A.P. Van Peteghem

The tribological behaviour of engineering plastics during sliding friction investigated with small-scale specimens

Abstract For economical, ecological and even technical reasons for some years there has been a tendency to introduce self-lubricating materials for bearing applications. In this way external lubricants such as oil or grease can be excluded, the design can be simplified and maintenance cost can be reduced. Among the self-lubricating materials the so-called engineering plastics have increasing importance. Unfortunately, data on their friction and wear characteristics are very disparate and often there is a lack of general understanding of the physical phenomena involved. In the present paper some basic types of engineering plastics (PA, oil-filled PA PTFE-filled PETP, POM-H) are experimentally investigated by means of small-scale reciprocating tribotesting. The tribological behaviour is explained in correlation with the chemical and mechanical properties of the materials. The basic failure processes are described for mild wear conditions as well as for overload conditions.

Calculation of the impedance of noncylindrical pores Part I: Introduction of a matrix calculation method

In this paper a new method is proposed for the calculation of the impedance of arbitrary electrodes containing noncylindrical pores and/or having place-dependent impedances. The method is based on splitting up the pore and the surrounding material into N discs. For the equivalent circuit of each disc a transmission line with constant impedances is adopted. By matrix calculations the impedance of the porous electrode can be obtained. A comparison is made between this, very general, matrix method and a recursion method developed by Keiser et al. for purely capacitive interface behaviour of pores in an electrode material with negligible impedance. It is shown that the matrix method requires much smaller N-values owing to the use of transmission lines for each disc. This makes it more appropriate to be used in curve fitting procedures. Moreover, it is shown that the typical behaviour of the pore impedance at low penetration depths is much better simulated with the matrix method. Furthermore, an attempt is made to provide more general knowledge about the impedance behaviour of noncylindrical pores as a function of the penetration depth of the a.c. signal. Finally, the theory is enlarged using constant phase elements instead of capacities to describe the behaviour of the electrode/electrolyte interfaces.

The influence of the anodization temperature and voltage on the porosity of the anodization layer on aluminium

Abstract The pore-filling method is a known process for measuring the porosity of porous anodic oxide layers on aluminium. This method has been used to study the relationship between the anodizing conditions and the porosity of the porous anodic film. In a series of experiments, the influence of the anodization voltage and temperature on the porosity, of an anodization layer formed in 15 wt.% sulphuric acid was investigated. From these results a mathematical equation was deduced that gives the porosity as a function of the anodization voltage and temperature. In the second part of the study the height of the voltage jump, on which the thickness of the barrier layer at the start of the re-anodization process depends, was investigated as a function of the anodization voltage. A linear relationship was found between the voltage jump and the anodization voltage.

Calculation of the impedance of noncylindrical pores Part II: Experimental verification on pores drilled into stainless steel

The method of one-dimensional modelling of the pore impedance is validated on pores drilled in AISI 316 stainless steel. Impedance measurements were carried out in 0.511 sulphuric acid solution. Experimentally, it is found that, for cylindrical pores with a radius/pore length-ratio smaller than 0.5, the one-dimensionality condition is satisfied. Furthermore, impedance measurements on noncylindrical, scaled-up pores provide evidence that a geometric constant Ag has to be introduced, as in Part I. In accordance with this theory it is found that for pores with narrowing cross section Ag is less than 1/3, while for pores with a broadening cross section Ag is greater than 1/3. Moreover, it is shown that the matrix method, presented in Part I, provides a way of calculating the impedance of a non-cylindrical pore by using a highly reduced number of discs, N, compared to the recursion method of Keiser et al. This is an advantage in fitting procedures.

Extraction of multi-component system of metals from simulated and industrial effluents by liquid surfactant membranes (LSM)

Abstract A study of metal extraction from a multi-component system was carried out by applying a liquid surfactant membrane (LSM) process. The system was an industrial effluent which contains a mixture of metallic species such as Na(I), Mn(II), Co(II), Ni(II), Fe(III) and Cr(III). Concurrently, an analogous research programme was conducted on model solutions reproducing nearly the same physical and chemical composition as the industrial feed. The main goal behind this endeavour was to elucidate the effect of impurities and other components co-existing alongside the metallic species in the industrial feed. Di-2-ethylhexyl phosphoric acid (D2EHPA) was used as a carrier with tri-n-butyl phosphate (TBP). The latters influence on the kinetics of metal extraction occupied a major part of the investigations. It was found that an optimal composition of the extracting mixture exists (5.5 vol.% D2EHPA + 0.5 vol.% TBP) at which a maximum extraction efficiency can be achieved. It has also been revealed that, although the selectivity is relatively poor, it can be improved by varying the concentration of TBP in the membrane phase. The raffinate kinetic profiles of the simulated solutions have been shown to fit those of the industrial effluent adequately.

Experimental study by means of thin layer activation of the humidity influence on the fretting wear of steel surfaces

Wear processes in general are influenced by the surrounding atmosphere, humidity especially. For fretting wear several authors have examined the influence of humidity on the wear scar morphology, the wear volume and the composition of the wear debris. No agreement between the different results has been found, yet. In the present study, the influence of the humidity in the ambient air on the fretting process is examined. The friction, the wear volume and material transfer are measured for the fretting of a bearing steel ball against a flat steel specimen under gross-slip regime, in running-in as well as steady state conditions. The wear and material transfer are quantitatively measured with Thin Layer Activation method. It has been observed that high ambient humidity inhibits adhesion and accompanying material transfer. As a result, high humidity has a beneficial influence on fretting wear, in opposition to generally accepted hypothesis.

Kinetic study of the emulsion breakage during metals extraction by liquid surfactant membranes (LSM) from simulated and industrial effluents

Abstract A study on emulsion stability during metals extraction by liquid surfactant membranes (LSM) process has been conducted. The investigations were focused on the emulsion performance during metals permeation from simulated and industrial effluents. To examine the internal aqueous phase leakage which is a symptom of emulsion instability, the internal tracer technique has been applied. The effect of the membrane composition and particularly tri-n-butyl phosphate (TBP) on the stability has been investigated extensively. It has been revealed that, there exists an optimal range of TBP concentrations where a maximum metal extraction and minimum emulsion instability can be achieved. The internal phase leakage model proposed by Boyadzhiev and co-workers has been modified to meet the present system hydrodynamical conditions. The experimental data fit quite well the proposed model.

Electrostatic Splitting of the Emulsion Used in Liquid Surfactant Membranes Process for Metals Separation

Abstract A study has been carried out to investigate the effect of different parameters on the splitting process of the emulsion used in the liquid surfactant membranes process for metals separation. The influence of electric field strength, frequency of the applied ac voltage, and membrane recycling on emulsion stability have been studied. The investigations have revealed that there exists a unique optimal frequency range in which the best separation of the emulsion phases is obtained. The effect of the membrane composition and the variation of tri-n-butyl phosphate (TBP) concentration has been shown to vary linearly with the splitting efficiency. This finding correlates quite well with Sadek and Hendricks equation on the force causing coalescence in an electric field.

Sorbitan monooleate (Span 80) decomposition during membrane ageing. A kinetic study

Abstract A study has been carried out to investigate the kinetics of surfactant decomposition during ageing of the membrane (organic phase) of the emulsion used in an liquid surfactant membrane (LSM) process for metals separation. At this stage of research, the process of Span 80 hydrolysis has been exclusively studied in the membrane phase, and thus, the effect of the carrier di-2-ethylhexyl phosphoric acid (D2EHPA) on the rate of hydrolysis. The effect of tri-n-butyl phosphate (TBP) and the membrane composition on the kinetics of Span 80 hydrolysis have been studied as well. It has been found that the bimolecular irreversible second-order kinetic model describes fairly well the experimental data. A numerical best-fit routine of a polynomial form has been exploited to modify the final model, which takes into account the TBP effect on the result rate of the proposed model.

The estimation of the charge transfer resistance by graphical analysis of inclined semicircular complex impedance diagrams

Recently a graphical method for the determination of the charge transfer resistance, obtained from impedance measurements, was developed. The original method was limited to perfect, semicircular Nyquist plots. Extension to inclined or depressed semicircular arcs is made possible by a few simple mathematical and graphical manipulations.