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Coatings 2013, 3(4), 243-252; doi:10.3390/coatings3040243
Abstract: The hardness, heat conductivity and low friction coefficient of microcrystalline diamond make it a suitable candidate for tribological applications. However, its roughness and high deposition temperature pose significant obstacles to these applications. We have successfully grown nanocrystalline diamond on steel at 400 °C by hot-filament chemical vapor deposition by employing a CrN interfacial layer. Nanocrystalline diamond combines hardness and surface smoothness required in tribological applications. Microcrystalline diamond and carbon nanotubes can also be grown by controlling the deposition parameters. The fabricated films were characterized with Raman spectroscopy, X-ray diffraction (XRD), Transmission electron microscopy (TEM), and scanning electron microscopy (SEM).
The superior material properties of synthetic diamond, such as mechanical hardness, high thermal conductivity, high electrical resistivity and broad optical transparency, make it an excellent candidate for diverse coating applications. However, the high diffusion of carbon in iron leads to the formation of black carbon (graphite soot) on steel substrates and prevents the deposition of microcrystalline diamond on them . The use of silicon interlayer for the growth of adhered diamond films on steel substrates was reported [2,3]. Buijnsters et al.  fabricated high quality and well adhered diamond films on steel substrates using a thin interlayer of Si. Cubic silicon carbide (3C-SiC) was also used as an interlayer to produce well-facetted diamond films with good crystallinity [5,6] but the film delaminated upon post-deposition cooling to room temperature. Ralchenko et al.  reported the growth of diamond using a tungsten layer, but this films presented high compressive stress.
Ti-based ceramics such as Ti(C,N), (Ti,Al) were also employed [8,9,10] for growing diamond films on steel substrates. The films synthesized on steel substrates using Ti and TiC showed promising results [11,12,13,14]. Lin and Kuo  reported high adhesion and produced good-quality diamond films on steel using electroplated Ni interlayer. Borges et al.  also reported the growth of diamond films on stainless steel 304 substrates with high Cr content (18 wt %). They showed that by nitriding the substrates, the amount of CrN increased on its surface and, during the initial stages of diamond deposition, chromium carbide formed at the substrate due to its carburization facilitating the diamond grown.
Chromium and chromium composites are widely used as interlayers for producing adherent diamond coating on steel [13,17,18,19]. The use of CrN as an interlayer has been extensively studied [8,20,21,22,23,24,25,26,27]. Some authors have also used multilayer systems, such as Ni/Cu/Ti , Ni/diamond embedded crystallites/Cu  and TiC/Ti(C,N)/TiN  to improve diamond adhesion on steel. The Ni/Cu/Ti interlayer system  enabled the growth of good quality diamond films, with good adhesion being confirmed by a 200 N load indentation using a Rockwell indenter. The multilayer systems consisting TiC/Ti(C,N)/TiN  were successful in initiating diamond nucleation and diamond crystal growth.
Table 1 presents a summary of the interlayers and interlayer systems used for diamond deposition on steels. The table also provides information on the diamond film quality, this meaning an average classification of the diamond film uniformity, Raman quality, crystallinity, film stress and adhesion of the film to the substrate. Globally, CrN seems to act as an excellent buffer layer. This also solves the C intake by iron that prevents diamond growth on its surface. Taking advantage of CrN as a buffer layer, we hereby report the synthesis of microcrystalline diamond (MCD), nanocrystalline diamond (NCD), and carbon nanotubes (CNTs) on steel at 400 °C by hot-filament chemical vapor deposition (HFCVD).
2. Experimental Section
The films were grown on 25 mm2 Cr2N-coated steel substrates in the HFCVD reactor that was previously described in detail . The steel samples (AISI P20 modified; composition (wt %): C 0.37; Si 0.30; Mn 1.40; Cr 2.00; Mo 0.20; Ni 1.00; S ≤ 0.010) were pre-coated with a commercial grade sputtered chromium nitride ~2 μm thick film. Prior to the diamond deposition, the substrates were seeded with diamond powder < 0.1 µm. After seeding, the substrates were ultrasonically cleaned in methanol for 20 min, then dried in air at room temperature and placed on a Mo substrate holder integrated with a pyrolytic graphite heater. The chamber was evacuated to 8 × 10−4 Pa (6 × 10−6 Torr) before admitting the reactive gas mixture consisting of 2% CH4 and 98% H2 with 500 ppm H2S. The total gas pressure was kept at 2700 Pa (20 Torr) during growth. The temperature of the rhenium filament was kept at 2500 °C and it was positioned at 8–9 mm above the substrates. The substrate temperature was kept at 400–500 °C  and the films were grown for 2–4 h (see Table 2).
|Material Grown||Substrate Temperature||Deposition Time|
|MCD||400 °C||2 h|
|NCD||400 °C||4 h|
|CNTs||500 °C||2 h|
The samples were characterized by Scanning Electron Microscopy (SEM Model 35 CF microscope, JEOL, Tokyo, Japan), TEM (LEO 922 OMEGA microscope operating at an accelerating voltage of 200 kV, Carl Zeiss, Jena, Germany), and Raman spectroscopy (ISA J Y Model T64000, HORIBA, Kyoto, Japan) using the 514.5 nm line of Ar-ion laser. The crystal structure was determined by θ–2θ scans obtained by the X-ray powder diffractometry method (Siemens D5000 diffractometer, Cu Kα ~1.5405 Å, Siemens, Munich, Germany).
3. Results and Discussion
Raman spectroscopy was used to characterize the carbon films synthesized on steel substrates using CrN as buffer layer (see Figure 1). The sharp peak at around 1333 cm−1 in Figure 1a corresponds to the stretching vibration of the tetrahedrally (sp3) bonded diamond lattice that is a conclusive characteristic of MCD structure . In Figure 1b, the bands at 1336 cm−1 and 1586 cm−1 are characteristic of sp3 hybridized carbon (diamond) and sp2 hybridized carbon (graphite), respectively. The band at ~1360 cm−1 corresponds to disorder-induced in sp2 C and is known as D-band, while the bands at 1136 and 1477 cm−1 together indicate the presence of transpolyacetylene (TPA) . These Raman features in Figure 1b indicate the fabrication of NCD films. Hence, MCD and NCD films were obtained on the CrN-covered steel substrates at 400 °C, depending on the deposition time. However, when the substrate temperature is increased to 500 °C, the Raman signatures change significantly as shown in Figure 1c. There is no indication of sp3 C, the TPA bands disappear, well-defined D and G bands are present, and new low-frequency bands appear (Figure 1c). The bands in the 100–400 cm−1 range are the radical breathing modes (RBM) of single-wall CNTs [32,33]. The diameter of an isolated single wall CNT is inversely proportional to the RBM frequency (ω), d(nm) = A/ω(cm−1), where d = diameter of single-wall CNTs, A is the proportionality constant equal to 223.75 .Using this equation, the diameters of CNTs were calculated to be 1.0 nm for the RBM peak at 222.8 cm−1, 0.8 nm for the RBM peak at 283 cm−1, and 0.6 nm for the RBM peak at 396.9 cm−1. Moreover, the ratio of intensity of the G-band to the D-band (IG/ID) gives an indication of the quality of the CNTs. The higher the intensity ratio is, the better quality nanotubes film . In the present case, IG/ID > 1, revealing that the fabricated CNTs are not highly disordered CNTs . Hence, the Raman spectrum shows that single-wall CNTs were fabricated on CrN/steel at a substrate temperature of 500 °C.
SEM images (Figure 2) reveal the evolution of the carbon films’ microstructure as a function of temperature and time. Figure 2a,b clearly shows the grainy structure of the MCD films, with crystallite sizes around 0.5–1.0 μm. The NCD films (Figure 2c,d) are composed of micro scale size clusters, which in turn are made of nanocrystallites and nanofibers. In the case of CNT films, Figure 2e,f, the SEM images show uniformly coated spaghetti-like carbon nanotubes films. The inset in Figure 2f shows the TEM image of a single-wall CNT. Multi- and single-wall CNTs, large and small, co-exist in these CNT films.
XRD diffractograms (Figure 3) help to elucidate the material transformations that take place in the substrate surface that lead the formation of CNTs. The XRD of the original Cr2N/steel substrate shows the Cr2N (101), Cr2N (110) and Fe (110) peaks (Figure 3a), as identified using standard JCPDS data (Joint Committee on Powder Diffraction Standards). Under CVD conditions at 400 °C for 2 h, the synthesis of MCD (peak ~44°) takes place and the Cr2N is partially transformed into CrN (Figure 3b) . After 4 h at 400 °C, an NCD layer covers the MCD, and Cr2N completely disappears into CrN (Figure 3c). At higher temperatures (500 °C) there is enough thermal energy for the C containing species to react with CrN to yield defect-C3N4, C6N2, and free Cr (Figure 3c) [25,36,37]. As a result, Cr nanoparticles become available at the substrate surface to act as catalysts for the synthesis of CNTs .
From the above discussion, it is concluded that the Cr2N buffer layer is useful for the synthesis of micro- and nano-crystalline diamond films at temperatures around 400 °C, but it yields catalyst particles for the synthesis of CNTs at higher temperatures. In previous works [39,40,41,42], polycrystalline diamond was deposited at higher temperatures on the same substrate interlayer system, although not using H2S. These samples were typically deposited at a surface temperature of about 750 °C which led to high compressive stress of the grown films, but no CNTs were detected. In the present case, H2S leads to the formation of C-S radicals in the heat-activated volume around the filament, which turns back into H2S in the cooler region above the substrate, while leaving C atoms deposited on the substrate surface . This process results in additional C-transport mechanism to the substrate that enhances the deposition of C atoms. Moreover, at the growing surface, sulfur acts as a cross-linker that promotes the formation of graphitic bonds at the growing surface before it is released back to the vapor phase .
It has been shown that it is possible to grow a nanocrystalline diamond coating on steel at 400 °C by HFCVD using Cr2N as buffer layer and trace amounts of H2S. Microscrystalline diamond and carbon nanotubes can also be grown by controlling the deposition parameters. However, nanocrystalline diamond provides a smooth and hard surface unlike microcrystalline diamond or carbon nanotubes. The substrate temperature and deposition time are the critical parameters to control the type of carbon material that is grown. This process can enable the fabrication of nanodiamond-coated tools and other tribological applications.
This research was carried out under the auspices of the Institute for Functional Nanomaterials (NSF Cooperative Agreement 1002410), GK-12 (NSF Grant 0841338), PR NASA EPSCoR (NASA Cooperative Agreement NNX13AB22A). V.F. Neto acknowledges the “Fundação para a Ciencia e Tecnologia” (Portugal) for his post-doctoral research grant (SFRH/BPD/44978/2008). We also want to acknowledge the help of William Pérez (Raman Spectroscopy), Javier Palomino Garate (Transmission Electron Microscopy), and Josué Ortiz (Scanning Electron Microscopy).
Conflicts of Interest
The authors declare no conflict of interest.
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