Tianmin Shao

Impact wear performances of Cr3C2NiCr coatings by plasma and HVOF spraying

Abstract Three kinds of Cr 3 C 2 25%NiCr coatings prepared by APS-Ar H 2 , APS-Ar He and HVOF spraying, respectively, have been studied with a self-made impact wear rig. The wear resistant performances of Cr 3 C 2 25%NiCr coatings were tested under different impact stress. The wear losses and worn surface profiles are measured, and the corresponding wear mechanisms were studied with Scanning Electronic Microscope (SEM). It is found that wear resistance of Cr 3 C 2 25%NiCr coatings under different impact conditions increases in the order of APS-Ar H 2 APS-Ar He and HVOF spraying. The HVOF sprayed Cr 3 C 2 25%NiCr coatings show the best impact wear resistance due to the denser structure and fewer defects. The wear mechanisms of Cr 3 C 2 25%NiCr coatings under impact condition is impact fatigue, which consists of two wear processes of plastic smearing and breaking-off. The plastic smearing produces plastic deformation of the thin layer in the outer coatings and then causes microcracks in the near subsurface due to stress accumulation. There are horizontal and perpendicular microcracks. The former commonly nucleate from the boundary of lamellae, while the latter may occur from the sub-microcracks inside lamellae. The propagating and merging of both horizontal and perpendicular microcracks finally give rise to the breaking-off of coatings. The cohesive defects such as stratification, porosity and microcracks in the lamellae have much effects on the wear resistance of coatings.

A method of micro laser surface texturing based on optical fiber focusing

In the present paper, optical fibers are used as focusing unit in the process of laser surface texturing, which can effectively decrease the area of the focused facular point. An original experimental equipment was built and described. Micro-textures of parallel grooves and meshed textures were produced on silicon surface. Extensive experiments proved that micro-marking of 2–3 μm could be etched with proper focusing distance.

Effect of contact deformation on contact electrification: a first-principles calculation

The effect of contact deformation on contact electrification of metallic materials was studied by the first-principles method. The results of charge population and the densities of states of the deformed contact models demonstrated that the magnitude of the transferred charge increased with deformation. The mechanism of the effect of deformation was investigated by studying the electronic properties of the deformed surface slabs. The results showed that crystal deformation led to a change in the electrostatic potential of the metal, where the number of nearly free electrons and unoccupied orbitals for charge transfer increased, and their energy barrier decreased.

Mechanical Properties and Thermal Stability of TiN/Ta Multilayer Film Deposited by Ion Beam Assisted Deposition

TiN/Ta multilayer film with a modulation period of 5.6 nm and modulation ratio of 1 : 1 was produced by ion beam assisted deposition. Microstructure of the as-deposited TiN/Ta multilayer film was observed by transmission electron microscopy and mechanical properties were investigated. Residual stress in the TiN/Ta multilayer film was about 72% of that of a TiN monolayer film with equivalent thickness deposited under the same conditions. Partial residual stress was released in the Ta sublayers during deposition, which led to the decrease of the residual stress of the TiN/Ta multilayer film. Nanohardness (H) of the TiN/Ta multilayer film was 24 GPa, 14% higher than that of the TiN monolayer film. It is suggested that the increase of the nanohardness is due to the introduction of the Ta layers which restrained the growth of TiN crystal and led to the decrease of the grain size. A significant increase (3.5 times) of the / (E elastic modulus) value indicated that the TiN/Ta multilayer film has higher elasticity than the TiN monolayer film. The (critical load in nano-scratch test) value of the TiN monolayer film was 45 mN, which was far lower than that of the TiN/Ta multilayer film (around 75 mN). Results of the indentation test showed a higher fracture toughness of the TiN/Ta multilayer film than that of the TiN monolayer film. Results of differential scanning calorimetric (DSC) and thermo gravimetric analysis (TGA) indicate that the TiN/Ta multilayer film has better thermal stability than the TiN monolayer film.

Laser etching of groove structures with micro-optical fiber-enhanced irradiation

A microfiber is used as a laser-focusing unit to fabricate a groove structure on TiAlSiN surfaces. After one laser pulse etching, a groove with the minimum width of 265 nm is manufactured at the area. This technique of microfabricating the groove in microscale is studied. Based on the near-field intensity enhancement at the contact area between the fiber and the surface during the laser irradiation, simulation results are also presented, which agree well with the experimental results.

Improvement on thermal stability of TiAlSiN coatings deposited by IBAD

ABSTRACT TiAlSiN coatings were deposited on Si substrates by ion beam-assisted deposition. The oxidation resistance and mechanism of TiAlSiN at 800°C, 900°C, and 1000°C under a nitrogen atmosphere were discussed and compared with those of TiAlSiN under argon and air atmospheres. The microstructure, bonding characteristics, and phase transition of the coatings were systematically investigated. The results indicated that nitrogen atmosphere effectively inhibited oxidation of the TiAlSiN coatings. The key reason for this improvement was that nitrogen maintained Ti–N and Si–N chemical bonds, and a dense Al2O3 layer was much more easily generated under the nitrogen atmosphere. We used first-principles calculations to investigate the electronic structures and bonding characteristics of TiAlSiN in this work.

Fabrication of micro grooves on silicon by micro optical fiber enhanced pulse laser irradiation

We report a microfabrication technique on micrometer to submicrometer scales using optical fiber enhanced irradiation. Micro quartz fibers with diameters of less than 5 μm were prepared and used as the focusing unit. Micro grooves with widths of microns and submicrons were fabricated. The influence of processing parameters such as fiber diameter, laser pulse energy and laser wavelength on groove features was studied. Finite difference time domain (FDTD) solutions were used to simulate the fiber optical field. Comparison between the results of simulation and experiment were made and discussed.

Laser induced twin-groove surface texturing based on optical fiber modulation

A new and simple twin-groove texture method is carried out by using an optical fiber as a focus unit in a laser surface texturing process. A laser pulse produces a set of twin grooves, which has 1–2 μm width, much slimmer than the 3 μm width of the grooves etched at the fibers focus point. The length–width ratio of the ultra-long twin grooves is over 2000. Micro-textures of twin grooves and meshed textures were produced on Si and (Ti, Al, Si)N hard coating, whose characters were strongly affected by the light diffraction behind the fibers. Finite-element simulations show that the energy fields in the fiber etching system are following Fresnel diffraction rules, which modulate the input energy to be localized on the coating surface.