- Email: [email protected]
Adhesion of amorphous diamond-like film on sputtered hardmetal (WC-Co) cutting tools
Diamond and Related Materials 5 (1996) 186-189
Adhesion of amorphous diamond-like film on sputtered hardmetal (WC-Co) cutting tools M. Hakovirta Accelerator Laboratory, P.O. Box 9, FIN-0001 4 University of Helsinki, Finland
Received 10 October 1995; accepted in final form 17 November 1995
Abstract Adhesion between (WC-Co) hardmetal (6 wt.% Co) and hydrogen-free, amorphous diamond-like film (78% sp3-bondings) produced by the filtered pulsed arc-discharge method has been investigated. The poor adhesion between cobalt and diamond-like film in the case of WC-Co samples was solved by etching cobalt (after a Co sputtering on the hardmetal surface of 0.8 wt.%) from the hardmetal surface using an argon sputtering ion gun (sputtering yield for cobalt is five times higher than for tungsten carbide). A conventional scratch tester was used to test adhesion between the diamond-like film and hardmetal substrate. Depending on the deposition parameters, the samples without etching by sputtering had critical load values (the load at which the coating is stripped from the substrate) from 4.9 to 8.8 N. With similar deposition parameters the samples etched by argon ion sputtering had critical load values from 10.8 to 21.6 N. Wear resistance and coefficient of friction were measured with a pin-on-disc tester. Film thickness, the quality of the film and the thickness of the adhesion layer produced with the high-energy carbon plasma, were the major factors affecting the adhesion. The best adhesion values measured with a scratch tester were obtained from diamond-like film of intermediate thickness (- 1.0 pm) and intermediate quality (-68% sp3-bondings). In cutting tests on aluminium, (G-AlSil2(Cu)P and AlMgSiT5) diamond-like films deposited on cutting tools (critical load > 10 N) showed no delamination or observable wear and much lower sticking of aluminium to the tool surface than with plain WC-Co cutting tools. Keywords: Adhesion; Sputtering; Amorphous diamond-like
film; Hardmetal
1. Introduction The properties of diamond-like films such as low coefficient of friction, high-wear resistance and high thermal conductivity make diamond-like films an attractive protective film material for cutting tools. For machining nonferrous alloys and composites with very hard components diamond-like film can be especially useful [ 11. In this paper, the adhesion and other properties of amorphous diamond-like film produced by the pulsed arc-discharge method on commercial hardmetal pieces are reported and the characteristics of surface treatment techniques and deposition parameters are discussed [ 21. WC-Co hardmetal has good properties for different cutting applications and is widely used. Diamond-like film has good adhesion to WC, but does not adhere well to the cobalt binder in WC-Co hardmetal [ 1,3]. Previously, the effect of radiation-enhanced outdiffusion during ion implantation was investigated [ 41. According to the theory, the outdiffusion is much higher in Co than in WC. This decreases mixing of the intermediate 0925-9635/96/$15.000 1996Elsevier Science S.A. All rights reserved SSDZ 0925-9635(95)00497-l
layer and therefore results in lower adhesion between diamond-like film and Co compared with adhesion between diamond-like film and WC. These unfavourable characteristics of Co can be reduced by using an intermediate layer structure or by increasing the active area of WC on the surface of the hardmetal using chemical etching [ 11. In addition, internal stress is a factor that has to be taken into account. The compressive stresses are generated during the deposition of diamond-like film. High compressive stress limits the thickness of the diamond-like film layer that can be deposited [3]. Another practical problem reducing adhesion is the difference between the thermal expansion coefficients of WC-Co hardmetal and the diamond-like coating. This causes thermal stresses to be generated during the production of diamond-like coatings and during machining. Thermal stresses are especially important when using high-temperature CVD techniques for depositing diamond-like films on hardmetal cutting tools. Using the pulsed arc-discharge method an extremely good adhesion has been obtained between diamond-like film and silicon [3]. The characteristics of the method,
187
A4. HakovirtalDiamond and Related Materials 5 (1996) 186-189
such as high quality of the coatings and deposition at room temperature, make the filtered pulsed arc-discharge method an attractive alternative for coating hardmetal pieces with a diamond-like film [5,6]. In this paper, the idea of anchoring diamond-like film onto tungsten carbide grains on the hardmetal surface is used. The tungsten carbide grains were exposed on the hardmetal surface using a sputtering ion gun for etching [ 71. This is possible because cobalt has an Ar ion sputtering yield five times higher than tungsten carbide [8]. Etching by sputtering is a very convenient method because Ar ion sputtering is also used (with lower anode-cathode voltage) for substrate cleaning purposes.
400
I @I
200
I
-
2. Experimental details Diamond-like films were deposited on commercial (WC-Co) hardmetal cutting tool samples. The samples were from Seco Tools and their commercial trademark was HX. The samples had 6 wt.% of Co, 93.5 wt.% of WC and 0.5 wt.% of TaC. The WC grain size was l-2 urn. The Vickers hardness value was 1640. Hardmetal (WC-Co) pieces (12.5 x 12.5 mm2) were first mechanically abraded with Sic emery paper down to 1000 grade and then polished with a 1 urn diamond paste. The average roughness of the surface was at this stage about 0.05 urn (Fig. l(a)). After polishing, the surface was ultrasonically cleaned in acetone and ethyl alcohol and dried in nitrogen gas. In order to get rid of excess gas inside the hardmetal surface the samples went through heat treatment. Samples were heated in a vacuum of 100 uPa at a temperature of 650 “C for 1 h. After this the samples were Ar ion sputtered with a sputtering ion gun for 1 h with an anode-cathode voltage of 4200 V. Because the sputtering etch rate of WC is much smaller than the sputtering etch rate of cobalt, argon ion sputtering results in selective etching of Co from the WC-Co surface and leaves Co-free WC grains on the surface. The diamond-like coating was deposited in 100 uPa vacuum first with a higher carbon plasma energy (140 eV) and after that with a lower plasma energy to produce a high-quality diamond-like film (78% sp3bondings) [9-l 11. To compare the adhesion values of diamond-like films deposited on polished and argon ion sputtered WC-Co samples, the same deposition procedure was performed for both types of samples. The hydrogen concentration of the diamond-like films was measured by FRES (forward recoil spectroscopy) and was ~0.1 at.%. The sp2/sp3-bonding ratio was measured with the ESCA (electron spectroscopy for chemical analysis). The friction and wear tests were performed using a pin-on-disc device. During the tests the relative humidity was kept at 50% at 25 “C. A conventional scratch tester was used to test the adhesion of diamond-
I Cd)
I
00
0.5
Distance
I
1.0
(mm)
Fig. 1. Profilometer traces from (a) polished hardmetal; (b) surface after 1 h argon sputtering pretreatment; (c) wear track of pin-on-disc wear test with an Al,O, pin; a 1.6 N load and 1 x lo5 revolutions against hardmetal coated with diamond-like film; (d) wear tests with an Al,O, pin, a 1.6 N load and 1 x 10s revolutions against hardmetal etched by argon sputtering.
like film deposited on hardmetal substrate. Optical micrographs were taken to investigate the topography of the polished hardmetal after 1 h Ar ion sputtering and to compare the amount of diamond-like film delamination in the scratch tester measurements. The amount of Co on the surface of hardmetal after 1 h sputtering pretreatment was measured by using AES (Auger electron spectroscopy). Turning tests against aluminium were performed to see if any film delamination occurred in the coated samples during machining, and to see if there was any difference in the sticking of aluminium to uncoated hardmetal pieces compared with the diamondlike film coated samples.
3. Results The thickness of the diamond-like films varied from 200 nm to 2.5 urn. The roughness value of the hardmetal
188
M. HakovirtalDiamond and Related Materials 5 (1996) 186-189
samples was about 0.15 urn after 1 h of argon ion sputtering. One can see from profilometer traces the WC grains protruding from the cobalt binding material (Fig. l(b)). Optical micrographs from a polished WC-Co hardmetal and from the hardmetal that has been argon ion sputtered for one hour can be seen in Fig. 2. The AES measurements show that after the sputtering procedure the Co concentration on WC-Co surface dropped from 6 to 0.8 wt.%. In friction and wear measurements with the pin-ondisc device an A1203 (ball diameter d= 6 mm) pin was used. The load was 1.6 N and the number of revolutions was 1 x 105. No wear tracks were observed in the case of the Al,Os pin against diamond-like coating. For comparison, the friction and wear was measured for a sputtered WC-Co sample. The difference between wear tracks caused by the A&O3 pin on the diamond-like film and on the hardmetal can be seen in Figs. l(c) and l(d). Some of the sharp peaks in the profilometer traces (Fig. 1(c)) are due to microscopic graphite particles that are produced during film deposition [ 121. The majority of the peaks are WC grains protruding from the hardmetal surface. The friction between the A&O3 pin and the diamond-like film on WC-Co hardmetal was 0.08; on the WC-Co hardmetal it was 0.55. The adhesion tests were performed with a scratch tester. The tests were done on diamond-like films with different thicknesses and qualities to investigate the influence of compressive stresses inside the diamond-like films on adhesion. Improved adhesion was measured for samples that were etched using Ar ion sputtering. When comparing samples that had l-urn thick diamond-like coatings produced with similar deposition parameters, the largest critical load value for a sample without argon ion sputtering was 6.9 N, whereas for samples with sputtering pretreatment it was 14.7 N. A significant difference
was seen between these samples upon looking at the delamination of the diamond-like films after the scratch tester’s diamond stylus had penetrated the film (Fig. 3). A large degree of delamination was also observed in Ar ion sputtering pretreated samples coated with thick (> 1 urn) diamond-like film. As the thickness increased, the delamination increased. This is due to compressive stresses inside diamond-like film. Compressive stresses were measured earlier [ 131. Depending on the process parameters and the coating thickness (0.7-2.1 pm), the stress values varied from 2.4 to 8.5 GPa. In scratch tester measurements, the thickness of the adhesion layer (the first diamond-like carbon layer, which is produced with high-energy carbon plasma) was found to influence the adhesion. The adhesion layer was produced using a higher carbon plasma energy (140 eV) in order to get a larger range for the implanted ions. If
(4
(b)
Fig. 2. Optical micrographs of (a) polished hardmetal hardmetal after 1 h argon sputtering pretreatment.
and
(b)
Fig. 3. (a) A scratch from a scratch tester on a hardmetal sample coated with diamond-like film (1 pm thick) of intermediate quality (- 68% sp3-bondings) and without sputtering pretreatment. (b) A scratch on a similar sample with etching by argon ion sputtering.
M. HakovirtalDiamond and Related Materials 5 (1996) 186-189
189
hardmetal pieces was large. On the tip of the cutting tool coated with a diamond-like film there was only a small trace of aluminium (AlMgSiTS), but in the case of the uncoated cutting tool the degree of sticking of aluminium was much higher (Fig. 4). 4. Discussion Adhesion is always a key factor in making thin films to be used in cutting applications. Etching by argon ion sputtering was found to improve adhesion between hardmetal and diamond-like coating produced by the filtered pulsed arc-discharge method. The WC grains on the surface of argon ion sputtered hardmetal substrate work as anchors, i.e. diamond-like film adheres to them. This seems to reduce the effect of the compressive stresses inside the film. A clear correlation could be seen between the adhesion and the quality and the thickness of the diamond-like film. This is due to the compressive stress inside the diamond-like film which increases with growing thickness and increasing sp3-bonding fraction. The thickness of the first diamond-like film layer (produced by high-energy carbon plasma) was also found to have an influence on adhesion. If this layer was too thick (> 100 nm), the critical load values were lower than with samples with a thinner (- 100 nm) adhesion layer. The results from turning tests show that the adhesion of diamond-like film on hardmetal cutting tools is sufficient if the critical load values are > 10 N. They also showed that in a real turning situation the adhesion of aluminium to hardmetal cutting tools coated with diamond-like film is noticeably lower than to a normal WC-Co hardmetal cutting tool surface.
(4
(b) Fig. 4. Adhesion of aluminium hardmetal piece; (b) a hardmetal
(AIMgSiT5) in turning tests to (a) a piece coated with a diamond-like film.
this layer was too thick (> 100 nm) a high degree of delamination of the film occurred during scratch measurements. It seems that too thick a layer of poor-quality diamond-like film (-47% sp3-bondings) is not a good foundation for a good quality film. The best critical load values in scratch tester adhesion tests were measured for samples that were coated with - l-urn thick diamond-like coatings of intermediate quality (- 68% sp3-bondings). The critical load values in these samples exceeded 20 N. In these samples the thickness of the adhesion layer produced with a high-energy carbon plasma was about 100 nm. In turning tests both diamondlike film coated samples and samples without diamondlike film coating were used in a cutting procedure against aluminium (G-AlSil2(Cu)P and AlMgSiT5) with a cutting speed of 3.5 m s-i for 5 min. There was no delamination in the diamond-like film-coated samples with critical load values > 10 N. In addition, the difference between the adhesion of aluminium to coated and uncoated
References Fan, X. Chan, K. Jagannadham and J. Narayan, J. Mater. Res., 9 (1994)2850. 121 D.R. McKenzie, D. Muller and B.A. Pailthorpe, Phys. Rev. Left., 67 (1991)773. c31 X. Chen and J. Narayan, _r. A&. Phys., 74 (1993) 4168. 141 A. Anttila and M. Hautala, Appl. Phys., 19 (1979) 199. 151 A. Anttila, J.-P. Hirvonen and J. Koskinen, US patent no. 5078848, 1992. 61 A. Anttila and J. Koskinen, FIN patent no. 89725, 1993. 71 A. Anttila, R. Lappalainen, J. Sale and M. Hakovirta, Surf. Coat. Technol., in press. 81 J.F. Ziegler and J.P. Biersack, TRIM-92 computer code, private communication.; J.F. Ziegler, J.P. Biersack and IJ. Littmark, The Stopping and Ranges of Ions in Matter, Vol. 1, Pergamon, New York, 1985. Phys. C91 M. Hakovirta, J. Salo, A. Anttila and R. Lappalainen, Left. A, in press. Cl01 A. Anttila, J. Salo and R. Lappalainen, Appl. Phys. A, in press. ctt1 I. Koponen, M. Hakovirta and R. Lappalainen, J. Appl. Phys., in press. 1121 M. Hakovirta, J. Salo, A. Anttila and R. Lappalainen, Diamond. Relat. Mater., 4 (1995) 1335. Mater. Lett., 24 Cl31 A. Anttila, J. Salo and R. Lappalainen, (1995) 153.
Cl1 W.D.
Adhesion of amorphous diamond-like film on sputtered hardmetal (WC-Co) cutting tools M. Hakovirta Accelerator Laboratory, P.O. Box 9, FIN-0001 4 University of Helsinki, Finland
Received 10 October 1995; accepted in final form 17 November 1995
Abstract Adhesion between (WC-Co) hardmetal (6 wt.% Co) and hydrogen-free, amorphous diamond-like film (78% sp3-bondings) produced by the filtered pulsed arc-discharge method has been investigated. The poor adhesion between cobalt and diamond-like film in the case of WC-Co samples was solved by etching cobalt (after a Co sputtering on the hardmetal surface of 0.8 wt.%) from the hardmetal surface using an argon sputtering ion gun (sputtering yield for cobalt is five times higher than for tungsten carbide). A conventional scratch tester was used to test adhesion between the diamond-like film and hardmetal substrate. Depending on the deposition parameters, the samples without etching by sputtering had critical load values (the load at which the coating is stripped from the substrate) from 4.9 to 8.8 N. With similar deposition parameters the samples etched by argon ion sputtering had critical load values from 10.8 to 21.6 N. Wear resistance and coefficient of friction were measured with a pin-on-disc tester. Film thickness, the quality of the film and the thickness of the adhesion layer produced with the high-energy carbon plasma, were the major factors affecting the adhesion. The best adhesion values measured with a scratch tester were obtained from diamond-like film of intermediate thickness (- 1.0 pm) and intermediate quality (-68% sp3-bondings). In cutting tests on aluminium, (G-AlSil2(Cu)P and AlMgSiT5) diamond-like films deposited on cutting tools (critical load > 10 N) showed no delamination or observable wear and much lower sticking of aluminium to the tool surface than with plain WC-Co cutting tools. Keywords: Adhesion; Sputtering; Amorphous diamond-like
film; Hardmetal
1. Introduction The properties of diamond-like films such as low coefficient of friction, high-wear resistance and high thermal conductivity make diamond-like films an attractive protective film material for cutting tools. For machining nonferrous alloys and composites with very hard components diamond-like film can be especially useful [ 11. In this paper, the adhesion and other properties of amorphous diamond-like film produced by the pulsed arc-discharge method on commercial hardmetal pieces are reported and the characteristics of surface treatment techniques and deposition parameters are discussed [ 21. WC-Co hardmetal has good properties for different cutting applications and is widely used. Diamond-like film has good adhesion to WC, but does not adhere well to the cobalt binder in WC-Co hardmetal [ 1,3]. Previously, the effect of radiation-enhanced outdiffusion during ion implantation was investigated [ 41. According to the theory, the outdiffusion is much higher in Co than in WC. This decreases mixing of the intermediate 0925-9635/96/$15.000 1996Elsevier Science S.A. All rights reserved SSDZ 0925-9635(95)00497-l
layer and therefore results in lower adhesion between diamond-like film and Co compared with adhesion between diamond-like film and WC. These unfavourable characteristics of Co can be reduced by using an intermediate layer structure or by increasing the active area of WC on the surface of the hardmetal using chemical etching [ 11. In addition, internal stress is a factor that has to be taken into account. The compressive stresses are generated during the deposition of diamond-like film. High compressive stress limits the thickness of the diamond-like film layer that can be deposited [3]. Another practical problem reducing adhesion is the difference between the thermal expansion coefficients of WC-Co hardmetal and the diamond-like coating. This causes thermal stresses to be generated during the production of diamond-like coatings and during machining. Thermal stresses are especially important when using high-temperature CVD techniques for depositing diamond-like films on hardmetal cutting tools. Using the pulsed arc-discharge method an extremely good adhesion has been obtained between diamond-like film and silicon [3]. The characteristics of the method,
187
A4. HakovirtalDiamond and Related Materials 5 (1996) 186-189
such as high quality of the coatings and deposition at room temperature, make the filtered pulsed arc-discharge method an attractive alternative for coating hardmetal pieces with a diamond-like film [5,6]. In this paper, the idea of anchoring diamond-like film onto tungsten carbide grains on the hardmetal surface is used. The tungsten carbide grains were exposed on the hardmetal surface using a sputtering ion gun for etching [ 71. This is possible because cobalt has an Ar ion sputtering yield five times higher than tungsten carbide [8]. Etching by sputtering is a very convenient method because Ar ion sputtering is also used (with lower anode-cathode voltage) for substrate cleaning purposes.
400
I @I
200
I
-
2. Experimental details Diamond-like films were deposited on commercial (WC-Co) hardmetal cutting tool samples. The samples were from Seco Tools and their commercial trademark was HX. The samples had 6 wt.% of Co, 93.5 wt.% of WC and 0.5 wt.% of TaC. The WC grain size was l-2 urn. The Vickers hardness value was 1640. Hardmetal (WC-Co) pieces (12.5 x 12.5 mm2) were first mechanically abraded with Sic emery paper down to 1000 grade and then polished with a 1 urn diamond paste. The average roughness of the surface was at this stage about 0.05 urn (Fig. l(a)). After polishing, the surface was ultrasonically cleaned in acetone and ethyl alcohol and dried in nitrogen gas. In order to get rid of excess gas inside the hardmetal surface the samples went through heat treatment. Samples were heated in a vacuum of 100 uPa at a temperature of 650 “C for 1 h. After this the samples were Ar ion sputtered with a sputtering ion gun for 1 h with an anode-cathode voltage of 4200 V. Because the sputtering etch rate of WC is much smaller than the sputtering etch rate of cobalt, argon ion sputtering results in selective etching of Co from the WC-Co surface and leaves Co-free WC grains on the surface. The diamond-like coating was deposited in 100 uPa vacuum first with a higher carbon plasma energy (140 eV) and after that with a lower plasma energy to produce a high-quality diamond-like film (78% sp3bondings) [9-l 11. To compare the adhesion values of diamond-like films deposited on polished and argon ion sputtered WC-Co samples, the same deposition procedure was performed for both types of samples. The hydrogen concentration of the diamond-like films was measured by FRES (forward recoil spectroscopy) and was ~0.1 at.%. The sp2/sp3-bonding ratio was measured with the ESCA (electron spectroscopy for chemical analysis). The friction and wear tests were performed using a pin-on-disc device. During the tests the relative humidity was kept at 50% at 25 “C. A conventional scratch tester was used to test the adhesion of diamond-
I Cd)
I
00
0.5
Distance
I
1.0
(mm)
Fig. 1. Profilometer traces from (a) polished hardmetal; (b) surface after 1 h argon sputtering pretreatment; (c) wear track of pin-on-disc wear test with an Al,O, pin; a 1.6 N load and 1 x lo5 revolutions against hardmetal coated with diamond-like film; (d) wear tests with an Al,O, pin, a 1.6 N load and 1 x 10s revolutions against hardmetal etched by argon sputtering.
like film deposited on hardmetal substrate. Optical micrographs were taken to investigate the topography of the polished hardmetal after 1 h Ar ion sputtering and to compare the amount of diamond-like film delamination in the scratch tester measurements. The amount of Co on the surface of hardmetal after 1 h sputtering pretreatment was measured by using AES (Auger electron spectroscopy). Turning tests against aluminium were performed to see if any film delamination occurred in the coated samples during machining, and to see if there was any difference in the sticking of aluminium to uncoated hardmetal pieces compared with the diamondlike film coated samples.
3. Results The thickness of the diamond-like films varied from 200 nm to 2.5 urn. The roughness value of the hardmetal
188
M. HakovirtalDiamond and Related Materials 5 (1996) 186-189
samples was about 0.15 urn after 1 h of argon ion sputtering. One can see from profilometer traces the WC grains protruding from the cobalt binding material (Fig. l(b)). Optical micrographs from a polished WC-Co hardmetal and from the hardmetal that has been argon ion sputtered for one hour can be seen in Fig. 2. The AES measurements show that after the sputtering procedure the Co concentration on WC-Co surface dropped from 6 to 0.8 wt.%. In friction and wear measurements with the pin-ondisc device an A1203 (ball diameter d= 6 mm) pin was used. The load was 1.6 N and the number of revolutions was 1 x 105. No wear tracks were observed in the case of the Al,Os pin against diamond-like coating. For comparison, the friction and wear was measured for a sputtered WC-Co sample. The difference between wear tracks caused by the A&O3 pin on the diamond-like film and on the hardmetal can be seen in Figs. l(c) and l(d). Some of the sharp peaks in the profilometer traces (Fig. 1(c)) are due to microscopic graphite particles that are produced during film deposition [ 121. The majority of the peaks are WC grains protruding from the hardmetal surface. The friction between the A&O3 pin and the diamond-like film on WC-Co hardmetal was 0.08; on the WC-Co hardmetal it was 0.55. The adhesion tests were performed with a scratch tester. The tests were done on diamond-like films with different thicknesses and qualities to investigate the influence of compressive stresses inside the diamond-like films on adhesion. Improved adhesion was measured for samples that were etched using Ar ion sputtering. When comparing samples that had l-urn thick diamond-like coatings produced with similar deposition parameters, the largest critical load value for a sample without argon ion sputtering was 6.9 N, whereas for samples with sputtering pretreatment it was 14.7 N. A significant difference
was seen between these samples upon looking at the delamination of the diamond-like films after the scratch tester’s diamond stylus had penetrated the film (Fig. 3). A large degree of delamination was also observed in Ar ion sputtering pretreated samples coated with thick (> 1 urn) diamond-like film. As the thickness increased, the delamination increased. This is due to compressive stresses inside diamond-like film. Compressive stresses were measured earlier [ 131. Depending on the process parameters and the coating thickness (0.7-2.1 pm), the stress values varied from 2.4 to 8.5 GPa. In scratch tester measurements, the thickness of the adhesion layer (the first diamond-like carbon layer, which is produced with high-energy carbon plasma) was found to influence the adhesion. The adhesion layer was produced using a higher carbon plasma energy (140 eV) in order to get a larger range for the implanted ions. If
(4
(b)
Fig. 2. Optical micrographs of (a) polished hardmetal hardmetal after 1 h argon sputtering pretreatment.
and
(b)
Fig. 3. (a) A scratch from a scratch tester on a hardmetal sample coated with diamond-like film (1 pm thick) of intermediate quality (- 68% sp3-bondings) and without sputtering pretreatment. (b) A scratch on a similar sample with etching by argon ion sputtering.
M. HakovirtalDiamond and Related Materials 5 (1996) 186-189
189
hardmetal pieces was large. On the tip of the cutting tool coated with a diamond-like film there was only a small trace of aluminium (AlMgSiTS), but in the case of the uncoated cutting tool the degree of sticking of aluminium was much higher (Fig. 4). 4. Discussion Adhesion is always a key factor in making thin films to be used in cutting applications. Etching by argon ion sputtering was found to improve adhesion between hardmetal and diamond-like coating produced by the filtered pulsed arc-discharge method. The WC grains on the surface of argon ion sputtered hardmetal substrate work as anchors, i.e. diamond-like film adheres to them. This seems to reduce the effect of the compressive stresses inside the film. A clear correlation could be seen between the adhesion and the quality and the thickness of the diamond-like film. This is due to the compressive stress inside the diamond-like film which increases with growing thickness and increasing sp3-bonding fraction. The thickness of the first diamond-like film layer (produced by high-energy carbon plasma) was also found to have an influence on adhesion. If this layer was too thick (> 100 nm), the critical load values were lower than with samples with a thinner (- 100 nm) adhesion layer. The results from turning tests show that the adhesion of diamond-like film on hardmetal cutting tools is sufficient if the critical load values are > 10 N. They also showed that in a real turning situation the adhesion of aluminium to hardmetal cutting tools coated with diamond-like film is noticeably lower than to a normal WC-Co hardmetal cutting tool surface.
(4
(b) Fig. 4. Adhesion of aluminium hardmetal piece; (b) a hardmetal
(AIMgSiT5) in turning tests to (a) a piece coated with a diamond-like film.
this layer was too thick (> 100 nm) a high degree of delamination of the film occurred during scratch measurements. It seems that too thick a layer of poor-quality diamond-like film (-47% sp3-bondings) is not a good foundation for a good quality film. The best critical load values in scratch tester adhesion tests were measured for samples that were coated with - l-urn thick diamond-like coatings of intermediate quality (- 68% sp3-bondings). The critical load values in these samples exceeded 20 N. In these samples the thickness of the adhesion layer produced with a high-energy carbon plasma was about 100 nm. In turning tests both diamondlike film coated samples and samples without diamondlike film coating were used in a cutting procedure against aluminium (G-AlSil2(Cu)P and AlMgSiT5) with a cutting speed of 3.5 m s-i for 5 min. There was no delamination in the diamond-like film-coated samples with critical load values > 10 N. In addition, the difference between the adhesion of aluminium to coated and uncoated
References Fan, X. Chan, K. Jagannadham and J. Narayan, J. Mater. Res., 9 (1994)2850. 121 D.R. McKenzie, D. Muller and B.A. Pailthorpe, Phys. Rev. Left., 67 (1991)773. c31 X. Chen and J. Narayan, _r. A&. Phys., 74 (1993) 4168. 141 A. Anttila and M. Hautala, Appl. Phys., 19 (1979) 199. 151 A. Anttila, J.-P. Hirvonen and J. Koskinen, US patent no. 5078848, 1992. 61 A. Anttila and J. Koskinen, FIN patent no. 89725, 1993. 71 A. Anttila, R. Lappalainen, J. Sale and M. Hakovirta, Surf. Coat. Technol., in press. 81 J.F. Ziegler and J.P. Biersack, TRIM-92 computer code, private communication.; J.F. Ziegler, J.P. Biersack and IJ. Littmark, The Stopping and Ranges of Ions in Matter, Vol. 1, Pergamon, New York, 1985. Phys. C91 M. Hakovirta, J. Salo, A. Anttila and R. Lappalainen, Left. A, in press. Cl01 A. Anttila, J. Salo and R. Lappalainen, Appl. Phys. A, in press. ctt1 I. Koponen, M. Hakovirta and R. Lappalainen, J. Appl. Phys., in press. 1121 M. Hakovirta, J. Salo, A. Anttila and R. Lappalainen, Diamond. Relat. Mater., 4 (1995) 1335. Mater. Lett., 24 Cl31 A. Anttila, J. Salo and R. Lappalainen, (1995) 153.
Cl1 W.D.