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Effect of adhesion strength of DLC to steel on the coating erosion mechanism
Diamond and Related Materials 10 Ž2001. 1824᎐1828
Effect of adhesion strength of DLC to steel on the coating erosion mechanism I.Sh. Trakhtenberg, A.B. VladimirovU , S.A. Plotnikov, A.P. Rubshtein, V.B. Vykhodets, O.M. Bakunin Metal Physics Institute, 18 S. Ko¨ ale¨ skaya St., 620219 Ekaterinburg, Russia
Abstract The wear of DLCs deposited onto steel substrates using a graphite arc-pulse sputtering technique in a corundum particle jet was studied. Two sample sets had different adhesion strength to the substrate due to different adhesive sublayer structures. It was found that the DLC itself does not wear, so that coating destruction occurs due to peeling. Analysis of the wear results for coatings having different Ž0.4᎐2.6 m. thickness revealed that peeling is a result of two basic crack systems: Ži. from the DLC surface inside the coating; and Žii. along the DLC᎐substrate interface. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon; Arc pulse; Adhesion strength; Erosion mechanism
1. Introduction Wear-resistant hard diamond-like carbon coatings ŽDLCs., whose friction coefficient is lower than that of many other materials, show promise in terms of favorable modification of the surface of cutting tools and components of mechanical systems w1,2x. This paper deals with the destruction of DLCs under multiple impacts of hard particles, or as a result of the so-called erosion. A specific feature of this process in any coated material is that impacts are delivered, not to a solid material, but to a material with a special Žcoating᎐substrate . plane. It is in this plane that mechanical characteristics change abruptly and the atomic binding force, which strongly depends on the coating deposition conditions, weakens compared to a solid material. The goal of this work was to ascertain the main physical factors Žproperties of the coating itself or the
U
Corresponding author. Tel.: q7-3432-499366; fax: q7-3432745244. E-mail address: [email protected] ŽA.B. Vladimirov..
conjugate plane between the coating and the substrate . determining the durability of the coating on the surface of the base material under external mechanical effects. The DLC᎐steel system is one of the most promising for practical applications. For this reason, the present study was concerned with this system.
2. Experimental technique DLCs were deposited on steel in an UVNIIPA-1-001 vacuum installation, which had a working chamber with three sources for: Ž1. cleaning of the surface by bombardment with ions; Ž2. arc sputtering of the metal; and Ž3. deposition of DLC by vacuum pulse sputtering of graphite. Two sets of samples, having a similar collection of DLC thickness values, were prepared. Adhesion strength of the DLC to steel was to be different for the sets of samples and was the same within a single set. To meet these requirements, test samples were prepared as follows. Samples entering the first set were sputtered in one vacuum cycle. The substrate was cleaned by ion
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 4 3 0 - 7
I.Sh. Trakhtenberg et al. r Diamond and Related Materials 10 (2001) 1824᎐1828
bombardment and an adhesion sublayer Ž100-nm Ti. was sputtered on the substrate. Finally, a DLC film Ž400 nm thick. was sputtered on the sublayer. The second set of samples was prepared analogously. The only distinction was the structure of the adhesion sublayer, which consisted of 30-nm Ti and 70-nm TiC. Our investigations w3x showed that the adhesion strength of DLC to steel should be much stronger in the second than in the first set of samples. Similarity of DLC in the two sets was confirmed by comparison of their Raman spectra. Subsequently, some samples from both sets were placed in the sputtering installation. Air-adsorbed gases were removed from the surface of these samples by ion bombardment and DLC of a preset thickness was sputtered on the samples. Thus, we obtained two sets of samples with different adhesion strength of the DLC to steel, but an identical structure and similar properties of the DLCs themselves, since they were sputtered at the same time. Reference samples having a surface area of ; 10 cm2 , which served to determine the thickness of the sputtered layer by the weighing method Ž"0.1 mg., were deposited simultaneously with the test samples. Simultaneous ‘sputtering’ of samples from sets I and II guaranteed against systematic differences in thickness and properties of the DLCs arising from unknown dissimilarity of sputtering conditions in more than one vacuum cycle. Samples from both sets and an uncoated reference sample of steel were placed in an abrasive test installation developed on the basis of a rotor-type accelerator of hard particles w4x. Corundum particles of 100 m in diameter traveling at a speed of 10 m sy1 struck the surface of the samples vertically. The samples were weighed regularly after a certain dose of impacts by hard particles in order to construct the dependence ⌬ m s fŽ N ., where ⌬ m is the weight loss of a sample and N is the dose or the number of impacts Žparticles cmy2 .. A relatively high accuracy and reproducibility of the experimental conditions are worth noting. Specifically, the mean wear intensity of the substrate material Ž⭸Ž ⌬ m.r⭸N . was reproduced to within "2.5%, irrespective of whether it was determined using a steel reference sample or from a section of the ⌬ m s fŽ N . curve for sputtered samples after the DLC was completely stripped. Numerous experiments w4,5x showed that very strong coatings did not have any traces of wear during the initial test period, or in operation under actual service conditions, but then they exfoliated from the substrate. Therefore, without looking into the mechanism by which DLCs exfoliate from the substrate under the action of abrasive particles, we analyzed the experimental results using one characteristic, namely the dose of impacts delivered to the coating material, Nc ,
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necessary for complete removal of a coating having thickness d. If the wear process continues long after the DLC has been removed, it is possible to determine J 0 s ⭸mr⭸N s const from a section of the curve ⌬ m s fŽ N . of the coating exfoliation. In our experiments, the J 0 value coincided with the ⭸mr⭸N value, which was determined for the reference sample Žuncoated substrate . to within 3%. If ⌬ N is defined as a length cut off on the abscissa axis by the actual dependence ⌬ m s fŽ N . after the coating exfoliation, it is easy to show Žas follows from a simple rearrangement of dependences in w4x. that Nc s ⌬ N q m crJ0 , where m c is the weight of the deposited hard coating.
3. Results Fig. 1 presents the experimental dependence of ⌬ m s fŽ N . for samples from both sets. The dependence of Nc s fŽ d . Žwhere d is the coating thickness . is shown in Fig. 2. From this figure, it is evident that strong adhesion causes, not only an increase in wear resistance of the DLC, but also alters the wear-resistant behavior depending on the thickness of the coating. A different
Fig. 1. Weight of sample loss versus number of abrasive particle impacts for DLCs on steel. The open and solid circles correspond to the first and second sample sets, respectively. The digital near line is the thickness of the sample.
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Fig. 2. Ža. Dependence of number of abrasive particle impacts Nc on coating thickness for first Žsolid circle. and second Žopen square. sample sets. Žb. Dependence of 1rNc on coating thickness for second sample set.
dependence behavior ŽFig. 2. is especially pronounced for thin coatings Ž dª 0.. If adhesion is weak, very thin coatings separate almost immediately from the substrate. If adhesion of the coating to the substrate is strong and dª 0, Nc is not zero. This provides a good outlook for the practical application of thin protective DLCs.
4. Discussion Let us analyze the aforementioned findings in the attempt to retrieve information about the main processes responsible for the observed features of erosion in the DLC᎐steel system. The most significant point Žas follows from the behavior of the curves in Fig. 1. is that ‘something happens’ in the coating exposed to multiple impacts of hard particles and, as a result, the coating peels off. Actually, exfoliation takes place during bombardment and proceeds as peeling of separate pieces of the coating. Weight measurements and examination of the samples by Rutherford backscattering of deuterons show that the thickness of exfoliated fragments is nearly equal to the initial thickness of the DLCs at different stages of the erosion process. Therefore, the main cause for the removal of the DLCs from the steel surface is not uniform wear of the coatings, but the development of a system of cracks under impacts by hard particles. In this connection, it is important to understand the space᎐force situation arising in the samples under the particular experimental conditions adopted in our work:
normal impacts by corundum particles of 50 m in radius traveling at a speed of 10 m sy1 . Calculations performed for these conditions in terms of our model w6x yielded the following result. The largest impact force is applied to an area of radius ␦ s ar '3 s 2.6 m Žwhere a is the radius of the contact spot w7x.. The mean pressure in this area is approximately 40 GPa. Normal compression stresses zz , which are several times larger than tangential stresses, are most significant at the DLC᎐steel interface upon impact. It is noteworthy that < zz < ) T Žwhere T is the yield strength of the steel. within ␦ 2 . This means that, the thinner the coating, the more intensive is the plastic deformation which takes place under the coating upon impact. When the coating thickness d increases, < zz < decreases almost linearly at the DLC᎐steel interface at d- ␦ and the dependence zz s fŽ d . is much weaker at d) ␦. From the discussion above, it follows that plastic deformation caused by impacts of hard particles should lead to the appearance of cavities in the DLC᎐steel plane. This process should increase as coating adhesion to the substrate decreases. Thus, failure of the coating᎐substrate system can be visualized as a simple scheme involving two independent processes: propagation of cracks from the surface of the coating to the substrate, and formation of cavities at the coating᎐substrate interface. Fig. 3 schematically depicts a system of cracks developing in the DLCs exposed to hard particles. An impact of a hard particle causes the formation of a
I.Sh. Trakhtenberg et al. r Diamond and Related Materials 10 (2001) 1824᎐1828
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necessarily inhomogeneous, the probability of those processes is multiplied. As a result, the coating peels off in fragments and a dose interval of N impacts, where exfoliation takes place, appears. Therefore, values of Nc , l 0 , etc., suppose that we deal with averaged characteristics. To get a better insight into ‘secondary’ physical factors responsible for the fragmentary exfoliation, we need Žand plan. to perform a separate study. We use these values in the analysis being well aware of their average character. Let us consider the exfoliation process in the coating᎐substrate plane. If S is the area of a cavity formed by one impact, the coating will peel off provided the condition S = N s 1 is fulfilled. Obviously, S should depend on the coating thickness d, because stresses in this plane decrease with increasing thickness of the coating w6x. Suppose that the dependence S s S0 Ž1 y ␣ d . is linear. Then, in the case of good adhesion, the dependence 1 s S0 Ž 1 y ␣ d . Nc
Fig. 3. Scheme of crack propagation leading to coating failure.
primary crack of length l 0 . Assume that this crack propagates upon subsequent impacts at a rate V s ⭸lr⭸N. Then the number of impacts required for the crack to reach the coating᎐substrate plane will be: Nc s N0 q
Ž dy l 0 . V
Ž1.
where N0 ; lr␦2 is the dose at which each part of the coating surface experiences one impact. The situation does not change for coatings of thickness dF l 0 : a crack reaches the coating᎐substrate surface even upon the first impact by a particle. Therefore, when dF l 0 , Nc ( N0 . If d) l 0 , propagation of cracks through the coating is governed by Eq. Ž1.. In our experiments, N0 ; 5 = 10 6 cmy2 and, as evident in Fig. 2, this value can be neglected for coatings whose thickness exceeds the length of a primary crack. If a system of cavities covers almost all the coating᎐substrate plane, the moment cracks reach this plane, the coating will peel off. In this case Žweak adhesion. the dependence Nc s fŽ d . should follow Eq. Ž1.. This is actually observed in one series of our experimental results ŽFig. 2a.. If cavities are absent in the coating᎐substrate plane Žstrong adhesion., the coating will not peel off, even if cracks propagate down to this plane. A crack has to ‘wait’ until conditions necessary for exfoliation of the coating are formed in the plane. Since secondary processes and structuralrgeometrical conditions are
Ž2.
should hold. This is actually observed in the experiments ŽFig. 2b.. Fig. 3 exemplifies a very thick Ž d4 ␦ . coating. In this case, the coating᎐substrate interface should not experience any forces and the coating should behave almost as a solid material. The wear rate should be constant, J s ⭸dr⭸N s const, due to the formation of spalling particles after the integration of impactinduced cracks. It would be reasonable to expect that the spalling mechanism Žquasi-homogeneous wear J s ⭸dr⭸N s const. operates in coatings with the highest wear resistance Žwith increasing Nc .. We have already drawn attention to this circumstance above in the discussion of results obtained for samples with d) ␦. Since the shape of the dependence Nc s fŽ d . is in agreement with our assumptions ŽEqs. Ž1. and Ž2. for weak and strong adhesion, respectively., it is possible to retrieve information about the main processes that take place in DLCs on steel during bombardment with hard particles. Processing of data for weak-adhesion samples Žset I. gave the value l 0 s 4 = 10y5 cm. Physically, in the given impact conditions, the length of the primary crack, l 0 , should mostly characterize the strength properties of the coating material and be dependent on the substrate material. This inference proves to be true, since previously we estimated l 0 ; 0.4 m from erosion tests of DLCs on aluminum performed under exactly the same conditions w8x. For the series of DLC samples on steel which had the dependence Nc ; d, we obtained, in accordance with Eq. Ž1., V s ⭸dr⭸N s 7 = 10y1 3 cm3 particley1.
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I.Sh. Trakhtenberg et al. r Diamond and Related Materials 10 (2001) 1824᎐1828
This result, which immediately follows from the experimental dependence Nc s fŽ d ., lends itself to a less unambiguous interpretation if the calculated effective area of one particle ␦ 2 is taken into account. Thus, each subsequent impact delivered to one and the same spot increases the crack length for ⌬ l s Vr␦2 s 4 = 10y6 cm. In other words, the increment in the crack length upon each impact is much less than the length of the primary crack. It should be emphasized that quantitative dependences, which were established above for the crack propagation, are significant for coatings protecting against an aggressive environment. Irrespective of the quality of the coating adhesion to the substrate, a system of cracks Žand consequently, penetrability of the aggressive environment to the surface of the protected material. is determined by the parameters in Eq. Ž1.. The following dependence holds for samples from set II with d- ␦ ŽFig. 2b.: 1 s 1.1= 10y8 Ž 1 y 4 = 10 3d . Nc
cm2 particley1
Ž3.
As was mentioned above, peeling of the coating on these samples is limited by the formation of cavities under the coating. Most probably, cavities appear as a result of plastic deformation Ždenting of steel under the coating upon impact and separation of the strong, elastic DLC when it straightens after the particle bounces off.. Eq. Ž3. suggests that the diameter Ž; 0.6 = 10y4 cm. of a cavity under a thin Ž dª 0. coating is within ␦ Žnearly four-fold smaller. and decreases with increasing thickness of the coating at d- ␦, almost in the same manner as compression stresses diminish in the coating᎐substrate plane. Finally, it must not be overlooked that, at d) ␦, values of Nc ŽFig. 2a,b. deviate from the dependence in Eq. Ž1. and, especially, Eq. Ž2.. We are left with the assumption that the observed deviation is due to spalling at a rate of J ; ⌬ drN involved in the wear process. This assumption is supported by the fact that J I s 5 = 10y1 4 Ž"30%. and J II s 6 = 10y1 4 cm3 particley1 Ž"10%. coincide within the error. Therefore, it may be inferred that expansion of the spalling zone is much slower than propagation of the vertical cracks Ž JrVI ; 0.1.. 5. Conclusion The ratio between the size of the effective area of an
impact on the material Žcontact spot area, mean pressure in the spot. and the coating thickness d is a decisive parameter in the formation of DLC protective properties. When d4 ␦, the coating behaves as a solid material. DLC wears due to the formation of cracks, which integrate together into a continuous, spatially closed surface, i.e. the surface of a particle ready to separate Žthe spalling mechanism.. When a coating is thin, the special coating᎐substrate plane gains in importance. As soon as the cracks reaches this plane, the coating peels off if an interface has been formed between the coating and the substrate in this plane owing to low adhesion strength. However, if adhesion strength is great, the coating will not separate until a cavity appears near the coating᎐substrate plane. Obviously, in these two cases the separation kinetics of the coating are different. The adhesion quality is very important for extremely thin coatings. If adhesion strength is low, the coating separates immediately. If adhesion strength to the substrate is high, the coating is preserved until a system of cavities is formed under the coating. In the case of DLCs on metal, the appearance of a cavity under the coating will depend, when plastic deformation under the coating is considered, on the strength of the substrate material, even if adhesion between the coating and the substrate is perfect w8x. Consequently, wear resistance of coatings, especially those on soft materials, cannot be improved limitlessly through refinement of the DLC sputtering technology. References w1x Y. Lifshitz, Diamond Relat. Mater. 8 Ž1999. 1659᎐1676. w2x A. Grill, V. Patel, Diamond Relat. Mater. 2 Ž1992. 597᎐605. w3x I.Sh. Trakhtenberg, S.A. Plotnikov, I.N. Korneev, A.P. Rubshtein, A.B. Vladimirov, O.M. Bakunin, Proceedings of the of ADCrFCT 99 conference, Tsukuba, Japan, 31 August᎐3 September 1999, Ž1999. 623᎐626. w4x S.D. Gorpinchenko, S.M. Klotsman, E.V. Kuzmina, S.A. Plotnikov, I.Sh. Trakhtenberg, Surf. Coat. Technol. 47 Ž1991. 201᎐208. w5x S.D. Gorpinchenko, S.M. Klotsman, E.V. Kuzmina, S.A. Plotnikov, I.Sh. Trakhtenberg, Diamond Relat. Mater. 1 Ž1992. 619᎐622. w6x V.V. Kondratyev, I.Sh. Trakhtenberg, A.V. Gapontsev, Fiz. Metall. Metallovedenie 90 Ž4. Ž2000. 1᎐7. w7x Y.V. Kolesnikov, E.M. Morozov, Mekhanika Kontaktnogo Razrusheniya, M. Nauka, 1989. w8x I.Sh. Trakhtenberg, A.B. Vladimirov, O.M. Bakunin, S.A. Plotnikov, I.N. Korneev, L.G. Korshunov, Proceedings of the ADCrFCT 99 conference, Tsukuba, Japan, 31 August᎐3 September 1999, Ž1999. 633᎐636.
Effect of adhesion strength of DLC to steel on the coating erosion mechanism I.Sh. Trakhtenberg, A.B. VladimirovU , S.A. Plotnikov, A.P. Rubshtein, V.B. Vykhodets, O.M. Bakunin Metal Physics Institute, 18 S. Ko¨ ale¨ skaya St., 620219 Ekaterinburg, Russia
Abstract The wear of DLCs deposited onto steel substrates using a graphite arc-pulse sputtering technique in a corundum particle jet was studied. Two sample sets had different adhesion strength to the substrate due to different adhesive sublayer structures. It was found that the DLC itself does not wear, so that coating destruction occurs due to peeling. Analysis of the wear results for coatings having different Ž0.4᎐2.6 m. thickness revealed that peeling is a result of two basic crack systems: Ži. from the DLC surface inside the coating; and Žii. along the DLC᎐substrate interface. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon; Arc pulse; Adhesion strength; Erosion mechanism
1. Introduction Wear-resistant hard diamond-like carbon coatings ŽDLCs., whose friction coefficient is lower than that of many other materials, show promise in terms of favorable modification of the surface of cutting tools and components of mechanical systems w1,2x. This paper deals with the destruction of DLCs under multiple impacts of hard particles, or as a result of the so-called erosion. A specific feature of this process in any coated material is that impacts are delivered, not to a solid material, but to a material with a special Žcoating᎐substrate . plane. It is in this plane that mechanical characteristics change abruptly and the atomic binding force, which strongly depends on the coating deposition conditions, weakens compared to a solid material. The goal of this work was to ascertain the main physical factors Žproperties of the coating itself or the
U
Corresponding author. Tel.: q7-3432-499366; fax: q7-3432745244. E-mail address: [email protected] ŽA.B. Vladimirov..
conjugate plane between the coating and the substrate . determining the durability of the coating on the surface of the base material under external mechanical effects. The DLC᎐steel system is one of the most promising for practical applications. For this reason, the present study was concerned with this system.
2. Experimental technique DLCs were deposited on steel in an UVNIIPA-1-001 vacuum installation, which had a working chamber with three sources for: Ž1. cleaning of the surface by bombardment with ions; Ž2. arc sputtering of the metal; and Ž3. deposition of DLC by vacuum pulse sputtering of graphite. Two sets of samples, having a similar collection of DLC thickness values, were prepared. Adhesion strength of the DLC to steel was to be different for the sets of samples and was the same within a single set. To meet these requirements, test samples were prepared as follows. Samples entering the first set were sputtered in one vacuum cycle. The substrate was cleaned by ion
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 4 3 0 - 7
I.Sh. Trakhtenberg et al. r Diamond and Related Materials 10 (2001) 1824᎐1828
bombardment and an adhesion sublayer Ž100-nm Ti. was sputtered on the substrate. Finally, a DLC film Ž400 nm thick. was sputtered on the sublayer. The second set of samples was prepared analogously. The only distinction was the structure of the adhesion sublayer, which consisted of 30-nm Ti and 70-nm TiC. Our investigations w3x showed that the adhesion strength of DLC to steel should be much stronger in the second than in the first set of samples. Similarity of DLC in the two sets was confirmed by comparison of their Raman spectra. Subsequently, some samples from both sets were placed in the sputtering installation. Air-adsorbed gases were removed from the surface of these samples by ion bombardment and DLC of a preset thickness was sputtered on the samples. Thus, we obtained two sets of samples with different adhesion strength of the DLC to steel, but an identical structure and similar properties of the DLCs themselves, since they were sputtered at the same time. Reference samples having a surface area of ; 10 cm2 , which served to determine the thickness of the sputtered layer by the weighing method Ž"0.1 mg., were deposited simultaneously with the test samples. Simultaneous ‘sputtering’ of samples from sets I and II guaranteed against systematic differences in thickness and properties of the DLCs arising from unknown dissimilarity of sputtering conditions in more than one vacuum cycle. Samples from both sets and an uncoated reference sample of steel were placed in an abrasive test installation developed on the basis of a rotor-type accelerator of hard particles w4x. Corundum particles of 100 m in diameter traveling at a speed of 10 m sy1 struck the surface of the samples vertically. The samples were weighed regularly after a certain dose of impacts by hard particles in order to construct the dependence ⌬ m s fŽ N ., where ⌬ m is the weight loss of a sample and N is the dose or the number of impacts Žparticles cmy2 .. A relatively high accuracy and reproducibility of the experimental conditions are worth noting. Specifically, the mean wear intensity of the substrate material Ž⭸Ž ⌬ m.r⭸N . was reproduced to within "2.5%, irrespective of whether it was determined using a steel reference sample or from a section of the ⌬ m s fŽ N . curve for sputtered samples after the DLC was completely stripped. Numerous experiments w4,5x showed that very strong coatings did not have any traces of wear during the initial test period, or in operation under actual service conditions, but then they exfoliated from the substrate. Therefore, without looking into the mechanism by which DLCs exfoliate from the substrate under the action of abrasive particles, we analyzed the experimental results using one characteristic, namely the dose of impacts delivered to the coating material, Nc ,
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necessary for complete removal of a coating having thickness d. If the wear process continues long after the DLC has been removed, it is possible to determine J 0 s ⭸mr⭸N s const from a section of the curve ⌬ m s fŽ N . of the coating exfoliation. In our experiments, the J 0 value coincided with the ⭸mr⭸N value, which was determined for the reference sample Žuncoated substrate . to within 3%. If ⌬ N is defined as a length cut off on the abscissa axis by the actual dependence ⌬ m s fŽ N . after the coating exfoliation, it is easy to show Žas follows from a simple rearrangement of dependences in w4x. that Nc s ⌬ N q m crJ0 , where m c is the weight of the deposited hard coating.
3. Results Fig. 1 presents the experimental dependence of ⌬ m s fŽ N . for samples from both sets. The dependence of Nc s fŽ d . Žwhere d is the coating thickness . is shown in Fig. 2. From this figure, it is evident that strong adhesion causes, not only an increase in wear resistance of the DLC, but also alters the wear-resistant behavior depending on the thickness of the coating. A different
Fig. 1. Weight of sample loss versus number of abrasive particle impacts for DLCs on steel. The open and solid circles correspond to the first and second sample sets, respectively. The digital near line is the thickness of the sample.
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Fig. 2. Ža. Dependence of number of abrasive particle impacts Nc on coating thickness for first Žsolid circle. and second Žopen square. sample sets. Žb. Dependence of 1rNc on coating thickness for second sample set.
dependence behavior ŽFig. 2. is especially pronounced for thin coatings Ž dª 0.. If adhesion is weak, very thin coatings separate almost immediately from the substrate. If adhesion of the coating to the substrate is strong and dª 0, Nc is not zero. This provides a good outlook for the practical application of thin protective DLCs.
4. Discussion Let us analyze the aforementioned findings in the attempt to retrieve information about the main processes responsible for the observed features of erosion in the DLC᎐steel system. The most significant point Žas follows from the behavior of the curves in Fig. 1. is that ‘something happens’ in the coating exposed to multiple impacts of hard particles and, as a result, the coating peels off. Actually, exfoliation takes place during bombardment and proceeds as peeling of separate pieces of the coating. Weight measurements and examination of the samples by Rutherford backscattering of deuterons show that the thickness of exfoliated fragments is nearly equal to the initial thickness of the DLCs at different stages of the erosion process. Therefore, the main cause for the removal of the DLCs from the steel surface is not uniform wear of the coatings, but the development of a system of cracks under impacts by hard particles. In this connection, it is important to understand the space᎐force situation arising in the samples under the particular experimental conditions adopted in our work:
normal impacts by corundum particles of 50 m in radius traveling at a speed of 10 m sy1 . Calculations performed for these conditions in terms of our model w6x yielded the following result. The largest impact force is applied to an area of radius ␦ s ar '3 s 2.6 m Žwhere a is the radius of the contact spot w7x.. The mean pressure in this area is approximately 40 GPa. Normal compression stresses zz , which are several times larger than tangential stresses, are most significant at the DLC᎐steel interface upon impact. It is noteworthy that < zz < ) T Žwhere T is the yield strength of the steel. within ␦ 2 . This means that, the thinner the coating, the more intensive is the plastic deformation which takes place under the coating upon impact. When the coating thickness d increases, < zz < decreases almost linearly at the DLC᎐steel interface at d- ␦ and the dependence zz s fŽ d . is much weaker at d) ␦. From the discussion above, it follows that plastic deformation caused by impacts of hard particles should lead to the appearance of cavities in the DLC᎐steel plane. This process should increase as coating adhesion to the substrate decreases. Thus, failure of the coating᎐substrate system can be visualized as a simple scheme involving two independent processes: propagation of cracks from the surface of the coating to the substrate, and formation of cavities at the coating᎐substrate interface. Fig. 3 schematically depicts a system of cracks developing in the DLCs exposed to hard particles. An impact of a hard particle causes the formation of a
I.Sh. Trakhtenberg et al. r Diamond and Related Materials 10 (2001) 1824᎐1828
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necessarily inhomogeneous, the probability of those processes is multiplied. As a result, the coating peels off in fragments and a dose interval of N impacts, where exfoliation takes place, appears. Therefore, values of Nc , l 0 , etc., suppose that we deal with averaged characteristics. To get a better insight into ‘secondary’ physical factors responsible for the fragmentary exfoliation, we need Žand plan. to perform a separate study. We use these values in the analysis being well aware of their average character. Let us consider the exfoliation process in the coating᎐substrate plane. If S is the area of a cavity formed by one impact, the coating will peel off provided the condition S = N s 1 is fulfilled. Obviously, S should depend on the coating thickness d, because stresses in this plane decrease with increasing thickness of the coating w6x. Suppose that the dependence S s S0 Ž1 y ␣ d . is linear. Then, in the case of good adhesion, the dependence 1 s S0 Ž 1 y ␣ d . Nc
Fig. 3. Scheme of crack propagation leading to coating failure.
primary crack of length l 0 . Assume that this crack propagates upon subsequent impacts at a rate V s ⭸lr⭸N. Then the number of impacts required for the crack to reach the coating᎐substrate plane will be: Nc s N0 q
Ž dy l 0 . V
Ž1.
where N0 ; lr␦2 is the dose at which each part of the coating surface experiences one impact. The situation does not change for coatings of thickness dF l 0 : a crack reaches the coating᎐substrate surface even upon the first impact by a particle. Therefore, when dF l 0 , Nc ( N0 . If d) l 0 , propagation of cracks through the coating is governed by Eq. Ž1.. In our experiments, N0 ; 5 = 10 6 cmy2 and, as evident in Fig. 2, this value can be neglected for coatings whose thickness exceeds the length of a primary crack. If a system of cavities covers almost all the coating᎐substrate plane, the moment cracks reach this plane, the coating will peel off. In this case Žweak adhesion. the dependence Nc s fŽ d . should follow Eq. Ž1.. This is actually observed in one series of our experimental results ŽFig. 2a.. If cavities are absent in the coating᎐substrate plane Žstrong adhesion., the coating will not peel off, even if cracks propagate down to this plane. A crack has to ‘wait’ until conditions necessary for exfoliation of the coating are formed in the plane. Since secondary processes and structuralrgeometrical conditions are
Ž2.
should hold. This is actually observed in the experiments ŽFig. 2b.. Fig. 3 exemplifies a very thick Ž d4 ␦ . coating. In this case, the coating᎐substrate interface should not experience any forces and the coating should behave almost as a solid material. The wear rate should be constant, J s ⭸dr⭸N s const, due to the formation of spalling particles after the integration of impactinduced cracks. It would be reasonable to expect that the spalling mechanism Žquasi-homogeneous wear J s ⭸dr⭸N s const. operates in coatings with the highest wear resistance Žwith increasing Nc .. We have already drawn attention to this circumstance above in the discussion of results obtained for samples with d) ␦. Since the shape of the dependence Nc s fŽ d . is in agreement with our assumptions ŽEqs. Ž1. and Ž2. for weak and strong adhesion, respectively., it is possible to retrieve information about the main processes that take place in DLCs on steel during bombardment with hard particles. Processing of data for weak-adhesion samples Žset I. gave the value l 0 s 4 = 10y5 cm. Physically, in the given impact conditions, the length of the primary crack, l 0 , should mostly characterize the strength properties of the coating material and be dependent on the substrate material. This inference proves to be true, since previously we estimated l 0 ; 0.4 m from erosion tests of DLCs on aluminum performed under exactly the same conditions w8x. For the series of DLC samples on steel which had the dependence Nc ; d, we obtained, in accordance with Eq. Ž1., V s ⭸dr⭸N s 7 = 10y1 3 cm3 particley1.
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I.Sh. Trakhtenberg et al. r Diamond and Related Materials 10 (2001) 1824᎐1828
This result, which immediately follows from the experimental dependence Nc s fŽ d ., lends itself to a less unambiguous interpretation if the calculated effective area of one particle ␦ 2 is taken into account. Thus, each subsequent impact delivered to one and the same spot increases the crack length for ⌬ l s Vr␦2 s 4 = 10y6 cm. In other words, the increment in the crack length upon each impact is much less than the length of the primary crack. It should be emphasized that quantitative dependences, which were established above for the crack propagation, are significant for coatings protecting against an aggressive environment. Irrespective of the quality of the coating adhesion to the substrate, a system of cracks Žand consequently, penetrability of the aggressive environment to the surface of the protected material. is determined by the parameters in Eq. Ž1.. The following dependence holds for samples from set II with d- ␦ ŽFig. 2b.: 1 s 1.1= 10y8 Ž 1 y 4 = 10 3d . Nc
cm2 particley1
Ž3.
As was mentioned above, peeling of the coating on these samples is limited by the formation of cavities under the coating. Most probably, cavities appear as a result of plastic deformation Ždenting of steel under the coating upon impact and separation of the strong, elastic DLC when it straightens after the particle bounces off.. Eq. Ž3. suggests that the diameter Ž; 0.6 = 10y4 cm. of a cavity under a thin Ž dª 0. coating is within ␦ Žnearly four-fold smaller. and decreases with increasing thickness of the coating at d- ␦, almost in the same manner as compression stresses diminish in the coating᎐substrate plane. Finally, it must not be overlooked that, at d) ␦, values of Nc ŽFig. 2a,b. deviate from the dependence in Eq. Ž1. and, especially, Eq. Ž2.. We are left with the assumption that the observed deviation is due to spalling at a rate of J ; ⌬ drN involved in the wear process. This assumption is supported by the fact that J I s 5 = 10y1 4 Ž"30%. and J II s 6 = 10y1 4 cm3 particley1 Ž"10%. coincide within the error. Therefore, it may be inferred that expansion of the spalling zone is much slower than propagation of the vertical cracks Ž JrVI ; 0.1.. 5. Conclusion The ratio between the size of the effective area of an
impact on the material Žcontact spot area, mean pressure in the spot. and the coating thickness d is a decisive parameter in the formation of DLC protective properties. When d4 ␦, the coating behaves as a solid material. DLC wears due to the formation of cracks, which integrate together into a continuous, spatially closed surface, i.e. the surface of a particle ready to separate Žthe spalling mechanism.. When a coating is thin, the special coating᎐substrate plane gains in importance. As soon as the cracks reaches this plane, the coating peels off if an interface has been formed between the coating and the substrate in this plane owing to low adhesion strength. However, if adhesion strength is great, the coating will not separate until a cavity appears near the coating᎐substrate plane. Obviously, in these two cases the separation kinetics of the coating are different. The adhesion quality is very important for extremely thin coatings. If adhesion strength is low, the coating separates immediately. If adhesion strength to the substrate is high, the coating is preserved until a system of cavities is formed under the coating. In the case of DLCs on metal, the appearance of a cavity under the coating will depend, when plastic deformation under the coating is considered, on the strength of the substrate material, even if adhesion between the coating and the substrate is perfect w8x. Consequently, wear resistance of coatings, especially those on soft materials, cannot be improved limitlessly through refinement of the DLC sputtering technology. References w1x Y. Lifshitz, Diamond Relat. Mater. 8 Ž1999. 1659᎐1676. w2x A. Grill, V. Patel, Diamond Relat. Mater. 2 Ž1992. 597᎐605. w3x I.Sh. Trakhtenberg, S.A. Plotnikov, I.N. Korneev, A.P. Rubshtein, A.B. Vladimirov, O.M. Bakunin, Proceedings of the of ADCrFCT 99 conference, Tsukuba, Japan, 31 August᎐3 September 1999, Ž1999. 623᎐626. w4x S.D. Gorpinchenko, S.M. Klotsman, E.V. Kuzmina, S.A. Plotnikov, I.Sh. Trakhtenberg, Surf. Coat. Technol. 47 Ž1991. 201᎐208. w5x S.D. Gorpinchenko, S.M. Klotsman, E.V. Kuzmina, S.A. Plotnikov, I.Sh. Trakhtenberg, Diamond Relat. Mater. 1 Ž1992. 619᎐622. w6x V.V. Kondratyev, I.Sh. Trakhtenberg, A.V. Gapontsev, Fiz. Metall. Metallovedenie 90 Ž4. Ž2000. 1᎐7. w7x Y.V. Kolesnikov, E.M. Morozov, Mekhanika Kontaktnogo Razrusheniya, M. Nauka, 1989. w8x I.Sh. Trakhtenberg, A.B. Vladimirov, O.M. Bakunin, S.A. Plotnikov, I.N. Korneev, L.G. Korshunov, Proceedings of the ADCrFCT 99 conference, Tsukuba, Japan, 31 August᎐3 September 1999, Ž1999. 633᎐636.