Influence of chromium buffer layer on Cr/ta-C composite films

Transcription

1 Influence of chromium buffer layer on Cr/ta-C composite films Y. Liu 1,X.Yu* 1,L.Ma 1, C. B. Wang 1 and M. Hua 2 Cr buffer layers of six different thicknesses (100, 200, 300, 400, 500 and 600 nm) were respectively deposited on silicon substrate so as to constitute a series of Cr implantation layer/cr buffer layer/tetrahedral amorphous carbon (ta-c) films. Analysis by SEM suggests that the implantation of Cr layer favourably improves the interfacial transition between ta-c film and Si substrate. X-ray diffraction detection reveals that the difference in thickness of Cr buffer layer does not significantly influence the crystalline structure. Raman result shows that the ductility of Cr buffer layer favours to remit stress of the ta-c film. Increase in Cr buffer layer thickness results in the reduction of nanohardness value. However, its elastic modulus and adhesion exhibit an initial increase followed by a decreasing fluctuation. Result of the study shows the accomplishment of superior mechanical properties for the ta-c film with 200 nm thick Cr buffer layer. Keywords: Carbon film, Chromium layer, Mechanical properties, Structure Introduction Tetrahedral amorphous carbon film (ta-c film) is a special kind of hydrogen free diamond-like carbon film. It contains over 60% of sp 3 -C bonding and subsequently endues its remarkable property with super hardness almost similar to diamond. 1,2 The sp 2 -C bonds improve its tribological behaviour and bring wider applicable range 3 5 as compared with diamond film. Among various film deposition techniques, filtered cathodic vacuum arc (FCVA) system with metal vapour vacuum arc (MEVVA) source is an approach particularly favourable for synthesising ta-c film on sliding mechanical components and yet sustains a green manufacturing environment. 5,6 Unfortunately, the sp 3 -C bonds formed in the process tends to induce compressive stress as high as dozens of GPa. This subsequently leads to poor adhesion onto the substrate and hence limits its fields of application. 6,7 Researchers have thus been working to alleviate such drawback by either sputter cleaning the substrate surface or suitably synthesising transitional interlayer(s). 2 6 Similarly, we deposited a series of ta-c films on silicon wafer, 9Cr18 and high strength substrate respectively, and compared their microstructures and tribological properties Our relevant studies included implanting carbon ions before the deposition 9 and employing a single chromium 10 or titanium interlayer 7 to improve film to substrate adhesion. The introduction of the implantation layer or interlayer is expected to enhance the interface compatibility and to improve the film to 1 School of Engineering and Technology, China University of Geosciences, Beijing , China 2 MBE Department, City University of Hong Kong, Hong Kong *Corresponding author, yuxiang690625@yahoo.com.cn. substrate adhesion to a certain extent. Typically, ta-c films with carbon implanted layer give nanohardness and critical load L c ranging to 40 GPa and 40 mn respectively, while its counterpart with chromium or titanium interlayer give nanohardness and critical load correspondingly ranging to 50 GPa and 50 mn. This obviously shows the advantage of Cr implantation/ interlayers in improving the mechanical performance of the films. Generally, a single implanted and/or interlayer between ta-c and its substrate may be difficult to meet the requirements of industrial applications in a severely harsh servicing environment. Although some available literatures have reported the influence of interfacial layer and buffer layer on the adhesion, report on the influence of Cr interlayer and buffer layer together on the microstructural and mechanical properties of ta-c films is still rather lacking. In this article, we have prepared different sets of Cr implantation/cr buffer/ta-c layer to improve the ta-c adherent property by changing the thickness of Cr buffer layer. The study allows identifying an effective approach for improving the adhesion and, at the same time, maintaining the superhard character of the films. Experimental The magnetic FCVA system, which was integrating with MEVVA source and S bend filter, was connected with two 90u bends 7,10 and used for producing plasma from graphite and chromium cathode targets. The targets were 99?999% pure and had a uniform diameter of 10 cm. An S type magnetic filter was used to filter both neutral particles and large particles, which were yielded as a result of arc ion plating. A chromium layer was first implanted on to the substrate of silicon wafers with the MEVVA source set at 10 kv. Cr buffer layers with thickness of 100, 200, 300, 400, 500 and 600 nm were ß 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 3 November 2012; accepted 17 December Surface Engineering 2013 VOL 29 NO 4 DOI / Y

2 1 Cross-sectional morphology of ta-c film with structure of substrate/cr implantation/cr buffer/ta-c layer then respectively deposited by activating the Cr target under different periods. Finally, a ta-c layer was deposited with an activated C target under base pressure at Pa, MEVVA source pulse current at 100 A, pulse amplitude at 2?5 ms, pulse frequency at 25 pulses/s and substrate bias voltage at 280 V. A nanoindenter was used to measure the nanohardness and elastic modulus of the film. The value of nanohardness was obtained as the average of four indentations: the depth of each indentation was limited to 50 nm so as to alleviate the influence of substrate on the measured hardness value of the Cr/ta-C film. The adhesion between the composite film and Si substrate was analysed by a nanoscratch tester. A scanning electron microscope (SEM) equipped with an energy dispersive spectrometer analyser was used to analyse the cross-section morphology of the ta-c film. The crystalline structure of the differently fabricated ta-c films was studied and compared by an X-ray diffractometer (XRD). The bonding structure of the films was investigated by a laser Raman spectroscope. Impact test was performed to evaluate the impact property of the films. The test involved impressing a 6 mm diameter Si 3 N 4 ceramic ball on the specimen under a normal load of 2?5 kgf for 1? times, and their corresponding impacted area was analysed by SEM. Results and discussion Fracture morphology Figure 1 shows a typical cross-sectional SEM image of ta-c film. It exhibits that the adhesion strength and hardness of the composite film are favourably improved and hardened by its compact structure. 12 Analysis of C, Cr and Si distributions across the film thickness, from the film surface down to the datum surface of the substrate, shows that the elements of C and Cr vary remarkably from layer to layer. Typically, the abundant level of C element in the ta-c layer first increases and then decreases. Although the Cr element in the Cr buffer layer also increases first and then decreases, the rise in its Cr implantation layer is very small. A sudden rise in Si abundance is also observed at the vicinity of the substrate. Furthermore, sign of mutual diffusion of C and Cr elements at the layer interfacial boundary of ta-c layer and the most top Cr buffer layer, and Cr and Si elements at the layer interfacial boundary of the most bottom Cr buffer layer and the top surface of Si wafer is 2 X-ray diffraction patterns for ta-c films with structure of substrate/cr implantation layer/cr buffer layer (x nm)/ta-c clearly identified (Fig. 1). Such diffusion results in distribution gradient effect at the vicinities between layer interfaces and serves to enhance the interfacial adhesion and strength of the films. Generally, the existence of Cr implants in the substrate improves the interface compatibility between the layer and the substrate as a result of the effect of hot spike pin at high voltage. X-ray diffractometer spectra The variation of XRD patterns of the films and different Cr buffer layer thicknesses x is shown in Fig. 2. The XRD spectrum of ta-c film with x50 shows the Cr peaks appearing at (110) plane. The introduction of Cr buffer layer shifts the chromium peaks of the Cr/ta-C composite films to appear to the diffraction angle 2h at 44?4 and 64?6u, which is corresponding to (110) and (200) planes respectively. Simultaneously, Cr 3 C 2 peak with a preferred orientation (140) is also identified to occur at 2h<35u, implying that there may be some reaction of C ions with Cr ions at the boundary layer between the ta-c and Cr buffer layer. 11 With increasing Cr buffer layer thickness x, stronger intensity of Cr peaks is observed, typically at Cr peak (110) when thicker Cr buffer layer is accomplished. However, there is not any evidence that the variation of Cr buffer layer thickness would affect the crystalline structure of ta-c films. Raman result versus intrinsic stress Detection of the distribution of amorphous carbon compounds in relation to different x values in ta-c films was conducted using a laser Raman spectrometer. Result of Gaussian fitting 8 allows a classic bonding structure of the ta-c film to be identified from the typical Raman spectrum with x5200 nm. A sp 2 bonding stretching vibration peak (G peak) is observed,1560 cm 21 and its counterpart of sp 2 symmetric vibration (D peak) is locating at 1390 cm 21. The corresponding G peak position and intrinsic stress of the prepared films as a function of Cr buffer layer thickness is illustrated in Fig. 3. Augment of Cr buffer layer thickness up-shifts the G peak position of the ta-c layer from 1558?3 to 1567?8 cm 21 and downshifts its counterpart intrinsic Surface Engineering 2013 VOL 29 NO 4 277

3 3 G peak shift and intrinsic stress of ta-c film as function of Cr buffer layer thickness stresses from 2?78 to 1?89 GPa. This phenomenon serves as evidence indicating that there is link between G peak shift and stress and verifying that compressive stresses cause peak shifting upward to higher wave number. 12 The Raman result suggests that suitably varying the thickness of the film can effectively adjust its corresponding internal stress. Such phenomenon can be suitably applied for modifying interfacial stress patterns of the layers. Influence of Cr buffer layer thickness on nanohardness and elastic modulus The two curves in Fig. 4 exhibit the effect of Cr buffer layer thickness on the nanohardness and elastic modulus. Generally, the measured values of nanohardness and elastic modulus of ta-c films illustrate a decreasing tendency with increase in x value. The nanohardness of ta-c films initially fluctuates between 58 and 68 GPa and then decreases from 68 GPa for the ta-c film with x5100 nm to 58 GPa for the ta-c film with x5600 nm. The elastic modulus value increases initially from 550 GPa for the ta-c film with x5100 nm to a maximum value 570 GPa for the ta-c film with x5200 nm, and then gradually decreases to a minimum value of,410 GPa for the ta-c film with x5600 nm. The hardness and modulus of the ta-c layer is more dependent on the magnitude of bias voltage and thickness of ta-c layer itself (which 5 Critical load as function of Cr buffer layer thickness usually causes obvious change in the residual stresses) than on the thickness of Cr buffer layer through the stress change due to the absorbing capability 13 of carbon ions as observed from the Raman result. Obviously, this study does not belong to the first case. As the study 9 indicates that the nanohardness (75 GPa) and elastic modulus (740 GPa) of the ta-c layer is much higher than the nanohardness (26 GPa) and elastic modulus (158 GPa) of the Cr buffer layer, it may thus suggest that the formation of compound layers (Fig. 1) may play a leading role for such dependency. Influence of Cr buffer layer thickness on adhesion strength Adhesion of ta-c composite film on substrate is both an important mechanical property and an essential parameter determining its service lifetime when the film is working under severe conditions. The adhesion of the films is evaluated by a nanoscratcher with critical load L c that causes abrupt change in a frictional detection process of nanoindenter scratching. The comparison of the variation of adhesion force between film and substrate with different Cr buffer layer thicknesses is shown in Fig. 5. Its trend suggests that the L c value rises initially with the x value increasing from 300 mn at x5100 nm to a maximum 320 mn at x5200 nm, and then decreases gradually to a minimum 250 mn at x5600 nm. The use of a relatively softer Cr buffer layer facilitates the absorption of striking energy during depositing ta-c layer 14 and subsequently allows rectifying the high stress in ta-c layer and minimising the poor interfacial compatibility among the layers. Consequently, an optimal thickness, typically x5200 nm, of Cr buffer layer is surely favourable for acquiring a good film to substrate adhesion. Figure 5 also illustrates that further increase in the thickness from x5200 nm onward usually accompanies with lower adhesion of the film simply because of its diminishing hardness and scratch resistance. 4 Influence of Cr buffer layer thickness on measurement values of nanohardness and elastic modulus of ta-c film Influence of Cr buffer layer thickness on impact resistance Figure 6 compares the SEM images of impact induced morphologies for the ta-c films with the four different Cr buffer layer thicknesses and after 1? times of 278 Surface Engineering 2013 VOL 29 NO 4

4 6 Morphologies of impact after 1? impacts for ta-c films with structure of substrate/cr implantation layer/cr buffer layer(x nm)/ta-c: a x50; b x5200; c x5300; d x5500 impact. A distinct white area is clearly observed around its indentation cave for the ta-c film without any Cr buffer layer (Fig. 6a). The white area is usually circular in most cases, and it is representing the delaminated region. Owing to the high tensile stress arisen during impact testing, the impact resistance of ta-c film with x5100 nm thick Cr buffer layer is normally poor. 15 Although sign of microcracks is observed at the central zone and there are two delaminated zones to be seen in the peripheral zone for the ta-c film with x5200 nm thick Cr buffer layer (Fig. 6b), it seems to give a much better strain reduced pattern when compared with its counterpart with x5100 nm. Although more microcracks are found at the central zone and also more delaminated zones are observed in the peripheral zone, ta-c film with x5300 nm Cr buffer layer (Fig. 6c) seemingly gives similar pattern to its counterpart as shown in Fig. 6b. The ta-c film with x5500 nm Cr buffer layer (Fig. 6d) shows a clear cave with no visible delaminated region around peripheral zone, but suffering from severe microcracks and with delaminated region at central zone. A comparison of the above results suggests that excessively thick Cr buffer layer tends to decrease film hardness and to diminish impact resistance although embedding of softer buffer layer in ta-c film can improve the film toughness and reduce the delamination to certain degree. 16 As the experimental results demonstrate that the ta-c film with x5200 nm thick Cr buffer layer tends to exhibit best impact resistance, the achievement of optimal mechanical properties and the better matching with its interbedded affinity (which is responsible for the delaminated discrepancy between Fig. 6b and c) require an adequately thick Cr buffer layer to be interlaid. Conclusion The ta-c films with Cr buffer layers of different thicknesses have been deposited using an FCVA system with MEVVA source. Observation using SEM has revealed that the Cr/ta-C composite film possesses a compact structure of Cr implantation/cr buffer/ta-c layer across the thickness of the film (i.e. from the substrate to the top of a film). The Cr buffer layers are favourable to improve the adhesion property between both the substrate to layer and layer to layer. Result of XRD has illustrated that the crystalline phase pattern seems independent of the variation of Cr buffer layer thickness. Raman analysis has reflected that the Cr buffer thickness is closely related to the stress induced in the ta-c film. The Cr buffer layer can effectively modulate the internal stress pattern and improve the impact resistance of the ta-c films. The composite film of 200 nm thick Cr buffer layer exhibits better mechanical properties when compared with its counterparts of other thicknesses of Cr buffer layers within the domain of our studies. Acknowledgements This work was supported by National Natural Science Foundation of China with grant nos and , the Fundamental Research Funds for the Central Universities (grant nos. 2011YXL019, 2010ZY40 and 2010ZY51), Program for New Century Excellent Talents in University (grant no. NCET ) and A Foundation for the Author of National Excellent Doctoral Dissertation of China (grant no. FANEDD ). Surface Engineering 2013 VOL 29 NO 4 279

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