- Email: [email protected]
metal composite coatings used for dry machining and other industrial applications
Surface and Coatings Technology 123 (2000) 84–91 www.elsevier.nl/locate/surfcoat
Current Industrial Practices
Performance of MoS /metal composite coatings used for dry 2 machining and other industrial applications k N.M Renevier a, *, N. Lobiondo b, V.C Fox a, D.G Teer a, J. Hampshire a a Teer Coatings Ltd., 290–293 Hartlebury Trading Estate, Hartlebury, Worcestershire DY10 4JB, UK b Multi-Arc Inc, 1598 East Lincoln, Madison Heights, MI 48071, USA Received 13 April 1999; received in revised form 21 July 1999
Abstract As previously reported (Fox et al., Proc. PSE Conf., Garmisch Partenkirchen, 14–18 September, 1998), the properties of MoS coatings can be improved by the co-deposition of a small amount of titanium. These MoS /Ti coatings, known as 2 2 MoST@ produced by closed field unbalanced magnetron sputtering, are harder, much more wear resistant and less sensitive to atmospheric water vapour. These coatings have given excellent industrial results for a wide range of cutting and forming applications. Two forms of these MoS /titanium composite coatings have been developed: MoS /titanium composite ( low titanium, 2 2 10 at%) and MoS /titanium composite (high titanium, 20 at%). 2 The MoS /titanium composite ( low titanium) exhibits a coating hardness of 500 HV, a coefficient of friction of 0.02 during 2 100 N applied load reciprocating wear testing, and a low wear rate, while the MoS /titanium composite (high titanium) exhibits 2 a coating hardness similar to that of TiN, a coefficient of friction of 0.04 during 100 N applied load reciprocating wear testing, and an extremely low wear rate. The choice of coatings is dependent upon the application. Recent industrial performance data related to the characteristics of these MoS /titanium composite (high titanium) self-lubricant coatings, which are utilised now in large-scale production, are 2 presented. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Cutting; Forming; Low friction; MoS ; Solid lubricant; Unbalanced magnetron 2
1. Introduction There is currently an increase in demand for dry machining [2]. Over the last century, the production engineer has required higher production rates and has often had to machine new materials of higher strength. Metallurgists have responded with new cutting, forming tools and components with improved performance. The development of tool material from carbon tool steel, high-speed steel, carbides, ceramics and nitrides, together with techniques of applying wear-resistant coatings, has allowed cutting, forming tools or components k Paper presented at the 26th International Conference on Metallurgical Coatings and Thin Films, April 12–15, 1999, San Diego, CA, USA * Corresponding author. Tel.: +44-1299-251-399; fax: +44-1299-250-171. E-mail address: [email protected] (N.M Renevier)
to be used at increased cutting or forming speed with increased lifetimes. This has forced machine tool manufacturers to develop their machines so as to be capable of making full use of the new tool materials through increased metal removal, forming rates and improved productivity. New materials have been developed, particularly in the aerospace, automotive and medical industries, and these new alloys are often more difficult to machine. So, although these materials may be lighter, stronger and more wear resistant, they suffer from poor machinability, thus creating unfavourable conditions on the cutting or edge of the tool which leads to a reduction in tool life and in some cases premature tool failure. Although thin hard films were introduced into the cutting and forming tools market in the 1960s to arrest or slow down the diffusion wear in carbide tools, the real impact in the high-speed steel tool area has taken
0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 42 4 - 7
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
place over the last decade, particularly with developments in the physical deposition (PVD) process. Whilst the major successes in coatings for industrial applications are titanium nitride ( TiN ), titanium carbide ( TiC ), titanium carbonitride ( TiCN ), titanium aluminium nitride ( TiAlN ), these coatings are not totally successful for all applications for several reasons [3], where a total systems approach needs to be employed in surface engineering. For centuries, liquid lubricants were used to limit the contact pressure and facilitate sliding. Contact is inevitable, and lubricants are used to reduce the temperature produced by friction. But, in many cases the presence of liquid is not recommended or forbidden for contamination reasons. Effects are particularly important in vacuum or at high temperature, for food, medical apparatus, or the nuclear industry. Lubrication costs are often high and therefore uneconomic. An additional problem with liquid lubricant is the efficient distribution of the lubricant on the required surface. Dry machining can appear a risky solution to this problem, as users are reluctant to damage their tools under dry conditions. However, the risks of damage to the tools by using solid lubricants in place of liquid lubricants are minimal. This is because, in effect the solid lubricant coating creates a third component within the system, which gives some advantages of a liquid lubricant. In some cases the use of a solid lubricant deposited where a traditional lubricant cannot perform allows an increase in the performance of tools and components. In future, solid lubrication may be imposed by legislation (for example for environmental protection) and also for cost savings (as the treatment of used lubricants is very expensive). MoS coatings have been proposed for the improve2 ment in the performance of cutting tools, and such coatings have been shown to give significant improvements [4]. At Teer Coatings core strategies of innovation and continuous improvement are being used to address these preoccupations. Hence, two new complementary types of coating were developed, known as MoST@ [1,5] and Graphit-iC@ [6,7]. As previously reported [1], MoST@ coatings are MoS /titanium composite coatings formed 2 by co-deposition of small amounts of titanium in a MoS based matrix. MoS /titanium composite coatings, 2 2 produced by closed field unbalanced magnetron sputtering, are harder, much more wear resistant and less sensitive to atmospheric water vapour than MoS . These 2 coatings have given excellent industrial results in a wide range of cutting and forming applications. The properties are dependent on the titanium content. Two successful forms of the range of MoST@ coatings have been developed: MoS /titanium composite ( low titanium) 2 and MoS /titanium composite (high titanium). 2 The MoS /titanium composite ( low titanium) exhibits 2
85
a coating hardness of 500 HV, a coefficient of friction of 0.02 during 100 N applied load reciprocating wear testing, and a low wear rate, while the MoS /titanium 2 composite (high titanium) exhibits a coating hardness similar to that of TiN, a coefficient of friction of 0.04 during 100 N applied load reciprocating wear testing, and an extremely low wear rate. Recent industrial performance data related to the characteristics of these MoS /titanium composite (high 2 titanium) self-lubricant coatings are reported in the range of cutting, forming tools and components, which are utilised today in large-scale production.
2. Background MoS coatings are often deposited using RF sputter2 ing [8–11], producing MoS coatings with a duplex 2 structure consisting of a dense, coherent film of about 100 nm followed by a loose, columnar, powdery film which is easily removed [12]. Most of these coatings were suitable only in vacuum and at 0% humidity. DC magnetron sputtering is used for the deposition of MoS coatings in this study. The quality of the 2 coatings is improved by applying a negative potential to the substrates so that the growing film is bombarded by energetic ions, densifying the structure and improving adhesion. Unbalancing the magnetic field of the magnetron [13] increases the intensity of the bombardment and further improvements are achieved by using a multimagnetron system in which unbalanced magnetrons are used in the closed field arrangement [13,14]. Unlike RF techniques, such methods give dense, coherent coatings. MoS properties degrade when in humid air, causing 2 an increase in friction coefficient and a decrease in lifetime. To reduce the water vapour content in the vacuum chamber a titanium target was sputtered during ion cleaning of the substrates prior to deposition to produce a gettering effect. The titanium was then used to deposit an interlayer which led to an improvement in coating adhesion (critical load above 120 N ). A natural progression of this work led to incorporation of titanium into the coating itself, resulting in improved friction (m of 0.02–0.1 at 40% humidity) and wear properties. These MoS /metal composite coatings were hard (1000– 2 2000 HV ) and also less sensitive to water vapour than pure MoS coatings. 2 Properties of the original MoS /metal composite 2 coating [MoS /titanium composite ( low titanium, 2 10 at% and high titanium, 20 at%)] have been previously reported [15]. These coatings are not only suitable in vacuum and at 0% humidity, but also up to 50% humidity, allowing terrestrial uses. These MoS compos2 ite coatings have been tested in a variety of industrial applications, showing excellent results for a wide range of cutting, forming and component applications. This paper reports industrial results from MoS /titanium 2 composite (high titanium, 20 at%) coating.
86
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
A resume of laboratory test results using microhardness and nanohardness testing, scratch adhesion testing, pin on disc and reciprocating friction and wear tests is presented in an attempt to be au fait with the tribological properties. The structures of the coatings have been extensively studied by a variety of techniques, including optical microscopy, transmission electron microscopy ( TEM ), X-ray diffraction ( XRD) and scanning electron microscopy (SEM ) and a summary of this work is reported to understand the structure of such complex coatings. 2.1. Deposition system MoS /titanium composite coatings were deposited by 2 DC magnetron sputtering using standard Teer CFUBMSIP equipment [16 ] (Fig. 1). The magnetrons within the coating chamber were arranged so that three MoS targets and one titanium target were used and the 2 substrates rotated between the targets. The amount of metal content was controlled by the power applied to the targets. To reduce the water vapour content in the vacuum chamber a titanium target was sputtered, under argon during ion cleaning of the substrates prior to deposition to produce a gettering effect. The titanium target was then used to deposit a 100 nm interlayer, which led to an improvement in coating adhesion. Titanium was used as an interlayer to improve the load bearing capacity and the adherence properties. This step is followed by a second interlayer by sputtering from two MoS targets and the titanium 2 target simultaneously, the power to the titanium target is gradually reduced. This layer consists of a 200 nm mixed MoS /titanium in a multilayer structure. 2 The main bulk of the coatings of about 800 nm, known as MoST@ coatings in the case of titanium
(a)
addition, are not multilayer coatings (as previously described [1]). The titanium target is switched off at the end of the process to produce a 50 nm pure MoS coating for 2 coloration. 2.2. Laboratory testing and results 2.2.1. Scratch testing Scratch testing on the MoS /titanium composite coat2 ings has produced results showing that the coatings undergo no failure up to a load of 120 N [15] using a 0.2 mm Rockwell diamond tip. Optical examination of the coatings after scratch testing has confirmed this result. 2.2.2. Pin on disc testing As was observed in the previous study on the MoS /titanium composite (high titanium) coating [15], 2 as the load increases the friction coefficient decreases and the wear rate increases. The specific wear rate for the MoS /titanium composite (high titanium) coating 2 has been calculated at around 4×10−17 m3(Nm)−1 after 1 h test performed at 80 N in dry conditions (40% relative humidity) with a linear speed of 200 mm/s (477 rpm, 8 mm diameter) and a 5 mm diameter WC-4% Co ball showing a 0.06 coefficient of friction. Preliminary results from pin on disc testing (5 N and 10 N ) at 350°C have indicated that no loss in properties is evident up to this temperature compared to those obtained at room temperature. 2.2.3. Reciprocating wear testing At 41% humidity and under a 100 N load using the same ball characteristics, the coating survives for 9999 cycles, the limit of the test. The friction coefficient is 0.043 and the wear after 9999 cycles is 0.5 mm. In order to demonstrate the differences between MoS and 2
(b)
Fig. 1. (a) Schematic representation and (b) general view of a four magnetron coating chamber configured with closed field unbalanced magnetron sputter ion plating (CFUBMSIP).
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
87
These results were confirmed by extensive nanoindentation tests on the MoS /titanium composite (high 2 titanium) coating, confirming the Fischerscope measurements. 2.3. Coating analysis
(a)
(b) Fig. 2. Results of (a) MoS and (b) MoST coatings tested at 40% 2 humidity, under water and under oil. Number of cycles, coefficient of friction, track width and wear rate are reported.
MoS /titanium composite coatings, the MoS /titanium 2 2 composite (high titanium) coating was tested under water at a load of 100 N where the coating survived for 3000 cycles with a friction coefficient ranging from 0.03 at the start of the test to 0.05 just before failure of the coating (32 cycles for pure MoS coatings). The 2 MoS /titanium composite was also tested under oil 2 where the coating survived for 10 000 cycles with a friction coefficient characteristic of the oil rather than the coatings (155 cycles for pure MoS ). Tested under 2 oil at low load ( less than 10 N, with the same ball characteristics), the coating presents no failure and the wear is lower than in dry conditions. Results are reported in Fig. 2. 2.2.4. Nanoindentation and microindentation Dynamic hardness measurements were performed with a Fischerscope Dynamic Hardness Tester at 4 mN load. The test gave a spread of results depending on the titanium concentration in different coatings, but all of the tests gave a value for coating hardness of above 15 GPa with values ranging up to 21 GPa (4 GPa for pure MoS ). Young’s modulus of 138 GPa has been 2 recorded, much higher than typical values for pure MoS coatings (around 70 GPa). 2
2.3.1. Chemical composition Auger electron spectroscopy (AES ) analysis was performed to determine the composition [17]. Sputtered neutral mass spectroscopy (SNMS) analysis and Rutherford backscattering (RBS) have failed to detect any multilayers, more details will be presented in Ref. [17]. 2.3.2. XRD XRD measurements were performed at Birmingham University using Phillips equipment with a Cu Ka line source (0.154046 nm and 0.154439 nm) and a goniometer PW3020. Measurements were reported only between 10 and 60° because of the very weak intensities of the peak after 60°. Analysis of MoS /titanium composite 2 coatings by XRD [17] (Fig. 3) revealed only a very broad band pattern indicating a structure consisting of quasi-amorphous, highly strained MoS (Fig. 3). For 2 comparison, XRD analysis was performed on pure titanium and pure MoS coatings. Several peaks were 2 evident for the MoS coating, indicating that the coating 2 is at least partly crystalline. None of these peaks occur on analysis of the MoS /titanium composite coating, 2 and so it would appear that the addition of Ti to the coating is inhibiting the formation of crystalline MoS . 2 The pure MoS coating is crystalline and the strongest 2 peak was found in the (002) plane compared with the (103) plane. On addition of metal, in this example titanium, the peak corresponding to the (002) plane disappears progressively and at the same time the band
Fig. 3. X-Ray diffraction pattern showing (a) pure MoS , (b) 2 MoST@, (c) MoS +titanium multilayer, (d) pure titanium. 2
88
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
corresponding to the (103) plane appears stronger than previously. The broad diffraction lines are shifted to lower diffraction angles as the metal content is increased. The corresponding lattice parameter enhancement is consistent with an expansion of the lattice as the metal content increases. Only at high metal concentrations was it possible to detect the titanium peak of the (002) plane. 2.3.3. TEM Extensive TEM analysis of the MoS /titanium com2 posite coating has been carried out at Birmingham University and Northumbria University. TEM crosssections showing MoS /titanium composite coating are 2 presented in Ref. [17]. 2.3.4. SEM A fractograph of a SEM cross-section of MoS / 2 titanium composite coating on top of a TiN coating is shown in Fig. 4. The coating was deposited onto tool steel substrate. From the fractograph it can be seen that the coating is dense, compact, non-columnar and adherent.
3. Industrial testing results and discussion A large number of industrial results have been obtained for the MoS /titanium composite coating 2 showing very big improvements for many applications in comparison with conventional hard coatings. Better results were obtained when MoS /titanium composite 2 coating is supported by a hard coating; the exact reasons are unknown, but some investigations are under way. The following results are the most recent. 3.1. Cutting tools applications 3.1.1. Drilling in dry and lubricated conditions Drilling tests were carried out. High-speed steel M6 tap drills have been coated with TiN, TiCN ( low carbon content), TiAlN (high aluminium content), TiN+MoS / 2
Fig. 4. Fractograph of TiN+MoST coating on tool steel substrate.
Fig. 5. Number of cutting holes produced (a) at 10 m/min in dry conditions, (b) at 20 m/min in dry conditions, (c) at 20 m/min lubricated with an oil mixture. TiCN (1): low carbon content, TiAlN: high aluminium content, TiCN+MoST: TiCN with low carbon content.
titanium composite and TiCN ( low carbon content)+ MoS /titanium composite. These tap drills were tested 2 under dry conditions and lubricated (mixed oil and watersoluble by 20%) to drill 5.5 mm through holes into AISI 400 stainless steel (11.7 mm thick, HRB 60–70). These tests were performed at 10 m/min (corresponding to 530 rpm) and at 20 m/min (corresponding to 1061 rpm). The number of holes produced was as recorded. Use of MoS /titanium composite ( Fig. 5) in dry 2 conditions on the top of hard coatings such as TiN and TiCN has increased the number of holes produced by twice in the case of TiN and by 4.1–4.8 times for TiCN. Unfortunately TiAlN coating, usually good for dry machining, was not successful for this application compared with the other coatings. The use of MoS /titanium composite under mixed 2 oil on top of a TiCN hard coating was improved by 2.4 times the lifetime of the drill. The poor result obtained for the TiN+MoST running under mixed oil (water-soluble) is puzzling and will be reinvestigated. 3.1.2. End milling applications Carbide end mills 12 mm×4 mm×25 mm have been coated with TiCN and TiCN+MoS /titanium compos2 ite and were tested by The Institute of Advanced Manufacturing Sciences Inc. for Multi-Arc Inc. These end mills were tested under dry conditions and lubricated ( Trim Sol: mixed oil and water-soluble by 5%) at a cutting speed of 150 m/min and a feed rate of 0.04 mm/turn onto AISI 304 stainless steel. The cutting depth was 4 mm, while the radial depth cut was 3 mm and the axial depth cut was 5 mm. Average milling force, tool wear and surface finishing were recorded. It has also been observed that MoS /titanium com2 posite (Fig. 6) coatings running in dry conditions deposited on top of a TiCN hard coating offer an increase in
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
89
The uncoated ejector pins produced 2000 shots before failure of the pins occurred, using the MoS /titanium 2 composite coating on the pins increased the number of shots produced to 100 000 shots. This is a 50 times improvement and a big cost saving is reported. 3.2.2. Perforation and piercing applications
(a)
(b)
Fig. 6. Average resultant force (a) and surface finishing (b) while end milling AISI 304 stainless steel using TiCN (using coolant) and TiCN+MoST (dry and using coolant) coated inserts.
milling distance and a reduction in average milling force during the test and also an improvement in surface finish as compared with the TiCN coatings alone. Both effects increase the productivity and the quality of the final product. However, MoS /titanium composite (Fig. 6) coating 2 deposited on top of a TiCN hard coating and running under mixed oil has not increased the end mill distance or the average milling force compared with the TiCN coating alone. This seems to show the limit of the coating under mixed lubricated (oil water-soluble) conditions. The water content in the lubricant is probably too high; it should be remembered that MoS /titanium 2 composite is a MoS based coating, which is dramati2 cally affected by water during use.
3.2.2.1. Test 1 Rapid wear occurred when AISI D2 punches (60–62 HRC ) were used for the perforating application. Punches were coated with the MoS /tita2 nium composite coating and compared with the previously sprayed MoS coating and uncoated punches 2 used by the company. The application for which the punches were tested was to perforate 1.2 mm thick 409 stainless plate for filters without lubricant, running at 250 pressings per minute. Previously the company had used a sprayed MoS 2 coating which enabled the punches to last for 1 day (around 80 000 pressings) without lubricant. With this sprayed coating the punch needed two to four regrinds to survive for 1 day, after which the punch was unable to function. With MoS /titanium composite coating, the 2 punches were still performing after 320 000 pressings (4 days) without the need for regrinding. The company reported a big cost saving by using the MoS /titanium 2 composite coated punches. 3.2.2.2. Test 2 Punches made from AISI M2 steel were coated with the MoS /titanium composite and 2 TiCN+MoS /titanium composite coatings. TiCN coat2 ing was deposited by arc evaporation (3–3.5 mm) and MoS /titanium composite by magnetron sputtering 2 (1.2 mm). The application for which the punches were tested was to pierce 1.2 mm thick AISI 409 stainless plate for filters without lubricant, running at 250 pressings per minute. Results are shown in Fig. 7. In the piercing application, an uncoated tool was able to pierce 5000 parts before tool failure. The use of a TiCN coating increased
3.2. Forming applications 3.2.1. Coating ejector pins The surface of ejector pins used for plastic moulds was coated with the MoS /titanium composite coating. 2 The exact conditions used during the process are unknown, but the results obtained represent comparative tests to the uncoated one. The number of shots performed with uncoated and coated pins was recorded.
Fig. 7. Results of MoST@ piercing application on AISI D2 steel.
90
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
the number of parts to 6000, and depositing MoS /titanium composite on top of TiCN further 2 increased the number of parts to 27 000. 3.2.2.3. Test 3 PS4 (CPM@ M4 of Dayton Progress Corporation, USA) (61–63 HRC ) punches were coated with MoS /titanium composite (1.2 mm) on top of TiCN 2 (3–3.5 mm of TiCN produced by arc evaporation) coating and used to perforate 12 mm HSLA steel with watersoluble lubricant (20%). Results are shown in Fig. 8. In the piercing application, an uncoated tool was able to pierce 15 000 parts before tool failure. The use of a TiCN coating increased the number of parts to 50 000, and depositing MoS /titanium composite on top of TiCN further 2 increased the number of parts to 200 000.
Fig. 9. Results of MoST@ punching application on AISI M2 steel.
3.2.3. Fine blanking stainless steel Results have been reported of punches coated with MoS /titanium composite on top of a TiCN coating. 2 The punches were made from AISI M2 steel (64 HRC ) and were coated with 3 to 3.5 mm of TiCN by arc evaporation followed by 1.2 mm of MoS /titanium 2 composite. 3.2.3.1. Test 1 The punches were tested on fine blanking 409 stainless steel without lubricant. In this application the number of parts produced with TiCN coatings increased from 3000 to 12 000 parts with the addition of MoS /titanium composite on top 2 of TiCN coatings (Fig. 9). 3.2.3.2. Test 2 The punches were tested on fine blanking 403 stainless steel with a water-soluble lubricant (20% water). In this application the number of parts produced with TiCN coatings increased from 220 000 to 600 000 parts with the addition of MoS /titanium composite on 2 the top ( Fig. 10).
Fig. 10. Number of parts produced with MoST@ in lubricated conditions (water-soluble 20%).
3.2.4. Drawing stainless steel applications Results have been reported of tools coated with MoS /titanium composite on top of a CrN coating. The 2 tools were made from AISI M2 steel (64 HRC ) and were coated with 3 to 3.5 mm of TiCN by arc evaporation followed by 1.2 mm of MoS /titanium composite. The 2 application for which the tools were tested was to draw 0.5 mm thick AISI 430 stainless steel under chlorinated oil used as lubricant. Results are shown in Fig. 11. In the piercing application, an uncoated tool was able to pierce 15 000 parts before tool failure. The use of a CrN coating increased the number of parts to 50 000, and depositing MoS /titanium composite on top of CrN further 2 increased the number of parts to 100 000.
4. Conclusions and recommendations
Fig. 8. Number of parts produced with MoST@ in lubricated conditions (water-soluble 20%).
Results show that product performance and surface finish of cutting and productivity of forming can be improved by using a solid lubricant such as MoS /titanium composite coatings. 2
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
91
hard thin film characteristics, as is evident from numerous publications reported elsewhere. A hard coating under MoS /titanium composite seems to be the best 2 choice. The exact reasons are unknown, but the improvement is probably associated with the additional support offered by the hard coating. MoS /titanium composite is a good candidate for a 2 clean world process, and gives greater productivity in machining, forming and for components. MoS / 2 titanium composite coating has great potential for success in dry machining, but the reduced wear resistance under oil is a limitation, which needs further study. Fig. 11. Number of parts produced with MoST@ in lubricated conditions.
It is important to establish the failure mechanisms and failure mode in a metal-cutting or metal/plasticforming or component situation in order to apply the MoS /titanium composite coatings effectively on top of 2 a hard coating. Design and substrate materials play an important role, which will be studied later (combination of tool design, manufacturing quality and appropriate coating such as MoS /titanium composite deposited on 2 a hard thin film in an attempt to improve tool performance life). Hard coatings such as TiN, TiCN, TiAlN may increase tool performance (for example cutting speed) and lifetime by arresting or slowing down certain types of wear. However, these coatings retain a high coefficient of friction and require lubricant. When cutting speeds are increased, the effects of liquid lubricants are reduced. Furthermore, reductions in friction result in decreased cutting force and tool temperatures, which reduces resistance to adhesion or local welding which may improve the quality of the product by replacing traditional lubricant by a solid lubricant such as MoS /titanium compos2 ite. Under most rubbing conditions MoS /titanium 2 composite coatings have a much lower wear rate than traditional hard coatings. They also have very low friction which allows the component to be used at high speed, lowering the frictional forces and the temperature. In cold forming applications use of the MoS /titanium composite coating has allowed us to 2 reduce the load applied during the process, which has also resulted in a decrease in energy consumption. A big cost saving has been reported using MoS /titanium composite coatings, in particular in 2 mould industries. MoS /titanium composite coatings, in contrast to 2 most other coatings, have allowed either the elimination or reduction of the lubricant quantity used; this big cost saving is because many tools used for forming require the old oil to be cleaned off before they can be used again. Work in dry conditions reduces this requirement. Considerable attention has previously been given to
Acknowledgements The authors would like to express their gratitude to the Department of Trade and Industry of the United Kingdom for their support through a SPUR award in the financing of this project. The authors would also like to thank the following people and companies. Dr. R. Gilmore of ISPRA (Italy), Dr. V Rigato of Thin Films (Italy), Nachi Fujikoshi Steel Corporation (Japan), Adachi New Industrial Company Ltd. (Japan), Third Millenium Ltd. ( UK ) and Dayton Progress Corporation ( USA).
References [1] V.C. Fox, N.M. Renevier, D.G. Teer, J. Hampshire, V. Rigato, Proc. PSE Conf., Garmisch Partenkirchen, 14–18 September, (1998). [2] T. Cselle, Manufact. Eng. (1995) 77–80. [3] B. Lefler, M. Svensson, L.G. Lord, in: Behaviour of Materials in Machining, IOM Conf. Proc., Stratford (1998) 3–8. [4] J. Rechberger, P. Brunner, R. Dubach, Surf. Coat. Technol. 62 (1993) 393–398. [5] D.G. Teer, V. Bellido-Gonzales, J. Hampshire, MoS /titanium 2 coatings, UK Patent GB9514773.2, 19/07/1995; EU Patent 0842306. [6 ] D. Camino, A.H.S. Jones, D. Mercs, D.G. Teer, Vacuum 52 (1999) 125–131. [7] D.G. Teer, D. Camino, A. Jones, Carbon coatings, UK Patent GB9622568.5, 02/12/1997. [8] T. Le Mogne, C. Donnet, J.M. Martin, A. Tonck, N. Millard Pinard, J. Vac. Sci. Technol. A 12 (1994) 1998. [9] G. Jayaram, N. Doraiswamy, L.D. Marks, M.R. Hilton, Surf. Coat. Technol. 68/69 (1994) 439–445. [10] B. Window, N. Savvides, J. Vac. Sci. Technol. A 4 (1986) 196. [11] D.G. Teer, Surf. Coat. Technol. 39/40 (1989) 565. [12] T. Spalvins, J. Vac. Sci. Technol. A 5 (2) (1987) 212–219. [13] G. Jayaram, L.D. Marks, M.R. Hilton, Surf. Coat. Technol. 76/ 77 (1995) 393–399. [14] D.P. Monaghan, D.G. Teer, K.C. Laing, I. Efeoglu, R.D. Arnell, Surf. Coat. Technol. 59 (1993) 21–25. [15] D.G. Teer, J. Hamphire, V. Fox, V. Bellido-Gonzalez, Surf. Coat. Technol. 94/95 (1997) 572–577. [16 ] D.G. Teer, Magnetron sputter ion plating, US Patent 5.556.519, 1996; UK Patent GB2258343B. [17] N.M. Renevier, V.C. Fox, D.G. Teer, J. Hampshire, paper presented at ICTCM’99, San Diego, 12–14 April, 1999.
Current Industrial Practices
Performance of MoS /metal composite coatings used for dry 2 machining and other industrial applications k N.M Renevier a, *, N. Lobiondo b, V.C Fox a, D.G Teer a, J. Hampshire a a Teer Coatings Ltd., 290–293 Hartlebury Trading Estate, Hartlebury, Worcestershire DY10 4JB, UK b Multi-Arc Inc, 1598 East Lincoln, Madison Heights, MI 48071, USA Received 13 April 1999; received in revised form 21 July 1999
Abstract As previously reported (Fox et al., Proc. PSE Conf., Garmisch Partenkirchen, 14–18 September, 1998), the properties of MoS coatings can be improved by the co-deposition of a small amount of titanium. These MoS /Ti coatings, known as 2 2 MoST@ produced by closed field unbalanced magnetron sputtering, are harder, much more wear resistant and less sensitive to atmospheric water vapour. These coatings have given excellent industrial results for a wide range of cutting and forming applications. Two forms of these MoS /titanium composite coatings have been developed: MoS /titanium composite ( low titanium, 2 2 10 at%) and MoS /titanium composite (high titanium, 20 at%). 2 The MoS /titanium composite ( low titanium) exhibits a coating hardness of 500 HV, a coefficient of friction of 0.02 during 2 100 N applied load reciprocating wear testing, and a low wear rate, while the MoS /titanium composite (high titanium) exhibits 2 a coating hardness similar to that of TiN, a coefficient of friction of 0.04 during 100 N applied load reciprocating wear testing, and an extremely low wear rate. The choice of coatings is dependent upon the application. Recent industrial performance data related to the characteristics of these MoS /titanium composite (high titanium) self-lubricant coatings, which are utilised now in large-scale production, are 2 presented. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Cutting; Forming; Low friction; MoS ; Solid lubricant; Unbalanced magnetron 2
1. Introduction There is currently an increase in demand for dry machining [2]. Over the last century, the production engineer has required higher production rates and has often had to machine new materials of higher strength. Metallurgists have responded with new cutting, forming tools and components with improved performance. The development of tool material from carbon tool steel, high-speed steel, carbides, ceramics and nitrides, together with techniques of applying wear-resistant coatings, has allowed cutting, forming tools or components k Paper presented at the 26th International Conference on Metallurgical Coatings and Thin Films, April 12–15, 1999, San Diego, CA, USA * Corresponding author. Tel.: +44-1299-251-399; fax: +44-1299-250-171. E-mail address: [email protected] (N.M Renevier)
to be used at increased cutting or forming speed with increased lifetimes. This has forced machine tool manufacturers to develop their machines so as to be capable of making full use of the new tool materials through increased metal removal, forming rates and improved productivity. New materials have been developed, particularly in the aerospace, automotive and medical industries, and these new alloys are often more difficult to machine. So, although these materials may be lighter, stronger and more wear resistant, they suffer from poor machinability, thus creating unfavourable conditions on the cutting or edge of the tool which leads to a reduction in tool life and in some cases premature tool failure. Although thin hard films were introduced into the cutting and forming tools market in the 1960s to arrest or slow down the diffusion wear in carbide tools, the real impact in the high-speed steel tool area has taken
0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 42 4 - 7
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
place over the last decade, particularly with developments in the physical deposition (PVD) process. Whilst the major successes in coatings for industrial applications are titanium nitride ( TiN ), titanium carbide ( TiC ), titanium carbonitride ( TiCN ), titanium aluminium nitride ( TiAlN ), these coatings are not totally successful for all applications for several reasons [3], where a total systems approach needs to be employed in surface engineering. For centuries, liquid lubricants were used to limit the contact pressure and facilitate sliding. Contact is inevitable, and lubricants are used to reduce the temperature produced by friction. But, in many cases the presence of liquid is not recommended or forbidden for contamination reasons. Effects are particularly important in vacuum or at high temperature, for food, medical apparatus, or the nuclear industry. Lubrication costs are often high and therefore uneconomic. An additional problem with liquid lubricant is the efficient distribution of the lubricant on the required surface. Dry machining can appear a risky solution to this problem, as users are reluctant to damage their tools under dry conditions. However, the risks of damage to the tools by using solid lubricants in place of liquid lubricants are minimal. This is because, in effect the solid lubricant coating creates a third component within the system, which gives some advantages of a liquid lubricant. In some cases the use of a solid lubricant deposited where a traditional lubricant cannot perform allows an increase in the performance of tools and components. In future, solid lubrication may be imposed by legislation (for example for environmental protection) and also for cost savings (as the treatment of used lubricants is very expensive). MoS coatings have been proposed for the improve2 ment in the performance of cutting tools, and such coatings have been shown to give significant improvements [4]. At Teer Coatings core strategies of innovation and continuous improvement are being used to address these preoccupations. Hence, two new complementary types of coating were developed, known as MoST@ [1,5] and Graphit-iC@ [6,7]. As previously reported [1], MoST@ coatings are MoS /titanium composite coatings formed 2 by co-deposition of small amounts of titanium in a MoS based matrix. MoS /titanium composite coatings, 2 2 produced by closed field unbalanced magnetron sputtering, are harder, much more wear resistant and less sensitive to atmospheric water vapour than MoS . These 2 coatings have given excellent industrial results in a wide range of cutting and forming applications. The properties are dependent on the titanium content. Two successful forms of the range of MoST@ coatings have been developed: MoS /titanium composite ( low titanium) 2 and MoS /titanium composite (high titanium). 2 The MoS /titanium composite ( low titanium) exhibits 2
85
a coating hardness of 500 HV, a coefficient of friction of 0.02 during 100 N applied load reciprocating wear testing, and a low wear rate, while the MoS /titanium 2 composite (high titanium) exhibits a coating hardness similar to that of TiN, a coefficient of friction of 0.04 during 100 N applied load reciprocating wear testing, and an extremely low wear rate. Recent industrial performance data related to the characteristics of these MoS /titanium composite (high 2 titanium) self-lubricant coatings are reported in the range of cutting, forming tools and components, which are utilised today in large-scale production.
2. Background MoS coatings are often deposited using RF sputter2 ing [8–11], producing MoS coatings with a duplex 2 structure consisting of a dense, coherent film of about 100 nm followed by a loose, columnar, powdery film which is easily removed [12]. Most of these coatings were suitable only in vacuum and at 0% humidity. DC magnetron sputtering is used for the deposition of MoS coatings in this study. The quality of the 2 coatings is improved by applying a negative potential to the substrates so that the growing film is bombarded by energetic ions, densifying the structure and improving adhesion. Unbalancing the magnetic field of the magnetron [13] increases the intensity of the bombardment and further improvements are achieved by using a multimagnetron system in which unbalanced magnetrons are used in the closed field arrangement [13,14]. Unlike RF techniques, such methods give dense, coherent coatings. MoS properties degrade when in humid air, causing 2 an increase in friction coefficient and a decrease in lifetime. To reduce the water vapour content in the vacuum chamber a titanium target was sputtered during ion cleaning of the substrates prior to deposition to produce a gettering effect. The titanium was then used to deposit an interlayer which led to an improvement in coating adhesion (critical load above 120 N ). A natural progression of this work led to incorporation of titanium into the coating itself, resulting in improved friction (m of 0.02–0.1 at 40% humidity) and wear properties. These MoS /metal composite coatings were hard (1000– 2 2000 HV ) and also less sensitive to water vapour than pure MoS coatings. 2 Properties of the original MoS /metal composite 2 coating [MoS /titanium composite ( low titanium, 2 10 at% and high titanium, 20 at%)] have been previously reported [15]. These coatings are not only suitable in vacuum and at 0% humidity, but also up to 50% humidity, allowing terrestrial uses. These MoS compos2 ite coatings have been tested in a variety of industrial applications, showing excellent results for a wide range of cutting, forming and component applications. This paper reports industrial results from MoS /titanium 2 composite (high titanium, 20 at%) coating.
86
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
A resume of laboratory test results using microhardness and nanohardness testing, scratch adhesion testing, pin on disc and reciprocating friction and wear tests is presented in an attempt to be au fait with the tribological properties. The structures of the coatings have been extensively studied by a variety of techniques, including optical microscopy, transmission electron microscopy ( TEM ), X-ray diffraction ( XRD) and scanning electron microscopy (SEM ) and a summary of this work is reported to understand the structure of such complex coatings. 2.1. Deposition system MoS /titanium composite coatings were deposited by 2 DC magnetron sputtering using standard Teer CFUBMSIP equipment [16 ] (Fig. 1). The magnetrons within the coating chamber were arranged so that three MoS targets and one titanium target were used and the 2 substrates rotated between the targets. The amount of metal content was controlled by the power applied to the targets. To reduce the water vapour content in the vacuum chamber a titanium target was sputtered, under argon during ion cleaning of the substrates prior to deposition to produce a gettering effect. The titanium target was then used to deposit a 100 nm interlayer, which led to an improvement in coating adhesion. Titanium was used as an interlayer to improve the load bearing capacity and the adherence properties. This step is followed by a second interlayer by sputtering from two MoS targets and the titanium 2 target simultaneously, the power to the titanium target is gradually reduced. This layer consists of a 200 nm mixed MoS /titanium in a multilayer structure. 2 The main bulk of the coatings of about 800 nm, known as MoST@ coatings in the case of titanium
(a)
addition, are not multilayer coatings (as previously described [1]). The titanium target is switched off at the end of the process to produce a 50 nm pure MoS coating for 2 coloration. 2.2. Laboratory testing and results 2.2.1. Scratch testing Scratch testing on the MoS /titanium composite coat2 ings has produced results showing that the coatings undergo no failure up to a load of 120 N [15] using a 0.2 mm Rockwell diamond tip. Optical examination of the coatings after scratch testing has confirmed this result. 2.2.2. Pin on disc testing As was observed in the previous study on the MoS /titanium composite (high titanium) coating [15], 2 as the load increases the friction coefficient decreases and the wear rate increases. The specific wear rate for the MoS /titanium composite (high titanium) coating 2 has been calculated at around 4×10−17 m3(Nm)−1 after 1 h test performed at 80 N in dry conditions (40% relative humidity) with a linear speed of 200 mm/s (477 rpm, 8 mm diameter) and a 5 mm diameter WC-4% Co ball showing a 0.06 coefficient of friction. Preliminary results from pin on disc testing (5 N and 10 N ) at 350°C have indicated that no loss in properties is evident up to this temperature compared to those obtained at room temperature. 2.2.3. Reciprocating wear testing At 41% humidity and under a 100 N load using the same ball characteristics, the coating survives for 9999 cycles, the limit of the test. The friction coefficient is 0.043 and the wear after 9999 cycles is 0.5 mm. In order to demonstrate the differences between MoS and 2
(b)
Fig. 1. (a) Schematic representation and (b) general view of a four magnetron coating chamber configured with closed field unbalanced magnetron sputter ion plating (CFUBMSIP).
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
87
These results were confirmed by extensive nanoindentation tests on the MoS /titanium composite (high 2 titanium) coating, confirming the Fischerscope measurements. 2.3. Coating analysis
(a)
(b) Fig. 2. Results of (a) MoS and (b) MoST coatings tested at 40% 2 humidity, under water and under oil. Number of cycles, coefficient of friction, track width and wear rate are reported.
MoS /titanium composite coatings, the MoS /titanium 2 2 composite (high titanium) coating was tested under water at a load of 100 N where the coating survived for 3000 cycles with a friction coefficient ranging from 0.03 at the start of the test to 0.05 just before failure of the coating (32 cycles for pure MoS coatings). The 2 MoS /titanium composite was also tested under oil 2 where the coating survived for 10 000 cycles with a friction coefficient characteristic of the oil rather than the coatings (155 cycles for pure MoS ). Tested under 2 oil at low load ( less than 10 N, with the same ball characteristics), the coating presents no failure and the wear is lower than in dry conditions. Results are reported in Fig. 2. 2.2.4. Nanoindentation and microindentation Dynamic hardness measurements were performed with a Fischerscope Dynamic Hardness Tester at 4 mN load. The test gave a spread of results depending on the titanium concentration in different coatings, but all of the tests gave a value for coating hardness of above 15 GPa with values ranging up to 21 GPa (4 GPa for pure MoS ). Young’s modulus of 138 GPa has been 2 recorded, much higher than typical values for pure MoS coatings (around 70 GPa). 2
2.3.1. Chemical composition Auger electron spectroscopy (AES ) analysis was performed to determine the composition [17]. Sputtered neutral mass spectroscopy (SNMS) analysis and Rutherford backscattering (RBS) have failed to detect any multilayers, more details will be presented in Ref. [17]. 2.3.2. XRD XRD measurements were performed at Birmingham University using Phillips equipment with a Cu Ka line source (0.154046 nm and 0.154439 nm) and a goniometer PW3020. Measurements were reported only between 10 and 60° because of the very weak intensities of the peak after 60°. Analysis of MoS /titanium composite 2 coatings by XRD [17] (Fig. 3) revealed only a very broad band pattern indicating a structure consisting of quasi-amorphous, highly strained MoS (Fig. 3). For 2 comparison, XRD analysis was performed on pure titanium and pure MoS coatings. Several peaks were 2 evident for the MoS coating, indicating that the coating 2 is at least partly crystalline. None of these peaks occur on analysis of the MoS /titanium composite coating, 2 and so it would appear that the addition of Ti to the coating is inhibiting the formation of crystalline MoS . 2 The pure MoS coating is crystalline and the strongest 2 peak was found in the (002) plane compared with the (103) plane. On addition of metal, in this example titanium, the peak corresponding to the (002) plane disappears progressively and at the same time the band
Fig. 3. X-Ray diffraction pattern showing (a) pure MoS , (b) 2 MoST@, (c) MoS +titanium multilayer, (d) pure titanium. 2
88
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
corresponding to the (103) plane appears stronger than previously. The broad diffraction lines are shifted to lower diffraction angles as the metal content is increased. The corresponding lattice parameter enhancement is consistent with an expansion of the lattice as the metal content increases. Only at high metal concentrations was it possible to detect the titanium peak of the (002) plane. 2.3.3. TEM Extensive TEM analysis of the MoS /titanium com2 posite coating has been carried out at Birmingham University and Northumbria University. TEM crosssections showing MoS /titanium composite coating are 2 presented in Ref. [17]. 2.3.4. SEM A fractograph of a SEM cross-section of MoS / 2 titanium composite coating on top of a TiN coating is shown in Fig. 4. The coating was deposited onto tool steel substrate. From the fractograph it can be seen that the coating is dense, compact, non-columnar and adherent.
3. Industrial testing results and discussion A large number of industrial results have been obtained for the MoS /titanium composite coating 2 showing very big improvements for many applications in comparison with conventional hard coatings. Better results were obtained when MoS /titanium composite 2 coating is supported by a hard coating; the exact reasons are unknown, but some investigations are under way. The following results are the most recent. 3.1. Cutting tools applications 3.1.1. Drilling in dry and lubricated conditions Drilling tests were carried out. High-speed steel M6 tap drills have been coated with TiN, TiCN ( low carbon content), TiAlN (high aluminium content), TiN+MoS / 2
Fig. 4. Fractograph of TiN+MoST coating on tool steel substrate.
Fig. 5. Number of cutting holes produced (a) at 10 m/min in dry conditions, (b) at 20 m/min in dry conditions, (c) at 20 m/min lubricated with an oil mixture. TiCN (1): low carbon content, TiAlN: high aluminium content, TiCN+MoST: TiCN with low carbon content.
titanium composite and TiCN ( low carbon content)+ MoS /titanium composite. These tap drills were tested 2 under dry conditions and lubricated (mixed oil and watersoluble by 20%) to drill 5.5 mm through holes into AISI 400 stainless steel (11.7 mm thick, HRB 60–70). These tests were performed at 10 m/min (corresponding to 530 rpm) and at 20 m/min (corresponding to 1061 rpm). The number of holes produced was as recorded. Use of MoS /titanium composite ( Fig. 5) in dry 2 conditions on the top of hard coatings such as TiN and TiCN has increased the number of holes produced by twice in the case of TiN and by 4.1–4.8 times for TiCN. Unfortunately TiAlN coating, usually good for dry machining, was not successful for this application compared with the other coatings. The use of MoS /titanium composite under mixed 2 oil on top of a TiCN hard coating was improved by 2.4 times the lifetime of the drill. The poor result obtained for the TiN+MoST running under mixed oil (water-soluble) is puzzling and will be reinvestigated. 3.1.2. End milling applications Carbide end mills 12 mm×4 mm×25 mm have been coated with TiCN and TiCN+MoS /titanium compos2 ite and were tested by The Institute of Advanced Manufacturing Sciences Inc. for Multi-Arc Inc. These end mills were tested under dry conditions and lubricated ( Trim Sol: mixed oil and water-soluble by 5%) at a cutting speed of 150 m/min and a feed rate of 0.04 mm/turn onto AISI 304 stainless steel. The cutting depth was 4 mm, while the radial depth cut was 3 mm and the axial depth cut was 5 mm. Average milling force, tool wear and surface finishing were recorded. It has also been observed that MoS /titanium com2 posite (Fig. 6) coatings running in dry conditions deposited on top of a TiCN hard coating offer an increase in
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
89
The uncoated ejector pins produced 2000 shots before failure of the pins occurred, using the MoS /titanium 2 composite coating on the pins increased the number of shots produced to 100 000 shots. This is a 50 times improvement and a big cost saving is reported. 3.2.2. Perforation and piercing applications
(a)
(b)
Fig. 6. Average resultant force (a) and surface finishing (b) while end milling AISI 304 stainless steel using TiCN (using coolant) and TiCN+MoST (dry and using coolant) coated inserts.
milling distance and a reduction in average milling force during the test and also an improvement in surface finish as compared with the TiCN coatings alone. Both effects increase the productivity and the quality of the final product. However, MoS /titanium composite (Fig. 6) coating 2 deposited on top of a TiCN hard coating and running under mixed oil has not increased the end mill distance or the average milling force compared with the TiCN coating alone. This seems to show the limit of the coating under mixed lubricated (oil water-soluble) conditions. The water content in the lubricant is probably too high; it should be remembered that MoS /titanium 2 composite is a MoS based coating, which is dramati2 cally affected by water during use.
3.2.2.1. Test 1 Rapid wear occurred when AISI D2 punches (60–62 HRC ) were used for the perforating application. Punches were coated with the MoS /tita2 nium composite coating and compared with the previously sprayed MoS coating and uncoated punches 2 used by the company. The application for which the punches were tested was to perforate 1.2 mm thick 409 stainless plate for filters without lubricant, running at 250 pressings per minute. Previously the company had used a sprayed MoS 2 coating which enabled the punches to last for 1 day (around 80 000 pressings) without lubricant. With this sprayed coating the punch needed two to four regrinds to survive for 1 day, after which the punch was unable to function. With MoS /titanium composite coating, the 2 punches were still performing after 320 000 pressings (4 days) without the need for regrinding. The company reported a big cost saving by using the MoS /titanium 2 composite coated punches. 3.2.2.2. Test 2 Punches made from AISI M2 steel were coated with the MoS /titanium composite and 2 TiCN+MoS /titanium composite coatings. TiCN coat2 ing was deposited by arc evaporation (3–3.5 mm) and MoS /titanium composite by magnetron sputtering 2 (1.2 mm). The application for which the punches were tested was to pierce 1.2 mm thick AISI 409 stainless plate for filters without lubricant, running at 250 pressings per minute. Results are shown in Fig. 7. In the piercing application, an uncoated tool was able to pierce 5000 parts before tool failure. The use of a TiCN coating increased
3.2. Forming applications 3.2.1. Coating ejector pins The surface of ejector pins used for plastic moulds was coated with the MoS /titanium composite coating. 2 The exact conditions used during the process are unknown, but the results obtained represent comparative tests to the uncoated one. The number of shots performed with uncoated and coated pins was recorded.
Fig. 7. Results of MoST@ piercing application on AISI D2 steel.
90
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
the number of parts to 6000, and depositing MoS /titanium composite on top of TiCN further 2 increased the number of parts to 27 000. 3.2.2.3. Test 3 PS4 (CPM@ M4 of Dayton Progress Corporation, USA) (61–63 HRC ) punches were coated with MoS /titanium composite (1.2 mm) on top of TiCN 2 (3–3.5 mm of TiCN produced by arc evaporation) coating and used to perforate 12 mm HSLA steel with watersoluble lubricant (20%). Results are shown in Fig. 8. In the piercing application, an uncoated tool was able to pierce 15 000 parts before tool failure. The use of a TiCN coating increased the number of parts to 50 000, and depositing MoS /titanium composite on top of TiCN further 2 increased the number of parts to 200 000.
Fig. 9. Results of MoST@ punching application on AISI M2 steel.
3.2.3. Fine blanking stainless steel Results have been reported of punches coated with MoS /titanium composite on top of a TiCN coating. 2 The punches were made from AISI M2 steel (64 HRC ) and were coated with 3 to 3.5 mm of TiCN by arc evaporation followed by 1.2 mm of MoS /titanium 2 composite. 3.2.3.1. Test 1 The punches were tested on fine blanking 409 stainless steel without lubricant. In this application the number of parts produced with TiCN coatings increased from 3000 to 12 000 parts with the addition of MoS /titanium composite on top 2 of TiCN coatings (Fig. 9). 3.2.3.2. Test 2 The punches were tested on fine blanking 403 stainless steel with a water-soluble lubricant (20% water). In this application the number of parts produced with TiCN coatings increased from 220 000 to 600 000 parts with the addition of MoS /titanium composite on 2 the top ( Fig. 10).
Fig. 10. Number of parts produced with MoST@ in lubricated conditions (water-soluble 20%).
3.2.4. Drawing stainless steel applications Results have been reported of tools coated with MoS /titanium composite on top of a CrN coating. The 2 tools were made from AISI M2 steel (64 HRC ) and were coated with 3 to 3.5 mm of TiCN by arc evaporation followed by 1.2 mm of MoS /titanium composite. The 2 application for which the tools were tested was to draw 0.5 mm thick AISI 430 stainless steel under chlorinated oil used as lubricant. Results are shown in Fig. 11. In the piercing application, an uncoated tool was able to pierce 15 000 parts before tool failure. The use of a CrN coating increased the number of parts to 50 000, and depositing MoS /titanium composite on top of CrN further 2 increased the number of parts to 100 000.
4. Conclusions and recommendations
Fig. 8. Number of parts produced with MoST@ in lubricated conditions (water-soluble 20%).
Results show that product performance and surface finish of cutting and productivity of forming can be improved by using a solid lubricant such as MoS /titanium composite coatings. 2
N.M Renevier et al. / Surface and Coatings Technology 123 (2000) 84–91
91
hard thin film characteristics, as is evident from numerous publications reported elsewhere. A hard coating under MoS /titanium composite seems to be the best 2 choice. The exact reasons are unknown, but the improvement is probably associated with the additional support offered by the hard coating. MoS /titanium composite is a good candidate for a 2 clean world process, and gives greater productivity in machining, forming and for components. MoS / 2 titanium composite coating has great potential for success in dry machining, but the reduced wear resistance under oil is a limitation, which needs further study. Fig. 11. Number of parts produced with MoST@ in lubricated conditions.
It is important to establish the failure mechanisms and failure mode in a metal-cutting or metal/plasticforming or component situation in order to apply the MoS /titanium composite coatings effectively on top of 2 a hard coating. Design and substrate materials play an important role, which will be studied later (combination of tool design, manufacturing quality and appropriate coating such as MoS /titanium composite deposited on 2 a hard thin film in an attempt to improve tool performance life). Hard coatings such as TiN, TiCN, TiAlN may increase tool performance (for example cutting speed) and lifetime by arresting or slowing down certain types of wear. However, these coatings retain a high coefficient of friction and require lubricant. When cutting speeds are increased, the effects of liquid lubricants are reduced. Furthermore, reductions in friction result in decreased cutting force and tool temperatures, which reduces resistance to adhesion or local welding which may improve the quality of the product by replacing traditional lubricant by a solid lubricant such as MoS /titanium compos2 ite. Under most rubbing conditions MoS /titanium 2 composite coatings have a much lower wear rate than traditional hard coatings. They also have very low friction which allows the component to be used at high speed, lowering the frictional forces and the temperature. In cold forming applications use of the MoS /titanium composite coating has allowed us to 2 reduce the load applied during the process, which has also resulted in a decrease in energy consumption. A big cost saving has been reported using MoS /titanium composite coatings, in particular in 2 mould industries. MoS /titanium composite coatings, in contrast to 2 most other coatings, have allowed either the elimination or reduction of the lubricant quantity used; this big cost saving is because many tools used for forming require the old oil to be cleaned off before they can be used again. Work in dry conditions reduces this requirement. Considerable attention has previously been given to
Acknowledgements The authors would like to express their gratitude to the Department of Trade and Industry of the United Kingdom for their support through a SPUR award in the financing of this project. The authors would also like to thank the following people and companies. Dr. R. Gilmore of ISPRA (Italy), Dr. V Rigato of Thin Films (Italy), Nachi Fujikoshi Steel Corporation (Japan), Adachi New Industrial Company Ltd. (Japan), Third Millenium Ltd. ( UK ) and Dayton Progress Corporation ( USA).
References [1] V.C. Fox, N.M. Renevier, D.G. Teer, J. Hampshire, V. Rigato, Proc. PSE Conf., Garmisch Partenkirchen, 14–18 September, (1998). [2] T. Cselle, Manufact. Eng. (1995) 77–80. [3] B. Lefler, M. Svensson, L.G. Lord, in: Behaviour of Materials in Machining, IOM Conf. Proc., Stratford (1998) 3–8. [4] J. Rechberger, P. Brunner, R. Dubach, Surf. Coat. Technol. 62 (1993) 393–398. [5] D.G. Teer, V. Bellido-Gonzales, J. Hampshire, MoS /titanium 2 coatings, UK Patent GB9514773.2, 19/07/1995; EU Patent 0842306. [6 ] D. Camino, A.H.S. Jones, D. Mercs, D.G. Teer, Vacuum 52 (1999) 125–131. [7] D.G. Teer, D. Camino, A. Jones, Carbon coatings, UK Patent GB9622568.5, 02/12/1997. [8] T. Le Mogne, C. Donnet, J.M. Martin, A. Tonck, N. Millard Pinard, J. Vac. Sci. Technol. A 12 (1994) 1998. [9] G. Jayaram, N. Doraiswamy, L.D. Marks, M.R. Hilton, Surf. Coat. Technol. 68/69 (1994) 439–445. [10] B. Window, N. Savvides, J. Vac. Sci. Technol. A 4 (1986) 196. [11] D.G. Teer, Surf. Coat. Technol. 39/40 (1989) 565. [12] T. Spalvins, J. Vac. Sci. Technol. A 5 (2) (1987) 212–219. [13] G. Jayaram, L.D. Marks, M.R. Hilton, Surf. Coat. Technol. 76/ 77 (1995) 393–399. [14] D.P. Monaghan, D.G. Teer, K.C. Laing, I. Efeoglu, R.D. Arnell, Surf. Coat. Technol. 59 (1993) 21–25. [15] D.G. Teer, J. Hamphire, V. Fox, V. Bellido-Gonzalez, Surf. Coat. Technol. 94/95 (1997) 572–577. [16 ] D.G. Teer, Magnetron sputter ion plating, US Patent 5.556.519, 1996; UK Patent GB2258343B. [17] N.M. Renevier, V.C. Fox, D.G. Teer, J. Hampshire, paper presented at ICTCM’99, San Diego, 12–14 April, 1999.