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Influence of chromium content on the dry machining performance of cathodic arc evaporated TiAlN coatings
Wear 254 (2003) 185–194
Influence of chromium content on the dry machining performance of cathodic arc evaporated TiAlN coatings S.G. Harris a,∗ , E.D. Doyle a , A.C. Vlasveld a , J. Audy a , J.M. Long b , D. Quick c a
c
Swinburne University of Technology, John Street Hawthorn, Vic., Australia b Deakin University, Pigdons Road, Geelong, Vic., Australia Ford Motor Company of Australia, Powertrain Operations, North Shore Road, Norlane, Vic., Australia Received 31 May 2002; received in revised form 4 October 2002; accepted 29 October 2002
Abstract Physical vapour deposition (PVD) titanium aluminium nitride coated cutting tools are used extensively in global manufacturing for reducing production costs and improving productivity in a number of aggressive metal-cutting operations, namely, dry and high-speed machining. In this investigation, the performance of Ti1−x Alx N and Ti1−x−y Alx Cry N coatings was assessed on Co-HSS twist drills used to machine grey cast iron. The failure criterion for drills was defined as a critical sized flank wear land at the outer corners of the drills. Using this criterion, the average tool life of uncoated twist drills was increased by factors of 2.5, 3.0 and 3.0 by Ti0.59 Al0.41 N, Ti0.27 Al0.19 Cr0.54 N and Ti0.21 Al0.14 Cr0.65 N coatings, respectively. Notwithstanding the similar increase in average tool life, the Ti1−x−y Alx Cry N coatings produced more consistent results than the Ti1−x Alx N coated drills with standard deviations of 67, 3 and 19 holes, respectively. This result has significant practical implications in manufacturing, since drills are not replaced on an individual basis, but rather on a preset tool change frequency. The present paper discusses the performance of Ti1−x Alx N and Ti1−x−y Alx Cry N coated drills in terms of average and practical drill life and concludes with remarks on the characterisation of PVD coatings and their significance on the performance of Co-HSS twist drills when dry machining grey cast iron. © 2002 Elsevier Science B.V. All rights reserved. Keywords: PVD; Cathodic arc evaporation; TiAlN; TiAlCrN; Dry machining; Wear
1. Introduction The use of hard wear resistant physical vapour deposition (PVD) coatings on cutting tools is now widespread in global manufacturing for reducing production costs and improving productivity, all of which is essential if industry is to remain economically competitive. Since the late 1980s, titanium aluminium nitride (TiAlN) PVD coatings have provided manufacturers with opportunities to improve cutting tool performance in aggressive machining operations [1]. Of particular interest to automotive manufacturers has been the potential to increase cutting speeds and feed rates when machining grey cast iron and aluminium–silicon alloys, which account for the majority of materials used in the manufacture of engine components for the popular car and truck markets [2]. The improved performance of TiAlN coated cutting tools is generally acknowledged to be due to the fact that this coating is able to maintain high hardness and resistance to oxidation at higher operating temperatures than tradi∗ Corresponding author. Tel.: +61-3-5250-1083; fax: +61-3-5250-1110. E-mail address: [email protected] (S.G. Harris).
tional PVD coatings, namely, titanium nitride (TiN) and titanium carbonitride (TiCN) [3]. The improved performance is a result of the formation of a well-adhered stable passive double oxide layer, which forms by outward diffusion of aluminium to form an Al-rich oxide layer at the topmost surface and inward diffusion of oxygen to form a Ti-rich oxide layer at the substrate interface [4–6]. This double oxide layer has been shown to reduce the rate of further oxidation by inhibiting oxygen diffusion to the underlying coating [7]. The success of PVD TiAlN coatings in cutting tool applications has prompted the recent development of more advanced quaternary nitride coatings, namely, TiAlVN, TiAlZrN, TiAlCN and TiAlCrN [8–11]. Of particular interest to automotive manufacturers, concerned with aggressive machining operations, has been the incorporation of chromium into TiAlN coatings. Additions of as much as 3 at.% chromium has been shown to improve the oxidation resistance of the coating up to 920 ◦ C, which represents a 50 ◦ C increase above the oxidation resistance of TiAlN (870 ◦ C) [5]. The increased oxidation resistance has been shown by Smith et al. [3] to improve the dry drilling performance of coated cutting tools used to machine grey cast iron.
0043-1648/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 2 ) 0 0 2 9 0 - 9
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In recent years, the growth of the PVD coating industry has seen an increase in the number of coatings available for commercial cutting tool applications. This increase has seen a trend toward coatings that are designed for specific applications where the grade of workpiece, the type of cutting tool and the operating parameters, namely, cutting speed, feed rate and levels of cutting fluid have an influence on the selection of the PVD coating. As such, an important part of any investigation into the performance of coated cutting tools is an understanding of the machinability of the work material, in particular, the interaction between the chip and cutting edge. The grey cast iron typically used for the manufacture of automotive components is characterised by a microstructure consisting of a continuous network of flake graphite embedded in a predominantly pearlitic matrix phase. Such a microstructure is specifically tailored to achieve the necessary tensile strength and wear resistance requirements of automotive engine blocks, while retaining a high degree of machinability. The free-machining characteristics of grey cast iron result from a randomly distributed network of soft flake graphite that interrupts the continuity of the metal matrix. With negligible strength and hardness, the graphite flakes provide planes of weakness throughout the cast iron that facilitate the formation of thin discontinuous chips during metal cutting. Since the chips are not continuous, the length of contact on the rake face is short and the cutting force and power consumption are low [12]. The presence of free graphite at the cutting edge also provides a source of self-lubrication which accounts for the generally good machinability of grey cast iron [13]. In this investigation, the dry machining performance of multilayer CrN/Ti1−x Alx N and CrN/Ti1−x−y Alx Cry N/ Ti1−x Alx N coatings were assessed when drilling into a workpiece of automotive grade grey cast iron. The coatings were deposited using a multi-source cathodic arc system comprising five vertically mounted cathodic arc sources. The CrN/Ti1−x Alx N and CrN/Ti1−x−y Alx Cry N/Ti1−x Alx N coatings were deposited using three Cr cathode targets and two Ti0.5 Al0.5 cathode targets. The objective of the present study was to compare the surface roughness, adhesion, hard-
ness, composition and elastic modulus of the coatings using a number of quantitative techniques. The results of cutting tool performance tests are reported for dry drilling automotive grade grey cast iron with 6.8 mm diameter cobalt high-speed steel (Co-HSS: M35) split point twist drills at a cutting speed of 38.5 m/min and a feed rate of 0.28 mm/rev.
2. Experimental procedure 2.1. Coating deposition A multi-source cathodic arc coating system was used in the present study to deposit multilayer CrN/Ti1−x Alx N and CrN/Ti1−x−y Alx Cry N/Ti1−z Alz N coatings. The coating chamber comprised of five vertically mounted cathodic arc sources (120 mm diameter) that surrounded a substrate holder configured in three-fold planetary rotation. The average distance between the arc sources and the substrate holder was 180 mm. The high-purity cathode targets used for the deposition of the coatings included three Cr targets (>99.9% purity) and two Ti0.5 Al0.5 targets (>99.9% purity). The position of these targets relative to the coating chamber is shown in Fig. 1. The substrate materials coated in the present study were mirror polished 30-mm diameter cobalt high-speed steel (Co-HSS: M35) coupons and 6.8-mm diameter Co-HSS (M35) split point twist drills. Prior to coating, the substrates were first cleaned using a water based cleaning line in which tools were washed in alkaline soap solution, while at the same time being subjected to ultrasonic and mechanical agitation. This was followed by a rinse in de-ionised water and drying at 110 ◦ C (383 K). Substrates were loaded into the coating chamber which was subsequently pumped down to a base pressure of 5 × 10−3 Pa and heated to 250 ◦ C (523 K). Introducing argon gas to a pressure of 5 Pa while biasing the substrates to a −1000 V potential resulted in the initiation of a glow discharge that was used to etch the argon ion substrates. Following argon ion etching, the negative substrate bias potential was reduced to −800 V and the
Fig. 1. A schematic diagram of the multi-cathode PVD system used for the deposition of Ti1−x Alx N and Ti1−x−y Alx Cry N coatings in the present study.
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Table 1 Process details for CAE PVD coatings Ti1−x Alx N
Ti1−x−y Alx Cry N (Cr arc current, 80 A) 10−3
Base pressure, 5 × Pa Temperature, 250 ◦ C Ar ion etch Time, 33 min Pressure, 5 Pa Substrate bias, −1000 V Pump down Pressure, 4 × 10−3 Pa CAE metal ion etch 1× Cr target Arc current, 80 A Time, 12 min Pressure, 8 × 10−1 Pa Substrate bias, −800 V Coating I, CrN 3× Cr targets Arc current, 80 A Time, 20 min Pressure, 8 × 10−1 Pa Substrate bias, −200 V Coating II, Ti1−x Alx N 2× Ti0.5 Al0.5 targets Ti0.5 Al0.5 arc current, 85 A Time, 60 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
10−3
Base pressure, 5 × Temperature, 250 ◦ C
Ti1−x−y Alx Cry N (Cr arc current, 120 A) Base pressure, 5 × 10−3 Pa Temperature, 250 ◦ C
Pa
Time, 33 min Pressure, 5 Pa Substrate bias, −1000 V
Time, 33 min Pressure, 5 Pa Substrate bias, −1000 V
Pressure, 4 × 10−3 Pa
Pressure, 4 × 10−3 Pa
1× Cr target Arc current, 80 A Time, 12 min Pressure, 8 × 10−1 Pa Substrate bias, −800 V
1× Cr target Arc current, 120 A Time, 12 min Pressure, 8 × 10−1 Pa Substrate bias, −800 V
3× Cr targets Arc current, 80 A Time, 20 min Pressure, 8 × 10−1 Pa Substrate bias, −200 V
3× Cr targets Arc current, 120 A Time, 20 min Pressure, 8 × 10−1 Pa Substrate bias, −200 V
3× Cr, 2× Ti0.5 Al0.5 targets Ti0.5 Al0.5 arc current, 85 A Cr arc current, 80 A Time, 45 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
3× Cr, 2× Ti0.5 Al0.5 targets Ti0.5 Al0.5 arc current, 85 A Cr arc current, 120 A Time, 45 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
2× Ti0.5 Al0.5 targets Arc current, 85 A Time, 15 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
2× Ti0.5 Al0.5 targets Arc current, 85 A Time, 15 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
Coating III, Ti1−z Alz N
substrates were subjected to a metal ion etch using Cr ions generated from three cathodic arc sources. A base coating of CrN was then deposited for a period of 20 min using three Cr cathodes and a substrate bias potential of −200 V. The latter was reduced to −100 V for the deposition of the Ti1−x Alx N and Ti1−x−y Alx Cry N coatings which were deposited from two Ti0.5 Al0.5 and three Cr cathode targets operating simultaneously. The composition of Cr in the Ti1−x−y Alx Cry N coatings was varied by depositing the coatings at different Cr arc currents, namely, 80 and 120 A. A top coating of Ti1−z Alz N was then deposited for 15 min at an arc current of 85 A and a substrate bias voltage of −100 V. A more detailed list of deposition process parameters is given in Table 1. 2.2. Evaluation of coating properties The PVD coatings were characterised using a number of quantitative and qualitative techniques which are described briefly in the following section. The surface roughness of mirror polished coupons was measured before and after coating deposition using a Mahr Perthometer Concept surface profilometer (Stylus: FRW-750, traverse length: 5.60 mm). Quantitative values of the elemental composi-
tion of the coatings were obtained as a function of coating depth using glow-discharge optical emission spectroscopy (GD-OES) using a Leco GDS-850A GD-OES spectrometer [14,15]. The coating–substrate adhesion was characterised using the Daimler–Benz indentation test [16]. The latter semi-quantitative test for coating substrate adhesion used a Rockwell C indenter to produce spherical indentations in coated coupons at a load of 500 N. The thickness of the coatings was measured by fracture metallography of cross-sections obtained by brittle fracture of coupons cooled to liquid nitrogen temperatures. Surface topography was observed using optical and scanning electron microscopy (SEM), which also enabled measurement of macroparticle and surface pitting characteristics, namely, the size, shape and distribution of these coating defects. The SEM was fitted with energy-dispersive X-ray spectroscopy (EDS) for compositional analysis of coated surfaces. 2.3. Cutting tool testing The dry machining performance of Ti1−x Alx N and Ti1−x−y Alx Cry N coated cutting tools was assessed when drilling a workpiece of automotive grade grey cast iron
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Fig. 2. Scanning electron micrographs of the grey cast iron workpiece showing (a) the distribution of flake graphite within the predominantly pearlitic matrix and (b) the presence of small amounts of free ferrite which can be seen adjacent to the graphite flakes.
Fig. 3. A schematic of a twist drill showing the method used to measure outer corner flank wear lands from a fixed reference point. Drills were deemed to have failed when the outer corner wear land reached 75% of the total margin width.
with 6.8-mm Co-HSS (M35) split point twist drills (refer to Table 2 for the workpiece and twist drill compositions). A scanning electron micrograph of the microstructure of the grey cast iron workpiece is shown in Fig. 2. The method used to evaluate the performance of the drills was to measure the progression of the outer corner flank wear lands using an optical microscope (25× magnification), and deem the drills to have failed when the wear land reached 75% of the total margin width (Fig. 3). This method of wear measurement is one not commonly reported in the literature, however, from a practical viewpoint it is a method particularly well suited to industry since it indicates directly the extent of drill wear and hence the appropriate time at which the drill should be re-sharpened [2]. The more common assessment of drill performance by screech failure, while being unambiguous, is of little practical value in indicating Table 2 Nominal workpiece and substrate compositions Grey cast iron workpiece Element wt.%
C 3.25
Si 1.95
Mn 0.7
Co-HSS substrate (M35) Element C Cr Mo wt.% 0.92 4.15 5.00
Cr 0.3
Cu 0.55
Sn 0.025
S 0.025
Ti 0.04
Fe Bal.
V 1.80
W 6.25
Co 4.75
Mn 0.30
Si 0.28
Fe Bal.
when drills need to be re-sharpened. Drill test conditions were chosen so that a practical tool life was achieved from both uncoated and coated drills, with wear patterns typical of those experienced at more conventional cutting speeds and feed rates. As such, a cutting speed of 38.5 m/min and a feed rate of 0.28 mm/rev were chosen for drilling blind holes without cutting fluid to a depth of 20.4 mm (3.0× diameter) in purpose-cast ingots of grey cast iron (500 mm × 290 mm × 50 mm).
3. Results and discussion 3.1. Glow-discharge optical emission spectroscopy (GD-OES) Quantitative values of the elemental composition of the Ti1−x Alx N and Ti1−x−y Alx Cry N coatings were obtained using a Leco GDS-850A GD-OES spectrometer. The results are given in Table 3. The atomic ratio between Ti and Al increased marginally with increasing Cr content, from 58.6 at.% Ti/41.4 at.% Al for the Ti0.59 Al0.41 N coatings to 59.9 at.% Ti/40.1 at.% Al for the Ti0.21 Al0.14 Cr0.65 N coatings (Fig. 4). Although this is a relatively small shift between coatings, it is a significant deviation from the 50.0 at.% Ti/50.0 at.% Al ratio of the Ti0.5 Al0.5 cathode targets. By
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Table 3 Mechanical properties of CAE PVD coatings Ti0.59 Al0.41 N
Ti0.27 Al0.19 Cr0.54 N
Ti0.21 Al0.14 Cr0.65 N
Thickness (m)
1.6
2.4
2.6
Roughness Ra a Rz(ISO) a R3z a Daimler–Benz adhesion ranking
0.10 1.47 0.75 1
0.14 1.78 1.08 1
0.15 1.88 1.10 1
Atomic percent of major elements, (at.%)b Ti Al Ti/Al ratio Cr N a b
22 15 58.6/41.4 – 63
12 8 59.5/40.5 24 56
9 6 59.9/40.1 29 55
Roughness of polished Co-HSS coupons prior to coating was 0.06 m Ra , 0.68 m Rz(ISO) and 0.49 m R3z . Elemental composition obtained by GD-OES.
Fig. 4. GD-OES depth profiles of the Ti0.27 Al0.19 Cr0.54 N (above) and Ti0.21 Al0.14 Cr0.65 N (below) coatings deposited with Cr arc currents of 80 and 120 A, respectively.
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applying a negative bias potential to the substrates during coating deposition, the ionised Ti, Cr and Al atoms were accelerated into the substrate causing re-sputtering of the growing film. The preferential loss of Al was a result of the atomic mass difference between Al (27 amu), Ti (48 amu) and Cr (52 amu) ions, which was sufficient to cause preferential sputtering of Al by the heavier Ti and Cr ions. It is evident from the GD-OES composition profiles (Fig. 4) that the coatings were over-stoichiometric and that the nitrogen content in the upper 0.4 m of the Ti1−x−y Alx Cry N coatings was as high as 65 at.%. It is suggested that the latter resulted from excess nitrogen in the coating chamber during the final stages of coating. During the deposition of the Ti1−x−y Alx Cry N coatings the consumption of nitrogen was highest when the two TiAl and three Cr arcs were active. When the Cr arcs were extinguished for the final 10 min of the coating cycle the consumption of nitrogen was reduced while the chamber pressure remained at 8 × 10−1 Pa. The result was an excess of nitrogen in the chamber which in turn resulted in an excess nitrogen in the surface region of the Ti1−x−y Alx Cry N coatings. The influence of nitrogen pressure on the stoichiometry of PVD coatings has yet to be established for the present operating configuration and, as such, forms part of ongoing research into the deposition of PVD coatings with various arc configurations. The presence of oxygen at the coating surface can be attributed to an oxide layer on the surface of the coatings.
In the present study, the roughness of mirror polished Co-HSS coupons increased from 0.68 to 1.47 m Rz(ISO) by the deposition of the Ti0.59 Al0.41 N coating. With a Cr arc current of 80 A, the Ti0.27 Al0.19 Cr0.54 N coatings increased the roughness of polished coupons to 1.78 m Rz(ISO) . An increase in the Cr arc current to 120 A (Ti0.21 Al0.14 Cr0.65 N coatings) resulted in an increase in roughness to 1.88 m Rz(ISO) . The 54% increase in surface roughness of polished coupons by Ti0.59 Al0.41 N coatings can be explained by a large number of macroparticles and pitting defects, which can be seen in SEM micrographs of the coating surface (Fig. 5a) and in surface roughness profiles (Fig. 6). The further increase in roughness by the incorporation of Cr into the coatings was assumed to be associated with an increase
3.2. Surface roughness characterisation The results of quantitative surface roughness tests on uncoated and PVD coated Co-HSS coupons are reported in Table 3, which shows an increase in roughness for the Ti1−x−y Alx Cry N coatings, compared with Ti1−x Alx N. Of the various parameters used for characterising the roughness of PVD coated surfaces, the arithmetic average roughness, Ra , is one of the most widely used in recent publications [17,18]. It is calculated as the area between the roughness profile and its mean line, or the integral of the absolute value of roughness profile height over the evaluation length. The main limitation of Ra is that it gives no indication of the surface texture, namely, macroparticles or pitting defects that can have a significant influence on the performance of the coating. A more indicative assessment of surface texture is given by Rz(ISO) . The latter is calculated from the average height of the five highest peaks plus the average depth of the five deepest valleys in a given profile. The affect on surface roughness by macroparticles and pitting defects can be further characterised using R3zi , which indicates the height from the third highest peak to the third lowest valley within one sample length. The average of the R3zi values over a given profile length is given by the parameter R3z , which has much the same purpose as Rz(ISO) except that less extreme peaks and valleys are measured.
Fig. 5. SEM micrographs of polished Co-HSS coupons coated with: (a) Ti0.59 Al0.41 N, (b) Ti0.27 Al0.19 Cr0.54 N and (c) Ti0.21 Al0.14 Cr0.65 N showing the increase in the size and number of macroparticles with increasing Cr content.
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Fig. 6. Surface profiles of CAE PVD coatings measured using a Mahr Perthometer surface profilometer showing the increase in the size and number of macroparticles with increasing Cr content.
in the number of Cr macroparticles and associated pitting defects (Fig. 5b and c). However, compositional analysis of the macroparticles using energy-dispersive spectroscopy showed the increase in surface roughness resulted from an increase in the size and number of TiAl macroparticles. It is therefore, suggested that the changes are due to changes in the nitrogen gas partial pressure brought about by the incorporation of Cr ions into the plasma stream. Increasing
the cation density of the plasma flux reduced the amount of reactive gas reaching the cathode surface, which led to a reduction in cathode target poisoning and an increase in the size and number of macroparticles emitted from the arc spot. This has been reported elsewhere [19,20] and was attributed to the formation of a thin nitride layer on the surface of the cathode which increases the melting temperature of the cathode surface.
Fig. 7. SEM micrographs of Rockwell ‘C’ indentations in Co-HSS coupons coated with (a and b) Ti0.27 Al0.19 Cr0.54 N and (c and d) Ti0.21 Al0.14 Cr0.65 N. Micrographs of the circumference regions of the indentations are shown in (a) and (c) at a magnification of 1500 times.
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Fig. 8. Outer corner wear of 6.8 mm diameter Co-HSS (M35) split point twist drills as a function of the number of holes drilled. Cutting speed: 38.5 m/min (1800 rpm), feed rate: 0.28 mm/rev, workpiece: grey cast iron, coolant: none.
3.3. Coating adhesion The results of Rockwell C indentation tests (Fig. 7) show no evidence of cracking or delamination of the coatings in the regions adjacent to the indentations produced using loads of 500 N. According to the Daimler–Benz indentation characterisation scale, the coatings showed excellent adhesion and subsequently received adhesion quality rankings of 1 (on the scale of 1–6). Even after increasing the indenter load to 1500 N the coatings continued to resist cracking and delamination. While this result indicates excellent coating ad-
hesion under the conditions of this test, the suitability of the Daimler–Benz indentation test for assessing the adhesion and/or the significance of adhesion in the performance of cutting tools in aggressive machining applications remains relatively unknown. 3.4. Cutting tool performance The results of cutting tool performance tests are shown in Figs. 8 and 9. The performance of the Ti0.59 Al0.41 N coated drills were in line with prior research in dry drilling grey cast
Fig. 9. Comparison of drill life between uncoated and Ti0.59 Al0.41 N, Ti0.27 Al0.19 Cr0.54 N and Ti0.21 Al0.14 Cr0.65 N coated 6.8 mm diameter Co-HSS (M35) split point twist drills when dry drilling grey cast iron. Cutting speed: 38.5 m/min (1800 rpm), feed rate: 0.28 mm/rev.
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iron using coated Co-HSS twist drills [2,21]. In the present study, the failure criterion for drills was defined as a critical sized (75%) flank wear land at the outer corners of the drills (refer to Fig. 3). This method of wear measurement is a practical technique particularly well suited to industry since it indicates directly the extent of drill wear and hence the appropriate time at which the drill should be re-sharpened. Using this criterion, the average tool life of uncoated twist drills was increased from 128 to 319 holes by the Ti0.59 Al0.41 N coating (Fig. 9). Notwithstanding this significant increase in performance, the scatter in the results for the coated drills was substantial and resulted in a standard deviation of 29 holes. In the case of the Ti1−x−y Alx Cry N coated drills, the average tool life of uncoated drills was increased to 376 and 371 holes, respectively from Ti0.27 Al0.19 Cr0.54 N and Ti0.21 Al0.14 Cr0.65 N coatings. In contrast to the Ti0.59 Al0.41 N coated drills, the corresponding standard deviations were 3 and 19 holes, respectively. From a practical viewpoint this is a significant result since, in manufacturing, drills are removed from production after a preset tool life, which is determined from the average tool life less a factor based on the statistical variability in results. The 95% confidence rule, which predicts 95% of results lie within two standard deviations from the empirically determined mean, can be used to define a practical tool life, namely, the mean tool life minus two times the standard deviation (Fig. 9). Notwithstanding an average tool life of 319 holes, the variability in results for the Ti0.59 Al0.41 N coated drills restricted the practical tool life to 185 holes (58% of the average tool life). In contrast, the low scatter in the results for the Ti0.27 Al0.19 Cr0.54 N coated drills allowed these drills to be used with a practical tool life of 369 holes, which represented 98% of the average tool life. Not only is this a significant increase in the practical tool life of coated drills, but it significantly reduces the amount of ‘wasted’ tool life which is lost from drills removed prior to reaching their maximum potential tool life.
4. Concluding remarks In the present investigation, a number of physical properties of Ti1−x Alx N and Ti1−x−y Alx Cry N coatings were characterised with respect to adhesion, roughness and composition, and their performance in dry machining of grey cast iron was assessed when drilling with Co-HSS twist drills. The high chromium containing Ti1−x−y Alx Cry N coatings were deposited using a multi-source cathodic arc system with a method of substrate rotation which was successful at depositing coatings which showed excellent adhesion. In terms of roughness, it was evident that coating roughness increased with increasing levels of chromium which, in the present study, was attributed to changes to the nitrogen gas partial pressure. Notwithstanding the increase in roughness, the average tool life was improved for the
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Ti1−x−y Alx Cry N coated twist drills. The improved performance of the Ti1−x−y Alx Cry N coated drills compared with the Ti0.59 Al0.41 N coated drills was due, in part, to the improved oxidation resistance of the Ti1−x−y Alx Cry N coatings and partly due to the multilayered structure in the chromium containing coatings. The addition of chromium also had implications for the consistency of drill test results, with standard deviations of 67, 3 and 19 holes, respectively for the Ti0.59 Al0.41 N, Ti0.27 Al0.19 Cr0.54 N and Ti0.21 Al0.14 Cr0.65 N coated twist drills. This result has significant practical implications in manufacturing, since drills are not replaced on an individual basis, but rather on a preset tool change frequency. Improving the consistency in drill performance allows manufacturers to utilise a higher proportion of drill life and reduce ‘wasted’ drill life of drills removed from production prior to reaching their maximum potential tool life.
Acknowledgements The authors wish to thank the Ford Motor Company of Australia for its support of the ongoing research and to thank Surface Technology Coatings and Sutton Tools for providing surface coatings and test facilities. Also the research work was partially carried out at CSIRO, Australia with the assistance of Mr Stefan Gulizia. References [1] W.-D. Münz, J. Vac. Sci. Technol. A 4 (1986) 2717. [2] S.G. Harris, E.D. Doyle, A.C. Vlasveld, P.J. Dolder, Surf. Coat. Technol. 133–134 (2000) 383–388. [3] I.J. Smith, D. Gillibrand, J.S. Brooks, W.-D. Münz, Surf. Coat. Technol. 90 (1997) 164–171. [4] D. McIntyre, J.E. Greene, G. Håkansson, J.-E. Sundgren, W.-D. Münz, J. Appl. Phys. 67 (1990) 1542. [5] L.A. Donohue, I.J. Smith, W.-D. Münz, I. Petrov, J.E. Greene, Surf. Coat. Technol. 94–95 (1997) 226–231. [6] L. Hultman, Vacuum 57 (2000) 1–30. [7] D.-Y. Wang, Y.-W. Li, C.-L. Chang, W.-Y. Ho, Surf. Coat. Technol. 114 (1999) 109–113. [8] O. Knotek, W.-D. Münz, J. Vac. Sci. Technol. A 5 (4) (1987) 2173. [9] L.A. Donohue, J. Cawley, J.S. Brooks, W.-D. Münz, Surf. Coat. Technol. 74–75 (1995) 123–134. [10] I.J. Smith, W.-D. Münz, L.A. Donohue, I. Petrov, J.E. Greene, Surf. Eng. 14 (1) (1998) 37–41. [11] E. Pflüger, A. Schröer, P. Voumard, L.A. Donohue, W.-D. Münz, Surf. Coat. Technol. 115 (1999) 17–23. [12] R.O. Marwanga, R.C. Voigt, P.H. Cohen, AFS Trans., vol. 107, Paper 99–80, 1999, pp. 595–607. [13] J.R. Davis, ASM Specialty Handbook: Cast Irons, vol. 494, Materials Park, OH, 1996. [14] J.M. Long, in: Proceedings of the 10th Asia–Pacific Conference on Non-Destructive Testing, Brisbane, Australia, 17–21 September 2001, (R. Diederichs, NDTnet: http://www.ndt.net/article/apcndt01/). [15] J.M. Long, in: E. Pereloma, K. Raviprasad (Eds.), Proceedings of Engineering Materials, Melbourne, 23–26 September 2001, Institute of Materials Engineering, Australia, pp. 87–92.
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[16] Daimler-Benz Adhesion Test, Verein Deutscher Ingenieure (VDI)— Richlinie 3198, 1992, p. 7. [17] C.J. Tavares, L. Rebouta, E. Alves, A. Cavaleiro, P. Goudeau, J.P. Riviere, A. Declemy, Thin Solid Films 377–378 (2000) 425–429. [18] P.E. Hovsepian, B.D. Lewis, W.-D. Münz, A. Rouzaud, P. Juliet, Surf. Coat. Technol. 116–119 (1999) 727–734.
[19] H.-D. Steffens, M. Mack, K. Moehwald, K. Reichel, Surf. Coat. Technol. 46 (1991) 65–74. [20] M. Ives, J.S. Brooks, J. Cawley, W. Burgmer, Surf. Coat. Technol. 49 (1991) 244–252. [21] S.G. Harris, E.D. Doyle, A.C. Vlasveld, P.J. Dolder, Surf. Coat. Technol. 146–147 (2001) 305–311.
Influence of chromium content on the dry machining performance of cathodic arc evaporated TiAlN coatings S.G. Harris a,∗ , E.D. Doyle a , A.C. Vlasveld a , J. Audy a , J.M. Long b , D. Quick c a
c
Swinburne University of Technology, John Street Hawthorn, Vic., Australia b Deakin University, Pigdons Road, Geelong, Vic., Australia Ford Motor Company of Australia, Powertrain Operations, North Shore Road, Norlane, Vic., Australia Received 31 May 2002; received in revised form 4 October 2002; accepted 29 October 2002
Abstract Physical vapour deposition (PVD) titanium aluminium nitride coated cutting tools are used extensively in global manufacturing for reducing production costs and improving productivity in a number of aggressive metal-cutting operations, namely, dry and high-speed machining. In this investigation, the performance of Ti1−x Alx N and Ti1−x−y Alx Cry N coatings was assessed on Co-HSS twist drills used to machine grey cast iron. The failure criterion for drills was defined as a critical sized flank wear land at the outer corners of the drills. Using this criterion, the average tool life of uncoated twist drills was increased by factors of 2.5, 3.0 and 3.0 by Ti0.59 Al0.41 N, Ti0.27 Al0.19 Cr0.54 N and Ti0.21 Al0.14 Cr0.65 N coatings, respectively. Notwithstanding the similar increase in average tool life, the Ti1−x−y Alx Cry N coatings produced more consistent results than the Ti1−x Alx N coated drills with standard deviations of 67, 3 and 19 holes, respectively. This result has significant practical implications in manufacturing, since drills are not replaced on an individual basis, but rather on a preset tool change frequency. The present paper discusses the performance of Ti1−x Alx N and Ti1−x−y Alx Cry N coated drills in terms of average and practical drill life and concludes with remarks on the characterisation of PVD coatings and their significance on the performance of Co-HSS twist drills when dry machining grey cast iron. © 2002 Elsevier Science B.V. All rights reserved. Keywords: PVD; Cathodic arc evaporation; TiAlN; TiAlCrN; Dry machining; Wear
1. Introduction The use of hard wear resistant physical vapour deposition (PVD) coatings on cutting tools is now widespread in global manufacturing for reducing production costs and improving productivity, all of which is essential if industry is to remain economically competitive. Since the late 1980s, titanium aluminium nitride (TiAlN) PVD coatings have provided manufacturers with opportunities to improve cutting tool performance in aggressive machining operations [1]. Of particular interest to automotive manufacturers has been the potential to increase cutting speeds and feed rates when machining grey cast iron and aluminium–silicon alloys, which account for the majority of materials used in the manufacture of engine components for the popular car and truck markets [2]. The improved performance of TiAlN coated cutting tools is generally acknowledged to be due to the fact that this coating is able to maintain high hardness and resistance to oxidation at higher operating temperatures than tradi∗ Corresponding author. Tel.: +61-3-5250-1083; fax: +61-3-5250-1110. E-mail address: [email protected] (S.G. Harris).
tional PVD coatings, namely, titanium nitride (TiN) and titanium carbonitride (TiCN) [3]. The improved performance is a result of the formation of a well-adhered stable passive double oxide layer, which forms by outward diffusion of aluminium to form an Al-rich oxide layer at the topmost surface and inward diffusion of oxygen to form a Ti-rich oxide layer at the substrate interface [4–6]. This double oxide layer has been shown to reduce the rate of further oxidation by inhibiting oxygen diffusion to the underlying coating [7]. The success of PVD TiAlN coatings in cutting tool applications has prompted the recent development of more advanced quaternary nitride coatings, namely, TiAlVN, TiAlZrN, TiAlCN and TiAlCrN [8–11]. Of particular interest to automotive manufacturers, concerned with aggressive machining operations, has been the incorporation of chromium into TiAlN coatings. Additions of as much as 3 at.% chromium has been shown to improve the oxidation resistance of the coating up to 920 ◦ C, which represents a 50 ◦ C increase above the oxidation resistance of TiAlN (870 ◦ C) [5]. The increased oxidation resistance has been shown by Smith et al. [3] to improve the dry drilling performance of coated cutting tools used to machine grey cast iron.
0043-1648/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 2 ) 0 0 2 9 0 - 9
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In recent years, the growth of the PVD coating industry has seen an increase in the number of coatings available for commercial cutting tool applications. This increase has seen a trend toward coatings that are designed for specific applications where the grade of workpiece, the type of cutting tool and the operating parameters, namely, cutting speed, feed rate and levels of cutting fluid have an influence on the selection of the PVD coating. As such, an important part of any investigation into the performance of coated cutting tools is an understanding of the machinability of the work material, in particular, the interaction between the chip and cutting edge. The grey cast iron typically used for the manufacture of automotive components is characterised by a microstructure consisting of a continuous network of flake graphite embedded in a predominantly pearlitic matrix phase. Such a microstructure is specifically tailored to achieve the necessary tensile strength and wear resistance requirements of automotive engine blocks, while retaining a high degree of machinability. The free-machining characteristics of grey cast iron result from a randomly distributed network of soft flake graphite that interrupts the continuity of the metal matrix. With negligible strength and hardness, the graphite flakes provide planes of weakness throughout the cast iron that facilitate the formation of thin discontinuous chips during metal cutting. Since the chips are not continuous, the length of contact on the rake face is short and the cutting force and power consumption are low [12]. The presence of free graphite at the cutting edge also provides a source of self-lubrication which accounts for the generally good machinability of grey cast iron [13]. In this investigation, the dry machining performance of multilayer CrN/Ti1−x Alx N and CrN/Ti1−x−y Alx Cry N/ Ti1−x Alx N coatings were assessed when drilling into a workpiece of automotive grade grey cast iron. The coatings were deposited using a multi-source cathodic arc system comprising five vertically mounted cathodic arc sources. The CrN/Ti1−x Alx N and CrN/Ti1−x−y Alx Cry N/Ti1−x Alx N coatings were deposited using three Cr cathode targets and two Ti0.5 Al0.5 cathode targets. The objective of the present study was to compare the surface roughness, adhesion, hard-
ness, composition and elastic modulus of the coatings using a number of quantitative techniques. The results of cutting tool performance tests are reported for dry drilling automotive grade grey cast iron with 6.8 mm diameter cobalt high-speed steel (Co-HSS: M35) split point twist drills at a cutting speed of 38.5 m/min and a feed rate of 0.28 mm/rev.
2. Experimental procedure 2.1. Coating deposition A multi-source cathodic arc coating system was used in the present study to deposit multilayer CrN/Ti1−x Alx N and CrN/Ti1−x−y Alx Cry N/Ti1−z Alz N coatings. The coating chamber comprised of five vertically mounted cathodic arc sources (120 mm diameter) that surrounded a substrate holder configured in three-fold planetary rotation. The average distance between the arc sources and the substrate holder was 180 mm. The high-purity cathode targets used for the deposition of the coatings included three Cr targets (>99.9% purity) and two Ti0.5 Al0.5 targets (>99.9% purity). The position of these targets relative to the coating chamber is shown in Fig. 1. The substrate materials coated in the present study were mirror polished 30-mm diameter cobalt high-speed steel (Co-HSS: M35) coupons and 6.8-mm diameter Co-HSS (M35) split point twist drills. Prior to coating, the substrates were first cleaned using a water based cleaning line in which tools were washed in alkaline soap solution, while at the same time being subjected to ultrasonic and mechanical agitation. This was followed by a rinse in de-ionised water and drying at 110 ◦ C (383 K). Substrates were loaded into the coating chamber which was subsequently pumped down to a base pressure of 5 × 10−3 Pa and heated to 250 ◦ C (523 K). Introducing argon gas to a pressure of 5 Pa while biasing the substrates to a −1000 V potential resulted in the initiation of a glow discharge that was used to etch the argon ion substrates. Following argon ion etching, the negative substrate bias potential was reduced to −800 V and the
Fig. 1. A schematic diagram of the multi-cathode PVD system used for the deposition of Ti1−x Alx N and Ti1−x−y Alx Cry N coatings in the present study.
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Table 1 Process details for CAE PVD coatings Ti1−x Alx N
Ti1−x−y Alx Cry N (Cr arc current, 80 A) 10−3
Base pressure, 5 × Pa Temperature, 250 ◦ C Ar ion etch Time, 33 min Pressure, 5 Pa Substrate bias, −1000 V Pump down Pressure, 4 × 10−3 Pa CAE metal ion etch 1× Cr target Arc current, 80 A Time, 12 min Pressure, 8 × 10−1 Pa Substrate bias, −800 V Coating I, CrN 3× Cr targets Arc current, 80 A Time, 20 min Pressure, 8 × 10−1 Pa Substrate bias, −200 V Coating II, Ti1−x Alx N 2× Ti0.5 Al0.5 targets Ti0.5 Al0.5 arc current, 85 A Time, 60 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
10−3
Base pressure, 5 × Temperature, 250 ◦ C
Ti1−x−y Alx Cry N (Cr arc current, 120 A) Base pressure, 5 × 10−3 Pa Temperature, 250 ◦ C
Pa
Time, 33 min Pressure, 5 Pa Substrate bias, −1000 V
Time, 33 min Pressure, 5 Pa Substrate bias, −1000 V
Pressure, 4 × 10−3 Pa
Pressure, 4 × 10−3 Pa
1× Cr target Arc current, 80 A Time, 12 min Pressure, 8 × 10−1 Pa Substrate bias, −800 V
1× Cr target Arc current, 120 A Time, 12 min Pressure, 8 × 10−1 Pa Substrate bias, −800 V
3× Cr targets Arc current, 80 A Time, 20 min Pressure, 8 × 10−1 Pa Substrate bias, −200 V
3× Cr targets Arc current, 120 A Time, 20 min Pressure, 8 × 10−1 Pa Substrate bias, −200 V
3× Cr, 2× Ti0.5 Al0.5 targets Ti0.5 Al0.5 arc current, 85 A Cr arc current, 80 A Time, 45 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
3× Cr, 2× Ti0.5 Al0.5 targets Ti0.5 Al0.5 arc current, 85 A Cr arc current, 120 A Time, 45 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
2× Ti0.5 Al0.5 targets Arc current, 85 A Time, 15 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
2× Ti0.5 Al0.5 targets Arc current, 85 A Time, 15 min Pressure, 8 × 10−1 Pa Substrate bias, −100 V
Coating III, Ti1−z Alz N
substrates were subjected to a metal ion etch using Cr ions generated from three cathodic arc sources. A base coating of CrN was then deposited for a period of 20 min using three Cr cathodes and a substrate bias potential of −200 V. The latter was reduced to −100 V for the deposition of the Ti1−x Alx N and Ti1−x−y Alx Cry N coatings which were deposited from two Ti0.5 Al0.5 and three Cr cathode targets operating simultaneously. The composition of Cr in the Ti1−x−y Alx Cry N coatings was varied by depositing the coatings at different Cr arc currents, namely, 80 and 120 A. A top coating of Ti1−z Alz N was then deposited for 15 min at an arc current of 85 A and a substrate bias voltage of −100 V. A more detailed list of deposition process parameters is given in Table 1. 2.2. Evaluation of coating properties The PVD coatings were characterised using a number of quantitative and qualitative techniques which are described briefly in the following section. The surface roughness of mirror polished coupons was measured before and after coating deposition using a Mahr Perthometer Concept surface profilometer (Stylus: FRW-750, traverse length: 5.60 mm). Quantitative values of the elemental composi-
tion of the coatings were obtained as a function of coating depth using glow-discharge optical emission spectroscopy (GD-OES) using a Leco GDS-850A GD-OES spectrometer [14,15]. The coating–substrate adhesion was characterised using the Daimler–Benz indentation test [16]. The latter semi-quantitative test for coating substrate adhesion used a Rockwell C indenter to produce spherical indentations in coated coupons at a load of 500 N. The thickness of the coatings was measured by fracture metallography of cross-sections obtained by brittle fracture of coupons cooled to liquid nitrogen temperatures. Surface topography was observed using optical and scanning electron microscopy (SEM), which also enabled measurement of macroparticle and surface pitting characteristics, namely, the size, shape and distribution of these coating defects. The SEM was fitted with energy-dispersive X-ray spectroscopy (EDS) for compositional analysis of coated surfaces. 2.3. Cutting tool testing The dry machining performance of Ti1−x Alx N and Ti1−x−y Alx Cry N coated cutting tools was assessed when drilling a workpiece of automotive grade grey cast iron
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Fig. 2. Scanning electron micrographs of the grey cast iron workpiece showing (a) the distribution of flake graphite within the predominantly pearlitic matrix and (b) the presence of small amounts of free ferrite which can be seen adjacent to the graphite flakes.
Fig. 3. A schematic of a twist drill showing the method used to measure outer corner flank wear lands from a fixed reference point. Drills were deemed to have failed when the outer corner wear land reached 75% of the total margin width.
with 6.8-mm Co-HSS (M35) split point twist drills (refer to Table 2 for the workpiece and twist drill compositions). A scanning electron micrograph of the microstructure of the grey cast iron workpiece is shown in Fig. 2. The method used to evaluate the performance of the drills was to measure the progression of the outer corner flank wear lands using an optical microscope (25× magnification), and deem the drills to have failed when the wear land reached 75% of the total margin width (Fig. 3). This method of wear measurement is one not commonly reported in the literature, however, from a practical viewpoint it is a method particularly well suited to industry since it indicates directly the extent of drill wear and hence the appropriate time at which the drill should be re-sharpened [2]. The more common assessment of drill performance by screech failure, while being unambiguous, is of little practical value in indicating Table 2 Nominal workpiece and substrate compositions Grey cast iron workpiece Element wt.%
C 3.25
Si 1.95
Mn 0.7
Co-HSS substrate (M35) Element C Cr Mo wt.% 0.92 4.15 5.00
Cr 0.3
Cu 0.55
Sn 0.025
S 0.025
Ti 0.04
Fe Bal.
V 1.80
W 6.25
Co 4.75
Mn 0.30
Si 0.28
Fe Bal.
when drills need to be re-sharpened. Drill test conditions were chosen so that a practical tool life was achieved from both uncoated and coated drills, with wear patterns typical of those experienced at more conventional cutting speeds and feed rates. As such, a cutting speed of 38.5 m/min and a feed rate of 0.28 mm/rev were chosen for drilling blind holes without cutting fluid to a depth of 20.4 mm (3.0× diameter) in purpose-cast ingots of grey cast iron (500 mm × 290 mm × 50 mm).
3. Results and discussion 3.1. Glow-discharge optical emission spectroscopy (GD-OES) Quantitative values of the elemental composition of the Ti1−x Alx N and Ti1−x−y Alx Cry N coatings were obtained using a Leco GDS-850A GD-OES spectrometer. The results are given in Table 3. The atomic ratio between Ti and Al increased marginally with increasing Cr content, from 58.6 at.% Ti/41.4 at.% Al for the Ti0.59 Al0.41 N coatings to 59.9 at.% Ti/40.1 at.% Al for the Ti0.21 Al0.14 Cr0.65 N coatings (Fig. 4). Although this is a relatively small shift between coatings, it is a significant deviation from the 50.0 at.% Ti/50.0 at.% Al ratio of the Ti0.5 Al0.5 cathode targets. By
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Table 3 Mechanical properties of CAE PVD coatings Ti0.59 Al0.41 N
Ti0.27 Al0.19 Cr0.54 N
Ti0.21 Al0.14 Cr0.65 N
Thickness (m)
1.6
2.4
2.6
Roughness Ra a Rz(ISO) a R3z a Daimler–Benz adhesion ranking
0.10 1.47 0.75 1
0.14 1.78 1.08 1
0.15 1.88 1.10 1
Atomic percent of major elements, (at.%)b Ti Al Ti/Al ratio Cr N a b
22 15 58.6/41.4 – 63
12 8 59.5/40.5 24 56
9 6 59.9/40.1 29 55
Roughness of polished Co-HSS coupons prior to coating was 0.06 m Ra , 0.68 m Rz(ISO) and 0.49 m R3z . Elemental composition obtained by GD-OES.
Fig. 4. GD-OES depth profiles of the Ti0.27 Al0.19 Cr0.54 N (above) and Ti0.21 Al0.14 Cr0.65 N (below) coatings deposited with Cr arc currents of 80 and 120 A, respectively.
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applying a negative bias potential to the substrates during coating deposition, the ionised Ti, Cr and Al atoms were accelerated into the substrate causing re-sputtering of the growing film. The preferential loss of Al was a result of the atomic mass difference between Al (27 amu), Ti (48 amu) and Cr (52 amu) ions, which was sufficient to cause preferential sputtering of Al by the heavier Ti and Cr ions. It is evident from the GD-OES composition profiles (Fig. 4) that the coatings were over-stoichiometric and that the nitrogen content in the upper 0.4 m of the Ti1−x−y Alx Cry N coatings was as high as 65 at.%. It is suggested that the latter resulted from excess nitrogen in the coating chamber during the final stages of coating. During the deposition of the Ti1−x−y Alx Cry N coatings the consumption of nitrogen was highest when the two TiAl and three Cr arcs were active. When the Cr arcs were extinguished for the final 10 min of the coating cycle the consumption of nitrogen was reduced while the chamber pressure remained at 8 × 10−1 Pa. The result was an excess of nitrogen in the chamber which in turn resulted in an excess nitrogen in the surface region of the Ti1−x−y Alx Cry N coatings. The influence of nitrogen pressure on the stoichiometry of PVD coatings has yet to be established for the present operating configuration and, as such, forms part of ongoing research into the deposition of PVD coatings with various arc configurations. The presence of oxygen at the coating surface can be attributed to an oxide layer on the surface of the coatings.
In the present study, the roughness of mirror polished Co-HSS coupons increased from 0.68 to 1.47 m Rz(ISO) by the deposition of the Ti0.59 Al0.41 N coating. With a Cr arc current of 80 A, the Ti0.27 Al0.19 Cr0.54 N coatings increased the roughness of polished coupons to 1.78 m Rz(ISO) . An increase in the Cr arc current to 120 A (Ti0.21 Al0.14 Cr0.65 N coatings) resulted in an increase in roughness to 1.88 m Rz(ISO) . The 54% increase in surface roughness of polished coupons by Ti0.59 Al0.41 N coatings can be explained by a large number of macroparticles and pitting defects, which can be seen in SEM micrographs of the coating surface (Fig. 5a) and in surface roughness profiles (Fig. 6). The further increase in roughness by the incorporation of Cr into the coatings was assumed to be associated with an increase
3.2. Surface roughness characterisation The results of quantitative surface roughness tests on uncoated and PVD coated Co-HSS coupons are reported in Table 3, which shows an increase in roughness for the Ti1−x−y Alx Cry N coatings, compared with Ti1−x Alx N. Of the various parameters used for characterising the roughness of PVD coated surfaces, the arithmetic average roughness, Ra , is one of the most widely used in recent publications [17,18]. It is calculated as the area between the roughness profile and its mean line, or the integral of the absolute value of roughness profile height over the evaluation length. The main limitation of Ra is that it gives no indication of the surface texture, namely, macroparticles or pitting defects that can have a significant influence on the performance of the coating. A more indicative assessment of surface texture is given by Rz(ISO) . The latter is calculated from the average height of the five highest peaks plus the average depth of the five deepest valleys in a given profile. The affect on surface roughness by macroparticles and pitting defects can be further characterised using R3zi , which indicates the height from the third highest peak to the third lowest valley within one sample length. The average of the R3zi values over a given profile length is given by the parameter R3z , which has much the same purpose as Rz(ISO) except that less extreme peaks and valleys are measured.
Fig. 5. SEM micrographs of polished Co-HSS coupons coated with: (a) Ti0.59 Al0.41 N, (b) Ti0.27 Al0.19 Cr0.54 N and (c) Ti0.21 Al0.14 Cr0.65 N showing the increase in the size and number of macroparticles with increasing Cr content.
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Fig. 6. Surface profiles of CAE PVD coatings measured using a Mahr Perthometer surface profilometer showing the increase in the size and number of macroparticles with increasing Cr content.
in the number of Cr macroparticles and associated pitting defects (Fig. 5b and c). However, compositional analysis of the macroparticles using energy-dispersive spectroscopy showed the increase in surface roughness resulted from an increase in the size and number of TiAl macroparticles. It is therefore, suggested that the changes are due to changes in the nitrogen gas partial pressure brought about by the incorporation of Cr ions into the plasma stream. Increasing
the cation density of the plasma flux reduced the amount of reactive gas reaching the cathode surface, which led to a reduction in cathode target poisoning and an increase in the size and number of macroparticles emitted from the arc spot. This has been reported elsewhere [19,20] and was attributed to the formation of a thin nitride layer on the surface of the cathode which increases the melting temperature of the cathode surface.
Fig. 7. SEM micrographs of Rockwell ‘C’ indentations in Co-HSS coupons coated with (a and b) Ti0.27 Al0.19 Cr0.54 N and (c and d) Ti0.21 Al0.14 Cr0.65 N. Micrographs of the circumference regions of the indentations are shown in (a) and (c) at a magnification of 1500 times.
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Fig. 8. Outer corner wear of 6.8 mm diameter Co-HSS (M35) split point twist drills as a function of the number of holes drilled. Cutting speed: 38.5 m/min (1800 rpm), feed rate: 0.28 mm/rev, workpiece: grey cast iron, coolant: none.
3.3. Coating adhesion The results of Rockwell C indentation tests (Fig. 7) show no evidence of cracking or delamination of the coatings in the regions adjacent to the indentations produced using loads of 500 N. According to the Daimler–Benz indentation characterisation scale, the coatings showed excellent adhesion and subsequently received adhesion quality rankings of 1 (on the scale of 1–6). Even after increasing the indenter load to 1500 N the coatings continued to resist cracking and delamination. While this result indicates excellent coating ad-
hesion under the conditions of this test, the suitability of the Daimler–Benz indentation test for assessing the adhesion and/or the significance of adhesion in the performance of cutting tools in aggressive machining applications remains relatively unknown. 3.4. Cutting tool performance The results of cutting tool performance tests are shown in Figs. 8 and 9. The performance of the Ti0.59 Al0.41 N coated drills were in line with prior research in dry drilling grey cast
Fig. 9. Comparison of drill life between uncoated and Ti0.59 Al0.41 N, Ti0.27 Al0.19 Cr0.54 N and Ti0.21 Al0.14 Cr0.65 N coated 6.8 mm diameter Co-HSS (M35) split point twist drills when dry drilling grey cast iron. Cutting speed: 38.5 m/min (1800 rpm), feed rate: 0.28 mm/rev.
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iron using coated Co-HSS twist drills [2,21]. In the present study, the failure criterion for drills was defined as a critical sized (75%) flank wear land at the outer corners of the drills (refer to Fig. 3). This method of wear measurement is a practical technique particularly well suited to industry since it indicates directly the extent of drill wear and hence the appropriate time at which the drill should be re-sharpened. Using this criterion, the average tool life of uncoated twist drills was increased from 128 to 319 holes by the Ti0.59 Al0.41 N coating (Fig. 9). Notwithstanding this significant increase in performance, the scatter in the results for the coated drills was substantial and resulted in a standard deviation of 29 holes. In the case of the Ti1−x−y Alx Cry N coated drills, the average tool life of uncoated drills was increased to 376 and 371 holes, respectively from Ti0.27 Al0.19 Cr0.54 N and Ti0.21 Al0.14 Cr0.65 N coatings. In contrast to the Ti0.59 Al0.41 N coated drills, the corresponding standard deviations were 3 and 19 holes, respectively. From a practical viewpoint this is a significant result since, in manufacturing, drills are removed from production after a preset tool life, which is determined from the average tool life less a factor based on the statistical variability in results. The 95% confidence rule, which predicts 95% of results lie within two standard deviations from the empirically determined mean, can be used to define a practical tool life, namely, the mean tool life minus two times the standard deviation (Fig. 9). Notwithstanding an average tool life of 319 holes, the variability in results for the Ti0.59 Al0.41 N coated drills restricted the practical tool life to 185 holes (58% of the average tool life). In contrast, the low scatter in the results for the Ti0.27 Al0.19 Cr0.54 N coated drills allowed these drills to be used with a practical tool life of 369 holes, which represented 98% of the average tool life. Not only is this a significant increase in the practical tool life of coated drills, but it significantly reduces the amount of ‘wasted’ tool life which is lost from drills removed prior to reaching their maximum potential tool life.
4. Concluding remarks In the present investigation, a number of physical properties of Ti1−x Alx N and Ti1−x−y Alx Cry N coatings were characterised with respect to adhesion, roughness and composition, and their performance in dry machining of grey cast iron was assessed when drilling with Co-HSS twist drills. The high chromium containing Ti1−x−y Alx Cry N coatings were deposited using a multi-source cathodic arc system with a method of substrate rotation which was successful at depositing coatings which showed excellent adhesion. In terms of roughness, it was evident that coating roughness increased with increasing levels of chromium which, in the present study, was attributed to changes to the nitrogen gas partial pressure. Notwithstanding the increase in roughness, the average tool life was improved for the
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Ti1−x−y Alx Cry N coated twist drills. The improved performance of the Ti1−x−y Alx Cry N coated drills compared with the Ti0.59 Al0.41 N coated drills was due, in part, to the improved oxidation resistance of the Ti1−x−y Alx Cry N coatings and partly due to the multilayered structure in the chromium containing coatings. The addition of chromium also had implications for the consistency of drill test results, with standard deviations of 67, 3 and 19 holes, respectively for the Ti0.59 Al0.41 N, Ti0.27 Al0.19 Cr0.54 N and Ti0.21 Al0.14 Cr0.65 N coated twist drills. This result has significant practical implications in manufacturing, since drills are not replaced on an individual basis, but rather on a preset tool change frequency. Improving the consistency in drill performance allows manufacturers to utilise a higher proportion of drill life and reduce ‘wasted’ drill life of drills removed from production prior to reaching their maximum potential tool life.
Acknowledgements The authors wish to thank the Ford Motor Company of Australia for its support of the ongoing research and to thank Surface Technology Coatings and Sutton Tools for providing surface coatings and test facilities. Also the research work was partially carried out at CSIRO, Australia with the assistance of Mr Stefan Gulizia. References [1] W.-D. Münz, J. Vac. Sci. Technol. A 4 (1986) 2717. [2] S.G. Harris, E.D. Doyle, A.C. Vlasveld, P.J. Dolder, Surf. Coat. Technol. 133–134 (2000) 383–388. [3] I.J. Smith, D. Gillibrand, J.S. Brooks, W.-D. Münz, Surf. Coat. Technol. 90 (1997) 164–171. [4] D. McIntyre, J.E. Greene, G. Håkansson, J.-E. Sundgren, W.-D. Münz, J. Appl. Phys. 67 (1990) 1542. [5] L.A. Donohue, I.J. Smith, W.-D. Münz, I. Petrov, J.E. Greene, Surf. Coat. Technol. 94–95 (1997) 226–231. [6] L. Hultman, Vacuum 57 (2000) 1–30. [7] D.-Y. Wang, Y.-W. Li, C.-L. Chang, W.-Y. Ho, Surf. Coat. Technol. 114 (1999) 109–113. [8] O. Knotek, W.-D. Münz, J. Vac. Sci. Technol. A 5 (4) (1987) 2173. [9] L.A. Donohue, J. Cawley, J.S. Brooks, W.-D. Münz, Surf. Coat. Technol. 74–75 (1995) 123–134. [10] I.J. Smith, W.-D. Münz, L.A. Donohue, I. Petrov, J.E. Greene, Surf. Eng. 14 (1) (1998) 37–41. [11] E. Pflüger, A. Schröer, P. Voumard, L.A. Donohue, W.-D. Münz, Surf. Coat. Technol. 115 (1999) 17–23. [12] R.O. Marwanga, R.C. Voigt, P.H. Cohen, AFS Trans., vol. 107, Paper 99–80, 1999, pp. 595–607. [13] J.R. Davis, ASM Specialty Handbook: Cast Irons, vol. 494, Materials Park, OH, 1996. [14] J.M. Long, in: Proceedings of the 10th Asia–Pacific Conference on Non-Destructive Testing, Brisbane, Australia, 17–21 September 2001, (R. Diederichs, NDTnet: http://www.ndt.net/article/apcndt01/). [15] J.M. Long, in: E. Pereloma, K. Raviprasad (Eds.), Proceedings of Engineering Materials, Melbourne, 23–26 September 2001, Institute of Materials Engineering, Australia, pp. 87–92.
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[16] Daimler-Benz Adhesion Test, Verein Deutscher Ingenieure (VDI)— Richlinie 3198, 1992, p. 7. [17] C.J. Tavares, L. Rebouta, E. Alves, A. Cavaleiro, P. Goudeau, J.P. Riviere, A. Declemy, Thin Solid Films 377–378 (2000) 425–429. [18] P.E. Hovsepian, B.D. Lewis, W.-D. Münz, A. Rouzaud, P. Juliet, Surf. Coat. Technol. 116–119 (1999) 727–734.
[19] H.-D. Steffens, M. Mack, K. Moehwald, K. Reichel, Surf. Coat. Technol. 46 (1991) 65–74. [20] M. Ives, J.S. Brooks, J. Cawley, W. Burgmer, Surf. Coat. Technol. 49 (1991) 244–252. [21] S.G. Harris, E.D. Doyle, A.C. Vlasveld, P.J. Dolder, Surf. Coat. Technol. 146–147 (2001) 305–311.