Electron energy-loss spectroscopy study of thin film hafnium aluminates for novel gate dielectrics

We have used conventional high‐resolution transmission electron microscopy and electron energy‐loss spectroscopy (EELS) in scanning transmission electron microscopy to investigate the microstructure and electronic structure of hafnia‐based thin films doped with small amounts (6.8 at.%) of Al grown on (001) Si. The as‐deposited film is amorphous with a very thin (～0.5 nm) interfacial SiO_x layer. The film partially crystallizes after annealing at 700 °C and the interfacial SiO₂‐like layer increases in thickness by oxygen diffusion through the Hf‐aluminate layer and oxidation of the silicon substrate. Oxygen K‐edge EELS fine‐structures are analysed for both films and interpreted in the context of the films’ microstructure. We also discuss valence electron energy‐loss spectra of these ultrathin films.     

In situ dynamic HR-TEM and EELS study on phase transitions of Ge2Sb2Te5 chalcogenides

The phase transition phenomena of Ge2Sb2Te5 chalcogenides were investigated by in situ dynamic high-resolution transmission electron microscopy (HR-TEM) and electron energy loss spectroscopy (EELS). A 300kV field emission TEM and a 1250kV high voltage TEM were employed for the in situ heating experiments from 20 to 500 degrees C for undoped and 3wt% nitrogen-doped Ge2Sb2Te5 thin films deposited by DC sputtering. Crystallization of amorphous Ge2Sb2Te5 to its cubic structure phase started at 130 degrees C and then rapid crystal growth developed from cubic to hexagonal phase in the range of 130-350 degrees C; finally, the hexagonal crystals started to melt at 500 degrees C. For nitrogen-doped Ge2Sb2Te5, its crystallization from amorphous film occurred at higher temperature of ca. 200 degrees C, and the cubic and hexagonal phases were usually formed simultaneously without significant growth of crystals at further heating to 400 degrees C. EELS measurements showed that the electronic structures of Ge, Sb and Te stayed almost the same regardless of the amorphous, FCC and hexagonal phases. The nitrogen doped in Ge2Sb2Te5 was confirmed to exist as a nitride. Also, the doped nitrogen distributed homogeneously in both amorphous and crystalline phases. Localization of doped nitrogen was not found in the grain boundary of crystallized phases. The dynamic process of phase transition was enhanced by high-energy electron irradiation. Peeling of atomic layers in nitrogen-doped Ge2Sb2Te5 film was detected during heating assisted with electron beam irradiation.     

Micellar calcium borate as an antiwear additive

The mechanisms of action of a new generation of antiwear additives is studied here by means of energy‐filtering transmission electron microscopy (EFTEM) carried out on the wear particles generated during friction tests between two ferrous surfaces (under boundary lubrication conditions). This paper deals with the structural and physico‐chemical changes that colloidal particles, calcium carbonate (CC) and calcium borate (CB) overbased salicylates detergents, have undergone during the build‐up of the interfacial antiwear tribofilm. EFTEM allowed us to investigate the nature of wear fragments originating from the film, stemming from CC and CB micelles, and to make a comparison regarding the tribofilm formation mechanisms. It appears that the CC wear debris are mainly crystalline and contain a high concentration of iron (as abrasive iron oxide Fe₂O₃), limiting their antiwear action. Consequently, CC micelles do not lead to an effective protective tribofilm. On the other hand, CB micelles do have an antiwear action, which we explained by the formation of a glassy iron borate tribofilm during the friction tests. Many of the CB wear fragments are composed of this amorphous material containing very small crystallites of residual calcite. Boron (contained in the CB micelles) is responsible for the presence of amorphous zones of the film and acts as a glass former, in a comparable way to phosphorus in zinc dithiophosphate. This revised version was published online in July 2006 with corrections to the Cover Date.     

The pairing-dependent effects of laser surface texturing on micro tribological behavior of amorphous carbon film

In this paper, a kind of textured amorphous carbon film with the pattern of micro dots matrix was developed by irradiating amorphous carbon film with Nd–yttrium aluminum garnet laser system. Confirmed by the characterizations is that the produced micro dots are protuberant and in nanocrystalline graphite phase with a porous structure and reduced hardness. The micro tribological behavior of textured film was studied experimentally using steel balls and Si₃N₄ balls as the counter body. It turns out that the influences of laser treatment on the tribological performance of amorphous carbon film are strongly dependent on the friction pairs. By specially probing into the effects of localized micro graphite bulges, possible friction reduction mechanisms are discussed.     

Mechanical and tribological properties of amorphous carbon films

The mechanical and tribological properties of amorphous carbon films have been studied in more detail in recent years because these films (a) can be deposited near room temperature, thus allowing film deposition on common engineering alloys (i.e., aluminum and steel) without altering their mechanical properties, and (b) are smooth and conform to surface roughness of the substrate, thus requiring no post deposition processing. In addition, amorphous carbon films exhibit low unlubricated sliding friction in contact with steel and ceramics which is comparable to that of steel against steel in a lubricated contact. The wear resistance of these films is also better than Ti‐based hard coatings. Further improvement in film tribological properties can be achieved by modifying film chemical composition. Because of these attractive features, amorphous carbon films have been evaluated in several applications including automotive, electronic and biomedical engineering. However, environmental factors such as oxygen and humidity have been found to influence tribological properties significantly. This paper reviews the current understanding of the tribological properties of both hydrogenated and non‐hydrogenated amorphous carbon films, the mechanisms responsible for low friction coefficient and identifies areas that require further research. This revised version was published online in June 2006 with corrections to the Cover Date.     

Characterization of the μ and P phase precipitates in the CMSX‐4 single crystal superalloy

A combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning‐transmission electron microscopy (STEM) using high‐angle annular‐dark‐field (HAADF) imaging, focussed ion beam‐ scanning electron microscopy (FIB‐SEM) tomography, selected area electron diffraction with beam precession (PED), as well as spatially resolved energy‐dispersive X‐ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), was used to investigate topologically close‐packed (TCP) phases, occurring in the CMSX‐4 superalloy subjected to high temperature annealing and creep deformation. Structural and chemical analyses were performed to identify the TCP phases and provide information concerning the compositional partitioning of elements between them. The results of SEM and FIB‐SEM tomography revealed the presence of merged TCP particles, which were identified by TEM and PED analysis as coprecipitates of the μ and P phases. Inside the TCP particles that were several micrometres in size, platelets of alternating μ and P phases of nanometric width were found. The combination of STEM‐HAADF imaging with spatially resolved EDS and EELS microanalysis allowed determination of the significant partitioning of the constituent elements between the μ and P phases.     

Combining Ar ion milling with FIB lift-out techniques to prepare high quality site-specific TEM samples

Focused ion beam (FIB) techniques can prepare site‐specific transmission electron microscopy (TEM) cross‐section samples very quickly but they suffer from beam damage by the high energy Ga⁺ ion beam. An amorphous layer about 20–30 nm thick on each side of the TEM lamella and the supporting carbon film makes FIB‐prepared samples inferior to the traditional Ar⁺ thinned samples for some investigations such as high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). We have developed techniques to combine broad argon ion milling with focused ion beam lift‐out methods to prepare high‐quality site‐specific TEM cross‐section samples. Site‐specific TEM cross‐sections were prepared by FIB and lifted out using a Narishige micromanipulator onto a half copper‐grid coated with carbon film. Pt deposition by FIB was used to bond the lamellae to the Cu grid, then the coating carbon film was removed and the sample on the bare Cu grid was polished by the usual broad beam Ar⁺ milling. By doing so, the thickness of the surface amorphous layers is reduced substantially and the sample quality for TEM observation is as good as the traditional Ar⁺ milled samples.     

Boundary Lubrication by Pure Crystalline Zinc Orthophosphate Powder in Oil

The aim of this study is to assess the tribological behavior of pure crystalline zinc orthophosphate under boundary lubrication in order to model zinc phosphate-based anti-wear additives. Boundary films were generated from α-Zn₃(PO₄)₂ powder dispersed in poly-alpha-olefin oil, at ambient temperature, by means of a steel sphere-on-flat contact in reciprocating motion. Electrical contact resistance and friction coefficient evolutions enable an understanding of the tribological behavior of crystalline zinc orthophosphate at the sliding interface. A conductive atomic force microscope (C-AFM) equipped with a current sensing setup, Raman spectroscopy, and nano-indentation were used to characterize the resulting film. When involved in a tribological contact, zinc orthophosphate powder forms a continuous patchy adherent film, changing its structure to amorphous orthophosphate, on both sliding steel surfaces. Morphological and mechanical properties of the film are discussed with respect to the ZDTP tribofilm models.     

Tribological properties of nickel-base nanocrystalline coatings sputter-deposited on austenitic stainless steel

500 nm-thick films are deposited on austenitic stainless steel by neutral (Ar⁺) or reactive (N⁺) ion beam sputtering of Ni or NiTi targets, with (or without) high energy 160 keV-Ar⁺ ion beam assistance. Most of the time the coatings are nanocrystalline and induce a large (excellent in some conditions) increase of the wear resistance. Only Ar⁺ ion beam sputtering of a NiTi target gives an amorphous deposit which does not improve the substrate tribological properties. The hardness and wear resistance of ion beam assisted films are larger than those obtained with non-ion beam assisted coatings. The presence of a hard TiN phase inside a ductile Ni phase, of grains with preferential orientation beneficial to slip, as well as film densification are the main factors which increase the wear resistance. The best results are obtained when the structure is composed of two phases, Ni and TiN. The TiN phase strengthens the already good tribological Ni properties and the Ni ductility induces mechanical accommodation during the friction process.     

Tribological behaviour of protic ionic liquids with ammonium salts modified LABSA as lubricants and additives

Two typical protic ionic liquids with ammonium salts modified linear alkylbenzene sulfonic acid (LABSA) were synthesised, and their tribological behaviours used as both lubricants compared with PAO10 and additives in PAO10 were evaluated with an Optimol SRV‐I oscillating reciprocating friction and wear tester. The results show that the two novel protic ionic liquids with ammonium salts modified LABSA exhibited excellent friction‐reducing and anti‐wear properties as lubricants and can also significantly improve the tribological performance of the base fluids as friction modifier additives. The worn surfaces were characterised by scanning electron microscope and X‐ray photoelectron spectroscopy, indicating that the excellent anti‐wear and friction‐reducing performance could attributed to the boundary lubrication films that could contain both the tribochemical film composed of organic amine decomposed from the protic ionic liquids with ammonium salts and the stable chemical absorbed film through the interactions between the sulfonate anions and surface metallic atoms during the sliding process. Copyright © 2012 John Wiley & Sons, Ltd.     

The synthesis and tribological behaviour of boron‐nitrogen‐modified fatty acid water‐based lubricant additives

A new type of boron‐nitrogen‐midified fatty acid as a water‐based lubricant additive was prepared and the chemical structure characterised by infrared spectroscopy. The tribological properties of the additive in water were evaluated using friction testers. The morphologies and tribochemical species of worn surfaces were analysed by means of X‐ray photoelectron spectroscopy (XPS). The results show that the additive is very good at increasing the losdcarrying capacity and the antiwear and friction‐reducing properties of wter. A lubrication mechanism is inferred in which a lhigh‐strength adsorption film and a tribochemical reaction film form on the rubbing surfaces as a resuly of the carrier effect of long‐chain fatty acid molecules, the high reaction activities of nitrogen, and the electron deficiency of boron, as well as the interactions of these factors.     

Nano-mechanical and tribological characteristics of ultra-thin amorphous carbon film investigated by afm

The mechanical as well as tribological characteristics of coating films as thin as a few nm become more crucial as applications in micro-systems grow. Especially, the amorphous carbon film has a potential to be used as a protective layer for micro-systems. In this work, quantitative evaluation of nano-indentation, scratching, and wear tests were performed on the 7nm thick amorphous carbon film using an Atomic Force Microscope (AFM). It was shown that AFM-based nano-indentation using a diamond coated tip can be feasibly utilized for mechanical characterization of ultra-thin films. Also, it was found that the critical load where the failure of the carbon film occurred was about 18μN by the ramp load scratch test. Finally, the wear experimental results showed that the quantitative wear rate of the carbon film ranged 10^-9~10^-8 mm³/N cycle. These experimental methods can be effectively utilized for a better understanding the mechanical and tribological characteristics at the nano-scale.     

Friction and Vibration of Brake Friction Materials Reinforced with Chopped Glass Fibers

A diamond-like carbon (DLC) film exhibits excellent tribological properties. This type of film has an amorphous structure that is generally composed of hydrogen and carbon atoms, and it is the structure of sp²- and sp³-hybridized orbital carbon which brings about the extraordinary tribological properties of the DLC film. It is known that heating causes structural changes in a DLC film, and pre-heat treatment greatly affects the various properties of a DLC film. In this study, we focus on the effects of pre-heat treatment on the friction and wear properties of a hydrogenated DLC film and discuss the structural changes in the film. After pre-heat treatment, the tribological properties were evaluated using a ball-on-disk sliding tester. Our findings indicated that the friction and wear properties of the DLC film were improved by pre-heating up to 500 °C. An as-deposited DLC film had a friction coefficient of approximately 0.15, whereas it was approximately 0.03 for a film pre-heated at 500 °C. The structure of the DLC film was analyzed using micro-laser Raman spectroscopy. The analytic results of the Raman spectroscopy of the film surface showed that the G peak position had shifted toward a higher wave number. This result suggested that hydrogen had evolved from the DLC film because of pre-heat treatment. The half bandwidth of the G peak shifted toward a lower wave number with increases in the pre-heating temperature. This indicated that graphitization of the DLC film had been induced by pre-heat treatment. From these findings, we consider that the hydrogen evolution induced structural changes. Line analysis using micro-laser Raman spectroscopy was performed on a cross section of the pre-heated DLC film. The line analysis showed structural changes which were induced by hydrogen evolution, on the top of the DLC film. On the other hand, hydrogen evolution and graphitization were prevented inside the film, indicating that a gradient structure had been generated by pre-heat treatment. The low friction coefficient of the pre-heated DLC film was caused by graphitization of the DLC film surface. The graphite layer on the top of the film would induce lower shearing resistance at the sliding interface. This gradient structure of the DLC film plays an important role in improving the tribological properties of the pre-heated DLC film.     

Mechanism of boundary film formation from n‐hexadecane

It is widely acknowledged that the process of surface film formation by tribological additives is very complex, because even saturated hydrocarbons — as major components of lubricating oils — undergo chemical changes under boundary lubrication conditions. To gain a better understanding of the tribochemical changes of paraffinic hydrocarbons, research was carried out using n‐hexadecane, which is widely used as a low‐viscosity model base oil. It has been hypothesised that the interactions between the products of n‐hexadecane triboreactions and steel surfaces are mostly initiated by the mechanical action of the system. Wear tests were performed, using a pin‐on‐disc machine. Fourier transform infrared microspectrophotometry and X‐ray photoelectron spectroscopy were used to analyse the wear tracks on the steel discs. Based on the analytical data obtained, a model of boundary film formation from aliphatic hydrocarbons is proposed.     

Gallium-enhanced phase contrast in atom probe tomography of nanocrystalline and amorphous Al-Mn alloys

Over a narrow range of composition, electrodeposited Al-Mn alloys transition from a nanocrystalline structure to an amorphous one, passing through an intermediate dual-phase nanocrystal/amorphous structure. Although the structural change is significant, the chemical difference between the phases is subtle. In this study, the solute distribution in these alloys is revealed by developing a method to enhance phase contrast in atom probe tomography (APT). Standard APT data analysis techniques show that Mn distributes uniformly in single phase (nanocrystalline or amorphous) specimens, and despite some slight deviations from randomness, standard methods reveal no convincing evidence of Mn segregation in dual-phase samples either. However, implanted Ga ions deposited during sample preparation by focused ion-beam milling are found to act as chemical markers that preferentially occupy the amorphous phase. This additional information permits more robust identification of the phases and measurement of their compositions. As a result, a weak partitioning tendency of Mn into the amorphous phase (about 2 at%) is discerned in these alloys.     

An EELS-based study of the effects of pyrolysis on natural carbonaceous materials used for activated charcoal preparation

Electron energy‐loss spectroscopy (EELS) has been used to characterize the electronic structure of charcoal phases at the nanoscale, thus demonstrating that the technique can be applied to environmental science. Activated charcoal is extensively used to remove pollutants from liquid and gaseous sewage. It is mainly obtained by activation of coke or charcoal produced from ligneous precursors. The present study concerns the use of by‐products of local Caribbean agriculture, such as sugar cane bagasse, fruit stones and seeds, for use as activated charcoal precursors. Charcoal phases are prepared by high‐temperature pyrolysis of lignocellulosic raw materials under a nitrogen gas flow. With the aim of optimizing the pyrolysis temperature and duration and oxygen content, the concentration of carbon sp² hybridized chemical bonds and structural ordering have been followed by EELS for different treatment temperatures. To quantify the carbon sp² content, near edge structure (NES) at the carbon K edge has been measured to determine the strength of π → π* and 1s → π* transitions. Three precursors of plant origin, shells of Terminalia catappa and Acrocomia karukerana and seeds of Psidium guajava, with the pyrolysis temperatures between 600 and 900 °C, were investigated. The fraction of carbon sp² bonding is found to increase when the temperature rises from 600 °C to the range 700–750 °C and becomes stable at higher temperatures. For temperatures in excess of 700 °C, structural ordering probably occurs and well‐defined 1s → σ* NES is present, whose intensity increases with increasing preparation temperature. For the highest temperature of around 900 °C, the structure of the final product is less well organized than graphitized carbon but a few per cent of a highly ordered phase is found.     

Smart acquisition EELS

We have developed a novel acquisition methodology for the recording of electron energy loss spectra (EELS) using a scanning transmission electron microscope (STEM): “Smart Acquisition”. Smart Acquisition allows the independent control of probe scanning procedures and the simultaneous acquisition of analytical signals such as EELS. The original motivation for this work arose from the need to control the electron dose experienced by beam-sensitive specimens whilst maintaining a sufficiently high signal-to-noise ratio in the EEL signal for the extraction of useful analytical information (such as energy loss near edge spectral features) from relatively undamaged areas. We have developed a flexible acquisition framework which separates beam position data input, beam positioning, and EELS acquisition. In this paper we demonstrate the effectiveness of this technique on beam-sensitive thin films of amorphous aluminium trifluoride. Smart Acquisition has been used to expose lines to the electron beam, followed by analysis of the structures created by line-integrating EELS acquisitions, and the results are compared to those derived from a standard EELS linescan. High angle annular dark-field images show clear reductions in damage for the Smart Acquisition areas compared to the conventional linescan, and the Smart Acquisition low loss EEL spectra are more representative of the undamaged material than those derived using a conventional linescan. Atomically resolved EELS of all four elements of CaNdTiO show the high resolution capabilities of Smart Acquisition.     

Chemical structure characterization of the boundary lubrication film using X-ray photoelectron spectroscopy and scanning Auger microprobe techniques

The chemical structure of the solid reaction film formed on the metal surface under boundary lubrication for a practical rubbing system is characterized using X-ray photoelectron spectroscopy and scanning Auger microprobe techniques in this paper. Analysis of the spectra suggests that the boundary lubrication film can be qualitatively divided into three layers,i.e. it consists of an adsorbed-reaction film, a mixed-reaction film and a diffusion reaction film. The significant experimental results are that the chemical states of the elements in the film are strongly dependent on the lubrication regime and their position within the lubrication film, with which some tribological phenomena can be reasonably explained.     

Tribological Behavior of Amorphous and Crystalline Overbased Calcium Sulfonate as Additives in Lithium Complex Grease

The aim of this study was to examine the tribological behavior of amorphous overbased calcium sulfonate (AOBCS) and crystalline overbased calcium sulfonate (COBCS, transformed from the AOBCS) as additives in lithium complex grease. The transformation product of the calcium carbonate polymorph from AOBCS was calcite, as determined by Fourier transform infrared spectroscopy. Tribological properties were evaluated by an oscillating reciprocating friction and wear tester and a four-ball tester. The results showed that the addition of COBCS can dramatically improve both the antiwear performance and the friction-reducing and load-carrying properties of the base grease. However, improvement of the tribological properties of the base grease by AOBCS was highly dependent on the concentrations added and the loads applied. The tribological properties of the base grease were improved more by the addition of COBCS than by the addition of AOBCS. X-ray photoelectron energy spectrometry and thermogravimetric analysis revealed that both AOBCS and COBCS underwent complicated tribochemical reactions in the base grease and that chemically reactive films consisting of CaCO₃, CaO, iron oxide and organic compounds were formed on the worn surfaces. Taken together with the results of the tribo-tests, we suggest that transformation of the calcium carbonate polymorphs was the main factor in improving the tribological properties of lithium complex grease. The transformation of calcium carbonate polymorphs can broaden the application of AOBCS as an extreme pressure/antiwear additive in greases under boundary lubrication conditions.     

Additives for Rape Seed Oil 1: Preparation and Tribological Behaviour of a Sulphurised Rape Seed Oil Lubricant Additive

A new type of environmentally friendly lubricant additive ‐ a sulphurised rape seed oil additive ‐ was prepared, and the chemical structure characterised using infrared spectroscopy. The tribological properties of the additive in a rape seed base oil were evaluated using a friction tester. The morphologies and tribochemical species of worn surfaces were analysed by means of X‐ray photoelectron spectroscopy. The results show that the additive increased the load‐carrying capacity and improved the antiwear and friction‐reducing properties of the rape seed oil. The inferred lubrication mechanism is that a high‐strength adsorption film and a tribochemical reaction film form on rubbing surfaces due to the carrier effect of the long‐chain rape seed oil molecules, the high reactivity of sulphur, and their synergism.