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Vol. 8. Issue 3.
Pages 3244-3250 (May - June 2019)
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Vol. 8. Issue 3.
Pages 3244-3250 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.05.012
Open Access
Application of nitrogen doped bamboo-like carbon nanotube for development of electrically conductive lubricants
László Vanyoreka,
Corresponding author

Corresponding author.
, Dávid Kissb, Ádám Prekoba, Béla Fisera,c, Attila Potykad, Géza Némethd, László Kuzselae, Dirk Dreesf, Attila Trohákg, Béla Viskolcza
a Institute of Chemistry, University of Miskolc, 3515 Miskolc-Egyetemváros, Hungary
b Higher Education and Industrial Cooperation Center, University of Miskolc, 3515 Miskolc-Egyetemváros, Hungary
c Ferenc Rákóczi II, Transcarpathian Hungarian Institute, Beregszász, Transcarpathia 90200, Ukraine
d Institute of Machine and Product Design, University of Miskolc, 3515 Miskolc-Egyetemváros, Hungary
e Institute of Materials Science and Technology, University of Miskolc, 3515 Miskolc-Egyetemváros, Hungary
f Falex Tribology NV, Wingepark 23, 3110 Rotselaar, Belgium
g Department of Automation and Infocommunication, 3515 Miskolc-Egyetemváros, Hungary
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The aim of this work was to examine the applicability of nitrogen-doped bamboo-shaped carbon nanotubes (BCNTs) as conductive additive in bearing grease. To synthesize BCNTs, catalytic chemical vapour deposition (CCVD) method was used by applying butylamine as nitrogen containing carbon source. During the preparation of the polydimethylsiloxane (PDMS)-based lubricants BCNT (1.5wt% or 3wt%) and Li-soap (5wt% or 10wt%, lithium-stearate) or colloidal silicon dioxide (1wt% and 1.5wt% Aerosil 200) was added and mixed by using a highly efficient ultrasonic technique. By adding lithium-soap to the BCNT loaded greases their lubricity improved. The electrical conductivity of greases was measured in stationary state and in rotating bearing (during operation) by using an in-house developed instrument. The nanotube containing samples have shown good electrical conductivity (7–18.5mS). The friction torque was also calculated based on the measurements of our in-house developed instrument. Efficient friction has been achieved with the 1.5wt% BCNT loaded samples (6.1 and 5.1Nmm). Thus, small amount of BCNT is enough to develop a conductive grease formula. All in all, the 3% BCNT and 1.0% colloidal SiO2 containing PDMS base-oil with 50mm/s viscosity is well suitable for loading of ball bearings.

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Electric motors are more and more widespread, therefore, problems related to their operation, such as electrical discharges in the ball bearings, are becoming increasingly prevalent. The generated electric sparks can cause damage by melting some points of the metal-surfaces in the bearing, which lead to uneven raceways, and the early degradation of bearings [1]. This problem can be avoided by applying electrically conductive lubricants. Nowadays, various conductive components are used in the greases such as ionic liquids, copper, silver, carbon black and graphite [2–4]. Zhang et al. used reduced graphene oxide in hyperbranched polyamine-ester base oil and thus, achieved a solvent-free novel lubricant [5]. Several complex, carbon based electrically conductive materials have been intensively applied as nano-additives in conductive greases, including nanosized Al coated graphene, nanosilver-polydopamine coated carbon nanospheres, NiCo layered double hydroxides/graphene oxide, graphene/lanthanum oxide KOH/N-graphene [6–10]. The surface polarity and the particle size of the conductive additives will influence the homogeneity and stability of the produced greases. Highly homogenous dispersibility can be achieved in the base-oils by using nanosized conductive materials as additives. Carbon nanotubes are remarkable electrically conductive nanostructured materials [11]. Their electronic properties can be tuned by incorporating nitrogen atoms (doping) into the graphitic structure of nanotube sidewalls [12–14]. The N-doped carbon materials are well suited in various systems, such as lithium ion batteries, fluorescent bioimaging, and supercapacitors [15–17]. Nitrogen doping can easily be accomplished by using nitrogen containing carbon sources (amines) in the CCVD (catalytic chemical vapour deposition) synthesis of nanotubes [18–22]. Four types of nitrogen can be found in the N-doped carbon nanotubes: pyridinic, pyrrolic, quaternary and oxidized pyridinic [23]. The structure of the nanotubes is also affected by the nitrogen incorporation which will led to the formation of the so-called N-doped bamboo-shaped carbon nanotubes (BCNTs) [13]. Nitrogen-doped BCNTs have been characterized by scanning tunneling spectroscopy [24]. By comparing the tunneling spectra of multi-walled carbon nanotubes (MWCNTs) and N-doped BCNTs, it can be seen that, the nitrogen-doped system has an additional electronic feature at ∼0.18eV, while in case of the MWCNTs the valence and conduction band appear symmetric about the Fermi level. It can be also noted that the presence of an electronic density of states (DOS) at the Fermi energy indicates that the electronic features of N-doped materials is similar to metals [15,25–27]. The degree of electrical conductivity of BCNTs can be tuned within wide range by modifying the nitrogen content [28,29]. The amount of incorporated nitrogen atoms can be increased by adjusting the CCVD synthesis parameters (e.g. temperature, type of carbon source, catalyst). Increasing synthesis temperature will lead to decreasing nitrogen content [30]. The electronic conductivity of BCNTs is highly depend on the amount of incorporated nitrogen atoms, in this sense their conductivity is customizable by varying the synthesis temperature [31]. Another advantage of the N-doped BCNTs is their oxidizable structure. Nitrogen-doped BCNTs contain several carboxyl and hydroxyl groups on their walls. Due to the above-mentioned remarkable properties of the BCNTs, are promising conductive additives for grease. Hydrogen bonds, secondary oxygen bridges and other interactions can be formed between the CNT filler and the base oil [32,33]. Thus, CNTs are well connected to the structure of the base oil creating a stable dispersion, and homogeneous grease.


Nitrogen doped carbon nanotubes were synthetized by applying butylamine (C4H11N, Merck) as carbon source, nickel(II)-nitrate hexahydrate (Ni(NO3)2×6H2O, Aldrich) and magnesium oxide (MgO, Reanal) as catalysts for the CCVD process, nitrogen (Messer, 99.995%) was used as carrier gas. Silicone oils with 5000mm/s and 50mm/s viscosity, lithium-stearate (C18H35LiO2, Aldrich) and colloidal silicon dioxide (Aerosil 200, Sigma Aldrich) were used as base-oil, soap and thickener additive of greases, respectively.

2.2Methods2.2.1CCVD synthesis of the BCNTs

The synthesis of BCNTs was carried out by using the Catalytic Chemical Vapour Deposition (CCVD) method using a previously optimized synthetic procedure [20]. Nickel containing (5wt%) magnesium oxide (2.00g) was placed into a quartz reactor in tube furnace, which was heated up to 750°C temperature. The synthesis time was 20min within which the carbon source (butylamine) was dosed by a syringe pump (6mlh−1) into the reactor and vaporized to carry onto the catalyst bed by nitrogen (100mlmin−1). The cycle was repeated 20 times. The nickel containing catalyst was removed by hydrochloride acid from the nanotubes. The purity of BCNTs was confirmed by thermogravimetric analysis and the carbon content was found to be 95.6%.

2.2.2Preparation of BCNT containing conductive bearing greases

Conductive greases were produced by high energy ultrasound technology. Carbon nanotubes were dispersed in polydimethylsiloxane by using Hielscher UIP1000hdt tip homogenizer (340W/19.42kHz, 2min) from Hielscher Ultrasonics GmbH. Ultrasonic cavitation leads to homogenous greases with great stability. The highly intensive sonication of greases results alternating high-pressure and low-pressure cycles in the base-oils. During the low-pressure cycle, small vacuum bubbles are forming in the liquid. The volume of the bubbles will reach a maximum where they cannot absorb more energy and collapse during the high-pressure cycle. At the point of collapse, very high temperatures (∼5000K) and pressures (∼2000atm) are reached locally. The cavitation can be achieved in liquid phase jets (up to 280m/s velocity), which is more than enough to produce very homogenous and stable greases. The total weight of the grease samples was 100g, within which silicone oil (polydimethylsiloxane, PDMS) was used as basic component. The BCNT content was 1.5wt% or 3wt%, while as soap to increase the lubricity of the samples, 5% or 10% of lithium-stearate was applied. The colloidal silicon dioxide containing samples were prepared by using 1.5wt% or 3wt% BCNT, 1.5wt% or 2.5wt % SiO2 (Aerosil), while the base oil was Wacker 5000 silicone oil (viscosity: 5000mm/s).

2.2.3Characterization techniques

Morphology, diameters and structure of the BCNTs were studied by FEI Technai G2-20X Twin high-resolution transmission electron-microscopy (HRTEM) operating at an accelerating voltage of 200kV and Philips CM 10 (100kV) transmission electron-microscopy (TEM). The purity (carbon content) of the carbon nanotube sample was measured by thermogravimetric analysis (TGA), by using of Netzsch Tarsus TG 209 thermo-microbalance. Nitrogen (4.5) and oxygen (5.0) mixture was applied as oxidative atmosphere in the TGA measurements. The flow rate was set to 6mlmin−1 and 14mlmin−1, for the oxygen and nitrogen, respectively. The heating rate was 10°Cmin−1, in the 35–800°C temperature range. The sample preparation was carried out by dropping from aqueous suspension of the samples onto a copper grid (300 Mesh, only carbon from Ted Pella). The binding-type of incorporated nitrogen atoms was studied by SPECS X-ray photoelectron spectroscopy (XPS) with Phoibus 150 MCD nine analyzer.

The frictional properties of carbon nanotube loaded bearing grease were measured by a Basalt-N2 equipment, where Cr6 steel plate and a steel cylinder with 6mm diameter and 16mm length was used as substrate and counter material. During the measurement 250 cycles was performed at room temperature by applying 5N load, 60MPa contact pressure and 20mm/s maximum sliding speed. The applied load (Fn), and tangential force (Ft) was used to calculate the coefficient of friction by using an Ft/Fn formula.

The dropping points of the greases were measured based on the ASTM D566 – 17 standard. The test tube was placed in a silicone oil bath which was heated at a rate of 5–6°C per minute, while being stirred.

The electrical conductivity was determined in stationary state by applying Orion Versa Star Pro conductivity meter (Thermo Scientific).

For the friction and electrical conductivity measurements, 100mg grease sample was loaded in single row radial ball bearing (608 LBnV; OD: 22mm). The conductivity of greases in the ball bearings during operation was determined at 2000min−1 rotational speed by using an in-house developed system (Fig. 1).

Fig. 1.

In-house developed system to determine electrical conductivity and torque friction of greases in ball bearings.


A CompactRIO (NI cRIO-9031) was used to control the measurement procedure. The system is equipped with a constant current source (NI-9265), a high precision analog input module (NI-9219) and a digital I/O module (NI-9403). The procedure was carried out by using a pair of bearings (608LB). The original lubricant was replaced with the tested mixture, using a new pair of bearings for each test.

A rotary encoder is used to measure the rotation speed of the DC motor, which is controlled through power electronics. A fan wheel is attached to the motor to simulate real world load scenarios. The measurement process is automated, it can be done at specific speed(s) manually.

The system used 4-wire measurement method. The source drives constant current (10mA by default, can be modified by the user) through the bearings and measures the voltage drop on them. The controller calculates the resistance of the bearings and displays it on a graph and saving the data at the same time. Measurements are done continuously for 5s, calculating average, minimum and maximum values for resistance.

In order to determine the lubrication and the applicability of BCNT loaded grease in ball bearing friction torque (Md) was calculated based on the free rotation time of a steel flywheel after switching off the engine (Eq. (1)):

where td was the free rotation time of the flywheel, ω was the angular speed (rotational-speed×2π). The mass moment of inertia of the flywheel, which also includes the inertia of the silicone shield of the ball bearing is 0.7×10−3kgm2. The rotational-speed during the tests was set to 33.33s−1 (2000min−1). Polydimethylenesiloxane based bearing grease, one of the most widely employed non-conductive lubricant, was used as reference and its friction torque was measured (4.9Nmm) for comparative purposes.

3Results and discussion3.1Characterization of BCNTs

The purified nitrogen doped carbon nanotubes were examined by transmission electron microscopy (Fig. 2a). On the TEM picture were not visible impurities, which origin from the CCVD catalyst, and the sample not included amorphous carbon forms. Diameter of nanotubes were measured by applying of ImageJ software, based on the scalebar of TEM picture. The outer diameters of BCNTs were measured between 8 and 44nm, the average diameter was 18.8±7.4nm (Fig. 2b). The purity, namely the carbon content of the purified BCNT sample was 92.6%, based on the thermogravimetric analysis (Fig. 2c).

Fig. 2.

TEM image (a), size distribution of the outer diameters of the nanotubes (b), and TG-DTG curves of the purified BCNT sample (c).


The bamboo-shaped structure of the BCNTs is visible on the HRTEM images (Fig. 3a). The average diameter of the carbon nanotubes was 18.5nm. The schematic illustration of the bamboo-like structure also shows several edges on the wall of nanotubes (Fig. 3b). In the synthetized BCNTs, based on the deconvoluted N 1s band of the XPS spectrum, three types of nitrogen atom can be differentiated: pyridinic, graphitic (quaternary) and oxidized nitrogen with a binding energy at 398.5eV, 401.1eV and 404.9eV, respectively (Fig. 3c). The incorporation of these nitrogen atoms into the system will led to structural distortion (bamboo-shape, edges) and electronic property change. The formed edges can serve as adsorption sites for base-oil molecules and soaps, which will enhance the dispersion stability of BCNT containing grease.

Fig. 3.

HRTEM image (a), graphical illustration of the bamboo-like structure (b), and deconvoluted N 1s band of the incorporated C-N chemical binding on XPS spectrum of BCNTs (c).

3.2Coefficients of frictions of BCNT loaded greases

In case of the soap-free greases the loading of BCNTs lead to increasing coefficients of friction (Fig. 4). Thus, to achieve proper lubrication Li-soap has to be used in the samples. The nanotube free soap containing lubricants has similar degree of friction as the BCNT contained samples. The coefficients of friction were very similar in case of the four BCNT and Li-soap contained greases.

Fig. 4.

Coefficients of friction of the prepared lubricant samples.

3.3Tests of the greases

Due to the area of applications (in ball bearings) of the prepared greases, it is necessary to reach high dropping point (>140°C) during operation. The dropping points were measured based on the ASTM D566 – 17 standard. By using BCNT as additive in the samples, the dropping point decreased, therefore, new composition was made. In order to improve the dropping points, high viscosity silicone oil (5000mm2/s) and fumed silica was used as thickener. These further additives increased the dropping point and it is reached >150°C (Fig. 5a).

Fig. 5.

Dropping point (a), electrical conductivity of the lubricants in stationary state (b) and in rotating bearing, during operation (c), friction torque in rotating bearing (d).


The conductivity of the lithium soap/BCNT and the SiO2 containing samples was measured in stationary state. It was found to be relatively high, >5mS in each case (Fig. 5b). The sample with 3% BCNT/5% Li-soap has a ∼9mS conductivity in stationary state. The 1.5wt% BCNT containing silica aerogel filled grease showed the smallest electrical conductivity in stationary state. In case of the 3% nanotube/silica loaded grease reached 4.5mS conductivity, which was higher than in the previous case.

The electrical conductivity was also measured during operation of the ball bearing (Fig. 5c). The conductivity increased in each case compare to the stationary state measurements. The maximum (31.5mS) was reached with the 3% BCNT containing, silica loaded lubricant. By increasing amount of Li-stearate the conductivity of the four samples decreased; thus, it is necessary to optimize the amount of the soap. Friction torque was measured during the operation of the bearing with the selected lubricants (Fig. 5d). The 3% BCNT containing samples are good conductive greases, but their friction torque is higher (14.2 and 16.1Nmm) than the 1.5% nanotube loaded samples (6.1 and 5.2Nmm). The 1.5% BCNT samples are similar to the non-conductive reference bearing grease in terms of friction torque (4.9Nmm). The SiO2 filled lubricants showed relative high friction torque, 12 and 13.6Nmm. The sample which contain 1.5% BCNT and 5% soap can be a perfect candidate as conductive bearing grease, but the dropping point is much lower than expected. All in all, the 3% BCNT and 1.0% SiO2 containing grease was well suitable for loading of ball bearing.


High energy ultrasonication was applied to produce homogeneous, stable and electrical-conductive bearing greases. Bamboo-shaped carbon nanotubes (BCNTs) were used as conductive additives for greases. Due to nitrogen incorporation into the structures, extraordinary electronic and structural properties developed which lead to good adsorption behavior and electric conductivity. Relatively small amount of BCNTs, 1.5wt% is enough to reach good electrical conductivity of greases (>14mS). The friction torque of the 1.5wt% nanotube and 5wt% soap containing conductive grease sample (6.1 Nmm) was similar to the non-conductive grease, which is widely used nowadays for loading of the ball bearings. Dropping points of the soap and BCNT containing samples were very low, thus the composition of the lubricants was modified. By using colloidal silicon dioxide and 3wt% BCNT in high viscosity (5000mm/s) silicone oil resulted high dropping point, 153°C. All of these effects are based on the ability of dispersed silica particles (aerosil) to form a network of aggregates via hydrogen bridges and/or van der Waals interactions in silicone oil. The nitrogen atoms and the oxidized functional groups in the added N-doped BCNTs further increase the possibility of interactions. Thus, a reversible, three-dimensional lattice structure could form and the viscosity of the SiO2-PDMS-BCNT will increase. However, the friction coefficient (12Nmm) was higher than in case of other samples, the electrical conductivity in bearings during operation was 31.5mS. All in all, the 3% BCNT and 1.0% SiO2 containing grease has the best characteristics to apply in ball bearings. Consequently, BCNT is well suited for the development of a conductive grease formula.

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.


This research was supported by the European Union and the Hungarian State, co-financed by the European Regional Development Fund in the framework of the GINOP-2.3.4-15-2016-00004 project, aimed to promote the cooperation between the higher education and the industry. The authors thank the co-operations for the Robert Bosch Energy and Body Systems Ltd.

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