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Friction and wear testing of multilayer coatings on carbide substrates for dry machining applications
Surface and Coatings Technology 155 (2002) 37–45
Friction and wear testing of multilayer coatings on carbide substrates for dry machining applications W. Grzesik*, Z. Zalisz, P. Nieslony Department of Manufacturing Engineering and Production Automation, Technical University of Opole, 5th Mikolajczyka St., P.O. Box 321, 45-233 Opole, Poland Received 9 July 2001; accepted in revised form 22 January 2002
Abstract The principle aim of this paper is to investigate three wear-protective coatings with multilayer structures, which are frequently used in the cutting tool industry and to assess their frictional behaviour under the test conditions equivalent to those for the cutting of medium carbon steel. A modified pin-on-disc tester was used to conduct experiments in which both the friction coefficient and the linear wear of the tribo-pair were recorded vs. sliding distance. The volumetric wear rate was proposed as a parameter for quantitative comparison of the wear resistance of the tribo-pairs tested. It was found that the principle stage of the specimen wear takes place during the first 200–240 s of sliding independently of the applied speed; obviously, after this stage the intensity of wear decreases drastically. The maximum values of specific wear rate observed at the lowest sliding speeds applied decrease significantly with the elevation of the sliding speed for all tribo-pairs, especially for coated specimens. The use of coatings with an Al2 O3 intermediate layer at the lower sliding speed of approximately 0.5 mys deteriorates the wear resistance of the coated specimens, but its protective role becomes more important at the high speed conditions in comparison to an uncoated carbide specimen. The maximum wear resistance was revealed in the case of a carbon steel–TiCyTiN coating tribo-pair for sliding speeds ranging from 0.5 to 3.0 mymin. It is concluded that the proposed methodology for experimental friction and wear quantification provides the new knowledge for multilayer coating structures on the cutting tool wear resistance and can be helpful in the optimum selection of coated tools for the dry cutting of steels. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Wear; Friction; Cylinder-on-disc; CVD multilayer coatings
1. Introduction The prediction and control of friction and wear is one of the most essential problems emerging in the design of cutting operations w1x. Metal machining is characterised by extreme contact conditions at the tool–workpiece and the tool–chip interfaces including high contact pressure and temperature, and especially highly active and freshly generated surfaces. Usually it is not possible to reproduce in full-scale the contact conditions in machining using conventional pin-on-disc testing because the wear mechanisms involved are not relevant to that observed in machining. In general, modified pinon-disc (Fig. 1) w2,3x, pin-on ring (crossed cylinders) w4,7x and ball-on-disc w5x test devices have been devel*Corresponding author. Tel.: q48-77-4006-290; fax: q48-774006-342. E-mail address: [email protected] (W. Grzesik).
oped in order to perform sliding wear tests that simulate dry machining when using coated cutting tool materials. In particular, Hedenqvist and Olsson w4x have shown that the coating tested and the substrate should be considered as one element termed as a coatingysubstrate system. Recently, Lebrun et al. w3x have carried out extensive friction experiments using a special test device including the thermal and mechanical outputs of the plain–plain contact tribo-system. In both this and in Refs. w8,9x the vital role of the frictional heat flux (friction mechanical dissipation) in the frictional behaviour of the work (chip)-coatingy substrate system was confirmed. In the sliding wear test presented in Fig. 2a a cylindrical test pin of 5 mm in diameter is mounted vertically on the tool holder and the cylindrical counter material is fixed in the lathe mandrel. A spring loading system is used to apply the
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W. Grzesik et al. / Surface and Coatings Technology 155 (2002) 37–45
selected metal-on-coating and metal-on-WCyCo carbide pairs that reproduce orthogonal cutting conditions at low, throughout moderate, up to high sliding speeds, respectively. 2. Experimentation 2.1. Test equipment
Fig. 1. A scheme of apparatus for modified pin-on-disc testing w2x.
normal force. In addition, continuous introduction of a fresh surface of work material, which is an important feature of a wear test for simulation of the cutting tool wear, is performed by engaging the feed motion of the lathe. During the test, friction and surface damage of the coating are controlled by the properties of the counter material together with the normal force and sliding speed. As can be seen in Fig. 2b, in this study the sliding wear of four different PVD coatings, i.e. CrN, TiN, (TiAl)N and Ti(C,N) on HSS (ASP 30) was investigated. The experimental results obtained, shown in Fig. 2b, revealed that at a low sliding speed wear is governed mainly by the coating hardness, i.e. the hardest Ti(C,N) coating displays the lowest wear rate (the hardness of the coatings increases in the order CrN– TiN–(Ti,Al)N–Ti(C,N). On the other hand, at the highest speed of 90 mymin the (Ti,Al)N coating outperformed the three other coatings tested. This can be attributed to a low oxidation rate of this coating and good tribological properties of the thin oxide layer formed on the top coating surface. In contrast, the low oxidation rate of the CrN coating probably explains a relatively good performance of this film at the sliding speed of approximately 90 mymin w4x. A modified approach to the use of a long sliding distance-friction-laboratory testing, which employs a special equipment layout based on line type of contact between the cylindrical specimen (round insert) and a flat rotating disc, is shown in Fig. 3. Experiments were designed to compare the frictional behaviour of the
As mentioned above, uncoated and coated carbides sliding against the carbon steel disc were tested at ambient under dry friction conditions using a modified tribo-tester equipped with control and computer DAQ systems. The functional structure of the experimental set-up and instrumentation for process measurement and control are presented in Fig. 4. The contact conditions in the investigations were generated by pressing the clearance face of a round tool insert to the flat face surface of the rotating steel disc, as shown in Fig. 3, thus the line type of contact was always kept at the beginning of the tribo-test performed w7x. The main features of the device used are as follows w10x: a the inputs: the normal load Fn and the sliding speed vs can be changed continuously and precisely stabilised by means of a pneumatic load and steeples power regulation systems, respectively. b the outputs: the friction force Ff, the linear wear of coupling materials Dl and the contact temperature of the specimen Tc are measured and analysed in real-time mode as functions of the sliding distance, d, or equivalently of time, t. Finally, the coefficient of sliding friction m, defined as the ratio of the variable friction force to the constant normal load, is continuously computed and recorded. c the tribo-tester operates under the control of specially designed computer software, which was installed in the memory of a PC computer equipped with a 16-channel AyD converter. 2.2. Materials A normalised medium carbon C45 steel equivalent to AISI 1045 (UTSs670 MPa; 35 HRC), frequently used
Fig. 2. Schematic illustration of the pin-on-ring device for sliding wear test (a) with test pin (at A) and counter material (at B). Wear of selected PVD coatings deposited on HSS test pins as a function of sliding speed when run against carbon steel (b) w6x.
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in the friction tests performed. The total thicknesses of these coatings were equal to 8, 8 and 10 mm, respectively. As a reference, conventional (uncoated) ISO-P30 cemented carbide was used. 2.3. Test procedure
Fig. 3. A scheme of the tribo-pair configuration used in this investigation.
in comparative machinability tests, was selected as a countermaterial. Three different states of the disc face surfaces were examined: fresh grounded, run-in (worn) after one trial and run-in (worn) after two trials. The mean diameter of the disc–pin contact was assumed to be equal to 79.7 mm. The test pins were rounded inserts of 25.4 diameter and of RNMG 250900 ISO designation. Three multilayered CVD coatings: two-layer TiCyTiN (NT35), three-layer TiCyAl2O3 yTiN (NT15) and fourlayer TiCyTi(C,N)yAl2 O3 yTiN (NT20) were included
Sliding friction tests were carried out with four material combinations, keeping the normal force at Fns 50 N. The sliding speed was varied from 0.5 to 3.0 my s, which corresponds to the cutting speeds normally employed when cutting carbon steels using uncoated and carbide tools. For each tribo-pair, the sliding distance was selected to be equivalent to a test time (tool life) of 20 min (1200 s). Prior to each test the working surfaces of specimens were cleaned ultrasonically with tetrachloromethane (carbon tetrachloride) and dried in a warm air stream. Three trials were conducted for each materialysliding speed combination. To quantify the friction behaviour of tool materials, the friction force was measured, and by using a DAQ system instantaneous values of the coefficient of friction were computed and recorded for certain intervals of time (;4 s). Light optical microscopy (LOM) was used to examine mating surfaces and to characterise the dominant wear mechanisms of the tool materials. In this investigation,
Fig. 4. Equipment layout for the cylinder-on-disc tester with load and temperature control.
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Fig. 5. Recordings of friction coefficient vs. sliding distance for P30 uncoated carbide-AISI 1045 steel pair.
Fig. 6. Recordings of friction coefficient vs. sliding distance for NT35, TiCyTiN coating-AISI 1045 steel pair.
an Olympus PME3 optical microscope equipped with a JVC TK-1281 colour video camera was employed. The selected wear scars produced on coated inserts (specimens) were visualised and the video signals were transmitted to a PC computer. Subsequently, the raw images were processed and dimensioned by means of a computer image processing system developed earlier by the authors w8x. The depth of the scar in the centre was measured by means of a light cross-sectional method using a double optical microscope.
Then, at the intermediate and higher speeds the following tens of meters of sliding result in the decrease of friction, which reaches the local minimum value promptly at a lower sliding speed, especially in the presence of Al2O3 ceramic layer. This sequence of friction variations that occurs during the primary 50– 100 m of sliding seems to be a typical case for all trials, independent of the coatings tested. Further tribological behaviour of the tribo-pairs tested depends significantly on the coating composition. The TiCyTiN coated and uncoated carbides showed a slow increase of the friction coefficient until the stabilisation of m was reached. In
3. Determination of frictional responses of the tribosystems In order to characterise the frictional interaction for the selected tribo-pairs, the friction force was measured with a frequency of 0.1 sy1 and converted into the coefficient of friction, which was stored in the computer memory as a function of the sliding distance d (equivalent to test time t). The selected relationships obtained for 60 trials were plotted in Figs. 5–10 in order to show the characteristic tendencies in frictional behaviour of the four tribo-contacts. Variations of the friction coefficient obtained for three different sliding speeds vss0.5, 1.5 and 3. 0 mys are presented in Figs. 5–10. As can be seen from these figures, frictional behaviour differs with the sliding speed applied and with the presence or absence of the protective layers, especially an intermediate Al2O3 ceramic layer in the multilayer coating structure. In general, a rapid increase of the friction coefficient, reaching the local maximum value at the lowest speed, of 0.5 mys was detected at the primary 50 m of sliding, independently of the tribo-pair tested (Figs. 5–8).
Fig. 7. Recordings of friction coefficient vs. sliding distance for NT15, TiCyAl2O3yTiN coating-AISI 1045 steel pair.
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Fig. 8. Recordings of friction coefficient vs. sliding distance for NT20, TiCyTi(C,N)yAl2 O3 yTiN coating-AISI 1045 steel pair.
Fig. 9. Recordings of friction coefficient vs. sliding distance for tested cutting tool materials. Sliding speeds0.5 mys.
particular, for the sliding speed of 0.5 mymin, the values of friction coefficient for TiCyTiN coated and uncoated carbides (Figs. 5 and 6) increased up to 0.5 during the first 50–70 m of sliding and next decreased rapidly to the local minimum of 0.35, then it slowly grew up and tended to stabilise at the value of approximately 0.42%0.45 after 200 m of sliding. There is a different frictional behaviour of multilayer coatings containing an intermediate A2O3 ceramic layer (Figs. 7 and 8). For the sliding speed of 0.5 mymin, the rapid increase in the friction coefficient up to 0.6 after the first 100 m of sliding distance was observed, which was preceded temporarily by the local minimum. Such a high value of the friction coefficient was previously reported by Czichos et al. for the steel–ceramic rubbing pair at a moderate sliding speed w11x. In these cases, the values of the friction coefficient decrease with sliding distance from the maximum value and tend to stabilise at approximately 0.45–0.48 when passing the running-in phase. The differences in ‘final’ values recorded can manifest the fact that at the end of all these tests Al2O3 particles torn from the cutting edge influence the behaviour of the contact zone. At the highest speed of 3.0 mys, the tribo-pairs containing both three- and four-layer hard films perform similarly to P30 uncoated carbide and two-layer coating. As a consequence, all the materials tested showed a gradual growth of the friction coefficient and its stabilisation after approximately 700 m of sliding distance at a practically constant value of 0.4. This probably results from the changes in the friction forces due to the substantial thermal softening of a carbon steel countermaterial. The effect of coating structure on the friction coefficient observed at the lowest and the highest sliding
speeds applied is presented in Figs. 9 and 10, respectively. The experimental evidence shows that the influence of the preparation of the disc surface on the frictional process seems to be of secondary importance. 4. Measurement and quantification of wear The linear wear of the coupled materials was measured and recorded as a function of the sliding distance. The selected relations are shown simultaneously with the friction coefficient in Figs. 11 and 12 in order to explain the tendencies in appropriate wear kinetics and to supplement the earlier analysis. In general, it arises
Fig. 10. Recordings of friction coefficient vs. sliding distance for tested cutting tool materials. Sliding speeds3.0 mys.
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Fig. 11. Recordings of a couple linear wear vs. sliding distance for tested cutting tool materials. Sliding speeds0.5 mys. RI—runningin.
from the plots that the majority of the frictional wear took place during the primary 100–120 m of sliding distance at a low speed (Fig. 11) and during the first 600–700 m of sliding distance at the highest speed applied (Fig. 12). Notice that this is a time period of 200–240 s in both cases. After this period of time the contact pressure decreases to a certain critical low value when the increment of wear becomes very small. The only exception concerns the uncoated specimen, which occurs at the speed of 3.0 mys where the significant rise of wear is observed at the test end. Notice that the
negative slopes in the graphs in Figs. 11 and 12 can be a result from the influence of thermal expansion of the mating materials when the contact temperature increases due to the frictional heat dissipation. From this point of view the running-in stage of the process should be finished after approximately 240 s in each test. The intensity of the wear of a couple was visually higher for lower sliding speeds, especially in the presence of Al2O3 ceramic layer. This can suggest that the protective coatings were fully removed after the first 50–120 m of sliding distance because of the severe abrasive action, likely, after this period of wear the metal-uncoated carbide pair was only involved in the sliding contact (Fig. 13a). After the sliding tests, optical microscopic examinations of the wear scars produced on the cylindrical surfaces of the inserts were carried out in order to select the prevailing wear mechanisms of coatings and the substrate. Roughly, three types of wear mechanisms of the coatings can be distinguished. They are as follows: 1. Severe abrasive wear of the specimen materials performed under the elastic contact conditions at the lowest sliding speed applied, which was specially intensive in the presence of Al2O3 loose particles or partially settled in the counterface. In particular, loose particles agglomerated in lumps close to the scar edge were observed in this case, as shown in Fig. 13a. 2. Both severe abrasive wear of the specimen material in which superficial layer of the disc is armed with the ceramic particles, which is not yet thermally plastified, and mild wear caused by plastic flow and attrition in contact with the superficially plastified disc face. The coating layers with marks of both wear mechanisms have been observed at the moderate sliding speed applied, as illustrated in Fig. 13b. 3. Moderate wear of the protective layers resulting from both plastic flow and attrition occurring in contact with the superficially plastified countermaterial. The exposed Al2O3 layer is observed at the highest sliding speed selected, as shown in Fig. 13c. The length and width of the wear scar produced on the tip were measured by an optical scope, and its depth was assessed by means of a light cross-sectional method. The results obtained were used to compute the specific volumetric wear rate given by the following equation: ks
Fig. 12. Recordings of a couple linear wear vs. sliding distance for tested cutting tool materials. Sliding speeds3.0 mys. RI—runningin.
V d=Fn
(1)
where V is the volume of the removed material in mm3. k, expressed in mm3 y(m N), was used as a factor, which allowed comparison of the wear resistance of coatings tested in relation to the reference. The values
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Fig. 13. Typical wear scars observed at different sliding speeds: (a) vss0.5 mys; (b) vss1.5 mys; (c) vss3.0 mys. Coating: multilayer TiCyTiCNyAl2O3 yTiN.
of the wear rates determined for the sliding time equal to 1200 s by means of Eq. (1) are listed in Table 1. The influence of the sliding speed on both the amount of wear and the wear rates when keeping the normal load at 50 N, is shown in Figs. 14 and 15, respectively. Several valuable remarks arising from the wear quantification are listed below, namely: –
–
high specific wear rates recorded at lower sliding speeds decrease with the elevation of the sliding speed for all tribo-pairs. at the lowest sliding speed the wear rates of 9.08=10y7 mm3 y(mN) for three- and 8.05=10y7 mm3 y(mN) for four-layer coatings including ceramic Al2O3 layer exceed those calculated for other specimen materials tested; i.e. is equal to
–
–
–
4.16=10y7 mm3 y(mN) and 6.37=10y7 mm3 y (mN) for TiCyTiN coating and uncoated specimen, respectively. in contrast, at the highest sliding speed the wear rates of 2.07=10y8 for three- and 2.03=10y8 mm3 y(mN) for four-layer coatings including ceramic Al2O3 layer are lower than those determined for uncoated specimen 1.01=10y7 mm3 y(mN) and TiCyTiN coating 1.4=10y8 mm3 y(mN). the minimum volumetric wear rate occurs when carbon steel with the TiCyTiN coating coupled. As can be seen in Fig. 15, this effect occurs in the full range of the sliding speeds applied, i.e. from 0.5 to 3.0 mymin. coatings containing the intermediate ceramic layer indicate lower wear resistance, especially in com-
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Table 1 Specification of the wear rates for tool materials tested after time ts1200 s Sliding speed vs, mys
Specimen material
(sliding distance d)
P30
0.5 1.0 1.5 2.0 3.0
–
(600) (1200) (1800) (2400) (3600)
y7
y3
TiCyTi(C,N)yAl2 O3 yTiN
TiCyAl2O3 yTiN
TiCyTiN y3
y7
V=10 (mm3)
k=10 (mm3y(m N))
V=10 (mm3)
k=10 (mm3y(m N))
V=10 (mm3)
k=10 (mm3y(m N))
V=10y3 (mm3)
k=10y7 (mm3y(m N))
19.2 16.6 21.0 18.6 18.2
6.37 2.77 2.33 1.56 1.01
12.5 12.7 9.9 3.5 2.6
4.16 2.11 0.80 0.38 0.14
27.0 26.8 31.5 14.7 4.8
9.08 4.46 3.51 1.22 0.27
24.0 22.8 16.1 8.6 4.2
8.05 3.81 1.79 0.72 0.23
parison to TiCyTiN layered carbide. This evidence was observed for all the sliding speeds applied, as shown in Fig. 15. the material volume removed during one second from the coated specimens decreases distinctly with the increase of the sliding speed. On the other hand, this factor seems to be practically independent of the sliding speed and the relevant sliding distance for the uncoated carbide specimen.
5. Discussion The experimental results show that at the low sliding speed of 0.5 mys the use of the ceramic Al2O3 layer does not improve or even somewhat deteriorates the wear resistance of the coated inserts. Relatively high values of the friction coefficient of 0.6 suggest the occurrence of high contact shear stresses. Moreover, distinct wear rates can be related to the intensive abrasive action of loose ceramic particles at the contact surfaces. These hard particles are torn out from the trailing edge of the crater formed and then they remove intensively the exposed substrate material.
Fig. 14. Dependence of volumetric wear rate on pin material and sliding speed rate. Pin materials: (1) P30 uncoated carbide; (2) TiCyTiN on carbide; (3) TiCyAl2O3yTiN on carbide; (4) TiCy(C,N)yAl2O3 yTiN on carbide.
y3
y7
However, it should be pointed out that at the highest sliding speed applied of 3.0 mys the coefficient of friction is reduced to approximately 0.4, independently of the coating structure. Moreover, the modification of wear mechanism due to the thermal plastification of the counter material diminishes significantly wear rates of the specimen. As a result, the protective role of multilayer coatings at higher sliding speed conditions becomes clear in comparison to the uncoated P30 specimen. According to the tendencies observed in these investigations it seems that the minimum wear rate has not yet been achieved for the coatings with the intermediate Al2O3 layer. The attainment of the minimum probably needs the use of higher sliding speed than the maximum one applied, i.e. substantially higher than 3.0 mys. For the applied conditions the minimum wear rates were found for the P30-TiCyTiN coating system against carbon steel, independently of the sliding speed used. This is in contrast to the experimental results obtained for the case when testing TiCyTiN hard film on HSS substrate at the sliding speed equal to 90 mymin w4x. The best performance of the P30-TiCyTiN coating
Fig. 15. Comparison of the wear rates for tool materials tested at different sliding speeds. Pin materials: (1) P30 uncoated carbide; (2) TiCyTiN on carbide; (3) TiCyAl2O3yTiN on carbide; (4) TiCyTi(C,N)yAl2 O3 yTiN on carbide.
W. Grzesik et al. / Surface and Coatings Technology 155 (2002) 37–45
system can be explained in terms of the hot hardness of the coating materials tested. The comparison of microhardness for different types of coatings indicate that titanium carbide and titanium carbonitride outperform both aluminium oxide and titanium nitride w12,13x. For example, at approximately 600 8C hot hardness for (Ti,C)N and TiN thin layer is approximately equal to 2000 and 1250 HV, respectively w13x. The tool wear mechanisms by plastic flow and attrition on ceramic coating layers at the high and by plucking at the lower speed conditions were reported by Ezugwu et al. w14x. 6. Conclusions (i) Majority of the specimens wear takes place during the first 200–240 s of sliding independently on the applied speed, after this time period the intensity of wear decreases drastically. (ii) Maximum values of specific wear rate observed at the lowest sliding speeds applied decrease significantly with the elevation of the sliding speed for all tribo-pairs, especially for coated specimens (factor of 30%35 for vs3.0 mys). (iii) The use of coatings with an Al2O3 intermediate layer at lower sliding speed of approximately 0.5 mys deteriorates the wear resistance of the coated specimens, but its protective role becomes incontestable at high speed conditions in comparison to an uncoated carbide specimen. (iv) The maximum wear resistance was revealed in the case of a carbon steel–TiCyTiN coating tribo-pair in the full range of the sliding speeds applied, i.e. from 0.5 to 3.0 mymin. (v) The importance of the proposed methodology of the experimental friction and wear quantification lies in providing the new knowledge on the influence of multilayer coating structures on the cutting tool wear resistance. Moreover, it can assist the optimum selection of coated tools for the dry cutting of steels.
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References w1x K. Holmberg, A. Matthews, H. Ronkainen, Coatings tribologycontact mechanisms and surface design, Tribol. Int. 32 (1–3) (1998) 107–120. w2x M.O. Olsson, S. Soderberg, ¨ S. Jacobson, S. Hogmark, Simulation of cutting tool wear by a modified pin-on-disc test, Int. J Mach. Tools Manufact. 29(3), 377–90. w3x M. Meiller, J.L. Lebrun, M. Touratier, D. Ryckelnck, Friction law for toolyworkpiece contact area in dry machining, in: Proceedings of the International Workshop on Friction and Flow Stress in Cutting and Forming, ENSAM-Paris, Paris, 2000, pp. 101–109. w4x P. Hedenquist, M. Olsson, Sliding wear testing of coated cutting tool materials, Tribol. Int. 23 (3) (1991) 143–150. w5x U. Wiklund, O. Wanstrand, ¨ M. Larsson, S. Hogmark, Evaluation of new multilayered physical vapour deposition coatings in sliding contact, Wear 236 (1999) 88–95. w6x S. Hogmark, P. Hedenqvist, S. Jacobson, Tribological properties of thin hard coatings: demands and evaluation, Surf. Coat. Technol. 90 (1997) 247–257. w7x K. Holmberg, A. Matthews, in: D. Dowson (Ed.), Coating Tribology. Properties, Techniques and Applications in Surface Engineering, Tribology Series, 28, Elsevier, Amsterdam, 1998. w8x W. Grzesik, The role of coatings in controlling the cutting process when turning with coated indexable inserts, J. Mater. Proc. Technol. 79 (1998) 133–143. w9x W. Grzesik, The influence of thin hard coatings on frictional responses in the orthogonal cutting process, in: Proceedings of the International Workshop on Friction and Flow Stress in Cutting and Forming, ENSAM-Paris, Paris, 2000, pp. 83–90. w10x W. Grzesik, Z. Zalisz, P. Nieslony, Assessing the ability of metal-on-coating pairs to modify frictional responses with application to the cutting process, in: M. Hadfield, C. Ciantar (Eds.), 1st International Conference on Tribology in Environmental Design 2000, Professional Engineering Publishing Ltd, Bury St Edmunds and London, 2000, pp. 147–154. w11x H. Czichos, S. Becker, J. Lexow, Multilaboratory tribotesting: results from the versatile advanced materials and standards programme on wear test methods, Wear 114 (1987) 109–130. w12x D.A. Stephenson, J.S. Agapiou, Metal Cutting Theory and Practice, Marcel Dekker, New York, 1997. w13x F. Klocke, T. Krieg, Coated tools for metal cutting-features and applications, Ann. CIRP 48y2 (1999) 515–525. w14x E.O. Ezugwu, K.A. Olajire, A. Jawaid, Wear performance of multilayer-coated carbide tools, Mach. Sci. Technol. 5 (1) (2001) 115–129.
Friction and wear testing of multilayer coatings on carbide substrates for dry machining applications W. Grzesik*, Z. Zalisz, P. Nieslony Department of Manufacturing Engineering and Production Automation, Technical University of Opole, 5th Mikolajczyka St., P.O. Box 321, 45-233 Opole, Poland Received 9 July 2001; accepted in revised form 22 January 2002
Abstract The principle aim of this paper is to investigate three wear-protective coatings with multilayer structures, which are frequently used in the cutting tool industry and to assess their frictional behaviour under the test conditions equivalent to those for the cutting of medium carbon steel. A modified pin-on-disc tester was used to conduct experiments in which both the friction coefficient and the linear wear of the tribo-pair were recorded vs. sliding distance. The volumetric wear rate was proposed as a parameter for quantitative comparison of the wear resistance of the tribo-pairs tested. It was found that the principle stage of the specimen wear takes place during the first 200–240 s of sliding independently of the applied speed; obviously, after this stage the intensity of wear decreases drastically. The maximum values of specific wear rate observed at the lowest sliding speeds applied decrease significantly with the elevation of the sliding speed for all tribo-pairs, especially for coated specimens. The use of coatings with an Al2 O3 intermediate layer at the lower sliding speed of approximately 0.5 mys deteriorates the wear resistance of the coated specimens, but its protective role becomes more important at the high speed conditions in comparison to an uncoated carbide specimen. The maximum wear resistance was revealed in the case of a carbon steel–TiCyTiN coating tribo-pair for sliding speeds ranging from 0.5 to 3.0 mymin. It is concluded that the proposed methodology for experimental friction and wear quantification provides the new knowledge for multilayer coating structures on the cutting tool wear resistance and can be helpful in the optimum selection of coated tools for the dry cutting of steels. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Wear; Friction; Cylinder-on-disc; CVD multilayer coatings
1. Introduction The prediction and control of friction and wear is one of the most essential problems emerging in the design of cutting operations w1x. Metal machining is characterised by extreme contact conditions at the tool–workpiece and the tool–chip interfaces including high contact pressure and temperature, and especially highly active and freshly generated surfaces. Usually it is not possible to reproduce in full-scale the contact conditions in machining using conventional pin-on-disc testing because the wear mechanisms involved are not relevant to that observed in machining. In general, modified pinon-disc (Fig. 1) w2,3x, pin-on ring (crossed cylinders) w4,7x and ball-on-disc w5x test devices have been devel*Corresponding author. Tel.: q48-77-4006-290; fax: q48-774006-342. E-mail address: [email protected] (W. Grzesik).
oped in order to perform sliding wear tests that simulate dry machining when using coated cutting tool materials. In particular, Hedenqvist and Olsson w4x have shown that the coating tested and the substrate should be considered as one element termed as a coatingysubstrate system. Recently, Lebrun et al. w3x have carried out extensive friction experiments using a special test device including the thermal and mechanical outputs of the plain–plain contact tribo-system. In both this and in Refs. w8,9x the vital role of the frictional heat flux (friction mechanical dissipation) in the frictional behaviour of the work (chip)-coatingy substrate system was confirmed. In the sliding wear test presented in Fig. 2a a cylindrical test pin of 5 mm in diameter is mounted vertically on the tool holder and the cylindrical counter material is fixed in the lathe mandrel. A spring loading system is used to apply the
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 0 4 0 - 3
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W. Grzesik et al. / Surface and Coatings Technology 155 (2002) 37–45
selected metal-on-coating and metal-on-WCyCo carbide pairs that reproduce orthogonal cutting conditions at low, throughout moderate, up to high sliding speeds, respectively. 2. Experimentation 2.1. Test equipment
Fig. 1. A scheme of apparatus for modified pin-on-disc testing w2x.
normal force. In addition, continuous introduction of a fresh surface of work material, which is an important feature of a wear test for simulation of the cutting tool wear, is performed by engaging the feed motion of the lathe. During the test, friction and surface damage of the coating are controlled by the properties of the counter material together with the normal force and sliding speed. As can be seen in Fig. 2b, in this study the sliding wear of four different PVD coatings, i.e. CrN, TiN, (TiAl)N and Ti(C,N) on HSS (ASP 30) was investigated. The experimental results obtained, shown in Fig. 2b, revealed that at a low sliding speed wear is governed mainly by the coating hardness, i.e. the hardest Ti(C,N) coating displays the lowest wear rate (the hardness of the coatings increases in the order CrN– TiN–(Ti,Al)N–Ti(C,N). On the other hand, at the highest speed of 90 mymin the (Ti,Al)N coating outperformed the three other coatings tested. This can be attributed to a low oxidation rate of this coating and good tribological properties of the thin oxide layer formed on the top coating surface. In contrast, the low oxidation rate of the CrN coating probably explains a relatively good performance of this film at the sliding speed of approximately 90 mymin w4x. A modified approach to the use of a long sliding distance-friction-laboratory testing, which employs a special equipment layout based on line type of contact between the cylindrical specimen (round insert) and a flat rotating disc, is shown in Fig. 3. Experiments were designed to compare the frictional behaviour of the
As mentioned above, uncoated and coated carbides sliding against the carbon steel disc were tested at ambient under dry friction conditions using a modified tribo-tester equipped with control and computer DAQ systems. The functional structure of the experimental set-up and instrumentation for process measurement and control are presented in Fig. 4. The contact conditions in the investigations were generated by pressing the clearance face of a round tool insert to the flat face surface of the rotating steel disc, as shown in Fig. 3, thus the line type of contact was always kept at the beginning of the tribo-test performed w7x. The main features of the device used are as follows w10x: a the inputs: the normal load Fn and the sliding speed vs can be changed continuously and precisely stabilised by means of a pneumatic load and steeples power regulation systems, respectively. b the outputs: the friction force Ff, the linear wear of coupling materials Dl and the contact temperature of the specimen Tc are measured and analysed in real-time mode as functions of the sliding distance, d, or equivalently of time, t. Finally, the coefficient of sliding friction m, defined as the ratio of the variable friction force to the constant normal load, is continuously computed and recorded. c the tribo-tester operates under the control of specially designed computer software, which was installed in the memory of a PC computer equipped with a 16-channel AyD converter. 2.2. Materials A normalised medium carbon C45 steel equivalent to AISI 1045 (UTSs670 MPa; 35 HRC), frequently used
Fig. 2. Schematic illustration of the pin-on-ring device for sliding wear test (a) with test pin (at A) and counter material (at B). Wear of selected PVD coatings deposited on HSS test pins as a function of sliding speed when run against carbon steel (b) w6x.
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in the friction tests performed. The total thicknesses of these coatings were equal to 8, 8 and 10 mm, respectively. As a reference, conventional (uncoated) ISO-P30 cemented carbide was used. 2.3. Test procedure
Fig. 3. A scheme of the tribo-pair configuration used in this investigation.
in comparative machinability tests, was selected as a countermaterial. Three different states of the disc face surfaces were examined: fresh grounded, run-in (worn) after one trial and run-in (worn) after two trials. The mean diameter of the disc–pin contact was assumed to be equal to 79.7 mm. The test pins were rounded inserts of 25.4 diameter and of RNMG 250900 ISO designation. Three multilayered CVD coatings: two-layer TiCyTiN (NT35), three-layer TiCyAl2O3 yTiN (NT15) and fourlayer TiCyTi(C,N)yAl2 O3 yTiN (NT20) were included
Sliding friction tests were carried out with four material combinations, keeping the normal force at Fns 50 N. The sliding speed was varied from 0.5 to 3.0 my s, which corresponds to the cutting speeds normally employed when cutting carbon steels using uncoated and carbide tools. For each tribo-pair, the sliding distance was selected to be equivalent to a test time (tool life) of 20 min (1200 s). Prior to each test the working surfaces of specimens were cleaned ultrasonically with tetrachloromethane (carbon tetrachloride) and dried in a warm air stream. Three trials were conducted for each materialysliding speed combination. To quantify the friction behaviour of tool materials, the friction force was measured, and by using a DAQ system instantaneous values of the coefficient of friction were computed and recorded for certain intervals of time (;4 s). Light optical microscopy (LOM) was used to examine mating surfaces and to characterise the dominant wear mechanisms of the tool materials. In this investigation,
Fig. 4. Equipment layout for the cylinder-on-disc tester with load and temperature control.
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Fig. 5. Recordings of friction coefficient vs. sliding distance for P30 uncoated carbide-AISI 1045 steel pair.
Fig. 6. Recordings of friction coefficient vs. sliding distance for NT35, TiCyTiN coating-AISI 1045 steel pair.
an Olympus PME3 optical microscope equipped with a JVC TK-1281 colour video camera was employed. The selected wear scars produced on coated inserts (specimens) were visualised and the video signals were transmitted to a PC computer. Subsequently, the raw images were processed and dimensioned by means of a computer image processing system developed earlier by the authors w8x. The depth of the scar in the centre was measured by means of a light cross-sectional method using a double optical microscope.
Then, at the intermediate and higher speeds the following tens of meters of sliding result in the decrease of friction, which reaches the local minimum value promptly at a lower sliding speed, especially in the presence of Al2O3 ceramic layer. This sequence of friction variations that occurs during the primary 50– 100 m of sliding seems to be a typical case for all trials, independent of the coatings tested. Further tribological behaviour of the tribo-pairs tested depends significantly on the coating composition. The TiCyTiN coated and uncoated carbides showed a slow increase of the friction coefficient until the stabilisation of m was reached. In
3. Determination of frictional responses of the tribosystems In order to characterise the frictional interaction for the selected tribo-pairs, the friction force was measured with a frequency of 0.1 sy1 and converted into the coefficient of friction, which was stored in the computer memory as a function of the sliding distance d (equivalent to test time t). The selected relationships obtained for 60 trials were plotted in Figs. 5–10 in order to show the characteristic tendencies in frictional behaviour of the four tribo-contacts. Variations of the friction coefficient obtained for three different sliding speeds vss0.5, 1.5 and 3. 0 mys are presented in Figs. 5–10. As can be seen from these figures, frictional behaviour differs with the sliding speed applied and with the presence or absence of the protective layers, especially an intermediate Al2O3 ceramic layer in the multilayer coating structure. In general, a rapid increase of the friction coefficient, reaching the local maximum value at the lowest speed, of 0.5 mys was detected at the primary 50 m of sliding, independently of the tribo-pair tested (Figs. 5–8).
Fig. 7. Recordings of friction coefficient vs. sliding distance for NT15, TiCyAl2O3yTiN coating-AISI 1045 steel pair.
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Fig. 8. Recordings of friction coefficient vs. sliding distance for NT20, TiCyTi(C,N)yAl2 O3 yTiN coating-AISI 1045 steel pair.
Fig. 9. Recordings of friction coefficient vs. sliding distance for tested cutting tool materials. Sliding speeds0.5 mys.
particular, for the sliding speed of 0.5 mymin, the values of friction coefficient for TiCyTiN coated and uncoated carbides (Figs. 5 and 6) increased up to 0.5 during the first 50–70 m of sliding and next decreased rapidly to the local minimum of 0.35, then it slowly grew up and tended to stabilise at the value of approximately 0.42%0.45 after 200 m of sliding. There is a different frictional behaviour of multilayer coatings containing an intermediate A2O3 ceramic layer (Figs. 7 and 8). For the sliding speed of 0.5 mymin, the rapid increase in the friction coefficient up to 0.6 after the first 100 m of sliding distance was observed, which was preceded temporarily by the local minimum. Such a high value of the friction coefficient was previously reported by Czichos et al. for the steel–ceramic rubbing pair at a moderate sliding speed w11x. In these cases, the values of the friction coefficient decrease with sliding distance from the maximum value and tend to stabilise at approximately 0.45–0.48 when passing the running-in phase. The differences in ‘final’ values recorded can manifest the fact that at the end of all these tests Al2O3 particles torn from the cutting edge influence the behaviour of the contact zone. At the highest speed of 3.0 mys, the tribo-pairs containing both three- and four-layer hard films perform similarly to P30 uncoated carbide and two-layer coating. As a consequence, all the materials tested showed a gradual growth of the friction coefficient and its stabilisation after approximately 700 m of sliding distance at a practically constant value of 0.4. This probably results from the changes in the friction forces due to the substantial thermal softening of a carbon steel countermaterial. The effect of coating structure on the friction coefficient observed at the lowest and the highest sliding
speeds applied is presented in Figs. 9 and 10, respectively. The experimental evidence shows that the influence of the preparation of the disc surface on the frictional process seems to be of secondary importance. 4. Measurement and quantification of wear The linear wear of the coupled materials was measured and recorded as a function of the sliding distance. The selected relations are shown simultaneously with the friction coefficient in Figs. 11 and 12 in order to explain the tendencies in appropriate wear kinetics and to supplement the earlier analysis. In general, it arises
Fig. 10. Recordings of friction coefficient vs. sliding distance for tested cutting tool materials. Sliding speeds3.0 mys.
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Fig. 11. Recordings of a couple linear wear vs. sliding distance for tested cutting tool materials. Sliding speeds0.5 mys. RI—runningin.
from the plots that the majority of the frictional wear took place during the primary 100–120 m of sliding distance at a low speed (Fig. 11) and during the first 600–700 m of sliding distance at the highest speed applied (Fig. 12). Notice that this is a time period of 200–240 s in both cases. After this period of time the contact pressure decreases to a certain critical low value when the increment of wear becomes very small. The only exception concerns the uncoated specimen, which occurs at the speed of 3.0 mys where the significant rise of wear is observed at the test end. Notice that the
negative slopes in the graphs in Figs. 11 and 12 can be a result from the influence of thermal expansion of the mating materials when the contact temperature increases due to the frictional heat dissipation. From this point of view the running-in stage of the process should be finished after approximately 240 s in each test. The intensity of the wear of a couple was visually higher for lower sliding speeds, especially in the presence of Al2O3 ceramic layer. This can suggest that the protective coatings were fully removed after the first 50–120 m of sliding distance because of the severe abrasive action, likely, after this period of wear the metal-uncoated carbide pair was only involved in the sliding contact (Fig. 13a). After the sliding tests, optical microscopic examinations of the wear scars produced on the cylindrical surfaces of the inserts were carried out in order to select the prevailing wear mechanisms of coatings and the substrate. Roughly, three types of wear mechanisms of the coatings can be distinguished. They are as follows: 1. Severe abrasive wear of the specimen materials performed under the elastic contact conditions at the lowest sliding speed applied, which was specially intensive in the presence of Al2O3 loose particles or partially settled in the counterface. In particular, loose particles agglomerated in lumps close to the scar edge were observed in this case, as shown in Fig. 13a. 2. Both severe abrasive wear of the specimen material in which superficial layer of the disc is armed with the ceramic particles, which is not yet thermally plastified, and mild wear caused by plastic flow and attrition in contact with the superficially plastified disc face. The coating layers with marks of both wear mechanisms have been observed at the moderate sliding speed applied, as illustrated in Fig. 13b. 3. Moderate wear of the protective layers resulting from both plastic flow and attrition occurring in contact with the superficially plastified countermaterial. The exposed Al2O3 layer is observed at the highest sliding speed selected, as shown in Fig. 13c. The length and width of the wear scar produced on the tip were measured by an optical scope, and its depth was assessed by means of a light cross-sectional method. The results obtained were used to compute the specific volumetric wear rate given by the following equation: ks
Fig. 12. Recordings of a couple linear wear vs. sliding distance for tested cutting tool materials. Sliding speeds3.0 mys. RI—runningin.
V d=Fn
(1)
where V is the volume of the removed material in mm3. k, expressed in mm3 y(m N), was used as a factor, which allowed comparison of the wear resistance of coatings tested in relation to the reference. The values
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Fig. 13. Typical wear scars observed at different sliding speeds: (a) vss0.5 mys; (b) vss1.5 mys; (c) vss3.0 mys. Coating: multilayer TiCyTiCNyAl2O3 yTiN.
of the wear rates determined for the sliding time equal to 1200 s by means of Eq. (1) are listed in Table 1. The influence of the sliding speed on both the amount of wear and the wear rates when keeping the normal load at 50 N, is shown in Figs. 14 and 15, respectively. Several valuable remarks arising from the wear quantification are listed below, namely: –
–
high specific wear rates recorded at lower sliding speeds decrease with the elevation of the sliding speed for all tribo-pairs. at the lowest sliding speed the wear rates of 9.08=10y7 mm3 y(mN) for three- and 8.05=10y7 mm3 y(mN) for four-layer coatings including ceramic Al2O3 layer exceed those calculated for other specimen materials tested; i.e. is equal to
–
–
–
4.16=10y7 mm3 y(mN) and 6.37=10y7 mm3 y (mN) for TiCyTiN coating and uncoated specimen, respectively. in contrast, at the highest sliding speed the wear rates of 2.07=10y8 for three- and 2.03=10y8 mm3 y(mN) for four-layer coatings including ceramic Al2O3 layer are lower than those determined for uncoated specimen 1.01=10y7 mm3 y(mN) and TiCyTiN coating 1.4=10y8 mm3 y(mN). the minimum volumetric wear rate occurs when carbon steel with the TiCyTiN coating coupled. As can be seen in Fig. 15, this effect occurs in the full range of the sliding speeds applied, i.e. from 0.5 to 3.0 mymin. coatings containing the intermediate ceramic layer indicate lower wear resistance, especially in com-
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Table 1 Specification of the wear rates for tool materials tested after time ts1200 s Sliding speed vs, mys
Specimen material
(sliding distance d)
P30
0.5 1.0 1.5 2.0 3.0
–
(600) (1200) (1800) (2400) (3600)
y7
y3
TiCyTi(C,N)yAl2 O3 yTiN
TiCyAl2O3 yTiN
TiCyTiN y3
y7
V=10 (mm3)
k=10 (mm3y(m N))
V=10 (mm3)
k=10 (mm3y(m N))
V=10 (mm3)
k=10 (mm3y(m N))
V=10y3 (mm3)
k=10y7 (mm3y(m N))
19.2 16.6 21.0 18.6 18.2
6.37 2.77 2.33 1.56 1.01
12.5 12.7 9.9 3.5 2.6
4.16 2.11 0.80 0.38 0.14
27.0 26.8 31.5 14.7 4.8
9.08 4.46 3.51 1.22 0.27
24.0 22.8 16.1 8.6 4.2
8.05 3.81 1.79 0.72 0.23
parison to TiCyTiN layered carbide. This evidence was observed for all the sliding speeds applied, as shown in Fig. 15. the material volume removed during one second from the coated specimens decreases distinctly with the increase of the sliding speed. On the other hand, this factor seems to be practically independent of the sliding speed and the relevant sliding distance for the uncoated carbide specimen.
5. Discussion The experimental results show that at the low sliding speed of 0.5 mys the use of the ceramic Al2O3 layer does not improve or even somewhat deteriorates the wear resistance of the coated inserts. Relatively high values of the friction coefficient of 0.6 suggest the occurrence of high contact shear stresses. Moreover, distinct wear rates can be related to the intensive abrasive action of loose ceramic particles at the contact surfaces. These hard particles are torn out from the trailing edge of the crater formed and then they remove intensively the exposed substrate material.
Fig. 14. Dependence of volumetric wear rate on pin material and sliding speed rate. Pin materials: (1) P30 uncoated carbide; (2) TiCyTiN on carbide; (3) TiCyAl2O3yTiN on carbide; (4) TiCy(C,N)yAl2O3 yTiN on carbide.
y3
y7
However, it should be pointed out that at the highest sliding speed applied of 3.0 mys the coefficient of friction is reduced to approximately 0.4, independently of the coating structure. Moreover, the modification of wear mechanism due to the thermal plastification of the counter material diminishes significantly wear rates of the specimen. As a result, the protective role of multilayer coatings at higher sliding speed conditions becomes clear in comparison to the uncoated P30 specimen. According to the tendencies observed in these investigations it seems that the minimum wear rate has not yet been achieved for the coatings with the intermediate Al2O3 layer. The attainment of the minimum probably needs the use of higher sliding speed than the maximum one applied, i.e. substantially higher than 3.0 mys. For the applied conditions the minimum wear rates were found for the P30-TiCyTiN coating system against carbon steel, independently of the sliding speed used. This is in contrast to the experimental results obtained for the case when testing TiCyTiN hard film on HSS substrate at the sliding speed equal to 90 mymin w4x. The best performance of the P30-TiCyTiN coating
Fig. 15. Comparison of the wear rates for tool materials tested at different sliding speeds. Pin materials: (1) P30 uncoated carbide; (2) TiCyTiN on carbide; (3) TiCyAl2O3yTiN on carbide; (4) TiCyTi(C,N)yAl2 O3 yTiN on carbide.
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system can be explained in terms of the hot hardness of the coating materials tested. The comparison of microhardness for different types of coatings indicate that titanium carbide and titanium carbonitride outperform both aluminium oxide and titanium nitride w12,13x. For example, at approximately 600 8C hot hardness for (Ti,C)N and TiN thin layer is approximately equal to 2000 and 1250 HV, respectively w13x. The tool wear mechanisms by plastic flow and attrition on ceramic coating layers at the high and by plucking at the lower speed conditions were reported by Ezugwu et al. w14x. 6. Conclusions (i) Majority of the specimens wear takes place during the first 200–240 s of sliding independently on the applied speed, after this time period the intensity of wear decreases drastically. (ii) Maximum values of specific wear rate observed at the lowest sliding speeds applied decrease significantly with the elevation of the sliding speed for all tribo-pairs, especially for coated specimens (factor of 30%35 for vs3.0 mys). (iii) The use of coatings with an Al2O3 intermediate layer at lower sliding speed of approximately 0.5 mys deteriorates the wear resistance of the coated specimens, but its protective role becomes incontestable at high speed conditions in comparison to an uncoated carbide specimen. (iv) The maximum wear resistance was revealed in the case of a carbon steel–TiCyTiN coating tribo-pair in the full range of the sliding speeds applied, i.e. from 0.5 to 3.0 mymin. (v) The importance of the proposed methodology of the experimental friction and wear quantification lies in providing the new knowledge on the influence of multilayer coating structures on the cutting tool wear resistance. Moreover, it can assist the optimum selection of coated tools for the dry cutting of steels.
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