Xuefeng Shu

Microstructure, corrosion and tribological and antibacterial properties of Ti-Cu coated stainless steel.

A Ti-Cu coated layer on 316L stainless steel (SS) was obtained by using the Closed Field Unbalanced Magnetron Sputtering (CFUBMS) system to improve antibacterial activity, corrosion and tribological properties. The microstructure and phase constituents of Ti-Cu coated layer were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and glow discharge optical emission spectrometry (GDOES). The corrosion and tribological properties of a stainless steel substrate, SS316L, when coated with Ti-Cu were investigated in a simulated body fluid (SBF) environment. The viability of bacteria attached to the antibacterial surface was tested using the spread plate method. The results indicate that the Ti-Cu coated SS316L could achieve a higher corrosion polarization resistance and a more stable corrosion potential in an SBF environment than the uncoated SS316L substrate. The desirable corrosion protection performance of Ti-Cu may be attributable to the formation of a Ti-O passive layer on the coating surface, protecting the coating from further corrosion. The Ti-Cu coated SS316L also exhibited excellent wear resistance and chemical stability during the sliding tests against Si3N4 balls in SBF environment. Moreover, the Ti-Cu coatings exhibited excellent antibacterial abilities, where an effective reduction of 99.9% of Escherichia coli (E.coli) within 12h was achieved by contact with the modified surface, which was attributed to the release of copper ions when the Ti-Cu coatings are in contact with bacterial solution.

Plastic characterization of metals by combining nanoindentation test and finite element simulation

Abstract Materials with the same elastic modulus E and representative stress and strain ( σ r , ɛ r ) present similar indentation–loading curves, whatever the value of strain hardening exponent n . Based on this definition, a good approach was proposed to extract the plastic properties or constitutive equations of metals from nanoindentation test combining finite element simulation. Firstly, without consideration of strain hardening, the representative stress was determined by varying assumed representative stress over a wide range until a good agreement was reached between the computed and experimental loading curves. Similarly, the corresponding representative strain was determined with different hypothetical values of strain hardening exponent in the range of 0–0.6. Through modulating assumed strain hardening exponent values to make the computed unloading curve coincide with that of the experiment, the real strain hardening exponent was acquired. Once the strain hardening exponent was determined, the initial yield stress σ y of metals could be obtained by the power law constitution. The validity of the proposed methodology was verified by three real metals: AISI 304 steel, Fe and Al alloy.

Ballistic resistance and energy absorption of honeycomb structures filled with reactive powder concrete prisms

Two kinds of innovative re-entrant and hexagonal cell honeycomb sandwich structures filled with reactive powder concrete were proposed, and the ballistic resistance and energy absorption of the sandwich structures were investigated by numerical simulations. The deformation and failure modes of the different structures were analyzed and evaluated in detail. The honeycomb sandwich structures filled with reactive powder concrete prisms improved the capacity of ballistic resistance and energy absorption significantly, compared to the normal reactive powder concrete plates and sandwich structures without reactive powder concrete prisms. The analysis shows that the auxetic re-entrant cell honeycomb sandwich structures have a better ballistic performance than the hexagonal cell honeycomb sandwich structures. The sandwich structures were subjected to impact by three kinds of projectiles: flat, hemispherical and conical nosed. The ballistic limit of the flat nosed projectile is the highest, while the impact performance of the conical and hemispherical nosed projectiles is obviously different from the flat nosed projectile, especially in a relative high velocity range. The sharper nose leads to a higher value of exit velocity and mass loss. In addition, effects of different design parameters on ballistic resistance were also studied by changing the thickness of honeycomb cell and face plates. Results indicate that the thickness of honeycomb walls and face plates have significant effect on the ballistic resistance and energy absorption in a relative low velocity range, while there are no big differences when the initial impact velocity exceeds 400u2009m/s.

Photocatalytic activities of N doped TiO2 coatings on 316L stainless steel by plasma surface alloying technique

Nitrogen doped titanium dioxide (N-TiO2) coatings were fabricated by oxidation of the TiNx coatings in air. TiNx coatings were prepared on stainless steel (SS) substrates by plasma surface alloying technique. The reference TiO2 sample was also deposited by oxidation of the Ti coatings in air. The as-prepared coatings were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and ultra violet-visible absorption spectroscopy (UV-Vis). The formation of anatase type TiO2 is confirmed by XRD. SEM measurement indicates a rough surface morphology with sharp, protruding modules after annealing treatment. The band gap of the N-doped sample is reduced from 3.25 eV to 3.08 eV compared with the undoped one. All the N-doped samples show red shift in photoresponse towards visible region and improved photocurrent density under visible irradiance is observed for the N-doped samples. The photocatalytic activity was evaluated via the photocatalytic oxidation of methylene blue (MB) in aqueous under visible light irradiation. The results reveal that the N-doped samples extend the light absorption spectrum toward the visible region. The degradation rate of N-TiO2 is 20% in visible irradiation for 150 min.

Strain-induced phase transformation behavior of stabilized zirconia ceramics studied via nanoindentation

To study the tetragonal-to-monoclinic (T-M) phase transformation behavior under different strain rates and indentation depths, nanoindentation tests were performed on stabilized zirconia ceramics with Continuous Stiffness Measurements. The results indicate decreased phase transformation velocities at both lower and higher strain rates, but increased velocity under medium strain rate during loading. The phase transformation process is sensitive to Ṗ/P but the final volume fractions are almost identical (45%). Furthermore, most of the phase transformation is completed during a short initial time followed by slight linear increase of the M-phase volume fraction with holding time. The phase transformation continuously slowed with increasing indentation depth when indented with a constant strain rate.

Assessment of titanium metallization thin film deposited on alumina substrate: Microstructure and nano-indentation

Surface titanium (Ti) metallization was conducted on alumina (Al2O3) through chemical vapor deposition (CVD) method derived from non-contact pack cementation. The effects of different deposition temperature (1000u202f°C, 1050u202f°C, and 1100u202f°C) were examined in this scenario. The morphology, phase composition, and interfacial defects of the resulting films were systematically investigated through scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction. The nanomechanical characterization of the proposed thin films was evaluated by conducting nano-indentation tests at different depths. The results revealed that uniform Ti films were coated on the Al2O3 substrate. During coating, the atoms on the matrix surface were driven to form different structure due to different deposition temperature, leading to disparate morphologies of the surface and the interface, which consequently influenced the binding force between the film and the substrate. Moreover, the nanomechanical properties were found to be related to the internal and interface structure. Decreased modulus and hardness were obtained for metallization films treated at 1050u202f°C, and plastic deformation was the main deformation pattern.

Bacteria adherence properties of nitrogen-doped TiO2 coatings by plasma surface alloying technique

In order to obtain a high-performance surface on 316L stainless steel (S. S) that can meet the requirements in medical material field environment, nitrogen-doped titanium dioxide (TiO2−xNx) was synthesized by oxidative annealing the resulted TiNx coatings in air. Titanium nitride coatings on 316L S. S were obtained by plasma surface alloying technique. The as-prepared coatings were characterized by X-ray diffraction, glow discharge optical emission spectrometer (GDOES), scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The bacteria adherence property of the TiO2−xNx coatings on S. S on the oral bacteria Streptococcus Mutans was investigated and compared with that of S. S by fluorescence microscopy. The mechanism of the bacteria adherence was discussed. The results show that the TiO2−xNx coatings are composed of anatase crystalline structure. SEM measurement indicates a rough surface morphology with three-dimensional homogenous protuberances after annealing treatment. Because of the photocatalysis and positive adhesion free energy, the TiO2−xNx coatings inhibit the bacteria adherence.

Experimental investigation on the yield behavior of PMMA

Quasi-static compression, tension and combined shear–compression tests were conducted to characterize the mechanical behavior of PMMA. The cylinder samples and dog-bone samples were used for uniaxial testing. The shear–compression samples of four different angles (15°, 30°, 45°, and 50°) were used to obtain the multiaxial stress state instead of complex loading equipment. The yield behaviors of PMMA cannot be described by the Tresca or Mises criterion due to the effects of both the first invariant of the stress and the third invariant of deviatoric stress. A criterion is proposed that includes the first invariant of the stress and the third invariant of deviatoric stress in this paper. In other word, the effects of hydrostatic pressure and Lode angle are considered. The proposed yield criterion has the capability to precisely capture the experimental yield loci of PMMA.

Shear properties of isotropic conductive adhesive joints under different loading rates

ABSTRACT Epoxy-based conductive adhesives have been widely used in the electronic field given the lead-free development of electronic packaging. The conductive adhesive joints must be subjected to shear loads during the service of electronic products considering the mismatch in mechanical properties between packaged chip and substrate. In this study, INSTRON 5544 universal material testing machine was used for tensile–shear tests of isotropic conductive adhesive joint specimens, which were prepared using pure copper plate adherend in the form of single-lap joints. Four loading rates, that is, 0.05, 0.5, 5, and 10 mm/min, were adopted. The relationship between shear load and displacement of two overlapping copper plates is deduced from a mechanical perspective. A mechanical model of the conductive adhesive shear specimen was developed by introducing dimensionless parameters, which are obtained from interfacial fracture energy and shear strength, to interpret the effect of loading rate on the shear properties of the conductive adhesive specimen considering the loading rate. Results show that this model can effectively reflect the relationship between shear load and displacement in the range of 0.05–10 mm/min.

CHARACTERISTICS OF TiNi THIN FILMS DEPOSITED BY MAGNETRON SPUTTERING SYSTEM WITH OPTICAL EMISSION SPECTROSCOPY MONITOR

TiNi composite thin films were fabricated using a closed-field unbalanced magnetron sputtering system equipped with optical emission spectroscopy monitor (OEM). The thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and nanoindentation. Results show that the TiNi films are amorphous, and their composition varies approximately linearly with the OEM value. Thus, the film composition could be controlled by in situ real-time OEM. The structure of the single B2 parent phase was observed in the annealed TiNi film. The hardness and elastic modulus of the films increased because of the precipitation of the Ti3Ni4 phase in the single B2 parent phase.