Evaluation of the nanomechanical and nanotribological properties of extremely thin diamond-like carbon films by atomic force microscopy

Transcription

1 Evaluation of the nanomechanical and nanotribological properties of extremely thin diamond-like carbon films by atomic force microscopy S. Miyake and M. Wang Department of Innovative System Engineering, Nippon Institute of Technology, Saitama 35-85, Japan Department of Research and Development, OSG Corporation, Aichi -85, Japan Extremely thin (approximately.8-, -, -, 5-, and -nm-thick) diamond-like carbon (DLC) films were deposited by filtered cathodic vacuum arc (FCVA) deposition and electron cyclotron resonance-chemical vapor deposition (ECR-CVD). The nanoindentation hardness and nanowear resistance of DLC films were evaluated by atomic force microscopy (AFM). The results of the nanoindentation and wear tests reveal differences in the wear resistance of the films and the superior mechanical properties of the FCVA-DLC films. The nanoscratch properties of DLC films were also studied. Upon scratching in a selected direction, the friction coefficient and wear depth of the FCVA-DLC films rapidly increased at a critical load, while these parameters gradually increased for the ECR-CVD-DLC films (even beyond the critical load). Keywords: AFM; extremely thin DLC films; nanomechanical property; nanotribological property; nanoscratch. Introduction Nanotribology is a key research area in the information industry. It is important in micromachines, magnetic recording head-media interfaces, large-scale integration (LSI) manufacturing systems, and mechanisms for use in space []. In these fields, atomic-scale wear and minimal fluctuations in friction can degrade equipment performance. Surface modification seems to be a useful way to reduce these problems []. Hence, methods for the modification of extremely thin surface layers play an important role in realizing highly reliable magnetic storage devices [, 3 5]. Diamond-like carbon (DLC) films are currently employed as magnetic disk protective layers [6 8]. nce DLC films have excellent mechanical properties, they have attracted great attention and practical application for these materials has been developed in various fields. DLC films are high-density amorphous carbon films showing metastable characteristics similar to those of diamond; they can be formed by various methods [9, ]. Furthermore, they have excellent tribological properties such as low friction coefficient and excellent wear resistance [, ]. A reduction in the effective magnetic spacing at the magnetic head-disk interface requires a reduction in the thickness of the protective film, which otherwise will lead to magnetic loss. Therefore, extremely thin protective layers have become important for ensuring the reliability of magnetic disk equipment [, ]. Although DLC films are currently used as magnetic disk protective layers [, 5, 3, ], it is difficult to maintain the mechanical endurance of a protective film if its thickness is reduced to approximately. nm, which equals approximately several atomic layers [3, ]. When considering such thin films, the failure characteristics of the atomic bonding during friction and wear should be taken into account [5, 6]. The maintenance of tribological endurance when only several atomic layers of the protective film are involved is difficult. To this end, the mechanical properties of a few atomic protective layers should be evaluated. Atomic force microscopy (AFM) is a useful technique for tribological evaluation at the atomic scale, enabling a new understanding of the friction and wear mechanisms; the method has been applied in nanoscale testing [7 ]. The socalled microwear or nanowear properties can be evaluated by AFM nanoscratch testing. Scratch parameters such as applied load, friction force, and scratched groove profile are experimentally evaluated. In general, in a scratch test, a diamond tip moves over a sample under a normal load, which is then increased until a failure is observed. The corresponding load, called the critical load, is obtained at this point. In scratch tests [], tip geometry effects on the deformation behavior of materials have been investigated using a diamond tip. Some of this study was done at the nanometer scale by means of molecular dynamics (MD) simulations [3 5], but the tips used were ideally sharp, three-sided pyramids. nce the hemisphere at the top of the pyramidal diamond tip affects the nanoscratch properties, the effects of tip geometry on the scratch properties vary at different scratch depths; these effects cannot be ignored in atomic-scale evaluations. An ultrathin layer of protective film is applied to the medium surface and is then typically overcoated with a protective film (thinner than 5 nm) to realize a high magnetic storage. Nowadays, electron cyclotron resonance chemical vapor deposition (ECR-CVD) [6] is commonly applied to deposit thin films, and filtered cathodic vacuum arc (FCVA) ta-c [6 8] thin films are expected to be applied to magnetic disks. This chapter focuses on the nanoscratch properties and nanometer-scale mechanical properties of extremely thin DLC films. In section, nanoindentation hardness and nanometer-scale wear tests were successfully performed to evaluate the surface of the films after wear and clarify the mechanical properties of the extremely thin DLC films. The section focuses on the evaluation of the nanoscratch properties of extremely thin (ca nm-thick) DLC films deposited by FCVA and ECR-CVD. 97 FORMATEX

2 . Specimens and Experimental Methods. Specimens and surface analysis Extremely thin protective DLC films were deposited on () wafers by FCVA and ECR-CVD [6 8]. The target thicknesses of the deposits were.8,.,., 5., and nm. The thickness was varied by changing the deposition time. The thickness of the DLC deposits was confirmed by transmission electron microscopy (TEM), and the deposition rate was calculated, showing that DLC films with thicknesses of.8,,, and 5 nm were successfully deposited [6 8]. The surface roughness (Ra) of the thin films was evaluated by AFM using a carbon nanotube tip. The roughness was as low as.9. nm similar to that of the substrate. The DLC films properties, including structure, composition and actual thickness were evaluated by Raman spectroscopy, TEM, and Auger electron spectroscopy (AES). To estimate the actual thickness of the deposits, depth profiles of the film composition were evaluated by AES for samples with thicknesses of,, and nm. Etching using an acceleration voltage of kv and argon ions with a current density of about.3 A/m was performed for s. The interface was qualitatively analyzed using a 5 6 Pa and kev electron beam. Depth analysis was performed by repeated etching under the same conditions.. Nanoindentation evaluation method The nanoindentation properties of the films were evaluated by AFM to understand their physical characteristics at a near-atomic scale. To determine the hardness of the extremely thin films obtained in this study, nanoindentation tests were performed by indenting a diamond-type probe (r = 5 6 nm, cube corner: 9 ) [9 3] into the specimen surface. The load applied during the experiment varied from to μn, and both the loading and unloading times were 5 s [3]. The resulting load penetration depth gives insight into the response of the material to mechanical stress from which parameters such as the hardness can be determined. Fig. shows the evaluation method. The hardness was evaluated from the plastic deformation depth, which was determined from the point of intersection of the straight line fitted to the appropriate unloading curve and the x-axis and is assumed to be equal to the contact depth [3]..3 Methods to evaluate the mechanical properties The nanometer-scale wear-resistance properties of DLC films were investigated by AFM using a diamond tip [], and the dependence of the nanowear properties of the deposits on the film thickness was evaluated. A Berkovich diamond indenter (i.e., a triangular pyramid) with μn load was used in the test. The radius of the indenter tip was approximately nm. During the experiment, the wear test area was 5 5 nm, and the observed area was nm, which is larger than the sliding area [8, 3].. Nanoscratch evaluation method To evaluate the nanometer-scale mechanical properties of DLC films, a nanoscratch test was performed using an atomic force microscope (Digital Instruments Nanoscope III with Hysitron indentation system). In the scratch tests, a diamond indenter slides on a specimen, generating a groove. Three factors contribute to the generated friction force: ) the adhesion force occurring at the elastic contact areas, ) the plowing force, and 3) the removal of a wedge or chip. In the nanoscratch process, the size of the wedge in front of the diamond tip is comparable to the scratch depth, so the effect of the wedge on friction cannot be neglected [33]. A diagram of the nanoscratch test and the measurement model are shown in Fig. [33], respectively. Scratch tests at μm length were captured using an atomic force microscope. The scratched surface profiles were then observed in the 5 5 μm test area. Scratch tests were performed to investigate the effect of a change in friction force on the load [33]. First, the maximum test load was set to, 5, and 3 μn by changing the scratch direction. The directions of a scratch can be described as the corn and edge of a Berkovich-type diamond indenter tip with nm radius, as shown in Fig. a. The examination was performed more than three times, and the mean and typical results were discussed. 3. Characterization of Extremely Thin Diamond-Like Carbon Films 3. Surface analysis of extremely thin protective DLC films Figure 3 shows the Raman spectra of DLC films prepared by FCVA and ECR-CVD. The thickness of the samples is the same. The typical spectrum of an amorphous hydrogenated (a-c:h) film has a G-band peak at approximately 55 cm and a D-band peak at approximately 35 cm, as shown in Fig. 3a [3]. The G-band peak is due to the motion of the sp carbon in the graphite plane, and the D-band peak is due to the disorder of the structure. In the FCVA film, the G-band peak intensity is low, and no significant D-band peak is observed, as shown in Fig. 3b. The background noise, caused by the polymeric structure in the film, is also low because the FCVA film was deposited by a hydrogen-free FORMATEX 975

3 process. These results show that the FCVA-DLC (ta-c) film contains a larger amount of sp3 bonds than the ECR-CVD- DLC (a-ch) film [3, 35]. Fig. Method for evaluating the nanoindentation. Fig. Nanoscratch test. Intensity FCVA-DLC -nm G-peak D-peak Raman shift, cm- Intensity ECR-CVD-DLC -nm G-peak D-peak Raman shift, cm- (a) FCVA-DLC nm (b) ECR-CVD-DLC nm Fig. 3 Raman spectra of -nm-thick DLC films. From the cross-sectional TEM images and AES depth profiles of FCVA and ECR-CVD-DLC films of various thicknesses, the actual thicknesses of the layers were evaluated. According to the AES spectra of the top surfaces of -nm-thick ECR-CVD-DLC and FCVA-DLC films, both materials mainly contained carbon and a trace amount of oxygen. Otherwise, no significant differences were observed [3]. The AES spectra of the top surfaces of -nm-thick DLC films are shown in Fig.. In both cases, C is the main component, with a small amount of O and. The actual thicknesses of the -nm-thick DLC films which, as for the standard thin films was confirmed by TEM images and the depth distributions obtained by AES as shown in Fig. 5 were measured. The results show that the FCVA-DLC and ECR-CVD-DLC films have actual thicknesses of 7 and 97 nm, respectively. The number of argon cycles required to etch the -nm-thick films was 333 for the FCVA-DLC and 85 for the ECR-CVD-DLC films. Intensity C O ev (a) -nm-thick FCVA-DLC Intensity ev (b) -nm-thick ECR-CVD-DLC Fig. AES spectra of the top surfaces of (a) -nm-thick FCVA-DLC and (b) -nm-thick ECR-CVD-DLC films. 976 FORMATEX

4 Then, the average argon etching times were calculated from these results [36]. The densities of the FCVA-DLC and ECR-CVD-DLC films were 3.3 and.9 g/cm 3, respectively (evaluated by Rutherford backscattering, RBS). The FCVA- DLC (ta-c) film contains a larger amount of sp 3 bonds compared with the ECR-CVD-DLC film [35, 37 39]. AES depth profiles of -nm-thick DLC films are shown in Fig. 6. The covalent radius of carbon is approximately.7 nm. Thus, a -nm-thick film contains approximately seven atoms. nce these films are very thin, the evaluation of their actual thickness is very difficult. In DLC films, the carbon concentration decreases near the target thickness of the layer. The real thickness of the -nm-thick FCVA-DLC film was estimated to be.78 nm, while that of the ECR-CVD- DLC film was determined to be.68 nm based on the depth at which the amounts of C and are nearly equal [3, 36]. Intensity C O.3 nm/cycle 333 cycle 6 8 Cycle (a) FCVA-DLC -nm Intensity C O.5 nm/cycle 85 cycle 6 8 Cycle (b) ECR-CVD-DLC -nm Fig. 5 AES depth profiles of -nm-thick DLC films. Relat. con., % (a) -nm-thick FCVA-DLC C O 3 5 Etching depth, nm Relat. con., % Etching depth, nm (b) -nm-thick ECR-CVD-DLC C O Fig. 6 AES depth profiles of -nm-thick DLC films: (a) FCVA-DLC and (b) ECR-CVD-DLC. The relation between actual thickness and target deposition thickness was evaluated by TEM and AES. After confirming the thickness of each DLC thin by TEM, the deposition rate was calculated, as shown in Fig. 7. The deposition rates showed that both deposition methods produced extremely thin DLC films with similar target thicknesses. The obtained DLC material thickness was several times the atomic diameter of carbon. In depth profiles of.-nm-thick DLC films, in which carbon reaches the same value as, the carbon depth is.9 and.6 nm for the FCVA-DLC and ECR-CVD-DLC films, respectively. These values are quite close to the. nm target thickness of the film. The Ra of the films was evaluated by AFM in the noncontact mode (SII SPA3HV) using a carbon nanotube tip. The FCVA-DLC films appeared to be free of microparticles and had a low surface roughness, as did the ECR-CVD- DLC films. The roughness of DLC films was as low as.9. nm and similar to that of the substrate. 3. Nanoindentation properties The nanoindentation curves of DLC films are shown in Fig. 8, where the -, 5-, and -nm-thick FCVA-DLC and ECR-CVD-DLC films are compared. As shown in Fig. 8a, the indentation depth of the ECR-CVD-DLC film is greater than that of the FCVA-DLC film. The FCVA-DLC film mainly shows elastic deformation, while the ECR-CVD-DLC film exhibits greater energy dissipation at the same indentation load [9, 3]. The nanoindentation hardness values determined at -μn load for DLC films deposited by FCVA and ECR-CVD are 57 and 5 GPa, respectively. The high value determined for the FCVA-DLC film corresponds to the reported hardness for a -nm-thick FCVA-DLC film deposited under similar conditions (59 GPa) [35]. The density of our FCVA-DLC film is 3.3 g/cm 3, which is close to the expected value of 3.37 g/cm 3. FORMATEX 977

5 9 8 7 Target thickness. nm. nm. nm. nm Etching speed Target thickness. nm. nm. nm. nm Etching speed 8 6 Relat. Con., % Etching cycle Relat. Con., % Etching cycle Etching depth, nm (a) FCVA-DLC 3 Etching depth, nm (b) ECR-CVD-DLC Fig. 7 AES depth profiles of DLC films with various target thicknesses. Under these conditions, the actual hardness of the films can be obtained, regardless of substrate effects. The FCVA- DLC film exhibits excellent resistance properties against nanometer-scale plastic deformation. The nanoindentation curves of films with a target thickness of 5 and nm are shown in Fig. 8b and 8c, respectively. In the nanoindentation loading curves of both DLC films, inflection points (arrows in Fig. 8b) are observed when the indentation depth exceeds the film thickness. As shown in Fig. 8b, the slope of the curve changes at indentation depths of about. and.8 nm. This indentation depth is shallower than the target thickness of 5 nm, which could be because of the influence of the substrate. The indentation curves of -nm-thick DLC films show the same trend. Therefore, the points of inflection correspond to the onset of nanometer-scale plastic deformations of the thin films due to damages at the surface [39]. Figure 9 shows the nanoindentation loading curves obtained at loads of,, 5 μn for -, 5-, and -nm-thick DLC films, respectively. As shown in Figs. 9a and 9b, for the FCVA-DLC and ECR-CVD-DLC films, even upon changing the film thickness, the relation between nanoindentation depth and load is almost consistent for indentation depths up to nm. In addition, within this range of indentation depths, the slopes of the ECR-CVD-DLC films are smaller than those of the FCVA-DLC films. It is possible to correctly approximate the relation between indentation depth (δ) and load (P) using the Hertz Elastic Contact formula in the nanoindentation curve. By performing a zero-point correction to the nanoindentation curve and substituting the radius of the diamond tip (R) into equation (), it is possible to calculate Young s modulus E for the indentation depth at low load: 3 P E = Rδ 3 E is obtained from the unloading curves of -nm-thick DLC films and has a value of 96 ± 3 GPa for the FCVA- DLC film and 39 ± 3 GPa for the ECR-CVD-DLC film. Fig. shows that E was estimated by the nanoindentation test at loads of,, 5 μn for -, 5-, and -nm-thick DLC films, respectively [36]. () Fig. 8 Nanoindentation curves of DLC films with different thicknesses: (a) -nm-thick (max load: μn), (b) 5-nm-thick (max load: μn), and (c) -nm-thick (max load: μn). 978 FORMATEX

6 5 -nm 5-nm -nm 5 -nm 5-nm -nm Load, μn 5 Load, μn Nanoindentation depth, nm (a) FCVA-DLC Nanoindentation depth, nm (b) ECR-CVD-DLC Fig. 9 Nanoindentation curves obtained at maximum loads of (a) μn for a -nm-thick film, (b) μn for a 5-nm-thick film, and (c) μn for a -nm-thick film. E, GPa ±3 -nm 5-nm -nm E, GPa ±3 -nm 5-nm -nm Nanoindentation depth, nm (a) FCVA-DLC Nanoindentation depth, nm (b) ECR-CVD-DLC Fig. Young s moduli estimated by unloading indentation curves of DLC films with different thicknesses: (a) -nm-thick (max load: μn), (b) 5-nm-thick (max load: μn), and (c) -nm-thick (max load: 5 μn).. Mechanical Properties of Extremely Thin Diamond-like Carbon Films. Dependence of the nanowear properties of extremely thin DLC films on load Figure shows the method applied to evaluate the nanowear as well as the nanowear and cross-sectional profiles of the () substrate. Fig. shows the nanowear profiles of 5-nm-thick FCVA-DLC and ECR-CVD-DLC films obtained at loads of and 5 μn. As shown in Fig. a, the wear depth is remarkably small in the FCVA-DLC film, and there is almost no wear at the surface of the 5-nm-thick FCVA-DLC film. On the other hand, in the case of the 5- nm-thick ECR-CVD-DLC film, a.7-nm-deep wear trace was observed at a load of 5 μn [36]. In this way, it is possible to investigate the differences in the wear-resistance properties of the FCVA-DLC and ECR-CVD-DLC films, even at film thicknesses of 5 nm as shown in Fig. b. Figure 3 shows the wear depths of extremely thin (ca. -, 5-, and -nm-thick) DLC films. A reference length for calculating Ry and Ra is.5 μm, and it becomes about 5 times smaller when the distance for calculating the wear depth is 5 nm [36]. Therefore, the rate of change of the wear shape is ten times larger. As shown in Fig., the average wear depth measured along the surface of a -nm-thick FCVA-DLC film at a load of 5 μn is. nm, demonstrating that the wear can be evaluated at the atomic level. The wear depth is remarkably small for the -nmthick FCVA-DLC film (ca.. nm at a load of 3 μn), but rather pronounced for the -nm-thick layer, even at low loads. In addition, the wear depth generated at a load of 3 μn was.7 nm, that is, the same as that observed for the 5- nm-thick film, and it did not increase further although the load increased. This value is one-third of the nm target thickness [36]. Nanoindentation tests show that the FCVA-DLC films exhibit high elastic modulus properties. As the thickness of the films decreases, the degree of deformation due to compression increases, which leads to an increment of the wear as a result of defects occurring on the outermost surface of the film. For the ECR-CVD-DLC films, the regular trend of the wear depth does not depend on the film thickness. nce the elastic moduli of the ECR-CVD-DLC films are low, the wear depth remarkably increases, even at low loads. The wear depth of the -nm-thick DLC layer is the shallowest, reaching a saturation value of about.8 nm at loads of μn and more. For the -nm-thick ECR-CVD-DLC and FCVA-DLC films, the wear depth saturates at a value of about.7 nm. This result is similar to that obtained from wear tests on nanometer multilayer films. The effect of preventing defect evolution during the nanometer-scale wear of extremely thin DLC films is thought to result from the boundary between the substrate and the DLC film [3, 9, 36]. FORMATEX 979

7 Fig. Nanowear evaluation method. Fig. Nanowear profiles of 5-nm-thick DLC films.. Dependence of the nanowear properties of extremely thin DLC films on the sliding cycle Wear depth of approximately 3 nm and protuberances with a height of nm were observed. The nanowear and crosssectional profiles of - and -nm-thick DLC films are shown in Figs. and 5, respectively. A wear of approximately 3 5 nm was observed in the ECR-CVD-DLC film, whereas no wear was seen for the FCVA-DLC film (Figs. a and 5a). The DLC films are extremely thin, and therefore, atomic-scale defects may affect their nanowear properties. The density of the FCVA-DLC films is higher than that of the ECR-CVD-DLC films, which contain hydrogen-terminated carbon networks. Thus, plastic deformation by friction stress is more easily caused in the latter case than in the former one. As shown in Fig. 6, the wear depth of a.8-nm-thick FCVA-DLC film rapidly increases with the number of sliding cycles, similar to what is observed on the bare surface. This suggests that the.8-nm-thick FCVA-DLC layer does not exhibit a superior nanowear resistance (see Fig. 6a). Nanowear depth dependence on the number of sliding cycles for various DLC films is shown in Fig. 7. The wear depths of - and -nm-thick FCVA-DLC films were extremely low (i.e., < nm), even after diamond-tip-scanning sliding cycles, as shown in Fig. 7a. In contrast, the wear depths of.8-, -, and -nm-thick ECR-CVD-DLC films nearly reached nm after one sliding cycle, and exceed their film thickness after a few cycles, see Fig. 7b. These results reveal the variations in wear resistance among the studied films and the excellent and superior wear resistance of FCVA-DLC films compared with ECR-CVD films. 5. Nanoscratch Properties of Extremely Thin Diamond-like Carbon Films 5. Dependence of the scratch properties on the scratch direction Figure 8a shows a cross-section AFM image of a scratch groove in a.-nm-thick ECR-CVD-DLC film [33]. The scratch depth in the corn direction is shallow at the beginning and then increases, while in the edge direction, it gradually increases. Figure 8b shows a cross-section AFM image of a scratch in a.-nm-thick FCVA-DLC film. The scratch depth in the corn direction rapidly increases at a certain inflection depth, so that a rough upheaval profile due to brittle fracture is observed in the vicinity of the scratch terminal. The wear debris formed as edges could be observed near the end of the scratched grooves in the corn direction. In the edge direction, the scratch depth gradually increases. 98 FORMATEX

8 .5.5 -nm 5-nm -nm.5.5 -nm 5-nm -nm 3 Load, μn (a) -, 5- and -nm-thick FCVA-DLC 3 Load, μn (b) -, 5- and -nm-thick ECR-CVD-DLC Fig. 3 Nanowear depth dependencies on load. Fig. Wear profiles of -nm-thick DLC films. Fig. 5 Wear profiles of -nm-thick DLC films. Fig. 6 Wear profiles of.8-nm-thick DLC films. The cross-section profiles and friction coefficients of.-nm-thick ECR-CVD-DLC and FCVA-DLC films as a function of the corn and edge directions are shown in Figs 9a and 9b [33]. The scratch profiles and friction coefficients showed good reproducibility during the three test runs. Scratching in the corn direction led to large differences in the properties of FCVA-DLC and ECR-CVD-DLC films. In addition, the wear depth and friction coefficient of the FCVA films rapidly increased from a certain depth. In the edge direction, the wear depths and friction coefficients of the ECR-CVD-DLC and FCVA-DLC films show a similar trend, that is, they both gradually increase [33]. 5. Differences between ECR-CVD-DLC and FCVA-DLC films To clarify the differences between the ECR-CVD-DLC and FCVA-DLC films, nanoscratch profiles were evaluated in the corn direction [33]. The scratch profiles and inflection depths of.8-nm-thick ECR-CVD-DLC and FCVA-DLC films are shown in Figs a and b, respectively. The scratch profiles of the ECR-CVD-DLC and FCVA-DLC films FORMATEX 98

9 were different. In the ECR-CVD-DLC films, the wear depth gradually increased, while in the FCVA-DLC films, a rapid increase in the wear depth was observed. Thus, the inflection points of the rapidly increasing wear depths were evaluated. The wear depth of the ECR-CVD-DLC films gradually increased, even beyond a critical load, while the wear depth of the FCVA-DLC films rapidly increased from this depth. Both inflection depths were approximately.9 nm, slightly deeper than the target film thickness of.8 nm nm -, -nm Sliding cycles (a) FCVA-DLC nm.8-nm -nm Sliding cycles (b) ECR-CVD-DLC Fig. 7 Nanowear depth dependence on the number of sliding cycles. Fig. 8 Scratch profiles dependence on the scratch direction. Figure shows inverted images that expand the initially scratched areas. At the beginning of the scratch test, no damage could be observed on the.6- and.-nm-thick FCVA-DLC films. The FCVA-DLC films exhibited a lower wear depth than the ECR-CVD-DLC films. A plastically deformed wear groove could be observed at an early stage in the ECR-CVD-DLC films, and an increase in depth (after reaching the desired film thickness) was also observed, as shown in Fig.. The grooves, that is, scratches, in the FCVA-DLC films were wider than those in the ECR-CVD-DLC films. The FCVA-DLC films do not readily plastically deform; instead, they exhibit large brittle fractures over the inflection depth. Figures and 3 show cross-section profiles and friction coefficients at various DLC film thicknesses. For.3-nmthick films (Fig. a), the properties of the ECR-CVD-DLC films were similar to those of the substrate. In contrast, the scratch depth and friction coefficient of the FCVA-DLC films were small at first, but increased faster than in the other sample after the first inflection point. Although.3 nm is smaller than the radius of a carbon atom, different increasing behaviors of the wear depth and friction coefficient could be observed in these DLC films. Figure b shows the relation between the scratch profiles and friction coefficients of.-nm-thick ECR-CVD-DLC and FCVA-DLC films. The effects on the DLC films are clearly distinct from those on the substrate. Once the wear depth reaches a critical value, a further rapid increase in wear depth is observed. This is particularly true for the FCVA-DLC films, while the wear depth of the ECR-CVD-DLC films gradually increases. As shown in Fig. 3a, the.-nm-thick DLC films show a behavior similar to that of the.-nm-thick layers regarding the scratch profiles and friction coefficients. The scratch depths and friction coefficients of these DLC films are clearly larger than those of the substrate. The scratch depth and friction coefficient of 5.-nm-thick FCVA-DLC films rapidly increase, as shown in Fig. 3b. From these results, it can be seen that the ECR-CVD-DLC films easily undergo plastic deformation, whereas the FCVA-DLC films do not. The latter materials rather develop brittle fractures, and the wear depth rapidly increases from a critical load. Figure shows the nanoscratch profiles and friction forces of.-nm-thick DLC films. In the case of the ECR- CVD-DLC layers, only adhesion forces act at the beginning because of the elastic contact. Plowing gradually occurs as 98 FORMATEX

10 Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.) the load increases. Once the friction force reaches the inflection point, sufficient film has been removed to reach the substrate. After passing the inflection point, the friction force increases rate decreases until a saturation point is reached. It is deduced that the plastic deformation resistance of is below that of DLC films. The inflection points were evaluated by the changes in scratch depth and friction coefficient. There are two inflection points, as shown in Fig. 5 [33]. At the first inflection point, labeled in Fig. 5b, the scratch depth and friction force increase rapidly. The wear depth nearly corresponds to the film thickness. Therefore, inflection is defined as the point at which the diamond tip penetrates the DLC film and arrives at the substrate. After inflection point, the main friction factor changes from the DLC film to the transition area between DLC film and substrate. The second inflection point, labeled in Fig. 5d, is defined as that the change from transition of DLC film and substrate transition, to stable scratching state in removal of the DLC coated substrate. Inflection point is characterized by saturation of the friction coefficient and a nearly constant increase in scratching depth with load. At the inflection point, the wear depths show similar dependences on the film thickness and correspond to the sum of the DLC film thickness and the depth of the inflection point of as shown in Fig. 6. The saturation of the friction coefficient on the DLCcoated substrate at inflection point seems to occur after the DLC film removal. Figure 6 shows the relation between scratch depth and film thickness at the inflection points on the nanoscratch profile. In the case of the ECR-CVD-DLC deposits, inflection points and are only observed at film thicknesses within. 5. and.6. nm, respectively. Thus, the deposition thickness has little effect on the mechanical properties of films thinner than.6 nm. In contrast, inflection points are clearly observed for the FCVA-DLC films, even at thicknesses below.6 nm, because these films have a greater resistance to plastic deformation than the ECR-CVD-DLC films. From these results, differences in the plastic deformation resistance of extremely thin (. -nm-thick) films can be evaluated. While it is difficult to evaluate the hardness of extremely thin DLC layers by the nanoindentation method, it is possible to do so by performing increasing-load nanoscratch tests, which also enables the evaluation of differences in the mechanical properties of extremely thin films deposited by different methods [6, 33, 36]. Fig. 9 Scratch profiles and friction coefficients Scratching profiles and inflection depth. Fig. dependence on the scratch direction (.-nm-thick) Fig Invert images of the scratch profiles ① 3 Position, μm ECR 6 8 FCVA Inflection point ② FCVA FCVA. nm 3. ECR..8 Start point ②...8 ECR-CVD. nm Inflection point ①.5 5 Friction coefficient, μ FCVA.6 nm Friction coefficient, μ ECR-CVD.6 nm Start point Position, μm Normal Force, μn (a).3 nm..8 Fig. FORMATEX 5 3 Normal Force, μn 5 (b). nm Scratch profiles and friction coefficients. 983

11 6 8 Friction coefficient, μ Start point Inflection point ECR 3 5 Position, μm FCVA 3 5 Normal Force, μn (a). nm 6 8 Friction coefficient, μ Start point Inflection point ECR 3 5 Position, μm FCVA 3 5 Normal Force, μn (b) 5. nm Normal force, μn Fig. 3 Scratch profiles and friction coefficients. Fig. Nanoscratch profiles and forces of -nm-thick DLC films ECR ECR ECR 3 3 Position, μm 5 3 Position, μm Friction force, μn Start point Inflection point (a) ECR-CVD-DLC 6 8 Friction force, μn Start point Inflection point FCVA FCVA FCVA Normal force, μn (b) FCVA-DLC Fig. 5 Model of the inflection points. Fig. 6 Inflection points of the nanoscratch profiles. 7. Conclusions In this chapter, we reported on the application of AFM to evaluate mechanical properties and nanoscale wear properties of extremely thin DLC films deposited by FCVA and ECR-CVD. According to the AFM results, no significant differences were observed between the nanoindentation curves of extremely thin DLC films deposited by FCVA-CVD and ECR-CVD, although it was difficult to evaluate the hardness of DLC films with a thickness of a few nm by this nanoindentation test. In contrast, nm wear tests were successfully carried out to evaluate the surface of DLC films after wear and clarify their mechanical properties. Hence, we have shown that AFM plays a vital role in the nanomechanical evaluation of extremely thin DLC films used for hard magnetic disks; it is possible to evaluate the nanomechanical and nanotribological properties of such films by applying this method. The procedure reported herein can also be applied in future studies. Acknowledgements This study was partly supported by the Storage Research Consortium (SRC). References [] Miyake S, Kaneko R. Microtribological properties and potential applications of hard, lubricating coatings. Thin Solid Films. 99; ( ): [] Miyake S, Kaneko R, Kikuya Y, Sugimoto I. Microtribological studies on fluorinated carbon film, Transactions of the American Society of Mechanical Engineers. Journal of Tribology. 99; 3 (): [3] Bhushan B. Tribology and Mechanics of Magnetic Storage Devices. Springer-Verlag, New York, 99. [] Miyake S, Wang M. Nanotribology of magnetic disks, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology. American Scientific Publishers. ; 9: [5] Miyake S, Wakatsuki Y, Wang M, Matsunuma S. Amplitude dependence of the lateral-vibration wear test for perpendicular recording magnetic disks treated by heat curing. Japanese Journal of Applied Physics. 5; (5A): FORMATEX

12 [6] Miyake S, Kaneko R, Miyamoto T. Micro and macrotribological improvement of CVD carbon film by the inclusion of silicon. Diamond Films Technol. 99; (): 5 7. [7] Miyake S. Microtribology of carbonaceous films, approach to atomic scale zero wear. Transactions of the Materials Research Society of Japan. B, 99; 5B: [8] Miyake S, Miyamoto T, Kaneko R. Microtribological improvement of carbon film by silicon inclusion and fluorination. Wear. 993; 68 ( ): [9] Enomoto Y, Miyake S. Tribology of Thin Films. University of Tokyo Press, Tokyo, 99, ISBN-3-65-X. [] Aisenberg S, Chabot R. Ion-beam deposition of thin films of diamondlike carbon. Journal of Applied Physics. 97; (7): [] Weiler M, Sattel S, Giessen T, Jung K, Ehrhardt H, Veerasamy VS, Robertson J. Preparation and properties of highly tetrahedral hydrogenated amorphous carbon. Physical Review B. 996; 53 (3): [] Miyake, S. Nanometer scale processing by tribological action and its potential applications. 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Three-dimensional molecular dynamics analysis of processing using a pin tool on the atomic scale. Nanotechnology. ; (3): [] Mulliah D, Kenny SD, Smith R, Sanz-Navarro CF. Molecular dynamic simulations of nanoscratching of silver (). Nanotechnology. ; 5 (3): 3 9. [5] Zhang JJ, Sun T, Yan YD, Liang YC, Dong S. Molecular dynamics study of groove fabrication process using AFM-based nanometric cutting technique. Applied Physics A: Materials Science & Processing. 9; 9: [6] Yamamoto T, Toyoguchi T, Honda F. Ultrathin amorphous C:H overcoats by pcvd on thin film media. IEEE Transactions on Magnetics. ; 36, : 5 9. [7] Robertson J. Ultrathin carbon coatings for magnetic storage technology, Thin Solid Films. ; 383, ( ): [8] Miyake S, Wang M. Mechanical properties of extremely thin B-C-N protective layer deposited with helium addition. Japanese Journal of Applied Physics. ; 3 (6A): [9] Miyake S. Improvement of mechanical properties of nanometer period multilayer films at interfaces of each layer. Journal of Vacuum Science & Technology. 3; B (): [3] Farhat, ZN, Ding, Y, Northwood DO, Alps AT. Nanoindentation and friction studies on Ti-based nanolaminated films. Surface and Coatings Technology. 997; 89: 3. [3] Miyake S, Saito T, Wang M, Watanabe S. Tribological properties of extremely thin protective carbon nitride films deposited on magnetic disk by complex treatment. Proceedings of the Institution of Mechanical Engineers: Journal of Engineering Tribology. Part J. 6;, : [3] Miyake S, Kim J. Nanoprocessing of carbon and boron nitride nanoperiod multilayer films. Japanese Journal of Applied Physics. 3; : L3 L5. [33] Miyake S, Yamazaki S. Nanoscratch properties of extremely thin diamond-like carbon films. Wear. 3; 35: [3] Ferrari AC. Determination of bonding in diamond-like carbon by raman spectroscopy. Diamond and Related Materials. ; : [35] Lemoinea P, Quinna JP, Maguirea PD, Zhaob JF, McLaughlina JA. Intrinsic mechanical properties of ultra-thin amorphous carbon layers. 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