H.W. Wang

XPS studies of MoS2 formation from ammonium tetrathiomolybdate solutions

Abstract Preparation of MoS 2 by acidification of ammonium tetrathiomolybdate solution has been studied by X-ray photoelectron spectroscopy (XPS). The precipitates formed from the solution are mainly composed of MoS 3 , the Mo 3d chemical shift of which is approximately 4.2eV. Heat treatments of the above product at 450 and 850°C, in a vacuum of 1.33 × 10 −4 Pa, lead largely to formation of MoS 2 , with typical chemical shifts of abouth 1.2eV. The MoS 2 formed consists of nano-sized, crystalline particles of hexagonal (2H-type) structure. The validity of MoS 2 formation by this route is confirmed by comparative study, namely the decomposition of ATT solids under the same respective heat treatment conditions. Apart from these main aspects, origins of minor XPS peaks are also considered.

The corrosion behaviour of macroparticle defects in arc bond-sputtered CrN/NbN superlattice coatings

The investigation concerned the corrosion behaviour of macroparticle and growth defects in PVD CrN/NbN superlattice coatings formed by are bond-sputtering (ABS) process on a mild steel BS6323. The electrochemical behaviour of the coatings was firstly studied by potentiodynamic polarising in de-aerated 0.5 M (Na2CO3-NaHCO3) buffer and 5% NaCl solutions, respectively. The coating and defects were then examined in planar view and cross-section by scanning electron microscopic analyses and the results were compared with those prior to the electrochemical measurement. It is found that the overall coating/substrate corrosion process is closely related to the deleterious effect of the macroparticles and growth defects in the PVD coatings. It is further demonstrated that for through thickness macroparticle inclusions, corrosion initiates by galvanic or crevice corrosion between the defect and the coating matrix, subsequently permitting solution access to those defects with eventual substrate pitting and corrosion at the coating/substrate interface. On the basis of the experimental findings and the macroparticle formation theory, the mechanisms of the growth defect-related coating/substrate corrosion are finally proposed

The influence of low concentrations of chromium and yttrium on the oxidation behaviour, residual stress and corrosion performance of TiAlN hard coatings on steel substrates

Ti0.43Al0.52Cr0.03Y0.02N films, which have been shown to exhibit a fine grain near equiaxed microstructure were found to exhibit a compressive residual stress of - 6.5 GPa in contrast to conventional columnar Ti0.44Al0.53Cr0.03N coatings which demonstrated - 3.8 GPa compressive stress. Novel coatings with this modified microstructure were also found to possess improved resistance to both dry oxidation and wet aqueous corrosion. Glancing angle parallel beam geometry X-ray diffraction (GAXRD) studies showed that in conventional Ti0.44Al0.53Cr0.03N films, severe oxidation initiated above 850 degrees C whilst oxidation of Ti0.43Al0.52Cr0.03Y0.02N started close to 950 degrees C. In an alkaline aqueous medium, Ti0.43Al0.52Cr0.03Y0.02N coatings deposited on steel showed an extended passive potential range and a significantly lower passive current compared with Ti0.44Al0.53Cr0.03N films of similar thickness. A similar improvement was evident in sulphuric acid where yttrium containing coatings passivated at high potential (Ti0.44Al0.53Cr0.03N films did not passivate). These effects may be ascribed to reduced porosity in the fine-grained Ti0.43Al0.52Cr0.03Y0.02N as well as the well-known effects of low concentrations of yttrium on high-temperature oxidation performance

Formation and characterization of self-lubricating MoS2 precursor films on anodized aluminium

Abstract A duplex anodizing process was used to form self-lubricating MoS 2 precursor films on aluminium. This encompasses an initial formation of a porous alumina film on aluminium, by anodizing in sulphuric acid, and a subsequent development of the MoS 2 precursors within the pores, by another anodizing treatment in ammonium tetrathiomolybdate electrolyte. The films were then fully examined by optical and electron microscopy, and energy dispersive X-ray analysis. The lubricant was also studied using X-ray diffraction, X-ray photoelectron spectroscopy, and selected area electron diffraction. Furthermore, the incorporation process of the solid lubricant within the pores is elucidated, and a model advanced, on the basis of the features of the V – t curves of re-anodizing and the determined morphology, microstructure and composition of the film and lubricant. Finally, the wear properties and microhardness of the film are briefly reported.

Wear associated with growth defects in combined cathodic arc/unbalanced magnetron sputtered CrN/NbN superlattice coatings during erosion in alkaline slurry

The erosive wear associated with growth defects was studied in deaerated 0.5 M (Na2CO3-NaHCO3) buffer solutions containing 150-200 mum alumina particles on a rotating cylinder erosion-corrosion system for PVD CrN/NbN superlattice coatings grown by the are bond sputtering (ABS) process on mild steel BS6323. Corrosion was minimised by selective control of the samples potential, according to potentiodynamic polarisation, during the slurry erosive wear test. The morphology of the coatings, particularly of the defects, was examined in planar and cross-section views by means of scanning electron microscopy before and after the test. It is found that wear of the coating is typically preferential to the defects, progressing from the particle exterior top to the interior with erosion time, while the coating matrix (areas free of such defects) is largely intact, after 24 h prolonged exposure to erosion

The erosive wear of mild and stainless steels under controlled corrosion in alkaline slurries containing alumina particles

The erosive wear in an alkaline slurry containing alumina particles of three typical engineering materials, the mild steel BS 6323 (Fe-C), the AISI 410 stainless steel (Fe-Cr-C), and the AISI 304 stainless steel (Fe-Cr-Ni), was carried out, by means of rotating cylinder, three-electrode erosion-corrosion test, with a view to investigation into the roles of the typical elements and the mechanical and chemical properties in the erosive wear under simultaneous controlled corrosion. The total weight loss of erosion-corrosion was obtained by precision weighing and the result was compared and interpreted, for each material, by a full microscopical examination of the erosion-corrosion scars using scanning electron microscopy (SEM). It was found that the overall performance under erosion-corrosion in an descending order was the stainless steels AISI 304, AISI 410, and the mild steel, although the precise difference in performance was dependent upon the process conditions. Such a ranking of performance was not in total consistence with that expected only from the mechanical or the chemical property differences of the materials concerned. The individual contribution of each erosion and corrosion process was thus further separated through corrosion charge conversion using the Faradays second law and the results were interpreted by discussion, on basis of the experimental and microscopical evidences, of the main factors that influenced the mechanical and wear behaviour, in conjunction with those influencing corrosion and passivity. Finally, schematic diagrams were proposed to outline the typical erosion and corrosion features thus obtained for all the three materials during erosion-corrosion.

Development and tribological assessment of self-lubricating anodic films on aluminium

Abstract The self-lubricating, porous anodic films formed on aluminium by a re-anodizing treatment in ammonium tetrathiomolybdate electrolyte is examined by transmission electron microscopy and X-ray photoelectron spectroscopy. Precipitation of mainly amorphous MoS 3 , or a mixture of MoS 2 and S, is revealed, which leads to a significantly reduced coefficient of friction.

Synthesis of molybdenum disulphide by acidification of ammonium tetrathiomolybdate solutions

Generally categorized in a large family of transition metal dichalcogenide layer crystals, molybdenum disulfide (MoS2) has been the subject of significant research interest for applications in non-aqueous lithium batteries [1], catalytic hydrodesulphurization of petroleum [2], and antiwear achievement by reduction in friction [3-5]. The successful application of this semiconductor compound originates largely from the sandwich interlayer structure, loosely bound by Van der Waals forces, evidenced by easy cleavage in the [0 0 1] direction along which the (S-Me-S) layers are stacked to form the crystal [6]. Apart from its natural state, known as molybdenite, crystalline MoS2 can also be synthesized by hightemperature solid state reaction between stoichiometrically mixed molybdenum and sulphur powders in vacuum [7-9], thermal decomposition of ammonium tetrathiomolybdate ((NH4)2MoS4) [10, 11] or amorphous molybdenum trisulfide (MoS3) [9, 12], and by thermally assisted transformation of amorphous MoS2 powders [13]. Synthetic MoS2 can also be prepared by sputtering [14], electrodeposition [15] and chemical deposition techniques [16]. The present letter reports formation of MoS2, by appropriate thermal treatment of precipitates resulting from acidification of ammonium tetrathiomolybdate solution. As mentioned previously, although MoS2 can be prepared from tetrathiomolybdate by thermal decomposition, the current work is of practical interest in that it produces evidence for in situ formation of MoS2 during anodizing of aluminium and its alloys in thiomolybdate electrolyte. Compared with the extensive research on the formation of alumina by anodizing, the formation of MoS2 during anodization has been rarely reported. Skeldon and Thompson [17] recently proposed to overcome the general poor surface retention of MoS2 by incorporating it, by anodizing, into a pre-anodized, hard porous alumina film on aluminium for tribological applications. The main anodic reaction during anodizing of aluminium results in proton generation at the pore base/electrolyte interface as follows

Simplifying the erosion–corrosion mechanism map for erosion of thin coatings in aqueous slurries

Abstract A significant advance in the study of erosive wear in recent years has been the construction of erosion mechanism maps showing the transitions between the erosion–corrosion regimes as a function of the main process parameters. The various corrosion regimes, ranging from active to passive conditions, have been incorporated on the map. Hence, the map is more complex than that in dry oxidizing conditions where the predominant corrosion process is film growth. The object of this work has been to study the combined effects of increasing erodent velocity and applied potential on the erosion of PVD coatings, i.e., Ti 2 N and CrN in aqueous conditions. The substrate was mild steel, the particles alumina and the solution, a carbonate–bicarbonate buffer. Mechanisms of erosion–corrosion were identified on the basis of the results. The erosion mechanism maps generated from the results provided a means of differentiating between erosion–corrosion behaviour of the coating and the substrate. Significant shifts in the boundaries were identified for different coatings. The stages in the construction of such maps, in addition to the various approaches towards simplifying the nomenclature and the format, are described in this paper.

Erosion of PVD TiN coatings under simultaneous corrosion in sodium carbonate/bicarbonate buffer slurries containing alumina particles

Following a preceding report, this paper summarises preliminary results for the study of PVD TiN coatings prepared on mild steel, BS6323, under potentiodynamically controlled corrosion during erosion in an alkaline slurry containing alumina particles. The erosion term was obtained when the total erosion-corrosion loss had been measured by sample weighing and the corrosion term obtained as described previously. The erosive wear tracks were examined, in conjunction with investigation into evidence of corrosion damage, by scanning electron microscopy. The significant, superior erosion resistance of the coating to that of the uncoated mild steel and AISI 304 stainless steel mainly resulted from the exceedingly high coating strength and its stability under the simultaneous corrosion attack. The erosion term was compared with the previously measured corrosion term, and their respective contributions to the overall erosion-corrosion damage evaluated for each sample.