Fractal analysis of the evolution of friction surfaces of galvanic AuNi coatings

Abstract

Gold based galvanic coatings are widely used for ensuring stable low contact resistance, high corrosion resistance, and thermal stability of both internal and external conductors in modern coaxial RF connectors [1]. Since the exploitation of these connectors involves numerous connect/disconnect cycles, wear resistance is also among the important performance characteris tics of galvanic coatings. The mechanisms of wear in these coatings can be studied in detail and measures capable of increasing the wear resistance can be pro posed based on an analysis of the morphology of fric tion surfaces, which continuously evolve in the course of tribological testing. However, considerable difficul ties are encountered in formulating the quantitative criteria of wear resistance, since the traditional char acteristics of surface morphology depend on the scale of measurements and the resolving power of a micro scope used for the examination of friction surfaces.In recent years, tribological analysis has widely employed a fractal approach, which is invariant with respect to scaling [2][3][4]. This approach is based on determining the so called fractal dimension, which, in contrast to other characteristics of surface roughness, allows the variation of a material surface relief to be quantitatively described simultaneously on various scaling levels [5,6]. The fractal analysis of friction sur faces traditionally employs images obtained using optical microscopy [2] and scanning electron micros copy [3]. However, since the principles of imaging in these techniques are based on rather complex electro magnetic and diffraction phenomena, it is sometimes difficult to establish whether an observed element of the microscopic image of the given surface microrelief represents a protrusion or a depression.In this context, an optimum tool for correct inves tigation of the morphology of friction surfaces is offered by the atomic force microscopy (AFM). Indeed, the AFM is insensitive to changes in the opti cal and electronic properties, provides information about the true topography of the sample surface, and allows real dimensions of details of the surface microrelief to be determined at high resolution in three dimensions. In combination with the AFM, fractal analysis can provide reliable evaluation of the evolution of the state of a solid surface in the course of wear. The present investigation was aimed at assessing the possibility of using the fractal analysis of AFM images for a quantitative description of the evolution of the morphology of friction surfaces of galvanic AuNi coatings.Samples of 3 µm thick AuNi (95 wt % Au-5 wt % Ni) coatings were electrolytically deposited onto sub strate plates made of beryllium bronze (commercial BrB2 grade). Prior to depositing the main coating, the substrate was electrolytically covered with a NiB buffer layer with a thickness of about 5 µm.The wear tests were carried out on a UMT 1 fric tion machine in the rod on disk configuration at a load of 1.85 N and a sliding velocity of 0.5 m/s. The counterbodi...

Development and characterization of magnetron sputtered self-lubricating Au-Ni/a-C nano-composite coating on CuCrZr alloy substrate

“…Au coatings are commonly used as contact materials to ensure stability and low contact resistance upon sliding contact in the electronics industry [1][2][3][4]. Soft Au which is not alloyed is one of the best metals with a high electrical conductivity, and it shows excellent corrosion resistance under most working conditions.…”

“…In high-reliability systems, high-performance Au coatings are required, and alloying Au with a small amount of other metal elements, such as Co and Ni, can generate harder Au coatings which exhibit distinct performance improvement [1,[6][7][8]. Hard Au coatings are generally obtained from acidic-cyanide baths in which nickel or cobalt salts are added as the hardening/brightening agents [3,4,[9][10][11][12]. Although Au-Ni and Au-Co alloy coatings exhibit a higher wear resistance compared to soft Au, adding solid lubricant materials into alloy coatings is beneficial for decreasing the wear rate by minimizing the adhesive wear.…”

Characterizations of thermal stability and electrical performance of Au-Ni coating on CuCrZr substrate for high vacuum radio-frequency contact application

“…Hard Au containing a small amount Ni or Co can have significantly higher hardness than pure Au [15][16][17][18][19] and consequently exhibit higher wear performance. Replacement of a pure Au coating with a hard Au coating may increase the working reliability and prolong the lifetime of an RF contact.…”