Effect of MoS2/PTFE coatings on performance of Si3N4/TiC ceramics in dry sliding against WC/Co

To enhance the tribological performance of Si₃N₄/TiC ceramics, MoS₂/PTFE composite coatings were deposited on the ceramic substrate through spraying method. The micrographs and basic properties of the MoS₂/PTFE coated samples were investigated. Dry sliding friction experiments against WC/Co ball were performed with the coated ceramics and traditional ones. These results showed that the composite coatings could significantly reduce the friction coefficient of ceramics, and protect the substrate from adhesion wear. The primary tribological mechanisms of the coated ceramics were abrasive wear, coating spalling and delamination, and the tribological property was transited from slight wear to serious wear with the increase of load because of the lower surface hardness and shear strength. The possible mechanisms for the effects of MoS₂/PTFE composite coatings on the friction performance of ceramics were discussed.     

Tribological behavior of the polyamide composite coating filled with different fillers under dry sliding

The polyamide (PA) composite coating filled with the particles of microsized MoS₂, microsized graphite, and nano‐Al₂O₃, respectively, were prepared by flame spraying. The friction and wear characteristics of the PA coating and composite coating filled with the varied content of filler under dry sliding against stainless steel were comparatively investigated using a block‐ring tester. The morphologies of the worn surfaces and transfer films on the counterpart steel ring were observed on a scanning electron microscope. The result showed that the addition of fillers to the composite coatings changed significantly the friction coefficient and wear rate of the coatings. The composite coatings filled with a low level content of fillers showed lower wear rate than did pure PA coating under dry sliding; especially the MoS₂/PA composite coating had the lowest wear rate among these composite coatings. The composite coatings with a high level content of fillers had higher wear rate than did pure PA coating, except of the Al₂O₃/PA composite coating. The bonding strengths between the polymer matrix and fillers changed with the content of the fillers, which accounted for the differences in the tribological properties of the composite coatings filled with the varied content fillers. On the other hand, the difference in the friction and wear behaviors of the composite coatings and pure coating were attributed to the difference in their worn surface morphologies and transfer film characteristics. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Enhanced tribological properties of Y/MoS2 composite coatings prepared by chemical vapor deposition

Chemical vapor deposition (CVD) is an efficient approach to prepare coatings on complex cutting tools. However, MoS₂ with self-lubrication ability and excellent tribological properties fabricated by CVD have been rarely reported in literature. The aim of this study was to deposit pure MoS₂ coatings and yttrium (Y) doped MoS₂ (Y/MoS₂) composite coatings on cemented carbide blades coated with titanium nitride by CVD. The structural and mechanical properties of the coatings were examined by scanning electron microscopy (SEM) and nanoindentation, respectively. The results demonstrated that the microstructure of Y/MoS₂ composite coatings was denser than that of the pure MoS₂ coating. The hardness and the adhesional properties were significantly enhanced for the Y/MoS₂ composite coatings. The tribological performance of the as-deposited coatings were investigated under atmospheric environment. Y/MoS₂ compostite coatings demonstrated an enhanced tribological performance with a stable and low coefficient of friction (COF) over the entire sliding time. In contrast, the COF of pure MoS₂ coating dramatically increased to value above 0.3 after a sliding time of only 30 min. Additionally, the Y/MoS₂ composite coatings showed a decreased wear rate (8.36 ± 0.29 × 10⁻⁷ mm³/Nm) compared to the pure MoS₂ coatings (3.41 ± 0.48 × 10⁻⁵ mm³/Nm) thus reflecting an improvement by two order of magnitude.     

Wear-resistant ceramic coatings deposited by liquid thermal spraying

As a surface strengthening and surface modification technology of materials, liquid thermal spray technology has been used in many fields, such as wear and friction reduction, corrosion resistance, and high-temperature oxidation resistance. This article reviews the progress of liquid thermal sprayed coating in wear resistance as well as friction reduction in recent years. The influences of microstructure, composition, phase structure and mechanical properties on the tribological properties of typical coatings (including ceramic coatings and multiphase composite coatings) are investigated. The tribological properties of the coating are determined by the coating characteristics (including microstructure, porosity, mechanical properties, etc.) and the service conditions (working temperature, lubrication state, etc.). Typical ceramic wear-resistant coatings include Al₂O₃, YSZ, HA coatings, etc. The tribological properties of the coating can be significantly improved through process optimization and heat treatment. The comparison of nanostructured and microstructured ceramic-based coating reveals that nanostructured coating reduces wear by absorbing stress. The interaction between different constituent phases improves wear resistance and reduces wear in composite coatings. Finally, various challenges faced by liquid thermal spray are pointed out, and future research focuses are proposed.     

Effect of specimen orientation and heat treatment on electroless Ni-PTFE-MoS<Subscript>2</Subscript> composite coatings

In this study, the effects of simultaneous co-deposition of polytetrafluoroethylene (PTFE) and MoS₂ particles on tribological properties of electroless nickel (EN) coating were studied. The influences of specimen orientation and heat treatment on EN-PTFE-MoS₂ composite coatings were also investigated. Scanning electron microscopy was used to study the morphology of coatings and the distributions of the lubricant particles in the deposits. Chemical analyses of coatings were done by electron dispersive spectrometry. The phases of the coatings were identified by X-ray diffraction utilizing CuKα radiation. Wear and friction properties of the coatings were also determined by pin-on-disk wear tester. The wear investigations showed that the EN-PTFE-MoS₂ composite coating performs better than EN-PTFE and EN-MoS₂ coatings in terms of friction coefficient and wear resistance. PTFE and MoS₂ contents of the EN-PTFE-MoS₂ coating were increased by changing the specimen orientation from vertical to horizontal configuration, which leads to enhancement in tribological properties of the coating. After heat treatment, the wear rate of EN matrix composite coating decreased with corresponding change in phase structure.     

Microstructural,mechanical and tribological properties of carbon nanotubes reinforced plasma sprayed molybdenum disulphide composite coatings

The development of 1-Dimensional (1D) and 2-Dimensional (2D) materials have gained considerable attention towards achieving solid-state lubricity. Herein, we present the effect of carbon nanotubes (1D) reinforcement into the molybdenum disulphide (2D) coatings. Plasma sprayed MoS₂ coatings reinforced with 2-4 wt% CNTs were fabricated using shroud plasma spraying over steel substrates. The shroud attachment envelops the plasma plume and cut down its exposure to surroundings, which minimizes the oxidation of MoS₂ powder during spraying. The microstructural analysis revealed the presence of MoS₂ and CNTs in the composite coating. The mechanical hardness and elastic modulus of MoS₂ coating improved by 2–3 folds in the composite coating. In tribological performance, the coefficient of friction (COF) decreased from 0.13 to 0.07 in M2C coating. The wear weight loss was estimated as 0.89 ± 0.07 mg, 0.18 ± 0.02 mg and 0.39 ± 0.03 mg for M, M2C and M4C coatings respectively. It can be attributed that tubular CNTs acted as bearing on MoS₂ layers. This work opens an impressive stepping for the synergistic mixture of 1D (CNTs) and 2D (MoS₂) material to obtain high-quality wear-resistant coatings.     

Wear and corrosion properties of electroless nickel composite coatings with PTFE and/or MoS2 particles

This study focuses on the effect of co-deposition of PTFE and/or MoS₂ particles on the morphology, wear, and corrosion properties of electroless nickel coating. The composite coating of EN–PTFE–MoS₂ was heat treated at different temperatures for hardness investigations. The surface morphology of coatings was characterized by scanning electron microscopy. Pin-on-disk and potentiodynamic polarization tests were used to study the tribological and corrosion properties of the coatings, respectively. Results of hardness analysis revealed that the hardness of electroless nickel coatings was increased by the heat treatment, and its maximum was gained at 400°C. Wear investigations showed that simultaneous co-deposition of the PTFE and MoS₂ particles into the nickel coating increased the wear resistance of the coating by about 30% and reduced the average value of friction coefficient to 0.25 from 0.65. Corrosion studies illustrated that simultaneous co-deposition of the PTFE and MoS₂ particles led to reduction in corrosion resistance by 10 and 5 times that of EN coating in brine and acidic solution, respectively.     

Mechanical and tribological properties of nano/micro composite alumina coatings fabricated by atmospheric plasma spraying

Adding nano particles can significantly improve the mechanical properties and wear resistance of thermal sprayed Al₂O₃ coating. However, it still remains a challenge to uniformly incorporate nano particles into traditional coatings due to their bad dispersibility. In the present work, nanometer Al₂O₃ (n-Al₂O₃) powders modified by KH-560 silane coupling agent were introduced into micrometer Al₂O₃ (m-Al₂O₃) powders by ultrasonic dispersion to afford nano/micro composite feedstock, and then four resultant coatings (weight fraction of n-Al₂O₃: 0%, 3%, 5% and 10%) were fabricated by atmospheric plasma spraying. The features and constitutes of feedstock and as-sprayed coatings, as well as their porosity, bonding strength, microhardness and frictional behaviors were investigated in detail. Results show that the nano/micro composite feedstock with uniform microstructure can be better melted in the spraying process, thereby obtaining coatings with denser microstructure, higher hardness and bonding strength. Added n-Al₂O₃ has no obvious effect on the friction coefficient of composite coatings, whereas can improve their wear-resistant and reduce the worn degree of counterpart. The wear mechanism of traditional coating is brittle fracture and lamellar peeling, while that of composite coating with weight fraction of n-Al₂O₃ of 10% is adhesive wear.     

The superlattice structure and self-adaptive performance of C–Ti/MoS2 composite coatings

To improve the self-adaptability of MoS₂ coating in different environments, the coatings were doped with functional C and Ti by unbalanced magnetron sputtering system. The clear superlattice structure with minimal modulation period was investigated by High Resolution Transmission Electron Microscope (HRTEM). The co-doped coatings have better mechanical properties due to the special structure and the formation of C–Mo, Ti–S and Ti–O bonds, and better lubrication performance in both high humidity and vacuum than those of the single-doped ones. The doped Ti not only facilitates the formation of the MoS₂ (002) basal plane, but also improves the oxidation resistance of the composite film. The degree of friction-induced graphitization on the wear tracks and the quality of transfer films on the wear scars are key factors affecting the lubrication performance of the composite film. In the high-humidity environment, the reasonable doping elements can promote the formation the high-quality transfer film by interacting with H₂O water molecules, which will benefit the lubrication of the coating better. Our findings deepen the understanding of MoS₂ composite coating and provide a new solution for improving the self-adaptability of the coating.     

Structure and properties of nanostructured ceramic matrix composite coatings prepared in-situ by reactive plasma spraying micro-sized Al–Fe2O3–Cr2O3 powders

Nanostructured FeAl₂O₄-based ceramic matrix composite coatings were prepared in-situ by reactive plasma spraying micro-sized Al–Fe₂O₃ and Al–Fe₂O₃–Cr₂O₃ powders. The microstructure, toughness, Vickers hardness, and adhesive strength of these coatings were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that both the coatings exhibited a nanostructured microstructure. The grains of coating AFC sprayed with Al–Fe₂O₃–Cr₂O₃ powders are finer than those of the coating AF sprayed with Al–Fe₂O₃ powders. The composite nano-coating sprayed with Al–Fe₂O₃–Cr₂O₃ powders exhibited higher hardness and better wear resistance compared with those of the composite nano-coating sprayed with Al–Fe₂O₃ powders. The adhesive strength, toughness, and wear resistance of the composite coating sprayed with Al–Fe₂O₃–Cr₂O₃ powders were significantly enhanced compared with those of the composite coating sprayed with Al–Fe₂O₃ powders, which were attributed to the Cr₂O₃ addition.     

Friction behavior of TiN–MoS2/PTFE composite coatings in dry sliding against SiC

To improve the dry friction behavior of traditional hard coatings, MoS₂/PTFE lubricating coatings were prepared on the PVD TiN-coated cemented carbide using spray method. The influences of MoS₂/PTFE lubricating coatings on the primary characteristics of TiN coatings were investigated. Reciprocating sliding tests were carried out with the TiN–MoS₂/PTFE coated specimen (T-M-P) under dry sliding conditions, and the tribological behaviors were compared to those of the TiN-coated one (T-N). The test results indicated that the adhesion force of coatings with substrate for T-M-P specimen increased, the surface micro-hardness, roughness and friction coefficient significantly decreased. Meanwhile, the surface adhesions and abrasion grooves of T-M-P specimen were reduced, and the main wear forms of T-M-P were abrasion wear and coating delamination. The MoS₂/PTFE lubricating coatings can be considered effective to improve the friction properties of traditional hard coatings.     

Study of corrosion and friction reduction of electroless Ni–P coating with molybdenum disulfide nanoparticles

One composite coating of Ni–P alloys containing MoS₂ nanoparticles was prepared by electroless technique based on the better friction reduction ability of MoS₂ and better anticorrosion property of electroless Ni–P alloys on carbon steel surfaces. Electrochemical method—that is, using Tafel polarization curves—was carried out in order to study the corrosion performance of the coating. The results indicate that the anticorrosion ability of the composite coating was decreased because of the addition of nano-MoS₂ particles. The corrosional surfaces were studied and analyzed through scanning electron microscopy (SEM). The corrosion mechanism of the composite coatings was mainly ascribed to the formation of microcells around the nanosized MoS₂ particles, and the active ion-like Cl⁻ destroyed the surface film and induced the corrosion on the inside part of the coating. The friction coefficient of electroless composite coatings was measured by end-facing tribometer. It was found that the friction coefficient of the Ni–P–(nano-MoS₂) composite coating decreased greatly compared with those of Ni–P electroless coatings.     

Deposition and structural analyses of molybdenum-disulfide (MoS2)–amorphous hydrogenated carbon (a-C:H) composite coatings

In this study we developed composite coatings consisting of amorphous hydrogenated carbon (a-C:H) and molybdenum-disulfide (MoS₂), and clarified their microstructure. In addition, we interpreted the tribological properties of the composite coatings in the viewpoint of a deposition-induced microstructural modification. The coatings were produced by the hybrid deposition technique of RF-generated methane and argon plasma and DC magnetron co-sputtering of MoS₂ target. The deposition parameter investigated in this study was methane flow rate. Structural analyses were performed using a transmission electron microscope (TEM) and an atomic force microscope (AFM). Friction tests were conducted using a ball-on-disk type tribometer. From an electron micrograph, it was confirmed that nano-clusters were embedded into an amorphous carbon host matrix. Surface roughness of the composite coating was ～ 0.25 nm in R_a compared to 5.0 nm in R_a of sputtered MoS₂. The concentration measurements were performed, and the results show that the sulfur and molybdenum concentration ratio, [S]/[Mo], is ～ 0.9, which indicates that the amount of sulfur was reduced due to the discharged plasma. In friction tests, composite coatings showed high friction in a vacuum condition. It was considered that lubricant MoS₂ lamellar structures showing super-low friction in a vacuum condition during friction could not be formed between ball and coating during friction because of the lack of sulfur in embedded clusters.     

Effect of microstructure and mechanical properties on wear behavior of plasma-sprayed Cr2O3-YSZ-SiC coatings

In this study, the microstructure and mechanical properties of the atmospheric plasma-sprayed Cr₂O₃ (C), Cr₂O₃-20YSZ (CZ), and Cr₂O₃-20YSZ-10SiC (CZS) coatings were evaluated and also compared with each other, so as to explain the coatings wear behavior. Microstructural evaluations included X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and porosity measurements. Mechanical tests including bonding strength, fracture toughness, and micro-hardness tests were used to advance our understanding of the correlation between the coatings properties and their wear behavior. The sliding wear test was conducted using a ball-on-disk configuration against an alumina counterpart at room temperature. Addition of multimodal YSZ and subsequent SiC reinforcements to the Cr₂O₃ matrix resulted in an increase in the fracture toughness and Vickers micro-hardness, respectively. It was found that the composite coatings had comparable coefficients of friction with pure Cr₂O₃ coatings. When compared with the C coating, the CZ and CZS composite coatings with higher fracture toughness exhibited superior wear resistance. Observation of the wear tracks of the coatings indicated that the lower wear rates of the CZ and CZS coatings were due to the higher plastic deformation of the detached materials. In fact, improvement in the wear resistance of the composite coatings was attributed to a phase transformation toughening mechanism associated with tetragonal zirconia which created more ductile tribofilms during the wear test participated in filling the pores of coatings.     

Microstructure of HVOF-sprayed Ag–BaF2⋅CaF2–Cr3C2–NiCr coating and its tribological behavior in a wide temperature range (25 °C to 800 °C)

Ag–BaF₂?CaF₂–Cr₃C₂–NiCr composite powders were prepared by physically blending commercial BaF₂?CaF₂–Cr₃C₂–NiCr and Ag powders. Ag–BaF₂?CaF₂–Cr₃C₂–NiCr composite coatings were deposited on Inconel 718 alloy substrate by high velocity oxy-fuel (HVOF) spraying. The friction and wear behavior of the coatings under dry sliding against Si₃N₄ balls from 25 °C to 800 °C was evaluated with a ball-on-disk high temperature tribometer. The microstructure and composition of the samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectrometer. Results showed that the composite coatings were mainly composed of hard phase of Cr₃C₂, binder phase of NiCr, high-temperature lubrication phase of fluorides and low-temperature lubrication phase of Ag. The fluorides existed in the forms of both crystal particles and amorphous state, while the silver featured as typical thermally sprayed splats. Due to the high flame temperature, some fluorides have been oxidized to chromates and around 30 wt% of Ag was lost during spraying. In addition, it was found that Ag content had an important influence on the composite coating, and an appropriate dosage of metallic silver could effectively improve the tribological performance of the coating. The generation of AgCrO₂ at moderate (500 °C and 650 °C) temperature and BaCrO₄ at high temperature (800 °C) could contribute to the decline in friction coefficients and wear rates of Ag–BaF₂?CaF₂–Cr₃C₂–NiCr coatings.     

Vertically aligned,polypyrrole encapsulated MoS2/graphene composites for high-rate LIBs anode

Ultrathin MoS₂ nanosheets were vertically anchored on the reduced graphene oxide (MoS₂/rGO) via hydrothermal method. To further engineering the surface conductivity, ultrathin polypyrrol (PPy) layer was coated on the MoS₂/rGO composite via in situ polymerization to form a bi-continuous conductive network with a sandwich-like structure. The graphene nanosheets and the PPy coating can facilitate the electrons transfer rate, while the ultrathin MoS₂ nanosheets can enhance the utilization efficiency of the active materials. The obtained MoS₂/rGO-10 composite exhibits high reversible specific capacity (970?mAh?g^?1 at 0.1?A?g^?1) and rate capability (capacity retention of 64% at 3.2?A?g^?1). Moreover, the PPy@MoS₂/rGO hybrids reveal lower specific capacity but better rate capability, and a “trade-off” effect between electrons and ions transfer resistance was observed. This easy-scalable PPy surface conductivity engineering strategy may be applied in the preparation of high-performance LIBs active materials.     

Effect of MoS2 content on friction and wear properties of Mo and S co-doped CrN coatings at 25–600 °C

With increasingly harsh working environments for mechanical systems and the rapid development of various high-tech industries, requirements for the stable operation of mechanical systems are increasing in a wide temperature range. Mo and S co-doped CrN coatings with different MoS₂ contents were prepared via unbalanced magnetron sputtering to provide better friction properties to the coatings at high temperatures. Scanning electron microscopy and nanoindentation were adopted to analyze the microstructure and mechanical performance. The mechanical performance of the coatings was enhanced by increasing the MoS₂ content, however, excessive MoS₂ reduced the mechanical properties of the coatings. Besides, the adhesion of the coatings first increased and then decreased rapidly with the increase of the MoS₂ content. In addition, the residual stress of the coating first decreased and then increased upon increasing the MoS₂ content. The high-temperature tribological behavior of the coatings was measured from room temperature (25 °C) to 600 °C. The CrN/MoS₂-0.6A coating was found to exhibit low friction and wear coefficient at room temperature and relatively good comprehensive properties at high temperature. This study provides a feasible design for engineering applications and lays the foundations for the preparation of coatings with superior high-temperature friction properties.     

Microstructures and tribological properties of vacuum plasma sprayed B4C–Ni composite coatings

A promising wear resistant coating has been fabricated via vacuum plasma spray (VPS) technique by using electroless plating composite powders comprised of B₄C and different amounts of Ni (10 and 20 vol.%). Tribological evaluation from the ball-on-disk test showed that the wear resistance of the composite coatings was superior to that of the pure B₄C coating, and the composite deposit containing 10 vol.% Ni demonstrated the optimum tribological properties. This mainly attributed to the more uniform microstructures of the composite coatings, and the higher thermal conductivity of the composite coating also contributed to its distinguished wear behaviors. For the coatings investigated, the dominant wear mechanism was determined to be oxidation and the formation of a transfer layer on the worn surface.     

Tribochemistry of TiN films sliding against ceramic counterparts in vacuum condition and N2 atmosphere

With the rapid development of aerospace technology, the tribological performance of moving parts under extreme operating conditions has attracted a great deal of attention and interest. The application of solid lubricant coatings has become a major means of improved performance to ensure stable operation. Although molybdenum disulfide (MoS₂) and diamond-like carbon (DLC) films have excellent low coefficients of friction, they are prone to failure in vacuum because they cannot overcome the challenges of assembly in atmospheric environments. Surprisingly, unexpected results were obtained in this study using conventional nitride films. Specifically, the friction coefficient of TiN/SiC friction pair in vacuum is 0.21 and the wear rate is 8.8 × 10^?7 mm³/mN. The relatively stable friction coefficient is mainly attributed to the formation of carbonaceous lubricating layer at the interface, which is the decisive factor in reducing wear. The friction coefficient of TiN/WC friction pair under N₂ atmosphere is 0.31 and the wear rate is 4.5 × 10^?7 mm³/mN. It can be summarized as follows: first, the mechanochemical induced chemical reaction of the interface, and secondly, the thermally excited nitrogen atoms saturate the dangling bonds of the transfer film. The results further reveal the friction mechanism of TiN films with advanced ceramic materials under harsh conditions and suggest a guide for engineering applications.     

A comparative investigation on microstructure evolution and wear resistance of in situ synthesized NbC,WC, TaC reinforced Mo2FeB2 coatings by laser cladding

To improve the microhardness and wear resistance of Mo₂FeB₂ coatings, composite coatings were prepared by laser cladding using in situ synthesized NbC, WC, and TaC. The influence of different carbides on the morphology, microstructure, microhardness, residual stress, and tribological properties of the composite coatings was investigated. The results showed various microstructural morphologies in different composite coatings. Apparent herringbone structures were observed in most coatings except for the Mo₂FeB₂/TaC composite coating and a eutectic structure was formed in the Mo₂FeB₂/WC composite coating. In addition, the heat-affected zone was typically composed of acicular martensite and lath martensite. The microhardness of the Mo₂FeB₂/WC composite coating increased to 1543.6 HV_0.5 compared with 985.7 HV_0.5 observed for the Mo₂FeB₂ coating. Tensile stress existed in the coating, bonding zone, and heat-affected zone, whereas the substrate exhibited compressive stress. The Mo₂FeB₂/WC composite coating exhibited the lowest tensile stress (298 MPa). The Mo₂FeB₂/WC composite coating containing WC and the W₂C phase had the lowest coefficient of friction (0.38) and wear rate (3.90 × 10^?5 mm³/Nm), indicating its excellent tribological properties. Moreover, the wear mechanism of the Mo₂FeB₂ coating is severe adhesive and abrasive wear. The adhesive wear mechanism was mitigated by the formation of in situ synthesized NbC, WC, and TaC. The wear mechanism of the Mo₂FeB₂/WC composite coating was only a slight abrasive wear.