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Wear mechanisms of WC-Co cemented carbide tools and PVD coated tools used for shearing Cu-alloy wire in zipper production
Wear 420–421 (2019) 96–107
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Wear journal homepage: www.elsevier.com/locate/wear
Wear mechanisms of WC-Co cemented carbide tools and PVD coated tools used for shearing Cu-alloy wire in zipper production
T
J. Heinrichsa, , H. Mikadob, A. Kawakamib, U. Wiklunda, S. Kawamurab, S. Jacobsona ⁎
a b
Applied Materials Science, Uppsala University, Sweden Machinery and Engineering Group, YKK Corporation, Japan
ARTICLE INFO
ABSTRACT
Keywords: Cemented carbide Cutting Shearing Wear Cu-alloy
To form the individual elements, that together form a zipper, a pre-formed Cu-alloy wire is sheared using cemented carbide tools. The wear caused by the relatively soft copper alloy on the much harder tool is generally quite slow. However, millions of elements are to be sheared so eventually the wear becomes unacceptable and the tool needs to be exchanged. To improve product quality, as well as minimize down time and material consumption, the tool life needs to be prolonged. To achieve this the wear process needs to be better understood. Uncoated tools used for an increasing number of shearing events have been studied in detail using high resolution SEM and EDS, to map the propagating wear and get an insight into the wear mechanisms. Transfer of material from the Cu-alloy to the tool occurs and the wear is highly concentrated to specific areas. This wear occurs on a very fine scale, limited to within individual WC grains at each event. Tools coated with PVD CrC and PVD CrN have been studied for comparison with the uncoated cemented carbide. Both coatings successfully protect the cemented carbide tool from wear, however occasional flaking occurs and then the cemented carbide becomes exposed and subsequently worn. The differences in performance and wear mechanisms between the uncoated and coated tools are discussed, with focus on the capability of the coatings to prolong the tool life.
1. Introduction The production of a functioning zipper includes several complicated forming and cutting process steps. One of these steps involves cutting of the individual elements forming the zipper from a pre-formed wire. This is known as a shearing process, comprising a shearing die, through which the pre-formed wire is fed, and a shearing punch that makes the actual cut. High quality metal zippers are often made from Cu-alloys. Cemented carbides are commonly used as tool material for both the die and the punch. The combination of hard ceramic tungsten carbide particles in a softer metallic (Co) matrix is often a good combination to achieve low wear rates in production. Still, the wear of the punch is a limiting factor, since it deteriorates the quality of the zipper elements and necessitates production stand still and exchange of tools. Thus, there is a desire to prolong the tool life of the punch to minimize down time, materials consumption and to improve the element quality. Studies on the wear of tools used in Cu-alloy forming processes are not frequently published. In particular, there has been very limited research on the tribological performance of uncoated and coated
⁎
cemented carbide tools used for shearing Cu-alloy wires. Some work has been published on related topics; wear of cemented carbide tools in Cu-alloy wire drawing [1] and in reciprocating sliding contact with Cualloy [2], where mainly adhesive wear was observed. Another application that has recently received attention is cutting of Cu-alloys. Due to new legislation, the amount of Pb in Cu-alloys is being reduced. The presence of Pb is important for the machinability, where part of the effect is due to that Pb accumulating on the Cu-alloy surface decreases friction and wear [3,4]. Thus studies on how to improve the machinability of Pb-free Cu alloys, to avoid high wear of the commonly used cemented carbide tools, have recently been reported [4–8]. The worn cemented carbide tool inserts mainly show crater wear [5,6], where Co is removed by diffusing into the passing Cu-alloy chip and WC grains are subsequently being removed [6]. Minor cross-diffusion of Cu and Zn to the Co binder also occurs [6]. The potential of applying thin hard coatings on cemented carbide tools for copper machining is investigated in several papers, and especially DLC, (Ti,V,Zr,Hf,Nb,Ta)N and CrN have shown good results [1,4–8]. Another area of recent interest is to replace the Co, the most commonly used binder in cemented carbides, with the purpose of reducing wear. Recently, the use of FeAl
Corresponding author. E-mail address: [email protected] (J. Heinrichs).
https://doi.org/10.1016/j.wear.2018.12.075 Received 24 October 2018; Received in revised form 21 December 2018; Accepted 21 December 2018 Available online 24 December 2018 0043-1648/ © 2019 Elsevier B.V. All rights reserved.
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underneath and then sheared to a set length by a shearing punch and die. Initially the contact between the wire and the punch is characterized by the shearing action. Eventually the wire fractures and sporadic sliding contacts between the fracture surface of the wire and the punch occur until completion of the stroke. The shearing process is further explained in [10]. Four uncoated cemented carbide tools were tested, each used to cut a predetermined number of elements, ranging from 1000–25,000,000. The latter number is about the expected life time of a shearing punch. One uncoated tool, prepared in the same way, was kept as an unused reference. Two types of PVD coatings, CrC and CrN, were evaluated in a similar way as the uncoated tools. The full testing scheme is presented in Table 2.
Table 1 Nanoindentation hardness, based on at least ten 100 nm deep indents. Material
H [GPa]
Number of indents
Co WC CrN CrC Cu-alloy
8.3 ± 0.56 31 ± 3.1 19 ± 1.1 25 ± 1.0 2.7 ± 0.23
10 22 34 39 128
binder in cutting tools for dry machining of oxygen-free copper was investigated, with good results [9]. These were attributed to the lower tendency of FeAl to oxidize, compared to Co, when in contact with Cu. Despite the substantial work performed, the understanding of the wear of cemented carbides in contact with Cu-alloys is still not complete. To prolong the life of the shearing punch used in zipper production, it is important to know how and where the wear starts and propagates. The present paper adds to the knowledge about this process by studying shearing punches at different stages of their life time, i.e. after cutting different amounts of elements, under the same conditions as in actual production. Based on the findings, the possibility to optimize the cemented carbide grade for shearing the copper alloy work material is discussed. The potential of prolonging the tool life even further by the use of PVD coatings is also evaluated.
2.3. Post test surface characterization After testing, all tools, including the reference, were cleaned first with acetone and then in ethanol in an ultrasonic bath for 3 min, respectively. Then all tools were analyzed using white light interference profilometry (WYKO NT1100), high resolution FEG-SEM (Zeiss Merlin) and Energy Dispersive X-ray Spectroscopy (EDS; Oxford X-max). Selected tools were analyzed also in cross section, prepared by Focused Ion Beam (FIB; FEI Strata DB235). 3. Results
2. Experimental
3.1. Wear measurement
2.1. Materials
The surface profile of each tool was measured along the symmetry line stretching over the cutting edge, as illustrated by the sketch in Fig. 2, which depicts the region adjacent to the cutting zone on the tool. The surface profiles are rendered in Fig. 2 and corresponding wear data is plotted in Fig. 3. The unworn reference tool displays a relatively smooth surface with a quite sharp cutting edge. During early use, shown by the tool having cut 1000 elements, only a slight smoothening of the cemented carbide occurs. After cutting 1,000,000 elements, almost 7 µm has been worn off at the cutting edge, which corresponds to slightly less than two layers of WC grains. The edge has also become slightly rounded. This wear gradually increases so that after cutting 25,000,000 elements some 45 µm have been removed at the very edge. The worn area then extends about 300 µm in from the cutting edge. All PVD coated tools showed much lower wear rates than the uncoated. The CrC coated tools used for cutting 4,250,000 and 8,500,000 elements respectively, both show low wear with similar profiles, with the exception that the tool used for cutting 8,500,000 elements is more worn at the very edge, Fig. 2. The deepest wear is found at the edge and the last indication of very slight wear is observed about 180 µm from the cutting edge. Still, 10 µm from the edge the wear depth is only about 1.5 µm for both tools. On the CrC coated tool used to cut 25,000,000 elements, the indications of wear again stretches to about 180 µm from
The studied tools are made of cemented carbide comprising 85 wt% WC particles, with an average grain size of 4 µm, in 15 wt% Co binder. The tools are finished by grinding, to an Ra value of 0.5 µm. Tools were used either uncoated or PVD coated with CrC or CrN. Prior to coating deposition each cutting edge went through a blast polishing process, using diamond particles, to improve the coating adhesion. The coatings have been used as deposited, without any post treatment. The coating thickness was measured using a calo tester to 2.2 µm (CrC) and 3.0 µm (CrN). The zipper elements were cut from a preformed Cu alloy wire with 15 wt% Zn. The hardness of the materials was measured using nanoindentation, with a Berkovich diamond tip and an indentation depth of 100 nm, see Table 1. To allow measurement of the individual constituents of the cemented carbide, rather than more random degrees of composite hardness, the position of each indent was verified using scanning electron microscopy (SEM). 2.2. Wear testing in production machine The wear tests were conducted using an actual production machine, Fig. 1. The preformed Cu-alloy wire is fed intermittently from
Fig. 1. Schematic drawings of a) the shearing process in the machine, b) the process in cross section and c) the cut element. 97
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removed in an area stretching to about 300 µm from the cutting edge, indicating contact with the counter material within this area. After producing 1,000,000 elements, the cutting edge has become slightly rounded and a new type of scratch pattern has appeared in the area adjacent to the cutting edge. After cutting 10,000,000 elements, the cutting edge is clearly worn, and a transition to a less worn area is visible about 300 µm from the cutting edge. After 25,000,000 elements, the presumed lifetime of an uncoated tool, the wear has continued even further. Besides the heavily worn area, discernible already after 10,000,000 elements, some occasional large scale transfer of Cu alloy is observed right below the transition line. These two regions of the tool are clearly visible also using light optical microscopy, Fig. 4. The region of the tool showing more extensive wear corresponds to the region experiencing shearing contact with the Cu-alloy, Fig. 5. Below the transition line only sporadic sliding contact against the fractured Cu-alloy surface occurs, resulting in the lower wear. At higher magnification, more details of the wear process are revealed, Fig. 6. The reference tool exhibits the typical surface microstructure caused by grinding, characterized by abrasive scratches with crushed WC grains and WC fragments embedded into the surface. After cutting 1000 elements, the structure is still characterized by crushed WC grains and numerous fragments. However, it has become smoother and foreign material can be observed on the surface, as medium gray areas with sub-micron scratches. After cutting 1,000,000 elements, fewer crushed grains and fragments are present, although locally they are still common. The areas of foreign material are still present and the tool has now, after the initial smoothening, again become rougher. The surface now shows parallel valleys and ridges, running in the sliding direction. After cutting 10,000,000 elements, crushed grains are no longer present. The microstructure shows a clear, polished-like
Table 2 Included tools tested in actual production machine. Material
Number of cut elements
Uncoated cemented carbide
0 (reference) 1000 1,000,000 10,000,000 25,000,000 4,250,000 8,500,000 25,000,000 50,000,000
CrC CrN
the cutting edge. However, the wear now has increased drastically in a region stretching to 70 µm from the edge. Very close to the edge, the wear depth approaches that of the uncoated tool used to cut 10,000,000 elements. The CrN coating on the tool used to cut 50,000,000 elements is barely worn at all and still covers the cutting edge. The slight curvature in its profile is a result of the pretreatment, where the tool is blast polished to improve coating adhesion. 3.2. Appearance of worn uncoated cemented carbide tools The wear surface appearance of all tools was studied in the SEM, Fig. 4. The unused reference is characterized by grinding scratches. It also has a dark appearance caused by carbon residues that adhere during the grinding process and still remain after the ultrasonic cleaning. After cutting 1000 elements, the adhered carbon has been
Fig. 2. Surface profiles over the cutting edge and the adjacent surface for each included tool, measured along 400 µm of the centerline of the tool, as indicated in the sketch. a) The recorded wear depth for all tools. b) The area framed in (a) with a magnified depth scale to better visualize the limited wear of the slightly used and the coated tools.
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Fig. 3. Wear depth at the cutting edge (hollow markers) and 10 µm from the cutting edge (filled markers). Data originating from the surface profiles in Fig. 2, where the first measurement point in each profile is used to define “at the cutting edge”. * indicates that the coating is worn through in the area where the measurement is performed.
Fig. 4. Overview images of the uncoated cemented carbide tool flank after different numbers of elements cut, as indicated in the images a)-e) (the cutting edges are seen on the top part of the images, SEM; 3 kV). f) Light optical image of the tool used to cut 25,000,000 elements is included for comparison. The area imaged in SEM is indicated.
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Fig. 5. Surface appearance of the cut element and the corresponding area of the shearing tool. Top left; the cut element with its sheared (I) and fractured (II) regions, imaged using the same magnification as the tool, top right (LOM). Bottom row; detail of the surface appearances in the regions I and II of the cut element (SEM; 3 kV).
appearance with distinct WC and Co phases. Relatively large parallel valleys and ridges still run across the microstructure, but the surface is now substantially smoother along the sliding direction than perpendicular to it. After cutting 25,000,000 elements, further smoothening has occurred. Besides the fine ridges and valleys, it has a very finely polished appearance. In the region showing the transition from shearing contact to (occasional) sliding contact, the tool used to cut 25,000,000 elements exhibits all the above reported wear details, Fig. 7. In the region that has experienced occasional sliding, the area is smooth without ridges and valleys. However, some crushed and fractured WC grains and some larger transferred particles are present, in similarity with what was observed on the slightly used tools. When passing the boundary into the shearing contact region, the previously observed pattern with ridges and valleys immediately emerges. When continuing further from the boundary towards the cutting edge, less fractured grains are observed, while the ridge and valley pattern continues all the way to the very cutting edge, Fig. 7c and d. The cutting edge itself is, however, smooth and rounded. No loss of whole grains was observed at any position. Cu and Zn has become transferred to the cemented carbide tool already after cutting 1 000 elements, as shown by the elemental analysis in Fig. 8. Oxygen is present in the same areas as the Cu alloy, indicating that the transferred material is partly oxidized. This initial transfer seems to have a more or less stochastic distribution over the surface. However, with increasing number of cut elements, the transfer
becomes more distinct and is mainly located in between WC grains. Also the character of the cemented carbide changes, from being a mixture of WC and Co to distinct and clearly separated phases. The intensity of Co is also reduced, indicating that the amount of Co in the surface structure is reduced as the number of cuts increases. To further study the transferred material and the wear, FIB cross sectioning was performed in two areas where presence of transferred alloy had been confirmed by EDS, Fig. 9. Although clearly indicated by the top view EDS analysis, high magnification was required to find the transferred material in the cross sections, as in Fig. 9b and d. In these, the transferred material is visible as thin dark lines underneath the protective layer of platinum. Note that the transferred material is located on top of the Co binder but not on WC grains, as also indicated by the elemental analysis in Fig. 8. The worn surface is very smooth in the sliding direction, Fig. 8a and b, similar to a polished surface. The microstructure is not noticeably affected below the very surface and there is no apparent preferential removal of binder. 3.3. Appearance of worn PVD coated cemented carbide tools The wear protective capacity of two types of PVD coatings, CrC and CrN, was tested, according to Table 2. After testing, all tools were analyzed using the SEM, Fig. 10. After cutting 4,250,000 elements, the CrC coating has been removed from the very cutting edge and thereby exposed the cemented carbide 100
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Fig. 6. Details of the uncoated cemented carbide shearing tools, all showing a region along the central line about 100 µm from the cutting edge. From a)-e) they have been used to cut an increasing number of elements, from 0 to 25 million, as indicated in the images (SEM; 3 kV).
substrate. This worn area, with exposed cemented carbide, grows in the direction from the cutting edge with increasing number of cut elements. After 25,000,000 cuts, the coating has been worn off from a minimum of 20 µm and up to 320 µm from the cutting edge, Fig. 10c. For the CrN coated tool, the coating is still covering the major part of the cutting edge after 50,000,000 cuts. However, wear exposing the cemented carbide substrate has occurred locally, exemplified by the brighter area to the far right in Fig. 10d. Besides that, some brighter material can be seen adhered on top of the seemingly intact coating. In higher magnification, Fig. 11, a fine scratch pattern is visible on the CrC coated surface. These are not sharp abrasive scratches, but rather similar to the ridges and valleys observed on the worn uncoated cemented carbide. The typical topographical pattern from the PVD deposition process, which was observed in areas outside the contact, is almost worn away. This indicates that the coating has become substantially worn. Some occasional material transfer to the coating can also be noted. Regions in which the cemented carbide substrate is
exposed exhibit a wear pattern similar to that on the uncoated tools. This is valid also for the scattered areas on the CrN coated tool where the substrate is exposed, as exemplified in Fig. 11d. In areas where the CrN coating is still present, the wear is very mild. A shallow pattern, a characteristic result of the PVD process, is still visible but somewhat modified by occasional small scratches and mild polishing of high areas into plateaus, Fig. 11c. The shallow pattern also causes some material to adhere, appearing light gray in the SEM image. The transfer of work material is more extensive in areas of exposed cemented carbide than in coated areas, as evident from the elemental mapping in Fig. 12a. The transferred material is located in between WC grains, thereby partly covering the Co, and composed of both Cu and Zn, as also observed on the uncoated tools. Contrastingly, the transfer located on top of the intact CrN coating is rich in the alloying element Zn, and partly oxidized, Fig. 12b. Similar observations were made when analyzing transferred material located on top of intact CrC coating (results not shown here). 101
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Fig. 7. Details of the uncoated cemented carbide tool after cutting 25,000,000 elements. a) Transition region, about 300 µm from the cutting edge, b) detail showing the framed area in (a), c) cutting edge, d) detail showing the framed area in (c). (SEM; 3 kV).
To reveal more details of the wear of the CrC coating, FIB cross sections were prepared in areas with slightly worn coating as well as in the transition area between remaining coating and exposed substrate, Fig. 13. Wherever the coating still remains, it is smooth and even, although it shows distinct traces of wear in top view. The substrate underneath the coating is characterized by cracked and fractured WC grains and fragments. The cross section in the transition area, Fig. 13d, including both CrC coating and exposed cemented carbide, demonstrates the dramatic increase in wear that occurs as soon as the substrate becomes exposed. The exposed cemented carbide directly adjacent to the coated area shows a wear depth corresponding to several layers of WC grains, and the WC grains show no signs of cracks.
to several layers of WC grains. This means that all traces from the preceding grinding should have been removed. Thus, the observed pattern must be caused by the sliding process. However, since the Cu alloy is so much softer than both the Co and the WC, it should be unable to cause scratches. Further, there are no hard particles in the system, capable of scratching the cemented carbide. One possibility could be that hard wear particles, in the form of WC grains or grain fragments, follow the work material and cause scratches downstream of the worn area. However, no loss of individual WC grains or larger pieces of the composite has been observed. Further, if produced by hard particles, the ridges and valleys should be expected to change shape and size as particles pass first one phase and then the other. In other words, hard particles should generally cause preferential wear of the softer Co, which has not been observed. The cemented carbide, including both WC and Co, is indeed gradually worn in contact with the passing Cu alloy. Both phases wear at the same rate, resulting in a very smooth wear surface. This, together with the low wear rate, indicates that it is worn on the atomic, or at least sub-micron, scale. Most likely, this wear is caused by chemical interactions and, thus, mechanical hardness has very little influence. During the shearing process, Cu and Zn become transferred to the tools, preferentially sticking on top of Co regions. This preference could be due to that the transfer occurs by chemical interactions with the Co. It could also be due to mechanical interlocking, but since the Co is not preferentially removed the transferred material is very thin and any effect of interlocking must be limited.
4. Discussion 4.1. Wear of uncoated cemented carbide On the uncoated cemented carbide tool, the originally rough ground surface, characterized by cracked and fractured WC grains, gradually becomes smooth and polished-like during use. Cracked and fractured grains were not only observed before use, but also remained after cutting 1000 and 1,000,000 elements. However, the surface profiles in Fig. 2 shows that the total wear depth after 1,000,000 is only 1 µm in the imaged area. This corresponds to just parts of the thickness of a single layer of WC grains. Thus, the crushed grains found after wear were probably crushed already when the tool was ground. After cutting 10,000,000 elements, no crushed grains remain. The worn surface is rather characterized by intact WC grains evenly distributed in the binder phase. There are however ridges and valleys running in the sliding direction after cutting 10,000,000 elements. According to the surface profiles, Fig. 2, the wear depth in the imaged area is 12 µm, corresponding
4.2. Wear of PVD coated cemented carbide tools Two hundred µm from the cutting edge, the CrC coated tool used to cut 25,000,000 elements is almost unworn. However, going closer to the cutting edge, at about 70 µm from the cutting edge the wear 102
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Fig. 8. Elemental mapping of the uncoated cemented carbide tools, about 100 µm from the cutting edge, after cutting a) 1000 and b) 25,000,000 elements (SEM and EDS; 3 kV).
increases drastically. Referring to the surface profiles in Fig. 2, this corresponds to a position where the wear depth reaches 2 µm, which is about the original thickness of the coating. Thus, the rapid wear corresponds to the wear of the unprotected cemented carbide substrate. As evident from the SEM (Fig. 10), the distance of 70 µm from the cutting
edge is valid for the position of the selected profile, but actually varies substantially along the edge line. The two CrC tools used for fewer cuts show no rapid wear depth increase, except for at the very cutting edge, but these are still protected by the coating on the tool flank. For both tools the wear depth is only about 1.5 µm measured 10 µm from the 103
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Fig. 9. Wear surface and cross section appearance of the worn cemented carbide tool after cutting 25,000,000 elements. a) Cross section along the sliding direction, b) detail of (a), c) cross section across the sliding direction and d) detail of (c). (SEM; combination of images in top view (untilted, 3 kV) and cross section (a, b 3 kV and c, d 5 kV).
Fig. 10. Overview of the worn PVD coated cemented carbide tools. Coating type and number of cut elements as indicated in the figure. (SEM; 3 kV).
cutting edge, Fig. 3. This low depth is the effect of the slow gradual wear of the coating, which has still not become worn through. (The depth profile most probably also includes a slight effect from the edge polishing before coating.)
The scratches in the CrC coating indicate that a gradual wear takes place. The coating is however much harder than the Cu alloy, and should not be mechanically scratched by the passing work material. Again, there is a possibility that WC grains from the exposed substrate, 104
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Fig. 11. Detail images of used PVD coated cemented carbide tools. a) Intact, but worn, CrC coating after cutting 25,000,000 elements, b) exposed and worn substrate, previously coated by CrC, after cutting 25,000,000 elements, c) mildly worn CrN coating after cutting 50,000,000 elements and d) exposed and subsequently worn cemented carbide substrate, previously coated by CrN, after cutting 50,000,000 elements. (SEM; 3 kV).
sliding. No particles harder than the tool surfaces have been found and these hills and valleys are not showing the typical characteristics of abrasive scratches. This suggests an influence of chemical wear, as mentioned previously. This assumption is strengthened by the observed improved wear resistance of the CrN coating, although being significantly softer than the CrC coating. Thus, it seems that both cemented carbide and CrC are chemically affected by the Cu alloy in a similar manner, while CrN is not. However, this needs to be investigated further.
upstream of the intact coating, could get caught in the passing Cu alloy and subsequently cause scratches in the coating. This is however contradicted by the fact that there have been no observations of whole grains being removed in the exposed areas. The scratches are again very fine and polished-like, indicating that some form of chemical wear could dominate also here. The CrC coating is even and micrometer thick all the way to the transition line where the substrate is exposed. This indicates that the coating has fractured and flaked off, rather than being gradually removed by the scratching wear. The flaking might be caused by too poor surface treatment of the substrate before coating. This is also suggested by the high presence of cracked and fractured WC grains and fragments in the top layer of the substrate, as visible in the cross sections in Fig. 13. The CrN coated tool showed barely any wear in the surface profile measurement, Figs. 2 and 3. However, the SEM reveals occasional exposure of substrate, leading to dramatically increased wear rate. The intact, coated surface in Fig. 11 shows very limited scratching and no gradual wear. The coated surface seems almost unaffected by the contact with the Cu alloy, with the exception of some scattered transferred material. Bearing in mind that this tool has been used twice as long as any of the other tools and still shows such very limited wear, it must be considered a very promising candidate, although it is sensitive to flaking. The cemented carbide substrate was treated in the same way irrespective of coating material, meaning that poor surface treatment of the substrate could have caused the occasional flaking also here. The uncoated cemented carbide and the CrC coated tools show partially similar wear appearance. Both materials are considered the hardest component in their respective system, nevertheless a wear pattern of hills and valleys is gradually generated in the direction of
5. Conclusions Tools used for shearing the Cu alloy wire used for zipper production, have been studied at different stages of their lifetime to investigate the wear mechanisms. The increased understanding of the wear mechanisms is believed to ultimately help prolonging the tool life. Additionally, the potential of using two types of PVD coatings to prolong tool life has been evaluated.
• With increasing number of cut elements the originally rough ground • • • • 105
tool surface with cracked and fractured WC grains and smeared out Co is gradually transformed into smooth and”polished-like” surface. The wear of WC grains is gradual and shallow (only minute parts of grains removed), no indication of loss of whole grains was found. No preferential removal of Co was observed, despite the vast hardness difference between Co and WC. The Cu alloy work material becomes transferred to the cemented carbide tool, initially in the form of micrometer-sized patches and subsequently as thin layers, located mainly on top of Co. The alloying element Zn is preferentially transferred to the coated
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Fig. 12. Surface appearance and elemental analysis of used PVD coated tools, 120 µm from the edge; a) CrC coating (left part) and exposed substrate (right part) after cutting 25,000,000 elements and b) CrN coating after cutting 50,000,000 elements. (SEM and EDS; 3 kV).
•
• CrN is very effective in reducing the tool wear and the wear of the
tools but not to the uncoated. The CrC coating substantially reduces tool wear. However, after extensive use it suffers from flaking which begins early on at the cutting edge. Where still present, it is gradually worn, and displays small-scale ridges and valleys.
• 106
coating itself is very limited. Occasional local coating removal occurs, leading to subsequent rapid wear of the exposed cemented carbide. Coatings have successfully been used to increase tool life time.
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Fig. 13. Worn PVD CrC coated cemented carbide tools after cutting 25,000,000 elements. a) Combination of top view (untilted) and cross section along the sliding direction, b) detail of smooth coating on top of cracked grains in the cemented carbide substrate (framed area in (a)). c) Top view and d) cross section across the sliding direction, along the line indicated in (c). (SEM; a-c: 3 kV, d: 5 kV.).
•
However, if worn through or flaked off, the substrate becomes unprotected and the wear rate increases in such areas. Irrespective of initially being coated or not, the worn surface of the cemented carbide shows a similar character, with smooth, even wear of WC and Co and transfer of Cu alloy.
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Wear mechanisms of WC-Co cemented carbide tools and PVD coated tools used for shearing Cu-alloy wire in zipper production
T
J. Heinrichsa, , H. Mikadob, A. Kawakamib, U. Wiklunda, S. Kawamurab, S. Jacobsona ⁎
a b
Applied Materials Science, Uppsala University, Sweden Machinery and Engineering Group, YKK Corporation, Japan
ARTICLE INFO
ABSTRACT
Keywords: Cemented carbide Cutting Shearing Wear Cu-alloy
To form the individual elements, that together form a zipper, a pre-formed Cu-alloy wire is sheared using cemented carbide tools. The wear caused by the relatively soft copper alloy on the much harder tool is generally quite slow. However, millions of elements are to be sheared so eventually the wear becomes unacceptable and the tool needs to be exchanged. To improve product quality, as well as minimize down time and material consumption, the tool life needs to be prolonged. To achieve this the wear process needs to be better understood. Uncoated tools used for an increasing number of shearing events have been studied in detail using high resolution SEM and EDS, to map the propagating wear and get an insight into the wear mechanisms. Transfer of material from the Cu-alloy to the tool occurs and the wear is highly concentrated to specific areas. This wear occurs on a very fine scale, limited to within individual WC grains at each event. Tools coated with PVD CrC and PVD CrN have been studied for comparison with the uncoated cemented carbide. Both coatings successfully protect the cemented carbide tool from wear, however occasional flaking occurs and then the cemented carbide becomes exposed and subsequently worn. The differences in performance and wear mechanisms between the uncoated and coated tools are discussed, with focus on the capability of the coatings to prolong the tool life.
1. Introduction The production of a functioning zipper includes several complicated forming and cutting process steps. One of these steps involves cutting of the individual elements forming the zipper from a pre-formed wire. This is known as a shearing process, comprising a shearing die, through which the pre-formed wire is fed, and a shearing punch that makes the actual cut. High quality metal zippers are often made from Cu-alloys. Cemented carbides are commonly used as tool material for both the die and the punch. The combination of hard ceramic tungsten carbide particles in a softer metallic (Co) matrix is often a good combination to achieve low wear rates in production. Still, the wear of the punch is a limiting factor, since it deteriorates the quality of the zipper elements and necessitates production stand still and exchange of tools. Thus, there is a desire to prolong the tool life of the punch to minimize down time, materials consumption and to improve the element quality. Studies on the wear of tools used in Cu-alloy forming processes are not frequently published. In particular, there has been very limited research on the tribological performance of uncoated and coated
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cemented carbide tools used for shearing Cu-alloy wires. Some work has been published on related topics; wear of cemented carbide tools in Cu-alloy wire drawing [1] and in reciprocating sliding contact with Cualloy [2], where mainly adhesive wear was observed. Another application that has recently received attention is cutting of Cu-alloys. Due to new legislation, the amount of Pb in Cu-alloys is being reduced. The presence of Pb is important for the machinability, where part of the effect is due to that Pb accumulating on the Cu-alloy surface decreases friction and wear [3,4]. Thus studies on how to improve the machinability of Pb-free Cu alloys, to avoid high wear of the commonly used cemented carbide tools, have recently been reported [4–8]. The worn cemented carbide tool inserts mainly show crater wear [5,6], where Co is removed by diffusing into the passing Cu-alloy chip and WC grains are subsequently being removed [6]. Minor cross-diffusion of Cu and Zn to the Co binder also occurs [6]. The potential of applying thin hard coatings on cemented carbide tools for copper machining is investigated in several papers, and especially DLC, (Ti,V,Zr,Hf,Nb,Ta)N and CrN have shown good results [1,4–8]. Another area of recent interest is to replace the Co, the most commonly used binder in cemented carbides, with the purpose of reducing wear. Recently, the use of FeAl
Corresponding author. E-mail address: [email protected] (J. Heinrichs).
https://doi.org/10.1016/j.wear.2018.12.075 Received 24 October 2018; Received in revised form 21 December 2018; Accepted 21 December 2018 Available online 24 December 2018 0043-1648/ © 2019 Elsevier B.V. All rights reserved.
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underneath and then sheared to a set length by a shearing punch and die. Initially the contact between the wire and the punch is characterized by the shearing action. Eventually the wire fractures and sporadic sliding contacts between the fracture surface of the wire and the punch occur until completion of the stroke. The shearing process is further explained in [10]. Four uncoated cemented carbide tools were tested, each used to cut a predetermined number of elements, ranging from 1000–25,000,000. The latter number is about the expected life time of a shearing punch. One uncoated tool, prepared in the same way, was kept as an unused reference. Two types of PVD coatings, CrC and CrN, were evaluated in a similar way as the uncoated tools. The full testing scheme is presented in Table 2.
Table 1 Nanoindentation hardness, based on at least ten 100 nm deep indents. Material
H [GPa]
Number of indents
Co WC CrN CrC Cu-alloy
8.3 ± 0.56 31 ± 3.1 19 ± 1.1 25 ± 1.0 2.7 ± 0.23
10 22 34 39 128
binder in cutting tools for dry machining of oxygen-free copper was investigated, with good results [9]. These were attributed to the lower tendency of FeAl to oxidize, compared to Co, when in contact with Cu. Despite the substantial work performed, the understanding of the wear of cemented carbides in contact with Cu-alloys is still not complete. To prolong the life of the shearing punch used in zipper production, it is important to know how and where the wear starts and propagates. The present paper adds to the knowledge about this process by studying shearing punches at different stages of their life time, i.e. after cutting different amounts of elements, under the same conditions as in actual production. Based on the findings, the possibility to optimize the cemented carbide grade for shearing the copper alloy work material is discussed. The potential of prolonging the tool life even further by the use of PVD coatings is also evaluated.
2.3. Post test surface characterization After testing, all tools, including the reference, were cleaned first with acetone and then in ethanol in an ultrasonic bath for 3 min, respectively. Then all tools were analyzed using white light interference profilometry (WYKO NT1100), high resolution FEG-SEM (Zeiss Merlin) and Energy Dispersive X-ray Spectroscopy (EDS; Oxford X-max). Selected tools were analyzed also in cross section, prepared by Focused Ion Beam (FIB; FEI Strata DB235). 3. Results
2. Experimental
3.1. Wear measurement
2.1. Materials
The surface profile of each tool was measured along the symmetry line stretching over the cutting edge, as illustrated by the sketch in Fig. 2, which depicts the region adjacent to the cutting zone on the tool. The surface profiles are rendered in Fig. 2 and corresponding wear data is plotted in Fig. 3. The unworn reference tool displays a relatively smooth surface with a quite sharp cutting edge. During early use, shown by the tool having cut 1000 elements, only a slight smoothening of the cemented carbide occurs. After cutting 1,000,000 elements, almost 7 µm has been worn off at the cutting edge, which corresponds to slightly less than two layers of WC grains. The edge has also become slightly rounded. This wear gradually increases so that after cutting 25,000,000 elements some 45 µm have been removed at the very edge. The worn area then extends about 300 µm in from the cutting edge. All PVD coated tools showed much lower wear rates than the uncoated. The CrC coated tools used for cutting 4,250,000 and 8,500,000 elements respectively, both show low wear with similar profiles, with the exception that the tool used for cutting 8,500,000 elements is more worn at the very edge, Fig. 2. The deepest wear is found at the edge and the last indication of very slight wear is observed about 180 µm from the cutting edge. Still, 10 µm from the edge the wear depth is only about 1.5 µm for both tools. On the CrC coated tool used to cut 25,000,000 elements, the indications of wear again stretches to about 180 µm from
The studied tools are made of cemented carbide comprising 85 wt% WC particles, with an average grain size of 4 µm, in 15 wt% Co binder. The tools are finished by grinding, to an Ra value of 0.5 µm. Tools were used either uncoated or PVD coated with CrC or CrN. Prior to coating deposition each cutting edge went through a blast polishing process, using diamond particles, to improve the coating adhesion. The coatings have been used as deposited, without any post treatment. The coating thickness was measured using a calo tester to 2.2 µm (CrC) and 3.0 µm (CrN). The zipper elements were cut from a preformed Cu alloy wire with 15 wt% Zn. The hardness of the materials was measured using nanoindentation, with a Berkovich diamond tip and an indentation depth of 100 nm, see Table 1. To allow measurement of the individual constituents of the cemented carbide, rather than more random degrees of composite hardness, the position of each indent was verified using scanning electron microscopy (SEM). 2.2. Wear testing in production machine The wear tests were conducted using an actual production machine, Fig. 1. The preformed Cu-alloy wire is fed intermittently from
Fig. 1. Schematic drawings of a) the shearing process in the machine, b) the process in cross section and c) the cut element. 97
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removed in an area stretching to about 300 µm from the cutting edge, indicating contact with the counter material within this area. After producing 1,000,000 elements, the cutting edge has become slightly rounded and a new type of scratch pattern has appeared in the area adjacent to the cutting edge. After cutting 10,000,000 elements, the cutting edge is clearly worn, and a transition to a less worn area is visible about 300 µm from the cutting edge. After 25,000,000 elements, the presumed lifetime of an uncoated tool, the wear has continued even further. Besides the heavily worn area, discernible already after 10,000,000 elements, some occasional large scale transfer of Cu alloy is observed right below the transition line. These two regions of the tool are clearly visible also using light optical microscopy, Fig. 4. The region of the tool showing more extensive wear corresponds to the region experiencing shearing contact with the Cu-alloy, Fig. 5. Below the transition line only sporadic sliding contact against the fractured Cu-alloy surface occurs, resulting in the lower wear. At higher magnification, more details of the wear process are revealed, Fig. 6. The reference tool exhibits the typical surface microstructure caused by grinding, characterized by abrasive scratches with crushed WC grains and WC fragments embedded into the surface. After cutting 1000 elements, the structure is still characterized by crushed WC grains and numerous fragments. However, it has become smoother and foreign material can be observed on the surface, as medium gray areas with sub-micron scratches. After cutting 1,000,000 elements, fewer crushed grains and fragments are present, although locally they are still common. The areas of foreign material are still present and the tool has now, after the initial smoothening, again become rougher. The surface now shows parallel valleys and ridges, running in the sliding direction. After cutting 10,000,000 elements, crushed grains are no longer present. The microstructure shows a clear, polished-like
Table 2 Included tools tested in actual production machine. Material
Number of cut elements
Uncoated cemented carbide
0 (reference) 1000 1,000,000 10,000,000 25,000,000 4,250,000 8,500,000 25,000,000 50,000,000
CrC CrN
the cutting edge. However, the wear now has increased drastically in a region stretching to 70 µm from the edge. Very close to the edge, the wear depth approaches that of the uncoated tool used to cut 10,000,000 elements. The CrN coating on the tool used to cut 50,000,000 elements is barely worn at all and still covers the cutting edge. The slight curvature in its profile is a result of the pretreatment, where the tool is blast polished to improve coating adhesion. 3.2. Appearance of worn uncoated cemented carbide tools The wear surface appearance of all tools was studied in the SEM, Fig. 4. The unused reference is characterized by grinding scratches. It also has a dark appearance caused by carbon residues that adhere during the grinding process and still remain after the ultrasonic cleaning. After cutting 1000 elements, the adhered carbon has been
Fig. 2. Surface profiles over the cutting edge and the adjacent surface for each included tool, measured along 400 µm of the centerline of the tool, as indicated in the sketch. a) The recorded wear depth for all tools. b) The area framed in (a) with a magnified depth scale to better visualize the limited wear of the slightly used and the coated tools.
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Fig. 3. Wear depth at the cutting edge (hollow markers) and 10 µm from the cutting edge (filled markers). Data originating from the surface profiles in Fig. 2, where the first measurement point in each profile is used to define “at the cutting edge”. * indicates that the coating is worn through in the area where the measurement is performed.
Fig. 4. Overview images of the uncoated cemented carbide tool flank after different numbers of elements cut, as indicated in the images a)-e) (the cutting edges are seen on the top part of the images, SEM; 3 kV). f) Light optical image of the tool used to cut 25,000,000 elements is included for comparison. The area imaged in SEM is indicated.
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Fig. 5. Surface appearance of the cut element and the corresponding area of the shearing tool. Top left; the cut element with its sheared (I) and fractured (II) regions, imaged using the same magnification as the tool, top right (LOM). Bottom row; detail of the surface appearances in the regions I and II of the cut element (SEM; 3 kV).
appearance with distinct WC and Co phases. Relatively large parallel valleys and ridges still run across the microstructure, but the surface is now substantially smoother along the sliding direction than perpendicular to it. After cutting 25,000,000 elements, further smoothening has occurred. Besides the fine ridges and valleys, it has a very finely polished appearance. In the region showing the transition from shearing contact to (occasional) sliding contact, the tool used to cut 25,000,000 elements exhibits all the above reported wear details, Fig. 7. In the region that has experienced occasional sliding, the area is smooth without ridges and valleys. However, some crushed and fractured WC grains and some larger transferred particles are present, in similarity with what was observed on the slightly used tools. When passing the boundary into the shearing contact region, the previously observed pattern with ridges and valleys immediately emerges. When continuing further from the boundary towards the cutting edge, less fractured grains are observed, while the ridge and valley pattern continues all the way to the very cutting edge, Fig. 7c and d. The cutting edge itself is, however, smooth and rounded. No loss of whole grains was observed at any position. Cu and Zn has become transferred to the cemented carbide tool already after cutting 1 000 elements, as shown by the elemental analysis in Fig. 8. Oxygen is present in the same areas as the Cu alloy, indicating that the transferred material is partly oxidized. This initial transfer seems to have a more or less stochastic distribution over the surface. However, with increasing number of cut elements, the transfer
becomes more distinct and is mainly located in between WC grains. Also the character of the cemented carbide changes, from being a mixture of WC and Co to distinct and clearly separated phases. The intensity of Co is also reduced, indicating that the amount of Co in the surface structure is reduced as the number of cuts increases. To further study the transferred material and the wear, FIB cross sectioning was performed in two areas where presence of transferred alloy had been confirmed by EDS, Fig. 9. Although clearly indicated by the top view EDS analysis, high magnification was required to find the transferred material in the cross sections, as in Fig. 9b and d. In these, the transferred material is visible as thin dark lines underneath the protective layer of platinum. Note that the transferred material is located on top of the Co binder but not on WC grains, as also indicated by the elemental analysis in Fig. 8. The worn surface is very smooth in the sliding direction, Fig. 8a and b, similar to a polished surface. The microstructure is not noticeably affected below the very surface and there is no apparent preferential removal of binder. 3.3. Appearance of worn PVD coated cemented carbide tools The wear protective capacity of two types of PVD coatings, CrC and CrN, was tested, according to Table 2. After testing, all tools were analyzed using the SEM, Fig. 10. After cutting 4,250,000 elements, the CrC coating has been removed from the very cutting edge and thereby exposed the cemented carbide 100
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Fig. 6. Details of the uncoated cemented carbide shearing tools, all showing a region along the central line about 100 µm from the cutting edge. From a)-e) they have been used to cut an increasing number of elements, from 0 to 25 million, as indicated in the images (SEM; 3 kV).
substrate. This worn area, with exposed cemented carbide, grows in the direction from the cutting edge with increasing number of cut elements. After 25,000,000 cuts, the coating has been worn off from a minimum of 20 µm and up to 320 µm from the cutting edge, Fig. 10c. For the CrN coated tool, the coating is still covering the major part of the cutting edge after 50,000,000 cuts. However, wear exposing the cemented carbide substrate has occurred locally, exemplified by the brighter area to the far right in Fig. 10d. Besides that, some brighter material can be seen adhered on top of the seemingly intact coating. In higher magnification, Fig. 11, a fine scratch pattern is visible on the CrC coated surface. These are not sharp abrasive scratches, but rather similar to the ridges and valleys observed on the worn uncoated cemented carbide. The typical topographical pattern from the PVD deposition process, which was observed in areas outside the contact, is almost worn away. This indicates that the coating has become substantially worn. Some occasional material transfer to the coating can also be noted. Regions in which the cemented carbide substrate is
exposed exhibit a wear pattern similar to that on the uncoated tools. This is valid also for the scattered areas on the CrN coated tool where the substrate is exposed, as exemplified in Fig. 11d. In areas where the CrN coating is still present, the wear is very mild. A shallow pattern, a characteristic result of the PVD process, is still visible but somewhat modified by occasional small scratches and mild polishing of high areas into plateaus, Fig. 11c. The shallow pattern also causes some material to adhere, appearing light gray in the SEM image. The transfer of work material is more extensive in areas of exposed cemented carbide than in coated areas, as evident from the elemental mapping in Fig. 12a. The transferred material is located in between WC grains, thereby partly covering the Co, and composed of both Cu and Zn, as also observed on the uncoated tools. Contrastingly, the transfer located on top of the intact CrN coating is rich in the alloying element Zn, and partly oxidized, Fig. 12b. Similar observations were made when analyzing transferred material located on top of intact CrC coating (results not shown here). 101
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Fig. 7. Details of the uncoated cemented carbide tool after cutting 25,000,000 elements. a) Transition region, about 300 µm from the cutting edge, b) detail showing the framed area in (a), c) cutting edge, d) detail showing the framed area in (c). (SEM; 3 kV).
To reveal more details of the wear of the CrC coating, FIB cross sections were prepared in areas with slightly worn coating as well as in the transition area between remaining coating and exposed substrate, Fig. 13. Wherever the coating still remains, it is smooth and even, although it shows distinct traces of wear in top view. The substrate underneath the coating is characterized by cracked and fractured WC grains and fragments. The cross section in the transition area, Fig. 13d, including both CrC coating and exposed cemented carbide, demonstrates the dramatic increase in wear that occurs as soon as the substrate becomes exposed. The exposed cemented carbide directly adjacent to the coated area shows a wear depth corresponding to several layers of WC grains, and the WC grains show no signs of cracks.
to several layers of WC grains. This means that all traces from the preceding grinding should have been removed. Thus, the observed pattern must be caused by the sliding process. However, since the Cu alloy is so much softer than both the Co and the WC, it should be unable to cause scratches. Further, there are no hard particles in the system, capable of scratching the cemented carbide. One possibility could be that hard wear particles, in the form of WC grains or grain fragments, follow the work material and cause scratches downstream of the worn area. However, no loss of individual WC grains or larger pieces of the composite has been observed. Further, if produced by hard particles, the ridges and valleys should be expected to change shape and size as particles pass first one phase and then the other. In other words, hard particles should generally cause preferential wear of the softer Co, which has not been observed. The cemented carbide, including both WC and Co, is indeed gradually worn in contact with the passing Cu alloy. Both phases wear at the same rate, resulting in a very smooth wear surface. This, together with the low wear rate, indicates that it is worn on the atomic, or at least sub-micron, scale. Most likely, this wear is caused by chemical interactions and, thus, mechanical hardness has very little influence. During the shearing process, Cu and Zn become transferred to the tools, preferentially sticking on top of Co regions. This preference could be due to that the transfer occurs by chemical interactions with the Co. It could also be due to mechanical interlocking, but since the Co is not preferentially removed the transferred material is very thin and any effect of interlocking must be limited.
4. Discussion 4.1. Wear of uncoated cemented carbide On the uncoated cemented carbide tool, the originally rough ground surface, characterized by cracked and fractured WC grains, gradually becomes smooth and polished-like during use. Cracked and fractured grains were not only observed before use, but also remained after cutting 1000 and 1,000,000 elements. However, the surface profiles in Fig. 2 shows that the total wear depth after 1,000,000 is only 1 µm in the imaged area. This corresponds to just parts of the thickness of a single layer of WC grains. Thus, the crushed grains found after wear were probably crushed already when the tool was ground. After cutting 10,000,000 elements, no crushed grains remain. The worn surface is rather characterized by intact WC grains evenly distributed in the binder phase. There are however ridges and valleys running in the sliding direction after cutting 10,000,000 elements. According to the surface profiles, Fig. 2, the wear depth in the imaged area is 12 µm, corresponding
4.2. Wear of PVD coated cemented carbide tools Two hundred µm from the cutting edge, the CrC coated tool used to cut 25,000,000 elements is almost unworn. However, going closer to the cutting edge, at about 70 µm from the cutting edge the wear 102
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Fig. 8. Elemental mapping of the uncoated cemented carbide tools, about 100 µm from the cutting edge, after cutting a) 1000 and b) 25,000,000 elements (SEM and EDS; 3 kV).
increases drastically. Referring to the surface profiles in Fig. 2, this corresponds to a position where the wear depth reaches 2 µm, which is about the original thickness of the coating. Thus, the rapid wear corresponds to the wear of the unprotected cemented carbide substrate. As evident from the SEM (Fig. 10), the distance of 70 µm from the cutting
edge is valid for the position of the selected profile, but actually varies substantially along the edge line. The two CrC tools used for fewer cuts show no rapid wear depth increase, except for at the very cutting edge, but these are still protected by the coating on the tool flank. For both tools the wear depth is only about 1.5 µm measured 10 µm from the 103
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Fig. 9. Wear surface and cross section appearance of the worn cemented carbide tool after cutting 25,000,000 elements. a) Cross section along the sliding direction, b) detail of (a), c) cross section across the sliding direction and d) detail of (c). (SEM; combination of images in top view (untilted, 3 kV) and cross section (a, b 3 kV and c, d 5 kV).
Fig. 10. Overview of the worn PVD coated cemented carbide tools. Coating type and number of cut elements as indicated in the figure. (SEM; 3 kV).
cutting edge, Fig. 3. This low depth is the effect of the slow gradual wear of the coating, which has still not become worn through. (The depth profile most probably also includes a slight effect from the edge polishing before coating.)
The scratches in the CrC coating indicate that a gradual wear takes place. The coating is however much harder than the Cu alloy, and should not be mechanically scratched by the passing work material. Again, there is a possibility that WC grains from the exposed substrate, 104
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Fig. 11. Detail images of used PVD coated cemented carbide tools. a) Intact, but worn, CrC coating after cutting 25,000,000 elements, b) exposed and worn substrate, previously coated by CrC, after cutting 25,000,000 elements, c) mildly worn CrN coating after cutting 50,000,000 elements and d) exposed and subsequently worn cemented carbide substrate, previously coated by CrN, after cutting 50,000,000 elements. (SEM; 3 kV).
sliding. No particles harder than the tool surfaces have been found and these hills and valleys are not showing the typical characteristics of abrasive scratches. This suggests an influence of chemical wear, as mentioned previously. This assumption is strengthened by the observed improved wear resistance of the CrN coating, although being significantly softer than the CrC coating. Thus, it seems that both cemented carbide and CrC are chemically affected by the Cu alloy in a similar manner, while CrN is not. However, this needs to be investigated further.
upstream of the intact coating, could get caught in the passing Cu alloy and subsequently cause scratches in the coating. This is however contradicted by the fact that there have been no observations of whole grains being removed in the exposed areas. The scratches are again very fine and polished-like, indicating that some form of chemical wear could dominate also here. The CrC coating is even and micrometer thick all the way to the transition line where the substrate is exposed. This indicates that the coating has fractured and flaked off, rather than being gradually removed by the scratching wear. The flaking might be caused by too poor surface treatment of the substrate before coating. This is also suggested by the high presence of cracked and fractured WC grains and fragments in the top layer of the substrate, as visible in the cross sections in Fig. 13. The CrN coated tool showed barely any wear in the surface profile measurement, Figs. 2 and 3. However, the SEM reveals occasional exposure of substrate, leading to dramatically increased wear rate. The intact, coated surface in Fig. 11 shows very limited scratching and no gradual wear. The coated surface seems almost unaffected by the contact with the Cu alloy, with the exception of some scattered transferred material. Bearing in mind that this tool has been used twice as long as any of the other tools and still shows such very limited wear, it must be considered a very promising candidate, although it is sensitive to flaking. The cemented carbide substrate was treated in the same way irrespective of coating material, meaning that poor surface treatment of the substrate could have caused the occasional flaking also here. The uncoated cemented carbide and the CrC coated tools show partially similar wear appearance. Both materials are considered the hardest component in their respective system, nevertheless a wear pattern of hills and valleys is gradually generated in the direction of
5. Conclusions Tools used for shearing the Cu alloy wire used for zipper production, have been studied at different stages of their lifetime to investigate the wear mechanisms. The increased understanding of the wear mechanisms is believed to ultimately help prolonging the tool life. Additionally, the potential of using two types of PVD coatings to prolong tool life has been evaluated.
• With increasing number of cut elements the originally rough ground • • • • 105
tool surface with cracked and fractured WC grains and smeared out Co is gradually transformed into smooth and”polished-like” surface. The wear of WC grains is gradual and shallow (only minute parts of grains removed), no indication of loss of whole grains was found. No preferential removal of Co was observed, despite the vast hardness difference between Co and WC. The Cu alloy work material becomes transferred to the cemented carbide tool, initially in the form of micrometer-sized patches and subsequently as thin layers, located mainly on top of Co. The alloying element Zn is preferentially transferred to the coated
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Fig. 12. Surface appearance and elemental analysis of used PVD coated tools, 120 µm from the edge; a) CrC coating (left part) and exposed substrate (right part) after cutting 25,000,000 elements and b) CrN coating after cutting 50,000,000 elements. (SEM and EDS; 3 kV).
•
• CrN is very effective in reducing the tool wear and the wear of the
tools but not to the uncoated. The CrC coating substantially reduces tool wear. However, after extensive use it suffers from flaking which begins early on at the cutting edge. Where still present, it is gradually worn, and displays small-scale ridges and valleys.
• 106
coating itself is very limited. Occasional local coating removal occurs, leading to subsequent rapid wear of the exposed cemented carbide. Coatings have successfully been used to increase tool life time.
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Fig. 13. Worn PVD CrC coated cemented carbide tools after cutting 25,000,000 elements. a) Combination of top view (untilted) and cross section along the sliding direction, b) detail of smooth coating on top of cracked grains in the cemented carbide substrate (framed area in (a)). c) Top view and d) cross section across the sliding direction, along the line indicated in (c). (SEM; a-c: 3 kV, d: 5 kV.).
•
However, if worn through or flaked off, the substrate becomes unprotected and the wear rate increases in such areas. Irrespective of initially being coated or not, the worn surface of the cemented carbide shows a similar character, with smooth, even wear of WC and Co and transfer of Cu alloy.
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