Papers by Author: Xiang Yu

Abstract: The combination of diamond-like carbon (DLC) coatings and fluid lubrication is an efficacious method to improve the performance and service life of the friction-pairs working under the severe conditions, but the synergistic effect of DLC coatings and lubricant additives have not been clearly unveiled. The synergistic effect of Cr-doped DLC coatings with different Cr content and the lubricant additives of ZDDP, MoDTC, and T307 on the wear of DLC-coated stainless steel sample was studied, and it was found that the wear resistance of DLC-coated stainless steel sample can be further improve by the combination of DLC coatings and fluid lubrication; doping DLC coatings with Cr at an optimum level is beneficial for the wear resistance of DLC-coated stainless steel sample lubricated by PAO, PAO+ZDDP or PAO + MoDTC, but DLC coatings with a moderate Cr content is harmful to the wear resistance of DLC-coated stainless steel sample lubricated by PAO+T307. The synergistic effect of T307 and undoped DLC coatings or Cr-doped DLC coatings with a Cr content of 23.3 % is the best, while the three studied lubricant additives are harmful for the wear resistance of DLC coatings with a Cr content of 2.4 %.

Abstract: Influence of the process parameters of a reactive mid-frequency dual-magnetron sputtering on surface defects of Cr_xO_y film was investigated. The forming mechanisms of the observed droplets and craters were analyzed. The optimal parameter combination for accomplishing fewer surface defects was explored; and the optimized process parameters evidently minimized the surface defects of the film. In the condition of the target current of 16 A, the gases pressure of 0.31 Pa, and the bias voltages in a range of -120~-240 V, an optimized Cr_xO_y film has been synthesized with surface defect density low to 78 defect/mm^-2.

Abstract: The electrical properties of ZnO-Pr6O11-based varistors, which composed of (98.5-x)mol% ZnO +0.5mol% Pr6O11 +1.0mol% Co3O4 +xmol% Cr2O3 (x=0.0, 0.25, 0.5, 1.0, 2.0), were investigated as a function of sintering temperature in the range of 1250-1400°C for 2 h. At the same sintering temperature, the nonlinear exponents increased with the increase of Cr2O3 doping amount no more than 0.5mol%, but decreased with more Cr2O3 doped. With the same doping level of Cr2O3, the samples' varistor voltage decreased with the sintering temperature increased; and the samples' nonlinear exponents increased first with the sintering temperature increased to 1300 °C, but decreased when the temperature increased further. Conclusively, the electrical properties of ZnO-Pr6O11-Co3O4- Cr2O3 varistors were strongly affected by the sintering temperature and the doping level of Cr2O3, and the samples, doped with 0.5 mol% Cr2O3 and sintered at 1300 °C, exhibited the optimum nonlinear performance.

Abstract: This work investigated the wear behavior of cemented carbide cermet YG8B under different concentrations of abrasive slurries using a modified ASTM B611 wet sand rubber rimmed wheel test system with a load of 225 N. The angular sand abrasives involved in carborundum, corundum and silica with particle size of about 350 μm, and were dispersed into water with different mass fractions. It was found that, with increasing sand concentration in slurry, the wear loss of the cermet samples increased, and with the increase of duration time, the wear loss of the cermet samples also increased. Under the same conditions, the cermet samples presented the biggest wear loss with carborundum as abrasive. On the basis of the observations on the worn surfaces by scanning electron microscope and 3D white-light interfering surface profiler, the wear mechanism of the cermet samples was proposed, which includes in extensive plastic deformation, groove, fracture and pullout.

Abstract: Ti-doped DLC films were synthesized from C2H2 and Ti by ion beam deposition and magnetron sputtering, and the influence of Ti target current on the structures and the friction coefficients of Ti-doped DLC films were studied by SEM, AES, XPS, a ball-on-disk friction tester. The Ti content in Ti-doped DLC films is below 1at% when Ti target currents rise up to 3.5 A; but when Ti target current exceeds 3.5 A, Ti content in the films is obviously increased with increasing Ti target current. Ti-doped DLC films have a smooth dense surface with several particles dispersed in it and the surface morphology deposited with different Ti target currents is similar. With the augment of Ti target current, the ratio of sp3-C to sp2-C is almost a constant first, and then an obvious rise followed by an abrupt decrease is found; but only when Ti target current is above 5A, the ratio of TiC-C to sp2-C is obviously increased with Ti target current. The lowest friction coefficients are obtained for the Ti-doped DLC films with the highest ratio of sp3-C to sp2-C and a low ratio of TiC-C to sp2-C.

Abstract: In this work, we have investigated the influence of the pulsed substrate bias voltage on the evolution of the surface morphology and sp3 hybridization degree of Ag containing diamond-like carbon (Ag-DLC) films deposited by using a mid-frequency dual-magnetron sputtering system. The unipole substrate bias voltage at 0 V, -60 V, -100 V and -150 V, respectively, was employed on AISI 440 substrate with the duty ratio uniformly set at 70%. The surface morphology was observed by AFM and the hybridization degree of the DLC films was performed respectively using Raman Spectroscopy and Rockwell C indenter. In these samples, the surface morphology and sp3 hybridization degree of Ag-DLC films show a certain dependence on the pulsed substrate bias applied. The evolution tendency of the surface morphology is found different with that of the sp3 hybridization degree of the DLC films under action of the pulsed substrate bias. These phenomena imply that the energetic particles may induce a balance between recombination of the particles and change of the internal stress in the DLC films.

Abstract: W-doped DLC films were synthesized from ethyne and tungsten by ion beam deposition and magnetron sputtering, and the influence of W target current on the structures and the properties of W-doped DLC films were studied. There exist some defects smaller than 3micron in W-doped DLC films and the influence of W target current on the defects is unobvious. The W content in the films is tardily increased with W target current below 3.5A, and then acutely rises with W target current. When target current is below 3.5A, the ratio of sp3-C to sp2-C is first decreased and then increased with the rise of target current, and the ratio of WC-C to sp2-C is close to 0; but when the target is above 3.5A, the ratio of sp3-C to sp2-C is decreased and the ratio of WC-C to sp2-C is augmented with further increasing target current. The hardness and the modulus is first decreased with target current and the minimum value is reached for the W-doped DLC films deposited with a target current of 2.6A. The W-doped DLC films deposited with a target current of 2.6A exhibit the best film-substrate adhesion. The W-doped DLC films deposited with a low target current exhibit a friction coefficient while the wear resistance of the W-doped DLC films deposited with a medium target current of 2.6A is best.

Abstract: W-doped DLC films were synthesized from CH4 and W by ion beam deposition and magnetron sputtering, and the influence of W target current on the surface morphology and the mechanical properties of W-doped DLC films deposited were studied. The W-doped DLC films in the study have a smooth dense surface with several particles of about 2micron. The hardness, the modulus, and the film-substrate adhesion of the films are increased with the rise of W target current and the critical load of the scratch test for all the W-doped DLC films is above 70N. The friction coefficient of W-doped DLC films is increased with the increase of W target current while the lowest wear rate is obtained when W target current is 1A.

Abstract: Aiming at improving their tribological behaviors, adhesion of diamond-like carbon (DLC) films on high-speed steel was investigated by varying doping silver (Ag) contents in a mid-frequency dual-magnetron sputtering system. Scratch testing was performed on the Ag-DLC films under a progressive normal load from 3 to 80 N, along with a Rockwell C indenter at a relative displacement speed for a Rockwell testing. A microscopic analysis of the scratch evolution under a progressive normal load permits identification of the various traces and the damage mechanisms of the films. A process of the film adhesion failure typically in turn appears like this, germination of the cracks along the longitudinal edges of the scratch traces; propagation in front of the indenter; and, detachment in the subsurface by shearing. An Ag-DLC film of 15.2 at% Ag exhibits a superior adherence on the steel substrate as compared with the three Ag DLC films of the other doping silver contents.