P. Morgen

Tribological properties of automotive disc brakes with solid lubricants

Abstract In automotive brake systems, high temperatures and pressures are generated at the contacting surfaces. This affects the pad and disc materials, creating a friction film between the surfaces, which consists of wear particles and volatile reactants from the pad and disc. To acquire suitable tribological properties, a pad matrix contains up to 20 different ingredients, mainly selected from experience. In the present report the effect of solid lubricants has been studied in three different pad matrices with relatively few components as compared to commercial brake pads. The components are Cu 2 S, PbS and Sb 2 S 3 , which are known to modify and stabilise the friction coefficient. The friction coefficient and wear rates of the pads are examined on a dynamometer, which simulates series of real-life car brake events. Two different energy levels and two ambient temperatures are included. After these tests, the brake disc surfaces are analysed with energy dispersive X-ray (EDX) and Auger electron spectroscopy (AES) in combination with argon ion sputtering to study the microscopic lateral and in-depth distribution of elements on and below the surface, looking for traces of the friction film. These experiments are used to discuss the correlation between the tribological properties and the external variables — braking temperature, solid lubricants and pad matrix.

Room temperature adsorption of Cs on Si(111)-(7 × 7) studied by optical second-harmonic generation

Abstract Room temperature adsorption of Cs on Si(111)-(7 × 7) has been followed continuously with second-harmonic generation during the evaporation process at several photon energies. The resonances of the clean (7 × 7) surface decay and new resonances involving Cs-induced states are observed below the monolayer coverage. The resonances at different coverages are discussed in terms of surface and interface states as they are known from photoemission, with good agreement between the techniques. Continued evaporation at monolayer coverage leads to an increasing polarizability perpendicular to the surface which — if the evaporation is stopped — rapidly decays to a lower saturation level. This is explained in terms of multipole plasmons in a thin metallic Cs film, of a thickness just above a monolayer, sustained during continuous evaporation.

Plasmon Excitations in Thin Alkali Metal Films on Si(111)7 × 7

The growth of thin Cs and K films on Si(111)7 × 7 kept at 170 K has been followed by second harmonic generation (SHG) during continuous deposition. Initially, both types of alkali atoms adsorb at dangling bond sites, in a similar way as at room temperature. At low temperature the completion of one overlayer is followed by multilayer growth. This causes orders of magnitude increase of SHG due to excitations of plasmons. A fairly sharp transition temperature below which multilayers can exist was found at 260 K for both metals. For Cs the variations in SHG with exposure can be explained in terms of coverage variations of the resonance frequency. The variations of SHG during deposition of K indicate the formation of islands.

Room-temperature deposition and growth of Au on clean and oxygen passivated Si(111) surfaces investigated by optical second-harmonic generation

Room-temperature deposition and growth of Au on Si(111) is investigated by means of optical second-harmonic generation. The process on the clean surface is compared under identical conditions to that of the oxygen passivated surface. Ordered Au/Si interfaces occur in both cases. For the surface the interface ordering starts after deposition of 4 monolayers, completing the formation of a phase with some dissolved Si, after which a continuous Au film grows between the substrate and the mixed phase. Oxygen passivation causes interface ordering from a lower Au coverage and a considerably higher degree of interface order. Oscillating second-harmonic generation intensities versus coverage with periods in the 12 - 17 monolayer range show that quantum well states formed in the Au film are responsible for the second-harmonic signal. The annealing behaviours of the Au/Si structures are also studied, and discussed with the inclusion of photoemission results.

Deposition and Growth of Ag on Si(111) Surfaces Studied by Optical Second-Harmonic Generation

Optical second-harmonic generation has been used to follow in situ the growth of thin Ag films deposited on Si(111) under various substrate conditions and temperatures. This method is particularly sensitive to the microscopic structure of the deposited Ag films. Thus, island formation is found to enhance the second-harmonic signal, and strongly temperature-dependent island formations were indeed observed at lower Ag coverages. For a film thickness above 25 monolayers, identical film roughness characteristics were found for growth temperatures between 200 and 450 K and for different types of substrate reconstruction. A layer-by-layer growth mode can be obtained for low substrate temperatures or for surface passivation by oxygen or hydrogen. In such cases the second-harmonic signal oscillates. The period of oscillation is 10 monolayers These oscillations are due to the formation of quantum-well states in the film.

Tribological boron concentration profiles at hard steel surfaces studied by SIMS

The samples for this analysis are hard steel machine elements. They have been treated with boron as an additive of the lubricant during part of the contact process in a tribological test rig, in order to create wear resistant surfaces. After the tribological procedure the surfaces are examined for their in-depth concentration profiles of oxygen, carbon, sulphur, iron and boron. This analysis is done with SIMS, using positively charged O 2 and Cs ion beams for erosion, and controlled 11 B implantation in the base material for calibration of sputter depth and boron concentration. For the present samples the native surface oxide concentration profile is found to enhance the signals considerably at the beginning of the SIMS measurements. This effect is discussed and tentatively corrected for through the observed 12 C profiles. Comparison with TRIM-simulated profiles for the implanted samples shows that the suggested correction procedure clearly improves the results for the boron profiles. It therefore becomes possible to quantify the shallow boron concentration profiles and to relate them to the tribological properties of the surface

The Pt/Si(111) interface and the properties of thin Pt layers on Si

Thin Pt- and Pt-silicide films on Si are currently used in microelectronics as Schottky diodes with high values of the electrical barrier (similar 0.8eV). Such films also have metallic or near metallic sheet conductivities and are suited for interconnects. The choice of Pt is indicated by the reaction between Pt and Si which seems easy to control, at low temperatures, enabling so-called self-alignment of the silicide pattern. This is somewhat unexpected, however, when considering the bulk thermodynamic phase diagram of these elements. From this, processing temperatures of over 830°C should be needed for reaching the eutectic temperature and an even higher temperature (1210°C) needed to reach the most stable silicide composition, PtSi. Many times, studies of the Pt/Si system under UHV conditions of processing and purity have been interpreted as forming silicide at the Si surface, even at room temperature, However, it was noted that no bulk silicide is grown from thin Pt deposits by heating in UHV. Normally, an inhomogeneous composition profile is found with the top surface being enriched in silicon. In the present studies Pt was deposited slowly on the Si(111) 7 × 7 surface. This was done to follow its reaction and in-depth distribution profile during deposition, and the formation of the Schottky barrier in this step. Subsequent annealing was then studied. Further experiments studied the sensitivity of annealing to impurities (C and O), and to oxidation. It was found that during deposition of Pt some Si atoms are retained in the top surface, bonding with or imbedding themselves in the Pt with a silicide-like appearance of the valence band, Si(2p) core levels, and the Si (L2,3 VV) Auger spectrum (XAES). However the distribution of Pt and Si below the surface during deposition is indicative of an almost homogeneous Pt film, with some signs of a combined island and layer growth, past the first monolayer of Pt. The dislodging of Si atoms occurs during completion of the first monolayer, and the reaction starts only after this is completed. Impurities in a Pt film, present before annealing, were found to create a homogeneous, stable metallic phase. Oxidation of various Pt/Si structures differs from that of clean Si surfaces at intermediate temperatures, where evidence for the formation of an oxide with (weak) features of SiO2 is obtained.

The Au/Si(111) system studied by optical second-harmonic generation

The room temperature growth of Au on Si(111) 7 × 7 has been followed continuously with second-harmonic generation, and at selected coverages with low energy electron diffraction and Auger electron spectroscopy, during the evaporation of the first 15 monolayers. A fast decay of the second-harmonic signal in the submonolayer regime is associated with disordering of the 7 × 7 structure. This is followed by a local maximum at about 1 monolayer coverage. The origin of this maximum is not yet fully understood. In the range from 2 to 5 monolayers, the signal variations reflect the formation of Au islands with areas of free Si in between. Above 5 monolayers second-harmonic generation shows increasing order in the system which is ascribed to the creation of an ordered Au/Si interface, starting when a closed Au layer is formed. Annealing of the room temperature deposited Au layer leads to interface ordering even at low temperatures where no superstructure is observable with low energy electron diffraction.

Experiments with extrinsic Si(111) surfaces : Cs adsorption at room temperature on Si(111) terminated with hydrogen and oxygen

Abstract The characteristic intrinsic properties of silicon surfaces are strongly related to the dangling bonds, and their presence again are due to the different possible reconstructions. It is possible to create surfaces on which the dangling bonds are removed or changed by adsorption of gas atoms or metal atoms. Such surfaces may exhibit new, tailorable properties, making them interesting for device applications. In this report, the properties of hydrogen and oxygen terminated n-type Si(111) surfaces are compared with respect to Cs adsorption, and seen against those of the Si(111) 7 × 7 surface. The techniques include core level and valence band photoemission with synchrotron radiation at the Aarhus storage ring, spectroscopic optical second harmonic generation and LEED. It is found that the hydrogen terminated (1 × 1) surface has practically no free dangling bonds and is unpinned. As a result the position of the band edge is strongly lowered with Cs adsorption. This surface is stable against Cs adsorption and shows a highly enhanced electron emission upon radiation with photons as a result of the lowered workfunction. The (monolayer) oxygen terminated surface is electronically stable and shows a pinning of the surface potential resisting the movement of the band edge with adsorption of Cs, despite a reaction between Cs and oxygen. For the clean surface direct interactions between Cs and Si surface atoms create new interface states at the Fermi level. A number of less significant differences between these surfaces are also pointed out.

Interfaces and interface properties of the Au/Si(111) system

The Au/Si(111) interface is studied as it forms during Au deposition on the 7×7 surface and subsequent, thermal processing. The morphology and composition of the surface region is characterised, with XPS and XAES, together with the order of the buried interface, using LEED and optical second harmonic generation (SHG). Oxidation and the effects of a thin natural oxide layer are also studied. The adsorption of the first monolayer of Au on the 7×7 surface releases Si atoms and saturates the rest atom dangling bonds. The interface, buried under thicker Au deposits, stays ordered, even after annealing. Heating of deposited Au layers to above the eutectic point produces a melt, which results in constant surface concentrations, after cooling, until the melting point of Au is reached.