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Determination of adhesion strength between a carbon film and a polymeric substrate
116
Diamond and Related Materials, 3 (1993) 116-118
Determination of adhesion strength between a carbon film and a polymeric substrate M . A. V o r o n k i n , S. N . D u b * , I. N . L u p i c h a n d B. A. M a s l y u k Institute for Superhard Materials of the Ukrainian Academy of Sciences, 254153 Kiev (Ukraine) (Received August 14, 1992; accepted in final form April 7, 1993)
Abstract The cracking and decohesion of amorphous hydrogenated carbon films on polymeric substrates are studied by tensile testing. Depending on adhesion strength the carbon film decohesion is found to occur either at the film-substrate interface or across the substrate. It is suggested that a ratio of the area of fracture across the substrate to the whole film area should be used for quantitative estimation of adhesion.
1. Introduction The method of scratching by Vickers [1] or Rockwell [2, 3] indenters is widely used for estimation of adhesion between a film and substrate. The adhesion strength is determined from a load which causes film decohesion from the substrate. In Vickers hardness tests, delamination of the film is sometimes observed; this allows the interfacial fracture energy to be measured [4, 5]. However, we observed no delamination of amorphous hydrogenated carbon ( a - C : H ) films on a soft polymeric substrate in scratching and indentation tests at loads of 2 N or less. We succeeded in estimating the adhesion between the a - C : H film and polymeric substrate by tensile testing. As the specimen undergoes large deformations during the testing, film cracking and decohesion from the substrate can be observed. Furthermore, the decohesion occurs partially at the film-substrate interface and partially across the substrate. We used a ratio of fracture area across the substrate to the whole film area to estimate the adhesion strength.
2. Experimental details a-C : H films up to 10 ~tm in thickness were deposited onto a poly(ethylene terephthalate) (PET) substrate 100 Ixm thick. The low temperature deposition method for carbon film from methane in a low frequency glow discharge plasma was used. The substrate temperature was 80 °C or less. The chamber pressure was maintained at 8 Torr. *Author to whom correspondence should be addressed.
0925-9635/93/$6.00
Tensile tests were carried out using a Heckert FP-10 universal testing machine at a cross-head speed of 2 m m m i n - 1. The samples were 50 m m long and 10 m m wide. The grippers spacing (gauge length) was 30 mm. In tests, this distance increased to 45 mm. After testing, the specimen surface was examined using optical and electron microscopes. The density of the films was determined by gravimetry. The hardness and elastic modulus of the films were determined using a N a n o Indenter II ~ (Nano Instrument Inc., Knoxville, TN, USA) mechanical properties microprobe [6]. The Berkovich indenter was used to produce indentations. All tests were performed at a peak load of 1 m N at a loading rate of 70 IxN s - 1. The hold time at peak load was 20 s.
3. Results and discussion Transparent yellow-brown films from 0.4 to 10 lam in thickness were obtained. Their density ranged from 0.9 to 1.2 g cm -3, the hardness varied between 0.6 and 1.4 G P a and the elastic modulus between 5 and 10 GPa. In terms of their properties, the films belong to the class of "soft" diamond-like carbon (DLC) films [7]. The substrate hardness under these test conditions is 0.36 GPa. 3.1. Effect o f a-C : H film thickness on its cracking and decohesion a - C : H films 0.4-2.3 Ixm in thickness were deposited under the same conditions. At the initial stage of tension there are parallel cracks which propagate perpendicu-
© 1993 - - Elsevier Sequoia. All rights reserved
M. A. Voronkin et al. / Adhesion between C and a polymeric substrate
larly to the tensile direction and cross the whole film. The distance between these cracks decreases linearly with decreasing film thickness. Further tension causes specimen contraction resulting in additional fracture of the film. The film either peels off the substrate or breaks parallel to the tensile direction. A thin film (0.4 jam), as shown in Fig. l(a), fails readily in the direction parallel to that of tension, so its decohesion from the substrate is insignificant. It is easier for a thicker (2.3 lam) film to peel off rather than to fail (Fig. l(b)). That is why a large part of the film exhibited decohesion during the tension. Thus the film fracture and decohesion are affected by both its adhesion and its thickness. In tension a film with low adhesion and a thickness of less than 0.5 gm breaks into fine fragments but does not peel off the substrate. Therefore estimation of the adhesion in tension requires that the thickness of the a - C : H film should be more than 2 jam. Further studies were carried out on films 5-10 jam in thickness.
117
Fig. 2. Optical micrograph showing carbon film cracking and decohesion from the polymeric substrate. Parallel cracks are oriented transverse to the tensile direction. Note the new crack in the centre which propagates from a defect at the surface of the film.
3.2. Cracking and decohesion o f a-C : H films as a f u n c t i o n o f adhesion
(a)
(b) Fig. 1. The cracking and decohesion of carbon films on a polymeric substrate with different film thicknesses: (a) 0.4 gm; (b) 2.3 p.m.
In the case of low adhesion the film breaks into fragments hundreds of micrometres in width. The delamination of the film from the substrate (Fig. 2) is initiated from opposite sides of the fragments which are transverse to the tensile direction. The delamination occurs through the development of half-elliptical cracks. The cracks from the opposite sides of a fragment coalesce and then propagate towards the specimen edges. As a result, the film peels off completely. Observation of the substrate surface after the film delamination shows that the film peels off only at the film substrate interface, i.e. there is no substrate fracture (Fig. 3(a)). For a - C : H film with medium adhesion the distance between parallel cracks is an order of magnitude smaller. The delamination is initiated at the substrate from two opposite sides of the fragments; then the crack reaches the film-substrate interface (Fig. 3(b)). At higher adhesion a larger zone of crack propagation across the substrate can be observed. Thus in this case the film comes off the substrate in the form of long narrow bands (Fig. 4) and the decohesion occurs partially along the film-substrate interface and partially across the polymeric substrate. For a - C : H film with high adhesion the long narrow bands which formed at the initial stage of tension break into short fragments during further tension, and the fragments crawl over each other (Fig. 5). The decohesion takes place completely across the polymeric substrate. In order to estimate adhesion we suggest that the ratio between the area of decohesion across the substrate to the whole film area should be used. The ratio can be given by A = (Sl--$2)/S
1
118
M. A. Voronkin et al. / Adhesion between C and a polymeric substrate
(a/ Fig. 5. The surface of the substrate after carbon film decohesion for a specimen with high adhesion. Decohesion occurs only across the substrate.
where $1 is the mean width of the film fragment and 5 2 is the mean size of a zone in which the delamination occurs at the film-substrate interface (Fig. 4). In the case of low adhesion, i.e. of the delamination at the filmsubstrate interface, this ratio is equal to zero. For a film with high adhesion when the delamination occurs entirely across the substrate, the ratio is equal to unity.
4. Conclusions (b) Fig. 3. The surface of the substrate after carbon film decohesion for specimens with (a) low adhesion and (b) medium adhesion. Depending on the adhesion, delamination of the carbon film occurs completely along the film-substrate interface or partly along the film-substrate interface and partly across the substrate.
It is found that during tensile tests delamination of the film from the substrate takes place. Depending on adhesion strength, delamination of the carbon film occurs completely along the film-substrate interface or partly along the film-substrate interface and partly across the substrate. For quantitative estimation of the adhesion strength between a carbon film and a polymeric substrate we propose the use of the ratio of the area of delamination across the substrate to the whole film area.
Acknowledgment The authors thank Monsanto Chemical Company for their support.
References
Fig. 4. Part of a film delaminated from the polymeric substrate in a tensile test. The decohesion of the film began across the substrate and then along the film-substrate interface in the central part of the band.
1 S. Noda, H. Doi, N. Yamamoto and T. Hioki, J. Mater. Sci. Lett., 5 (1986) 381. 2 M. T. Laugier, J. Mater. Sci., 21 (1986) 2269. 3 P. J. Burnett and D. S. Rickerby, J. Mater. Sci., 23 (1988) 2429. 4 D. B. Marshall and A. G. Evans, J. Appl. Phys., 56 (1984) 2632. 5 L. G. Rosenfeld, J. E. Ritter, T. J. Lardner and M. R. Lin, J. Appl. Phys., 67 (1990) 3291. 6 W. C. Oliver and C. J. McHargue, Thin Solid Films, 161 0988) 117. 7 J. Robertson, Surf. Coat. Technol., 50 (1992) 185.
Diamond and Related Materials, 3 (1993) 116-118
Determination of adhesion strength between a carbon film and a polymeric substrate M . A. V o r o n k i n , S. N . D u b * , I. N . L u p i c h a n d B. A. M a s l y u k Institute for Superhard Materials of the Ukrainian Academy of Sciences, 254153 Kiev (Ukraine) (Received August 14, 1992; accepted in final form April 7, 1993)
Abstract The cracking and decohesion of amorphous hydrogenated carbon films on polymeric substrates are studied by tensile testing. Depending on adhesion strength the carbon film decohesion is found to occur either at the film-substrate interface or across the substrate. It is suggested that a ratio of the area of fracture across the substrate to the whole film area should be used for quantitative estimation of adhesion.
1. Introduction The method of scratching by Vickers [1] or Rockwell [2, 3] indenters is widely used for estimation of adhesion between a film and substrate. The adhesion strength is determined from a load which causes film decohesion from the substrate. In Vickers hardness tests, delamination of the film is sometimes observed; this allows the interfacial fracture energy to be measured [4, 5]. However, we observed no delamination of amorphous hydrogenated carbon ( a - C : H ) films on a soft polymeric substrate in scratching and indentation tests at loads of 2 N or less. We succeeded in estimating the adhesion between the a - C : H film and polymeric substrate by tensile testing. As the specimen undergoes large deformations during the testing, film cracking and decohesion from the substrate can be observed. Furthermore, the decohesion occurs partially at the film-substrate interface and partially across the substrate. We used a ratio of fracture area across the substrate to the whole film area to estimate the adhesion strength.
2. Experimental details a-C : H films up to 10 ~tm in thickness were deposited onto a poly(ethylene terephthalate) (PET) substrate 100 Ixm thick. The low temperature deposition method for carbon film from methane in a low frequency glow discharge plasma was used. The substrate temperature was 80 °C or less. The chamber pressure was maintained at 8 Torr. *Author to whom correspondence should be addressed.
0925-9635/93/$6.00
Tensile tests were carried out using a Heckert FP-10 universal testing machine at a cross-head speed of 2 m m m i n - 1. The samples were 50 m m long and 10 m m wide. The grippers spacing (gauge length) was 30 mm. In tests, this distance increased to 45 mm. After testing, the specimen surface was examined using optical and electron microscopes. The density of the films was determined by gravimetry. The hardness and elastic modulus of the films were determined using a N a n o Indenter II ~ (Nano Instrument Inc., Knoxville, TN, USA) mechanical properties microprobe [6]. The Berkovich indenter was used to produce indentations. All tests were performed at a peak load of 1 m N at a loading rate of 70 IxN s - 1. The hold time at peak load was 20 s.
3. Results and discussion Transparent yellow-brown films from 0.4 to 10 lam in thickness were obtained. Their density ranged from 0.9 to 1.2 g cm -3, the hardness varied between 0.6 and 1.4 G P a and the elastic modulus between 5 and 10 GPa. In terms of their properties, the films belong to the class of "soft" diamond-like carbon (DLC) films [7]. The substrate hardness under these test conditions is 0.36 GPa. 3.1. Effect o f a-C : H film thickness on its cracking and decohesion a - C : H films 0.4-2.3 Ixm in thickness were deposited under the same conditions. At the initial stage of tension there are parallel cracks which propagate perpendicu-
© 1993 - - Elsevier Sequoia. All rights reserved
M. A. Voronkin et al. / Adhesion between C and a polymeric substrate
larly to the tensile direction and cross the whole film. The distance between these cracks decreases linearly with decreasing film thickness. Further tension causes specimen contraction resulting in additional fracture of the film. The film either peels off the substrate or breaks parallel to the tensile direction. A thin film (0.4 jam), as shown in Fig. l(a), fails readily in the direction parallel to that of tension, so its decohesion from the substrate is insignificant. It is easier for a thicker (2.3 lam) film to peel off rather than to fail (Fig. l(b)). That is why a large part of the film exhibited decohesion during the tension. Thus the film fracture and decohesion are affected by both its adhesion and its thickness. In tension a film with low adhesion and a thickness of less than 0.5 gm breaks into fine fragments but does not peel off the substrate. Therefore estimation of the adhesion in tension requires that the thickness of the a - C : H film should be more than 2 jam. Further studies were carried out on films 5-10 jam in thickness.
117
Fig. 2. Optical micrograph showing carbon film cracking and decohesion from the polymeric substrate. Parallel cracks are oriented transverse to the tensile direction. Note the new crack in the centre which propagates from a defect at the surface of the film.
3.2. Cracking and decohesion o f a-C : H films as a f u n c t i o n o f adhesion
(a)
(b) Fig. 1. The cracking and decohesion of carbon films on a polymeric substrate with different film thicknesses: (a) 0.4 gm; (b) 2.3 p.m.
In the case of low adhesion the film breaks into fragments hundreds of micrometres in width. The delamination of the film from the substrate (Fig. 2) is initiated from opposite sides of the fragments which are transverse to the tensile direction. The delamination occurs through the development of half-elliptical cracks. The cracks from the opposite sides of a fragment coalesce and then propagate towards the specimen edges. As a result, the film peels off completely. Observation of the substrate surface after the film delamination shows that the film peels off only at the film substrate interface, i.e. there is no substrate fracture (Fig. 3(a)). For a - C : H film with medium adhesion the distance between parallel cracks is an order of magnitude smaller. The delamination is initiated at the substrate from two opposite sides of the fragments; then the crack reaches the film-substrate interface (Fig. 3(b)). At higher adhesion a larger zone of crack propagation across the substrate can be observed. Thus in this case the film comes off the substrate in the form of long narrow bands (Fig. 4) and the decohesion occurs partially along the film-substrate interface and partially across the polymeric substrate. For a - C : H film with high adhesion the long narrow bands which formed at the initial stage of tension break into short fragments during further tension, and the fragments crawl over each other (Fig. 5). The decohesion takes place completely across the polymeric substrate. In order to estimate adhesion we suggest that the ratio between the area of decohesion across the substrate to the whole film area should be used. The ratio can be given by A = (Sl--$2)/S
1
118
M. A. Voronkin et al. / Adhesion between C and a polymeric substrate
(a/ Fig. 5. The surface of the substrate after carbon film decohesion for a specimen with high adhesion. Decohesion occurs only across the substrate.
where $1 is the mean width of the film fragment and 5 2 is the mean size of a zone in which the delamination occurs at the film-substrate interface (Fig. 4). In the case of low adhesion, i.e. of the delamination at the filmsubstrate interface, this ratio is equal to zero. For a film with high adhesion when the delamination occurs entirely across the substrate, the ratio is equal to unity.
4. Conclusions (b) Fig. 3. The surface of the substrate after carbon film decohesion for specimens with (a) low adhesion and (b) medium adhesion. Depending on the adhesion, delamination of the carbon film occurs completely along the film-substrate interface or partly along the film-substrate interface and partly across the substrate.
It is found that during tensile tests delamination of the film from the substrate takes place. Depending on adhesion strength, delamination of the carbon film occurs completely along the film-substrate interface or partly along the film-substrate interface and partly across the substrate. For quantitative estimation of the adhesion strength between a carbon film and a polymeric substrate we propose the use of the ratio of the area of delamination across the substrate to the whole film area.
Acknowledgment The authors thank Monsanto Chemical Company for their support.
References
Fig. 4. Part of a film delaminated from the polymeric substrate in a tensile test. The decohesion of the film began across the substrate and then along the film-substrate interface in the central part of the band.
1 S. Noda, H. Doi, N. Yamamoto and T. Hioki, J. Mater. Sci. Lett., 5 (1986) 381. 2 M. T. Laugier, J. Mater. Sci., 21 (1986) 2269. 3 P. J. Burnett and D. S. Rickerby, J. Mater. Sci., 23 (1988) 2429. 4 D. B. Marshall and A. G. Evans, J. Appl. Phys., 56 (1984) 2632. 5 L. G. Rosenfeld, J. E. Ritter, T. J. Lardner and M. R. Lin, J. Appl. Phys., 67 (1990) 3291. 6 W. C. Oliver and C. J. McHargue, Thin Solid Films, 161 0988) 117. 7 J. Robertson, Surf. Coat. Technol., 50 (1992) 185.