Self-lubricating Mechanism Research Articles

Influence of Lubricants on Wear and Self-Lubricating Mechanisms of Ni3Al Matrix Self-Lubricating Composites

A research is conducted on the possible beneficial synergistic effects of multiple additives in self-lubricating composites for use in high temperature friction and wear-related mechanical assemblies. Dry sliding tribological tests of Ni3Al matrix self-lubricating composites (NMSCs) on a HT-1000 ball-on-disk high-temperature tribometer are undertaken against Si3N4 at 25-800 °C. The results show that the subsurface microstructures beneath wear scar of NMSCs change with addition of different lubricants, which have great effects on tribological mechanisms and tribological performances. NMSC with addition of MoS2 and Ti3SiC2 exhibits distinct subsurface microstructure beneath wear scar and excellent tribological performance among all samples.

Tribological properties of phenolic resin bonded abrasive tools filled with two inclusion complexes of β-cyclodextrin and sulfur-containing additives

In this work, three kinds of phenolic resin bonded abrasive tool specimens containing β-cyclodextrin or its two complexes with dialkyl pentasulfide and sulfurized isobutylene at different contents were prepared by a cold compression method, respectively. The tribological properties of these abrasive tool specimens at different speeds and loads were investigated under water lubrication. The results indicated that filling of the complexes could improve the friction and wear performance of the abrasive tools. The minimum friction coefficient and wear rate were obtained when the content of the filler was about 15–20 wt.%. The friction coefficient of the abrasive tool decreased with the increase of speed and load, while the wear rate showed a reverse trend. X-ray photoelectron spectroscopy (XPS) analysis was employed to investigate the self-lubricating mechanism of the abrasive tool. It was found that the additives were released along with the decomposition of the complexes. The enhanced tribological properties of these abrasive tools were believed to stem from the formation of a self-lubricating layer that constituted by sulfide film and carbon deposited film, which had significant influence on the phenolic resin transfer process from substrate to the counter steel surface, and also had functions of anti-friction, anti-wear, and an improvement of surface quality.

Tribological properties of NiCr–Al2O3 cermet-based composites with addition of multiple-lubricants at elevated temperatures

The tribological properties of NiCr-40wt% Al2O3 (NC40A) cermet-based composites containing SrSO4 and other lubricant (graphite, MoS2 and Ag) against alumina ball were evaluated to identify their self-lubrication mechanisms from room temperature to 800°C. The composites demonstrated distinct improvements in effectively reducing friction and wear, as compared to NC40A cermet. The best results were observed for NC40A–10SrSO4–10Ag composite, which exhibited satisfactory reproducibility of friction coefficient over a wide temperature range (200–800°C) through high temperature cyclic friction tests due to the formation of synergistic lubricating films SrAl4O7, NiCr2O4 and Ag on the contact surface.

A calculated model for the porosity effect of a self-lubricating composite on its lubrication property

The fluid lubricating film of a soft solid lubricant on a porous composite surface after a sliding process is analyzed by scanning electron microscopy. The results show that the self-lubrication mechanism of the porous composite can also result in a conventional hydrodynamic lubrication.A calculated model for porous self-lubricating composites with realistically rough surfaces is developed. The results show that the lubrication property is affected by the porosity. This effect does not change with the surface morphology even though the morphology promotes a hydrodynamic lubrication effect. The result indicates that the optimum porosity of the composite improves the lubricating properties. Therefore, the optimization of the composite microstructure can improve the lifespan of the mechanical component.

Research on Lubrication and Wear-Resistant Mechanism of Ni60A/MoS<sub>2</sub> Composite Coating

The influence of MoS2 lubrication phase on the tribological properties of the Ni60A/MoS2 composite coating was conducted on UMT-2 micro-wear testing machine (USA), discussing the self-lubricating effect and mechanism. The result shows that with the increasing content of MoS2, the friction coefficient of the coating which changed with the increasing content of the MoS2 presents firstly decreases then increases, and the value reach the minimum when the quality percent of MoS2 wrapped with Nickel is 35%. Low-friction property of the Ni60A/MoS2 composite coating is due to the forming of MoS2 lubricating film in friction surface. The decreasing of the friction coefficient of the coating is in proportion to the coverage area of MoS2 lubricating film.

Effect of Aluminum Addition to Cu-10wt%Sn Bearing on its Pin-on-Disc Wear Behavior

The objective of this research was to study effect of sintering time, sintering temperature and aluminum addition on pin-on-disc wear behavior of Cu-10wt%Sn bearing that was produced from powder metallurgy method. It was found that, for the as-sintered samples, the larger addition of aluminum, the greater reduction in density and hardness in all sintering conditions. Density and hardness of samples sintered at 900°C decreased with increasing sintering time. Density and hardness of samples sintered at 830°C increased with increasing sintering time. Maximum hardness was obtained when the samples were sintered at 900°C for 30 min or sintered at 830oC for 60 min. Heat treating the samples by annealing at 750oC for 1 h, and quenching in water improved the hardness of the samples alloyed with aluminum significantly. Hardness of 5wt%Al sample increased by 37% to 69.2 HV. Wear mode of samples without aluminum addition was abrasive with accumulation of α-phase wear debris on worn pin surface. For samples with aluminum addition, wear mode was abrasive-oxidative producing clean worn surface which was preferred for self-lubrication mechanism of this type of bearing.

Influences of SrSO<sub>4</sub> and Ag on High Temperature Tribological Properties of Spark-Plasma-Sintered ZrO<sub>2</sub>(Y<sub>2</sub>O<sub>3</sub>)-Al<sub>2</sub>O<sub>3</sub> Composites

ZrO2(Y2O3)-Al2O3-SrSO4 nanocomposites incorporated with and without Ag addition have been fabricated by spark plasma sintering (SPS) to evaluate their friction and wear properties in sliding against alumina ball from room temperature to 600 oC. X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy were used to investigate microstructure and self-lubrication mechanisms of nanocomposites after wear tests at different temperatures. The ZrO2(Y2O3)-Al2O3-SrSO4 nanocomposite exhibits low and stable friction coefficients of 0.2 to 0.3 and wear rates in the order of 10-6 mm3/Nm at high temperatures. With the addition of Ag into the composite, the intermediate temperature lubricating property is greatly improved. Plastic deformation of SrSO4 and sliver during sliding plays an important role in formation of lubricating films on worn surfaces of nanocomposites. These lubricating films reduce the friction and wear of the ZrO2(Y2O3)-Al2O3 matrix composites.

Study on Dynamic Friction Process of Sweat Self-Lubricating Composite at Elevated Temperature Based on CA Method

In the paper, a three-dimensional model is developed with cellular automaton method based on a series of experiments and the self-lubricating mechanism analysis. The model is used to investigate the dynamic friction process of sweat self-lubricating composite during high temperature sliding. The sliding friction coefficient and the distribution of lubricant on the worn surface of the composite are calculated out by using the cellular automaton (CA) model. The agreement between the computing and experimental results shows the model developed in the paper can be effectively used to study the friction behaviors of the sweat self-lubricating composite at high temperature.

Microstructure and tribological properties of ZrO2(Y2O3) matrix composites doped with different solid lubricants from room temperature to 800 °C

In this paper, spark plasma sintering is employed to synthesize a variety of self-lubricating ZrO2(Y2O3) matrix composites by tailoring the chemical compositions. The additives of BaF2, CaF2, Ag, Ag2O, Cu2O, BaCrO4, BaSO4, SrSO4 and CaSiO3 were incorporated into the ceramic matrix to evaluate their potentials as effective high-temperature solid lubricants from room temperature to 800°C by using a ball-on-block friction and wear tester in sliding against alumina ball. For a comparative study, ZrO2(Y2O3) matrix composites incorporated with and without graphite or MoS2 were also investigated under the identical test conditions. The relationships among composition, microstructure and tribological properties of the composites are investigated to identify their self-lubrication mechanisms. The unlubricated ZrO2(Y2O3)–Al2O3 (TZ3Y20A) ceramics have a friction coefficient of 1.15 and a wear rate in the order of 10−4mm3/Nm at 800°C, while the TZ3Y20A–SrSO4 composites exhibit a steady-state friction coefficient of less than 0.2 and a wear rate in the order of 10−6mm3/Nm from room temperature to 800°C. Surface fatigue and brittle fracture are considered as the dominating wear mechanism of unmodified TZ3Y20A ceramics at 800°C. However, for self-lubricating TZ3Y20A–SrSO4 composites, the formation, plastic deformation and effective spreading of SrSO4 lubricating films are the most important factor to reduce friction and wear rate over a wide temperature range.

Dry-sliding self-lubricating ceramics: CuO doped 3Y-TZP

Dense 8mol% CuO doped 3Y-TZP ceramics prepared by pressureless sintering at 1500°C exhibits a good wear-resistance (specific wear rate k<10−6mm3N−1m−1) and promisingly low friction (coefficient of friction f=0.2–0.3) when sliding against an alumina ball under unlubricated conditions. It was recognized that a self-lubricating mechanism is the most important contribution to the reduction of friction. During operation of the tribosystem, a soft interfacial patchy layer is generated in the contact area. As confirmed by calculations, based on a deterministic friction model, this soft interfacial patchy layer reduces friction. It was demonstrated that the presence of copper oxide is important for the formation of such an interfacial layer. The mechanism of the transition from mild to severe wear was also investigated. Detachment of a top layer in the wear track was proven to be the main reason for this tribological change.

HIGH TEMPERATURE TRIBOLOGICAL PROPERTIES OF SPARK-PLASMA-SINTERED Al2O3-SrSO4 SELF-LUBRICATING NANOCOMPOSITES INCORPORATED WITH AND WITHOUT Ag ADDITION

Spark plasma sintering (SPS) is employed to fabricate self-lubricating Al 2 O 3- SrSO 4 nanocomposites incorporated with and without Ag addition. The friction and wear properties have been evaluated using a high temperature friction and wear tester from room temperature to 600°C in dry sliding against alumina ball. X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) analyzer were used to investigate microstructure and self-lubrication mechanisms of nanocomposites after wear tests at different temperatures. Al 2 O 3- SrSO 4 nanocomposites with optimum compositional combinations exhibit low and stable friction coefficients of 0.22 and wear rates in the order of 10-5 mm3/Nm at high temperatures. At low temperature, the Ag additives in the composite form a discontinuous lubricating film to effectively reduce friction and wear. With increasing test temperature, plastic deformation of SrSO 4 during sliding plays an important role in formation of lubricating films on worn surfaces to reduce the friction and wear.

Synthesis and characterization of CrN/Mo 2N multilayers and phases of Molybdenum nitride

CrNx, MoNx, CrOx, MoOx and CrN/Mo 2N multilayer films were deposited by reactive sputtering. Characterization of the coatings was carried out using XRD, XPS, optical profilometer, and nano-indentation. The oxidation of the thin films was studied as a function of annealing temperature. Tribological characterization of the Cr 2O 3 and the Mo-oxides was carried out as a function of annealing temperature. The low friction behavior of MoOx and MoNx was demonstrated. The deposition of CrNx and MoNx was also characterized and multilayer deposition of CrN and Mo 2N was carried out. The hardness of Mo 2N and CrN and the multilayers were shown to lie in the 20–30 GPa hardness regime. The multilayers are stable up to 500 °C in air and up to 1000 °C in argon. Initial friction testing on the multilayer coatings showed an activated self-lubricating mechanism.

Elevated temperature diffusion self-lubricating mechanisms of a novel cermet sinter with orderly micro-pores

A special self-lubricating mechanism of a novel kind of cermet sinter consisting of orderly micro-pores has been investigated at elevated-temperature. Solid lubricant can be stored in these orderly micro-pores and can diffuse to frictional surfaces to provide lubrication. Experiments show that when soft metal, such as Pb or Ag, is used as the solid lubricant the frictional coefficient of the cermet can be decreased about 60–65% compared with no such additives. Because of the controlled porosity (e.g. less than 20%), the side effects of such a micro-pore structure on the material mechanical properties (e.g. crushing strength) can be very limited. Obviously, because of the excellent lubricating property and the impressive loading capacity, such a kind of self-lubricating cermet sinter will have extensive potential uses, such as for heavily loaded bearings in metallurgy equipments. By using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX), the differences between the diffusing processes of Pb and Ag are observed and discussed to give more details on this self-lubricating mechanism.

Self-lubricating mechanisms via the in situ formed tribofilm of sintered ceramics with CaF2 additions when sliding against hardened steel

Abstract Sliding wear tests against 45# hardened steel were performed on Al2O3/TiC ceramic composites with the additions of CaF2 solid lubricants. The friction coefficient and wear rates were measured. The wear surface features were examined by scanning electron microscopy. Results showed that the friction coefficient and the wear rate decreased with the increase of CaF2 content up to 10 vol%, with further increases in CaF2 content, the wear rate increased rapidly. The mechanisms responsible were determined to be an in-situ formed self tribofilm between the ring-block sliding couple. SEM observations showed that two types of tribofilms are formed on the wear surface depending on the CaF2 content. A dense self tribofilm with a smooth surface associated with small friction coefficient and low wear rate was formed by the releasing and smearing of CaF2 solid lubricants on the wear surface when with 10 vol% CaF2 content. This dense self tribofilm acted as solid lubricant film between the sliding couple, and thus significantly reduced the friction coefficient and the wear rate. Breakdown of the tribofilm on the surface associated with a large wear rate was observed on samples with 15 vol% CaF2 content. This is due to the large degradation of mechanical properties of the composite with higher CaF2 contents.

Self-lubricating behaviours of ceramic tools in dry cutting

An in situ reaction self-lubricating ceramic tool (i.e. Al2O3/TiB2) and a self-lubricating ceramic tool with the addition of solid lubricants (i.e. Al2O3/TiC/CaF2) were produced by hot pressing. The lubricating behaviour of these self-lubricating tools in dry cutting processes were studied. It was shown that self-tribofilms, which contributed to the decrease of the friction coefficient at the tool-chip interface, were consistently formed on the tool rake surface for these self-lubricating tools. The formation process and composition of these self-tribofilms were determined and the self-lubricating mechanisms were advanced. Cutting speed was found to have a profound effect on the self-lubricating behaviours. These self-lubricating tools can be used to reduce the friction coefficient between the tool-chip interface during dry cutting.

Self-lubrication mechanism via the in situ formed lubricious oxide tribofilm

In situ formation of tribofilms is indispensable to reduce the wear volume and friction coefficient in dry. These tribofilms must be robust even in working at high normal pressure and sliding velocity between work materials and tools. Although TiN and TiC ceramic coating films are frequently utilized as a protective coating for dies and cutting tools, they often suffer from severe, adhesive wearing in dry forming and machining. Chlorine ion implantation assists lubricious oxide film to be in situ formed during wearing. At the presence of chlorine atoms in the inside of TiN or TiC films, in situ formation of lubricous intermediate titanium oxides with TiO and Ti n O 2 n−1 is sustained to preserve low frictional and wearing state. The fundamental process of this self-lubrication mechanism on the tribofilms, is discussed together with microscopic analysis and observation of worn surface and wear debris. The self-lubrication process works well in dry machining in order to reduce the flank wear of cutting tools even in the higher cutting speed range up to 500 m/min.

Self-Lubrication of Chlorine-Implanted Titanium Nitride Coating

Chlorine implantation into TiN coatings decreases the wear loss and the friction coefficient. Even by low-dose chlorine implantation, the wear volume is decreased by three orders of magnitude or more, and the friction coefficient becomes <0.1. This self-lubrication mechanism is related to the presence and mobility of implanted chlorine atoms inside the columnar TiN microstructure. According to observations of chlorine-implanted TiN coatings using high-resolution transmission electron microscopy, the chlorine atoms are present in the damaged region, where TiN is composed of nanosized grains. From these data, a self-lubrication mechanism is proposed with chlorine catalyzing the oxidation of titanium and leading to the formation of some tribological reaction product.

Characterization of tribo-layers on self-lubricating plasma-assisted chemical-vapor-deposited TiN coatings

Recently, several new solid lubricants and modern lubrication concepts have been developed to achieve lower friction and wear and thus longer lifetime in severe tribological applications. The aim of this study is to characterize tribo-layers formed during ball-on-disc testing on low-friction, Cl-containing TiN coatings deposited by plasma assisted chemical vapor deposition and to clarify their formation mechanism. Characterization of the transfer layers was done by optical microscopy, optical profilometry, Raman spectroscopy, Auger electron spectroscopy and X-ray photoelectron spectroscopy. Differential scanning calorimetry was used to provide information on the chlorine-influenced chemical reactions of the coatings in ambient air. Iron oxide layers of a thickness in the nm-range have been found on low-chlorine containing TiN coatings (<3 at.% Cl) showing friction coefficients of approximately 0.8, whereas on TiN coatings with higher chlorine contents (>3 at.% Cl) rutile layers were preferably formed, resulting in friction coefficients below 0.2. This self-lubrication mechanism can be explained by the in-situ formation of easy-shearable titanium oxides in the contact zone in the presence of humidity and oxygen.

Friction behaviour of solid oxide lubricants as second phase in α-Al 2O 3 and stabilised ZrO 2 composites

The influence of metal oxide additives within alumina (α-Al 2O 3) and yttria-stabilised tetragonal zirconia (Y-TZP) matrices was studied with respect to the tribological behaviour of the composites. The solid lubricants CuO, ZnO, MgO, MnO 2 and B 2O 3 were added in sufficiently small quantities (1 or 5 wt.%) to preserve the mechanical properties of the matrix phase without increasing the specific wear rate. The additives selected were an attempt to provide the materials with a self-lubricating mechanism and therefore decrease the coefficient of dry-friction. The ceramics were prepared by means of wet-ball milling powders, compaction by isostatic pressing and densification by sintering. Near-full densities (98–99%) were obtained for all ceramics except that containing CuO (92%). Tribological behaviour of the various composite compositions was tested on a pin-on-disk assembly using both α-Al 2O 3 and ZrO 2 as counter surface. Friction reductions of almost 40% for the CuO composite was obtained while the influence of the other oxide additives were less pronounced.

Self-lubrication mechanism of chlorine implanted TiN coatings

Different from the conventional physical modifications, significant reduction of wear and friction in severe dry conditions can be accommodated to titanium nitride (TiN) coating via the chlorine ion implantation. High friction coefficient with μ=0.8–1.2 for the as-deposited TiN is reduced to be less than 0.2 at room temperature. Titanium mono-oxide (TiO) and oxides with oxygen deficiency or Magnèli phase with Ti n O 2 n−1 , were formed inside the wear track of Cl-implanted TiN coating. Due to the shear deformability of titanium mono-oxide and crystallographic shearing planes in this Magnèli phase, vicinity of the Cl-implanted TiN surface can be elasto-plastically deformed, resulting in reduction of shear stress, wear and friction. Micro-X-ray photoelectron spectroscopy (XPS) measurement as well as high-resolution transparent electron microscopy (HRTEM), were an effective tool to describe local surface reaction taking place inside and outside of the wear track. Oxidation process of TiN during wear is drastically changed at the presence of Cl-atoms on the surface. Cl-atoms diffuse from the inside of TiN to the surface to accelerate the formation of titanium oxides, and to escape out of the system together with oxide debris. Both wear volume and friction coefficient, are preserved to be as low as or lower than diamond like carbon (DLC) coatings. This preferable tribological property comes from self-lubrication mechanism of the Cl-implanted TiN due to significant change of surface reaction by the effect of Cl-atoms.