Q. Wei

Evolution and microstructure of shear bands in nanostructured Fe

Shear band development in consolidated nanocrystalline and ultrafine-grained Fe has been monitored as a function of overall strain from the onset of plastic deformation. The deformation mechanisms of the grains inside the shear bands, the origin of the inhomogeneous deformation, and the propensity for shear localization in nanostructures are explained based on microstructural information acquired using transmission electron microscopy.

Structural characteristics of AlN films deposited by pulsed laser deposition and reactive magnetron sputtering: A comparative study

Aluminum nitride films have been deposited on Si(111) substrates at different substrate temperatures using two techniques; pulsed laser deposition or reactive magnetron sputtering. The films deposited by either of the techniques have been characterized by x-ray diffraction and transmission electron microscopy to determine the crystalline quality, grain size, and epitaxial growth relation with respect to the substrate. The bonding characteristics and the residual stresses present in the films have been evaluated using Raman and Fourier transform infrared spectroscopy. Secondary ion mass spectrometry has been performed to determine the nitrogen stoichiometry and the presence of impurities such as oxygen and silicon. The adhesion strength of the AlN films to the silicon substrate and the wear resistance have been determined by scratch test and a specially designed microscopic wear test. A comparison of the different characteristic features associated with the AlN films deposited by pulsed laser deposition or...

Microstructure and mechanical properties of tantalum after equal channel angular extrusion (ECAE)

We have investigated the microstructure and mechanical properties of equal channel angular extruded (ECAE) Ta. Mechanical properties were measured both under quasi-static loading and dynamic loading (in the latter case, the compression Kolsky bar technique was employed to attain strain rates of ∼10 3 s −1 ). It is shown that four passes of ECAE with route C at room temperature, which results in an equivalent strain of ∼4.64, increases the strength of Ta by a factor of 2–3 under quasi-static loading, and by a factor of more than 1.5 under dynamic loading. Under quasi-static loading, the ECAE processed samples exhibit almost elastic-perfect plastic behavior; under dynamic loading, slight softening is observed, presumably due to adiabatic heating. It is found that ECAE decreases the strain rate sensitivity. Comparison of the X-ray diffraction (XRD) between the un-processed and ECAE processed Ta indicates significant broadening of the XRD peaks in the ECAE processed sample. Transmission electron microscopy reveals textured, elongated substructures with an average size of about 200 nm, and the substructures are separated by small angle grain boundaries. This work shows the potential for the production of ultra-fine grained or even nano-structured refractory metals with high melting points by using severe plastic deformation. Signs indicating increased shear localization tendancy were observed at high strain rates.

Plastic flow localization in bulk tungsten with ultrafine microstructure

Shear localization is demonstrated in bulk tungsten (W) of commercial purity under dynamic uniaxial compression. Microstructure refinement via severe plastic deformation was the strategy used to induce this unusual deformation mode for W. The ultrafine microstructure achieved in bcc materials leads to elevated strength and ductility, as well as reduced strain hardening and strain rate hardening, thus enhancing the propensity for adiabatic plastic flow localization.

Preparation and mechanical properties of composite diamond-like carbon thin films

We have investigated mechanical properties of diamond-like carbon (DLC) thin films, particularly the internal compressive stress and ways to alleviate it. Foreign atoms such as copper, titanium, and silicon were incorporated into the DLC films during pulsed laser deposition. The chemical composition of the doped films was determined using Rutherford backscattering spectrometry (RBS) and x-ray photoelectron spectroscopy (XPS). Optical microscopy of the doped films showed that DLC films containing Cu exhibit much less particulate density as compared to the films containing Ti and Si. Visible Raman spectroscopy was used to characterize the films. The effect of dopants on the Raman spectrum was analyzed in terms of peak shape and position. Optical microscopy of the pure DLC of a certain thickness showed severe buckling. The mechanisms of adhesion associated with DLC coatings were discussed. Qualitative scratch tests on the specimens showed that pure DLC films have relatively poor adhesion due to a large compr...

Mechanical properties of diamond-like carbon composite thin films prepared by pulsed laser deposition

We have investigated the mechanical properties of diamond-like carbon (DLC) thin films that contain foreign atoms. The DLC films were prepared by pulsed laser deposition. A novel target design was adopted to incorporate foreign atoms into the DLC films during film deposition. Copper, titanium and silicon are chosen as the dopants. The chemical composition of the doped films was determined using Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy and calibrated extrapolation. Experimental results of both visible and UV Raman are presented and discussed in terms of peak shape and position. The effect of dopants on the Raman spectrum is also analyzed. Optical microscopy of the pure DLC of a certain thickness showed severe buckling. A brief review of the theoretical background of adhesion is given and the possible mechanisms of adhesion that may work in DLC coatings are discussed. Qualitative scratch tests on the specimens show that pure DLC has quite poor adhesion due to the large compressive stress, while the doped DLC films exhibit much improved adhesion. Wear tests show improved wear resistance in the doped DLC coatings. Nanoindentation results give an average hardness above 40 GPa and effective Youngs modulus above 200 GPa for pure DLC. The copper doped DLC films showed slightly decreased hardness and Youngs modulus as compared to pure DLC films. Ti and Si can reduce the hardness and Youngs modulus more than Cu. All these can be understood by analyzing the internal stress reduction as derived from Raman G-peak shift to lower wavenumbers. A preliminary model of the stress reduction mechanism is discussed.

Superhard diamondlike carbon: preparation, theory, and properties

Abstract One of the many forms of carbon, diamondlike carbon (DLC) or tetrahedral amorphous carbon (ta-C) consists mainly of sp3 bonded carbon atoms. If properly prepared, DLC can have properties that rival those of crystalline diamond. The beneficial properties of DLC stem from the continuous rigid random networks of sp3 carbon atoms, and the properties can essentially be tailored by controlling the sp3/sp2 ratio. Techniques that have been successfully used to prepare high quality DLC coatings or thin films include pulsed laser ablation, filtered cathodic vacuum arc deposition, and mass selected ion beam deposition. Diamondlike carbon coatings that possess properties close to diamond in terms of hardness, atomic smoothness, infrared transparency, and chemical inertness can be processed easily with these techniques. In the past decade, tremendous progress has been made in experimental and theoretical investigations of hydrogen free DLC. Experimental and commercial applications in areas including microelectronics, microtribology, and biomedical technologies have been demonstrated. Potential applications include sensors, flat panel displays (field emitters), and photodiodes. Past and recent developments in synthesis and processing, properties, and modelling of hydrogen free superhard amorphous DLC are comprehensively reviewed. The techniques of fabrication, theoretical modelling, physical and mechanical characterisation, properties, and present and potential applications of DLC are discussed.

Improvement of wear resistance of pulsed laser deposited diamond-like carbon films through incorporation of metals

We have investigated the characteristics of diamond-like carbon (DLC), DLC doped with Cu, and DLC doped with Ti deposited by a sequential pulsed laser ablation of two targets. The composition of these films was determined by Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy and transmission electron microscopy studies showed typical features of DLC with a high fraction of sp 3 bonded carbon in the doped films as well as in the undoped films. Wear resistance measurements made on the samples by means of the ‘crater grinding method’ showed that DLC2.75% Ti has the highest wear resistance, while that of pure DLC has the lowest amongst the samples. Careful analysis of the Raman data indicates a significant shift to shorter wavelength with the addition of metal, which means that the compressive stress in the DLC films has been reduced. We envisaged that the reduction in the compressive stress promotes the wear resistance of the coatings. The XPS studies showed evidence for the formation of Ti‐C bonding in the Ti doped sample. Thus metal-doped DLC coatings are expected to improve the tribological properties and enhance the performance of components coated with metal-doped DLC.

Uncovering high-strain rate protection mechanism in nacre

Under high-strain-rate compression (strain rate ∼103 s−1), nacre (mother-of-pearl) exhibits surprisingly high fracture strength vis-à-vis under quasi-static loading (strain rate 10−3 s−1). Nevertheless, the underlying mechanism responsible for such sharply different behaviors in these two loading modes remains completely unknown. Here we report a new deformation mechanism, adopted by nacre, the best-ever natural armor material, to protect itself against predatory penetrating impacts. It involves the emission of partial dislocations and the onset of deformation twinning that operate in a well-concerted manner to contribute to the increased high-strain-rate fracture strength of nacre. Our findings unveil that Mother Nature delicately uses an ingenious strain-rate-dependent stiffening mechanism with a purpose to fight against foreign attacks. These findings should serve as critical design guidelines for developing engineered body armor materials.

Microcompression of nanocrystalline nickel

Microcompression is a technique that was developed as a means to probe the properties of micrometer-sized specimens using a modification of a conventional nanoindentation system. We use this technique to present the first uniaxial compressive data on electrodeposited nanocrystalline nickel (a material system where the grain size is much smaller than the specimen size). The compression-tension asymmetry of this nanocrystalline material is also discussed.