184717236. E-mail address: [email protected] (J. Heinrichs).
http://dx.doi.org/10.1016/j.wear.2014.04.006 0043-1648/& 2014 Elsevier B.V. All rights reserved.
surfaces and surface treatments have been proposed and evaluated. Some ceramic counter surfaces, including alumina and silicon nitride, have been tested with poor results [1–3]. This has been attributed to tribochemical reactions, and for alumina it is due to the formation of titanium aluminides, along with various titanium oxides [1]. Steel counter surfaces have proven to work better than alumina, both when it comes to friction and wear, however still causing adhesive transfer to the same [2]. A carbon based counter surface, polytetrafluoroetylene (PTFE), has on the other hand proven very effective in preventing adhesive transfer [2]. Initial transfer of PTFE to the titanium surface causes a low friction PTFEPTFE sliding contact. Investigations where the titanium surface has been coated to prevent metal to metal contact during sliding has also been reported in the literature. Thermal oxidation or anodization, to form a thick TiO2-oxide, protects the surface efficiently from transfer, however with a relatively high friction [4,5]. Combining the anodizing with addition of PTFE or MoS2 combines surface protection of the oxide with an easily shared layer, which lowers the friction [5]. However, although adhesive transfer of titanium has
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been a thoroughly researched topic, the initiation of transfer is still to be investigated. The objective of the present work is to increase the understanding of the mechanisms behind the initial transfer of titanium in sliding contact with different tool surfaces during forming, of interest for the manufacturing industry. The system is simplified by studying a contact with commercially pure titanium, without any significant amounts of alloying elements. Neither is coating or surface treatment of the titanium taken into account, due to the extensive plastic deformation and surface expansion often associated with forming processes, and like any solid lubrication of the titanium, they would also have to be removed in a later stage during production and is thereby undesired. Four different counter surfaces are studied; cold work tool steel and cemented carbide, representing commonly used tool materials, an alumina coating, representing a traditional coating for cutting inserts, and a DLC coating, representing a coating that previously have proven successful in galling prevention with aluminium and stainless steel [8,9]. The contact is studied in detail by performing well controlled in situ tests in a scanning electron microscope (SEM). The contact is limited to a single titanium asperity (Ø≈100 mm) in sliding contact with a tool material under a load sufficient to cause plastic deformation of the titanium. Transfer particles can be directly observed and correlated to surface features of the tool material and the friction response in the system. Although being a radically simplified test, it mimics central parts of the contact situation in forming, and gives unique insights into the very initial friction and wear mechanisms. This method has recently proven successful in studying the phenomena of initial material transfer of aluminium and stainless steel [8–11].
2. Experimental 2.1. Materials Titanium Grade 2, i.e. commercially pure titanium (499% Ti), with a hardness of 145 HV, was selected to represent the work material in the present study. Cylinders (Ø2.9 mm, length 15– 20 mm) were manufactured by turning, followed by shaping a tip in one end. The final preparation of the tip was grinding with 1000 followed by 4000 grit SiC grinding paper. The investigated tool materials include nitrogen alloyed powder metallurgical cold work tool steel (Uddeholms Vancron 40, wt%: 1.1 C, 1.8 N, 0.5 Si, 8.5 V, 4.5 Cr, 0.4 Mn, 3.2 Mo, 3.7 W, bal. Fe), cemented carbide (94% WC, 6% Co), chemical vapour deposited alumina (Al2O3) and physical vapour deposited diamond-like carbon (DLC), the latter two coatings deposited on cemented carbide and tool steel substrates, respectively. All tool materials were carefully polished, to be able to study differences in tool material chemistry rather than the effect of micro-scale surface roughness. The final polishing step comprised mechanical polishing with 1 μm diamond paste, resulting in mirror-like surface finish with a surface roughness of Rz o 50 nm. The surface appearance after the final polishing was imaged using atomic force microscopy (AFM; PSIA XE150), see Fig. 1. All four samples showed small surface irregularities, in sizes up to single nanometres. Further, the alumina surface showed the presence of nano scratches and thermally induced cracks, and the tool steel displayed somewhat larger protrusions (10–15 nm), deriving from carbonitride particles present in the microstructure.
Fig. 1. AFM topography images showing the surface features over an area of 20 μm 20 μm of the polished tool material flats. (a) Cemented carbide, (b) alumina, (c) tool steel and (d) DLC.
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Before testing, all surfaces were cleaned in an ultrasonic bath with acetone, followed by ethanol, for 3 min, respectively, then rinsed in ethanol and dried in compressed air.
2.2. Method In the sliding tests, a sharp tip with a small flat end surface (ØE 50–100 mm), representing the work material, slides against a flat surface, representing the tool, see Fig. 2. The contact conditions result in plastic deformation of the work material against the harder tool surface already during the initial static loading, thereby simulating forming or machining. The sliding tests were performed in situ in an SEM (Leo 440). The limited and welldefined contact (ØE100 mm after the first static loading), along with the possibility to study the sliding in the SEM, makes it possible to correlate local surface variations to events of work material transfer and friction fluctuations [10]. The titanium tip was tested against each tool material flat, in three parallel tracks, with 1, 5 and 10 passages in the same track, while the friction force and normal load were recorded. All tests were run unlubricated, with a load of 3 N, sliding speed of 2.5 mm/ min and a sliding distance of 2.5–2.8 mm for each passage. Since the testing was performed in an SEM, room temperature and low pressure (1 10 5 Torr) conditions prevailed.
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3. Results 3.1. Friction behaviour The friction curves recorded for the sliding between the titanium tip and the four flat samples are shown in Fig. 3 (each showing 10 repeated passages along a single track). For the cemented carbide the friction coefficient was low during the first passage, slightly above 0.2. However, it increased to about 0.75 already during the second passage. The friction increased even further for each passage, reaching 1.0 during the 10 passages. The trend was similar against the alumina. However, the friction increase between successive passages was more gradual, and the final friction coefficient was just above 0.7. Also against the tool steel the friction increased for each passage. However, the increase was more moderate, reaching a friction coefficient of about 0.4 during the 10th passage. The DLC coated tool steel showed a completely different behaviour. During the first passage the friction was as low, or even lower, as for the other materials, but here it remained low during all 10 passages. No tendency to friction increase could be observed, even in the magnified scale, in this very initial sliding test. However, when a scratch was intentionally introduced in the DLC coating across the tip sliding direction, the friction increased steeply when passing this scratch, see Fig. 4. The friction then remained at a high level for some distance during the subsequent sliding against the polished surface. The high friction region extended for longer and longer sliding distances with each passage.
2.3. Post test characterisation 3.2. Work material transfer The direct observation in the SEM during testing was supplemented by higher resolution studies of the surfaces using FEGSEM (Zeiss Ultra 55, Zeiss 1550, Zeiss Merlin) and cross-section studies using focused ion beam (FIB; FEI Strata DB235) together with transmission electron microscopy (TEM; JEOL JEM-2100F). The SEM micrographs were taken using 3 kV (to achieve a shallow information depth) and no tilt, unless otherwise stated. The elemental composition of the transfer films was analyzed in the TEM using energy dispersive X-ray spectroscopy (EDS). To analyze the oxygen content in the transferred material and along the interface electron energy-loss spectroscopy (EELS), which is better suited for analysis of light elements, was utilized in scanning TEM (STEM) mode. To make the interpretation of the result easier, spectrum imaging (SI) was used together with STEM–EELS.
Material transferred from the titanium tip was observed on all tool material surfaces, except for the polished DLC surface. The cemented carbide showed extensive material transfer already after the first passage, see Fig. 5. Large transfer particles (several micrometres long) were observed. After 5 passages substantially more material had transferred, resulting in a large share of the surface being covered by titanium. The alumina surface showed only occasional transfer particles of some micrometers after a single passage, see Fig. 6. However, after 5 passages the transfer was quite extensive. The transfer events are mainly associated to the small nano-scratches in the polished surface. The tool steel surface was almost bare after the first passage of the Ti tip, see Fig. 7. After 10 passages transfer particles were easily observed, primarily on top of or in front of the slightly protruding carbonitride particles, see Fig. 7b. The transferred titanium particles were considerably smaller than those found on both cemented carbide and alumina. When studying the fully polished DLC surface in the SEM, the track was barely visible even after 10 passages, and no transfer could be observed. However, when testing the DLC surface with a small scratch introduced, titanium was immediately transferred to the scratch, as well as to the polished surface following the scratch, as shown in Figs. 8 and 9a. The transfer extended further and further from the scratch with each passage. After 10 repeated passages, transferred material was found along almost the full length of the track. The transfer was now extensive on the polished surface, see Fig. 9b, resembling the transfer to the polished cemented carbide surface shown in Fig. 5. In contrast, no material transfer was observed before passing the scratch. 3.3. Morphology and composition of the transfer films
Fig. 2. The test equipment, including the titanium tip and the tool material flat, to be operated in the SEM.
To further investigate the morphology and composition of the transferred titanium layers, cemented carbide was chosen for more
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Fig. 3. Friction recordings for10 passages along single tracks on the four materials. Each curve represents one passage of the titanium tip. (a) Cemented carbide, (b) alumina, (c) tool steel and (d) DLC, with a magnified scale inserted. (e) The mean and maximum friction coefficient during each individual passage, for all combinations.
Fig. 4. (a) Friction recordings for 10 passages in the same track for intentionally scratched DLC. The tip reaches the scratch edge, exemplified in (b) after about 0.6 mm sliding.
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Fig. 5. Appearance of the cemented carbide surface partly covered by transferred titanium (darker) after (a) 1 passage and (b) 5 passages of the Ti tip sliding from left to right. (Note that the magnification is the same in both SEM micrographs).
Fig. 6. Alumina coating surface after sliding contact against Ti (from left to right). (a) After 1 passage, showing occasional transfer particles (brighter), and (b) 5 passages. (SEM).
Fig. 7. SEM images showing the tool steel surface (a) practically unaffected after 1 passage and (b) with considerable amount of transfer particles after 10 passages of the Ti tip (sliding from left to right).
detailed studies in the TEM. Electron transparent cross section samples were prepared using FIB. They were prepared from the contact tracks exposed to 1 and 5 passages, oriented along the sliding direction and positioned according to Fig. 10. The material transferred after one passage formed micrometer sized smooth patches on top of the otherwise bare cemented carbide, see Fig. 11. After five passages the transferred material formed larger, smooth patches, however showing some steps, as the one observed
in Fig. 11b. No such steps could be observed after one passage. In these samples, both types of transfer layers showed a maximum thickness of slightly above 100 nm. The interface between the transferred material and the cemented carbide was very dense and showed no pores or cracks, as exemplified in Fig. 12. No indications of titanium diffusing into the Co or WC phases or formation of new phases could be found with EDS analysis in STEM, Fig. 13.
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Interestingly, the degree of oxidation of the transferred titanium was found to be extremely low, as shown in Fig. 14.
4. Discussion The initial friction in unlubricated sliding contact between titanium and four carefully polished tool surfaces was investigated in situ in an SEM. Interestingly, the transfer from the titanium to the tool surface was found to be the primary factor in determining the friction level. On the first passage of the Ti tip, i.e. against the virgin polished tool surface, the friction was low and stable against all tested materials (tool steel, cemented carbide, alumina and DLC). Substantial transfer occurred to the cemented carbide surface, and this contact was also associated with the highest initial friction level (μ E0.25). Against the other three surfaces, no or very limited transfer was found after the first passage and the friction was exceptionally low for unlubricated contact involving Ti (μ E0.11–0.15). On subsequent passages the friction level gradually escalated against all surfaces except against the DLC. Correspondingly, the DLC stayed virtually free from transfer, while the friction escalation on the others was associated with increasing levels of transfer, and the subsequent increasingly rougher surfaces. When testing against the scratched DLC surface, the rough scratch in itself caused local transfer of titanium (as expected). More interestingly, it also caused transfer to the smooth polished surface after passing the scratch. This indicates that the removal
of a layer from the titanium surface “activates” the contact. The mechanical disruption of the sliding surface probably breaks the titanium oxide film and thereby exposes the reactive metallic titanium, making it more prone to adhere to the mating tool surface. When activated in this way, there will be transfer also to the smooth DLC surface, with a corresponding friction increase. A very similar behaviour of aluminium on DLC was reported in Ref. [8], which also elaborates on the different mechanisms of transfer. Consequently, the difference in friction characteristics between the different tool surfaces is to a large extent controlled by the fine-scale micro topography of the polished surfaces. While the DLC surface shows a smooth and defect free surface, the tool steel, cemented carbide and alumina surfaces all show local surface irregularities due to small surface steps in connection to boundaries between the different phases (i.e. carbides, carbonitrides and martensitic matrix in the tool steel, and WC hard phase and Co binder phase in the cemented carbide) or surface defects (pits and cracks in the CVD alumina). Besides a smoother topography after polishing, the DLC surface is believed to show a low (chemical) adhesion tendency towards titanium and titanium oxide, but that is still to be confirmed. Cemented carbide was studied in more detail because of its very intriguing friction and transfer behaviour. The initial friction coefficient was the highest recorded against the polished tool material surfaces. However, the really exceptional part was the extreme friction increase between the first and second passage. During the first passage the titanium tip meets the virgin, polished cemented carbide surface and a lot of material becomes adhered (naturally involving intense local shearing of titanium), however the corresponding friction is still relatively low. During the next passage, when the tip experiences sliding over the cemented carbide surface partly covered by transferred titanium, the friction level triples and the transfer escalates. This may have three possible explanations;
the titanium adheres stronger to the already transferred titanium,
the titanium tip is scratched by the transferred material, adding a larger ploughing term to the friction, or
the titanium tip changes in character.
Fig. 8. Surface appearance of the polished and intentionally scratched DLC surface after 1, 5 and 10 passages of the titanium tip (sliding from left to right). Note the total absence from transfer in front of the scratch. (SEM).
The TEM studies neither confirm nor contradict these explanations, however some important characteristics are revealed. It is possible that the adhesion between the titanium tip and the transferred titanium is stronger than between titanium and cemented carbide. However, the transferred film did not seem to
Fig. 9. The DLC surface, with an intentional scratch, after sliding contact with the Ti tip, sliding from left to right. (a) After 1 passage, about 50 mm after passing the scratch. (b) After 10 passages, about 120 mm after passing the scratch. (SEM).
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Fig. 10. Position of the electron transparent cross-sections prepared from the cemented carbide sample after (a) 1 passage (12 kV) and (b) 5 passages (3 kV, tilt 52 degrees). Ti tip sliding from left to right. (SEM).
Fig. 11. Cross-sections of the transfer film on cemented carbide after (a) 1 passage and (b) 5 passages (TEM).
Fig. 12. High magnification TEM image of the dense interface between transferred work material and cemented carbide after 1 passage of the Ti tip.
grow thicker from the first to the fifth passage, implying that titanium is not primarily added on top of already transferred material, but rather to the cemented carbide surface. The absence of pores and cracks in that interface also implies strong adhesion between the two. The increase in roughness cannot be disregarded. The polished surface showed occasional single nanometer sized roughness, while the transferred particles protrude up to 100 nm from the surface and do act to scratch the tip, thereby increasing the mechanical interaction between the surfaces and thus also increasing the friction. The titanium tip might also change in character, becoming
rougher from the scratching, work harden in the contact, growing a thicker oxide, etc. However, although titanium is known to oxidize rapidly, no significant amount of oxide was incorporated into the transferred material. Since the material being transferred from the tip is mainly metallic, the tip itself cannot be significantly oxidized. Conclusively, the amount of transfer and the topographical character of the surfaces vary between the materials and with the number of passages. As a very simplified rule, we can note that the more transfer, the higher the friction. At this stage, we cannot say how much of this increase is due to an increased ploughing term (rougher surfaces) and how much is due to an increased adhesive term, i.e. due to the change of contacting materials. However, based on the low initial friction levels, we can draw the important conclusion that it is not high friction that leads to transfer, but transfer that leads to high friction!
5. Conclusions The very initial transfer and friction behaviour of titanium onto unlubricated, well-polished flat tool material surfaces has been studied. The initial friction was found to be low and stable against all tested materials (tool steel, cemented carbide, alumina and DLC). On subsequent passages the friction level gradually escalated against all surfaces except the DLC.
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Fig. 13. STEM–EDS analysis of cross-sections of cemented carbide after 1 passage and 5 passages, respectively.
Fig. 14. STEM–EELS spectrum imaging acquired from the interface region between transferred titanium and cemented carbide surface after 1 passage.
The DLC stayed virtually free from transfer, while the friction escalation on the others was associated to increasing levels of transfer, and the subsequent increasingly rougher surfaces. A scratch across the DLC surface caused local transfer and also transfer to the smooth polished surface after the scratch. Based on these very initial tests with a well-defined and limited contact, a well-polished DLC coating seems promising in preventing titanium transfer. It is not high friction that leads to transfer, but transfer that leads to high friction.
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