论文2014阅读次数 [1132] 发布时间 :2015-04-01 17:20:30
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9. 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.
8. Magnetism induced by excess electrons trapped at diamagnetic edge-quantum well in multi-layer grapheme
Applied Physics Letters, 105, 042402 (2014) (PDF-File)
Xi Zhang, Chao Wang, Chang Q Sun, and Dongfeng Diao*
In this paper, we clarified a robust mechanism of magnetism generated by excess electrons captured by edge-quantum well of
diamagnetic armchair edges. Consistency between density functional theory calculations and electron cyclotron resonance experiments verified that: (1) Multi-layer armchair nanoribbons are stable with proper amounts of excess electrons which can provide net spin; (2) Since under-coordination induces lattice relaxation and potential well modulation, electrons tend to be trapped at edges; and (3) Neither large amount of excess electrons nor positive charges can induce magnetism. This work shed light on the development of grapheme devices in its magnetic applications.
7. 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 and
magnetical 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.
6. 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.
5. 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, and 800 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 at 400 V 200W, whereas the lowest friction coefficient of 0.12 of CNx coatings is achieved at 800 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.
4. 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 lead
to 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 is
caused by the magnetization rotation.
3. 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 to
increase 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.
2. 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.
1. Experimenal study on Load Capacity of Nanoparticles-laden Gas Film in Thrust Bearing
Ind. Lubr. Tribol., accepted for publication.
Zhiru Yang, Dongfeng Diao*, Hongyan Fan, Xue Fan
