Radial nanofretting behaviors of ultrathin carbon nitride film on silicon substrate

Tribology International 44 (2011) 1400–1406

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Radial nanofretting behaviors of ultrathin carbon nitride film on silicon substrate Jiaxin Yu a, Shuang Zhang a, Linmao Qian a,n, Jun Xu b, Wangyu Ding b,1, Zhongrong Zhou a a b

Tribology Research Institute, National Traction Power Laboratory, Southwest Jiaotong University, Chengdu 610031, PR China State Key Laboratory of Materials Modification by Laser, Ion, Electron Beams, Dalian University of Technology, Dalian 116024, PR China

a r t i c l e in f o

a b s t r a c t

Article history: Received 4 May 2010 Received in revised form 28 September 2010 Accepted 28 September 2010 Available online 7 October 2010

With a nanoindenter, the radial nanofretting behaviors of amorphous ultrathin carbon nitride (a-CNx) film on the silicon substrate were investigated by a spherical diamond indenter. The experimental results indicate that the radial nanofretting damage on a-CNx film usually successively experiences the buckling, cracking and detachment of film. These damages can be easily detected by the variation in the apparent contact stiffness. Generally, the initial increase in the contact stiffness indicates the buckling of film; the following sharp decrease in the contact stiffness reveals the initiation and propagation of circular cracks in film; the final stable contact stiffness implies the detachment of film. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Radial nanofretting Nanoindentation Carbon nitride film Si(1 0 0)

1. Introduction Nanofretting refers to a cyclic motion of contact interfaces, where the local relative displacement reveals a non-uniform distribution in the contact area and the displacement amplitude is in the nanometer scale [1–3]. According to the different motion directions, nanofretting can be assorted as tangential and radial nanofretting. The tangential nanofretting runs along the tangential direction and the radial nanofretting runs along the vertical direction of contact interfaces. During radial nanofretting, the surfaces usually stick together at the central contact region and the micro-slip appears on the edge of the contact area. As a result, the radial nanofretting damages are mainly generated on the edge of the contact area [2]. Due to the temperature variation and mechanical vibration, the radial nanofretting potentially exists at the contact interfaces of micro/nanoelectromechanical systems (MEMS/NEMS) sustained by alternating stress such as structural girder, coating, gemel, micro-bearing, microswitch and micromotor [4,5]. For example, in a radio frequency MEMS, the radial nanofretting may induce the oxidation and fatigue of contact interfaces, which may finally result in the failure of the contact switches [6]. Because of good wear resistance, low friction coefficient and chemical inertness, the ultrathin hard films, such as diamond-like carbon (DLC) and

n

Corresponding author. Tel.: + 86 28 87600687. E-mail address: [email protected] (L. Qian). 1 Present address: Institute of Optoelectronic Materials and Device, Dalian Jiaotong University, Dalian 116028, PR China 0301-679X/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2010.09.014

amorphous carbon nitride (a-CNx) films, have been considered as good candidates for the protective coatings in MEMS/NEMS [7–9]. However, their efficacy in resisting the nanofretting damage of the silicon substrate needs to be verified. After the concept of nanofretting was proposed by Zhou and Qian [10] in 2003, only a few papers were found to discuss the radial nanofretting behaviors of materials [2,11]. In 2008, Zhang et al. [11] reported that the hard CrNx film could effectively improve the anti-pressure ability of 40Cr substrate in radial nanofretting. Recently, Qian et al. [2] investigated the radial nanofretting behaviors of four typical structural materials in MEMS (polycrystalline copper, monocrystal silicon and nickel titanium shape memory alloys) under high load by a Berkovich diamond tip. The results indicated that both the contact stiffness and the projected area of the indents on four materials attained to constants after the initial increase with the increase in the number of cycles. The force vs. displacement curve exhibited a hysteresis loop due to the energy dissipation in nanofretting cycle. However, the radial nanofretting damage on four materials revealed various behaviors. The typical nanofretting damage in copper is identified as the pileup of the wrinkles on the edge of indents, while the damage in silicon is characterized as the initiation and propagation of the cracks on the edge of the plastic zone of indents. Despite all that, the radial nanofretting behaviors of ultrathin protective coating on silicon have not been well addressed. In this paper, the radial nanofretting tests were performed on the ultrathin a-CNx film and its silicon substrate. The apparent contact stiffness of two materials against spherical diamond indenter in radial nanofretting was measured. The deformation

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and damage of the materials were characterized by a scanning electron microscope. Based on the experimental results and analysis, the variation of the apparent contact stiffness was suggested to determine the nanofretting damage of ultrathin film.

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C1s 284.73

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Binding energy (eV) Fig. 2. XPS spectra of the a-CNx film: (a) C1s and (b) N1s.

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Friction force (mN)

With high purity graphite (99.9%) as the target, the a-CNx films were deposited onto the Si(1 0 0) substrate by a microwave electron cyclotron resonant plasma enhanced unbalance magnetron sputtering system. The Si(1 0 0) substrate is p-type and of 500 mm in thickness. Before loading into the vacuum chamber, the silicon surfaces were successively cleaned with acetone and anhydrous alcohol. Prior to deposition, the Si(1 0 0) substrate was cleaned using ion sputtering for 10 min at an argon flow rate of 20 sccm with a substrate bias of  400 V. During the sputtering process, the target power was set at 250 W, the flow rate of high purity nitrogen was kept at 25 sccm and the gas pressure was 0.2 Pa. The total depositing time was 120 min. With a surface profilometer (AMBIOS XP-2, Ambios Technology Inc, USA), the thickness of the deposited film was measured as 120 nm. Fig. 1 shows the typical AFM images of the Si(1 0 0) substrate and the a-CNx film. It was found that the a-CNx film was composed of fine grains with a diameter of about 30 nm. The root-mean-square roughness of the silicon substrate and film surface was 0.23 and 0.51 nm, respectively. Clearly, the deposition of the a-CNx film shows very limited effect on the variation of the surface quality of the silicon substrate. To characterize the chemical structure of the a-CNx film, the C1s and N1s spectra of the film were detected by an X-ray photoelectron spectroscope (XPS, XSAM 800, KRAGOS, UK). As shown in Fig. 2, the three peaks in the deconvoluted C1s spectrum can be attributed to sp2 C–N, sp3 C–N and C ¼C bonds, and the two peaks in the deconvoluted N1s spectrum can be assigned to N-sp3 C and N-sp2 C bonds [12]. With a nanoindenter (CSEM Instruments SA, Switzerland), the elastic modulus of the silicon substrate and the a-CNx film were measured as 130 and 104 GPa, respectively. The nanoindentation hardness of the a-CNx film was measured as 2.7 GPa. In order to determine the adhesive strength of the a-CNx film on silicon substrate, the nanoscratch tests were performed on the a-CNx film through linear loading from 0.3 to 100 mN. The scratch distance was 2 mm and the radius of diamond tip was 2 mm. As shown in Fig. 3, with the increase in the normal load from 0.3 to 70 mN, the friction force increases linearly. When the normal force attains a critical value of 70 mN, the friction force reveals

286.31

Intensity

2.1. Preparation and characterization of the a-CNx films

Intensity

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Detachment of film

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Fig. 1. AFM images of Si(1 0 0) and the a-CNx film; the scan area is 1 mm  1 mm.

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Fig. 3. Friction force vs. normal load curve during a nanoscratch on the a-CNx film; the inset optical image reveals three repeated nanoscratches on the a-CNx film obtained by linear loading from 0.3 to 100 mN.

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a sharp jump, indicating the detachment of the a-CNx film from the silicon substrate. After that, the friction force increases linearly again. Therefore, the critical normal load corresponding to the detachment of the film was 70 mN under the given conditions. 2.2. Experimental methods All the radial nanofretting experiments were carried out by a nanoindenter. The schematic illustration in Fig. 4(a) shows the cyclic radial movement in the a-CNx film by a spherical diamond indenter. It is known that a-CNx film is often highly stressed and easily delaminated in single indentation by a Berkovich indenter [13]. In order to avoid the delamination of the a-CNx film in the initial stage of radial nanofretting, a spherical diamond indenter with a curvature radius of 20 mm was used in the nanofretting test, as shown in Fig. 4(b). Three peak normal forces (or Fmax), 50, 100 and 200 mN, were used in the radial nanofretting tests. At each test, the normal force varied from 1 mN to Fmax for 1–1000 cycles. While the number of nanofretting cycle N was below 200, the loading rate was adjusted as 30 s loading period, 2 s delay at the peak load and 30 s unloading period in each nanofretting cycle. When the number of nanofretting cycle was 1000, both the loading and unloading time was adjusted to 2 s and the delay at

peak load was kept 1 s. Before and after the radial nanofretting tests, the thermal drift of the nanoindenter was calibrated according to the method in Ref. [14]. The nanofretting results were then revised following the average value of two calibrations. All the radial nanofretting experiments were performed under unlubricated condition and a relative humidity of 50–60%. At each condition, the radial nanofretting tests were repeated at least three times to ensure the repetition of data. After the tests, the radial nanofretting damage on silicon and the a-CNx film was characterized by a scanning electron microscope (SEM, Quanta 200, Philips-FEI, Holland).

3. Experimental results and discussion 3.1. Radial nanofretting force vs. displacement curves Fig. 5 shows the normal force F vs. displacement d (or F–d) curves in radial nanofretting of the diamond tip against Si(1 0 0) and a-CNx film corresponding to various numbers of nanofretting cycles N and peak normal forces Fmax, respectively. To plot the curves clearly, the start point of the F–d curves were shifted to different values of d. Since the displacement amplitudes of the F–d curves are in nanoscale (100–460 nm), the cyclic movements between diamond tip and samples are typical radial nanofretting. Their displacement amplitudes are much smaller than those obtained in the radial fretting of a 52100 steel ball against 1045 steel plate (4–10 mm) [15]. Different from the radial nanofretting test on silicon with Berkovich indenter [2], the deformation of Si(1 0 0) and the a-CNx film by the spherical indenter is almost recoverable and the shape of the F–d curves exhibits a small hysteresis loop, suggesting the possible energy dissipation in indentation. To show the variation in F–d curves with the increase in N, the F–d curves of two samples in the 1st and 200th cycles were plotted together as shown in Fig. 6. One can see that the F–d curve of the Si(1 0 0) in the 1st cycle is very similar to that in the 200th cycle. On the contrary, the F–d curves of the a-CNx film exhibit a large variation with the increase in N. Fig. 7 reveals the variation of the maximum radial displacement dt in a nanofretting cycle of the diamond tip against Si(1 0 0) and the a-CNx film with the increase in N under various Fmax. For Fmax below 100 mN, the dt on the a-CNx film only shows a little decrease with the increase in N. However, as Fmax increases to 200 mN, the dt on the a-CNx film first decreases from 470 nm in the first cycle to 409 nm in the 170th cycle, and then increases and reaches a constant value of 430 nm in the 190th cycle. As a comparison, the dt on Si(1 0 0) almost keeps stable with the increase in N at a Fmax of 200 mN. It was also noted that the maximum radial displacement dt on Si(1 0 0) is always lower than that on the a-CNx film under the same conditions. 3.2. Variation of the apparent contact stiffness in radial nanofretting

Fig. 4. (a) Schematic illustration showing the radial nanofretting test and (b) SEM image of the spherical diamond indenter with a curvature radius of 20 mm.

It is known that the initial slope of the unloading part of the F–d curve represents the contact stiffness S between the spherical diamond tip and specimens [16]. Fig. 8 shows the variation in the apparent contact stiffness of the diamond tip against Si(1 0 0) and the a-CNx film with the increase in N under various Fmax. As shown in Fig. 8(a), due to its relatively lower elastic modulus, the a-CNx film shows smaller contact stiffness than silicon substrate in the initial nanofretting cycles [16,17]. Moreover, compared to the constant S on silicon substrate, the S on the a-CNx film shows a large variation with the increase in N for Fmax ¼200 mN. Clearly, such variation in S should not be attributed to the silicon

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Fig. 5. F–d curves in radial nanofretting of the diamond tip against Si(1 0 0) and the a-CNx film corresponding to various cycling number and peak normal forces. (a) Si(1 0 0) and Fmax ¼ 200 mN; (b) a-CNx film and Fmax ¼ 50 mN; (c) a-CNx film and Fmax ¼ 100 mN and (d) a-CNx film and Fmax ¼200 mN.

substrate but to the a-CNx film on it. Fig. 8(b) shows the effect of Fmax on the S of the diamond tip against the a-CNx film. For Fmax below 100 mN, the S on a-CNx film reveals a little increase with the increase in N, which is just opposite to the variation in dt. However, for Fmax ¼200 mN, the S on the a-CNx film first increases slowly from 0.61  106 to 0.73  106 N/m, and then decreases sharply to a constant value of 0.64  106 N/m. Combining with the results in Fig. 7, it can be deduced that the variation of both dt and S on the a-CNx film might be related to the deformation and damage of the film.

3.3. Radial nanofretting damage of materials After the radial nanofretting tests, the damage on the surfaces was characterized by a SEM. No damage was found on the silicon surface even after 1000 indentation cycles at a Fmax of 200 mN. On the contrary, the radial nanofretting damage on the a-CNx film reveals a strong dependence on the N and Fmax, as shown in Fig. 9. For Fmax ¼50 mN, the damage on the a-CNx film is too weak to be observed. For Fmax ¼100 mN, the film around the contact area bulged upwards with the increase in N, indicating the buckling

deformation of film. With the increase in N for Fmax ¼200 mN, the film around the contact area firstly bulged upwards, then the ringlike crack initiated and propagated in the film, and finally the film detached from the substrate. This method may also be used to determine the radial nanofretting damage of film with different thicknesses. For those films with high residual stress, a lower applied load and blunter indenter may be used to avoid the delamination of film in single indentation. Based on the experimental results shown in Figs. 8 and 9, the radial nanofretting damage process of the a-CNx film was schematically shown in Fig. 10. Accompanying the variation of the apparent contact stiffness with the increase in N, the radial nanofretting damage on the a-CNx film may experience the following three stages in turn: (1) buckling of the film (process 1–3); (2) initiation and propagation of the ring-like crack at the edge of the buckled film (processes 3,4) and (3) detachment of the film from the substrate (process 5). Firstly, during the radial nanofretting, the mutual contact pressure produces normal compression as well as tangential displacements. Since the materials of the contact pairs here are dissimilar, the tangential displacements would be different so that slip will take place. We might expect therefore a central

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Fig. 8. Apparent contact stiffness of Si(1 0 0) and a-CNx film plotted as the function of the number of nanofretting cycles; (a) comparison of the apparent contact stiffness on Si(1 0 0) and a-CNx film under Fmax ¼200 mN and (b) comparison of the apparent contact stiffness on a-CNx film under various Fmax.

region where the surfaces stick together and regions of slip towards the edge of the contact. Such radial micro-slip in the nanofretting test will further induce the deformation of the a-CNx film and silicon substrate, exerting an outward acting pressure on the film around the contact area, as shown in the insert picture of Fig. 10 [18,19]. On the other hand, the difference between the elastic modulus of the a-CNx film and silicon substrate will result in their different elastic deformation under indentation [20]. Hence, with the increase in the indentation force, the compressive stress in the film will increase. When the sum of the compressive stress and the residual stress exceeds the critical buckling stress, an interfacial crack may nucleate between the film and the substrate and propagate with the increase in the cycling number N. Since the radial micro-slip occurs on the edge of the contact area, the interfacial crack is therefore usually nucleated outside the central stick region, as shown in Fig. 9. Once the crack grows long enough, the buckling of the film appears. At this stage, the a-CNx film in the central area of indent still adheres to the silicon substrate, and the upwards annular buckled film may serve as a spring during indentation because of the bending stress. Compared to the original flat surface, the indenter needs higher force to indent the same depth into the buckled

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Fig. 9. SEM images of the radial nanofretting damage on the a-CNx film after various cycling number and peak normal forces.

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2 4 4 5 Vi σr 1

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Number of cycle N 5 Fig. 10. Schematic illustration showing the radial nanofretting damage process of a film/substrate system; variation in the apparent contact stiffness was plotted together for comparison.

sample, where the additional indentation force is used to balance the bending stress of the buckled film. Therefore, interfacial crack does not lead to an abrupt decrease in apparent contact stiffness, but induce a slight increase in apparent contact stiffness by means of the buckling of the film as shown in Fig. 8. With the increase in N, the film is more buckled, the bending stress is higher and the corresponding apparent contact stiffness is larger. In the second stage, the ring-like cracks initiate and propagate at the edge of the buckled film. During the radial nanofretting process, the bending stress of the buckled film will increase with the increase in N or Fmax. When the bending stress reaches the critical value, the cracks will initiate and propagate instantaneously at the edge of the buckled film and form a ring-like crack as shown in Fig. 10. This results in the separation of the film

around the contact area from the bulk film and the substrate via cracking through films. In this case, the bending stresses which partly support the indenter are suddenly removed, resulting in an abrupt decrease in the apparent contact stiffness S as shown in Fig. 8. In the third stage, the ring-like cracks continued to grow during the subsequent nanofretting cycles and the buckled film finally detached from the substrate. At this condition, the indenter indents the film/substrate system as the beginning, no additional stress occurs during indentation, resulting in the steadiness in the apparent contact stiffness of the film/substrate system with the increase in the cycling number. Obviously, the apparent contact stiffness between the spherical diamond indenter and the a-CNx film is very sensitive to the

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deformation and damage of the film. The increase in the contact stiffness usually indicates the buckling of the film; the following sharp decrease in the contact stiffness reveals the initiation and propagation of circular cracks in film; the final stable contact stiffness suggests the detachment of film. Therefore, from the variation in the apparent contact stiffness in radial nanofretting, the detailed damage process of ultrathin film can be easily determined, which obviates the need for imaging the residual indent.

4. Conclusions The radial nanofretting behaviors of the a-CNx film on Si(1 0 0) substrate were investigated by a nanoindenter. The main conclusions can be summarized as follows: (1) Under the given conditions, the a-CNx film shows worse wear resistance against radial nanofretting than compared with silicon substrate. (2) The radial nanofretting damage on the a-CNx film was found to start from the buckling of the film, followed by the initiation and propagation of the ring-like crack at the edge of the buckled film, and finish at the detachment of the film from the substrate. (3) The apparent contact stiffness is very sensitive to the deformation and damage of the film. The increase in the contact stiffness usually indicates the buckling of film; the following sharp decrease in the contact stiffness reveals the initiation and propagation of circular cracks in film; the final stable contact stiffness suggests the detachment of film.

Acknowledgement The authors are grateful for the financial support from the Natural Science Foundation of China (90923017, 50625515, 50821063, 60576022).

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