C.M. Lepienski

The effect of accelerated aging on the surface mechanical properties of polyethylene

The surface mechanical properties of polyethylene subjected to accelerated aging in Weathering Tester (QUV) and weatherometer (WOM) chambers were determined by a nanoindentation technique. It was observed that the apparent surface hardness was three times greater than that of the bulk after 1600 h of exposure in the WOM chamber, and a similar increase was also observed after 800 h of exposure in the QUV chamber. The elastic modulus at tip penetration depths of 500 nm also shows a significant increase from 400 to 2000 MPa for samples aged in the QUV chamber during 800 h. For aging times of 200 h and lower the variations in surface mechanical properties are small and restricted to a thin layer, with thickness lower than 1 μm for samples aged in both WOM or QUV chambers. The modifications in nanomechanical properties were correlated with the corresponding chemical processes due to aging, measured through the carbonyl index profile obtained from ATR–FTIR. An interpretation for the surface cracking of aged LDPE based on differential increases in elastic modulus at different depths is presented.

Electric field distribution and near-surface modifications in soda-lime glass submitted to a dc potential

Abstract The laser-induced pressure pulse method (LIPP) is employed to investigate the electric field distribution in soda-lime glass samples after application of a dc potential (dc poling). The modifications of near-surface composition resulting from the dc polarization procedure were determined by means of three nuclear techniques for near-surface analysis: Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERD) and nuclear reaction analysis. An alkali-depleted layer is formed near the anode. This depletion layer persists for a long time after the dc poling. The electric field in this region is of the order of the intrinsic dielectric breakdown, and its configuration depends on the penetration, or not, of hydrogen. Based on these results, the conduction mechanism in the alkali-depleted layer is discussed. It is suggested that when hydrogen blocking electrodes are used ionized electrons are probably the charge carriers in the alkali-depleted layer.

Improvement of tribological properties of Ti6Al4V by nitrogen plasma immersion ion implantation

Plasma immersion nitrogen implantation of Ti6Al4V (TAV) alloy was carried out to improve the surface tribological properties of test samples for artificial heart valves. Our results show that a good implantation with nitrogen peak concentration of 40% was achieved but with only approximately 50 nm implanted layer, after 60 min of treatment. Longer treatments showed no improvement in the retained dose, probably due to sputtering effects. However, both significant reduction of friction coefficient and increase in hardness was seen even for such a shallow implantation. Furthermore, the hardness improvement extended to regions much deeper than the implanted layer. Improvements of the wear resistance of such nitrogen implanted Ti alloy is expected, increasing considerably the useful lifetime of components made of TAV which is finding widespread use in biomedical and aerospace applications.

ALTERNATING ELECTRICAL CONDUCTIVITY OF POLYANILINE

The ac complex conductivity σ*(f) of polyaniline (PAN) films at different doping levels and different temperatures, in the 1–100 KHz frequency range, are reported. The results are typical of a disordered medium where the real component of ac conductivity is frequency independent at low frequencies, rising for higher values of frequencies. In order to interpret both the real and the imaginary components of σ*(f), we developed a model which considers the doped PAN as a disordered insulating matrix, sprinkled with conductive islands generated by doping, as indicated by energy dispersed x-ray microanalysis. The conduction through the insulating matrix obeys the random free energy barrier model, while in the conductive islands a metallic frequency-independent conductivity is considered. From the fittings we obtained the activation energy value of the maximum energy barrier of the doping mechanism and estimated the concentration of hopping sites.The ac complex conductivity σ*(f) of polyaniline (PAN) films at different doping levels and different temperatures, in the 1–100 KHz frequency range, are reported. The results are typical of a disordered medium where the real component of ac conductivity is frequency independent at low frequencies, rising for higher values of frequencies. In order to interpret both the real and the imaginary components of σ*(f), we developed a model which considers the doped PAN as a disordered insulating matrix, sprinkled with conductive islands generated by doping, as indicated by energy dispersed x-ray microanalysis. The conduction through the insulating matrix obeys the random free energy barrier model, while in the conductive islands a metallic frequency-independent conductivity is considered. From the fittings we obtained the activation energy value of the maximum energy barrier of the doping mechanism and estimated the concentration of hopping sites.

AC conduction of poly(o-methoxyaniline)

Abstract Chemically synthesized poly(o-methoxyaniline) was dissolved in N-methyl pirrolidinone and cast in the form of flexible, free-standing films. The electrical properties of these films were investigated by ac conductivity measurements in the frequency range from 10 −2 to 10 5 Hz, as a function of the doping level and temperature. For high doping levels the conductivity is frequency independent. However, for low doping levels the conductivity increases significantly in the high frequency range. The results are discussed in terms of a hopping model based on a method of distribution of transition rates.

Factors limiting the measurement of residual stresses in thin films by nanoindentation

Abstract Experimental results and finite element simulations are presented to examine whether a recently developed nanoindentation method for measuring residual stresses in bulk, monolithic materials can be applied to thin films. The method is based on indentation with spherical indenters in the elastic–plastic transition regime. Experiments were performed on 7 μm copper films and 5 μm chromium films deposited on (100) silicon substrates using indenters with radii of 30, 69 and 122 μm. The biaxial stresses in the films, which were controlled by processing, were verified by X-ray techniques. Nanoindentation measurement of the film stresses proved difficult due to practical considerations arising from surface roughness, substrate effects and problems in producing an appropriate reference specimen for comparison. Finite element simulations showed that the substrate problems can be alleviated by using an indenter with a radius of the order of (or smaller than) the film thickness. However, the other difficulties pose serious obstacles to the practical implementation of the method to thin film residual stress measurement.

Surface improvements of industrial components treated by plasma immersion ion implantation (PIII): results and prospects

Abstract The major drive for PIII research in recent years has been the widespread use of plasma-based ion implantation in industries aiming at attaining high value-added components. After achieving the domain of the complete basic PIII cycle, we started to pursue the implementation of this process in various types of industrial components. A DC glow discharge source with a controlled plasma floating potential was used in a 100-l PIII system driven by a 30-kV peak voltage, 50 μs duration, up to 1.1 kHz pulse power source, in order to process the components which were provided by regional companies, spanning from machinery tools to prosthesis. The industrial components were set-up in the PIII chamber as received from the companies, after a simple cleaning procedure. In this phase, only nitrogen implantation was performed. The required processing times were typically from 60 to 120 min and the components were treated either individually or in batches. Fast steel drill bits, knife blades for wood cutting, tools for odontological applications, molds made of fast steel, a prosthesis made of Ti alloy, etc., have been three-dimensionally implanted successfully. Next, improvements in the PIII ongoing system included: a 10-kW pulser with up to 60 kV capability, turbo-pump, refrigerated walls, auxiliary heating of the components, a larger chamber and a magnetron sputtering source for hybrid treatments.

Mechanical properties of Ag-doped top-seeded melt-grown YBCO pellets

In the present work we report the mechanical properties of ca(b)-planes of Ag-doped top-seeded melt-grown YBaCuO (YBCO) pellets at different concentrations. Hardness and elastic modulus were obtained by instrumented indentation and fracture toughness by conventional Vickers indentation. Hardness profiles for both planes indicated values between 7-8 GPa at deep tip penetration. Significant differences in elastic modulus were observed as a function of Ag content for the ab-plane while no difference were seen for the ca(b)-plane. Doping with 5 wt. % Ag2O increases the hardness and elastic modulus for the ab-plane in relation to the undoped sample due to Ag solid-solution hardening. Indentation fracture toughness rises with Ag doping for the ab-plane. Intensive plastic deformation was observed in ca(b) plane for conventional Vickers indentation.

Mechanical properties of layered InSe and GaSe single crystals

The mechanical properties of InSe and GaSe single crystals have been studied by means of nanoindentation tests. Both bulk crystals are well ordered and present a predominant γ-type interlayer stacking sequence as determined by x-ray diffraction and transmission electron microscopy measurements. The course of plastic deformation induced in the crystals by application of a definite shear stress through the penetration of a Berkovich tip indicates that the deformation occurs predominantly by pop-in events along easy slip directions having a fairly elastic character between displacements. Hardness anisotropy along crystal axes is clearly seen and the measured elastic modulus presents a discrepancy smaller than 5% in comparison with theoretical calculations performed using previous experimental values of the elastic constants.

Improvements of ultra-high molecular weight polyethylene mechanical properties by nitrogen plasma immersion ion implantation

Nitrogen Plasma Immersion Ion Implantation (PIII) has been used to modify the surface chemical structure of Ultra High Molecular Weight Polyethylene (UHMWPE). Grinding and polishing processes based on abrasive papers and alumina pastes have been evaluated with regard to their results on the improvement of polymer surface roughness, which has shown to be of crucial importance for hardness characterization. Raman spectroscopy, XPS, and Nanoindentation tests were used to characterize the modified surfaces. Experimental results has shown that UHMWPE surface mechanical properties such as hardness and elastic modulus can be improved by induced chain cross-linking between the macromolecules on the polymer surface caused by nitrogen PIII. The new material formed on the surface is Diamond Like Carbon (DLC). As a significant improvement in hardness was obtained by DLC synthesis on the treated surface, it is expected a dramatic improvement of abrasion resistance and overall durability of prostheses made with PIII treated UHMWPE.