Graphite Surface Layer Research Articles

Surface characterization of diamond-like amorphous carbon foils by (e,2e) spectroscopy and transmission electron energy loss spectroscopy

We have measured the spectral momentum densities of thin foils of diamond-like carbon using (e,2e) spectroscopy. Transmission electron energy loss spectra and (e,2e) spectra were measured before and after annealing a thin foil at around 900 degrees C and before and after thinning the foil using reactive ion etching in an argon-oxygen plasma. The valence band spectral momentum densities are compared with spherically averaged graphite and diamond band theory calculations. After annealing the surface sensitive (e,2e) data are closer to the graphite theory for the foil. Before annealing and also after plasma etching the (e,2e) data compare more favourably with the diamond theory. Bulk-sensitive transmission energy loss spectra for the annealed sample show a weak graphitic plasmon at around 6 eV energy loss which disappears after subsequent plasma etching. These measurements show that the diamond-like carbon films become graphitic only at the surface after annealing and that the graphitic surface layer can be easily removed by reactive ion etching.

Molecular dynamics simulation of thin film amorphous carbon growth

Abstract Molecular dynamics simulations of amorphous diamond thin film growth were carried out using a Stillinger-Weber potential parameterised to describe both sp2 and sp3 carbon bonding. The effects of the incident beam energy and angle and substrate temperature were investigated. It was found that in very thin films, not deep enough to allow the generation of compressive stress, atomic impacts lead to the formation of a graphitic surface layer. Results also indicate that the optimal conditions for the formation of tetrahedral sp3 structures involve low incident beam energy and low substrate temperatures.

Early stages of bump formation on the surface of ion-bombarded graphite

We investigate possible mechanisms for the formation of bumps, of atomic dimensions, in the surface layer of graphite under low-energy, single ion bombardment. We perform a Molecular Dynamics simulation and analyse the results in view of recent experimental data obtained with the Scanning Tunnelling Microscope. Only the early stages of bump formation are described. The depth profile of momentum deposited in the cascade is evaluated and used, in conjunction with the atomic density profile, to characterise interlayer distortions. The surface bumps appear under the presence of stresses developed in the near-surface layers of the target, due to defects created by the collision cascade. The bump originates around the positions of vacancies and interstitials, and is stabilised by the presence of interlayer interstitials. We have also produced a video film that helps in the visualisation of the time evolution of topographical features.

X-Ray Photoelectron Spectroscopic Studies of Palladium Dispersed on Carbon Surfaces Modified by Ion Beams and Plasmatic Oxidation

The effects of ion bombardment and r.f. plasma oxidation of graphite surfaces on subsequent growth and electronic properties of vacuum deposited palladium clusters have been investigated by methods of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy with X-ray excitation (XAES). Due to the significantly increased density of surface defects on which the nucleation process occurs the bulk value of the Pd 3d core level binding energy is achieved at higher surface coverage by palladium on bombarded surfaces than on ordered graphite. Angle resolved photoelectron spectra of oxidized graphite surfaces reveal significant embedding of oxygen in graphite surface layers. The C 1s and O 1s photoelectron spectra are consistent with presence of two major oxygen species involving C-O and C=O type linkages which are not homogeneously distributed within the graphite surface layers. Two effects were observed on oxidized surfaces: an increase of palladium dispersion and interaction of the metal clusters with surface oxygen groups. Using the simple interpretation of the modified Auger parameter the relaxation and chemical shift contributions to the measured Pd core level shifts are estimated. In the region of low surface coverage by palladium the effect of palladium-oxygen interaction on Pd core level binding energy exceeds the effects of increased dispersity.

Characterization of laser-irradiated surfaces of a polycrystalline diamond film with an atomic force microscope

A polycrystalline diamond film was etched down to a depth corresponding to the grain size with 193 nm excimer laser radiation. Below a graphitic surface layer (GL) we have observed a highly stable layer (SL) which is conducting. Atomic force microscope measurements were made in order to characterize the topography and microfriction of these two surface layers. Features of the diamond facet structure are still visible in spite of smoothening by the laser etch process. An approximation for the friction coefficient of both surface layers yields values of between 0.01 and 0.1. The friction pictures of the GL show a local period length of about 70 nm, which could be a typical diameter for micrographitic grains. The SL is more disrupted than the GL and the appearance of a period length of about 70 nm has faded discernibly.

Depth-profiling of beryllium on graphite with ion beams

The surface layer of beryllium-coated graphite was analysed by backscattering and nuclear reaction methods using a 2.5 MeV accelerator. For the Be analysis the nuclear reactions 9Be(p, d) 8Be, 9Be(p, α) 6Li, 9Be(d, p) 10Be, 9Be(d, t) 8Be, 9Be(d, α) 7Li and 9Be( 3He, p) 11B are compared with backscattering of 4He + and protons. For the analysis of carbon, which provides complementary information to the depth profiling of Be, and for the analysis of oxygen, which is the third element of importance in the system, elastic backscattering of 4He + and protons is applied. It turns out that the backscattering analysis with protons of 1.3 MeV presents the best compromise for a quick and straightforward measurement of all three elements. The other methods have their merits for a more detailed analysis.

Mechanical properties of nitrogen-implanted 18W4Cr1V bearing steel

Nitrogen was implanted into 18W4Cr1V bearing steel (where the composition is in approximate weight per cent) to doses of 10 14−10 17 ions cm −2 at 400 keV. The specimens were analyzed using the Rutherford backscattering and the X-ray photoelectron spectroscopy techniques and then tested on a pin-on-disc wear machine. There was a thick non-metallic film on the high dose specimens which was shown by the X-ray photoelectron spectroscopy examination to be graphite. Friction and wear were significantly decreased and hardness was increased by a factor of 4. These changes were observed to depend on the nitrogen dose and the applied load. The formation of a graphite surface layer is considered to be due to vacuum carburization and the increase in wear resistance is thought to be associated with nitride precipitation hardening.

The reaction of energetic O 2+, thermal O 2, and thermal O 2/ar + on graphite and the use of graphite for oxygen collector probes

The bombardment of graphite with energetic oxygen leads initially to a retention of oxygen within the graphite and subsequently to a reemission of CO and CO 2, increasing continuously with further implantation. After saturation of the graphite surface layer with oxygen, the reemission of CO and CO 2 reaches a steady state in which the implanted oxygen ion reacts to CO or CO 2 resulting in a chemical erosion yield close to 1. Since physical sputtering occurs simultaneously, the total erosion yield for graphite is nearly 1 for low energy oxygen and greater than 1 for energies above 500 eV. By a subsequent heating of the targets the retained oxygen is released completely in the form of CO and CO 2. Pyrolytic graphite collects nearly all the implanted oxygen until a critical irradiation dose is reached beyond which the retention behaviour decreases. This critical dose depends on the oxygen energy and on the irradiation temperature. These results were applied in developing a new method for measuring the oxygen impurity flux in the scrape-off region of fusion machines as demonstrated by exposed probes in TEXTOR under various discharge conditions.

Elemental and chemical-state mapping of boron carbide fracture surfaces

Samples of commercial B 4C and of reactive hot-pressed B 9C were fractured in situ in a scanning Auger microscopy (SAM) system. The B 4C fracture surfaces showed small areas of pore structures, as well as larger relatively smooth areas. Impurities (Cu, Si, S, Ca, N, O and Cr) were found in trace amounts in the pore areas. SAM maps were made of most of these impurities. The nitrogen was found by Auger lineshape in point Auger electron spectroscopy scans to be in the compound BN. Carbon was found on the surface in both boron carbide and as graphite. SAM was used to seperately map B in B 4C and B in BN, as well as C in B 4C and C in graphite, in a pore region. In the smooth areas of B 4C fracture surfaces fewer impurities were found. The carbon occured in B 4C and in localized carbon-rich areas which had graphite surface layers. In contrast, only B and C were found on the B 9C fracture surfaces; but the distributions of these elements were not homogenous. Carbon was found both in elemental form as graphite and chemically bound to boron as a carbide. In comparison with the graphite regions on the B 4C surfaces, the graphite areas on the B 9C fracture surfaces were not localized. The total surface area was nearly balanced between graphite and carbide regions. Argon ion bombardment revealed the graphite areas to be less than 100 nm thick.

Relaxation processes on graphitic surfaces. Part 1.—In the absence and presence of adsorbed 4He on Spheron and Grafoil with increasing temperature

An analysis of the time dependence (after-periods) of heat capacities measured for Spheron and Grafoil in the presence and absence of adsorbed 4He (and Ne) reveals the existence of slow first-order relaxation processes. For low temperatures these are endothermic but above 19.6 and 11.5 K for Spheron and Grafoil, respectively, they are exothermic. The rate at which these processes occur is determined by thermoelastic stresses associated with relatively small readjustments in the disordered surface geometry. In the absence of adsorbates the dimensional expansion in the c direction and contraction parallel to the surface can lead, respectively, to endothermic (dominant at low temperatures) and exothermic processes. In the presence of adsorbed 4He the superimposition of two endothermic processes is attributed to thermal expansion of the superimposed film and of the surface layers. The decrease, with increasing coverage of adsorbate, of the temperature above which the exothermic processes are observed is attributed to the effect of adsorbate on the structure of the surface layers of graphite. The increased deformation in the substrate lattice due to the interaction of the superimposed film results in the longer relaxation times and the higher ‘relaxed heat capacities’ of these processes.

The energetics of the epitaxial deposition of polyethylene onto the basal plane (0001) of graphite

AbstractPolymer‐substrate interaction energetics have been determined in order to describe the epitaxial deposition of polyethylene into graphite. The potential energy was calculated using van der Waal's interactions. A closed‐form representation of the potential was derived for a substrate which was considered infinite in planar area. Inspection of the relative polymer‐substrate energies for various positions above the substrate indicate a planar‐zigzag polyethylene chain segment deposits with its backbone parallel to the substrate. The chain axis preferentially orients in the (1120) directions of the graphite surface layer. Specifically, deposition occurs along the lines which intersect the nearest‐neighbor midpoints of the CC bonds in the surface layer.

The effect of cast iron graphites on friction and wear performance I: Graphite film formation on grey cast iron surfaces

Cast iron is assessed as a self-lubricating metal-base composite material. The formation of cast iron graphite films and the effective surface treatment of grey cast iron are discussed. It was found that the friction and wear behaviour of cast iron are influenced by the formation of a graphite surface layer. When cast iron surfaces are etched with Nital, they are found to be covered with a graphite film during the sliding friction process. This contributes to the protection of metallic contact points. The formation of graphite films during testing is assumed to be due to the non-elastic deformation of surface graphite. The effects of applied load, sliding velocity and lubricant are also discussed. The coefficient of friction increases with applied load and sliding velocity, and therefore lubrication becomes more important. These phenomena are discussed in terms of the formation of graphite films during the friction process.

Clean surface reactions between diamond and steel

THE rate of wear of diamond grinding on machining steel is known to be extremely high. For example, the wear rate of a diamond turning mild steel is 104 times greater than when turning brass of comparable hardness1. Several discussions of this wear conclude that it is due principally to the high local flash temperatures generated during machining and to the catalytic properties of the steel. Static experiments show that diamond graphitises at temperatures above 1,800K in vacuo, or above 1,100K in the presence of iron, and other experiments that flash temperatures of this order may be generated by machining. Hence, there is now general agreement that the wear proceeds by the graphitisation of the diamond; the surface layer of graphite being continually removed either by the abrasion of the steel or by solution in the steel2–7. However, little information is available on the actual conditions prevailing at the interface during machining. Therefore, we have studied the mechanism of wear more closely by making quantitative measurements of the wear rates of round-nosed diamond tools turning billets of mild steel under well defined conditions.

The adsorption and decomposition of ethylene on Ni(110)

The interaction of ethylene gas with a Ni(110) surface has been studied by a combination of modulated beam mass spectrometry and Auger electron spectroscopy techniques. At temperatures below 350°C, ethylene is chemisorbed with unity initial sticking coefficient up to a saturation coverage of one carbon atom per surface nickel atom. Above 150°C, hydrogen evolution is observed concurrent with the adsorption process, the amount of hydrogen liberated corresponding to complete dehydrogenation of the ethylene. Indirect evidence also suggests dehydrogenation down to 25°C. A physisorbed ethylene species is observed to coexist with the chemisorbed layer. Above 350°C formation of a graphitic surface layer is observed. The results obtained are consistent with earlier photoemission spectrometric measurements of Demuth and Eastman (Phys. Rev. Letters 32 (1974) 1123).

Radiation induced defects in diamond

Abstract A critical review of radiation damage in diamond is presented, focusing on important specific centres rather than attempting a review of the whole field. The proposed models for these centres are examined and often found wanting. Phenomena observed in ion bombardment of diamond is discussed and found to probably be due to amorphous or graphite surface layers. Throughout a number of suggestions of important experiments are given.