Flame synthesis of carbon nanostructures on Ni-plated hardmetal substrates
© Zhu et al; licensee Springer. 2011
Received: 15 December 2010
Accepted: 13 April 2011
Published: 13 April 2011
In this article, we demonstrate that carbon nanostructures could be synthesized on the Ni-plated YG6 (WC-6 wt% Co) hardmetal substrate by a simple ethanol diffusion flame method. The morphologies and microstructures of the Ni-plated layer and the carbon nanostructures were examined by various techniques including scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The growth mechanism of such carbon nanostructures is discussed. This work may provide a strategy to improve the performance of hardmetal products and thus to widen their potential applications.
Hardmetals are widely used for cutting tools and as wear resistant components . In order to improve the performance and the durability of the hardmetals, the application of coating is necessary. Carbon nanostructures such as nanocrystalline diamond, carbon nanofibers (CNFs), and carbon nanotubes (CNTs) are considered as the ideal coating or reinforcing candidates for the hardmetals due to their extraordinary mechanical, chemical, thermal, and electronic properties [2–4]. However, a poor adhesion of the coating on the hardmetal substrate is still the main limitation to the wide applications.
Many efforts have been made to improve the adhesion between the coating and the hardmetal substrate, the introduction of an interlayer was demonstrated as one of the most effective approaches to achieve this [5, 6]. Here, we chose metal Ni as an interlayer considering its several inherent advantages [7–10]: (1) the linear expansion coefficient of Ni is very close to that of the hardmetal substrate; (2) Ni possesses favorable wettability with carbon nanostructures and thus catalyzes their nucleation and growth; and (3) Ni is hardly influenced by the temperature in combustion flame due to its outstanding heat resistance.
In comparison to other traditional methods, flames can naturally provide a source of both reactive hydrocarbon gas and elevated temperatures for large-scale synthesis of carbon nanostructures at higher energy utilization rate and at lower cost [7–13]. For example, ethanol diffusion flame has been reported to synthesize CNTs and CNFs on carbon steels, low alloy steels, Ni-containing metals such as type 304 and YUS701 austenitic stainless steels, and pure copper [8, 12, 13].
In this work, we demonstrate that carbon nanostructures could be successfully deposited on the Ni-plated YG6 hardmetal substrates by a simple ethanol diffusion flame method. The characterization of the as-prepared carbon nanostructures was carried out using scanning electron microscope (SEM), X-ray diffraction (XRD), and Raman spectroscopy. The effects of flame-deposition time and the metal catalyst on the growth of these carbon nanostructures were also investigated.
The commercial YG6 hardmetal cutting tool inserts (WC-6 wt% Co, made by Zhuzhou cemented carbide corporation, Zhuzhou China style C116) were used as substrates. The electroplating bath composition includes nickel sulfate (NiSO4) 250 to 300 g/l, nickel chloride (NiCl2) 50 to 60 g/l, boric acid (H3BO3) 40 to 50 g/l, and some additives. The pH value of the electrolyte varied between 3.8 and 4.5, and the average J k (i.e., cathodic current density) was maintained at 2 A/dm2.
An Ni layer of approximately 20 μm in thickness was electro-deposited on the hardmetal YG6 surface, and the electroplating process was performed in the following sequence: mechanical grinding (on the diamond discs of 240# → 400# → 600# → 800#, respectively) → chemical deoiling and degreasing → electro cleaning → acid pickling → alkali cleaning → nickel plating.
The electroplating process was carried out for 40 min at temperature of 50 to 65°C.
Various techniques were used to characterize the carbon nanostructures grown on the surface of the hard metal. The particle characteristics (shape, size, and distribution) were examined by a Philip-XL30 FEG SEM. The composition and bonding information of the coating layer were obtained by XRD, recorded using a Rigaku D/max-IIIA (30 kV, 30 mA, Cu Kα) at a scanning rate of 0.5°/min in the 2θ range of 25 to 125°. Raman spectroscopy (Renishaw 2000, Ar laser wavelength 514 nm, 20 mW) was utilized to identify and analyze the microstructures of the coating.
Results and discussion
The thickness of the interlayer is very crucial to achieve a better adhesion between the coating layer and the substrate. If the interlayer is too thin, cobalt (Co) element contained in the substrate could re-diffuse from inside to outside at high flame temperature, thus reducing the effect of the interlayer. If the interlayer is too thick, the performance of the hardmetal tools could be degraded since the Ni interlayer possesses lower hardness and heat resistance than the hardmetal substrate. Therefore, the thickness of the interlayer is generally set ranging from several to several tens micrometers [5, 6].
where δ is the thickness of electro-plated layer (μm); C the electrochemical equivalent (g Ah-1), 1.095 for Ni; t the electrodeposition time; η k the current efficiency, here 85%; ρ the density of metal-plated layer (g cm-3), 8.908 for Ni. Therefore, the thickness of the Ni-plated layer is amounted to 20 μm.
When the time was extended to 60 min, the flame-deposited products display tube/wire-like morphology in both the center zone and the marginal zone (Figure 4c, d). In this study, it was found that the yield of the nanofibrous carbon materials increased with extending deposition time. According to Choi et al. , the carbon atoms existed in form of both CNTs and carbon nanoparticles in the initial stage, and the hydrogen atoms produced during incomplete combustion of ethanol began to etch the flame-deposited material after the generation of carbon nanoparticles. Consequently, CNTs gradually dominated the product as time progressing due to the higher stability of CNTs than that of carbon nanoparticles. It should be noted that it is difficult to detect the internal structure of the as-deposited carbon nanostructures within the resolution of SEM. A further examination by transmission electron microscopy (TEM) is needed to confirm whether the flame product is CNFs or CNTs or the combination of both.
By comparison of the three spectrums in Figure 5, it can be concluded that the highest peak at 44.5° is composed of both Ni and graphite. The 60-min sample shows higher intensity peaks of graphite than those of the 30-min sample, which is closely associated with a lager amount of carbonaceous material caused by the prolonged flame-deposition time. This is very consistent with the SEM observation as seen in Figure 4.
The Raman spectroscopic parameters for the flame-deposited carbon nanostructures.
(D + G)
I D/I G
The 30-min sample
The 60-min sample
Various growth mechanisms for carbon nanostructures, including nanocrystalline diamond, CNFs, and CNTs, have been proposed . In our present work, the existence of Ni particles at the tip of the nanofibrous carbon material shows good evidence of the metal-catalyzed growth [7–10]. It is well documented that Ni is capable of catalyzing the nucleation and growth of CNFs/CNTs due to a weak affinity for carbon [7–10]. However, different crystal planes of Ni exhibit different preferences for the epitaxial matches with carbon as well as different activities for the decomposition of the hydrocarbons . Based on the existing models for carbon nanomaterial synthesis [7–14, 16, 17], the growth process of the carbon nanostructures deposited on Ni-plated hardmetal substrates in the current ethanol diffusion flames could be divided into three stages. First, the fuel ethanol pyrolysed into abundant carbonaceous radical species such as C2, C3, C4, and CO, which precipitated on the active crystal planes of the catalytic Ni particles as mentioned above. Secondly, the pyrolytic hydrocarbons and carbon clusters deposited on the surface of the catalytic Ni particles, meanwhile the hydrocarbon products continued to decompose into other smaller carbon-containing substances. Thirdly, catalyzed by Ni particles, these carbon precursors diffused from one active crystal plane of the catalytic Ni particles to another and finally deposited in the form of CNFs/CNTs on the hardmetal substrate.
We have demonstrated in this work that the ethanol diffusion flame method could be used to synthesize carbon nanostructures on Ni-plated YG6 hardmetal substrates. The quality and the graphitization degree of the flame-deposited carbon nanostructures were significantly enhanced with the increase of deposition time. The characteristics (grain size, shape, and distribution) of the Ni catalyst had a crucial influence on the growth of the carbon nanostructures. In addition, due to the unsteady flame and carbon supply during combustion, inhomogeneous carbon nanostructures were fabricated eventually. These findings could provide a new insight for enhancing the performance of hardmetals by a simple flame method.
energy dispersive X-ray spectroscopy
- , ICDD:
International Centre for Diffration Data
scanning electron microscope
transmission electron microscopy
This work was supported by National Nature Science Foundation of China (no. 50971062) and Science and Technology Plan Project of Guangdong Province, China (no. 2008B01060033). Z. L. acknowledges the funding support by the Australian Research Council (DP0773977).
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