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Abrasive wear of multilayer κ-Al2O3–Ti(C,N) CVD coatings on cemented carbide
Wear 263 (2007) 74–80
Abrasive wear of multilayer -Al2O3–Ti(C,N) CVD coatings on cemented carbide M. Fallqvist a,∗ , M. Olsson a , S. Ruppi b a
Dalarna University, SE-781 88 Borl¨ange, Sweden b SECO Tools AB, SE-737 82 Fagersta, Sweden
Received 1 September 2006; received in revised form 29 December 2006; accepted 7 January 2007 Available online 23 May 2007
Abstract In the present study the wear resistance of -Al2 O3 –Ti(C,N) multilayer CVD coatings with different multilayer structures (8, 15, 32 layers of -Al2 O3 separated by thin Ti(C,N) layers) have been investigated using a micro-abrasion and a cutting test. The results show that the wear rate of the -Al2 O3 multilayer coatings tend to decrease with decreasing layer thickness in the micro-abrasion test and decrease with increasing layer thickness in the cutting tests. The reason for this is mainly due to the difference in wear behaviour depending on temperature. The results obtained are discussed in relation to the dominant wear mechanisms of the coatings which have been identified using scanning electron microscopy and energy dispersive X-ray spectroscopy. The potential of the micro-abrasion test in the characterisation of thin CVD coatings for cutting tool applications is discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; CVD coating; Al2 O3 ; Multilayer; Metal cutting
1. Introduction The wear and failure modes of metal cutting tools is a research area of great importance for the manufacturing industry. In metal cutting, progressive wear of the cutting tool occurs where the tool is in contact with the workpiece, i.e. on the rake face and the tool flank. The wear process is very complex, involving both mechanical and chemical interaction between the contacting surfaces and is to a large extent governed by the cutting forces, the cutting speed and the chemical composition of the tool and the workpiece materials, respectively. Cemented carbide is the dominant tool material for metal cutting operations using indexable inserts. For these, multilayer-coatings of hard refractory materials such as TiC, Ti(C,N), TiN and Al2 O3 , are today used in order to further enhance the performance of the cutting tool insert. In the machining of steel (iron based metals) Al2 O3 , showing a high chemical and thermal stability, plays an important role acting as a diffusion barrier protecting the underlying cemented carbide from dissolution
∗
Corresponding author. Tel.: +46 23 778651; fax: +46 23 778601. E-mail address: [email protected] (M. Fallqvist).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.01.113
thus reducing the crater wear on the tool rake face. However, due to the sliding of workpiece material against the rake and flank faces the relatively soft Al2 O3 is exposed to both adhesive and abrasive wear which limits its lifetime in many cutting applications [1–3]. Consequently, attempts have been made to improve the properties of CVD Al2 O3 coatings, e.g. through the deposition of different types of Al2 O3 multilayer coatings as well as phase and texture-controlled Al2 O3 coatings [4–7]. The aim of the present study is to investigate the abrasive wear resistance of -Al2 O3 –Ti(C,N) multilayer CVD coatings with different multilayer structures (8, 15, 32 layers of -Al2 O3 separated by thin Ti(C,N) layers, respectively) using a micro-abrasion test with the potential to evaluate the intrinsic wear resistance of the coatings [8–10]. The results are discussed in relation to the results obtained from continuous turning tests using quenched and tempered low alloyed steel, AISI 4340, as workpiece material. The identified wear mechanisms of the coatings have been identified using scanning electron microscopy and energy dispersive X-ray spectroscopy. The potential of the micro-abrasion test in the characterisation of thin CVD coatings for cutting tool applications is discussed.
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Table 1 CVD coatings investigated in the present study Coating
Coating thickness (m)
-A12 O3 -A12 O3 , 8 -Al2 O3 , 15 -Al2 O3 , 32
9 11 11 12
± ± ± ±
1 0.5 0.5 0.5
Individual -Al2 O3 layer thickness, λ (m) – 1.3 0.6 0.3
2. Experimental 2.1. Materials The CVD coatings investigated were deposited using a computer-controlled hot-wall CVD reactor. Commercial cemented carbide inserts (geometry SNUN 120412) with a composition of 85.5 wt% WC, 9 wt% (Nb,Ti,Ta)C and 5.5 wt% Co were used as substrates. Four different types of coatings were deposited; a -Al2 O3 single layer coating and -Al2 O3 –Ti(C,N) multilayer coatings with 8, 15 and 32 individual -Al2 O3 layers, respectively, where the thicker -Al2 O3 layers are separated by thinner (approximately 50 nm) Ti(C,N) layers, see Table 1. For all types of coatings a 2.5 m thick intermediate layer of Ti(C,N) was deposited directly on the cemented carbide substrate followed by the deposition of a bonding layer which promotes the nucleation and growth of -Al2 O3 . The -Al2 O3 –Ti(C,N) multilayer coatings were deposited at 990 ◦ C in a commercial CVD reactor. The -Al2 O3 was deposited from the system AlCl3 –CO2 –Ar–H2 –H2 S while the thin Ti(C,N) layers between the -Al2 O3 layers were deposited from the system TiCl4 –CH4 –H2 –N2 . The Ti(C,N) intermediate layer was deposited using moderate temperature CVD (MTCVD) at a temperature of about 860 ◦ C from the system TiCl4 –CH3 CN–HCl–N2 . 2.2. Experimental procedure 2.2.1. Mechanical testing The micro-hardness of the coatings was measured using a commercial micro-hardness tester (Leitz Wetzlar Durimet) and a load of 500 g. The hardness of the coatings was measured directly on the surface of the coating after careful polishing using 1 m diamond. Although the load used, 500 g, will result in a hardness value influenced by the underlying Ti(C,N) intermediate layer, the composite hardness value obtained still can be used in order to rank the different coatings (the depths of the indents were about 1/4 to 1/3 of the multilayer thickness). The resistance to scratch induced failure of the coatings was evaluated using a commercial scratch tester (CSM Instruments Revetest® ). The tests were performed using a 200 m radius Rockwell C diamond stylus and a continuously increasing normal load in the range 0–100 N using a load rate of 10 N/mm. During the scratching event the friction force and the acoustic emission (A.E.) were detected and post-test characterisation using scanning electron microscopy (SEM) was used to evaluate the failure modes of the coatings. Two different critical loads,
Fig. 1. Crater obtained in the micro-abrasion test as presented by a 3D optical profiling image, using an interference microscope. By measuring the volume of the crater the wear rate of the CVD coatings investigated could be calculated.
FN,C1 and FN,C2 , defined as the load for which continuous cracking in connection to the scratch and continuous exposure of the substrate material occurred, were determined. 2.2.2. Micro-abrasion tests A Precision Dimple Grinder (Gatan Model 656) was used to evaluate the micro-abrasion wear resistance of the coatings. The equipment, normally used for preparation of samples for transmission electron microscopy (TEM), consists of a stainless steel grinding wheel (radius 10 mm) rotating against a horizontally mounted sample which in the present study was fixed. In the micro-abrasion tests a slurry of 1 or 6 m diamond particles (Kemet® Liquid diamond Type WX XStr), a normal load of 20 g and a rotating speed of 220 rpm were used. In order to keep the resulting craters within the thickness of the -Al2 O3 multilayer coatings, the grinding time was set to 600 s for the finer 1 m and 120 s for the larger 6 m diamond particles. Threedimensional surface profiling using a white light 3D interference microscope (WYKO NT-2000) was used to measure the volume of the resulting wear craters, see Fig. 1. Prior to these measurements the samples were sputter coated with a thin layer of gold in order to increase the signal from the sample. 2.2.3. Cutting tests Cutting tests in the form of continuous turning of a plain carbon steel (AISI 4340) were performed in order to evaluate the performance of the coated inserts. The feed, depth of cut and cutting speed were 0.4 mm/rev, 2.5 mm and 200, 250 and 275 m/min, respectively. The cutting times were chosen to 1, 3, 6 and 10 min in order to follow the flank and crater wear. After cutting, the inserts were cleaned in HCl in order to remove the major part of adhered workpiece material. However, since part of the rake face, and especially the crater, still was covered with adhered workpiece material, the wear of the cutting tools was evaluated by measuring the tool flank using optical microscopy.
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Fig. 2. Cross-sections of the CVD coatings investigated -Al2 O3 –Ti(C,N) multilayer coatings with 8 (a), 15 (b) and 32 (c) individual -Al2 O3 layers, respectively. The -Al2 O3 layers appear dark and the thinner Ti(C,N) layers appear bright.
2.2.4. Wear characterisation After the micro-abrasion and cutting tests the worn inserts were subjected to a careful examination using SEM and energy dispersive X-ray spectroscopy (EDX) in order to evaluate the dominant wear mechanisms. 3. Results 3.1. Coating characteristics Fig. 2 shows the cross-sectional microstructure of the coatings investigated as observed in the SEM. 3.2. Mechanical properties The Vickers hardness values of the multilayer coatings ranged between 2390 and 2580HV0.5 , while the hardness of single layer was somewhat lower, 2000HV0.5 , see Table 2. The results of the scratch tests show that the critical normal load for substrate exposure, FN,C2 , increases in the order -Al2 O3 /8 < -Al2 O3 /15 < -Al2 O3 < -Al2 O3 /32, see Table 2. For the -Al2 O3 multilayer coatings, this trend was also observed above FN,C2 where the area of exposed substrate material was found to increase with decreasing number of individual layers, see Fig. 3. The relatively poor performance of the Al2 O3 /8 multilayer coating in the scratch test was also revealed by a relatively low FN,C1 -value, and a significantly higher A.E. intensity as compared with the other coatings, indicating the presence of extensive cracking within the coating for normal loads below FN,C2 .
3.3. Tribological properties 3.3.1. Micro-abrasion test Fig. 4 shows the wear rates of the coatings evaluated in the micro-abrasion tests. In all tests the standard deviation was approximately 10% of the mean value. As can be seen, the larger 6 m diamond particles result in a wear rate more than order of magnitude higher than for the finer 1 m diamond particles. For both types of abrasives the -Al2 O3 single layer coating shows a higher wear resistance as compared with the -Al2 O3 multilayer coatings, which show an increasing wear resistance with increasing number of individual layers. This result is somewhat surprising since multilayer coatings show considerably better performance than single layer coatings in many tribological applications and hence a thorough characterisation of the worn coatings was performed in order to determine the dominant wear mechanisms. Characterisation of the worn coatings using SEM shows that two different wear mechanisms can be distinguished; microcutting (due to the cutting action of the diamond particles) and micro-chipping (due to poor cohesive/adhesive strength of the coating). Fig. 5 shows representative examples of these mechanisms as illustrated by the worn surfaces of the 8layered -Al2 O3 multilayer coating, showing a high tendency to micro-chipping, and the 32-layered -Al2 O3 multilayer coating, showing a relatively low tendency to micro-chipping. As illustrated by the SEM micrographs, the tendency to micro-chipping is more pronounced for the larger 6 m abrasive particles which are believed to generate higher contact stresses during the abrasive contact. The -Al2 O3 single layer coating only displayed a
Table 2 Mechanical properties of the -Al2 O3 –Ti(C,N) multilayer coatings investigated Coating
Hardness, HV0.5 (kg/mm2 )
-A12 O3 -A12 O3 , 8 -Al2 O3 , 15 -Al2 O3 , 32
2000 2450 2390 2580
a b
± ± ± ±
70 130 120 130
Critical loada , FN ,C1 (N) 46 10 32 34
± ± ± ±
4 2 2 2
Normal load resulting in continues cracking as detected by acoustic emission. Normal load resulting in continues cohesive or adhesive failure resulting in substrate exposure.
Critical loadb , FN ,C2 (N) 75 65 70 85
± ± ± ±
5 5 5 5
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Fig. 3. Scratch induced surface failure at a normal load of 90 N of the CVD coatings investigated: (a) -Al2 O3 single layer, (b–d) -Al2 O3 –Ti(C,N) multilayer coatings with 8 (b), 15 (c) and 32 (d) individual layers, respectively.
limited tendency to micro-chipping. Since the wear rate of the coatings correlate with their tendency to micro-chipping it can be concluded that the cohesive/adhesive strength of the coating is of outmost importance in order to obtain a low wear rate in this type of test. 3.3.2. Metal cutting test Fig. 6 shows the flank wear rate, i.e. the increase in flank wear with cutting time, of the coated inserts in the metal cutting test. The data given represents the average of several cutting edges of each of the different inserts. As can be seen, the flank wear increases with increasing number of layers of the multilayered -Al2 O3 coated inserts for all cutting speeds investigated. However, the single layer -Al2 O3 coated inserts show the highest
wear, i.e. the opposite trend as observed in the micro-abrasion test. Observations of the worn cutting inserts in the SEM reveal different wear mechanisms in different regions, see Figs. 7 and 8. On the rake face, a crater is formed and within this crater the coating is mainly worn by superficial adhesive wear, i.e. localised plastic shearing at the surface, followed by fracture and detachment of thin sheared coatings fragments. This wear mechanism dominates in the entrance (with respect to the chip sliding direction) and central parts of the crater, see Fig. 8b. In the exit part of the crater the coating is mainly worn by abrasive wear, see Fig. 8a. On the flank face, the dominant wear mechanism is a combination of abrasive and adhesive wear, see Fig. 8c. Wear due to chipping was not observed in the cutting tests.
Fig. 4. Abrasive wear rate of -Al2 O3 single layer and -Al2 O3 –Ti(C,N) multilayer coatings with 8, 15 and 32 individual layers tested against 1 m (a) and 6 m (b) diamond particles in the micro-abrasion wear test.
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Fig. 5. SEM micrographs illustrating the differences in dominant wear mechanisms (micro-cutting and micro-chipping, respectively) for the 8-layered (a) and 32-layered (b) -Al2 O3 –Ti(C,N) multilayer coatings in the micro-abrasion test.
4. Discussion
Fig. 6. Flank wear rate of -Al2 O3 single layer and -Al2 O3 –Ti(C,N) multilayer (8, 15 and 32 individual layers) coated inserts in the cutting tests.
Although abrasion seems to be an important wear mechanism on the flank in the cutting test no direct correlation is obtained when comparing the results obtained in the micro-abrasion and cutting tests. The reason for this is partly due to the fact that the temperatures are significantly higher in the cutting test and partly due to the fact that, while the micro-abrasion test only test the -Al2 O3 coating, the cutting test will test the whole coating and substrate combination, i.e. the composite. The temperature effect is illustrated by the fact that while brittle micro-chipping of the coating plays an important role in the micro-abrasion test (as well as in the scratch test) no signs of any brittle coating failure (chipping, spalling, etc.) is observed in the cutting test. Hence, we believe that, while fracture mechanical failure mechanisms mainly controls the wear in the micro-abrasion test (as well as the surface failure in the scratch test) these mechanisms are of
Fig. 7. SEM micrographs showing the wear of a 15-layered -Al2 O3 –Ti(C,N) multilayer coated insert after cutting: (a) crater wear and (b) flank wear.
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Fig. 8. SEM micrographs showing (a) abrasive wear of -Al2 O3 on the exit side of the crater, (b) superficial adhesive wear of -Al2 O3 on the entrance side of the crater, and (c) abrasive wear of -Al2 O3 on the flank. The arrows in the micrographs indicate the flow direction of the chip (a and b) and the work piece material (c).
less importance at higher temperatures were the coatings behave more ductile. Nevertheless, the micro-abrasion test is believed to be a promising method in order to obtain information about the wear resistance of CVD coatings used in low temperature applications as well as CVD coatings on cutting tools where fracture mechanical failure mechanisms such as micro-chipping and spalling are of more importance, e.g. in intermittent cutting operations. In a previous study focusing on the wear behaviour of PVD coated tool steels, Rutherford et al. [11] showed that both micro-abrasion testing and scratch testing showed good correlation with results of milling tests, i.e. the flank wear rate of the tools. The increase in wear resistance with increasing number of individual -Al2 O3 layers observed in the micro-abrasion test is probably due to the fact that the volume fraction of Ti(C,N) in the coating will increase with increasing number of individual -Al2 O3 layers (Ti(C,N) being significantly harder than -Al2 O3 at room temperature) and that a finer multilayered structure will show a higher resistance to crack growth and thus micro-chipping. Also, the detrimental effect of any residual tensile stresses in the coating will be lower in the case of a finer multilayered structure [9]. Nevertheless, since all multilayer coatings are prone to chipping the single layer -Al2 O3 coating, mainly showing micro-cutting, display the lowest wear rate. The reason why the flank wear of the multilayered Al2 O3 inserts increases with increasing number of individual -Al2 O3 layers, i.e. the opposite behaviour as observed in the micro-abrasion test, is not yet fully understood. However, the results indicate that the strength of -Al2 O3 /Ti(C,N) interface regions may be critical at high temperatures. The situation
is complex and at least the following explanations, all being dependent on λ, the individual -Al2 O3 layer thickness, can be considered: (i) coating composition, (ii) interfacial adhesion and (iii) -Al2 O3 → ␣-Al2 O3 phase transformation. Since Ti(C,N) displays a more drastic drop in hot hardness as compared with Al2 O3 in the temperature range of interest (while the hardness of Ti(C,N) and Al2 O3 at RT is 2800HV and 2000HV, respectively, Ti(C,N) and Al2 O3 display almost the same hardness, i.e. 800HV, at 1000 ◦ C [12,13]) it is believed that the strengthening effect (observed at RT) of Ti(C,N) layers within the multilayer coatings will decrease with increasing temperature. Furthermore, since TEM studies have confirmed interfacial porosity in similar CVD -Al2 O3 multilayer coatings [5], it is believed that the presence of pores in the multilayer structure will lower the strength of the multilayer coatings and especially those with a large number of interfaces (large number of Ti(C,N) layers). Finally, the possible phase transformation of -Al2 O3 to ␣-Al2 O3 , associated with a volume decrease of about 8% and thus a reduced cohesive strength, may also play an important role in the cutting tests [14]. Although this phase transformation takes place above 1000 ◦ C and the temperature at the flank is somewhat lower (800 ◦ C as given by Ref. [2]) the outermost surface may be exposed to high flash temperatures which will promote the above phase transformation. Also, Lindulf et al. [15] have shown that a finer multilayer (high λ-value) structure will display a higher transformation rate as compared with coarser multilayer (low λ-value) structures and thus most
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probably a higher flank wear rate. Assuming that the temperature is about 800 ◦ C the relatively high flank wear rate of single layer -Al2 O3 coated inserts is probably due to a relatively low hot hardness in combination with a coarser microstructure as compared with the -Al2 O3 –Ti(C,N) multilayer coatings. 5. Conclusions In the present study the micro-abrasion resistance of a number of -Al2 O3 –Ti(C,N) multilayer coatings have been evaluated and compared with the performance in metal cutting. The main results are as follows: • In the micro-abrasion test, the wear rate of the -Al2 O3 multilayer coatings tend to decrease with decreasing layer thickness and the dominating wear mechanisms were found to be micro-cutting and micro-chipping. However, the single layer alumina coating, not showing any micro-chipping, displayed the overall lowest wear rate. • In the cutting test, the flank wear rate of the -Al2 O3 multilayer coatings tend to decrease with increasing layer thickness and the dominating wear mechanisms were found to be adhesive and abrasive wear. In this test, the single layer alumina coating displayed the overall highest flank wear rate. • The lack of correlation between the two tests are most probably due to the significantly higher temperatures in the cutting tests, higher temperature making the finer layered -Al2 O3 multilayer coatings less wear resistant than coarse layered -Al2 O3 multilayer coatings. Acknowledgement The authors gratefully acknowledge the financial support from the National Graduate School in Materials Science.
References [1] P.A. Dearnley, E.M. Trent, Wear mechanisms of coated carbide tools, Metals Technol. 9 (1982) 60–75. [2] P.A. Dearnley, Rake and flank wear mechanism of coated and uncoated cemented carbides, J. Eng. Mater. Technol. 107 (1985) 68–82. [3] S. Ruppi, B. H¨ogrelius, M. Huhtiranta, Wear characteristics of TiC, Ti(C,N) and Al2 O3 coatings in the turning of conventional and Ca-treated steels, Int. J. Refract. Metals Hard Mater. 16 (1998) 353–368. [4] M. Halvarsson, S. Ruppi, Microstructural investigations of CVD TiN/Al2 O3 multilayer coatings on cemented carbide, Int. J. Refract. Metals Hard Mater. 15 (1997) 169–178. [5] M. Halvarsson, J.E. Trancik, S. Ruppi, The microstructure of CVD -Al2 O3 multilayers separated by thin intermediate TiN or TiC layers, Int. J. Refract. Metals Hard Mater. 24 (2006) 32–38. [6] A. Larsson, S. Ruppi, Microstructure and properties of CVD ␥-Al2 O3 coatings, Int. J. Refract. Metals Hard Mater. 19 (2001) 515–522. [7] S. Ruppi, Deposition, microstructure and properties of texture-controlled CVD ␣-Al2 O3 coatings, Int. J. Refract. Metals Hard Mater. 23 (2005) 306–316. ˚ Kassman, S. Jacobsson, L. Ericsson, P. Hedenqvist, M. Olsson, A new [8] A. test method for the intrinsic abrasion resistance of thin coatings, Surf. Coat. Technol. 50 (1991) 75–84. [9] M. Nordin, S. Jacobson, A new tool for evaluation of intrinsic mechanical properties of multilayered coatings, Surf. Eng. 17 (2001) 465–471. [10] M.G. Gee, A. Gant, I. Hutchings, R. Bethke, K. Schiffman, K. Van Acker, S. Poulat, Y. Gachon, J. von Stebut, Progress towards standardisation of ball cratering, Wear 255 (2003) 1–13. [11] K.L. Rutherford, I.M. Hutchings, S.J. Bull, E.D. Doyle, Laboratory characterisation of the wear behaviour of PVD-coated tool steels and correlation with cutting tool performance, Surf. Coat. Technol. 80 (1996) 176–180. [12] H.L. Wang, M.H. Hon, Temperature dependence of ceramics hardness, Ceram. Int. 25 (1999) 267–271. [13] S. PalDey, S.C. Deevi, Single layer and multilayer wear resistant coatings of (Ti,Al)N: a review, Mater. Sci. Eng. A342 (2003) 58–79. [14] S. Vuorinen, L. Karlsson, Phase transformation in chemical vapourdeposited -alumina, Thin Solid Films 214 (1992) 132–143. [15] N. Lindulf, M. Halvarsson, H. Nord´en, S. Vuorinen, Microstructural investigation of the -Al2 O3 → ␣-Al2 O3 transformation in multilayer coatings of chemically vapour deposited -Al2 O3 , Thin Solid Films 253 (1994) 311–317.
Abrasive wear of multilayer -Al2O3–Ti(C,N) CVD coatings on cemented carbide M. Fallqvist a,∗ , M. Olsson a , S. Ruppi b a
Dalarna University, SE-781 88 Borl¨ange, Sweden b SECO Tools AB, SE-737 82 Fagersta, Sweden
Received 1 September 2006; received in revised form 29 December 2006; accepted 7 January 2007 Available online 23 May 2007
Abstract In the present study the wear resistance of -Al2 O3 –Ti(C,N) multilayer CVD coatings with different multilayer structures (8, 15, 32 layers of -Al2 O3 separated by thin Ti(C,N) layers) have been investigated using a micro-abrasion and a cutting test. The results show that the wear rate of the -Al2 O3 multilayer coatings tend to decrease with decreasing layer thickness in the micro-abrasion test and decrease with increasing layer thickness in the cutting tests. The reason for this is mainly due to the difference in wear behaviour depending on temperature. The results obtained are discussed in relation to the dominant wear mechanisms of the coatings which have been identified using scanning electron microscopy and energy dispersive X-ray spectroscopy. The potential of the micro-abrasion test in the characterisation of thin CVD coatings for cutting tool applications is discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; CVD coating; Al2 O3 ; Multilayer; Metal cutting
1. Introduction The wear and failure modes of metal cutting tools is a research area of great importance for the manufacturing industry. In metal cutting, progressive wear of the cutting tool occurs where the tool is in contact with the workpiece, i.e. on the rake face and the tool flank. The wear process is very complex, involving both mechanical and chemical interaction between the contacting surfaces and is to a large extent governed by the cutting forces, the cutting speed and the chemical composition of the tool and the workpiece materials, respectively. Cemented carbide is the dominant tool material for metal cutting operations using indexable inserts. For these, multilayer-coatings of hard refractory materials such as TiC, Ti(C,N), TiN and Al2 O3 , are today used in order to further enhance the performance of the cutting tool insert. In the machining of steel (iron based metals) Al2 O3 , showing a high chemical and thermal stability, plays an important role acting as a diffusion barrier protecting the underlying cemented carbide from dissolution
∗
Corresponding author. Tel.: +46 23 778651; fax: +46 23 778601. E-mail address: [email protected] (M. Fallqvist).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.01.113
thus reducing the crater wear on the tool rake face. However, due to the sliding of workpiece material against the rake and flank faces the relatively soft Al2 O3 is exposed to both adhesive and abrasive wear which limits its lifetime in many cutting applications [1–3]. Consequently, attempts have been made to improve the properties of CVD Al2 O3 coatings, e.g. through the deposition of different types of Al2 O3 multilayer coatings as well as phase and texture-controlled Al2 O3 coatings [4–7]. The aim of the present study is to investigate the abrasive wear resistance of -Al2 O3 –Ti(C,N) multilayer CVD coatings with different multilayer structures (8, 15, 32 layers of -Al2 O3 separated by thin Ti(C,N) layers, respectively) using a micro-abrasion test with the potential to evaluate the intrinsic wear resistance of the coatings [8–10]. The results are discussed in relation to the results obtained from continuous turning tests using quenched and tempered low alloyed steel, AISI 4340, as workpiece material. The identified wear mechanisms of the coatings have been identified using scanning electron microscopy and energy dispersive X-ray spectroscopy. The potential of the micro-abrasion test in the characterisation of thin CVD coatings for cutting tool applications is discussed.
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Table 1 CVD coatings investigated in the present study Coating
Coating thickness (m)
-A12 O3 -A12 O3 , 8 -Al2 O3 , 15 -Al2 O3 , 32
9 11 11 12
± ± ± ±
1 0.5 0.5 0.5
Individual -Al2 O3 layer thickness, λ (m) – 1.3 0.6 0.3
2. Experimental 2.1. Materials The CVD coatings investigated were deposited using a computer-controlled hot-wall CVD reactor. Commercial cemented carbide inserts (geometry SNUN 120412) with a composition of 85.5 wt% WC, 9 wt% (Nb,Ti,Ta)C and 5.5 wt% Co were used as substrates. Four different types of coatings were deposited; a -Al2 O3 single layer coating and -Al2 O3 –Ti(C,N) multilayer coatings with 8, 15 and 32 individual -Al2 O3 layers, respectively, where the thicker -Al2 O3 layers are separated by thinner (approximately 50 nm) Ti(C,N) layers, see Table 1. For all types of coatings a 2.5 m thick intermediate layer of Ti(C,N) was deposited directly on the cemented carbide substrate followed by the deposition of a bonding layer which promotes the nucleation and growth of -Al2 O3 . The -Al2 O3 –Ti(C,N) multilayer coatings were deposited at 990 ◦ C in a commercial CVD reactor. The -Al2 O3 was deposited from the system AlCl3 –CO2 –Ar–H2 –H2 S while the thin Ti(C,N) layers between the -Al2 O3 layers were deposited from the system TiCl4 –CH4 –H2 –N2 . The Ti(C,N) intermediate layer was deposited using moderate temperature CVD (MTCVD) at a temperature of about 860 ◦ C from the system TiCl4 –CH3 CN–HCl–N2 . 2.2. Experimental procedure 2.2.1. Mechanical testing The micro-hardness of the coatings was measured using a commercial micro-hardness tester (Leitz Wetzlar Durimet) and a load of 500 g. The hardness of the coatings was measured directly on the surface of the coating after careful polishing using 1 m diamond. Although the load used, 500 g, will result in a hardness value influenced by the underlying Ti(C,N) intermediate layer, the composite hardness value obtained still can be used in order to rank the different coatings (the depths of the indents were about 1/4 to 1/3 of the multilayer thickness). The resistance to scratch induced failure of the coatings was evaluated using a commercial scratch tester (CSM Instruments Revetest® ). The tests were performed using a 200 m radius Rockwell C diamond stylus and a continuously increasing normal load in the range 0–100 N using a load rate of 10 N/mm. During the scratching event the friction force and the acoustic emission (A.E.) were detected and post-test characterisation using scanning electron microscopy (SEM) was used to evaluate the failure modes of the coatings. Two different critical loads,
Fig. 1. Crater obtained in the micro-abrasion test as presented by a 3D optical profiling image, using an interference microscope. By measuring the volume of the crater the wear rate of the CVD coatings investigated could be calculated.
FN,C1 and FN,C2 , defined as the load for which continuous cracking in connection to the scratch and continuous exposure of the substrate material occurred, were determined. 2.2.2. Micro-abrasion tests A Precision Dimple Grinder (Gatan Model 656) was used to evaluate the micro-abrasion wear resistance of the coatings. The equipment, normally used for preparation of samples for transmission electron microscopy (TEM), consists of a stainless steel grinding wheel (radius 10 mm) rotating against a horizontally mounted sample which in the present study was fixed. In the micro-abrasion tests a slurry of 1 or 6 m diamond particles (Kemet® Liquid diamond Type WX XStr), a normal load of 20 g and a rotating speed of 220 rpm were used. In order to keep the resulting craters within the thickness of the -Al2 O3 multilayer coatings, the grinding time was set to 600 s for the finer 1 m and 120 s for the larger 6 m diamond particles. Threedimensional surface profiling using a white light 3D interference microscope (WYKO NT-2000) was used to measure the volume of the resulting wear craters, see Fig. 1. Prior to these measurements the samples were sputter coated with a thin layer of gold in order to increase the signal from the sample. 2.2.3. Cutting tests Cutting tests in the form of continuous turning of a plain carbon steel (AISI 4340) were performed in order to evaluate the performance of the coated inserts. The feed, depth of cut and cutting speed were 0.4 mm/rev, 2.5 mm and 200, 250 and 275 m/min, respectively. The cutting times were chosen to 1, 3, 6 and 10 min in order to follow the flank and crater wear. After cutting, the inserts were cleaned in HCl in order to remove the major part of adhered workpiece material. However, since part of the rake face, and especially the crater, still was covered with adhered workpiece material, the wear of the cutting tools was evaluated by measuring the tool flank using optical microscopy.
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Fig. 2. Cross-sections of the CVD coatings investigated -Al2 O3 –Ti(C,N) multilayer coatings with 8 (a), 15 (b) and 32 (c) individual -Al2 O3 layers, respectively. The -Al2 O3 layers appear dark and the thinner Ti(C,N) layers appear bright.
2.2.4. Wear characterisation After the micro-abrasion and cutting tests the worn inserts were subjected to a careful examination using SEM and energy dispersive X-ray spectroscopy (EDX) in order to evaluate the dominant wear mechanisms. 3. Results 3.1. Coating characteristics Fig. 2 shows the cross-sectional microstructure of the coatings investigated as observed in the SEM. 3.2. Mechanical properties The Vickers hardness values of the multilayer coatings ranged between 2390 and 2580HV0.5 , while the hardness of single layer was somewhat lower, 2000HV0.5 , see Table 2. The results of the scratch tests show that the critical normal load for substrate exposure, FN,C2 , increases in the order -Al2 O3 /8 < -Al2 O3 /15 < -Al2 O3 < -Al2 O3 /32, see Table 2. For the -Al2 O3 multilayer coatings, this trend was also observed above FN,C2 where the area of exposed substrate material was found to increase with decreasing number of individual layers, see Fig. 3. The relatively poor performance of the Al2 O3 /8 multilayer coating in the scratch test was also revealed by a relatively low FN,C1 -value, and a significantly higher A.E. intensity as compared with the other coatings, indicating the presence of extensive cracking within the coating for normal loads below FN,C2 .
3.3. Tribological properties 3.3.1. Micro-abrasion test Fig. 4 shows the wear rates of the coatings evaluated in the micro-abrasion tests. In all tests the standard deviation was approximately 10% of the mean value. As can be seen, the larger 6 m diamond particles result in a wear rate more than order of magnitude higher than for the finer 1 m diamond particles. For both types of abrasives the -Al2 O3 single layer coating shows a higher wear resistance as compared with the -Al2 O3 multilayer coatings, which show an increasing wear resistance with increasing number of individual layers. This result is somewhat surprising since multilayer coatings show considerably better performance than single layer coatings in many tribological applications and hence a thorough characterisation of the worn coatings was performed in order to determine the dominant wear mechanisms. Characterisation of the worn coatings using SEM shows that two different wear mechanisms can be distinguished; microcutting (due to the cutting action of the diamond particles) and micro-chipping (due to poor cohesive/adhesive strength of the coating). Fig. 5 shows representative examples of these mechanisms as illustrated by the worn surfaces of the 8layered -Al2 O3 multilayer coating, showing a high tendency to micro-chipping, and the 32-layered -Al2 O3 multilayer coating, showing a relatively low tendency to micro-chipping. As illustrated by the SEM micrographs, the tendency to micro-chipping is more pronounced for the larger 6 m abrasive particles which are believed to generate higher contact stresses during the abrasive contact. The -Al2 O3 single layer coating only displayed a
Table 2 Mechanical properties of the -Al2 O3 –Ti(C,N) multilayer coatings investigated Coating
Hardness, HV0.5 (kg/mm2 )
-A12 O3 -A12 O3 , 8 -Al2 O3 , 15 -Al2 O3 , 32
2000 2450 2390 2580
a b
± ± ± ±
70 130 120 130
Critical loada , FN ,C1 (N) 46 10 32 34
± ± ± ±
4 2 2 2
Normal load resulting in continues cracking as detected by acoustic emission. Normal load resulting in continues cohesive or adhesive failure resulting in substrate exposure.
Critical loadb , FN ,C2 (N) 75 65 70 85
± ± ± ±
5 5 5 5
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Fig. 3. Scratch induced surface failure at a normal load of 90 N of the CVD coatings investigated: (a) -Al2 O3 single layer, (b–d) -Al2 O3 –Ti(C,N) multilayer coatings with 8 (b), 15 (c) and 32 (d) individual layers, respectively.
limited tendency to micro-chipping. Since the wear rate of the coatings correlate with their tendency to micro-chipping it can be concluded that the cohesive/adhesive strength of the coating is of outmost importance in order to obtain a low wear rate in this type of test. 3.3.2. Metal cutting test Fig. 6 shows the flank wear rate, i.e. the increase in flank wear with cutting time, of the coated inserts in the metal cutting test. The data given represents the average of several cutting edges of each of the different inserts. As can be seen, the flank wear increases with increasing number of layers of the multilayered -Al2 O3 coated inserts for all cutting speeds investigated. However, the single layer -Al2 O3 coated inserts show the highest
wear, i.e. the opposite trend as observed in the micro-abrasion test. Observations of the worn cutting inserts in the SEM reveal different wear mechanisms in different regions, see Figs. 7 and 8. On the rake face, a crater is formed and within this crater the coating is mainly worn by superficial adhesive wear, i.e. localised plastic shearing at the surface, followed by fracture and detachment of thin sheared coatings fragments. This wear mechanism dominates in the entrance (with respect to the chip sliding direction) and central parts of the crater, see Fig. 8b. In the exit part of the crater the coating is mainly worn by abrasive wear, see Fig. 8a. On the flank face, the dominant wear mechanism is a combination of abrasive and adhesive wear, see Fig. 8c. Wear due to chipping was not observed in the cutting tests.
Fig. 4. Abrasive wear rate of -Al2 O3 single layer and -Al2 O3 –Ti(C,N) multilayer coatings with 8, 15 and 32 individual layers tested against 1 m (a) and 6 m (b) diamond particles in the micro-abrasion wear test.
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Fig. 5. SEM micrographs illustrating the differences in dominant wear mechanisms (micro-cutting and micro-chipping, respectively) for the 8-layered (a) and 32-layered (b) -Al2 O3 –Ti(C,N) multilayer coatings in the micro-abrasion test.
4. Discussion
Fig. 6. Flank wear rate of -Al2 O3 single layer and -Al2 O3 –Ti(C,N) multilayer (8, 15 and 32 individual layers) coated inserts in the cutting tests.
Although abrasion seems to be an important wear mechanism on the flank in the cutting test no direct correlation is obtained when comparing the results obtained in the micro-abrasion and cutting tests. The reason for this is partly due to the fact that the temperatures are significantly higher in the cutting test and partly due to the fact that, while the micro-abrasion test only test the -Al2 O3 coating, the cutting test will test the whole coating and substrate combination, i.e. the composite. The temperature effect is illustrated by the fact that while brittle micro-chipping of the coating plays an important role in the micro-abrasion test (as well as in the scratch test) no signs of any brittle coating failure (chipping, spalling, etc.) is observed in the cutting test. Hence, we believe that, while fracture mechanical failure mechanisms mainly controls the wear in the micro-abrasion test (as well as the surface failure in the scratch test) these mechanisms are of
Fig. 7. SEM micrographs showing the wear of a 15-layered -Al2 O3 –Ti(C,N) multilayer coated insert after cutting: (a) crater wear and (b) flank wear.
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Fig. 8. SEM micrographs showing (a) abrasive wear of -Al2 O3 on the exit side of the crater, (b) superficial adhesive wear of -Al2 O3 on the entrance side of the crater, and (c) abrasive wear of -Al2 O3 on the flank. The arrows in the micrographs indicate the flow direction of the chip (a and b) and the work piece material (c).
less importance at higher temperatures were the coatings behave more ductile. Nevertheless, the micro-abrasion test is believed to be a promising method in order to obtain information about the wear resistance of CVD coatings used in low temperature applications as well as CVD coatings on cutting tools where fracture mechanical failure mechanisms such as micro-chipping and spalling are of more importance, e.g. in intermittent cutting operations. In a previous study focusing on the wear behaviour of PVD coated tool steels, Rutherford et al. [11] showed that both micro-abrasion testing and scratch testing showed good correlation with results of milling tests, i.e. the flank wear rate of the tools. The increase in wear resistance with increasing number of individual -Al2 O3 layers observed in the micro-abrasion test is probably due to the fact that the volume fraction of Ti(C,N) in the coating will increase with increasing number of individual -Al2 O3 layers (Ti(C,N) being significantly harder than -Al2 O3 at room temperature) and that a finer multilayered structure will show a higher resistance to crack growth and thus micro-chipping. Also, the detrimental effect of any residual tensile stresses in the coating will be lower in the case of a finer multilayered structure [9]. Nevertheless, since all multilayer coatings are prone to chipping the single layer -Al2 O3 coating, mainly showing micro-cutting, display the lowest wear rate. The reason why the flank wear of the multilayered Al2 O3 inserts increases with increasing number of individual -Al2 O3 layers, i.e. the opposite behaviour as observed in the micro-abrasion test, is not yet fully understood. However, the results indicate that the strength of -Al2 O3 /Ti(C,N) interface regions may be critical at high temperatures. The situation
is complex and at least the following explanations, all being dependent on λ, the individual -Al2 O3 layer thickness, can be considered: (i) coating composition, (ii) interfacial adhesion and (iii) -Al2 O3 → ␣-Al2 O3 phase transformation. Since Ti(C,N) displays a more drastic drop in hot hardness as compared with Al2 O3 in the temperature range of interest (while the hardness of Ti(C,N) and Al2 O3 at RT is 2800HV and 2000HV, respectively, Ti(C,N) and Al2 O3 display almost the same hardness, i.e. 800HV, at 1000 ◦ C [12,13]) it is believed that the strengthening effect (observed at RT) of Ti(C,N) layers within the multilayer coatings will decrease with increasing temperature. Furthermore, since TEM studies have confirmed interfacial porosity in similar CVD -Al2 O3 multilayer coatings [5], it is believed that the presence of pores in the multilayer structure will lower the strength of the multilayer coatings and especially those with a large number of interfaces (large number of Ti(C,N) layers). Finally, the possible phase transformation of -Al2 O3 to ␣-Al2 O3 , associated with a volume decrease of about 8% and thus a reduced cohesive strength, may also play an important role in the cutting tests [14]. Although this phase transformation takes place above 1000 ◦ C and the temperature at the flank is somewhat lower (800 ◦ C as given by Ref. [2]) the outermost surface may be exposed to high flash temperatures which will promote the above phase transformation. Also, Lindulf et al. [15] have shown that a finer multilayer (high λ-value) structure will display a higher transformation rate as compared with coarser multilayer (low λ-value) structures and thus most
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probably a higher flank wear rate. Assuming that the temperature is about 800 ◦ C the relatively high flank wear rate of single layer -Al2 O3 coated inserts is probably due to a relatively low hot hardness in combination with a coarser microstructure as compared with the -Al2 O3 –Ti(C,N) multilayer coatings. 5. Conclusions In the present study the micro-abrasion resistance of a number of -Al2 O3 –Ti(C,N) multilayer coatings have been evaluated and compared with the performance in metal cutting. The main results are as follows: • In the micro-abrasion test, the wear rate of the -Al2 O3 multilayer coatings tend to decrease with decreasing layer thickness and the dominating wear mechanisms were found to be micro-cutting and micro-chipping. However, the single layer alumina coating, not showing any micro-chipping, displayed the overall lowest wear rate. • In the cutting test, the flank wear rate of the -Al2 O3 multilayer coatings tend to decrease with increasing layer thickness and the dominating wear mechanisms were found to be adhesive and abrasive wear. In this test, the single layer alumina coating displayed the overall highest flank wear rate. • The lack of correlation between the two tests are most probably due to the significantly higher temperatures in the cutting tests, higher temperature making the finer layered -Al2 O3 multilayer coatings less wear resistant than coarse layered -Al2 O3 multilayer coatings. Acknowledgement The authors gratefully acknowledge the financial support from the National Graduate School in Materials Science.
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