Abstract
In this study, thin films of aluminium nitride (AlN) are deposited on Nimonic 75 substrates using a 4 kJ Mather type plasma focus device (PFD) for 5, 10 and 15 focus shots. For the deposition of AlN films, a solid aluminium fitted anode was used instead of a copper anode. The PFD was operated with nitrogen gas at a pressure of 2 torr. X-ray diffraction results reveal the formation of a nanocrystalline AlN coating on the surface of the substrate. The crystallite size is dependent on the number of focus shots. The density of grains increased with an increase in the number of focus deposition shots, as illustrated by field emission scanning electron microscopy (FESEM). The FESEM images confirm the distribution of spherical grains for 15 focus shots. Energy dispersive X-ray spectroscopy spectra indicate the presence of expected constituent elements such as N and Al.
Introduction
The AlN is extensively used for industrial applications such as electronic substrate, a heat sink, electronic components, material for crucibles and vessels handling corrosive chemicals and molten metals, semiconductor equipment and reaction vessels for etching 9 due to its excellent corrosion resistance, 10 high dielectric resistivity(10 13 Ωcm), high electrical insulation, high hardness (Hv1400), 9 high thermal conductivity (up to 320 W mK−1), small thermal expansion coefficient, 11 high mechanical strength 12 and resistance against oxidation. 13 AlN can also be used as a coating on cutting tools, heating elements, anticorrosive coating, 14 hard coatings15,16 and high frequency surface acoustic wave devices. 17 Therefore, it is reasonable to develop AlN coating. Many deposition techniques, including dual ion beam sputtering, 17 reactive magnetron sputtering, 18 pulsed direct current reactive unbalanced magnetron sputtering 19 and low pressure chemical vapour deposition, 14 are employed to deposit AlN thin films.
The focused plasma phenomenon in a plasma focus device (PFD) occurs at the open end of coaxial electrodes when an electric pulse is applied across the electrodes by discharging a low inductance and high voltage capacitor. Because of the Lorentz force, the plasma sheath is accelerated along the coaxial electrodes. After the plasma sheath reaches the open end of the electrodes, the plasma is focused in a small cylindrical plasma column. Disruption of plasma column during radial compression induces an electric field that accelerates the ions and electrons in the opposite direction with very high velocities (v≥107 cm s−1). In this device, electrical energy upon discharge is converted into plasma energy, resulting in the formation of a short lived plasma (∼100 ns) but hot (1–2 keV) and dense (∼10 25 –1026 m−3) plasma column.16,22 Recently, high density plasma sources like PFDs have been used for the implantation of nitrogen, 24 titanium nitride, 25 nitriding of aluminium, 26 and tungsten nitride thin films. 27 As shown in Fig. 1, when a specimen is subjected to these ions, they penetrate to a depth of the surface layer well beyond the ion energy range from a few kiloelectron volts to a few megaelectron volts.20,21 Among deposition methods, the PFD technique offers several benefits. In the method, the reactants are cheap gases, the gas consumption is significantly small, and substrate heating is not required during the deposition process. Deposition time is very small as compared to direct current pulse or radio frequency glow discharge. It is operated under pressure conditions that can easily be maintained. 22 The use of PFD for coating shows excellent results in improving the hardness, 28 wear resistance, 29 elastic modulus 30 and corrosion performance. 22 The mechanism of the nanolayer coating by PFD is as follows: The disruption of the focused plasma due to sausage instability induces an electric field that disintegrates plasma to powerful ion and electron beams in the opposite direction along the z axis of the PFD. As shown in Fig. 1, the top of the anode bar is modified in such a manner that the aluminium disc fits on the anode tip. Therefore, high energy electrons interacting with the anode tip and ablated aluminium ions along with nitrogen ions move upwards and get deposited on the Nimonic alloy specimen as AlN nanoparticles. On the Nimonic alloy surface, the ions undergo a diffusion process due to the surface mobility they have owing to their kinetic energy to cluster together to form nanoparticles. The formation of nanoparticles is also possible during the gas phase dynamics as the ablated material moves from the anode tip to the specimen.
Schematic diagram of PFD
Afrashteh and Habibi 28 investigated nitrogen ion irradiation on aluminium targets by a 4 kJ PFD. They demonstrated that the hardness of the treated samples was improved approximately three times. Sadiq et al. 26 used a 1·8 kJ PFD for ion implantation of nitrogen on aluminium substrates. It was reported that the surface hardness of the samples was improved by 300%. Guo et al. 31 stated that the surface roughness of the AlN films improved with increasing substrate temperature and nitrogen concentration. Omrani et al. 22 evaluated the energy and density of nitrogen ions in the 4 kJ PFD (12·5 kV, operated at P = 3 mbar of nitrogen gas). They demonstrated that the energy and maximum density of nitrogen ions are 64 keV and 5·9×1013 cm−3 respectively.
The present work reports the results of AlN ion implantation into the Nimonic 75 samples using a 4 kJ PFD operated with nitrogen gas under various shots. Thin film characterisation techniques like X-ray diffractometer (XRD) and field emission scanning electron microscopy (FESEM) attached with energy dispersive X-ray spectroscopy (EDX) spectroscopy are employed.
Experimental
The experiments are performed in Amirkabir plasma focus facility, which is powered by a 40 μF capacitor bank, operating at 15 kV. 32 The measured total external inductance is 115 nH. The anode made of copper is 148 mm long and 20 mm thick. The outer electrode consists of six copper rods with an inner diameter of 44 mm. 32 Deposition of AlN film, aluminium fitted anode was used instead of the copper anode, as shown in Fig. 1. Samples of Nimonic 75 are in square shaped (size 20×20×1 mm3). Nimonic samples were treated with no. 200, 400, 600, 800, 1200, 1000 and 2000 emery papers and then polished in order to have a mirror surface and cleaned with acetone before ion implantation.
An axial sample holder is fitted with the top plate of the chamber to support the substrate samples. Then, the samples are cleaned with acetone before entering into the PFD vacuum chamber. The samples are mounted axially above the anode with a distance of 5 cm. This is obtained experimentally as an optimum distance for the device. 27 The chamber of the PFD is evacuated up to 6×10−2 torr pressure and was filled with high purity nitrogen gas. Optimum pressure with nitrogen at the applied voltage of 12 kV was determined as 2 torr. After every three shots, the vacuum chamber is evacuated in order to have pure gas, and then it is filled with nitrogen at the optimum pressure. The relativistic electrons emanated from the focus region cause ablation of aluminium placed at the anode tip as insert,21,23,33 as shown in Fig. 1. Synthesis of the AlN composite film and the Nimonic 75 substrate using PFD consists of four key processes: (i) direct implantation of energetic nitrogen ions into the Nimonic disc (these energetic ions may be a rapid increase in the temperature of the Nimonic 75 substrate surface, causing its local melting and evaporation at the surface), (ii) reaction of background nitrogen ions with evaporated Nimonic 75, (iii) reaction of nitrogen ions with ablated aluminium ions 20 and (iv) reaction of ablated aluminium with Nimonic atoms. 21
Results and discussions
Crystalline structure
Figure 2 shows the XRD patterns of high textured AlN films deposited for multiple (5, 10 and 15) focus shots. This analysis has been performed in the range of 20–90° using Cu Kα (λ = 1·5406 Å) radiation for this purpose. The XRD patterns exhibit the diffraction peaks related to different compounds such as AlN, AlNi3 and Cr2N, confirming the deposition of (AlN) film on the Nimonic substrate.
X-ray diffraction patterns of untreated and treated Nimonic 75
The XRD pattern of virgin material shows peaks at 44·06, 51·28 and 75·56°. This is due to Ni2. The diffraction peaks corresponding to AlN (200) and AlNi3 nanocrystalline planes are observed on all coated samples. The peak intensity of (002) plane reflection of Cr2N is observed for 15 focus shots, and the Cr2N (111) diffraction peak is observed on 10 and 15 shot samples. The intensity of the AlN (2 0 0) peak for the 15 shot sample at 43·88° is much higher than that of the other samples. The film deposited with five focus shots exhibit the emergence of AlN (2 0 0) at 2θ = 44·01° (refer to Joint Committee on Powder Diffraction Standards [JCPDS] card no. 00-025-1495) and AlNi3 (220) at 2θ = 75·41° (refer to JCPDS card no. 00-050-1265). For the ten shot sample, Cr2N (111) and Ni3N (002) at 2θ = 42·52, 34 AlN (2 0 0) at 2θ = 43·99° (refer to JCPDS card no. 00-025-1495) and AlNi3 (220) at 2θ = 75·32° (refer to JCPDS card no. 00-050-1265) peaks are illustrated. The XRD pattern for the film using 15 focus shots shows the diffraction peaks corresponding to Cr2N (002) at 2θ = 40·91, Cr2N (111) and Ni3N (002) at 2θ = 42·55, 34 AlN (2 0 0) at 2θ = 43·88° (refer to JCPDS card no. 00-025-1495) and AlNi3 (2 2 0) at 2θ = 75·31° (JCPDS card no. 00-050-1265) .
The crystallite size of the AlN (200) plane is estimated from XRD data using Scherers'formula
35
Figure 3 shows the variation in crystallite size of AlN (200) peak by increasing the number of focus shots at 2θ value of 44°. When the focus shots were increased from 5 to 15 shots, the AlN mean size was decreased from 46 to 29 nm. The smallest grain size was obtained for 15 focus shots.
Crystallite size of AlN (200) phase
The decrease in crystallite size is probably due to the recrystallisation or incorporation of ions and sputtering. The rise in ion beam energy and consequently the temperature of the sublayer by increasing the number of focus shots may be attributed with the recrystallisation of this phase.21,23
Scanning electron microscopy
The effect of increasing focus shots on the surface morphology AlN films deposited on Nimonic 75 has been investigated using FESEM. Figure 4 shows the FESEM microstructure of AlN films deposited for multiple (5, 10 and 15) focus shots and the unexposed Nimonic 75 substrate at 30 000 magnification. All the samples are placed at 5 cm in front of the central anode. Some fine scratches induced with manual polishing marks are observable in Fig. 4a for a pristine sample. The surface morphology of the film deposited for five focus shots. Figure 4b shows the presence of grains with dimensions of about 50–120 nm. The particles in the five shot sample do not cover the whole surface area, which shows less dense in the top surface layer. Figure 4c shows the rounded grain surface morphology of the film deposited for ten focus shots. For the sample exposed to ten focus shots, the average size of the grains is in the range of 50–100 nm. Figure 4d shows the formation of rounded grains of different sizes ranging from 50 to 80 nm, which are distributed over the entire surface. Figure 4d shows a dense granular surface morphology with identical distribution of different grains. The sample deposited with 15 focus shots is more compact than the 10 shot sample.
a bare Nimonic 75; b 5 shots; c 10 shots; d 15 shots
The agglomeration process arises upon the increase in the number of deposition shots, which can be attributed to more materials being ablated with an increasing number of shots. Moreover, the substrate temperature is associated with the ion dose; as the substrate temperature increases with ion dose increases, resulting in the growth of particles in the form of rounded grains as well as agglomerate. 21 It is concluded that after the formation of nanoparticles, they are deposited on the substrate surface in a fairly uniform manner. The results of SEM show that Nimonic alloy specimens (Area 20×20×1 mm3) are completely restructured with appearance of AlN nanosize structures.
Energy dispersive X-ray spectroscopy
The EDX spectra of the treated Nimonic 75 samples are shown in Fig. 5. The EDX spectra of the deposited films show the elements that are supposed to be present. This provides quantitative evidence for the presence of different elements such as N, Al, Ni, Cr and Fe. The spectra of different deposited film samples are similar in the sense that similar peaks can be identified in them with variation in peak intensities. The relative variation of the intensities of peaks corresponding to elements present in the film is due to the different thicknesses of films. The EDX results have confirmed the presence of nitrogen and aluminium in the film. The results obtained from the EDX analysis are tabulated in the form of Table 1.The nitrogen content in the film increases by increasing the number of focus deposition shots along the anode axis as the ion energy flux increases.
Energy dispersive X-ray spectroscopy spectra of a 5 shots, b 10 shots and c 15 shots
Energy dispersive X-ray spectroscopy elemental analysis of treated Nimonic surface by 5, 10 and 15 shots
Conclusion
In this investigation, deposition of AlN nanoparticles on the Nimonic substrate was accomplished using energetic nitrogen ions emitted during the radial collapse, and the aluminium ions ablated from the inserted material placed at the anode tip of the PFD was studied. Nimonic samples placed at 5 cm in front of the central anode were exposed to different numbers of focus shots (5, 10 and 15). The SEM images confirm the formation of a coating on the surface. According to the results of XRD, the growth of different compounds (AlN, AlNi3, Ni3N and Cr2N) confirm the formation of AlN film. The crystallite size of AlN is found to be 46, 33 and 29 nm for 5, 10 and 15 focus shots respectively. The presence of nitrogen and aluminium in the films is confirmed by the EDX results. As seen, the results show a compact and uniform surface morphology for 15 focus shots. Thus, PFDs are useful devices for the implantation of AlN nanoparticle on Nimonic 75.