Structure, morphology and chemical composition of sputter deposited nanostructured Cr-WS2 solid lubricant coatings

WS₂ and Cr-WS₂ nanocomposite coatings were deposited at different Cr contents (approximately 15-50 at.%) on silicon and mild steel substrates using an unbalanced magnetron sputtering system. X-ray diffraction (XRD) was used to study the structure of Cr-WS₂ coatings and the bonding structure of the coatings was studied using X-ray photoelectron spectroscopy (XPS). The characterization of different phases present in Cr-WS₂ coatings was carried out using micro-Raman spectroscopy. The XPS and Raman data indicated the formation of a thin layer of WO₃ on the surface of Cr-WS₂ coatings and the intensity of the oxide phase decreased with an increase in the Cr content, which was also confirmed using energy-dispersive X-ray analysis results. The surface morphologies of WS₂ and Cr-WS₂ coatings were examined using field emission scanning electron microscopy (FESEM) and atomic force microscopy. It has been demonstrated that incorporation of Cr in WS₂ strongly influences the structure and morphology of Cr-WS₂ coatings. The XRD and FESEM results suggested that increase in the Cr content of Cr-WS₂ coatings resulted in a structural transition from a mixture of nanocrystalline and amorphous phases to a complete amorphous phase. The cross-sectional FESEM data of WS₂ coating showed a porous and columnar microstructure. For the Cr-WS₂ coatings, a mixture of columnar and featureless microstructure was observed at low Cr contents (≤ 23 at.%), whereas, a dense and featureless microstructure was observed at high Cr contents. Detailed cross-sectional transmission electron microscopy (TEM) studies of Cr-WS₂ coatings prepared at Cr content ≤ 23 at.% indicated the presence of both nanocrystalline (near the interface) and amorphous phases (near the surface). Furthermore, high-resolution TEM data obtained from the nanocrystalline region showed inclusion of traces of amorphous phase in the nanocrystalline WS₂ phase. Potentiodynamic polarization measurements indicated that the corrosion resistance of Cr-WS₂ coatings was superior to that of the uncoated mild steel substrate and the corrosion rate decreased with an increase in the Cr content.     

Microstructure and mechanical properties of Al2O3-Cr2O3 composite coatings produced by atmospheric plasma spraying

Al₂O₃, Al₂O₃-Cr₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction. Electron probe microanalyzer was employed to investigate the polished and fractured surface morphologies of the coatings. Mechanical properties including microhardness, fracture toughness and bending strength were evaluated. The results indicate that the addition of Cr₂O₃ is conducive to the stabilization of α-Al₂O₃. Compared with the pure Al₂O₃ and Cr₂O₃ coatings, Al₂O₃-Cr₂O₃ composite coatings show lower porosities and denser structures. Heterogeneous nucleation of α-Al₂O₃ occurs over the isostructural Cr₂O₃ lamellae and partial solid solution of Al₂O₃ and Cr₂O₃ might be occurring as well. Furthermore, grain refining and solid solution strengthening facilitate the mechanical property enhancement of Al₂O₃-Cr₂O₃ composite coatings.     

Evaluation of adhesion and fatigue of MoS2-Nb solid-lubricant films deposited by pulsed-dc magnetron sputtering

The thin film deposition for tribological applications becomes more and more widespread. The tribological performance of the overlay coatings correlates with coating-substrate adhesion. Hence, it is important to measure adhesive strength. The scratch adhesion test for thin films is extensively used. In this work, MoS₂-Nb coatings deposited by closed-field unbalanced magnetron sputtering (CFUBMS) have been scratch tested in two modes. A multi-mode operation was used as sliding-fatigue, like multi-pass scratching in the same track at different fractions of critical load (unidirectional sliding) and a standard mode using progressive load operation. Failure mechanisms are discussed according to examination of response of very dense microstructure and the adhesion value. The critical load to the first failure (L_C1) was 15 N but the final adhesion value from the film and substrate interface was 120 N (L_C2) as function of the coating thinning. The coefficient of friction (COF) from the multi-scratch for MoS₂-Nb started at a very high value of around 0.067, 0.073, and 0.093 under 5, 8, and 15 N loads respectively and then drops to 0.006, 0.035, and 0.065 at the end of the 1000 cycles. The most significant finding in the test is that when the multi-scratch passes reached to 1000 cycles, micro scale fatigue failures disappeared.     

Mechanical and tribological properties of sputter deposited nanostructured Cr-WS2 solid lubricant coatings

Solid lubricant coatings of WS₂ and Cr-WS₂ (15-50 at.% Cr) prepared using an unbalanced magnetron sputtering system were evaluated for their mechanical and tribological properties. Nanoindentation results indicated that addition of Cr helped in improving the mechanical properties and the elastic recovery ability of Cr-WS₂ coatings. The adhesive strengths of Cr-WS₂ coatings were evaluated using a nanoscratch tester and from the nanoscratch profiles, critical load values and optical images, it was evident that the adhesion of Cr-WS₂ coatings increased with an increase in the Cr content. Further analysis of the nanoscratch data indicated that WS₂ coatings exhibited large amount of plastic deformation compared to Cr-WS₂ coatings which showed a combination of elastic-plastic deformation. However, micro-tribometer measurements at a load of 2 N showed that the tribological properties of Cr-WS₂ coatings deteriorated with an increase in the Cr content. For example, Cr-WS₂ coatings prepared at Cr content ≥ 33 at.% failed after a sliding distance of 1 m. On the other hand, WS₂ and Cr-WS₂ coatings prepared at low Cr contents (15-23 at.% Cr) exhibited a stable friction coefficient (50-60% relative humidity) in the range of 0.10-0.13 for a sliding distance of 14 m. Micro-Raman spectroscopy data of the worn films taken after a sliding distance of 14 m indicated the presence of WS₂ transfer films for WS₂ and Cr-WS₂ coatings prepared at low Cr contents. For Cr-WS₂ coatings with Cr content ≥ 33 at.%, the worn films consisted predominantly of WO₃. After an extended sliding distance of 50 m, Cr-WS₂ coatings (15-23 at.% Cr) outperformed WS₂ coating which failed after 20 m. Further, the coatings prepared at low Cr contents did not show any failure even after a sliding distance of 200 m. At a higher load of 7 N, Cr-WS₂ coating with 15 at.% Cr exhibited the best performance with a friction coefficient of 0.07 up to a sliding distance of 72 m. These results indicate that the amount of Cr in the WS₂ matrix needs to be controlled judiciously to obtain improved mechanical and tribological properties in Cr-WS₂ solid lubricant coatings.     

Mechanical and tribological performance of Al2O3-TiO2 coatings elaborated by flame and plasma spraying

Mechanical properties and wear rates of Al₂O₃-13 wt.% TiO₂ (AT-13) and Al₂O₃-43 wt.% TiO₂ (AT-43) coatings obtained by flame and atmospheric plasma spraying were studied. The feed stock was either ceramic cords or powders. Results show that the wear resistance of AT-13 coatings is higher than that of AT-43 and it seems that the effect of hardness on wear resistance is more important than that of toughness. Additionally, it was established that, according to conditions used to elaborate coatings and the sliding tribological test chosen, spray processes do not seem to have an important effect on the wear resistance of these coatings.     

Tribological properties of Cr- and Ti-doped MoS2 composite coatings under different humidity atmosphere

Solid-lubricant MoS₂ coatings have been successfully applied in high vacuum and aerospace environments. However, these coatings are very sensitive to water vapor and not suitable for applications in moist environments. In this work, Cr- and T-doped MoS₂ composite coatings were developed. The results demonstrated that these composite coatings are promising for applications in high humidity environments.MoS₂-Cr and MoS₂-Ti composite coatings with different Cr or Ti content were deposited on high speed steel substrate by unbalanced magnetron sputtering. The composition, microstructure, and mechanical properties of the as-deposited MoS₂-metal composite coatings were analyzed by energy dispersive analysis of X-ray (EDX), X-ray diffraction (XRD), and nanoindentation experiments. The tribological properties of the coatings were evaluated against an alumina ball under different relative humidity atmosphere using a ball-on-disc tribometer. The MoS₂-Cr and MoS₂-Ti coatings showed a maximum hardness of 7.5 GPa and 8.4 GPa at a dopant content of 16.6 at.% Cr or 20.2 at.% Ti, respectively. The tribological test results showed that, with a small amount of Cr and/or Ti doping, the tribological properties of MoS₂ coatings under humid atmosphere could be significantly improved. The optimum doping level was found to be around 10 at.% for both MoS₂-Cr coatings and MoS₂-Ti coatings to show the best tribological properties, with both the lowest friction coefficient and wear rate. The excellent tribological properties of the MoS₂-Cr and MoS₂-Ti coatings with an appropriate metal doping level in moist atmosphere are found due to their ability to form stable transfer layer on the surface of the counterbody, which supplies lubrication for the contact surface.     

In-situ TiB2 and Al2O3 formation by DC plasma spraying

In-situ plasma spraying (IPS) is a promising process to fabricate composite coatings with in-situ formed thermodynamically stable phases. In the present study, mechanically alloyed Al-12Si, B₂O₃ and TiO₂ powder was deposited onto an aluminum substrate using atmospheric plasma spraying (APS). It has been observed that, during the coating process, TiB₂ and Al₂O₃ are in-situ formed through the reaction between starting powders and finely dispersed in hypereutectic Al-Si matrix alloy. Also, obtained results demonstrate that in-situ reaction intensity strongly depends on spray conditions.     

Study of Al2O3 coatings on AISI 316 stainless steel obtained by controlled atmosphere plasma spraying (CAPS)

The tribological behaviour of Al₂O₃ coatings on AISI 316 stainless steel, obtained by the process of controlled atmosphere plasma spraying (CAPS), is studied in this work. Atmospheric plasma spraying (APS) and high pressure plasma spraying (HPPS) were applied in order to produce these coatings. The APS coatings exhibited lower microhardness values compared to the values of HPPS coatings. Regarding the HPPS coatings it was found that plasma composition, through its heat capacity, does influence the heat transfer to particles, and, consequently, their flattening and densification process, which govern coating properties. It was revealed that tribological behaviour of coatings was influenced by the applied spraying method too. Coatings from HPPS under high-enthalpy conditions led to worst wear behaviour. In general, properties, such as microstructure, microhardness, coefficient of friction and wear resistance depended on the processing conditions such as pressure and composition of the spraying chamber atmosphere.     

Micro-scale sliding contacts on Au and Au-MoS2 coatings

The microtribological properties of Au and Au-MoS₂ coatings were examined using a nanoindentation instrument. MoS₂ was chosen for this study as an additive to Au due to its positive influence on the mechanical and tribological properties. Reciprocating microscratch tests were performed using a diamond indenter with a tip radius of 50 μm and a range of normal loads between 0.2 mN and 5.0 mN. The friction and wear results, with respect to the two coatings, were correlated to different velocity accommodation modes and levels of adhesion. It was found that the addition of 20 mol% MoS₂ to Au reduced the adhesion and limiting friction and also improved the wear resistance significantly. This coating shows potential for applications in microcomponents and microswitches due to its wear resistance, relatively low friction and good electrical conductivity.     

Thermal conductivity and thermal cycle life of La2O3 and HfO2 doped ZrO2-Y2O3 coatings produced by EB-PVD

The effects of La₂O₃ and HfO₂ addition on thermal conductivity and thermal cycle life of EB-PVD YSZ coatings were investigated. La₂O₃ and HfO₂ were selected as additives, because they significantly suppress the sintering of YSZ. The developed coating showed low thermal conductivity as well as high resistance to sintering. Burner rig tests confirmed that the developed coating have a superior thermal insulating effect and have a longer life than that of a coating with conventional composition.     

Thermal shock behavior of nanostructured and conventional Al2O3/13 wt%TiO2 coatings fabricated by plasma spraying

The thermal shock behavior of three kinds of Al₂O₃/13 wt%TiO₂ coatings fabricated by plasma spraying was studied in this paper. One kind of those coatings was derived from conventional fused and crushed feedstock powder available commercially; the other two kinds of coatings were derived from nanostructured agglomerated feedstock powders. These two nano coatings possess moderate pores and pre-existing microcracks, they were composed of fused structure and three-dimensional net or skeleton-like structure. For conventional coatings, the pores and pre-existing cracks were bigger, sharp-point and mostly distributed between splats. Thermal shock tests for the three coatings were performed by water quenching method. Testing result showed the two kinds of nano coatings had much higher thermal shock resistance than the conventional coatings. The improved thermal shock resistance for nano coatings could attribute to their improved microstructure and crack propagation mode. The damage evolution and failure mechanism of coatings was quite different at thermal shock temperature of 650 °C and 850 °C, which was explained by a simple model. Different crack propagating modes in nanostructured and conventional coatings during thermal shock tests were due to their different microstructures in these two kinds coatings. The stress state of coating surfaces during the thermal cycles was also discussed in this paper.     

TEM and DFT investigation of CVD TiN/κ-Al2O3 multilayer coatings

This paper investigates the interfacial structure in hot-wall CVD TiN/κ-Al₂O₃ multilayer coatings using both HREM and DFT modeling. Two multilayers with different thicknesses of the TiN layers (50 and 600 nm) separating the κ-Al₂O₃ layers are analyzed. The general microstructure of the two multilayers is relatively similar. The TiN layer in the thicker TiN/κ-Al₂O₃ coating is thick enough to be several TiN grains high. This means that epitaxial columns, which are often found in the thinner TiN/κ-Al₂O₃ coatings, are not present. However, the orientation relationships at the TiN/κ-Al₂O₃ interfaces are the same in both multilayers. The HREM investigations show that κ-Al₂O₃ (001) planes can grow directly on flat (111) TiN faces, without any other phases or detectable amounts of impurities, such as sulphur, present. Where the TiN layers are more curved, γ-Al₂O₃ can be grown, at least partly stabilized by the cube-on-cube orientation relationship between γ-Al₂O₃ and the underlying TiN. The DFT calculations show very similar adsorption strengths for an O monolayer positioned on Ti-terminated TiC(111) and TiN(111) surfaces, with preferred adsorption in the fcc site. O adsorption on N-terminated TiN(111) is much weaker, with preferred adsorption in the top site. Calculated elastic-energy contributions yield a higher stability for κ-Al₂O₃ on TiN(111) than on TiC(111) and a higher stability for κ-Al₂O₃ than for α-Al₂O₃ on both TiC and TiN. This indicates that the observed higher stability of κ-Al₂O₃ on TiC(111) than on TiN(111) is not due to the lattice mismatch, while the preferred epitaxial growth of κ-Al₂O₃ over α-Al₂O₃ can be partly attributed to the mismatch.     

High-temperature deformation of ZrB2 ceramics with WC additive in four-point bending

This work investigated high-temperature four-point bending deformation of hot-pressed ZrB₂ ceramics with WC additive at 1900 °C for 0–60 min in argon atmosphere under a static load of 25 MPa. After 60 min bending deformation, about 8% strain was achieved without apparent macroscopic cracks, suggestive of superplastic behavior at 1900 °C. The tensile side and the compressive side of the deformed samples exhibited different microstructural characteristics. On the tensile side, many cavities were observed at the multigrain junctions and the fracture surface was relatively flat. On the compressive side very few cavities occurred and some large cracks were present on the fracture surface. The presence of a secondary W-containing phase formed by the addition of WC, prohibited the coalescence of cavities by its plastic deformation.     

Electrodeposition of sol-enhanced nanostructured Ni-TiO2 composite coatings

A novel electroplating method has been developed to produce nanocrystalline metal-matrix nano-structured composite coatings. A small amount of transparent TiO₂ sol was added into the traditional electroplating Ni solution, leading to the formation of nanocrystalline Ni-TiO₂ composite coatings. These coatings have a smooth surface. The Ni nodules changed from traditional pyramid-like shape to spherical shape. The grain size of Ni was also significantly reduced to the level of 50 nm. It was found that the amorphous anatase TiO₂ nano-particles (∼ 10 nm) were highly dispersed in the coating matrix. The microhardness was significantly increased from 320 HV₁₀₀ of the traditional Ni coating to 430 HV₁₀₀ of the novel composite coating with 3.26 wt.% TiO₂. Correspondingly, the wear resistance of the composite coating was improved by ∼ 50%.     

The effect of plasma spray parameters on the cavitation erosion of Al2O3-TiO2 coatings

This paper reports a study of how the choice of plasma spray parameters, used during deposition of Al₂O₃-13%TiO₂ coatings on carbon steel, influences the cavitation erosion properties of such coatings. The parameters studied are the power feeding rate and hydrogen flow rate. The surface and cross section of coatings before and after cavitation were also observed by scanning electron microscopy (SEM). The phases present in the coatings were characterized by X-ray diffraction method (XRD). The microscopic observations were used to study the inter-lamellar connection, porosity, unmelted particles and so on inside the coating. We also measured the roughness, microhardness, adhesion strength and cavitation erosion of the coatings. The XRD results showed that the coating includes different allotropes of Al₂O₃ such as α and γ. The cavitation erosion studies of the coatings were conducted by ultrasonic cavitation testing on the basis of ASTM G32 standard. It was found that cavitation erosion is accelerated around the unmelted particles and porosities. The results reveal that the cavitation resistance of the coating is determined by its microstructure and that increasing discontinuities (inside the coating) reduce its cavitation resistance. We have found that the coating obtained at hydrogen gas flow rate of 16 L/min and powder feeding rate of 20 g/min has the best cavitation resistance.     

Characterization and frictional behavior of nanostructured Ni–W–MoS2 composite coatings

Ni–W–MoS₂ composite coatings were obtained by pulse plating from a Ni–W electrolyte containing suspended MoS₂ particles. The coating composition, morphology, crystalline structure, microhardness and frictional behavior were studied as a function of MoS₂ concentration. The results obtained in this study indicate that co-deposited lubricant particles strongly influenced the composite Ni–W coating properties. It was found that increasing co-deposited MoS₂ diminished tungsten content in the coating and consequently increased the average grain size. Ni–W nanostructured coatings with high MoS₂ content have a porous sponge-like structure, high surface roughness and irregular frictional behavior. However, the friction coefficient of Ni–W coatings is reduced to half its value with low MoS₂ content.     

Deposition, microstructure and properties of texture-controlled CVD α-Al2O3 coatings

The influence of nucleation on the microstructure and properties of CVD Al₂O₃ was investigated. The experimental α-Al₂O₃ layers were deposited (a) without nucleation control and (b) with nucleation steps resulting in pronounced , and growth textures. The experimental layers were characterised using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Wear properties of the textured coatings were evaluated in turning. The chemistry of the nucleation surface appeared to be an important factor in pre-determining the phase content and growth textures of the Al₂O₃ layers. Optimised nucleation resulted in substantially improved wear properties and these kinds of α-Al₂O₃ layers were typically composed of relatively small, defect-free grains exhibiting no porosity. The textured α-Al₂O₃ layer showed the best wear resistance.     

Structural and electrical properties of Gd(Ni1/2Zr1/2)O3

The Gd(Ni_1/2Zr_1/2)O₃ (GNZ) ceramic is synthesized by the solid-state reaction technique. The X-ray diffraction pattern of the sample shows monoclinic phase at room temperature. The dielectric dispersion of the material is investigated in the temperature range from 303 K to 673 K and in the frequency range from 100 Hz to 1 MHz. The relaxation peak is observed in the frequency dependence of the loss tangent. The relaxation time at different temperatures is found to obey Arrhenius law having activation energy of 1.1 eV which indicates the hopping of ions at the lattice site and may be responsible for the dielectric relaxation of GNZ. The scaling behaviour of loss tangent suggests that the relaxation mechanism is temperature independent. The frequency dependent conductivity spectra follow the power law. In the impedance formalism, the Cole-Cole model is used to study the relaxation mechanism of GNZ.     

Microwave ECR plasma CVD of cubic Y2O3 coatings and their characterization

Yttrium oxide (Y₂O₃) thin films are deposited by microwave electron cyclotron resonance (ECR) plasma assisted metal organic chemical vapour deposition (MOCVD) process at a substrate temperature of 350 °C using indigenously developed metal organic precursors (2,2,6,6-tetra methyl-3,5-heptane dionate) yttrium, commonly known as Y(thd)₃ synthesized by ultrasound method. The deposited coatings are characterized by X-ray photoelectron spectroscopy, glancing angle X-ray diffraction, scanning electron microscopy, EDS and infrared spectroscopy. The characterization results indicate that it is possible to deposit non-porous coatings with excellent uniformity of a single phase cubic Y₂O₃ on various substrates by this process at reasonably low substrate temperature that is desirable in various manufacturing processes.     

Preparation and characteristic of spherical Li3V2(PO4)3

Spherical Li₃V₂(PO₄)₃ was synthesized by using N₂H₄ as reducer. The products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that single-phase, spherical and well-dispersed Li₃V₂(PO₄)₃ has been successfully synthesized in our experimental process. Electrochemical behaviors have been characterized by charge/discharge measurements. The initial discharge capacities of Li₃V₂(PO₄)₃ were 123 mAh g⁻¹ in the voltage range of 3.0–4.3 V and 132 mAh g⁻¹ in the voltage range of 3.0–4.8 V.