Preparation and tribological performances of Ni-P-multi-walled carbon nanotubes composite coatings

Multi-walled carbon nanotubes （MWNTs） were wet-milled in the presence of ammonia and cationic surfactant and then used as reinforcements to prepare Ni-P-MWNTs composite coatings by electroless plating. The tribological performances of the composite coatings under dry condition were investigated in comparison with 45 steel and conventional Ni-P coating, Micrographs show that short MWNTs with uniform length and open tips were obtained through the wet-milling process. The results of wear test reveal that the Ni-P-MWNTs composite coatings posses much better friction reduction and anti-wear performances when compared with 45 steel and Ni-P coating. Within the MWNTs content range of 0.74%-1.97%, the friction coefficient and the volume wear rate of the composite coatings decrease gradually and reach the minimum values of 0.08 and 6.22x10-15 m3/（N.m）, respectively. The excellent tribological performances of the composite coatings can be attributed to the introduction of MWNTs, which play both roles of reinforcements and solid lubricant during the wear process.

Fabrication of in-situ synthesized ceramic reinforced Ni-based alloy composite coatings by reactive braze coating processing

In-situ synthesized ceramic such as TiC,Cr7C3 and Cr5B3 reinforced Ni-based alloy composite coating was fabricated on the surface of mild steel substrate by reactive braze coating processing with colloidal graphite,Cr,Ni,ferro-boron,Si and titanium powders as the raw materials at low temperature of 1000℃,and a new kind of coating materials was developed.By means of SEM,EDS,XRD and surface hardness tester,the microstructures,phases,hardness and wear-resistance of the coating were analyzed,respectively.The results revealed that the coating was mainly composed of the ceramic in-situ synthesized reinforcement phases of TiC,Cr7C3 and Cr5B3 and the binder phases in-situ synthesized of Ni31Si12 and(Ni,Fe)solid solution;The ceramic reinforcement phases of TiC,Cr7C3 and Cr5B3 were randomly distributed in the binder phases of Ni31Si12 and(Ni,Fe)solid solution;The coating had about 15vol%pores and can possibly be applied as a self-lubrication coating;The coating and the substrate were integrated together by metallurgical bonding;The coating had a hardness up to 91-94HR15N.

Effect of surface roughness on the surface lubrication performance of galvanized steel sheets with a self-lubricated coating

Four kinds of galvanized steel sheets having different surface roughness values were used to prepare the steel sheets with a self-lubricated coating. The effects of surface roughness on the surface lubrication performance of the steel sheets were examined using a friction coefficient tester. Results revealed large dynamic friction coefficients for the galvanized steel sheets, which increased remarkably with surface roughness. Once the self-lubricated coating was applied, significant drops in the dynamic friction coefficients were measured. After the first stage of the friction test,the coefficients were almost unchanged, which reflected a weak dependence on the surface roughness of the self-lubricated steel sheets. However, the dynamic friction coefficients gradually increased as the test progressed, where these increase clearly correlated with the surface roughness of the self-lubricated steel sheets.

Preparation and Characterization of Plasma-Sprayed Ultrafine Chromium Oxide Coatings

Ultrafine chromium oxide coatings were prepared by plasma spraying with ultrafine feedstock. Processing parameters of plasma spraying were optimized. Optical microscope （OM） was used to observe the microstructure of the ultrafine chromium oxide coatings. Scanning electron microscopy （SEM） was used to observe the morphology and particle size of ultrafine powder feedstock as well as to examine the microstructure of the chromium oxide coating. In addition, hardness and bonding strength of the ultrafine chromium oxide coatings were measured. The results showed that the optimized plasma spraying parameters were suitable for ultrafine chromium oxide coating and the properties and microstructure of the optimized ultrafine chromium oxide coating were superior compared to conventional chromium oxide wear resistant coatings.

Effect of wear conditions on tribological properties of electrolessly-deposited Ni-P-Gr-SiC hybrid composite coating

The friction and wear properties of the electrolessly-deposited Ni-P-Gr-SiC composites were investigated. The effects of graphite content, load and rotation speed on the friction coefficient and wear resistance of the composite coatings were mainly investigated. The worn surface and cross section of the coatings were characterized by scanning electron microscopy and energy-dispersive X-ray analysis. The results show that the composite coatings reveal good antifriction and wear resistance due to the synergic effect of graphite and SiC particles. The formation of graphite-rich mechanically mixed layer （GRMML） on the surface of Ni-P-Gr-SiC coating contributes to the good tribological behavior of the wear counterparts and SiC particles play a load bearing role in protecting GRMML from shearing easily.

PTA clad (Cr,Fe)_7C_3/γ-Fe in situ ceramal composite coating

A wear-resistant （Cr, Fe）7C3/γ-Fe in situ ceramal composite coating was fabricated on the substrate of 0.45wt%C carbon steel by a plasma-transferred arc cladding process using the Fe-Cr-C elemental powder blends. The microstructure, microhardness, and dry-sliding wear resistance of the coating were evaluated. The results indicate that the microstructure of the coating, which was composed of （Cr, Fe）7C3 primary phase uniformly distributed in the γ-Fe, and the （Cr, Fe）7C3 eutectic matrix was metallurgically bonded to the 0.45wt%C carbon steel substrate. From substrate to coating, the microstructure of the coating exhibited an evident epitaxial growth character. The coating, indehiscent and tack-free, had high hardness and appropriate gradient. It had excellent wear resistance under the dry sliding wear test condition.

Microstructures and high-temperature self-lubricating wearresistance mechanisms of graphene-modified WC–12Co coatings

To reduce the friction coefficient of cobalt-cemented tungsten carbide(WC–12Co)wear-resistant coatings,graphene was compounded into WC–12Co powder via wet ball milling and spray granulation.Selflubricating and wear-resistant graphene coatings were prepared via detonation gun spraying.The presence,morphologies,and phase compositions of graphene in the powders and coatings that are obtained through different powder preparation processes were analyzed.The analysis was performed using the following technologies:energy-dispersive X-ray-spectroscopy(EDXS),scanning electron microscopy(SEM),transmission electron microscopy(TEM),and Raman spectroscopy.The mechanical properties of the coatings were studied using a microhardness tester and a universal drawing machine.The friction and wear properties of the coatings were studied using an SRV-4 friction and wear tester.The results showed that the graphene content in the WC–12Co coating modified with graphene was higher than that without modification;graphene was embedded in the structure in a transparent and thin-layer state.The adhesive strength of this coating at approximately 25°C was approximately 60.33 MPa,and the hardness was approximately 984 HV0.3.After high-temperature treatment,the adhesive strength and hardness of the graphene oxide(GO)/WC–12Co coating decreased slightly(the lowest adhesive strength of 53.16 MPa was observed after treatment at 400°C,and the lowest hardness of approximately 837 HV0.3 was observed after treatment at 300°C).Compared to the friction coefficient(0.6)of the WC–12Co coating obtained at room temperature,the friction coefficient of the GO/WC–12Co coating was decreased by approximately 50%of that value.The graphene-modified coating was continuously exposed to the wear tracks on the surface of the contacting materials during friction,and a lubricating film was formed in the microareas in which the wear tracks were present.The coating exhibited improved self-lubricating and wear-resistant effects compared to the unmodified WC–12Co coating.The results of this study demonstrated that graphene could be effective in self-lubrication and wear-reduction in a temperature range of 100–200°C,as a friction coefficient of 0.3 was maintained.

Micro-impact testing: A novel nano-/micro-tribological tool for assessing the performance of wear-resistant coatings under impact/fatigue conditions

A novel repetitive contact technique, micro-impact testing, has been developed forcoating evaluation. Impact test results have proved a more consistent measure of coating per-formance under impact/fatigue conditions than traditional tribological methods such as scratch orpin-on-disk testing. The operating principles of the technique are described with some illustrativeresults on hard and soft coatings. The technique can provide information on impact wear resis-tance, susceptibility to brittle fracture, and strength of interfacial adhesion to substrate.

Self-lubricating epoxy-based composite abradable seal coating eliminating adhesive transfer via hierarchical design

Aluminum-based composite abradable seal coatings are pivotal to improving the efficiency of aero engines or gas turbines.However,the adhesive transfer frequently occurs between metallic blade tips and aluminum-based composite coatings,resulting in engine vibration and even jam.Many past studies had tried to solve this problem by reducing coating hardness,improving lubrication,or strengthening blade tips,but all had failed.In this paper,we proposed a novel epoxy-based composite abradable seal coating,eliminating adhesive transfer by changing metal-to-metal scraping pair to metal-to-polymer scraping pair.The coating was developed via a hierarchical structure design.Large spherical pores were uniformly distributed in the continuous epoxy matrix with fine graphite dispersion.By adding 20 vol.%graphite and 50 vol.%hollow microspheres,a self-lubricating epoxy-based coating of 0.26 friction coefficient with thermal conductivity of 0.28 W/(m·K),coating HR15 Y hardness at 54.8,and bonding strength at 18.7 MPa can be reached.When the metallic blades scrape the epoxy-based composite coating,no adhesive transfer occurs.Besides,a smooth scraped surface is formed by pseudoplastic deformation.This epoxy-based composite abradable seal coating opens a new way to improve the efficiency and reliable operations of air engine compressors.

Effect of Produced Fluid on the Wear Behavior of Grade D Sucker Rod and N80 Type Tubing

Inuestigated wear tests of grade D sucker rod and N80 tubing with a produced liquid are the effect of the water cut of produced liquid on the wear rate and the wear coefficient of the friction pair. And presented here is a comparison of tribological performance of tubing/sucker rod with sprayed tubing/sucker rod and tubing/sprayed sucker rod. The results show that one of the main reasons for wear failure between the sucker rod and the tubing is a high water cut of the produced liquid. The wear rate increases greatly when the water cut ranges from 70% to 85%, and increases gradually when the water cut is less than 70% or more than 85%. The higher the water cut is, the greater the wear rate will be. SEM (scanning electron microscope) micrographs show that the worn surface of the tubing is smoother, with shallow grooves, when the water cut is 55%; when the water cut is 95%, there are broader and deeper grooves on the worn surface. The results also show that the wear rate of the sprayed wear-resistant coating of sucker rod/tubing is less than that of the unsprayed sucker rod/tubing.

Tribological properties of plasma-sprayed nickel alloy matrix self-lubricating coating at elevated temperatures

A nickel alloy matrix high-temperature self-lubricating coating(77.5 wt%(Ni-Cr-Mo-Al)-12.5 wt%Ag-10 wt%BaF2/CaF2)was prepared by plasma spraying technique.Results show that the Vickers microhardness of coating is(2.9±0.5)GPa,and the average bonding strength is about(35.6±0.5)MPa.The coating rubbing against Inconel 718 superalloy pin exhibits superior tribological performance with coefficient of friction(COF)of below 0.25 and wear rate of 8.2×10^(-5)-15.2×10^(-5)mm^(3)·N^(-1)·m^(-1)at a wide temperature range from 25 to 800℃,and the COF and the wear rate are slightly reduced with temperature increasing.The low COF and wear rate were attributed to the synergistic effects of Ag,BaF_(2)/CaF_(2),Ag2MoO_(4)and BaMoO_(4).

Self-Lubricating PEO Coating on an Al Alloy Produced by Vacuum Impregnation Post-treatment

The objective of this research was to develop a novel self-lubricating coating on an AA6061 aluminum alloy.Three coatings were prepared by the plasma electrolytic oxidation（PEO） process using 50-, 500-, and 1000-Hz pulsed direct current, respectively. The as-deposited coatings were then post-treated using two different methods, viz., ultrasonic vibration-aided vacuum oil impregnation（UVOI） and oil impregnation under ambient pressure（OIAP）. After posttreatment, an oil-containing, self-lubricating top layer was formed on the coatings. The effects of the coatings＇ surface morphologies and structures on their oil-holding capabilities were discussed. The results revealed that coatings prepared with higher frequency had a greater oil-holding capacity using OIAP post-treatment, while those prepared with lower frequency had a greater oil-containing capability using UVOI post-treatment. These phenomena are related to the morphologies of the coatings produced with various current modes. The tribological properties of the coatings before and after post-treatments were investigated by pin-on-disc sliding wear tests. Due to the formation of a lubricant-containing top layer, the post-treatment coatings had a lower friction coefficient and improved wear resistance compared with the asdeposited coatings. In addition, the coatings after UVOI treatment had better wear performance than those post-treated using the OIAP process. Among all coatings, the coating produced with a 50-Hz pulsed current followed by UVOI posttreatment achieved the lowest friction coefficient（0.03） and best wear resistance when sliding against a Si3N4 ceramic counterface. This study indicates that a novel self-lubricating coating can be prepared by a PEO process combined with vacuum oil impregnation post-treatment.

In-situ preparation of robust self-lubricating composite coating from thermally sprayed ceramic template

The self-lubricating ceramic coatings that can control friction and wear have attracted researchers’widespread attention.However,the poor interfacial bonding between lubricants and ceramics and the deterioration of mechanical properties due to a tribological design limit their practical applications.Here,a robust self-lubricating coating was fabricated by an in-situ synthesis of MoS_(2)/C within inherent defects of thermally sprayed yttria-stabilized zirconia(YSZ)coatings.The edge-pinning by noncoherent endows hybrid coatings with excellent interfacial strength,increasing their hardness(HV)and cohesive strength.Furthermore,owing to the formation of a well-covered robust lubricating film at a frictional interface,a coefficient of friction(COF)can be reduced by 79.6%to 0.15,and a specific wear rate(W)drops from 1.36×10^(−3) to 6.27×10^(−7) mm^(3)·N^(−1)·m^(−1).Combining outstanding mechanical properties and tribological performance,the hybrid coating exhibits great application potential in controlling friction and wear.Importantly,this strategy of introducing the target materials into the inherent defects of the raw materials to improve the relevant properties opens new avenues for the design and preparation of composite materials.

Tribological behavior of a Ni-WS2 composite coating across wide temperature ranges

In order to reduce the friction coefficient of a pure nickel coating and extend the lifetime of metal parts under extreme friction conditions,a series of Ni-based WS2-composite solid lubrication coating containing different WS2 concentrations were prepared on a 45#mild carbon steel substrate by electroplating.The cyclic voltammetry method was used to investigate the electroplating regulation of the Ni-WS2 composite coatings.X-ray diffraction(XRD) and scanning electron microscopy(SEM) were used to analyze the microstructures and wear surfaces of the composite coatings,the tribological properties and wear mechanisms of the composite coatings with different WS2 concentrations.The results show that the addition of WS2 can promote the cathode polarization of the electroplating process,and the polarization degree goes up with the increase in WS2 concentrations.The friction coefficient of Ni-composite coatings significantly decreases by the addition of WS2 particles.The lowest friction coefficient at room temperature is obtained at a value around 0.01-0.03 from the coating deposited in the electrolyte solution with a 30g·L^-1 WS2 concentration.The friction coefficient of the Ni-WS2 composite coating remains in 0.01-0.03 with the increase in temperature from room temperature to 300℃.When the temperature goes up to 500℃,the friction coefficient manifests a continuous increase to 0.12,because WS2 is gradually oxidized into WO3 and therefore loses its lubrication ability.

Mechanism of thermoviscoelasticity driven solid-liquid interface reducing friction for polymer alloy coating

High-temperature ablation is a common failure phenomenon that limits the service life of the transmission parts on heavy-duty machines used in heavy load,high temperature,high shock conditions due to in-sufficient supply of lubricating oil and grease.Traditional self-lubricating coatings prepared by inorganic,organic or organic-inorganic hybrid methods are prone to be oxidated at high temperatures to lose their friction reducing function,so that it is difficult to meet the engineering requirements of high-temperature lubrication.We design viscoelastic polymer coatings by a high-temperature self-lubricating and wear-resistant strategy.Polytetrafluoroethylene(PTFE,T_(m)=329℃)and polyphenylene sulfide(PPS,T_(g)=84℃,T_(m)=283℃)are used to prepare a PTFE/PPS polymer alloy coating.As the temperature increases from 25 to 300℃,the PTFE/PPS coating softens from glass state to viscoelastic state and viscous flow state,which is owing to the thermodynamic transformation characteristic of the PPS component.Additionally the friction coefficient(μ)decreased from 0.096 to 0.042 with the increasing of temperature from 25 to 300℃.The mechanism of mechanical deformation and surface morphology evolution for the PTFE/PPS coating under the multi-field coupling action of temperature(T),temperature–centrifugal force(T–F_(ω)),temperature–centrifugal force–shearing force(T–F_(ω)–F_(τ))were investigated.The physical model of“thermoviscoelasticity driven solid–liquid interface reducing friction”is proposed to clarify the self-lubricating mechanism determined by the high-temperature viscoelastic properties of polymers.The high-temperature adjusts the viscosity(η)of the coating,increases interface slipping and intensifies shear deformation(τ),reducing the friction coefficient.The result is expected to provide a new idea for designing anti-ablation coatings served in high temperature friction and wear conditions.