Morphological Growth of Sputtered MoS2 Films

Sputtered MoS₂ films from 300 Å to 20,000 Å thick were deposited on metal and glass surfaces. The substrate effects such as surface temperature, finish, pretreatment, and chemistry as they affect the film formation characteristics were investigated by optical, electron transmission, electron diffraction, and scanning electron microscopy. Substrate temperature and surface chemistry were found to be the prime variables as to the formation of a crystalline or amorphous film. The friction characteristics are strictly influenced by the type of film formed. Surface chemistry and surface pretreatment account for compound formation and corresponding grain growth, which directly affect the adhesion characteristics, resulting in poor adherence. The type of surface finish (topography) as related to scratches, impurities, inhomogeneities, etc., are favorable nucleation sites for the growth of isolated and complex nodules within the film, and various complex surface overgrowths on the film. These nodular growth features have progressively more undesirable effects on the film behavior as the film thickness increases.     

Frictional and morphological properties of AuMoS2 films sputtered from a compact target

AuMoS₂ films 0.02–1.2 μm thick were sputtered from a target compacted from 5 wt.% Au plus 95 wt.% MoS₂ to investigate the frictional and morphological film growth characteristics. The gold dispersion effects in MoS₂ films are of interest to increase the densification and strengthening of the film structure. Three microstructural growth stages were identified on the nano-micro-macrostructural level. During sliding both sputtered AuMoS₂ and sputtered MoS₂ films have a tendency to break within the columnar region. The remaining or effective film, about 0.2 μm thick, performs the lubrication. The AuMoS₂ films displayed a lower friction coefficient with a high degree of frictional stability and less wear debris generation compared with pure MoS₂ films. The more favorable frictional characteristics of the AuMoS₂ films are attributed to the effective film thickness and the high density packed columnar zone which has a reduced effect on the fragmentation of the tapered crystallites during fracture.     

Morphological and frictional behavior of sputtered MoS2 films

From the texture and growth patterns of sputtered MoS₂ films deposited onto substrates, three regions can be distinguised: (1) a ridge formation region, (2) an equiaxed transition zone and (3) a columnar-fiber-like structure. The lubricating properties of sputtered MoS₂ films can be visually identified with respect to optical changes before and after rubbing. The orientation of the surface microcrystallites is identified, and the change in optical properties is explained. In sliding contact the sputtered film tends to break up at the base of the columnar region. Effective lubrication occurs with the film remaining on the substrate. This film is 0.18–0.22 microm thick.     

Coatings for wear and lubrication

In this paper we review the recent advances in the tribological uses of r.f.-sputtered and ion-plated films of solid film lubricants (laminar solids, soft metals, organic polymers) and wear-resistant refractory compounds (carbides, nitrides, silicides). The sputtering and ion-plating potentials and the corresponding coatings formed are evaluated relative to the friction coefficient, wear endurance life and mechanical properties. The tribological and mechanical properties of each kind of film are discussed in terms of film adherence, coherence, density, grain si morphology, internal stresses and thickness and substrate conditions such as temperature, topography, chemistry and d.c. biasing. The ion-plated metallic films in addition to improved tribological properties also have better mechanical properties such as tensile strength and fatigue life.     

The X-ray photoelectron spectroscopy depth profiling and tribological characterization of ion- plated gold on various metals

An investigation was conducted to examine, by X-ray photoelectron spectroscopy analysis and depth profiling, the atomic nature of such tribological properties as friction and microhardness of ion-plated gold. Friction properties were measured with (1) a gold film, (2) the graded interface between gold and nickel substrate and (3) the nickel substrate. All sliding was conducted against hard silicon carbide pins in two processes. The first is the adhesive process in which friction arises primarily from adhesion between sliding surfaces, and the second is a non-adhesive process, namely abrasion, in which friction occurs as a result of the hard pin sliding against the film, indenting into it and plowing a series of grooves. The other substrates used in this study included copper and 440C stainless steel. Reference experiments were also conducted with vapor-deposited gold on the substrates. A vacuum environment is used in order to maximize the adhesion effect, while oil is used to minimize the adhesion effect. The results of the investigation indicate that the friction related to adhesion as well as the friction related to abrasion are influenced by the coating depth. The trends in friction behavior as a function of film depth are, however, just the opposite. The graded interface exhibited the highest adhesion and friction, while the graded interface resulted in the lowest abrasion and friction. The coefficient of friction due to abrasion is inversely related to the hardness. The greter the hardness of the surface, the lower are the abrasion and friction. The microhardness in the graded interface exhibited the highest hardness due to an alloy hardening effect. Almost no graded interface between the vapor- deposited gold film and the substrates was detected.     

Polyisoprene-nanostructured carbon composite – A soft alternative for pressure sensor application

Latest related research shows natural polyisoprene-nanostructured carbon composite (PNCC) as a promising piezoresistive material for soft pressure sensors. The main advantages of PNCC over conventional sensors are exceptional sensitivity in pressure range from 0.1 to 10 bar and possibility to be embedded into completely soft (hyper-elastic) structures.In this work we have elaborated a completely soft (hyper-elastic) PNCC pressure sensor prototype, made using functional multi-layer approach, when elementary layers of PNCC with different conductive filler concentration are cured together and form a uniform sensor body with integrated soft (hyper-elastic) electrodes. We proposed also a theoretical model to explain the observed positive piezoresistivity and used it for fitting of our experimental results.The prototype system elaborated could be used for counting interface events between sensor and external environmental factor. The achieved result could be a step towards the artificial skin, capable to sense non-destructive interaction with the external influence.     

Lubrication with Sputtered MoS2 Films

Sputtered MoS₂films (2000–6500 Å) were deposited on highly polished metal surfaces. These films have a low coefficient of friction (0.03–0.04) at speeds of 40–80 rpm and loads of 250–1000 grams. At loads of 250 grams the wear lives are over 0.5 million cycles, but at 1000 gram loads, it decreases to 38,000 cycles. Friction experiments and tensile tests have indicated that sputtered films have a strong adherence to metal surfaces. Electron transmission, diffraction and scanning electron microscopy show that these films have an extremely small particle size, less than 30 Å in diameter, and are very dense and free from observable pinholes. The high kinetic energy of these sputtered species, the submicroscopic particle size and the sputter-etched substrate surface is responsible for strong adhesion and cohesion of the sputtered film.     

Frictional and morphological characteristics of ion- plated soft metallic films

Ion-plated metallic films in contrast with films applied by other deposition techniques offer lower friction coefficients and longer endurance lives and exhibit a gradual increase in the friction coefficient after the film has been worn off. The friction coefficients of metallic films are affected by the degree of adherence, the thickness and the nucleation and growth characteristics. The effective film thickness for the minimum friction coefficient was established for gold and lead films. The nucleation and growth characteristics during ion plating lead to a fine continuous crystalline structure, which contributes to a lower friction coefficient.