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DLC coating on stainless steel by pulsed methane discharge in repetitive plasma focus
Applied Surface Science 303 (2014) 187–195
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DLC coating on stainless steel by pulsed methane discharge in repetitive plasma focus M. Hassan a,c,∗ , A. Qayyum b , S. Ahmad b , S. Mahmood c,d , M. Shafiq a , M. Zakaullah a , P. Lee c , R.S. Rawat c a
Department of Physics, Quaid-i-Azam University, 45320 Islamabad, Pakistan National Tokamak Fusion Program, 3329 Islamabad, Pakistan c Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, BLK7, 1 Nanyang Walk, Singapore 637616, Singapore d Department of Physics, University of Karachi, 75270 Karachi, Pakistan b
a r t i c l e
i n f o
Article history: Received 8 June 2013 Received in revised form 21 February 2014 Accepted 24 February 2014 Available online 12 March 2014 PACS: 52.59Hq 52.77Fv 61.05cp Keywords: DLC DPF XRD SEM Raman spectroscopy
a b s t r a c t Amorphous hydrogenated carbon (a-C:H)/diamond-like carbon (DLC) coatings have been achieved on AISI 304 stainless steel (SS) substrates by employing energetic ions emitted from a repetitive plasma focus operated in CH4 discharge. The Raman spectroscopy of the coatings exhibits the evolution of aC:H/DLC coatings with clearly observed D and G peaks centered about 1320–1360 and 1560–1620 cm−1 respectively. The diamond character of the coatings is influenced by the ion flux and repetition rate of the focus device. The repetitive discharge mode of plasma focus has led to the formation of a-C:H/DLC coatings in short duration of time. The coatings transform from a-C to a-C:H depending upon substrate angular position. X-ray diffraction (XRD) analysis confirms the formation of DLC coating owing to stress-induced restructuring in SS. The estimated crystallite size is found to be ∼40–50 nm. Field emission scanning electron micrographs exhibit a layered granular surface morphology of the coatings. The Vickers surface hardness of the DLC coated SS samples has been significantly improved. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Carbon can exist in three different chemical states, involving sp3 , sp2 and sp1 chemical co-ordination. Eventually, a variety of structures can be resulted, which may also contain other chemical elements. Mainly, there are two types of diamond-like structures; identified explicitly as hydrogenated amorphous carbon (a-C:H) or diamond-like hydrocarbons and non-hydrogenated amorphous carbon (a-C) or diamond-like carbon (DLC). Such a-C:H and aC coatings have received much scientific interest owing to their useful mechanical properties. Raman features of a-C:H coatings deposited at room temperature were discussed by Schwan et al. [1]. They showed that small clusters of condensed benzene rings in carbon films can be explained in the observed Raman scattering. Like diamond, the DLC coatings have also attained remarkable
∗ Corresponding author at: Department of Physics, Quaid-i-Azam University, 45320 Islamabad, Pakistan. Tel.: +92 3004237931. E-mail addresses: [email protected], [email protected] (M. Hassan). http://dx.doi.org/10.1016/j.apsusc.2014.02.142 0169-4332/© 2014 Elsevier B.V. All rights reserved.
popularity because of their superior physical properties such as high hardness, high strength, and better stability at high temperature, high optical transparency, high thermal conductivity, high dielectric strength, high chemical inertness and low coefficient of friction. Eventually, DLC coatings find their suggested use in the field of tribology (reduction of friction, wear, erosion, cavitation), bioengineering, both passive and active electronics elements and optics. A wide range of materials can be opted as a substrate for DLC coatings. The choice of AISI 304 stainless steel (SS) having high strength, excellent corrosion resistance and excellent formability makes it useful for various applications. Typical uses include architectural moldings and frames, kitchen equipment, as well as chemical, textile, paper and chemical industry processing equipment. AISI 304 stainless steel has been deposited with DLC coatings by physical vapor deposition and investigated for corrosion resistance [2]. Its tribological performance has been investigated by duplex surface treatment i.e. plasma electrolytic nitrocarburising and plasma-immersion ion-assisted deposition of DLC coatings [3,4] and sputter deposition [5].
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Table 1 Technical parameters of the repetitive plasma focus (NX2) device used for DLC coating on SS. Parameter
Specification
Parameter
Specification
Capacitance, C0 Operating charging voltage, V0 Maximum charging voltage, V Maximum stored energy, E Operating repetition rate, f Anode diameter, a
27.6 F (0.6 F × 46) 13.5 kV 15 kV 3.1 kJ 0.5 and 1.0 Hz 31 mm
Inductance of circuit, L0 Impedance, Z0 Maximum current, I0 T/4 time of short circuit, t Anode (copper) length, la
26 nH (L0 /C0 )1/2 = 31 m 430 kA 1.33 s 45 mm
The amorphous nature of DLC coatings allows the uniform coverage of surface. A variety of techniques like chemical vapor deposition (CVD) [6], pulsed laser ablation [7], cathodic arc plasma deposition [8], pulsed electrode surfacing [9] and sputtering by ion beam [10] were successfully applied to produce these coatings. Recently, the dense plasma focus (DPF) has been employed for the development of surface properties of various materials. The DPF is a simple and low cost pulsed coaxial plasma accelerator that produces a short lived plasma pinch of very high densities (∼1025 to 1026 m−3 ) and high temperatures (1–2 keV). The DPF is a bright source of multi-radiations such as highly energetic (25 keV to 8 MeV) high fluence ions, relativistic electrons, X-rays and neutrons [11–15]. The ion beam is mainly emitted in the forward direction (with respect to the electrode axis), within a cone with an aperture angle ranging from 30◦ to 70◦ depending on their energy [16]. These highly energetic ions have been used to grow crystalline TiO2 [17], to induce crystallization in amorphous lead zirconate titanate (PZT) films [18], to change the orientation of CdI2 films [19], to deposit nanocrystalline multiphase titanium oxycarbide thin films on Ti [20], to deposit TiN films on Ti [7], on Zr [21], on AISI-304 [22], to modify the surface of austenitic SS [23], and to improve the wear behavior of SS and Ti by nitrogen implantation [24]. Energetic ions emitted from low energy DPF (PF II) device have also been employed to deposit well-adhered TiN coatings on AISI 1010 and AISI 304 SS due to ion-induced strong heating of superficial layer of SS and diffusion of impinging ions [25]. The use of DPF device as an ion implanter has been found to be feasible that could perform nitriding in short time intervals. Heating rate of ∼15 K ns−1 , temperature gradients of 1500 K mm−1 and peak temperatures near the melting point played an important role in the surface modification process. The use of plasma focus for materials processing has demonstrated its interesting features of high deposition rates and good adhesion owing to its wide energy spectrum of radiations [16–18]. Various features and characteristics of plasma focus device that makes it a unique device for applications in materials processing and deposition are provided in detail in a recent review paper by Rawat [26]. The aim of the present work is to deposit DLC coatings on SS using ions of a repetitive DPF device designated as Nanyang X-ray source-2 or NX2 plasma focus device [13] for two different focus shot repetition rates as a function of angular position of deposition substrate with respect of anode axis. Raman spectroscopy, X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS) are used to investigate sp2 and sp3 bonding, structural and elemental changes respectively, whereas field emission scanning electron microscopy (FESEM) is used to study the surface morphology of the DLC coatings. 2. Experimental setup For the present experiment, the DPF device is operated in auto trigger mode with repetition rates of 0.5 and 1.0 Hz. The technical parameters of the device are given in Table 1. The discharge chamber is evacuated down to 10−4 mbar using turbo molecular pump, and is then filled with CH4 as working gas at an optimum pressure
of 2.5 mbar. The NX2 device employs a Cu anode having engraved tip in order to reduce the impurities introduced into plasma [11]. When the electrical energy stored in the capacitor bank (four modules CB1, . . ., CB4) is transferred to the electrode system by low inductance pseudo-spark gap (PSG) switches, the gas breakdown occurs initially across the surface of the insulator sleeve separating the anode and the cathode. The breakdown, under the influence of Lorentz forces, proceeds on to form uniform, axis symmetrical current sheath in inverse pinch phase and is then accelerated down the anode axis during axial rundown phase. Upon reaching the top of the anode, the current sheath collapses radially inward resulting in the formation of hot dense pinched plasma at the top of the anode during the final focus phase. The plasma temperature is sufficiently high to cause complete ionization of the working gas. CH4 , being the working gas, supplies ions in almost all ionization states [11]. The formation of hot dense plasma is followed by the onset of sausage (m = 0) instability. This instability enhances the induced electric field locally, and hence breaks the focus plasma column [27]. This leads to the acceleration of gaseous ions with very high energies toward the top of the chamber. The key plasma charged particles (energetic ions and electrons) and radiation (EUV, X-rays, neutrons, etc.) characteristics for plasma focus devices are well established through various experimental and simulation studies. The energies of instability-accelerated ions, which mostly move towards the top of the anode, are in the range of tens of keV to a few MeV [28,29]. It is also a well-established fact that the dimensions of coaxial electrode assembly, electrical energy stored in the capacitor bank, peak discharge current flowing through the plasmas, and the operating gas type do not affect many of the key characteristics such as plasma density, plasma temperature, current sheath speeds, radiation and ion energy spectra etc., of the optimized DPF devices indicating the unique universality in the DPF devices [26]. Hence for the methane filled NX2 DPF device operation the energy of the carbon and hydrogen ions will also be in the range of few tens of keV to a few MeV. More detailed information about ion beam characteristics of DPF devices have recently been estimated and reported by Lee and Saw [30,31]. Further details of NX2 DPF operation and the preliminary diagnostics can be found elsewhere [13]. The energetic ions emitted from the focus plasma are utilized for the formation of DLC coatings on AISI 304 samples. The important parameters are ion energy, ion flux which is controlled by distance or angle of deposition, operating voltage, filling pressure, shot to shot reproducibility and the repetition rate of the device that may strongly influence the physical characteristics of the coating at a particular deposition position. The NX2 device has been operated under optimum discharge conditions for CH4 plasma. The chamber pressure of NX2 is highly stable and hence the shot to shot reproducibility of this device, monitored and established by intense voltage peak in high voltage probe signal and sharp dip in current derivative Rogowski coil signal [13], is excellent. films were deposited on highly polished DLC 15 mm × 15 mm × 1 mm stainless steel-304 substrates with surface roughness average of approximately 30 A˚ as measured by TENCOR P-10 Surface Profiler. The SS samples were ultrasonically cleaned sequentially with de-ionized water, acetone and ethanol
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Fig. 1. Schematic diagram of repetitive plasma focus used for DLC coatings. Pseudo-spark switches PSG1 to PSG4 facilitate minimum inductance switching of current in repetitive mode discharge. AISI 304 samples are mounted in the processing chamber at ambient temperature.
for 15 min each at 30 ◦ C. Total three (03) samples were coated for a particular repetition rate. The samples were mounted at axial distance of 9 cm from the anode tip at different angular positions such as at 0◦ , 10◦ and 20◦ with respect to the anode axis as shown in Fig. 1. The estimated area of the nitride zone viewed from the top of the conical cross-section is ∼50 cm2 . Noticeably, the deposition of films such as TiN [12,22] along the anode axis at lower axial distance has been reported with significant surface damage by the last treatment shot because of the maximum energy and flux of ions along the axis. Eventually, the deposition of these films was conducted at different angular positions (referred as 0◦ , 10◦ and 20◦ ) with respect to the anode axis. The conditions for DLC coatings (substrate position, no. of shots) in the present work have been selected based on plenty of work conducted [20–22]. Since the DPF has well established ion profile (flux and energy distribution) in the conical geometry [16], the ion flux reduction with increasing conical angle affects the quality of the DLC coatings. The goal was to select the optimum angular position when deposited for two different repetition rates
(0.5 and 1.0 Hz). All depositions were carried out using 30 focus shots for these repetition rates of 0.5 and 1.0 Hz. Raman spectroscopy analysis of deposited carbon coatings is performed using a Renishaw MicroRaman spectrometer equipped with an argon ion laser that uses excitation wavelength of 514 nm. The spectrometer has spectral resolution of 1 cm−1 . The Raman spectra are acquired over the range of 10–2000 cm−1 in the back scattering geometry by measuring the inelastically scattered light from the sample. By fitting the peaks with Gaussian distribution, Raman spectra are also used to estimate the sp2 to sp3 ratio that determines the diamond character in the deposited layer. XRD analysis is used to investigate the possible changes in the microstructure of SS 304 samples. SEM along with EDS is employed to study the surface morphology and elemental composition of the coatings. SEM micrographs were acquired at a tube voltage of 5 kV, electron current of 10 A and working distance varying from 3 to 8 mm. EDS analysis was performed at a tube voltage of 10 kV, electron current of 20 A and working distance of 15 mm. Wilson Wolpert 401MVA Vickers micro-hardness tester is employed to determine
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Fig. 2. Raman spectra of the DLC coatings deposited at different angular positions (0◦ , 10◦ and 20◦ ) for repetition rates of (a) 0.5 Hz and (b) 1.0 Hz.
the micro-hardness as a function of indentation depth in the DLC coatings. 3. Results and discussion 3.1. Raman spectroscopy Raman spectroscopy is a standard non-destructive tool for the characterization of crystalline, nanocrystalline and amorphous carbons. Generally, Raman spectra of disordered graphite show two quite sharp modes; the G peak around 1500–1630 cm−1 and the D peak around 1330–1350 cm−1 [32,33]. The G peak is assigned to the E2g symmetric vibration mode of graphite layers of microdomain in the coatings [32,33]. Fig. 2 shows Raman spectra of the DLC coatings at different angular positions (0◦ , 10◦ and 20◦ ) for repetition rates of (a) 0.5 Hz and (b) 1.0 Hz. For both the deposition rates, Raman spectra
show the emergence of two broad bands, centered at around 1330–1360 cm−1 (D band) and 1560–1620 cm−1 (G band). The occurrence of D and G bands explicitly reports the presence of DLC coatings on the SS 304 substrates. The intensity and broadening of D and G bands enlighten the strength of sp2 and sp3 bonds. The broadening of bands can be assigned to the second order scattering or some disorder-induced vibrational mode [34]. The incorporation of gaseous impurities as well as those from electrodes, left unsputtered during coatings, leads to the broadening of such bands. The emergence of D and G Raman peaks for all samples points to the successful deposition of DLC coatings on SS substrates for 30 focus shot depositions at all three angular positions for both repetition rate operations. The quality of the DLC coatings, in terms of peak intensity ratio, depends on the spatial distribution of ion beams. Since plasma focus has been well investigated for the axial [12] and angular distribution of ion beams [35], the depositions done at 10◦ and 20◦ angular positions provides sufficient ion energy and flux to establish sp2 and sp3 bonds characterized by D and G bands. The advantage of the repetitive NX2 device over a single shot device is its higher repetitive rate performance unlike a conventional single shot DPF device usually operated in manual trigger mode [36] with shot interval of about 1–2 min. Substantial thickness (about one micron or so) of coatings can be achieved in a short duration while operating the DPF device in repetitive mode. However, in the repetitive mode, energetic ions bombardment may increase the substrate temperature if no cooling is arranged. The plasma focus device is pulsed plasma device and hence the substrate temperature variation upon its exposure to energetic pulsed ion beams is also a transient phenomenon with extremely high temperature rise rate (∼40 K ns−1 ) followed by rapid quenching [37]. So if the device is used in single shot mode with average shot interval of 1 min or so, then rise in average substrate temperature will be negligible even though there is intense transient heating. However, for repetitive operation the average substrate temperature was not negligible as estimated by Rawat et al. [17]. For 2.0 Hz repetition rate, they reported [17] the increase in average substrate temperature from room temperature of 295 K to about 310 K for 100 NX2 DPF device shots; while for low repetition rate of 0.2 Hz the average substrate temperature increase was only about 1–2 K. In the present experiment, two repetition rates 0.5 Hz and 1.0 Hz achieved the DLC coatings respectively in 60 and 30 s for 30 focus shots. The DLC coatings deposited for both rates show distinct compositional and morphological profile for particular angular positions. For 0.5 Hz rate (Fig. 2a), the D and G peaks are less intense due to the slower rate of ion energy/flux imparted at substrates at this rate. A high content of sp3 carbon bonding in the DLC films is believed to be responsible for the weak feature of the Raman peaks, because the Raman phonon line is more sensitive to the sp2 carbon bonding. However, for 1.0 Hz DPF operation rate (Fig. 2b), the temperature rise of few K due to faster deposition rate seems to assist the growth of DLC coatings with enhanced Raman D and G peaks. Rapid cooling of the sample surface between consecutive shots as discussed quantitatively by many authors [12,23,24] renders the surface less energetic. Increasing the shot to shot repetition rate raises the temperature of the substrate in the near surface region to few K, which further helps in formation of DLC deposition. Qualitatively, the intensity of D and G bands increases and broadening decreases for depositions at 1.0 Hz repetition rate. This increase in bands intensity confirms the reduction in the disordering level of the coatings which may be due to the increase of average substrate surface temperature and stress relaxation for higher repetition rate of 1.0 Hz. This reduction in disordering level may be attributed to the minimization of total free energy of the coatings and consequently results in the crystal growth. Thus, the band growth and broadening of the D and G bands strongly depend on the repetition
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Fig. 4. Typical X-ray diffractograms of the DLC coated SS samples at different angular positions (10◦ and 20◦ ) for 1.0 Hz repetition rate, showing (1 1 1) and (2 0 0) plane orientations of the ␥ phase, and (1 1 0) plane of restructured ␣-Fe phase in AISI 304 SS substrate.
Fig. 3. Variation of ID /IG ratio and G band position in the DLC coating as a function of angular position of the sample for repetition rates of (a) 0.5 Hz and (b) 1.0 Hz.
rate and hence the time-averaged substrate surface temperature rise owing to the successive focus shots. The D band to G band intensity ratio and G band peak positions have been commonly used for the characterization of amorphous DLC content and the induced stresses in the coatings respectively. Such compressive internal stresses in diamond and DLC coatings were related to the microstructure of the coatings [38]. Fig. 3 shows the plots of the band intensity ratio (ID /IG ) and the G band position (cm−1 ) as a function of angular position for two different repetition rates; 0.5 and 1.0 Hz of focus shots. The sp2 -bonded carbon trapped in the sp3 -bonded diamond crystallites or in the grain boundaries causes more lattice strains in the diamond coating. The plot in Fig. 3a shows that with change in angular position from 0◦ to 20◦ for 0.5 Hz repetition rate, the band intensity ratio increases suggesting relatively decreased sp3 content in the coating. Moreover, there is up-shift in the G band position showing the increased vibrational frequency under reduced stresses. The decrease in the number of four-fold coordinated carbon atoms (sp3 ) with the increased value of intensity ratio as well as G band peak position has well been demonstrated by many authors [39,40]. The plot in Fig. 3b shows that with change in angular position from 0◦ to 20◦ for 1.0 Hz repetition rate, the band intensity ratio decreases, that shows increased sp3 content in the coating. It is also supported by down-shift of the G band position owing to reduced vibrational frequency under applied stresses.
The DLC coatings have significant but varied amount of sp2 and sp3 bonding, possible clustering of similarly bonded carbon atoms to provide some short range order, and very small or large amount of hydrogen. The variation of ID /IG at different angular positions for two different repetition rates tracks the sp2 /sp3 content variation (or the DLC character) of the coatings. Thermal desorption of bonded hydrogen takes place during rapid heating of the coating surface. Eventually, hydrogenated amorphous carbon may transform to non-hydrogenated one and vice versa depending upon substrate position. For example for a particular repetition rate, DLC coatings evolve from a-C to a-C:H owing to less efficient thermal desorption of hydrogen at larger angular position exposed to reduced ions flux of DPF. Thermal desorption of DLC coatings in terms of their thermal stability has also been studied by Tallant et al. [41] who showed that DLC coatings are structurally stable up to about 260 ◦ C even in high humidity or boiling water. Thus, 1.0 Hz repetition rate and 10◦ angular position are suitable parameters for the quality DLC coatings. Lower angular position of 10◦ as compared to 20◦ favors hydrogen evolution that has a well-established causative relationship to the observed sp3 to sp2 conversion owing to increased ion fluence having same effect as that of heating. Notably, 0◦ angular position is not favorable owing to deprived surface uniformity, although having significant sp2 content for both repetition rates, and micro-hardness reported in later section. 3.2. XRD analysis Fig. 4 shows the typical XRD pattern of AISI 304 samples deposited with DLC coatings at 10◦ and 20◦ angular positions with respect to the anode axis, for the focus shot repetition rate of 1.0 Hz. For the pristine AISI 304 sample, the diffraction peaks appear at a 2 value of 43.5◦ and 50.4◦ respectively corresponding to (1 1 1) and (2 0 0) plane orientations of the ␥ phase of AISI 304 SS. The XRD spectra obtained clearly show the emergence of ␣-Fe (1 1 0) peak at 44.31◦ owing to stress induced restructuring/phase transformation in SS. Zeb et al. [42] also deposited amorphous DLC coatings on Si (1 0 0) using a low energy (1.45 kJ) dense plasma focus. They
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observed the XRD peak as asymmetric broad humps at 2 ≈ 44◦ and attributed it to the formation of lattice defects in the silicon after ion irradiation. DLC phase appearance is owing to the impact of short pulse (∼100 ns) instability accelerated energetic carbon ions which can be implanted on the substrate surface and also due to the condensation of the high temperature high density carbon plasma, from the dissociation of filling CH4 gas in DPF chamber, on the SS substrate typically lasting for few microseconds. As the deposition is conducted using multiple focus shots fired at repetition rates of 0.5 and 1.0 Hz, the very high flux energetic carbon and hydrogen ions from subsequent shots can cause the melting and subsequent rapid cooling of the carbon coated SS substrate surface. This results in the recrystallization of SS substrate surface along with that of carbon layer deposited up to previous focus shot due to the adsorption of energetic species (mostly carbon ions) onto the surface thus forming new phases [43]. The cooling of surface develops amorphous regions and defects within the bulk [44]. The DPF ion adsorbate-induced reconstruction yields new structures with the minimization of the surface free energy of substrate and the coating. This is also accompanied by the introduction of stresses to the system. The stresses have one major advantage of hardness improvement. The presence of tensile stresses in the DLC coatings is observed by the downshift of the carbon peak position in the XRD spectrum, when compared with the standard stress free data. This indicates a uniform stress developed normal to the corresponding crystal plane in the film during rapid cooling after transient temperature rise by ion irradiation. The tensile stresses in DLC coatings are owing to the incorporation of energetic (up to few hundreds of keV) carbon species interstitially (up to a few hundreds of nm depending on ion energy range) in addition to their presence as bonded C in sp2 and sp3 . Similar effect of N2 ion implantation on AISI 304 has revealed that the initial compression residual stresses were changed to tensile stresses after ion implantation. The residual stress gradient, induced in the implanted surfaces, was found to become more significant with the increase of ion beam fluence. The DPF ions induced stress evolution and transformation from compressive to tensile or vice versa have extensively been reported already [21,45,46]. From Fig. 4, it is clear that the DLC coating at 20◦ angular position has less stresses as compared to that achieved at 10◦ angular position due to low energy flux at 20◦ position. Our previous results on DPF based coatings strongly support this evidence [20,21]. The average crystallite size is calculated from the ␣-Fe (1 1 0) peak width using the Scherrer formula crystallite size =
K , ˇ cos
where K = 0.99 is the numerical constant, = 1.540598 A˚ is the wavelength of X-ray source, ˇ is the broadening (in radians) of the diffraction peak and is the Bragg’s angle. The average crystallite size of ␣-Fe in restructured SS 304 varies from ∼40 to 50 nm. 3.3. SEM/EDS Scanning electron microscopy (SEM) and energy dispersive Xray spectroscopy (EDS) were carried out for the morphological and elemental analysis of the coatings. Fig. 5 shows the SEM micrograph, EDS map, SEM dark image taken at working distance of 15 mm showing area selection for EDS map and spectrum acquisition, dark image of EDS map and EDS spectrum recorded for a typical sample with DLC coating achieved on 10◦ angular position for 1.0 Hz repetition rate. The coating appears golden in color when viewed visually. Such type of color appearance reports the amorphous, polymeric nature of the DLC coatings. From the SEM
micrograph taken at magnification of ×40,000 (Fig. 5a), the patchy or layered profile of the DLC coatings can be clearly identified. Since the deposition is done in multiple shots, there might be multilayers composed of nanoparticles. The layered regions are probably coagulated zones. The grain size of ∼40–60 nm with some size dispersion is observed on the DLC coating deposited at 10◦ angular position. Such granular surfaces with significant thermal coagulation effects have also been early reported in our previous work [24]. These coagulated regions are developed due to the high energy flux of instability accelerated ion beams emitted from the NX2 operated at high repetition rate usually at lower angular position of the substrate. This can be elaborated under the theory of charged clusters (TCCs) [47] and the thermal coagulation phenomenon. The DPF ions and/or radicals, having multiple charged states [15], nucleate in the gas phase to form charged nucleates, which are then deposited on the substrate to form coating. A large amount of energy transferred to the substrate via electronic excitations develops defects and consequently nanostructures of nm scale nucleates. The nucleates can grow either by reactions with reactive species in the gas phase or combining with very small uncharged clusters. These nucleates are grown by the ion-induced collision cascades and extend to a range of up to a few tens of nm for ion energies of several tens of keV to MeV [48]. The DLC coatings obtained at 20◦ angular position have almost identical morphology, whereas those at 0◦ have slight damage owing to the high energy ions. The energetic impacts from highly ionized hot dense CH4 plasma leads to the formation of sp3 -bonded carbon in a sp3 -bonded substrate surface matrix. The surface structure may have a hydrogen and sp3 hybridized carbon enriched surface. During multiple shots irradiation, the coatings developed with previous shots are hydrogenated by exposing them to the ionic and/or atomic hydrogen beams, leading to the transformation of sp2 hybridized carbon groups to sp3 hybridized groups at the coating surface. This depends on the H/C ratio in CH4 plasma at any angular position. Since carbon content is high at lower angular position (0◦ and 10◦ ), sp2 -enriched dominates at these positions. The implanted ions displace predominantly bonded hydrogen owing to lower threshold energy for hydrogen, compared to that of carbon [49]. The displaced hydrogen atoms can recombine with a lattice defect or form H2 molecules by atomic recombination. The H2 molecules are either trapped in internal voids or diffuse to the surface and desorb. This leads to the phase transition from polymer-like structure to the diamond-like structure with low hydrogen content and an increase in the coating density. Thus, the ion-induced compression of polymer-like sp3 bonded structure to a denser sp2 -enriched phase is resulted from strong thermal spikes during DPF ion irradiation. The EDS map of the coating exhibit the distribution of elements present in the coating using the imaging energy filters. By selecting the area of the SEM image for mapping and spectrum acquisition (Fig. 5c), the elemental distribution of the elements surface coating can be obtained. In mapping, an image of the sample will be made that is simply the intensity of a particular emission mapped in an XY raster. The intensity of the particular X-ray lines in the maps is qualitative measure of elemental distribution. Fig. 5d shows the SEM dark image used to collect the EDS map on the same scale. Fig. 5b shows the CK␣ map presenting the elemental distribution of carbon on the DLC coating surface. The intensity of the CK␣ in the maps is a qualitative measure of the elemental concentration. A homogeneous spatial distribution of the CK␣ emission indicates the spatially uniform carbon over the DLC coated SS sample surface macroscopically. Fig. 5e shows the EDS spectrum of the above mentioned coating. The incorporation of C content is evident in the spectrum, along with the presence of SS composition. The carbon content in pristine AISI 304 steel is 0.08 wt.%, which is much less than the values obtained for DLC coated SS. The EDS spectra demonstrate
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Fig. 5. (a) Typical SEM micrograph of the DLC coatings with coagulated regions due to ions assisted thermal effects, (b) CK␣ EDS map, (c) SEM dark image to select area for EDS map and spectrum acquisition, (d) dark image of the EDS map and (e) EDS spectrum of the DLC coatings deposited at the angular position of 10◦ for 1.0 Hz repetition rate.
the increase in carbon concentration depending on the angular position. For the DLC coatings obtained for 0.5 Hz repetition rate, the concentration of carbon varies from 10.1 at.% to 5.2 at.% (3.31 ± 1.22 wt.% to 2.12 ± 0.88 wt.%) when the sample position is shifted from 0◦ to 20◦ with respect to anode axis. Whereas, for the DLC coatings obtained for 1.0 Hz repetition rate, the carbon concentration is reduced from 14.21 at.% to 7.82 at.% (4.31 ± 1.22 wt.% to 2.81 ± 0.79 wt.%) when the sample position is shifted from 0◦ to 20◦ . The carbon concentration data has been provided in Table 2. It has been found that the C content is significantly reduced at higher angular position. The reduction in carbon content at the 20◦ angular position is due to the reduced ion fluence impinging at this position of the substrate. Moreover, the C content is found to reduce with the repetition rate reduction also, showing deprived quality of the DLC coatings due to lesser temperature assistance.
Thus, appearance of carbon content in sp2 -enriched phase shows that the 10◦ angular position and 1.0 Hz repetition rate provide the optimum deposition conditions in the present experiment. 3.4. Micro hardness measurements Fig. 6 shows the surface hardness (GPa) of DLC coatings as a function of the indentation depth (m) with different test loads on the sample surfaces treated at various angular positions for 1.0 Hz repetition rate. The maximum surface hardness is found for DLC coatings at 0◦ angular position. The typical micro-hardness of the DLC coatings is around 3 GPa. However, in the near surface region, it approaches to 6 GPa. It is noted that the relatively lower values of microhardness are obtained at higher angular position in spite of higher sp3 content in DLC coatings. This fact may be attributed to
Table 2 EDS data showing the carbon concentrations (at.% and wt.%) in DLC coated SS surfaces at different sample positions for two different repetition rates. Sample identification
C content (at.%) C content (wt.%)
Repetition rate 0.5 Hz
Repetition rate 1.0 Hz
0◦ position
10◦ position
20◦ position
0◦ position
10◦ position
20◦ position
10.1 3.31 ± 1.22
8.62 3.04 ± 0.92
5.2 2.12 ± 0.88
14.21 4.31 ± 1.22
11.86 3.78 ± 1.02
7.82 2.81 ± 0.79
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owing to stress induced disorder in the SS lattice, having crystallite size estimated to be ∼40–50 nm. The repetitive ion bombardment has played an important role in surface transient temperature rise and eventual less strained DLC coating in comparison with low repetition rate plasma focus ions bombardment. The surface microhardness of the coatings achieved at 10◦ angular position has been increased three times for repetition rate of 1.0 Hz.
Acknowledgements One of the authors (Dr. M. Hassan) is grateful to the NESCOM for providing financial support to carry out experimentation and materials analysis at Nanyang Technological University (NTU), Singapore. One of them, S. Mahmood would like to thank NIE, NTU for providing the research scholarship.
References
Fig. 6. Variation of surface microhardness of the SS samples coated with DLC at different angular positions for a repetition rate of 1.0 Hz. The maximum surface hardness is shown by the maximum stressed DLC coatings at 0◦ angular position.
the polymeric nature of the coatings owing to reduced ion fluence with lower charge states. An approximately three fold increase in the hardness value is evaluated for the typical DLC coated surface coatings at 0◦ angular position. The descending microhardness in the near surface region of the SS sample reports the implantation limit of carbon ions up to ∼2 m. Below this implanted zone, ion-induced stress hardening results due to quenching of surfaces under fast transient temperature rise by high repetition rate deposition. This eventually develops stressed regions not only due to temperature gradients but also due to concentration gradients, that positively contribute to the microhardness improvement up to a few m depth in the surfaces of DLC coated SS samples. Many researchers have reported such type of stress induced hardening by the collision cascades of energetic ions [45,49] and the incorporation of the carbon ions in the coatings by successive bombardment [7,12,20]. 4. Conclusions Raman spectroscopy results explicitly confirm the deposition of a-C:H/DLC coatings on the SS substrates. The occurrence of G band indicates the formation of graphitic sp2 microdomains while the D band is assigned to the bond angle disorder in the graphitelike microdomains, induced by linking with sp3 carbon atoms and finite crystallite size of sp2 microdomains. It has been found that the growth and the broadening of the bands is strongly influenced by the substrate angular position and hence the ions energy fluence. For a typical higher repetition rate of 1.0 Hz, carbon content is high as compared to deposition for 0.5 Hz repetition rate. However, the band intensity ratio decreases for higher angular position showing an increased sp3 (polymeric) content in the DLC coatings. It is accompanied by the down-shift of the G band position owing to reduced vibrational frequency under ion induced stresses. Thus, 10◦ angular position and 1.0 Hz repetition rate provide the optimum conditions for the present experiment regarding DLC quality. The DLC coatings transform from a-C to a-C:H when the sample position is shifted to higher angular position owing to less efficient thermal desorption of hydrogen bonded with carbon. XRD results explicitly evident the emergence of ␣-Fe (1 1 0) diffraction peak
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
DLC coating on stainless steel by pulsed methane discharge in repetitive plasma focus M. Hassan a,c,∗ , A. Qayyum b , S. Ahmad b , S. Mahmood c,d , M. Shafiq a , M. Zakaullah a , P. Lee c , R.S. Rawat c a
Department of Physics, Quaid-i-Azam University, 45320 Islamabad, Pakistan National Tokamak Fusion Program, 3329 Islamabad, Pakistan c Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, BLK7, 1 Nanyang Walk, Singapore 637616, Singapore d Department of Physics, University of Karachi, 75270 Karachi, Pakistan b
a r t i c l e
i n f o
Article history: Received 8 June 2013 Received in revised form 21 February 2014 Accepted 24 February 2014 Available online 12 March 2014 PACS: 52.59Hq 52.77Fv 61.05cp Keywords: DLC DPF XRD SEM Raman spectroscopy
a b s t r a c t Amorphous hydrogenated carbon (a-C:H)/diamond-like carbon (DLC) coatings have been achieved on AISI 304 stainless steel (SS) substrates by employing energetic ions emitted from a repetitive plasma focus operated in CH4 discharge. The Raman spectroscopy of the coatings exhibits the evolution of aC:H/DLC coatings with clearly observed D and G peaks centered about 1320–1360 and 1560–1620 cm−1 respectively. The diamond character of the coatings is influenced by the ion flux and repetition rate of the focus device. The repetitive discharge mode of plasma focus has led to the formation of a-C:H/DLC coatings in short duration of time. The coatings transform from a-C to a-C:H depending upon substrate angular position. X-ray diffraction (XRD) analysis confirms the formation of DLC coating owing to stress-induced restructuring in SS. The estimated crystallite size is found to be ∼40–50 nm. Field emission scanning electron micrographs exhibit a layered granular surface morphology of the coatings. The Vickers surface hardness of the DLC coated SS samples has been significantly improved. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Carbon can exist in three different chemical states, involving sp3 , sp2 and sp1 chemical co-ordination. Eventually, a variety of structures can be resulted, which may also contain other chemical elements. Mainly, there are two types of diamond-like structures; identified explicitly as hydrogenated amorphous carbon (a-C:H) or diamond-like hydrocarbons and non-hydrogenated amorphous carbon (a-C) or diamond-like carbon (DLC). Such a-C:H and aC coatings have received much scientific interest owing to their useful mechanical properties. Raman features of a-C:H coatings deposited at room temperature were discussed by Schwan et al. [1]. They showed that small clusters of condensed benzene rings in carbon films can be explained in the observed Raman scattering. Like diamond, the DLC coatings have also attained remarkable
∗ Corresponding author at: Department of Physics, Quaid-i-Azam University, 45320 Islamabad, Pakistan. Tel.: +92 3004237931. E-mail addresses: [email protected], [email protected] (M. Hassan). http://dx.doi.org/10.1016/j.apsusc.2014.02.142 0169-4332/© 2014 Elsevier B.V. All rights reserved.
popularity because of their superior physical properties such as high hardness, high strength, and better stability at high temperature, high optical transparency, high thermal conductivity, high dielectric strength, high chemical inertness and low coefficient of friction. Eventually, DLC coatings find their suggested use in the field of tribology (reduction of friction, wear, erosion, cavitation), bioengineering, both passive and active electronics elements and optics. A wide range of materials can be opted as a substrate for DLC coatings. The choice of AISI 304 stainless steel (SS) having high strength, excellent corrosion resistance and excellent formability makes it useful for various applications. Typical uses include architectural moldings and frames, kitchen equipment, as well as chemical, textile, paper and chemical industry processing equipment. AISI 304 stainless steel has been deposited with DLC coatings by physical vapor deposition and investigated for corrosion resistance [2]. Its tribological performance has been investigated by duplex surface treatment i.e. plasma electrolytic nitrocarburising and plasma-immersion ion-assisted deposition of DLC coatings [3,4] and sputter deposition [5].
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Table 1 Technical parameters of the repetitive plasma focus (NX2) device used for DLC coating on SS. Parameter
Specification
Parameter
Specification
Capacitance, C0 Operating charging voltage, V0 Maximum charging voltage, V Maximum stored energy, E Operating repetition rate, f Anode diameter, a
27.6 F (0.6 F × 46) 13.5 kV 15 kV 3.1 kJ 0.5 and 1.0 Hz 31 mm
Inductance of circuit, L0 Impedance, Z0 Maximum current, I0 T/4 time of short circuit, t Anode (copper) length, la
26 nH (L0 /C0 )1/2 = 31 m 430 kA 1.33 s 45 mm
The amorphous nature of DLC coatings allows the uniform coverage of surface. A variety of techniques like chemical vapor deposition (CVD) [6], pulsed laser ablation [7], cathodic arc plasma deposition [8], pulsed electrode surfacing [9] and sputtering by ion beam [10] were successfully applied to produce these coatings. Recently, the dense plasma focus (DPF) has been employed for the development of surface properties of various materials. The DPF is a simple and low cost pulsed coaxial plasma accelerator that produces a short lived plasma pinch of very high densities (∼1025 to 1026 m−3 ) and high temperatures (1–2 keV). The DPF is a bright source of multi-radiations such as highly energetic (25 keV to 8 MeV) high fluence ions, relativistic electrons, X-rays and neutrons [11–15]. The ion beam is mainly emitted in the forward direction (with respect to the electrode axis), within a cone with an aperture angle ranging from 30◦ to 70◦ depending on their energy [16]. These highly energetic ions have been used to grow crystalline TiO2 [17], to induce crystallization in amorphous lead zirconate titanate (PZT) films [18], to change the orientation of CdI2 films [19], to deposit nanocrystalline multiphase titanium oxycarbide thin films on Ti [20], to deposit TiN films on Ti [7], on Zr [21], on AISI-304 [22], to modify the surface of austenitic SS [23], and to improve the wear behavior of SS and Ti by nitrogen implantation [24]. Energetic ions emitted from low energy DPF (PF II) device have also been employed to deposit well-adhered TiN coatings on AISI 1010 and AISI 304 SS due to ion-induced strong heating of superficial layer of SS and diffusion of impinging ions [25]. The use of DPF device as an ion implanter has been found to be feasible that could perform nitriding in short time intervals. Heating rate of ∼15 K ns−1 , temperature gradients of 1500 K mm−1 and peak temperatures near the melting point played an important role in the surface modification process. The use of plasma focus for materials processing has demonstrated its interesting features of high deposition rates and good adhesion owing to its wide energy spectrum of radiations [16–18]. Various features and characteristics of plasma focus device that makes it a unique device for applications in materials processing and deposition are provided in detail in a recent review paper by Rawat [26]. The aim of the present work is to deposit DLC coatings on SS using ions of a repetitive DPF device designated as Nanyang X-ray source-2 or NX2 plasma focus device [13] for two different focus shot repetition rates as a function of angular position of deposition substrate with respect of anode axis. Raman spectroscopy, X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS) are used to investigate sp2 and sp3 bonding, structural and elemental changes respectively, whereas field emission scanning electron microscopy (FESEM) is used to study the surface morphology of the DLC coatings. 2. Experimental setup For the present experiment, the DPF device is operated in auto trigger mode with repetition rates of 0.5 and 1.0 Hz. The technical parameters of the device are given in Table 1. The discharge chamber is evacuated down to 10−4 mbar using turbo molecular pump, and is then filled with CH4 as working gas at an optimum pressure
of 2.5 mbar. The NX2 device employs a Cu anode having engraved tip in order to reduce the impurities introduced into plasma [11]. When the electrical energy stored in the capacitor bank (four modules CB1, . . ., CB4) is transferred to the electrode system by low inductance pseudo-spark gap (PSG) switches, the gas breakdown occurs initially across the surface of the insulator sleeve separating the anode and the cathode. The breakdown, under the influence of Lorentz forces, proceeds on to form uniform, axis symmetrical current sheath in inverse pinch phase and is then accelerated down the anode axis during axial rundown phase. Upon reaching the top of the anode, the current sheath collapses radially inward resulting in the formation of hot dense pinched plasma at the top of the anode during the final focus phase. The plasma temperature is sufficiently high to cause complete ionization of the working gas. CH4 , being the working gas, supplies ions in almost all ionization states [11]. The formation of hot dense plasma is followed by the onset of sausage (m = 0) instability. This instability enhances the induced electric field locally, and hence breaks the focus plasma column [27]. This leads to the acceleration of gaseous ions with very high energies toward the top of the chamber. The key plasma charged particles (energetic ions and electrons) and radiation (EUV, X-rays, neutrons, etc.) characteristics for plasma focus devices are well established through various experimental and simulation studies. The energies of instability-accelerated ions, which mostly move towards the top of the anode, are in the range of tens of keV to a few MeV [28,29]. It is also a well-established fact that the dimensions of coaxial electrode assembly, electrical energy stored in the capacitor bank, peak discharge current flowing through the plasmas, and the operating gas type do not affect many of the key characteristics such as plasma density, plasma temperature, current sheath speeds, radiation and ion energy spectra etc., of the optimized DPF devices indicating the unique universality in the DPF devices [26]. Hence for the methane filled NX2 DPF device operation the energy of the carbon and hydrogen ions will also be in the range of few tens of keV to a few MeV. More detailed information about ion beam characteristics of DPF devices have recently been estimated and reported by Lee and Saw [30,31]. Further details of NX2 DPF operation and the preliminary diagnostics can be found elsewhere [13]. The energetic ions emitted from the focus plasma are utilized for the formation of DLC coatings on AISI 304 samples. The important parameters are ion energy, ion flux which is controlled by distance or angle of deposition, operating voltage, filling pressure, shot to shot reproducibility and the repetition rate of the device that may strongly influence the physical characteristics of the coating at a particular deposition position. The NX2 device has been operated under optimum discharge conditions for CH4 plasma. The chamber pressure of NX2 is highly stable and hence the shot to shot reproducibility of this device, monitored and established by intense voltage peak in high voltage probe signal and sharp dip in current derivative Rogowski coil signal [13], is excellent. films were deposited on highly polished DLC 15 mm × 15 mm × 1 mm stainless steel-304 substrates with surface roughness average of approximately 30 A˚ as measured by TENCOR P-10 Surface Profiler. The SS samples were ultrasonically cleaned sequentially with de-ionized water, acetone and ethanol
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Fig. 1. Schematic diagram of repetitive plasma focus used for DLC coatings. Pseudo-spark switches PSG1 to PSG4 facilitate minimum inductance switching of current in repetitive mode discharge. AISI 304 samples are mounted in the processing chamber at ambient temperature.
for 15 min each at 30 ◦ C. Total three (03) samples were coated for a particular repetition rate. The samples were mounted at axial distance of 9 cm from the anode tip at different angular positions such as at 0◦ , 10◦ and 20◦ with respect to the anode axis as shown in Fig. 1. The estimated area of the nitride zone viewed from the top of the conical cross-section is ∼50 cm2 . Noticeably, the deposition of films such as TiN [12,22] along the anode axis at lower axial distance has been reported with significant surface damage by the last treatment shot because of the maximum energy and flux of ions along the axis. Eventually, the deposition of these films was conducted at different angular positions (referred as 0◦ , 10◦ and 20◦ ) with respect to the anode axis. The conditions for DLC coatings (substrate position, no. of shots) in the present work have been selected based on plenty of work conducted [20–22]. Since the DPF has well established ion profile (flux and energy distribution) in the conical geometry [16], the ion flux reduction with increasing conical angle affects the quality of the DLC coatings. The goal was to select the optimum angular position when deposited for two different repetition rates
(0.5 and 1.0 Hz). All depositions were carried out using 30 focus shots for these repetition rates of 0.5 and 1.0 Hz. Raman spectroscopy analysis of deposited carbon coatings is performed using a Renishaw MicroRaman spectrometer equipped with an argon ion laser that uses excitation wavelength of 514 nm. The spectrometer has spectral resolution of 1 cm−1 . The Raman spectra are acquired over the range of 10–2000 cm−1 in the back scattering geometry by measuring the inelastically scattered light from the sample. By fitting the peaks with Gaussian distribution, Raman spectra are also used to estimate the sp2 to sp3 ratio that determines the diamond character in the deposited layer. XRD analysis is used to investigate the possible changes in the microstructure of SS 304 samples. SEM along with EDS is employed to study the surface morphology and elemental composition of the coatings. SEM micrographs were acquired at a tube voltage of 5 kV, electron current of 10 A and working distance varying from 3 to 8 mm. EDS analysis was performed at a tube voltage of 10 kV, electron current of 20 A and working distance of 15 mm. Wilson Wolpert 401MVA Vickers micro-hardness tester is employed to determine
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Fig. 2. Raman spectra of the DLC coatings deposited at different angular positions (0◦ , 10◦ and 20◦ ) for repetition rates of (a) 0.5 Hz and (b) 1.0 Hz.
the micro-hardness as a function of indentation depth in the DLC coatings. 3. Results and discussion 3.1. Raman spectroscopy Raman spectroscopy is a standard non-destructive tool for the characterization of crystalline, nanocrystalline and amorphous carbons. Generally, Raman spectra of disordered graphite show two quite sharp modes; the G peak around 1500–1630 cm−1 and the D peak around 1330–1350 cm−1 [32,33]. The G peak is assigned to the E2g symmetric vibration mode of graphite layers of microdomain in the coatings [32,33]. Fig. 2 shows Raman spectra of the DLC coatings at different angular positions (0◦ , 10◦ and 20◦ ) for repetition rates of (a) 0.5 Hz and (b) 1.0 Hz. For both the deposition rates, Raman spectra
show the emergence of two broad bands, centered at around 1330–1360 cm−1 (D band) and 1560–1620 cm−1 (G band). The occurrence of D and G bands explicitly reports the presence of DLC coatings on the SS 304 substrates. The intensity and broadening of D and G bands enlighten the strength of sp2 and sp3 bonds. The broadening of bands can be assigned to the second order scattering or some disorder-induced vibrational mode [34]. The incorporation of gaseous impurities as well as those from electrodes, left unsputtered during coatings, leads to the broadening of such bands. The emergence of D and G Raman peaks for all samples points to the successful deposition of DLC coatings on SS substrates for 30 focus shot depositions at all three angular positions for both repetition rate operations. The quality of the DLC coatings, in terms of peak intensity ratio, depends on the spatial distribution of ion beams. Since plasma focus has been well investigated for the axial [12] and angular distribution of ion beams [35], the depositions done at 10◦ and 20◦ angular positions provides sufficient ion energy and flux to establish sp2 and sp3 bonds characterized by D and G bands. The advantage of the repetitive NX2 device over a single shot device is its higher repetitive rate performance unlike a conventional single shot DPF device usually operated in manual trigger mode [36] with shot interval of about 1–2 min. Substantial thickness (about one micron or so) of coatings can be achieved in a short duration while operating the DPF device in repetitive mode. However, in the repetitive mode, energetic ions bombardment may increase the substrate temperature if no cooling is arranged. The plasma focus device is pulsed plasma device and hence the substrate temperature variation upon its exposure to energetic pulsed ion beams is also a transient phenomenon with extremely high temperature rise rate (∼40 K ns−1 ) followed by rapid quenching [37]. So if the device is used in single shot mode with average shot interval of 1 min or so, then rise in average substrate temperature will be negligible even though there is intense transient heating. However, for repetitive operation the average substrate temperature was not negligible as estimated by Rawat et al. [17]. For 2.0 Hz repetition rate, they reported [17] the increase in average substrate temperature from room temperature of 295 K to about 310 K for 100 NX2 DPF device shots; while for low repetition rate of 0.2 Hz the average substrate temperature increase was only about 1–2 K. In the present experiment, two repetition rates 0.5 Hz and 1.0 Hz achieved the DLC coatings respectively in 60 and 30 s for 30 focus shots. The DLC coatings deposited for both rates show distinct compositional and morphological profile for particular angular positions. For 0.5 Hz rate (Fig. 2a), the D and G peaks are less intense due to the slower rate of ion energy/flux imparted at substrates at this rate. A high content of sp3 carbon bonding in the DLC films is believed to be responsible for the weak feature of the Raman peaks, because the Raman phonon line is more sensitive to the sp2 carbon bonding. However, for 1.0 Hz DPF operation rate (Fig. 2b), the temperature rise of few K due to faster deposition rate seems to assist the growth of DLC coatings with enhanced Raman D and G peaks. Rapid cooling of the sample surface between consecutive shots as discussed quantitatively by many authors [12,23,24] renders the surface less energetic. Increasing the shot to shot repetition rate raises the temperature of the substrate in the near surface region to few K, which further helps in formation of DLC deposition. Qualitatively, the intensity of D and G bands increases and broadening decreases for depositions at 1.0 Hz repetition rate. This increase in bands intensity confirms the reduction in the disordering level of the coatings which may be due to the increase of average substrate surface temperature and stress relaxation for higher repetition rate of 1.0 Hz. This reduction in disordering level may be attributed to the minimization of total free energy of the coatings and consequently results in the crystal growth. Thus, the band growth and broadening of the D and G bands strongly depend on the repetition
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Fig. 4. Typical X-ray diffractograms of the DLC coated SS samples at different angular positions (10◦ and 20◦ ) for 1.0 Hz repetition rate, showing (1 1 1) and (2 0 0) plane orientations of the ␥ phase, and (1 1 0) plane of restructured ␣-Fe phase in AISI 304 SS substrate.
Fig. 3. Variation of ID /IG ratio and G band position in the DLC coating as a function of angular position of the sample for repetition rates of (a) 0.5 Hz and (b) 1.0 Hz.
rate and hence the time-averaged substrate surface temperature rise owing to the successive focus shots. The D band to G band intensity ratio and G band peak positions have been commonly used for the characterization of amorphous DLC content and the induced stresses in the coatings respectively. Such compressive internal stresses in diamond and DLC coatings were related to the microstructure of the coatings [38]. Fig. 3 shows the plots of the band intensity ratio (ID /IG ) and the G band position (cm−1 ) as a function of angular position for two different repetition rates; 0.5 and 1.0 Hz of focus shots. The sp2 -bonded carbon trapped in the sp3 -bonded diamond crystallites or in the grain boundaries causes more lattice strains in the diamond coating. The plot in Fig. 3a shows that with change in angular position from 0◦ to 20◦ for 0.5 Hz repetition rate, the band intensity ratio increases suggesting relatively decreased sp3 content in the coating. Moreover, there is up-shift in the G band position showing the increased vibrational frequency under reduced stresses. The decrease in the number of four-fold coordinated carbon atoms (sp3 ) with the increased value of intensity ratio as well as G band peak position has well been demonstrated by many authors [39,40]. The plot in Fig. 3b shows that with change in angular position from 0◦ to 20◦ for 1.0 Hz repetition rate, the band intensity ratio decreases, that shows increased sp3 content in the coating. It is also supported by down-shift of the G band position owing to reduced vibrational frequency under applied stresses.
The DLC coatings have significant but varied amount of sp2 and sp3 bonding, possible clustering of similarly bonded carbon atoms to provide some short range order, and very small or large amount of hydrogen. The variation of ID /IG at different angular positions for two different repetition rates tracks the sp2 /sp3 content variation (or the DLC character) of the coatings. Thermal desorption of bonded hydrogen takes place during rapid heating of the coating surface. Eventually, hydrogenated amorphous carbon may transform to non-hydrogenated one and vice versa depending upon substrate position. For example for a particular repetition rate, DLC coatings evolve from a-C to a-C:H owing to less efficient thermal desorption of hydrogen at larger angular position exposed to reduced ions flux of DPF. Thermal desorption of DLC coatings in terms of their thermal stability has also been studied by Tallant et al. [41] who showed that DLC coatings are structurally stable up to about 260 ◦ C even in high humidity or boiling water. Thus, 1.0 Hz repetition rate and 10◦ angular position are suitable parameters for the quality DLC coatings. Lower angular position of 10◦ as compared to 20◦ favors hydrogen evolution that has a well-established causative relationship to the observed sp3 to sp2 conversion owing to increased ion fluence having same effect as that of heating. Notably, 0◦ angular position is not favorable owing to deprived surface uniformity, although having significant sp2 content for both repetition rates, and micro-hardness reported in later section. 3.2. XRD analysis Fig. 4 shows the typical XRD pattern of AISI 304 samples deposited with DLC coatings at 10◦ and 20◦ angular positions with respect to the anode axis, for the focus shot repetition rate of 1.0 Hz. For the pristine AISI 304 sample, the diffraction peaks appear at a 2 value of 43.5◦ and 50.4◦ respectively corresponding to (1 1 1) and (2 0 0) plane orientations of the ␥ phase of AISI 304 SS. The XRD spectra obtained clearly show the emergence of ␣-Fe (1 1 0) peak at 44.31◦ owing to stress induced restructuring/phase transformation in SS. Zeb et al. [42] also deposited amorphous DLC coatings on Si (1 0 0) using a low energy (1.45 kJ) dense plasma focus. They
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observed the XRD peak as asymmetric broad humps at 2 ≈ 44◦ and attributed it to the formation of lattice defects in the silicon after ion irradiation. DLC phase appearance is owing to the impact of short pulse (∼100 ns) instability accelerated energetic carbon ions which can be implanted on the substrate surface and also due to the condensation of the high temperature high density carbon plasma, from the dissociation of filling CH4 gas in DPF chamber, on the SS substrate typically lasting for few microseconds. As the deposition is conducted using multiple focus shots fired at repetition rates of 0.5 and 1.0 Hz, the very high flux energetic carbon and hydrogen ions from subsequent shots can cause the melting and subsequent rapid cooling of the carbon coated SS substrate surface. This results in the recrystallization of SS substrate surface along with that of carbon layer deposited up to previous focus shot due to the adsorption of energetic species (mostly carbon ions) onto the surface thus forming new phases [43]. The cooling of surface develops amorphous regions and defects within the bulk [44]. The DPF ion adsorbate-induced reconstruction yields new structures with the minimization of the surface free energy of substrate and the coating. This is also accompanied by the introduction of stresses to the system. The stresses have one major advantage of hardness improvement. The presence of tensile stresses in the DLC coatings is observed by the downshift of the carbon peak position in the XRD spectrum, when compared with the standard stress free data. This indicates a uniform stress developed normal to the corresponding crystal plane in the film during rapid cooling after transient temperature rise by ion irradiation. The tensile stresses in DLC coatings are owing to the incorporation of energetic (up to few hundreds of keV) carbon species interstitially (up to a few hundreds of nm depending on ion energy range) in addition to their presence as bonded C in sp2 and sp3 . Similar effect of N2 ion implantation on AISI 304 has revealed that the initial compression residual stresses were changed to tensile stresses after ion implantation. The residual stress gradient, induced in the implanted surfaces, was found to become more significant with the increase of ion beam fluence. The DPF ions induced stress evolution and transformation from compressive to tensile or vice versa have extensively been reported already [21,45,46]. From Fig. 4, it is clear that the DLC coating at 20◦ angular position has less stresses as compared to that achieved at 10◦ angular position due to low energy flux at 20◦ position. Our previous results on DPF based coatings strongly support this evidence [20,21]. The average crystallite size is calculated from the ␣-Fe (1 1 0) peak width using the Scherrer formula crystallite size =
K , ˇ cos
where K = 0.99 is the numerical constant, = 1.540598 A˚ is the wavelength of X-ray source, ˇ is the broadening (in radians) of the diffraction peak and is the Bragg’s angle. The average crystallite size of ␣-Fe in restructured SS 304 varies from ∼40 to 50 nm. 3.3. SEM/EDS Scanning electron microscopy (SEM) and energy dispersive Xray spectroscopy (EDS) were carried out for the morphological and elemental analysis of the coatings. Fig. 5 shows the SEM micrograph, EDS map, SEM dark image taken at working distance of 15 mm showing area selection for EDS map and spectrum acquisition, dark image of EDS map and EDS spectrum recorded for a typical sample with DLC coating achieved on 10◦ angular position for 1.0 Hz repetition rate. The coating appears golden in color when viewed visually. Such type of color appearance reports the amorphous, polymeric nature of the DLC coatings. From the SEM
micrograph taken at magnification of ×40,000 (Fig. 5a), the patchy or layered profile of the DLC coatings can be clearly identified. Since the deposition is done in multiple shots, there might be multilayers composed of nanoparticles. The layered regions are probably coagulated zones. The grain size of ∼40–60 nm with some size dispersion is observed on the DLC coating deposited at 10◦ angular position. Such granular surfaces with significant thermal coagulation effects have also been early reported in our previous work [24]. These coagulated regions are developed due to the high energy flux of instability accelerated ion beams emitted from the NX2 operated at high repetition rate usually at lower angular position of the substrate. This can be elaborated under the theory of charged clusters (TCCs) [47] and the thermal coagulation phenomenon. The DPF ions and/or radicals, having multiple charged states [15], nucleate in the gas phase to form charged nucleates, which are then deposited on the substrate to form coating. A large amount of energy transferred to the substrate via electronic excitations develops defects and consequently nanostructures of nm scale nucleates. The nucleates can grow either by reactions with reactive species in the gas phase or combining with very small uncharged clusters. These nucleates are grown by the ion-induced collision cascades and extend to a range of up to a few tens of nm for ion energies of several tens of keV to MeV [48]. The DLC coatings obtained at 20◦ angular position have almost identical morphology, whereas those at 0◦ have slight damage owing to the high energy ions. The energetic impacts from highly ionized hot dense CH4 plasma leads to the formation of sp3 -bonded carbon in a sp3 -bonded substrate surface matrix. The surface structure may have a hydrogen and sp3 hybridized carbon enriched surface. During multiple shots irradiation, the coatings developed with previous shots are hydrogenated by exposing them to the ionic and/or atomic hydrogen beams, leading to the transformation of sp2 hybridized carbon groups to sp3 hybridized groups at the coating surface. This depends on the H/C ratio in CH4 plasma at any angular position. Since carbon content is high at lower angular position (0◦ and 10◦ ), sp2 -enriched dominates at these positions. The implanted ions displace predominantly bonded hydrogen owing to lower threshold energy for hydrogen, compared to that of carbon [49]. The displaced hydrogen atoms can recombine with a lattice defect or form H2 molecules by atomic recombination. The H2 molecules are either trapped in internal voids or diffuse to the surface and desorb. This leads to the phase transition from polymer-like structure to the diamond-like structure with low hydrogen content and an increase in the coating density. Thus, the ion-induced compression of polymer-like sp3 bonded structure to a denser sp2 -enriched phase is resulted from strong thermal spikes during DPF ion irradiation. The EDS map of the coating exhibit the distribution of elements present in the coating using the imaging energy filters. By selecting the area of the SEM image for mapping and spectrum acquisition (Fig. 5c), the elemental distribution of the elements surface coating can be obtained. In mapping, an image of the sample will be made that is simply the intensity of a particular emission mapped in an XY raster. The intensity of the particular X-ray lines in the maps is qualitative measure of elemental distribution. Fig. 5d shows the SEM dark image used to collect the EDS map on the same scale. Fig. 5b shows the CK␣ map presenting the elemental distribution of carbon on the DLC coating surface. The intensity of the CK␣ in the maps is a qualitative measure of the elemental concentration. A homogeneous spatial distribution of the CK␣ emission indicates the spatially uniform carbon over the DLC coated SS sample surface macroscopically. Fig. 5e shows the EDS spectrum of the above mentioned coating. The incorporation of C content is evident in the spectrum, along with the presence of SS composition. The carbon content in pristine AISI 304 steel is 0.08 wt.%, which is much less than the values obtained for DLC coated SS. The EDS spectra demonstrate
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Fig. 5. (a) Typical SEM micrograph of the DLC coatings with coagulated regions due to ions assisted thermal effects, (b) CK␣ EDS map, (c) SEM dark image to select area for EDS map and spectrum acquisition, (d) dark image of the EDS map and (e) EDS spectrum of the DLC coatings deposited at the angular position of 10◦ for 1.0 Hz repetition rate.
the increase in carbon concentration depending on the angular position. For the DLC coatings obtained for 0.5 Hz repetition rate, the concentration of carbon varies from 10.1 at.% to 5.2 at.% (3.31 ± 1.22 wt.% to 2.12 ± 0.88 wt.%) when the sample position is shifted from 0◦ to 20◦ with respect to anode axis. Whereas, for the DLC coatings obtained for 1.0 Hz repetition rate, the carbon concentration is reduced from 14.21 at.% to 7.82 at.% (4.31 ± 1.22 wt.% to 2.81 ± 0.79 wt.%) when the sample position is shifted from 0◦ to 20◦ . The carbon concentration data has been provided in Table 2. It has been found that the C content is significantly reduced at higher angular position. The reduction in carbon content at the 20◦ angular position is due to the reduced ion fluence impinging at this position of the substrate. Moreover, the C content is found to reduce with the repetition rate reduction also, showing deprived quality of the DLC coatings due to lesser temperature assistance.
Thus, appearance of carbon content in sp2 -enriched phase shows that the 10◦ angular position and 1.0 Hz repetition rate provide the optimum deposition conditions in the present experiment. 3.4. Micro hardness measurements Fig. 6 shows the surface hardness (GPa) of DLC coatings as a function of the indentation depth (m) with different test loads on the sample surfaces treated at various angular positions for 1.0 Hz repetition rate. The maximum surface hardness is found for DLC coatings at 0◦ angular position. The typical micro-hardness of the DLC coatings is around 3 GPa. However, in the near surface region, it approaches to 6 GPa. It is noted that the relatively lower values of microhardness are obtained at higher angular position in spite of higher sp3 content in DLC coatings. This fact may be attributed to
Table 2 EDS data showing the carbon concentrations (at.% and wt.%) in DLC coated SS surfaces at different sample positions for two different repetition rates. Sample identification
C content (at.%) C content (wt.%)
Repetition rate 0.5 Hz
Repetition rate 1.0 Hz
0◦ position
10◦ position
20◦ position
0◦ position
10◦ position
20◦ position
10.1 3.31 ± 1.22
8.62 3.04 ± 0.92
5.2 2.12 ± 0.88
14.21 4.31 ± 1.22
11.86 3.78 ± 1.02
7.82 2.81 ± 0.79
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owing to stress induced disorder in the SS lattice, having crystallite size estimated to be ∼40–50 nm. The repetitive ion bombardment has played an important role in surface transient temperature rise and eventual less strained DLC coating in comparison with low repetition rate plasma focus ions bombardment. The surface microhardness of the coatings achieved at 10◦ angular position has been increased three times for repetition rate of 1.0 Hz.
Acknowledgements One of the authors (Dr. M. Hassan) is grateful to the NESCOM for providing financial support to carry out experimentation and materials analysis at Nanyang Technological University (NTU), Singapore. One of them, S. Mahmood would like to thank NIE, NTU for providing the research scholarship.
References
Fig. 6. Variation of surface microhardness of the SS samples coated with DLC at different angular positions for a repetition rate of 1.0 Hz. The maximum surface hardness is shown by the maximum stressed DLC coatings at 0◦ angular position.
the polymeric nature of the coatings owing to reduced ion fluence with lower charge states. An approximately three fold increase in the hardness value is evaluated for the typical DLC coated surface coatings at 0◦ angular position. The descending microhardness in the near surface region of the SS sample reports the implantation limit of carbon ions up to ∼2 m. Below this implanted zone, ion-induced stress hardening results due to quenching of surfaces under fast transient temperature rise by high repetition rate deposition. This eventually develops stressed regions not only due to temperature gradients but also due to concentration gradients, that positively contribute to the microhardness improvement up to a few m depth in the surfaces of DLC coated SS samples. Many researchers have reported such type of stress induced hardening by the collision cascades of energetic ions [45,49] and the incorporation of the carbon ions in the coatings by successive bombardment [7,12,20]. 4. Conclusions Raman spectroscopy results explicitly confirm the deposition of a-C:H/DLC coatings on the SS substrates. The occurrence of G band indicates the formation of graphitic sp2 microdomains while the D band is assigned to the bond angle disorder in the graphitelike microdomains, induced by linking with sp3 carbon atoms and finite crystallite size of sp2 microdomains. It has been found that the growth and the broadening of the bands is strongly influenced by the substrate angular position and hence the ions energy fluence. For a typical higher repetition rate of 1.0 Hz, carbon content is high as compared to deposition for 0.5 Hz repetition rate. However, the band intensity ratio decreases for higher angular position showing an increased sp3 (polymeric) content in the DLC coatings. It is accompanied by the down-shift of the G band position owing to reduced vibrational frequency under ion induced stresses. Thus, 10◦ angular position and 1.0 Hz repetition rate provide the optimum conditions for the present experiment regarding DLC quality. The DLC coatings transform from a-C to a-C:H when the sample position is shifted to higher angular position owing to less efficient thermal desorption of hydrogen bonded with carbon. XRD results explicitly evident the emergence of ␣-Fe (1 1 0) diffraction peak
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