Table of contents

Tribocorrosion covers the science of surface transformations resulting from the interaction of mechanical loading and chemical reactions that occur between elements of a tribosystem exposed to corrosive environments. Thus it combines the mechanical and chemical interactions of body, counterbody, interfacial medium, and environment including friction, lubrication, wear and tribologically activated chemical and electrochemical reactions. During friction the adhesive dissipation of energy is often influenced by chemical effects, while during wear the wear mechanisms of 'adhesion' as well as 'tribochemical reactions' are affected. The latter mechanism is of interest in mechanical engineering particularly, because it might prevent adhesion, which would bring about a much higher wear rate or even seizure. Nevertheless, in electronics as well as for MEMS and NEMS, chemical reactions are often avoided because they change the nature (e.g. conductivity, reactivity) of surfaces of sensors or alter the dimensions of parts within the nm- and μm-range by the growth of a reaction layer. Thus, depending on the structure and loading of the tribosystem, tribocorrosion phenomena could be either beneficial or detrimental.

In order to benefit from tribocorrosion phenomena it is crucial to control them as well as the nature of rubbed or worn surfaces, which have undergone tribological stresses. Unfortunately, the tribocorrosion mechanisms are still not well understood. Reasons for this stem not only from the complex interactions of chemical and mechanical factors but also from the experimental difficulty of characterizing mechanically and/or thermally induced surface phenomena occurring inside small contacts. Due to the fact that the duration of a single asperity contact is short and that it takes place directly at the interface of contacting bodies, even modern surface analysing techniques often fail. Many reactions could take place at the same time and the quantity of reaction products might be very small. In addition, metastable phases might be important inside a contact, which then transform into stable reaction products outside of the contact. Thus, in situ techniques are most suitable for tribocorrosion investigations. In the case of electronically conducting materials exposed to ionic conductive liquids, such as aqueous solutions, triboelectrochemical techniques offer interesting possibilities to control, in situ and in real time, the surface reactions occurring in a contact. However, further experimental developments are needed to obtain a clearer picture of the processes involved. Thus, tribocorrosion has been a point of basic research for more than 40 years and will remain as such in the near future.

This special cluster issue within Journal of Physics D: Applied Physics provides state-of-the-art information on tribocorrosion. Due to the fact that this research area is very large and complex, the given selection of papers will certainly not be representative of the entire field. The editors have tried, however, to offer a glimpse of what happens with metals, ceramics, and polymers when exposed to the combined action of friction and corrosion. Thus, this issue begins with a view on general and basic aspects of tribocorrosion. This is followed by papers which concentrate on the materials response to tribocorrosion and contributions related to 'abrasion' and 'surface fatigue' wear mechanisms. Finally the influence of tribocorrosion on certain applications is presented. This cluster should encourage natural as well as engineering scientists to carry out basic and applied research and should convince industrial chemists, physicists and mechanical engineers that there is hope.

Tribocorrosion involves mechanical and chemical/electrochemical interactions between surfaces in relative motion in the presence of a corrosive environment. Tribocorrosion phenomena are encountered in many technological areas where they cause damage to installations, machines and devices. Often tribocorrosion damage is a problem for safety or for human health. In other applications tribocorrosion phenomena are put to good use in manufacturing. The chemo-mechanical mechanisms of tribocorrosion are still incompletely understood, they involve the properties of contacting material surfaces, the mechanics of the contact and the corrosion conditions. Electrochemical methods are widely used for the study of tribocorrosion reactions. To yield information about synergistic and antagonistic mechanisms they must be applied in situ under strictly controlled mechanical conditions, using materials with well-characterized surface properties. Recent progress in modelling and understanding of tribocorrosion systems is discussed and some challenges and opportunities for future research are identified.

Tribochemistry can be defined as the chemical reactions between the surface and the lubricant molecules inside a sliding contact under boundary lubrication conditions. These reactions are important because they control the lubrication processes, and therefore they determine the reliability and durability of moving parts in machineries. In spite of its practical significance, our understanding of the nature of tribochemistry is limited, and to a large extent, empirical in nature, and mainly restricted to the steel–hydrocarbon system. As new materials emerge and are being applied to various applications, users are often hampered by the lack of knowledge of how these new materials can be effectively lubricated via tribochemical reactions, resulting in occasional premature failure and product recall.

There are many tribochemical reactions which occur in a sliding system, such as tribocorrosion, electro-chemistry, hydrolysis of ceramics and anti-wear additive film formation. However, the fundamental basis of lubrication in most of the practical systems starts with the reactions between materials surfaces and hydrocarbon base oils. Without this reaction, for example, an anti-wear additive such as zinc dithiophosphate would not be effective. Therefore, in this paper, we will focus on the effect of materials on the basic tribochemical reactions between hydrocarbons and surfaces.

This paper integrates existing information in the literature on the lubrication chemistry of various materials and provides a consistent framework in the context of tribochemistry. For discussion purposes, we will examine metals, semiconductors, and insulators. The preponderance of data in the metals–hydrocarbon systems suggests that thermochemistry and organometallic chemistry dominate the interfacial chemical reactions. For semiconductors and insulators, there is evidence that electrostatic charge induced electron emission plays an important role in the tribochemistry of these materials. Various measurement methods used to characterize tribochemical behaviours are introduced and discussed in terms of reactivity analysis and tribochemical mechanisms.

The influencing parameters considered in the present state of the art for determining the thickness of the lubricating film and the lubrication regime under tribological conditions are mostly basic physical, macro-geometrical or mechanical and effects such as surface charge and the associated pH, zeta-potential or solubility are not considered important. However, in this work we present how remarkably these electrochemical parameters affect the properties of the surface layer, the physico-chemical and load-carrying properties of the lubricating film, the tribochemical reactions and the tribolayer failure modes of water-lubricated contacts: more than 25 specific properties were shown/proved or were recognized as potential parameters resulting from surface-charge effects. Under boundary-lubrication conditions, this can result in a significantly different severity of the contact conditions and consequently in friction and wear compared with those predicted with conventional considerations and models. This seems to be especially important not only for water-based but also for other low-viscosity lubricants, which due to thin lubricating films provide poor and/or fragile film conditions. Accordingly, we suggest that a more complete set of tribo-electro-chemical effects should be considered and controlled in a lubricating film and that more tailored models, which also take into account some of these effects, should be developed to enable a more accurate prediction of the contact conditions in future boundary-lubricated systems.

A single crystal-tantalum (100) surface was anodically oxidized at 5.0 V (SHE) for 1 h in pH 8.4 borate buffer solution. Nano-scratching in solution at a constant normal force of 300 µN was performed on the tantalum (100) surface kept at 5.0 V (SHE) after anodic oxidation. Nano-scratching in air was also performed on the tantalum (100) surface after anodic oxidation. A stick–slip was always observed in lateral force versus lateral displacement curves during nano-scratching in solution. The degree of stick–slip, however, decreased with increasing scratching rate. The average friction coefficient obtained with nano-scratching in solution was significantly larger than that obtained with nano-scratching in air and increased linearly with the logarithm of time required for scratching. In contrast, the friction coefficient obtained with nano-scratching in air was independent of the time required for scratching. The protrusion at the scratching end of the groove produced by nano-scratching in solution was observed from the AFM image, while no protrusion was observed for nano-scratching in air. It has been proposed that the protrusion consists of anodic oxide accumulated on the moving front of the indenter tip during nano-scratching in solution and contributes to the increase in lateral force, i.e. friction coefficient.

Slurry impingement is a type of tribocorrosion resulting from simultaneous mechanical and electrochemical surface degradation. In order to understand the elementary process of slurry impingement and the effect of mechanical damage on passive films, electrochemical responses associated with passive film breakdown by particle impacts and following repassivation were measured during slurry impingement on Al and Al–1wt%Si alloy microelectrodes. Measurements were made using a slurry jet system which consists of a pump, a microelectrode, a potentiostat suited for measurements of fast transients of small currents and a high frequency data acquisition system. Current transients corresponding to separated single particle impacts have been successfully measured. The current transient sharply rises and gradually decays following a high field model of oxide growth. It is shown that pure Al repassivates faster after particle impact than the Al–1 wt% Si alloy. This type of erodent particle had an influence on the apparent repassivation rate of these electrodes, and both Al and Al–1 wt% Si alloy showed slower current decay after the impact of angular SiC particle than after the impact of spheroidal zirconia particle. The SiC particles made deep scars and scratches, and the zirconia particles made shallow depressions.

Chromium carbide/nickel based composites are applicable in many environments involving tribo-corrosion due to their combined ability to resist wear and corrosion. Hence, they are candidate materials for use either in bulk as surface coatings in crude oil (offshore) or in power and marine industries. The aim of this work was to study the effect of material parameters such as composition and surface roughness, together with test conditions such as abrasive particle concentration, applied potential, temperature and time of experiment on the performance of chromium carbide based cermets. Potentiodynamic and potentiostatic tests were carried out as part of this work. SEM studies were also conducted to establish the mechanisms of the material degradation processes. Finally, erosion–corrosion maps were constructed based on the results. Material wastage, synergy and regime maps were developed for these materials and demonstrated that the performance of the cermet depends on the interplay of material and process variables.

The tribocorrosion behaviour of a 316L austenitic stainless steel sliding against alumina was investigated under electrochemical control in sulfuric acid using a tribometer with a ball on flat configuration. Tests were conducted by applying either a passive potential (metal covered by an oxide film) or a cathodic potential (no passive film, negligible corrosion) to the steel. Friction, wear and anodic current were monitored. The near surface microstructure of wear tracks was analysed by transmission electron microscopy (TEM). Significantly higher wear was observed at the passive potential compared with the cathodic potential. Chemical reactions could not account for this difference in deterioration. TEM analysis revealed that the plastic behaviour of the metal, and thus its response to wear, depends on the prevailing electrochemical conditions, the passive potential showing larger deformation than the cathodic. This effect was attributed to the presence of the passive film that induces residual stresses and interferes with dislocation activity.

Electrochemical noise measurements were performed to investigate the intrinsically stochastic character of the tribocorrosion process. Unidirectional sliding tests (pin-on-disc) were performed using AISI 304L stainless steel sliding against corundum. Experiments were carried out in Cl⁻ and containing media under open-circuit and potentiostatic polarization conditions. The power spectral density (PSD) of the current and potential signals showed a strong dependence on the sliding frequency but did not depend significantly on the normal load between 5 and 20 N. The fluctuations of the tangential and normal loads were also recorded, and a critical comparison between the PSD of the electrochemical response and the PSD of the mechanical solicitation (load) is proposed. At high frequencies (f > 0.1 Hz), the PSD of current or potential fluctuations have significantly different shapes than the PSD of load signals: the electrochemical signal PSD is governed by the dynamic balance between local depassivation and repassivation which only depends on the kinetics of the electrochemical phenomena. For lower frequencies, a plateau is observed for both the electrochemical PSD and the load PSD. The electrochemical signal is then governed by the continuous depassivation induced by sliding which appears as a low frequency component. These results suggest that the electrochemical noise technique investigated in the frequency domain might be a promising electrochemical tool for successfully unfolding tribocorrosion signatures for material parings in sliding-corrosion tests.

Specific wear of rod cluster control assemblies (RCCA) in pressurized water nuclear reactors (PWR) results from contacts with their guides due to flow-induced vibration. Particular sliding impact contact conditions and the specific environment in PWR coupling temperature and solution chemistry involve original mechanisms of wear. A specific tribometer has been developed during recent years by FRAMATOME-ANP. This simulator operates in experimental conditions close to the sliding impact motion of RCCA assemblies against guides and under a similar environment (temperature, pressure and water chemistry). Numerous and various wear tests were realized in order to study the influence of the sliding impact parameters on wear loss and wear scars. Specific tests with continuous data acquisition were realized. Different wear scars were observed. An analogy with erosion was done. An erosion model was used to determine critical conditions concerning friction coefficient and incidence angle values leading to wear. Considering these critical conditions, it was possible to explain the observed difference of wear loss and facies.

With the increasing interest in metal-on-metal (MoM) joint implants, studies have been focused on their wear and corrosion behaviour. Integrated electrochemical tests have been conducted and are reported in this paper. The corrosion, wear and wear–corrosion behaviour for three materials (high carbon CoCrMo, low carbon CoCrMo and UNS S31603) have been discussed. Corrosion effects on the overall performance for the three materials are analysed. Two distinct regimes have been found for the three materials: (a) the running-in regime and (b) the steady state regime, in line with other research. Even in the steady state regime, 20%–30% of the material degradation can be attributed to corrosion-related damage. High carbon CoCrMo showed excellent corrosion, wear and corrosion–wear resistance and therefore it delivered the best overall performance in terms of a lower wear rate, a lower friction coefficient and a higher resistance to corrosion.

Metal-on-metal joint replacements have been considered an effective alternative to metal-on-polyethylene bearings. Various electrochemical and spectroscopic techniques have been used in studying the corrosion behaviour and surface compositions of these implant materials. X-ray photoelectron spectroscopy is one of the most popular methods of obtaining details regarding the near surface layers of metallic components. This paper is focused on the metal–protein interaction and the change of candidate material (HC CoCrMo, LC CoCrMo and UNS S31603) surfaces under tribological contact in simulated biological fluids. The relationship between wettability and corrosion (specifically tribocorrosion) in a protein-rich environment is also discussed. As a general conclusion, a metal–protein complex was found on the surfaces which grew and developed with the metal oxides (Cr₂O₃/CoO etc) as sliding progressed. It is believed that wettability, which is affected by the nature of the protein adsorption at the metal interface, can influence material corrosion and tribocorrosion behaviour.

Ultra high molecular weight polyethylene (UHMWPE) has been used as a bearing surface in joint replacement prostheses for over 40 years. Recently, highly crosslinked UHMWPE (HXPE) materials were introduced based on the finding that crosslinking reduces wear. These first generation HXPE materials were produced by irradiation followed by heating below the melting temperature (annealing) or above the melting temperature (remelting). Both classes of HXPE material have demonstrated greatly reduced wear. However, remelted HXPE materials have reduced fatigue strength while annealed HXPE materials may oxidize when exposed to oxygen. A second generation HXPE material was produced using a sequential irradiation and annealing process (SXL). SXL materials have crosslinking levels equivalent to those of first generation HXPE materials, have fatigue and mechanical strength characteristics of first generation annealed HXPE material and have an oxidation resistance equivalent to that of virgin (unprocessed) UHMWPE. This combination of properties makes SXL HXPE a preferred material for bearing surfaces of joint prostheses.

Magnetic x-ray microdiffraction uses the structural specificity of x-ray diffraction to probe complex magnetic structures at the length scales relevant to physical phenomena including domain dynamics and phase transitions. Conventional magnetic crystallography techniques such as neutron or x-ray diffraction lack this spatial resolution. The combination of both reciprocal space and real space resolution with a rich magnetic cross section allows new microscopy techniques to be developed and applied to magnetism at the scale of single domains. Potential applications include a wide range of magnetic problems in nanomagnetism, the interaction of strain, polarization and magnetization in complex oxides and spatially resolved studies of magnetic phase transitions. We present the physical basis for x-ray microdiffraction and magnetic scattering processes, review microdiffraction domain imaging techniques in antiferromagnetic and ferromagnetic materials and discuss potential directions for studies.

The structural, composition and optical absorption properties of Cd_1−xZn_xS (CdZnS) thin films fabricated by the reactive diffusion of Zn in CdS have been investigated. These ternary compounds were characterized by x-ray diffraction (XRD), energy dispersive x-ray fluorescence and optical absorption measurements. It is established that thermal annealing of Zn/CdS structure at temperature (400 °C) lower than the melting point of Zn (418 °C) results in the concentration distribution of Zn in CdS film described by an erfc-curve (D = 5 × 10⁻¹⁴ cm² s⁻¹) and characterizing the free impurity diffusion from a constant source. In contrast to this, the concentration profile of Zn in CdS film at higher annealing temperature (570 °C) is not described by the erfc-curve and shows a nearly stepped form, which is characteristic of reactive diffusion. XRD patterns of Zn/CdS structures annealed at 570 °C showed diffraction peaks of ternary CdZnS compounds. Analysis of the absorption spectrum of such films indicates formation of CdZnS composition with the largest value of energy band gap up to 2.64 eV, exceeding the band gap of CdS (2.43 eV). It is concluded that interdiffusion in Zn/CdS structures at temperatures exceeding the melting point of Zn was accompanied by formation of Cd_1−xZn_xS ternary compounds. The band gap of this variable band structure changes from 2.64 eV in the near surface region to 2.43 eV (CdS) in the inner region of the film.

A Nd³⁺: LaBO₃ crystal with dimensions ϕ20 × 15 mm³ has been grown by the Czochralski method from a flux of Li₃BO₃. The hardness of the Nd³⁺: LaBO₃ crystal is 802.6 VDH or 6.5 on the scale of Mohs. The absorption and emission spectra of Nd³⁺: LaBO₃ have been investigated. The absorption cross-section is 2.16 × 10⁻²⁰ cm² at 807 nm and the emission cross section is 6.78 × 10⁻¹⁹ cm² at 1057 nm. The fluorescence lifetime τ_f of Nd³⁺: LaBO₃ crystals with different Nd³⁺ concentration was determined.

In this paper we report the optical characterization of Ho³⁺-doped fluoroindate glasses with the following composition (40 − x)InF₃–20SrF₂–16BaF₂–20ZnF₂–2GdF₃–2NaF–xHoF₃ with x = 1.0, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 8.0 and 9.0 mol%. This investigation was done by means of absorption, fluorescence and lifetime measurements. The phenomenological intensity parameters Ω_λ (λ = 2, 4, 6) were calculated using the standard Judd–Ofelt theory. In order to evaluate potential applications of the ⁵I₇ → ⁵I₈ Ho³⁺ emission at 2.0 µm, the radiative lifetime, total transition rate and emission cross section were determined. The effect of Ho³⁺ concentration on the fluorescence intensity and lifetimes was analysed. It was concluded that the non-radiative losses of the ⁵I₇ level are mainly due to energy migration among Ho³⁺ ions followed by energy transfer to quenching impurities.

Optical properties of sol–gel synthesized boron co-doped germanosilicate(xB₂O₃ : 20GeO₂ : (80 − x)SiO₂) thin films (single layer) were investigated by varying the molar concentration of B₂O₃ for x = 5, 10 and 20 under various annealing temperatures, ranging from 500 to 1000 °C. Densification of the thin film was found to be influenced by the concentration of boron under various annealing temperatures. Iterative deposition of the dense xB₂O₃ : 20GeO₂ : (80 − x)SiO₂ (for x = 5, 10 and 20) thin film was used to fabricate a few micrometre thick single-mode slab waveguides at 1.55 µm. Refractive indices and the propagation loss values of the waveguide samples were determined by using the prism-coupler technique. Under ultra-violet illumination, the refractive index change (Δn) of the xB₂O₃ : 20GeO₂ : (80 − x)SiO₂ increases with increasing B₂O₃concentration, for both transverse electric and transverse magnetic. The values of Δn_TE are ∼0.009, ∼0.012 and ∼0.014, for x = 5, 10 and 20, respectively. The high values of Δn, which increase with the B₂O₃ concentration are probably caused by the larger stress present in the boron doped films.

A SiC/ZnO five-layer multi-layer on Si(111) substrate with a SiC buffer layer was fabricated using the radio frequency -magnetron sputtering technique. The as-deposited films were annealed in the temperature range 600–1000 °C under nitrogen ambient. With the increase in annealing temperature (Ta), especially from the Ta approximately 800 °C, the quality of the SiC buffer layer is improved, resulting in the enhancement of the crystal quality of ZnO. With the further increase in Ta up to 1000 °C, the crystal quality of ZnO becomes worse because the ZnO layers and SiC layers penetrate each other greatly and the interfaces between ZnO and SiC are very impacted and complicated, which can be demonstrated by the emergence of the new peaks (869, 890 and 927 cm⁻¹) in the Fourier transform infrared spectroscopy spectra. We have also discussed the photoluminescence properties of the sample in great detail. The optical properties of the sample are strongly dependent on the condition of the SiC buffer layer. Furthermore, with the increase in Ta, the near band edge emission has a tendency of redshift due to the enhancement of the penetration or interaction between the ZnO layers and SiC layers resulting in the band structure deformation or lattice deformation. The origin of the 416 nm peak is associated with the interface traps existing in the depletion regions between the ZnO–ZnO grain boundaries.

Slow oxidation of n-butane and n-decane in lean mixtures with oxygen under the action of a nanosecond pulsed discharge has been experimentally investigated. The discharge was excited by 25 ns pulses with an amplitude of 22 kV on high-voltage electrode and repetition frequency of 40 Hz in a 20 cm long quartz tube. The initial pressure of the mixtures was varied within the range from 0.8 to 10 Torr. The energy input in the discharge has been measured. The radiation intensities of the following molecular bands have been measured in time-resolved and integral modes: , , OH(A² Σ, v' = 0 → X² Π, v'' = 0), CO(B¹ Σ, v' = 0 → A¹ Π, v'' = 2). It has been demonstrated that, under the same experimental conditions in lean mixtures with oxygen, the times for the n–decane and n–butane oxidation are equal.

The bidimensional model of the electric arc is enhanced with the plasma–electrodes interaction to predict the properties and the energy distribution of an argon arc operating with current intensities between 100 and 200 A and electrode gaps of 10 and 20 mm.

An adaptive numerical insulation is applied to the cathode, to properly simulate its thermionic emission mechanism and overcome the dependence on empirical distributions of the current density at its tip.

The numerical results are quantitatively compared with the data obtained from calorimetric and spectroscopical measurements, performed on a device which generates a transferred arc between a water cooled copper anode and a thoriated tungsten cathode enclosed in a stainless steel chamber.

The calculation of the heat fluxes towards the electrodes permits to determine the amount of power delivered to each component of the arc system (the anode, the cathode assembly and the chamber) and to evaluate the overall efficiency of the process for different configurations.

The agreement between theory and data, over the range of parameters investigated, is sensible both in the temperature profiles and in the energy distributions. In such configurations, the conduction from the hot gas is the most relevant term in the overall heat transferred to the anode, but it is the electron transfer which rules the heat transfer in the arc attachment zone.

The arc attachment radius is also dependent on the process parameters and increases with the arc current (from approximately 5 mm at 100 A to 7 mm at 200 A) and the arc length. However the maximum heat flux reached on the axis decreases increasing the gap between the electrodes, although more power is delivered to the anode due to the radial spreading of the plasma.

A 10 mm 200 A argon arc releases to the anode about 2.6 kW, which corresponds to 75% of the total arc power available. If the arc is extended to 20 mm the power transferred rises by nearly 350 W, but the overall efficiency drops to 65% due to the increased losses towards the chamber.

The power delivered to the anode increases almost linearly with the arc current, presenting a slope of about 15 W A⁻¹, independent of the arc length.

By means of hollow cathode spectroscopy and atomic force microscopy the surface morphology and composition of SnO₂ thin film, modified with hexamethyldisilazane after rapid thermal annealing treatment (800–1200 °C), are investigated. Formation of crystalline structure is suggested at lower temperatures. Depolimerization, destruction and dehydration are developed at temperatures of 1200 °C. It is shown that the rapid thermal annealing treatment could modify both the surface morphology and the composition of the layer, thus changing the adsorption ability of the sensing layer. The results confirm the ability of hollow cathode emission spectroscopy for depth profiling of new materials especially combined with standard techniques.

Comparisons of fluid model predictions with various measurements in an inductively coupled plasma are reported. The gas chemistries used include pure Ar, mixtures of Ar and O₂ and mixtures of Ar, O₂ and Cl₂ for pressures ranging from 10 to 80 mTorr and plasma densities ranging from ∼10¹⁰–10¹¹ cm⁻³. Measurements of electron density, electron energy probability function, positive ion composition and wall flux and neutral radical concentration at the chamber wall are reported and compared with corresponding model predictions. The model treats electrons, ions and neutrals as fluids, under the assumption of quasineutrality. Power into electrons is from inductive coupling from an external coil and capacitive coupling is neglected. The non-isothermal, multicomponent reacting flow equations with jump boundary conditions are used to describe neutrals and are solved iteratively with the plasma and electromagnetic equations until a steady solution is found. Radical and ion composition predictions are shown to be in reasonable agreement for Ar and Ar/O₂ plasmas, but the corresponding predictions in mixtures of Ar, O₂ and Cl₂ are in only partial agreement with measurements, implying that the chemical reaction database for this chemistry requires further work.

We have generated a N₂ flowing discharge sustained by surface waves employing a 2.45 GHz high frequency source. The N₂ dissociation was studied in the discharge and post-discharge regions by optical emission spectroscopy (OES) as a function of the experimental parameters: discharge power (30–160 W) and absolute pressure (1–20 Torr), at 0.5 Slm⁻¹ flow rate. The N(2p^3 4S⁰) absolute density was measured in the discharge by actinometry. We have introduced the effect of the N₂(X¹ Σ_g⁺) vibrational temperature in the actinometry equation. Such a consideration was made based on the work of Catherinot and Sy (1979 Phys. Rev. A 20 1511) which presents a profound discussion about the quenching mechanism of the N(3p ⁴S⁰) by N₂(X¹ Σ_g⁺,v) states. The 811.5 nm Ar and 821.6 nm N lines, from 2p₉ → 1s₅ and 3p ⁴P⁰ → 3s ⁴P transitions have been utilized here. Both are easily observed in the surface wave discharge. Further, the NO chemical titration was carried out in the late post-discharge furnishing the N(2p³⁴S⁰) absolute density in that region. The extinction point of the 580.4 nm band of the N₂ 1st positive system was measured. The density values obtained by actinometry are validated by comparison with those measured in the post-discharge region. The two sets of data, function of pressure and discharge power, may be related by the mass conservation equation. The work provides the experimental N(2p³⁴S⁰) absolute density values as a function of power and pressure only by application of OES techniques. Moreover, we demonstrate here that the N(3p ⁴P⁰) state is quenched by N₂(X¹ Σ_g⁺, v ⩾ 2) molecules forming N₂(b ¹Π_u, v = 0) and N(⁴S) states, in agreement with Catherinot and Sy.

A theory is developed for the formation of a low pressure, current-free double layer just inside an upstream dielectric source chamber connected to a larger diameter, downstream metallic expansion chamber. The double layer is described using four groups of charged particle: thermal ions, mono-energetic accelerated ions flowing downstream, accelerated electrons flowing upstream and thermal electrons. The accelerated electrons are formed downstream from an initially nearly half-Maxwellian electron distribution. A fifth group of counter-streaming electrons formed by the reflection of the accelerated electrons from the sheath at the insulated end wall of the source chamber is used to enforce the condition that the double layer be current-free. The condition of particle balance upstream is found to determine the double layer potential. The double layer is found to disappear at very low pressures due to loss of ionization balance upstream and due to energy relaxation processes for ionizing electrons at higher pressures. The theory is found to be in good agreement with the experiments.

A new technique, the two-stage probe-spectroscopic diagnosis method, is proposed to diagnose indirectly the electron temperature in a narrow discharge plasma at low/middle pressure. The first stage includes the diagnosis of electron temperature T_e by the double probe method and measurement of the emission spectrum in an ancillary discharge tube. From its emission spectrum, a pair of spectral lines is selected in such a way that the ratio I_mq/I_nr of their intensities, I_mq and I_nr, is changed monotonically with gas pressure. The corresponding relation between the electron temperature T_e and the ratio I_mq/I_nr can be obtained in the ancillary discharge tube at certain pressure. During the second stage, the electron temperature in the diagnosed discharge plasma can be determined from its ratio I_mq/I_nr according to the above relation between T_e and I_mq/I_nr obtained in the ancillary discharge tube. As an example, the electron temperature in a high power flat He–Ne laser is diagnosed using this probe-spectrometry diagnosis technique. The experiments prove that the calculated electron temperature is obviously higher than the actual electron temperature in the low pressure or small size discharge. The principle, method, setup and results of this diagnostic technique are discussed in this paper.

Hysteresis and mode transitions in inductively coupled Ar–Hg plasma in the electrodeless induction lamp are studied at different discharge frequencies and under different matching conditions. It is observed that transition currents change at different frequencies and hysteresis exists not only between the starting and minimum maintaining currents of the electromagnetic mode (H mode) discharge but also between the starting and minimum maintaining currents of the electrostatic mode (E mode) discharge. The illuminance and global electrical parameters in the mode transitions are recorded. It is shown that the E to H mode transition is accompanied by increased plasma resistance and decreased plasma reactance, which results in a higher efficiency in the H mode. Under the same output voltage of the radio frequency source, mode transition can also be triggered by changing the matching condition. The emission spectra recorded before and after the E to H mode transition provide experimental evidence for the theory that the change of the electron energy distribution function plays an important role in the hysteresis effect.

Two new distributions of the statistical time delay of electrical breakdown in nitrogen are reported in this paper. The Gaussian and Gauss-exponential distributions of statistical time delay have been obtained on the basis of thousands of time delay measurements on a gas tube with a plane-parallel electrode system. Distributions of the statistical time delay are theoretically founded on binomial distribution for the occurrence of initiating electrons and described by using simple analytical and numerical models. The shapes of distributions depend on the electron yields in the interelectrode space originating from residual states. It is shown that a distribution of the statistical time delay changes from exponential and Gauss-exponential to Gaussian distribution due to the influence of residual ionization.

The plasma arc inside the F4 torch used for atmospheric plasma spraying is characterized by means of analytical and numerical methods. A simplified analytical model is formulated to understand the physical behaviour of the plasma arc. A three-dimensional numerical model is developed to simulate the realistic plasma arc flow inside the torch. At a given torch power and gas flow rate, possible combinations of the arc core radius and arc length are predicted. The thermodynamic principle of minimum entropy production is used to determine the combination of arc core radius and arc length, which corresponds to the actual physical situation of the arc inside the torch. The effect of arc current and gas flow rate on the plasma arc characteristics is clarified. The effect of hydrogen content in the plasma gas on its velocity and temperature profiles at the nozzle exit is shown. Predicted torch efficiencies are comparable to measured ones. The results of the numerical model are similar to that an analytical model. Previously published experimental and numerical results support part of the present results.

A free electron laser (FEL) amplifier with an elliptical waveguide and a planar wiggler is proposed. A set of coupled operating equations for this FEL operating in the Raman regime is derived using three-dimensional nonlinear theory. The numerical simulation code is developed, and the characteristics of this FEL amplifier are analysed numerically. Our numerical simulations are conducted to model a 34.6 GHz amplifier with an electron beam with an energy of 3.1 MeV. The results indicate that the elliptical waveguide is more suitable for a FEL with a planar wiggler system than a rectangular waveguide.

We show a structural modification technique which causes the fluctuated surface of multi-wall carbon nanotubes (MWNTs) by using hydrogen sulfide as a reaction-assist medium, which promotes the formation of sp³-hybridized orbitals of carbon and suppresses the formation of sp²-hybridized orbitals in the plasma process. We observed the structure of MWNTs synthesized with the addition of H₂S by means of high-resolution transmission electron microscopy and electron energy loss spectroscopy, and found the cornhusk-like surface fluctuations on the surface of MWNTs. In this study, we discuss the mechanism underlying this structural modification of carbon nanotubes precisely.

The paper presents a nanoindentation study of MgO single crystals before and after ion irradiation up to a fluence of 10²⁰ Ar⁺ m⁻². It is confirmed that crystalline MgO is a brittle material of comparatively high nanohardness, ranging from H = 12.5 to 14.5 GPa depending on surface orientation. The plastic deformation is based on a dislocation glide with formation of slip bands giving rise to piling up around the indent which is strongly related to the sample crystallography. Repeated loading–unloading cycles (multi-cycling) revealed the appearance of hysteresis loops that are related to nanofracturing. Irradiation with 100 keV Ar⁺ ions resulted in a reproducible hardness increase to a value of H = 19 GPa independent of surface orientation. The indentation modulus E = 285 GPa remained unaffected by ion irradiation. Furthermore, no hysteresis loop in the force–displacement curve was formed with multi-cycling after implantation. This is explained in terms of point defect assisted plasticity: defect pinning will decrease the dislocation mobility and hence increase the hardness and also cause the sample to become less brittle resulting in a large decrease in nanocrack formations. In contrast to the dislocation glide, the point defect contribution to plastic deformation is almost orientation independent. This picture is supported by the orientation independent hardness as well as by the observation that ion irradiated samples exhibit a reduction in, and more homogeneously distributed, piling up around the indent.

We report the structural evolution of diamond films in a plasma assisted hot filament chemical vapour deposition system with a variable substrate bias through a two-step process including deposition and subsequent modification. It is found that the diamond surfaces can be significantly modified after the treatment of hydrogen plasma. The surfaces with pyramidic structures form under the proper treated duration and temperature with a decrease in the content of amorphous carbon. As the treatment is prolonged, the surface presents ellipsoid structures while the diamond film becomes amorphous. Furthermore, 50 µm-long columns appear instead of the ellipsoids in a higher regime of the substrate temperature. These results suggest that the hydrogen plasma treatment and substrate temperature can significantly tune the surface topography of diamond films, and even give rise to chemical bonding variations with pronounced surface structures.

The dielectrophoretic behaviour of barium titanate (BaTiO₃) particles with a mean grain size of 3 µm was studied. Suspensions of the powdered ceramic in the concentration range 0.01–1.60% (w/v) were prepared in dilute aqueous solutions of NaCl and concentrated aqueous solutions of the amphoteric molecules HEPES (N-[2-hydroxyethyl] piperazine-N'4-[2-ethanesulfonic acid] and EACA (ε -aminocaproic acid). When suspended in water without ampholytes, the particles showed positive dielectrophoresis (DEP) over the whole frequency range (1 kHz–20 MHz), independent of the medium conductivity or applied voltage. When amphoteric molecules were added at a final concentration of up to 0.57 M, the particles showed positive DEP at all frequencies. When the concentration of ampholytes was increased to 0.71 M, the particles showed positive DEP at frequencies up to 100 kHz and voltages lower than 12 V_pk–pk at all electrode sizes. However, at 100 kHz, when the amplitude was increased to over 12 V_pk–pk, the particles started to display negative DEP at the smallest electrode size (20 µm) and moved away from the microelectrodes, accumulating in the gap between the electrodes. At the highest voltages used (16–20 V_pk–pk), the particles were seen moving upwards and remained stably levitated above the array. For frequencies larger than 100 kHz, the particles showed positive DEP only. It is shown that such behaviour cannot be expected on the basis of the dielectric properties of barium titanate and the suspending medium, and it is suggested that this behaviour may be caused by the fact that at high amphotere concentration and voltages the electric field across the particles surpasses the dielectric strength of the BaTiO₃ particles, resulting in a sudden drop in the particle's permittivity. The fact that not all particles showed negative DEP suggests a spread in the dielectric properties of barium titanate particles. Physical separation of barium titanate particles with presumably different dielectric properties was shown to be possible using a flow-through device.

In this work, a technique based on optical heating and electrical thermal sensing (OHETS) is developed to characterize the thermophysical properties of one-dimensional micro/nanoscale conductive and non-conductive wires. In this method, the to-be-measured thin wire is suspended over two electrodes and is irradiated with a periodically modulated laser beam. The laser beam induces a periodical temperature variation in the wire/tube, which will lead to a periodical change in its electrical resistance. A dc current is applied to the sample, and the resulting periodical voltage variation over the wire is measured and used to extract the thermophysical properties of the wire/tube. A 25.4 µm thick platinum wire is used as the reference sample to verify this technique. Sound agreement is obtained between the measured thermal conductivity and the reference value. Applying the OHETS technique, the thermal diffusivity of conductive single-wall carbon nanotube (SWCNT) bundles and non-conductive human hair and cloth fibres are measured. For non-conductive wires, a thin (∼nm) metallic film is coated at the outside of the wire for electrical thermal sensing. The measured thermal diffusivities for three different SWCNT bundles are 2.98 × 10⁻⁵ m² s⁻¹, 4.41 × 10⁻⁵ m² s⁻¹ and 6.64 × 10⁻⁵ m² s⁻¹. These values are much less than the thermal diffusivity of graphite in the layer direction. For human hair and microscale cloth fibres, our experiments show that their thermal diffusivities are at the level of 10⁻⁶ m² s⁻¹.

A leakage current model that includes space charge effects is developed for (Ba,Sr)TiO₃ (BST) thin films. Numerical simulation using this model has been performed to investigate the leakage mechanism in an epitaxial BST thin film after a hydrogen-containing forming gas anneal. The present work suggests that before the forming gas anneal the leakage mechanism in the BST film is probably the Schottky emission whereas after the forming gas anneal the leakage characteristics can be described by the modified Schottky equation of Simmons with effective barrier heights obtained from the model simulation.

Well-aligned graphitic nanocones were synthesized on polished pristine p-type (100) silicon wafers through plasma enhanced hot filament chemical vapour deposition. The field electron emission properties were investigated using anode probes of different diameters. Non-linearity was observed in the field electron emission (FEE) Fowler–Nordheim (FN) plots at high electric field. The numerical calculations based on the superposition of two types of characteristic emission sites are consistent with the experimental data. The non-linearity in the FN plot may be attributed to the non-uniform field enhancement factor (FEF) of the graphitic nanocones. At low electric field, electrons are emitted mainly from nanocones with large FEF, corresponding to a small slope magnitude in the FN plots. With the increasing electric field, the nanocones with small FEFs also contribute to the emission current, which results in a reduced average FEF and so a larger slope magnitude. If the difference in FEFs of the two types of emission sites is large, the emission sites with smaller FEF may not be able to participate in emission even at the highest electric field tested in the experiments and the FN plot remains a straight line determined presumably by the emission sites with larger FEF.

This paper reports the results of an investigation on combined torsional buckling of an individual multi-walled carbon nanotube (MWNT) under combined torque and axial loading. Here, a multiple shell model is adopted and the effect of van der Waals forces between two adjacent tubes is taken into account. According to the ratio of radius to thickness, MWNTs discussed in this paper are classified into three types: thin, thick and nearly solid. The critical shear stress and the combined buckling mode are calculated for three types of MWNTs under combined torque and axial loading. Results carried out show that the buckling mode (m, n) corresponding to the critical shear stress is unique, which is obviously different from the purely axial compression buckling of an individual MWNT. Numerical results also show that the critical shear stresses and the corresponding buckling modes of MWNTs under combined torque and axial loading are dependent on the axial loading form and the types of MWNTs. The new features and meaningful numerical results in the present work on combined buckling of MWNTs under combined torque and axial loading may be used as a useful reference for the designs of nano-drive devices and rotational actuators in which MWNTs act as basic elements.

The polyimide material Kapton has been studied by positron annihilation lifetime spectroscopy (PALS) and lifetime results for two grades of Kapton are presented. Positron transmission measurements using stacked Kapton foil, backed with Al, and with Ni, were performed and the model dependent Kapton positron absorption coefficient values are given. Expressions for determining positron source support foil contributions in PALS experiments are evaluated and discussed.

Highly (100)-oriented Ba(Zr_0.2Ti_0.8)O₃/La_0.7Ca_0.3MnO₃ (BZT/LCMO) heterostructure thin films have been grown on Si(100) and Pt(111)/Ti/SiO₂/Si(100) substrates prepared by pulsed laser deposition. The structure and surface morphology of the films have been characterized by x-ray diffraction and atomic force microscopy. The dielectric constant of the films changes significantly with applied dc bias field and the films have high tuneability of 59.1%, 52% and 59.4% at an applied field of 400 kV cm⁻¹, respectively, for the BZT film on Pt/Ti/SiO₂/Si and the BZT/LCMO heterostructure films on Si and Pt/Ti/SiO₂/Si substrates. The tuneability of the BZT/LCMO thin films on Pt-coated Si substrate was higher than that of the BZT thin films on Pt/Ti/SiO₂/Si and LCMO/Si substrates. The high tuneability has been attributed to the (100) texture of the films and larger grain sizes. The BZT thin film is an attractive candidate for tuneable microwave device applications.

In our experiments, carbon and hafnium films were deposited onto molybdenum substrates by dual ion beam sputtering and radio-frequency magnetron sputtering technology, respectively, and then the BaO layer was deposited on C/Mo and Hf/Mo substrates by a chemical method in order to simulate the working conditions of the grids contaminated by active electron-emission substances from the cathode. The composition and phase change of the samples were investigated after annealing from 600 to 950 °C. The results show that the carbon or hafnium film can effectively reduce accumulation of the cathode emitting substance BaO on the grid surface. The suppression mechanism of the electron emission from the grid is discussed.

Significant progress has been achieved during the last several years relating to experimental and theoretical aspects of terahertz (or submillimetre wave) Fourier transform spectroscopy of biological macromolecules. However, previous research in this spectral range has been focused on bio-materials in solid state since it was common opinion that high water absorption will obscure the spectral signatures of the bio-molecules in solutions. At the same time, the biological functions of DNA and proteins take place in water solutions. In this work, the spectra of DNA samples have been measured in liquid phase (gel) over the spectral range 10–25 cm⁻¹ and compared with spectra obtained from solid films. The results demonstrate that there is very little interference between the spectral features of the material under test and the water background except for the band around 18.6 cm⁻¹. Multiple resonances due to low frequency vibrational modes within biological macromolecules in solutions are unambiguously demonstrated. Higher level of sensitivity and higher sharpness of vibrational modes are observed in the liquid environment in comparison with the solid phase, with the width of spectral lines 0.3–0.5 cm⁻¹. Gel sample spectra are found to be polarization-dependent. The ability of THz spectroscopy to characterize samples in liquid phase could be very important since it permits examination of DNA interactions in real (wet) samples. One demonstrated example of practical importance is the ability to discriminate between spectral patterns for native and denaturated DNA.

Finite time thermodynamics is used to solve a new model of an extended Brayton cycle with variable-temperature heat reservoirs and finite size heat exchangers. The model takes into account external and internal entropy generation and handles heat recovery and heat leaks to the environment in a novel way. The extended system considerations are very important for minimizing entropy generation and maximizing second law efficiency, profit and ecological criterion. An optimization analysis was developed on this new model to determine its maximum power and minimum entropy generation, and amid the most important findings were the global maximum net power, global minimum entropy generation, optimum global heat exchangers size distribution, best working fluid specific heat ratio and optimal fluid heat capacities, some of these never having been published previously.

While travelling wave dielectrophoresis (twDEP) offers a promising method for the control of micro-sized particles suspended in liquids, particularly when the motion of particles along the length of the channel is sought without having to pump the liquid itself, it leads to a variety of complex dynamical regimes which need to be clearly understood for the design of efficient microfluidic devices targeting particular functions. In this paper, we describe the various dynamical regimes in terms of the forces acting on the particles, i.e. the conventional dielectrophoretic and twDEP force and torque, the viscous drag exerted by the fluid on the particle and the electrostatic and hydrodynamic particle–particle interactions. We also explore the variation of the dynamical regimes in three different configurations typical of microfluidic channels whose electrodes are embedded in the bottom wall. The first two configurations have different, i.e. aligned and staggered, electrode geometries, and the third configuration consists of aligned electrodes but energized at different potentials. For these purposes, we use our direct numerical simulation code based on the distributed Lagrange multiplier method for solving the equations of motion for both the fluid and the individual particles, and the point dipole model to compute the electrostatic forces. The model particles are chosen so as to have mechanical and electrical properties of yeast cells suspended in an aqueous solution. It is found that the motion of the particles not only depends significantly on the Clausius–Mossotti factor, which is a function of the electric properties of the fluid and the particles, but also on the specific configuration considered. Particularly, the spinning of particles plays a crucial role in the particle translations and interactions, but the direction of such spinning motion depends on the particular device configuration used.

The results of the interaction of 200 fs and 4 ns laser pulses produced by a Ti : Sapphire commercial laser system with various materials are reported. The drilling rates were measured as a function of laser pulse energy and material thickness. Saturation of the material removal rate was observed for high laser pulse energy. The dependence of the material removal rate on the sample thickness was measured. The results of numerical simulation of laser drilling with femtosecond and nanosecond laser pulses using a theoretical model which includes phase transition at the critical temperature are reported. The computed drilling rates are in agreement with the data of measurements. The effect of a suspected nanosecond pedestal pulse containing approximately 30% of total pulse energy on the rate of drilling with femtosecond laser pulses is numerically simulated and the computed results are discussed.