Yingying Fu

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.

Optimization of cubic GaN growth by metalorganic chemical vapor deposition based on residual strain relaxation

The reduction of residual strain in cubic GaN growth by inserting a thermoannealing process is investigated. It is found that the epilayer with smaller tensile strain is subject to a wider optimal “growth window.” Based on this process, we obtain the high-quality GaN film of pure cubic phase with the thickness of 4 μm by metalorganic chemical vapor deposition. The photoluminescence spectrum at room temperature shows the thick GaN layer has a near-band emission peak with a full width at half maximum of 42 meV which confirms its high crystal quality, further supported by the x-ray (002) diffraction measurement. A simplified model is demonstrated to interpret this strain effect on the growth process.

Mechanisms of the sidewall facet evolution in lateral epitaxial overgrowth of GaN by MOCVD

The lateral epitaxial overgrowth of GaN was carried out by low-pressure metalorganic chemical vapor deposition, and the cross section shape of the stripes was characterized by scanning electron microscopy. Inclined {11–2n} facets (n ≈ 1–2.5) were observed in the initial growth, and they changed gradually into the vertical {11–20} sidewalls in accordance with the process of the lateral overgrowth. A model was proposed utilizing diffusion equations and boundary conditions to simulate the concentration of the Ga species constituent throughout the concentration boundary layer. Solutions to these equations are found using the two-dimensional, finite element method. We suggest that the observed evolution of sidewall facets results from the variation of the local V/III ratio during the process of lateral overgrowth induced by the lateral supply of the Ga species from the SiNx mask regions to the growing GaN regions.

Friction and Wear Properties of Graphite-Like Carbon Films Deposited on Different Substrates with a Different Interlayer Under High Hertzian Contact Stress

Four types of graphite-like carbon (GLC) films were deposited on different substrates (Ti6Al4V, WC-27CrNi) with a different interlayer (TiC/Ti, TiC/Ti/TiN) using an unbalanced magnetron sputtering system. The effect of substrate and interlayer on the microstructure and properties of the studied GLC films was then investigated using different characterization techniques. The results show that both the substrate and interlayer had an obvious influence on the tribological properties of the studied GLC films even though there was no significant structural difference between these films. Specifically, a substrate with a high hardness was propitious to achieving superior tribological behaviors for carbon film even with a different interlayer. However, the interlayer had a distinct influence on the tribological properties of the carbon film deposited on different substrates, and this effect varied with the hardness property of the substrate. For a hard substrate, the wear rate and wear life were similar irrespective of the interlayer. For a soft substrate, the addition of a TiN interlayer improved the wear life sevenfold compared to the film with only a TiC/Ti interlayer, but the wear rate for a film with and without a TiN interlayer was approximately the same. The obvious discrepancy between wear life and wear rate for a carbon film deposited on soft substrate was closely related with the film adhesion strength and plastic deformation of the substrate materials. Based on these results, it can be concluded that the wear life is a better parameter than wear rate in terms of characterization of the wear resistance of carbon film once the applied load causes the plastic deformation of the substrate.

Investigation on the origin of crystallographic tilt in lateral epitaxial overgrown GaN using selective etching

The crystallographic tilt of the lateral epitaxial overgrown (LEO) GaN on sapphire Substrate with SiNx mask is investiaated by double crystal X-ray diffraction. Two wing peaks beside the GaN 0002 peak can be observed for the as-grown LEO GaN. During the selective etching of SiNx mask, each wing peak splits into two peaks, one of which disappears as the mask is removed, while the other remains unchanged. This indicates that the crystallographic tilt of the overgrown region is caused not only by the plastic deformation resulted from the bending of threading dislocations, but by the non-uniformity elastic deformation related with the GaN, SiNx interfacial forces. The widths of these two peaks are also studied in this paper

Structural characterization of epitaxial lateral overgrown GaN on patterned GaN/GaAs(001) substrates

Epitaxial lateral overgrown (ELO) cubic GaN (c-GaN) on SiO2 patterned GaN/GaAs(0 0 1) substrates by metalorganic vapor phase epitaxy was investigated using transmission electron microscopy and X-ray diffraction (XRD) measurements. The density of stacking faults (SFs) in ELO c-GaN was similar to6 x 10(8) cm(-2), while that in underlying GaN template was similar to5 x 10(9) cm(-2). XRD measurements showed that the full-width at half-maximum of c-GaN (0 0 2) rocking curve decreased from 33 to 17.8 arcmin, indicating the improved crystalline quality of ELO c-GaN. The mechanism of SF reduction in ELO c-GaN was also discussed

Relationship between bias voltage and microstructure as well as properties of CrAlYN films

In this work, a series of CrAlYN films doped with 1 at.% yttrium were deposited by unbalanced reactive magnetron sputtering under different bias voltages. The effects of bias voltage on microstructure and properties of the CrAlYN films were subsequently investigated. It is found that all the as-deposited films have similar chemical composition and crystalline structure. However, the bias voltage has significant impact on the mechanical properties and oxidation resistance of the resulting films. Namely, the film deposited at 100 V has the highest hardness and best oxidation resistance, which are mainly attributed to its denser structure and higher Al content than others. In addition, the film obtained at 100 V exhibits superior oxidation resistance even at 1000 °C, and good friction and wear properties at 600 and 800 °C, and the latter two are mainly ascribed to the formation of compact transfer layer on the worn surfaces. However, this film experienced obvious wear loss at low testing temperatures (i.e., 200 and 400 °C) due to the serious abrasive wear.

Atomic scale study of strain relaxation in Sn islands on Sn-induced Si(111)-(23×23) surface

Surface structure of the Sn islands 5 ML high, prepared on Si(111)-(2 root 3X2 root 3)-Sn substrate, is investigated by low temperature scanning tunneling microscopy/spectroscopy. Due to the elastic strain relaxation in the islands, the in-plane unit cell structure distorts and the apparent height of the surface atoms varies regularly to form an overall modulated strip structure. The quantum well states are observed to depend on the relative position within this structure, which implies the change of the surface chemical potential induced by the elastic strain relaxation as well.

Preparation and high-temperature tribological properties of CrAlVYN-Ag nanocomposite coatings

ABSTRACT CrAlVYN-Ag coatings were successfully deposited by introducing the elements of V and Ag into CrAlYN hard coatings. Their microstructure and mechanical property as a function of the annealing temperature were investigated from room temperature (RT) (∼25°C) to 800°C. Besides, the worn surfaces were analyzed after combined effects of temperature and friction to figure out the wear mechanisms at different temperatures. The coatings exhibited special surface morphologies and Ag diffusion after annealing at different temperatures. In addition, the x-ray diffraction results showed that the coatings suffered obvious oxidation once the temperature exceeded 600°C; as a result of this, the coating hardness decreased sharply. The friction coefficients were relatively high during the tribological tests from RT to 400°C because the abrasive wear mechanisms played a dominant role. The lower friction coefficients obtained at 600 and 700°C were mainly due to the self-lubricating mechanisms. However, the coating exhibited higher friction coefficient at 800°C, which was mainly ascribed to the severe oxidation wear of the coating.

Study on the microstructure and properties of graphite-like carbon films deposited by unbalanced magnetron sputtering

A series of graphite-like amorphous carbon films were deposited by unbalanced magnetron sputtering system with different sputtering currents. The microstructure, morphology, contact angle, and tribological properties of the resultant carbon films were subsequently investigated by means of different characterization techniques. Raman analysis shows that the resultant amorphous carbon films are dominated by sp2 sites, and the intensity ratio of the D and G peaks increases gradually from ∼4.1 to ∼5.4 with decrease in the sputtering current from 16 to 4 A, which is one order of magnitude larger than that of diamond-like carbon films with high sp3 content, indicating a more graphite-like structure. The surface of the resulting carbon films is composed of granular structure with different diameters and heights, and the root mean square roughness of the studied films monotonically increases from 3.5 nm at a sputtering current of 4 A to 8.2 nm at 16 A. The contact angle of graphite-like carbon films with water changes from 43.4° to 89.5° with decrease in the sputtering current. Besides, these graphite-like carbon films demonstrate superior tribological properties with high load-bearing capacity and low friction coefficient in humid atmosphere. The superior tribological behaviors of this graphite-like carbon film are promising for a wide range of applications such as gears, cutting tools, automobile parts, etc.