=2016 = 

20.Low-energy electron irradiation induced top-surface nanocrystallization of amorphous carbon film

Applied Surface Science 384 (2016) 341–347

Cheng Chena,b, Xue Fana, *, Dongfeng Diao, *

We report a low-energy electron irradiation method to nanocrystallize the top-surface of amorphouscarbon film in electron cyclotron resonance plasma system. The nanostructure evolution of the carbonfilm as a function of electron irradiation density and time was examined by transmission electron micro-scope (TEM) and Raman spectroscopy. The results showed that the electron irradiation gave rise to theformation of sp2nanocrystallites in the film top-surface within 4 nm thickness. The formation of sp2nanocrystallite was ascribed to the inelastic electron scattering in the top-surface of carbon film. Thefrictional property of low-energy electron irradiated film was measured by a pin-on-disk tribometer.The sp2nanocrystallized top-surface induced a lower friction coefficient than that of the original pureamorphous film. This method enables a convenient nanocrystallization of amorphous surface.

19. ECR sputtering and electron/ion alternative irradiation for multilayer carbon films fabrication with tunable layer thickness

Surface & Coatings Technology 296 (2016), 26–32

Peidong Xue, Lei Yang*, Dongfeng Diao*

In this work, we proposed a method of electron cyclotron resonance (ECR) sputtering and electron/ion alternative irradiation to formmultilayer carbon filmswith different single layer thicknesses. The soft electron irradiated layers and protective ion irradiated layers were deposited as the component layers. The transmission electron microscopy (TEM) and Raman spectra proved that the graphene sheets structure in electron irradiated layers could be preserved in multilayer carbon films. We found a rule of the dependence of the surface roughness, mechanical and tribological properties of themultilayer films on the single layer thickness. With the decrease of single layer thickness, the surface roughness decreased and the mechanical and tribological properties improved notably. The mechanisms of the long wear life with low friction coefficient were further discussed based on the hardness enhancement and the cracking length limitation. This work indicated that by decreasing the single layer thickness, ECR sputtering and electron/ion alternative irradiation can fabricate films with combined properties of the component materials, which can be expected for broad nanolayered surface science and engineering applications.

18. The adhesion behavior of carbon coating studied by re-indentation during in situ TEM nanoindentation

Applied Surface Science 362 (2016) 49–55(PDF-File)

Xue Fan, Dongfeng Diao∗

We report a nanoscale adhesion induced nano-response 

in terms of re-indentation during in situ trans-mission electronmicroscope (TEM) nanoindentation on the carbon coating with silicon substrate. Theadhesive force generated with nanoindentation was measured, and re-indentation phenomenon duringunloading with displacement sudden drop and external loading force change from tension to com-pression was found. The occurrence of re-indentation during unloading was ascribed to the adhesiveforce of the contact interface between the indenter and the coating surface. Adhesion energies releasedfor re-indentation processes were quantitatively analyzed from the re-indentation load–displacementcurves, and carbon coating reduced the impact of adhesion for silicon substrate. The adhesion inducednano-response of contact surfaces would affect the reliability and performance of nano devices.

=2015 = 

17. Restructured graphene sheets embedded carbon film by oxygen plasma etching and its tribological properties

Applied Surface Science 357 (2015) 771–776（PDF-File）

Meiling Guo, Dongfeng Diao*, Lei Yang, Xue Fan

An oxygen plasma etching technique was introduced for improving the tribological properties of thegraphene sheets embedded carbon (GSEC) film in electron cyclotron resonance plasma processing system. The nanostructural changing in the film caused by oxygen plasma etching was examined by transmission electron microscope, Raman spectroscopy and X-ray photoelectron spectroscopy, showing that the 3 nm thick top surface layer was restructured with smaller graphene nanocrystallite size as well as higher sp3bond fraction. The surface roughness, mechanical behavior and tribological properties of the original GSECand oxygen plasma treated GSEC films were compared. The results indicated that after the oxygen plasma treatment, the average roughness decreased from 20.8 ± 1.1 nm to 1.9 ± 0.1 nm, the hardness increased from 2.3 ± 0.1 GPa to 2.9 ± 0.1 GPa, the nanoscratch depth decreased from 64.5 ± 5.4 nm to 9.9 ± 0.9 nm,and the wear life increased from 930 ± 390 cycles to more than 15,000 frictional cycles. The origin of the improved tribological behavior was ascribed to the 3 nm thick graphene nanocrystallite film. This finding can be expected for wide applications in nanoscale surface engineering.

16. Stable and super-low friction of amorphous carbon nitride coatings in nitrogen gas by using two-step ball-on-disk friction test

Lubrication Science 27(2015), 137–149(PDF-File)

Pengfei Wang*, Masakatsu Sugo and Koshi Adachi

Effect of running-in process on friction behaviour of carbon nitride (CNx) coating in N2 gas stream was investigated with a newly introduced two-step ball-on-disk friction test, where the rubbed Si3N4 ball in the pre-sliding (step 1) was replaced by a new CNx-coated Si3N4 ball in the subsequent sliding stage under N2 gas (step 2). The two-step friction test is clarified to be a simple but effective technique for obtaining contact material combination of self-mated CNx coatings and for achieving stable and low frictions of CNx coatings. Friction coefficients of CNx/CNx in N2 gas stream decrease greatly from 0.07 without pre-sliding to less than 0.025 in two-step friction tests. The minimum friction coefficient of 0.004 was obtained by introducing 500 cycles of pre-sliding in ambient air. These stable and low frictions are attributed to the generation of self-mated CNx coatings and the formation of a lubricious layer on the disk surface.

15. Three-layered sandwich structured carbon film prepared by sputtering and ion/electron/ion alternative irradiation

Surface & Coatings Technology 278 (2015), 12-17(PDF-File)

Wenlei Zhang, Dongfeng Diao*, Xue Fan

The electron irradiated carbon film based on electron cyclotron resonance (ECR) sputtering technology has been proved tohave both high conductivity and paramagnetism. However, the relatively low hardness, wear life and high surface roughness value limit its application in Micro-electromechanical Systems (MEMS). To improve these properties, the sandwich structured carbon films with different modulation ratios were introduced on silicon (100) wafers by alternative irradiation (ion/electron/ion) technique in ECR sputtering system. The three-layered  anostructure of films was measured with a transmission electron microscopy (TEM). The surface roughness of the film can be controlled to 0.19 nm according to the measurement of atomic force microscope (AFM). The unique characteristics of electron irradiated layer remained stable after irradiation through the analyzing of Raman spectroscopy. The mechanical properties (including hardness, elastic modulus, fracture behavior) and wear life were improved significantly due to the sandwich structure. The results showed that the sandwich structured carbon film is a kind of functional material equipped with enhanced mechanical and tribological properties.

14.Experimental study on load capacity of nanoparticles-laden gas film in thrust bearing

Industrial Lubrication and Tribology，67（3），233–239（2015）

Zhiru Yang, Dongfeng Diao, Hongyan Fan，Xue Fan and Chao Wang

Purpose – The purpose of this paper is to study the load capacity of nanoparticles-laden gas film (NLGF) in thrust bearing.

Design/methodology/approach – SiO2 nanoparticles were added into gas to form an NLGF. The nanoparticles volume fraction in the film was controlled by a vibrator. The film thickness and the film pressure were measured by a micro cantilever displacement sensor and a membrane pressure sensor, respectively. The total load that makes the film thickness keeping constant was quantified, and then, the film load capacity was obtained.

Findings – The investigation shows that nanoparticles can enlarge the film load capacity remarkably; even a little amount of nanoparticles (0.01 per cent) could lead to a sharp rise. With the increase of nanoparticles volume fraction, load capacity increases. However, the increment of load capacity decreases gradually. In addition, the film pressure variation proves the enhancement effect of nanoparticles on the film load capacity.

Research limitations/implications – The paper is restricted to the findings based on NLGF, which is formed by dispersing SiO2 nanoparticles in gas film as an additive. The experimental results are applicable within the range of nanoparticles volume fraction of 0.01-0.33 per cent.

Originality/value – The fact that nanoparticles could enlarge the gas film load capacity is verified by experiment for the first time. This study reveals the corresponding relation between nanoparticles volume fraction and the film load capacity.

=2014 = 

13. Low frictions of self-mated CNx coatings in dry and humid inert gas environments

     Surface & Coatings Technology, 258, 1137 (2014) (PDF-File)

     Pengfei Wang , and Koshi Adachi.

The friction behavior of CNx coated Si3N4 disk sliding against CNx coated Si3N4 ball (denoted as CNx/CNx) in dry and humid inert gas environments (i.e. nitrogen, argon, and helium) is investigated in this study. The sliding contact of self-mated CNx coatings has been realized thanks to the promising two-step ball-on-disk friction test, where the rubbed Si3N4 ball in the running-in stage (step 1) is replaced by a new CNx coated Si3N4 ball in the subsequent stage (step 2). As a result, friction coefficients of less than 0.05 are obtained for self-mated CNx coatings in all three dry inert gas environmentswith relative humidity under 5%RH.Moreover, friction coefficients of less than 0.10 are obtained for the sliding contact of CNx/CNx in inert gas environmentswith relative humidity of larger than 35%RH. Especially, the lowest friction coefficient of 0.020 is achieved in the nitrogen gas environment with relative humidity of 37%RH. Furthermore, the sliding contact of self-mated CNx coatings survives after friction tests in humid nitrogen and argon gas environments. The low frictional performance and low sensitivity of friction to water vapor for the self-mated CNx coatings in inert gas environments are mainly attributed to the formation of a sp2 rich carbon tribo-layer on the mating surface. The pronounced frictional performance of selfmated CNx coatings has made them good candidates for the demanding industrial applications.

12. Frictional behavior of carbon film embedded with controlling-sized graphene nanocrystallites

   Tribology Letters, 55, 429（2014）(PDF-File)

   Cheng Chen, Dongfeng Diao*, Xue Fan, Lei Yang and Chao Wang

Graphene nanocrystallites embedded in amorphous carbon matrix can bring excellent tribological, electrical andmagnetical properties to the carbon films. But too large size of graphene nanocrystallite would lead to degradation of the tribological performance. So it is necessary to clarify the dependence of frictional behavior of the carbon film on graphene nanocrystallite size. In order to control the size, different electron irradiation densities were introduced during film growth in the electron cyclotron resonance plasma sputtering process. Frictional tests on the films were carried out with a Pin-on-Disk tribometer. The evolution of graphene nanocrystallite size along with electron irradiation density was examined by transmission electron microscopy and Raman spectroscopy. The results showed that the graphene nanocrystallite size increased with increasing of the electron irradiation density. The film with a graphene nanocrystallite size of 1.09 nm exhibited a low friction coefficient of 0.03 and a long wear life. When nanocrystallite size increased, the friction coefficient increased and the wear life decreased. Observation on transfer film revealed that the nanocrystallite in transfer film grew larger when initial size was 1.09 nm, and changed smaller when initial size was 1.67 nm. The results suggested that embedded grapheme nanocrystallite played an important role in the formation of transfer film, the initial size of graphene nanocrystallite strongly affected the frictional behavior of the film, and the graphene nanocrystallite needed to be controlled under a certain size in order to keep the good tribological performance.

11. Scratch Behavior of Re-structured Carbon Coating by Oxygen Plasma Etching Technology for Magnetic Disk Application

    Surface &Coatings Technology 251, 128 (2014)  (PDF-File)

    Meiling Guo, Dongfeng Diao*, Xue Fan, Lei Yang, Liwei Yu.

An oxygen plasma etching technology to prepare ultrathin carbon coatings for magnetic disk in electroncyclotron resonance(ECR) plasma system was developed. In the preparation process, as-deposited carbon coatings were etched by oxygen plasma to obtain re-structured carbon coatings. Scratch behaviors of the as-deposited and re-structured carbon coatings on silicon substrates were evaluated by atomic force microscope (AFM) scratch test. It was found that the scratch resistance of the re-structured carbon coating was improved. Then X-ray photoelectron spectroscopy (XPS) analysis showed that after oxygen plasma etching, the C_C (sp2) content decreased from 55% to 17%, the C\C (sp3) content increased from 33% to 58% and the C\O content increased from 7% to 18%. It indicated that the oxygen plasma etching induced the increase of the sp3 bonding carbon, which contributed to the improved scratch resistance. Further transmission electron microscope (TEM) observation was conducted and the rising of the scratch resistance was inferred to be caused by the formation of the interlayer cross-linking after preferentially breaking the sp2 bond. Based on the above results, the ultrathin re-structured carbon coatings by oxygen plasma etching were prepared on uncoated magnetic disk substrates and showed a better scratch behavior and a comparable surface roughness in comparison with the commercial magnetic disk.

10. Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride coatings

    Materials Chemistry and Physics, 145,434 (2014) (PDF-File)

    Pengfei Wang *, Takanori Takeno, Julien Fontaine, Masami Aono, Koshi Adachi, Hiroyuki Miki, and Toshiyuki Takagi.

Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride (CNx) coatings are investigated. CNx coatings are fabricated by a hybrid coating process with the combination of radio frequency plasma enhanced chemical vapor deposition (RF PECVD) and DC magnetron sputtering at various substrate bias voltage and target sputtering power in the order of 400 V 200W,400 V 100W,800 V 200W, and800 V 100W. The deposition rate, N/C atomic ratio, and hardness of CNx coatings as well as friction coefficient of CNx coating sliding against AISI 52100 pin in N2 gas stream decrease, while the residual stress of CNx coatings increases with the increase of substrate bias voltage and the decrease of target sputtering power. The highest hardness measured under single stiffness mode of 15.0 GPa and lowest residual stress of 3.7 GPa of CNx coatings are obtained at400 V 200W, whereas the lowest friction coefficient of 0.12 of CNx coatings is achieved at800 V 100W. Raman and XPS analysis suggest that sp3 carbon bonding decreases and sp2 carbon bonding increases with the variations in substrate bias voltage and target sputtering power. Optical images and Raman characterization of worn surfaces confirm that the friction behavior of CNx coatings is controlled by the directly sliding between CNx coating and steel pin. Therefore, the reduction of friction coefficient is attributed to the decrease of sp3 carbon bonding in the CNx coating. It is concluded that substrate bias voltage and target sputtering power are effective parameters for tailoring the structural and tribological properties of CNx coatings.

9. Contact Stress-Induced Micromagnetic Behavior in Magnetic Recording Disk

    Tribology Letters 54, 287 (2014)  (PDF-File)

    Lei Yang and Dongfeng Diao*

Stress plays an important role in the magnetic properties of ferromagnetic materials. Sliding contact in hard disk drives can leadto tribological failures of the disk in terms of data loss and demagnetization. However, the relationship between contact stress-induced magnetization changes and tribological failures of magnetic recording disk is rarely discussed. In this study, the contact stressinduced micromagnetic behavior in magnetic recording disk was investigated using micromagnetic simulation. A micromagnetic model including the magnetostriction effect into the Landau–Lifshitz–Gilbert equation was developed to simulate the stress effect on the magnetization changes. Then finite element analysis was used to calculate the critical stresses for the occurrence of data loss and demagnetization of perpendicular magnetic recording disk under sliding contact according to our previous experimental results. Based on these simulation results, it was found that the magnetic moment decreased by 8.9 % under the critical stress for data loss, and it rotated 55.7 under the critical stress for demagnetization. In addition, the simulated static domain structures when data loss and demagnetization occur were in agreement with the previously reported experimental results. Finally, the relationship between the contact stress-induced tribological failures and micromagnetic behavior of the magnetic disk was illustrated. It was proposed that data loss is caused by the magnetization reduction, while demagnetization iscaused by the magnetization rotation.

8. Lubrication Performance of Nanoparticles-Laden Gas Film in Thrust Bearing under Noncontact and Contact Conditions

   ASME,Trans., Journal of Tribology 136, 034505(2014)  (PDF-File)

   Hongyan Fan, Xue Fan, Zhiru Yang and Dongfeng Diao*

The nanoparticles-laden gas film (NLGF), which is formed by adding nanoparticles into the gas film, has a potential toincrease the load capacity of the gas film and to protect the surfaces of the bearing from severe contact damage. In order to explore the lubrication performance of NLGF, the load capacity in the noncontact state and the friction coefficient in the contact state were studied experimentally by a novel NLGF thrust bearing apparatus. The effects of nanoparticles concentration on the load capacity and the friction coefficient were investigated, respectively. The lubrication performance of NLGF in a 200 start-stop cyclic test was evaluated. The contact surfaces were analyzed by the surface profilometer, scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). The results showed that NLGF had the enhancement of the load capacity in the noncontact state and possessed the properties of friction reduction and surface protection in the contact state. An optimal nanoparticles concentration of 60 g/m3 was found, making NLGF have a relative high load capacity in the noncontact state and the lowest friction coefficient in the contact state. With the optimal concentration, the friction coefficient with NLGF kept a low value during the 200 start-stop cyclic test. Then the friction reduction mechanism of NLGF was discussed, and it was inferred that the surface of the disk was covered with a protective film formed by nanoparticles, leading to a lower shear force. This study opens new perspectives of adding nanoparticles into gas bearings to improve the lubrication performance.

7. Nanoparticles-Laden Gas Film in Aerostatic Thrust Bearing

   ASME,Trans., Journal of Tribology 136, 034501 (2014)   (PDF-File)

    Zhiru Yang, Dongfeng Diao*,   Xue Fan and Hongyan Fan

Nanoparticles-laden gas film (NLGF) was formed by adding SiO2 nanoparticles with volume fraction in the range of 0.014–0.330% and size of 30 nm into the air gas film in a thrust bearing. An effective viscosity of the gas-solid two phase lubrication media was introduced. The pressure distribution in NLGF and the load capacity of the thrust bearing were calculated by using the gassolid two phase flow model with the effective viscosity under the film thicknesses range of 15–60 lm condition. The results showed that the NLGF can increase the load capacity when the film thickness is larger than 30 lm. The mechanism of the enhancement effect of load capacity was attributed to the increase of the effective viscosity of the NLGF from the pure air film, and the novel lubrication media of the NLGF can be expected for the bearing industry application.

=2013 = 

6. Magnetic behavior of graphene sheets embedded carbon film originated from graphenenanocrystallite

   Applied Physics Letters 102, 052402 (2013)

   Chao Wang and Dongfeng Diao

We found paramagnetic behavior at 300K of graphene sheets embedded carbon (GSEC) film, which is deposited under lowenergy electron irradiation in electron cyclotron resonance plasma. The origin of the magnetic properties of GSEC film is ascribed to the formation of graphene nanocrystallite. With higher irradiation energy, the size of nanocrystallite barely changed, while the density in GSEC film became higher, leading to a dramatically increase of saturation magnetization and residual magnetism. This finding indicates that GSEC film with higher magnetization can be expected, which has the potential for magnetic and spintronics applications.

5. Coating NiTi archwires with diamond-like carbon films: reducing fluoride-induced corrosion and improving frictional properties
     J Mater Sci: Mater Med  24, 2287 (2013)

     S.Y. Huang, J.J. Huang, T. Kang, Dongfeng Diao*, and Y.Z. Duan

This study aims to coat diamond-like carbon (DLC) films onto nickel–titanium (NiTi) orthodontic archwires. The film protects against fluoride-induced corrosion and will improve orthodontic friction. ‘Mirror-confinement-type electron cyclotron resonance plasma sputtering’ was utilized to deposit DLC films onto NiTi archwires. The influence of a fluoride-containing environment on the surface topography and the friction force between the brackets and archwires were investigated. The results confirmed the superior nature of the DLC coating, with less surface roughness variation for DLC-coated archwires after immersion in a high fluoride ion environment. Friction tests also showed that applying a DLC coating significantly decreased the fretting wear and the coefficient of friction, both in ambient air and artificial saliva. Thus, DLC coatings are recommended to reduce fluoride-induced corrosion and improve orthodontic friction.

4. Nanoindentation Behavior of Amorphous Carbon Films Containing Nanocrystalline Graphite and Diamond clusters Prepared by Radio Frequency Sputtering

     Applied Surface Science 273, 816-823 (2013)

     Xue Fan, Dongfeng Diao*, Kenji Nose and T. Yoshida

Amorphous carbon (a-C) films were prepared by a radio-frequency sputtering method. Nano structures in the films werecontrolled by changing the ion irradiation energy and deposition temperature. It was found that nanocrystalline graphite and diamond clusters were embedded in the pure amorphous structure with sizes of approximately 5 nm. a-C films contained nanocrystalline graphite clusters (a-C:NCG) were obtained with the ion energy ranging from 50 to 120 eV and temperature in 300–370 K. a-C film contained nanocrystalline diamond clusters (a-C:NCD) was obtained with 120 eV at 570 K. Nanoindentation behaviors of these carbon films were compared with pure amorphous structured carbon film. The percentage of elastic recoveries of a-C:NCD, a-C, and a-C:NCG films were obtained to be 81.9%, 84.3%, and 87.5%, respectively. Pop-in steps with about 3 nm displacement appeared in loading curves for a-C:NCG film, and 10 nm for a-C:NCD film. These results showed that the nanoindentation behaviors of amorphous carbon film containing cross-linked nanocrystalline graphite clusters is better than that of diamond clusters.

3. Tribological Thermostability of Carbon Film with Vertically Aligned Grapheen Sheets

Tribology Letters 50, 305–311 (2013)

Cheng Chen and Dongfeng Diao*

Tribological thermostability of carbon film with vertically aligned graphene sheets was studied with annealing temperatures up to 1,750 C. The carbon film was deposited on silicon carbide substrate by electron cyclotron resonance plasma sputtering. Tribological thermostabilities of the carbon film in terms of friction coefficient, wear life, and nanoscratch depth were investigated by Pin-on-Disk tribometer and atomic force microscopy. The evolution of nanostructure of vertically aligned graphene sheets in the carbon film as a function of annealing temperature was examined by Raman spectroscopy and transmission electron microscopy. The results showed that the friction coefficient, wear life, and nanoscratch depth of the carbon film were thermally stable up to 1,250 C. When the annealing temperature was 1,500 C, the friction coefficient and the nanoscratch depth increased, the wear life decreased, but still all were of considerable values. These variations were attributed to the initiation of tubular-like structure originated from graphene sheets stacks. After annealing at 1,750 C, tribological performances degraded catastrophically due to the abundant formation of tubular-like structures and the appearance of a graphitic interlayer between the film and the substrate.

2. Evolution of Maximum Contact Stresses in Amorphous Carbon Coated Silicon During Sliding Wear Against Si3N4 Ball

Tianxiao Cai, Pengyu Zhang and Dongfeng Diao*

ASME,Trans., Journal of Tribology 135, 021401-10 (2013)

The evolution of the maximum contact stresses in amorphous carbon coated silicon during sliding wear against a Si3N4 ball was investigated. Amorphous carbon coating was prepared on a silicon substrate by the electron cyclotron resonance (ECR) plasma sputtering method. Surface morphologies of the coating and counterpart were measured by an atomic force microscope (AFM). The friction and wear behavior of the coating was studied by a ball-on-disk tribometer. The cross-sections of the wear tracks at different wear stages were observed with a scanning electron microscope (SEM). Maximum contact stresses with different coating thicknesses were calculated by the three-dimensional semi-analytical method (SAM). The results demonstrated that when taking surface asperities into consideration, maximum shear stress at the bonding interface and adjacent substrate showed a dramatic increase during wear and should be responsible for the initiation and propagation of the cracks observed at the final stage of sliding.

1. Frictional behavior of nanostructured carbon films

Dongfeng Diao*, Chao Wang, Xue Fan

Friction 1, 63-71(2013)

We propose a new path for preparing nanostructured carbon films (NCFs) by using electron cyclotron resonance (ECR) plasma sputtering with ion–electron hybrid irradiation for controlling the frictional behavior. The frictional behavior of the NCF was measured by using a pin-on-disk tribometer with a nanoprobe displacement sensor, and the transition curves of the friction coefficient and microdisplacement of the NCFs were examined. The friction mechanism was discussed by transmission electron microscopy (TEM) observation on the wear track. From the results, we found a new method to prepare NCFs, which has the potential to achieve low friction at the early stage of sliding contact. In addition, the technology of ECR plasma with ion–electron hybrid irradiation provides a new vision to rebuild a nanostructured surface from an original surface for controlling the frictional behavior.