Mechanisms of material transfer studied in situ in the SEM:

Wear 292–293 (2012) 49–60

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Mechanisms of material transfer studied in situ in the SEM: Explanations to the success of DLC coated tools in aluminium forming Jannica Heinrichs a,n, Mikael Olsson a,b, Staffan Jacobson a a b

˚ Tribomaterials group, The Angstr¨ om Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Materials Science, Dalarna University, SE-791 88 Falun, Sweden

a r t i c l e i n f o

abstract

Article history: Received 26 January 2012 Received in revised form 30 May 2012 Accepted 31 May 2012 Available online 9 June 2012

Metal forming constitutes a group of industrially important processes to form metallic components to net shape. When forming aluminium and other materials that tend to stick to the tools, problems occur. The transferred work material increases the friction, which increases the forming forces. Additionally, the transferred work material becomes hardened and then scratches the softer work material in subsequent forming. This process, known as galling, compromises the surface finish of the next pieces to be formed. This paper employs a newly developed technique to investigate the initial stages of transfer at high resolution in situ in the SEM. We show that the complex microscale processes involved can be distinguished into three classes: primary transfer, secondary transfer and damage activated transfer. The damage activated transfer constitutes a new fundamental tribological phenomenon, involving the activation and healing of a soft metal in sliding contact with a harder surface. Damage activation leads to transfer onto surfaces such as the polished DLC in this investigation, which would otherwise not see any transfer. These processes are important when forming aluminium, but are expected to be of general tribological significance, in sliding involving non-perfect lubricant films, especially for soft metals with protective surface oxides. & 2012 Elsevier B.V. All rights reserved.

Keywords: Galling Surface topography Carbon-based coatings Electron microscopy

1. Introduction Metal forming processes are widely used in industry. Components with reasonably uncomplicated geometries can be formed from many metallic materials to net shape in a single forming operation. If the demands on tolerances, material properties and surface finish are high, the forming can preferably be performed cold, which however will result in higher forming forces. When forming materials that tend to stick to the tools, problems occur. The transferred work material increases the friction in the tool to workpiece interface and thereby also the forming force required. This in turn limits the complexity of the shapes possible to form [1]. In addition to increasing the forming forces, the transferred work material becomes hardened, due to oxidation, work hardening and grain refinement, and then scratches the softer work material in subsequent forming. This process, known as galling, deteriorates the surface finish of the next pieces to be formed. When the surface finish becomes unacceptable, the production has to be interrupted and the tools reconditioned or exchanged.

n

Corresponding author. Tel.: þ46 184717236; fax: þ46 4713572. E-mail address: [email protected] (J. Heinrichs).

0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.05.033

Aluminium alloys are prone to adhere to forming tools [2]. The hardening of the transferred material is extensive and the relatively soft work material entering the tool can be severely scratched if the transfer builds up large, thick and uneven layers. The hardened aluminium can be removed from the tools by wet chemical etching. However, this causes production stops that are highly undesired and further the etching agent might etch the tool, although to a lesser degree. Several studies have been performed on the mechanism of sticking and transfer. However, the transfer may become quite substantial before galling is noticed as a rise in friction or impaired surface roughness of the final products [3]. On the microscopic level, the first transfer events might have taken place long before, so that the initial galling sites have become concealed by subsequent larger transfer events. This paper employs a newly developed technique to investigate the initial stages of transfer at very high resolution, as detailed in [4]. A sharp tip of work material is put into contact with a tool material under a controlled load, followed by a relative movement of the two surfaces. The sliding is directly observed in a Scanning Electron Microscope (SEM). With this well-controlled and limited contact, the observed movement and passages of surface features can be directly related to load variations, friction responses and material transfer. The passage of the tip involves a

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sliding distance determined by the diameter of its flattened end (typically 100–200 mm). This allows observations of transfer and other changes to the flat during very limited sliding distances, not easily attainable by other experimental techniques. In part one of this investigation, it was shown that the tested tool steel (Vancron 40) immediately shows transfer of aluminium, even if carefully polished [4]. The transfer takes place at different scales: on the scale of surface damage (filling up scratches, building up in front of ridges and protrusions around scratches) and on the scale of microstructure of the steel (on top of or in front of slightly protruding hard phase particles). However, the immediate effect on the friction usually passes unnoticed, since the influence from an isolated transfer event is usually insignificant compared to the friction of the whole system [3]. When measuring the friction of a small, localised contact, as employed in the present investigation, a single scratch or indent constitutes a large share of the contact area and may therefore have a huge impact on the measured friction. Even if the individual small-scale transfer events caused by roughness on the nano scale give too small effects on the friction to be detected, they can with advantage be studied with this new method. Their collective effect and variation with time are clearly noticeable. The approach for SEM investigations presented in [4] (employing low acceleration voltage and high magnification) clearly reveals their appearance and evolution. Diamond-like Carbon (DLC) includes a large family of coatings that due to their beneficial tribological properties are used in many applications [5]. When used properly, they offer low friction, high wear resistance and anti-sticking properties in many systems. Some of the publications on tribological evaluations of DLC coatings are summarised by Grill in [6] while Hauert in [7] describes applications where they have been successfully used. When it comes to aluminium forming, some DLC coatings have proven to perform extremely well and exhibit a beneficial antigalling behaviour [2]. In a lab test simulating the deformation and surface expansion in aluminium cold forging, the use of DLC (a–C:H or a–C:Me1) coated tool steel increased the life of fine polished tool samples by more than 2000% [8]. Other DLC coatings (a–C:H:Me) did not significantly influence the friction and tool life. In a study comparing the room and high temperature properties of W–DLC in sliding contact with aluminium, Gharam [9] showed that a thin layer of carbon is transferred to the aluminium pin at room temperature. At intermediate temperatures no such carbon transfer occurred which resulted in the opposite transfer of aluminium to the DLC and associated high friction. However, at even higher temperatures of 400 1C, a transfer layer of tungsten oxide was found, less transfer of aluminium occurred and low friction was observed. Ni et al. [10] performed a similar study, where room and high temperature properties of Cr–DLC coatings, hydrogenated as well as non-hydrogenated, were evaluated in pin-on-disc sliding against aluminium. Both coatings performed very well at room temperature, providing low friction and avoiding aluminium transfer. However, at elevated temperatures transfer of aluminium did occur to the non-hydrogenated coating and the hydrogenated coating was worn due to graphitization. DLC coatings have also been evaluated on cutting tools, intended to reduce aluminium transfer in dry machining. Cemented carbide substrates were coated with a non-hydrogenated a–C and tested in a pin-on-disc equipment as well as in a milling equipment. In the pin-on-disc test low friction was achieved and no transfer of aluminium was observed. From the milling tests a

1 a–C:H denotes DLC of amorphous carbon alloyed with hydrogen. In a–C:Me, the Me denotes alloying with a metal, a–C:X:Y denotes alloying with both elements X and Y.

reduced tendency to material transfer was reported and built-up edge formation could be prevented. This resulted in improved quality of the machined surface as well as in reduced cutting forces [11]. Other studies report that DLC coatings do not influence the friction and work material pick-up tendency, or even make it worse. In a punch test with high surface expansion, simulating cold massive forming, three DLC coatings were tested; a–C:H, a–C:H:Si and a–C. All three resulted in more material transfer and subsequent higher friction than the uncoated tool steel. However, the tested a–C:H showed a tendency to improve the surface finish of the formed products [12]. From these studies it is clear that addition of a DLC coating does not consistently decrease friction and prevent galling; the detailed properties and chemistry of the coating obviously matter. The mechanism behind the decisive differences between the transfer to a tool steel and to a DLC coating, is yet unknown but will be further investigated in this paper by comparisons of the very initial stages of material transfer. The surfaces are tested both as carefully polished and with intentional scratch damages to the polished surfaces. The complex conditions involved in transfer and galling are identified and broken down to a number of distinctive mechanisms, and the anti-galling properties of DLC are discussed in the light of this summary.

2. Experimental The contact conditions during aluminium forming have been mimicked in a test equipment mounted in a scanning electron microscope (SEM). A tip shaped cylinder that has been sliding against a flat tool surface has represented the work material. The load has been high enough to plastically deform the work material, in accordance with forming. The combination of load, hardness and geometry of the pin samples results in a very limited contact area, about 100–200 mm in diameter. The limited and well-defined contact makes it possible to correlate local differences on the surfaces to transfer of work material, frictional changes and tip deformation. The effects can be studied in real time at high resolution, with the SEM. 2.1. Materials 2.1.1. The work material/tip shaped material An industrially important AlMgSi alloy (6082) has been used as work material (see Table 1). The cylindrically shaped aluminium samples were manufactured from sheet metal by turning and subsequently a rough shaping of the tip. The final tip shape was formed by grinding with 1000 grit followed by 4000 grit SiC grinding paper. A sketch showing the final tip geometry is found in Fig. 1. Table 1 Nominal composition of aluminium alloy 6082 (wt%) [13]. Si

Mg

Mn

Fe

Cr

Cu

Zn

Others

Al

1.2

0.78

0.50

0.33

0.14

0.08

0.05

0.15

bal

Fig. 1. Sketch showing the tip shaped work material.

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2.1.2. The tool material/the flat The tool material tested is a tool steel coated with a sputtered DLC coating (thickness 2 mm and hardness 1500 HV0.05), which has proven to have good anti-galling properties in earlier testing [8]. This is compared with a nitrogen alloyed powdermetallurgical cold work tool steel (Vancron 40) presented in Part 1[4]. The coated tool steel flats (13 mm  13 mm  10 mm) were polished to a surface roughness of Ra 0.05 mm, using 1 mm abrasive diamond paste. The polished surface was studied in high resolution using Atomic Force Microscopy (AFM) (see Fig. 2). After polishing, some coated samples were given intentional scratches to represent local damage of a tool. The scratches were made using single SiC grains drawn perpendicular to the sliding direction used in the subsequent testing. These SiC grains caused the coating to flake in the vicinity of the scratch and plastic ploughing in the substrate steel (see Fig. 3).

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2.4. Post test characterisation The direct in situ studies were supplemented by high-resolution SEM studies of both flats and tips using a Zeiss Ultra 55 FEG-SEM and a LEO 1550 FEG-SEM. The element composition of the surfaces was analysed by energy dispersive X-ray spectroscopy (EDS) using an Oxford Aztec X-max system. Since the transferred material typically was very thin, mainly low primary electron

2.2. Description of the in situ manipulator The tests were performed in an in situ manipulator [14–16] installed in an SEM (LEO 440). The equipment operates under vacuum, so that the testing can be directly observed using the electron beam and a secondary electron detector. A work material tip is put into contact with a coated tool steel flat, as imaged with the SEM in Fig. 4. The tip is loaded by a spring and held stationary during testing, while the flat is moved. The normal force and friction force are continuously monitored.

Fig. 3. Well-polished DLC coating with one intentional scratch, showing spalling of the coating and exposure of the steel substrate.

2.3. Experiments in the in situ manipulator in the SEM All tests comprised sliding of the aluminium tips against the DLC coated and polished tool steel surfaces. The normal load was set to 3 N in all tests, the sliding distance was 4 mm for each passage and the sliding speed was 3 mm/min. All samples were cleaned in ethanol before testing. The aluminium tips were initially relatively sharp, but flattened on the first static contact. The flat area then grew during the sliding contact, especially during high friction conditions. The tests comprised 1, 5 or 10 passages along the same track over the surface. After each passage in the series, the tip was lifted out of contact, the flat was moved back and the tip again lowered at the original starting position. The passages were studied in the SEM, which facilitated direct observation of tip deformation and transfer of material to the tool surface. This allows for immediate correlation between the work material transfer and changes in surface appearance as well as influence on friction.

Fig. 4. In situ view of aluminium tip after three passages in sliding contact with polished DLC using SEM.

Fig. 2. Well-polished DLC surface imaged using AFM. Typical roughness height is 2 nm. It should be noted that the peaks are far from being as pointy as they appear in this height-to-width scaling ratio. The slope of the sharpest peaks is around 12 nm/100 nm. Scanned areas are (a) 10 mm  10 mm, and (b) 3.5 mm  3.5 mm.

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energy of 3 keV was used in the SEM, which limits the interaction depth. To complement the surface studies, cross-sections were made in the work material using Focused Ion Beam (FIB). With this technique the uppermost surface layers can be preserved and the cross-section can be positioned with micrometre accuracy.

3. Results 3.1. Friction behaviour The aluminium test pin was slid against polished DLC and against polished DLC with two intentional scratches, while the friction force and normal load was continuously recorded. The resulting friction behaviour is shown in Fig. 5. Against the fine polished flat, the initial friction coefficient was relatively low, and it remained on the low level during the full passage. The second passage in the same track gave very similar friction, seemingly unaffected by the previous contact. This steady low-friction behaviour continued over all 10 passages along the same track. The friction coefficient against the intentionally scratched DLC surface was initially at the same level as against that without scratches. However, when passing over the two scratches, the friction increased drastically. After passing the last scratch, the friction slowly decreased until reaching the initial level. On the second passage, the initial friction was again at the same level, while the increase when passing the scratches was more dramatic, reaching a maximum of 0.42. Again, the friction decreased after passing the last scratch, but now some additional 0.5 mm sliding distance was needed to reach the low friction level. All following passages reached roughly the same maximum friction when passing the scratches. However, the friction did not immediately decrease after passing the scratch, but stayed on a level m  0.35, before slowly returning to the initial value. The high friction level after the second scratch was maintained for gradually longer sliding distance with each passage (see Fig. 5(b)). When moving the tip to a new starting position, and making a new series of passages parallel to the previous, an almost identical behaviour was found.

3.2. Observations of material transfer The friction curves in Fig. 5 reveal that a number of different mechanisms must be active, depending on absence or presence of scratches, the sliding distance after passing the scratches and the number of passages. The areas to the right of the scratches are as polished as those to the left, but obviously cause considerably higher friction. The reasons behind this and the other features of the friction curves were investigated in the SEM during as well as after testing. 3.2.1. Fine polished DLC surfaces During the in-situ observations, which require high acceleration voltage for sufficient signals, no transfer of work material to the polished DLC coating could be detected. After testing, the surfaces were studied at higher magnification and using lower acceleration voltage, which offers a more surface sensitive imaging. Even with this technique, almost no transferred aluminium could be detected (see Fig. 6). However, it is clear that the passage of the pin leaves some traces on the DLC coating, albeit barely detectable even on using the in-lens detector, which provides the most surface sensitive imaging (Fig. 6a and c). At rare scattered rough spots it was possible to find some very small transferred particles, as exemplified in Fig. 7. 3.2.2. Effect of scratches on polished DLC surfaces The most obvious result from the in-situ observations is that the high friction measured while passing the intentional scratches is correlated to much more transfer than the sliding against the polished surface. The amount of aluminium transfer in connection to the scratches was registered before, during, as well as after testing. An overview of the whole testing area is shown in Fig. 8 and the appearance of the two scratches after selected number of passages is shown in Fig. 9. The first scratch, Fig. 9 (left column), is narrower and slowly becomes filled with aluminium. However, the transferred aluminium does not grow considerably above the coating height (1st scratch, passage 1, 6 and 7). It should also be noted that no aluminium is transferred to the well-polished surface in front of the scratch, while a significant amount is transferred after the scratch.

Fig. 5. Friction coefficient recorded for the aluminium tip sliding over the DLC coated tool steel flat. Each line represents an individual passage out of 10 in the same direction along the same track. (a) Polished DLC. (b) Polished DLC with two distinct intentional scratches perpendicular to the sliding track.

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Fig. 6. Appearance of polished DLC coating after 10 passages of the aluminium tip. Exactly the same area is imaged with 3 kV acceleration voltage using two different types of electron detectors giving different contrast. (a) and (c) In-lens detector, (b) and (d) secondary electron detector. (a) and (b) Overview over the full width of the track. (c) and (d) Details in the track.

The passage over the scratches not only causes these relatively thick transfer layers, but also results in small-scale transfer, downstream (to the right) of the scratch, as vaguely visible in Fig. 9. The area covered by this small-scale transfer grows in the sliding direction with the number of passages, as visible in Fig. 8. Note that none of the contact tracks show transferred aluminium in front of the first scratch, irrespective of the number of passages. The small-scale particles (or lumps) transferred to the right of the scratches were also studied at higher magnification. After one single passage of the aluminium tip, the surface is characterised by scattered sub-micrometre sized transfer particles (see Fig. 10(a)). The particle size, density and extent increases with increasing number of passages (compare Fig. 10a, b and c).

3.3. Appearance of pin surface

Fig. 7. One of the rare very small particles (I) that has become transferred at a local rough spot (II) on the polished DLC surface. SEM, 5 kV acceleration voltage, in-lens detector.

With every new passage, more aluminium adheres to the polished surface, but only after passing the scratch. The second scratch is wider and exposes more of the substrate (see Fig. 9 (right column)). Also this scratch gradually becomes filled with aluminium. Once filled, more transfer typically builds up on previously adhered aluminium and also grows out from the scratch, mainly against the sliding direction. However, this is not a steady growth, but material may also become removed. An example is pointed out, where a part that is added on the 6th passage disappears during the 7th.

The aluminium tip shows very limited macroscopic deformation while sliding over the polished low-friction surface (see Fig. 4). Contrastingly, it becomes heavily deformed during the high-friction sliding associated to the intentionally scratched sample (see Fig. 11). On the macroscopic level, it has become bent by the friction forces. On the microscopic level, the uppermost surface is flattened and sheared over large regions. Aluminium material is obviously transported from the front end towards the back of the tip end (see Fig. 11(b)). To learn more about the mechanisms behind the gradual friction decrease displayed after passing the scratches, one of the sheared zones of the tip as shown in Fig. 11(b) was studied in cross-section. It was found that the superficial aluminium grains are heavily deformed and elongated in the sliding direction (see Fig. 12). At the outermost surface the grains are much smaller, in the sub-micron range.

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Fig. 8. Intentionally scratched polished DLC flat after 1, 5 and 10 passages of the aluminium tip. The arrow indicates the start and end positions of the sliding path used for all passages. Note that the transfer of particles resulting in the bright tracks starts when passing the first scratch and then ends after a distance that increases with increasing number of passages. The boxed areas are shown at higher magnification in Fig. 10. (The slightly curved appearance of the 10 passages track is an artefact due to the image distortion occurring at the lowest magnifications). SEM secondary electron image with 3 kV acceleration voltage.

4. Discussion 4.1. Basics of the friction modifications The friction between the aluminium tip and the polished DLC surface was low and remarkably stable for being an unlubricated contact under a load high enough to immediately flatten the tip. The stable friction behaviour was also accompanied by a very stable contact interface; after the initial flattening of the tip, no further deformation was noticed, and almost no transfer of aluminium to the DLC was found. Contrastingly, the intentional scratches over the DLC surface brought about a much more complicated friction behaviour, and associated complicated changes of the contact interface. These changes are very interesting and will be discussed in detail. Obviously, the nature of the interface may affect the friction in various ways. A very smooth surface with low adhesive interaction will not cause much deformation of the softer surface and will result in a low friction coefficient. Sharp edges on the harder surface, as presented by the ridges and coating edges of the intentional scratches (Fig. 3), will cause substantial deformation and even wear of the aluminium tip and thereby a considerable addition to the friction. This is often denoted the ploughing component of friction. The worn off material is typically hardened by deformation hardening and oxidation, and will typically not be as smooth as the polished DLC. Therefore this transferred material will also add a ploughing friction component at subsequent passages (Fig. 9). Further, the contact has now shifted from aluminium or rather alumina against DLC to alumina against alumina. This will also modify the friction level. Finally, after passing the scratches or the transferred material filling the scratches, small-scale transfer will occur (Figs. 8 and 10). This will act in the same way as the larger transfer in the scratches; they will both add alumina on the DLC side, thereby reducing the DLC/alumina sliding, and also adding a ploughing component.

4.2. Details of the transfer mechanisms The present study clearly demonstrates the intricacy of the material transfer phenomenon. Obviously, it must involve several separate mechanisms that dominate under different conditions and may interact in various ways. Based on the vast number of publications including information about transfer of aluminium, (see [17–21]) and the present findings, including part one of this investigation [4], we propose a set of mechanisms. The mechanisms are organised into classes represented by simplified schematic

drawings and briefly described in Table 2, and further discussed below. On the finely polished DLC surface, transfer of aluminium was more or less negligible. This is in good agreement with several recent investigations, which have illuminated the excellent antisticking properties of smooth DLC coatings in contact with aluminium [3,8]. In the present investigation, wherever transferred material was found on the polished DLC, it was located to the rare scattered micrometre-sized surface defects, as exemplified in Fig. 7. However, the larger scale roughness presented by the intentional scratches totally changes both the friction and transfer situations. This distinctive change, even continuing when sliding over the polished DLC surface after passing the scratches (as clearly demonstrated in Figs. 8 and 9), is very intriguing. The changed behaviour includes several parts: large scale transfer directly to the scratched area, build up of transfer in front of the scratches and, most interestingly, transfer to the ‘‘perfect’’ surfaces after the scratches followed by a gradual diminishing of this transfer until it completely halts. On the following passage the same procedure repeats, with the exception that a longer sliding distance is needed for the transfer to end. This ‘‘activation’’ of the aluminium work material (causing transfer to the otherwise transfer free DLC) followed by ‘‘healing’’ of the work material surface (leading to that this transfer stops) makes up a new fundamental mechanism, that to the best of our knowledge never before has been reported. The whole process can be explained by the following set of mechanisms. When passing the scratch, the sliding surface of the aluminium tip will be mechanically damaged. On the first passage, the tip will meet the sharp edges left by spalled-off DLC coating fragments. This will scrape off relatively large aluminium lumps, which fill the scratch and also may build up transfer layers just in front of it (cf. Fig. 9). The scraped-off material will of course be missing from the tip, which will also loose its shape due to plastic deformation induced by sliding over the rough protrusions of the scratch. The plastic deformation and wear loss combined will result in a very uneven surface of the tip. When this roughened tip ‘‘lands’’ on the smooth surface after passing the scratch (as illustrated by Mechanism V in Table 2), the contact area will be very small and the surface oxide will be severely damaged or even partly missing. Note that aluminium oxide is a very brittle ceramic, here covering the soft, ductile aluminium metal. Further, the native oxide film is extremely thin (typically 2–3 nm). In a mild sliding situation, this chemically stable film, some 20 times harder than the underlying metal, will stay intact and separate the tool

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Fig. 9. The two scratches on the intentionally scratched polished DLC flat after different number of passages of the aluminium tip (as denoted in the micrographs). The white arrow indicates the sliding direction of the aluminium tip. An overview is given in Fig. 8. Imaged in SEM using 20 kV acceleration voltage.

surface from the reactive aluminium metal. However, we must expect the oxide to become severely damaged when being forced over a hard surface, whenever the roughness is significant in relation to the oxide film thickness [22]. This damage will activate the work material tip in two ways: 1) metallic aluminium will be exposed,

2) the uneven surface will experience high load concentrations resulting in extensive deformation in the contact against the smooth surface (flattening). Also this deformation may fracture the remaining or newly formed oxide layers. In both cases, the exposed metallic aluminium will be reactive and more prone to forming strong adhesive bonds with the

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Fig. 10. Appearance of polished DLC flat after different number of tip passages (from left to right), resulting in increasing number and size of transferred fragments. The tip has passed the intentional scratches before reaching the image areas, as indicated in Fig. 8. Each image shows an overview to the left and a higher magnification image of the same area to the right. The corresponding friction levels are given. SEM with acceleration voltage of 3 kV. (a) 1 passage, (b) 5 passages and (c) 10 passages.

Fig. 11. Aluminium tip surface after sliding against the DLC flat with two intentional scratches. The tip has passed both scratches and then slid against fine polished DLC with transferred aluminium from two preceding passages. The test was interrupted when the friction had fallen to m  0.25. (a) Overview. The arrow indicates the sliding direction of the counter material. (b) Close-up of the area indicated in (a).

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Fig. 12. Cross-section images of the aluminium tip surface after sliding against DLC with two intentional scratches. The whole cross-section, including segregated alloying element particles, is noticeably shear deformed in the sliding direction. The position of the cross-section is indicated in Fig. 11(b). The cross section was prepared using a FIB instrument. The same area is imaged using (a) secondary electrons and (b) ions.

surface. Both the bonding tendency and the extensive deformation will promote transfer to the DLC surface. Although the test is performed in vacuum, the transferred lumps become oxidised [23], which make them fully capable of deforming and scratching the aluminium on following passages. The transformation is due to their small size, the severe conditions leading to repeated breaking of their surface oxide and the high affinity of aluminium to oxidation. The healing of the rough, worn aluminium surface is proposed to include the following steps: 1) Sliding against the smooth hard DLC surface, showing intrinsic low-friction properties, will gradually smoothen the softer aluminium surface. The increasing contact area reduces the contact pressure and shear stresses at the sliding interface. 2) The reduced contact pressure and shear stresses will reduce the tendency of transfer from the aluminium surface, which will allow the formation of a stable, oxidised, sliding surface. 3) Observations of the contact surface on the aluminium pin reveal that the healing process first initiates at the front edge and then propagates backwards until a stable low-friction sliding condition is achieved. 4) During the stabilisation and once the sliding has stabilised, the surface material has time to develop. We have observed (a) the generation of a very fine grained more equiaxed surface layer, cf. Fig. 11, possibly due to a shear activated recrystallisation mechanism and (b) formation of a composite microstructure composed of deformed aluminium grains and presumably mixed with more oxidised material.

With increasing number of passages, the area showing transferred aluminium downstream the scratch grows in length, as does the degree of aluminium coverage. This is demonstrated by the extension of the tracks to the right of the scratches in Fig. 8. Correspondingly, this means that the healing process is gradually delayed with each passage. The delay is explained by that for each new passage; the work material pin has to pass not only over the scratches but also over the lumps transferred due to the damage activation in the previous passage. These lumps also roughen and damage the surface and therefore also cause damage activated transfer. As a consequence, the portion of the track that exhibits transfer will elongate for each passage.

Experimentally it was found that the healing process here requires about 330 mm sliding over the smooth DLC. This was estimated both from the friction measurements (Fig. 5) and the extension of the transfer tracks (Fig. 8). In the friction curves this distance is manifested by the gradually increasing sliding distance to reach the low level (on an average increasing with roughly 330 mm per passage). On the surface it is manifested as the stepwise growing part of the track showing transfer particles. Note, that this particular healing distance is only valid for this specific experiment, while we expect the mechanism to be general to similar conditions. 4.3. Requirements for transfer free sliding and healing of activated work material Even in the absence of a suitable lubricant, the sliding contact between a forming tool and aluminium can stay transfer free. This requires that the tool surface 1) is smooth enough to avoid primary transfer by mechanical scraping, 2) has a chemistry that does not initiate primary transfer by adhesive bonding, 3) is smooth enough not to activate the aluminium surface, 4) is smooth enough and offers low enough friction to rapidly induce healing of activated areas of the work material. In this way local damage etc. will not immediately induce large scale transfer, 5) can preserve its high smoothness and beneficial surface chemistry over long sliding distances against the work material. Obviously, if free from damage, the polished DLC meets the requirements (1)–(4), as demonstrated by keeping almost free from transfer and keeping the friction constant and low (m E0.18). In addition to its high smoothness and intrinsic lowfriction properties, the combination of a relatively high hardness and a relatively low elastic modulus could be assumed to be beneficial for keeping the wear rate low against the aluminium and its oxide. If this is true, it would also preserve the good properties over much larger times than investigated here, thereby fulfilling also requirement (5). Presence of the larger scale scratches dramatically deteriorated the situation, leading to

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Table 2 Schematic illustrations and short descriptions of the proposed mechanisms involved in the transfer of aluminium to tool surfaces in forming. The surface oxide is indicated as a contour on the tip for mechanisms V–VII. See text for detailed explanations and discussion. Transfer free sliding The friction is low, stable and does not change from passage to passage. The tip becomes smooth (shape adapts to the tool surface) and probably stabilized by tribologically stimulated growth of the oxide layer. Only observed for the polished DLC without intentional scratches.

Primary transfer I Transfer by scraping against permanent roughness. Ia Due to pits in the tool surface IIa Due to protrusions from the tool surface Relatively stable, permanent grip that will allow gradual growth towards the sliding direction. Maximum layer thickness influenced by height of roughness.

II Transfer by adhesive bonding to the tool material surface (permanent surface) May offer relatively permanent grip that allows gradual growth. For some materials the bonding strength will only be high enough on specific phases (as indicated by light ‘‘particle’’).

Secondary transfer III By scraping against already transferred material

IV By adhesive bonding to already transferred material Maximum thickness depends on strengths of primary and secondary bonds and cohesion of transferred lump in relation to the local stresses (due to friction and normal load) affecting the transferred lump.

V Damage activated transfer Transfer to surfaces due to work material damage caused by passage of roughness on the tool surface When passing hard rough elevations, the work material surface will become worn, deformed and rough by scratching, and the surface oxide will at least partially be removed. On continued sliding, the uneven work material surface will experience high load concentrations resulting in extensive deformation (flattening), possibly again resulting in damage to the oxide surface. The exposed metallic aluminium will be more reactive and probably more prone to forming strong bonds with the surface. These mechanisms – the activation of the work material – will result in strongly intensified transfer tendency after passing the rough area. Due to the activation of the work material, tool surfaces that do not exhibit roughness or phases with strong bonding, and therefore would not experience Mechanism II, will still suffer from transfer by adhesive bonding, as in Vb. In this way, the roughness on a surface will not only cause direct transfer (according to mechanisms I and III) but also a delayed transfer.

VI Secondary damage activated transfer The lumps transferred due to the damage activation will roughen the surface. On following passes over these areas, this roughness will also cause damage activated transfer. Due to this mechanism the affected area will grow with each passage.

VII Healing of damage activated work material The work material heals if it is allowed to slide some distance against a smooth surface with low adhesive forces. The healing ‘‘reverses the activation’’, meaning that it resets a stable smooth surface, which can slide without further transfer.

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primary, secondary and damage activated transfer. This resulted in a doubled coefficient of friction over a surface extending far from the scratch (ending at m E0.35), and a growing area covered by transferred material.

4.4. Comparisons with tool steel How does this compare to the finely polished Vancron 40 tool steel investigated in Part 1 [4]? Although the tool steel was finely polished, it displayed immediate pick-up of aluminium in the microscale. This primary transfer preferentially occurred on top of the protruding carbonitride particles, despite their extremely low heights (10–15 nm) and correspondingly low slopes. We could not definitely establish whether this primary transfer was due to scraping off or adhesive bonding or possibly damage activated adhesive bonding. The latter could result in transfer on the very local scale on top of the carbonitride particles, if their nano-roughness damages the native oxide film. Thereby, the metal would become activated and make strong bonds to the particle [4]. Regardless of the mechanism, once the primary transfer was established, it was followed by secondary transfer, growing with each sliding pass. During this process (10 passages of the tip) the coefficient of friction increased by a factor of 2.5, ending at m E0.44. In contrast to the DLC surface, the presence of the larger scale scratch only marginally affected the friction behaviour. This means that also in the finely polished condition, this tool steel does not meet all the requirements (1)–(4). It could be speculated that a polished tool steel with smaller hard phase particles could offer a more advantageous topography, thereby meeting requirements (1) and (3). It still has to be tested if the adhesive bonding and the friction properties can be good enough to fulfil requirements (2) and (4), and finally if the wear resistance is good enough to fulfil also requirement (5). Similar aspects could be discussed for alternative coatings, with a focus on their chemical affinity to aluminium, suitability to fine polishing and ability to preserve a good finish.

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The friction does not increase during the test on the polished DLC. By making intentional scratches on the polished DLC, the transfer behaviour was dramatically changed. We noted direct transfer on top of and in front of the scratch. This transfer gradually builds up during consecutive passages. The friction is high during the passage over this region. Further, the damage caused to the work material surface by the intentional scratches results in damage activated transfer down the sliding path. There would be no corresponding transfer to the polished surface in the absence of scratches. This transfer region is associated to an intermediate friction level. The damage activated transfer gradually ceases upon further sliding, i.e. the damaged work material ‘‘heals’’. During this gradual change, the friction also gradually falls back to the lowfriction level typical of the polished surface. On the following passage, the already transferred lumps damage the work material surface and hinder the healing process. In this way the healing is postponed until the tip leaves the previous damage activated transfer area. On consecutive passages this results in a gradual elongation of the transfer area, and an associated elongation of the region of intermediate friction level. Consequently the two contacting bodies follow very different routes; the damage caused to the aluminium part can quickly heal, while the tool surface steadily becomes more deteriorated by the transferred material. The great advantage of the DLC coating over the tool steel seems primarily to be due to its more advantageous topography after polishing, probably in combination with very weak adhesion tendency towards the aluminium oxide. This beneficial surface both avoids local transfer (such as that on the protruding carbonitride hard phases on the tool steel surface) and offers a gentle enough contact to allow healing if the work material surface has become locally damaged by the contact against local surface roughness.

Acknowledgements 5. Conclusions The present detailed in situ studies have substantially increased our understanding of the micro mechanisms involved in the initial stages of work material transfer, and the background to the differences between the investigated polished DLC coated steel and the polished tool steel, presented in Part 1 [4]. We have shown that the complex processes involved in the initial stages of transfer can be broken down into three classes: primary transfer, secondary transfer and damage activated transfer. The damage activated transfer constitutes a new fundamental tribological phenomenon, involving the activation and healing of a soft metal in sliding contact with a harder surface. These processes were shown to be important when forming aluminium, but are also expected to have a more general tribological significance. In all situations of sliding friction when the oxide layer on metals is damaged by the contact against the counter body, the friction and wear behaviour will be affected. It could be expected to be particularly important for light metals (which are soft and have hard oxides) and all situations involving nonperfect lubricant films. While the finely polished tool steel always immediately (after the first 100–200 mm sliding, corresponding to a single passage by the tip, or less) picks up aluminium (in the microscale), the polished DLC surface is virtually free from aluminium transfer after the 10 passage tests used here.

The authors would like to acknowledge the Swedish Foundation for Strategic Research for financial support via the programme Technical advancement through controlled tribofilms. Steertec Raufoss is acknowledged for the support of Jannica Heinrichs and for introducing the authors to the industrial aspects of aluminium forming. Professor Sture Hogmark is gratefully acknowledged for valuable comments on the manuscript.

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