Hongxuan Li

Preparation and characterization of hydrogenated diamond-like carbon films in a dual DC-RF plasma system

A dual direct current and radio frequency (DC-RF) plasma system was used to deposit hydrogenated diamond-like carbon (DLC) films from methane plasma. It has the advantages of separately controlling ion density and ion energy by RF power and DC bias, respectively, over conventional simply capacitive-coupled radio frequency plasma enhanced chemical vapour deposition system. Thus, the hydrogenated DLC films were obtained at different RF powers and DC biases, using CH4 plus Ar as the feedstock. The effects of RF power and DC bias on the structure and properties of the films were investigated by means of Fourier transformation infrared spectroscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and nano-indentation. The results were as follows: the sp3 content, hardness, and Youngs modulus of the DLC films increased with increasing RF power at a constant DC bias of ?200?V and reached the maximum values at an RF power of 300?W, after which they decreased with further increase of the RF power. The DC bias had a similar but greater effect on the structure and properties of the films, owing to a greater influence of the ion energy on the characteristics of the films than the ion current density. The film deposited at an RF power of 300?W and DC bias of ?200?V has the most diamond-like characteristics with maximum hardness, Youngs modulus, and sp3 content. Since both the ion current density and ion energy greatly affect the structure and characteristics of the DLC films, it is imperative to select proper processing parameters to obtain high quality DLC films.

The effect of duty cycle on the microstructure and properties of graphite-like amorphous carbon films prepared by unbalanced magnetron sputtering

The effect of duty cycle on the microstructure and properties of graphite-like amorphous carbon films prepared by unbalanced magnetron sputtering was investigated. The structure of the resultant carbon film is amorphous, as shown by high-resolution transmission electron microscopy. Raman analysis shows that the studied films are dominated by sp2 sites, and the intensity ratio of the D and G peaks ranges from 4.0 at a duty cycle of 20% to 6.0 at 50%, which is one order of magnitude larger than that of diamond-like carbon films, indicating an obvious increase in sp2 sites with duty cycle. The surface morphology was investigated by atomic force microscopy. The images show that the as-deposited carbon films have a very rough surface, and the maximum granular structure size is up to 180 nm in diameter and 50 nm in height. The hardness and internal stress of the resultant carbon films increase with increasing duty cycle, accompanied by an increase in sp2 fraction in the films, which is different from the diamond-like carbon films. In addition, the resultant carbon films show superior tribological properties with high load-bearing capacity and excellent wear resistance. The influence of duty cycle on the microstructure and properties is discussed in detail.

Effect of relative humidity on the tribological properties of hydrogenated diamond-like carbon films in a nitrogen environment

Hydrogenated diamond-like carbon (DLC) films were deposited on Si (100) wafers by a plasma enhanced chemical vapour deposition technique using CH4 plus Ar as the feedstock. The friction and wear properties of the resulting films under different relative humidities, ranging from 5% to 100%, in a nitrogen environment, were measured using a ball-on-disc tribometer, with Si3N4 balls as the counterparts. The friction surfaces of the films and Si3N4 balls were observed on a scanning electron microscope, and investigated by x-ray photoelectron spectroscopy. The results showed that the friction coefficient increased continuously from 0.025 to 0.09 with increase in relative humidity from 5% to 100%, while the wear rate of the films sharply decreased and reached a minimum at a relative humidity of 40%, then it increased with further increase of the relative humidity. The interruption of the transferred carbon-rich layer on the Si3N4 ball, and the friction-induced oxidation of the films at higher relative humidity were proposed as the main reasons for the increase in the friction coefficient. Moreover, the oxidation and hydrolysis of the Si3N4 ball at higher relative humidity, leading to the formation of a tribochemical film, which mainly consists of silica gel, on the friction surface, are also thought to influence the friction and wear behaviour of the hydrogenated DLC films.

Synthesis and characterization of titanium-containing graphite-like carbon films with low internal stress and superior tribological properties

Titanium-containing graphite-like carbon films were deposited on silicon substrates by an unbalanced magnetron sputtering system. The effect of titanium concentration on the film microstructure and properties was subsequently investigated by means of different characterization techniques. It is found that the current carbon films have a graphite-like structure with some fine titanium carbide particles dispersed in an amorphous carbon matrix. With increasing titanium concentration from 0?at% to ?9.6?at%, the sp2 concentration in the film shows a slight increase, while the hardness of the carbon films decreases evidently when a small quantity of titanium (?2.9?at%) is introduced into the film structure, but it does not suffer an obvious change with further increase in titanium concentration until the titanium concentration is up to ?9.6?at%. The increased hardness of the film with a titanium concentration of about 9.6?at% is probably due to the formation of specified dimension titanium carbide crystals in the amorphous carbon matrix. All the current carbon films have low internal stress and rough surface, and the doping of titanium has little influence on their internal stress and root mean square roughness. The friction coefficient of the films decreases distinctly as the titanium concentration increases from 0?at% to ?9.6?at%, but the wear rate does not increase evidently until the titanium concentration exceeds over ?6.2?at% in air tests. The titanium-containing graphite-like carbon films show a stable friction coefficient and extremely low wear under oil lubricated conditions. The unique graphite-like structure, the high hardness and elastic modulus ratio (H/E) and the easy formation of a transfer film are mainly responsible for the superior tribological properties of the resulting titanium-containing graphite-like carbon films.

Fullerene-like hydrogenated carbon films with super-low friction and wear, and low sensitivity to environment

A novel hydrogenated carbon film containing fullerene-like nanostructure was prepared by pulse bias-assisted plasma enhanced chemical vapour deposition, and the fullerene-like arrangement in the film was characterized by high resolution transmission electron microscopy. The as-prepared hydrogenated carbon film exhibited super-low friction and wear in both dry N2 and humid ambient atmospheres, and was superior to the conventional hydrogenated carbon films. These excellent tribological properties could be attributed to the unique fullerene-like nanostructure, which endows the film with some special chemical and physical features, such as high chemical inertness, hardness and elastic recovery owing to the closed, curved and caged graphite planes, and hence, improves the tribological properties of the hydrogenated carbon film.

Effects of pulse bias duty cycle on fullerenelike nanostructure and mechanical properties of hydrogenated carbon films prepared by plasma enhanced chemical vapor deposition method

Fullerenelike hydrogenated carbon films were produced by pulse bias-assisted rf inductively coupled plasma enhanced chemical vapor deposition (ICPECVD). The effects of pulse duty cycle on the microstructure and mechanical properties of the resultant films were investigated by means of high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, nanoindentation, and stress measurement. The low pulse duty cycle was found the key in the formation of fullerenelike structure in hydrogenated carbon films, and thus increased the hardness, elasticity, and internal stress of the films. The role of pulse duty cycle in evolution of fullerenelike structure was also discussed in terms of ion bombardment, hydrogen removal, and “annealing” effects.

Structure, Mechanical, and Tribological Properties of MoS2/a-C:H Composite Films

A series of hydrogenated amorphous carbon (a-C:H) films doped with molybdenum disulfide (MoS2) were deposited by medium frequency unbalanced magnetron sputtering with mixed Ar/CH4 gases of different volume ratios as the source gases. The effects of Ar/CH4 ratio on morphology, microstructure, mechanical, and tribological properties of the MoS2/a-C:H composite films were investigated. Results show that the content of MoS2 in the as-deposited films decreases with the decreasing Ar/CH4 ratio, and the highest Ar/CH4 ratio favors the formation of nanostructured films. Besides, the hardness and internal stress of the composite films first decrease and then increase with decreasing Ar/CH4 ratio. Furthermore, the film deposited at the highest Ar/CH4 ratio exhibits excellent antiwear ability in all test environments and shows promising potential as a solid lubricating film in aviation and space industries.

Effects of environmental molecular characteristics and gas–surface interaction on friction behaviour of diamond-like carbon films

The superlow friction behaviours of diamond-like carbon (DLC) films in three different inert environments (dry N2, CO2 and Ar gas) were investigated and compared. The friction of the DLC films in dry N2 and CO2 was superior to that in dry Ar, and was dependent on the environmental exposure time. The possible reason is that N2 and CO2 have the same molecular characteristic of lone pair electrons at both sides of the molecules, while Ar has no lone pair electrons. And a special gas–surface interaction due to π orbital–lone pair electrons interactions is present at the sliding interfaces of DLC films in dry CO2 and N2. A friction model in relation to environmental molecular characteristics and gas–surface interactions was proposed to explain the friction behaviours of DLC films.

Perspectives of friction mechanism of a-C:H film in vacuum concerning the onion-like carbon transformation at the sliding interface

A-C:H films with low friction and good wear resistance have long been regarded as a potential space lubricating film. However, its superlubricity mechanism and failure process in vacuum still remains to be improved. To clarify its friction mechanism, here, we systematically investigated the tribological property of a typical a-C:H film under a high vacuum environment. The results show that the extremely low friction coefficient lasts 2700 cycles under a contact pressure of 930 MPa, and the entire friction process can be divided into three stages. The friction coefficient was first stable with a low value after a short period of running-in, then it underwent an evident fluctuation period and further decreased to an extremely low value (0.005) until it abruptly failed. The structural evolution of the a-C:H film on a sliding interface for different periods through the entire friction process was characterized and a dynamic friction mechanism was established. The self-mated (a-C:H/a-C:H) friction process and hydrogen passivation contributed to the decrease of the friction coefficient in the early stages of sliding. Then, the emission of hydrogen became evident under high local stress and more dangling bonds were exposed on the worn surface, which leads to the wild adhesion wear between the sliding surfaces. The alternated process between an old film and new film is in consonance with the fluctuation of friction coefficient. Afterwards, carbon onions with a closed spherical shell structure are spontaneously formed on the worn surface in the absence of the hydrogen passivation effect, which further reduce the friction coefficient to an extremely low value. This study provides guidance to the further design of a new generation of a-C:H films with a special structure that exhibits a better tribological performance in a vacuum environment.

Preparation and properties of MoS2/a-C films for space tribology

MoS2/a-C composite films with various (Mo+S)/C ratios were deposited by medium frequency unbalanced magnetron sputtering. The effects of MoS2 doping on the microstructure, mechanical and vacuum tribological properties of the films were investigated. Results show that the sp2 carbon content in the film increases with increasing (Mo+S)/C ratio from 0 to 0.19, and MoS2 nanocrystallines are formed in film as (Mo+S)/C ratio increased to 0.07. Consequently, the composite film exhibits decreasing hardness (from 5.2 to 2.5?GPa) and elastic modulus (from 116 to 24?GPa). As the (Mo+S)/C ratio increases from 0 to 0.19, the friction coefficient of the films decreases from 0.18 to 0.008 and sliding time increases from 3?s to more than 3600?s in vacuum. This is mainly attributed to that the films with higher (Mo+S)/C ratios (at least 0.12) are inclined to form the lamellar MoS2 with low shear strength on the counterface. Furthermore, the composite film with (Mo+S)/C ratio of 0.12 has been investigated in various test environments (air, N2, vacuum). The average friction coefficient is lower than 0.035 and it exhibits little sensitivity to the test environment. However, the highest and lowest wear rate (about 4???10?7?mm3?Nm?1 and 0.8???10?7?mm3?Nm?1) are obtained in vacuum and N2, respectively. The environment dependence of the tribological behaviours is related with the lattice orientation of MoS2 crystalline and the graphitization degree of the film.