Tribological properties of spark-plasma-sintered ZrO2(Y2O3)-CaF2-Ag composites at elevated temperatures

Spark-plasma sintering is employed to synthesize self-lubricating ZrO₂(Y₂O₃) matrix composites with different additives of CaF₂ and Ag as solid lubricants by tailoring the composition and by adjusting the sintering temperature. The friction and wear behavior of ZrO₂(Y₂O₃) matrix composites have been investigated in dry sliding against an alumina ball from room temperature to 800 °C. The effective self-lubrication at different temperatures depends mainly on the content of various solid lubricants in the composites. The addition of 35 wt.% Ag and 30 wt.% CaF₂ in the ZrO₂(Y₂O₃) matrix can promote the formation of a well-covered lubricating film, and effectively reduce the friction and wear over the entire temperature range studied. The friction coefficients at low temperatures were at a minimum value for the composite containing 35 wt.% of silver. At this silver concentration, low and intermediate temperature lubricating properties are greatly improved without affecting high-temperature lubrication by the calcium fluoride in ZrO₂(Y₂O₃) matrix composites. The worn surfaces and transfer films formed during wear process have been characterized to identify the synergistic lubrication behavior of CaF₂ and Ag lubricants at different temperatures.     

MoS2/Ti low-friction coating for gears

The applicability of a multilayer composite surface coating in gears is discussed in this work, mainly in what concerns to gear efficiency at normal operating conditions and to scuffing load capacity. The average friction coefficient between gear teeth is discussed and compared with uncoated steel gears.The disulfide molybdenum/titanium (MoS₂/Ti) composite coating is studied and the deposition procedure is described.Several screening tests, like Rockwell indentations, ball cratering, pin-on-disc and reciprocating wear, were performed to evaluate the adhesion to the substrate, the tribological performance of this coating and his applicability in heavy loaded rolling-sliding contacts, such as found in gears.FZG gear efficiency tests were performed using type C gears in order to evaluate the influence of the surface coating in gear efficiency, for a wide range of operating conditions. These tests in conjunction with a numerical model for the energetic balance of the FZG gearbox allowed the determination of the average friction coefficient between gear teeth, taking into account the presence of the surface coating.FZG gear scuffing tests were also performed using type C gears in order to evaluate the coating anti-scuffing performance, which proved to be very significant.     

不同润滑条件下氧化铝陶瓷衬板的磨损机制

在干摩擦、水润滑、油润滑3种不同润滑条件下对氧化铝陶瓷进行摩擦磨损试验。利用SEM对磨损后的磨痕进行观察并进行显微组织结构分析,探讨不同介质下氧化铝陶瓷的磨损机制。结果表明:氧化铝陶瓷材料干摩擦条件下的磨损机制为大量的脆性剥落和大量的磨粒磨损,在水润滑条件下为较少的脆性剥落和轻微的磨粒磨损,在油润滑条件下为很少的脆性剥落和极微的磨粒磨损;液体润滑剂可使氧化铝陶瓷材料的磨损量大幅度降低,其中油润滑条件的减磨效果最为突出。     

Tribological behavior of plasma sprayed Cr2O3-3%TiO2 coatings

Tribological behaviors of plasma-sprayed conventional and nanostructured Cr₂O₃-3%TiO₂ ceramic coatings (i.e., CC3T and NC3T, respectively) using pin on disc type dry sliding and pot type slurry erosion test were investigated in the present work. The experimental results indicated that there were two main wear mechanisms, plastic smearing and adhesive tearing, in the worn coatings under dry sliding. Plastic smearing corresponded to a lower average friction coefficient and wear rate, while adhesive tearing corresponded to higher values. The erosive environment selected for the slurry erosion experiments include 10, 20 and 30% of SiO₂ slurry concentrations in water with particle size 75-106 μm. The main damage mechanism observed in all the coatings submitted to slurry erosion were the formation and propagation of brittle cracks resulting in the detachment of coating surface material. Microstructural investigation was also carried to investigate the wear and erosion mechanism of the coatings using FE-SEM and EDS analysis. Properties like microhardness and porosity were also investigated for these coatings. Tribological performance of NC3T was better as compared to CC3T as observed in the present work.     

MoS2 single sheet lubrication by molybdenum dithiocarbamate

The mechanisms by which Modtc reduces friction in the centirange under boundary lubrication have been investigated using analytical tribometry. First, the SRV friction test was coupled with energy-filtering TEM on wear fragments and spatially-resolved XPS inside the wear scars. Second, we performed UHV friction tests on Modtc tribofilms previously created on a large area. The overall data demonstrate that the mechanisms of friction-reduction by Modtc is attributed to the effect of sliding between single layers of MoS₂ only, and not to intra-sliding in MoS₂ 3-D crystal. Highly-dispersed MoS₂ sheets are present in a carbon matrix in the tribofilm material. The growth of the 2-D MoS₂ single sheets is thought to be formed by degradation of the Modtc molecule by electron transfer mechanisms activated by the friction process. The lubrication of the uncoated, stationary counterface is attributed to successive transfer of individual sheets towards the friction surface. Practically, in these conditions only a few per cent of dispersed MoS₂ is sufficient to lubricate at the same level as pure MoS₂.     

Tribological properties of ZrO2 (Y2O3)-Mo-BaF2/CaF2 composites at high temperatures

ZrO₂ (Y₂O₃) with different contents of BaF₂/CaF₂ and Mo were fabricated by hot pressed sintering, and the tribological behavior of the composites against SiC ceramic was investigated from room temperature to 1000 °C. It was found that the ZrO₂ (Y₂O₃)-5BaF₂/CaF₂-10Mo composite possessed excellent self-lubricating and anti-wear properties. The low friction and wear were attributed to enhanced matrix and BaMoO₄ formed on the worn surfaces.     

Study of tribofilms formed during dry sliding of Ta2AlC/Ag or Cr2AlC/Ag composites against Ni-based superalloys and Al2O3

Herein we show that dry sliding, in air, in the 25–550 °C temperature range, of the novel Ta₂AlC/20 vol.% Ag and Cr₂AlC/20 vol.% Ag composites against Ni-based superalloys (SAs) and alumina led to the formation of steady-state tribofilms whose thicknesses and compositions varied depending on sliding conditions. At elevated temperatures, under both isothermal and thermocyclic conditions, relatively thick (>0.5 μm) well-compacted “glaze” lubricious tribofilms were developed as a result of joint action of tribo-chemical and tribo-mechanical factors involving repeated tribo-oxidation, mixing, fracturing, sintering, etc. They were mainly composed of oxidized constituents from both counterparts (if slid vs. SA) or solely from MAX-Ag ones (vs. Al₂O₃) and possessed a fine multi-layered microstructure, i.e. a more oxidized thin outermost layer and a less oxidized carbide-containing thicker inner layer. During thermocycling the tribofilms adapted to 25–550 °C temperature variations and preserved their primary macro- and microstructure, hardness, good adhesion to the matrix and lubricating characteristics. Consequently, the tribological properties of MAX-Ag/SA tribocouples did not deteriorate, but slightly improved with sliding distance. A possible mechanism of tribofilm generation and their transformations at various temperatures is discussed. The effect of SA and alumina counterparts on the formation and degradation of the tribofilms are also discussed.     

Modern solid lubrication: Recent developments and applications of MoS2

An important and growing field of lubrication lies in the use of solid films, although they are in general more expensive than oils or greases, and require specialist attention both in mechanical design and in coating application techniques. In this paper, the general classification of solid lubricant types is reviewed, along with the reasons for choosing, and methods of depositing, solid lubricants, in particular MoS₂. The best‐performing and most flexible technique for making MoS₂ films is by physical vapour deposition (PVD), and the variants of that technology are considered. The intrinsically lubricating, lamellar structure of pure MoS₂ is described, along with a brief summary of its wear and failure modes. Present applications for lubrication by MoS₂ in spacecraft and dry machining are outlined, as are anti‐adhesive uses in extruding and moulding. The current state of the art of modification of MoS₂ films consists in the addition of dopants (co‐sputtering), in multilayering as a series of films, each fulfilling a specific task, or in stacking repeating nano‐metre‐scale films. Composite films of MoS₂ islands in a hard film matrix are also being developed.     

Tribological behavior of select MAX phases against Al2O3 at elevated temperatures

The layered M_n+1AC_n ternary carbides – MAX phases – Ta₂AlC, Ti₂AlC, Cr₂AlC and Ti₃SiC₂ were tested under dry sliding conditions against alumina at 550 °C and 3 N load (for a stress of ≈0.08 MPa) using a pin-on-disk tribometer. Ta₂AlC and Ti₂AlC exhibited low specific wear rates, SWRs, (≤1 × 10⁻⁶ mm³/N m), while the coefficients of friction, μ, were 0.9 and 0.6, respectively. At 0.4, μ of Ti₃SiC₂ was the lowest measured, but the SWR, at ≈2 × 10⁻⁴ mm³/N m, was high. With a μ of 0.44 and a SWR of 6 × 10⁻⁵ mm³/N m the Cr₂AlC sample was in between. No visible wear of Al₂O₃ counterparts was observed in all the tribocouples. Tribofilms, which were mainly comprised of X-ray amorphous oxides of the M and A elements and, in some cases, unoxidized grains of the corresponding MAX phases, were formed on the contact surfaces. The correlations between observed tribological properties and tribofilm characteristics are discussed.     

Effect of a surface film on the friction and wear properties of plasma-sprayed Al2O3---40%ZrO2 coatings

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.     

Super-low friction of MoS2 coatings in various environments

The ultra-low friction coefficient (typically in the 10⁻² range) of MoS₂-based coatings is generally associated with the friction-induced orientation of ‘easy-shear’ planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used and AES/XPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS₂ PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS₂ coatings deposited on AISI 52100 steel was studied in ultra-high vacuum (UHV: 5 × 10⁻⁸ Pa), high vacuum (HV: 10⁻³ Pa), dry nitrogen (10⁵ Pa) and ambient air (10⁵ Pa). ‘Super-low’ friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013–0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the ‘super-low’ friction behaviour (10⁻³ range) can be attributed to superlubricity, obtained for a particular combination of cystallographic orientation and the absence of contaminants, leading to a considerable decrease in the interfacial shear strength.     

Tribological characteristics of bonded MoS2 films evaluated in rolling-sliding contact in a vacuum

Tribological characteristics of bonded MoS₂ films have been evaluated in air, nitrogen and vacuum using a two-roller type rolling-sliding friction apparatus. Two conventional space-proven films, polyamideimide-bonded and sodium silicate-bonded, designated as film A and film B respectively, were tested at a speed of 998 rev min^-1 under a load of 100 N at slip ratios of 1–100%, the slip ratio being defined as (V₁ - V₂)/ V₁ × 100%, in which V₁ and V₂ was rotational speed of each roller.

The wear life of the film A was more slip ratio dependent than that of the film B. The wear life of the film A tested in a vacuum was the longest at 10% slip ratio but it reduced significantly at 1%. On the contrary, the best durability of the film B was obtained at 100% slip ratio (pure sliding). The wear life of the film A shortened considerably in air and nitrogen.

In general, a solid lubricant film applied to spur gears is subjected to rolling-sliding friction of 0-10% slip ratios. To improve the endurance life of the film A at 1% slip ratio, films prepared by changing content of pigments (MoS₂ and Sb₂0₃) were evaluated. The films with a pigment content of 5-15% showed a marked improvement in wear life at a slip ratio of 1%, while maintaining excellent durability at 10%.

The wear process of an improved film was monitored using a gap sensor. It appears to be dominated by fatigue.     

Tribological behaviour of sintered 316L stainless steel impregnated with MoS2 plain bearing

The mechanical and tribological properties of sintered 316L stainless steel impregnated with molybdenum disulfide (MoS₂) were investigated. Tests were carried out at room temperature for two specific ranges of PV value (1.1 and 1.8 MPa m/s). The results prove that the friction coefficient and the wear are strongly influenced by the addition level of MoS₂.

In this paper, MoS₂ powder was mixed with 316L powder before being processed via compacting and sintering steps. The microstructure, hardness, tensile strength and elongation at breaking point of the sintered specimens were evaluated. The friction and wear properties of the materials were examined by a partial plain bearing wear test rig under dry conditions at room temperature and in air. Although some of mechanical properties of the composite decreased with increasing MoS₂ amount, the MoS₂ was very effective in reducing the friction and wear of the composites. Particularly, the sintered 316L–15% MoS₂ materials at 1.1 PV value showed a reduction of friction coefficient by approximately 20–25% when compared to the sintered 316L specimen without addition of MoS₂. In addition, wear of specimen with addition of MoS₂ was also reduced to some extent (5–10% weight loss reduction) at this specific PV value.     

含硼水基润滑剂的制备及其性能研究

利用油酸、月桂酸、硼酸和醇胺合成出一种新型水基润滑剂,利用四球试验机考察了其摩擦学性能,并初步探讨了润滑剂的润滑机制。试验结果表明:由油酸、月桂酸复配后制备的油酸月桂酸醇胺酯的润滑性能优于油酸醇胺酯或月桂酸醇胺酯;硼酸可以显著提高它们的润滑性能,且当油酸、月桂酸、醇胺、硼酸的质量比为0.7∶0.1∶1∶0.6时,合成的水基润滑剂的润滑性能最佳。     

A comparison of oxidation and oxygen substitution in MoS2 solid film lubricants

Significant advancements in the production of low friction, long wear life, sputter-deposited MoS₂ lubricant coatings have been made in the last decade. The introduction of multi-layered coatings, the establishment of careful controls on doping during DC and magnetron sputter deposition, and the implementation of ion assisted deposition have resulted in lubricants with substantially longer wear lives (up to a factor of ten greater than in the early 1980s) and lower sliding friction coefficients. A major research effort, designed to improve the performance of solid lubricants, involved a number of laboratories during this time period, resulting in these major breakthroughs. However, even with this concentrated effort, the typical investigation involved making an educated guess, based on previous experience, of the deposition conditions, target compositions, or post treatments that might be expected to provide improved performance of resulting coatings. One notable discovery during this time period was that typical MoS₂ films contain large quantities (up to 20 atom %) of oxygen substituted for sulfur in individual crystal lattices. In this paper we will compare the effects of this oxygen substitution with the effects of oxidation which involves a change in the oxidation number of the central molybdenum atoms within the crystals. A discussion of the relationship(s) between chemistry and coating structure and tribological performance will be presented with emphasis on defect chemistry and multiple phase interactions. Speculations on the role of coating chemistry in determining coating performance in applications such as in ball bearings will be presented.     

Tribological properties of Ni3Al-Cr7C3 composite coating under water lubrication

The tribological properties of Ni₃Al-Cr₇C₃ composite coating under water lubrication were examined by using a ball-on-disc reciprocating tribotester. The effects of load and sliding speed on wear rate of the coating were investigated. The worn surface of the coating was analyzed using electron probe microscopy analysis (EPMA) and X-ray photoelectron spectroscopy (XPS). The results show the friction coefficient of the coating is decreased under water lubrication. The wear rate of the coating linearly increases with the load. At high sliding speed, the wear rate of the coating is dramatically increased and a large amount of the counterpart material is transferred to the coating worn surface. The low friction of the coating under water lubrication is due to the oxidizing of the worn surface in the wear. The wear mechanism of the coating is plastic deformation at low normal load and sliding speed. However, the wear mechanism transforms to microfracture and microploughing at high load with low sliding speed, and oxidation wear at high sliding speed. It is concluded that the contribution of the sliding speed to an increase in the coating wear is larger than that of the normal load.     

Effects of MoS2 concentration on friction

The effects of MoS₂ concentration on friction were investigated by using the ring compression test. An optimum concentration was found for friction. The optimum value is affected by specimen hardness and surface finish. A second optimum value also seems to exist for annealed (softer) 2011 aluminium alloy specimens.     

Direct visualization of sliding-induced tribofilm on Au/MoS<Subscript>2</Subscript> nanocomposite coatings by c-AFM

The nanoscale lubrication mechanism of nanocomposite Au/MoS₂ solid lubricant coatings has been studied by conductive atomic force microscopy (c-AFM). A direct visualization of the lubricating process suggests tribomechanical formation of a MoS₂ tribofilm to be a key mechanism. The sliding-induced tribofilm formation was visualized by a reduction in local friction and conductivity in nanoscale AFM images. The tribofilm was found to possess considerable crystallinity and orientation, which was not observed in the as-deposited coatings. The observed mechanism is broadly applicable to a range of nanocomposite metal/MoS₂ coatings.     

Morphological properties of sputtered MoS2 films

As the lubricating properties of MoS₂ are due to the sliding of lamellae, the crystallographic orientation and structural properties of these lamellae are important for the tribological behaviour. Thus an analysis of the growth morphology of sputtered MoS₂ films was carried out. It is shown that morphological properties can be influenced by the parameters of the deposition process; in particular, it is possible to prepare coatings in which the lamellae are oriented parallel to each other.     

Influence of Al2O3 reinforcement on the abrasive wear characteristic of Al2O3/PA1010 composite coatings

In the present study, the abrasive wear characteristics of Al₂O₃/PA1010 composite coatings were tested on the turnplate abrasive wear testing machine. Steel 45 (quenched and low-temperature tempered) was used as a reference material. The experimental results showed that when the Al₂O₃ particles have been treated with a silane coupling agent (γ-aminopropyl-triethoxysilane), the abrasive wear resistance of Al₂O₃/PA1010 composite coatings has a good linear relationship with the volume fraction of Al₂O₃ particles in Al₂O₃/PA1010 composite coatings and the linear correlation coefficient is 0.979. Under the experimental conditions, the size of Al₂O₃ particles (40.5-161.0 μm) has little influence on the abrasive wear resistance of Al₂O₃/PA1010 composite coatings. By treating the surface of Al₂O₃ particles with the silane coupling agent, the distribution of Al₂O₃ particles in PA1010 matrix is more homogeneous and the bonding state between Al₂O₃ particles and PA1010 matrix is better. Therefore, the Al₂O₃ particles make the Al₂O₃/PA1010 composite coatings have better abrasive wear resistance than PA1010 coating. The wear resistance of Al₂O₃/PA1010 composite coatings is about 45% compared with that of steel 45.