TEM investigation of MoSeC films

Abstract

Transition metal dichalcogenides (TMD) are widely used as self-lubricating material either as oil additive or directly as thin films. Magnetron sputtering is a deposition method allowing depositing such films with high density and adhesion. However, their spread use in practical applications is still hindered since their excellent sliding properties are deteriorated in the presence of humidity and under high contact pressures. MoSe 2 , one of the members of TMD family recently studied [1], has been co-sputtered with carbon in order to improve the mechanical and tribological properties when compared to pure MoSe 2 films.The films have been deposited by co-sputtering from a C target embedded with a different number of MoSe 2 pellets. Prior to the deposition of Mo-Se-C films a 300 nm Ti layer was deposited on the steel and Si substrates in order to improve the adhesion. The chemical composition was measured by electron probe microanalysis (EPMA). Cross-sectioned specimens were prepared from samples containing 38, 51 and 68 at.% of carbon (Se/Mo ratio 1.8, 1.8 and 2.0, respectively) by mechanical polishing and ion milling. The morphology and the chemical composition of the coatings were investigated by HRTEM and EDS in a Philips-CM300 microscope. To support TEM analysis, the coatings were tested by Raman spectroscopy (Ar + laser, 514.5 nm wavelength), X-Ray Photoelectron spectroscopy (XPSMg Kα radiation) and X-ray diffraction (XRD -Brag-Brentano configuration in glancing mode, Co Kα radiation) and their hardness was evaluated by depth sensing indentation (Fisherscope H100, maximum load 50 mN). The XPS spectra were taken after ion etching with Ar+ ions (3 KeV and 5 μA/cm 2 ) during 3 min.TEM micrographs show that film morphology strongly depends on the C content ( Fig. 1). By increasing the C content of the films, the MoSe 2 nanocrystallites became smaller as indicated by the increase of the width of the diffraction rings in SAED patterns. At the same time the film morphology became more homogeneous. The contrast observed in the Mo-Se-C layers, after about 200 nm thickness, can be attributed to the material porosity, probably related to the typical columnar morphology usually developed in TMD films. The lower part of the films looks more dense and homogeneous (low porosity). The films porosity clearly decreases with increasing C content ( Fig. 1 and 2); simultaneously, the columnar structure almost vanishes. This result complements the information obtained by previous SEM analysis of MoSe-C coatings published in Ref [1], (see the right part of the pictures in figure 1) where the aspect of low density is kept even for high C content films. Based on TEM analysis, it seems that the columnar growth is typical only for low carbon coatings and the dendritic growth described in Ref [2] and observed in figure 1 is probably only an artifact originated during sample breaking. The increase of the material density could promote the improvement of the hardness from 1.4 to 4.0 GPa with increasing C content in the range 38-68 at.%....

Effect of carbon concentration and argon flow rate on the microstructure and triboperformance of magnetron sputtered WS2/a-C coatings

“…HRTEM images of Fig. 5c show that the nanocrystalline WS 2 phases, with a platelet shape, are randomly oriented in an amorphous carbon matrix, see also [5,23,24,37,38]. TEM micrographs in Fig.…”

“…The sulfur stoichiometry also influences the crystallography and tribological behaviors of MeSC coating. As the sulfur content increases, the (002) lamellar crystallites become more pronounced and the size of grains increases in the form of dendritic-like branches [5,[22][23][24]. Voevodin [5] found in WS y C coatings when y = 0 WC nanoclusters surrounded by amorphous DLC.…”