Applications of ceramics for tribological components

Fine ceramics are expected to be applied as new materials to many tribological components because of their superior heat resistance and corrosion resistance and their high hardness. In this paper, wear and frictional test results from applications of these components in machines are given and the capabilities of fine ceramics are described.     

Studying tribological characteristics of alumina- and zirconia-based ceramics

Tribotests of ceramic specimens of various compositions (Al₂O₃-1% TiO₂, ZrO₂-5.3% Y₂O₃, Al₂O₃-15% (ZrO₂ + 5.3% Y₂O₃)) were carried out under dry friction conditions. It has been shown that all of the specimens have a high wear resistance, while the Al₂O₃-15% (ZrO₂ + 5.3% Y₂O₃) specimens have the lowest wear rate and the highest microhardness. This is due to the fine-grained structure of these specimens and their higher density compared to that of the other ceramic compositions.     

Laser decoating of DLC films for tribological applications

Damaged DLC coatings usually require remanufacturing of the entire coated components starting from an industrial chemical de-coating step. Alternatively, a complete or local coating repair can be considered. To pursue this approach, however, a local coating removal is needed as first operation. In this context, controlled decoating based on laser sources can be a suitable and clean alternative to achieve a pre-fixed decoating depth with high accuracy. In the present study, we investigated a laser-based decoating process executed on multilayered DLC films for advanced tribological applications (deposited via a hybrid PVD/PE-CVD technique). The results were acquired via multifocal optical digital microscopy (MF-ODM), which allowed high-resolution 3D surface reconstruction as well as digital profilometry of the lasered and unlasered surface. The study identifies the most critical process parameters which influence the effective decoating depth and the post-decoating surface roughness. In particular, the role of pulse overlap (decomposed along orthogonal directions), laser fluence, number of lasing passes and assist gas is discussed in text. A first experimental campaign was designed to identify the best conditions to obtain full decoating of the DLC + DLC:Cr layers. It was observed that decreasing the marking speed to 200 mm/s was necessary to obtain a sufficient pulse overlap and a nearly planar ablation profile. By operating with microsecond pulses and 1 J/cm² (fairly above the ablation threshold), less than 10 passes were needed to obtain full decoating of the lasered area with an etching rate of 1.1 μm/loop. Further experiments were then executed in order to minimise the roughness of the rest surface with the best value found at around 0.2 μm. Limited oxidation but higher R _a values were observed in Ar atmosphere.     

Spectromicroscopy of tribological films from engine oil additives. Part I. Films from ZDDP's

Antiwear films formed from pure neutral alkyl‐ and aryl‐ZDDP's, and a commercial ZDDP, have been studied with high resolution synchrotron‐based photoemission spectromicroscopy with a new instrument, MEPHISTO. Good P L‐edge XANES spectra have been taken on areas between 12 and 400 μm², and good images of phosphates and ZDDP have been obtained at ∼1 μm resolution on both smooth and rough steel. These spectra, and corresponding images, show immediately that both the chemistry and the morphology of the alkyl and aryl films are very different. The alkyl film contains a range of smaller and larger protective polyphosphate pads from a few to ∼25 μm² in area. We have shown that the chemistry of small and large pads are different. The large pads contain very long chain polyphosphate; while the smaller pads contain short chain polyphosphate. The aryl films contain ortho‐ or pyro‐phosphates, are much thinner and more uniform, with obviously more streaking from initial wear, and no obvious protective pad formation. Antiwear films generated from the commercial ZDDP, rubbed in base oil, show that the long chain polyphosphate is converted to ortho‐ or pyro‐phosphate, but the amount and distribution of phosphate does not change noticeably. The antiwear films are remarkably stable physically. This revised version was published online in July 2006 with corrections to the Cover Date.     

The tribological behavior of thiophosphates as additives in rapeseed oil

Two kinds of thiophosphate, tri-n-octyl thiophosphate and tri-n-octyl tetrathiophosphate, were synthesized. The tribological behavior of the synthetic compounds and tricresyl phosphate as additives in rapeseed oil for steel-steel frictional pair was investigated with a four-ball machine. The relationship between the additive structure and tribological properties was explored, while the lubrication mechanisms of the additives were investigated as well. Thus the worn surfaces of the steel ball lubricated with the additive-containing rapeseed oil were analyzed by means of X-ray photoelectron spectrometry and scanning electron microscopy, and the elemental compositions and distributions in the worn steel surfaces were observed and determined with an EPMA-810Q electron probe micro-analyzer. It has been found that the synthetic thiophosphates as additives in rapeseed oil at proper concentrations show better tribological properties than tricresyl phosphate. Tribochemical reactions were involved for steel–steel frictional pair lubricated with the rapeseed oil containing thiophosphates, with the formation of a boundary lubricating and protecting film composed of glyceride of rapeseed oil and tribochemical reaction products of the lubricants. This contributes to improve the tribological properties of the rapeseed oil base stock.     

Potential of kenaf fibres as reinforcement for tribological applications

This paper presents an attempt to use kenaf fibres as reinforcement for tribo-composite based on epoxy for bearing applications. Kenaf fibres reinforced epoxy (KFRE) composite was fabricated using a closed mould technique associated with vacuum system. Sliding wear and frictional behaviour of the composite were studied against polished stainless steel counterface using Block-On-Disc (BOD) machine at different applied loads (30–100 N), sliding distances (0–5 km) and sliding velocities (1.1–3.9 m/s). The effect of the fibre orientations, with respect to the sliding direction, was considered; these orientations are parallel (P-O), anti-parallel (AP-O) and normal (N-O). The morphology of the worn surfaces of the composite was studied using a scanning electron microscope (SEM). The result revealed that the presence of kenaf fibres in the composite enhanced the wear and frictional performance of the epoxy. Applied load and sliding velocity have less effect on the specific wear rate of the composite in all the three orientations. The composite exhibited better wear performance in N-O compared to P-O and AP-O.     

General aspects for tribological applications of hard particle coatings

Hard coatings, consisting of WC, TiC or Cr₃C₂ particles with a nickel or cobalt matrix were compared with conventional wear-resistant materials like hardened steel 100 Cr6, Ferro TiC P143, WC-Co hard metal and a widely used thermal spray layer NiCrBSi. The coating procedure was flame spraying and diffusion welding. Some layers were remelted using an electron beam to improve their microstructural properties, porosity and binding to the bulk material.

Wear tests were performed under different degrees of severity to qualify the resistance of the coating, using abrasive, sliding and impact test methods representing different wear mechanisms. It is shown that the benefit of the hard particle content depends on the acting loading situation. Under abrasive and sliding conditions the advantage of a high hardness level, i.e. a high concentration of hard phases, could be demonstrated. For impact loading, causing severe surface fatigue, homogeneous materials with high toughness, such as martensitic steels, are beneficial; followed by coatings with a high concentration of ductile matrix. In some cases, the weaknesses, such as brittleness and limited strength of binding to the bulk, could be improved by electron beam remelting.     

Effect of conditions of diamond grinding on tribological behavior of alumina-based ceramics

Characteristic morphologies of the surface of oxide ceramics, which are produced by grinding, have been identified and a relationship between conditions of diamond grinding and these morphologies has been revealed using the developed method of study. It has been found that the type of the characteristic morphology of the surface of ground oxide-ceramic specimens has a strong effect on the tribological behavior of these specimens. The prime role of the grinding subsystem, which is a component of the manufacturing system for producing ceramic friction members, in the formation of the mode of wear at the run-in stage has been established.     

Ageing impact on tribological properties of MoDTC-containing base oil

Since last decades, there is an industry-wide need for the comprehension of how degradation affects lubricant tribological performance.The lubricating performances of MoDTC-containing base oil, fresh and aged, have been assessed using reciprocating tribometer, and XPS and TEM have been used to characterize the tribofilm formed.An induction period increasing with the degradation time was evidenced on the friction behavior when Mo-containing base oil was subjected to the degradation process and the formation of a tribofilm rich in molybdenum oxisulphides has been documented.The results found are discussed to clarify the correlation between degradation time, tribological performance and tribofilm characterizations.     

The tribological behavior of diester-containing polysulfides as additives in mineral oil

The diester-containing polysulfides were synthesized and their anti-wear (AW) and friction-reducing behaviors as additives in HVIW H150 base oil were evaluated using a four-ball tester. The copper corrosion test was conducted to identify their anti-corrosion property. The nature of the film formed in the rubbed surface was investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Based on the experimental results, diester-containing polysulfides show good anti-corrosion properties and excellent AW abilities similar to that of ZDDP. The results of XPS analyses illustrate that an S-containing inorganic layer, generated by the reaction of diester-containing polysulfides with the metallic surfaces, mainly contains sulfate, sulfite and little sulfide, and there exists a polymer film protective layer over the inorganic layer surface.     

The tribological characteristics of JS material under lubrication of oil

A material composed of a steel backing, a sintered porous bronze middle layer and a layer of reinforced PTFE, which is named JS material, was prepared. The friction, wear and limiting PV values of this material under dry friction as well as the lubrication of number 20 mechanical oil were studied using a MPV-1500 friction tester. The worn surface of JS material and the transfer film formed on the counterface of carbon steel were investigated using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results show that the friction, wear and limiting PV values of JS material can be greatly improved with the lubrication of oil. The results of SEM and EPMA analyses indicate that, under dry friction conditions, the solid lubricant PTFE and Pb easily transfer to the steel counterface and results in the reduction of friction and wear; while under the lubrication of oil, little transference of PTFE and Pb to the steel surface occurs and very small friction and wear are achieved. Analyses of frictional surfaces also suggest that the Pb filler gets enrichment on the rubbing surfaces, which is beneficial in increasing the adhesion of the transfer film with the steel surface.     

High-pressure behavior and tribological properties of wind turbine gear oil

Different types of synthetic polyalphaolefin (PAO) oils and a mineral oil are considered in this study. High-pressure viscosity test was done and pressure-viscosity coefficient was measured for all sample oils. Results showed the better performance of PAO oils than the mineral oil. Authors also tested some other tribological properties such as low-temperature behavior, bulk property, frictional coefficient, and wear behavior, which are important for wind turbine gear oil. Low-temperature behavior and frictional property of PAO oils exhibited the better results. Study also showed that the prediction of low-temperature fluidity is possible using the sound velocity in the oil. Finally, the presence of polymethakrylate (PMA) absorbent in PAO oil exposed comparatively better results among all PAO oils.     

The tribological behaviors of hydroxyl-containing dithiocarbamate-triazine derivatives as additives in rapeseed oil

Two ashless and non-phosphorus hydroxyl-containing dithiocarbamate-triazine compounds, 2,4,6-tri[N,N-di-n-butyldithiocarmate-(2′-hydroxyl)-propionylthio]-1,3,5-s-triazine (LDBA) and 2,4,6-tri[N,N-di-i-octyldithiocarmate-(2′-hydroxyl)-propionylthio]-1,3,5-s-triazine (LDIOA), were prepared and their tribological behaviors as additives in rapeseed oil (RSO) were evaluated using a four-ball tester. Thermal degradation tests were conducted to identify their thermal stabilities using a thermo-gravimetric analyzer. The worn surfaces of the steel balls were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). The results indicate that the additives possess high thermal stabilities and good load-carrying capacities. Moreover, they both have good anti-wear and friction-reducing property at relatively high concentration (>1.5 wt%) and under all test loads. The results of XPS analyses illustrate that the prepared compounds as additives in RSO forms a protective film containing inorganic sulfide, sulfate, oxidized compounds and organic nitrogen-containing compounds on the metal surface during the sliding process.     

工程陶瓷在水压控制阀中的应用

分析了水压控制阀的关键技术问题,对工程陶瓷在解决这些问题时的独特优势进行了介绍,并对陶瓷应用于水压控制阀时的主要失效形式进行了论述。     

Study of preparation and tribological properties of rare earth nanoparticles in lubricating oil

Surface-modified mixed rare earth nanoparticles are prepared and their tribological performances as lubrication additives are evaluated using a four-ball friction and wear tester. The analytical results show that the average size of the particles is less than 30 nm and they exhibit excellent anti-wear, load-carrying and good friction-reducing capacities in base stock. Auger electron spectroscopy (AES) and X-ray photoelectron spectrometry (XPS) are performed to investigate their tribochemical mechanism and the results indicate that there is a boundary film composed with ferrous oxides, organic acid, rare earth oxides and complex of rare earth metals formed on the rubbed surface. The high concentration of ferrous oxides in the friction surface, which attributes to the oxidation catalysis of nano rare earth particles during the friction process, may be the main reason that leads to their excellent tribological properties.     

An investigation on tribological properties of graphite nanosheets as oil additive

Graphite nanosheets with average diameter of 500 nm and thickness about 15 nm were prepared by stirring ball milling. The tribological behavior of the graphite nanosheets as additive in paraffin oil were investigated with a four-ball and a pin-on-disk friction and wear tester. The wear surfaces of the steel ball lubricated with the additive-containing paraffin oil were analyzed by means of scanning electron microscopy (SEM). It has been found that the graphite nanosheets as additive in oil at proper concentration show better tribological properties than pure paraffin oil. The load-carrying capacity and antiwear ability of the lubricating oil were improved. Moreover, the friction coefficient of the lubricating oil was decreased by the addition of the graphite nanosheets. The optimal concentration of the additive in paraffin oil is about 1.0 × 10⁻² wt.%.     

The tribological properties of diethylammonium lanthanide tetrakis (diethyldithiocarbamate) in lubricating oil

The synthesis of four oil-soluble rare earth complexes of diethylammonium lanthanide tetrakis(diethyldithiocarbamate) (ReDTC) is described. The tribological properties of these rare earth complexes as additives in lubricating oil have been evaluated using a four-ball machine. The results show that these novel complexes have excellent extreme-pressure properties and selective antiwear properties in lubricating oil. The action mechanism of the rare earth complexes as additives in lubricating oil was investigated using X-ray photoelectron spectroscopy (XPS).     

A study of the tribological behaviour of heterocyclic derivatives as additives in rape seed oil and mineral oil

The tribological properties of two novel heterocyclic derivatives as additives in rape seed oil and in mineral oil were investigated using a four‐ball machine. The morphologies and chemical features of the worn steel surfaces were observed and examined by means of X‐ray photoelectron spectroscopy and scanning electron microscopy. Both heterocyclic compounds improved the load‐carrying and extreme‐pressure capacities and the friction‐reducing and antiwear properties of rape seed oil more than they improved those of the mineral oil. Surface analyses of the rubbed surfaces revealed the formation of a protective film containing FeSO₄, organosulphur compound, FeS₂, polymerised ester, and organonitrogen compound when the surfaces were lubricated by rape seed oil containing the additives. The protective film formed during sliding processes contributed to the increase in the wear resistance.