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Tribological properties of PVD titanium carbides
Vacuum 53 (1999) 57—60
Tribological properties of PVD titanium carbides M. Pancielejkoa,*, W. Prechta, A. Czyzniewskib a
Institute of Materials Science and Technology, Technical University of Koszalin, Raclawicka 15, 75-620 Koszalin, Poland b Technology Transfer Centre, Technical University of Koszalin, Raclawicka 15, 75-620 Koszalin, Poland
Abstract The influence of the acetylene flow rate on the tribological properties of TiC films produced by cathodic arc plasma deposition (CAPD) on the high-speed steel substrates was investigated. The following properties of films were tested: phase and chemical composition, microhardness, adhesion characterised by the scratch method, thickness, fracture morphology (by SEM), surface roughness, friction coefficient and abrasive wear (by ball-on-disk method). The flow rate of acetylene was changed between 20—250 sccm by constant substrate bias voltage of !70 V. The highest values of microhardness of coatings were obtained at a flow rate between 70 and 140 sccm. The maximum of 43 GPa value was achieved at a flow rate of 70 sccm. At flow rates of 20 and above 180 sccm, the microhardness was about 16 GPa. The friction coefficient reaches the value of 0.2, its decreasing is caused by the increase in flow rate of acetylene. Coating abrasive wear also decreases. The highest values of abrasive wear were obtained at the acetylene flow of 20 sccm, for which the dry friction coefficient was 0.54. The abrasive wear of the counter-specimen (ball) gets also bigger for higher values of the dry friction coefficient. By X-ray examination of crystalline structure, reflexes from (1 1 1)TiC and (2 2 0)TiC planes were recorded; they disappear at higher acetylene flow rates. This may be the result of an amorphisation process of the film structure. 1999 Elsevier Science Ltd. All rights reserved. Keywords: CAPD PVD; Titanium carbide; Tribological properties; Ball-on-disk
1. Introduction TiC films coated by PVD methods have revealed excellent tribological properties. They mainly depend on the particular technology, applied deposition parameters and the type of applied reactive carbonaceous gas. The most often used gases are methane, acetylene, ethene and ethane [1—9]. The microhardness of these films, measured by the authors [2—6,10], was mostly of the order of 35 GPa, seldom lower than 25 GPa [8,11]; but sometimes it was very high — i.e. over 40 GPa [1,9]. Adhesion characterised by critical load ¸ in the scratch method ranged ! from approx. 20 N to over 100 N [2,8,9]. The values of a friction coefficient obtained during the tests on wear resistance were ranged from 0.3 [4,5] to 0.6 [8]. They depended on measurement method, operating characteristic of devices and conditions of friction measurements [12,13].
* Corresponding author. Tel. 0048 94 342 7881; fax: 0048 94 342 6753; e-mail: [email protected]
TiC films produced by PVD methods, in view of their properties, have mainly been applied to coating of tools made of high-speed steel and tool edges of sintered carbides in order to increase their life [2,11,12,14]. Results of an investigation in the abrasive wear resistance of TiC films coated by a CAPD method on HSS substrates are presented in this work. TiC films were coated at an acetylene flow rate from 20 to 250 sccm and constant substrate bias voltage.
2. Experimental procedure TiC films were coated by CAPD method. Values of major process parameters are presented in Table 1. Acetylene of purity 99.8% was used as a reactive gas. Substrates were made of HSS type HS6-5-2 in disk shape (dia. 40 mm;4 mm, 63 HRC, R (0.05 lm). The preparation of the substrates for the process consisted in ultrasonic-aided cleaning with organic solvents and alkaline detergents. The next operation was the cleaning with trichloroethylene vapour. In the vacuum chamber the substrates were cleaned by argon glow discharge. A series
0042-207X/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 3 9 1 - 1
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M. Pancielejko et al. / Vacuum 53 (1999) 57—60
E The film morphology was examined by SEM, with a JSM-S1, Jeol instrument.
Table 1 Technological parameters of the TiC coating deposition Deposition parameter Residual total pressure (Pa) Acetylene flow rate (sccm) Cathode voltage (V) Cathode current (A) Negative bias voltage (V) Distance target—substrate (cm) Deposition rate (lm min\) Substrate temperature after ion cleaning (K) Minimal cooling time in vacuum (min)
Value
3. Results and discussion
4;10\ 20—250 20 86 !20—!250 20 0.2—0.4 523—573 60
of coating processes were carried out at stabilised flow rates of acetylene, ranging from 20 to 250 sccm at constant substrate bias voltage º "!70 V. The properties of the specimens coated with films to a thickness of about 4 lm were tested using the following devices and methods: E Adhesion characterisation was carried out by the scratch method using a Scratch Tester Revetest威 produced by CSEM. E The friction coefficient and wear resistance were determined using a ball-on-disk equipment. Applied parameters: linear sliding speed 0.1 m;s\, load 20 N, sliding length 2000 m, the counter-specimen was a ball made of 100Cr6 bearing steel (dia. 10 mm, 62 HRC), at relative humidity of 40$5% and ambient temperature 23$1°C. To determine the abrasive wear resistance a wear rate coefficient k was introduced, which comprises the volume of the worn groove profile, the sliding distance and the normal load. The formula for k is: » k" ¸;d
E
E E
E E
(mm m\ N\),
Experimental results of chemical composition analyses obtained by the EDAX method (at% Ti), microhardness lHV, adhesion characteristic (critical load ¸ ) and ! a wear rate coefficient k of TiC films obtained at varying flow rates of acetylene, i.e. from 20 to 250 sccm, and at constant substrate bias voltage º "!70 V is illustrated in Fig. 1. The maximum microhardness of 43 GPa was measured in films coated at a steady acetylene flow of 70 sccm. This value is close or higher than reference data [2—6,8,10,11]. When a flow rate was 20 or over 180 sccm, the film microhardness was approx. 16 GPa. Decrease in microhardness of films obtained at flows over 180 sccm is possible to result from the formation of polymer-type Ti-C : H films with high carbon content and low fraction of Ti atoms. This could be confirmed by analysis the titanium content in films by EDAX method as well as the authors’ previous tests [15]. The critical load ¸ which characterises the coating ! adhesion to substrates was for all films over 40 N. An marched increase of adhesion was observed at flow rates between 80 and 180 sccm, which may be connected with an increase slightly in film thickness. One may observe, that the wear rate coefficient k decreases at higher flow rates of acetylene over the whole measuring range, pointing to higher wear resistance with
(1)
where » is the wear volume (mm), ¸ is the load (N) d is the sliding distance (m). The microhardness was examined with a metallographic microscope Neophot 2, Carl Zeiss Jena equipped with a Hanemann’s microhardness tester, with applied loads of 65 or 100 g. The thickness measurements were carried out by the Callotest method. The chemical composition of the films was determined by the EDAX method (energy dispersive X-ray analysis). An EDAX X-ray microanalyser KEVEX DELTA, class V (USA) was installed in an electron microscope Jeol Superprobe JXA-733 (Japan). The film roughness and profiles of worn grooves while testing were measured using a Hommel—Tester T2000 profilometer. The crystalline structure was tested on a Dron 2 X-ray diffractometer using CrK radiation. ? @
Fig. 1. Titanium content (at% Ti) of coatings determined by EDAX, microhardness lHV (GPa), adhesion characterisation (critical load ¸ ! (N)) and wear coefficient k (10\ mm/m;N) at titanium carbide coatings, against acetylene flow rate.
M. Pancielejko et al. / Vacuum 53 (1999) 57—60
59
Fig. 2. Dry friction coefficient curves for specimens obtained at different acetylene flow rates ( f "20, 80 and 250 sccm) at the constant !& substrate bias voltage of º "!70 V.
increasing flow rate. The volume of a worn in a counterspecimen was also on the decrease when the acetylene flow was increased. The runs of friction coefficients while wear testing for films coated at three different acetylene flow rates and at constant substrate bias voltage are shown in Fig. 2. In the case of films coated at the lowest acetylene flow (20 sccm) the highest friction coefficients were obtained, however considerable fluctuations were observed. The lower values of friction coefficient, approx. 0.2, and distinctly smaller fluctuations were registered at the highest acetylene flow rate (250 sccm). The rapid decrease in friction coefficient from 0.6 to 0.25 for a film obtained at a flow rate of 80 sccm is likely to be caused by material transfer from the steel counter-specimen to the film. At the beginning of the wear test probably the material of the steel counter-specimen was rubbed into the hard, rough film surface (R approx. 0.3 lm) which effected an increase in the friction coefficient to approx. 0.6. It is necessary to carry on further investigations on the composition of wearing film surfaces and counter-specimens during abrasive wear tests in order to explain the phenomena occurring in a friction mechanism between a TiC film and a hardened steel counter-specimen. The SEM pattern of TiC film fracture morphology obtained at an acetylene flow of 30 sccm, revealed compact structure with visible columnar grains (Fig. 3a). Fractures of films, obtained at higher acetylene flow rates, did not reveal distinct crystalline structures (Fig. 3b). Rather they have a glassy amorphous character. Morphology of film fractures confirmed the thesis of good adhesion of TiC films to the substrate, because there were no visible chips, spallings or cracks on the film-substrate border. Fractured films did not separate from the substrate and effectively reproduced the substrate surfaces topography. X-ray examination of TiC films, revealed the reflexes from characteristic crystallographic planes, i.e. (1 1 1)TiC
Fig. 3. SEM pattern of TiC coatings fractures: (a) f "30 sccm, !& º "!70 V. Mag. ;3000, º"10 kV, (b) f "180 sccm, º " !& !70 V. Mag. ;3000, º"10 kV.
and (2 2 0)TiC were recorded, in accordance with works [1—7,10]. The reflexes disappeared at higher acetylene flows, due, possibly to an amorphisation of the film structure.
4. Conclusions (1) The friction coefficient between TiC films and hardened bearing steel counter-specimens fluctuates from 0.52 to 0.17 and goes along with an increase in an acetylene flow rate at the film deposition. (2) The lowest friction coefficients were below 0.2, the lowest wear rate coefficients were obtained for films at the highest acetylene flow rates. (3) It is necessary to carry out further investigations on the composition of wearing film surfaces and counter-specimens for abrasive wear tests in order to explain the phenomena occurring in the sliding friction mode of TiC film and a hardened steel counterspecimen.
Acknowledgements Authors of this work to thank Dr F. Fendrych from Institute of Physic of the Czech Academy of Sciences for EDAX investigations.
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M. Pancielejko et al. / Vacuum 53 (1999) 57—60
References [1] Knotek O, Lo¨fler F, Kra¨mer G. Surf Coat Technol 1993;61:320. [2] Sproul WD. J Vac Sci Technol 1986;A4(6):2874. [3] Jeong JI, Hong JH, Kang JS, Shin HJ, Lee YP. J Vac Sci Technol 1991;A9(5):2618. [4] Jeong JI, Braun M, Gudowska I. J Vac Sci Technol 1994; A12(3):733. [5] Deng J, Braun M. Surf Coat Technol 1994;70:49. [6] Martin PJ, Netterfield RP, Kinder TJ, Descoˆtes L. Surf Coat Technol 1991;49:239. [7] Kaufherr N, Fenske GR, Bush DE, Lin P, Deshpandey C, Bunshah RF. Thin Solid Films 1987;153:149. [8] Voevodin AA, Rebholz C, Schneider JM, Stevenson P, Matthews A. Surf Coat Technol 1995;73:185.
[9] Arrando F, Polo MC, Molera P, Esteve J. Surf Coat Technol 1994;68/69:536. [10] Karlson L. Residual stresses and mechanical properties of Ti(C,N) thin films deposited by arc evaporation. Licentiate Thesis no. 633. Linko¨ping Studies in Science and Technology, Linko¨ping, May 1997. [11] Nakamura K, Inagawa K. Thin Solid Films 1977;40:155. [12] Habig K-H. Tribology International, April 89. 1989;22(2):65. [13] Meier zu Ko¨ker G, Gross T, Santner E, Wear 1994;179:5. [14] Randhawa H. Wear resistance hard coatings for cutting tools. Strategies for Automation of Machining Materials and Processes, May 1987. [15] Precht W, Lunarska E, Czyzniewski A, Pancielejko M, Walkowiak W. Vacuum 1996;47(6—8):867.
Tribological properties of PVD titanium carbides M. Pancielejkoa,*, W. Prechta, A. Czyzniewskib a
Institute of Materials Science and Technology, Technical University of Koszalin, Raclawicka 15, 75-620 Koszalin, Poland b Technology Transfer Centre, Technical University of Koszalin, Raclawicka 15, 75-620 Koszalin, Poland
Abstract The influence of the acetylene flow rate on the tribological properties of TiC films produced by cathodic arc plasma deposition (CAPD) on the high-speed steel substrates was investigated. The following properties of films were tested: phase and chemical composition, microhardness, adhesion characterised by the scratch method, thickness, fracture morphology (by SEM), surface roughness, friction coefficient and abrasive wear (by ball-on-disk method). The flow rate of acetylene was changed between 20—250 sccm by constant substrate bias voltage of !70 V. The highest values of microhardness of coatings were obtained at a flow rate between 70 and 140 sccm. The maximum of 43 GPa value was achieved at a flow rate of 70 sccm. At flow rates of 20 and above 180 sccm, the microhardness was about 16 GPa. The friction coefficient reaches the value of 0.2, its decreasing is caused by the increase in flow rate of acetylene. Coating abrasive wear also decreases. The highest values of abrasive wear were obtained at the acetylene flow of 20 sccm, for which the dry friction coefficient was 0.54. The abrasive wear of the counter-specimen (ball) gets also bigger for higher values of the dry friction coefficient. By X-ray examination of crystalline structure, reflexes from (1 1 1)TiC and (2 2 0)TiC planes were recorded; they disappear at higher acetylene flow rates. This may be the result of an amorphisation process of the film structure. 1999 Elsevier Science Ltd. All rights reserved. Keywords: CAPD PVD; Titanium carbide; Tribological properties; Ball-on-disk
1. Introduction TiC films coated by PVD methods have revealed excellent tribological properties. They mainly depend on the particular technology, applied deposition parameters and the type of applied reactive carbonaceous gas. The most often used gases are methane, acetylene, ethene and ethane [1—9]. The microhardness of these films, measured by the authors [2—6,10], was mostly of the order of 35 GPa, seldom lower than 25 GPa [8,11]; but sometimes it was very high — i.e. over 40 GPa [1,9]. Adhesion characterised by critical load ¸ in the scratch method ranged ! from approx. 20 N to over 100 N [2,8,9]. The values of a friction coefficient obtained during the tests on wear resistance were ranged from 0.3 [4,5] to 0.6 [8]. They depended on measurement method, operating characteristic of devices and conditions of friction measurements [12,13].
* Corresponding author. Tel. 0048 94 342 7881; fax: 0048 94 342 6753; e-mail: [email protected]
TiC films produced by PVD methods, in view of their properties, have mainly been applied to coating of tools made of high-speed steel and tool edges of sintered carbides in order to increase their life [2,11,12,14]. Results of an investigation in the abrasive wear resistance of TiC films coated by a CAPD method on HSS substrates are presented in this work. TiC films were coated at an acetylene flow rate from 20 to 250 sccm and constant substrate bias voltage.
2. Experimental procedure TiC films were coated by CAPD method. Values of major process parameters are presented in Table 1. Acetylene of purity 99.8% was used as a reactive gas. Substrates were made of HSS type HS6-5-2 in disk shape (dia. 40 mm;4 mm, 63 HRC, R (0.05 lm). The preparation of the substrates for the process consisted in ultrasonic-aided cleaning with organic solvents and alkaline detergents. The next operation was the cleaning with trichloroethylene vapour. In the vacuum chamber the substrates were cleaned by argon glow discharge. A series
0042-207X/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 3 9 1 - 1
58
M. Pancielejko et al. / Vacuum 53 (1999) 57—60
E The film morphology was examined by SEM, with a JSM-S1, Jeol instrument.
Table 1 Technological parameters of the TiC coating deposition Deposition parameter Residual total pressure (Pa) Acetylene flow rate (sccm) Cathode voltage (V) Cathode current (A) Negative bias voltage (V) Distance target—substrate (cm) Deposition rate (lm min\) Substrate temperature after ion cleaning (K) Minimal cooling time in vacuum (min)
Value
3. Results and discussion
4;10\ 20—250 20 86 !20—!250 20 0.2—0.4 523—573 60
of coating processes were carried out at stabilised flow rates of acetylene, ranging from 20 to 250 sccm at constant substrate bias voltage º "!70 V. The properties of the specimens coated with films to a thickness of about 4 lm were tested using the following devices and methods: E Adhesion characterisation was carried out by the scratch method using a Scratch Tester Revetest威 produced by CSEM. E The friction coefficient and wear resistance were determined using a ball-on-disk equipment. Applied parameters: linear sliding speed 0.1 m;s\, load 20 N, sliding length 2000 m, the counter-specimen was a ball made of 100Cr6 bearing steel (dia. 10 mm, 62 HRC), at relative humidity of 40$5% and ambient temperature 23$1°C. To determine the abrasive wear resistance a wear rate coefficient k was introduced, which comprises the volume of the worn groove profile, the sliding distance and the normal load. The formula for k is: » k" ¸;d
E
E E
E E
(mm m\ N\),
Experimental results of chemical composition analyses obtained by the EDAX method (at% Ti), microhardness lHV, adhesion characteristic (critical load ¸ ) and ! a wear rate coefficient k of TiC films obtained at varying flow rates of acetylene, i.e. from 20 to 250 sccm, and at constant substrate bias voltage º "!70 V is illustrated in Fig. 1. The maximum microhardness of 43 GPa was measured in films coated at a steady acetylene flow of 70 sccm. This value is close or higher than reference data [2—6,8,10,11]. When a flow rate was 20 or over 180 sccm, the film microhardness was approx. 16 GPa. Decrease in microhardness of films obtained at flows over 180 sccm is possible to result from the formation of polymer-type Ti-C : H films with high carbon content and low fraction of Ti atoms. This could be confirmed by analysis the titanium content in films by EDAX method as well as the authors’ previous tests [15]. The critical load ¸ which characterises the coating ! adhesion to substrates was for all films over 40 N. An marched increase of adhesion was observed at flow rates between 80 and 180 sccm, which may be connected with an increase slightly in film thickness. One may observe, that the wear rate coefficient k decreases at higher flow rates of acetylene over the whole measuring range, pointing to higher wear resistance with
(1)
where » is the wear volume (mm), ¸ is the load (N) d is the sliding distance (m). The microhardness was examined with a metallographic microscope Neophot 2, Carl Zeiss Jena equipped with a Hanemann’s microhardness tester, with applied loads of 65 or 100 g. The thickness measurements were carried out by the Callotest method. The chemical composition of the films was determined by the EDAX method (energy dispersive X-ray analysis). An EDAX X-ray microanalyser KEVEX DELTA, class V (USA) was installed in an electron microscope Jeol Superprobe JXA-733 (Japan). The film roughness and profiles of worn grooves while testing were measured using a Hommel—Tester T2000 profilometer. The crystalline structure was tested on a Dron 2 X-ray diffractometer using CrK radiation. ? @
Fig. 1. Titanium content (at% Ti) of coatings determined by EDAX, microhardness lHV (GPa), adhesion characterisation (critical load ¸ ! (N)) and wear coefficient k (10\ mm/m;N) at titanium carbide coatings, against acetylene flow rate.
M. Pancielejko et al. / Vacuum 53 (1999) 57—60
59
Fig. 2. Dry friction coefficient curves for specimens obtained at different acetylene flow rates ( f "20, 80 and 250 sccm) at the constant !& substrate bias voltage of º "!70 V.
increasing flow rate. The volume of a worn in a counterspecimen was also on the decrease when the acetylene flow was increased. The runs of friction coefficients while wear testing for films coated at three different acetylene flow rates and at constant substrate bias voltage are shown in Fig. 2. In the case of films coated at the lowest acetylene flow (20 sccm) the highest friction coefficients were obtained, however considerable fluctuations were observed. The lower values of friction coefficient, approx. 0.2, and distinctly smaller fluctuations were registered at the highest acetylene flow rate (250 sccm). The rapid decrease in friction coefficient from 0.6 to 0.25 for a film obtained at a flow rate of 80 sccm is likely to be caused by material transfer from the steel counter-specimen to the film. At the beginning of the wear test probably the material of the steel counter-specimen was rubbed into the hard, rough film surface (R approx. 0.3 lm) which effected an increase in the friction coefficient to approx. 0.6. It is necessary to carry on further investigations on the composition of wearing film surfaces and counter-specimens during abrasive wear tests in order to explain the phenomena occurring in a friction mechanism between a TiC film and a hardened steel counter-specimen. The SEM pattern of TiC film fracture morphology obtained at an acetylene flow of 30 sccm, revealed compact structure with visible columnar grains (Fig. 3a). Fractures of films, obtained at higher acetylene flow rates, did not reveal distinct crystalline structures (Fig. 3b). Rather they have a glassy amorphous character. Morphology of film fractures confirmed the thesis of good adhesion of TiC films to the substrate, because there were no visible chips, spallings or cracks on the film-substrate border. Fractured films did not separate from the substrate and effectively reproduced the substrate surfaces topography. X-ray examination of TiC films, revealed the reflexes from characteristic crystallographic planes, i.e. (1 1 1)TiC
Fig. 3. SEM pattern of TiC coatings fractures: (a) f "30 sccm, !& º "!70 V. Mag. ;3000, º"10 kV, (b) f "180 sccm, º " !& !70 V. Mag. ;3000, º"10 kV.
and (2 2 0)TiC were recorded, in accordance with works [1—7,10]. The reflexes disappeared at higher acetylene flows, due, possibly to an amorphisation of the film structure.
4. Conclusions (1) The friction coefficient between TiC films and hardened bearing steel counter-specimens fluctuates from 0.52 to 0.17 and goes along with an increase in an acetylene flow rate at the film deposition. (2) The lowest friction coefficients were below 0.2, the lowest wear rate coefficients were obtained for films at the highest acetylene flow rates. (3) It is necessary to carry out further investigations on the composition of wearing film surfaces and counter-specimens for abrasive wear tests in order to explain the phenomena occurring in the sliding friction mode of TiC film and a hardened steel counterspecimen.
Acknowledgements Authors of this work to thank Dr F. Fendrych from Institute of Physic of the Czech Academy of Sciences for EDAX investigations.
60
M. Pancielejko et al. / Vacuum 53 (1999) 57—60
References [1] Knotek O, Lo¨fler F, Kra¨mer G. Surf Coat Technol 1993;61:320. [2] Sproul WD. J Vac Sci Technol 1986;A4(6):2874. [3] Jeong JI, Hong JH, Kang JS, Shin HJ, Lee YP. J Vac Sci Technol 1991;A9(5):2618. [4] Jeong JI, Braun M, Gudowska I. J Vac Sci Technol 1994; A12(3):733. [5] Deng J, Braun M. Surf Coat Technol 1994;70:49. [6] Martin PJ, Netterfield RP, Kinder TJ, Descoˆtes L. Surf Coat Technol 1991;49:239. [7] Kaufherr N, Fenske GR, Bush DE, Lin P, Deshpandey C, Bunshah RF. Thin Solid Films 1987;153:149. [8] Voevodin AA, Rebholz C, Schneider JM, Stevenson P, Matthews A. Surf Coat Technol 1995;73:185.
[9] Arrando F, Polo MC, Molera P, Esteve J. Surf Coat Technol 1994;68/69:536. [10] Karlson L. Residual stresses and mechanical properties of Ti(C,N) thin films deposited by arc evaporation. Licentiate Thesis no. 633. Linko¨ping Studies in Science and Technology, Linko¨ping, May 1997. [11] Nakamura K, Inagawa K. Thin Solid Films 1977;40:155. [12] Habig K-H. Tribology International, April 89. 1989;22(2):65. [13] Meier zu Ko¨ker G, Gross T, Santner E, Wear 1994;179:5. [14] Randhawa H. Wear resistance hard coatings for cutting tools. Strategies for Automation of Machining Materials and Processes, May 1987. [15] Precht W, Lunarska E, Czyzniewski A, Pancielejko M, Walkowiak W. Vacuum 1996;47(6—8):867.