Yuansheng Jin

A study on abrasive resistance of Ni-based coatings with a WC hard phase

To improve the wear resistance of the piston ring and cylinder liner for the automobiles in desert area applications, an experimental study has been made for Ni-based composite coatings deposited on traditional cast substrates by plasma spraying with laser post-treatment. Three Ni-WC composite coatings with varied contents of WC particles were tested. Tribological experiments were carried out under abrasive conditions on a commercial Falex-6 friction and wear test machine. The test results showed that the composite coatings with a WC hard phase have improved abrasive wear resistance in comparison with current ring/cylinder materials, and the best one is a laser-treated plasma-sprayed Ni60 + 60% WC coating. The wear mechanism of the composite coatings was discussed on the basis of microscopic observations of worn surfaces.

The analysis of the friction and wear mechanisms of plasma-sprayed ceramic coatings at 450°C

Abstract In this paper, the friction and wear mechanisms of plasma-sprayed ceramic coatings (Cr2O35%SiO23%TiO2, Cr3C212%Ni3%Cr, TiO2, Al2O3-(Metco), WC18%Co, PSZ, Al2O3 (made in China)) at 450 °C have been studied. It has been determined that the friction and wear behaviour, such as fatigue spalling, plastic deformation and adhesive transfer, of ceramic coatings is similar to that of metals. Under high temperature (450 °C), the dry friction coefficients of ceramic coatings were much higher than the lubricated friction coefficients of ceramic coatings. At the same time the wear mechanisms and the frictional surface topography under dry friction condition were much different from those under lubricated conditions. Adhesive transfer occurred easily under dry friction conditions. However, it was rather difficult for adhesive friction to take place when lubricated.

The friction and wear performance of plasma-sprayed ceramic coatings at high temperature

Abstract Dry, lubricated and pre-lubricated tribological behaviour of plasma-sprayed ceramic coatings under 450 °C was studied. Eight different ceramic powders are used to make the coatings. The results show that the dry friction coefficients of most couples are about 0.5. Only one couple shows a rather low friction coefficient (0.12, when the Hertz load is 0.20 GPa) and low wear rate in the dry friction process. This couple is expected to be used as a self-lubrication pair. The friction coefficient of all the couples is in the range 0.043–0.142 when lubrication is used. Additionally, the wear rate is strongly influenced by the used lubricant.

Impact wear performances of Cr3C2NiCr coatings by plasma and HVOF spraying

Abstract Three kinds of Cr 3 C 2 25%NiCr coatings prepared by APS-Ar H 2 , APS-Ar He and HVOF spraying, respectively, have been studied with a self-made impact wear rig. The wear resistant performances of Cr 3 C 2 25%NiCr coatings were tested under different impact stress. The wear losses and worn surface profiles are measured, and the corresponding wear mechanisms were studied with Scanning Electronic Microscope (SEM). It is found that wear resistance of Cr 3 C 2 25%NiCr coatings under different impact conditions increases in the order of APS-Ar H 2 APS-Ar He and HVOF spraying. The HVOF sprayed Cr 3 C 2 25%NiCr coatings show the best impact wear resistance due to the denser structure and fewer defects. The wear mechanisms of Cr 3 C 2 25%NiCr coatings under impact condition is impact fatigue, which consists of two wear processes of plastic smearing and breaking-off. The plastic smearing produces plastic deformation of the thin layer in the outer coatings and then causes microcracks in the near subsurface due to stress accumulation. There are horizontal and perpendicular microcracks. The former commonly nucleate from the boundary of lamellae, while the latter may occur from the sub-microcracks inside lamellae. The propagating and merging of both horizontal and perpendicular microcracks finally give rise to the breaking-off of coatings. The cohesive defects such as stratification, porosity and microcracks in the lamellae have much effects on the wear resistance of coatings.

The effectiveness of high temperature lubrication by in situ formation of graphite/MoS2 films

Abstract While investigating the problems of high temperature lubrication of adiabatic diesel engines, it was discovered that boundary lubricant films could be transformed into reacted films in an extremely short period under certain reaction kinetics after the oil films had lost their lubrication efficacy. The reacted films consisted of in situ formed graphite and MoS 2 . A performance index R is defined to characterize their formation ability and lubricating property for evaluating their effectiveness of high temperature lubrication. The index R , which is calculated through maximum and minimum friction coefficients in the formation process of the reacted films as well as the reaction time and duration of the films, is correlated to such parameters as temperature, pressure and sliding velocity.

The analysis of the chemical structure and properties of ceramic surface films in friction using scanning electron microscopy, auger electron spectroscopy and microregion X-ray diffraction

Abstract In this paper, the chemical surface film of a plasma-sprayed ceramic coating (Cr 3 C 2 , Al 2 O 3 13%TiO 2 ), formed by the reaction of the additives, calcium, sulphur and phosphorus, with a ceramic surface at high temperature, has been studied using scanning electron microscopy, Auger electron spectroscopy and microregion X-ray diffraction. Scanning electron microscopy was used to observe the surface film topography, Auger electron spectroscopy was used to analyse the change in the surface film elements with film depth and microregion X-ray diffraction was used to identify the chemical structure of the film. The results show that microregion X-ray diffraction is an effective means of studying the film structure; the ceramic surface, which is generally inert, can react with the additives, calcium, sulphur and phosphorus, to form chemical surface films which effectively decrease friction and wear.

Effect of a surface film on the friction and wear properties of plasma-sprayed Al2O340%ZrO2 coatings

Abstract In order to provide the replenishment of a surface film in high temperature lubrication, an in situ formed graphite film is very useful. In this work, five synthetic lubricants were employed to evaluate the effectiveness of graphite film generation on a ceramic coating under extreme conditions of high pressure and 320 °C. Combined lubrication of the liquid and solid film occurs, and a carbonaceous polymer is generated on the Al2O340%ZrO2 coating. Because of combined lubrication, the friction coefficient decreases to 0.04 from about 0.5 for dry friction. The main wear mechanisms involved smearing and breaking off of the ceramic coating for high temperature lubrication. Zin dialkyldithiophosphate (ZDDP) plays an important role in the anti-wear performance of oils, and mainly smearing wear is observed on the coatings lubricated with ZDDP-containing oils.

Sliding wear of cast-iron coatings and cast-iron-cast-iron pairs in lubricated contact

Abstract The friction coefficients and wear rates of cast-iron coatings and cast-iron-cast-iron pairs in lubricated contact at different temperatures were measured using an elevated temperature wear tester. The removal mechanism of materials and the behavior of coating, substrate microstructure and precipitation particles during wear were studied by scanning electron microscopy and energy dispersive X-ray analysis. It was shown that at higher temperatures the wear of electroplated chromium/niobium cast-iron pair was mainly caused by adhesion, but at lower temperatures the wear was a result of fatigue delamination which was controlled by the nucleation and propagation of fissures. Whether the temperature was high or low, the asperity on the infiltrated niobium layer was observed to cause abrasive wear of the liner.

The synergistic effects of Mo as an alloy element added to chromium carbide coatings on their lubrication properties at high temperatures

Abstract This paper studies the effects of Mo, added to a plasma-sprayed Cr 3 C 2 -25%NiCr coating, on the anti-friction performance of the coating at temperatures up to 850 °C. 7% Mo is mixed with Cr 3 C 2 -25%NiCr powder for spraying the Cr 3 C 2 -NiCr-Mo coating. The surface of this kind of coating is found to react with additive elements in synthetic oil in the friction process when sliding against a plasma-sprayed Cr 2 O 3 coating at high temperatures. The reacted surface film shows the effect of solid lubrication. Compared with the Cr 3 C 2 -NiCr coating, the Cr 3 C 2 -NiCr-Mo coating possesses much better anti-friction capability at high temperatures. Surface analysis using SEM, WDS and XPS shows that Mo plays a very important role in forming different kinds of solid lubricating films at different temperatures. The solid lubricating films formed are dominated by MoS 2 and graphite.

Investigation of matching characteristics of important frictional materials with lubricant antiwear additives

Abstract Matching experiments have been conducted, using a Falex tester, on the conventional lubricant antiwear additives of S agent, P agent and those containing both S and P agents. Also studied was a newly synthesized organo-boron antiwear additive for commonly used frictional materials ( e.g. steel, copper and cast iron). Load-velocity diagrams of the four lubricant antiwear additives and six pairs of frictional couples were correlated. The chemical structures of boundary films on the surfaces of some typical frictional couples have been analyzed using Auger electron spectroscopy and X-ray photoelectron spectroscopy.