- Email: [email protected]
Dry machining — commercial viability through filtered arc vapour deposited coatings
Surface and Coatings Technology 133᎐134 Ž2000. 383᎐388
Dry machining ᎏ commercial viability through filtered arc vapour deposited coatings S.G. Harris a,U , A.C. Vlasveld a , E.D. Doylea , P.J. Dolder b b
a Swinburne Uni¨ ersity of Technology, Mail 噛38, P.O. Box 218, John Street Hawthorn, Victoria, Australia Ford Motor Company of Australia, Powertrain Operations, North Shore Road, Norlane, Victoria, Australia
Abstract Critical to the economic viability of dry machining is justifying increased tool costs in terms of productivity and ecological savings achievable through the elimination of metal cutting fluids. The automotive industry is a large-scale commercial manufacturing sector in which emphasis is placed on reducing impact on the environment as well as seeking to reduce costs in manufacturing. This paper presents results for dry drilling fully pearlitic grey cast iron with uncoated and partially filtered arc deposited TiN and TiAlN coated Co-HSS split point twist drills using two methods to distinguish drill failure, namely audible screech and measurement of outer corner wear lands. The latter method proved to be of most relevance in manufacturing because it is more closely related to the industrial practice of on-condition assessment prior to drill resharpening. Using the outer corner wear method, TiN coatings Ž3.0 m thickness . achieved a modest increase in drill life of 1.4 times, compared with significant improvements of 2.6 times, achieved from a TiAlN top coating Ž0.8 m thickness . on a base coating of TiN Ž1.2 m thickness .. This is a novel result in that a thin top coating of the expensive TiAlN coating over the much cheaper TiN coating can provide a very cost-effective tool coating combination. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Dry machining ; TiN; TiAlN; Twist drills
1. Introduction The ever-increasing demand for more cost-effective machining processes with reduced ecological impact is forcing large scale automotive manufacturers to look for benefits to be gained from high speed andror dry machining through the exploitation of advanced surface coatings on cutting tools. This is a particularly challenging demand given that grey cast iron will continue to be the preferred workpiece material for cylinder block production in the popular car market for at least the next 5᎐10 years. In particular, the removal of
U
Corresponding author. Tel.: q613-9214-8987; fax: q613-92148264. E-mail address: [email protected] ŽS.G. Harris..
cutting fluids from such commercial machining operations requires improved high-temperature performance of cutting tools in order to sustain feed rates necessary to meet tight productivity requirements. Critical to understanding the effects of grey cast iron on tool life performance of uncoated and PVD-coated cutting tools in increasingly demanding machining operations is an awareness of the micro-constituents present in the workpiece and their effect on tool wear. The machinability of cast iron is often related directly to bulk hardness, owing to the relationship between hardness and abrasive wear w1x. The bulk hardness is often measured using the Brinell technique since the Brinell figure is made up of the complex sum of the resistance to plastic deformation of the individual micro-constituents w2x. However, hardness alone is not consistent as a measure of machinability and can only
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 8 9 5 - 1
384
S.G. Harris et al. r Surface and Coatings Technology 133᎐134 (2000) 383᎐388
be used to compare structurally similar materials w3x. The presence of as little as 5% free carbide in cast iron has a significant impact on machinability by markedly increasing the abrasive nature of the workpiece, whilst registering only a slight increase in the Brinell hardness test w3x. The microstructure of grey cast iron used for automotive cylinder blocks is specifically tailored for optimum in-service wear resistance and strength while retaining a high level of machinability. Alloying with graphitising elements, in conjunction with strict control of solidification and cooling rates during the casting process, enables the production of grey cast iron with a tight network of flake graphite within a predominantly pearlitic matrix. The latter, with its lamellar configuration of alternating layers of soft, low-carbon ferrite and hard cementite ŽFe 3 C., provides the best compromise in terms of machinability and wear resistance from the as-cast iron w2,3x. The presence of flake graphite provides discontinuities in the matrix that facilitate chip breakage and results in short, discontinuous chip formation providing excellent machinability. Clearly, in any investigation of dry machining of cast iron, it is important that the microstructure be fully characterised metallographically. From a manufacturing viewpoint the opportunity to reduce costs by reducing the volume of cutting fluid used to machine large numbers of cast iron engine components is of particular interest to the automotive industry. The primary roles of cutting fluid are lubrication at the toolrchiprworkpiece interface and removal of thermal energy from the cutting region. The success of advanced surface coatings for dry machining relies on high temperature oxidation resistance and an ability to maintain high hardness at elevated temperatures. The more traditional PVD coatings such as TiN have been shown to deteriorate significantly at temperatures typical of dry machining w4x. As low as 550⬚C w4x TiN forms a brittle oxide, TiO 2 w5x, which renders coated cutting tool performance closer to that of uncoated cutting tools. More recent developments in advanced surface coatings for dry machining have seen the emergence of titanium aluminium nitride, TiAlN. It has been identified by Wang et al. w6x that TiAlN resists oxidation up to 925⬚C, at which point the formation of a protective outer film of Al 2 O 3 is formed that creates a barrier against oxygen diffusion to the underlying coating. In addition, Smith et al. w7x have shown that surface finish of the TiAlN hard thin film coating can significantly affect the performance of the coating. The present study, in part, examines the issue of dry machining grey cast iron by trialing TiAlN coatings produced by partially filtered arc deposition ŽPFAD. and the possibility of a thin film top layer coating of TiAlN deposited onto TiN.
2. Experimental procedure 2.1. Coating deposition TiN and TiAlN coatings were deposited using a dual source filtered arc deposition system, comprising two arc evaporation sources that can be configured in either a partial or full filtration mode. The full filtration mode uses a plasma duct bent at 90⬚ to the deposition chamber w8x whereas partial filtration uses a straight plasma duct. Prior to coating deposition, 6.8-mm diameter Co-high speed steel split point twist drills were cleaned using a commercially operated water-based cleaning line. Following this, they were loaded into the coating chamber and pumped down to a base pressure of - 2 = 10y5 mbar. The drills were then subjected to a 20-min ion etch using a hot anode, cold cathode ion source, followed by heating to ; 350⬚C and a further 15-min ion etch. For both TiN and TiAlN coatings an initial metallic interlayer was deposited in an Ar atmosphere for 2 min. Nitrogen gas was subsequently introduced into the chamber to allow the deposition of TiN. For the TiN coatings this continued for 50 min after which the drills were cooled and removed from the system. In the case of TiAlN coatings the TiN was deposited for 25 min followed by TiAlN that was deposited for 35 min. 2.2. E¨ aluation of coating properties Properties of PVD coatings deposited on Co-HSS disks were evaluated using a number of quantitative and qualitative techniques, namely: 䢇
䢇
䢇
䢇
䢇
䢇
Surface profilometry using a ‘Talysurf 10’ profilometer to measure the arithmetic mean surface roughness Ž R a .. Daimler-Benz Indentation Test w9x using a Rockwell C indenter Ž500 N load. to assess coating adhesion to the Co-HSS substrate. Optical and scanning electron microscopy (SEM) for a visual examination of surface topography and measurement of macroparticle content. Measurement of coating thickness and grain morphology on cross-sections obtained by brittle fracture of coated substrates cooled to liquid nitrogen temperatures. Energy-dispersi¨ e spectroscopy (EDS) for compositional analysis of macroparticles and coated surface. Vickers hardness (30 kg load) was used to measure substrate hardness.
2.3. Drill testing The two most commonly used methods for assessing
S.G. Harris et al. r Surface and Coatings Technology 133᎐134 (2000) 383᎐388
385
Table 1 Nominal workpiece and substrate compositions Cast iron workpiece Element wt.% Co-HSS ŽHS6-5-2-5. substrate Element wt.%
C 3.25
Si 1.95
Mn 0.7
Cr 0.3
Cu 0.55
Sn 0.025
S 0.025
C 0.92
Cr 4.1
Mo 5.0
V 1.9
W 6.4
Co 4.8
Fe rem.
drill life are by measurement of corner wear on the outer margin of the drill and by audible screech, which is indicative of ultimate drill failure. The latter method of drill life testing presents a technique that requires little operator effort and delivers a clear-cut result for ultimate drill life. However, often the drills break or are unable to be resharpened, consequently from a commercial perspective, this technique is impractical for determining useable tool life of drills because in production drills are resharpened many times in the overall life of the drill. An additional downfall of screech testing is that the results of tests show large scatter making it difficult to distinguish between the performance of drills tested under different conditions. In the present investigation both of these methods were used to evaluate uncoated and partially filtered TiN and TiAlN coated drill performance when drilling a workpiece of as-cast automotive grey cast iron Žsee Table 1 for cutting tool substrate and workpiece compositions. without metal cutting fluids. The microstructure of the grey cast iron was one of fine graphite flakes embedded in a fully pearlitic matrix. The wear lands generated at the outer corners of the drills were periodically measured using a goniometer microscope at =20 magnification over complete tool life when drilling blind holes to a depth of 20.4 mm Ž3.0= diameter. in purpose-cast ingots Ž500 = 290 = 50 mm.. Drills were deemed to have failed the wear measurement test when the outer corner wear lands extended as far as the trailing edge of the drill margin. Screech failure was indicated by a loud audible sound that implied ultimate drill failure had been reached and that further drilling would result in catastrophic failure of the drill. As a means of accelerating the drill tests, excessively high speed and feed rates were initially investigated. These test conditions resulted in premature failure of the drills with wear modes not indicative of those observed at cutting speeds and feed rates normally used in commercial drilling operations with Co-HSS drills. Consequently, drill test conditions were elected to give a practicable tool life with wear patterns typical of those experienced at more conventional cutting speeds and feed rates. A cutting speed of 30 mrmin was chosen for the dry drilling tests, throughout which the feed per revolution remained constant at 0.22 mmrrev.
Ti 0.04
Fe rem.
3. Results and discussion 3.1. Thickness of partially filtered coatings According to SEM fractography of sectioned TiNand TiAlN-coated Co-HSS disks the coatings deposited by the partially filtered ŽPF. arc technique were dense, closely adhering coatings with a fine columnar grain structure. The SEM cross section of PF-TiN ŽFig. 1a. shows a mono-layer coating with total thickness of 3.0 m. In the case of PF-TiAlN the SEM cross-section ŽFig. 1b. reveals a two-phase coating consisting of PFTiN base coating Ž1.2 m thickness . and PF-TiAlN top
Fig. 1. SEM micrographs of fractured cross-sections of: Ža. PF-TiN showing a mono-layer coating with total thickness of 3.0 m; and Žb. PF-TiAlN coating showing a two-layer coating consisting of PF-TiN Ž1.2 m thickness . and PF-TiAlN top layer Ž0.8 m thickness ..
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S.G. Harris et al. r Surface and Coatings Technology 133᎐134 (2000) 383᎐388
addition to published results by other authors on the surface roughness of ‘pure’ arc deposited TiN and TiAlN w10,11x. The surface roughness after coating was increased from 0.02 m for uncoated polished disks, to 0.08 m for PF-TiN and to 0.15 m for PF-TiAlN. The results for partially filtered TiN and TiAlN affirm the SEM observations that identified the increase in size and number of macroparticles in TiAlN coatings. Munz ¨ et al. w12x identified a relationship between the melting temperature of cathode targets and the concentration of macroparticles in coatings deposited on steel substrates during cathodic arc enhanced metal ion etching. The number of macroparticles produced from Al, TiAl and Ti cathodes were compared and it was found that the number of macroparticles decreased with increasing cathode material melting temperature. The results from the current research into partially filtered cathodic arc TiN and TiAlN are in line with the findings by Munz ¨ et al. with respect to the relationship between macroparticle generation and the melting temperature of elements andror constituents of the cathode. To date the investigations by the present authors into TiAlN coatings for commercial application in dry drilling automotive grey cast iron have centred on partially filtered coatings. It is the intention of ongoing research to compare the performance of the smoother fully filtered coatings that contain significantly fewer macroparticles. Fig. 2. SEM micrographs showing surface topography and macroparticle concentrations in: Ža. PF-TiN; and Žb. PF-TiAlN coatings.
coating Ž0.8 m thickness .. The partial filtering configuration of the coating chamber gave deposition rates of 3.6 and 1.4 mrh for the TiN and TiAlN coatings, respectively. 3.2. Arithmetic mean surface roughness, R a The macroparticle concentration in partially filtered TiN and TiAlN coatings can be seen in Fig. 2a,b, which shows a greater number of macroparticles in the TiAlN coating ŽFig. 2b.. In general, the macroparticles in TiAlN had a greater concentration and larger maximum size than the equivalent macroparticles in the TiN coatings ŽFig. 2a.. The results of arithmetic mean surface roughness tests are reported in Table 2, in Table 2 Surface roughness Žm. Ž R a . Uncoated Co-HSS PF-TiN PF-TiAlN ‘Pure’ arc TiN ‘Pure’ arc TiAlN
0.02 0.08 0.15 0.1᎐0.3 w10x 0.3 w11x
3.3. Daimler᎐Benz adhesion The results of Rockwell C indentation of coated Co-HSS disks revealed that the TiN coating was apparently well adhered to the substrate. On the Daimler᎐Benz adhesion classification scale the TiN rated at the high-quality end with a ranking of between 2 and 3. Outside the indentation, cracking occurred in a random network located within a radial boundary 45% greater than the indentation. The SEM micrograph of the indented TiN-coated disk ŽFig. 3a. shows peripheral cracking with only small regions of delamination in close proximity to the cracks. Within the indent itself radial cracks extend from the centre but show no evidence of bulk delamination. In contrast, TiAlN ranked at the lower end of the adhesion quality scale with a ranking of between 4 and 5 Ž6 being the lowest quality ranking.. Peripheral cracking occurred outside the boundary of the indentation and like TiN it was confined to within a radius 45% greater than that of the indent. However, unlike TiN, cracking of the TiAlN coating showed a consistent pattern with an outer ring of cracking surrounding an inner region of bulk delamination ŽFig. 3b.. The inner region of delamination extended radially from the circumference of the indent by as much as 75 m, with
S.G. Harris et al. r Surface and Coatings Technology 133᎐134 (2000) 383᎐388
387
tool life the PF-TiN improved performance of uncoated drills by 1.4 times while the PF-TiAlN improved performance by 2.6 times. From a practical viewpoint this is a significant result since, in manufacturing, drills are not run to complete failure, rather they are resharpened at intervals based on on-condition assessment, that is, on the extent of outer corner wear. In seeking an explanation of the fact that PF-TiAlN coated drills outperformed PF-TiN coated drills, notwithstanding the reduced adhesion and increased surface roughness, the effect of coating hardness and oxidation resistance is considered in relation to cutting tool performance. PVD TiN coatings have a hardness of 2200᎐2500 kgfrmm2 , compared with PVD TiAlN with a hardness of 2500᎐3000 kgfrmm2 w13,14x. The increased hardness of TiAlN provides improved abrasive wear resistance against hard microstructural constituents in grey cast iron. In addition, the oxidation resistance of TiAlN is vastly superior to TiN at elevated temperatures typical of dry machining. The temperature achieved at the cutting edge in dry drilling cast iron has been shown to oxidise TiN coatings causing the formation of a brittle oxide ŽTiO 2 . that results in cutting tool performance more comparable to uncoated cutting tools w10x. In contrast TiAlN has been shown to form an outer film of Al 2 O 3 that provides a protective barrier against oxygen diffusion to the underlying coating, which enFig. 3. SEM micrographs of Rockwell ‘C’ indentations in Co-HSS coupons showing peripheral cracking of PF-TiN coating Ža. and bulk delamination in addition to peripheral cracking of the PF-TiAlN coating Žb..
the outer ring of cracking a further 65 m outside the delaminated section. EDS of the exposed areas confirmed that the coating had in fact delaminated between the TiN and the Co-HSS substrate, and not between the TiN and TiAlN top layer. 3.4. Drill testing The results of the two drill testing methods, namely outer corner wear and screech failure, for uncoated, PF-TiN and PF-TiAlN coated drills are shown in Fig. 4, in which drill failure was identified by complete outer corner wear, and by screech testing. In both the outer corner wear and screech tests, the results clearly show an improvement in the mean tool life for the PVDcoated drills compared to the uncoated drills. In addition, the relative increase in performance varied depending on the adopted method of tool life testing. In the case of screech testing of uncoated Co-HSS drills an increase of 1.7 times was achieved from PF-TiN coating and 2.2 times from PF-TiAlN coating. However, on the basis of outer corner wear measurement of
Fig. 4. Drill life performance comparison between PF-TiN and PFTiAlN coated drills as measured by outer corner wear and screech testing.
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S.G. Harris et al. r Surface and Coatings Technology 133᎐134 (2000) 383᎐388
hances cutting tool performance w11x. The observed results of drill testing support these findings with an improvement in tool life of nearly double for PFTiAlN-coated drills when compared with PF-TiN-coated drills that performed moderately better than uncoated drills. This is an appreciable improvement in tool life considering the coating thickness of PF-TiAlN was 0.8 m Žon top of 1.2 m TiN coating., compared to a PF-TiN coating thickness of 3.0 m. Prior research into PVD TiAlN coatings for high temperature and abrasive machining applications has utilised TiAlN coatings up to 4 m in thickness w15,16x. Provided these coatings have adequate adhesion to the substrate, through appropriate interlayers and deposition techniques, significant improvements in abrasive and high-temperature wear resistance are achievable. However, in light of the results of the present drill tests it would appear that advanced surface coatings can improve cutting tool performance in high temperature applications through multilayered coatings comprising a base layer of TiN on which a thin top layer coating is deposited. In terms of commercial application these coatings present a cost effective multilayer system in which the performance of relatively low-cost TiN coatings can be enhanced by thin high-performance top layer coatings that retain the significant benefits of diffusion barrier films.
4. Conclusion Two experimental methods were used to distinguish drill failure of uncoated and partially filtered arc deposited TiN- and TiAlN-coated Co-HSS split point twist drills, namely audible screech and measurement of outer corner wear lands when dry drilling fully pearlitic grey cast iron. Measuring outer corner drill wear proved to be of most relevance in manufacturing because it more closely related to the industrial practice of on-condition assessment prior to drill resharpening. Using this method, TiN coatings Ž3.0 m thickness. achieved a modest increase in drill life of 1.4
times, compared with 2.6 times, achieved from a TiAlN top coating Ž0.8 m thickness . on a base coating of TiN Ž1.2 m thickness ..
Acknowledgements The authors wish to extend their thanks to the Ford Motor Company of Australia for ongoing support of the research and to Surface Technology Coatings and Sutton Tools for providing surface coatings and test facilities. References w1x S. Coromant, Modern Metal Cutting ᎏ A Practical Handbook, Tofters Tryckeri AB, 1994, pp. II᎐25. w2x H. Angus, Cast Iron: Physical and Engineering Properties, 2nd edition, Butterworths, London, 1976, p. 333. w3x J.R. Davis, ASM Specialty Handbook: Cast Irons, vol. 494, Materials Park, OH, 1996. w4x W.-D. Munz, ¨ J. Vac. Sci. Technol. A4 Ž6. Ž1986. 2717᎐2725. w5x Y.K. Wang, X.Y. Cheng, W.M. Wang et al., Surf. Coat. Technol. 72 Ž1995. 71᎐77. w6x D.-Y. Wang, y.-W. Li, C.-L. Chang, W.-Y. Ho, Surf. Coat. Technol. 114 Ž1999. 109᎐113. w7x I.J. Smith, D. Gillibrand, J.S. Brooks, W.-D. Munz, ¨ S. Harvey, R. Goodwin, Surf. Coat. Technol. 90 Ž1997. 164᎐171. w8x P.J. Martin, R.P. Netterfield, T.J. Kinder, Thin Solid Films 193 Ž1990. 77. w9x Daimler᎐Benz Adhesion Test, Verein Deutscher Ingenieure ŽVDI.-Richlinie 3198 Ž1992. 7. w10x R.R. Aharonov, M. Chhowalla, S. Dhar, R.P. Fontana, Surf. Coat. Technol. 82 Ž1996. 334᎐343. w11x E. Erturk, ¨ H.J. Heuvel, H.G. Dederich, in: E. Broszeit, W.-D. Munz, ¨ H. Oechsner, R.T. Rie, G.K. Wolf ŽEds.., Plasma Surface Engineering, DGM, Informationsgesellschaft Verlag, Obserursel, Germany, 1988, p. 553. w12x W.-D. Munz, ¨ I.J. Smith, D.B. Lewis, S. Creasey, Vacuum 48 Ž5. Ž1997. 473᎐481. w13x W.D. Sproul, Surf. Coat. Technol. 81 Ž1996. 1᎐7. w14x M. Van Stappen, L.M. Stals, M. Kerkhofs, C. Quaeyhaegens, Surf. Coat. Technol. 74᎐75 Ž1995. 629᎐633. w15x H. Ronkainen, I. Nieminen, K. Holmberg et al., Surf. Coat. Technol. 49 Ž1991. 468᎐473. w16x B.F. Coll, P. Sathrum, R. Fontana, J.P. Peyre, D. Duchateau, M. Benmalek, Surf. Coat. Technol. 52 Ž1992. 57᎐64.
Dry machining ᎏ commercial viability through filtered arc vapour deposited coatings S.G. Harris a,U , A.C. Vlasveld a , E.D. Doylea , P.J. Dolder b b
a Swinburne Uni¨ ersity of Technology, Mail 噛38, P.O. Box 218, John Street Hawthorn, Victoria, Australia Ford Motor Company of Australia, Powertrain Operations, North Shore Road, Norlane, Victoria, Australia
Abstract Critical to the economic viability of dry machining is justifying increased tool costs in terms of productivity and ecological savings achievable through the elimination of metal cutting fluids. The automotive industry is a large-scale commercial manufacturing sector in which emphasis is placed on reducing impact on the environment as well as seeking to reduce costs in manufacturing. This paper presents results for dry drilling fully pearlitic grey cast iron with uncoated and partially filtered arc deposited TiN and TiAlN coated Co-HSS split point twist drills using two methods to distinguish drill failure, namely audible screech and measurement of outer corner wear lands. The latter method proved to be of most relevance in manufacturing because it is more closely related to the industrial practice of on-condition assessment prior to drill resharpening. Using the outer corner wear method, TiN coatings Ž3.0 m thickness . achieved a modest increase in drill life of 1.4 times, compared with significant improvements of 2.6 times, achieved from a TiAlN top coating Ž0.8 m thickness . on a base coating of TiN Ž1.2 m thickness .. This is a novel result in that a thin top coating of the expensive TiAlN coating over the much cheaper TiN coating can provide a very cost-effective tool coating combination. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Dry machining ; TiN; TiAlN; Twist drills
1. Introduction The ever-increasing demand for more cost-effective machining processes with reduced ecological impact is forcing large scale automotive manufacturers to look for benefits to be gained from high speed andror dry machining through the exploitation of advanced surface coatings on cutting tools. This is a particularly challenging demand given that grey cast iron will continue to be the preferred workpiece material for cylinder block production in the popular car market for at least the next 5᎐10 years. In particular, the removal of
U
Corresponding author. Tel.: q613-9214-8987; fax: q613-92148264. E-mail address: [email protected] ŽS.G. Harris..
cutting fluids from such commercial machining operations requires improved high-temperature performance of cutting tools in order to sustain feed rates necessary to meet tight productivity requirements. Critical to understanding the effects of grey cast iron on tool life performance of uncoated and PVD-coated cutting tools in increasingly demanding machining operations is an awareness of the micro-constituents present in the workpiece and their effect on tool wear. The machinability of cast iron is often related directly to bulk hardness, owing to the relationship between hardness and abrasive wear w1x. The bulk hardness is often measured using the Brinell technique since the Brinell figure is made up of the complex sum of the resistance to plastic deformation of the individual micro-constituents w2x. However, hardness alone is not consistent as a measure of machinability and can only
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 8 9 5 - 1
384
S.G. Harris et al. r Surface and Coatings Technology 133᎐134 (2000) 383᎐388
be used to compare structurally similar materials w3x. The presence of as little as 5% free carbide in cast iron has a significant impact on machinability by markedly increasing the abrasive nature of the workpiece, whilst registering only a slight increase in the Brinell hardness test w3x. The microstructure of grey cast iron used for automotive cylinder blocks is specifically tailored for optimum in-service wear resistance and strength while retaining a high level of machinability. Alloying with graphitising elements, in conjunction with strict control of solidification and cooling rates during the casting process, enables the production of grey cast iron with a tight network of flake graphite within a predominantly pearlitic matrix. The latter, with its lamellar configuration of alternating layers of soft, low-carbon ferrite and hard cementite ŽFe 3 C., provides the best compromise in terms of machinability and wear resistance from the as-cast iron w2,3x. The presence of flake graphite provides discontinuities in the matrix that facilitate chip breakage and results in short, discontinuous chip formation providing excellent machinability. Clearly, in any investigation of dry machining of cast iron, it is important that the microstructure be fully characterised metallographically. From a manufacturing viewpoint the opportunity to reduce costs by reducing the volume of cutting fluid used to machine large numbers of cast iron engine components is of particular interest to the automotive industry. The primary roles of cutting fluid are lubrication at the toolrchiprworkpiece interface and removal of thermal energy from the cutting region. The success of advanced surface coatings for dry machining relies on high temperature oxidation resistance and an ability to maintain high hardness at elevated temperatures. The more traditional PVD coatings such as TiN have been shown to deteriorate significantly at temperatures typical of dry machining w4x. As low as 550⬚C w4x TiN forms a brittle oxide, TiO 2 w5x, which renders coated cutting tool performance closer to that of uncoated cutting tools. More recent developments in advanced surface coatings for dry machining have seen the emergence of titanium aluminium nitride, TiAlN. It has been identified by Wang et al. w6x that TiAlN resists oxidation up to 925⬚C, at which point the formation of a protective outer film of Al 2 O 3 is formed that creates a barrier against oxygen diffusion to the underlying coating. In addition, Smith et al. w7x have shown that surface finish of the TiAlN hard thin film coating can significantly affect the performance of the coating. The present study, in part, examines the issue of dry machining grey cast iron by trialing TiAlN coatings produced by partially filtered arc deposition ŽPFAD. and the possibility of a thin film top layer coating of TiAlN deposited onto TiN.
2. Experimental procedure 2.1. Coating deposition TiN and TiAlN coatings were deposited using a dual source filtered arc deposition system, comprising two arc evaporation sources that can be configured in either a partial or full filtration mode. The full filtration mode uses a plasma duct bent at 90⬚ to the deposition chamber w8x whereas partial filtration uses a straight plasma duct. Prior to coating deposition, 6.8-mm diameter Co-high speed steel split point twist drills were cleaned using a commercially operated water-based cleaning line. Following this, they were loaded into the coating chamber and pumped down to a base pressure of - 2 = 10y5 mbar. The drills were then subjected to a 20-min ion etch using a hot anode, cold cathode ion source, followed by heating to ; 350⬚C and a further 15-min ion etch. For both TiN and TiAlN coatings an initial metallic interlayer was deposited in an Ar atmosphere for 2 min. Nitrogen gas was subsequently introduced into the chamber to allow the deposition of TiN. For the TiN coatings this continued for 50 min after which the drills were cooled and removed from the system. In the case of TiAlN coatings the TiN was deposited for 25 min followed by TiAlN that was deposited for 35 min. 2.2. E¨ aluation of coating properties Properties of PVD coatings deposited on Co-HSS disks were evaluated using a number of quantitative and qualitative techniques, namely: 䢇
䢇
䢇
䢇
䢇
䢇
Surface profilometry using a ‘Talysurf 10’ profilometer to measure the arithmetic mean surface roughness Ž R a .. Daimler-Benz Indentation Test w9x using a Rockwell C indenter Ž500 N load. to assess coating adhesion to the Co-HSS substrate. Optical and scanning electron microscopy (SEM) for a visual examination of surface topography and measurement of macroparticle content. Measurement of coating thickness and grain morphology on cross-sections obtained by brittle fracture of coated substrates cooled to liquid nitrogen temperatures. Energy-dispersi¨ e spectroscopy (EDS) for compositional analysis of macroparticles and coated surface. Vickers hardness (30 kg load) was used to measure substrate hardness.
2.3. Drill testing The two most commonly used methods for assessing
S.G. Harris et al. r Surface and Coatings Technology 133᎐134 (2000) 383᎐388
385
Table 1 Nominal workpiece and substrate compositions Cast iron workpiece Element wt.% Co-HSS ŽHS6-5-2-5. substrate Element wt.%
C 3.25
Si 1.95
Mn 0.7
Cr 0.3
Cu 0.55
Sn 0.025
S 0.025
C 0.92
Cr 4.1
Mo 5.0
V 1.9
W 6.4
Co 4.8
Fe rem.
drill life are by measurement of corner wear on the outer margin of the drill and by audible screech, which is indicative of ultimate drill failure. The latter method of drill life testing presents a technique that requires little operator effort and delivers a clear-cut result for ultimate drill life. However, often the drills break or are unable to be resharpened, consequently from a commercial perspective, this technique is impractical for determining useable tool life of drills because in production drills are resharpened many times in the overall life of the drill. An additional downfall of screech testing is that the results of tests show large scatter making it difficult to distinguish between the performance of drills tested under different conditions. In the present investigation both of these methods were used to evaluate uncoated and partially filtered TiN and TiAlN coated drill performance when drilling a workpiece of as-cast automotive grey cast iron Žsee Table 1 for cutting tool substrate and workpiece compositions. without metal cutting fluids. The microstructure of the grey cast iron was one of fine graphite flakes embedded in a fully pearlitic matrix. The wear lands generated at the outer corners of the drills were periodically measured using a goniometer microscope at =20 magnification over complete tool life when drilling blind holes to a depth of 20.4 mm Ž3.0= diameter. in purpose-cast ingots Ž500 = 290 = 50 mm.. Drills were deemed to have failed the wear measurement test when the outer corner wear lands extended as far as the trailing edge of the drill margin. Screech failure was indicated by a loud audible sound that implied ultimate drill failure had been reached and that further drilling would result in catastrophic failure of the drill. As a means of accelerating the drill tests, excessively high speed and feed rates were initially investigated. These test conditions resulted in premature failure of the drills with wear modes not indicative of those observed at cutting speeds and feed rates normally used in commercial drilling operations with Co-HSS drills. Consequently, drill test conditions were elected to give a practicable tool life with wear patterns typical of those experienced at more conventional cutting speeds and feed rates. A cutting speed of 30 mrmin was chosen for the dry drilling tests, throughout which the feed per revolution remained constant at 0.22 mmrrev.
Ti 0.04
Fe rem.
3. Results and discussion 3.1. Thickness of partially filtered coatings According to SEM fractography of sectioned TiNand TiAlN-coated Co-HSS disks the coatings deposited by the partially filtered ŽPF. arc technique were dense, closely adhering coatings with a fine columnar grain structure. The SEM cross section of PF-TiN ŽFig. 1a. shows a mono-layer coating with total thickness of 3.0 m. In the case of PF-TiAlN the SEM cross-section ŽFig. 1b. reveals a two-phase coating consisting of PFTiN base coating Ž1.2 m thickness . and PF-TiAlN top
Fig. 1. SEM micrographs of fractured cross-sections of: Ža. PF-TiN showing a mono-layer coating with total thickness of 3.0 m; and Žb. PF-TiAlN coating showing a two-layer coating consisting of PF-TiN Ž1.2 m thickness . and PF-TiAlN top layer Ž0.8 m thickness ..
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addition to published results by other authors on the surface roughness of ‘pure’ arc deposited TiN and TiAlN w10,11x. The surface roughness after coating was increased from 0.02 m for uncoated polished disks, to 0.08 m for PF-TiN and to 0.15 m for PF-TiAlN. The results for partially filtered TiN and TiAlN affirm the SEM observations that identified the increase in size and number of macroparticles in TiAlN coatings. Munz ¨ et al. w12x identified a relationship between the melting temperature of cathode targets and the concentration of macroparticles in coatings deposited on steel substrates during cathodic arc enhanced metal ion etching. The number of macroparticles produced from Al, TiAl and Ti cathodes were compared and it was found that the number of macroparticles decreased with increasing cathode material melting temperature. The results from the current research into partially filtered cathodic arc TiN and TiAlN are in line with the findings by Munz ¨ et al. with respect to the relationship between macroparticle generation and the melting temperature of elements andror constituents of the cathode. To date the investigations by the present authors into TiAlN coatings for commercial application in dry drilling automotive grey cast iron have centred on partially filtered coatings. It is the intention of ongoing research to compare the performance of the smoother fully filtered coatings that contain significantly fewer macroparticles. Fig. 2. SEM micrographs showing surface topography and macroparticle concentrations in: Ža. PF-TiN; and Žb. PF-TiAlN coatings.
coating Ž0.8 m thickness .. The partial filtering configuration of the coating chamber gave deposition rates of 3.6 and 1.4 mrh for the TiN and TiAlN coatings, respectively. 3.2. Arithmetic mean surface roughness, R a The macroparticle concentration in partially filtered TiN and TiAlN coatings can be seen in Fig. 2a,b, which shows a greater number of macroparticles in the TiAlN coating ŽFig. 2b.. In general, the macroparticles in TiAlN had a greater concentration and larger maximum size than the equivalent macroparticles in the TiN coatings ŽFig. 2a.. The results of arithmetic mean surface roughness tests are reported in Table 2, in Table 2 Surface roughness Žm. Ž R a . Uncoated Co-HSS PF-TiN PF-TiAlN ‘Pure’ arc TiN ‘Pure’ arc TiAlN
0.02 0.08 0.15 0.1᎐0.3 w10x 0.3 w11x
3.3. Daimler᎐Benz adhesion The results of Rockwell C indentation of coated Co-HSS disks revealed that the TiN coating was apparently well adhered to the substrate. On the Daimler᎐Benz adhesion classification scale the TiN rated at the high-quality end with a ranking of between 2 and 3. Outside the indentation, cracking occurred in a random network located within a radial boundary 45% greater than the indentation. The SEM micrograph of the indented TiN-coated disk ŽFig. 3a. shows peripheral cracking with only small regions of delamination in close proximity to the cracks. Within the indent itself radial cracks extend from the centre but show no evidence of bulk delamination. In contrast, TiAlN ranked at the lower end of the adhesion quality scale with a ranking of between 4 and 5 Ž6 being the lowest quality ranking.. Peripheral cracking occurred outside the boundary of the indentation and like TiN it was confined to within a radius 45% greater than that of the indent. However, unlike TiN, cracking of the TiAlN coating showed a consistent pattern with an outer ring of cracking surrounding an inner region of bulk delamination ŽFig. 3b.. The inner region of delamination extended radially from the circumference of the indent by as much as 75 m, with
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tool life the PF-TiN improved performance of uncoated drills by 1.4 times while the PF-TiAlN improved performance by 2.6 times. From a practical viewpoint this is a significant result since, in manufacturing, drills are not run to complete failure, rather they are resharpened at intervals based on on-condition assessment, that is, on the extent of outer corner wear. In seeking an explanation of the fact that PF-TiAlN coated drills outperformed PF-TiN coated drills, notwithstanding the reduced adhesion and increased surface roughness, the effect of coating hardness and oxidation resistance is considered in relation to cutting tool performance. PVD TiN coatings have a hardness of 2200᎐2500 kgfrmm2 , compared with PVD TiAlN with a hardness of 2500᎐3000 kgfrmm2 w13,14x. The increased hardness of TiAlN provides improved abrasive wear resistance against hard microstructural constituents in grey cast iron. In addition, the oxidation resistance of TiAlN is vastly superior to TiN at elevated temperatures typical of dry machining. The temperature achieved at the cutting edge in dry drilling cast iron has been shown to oxidise TiN coatings causing the formation of a brittle oxide ŽTiO 2 . that results in cutting tool performance more comparable to uncoated cutting tools w10x. In contrast TiAlN has been shown to form an outer film of Al 2 O 3 that provides a protective barrier against oxygen diffusion to the underlying coating, which enFig. 3. SEM micrographs of Rockwell ‘C’ indentations in Co-HSS coupons showing peripheral cracking of PF-TiN coating Ža. and bulk delamination in addition to peripheral cracking of the PF-TiAlN coating Žb..
the outer ring of cracking a further 65 m outside the delaminated section. EDS of the exposed areas confirmed that the coating had in fact delaminated between the TiN and the Co-HSS substrate, and not between the TiN and TiAlN top layer. 3.4. Drill testing The results of the two drill testing methods, namely outer corner wear and screech failure, for uncoated, PF-TiN and PF-TiAlN coated drills are shown in Fig. 4, in which drill failure was identified by complete outer corner wear, and by screech testing. In both the outer corner wear and screech tests, the results clearly show an improvement in the mean tool life for the PVDcoated drills compared to the uncoated drills. In addition, the relative increase in performance varied depending on the adopted method of tool life testing. In the case of screech testing of uncoated Co-HSS drills an increase of 1.7 times was achieved from PF-TiN coating and 2.2 times from PF-TiAlN coating. However, on the basis of outer corner wear measurement of
Fig. 4. Drill life performance comparison between PF-TiN and PFTiAlN coated drills as measured by outer corner wear and screech testing.
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hances cutting tool performance w11x. The observed results of drill testing support these findings with an improvement in tool life of nearly double for PFTiAlN-coated drills when compared with PF-TiN-coated drills that performed moderately better than uncoated drills. This is an appreciable improvement in tool life considering the coating thickness of PF-TiAlN was 0.8 m Žon top of 1.2 m TiN coating., compared to a PF-TiN coating thickness of 3.0 m. Prior research into PVD TiAlN coatings for high temperature and abrasive machining applications has utilised TiAlN coatings up to 4 m in thickness w15,16x. Provided these coatings have adequate adhesion to the substrate, through appropriate interlayers and deposition techniques, significant improvements in abrasive and high-temperature wear resistance are achievable. However, in light of the results of the present drill tests it would appear that advanced surface coatings can improve cutting tool performance in high temperature applications through multilayered coatings comprising a base layer of TiN on which a thin top layer coating is deposited. In terms of commercial application these coatings present a cost effective multilayer system in which the performance of relatively low-cost TiN coatings can be enhanced by thin high-performance top layer coatings that retain the significant benefits of diffusion barrier films.
4. Conclusion Two experimental methods were used to distinguish drill failure of uncoated and partially filtered arc deposited TiN- and TiAlN-coated Co-HSS split point twist drills, namely audible screech and measurement of outer corner wear lands when dry drilling fully pearlitic grey cast iron. Measuring outer corner drill wear proved to be of most relevance in manufacturing because it more closely related to the industrial practice of on-condition assessment prior to drill resharpening. Using this method, TiN coatings Ž3.0 m thickness. achieved a modest increase in drill life of 1.4
times, compared with 2.6 times, achieved from a TiAlN top coating Ž0.8 m thickness . on a base coating of TiN Ž1.2 m thickness ..
Acknowledgements The authors wish to extend their thanks to the Ford Motor Company of Australia for ongoing support of the research and to Surface Technology Coatings and Sutton Tools for providing surface coatings and test facilities. References w1x S. Coromant, Modern Metal Cutting ᎏ A Practical Handbook, Tofters Tryckeri AB, 1994, pp. II᎐25. w2x H. Angus, Cast Iron: Physical and Engineering Properties, 2nd edition, Butterworths, London, 1976, p. 333. w3x J.R. Davis, ASM Specialty Handbook: Cast Irons, vol. 494, Materials Park, OH, 1996. w4x W.-D. Munz, ¨ J. Vac. Sci. Technol. A4 Ž6. Ž1986. 2717᎐2725. w5x Y.K. Wang, X.Y. Cheng, W.M. Wang et al., Surf. Coat. Technol. 72 Ž1995. 71᎐77. w6x D.-Y. Wang, y.-W. Li, C.-L. Chang, W.-Y. Ho, Surf. Coat. Technol. 114 Ž1999. 109᎐113. w7x I.J. Smith, D. Gillibrand, J.S. Brooks, W.-D. Munz, ¨ S. Harvey, R. Goodwin, Surf. Coat. Technol. 90 Ž1997. 164᎐171. w8x P.J. Martin, R.P. Netterfield, T.J. Kinder, Thin Solid Films 193 Ž1990. 77. w9x Daimler᎐Benz Adhesion Test, Verein Deutscher Ingenieure ŽVDI.-Richlinie 3198 Ž1992. 7. w10x R.R. Aharonov, M. Chhowalla, S. Dhar, R.P. Fontana, Surf. Coat. Technol. 82 Ž1996. 334᎐343. w11x E. Erturk, ¨ H.J. Heuvel, H.G. Dederich, in: E. Broszeit, W.-D. Munz, ¨ H. Oechsner, R.T. Rie, G.K. Wolf ŽEds.., Plasma Surface Engineering, DGM, Informationsgesellschaft Verlag, Obserursel, Germany, 1988, p. 553. w12x W.-D. Munz, ¨ I.J. Smith, D.B. Lewis, S. Creasey, Vacuum 48 Ž5. Ž1997. 473᎐481. w13x W.D. Sproul, Surf. Coat. Technol. 81 Ž1996. 1᎐7. w14x M. Van Stappen, L.M. Stals, M. Kerkhofs, C. Quaeyhaegens, Surf. Coat. Technol. 74᎐75 Ž1995. 629᎐633. w15x H. Ronkainen, I. Nieminen, K. Holmberg et al., Surf. Coat. Technol. 49 Ž1991. 468᎐473. w16x B.F. Coll, P. Sathrum, R. Fontana, J.P. Peyre, D. Duchateau, M. Benmalek, Surf. Coat. Technol. 52 Ž1992. 57᎐64.