Tribological Characteristics of Nitrogen (N+) Implanted Iron

The effect of implantation of nitrogen ions (1.5 MeV) on the friction and wear characteristics of pure iron sliding against M-50 steel (unimplanted) was studied in a pin-on-disk sliding friction apparatus. Test conditions included room temperature (～25°C), a dry air atmosphere, a load of ½ kg (4.9 N), sliding velocities of 0.043 to 0.078 m/s (～15 to 25 rpm), a pure hydrocarbon lubricant (n-hexadecane), or a USP mineral oil and nitrogen ion implantation doses of 5 × 10¹⁵ and 5 × 10¹⁷ ions/cm².

No differences in wear rates were observed in the low-dose (5 × 10¹⁵ ions/cm²) experiments. In the high-dose experiments (5 × 10¹⁷ ions/cm²), small reductions in initial (～40 percent) and steady-state (～20 percent) wear rates were observed for nitrogen-implanted iron riders as compared with unimplanted controls. No differences in average friction coefficients were noted for either dose.

Auger electron spectroscopy combined with argon ion bombardment revealed a subsurface Gaussian nitrogen distribution with a maximum concentration of 6 atomic percent at a depth of 8 × 10^?7 m (0.8 μm). Similar analysis within the wear scar (～2.0 × 10^?5 m subsurface) of an implanted rider after 20 μm of wear yielded only background nitrogen concentration. No inward migration of nitrogen ions was observed.     

Comparison of the effects of the lubricant-molecule chain length and the viscosity on the friction and wear of diamond-like-carbon coatings and steel

For steel contacts it is usual for the longer molecular chain lengths of saturated linear hydrocarbons and their acids and alcohols to reduce the coefficient of friction in the boundary-lubrication regime. However, the effect of these lubricant properties on DLC contacts is still unknown. Since the boundary-lubrication mechanisms between DLC coatings and conventional additives do not appear to be as effective as with metals, other potential mechanisms, even though based on weaker interactions or the oil's physical and rheological properties, may thus be very relevant. In this study we focus on the influence of the base oil's chain length and viscosity on the friction and wear in DLC/DLC contacts, and we compare this behaviour with conventional steel/steel contacts, using several simple linear hydrocarbons, i.e., alkanes, and complex branched hydrocarbons, i.e., polyalphaolefins. The results show that in both the steel/steel and DLC/DLC contacts the wear decreases with a longer molecular chain length and a higher viscosity of the oil. However, in DLC/DLC contacts the coefficient of friction increases when oil with a longer molecular chain length or a higher viscosity is used, and decreases with the lower oil viscosity and shorter chain lengths, which is just the opposite to conventional steel/steel behaviour. These results are analysed and discussed in view of lubricant cohesive energy, surface tension, shear strength, viscosity and chain length.     

Tribological behavior of metal matrix composites

The wear and friction behavior of continuous graphite fiber reinforced metal matrix composites was investigated. Composite materials were tested against 4620 steel at 54 m s^?1 at room temperature in air without lubricant. The graphite fibers studied included rayon-, pitch- and polyacrilonitrile (PAN)-based fibers. Both high modulus and high strength PAN-based fibers were examined. The fibers were incorporated into copper- and silver-based alloys by means of a liquid metal infiltration technique. The results of this study indicate that the type of graphite fiber in the composite is the most significant factor in the wear and friction behavior of metal matrix composites. In some high modulus fiber tin-bronze composites the fiber fraction influences the wear rate but not the coefficient of friction. Neither the matrix alloy nor the composite tensile strength per se correlate with the friction and wear properties; however, there are specific trends for the various matrix alloys.     

Promotion of Anti-Wear Property of Sodium Stearate by Oxygen and Surface Active Agents

The anti-wear properties of a water-based lubricant were studied using a newly designed friction tester which enabled the rubbing with nascent surface under controlled atmospheres. Sodium stearate considerably reduced the wear of steel in oxygen rich atmospheres.

Supplementary adsorption tests were performed to identify the anti-wear film formed on the rubbed surface. Surface analysis by Fourier transform infrared spectroscopy (FTIR) revealed that metal stearate film was heavily formed on the rubbed surface in oxygen rich atmospheres. The measurement of metal elements in the protective film by atomic absorption spectroscopy clarified that the main constituent of the film was iron stearate. These results of adsorption tests could support that iron stearate might prevent the wear of steel in the wear test.

It was observed that a surface active agent prevented the formation of lubricative oxide film, but when used with sodium stearate, it synergistically promoted the anti-wear property of sodium stearate. This was attributed to the fact that the surface active agent improved the permeability of recrystallized sodium stearate to the rubbing area.     

Experimental Evaluation of Friction and Wear Properties of Solid Lubricant Coatings on SUS440C Steel in Liquid Nitrogen

The friction and wear properties of the prevailing different solid lubricant coatings (Ion-plated Au, Ion-plated Ag and RF-sputtered PTFE on SUS440C stainless steel) used in the bearings of high-speed cryogenic-turbo-pumps of liquid rocket engines were experimentally evaluated in liquid nitrogen immersed conditions. Also the above experiments were carried out with two newly proposed solid lubricant coatings of sputter-ion-plated MoSTi and a new ion-plated Pb on SUS440C stainless steel. The friction coefficient and wear rates of the coatings of ion-plated Au, ion-plated Ag, RF-sputtered PTFE, the new ion-plated Pb and MoS₂Ti-SIP (with coating thickness of 0.7±0.1 μm) on SUS440C steel against SUS440C stainless steel ball in liquid nitrogen were compared. Worn surfaces were examined microscopically with a microscope and a profilometer for understanding the mechanisms of friction and wear and transfer film lubrication in liquid nitrogen. It is found that the newly proposed solid lubricant coatings are showing promising results for their use in liquid nitrogen immersed conditions. The sputter-ion-plated MoSTi coating on SUS 440C steel shows a minimum value of friction coefficient (μ=0.015) and wear rate (w_c=0.56 × 10⁻⁶ mm³/N m ) in liquid nitrogen.     

Wear of metals at high sliding speeds

High speed sliding wear of AISI 1020 steel, AISI 304 stainless steel and commercially pure titanium (75A) was studied using a pin-on-ring geometry. All the tests were carried out in air without any lubricant. The sliding speed was 0.5–10.0 m s^?1 and the normal force was 49.0 N (5 kgf).The friction coefficient of all the materials tested decreased with the sliding speed; this appears to be a consequence of oxide formation. The wear rate of 304 stainless steel increased monotonically with speed, whereas the wear rate of 1020 steel and titanium first decreased and then increased and again decreased, with a maximum occurring at about 5 m s^?1. The complex variation of the wear rate as a function of speed is explained in terms of the dependence of the friction coefficient, hardness and toughness of the materials on temperature. Microscope examinations of the wear track, the sub-surface of worn specimens and the wear particles indicate that the wear mode was predominantly by subsurface deformation, crack nucleation and growth processes, i.e. the delamination process, similar to the low speed sliding wear of metals. Oxidative and adhesion theories proposed in the past to explain the high speed sliding wear of metals are found to be incompatible with the experimental observations.     

Tribological Properties of Ion-Implanted 52100 Steel

Titanium, argon, nitrogen, and iron were implanted in separate strips on a ferritic AISI E52100 cylinder. Three implant energies were chosen to obtain a continuous distribution from the surface to a depth of 120 nm and total doses were 4.10¹⁷ ions cm^?2. The friction coefficient and wear track topography were measured by sliding against a martensitic AISI 52100 steel ball in air, in a fully formulated lubricant and in highly purified hexadecane. In dry sliding, titanium reduced the friction coefficient to 0.32 and suppressed the near-surface cracking, [in agreement with previous work at Harwell and Naval Research Labs]. In hexadecane, titanium reduced the friction coefficient to 0.22, iron and nitrogen increased, it to 0.7 from 0.55, and all implants decreased wear. In the fully formulated lubricant, only the break-in pattern was modified. Auger measurements showed that carbon penetrated the material with all implants, creating TiC in the Ti-implanted strip and probably strengthening the other implanted areas by carbide precipitates and compressive stresses.     

无机盐阴离子对果胶水基润滑液减摩性能的影响

利用微摩擦磨损试验机评价果胶作为水基润滑添加剂的摩擦学性能,采用扫描电子显微镜分析磨痕表面形貌,并采用能量色散光谱仪分析摩擦磨损机制。同时还考察工业用水中存在的阴离子对含果胶水基润滑液摩擦学性能的影响。结果表明:在去离子水中加入少量的果胶就可显著改善水的摩擦学性能,并且随着果胶添加量的增大,减摩性能得到进一步提高,这主要是由于果胶在表面吸附形成的润滑膜起到良好的减摩效果;不同阴离子对果胶的摩擦学性能具有不同的影响,其影响的实质是盐析作用和吸附作用。     

Transfer Solid Lubrication of Aluminum Sliding Contacts

The effects of transfer from solid lubricant sticks of unfilled, glass-filled, and bronze-filled PTFE on the room-temperature wear and friction of trailing primary contacts of aluminum (6061 T6) rods in repetitive intermittent contacts were investigated in a ring-on-rod configuration. The materials of the ring countersurfaces upon which the solid lubricants transferred and against which the trailing aluminum rods wore included steel, aluminum, copper, and an oxide dispersion-strengthened copper alloy. This sliding of the unlubricated copper ring countersurfaces against the aluminum led to the roughening of the copper as large (> 1 mm) aluminum particles embedded themselves upon the countersurface, with consequent transitions in the aluminum wear rate and the coefficient of friction to values exceeding 6 × 10^{? 3} mm³/Nm and 0.6, respectively, after an incubation period of several initial contacts of lower wear rate and friction. The other ring countersurface materials resulted in similarly high aluminum rod wear rate and coefficient of friction, more nearly from the onset of sliding. The application of unfilled PTFE solid lubricant transfer reduced the aluminum's gouging of the copper countersurfaces and correspondingly reduced the aluminum rod wear rate and the coefficient of friction against the copper, as well as against all other countersurface materials, towards 2 × 10^?3 mm³/Nm and 0.3 or less, respectively. Glass- and bronze-filled PTFE transfer lubricants provided reductions in the wear rate of the aluminum rod comparable to or in some cases better than the unfilled PTFE, though the unfilled PTFE transfer lubricant in several cases provided better friction reduction.     

Effect of the structure and metal atom of dialkyldithiophosphate derivatives on the wear behaviour in steel‐zirconia contacts

Wear investigations concerning the different structures and metal atoms of metal dialkyldithiophosphates (MeDTPs) were conducted using a ball‐on‐disc apparatus. Steel ball bearings (3.175 mm in diameter) and counterface discs, made of hot pressed ZrO₂ partially stabilised by Y₂O₃, were used. The synthesised MeDTPs were made up of primary linear aliphatic alcohols with hydrocarbon lengths varying from 8 to 16 carbon atoms, and contained the following metals: zinc(II), iron(III), gallium(III), antimony(III), and copper(II). Tests were performed at room temperature. The sliding speed was set to a constant 0.03 m/s, and a constant 30 N load was used. The additives investigated were used as solutions in n‐hexadecane. The study also focused on the influence of additive concentration on wear. It was found that the effectiveness in reducing wear depends both on the metal atom and on the length of the hydrocarbon chain in the additive's molecules. The lowest wear volumes were observed for additives with n‐octyl and n‐tetradecyl hydrocarbon chains. The worst antiwear performance was observed for n‐dodecyl derivatives. For almost all additives, more concentrated solutions resulted in less wear. Some friction coefficient results are also presented.     

Bio-Fuel-Contaminated Lubricant and Hardening Effects on the Friction and Wear of AISI 1045 Steel

A study has been made on the wear and friction of hardened AISI 1045 steel using a tri-pin-on-disc type of friction and wear apparatus. During the investigation the linear pin wear, coefficient of friction and rise in pin specimen temperature were monitored and wear and friction curves plotted. Wear surfaces and mechanisms were investigated by means of optical microscopy. Analysis of used lubricating oil was performed using FTIR spectroscopy.

It was shown that the wear rate, type of wear and friction coefficient were influenced by contaminating the lubricant with bio-fuel as well as the surface hardening treatment. Corrosive wear and pits on the specimen surface were found when plain bio-fuel was used as lubricating oil. The results also confirmed that better wear resistance was obtained from the surface-hardened steel specimen with 4 % bio-fuel-contaminated lubricant.

Results from this study will be useful in material selection for tribological components in diesel engines running on vegetable fuel.     

An Investigation into Lubrication and Oxide Breakdown During Load-Carrying Capacity Testing

Changes occurring in the friction and wear mechanisms during a load-carrying capacity test, lubricated with cetane containing a carboxylic acid, were investigated. The changes in wear scar/track appearance and oxide coverage/composition were analyzed during every load stage.

The main conclusions were as follows:

• The breakdown in the protective oxide layer formed on the opposing steel surfaces was found to be the prerequisite for initiation of seizure.

• The seizure load achieved during load-carrying capacity testing quantifies the ability of the test fluid to prevent transition to the adhesive wear regime.

• The most severe surface damage was found to occur during the first few seconds after test initiation. Desorption of the adsorbed lubricant film and the subsequent removal of the naturally occurring thin oxide layer results in the initial period of adhesive wear.

• Partial recovery to a state of acceptable friction occurs after the period of initial seizure. During this period, the surface coverage by the adsorption lubricant molecules and the oxide coverage are sufficient to prevent severe adhesive wear from occurring. Wear is primarily a combination of oxidative, abrasive, and fatigue wear (all possible in the regions of mixed friction and boundary lubrication).

• Final lubricant breakdown and eventual seizure are initiated when the oxide removal rate exceeds the oxide formation rate resulting in severe adhesive wear followed by seizure.

     

Chain length effects in boundary lubrication

The effect of additive chain length on friction coefficient and transition temperature is investigated under different test conditions. Carboxylic acids and normal alcohols with 12 – 18 carbon atoms in the chain were used as additives in n-hexadecane base lubricant. It was found that the magnitude of the friction coefficient decreases as the additive chain length is increased. Matching of the additive chain length with the base lubricant had no effect on the friction coefficient. Similarly, the transition temperature at low sliding speeds increased with increasing chain length and was not affected by chain matching. At higher speeds, the acids containing 16 and 18 carbon atoms did not exhibit a transitional behavior within the studied temperature range. These results are discussed in terms of the separation distance between the contacting surfaces, the intermolecular dispersion forces and the structure of adsorbed layers. It is concluded that, at very low speeds, boundary lubrication is controlled by adsorbed monolayers, whereas at higher speeds, where partial elastohydrodynamic conditions prevail, ordered multimolecular layers may influence lubrication.     

The Influence of Lubrication on Head and Disk Wear

Magnetic disks are usually lubricated with fluorocarbon-type lubricants to reduce head and disk wear during the start/stop process of the disk rotation. In this paper, the influence of disk lubrication on the tribological characteristics of the head/disk interface is investigated by pin-on-disk wear tests and the head/disk friction tests.

The anti-wear performance of a lubricant is very high. For example, a lubricant coating of 8.4 × 10^?5 mg/cm² exhibits 1/20 of the ferrite pin wear rate of an unlubricated disk. For a lubricated disk, ferrite pin wear decreases at increased sliding velocities as high as 10 m/s, while pin wear increases rapidly with increased velocity for an unlubricated disk. The lubricant used here performs well in suppressing the wear increase caused by increased load. Regarding friction characteristics, however, an excessive amount of lubricant induces severe head/disk sticking, causing head crash. With respect to head/disk sticking, the upper-limit of the amount of lubricant is 8.4 × 10^?5 mg/cm².     

Study of the tribological behavior of S-(carboxylpropyl)-N-dialkyl dithiocarbamic acid as additives in water-based fluid

There has been a growing concern for the use of water-based fluids because of the worldwide interest in environmental issues. This has promoted the research and use of water-soluble additives as environmental friendly lubricants. A kind of potential water-soluble additive, S-(carboxylpropyl)-N-dialkyl dithiocarbamic acid was prepared in this work. The friction and wear behaviors of the synthesized compounds as an additive in water-based liquid were evaluated with a four-ball tester and a ring-on-block rig. The wear scar morphology of the ball and the chemical nature of the antiwear films generated on the steel counter face were investigated with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It was found that the synthesized additives had excellent antiwear, load-carrying and extreme pressure performance. The additive reacted with the counter-face metal and generated a surface protective film consisting of FeS, FeS₂, FeSO₄ and an absorbed compound containing N and acid.     

Lubricity characteristics of ethyl hexyl esters of mono‐, Di‐, and triethenoic acids

Ethyl hexyl esters of oleic, linoleic, and linolenic acids were synthesised and thermally polymerised to obtain products with viscosities in the range of hydrocarbon lubricants at 100°C (11–22 cSt). Molecular weight, elemental analysis, IR, ¹³C NMR, and intrinsic viscosity data showed that most of these derivatives are mixtures of monomers, dimers, and trimers and have linear and cyclic products with predominantly trans characteristics. The lubricity characteristics were determined on a friction and wear tester under conditions of thick‐film lubrication. A comparison was made with hydrocracked hydrocarbon lubricants of comparable viscosities at 100°C and comparable viscosity indices. It is inferred that all the esters maintained relatively thicker surface films and much lower friction coefficients than the hydrocarbon oils. The wear‐scar data show that the antiwear characteristics of polymerised ethyl hexyl oleate and linoleate are only slightly inferior to those of the hydrocarbon oils, but at higher temperatures their antiwear characteristics rapidly deteriorate while the friction coefficients markedly increase and become comparable to those of hydrocarbon oils. These studies are being undertaken with a view to selecting and modifying vegetable oils containing mixtures of fatty acids for obtaining esters of outstanding friction, wear, and film‐forming characteristics.     

石墨干膜润滑剂在碳钢表面摩擦磨损性能试验研究*

传统油或脂润滑剂在极端工况环境下无法满足碳钢类零件的减摩要求,采用干膜润滑剂是提高极端工况环境下碳钢表面摩擦磨损性能的可行性方法。采用超声波分散方法制备以石墨粉末为基体的干膜润滑剂,使用压力喷涂技术使其沉积在碳钢试件表面,在端面摩擦试验仪中开展干摩擦和石墨干膜润滑剂润滑下摩擦磨损性能对比性试验研究。试验结果表明：石墨干膜润滑剂在碳钢表面的沉积效果较好,沉积的石墨干膜润滑剂具有较好的润滑性能,可以有效地保护碳钢表面不被过度磨损;喷涂石墨干膜润滑剂的碳钢试件的工作寿命随着压力载荷和主轴转速的增大而缩短,负载和滑动速度的联合作用会加速涂层向稳定方向的过渡;磨损过程中形成的微观润滑剂颗粒会形成颗粒流润滑,适当添加石墨颗粒粉末可能会延长润滑剂正常发挥减摩作用的时间。制备的石墨干膜润滑剂为碳钢在极端工况环境下的减摩提供了支持。     

High speed tribological properties of graphite fiber/ Cu-Sn matrix composites

High speed dry friction experiments of graphite fiber/Cu-Sn matrix composites against steel were conducted at sliding velocities up to 235 m s^?1. The composite samples were prepared by the method of liquid metal infiltration. It has been determined that the friction coefficient and the wear rate depend on the amount of tin in the matrix, orientation of fibers relative to the sliding surface, the sliding velocity, the graphite grain size and the degree of liquid metal infiltration within the fibers. The increase in tin content leads to a decrease in both friction and wear due to an increase in matrix hardness. Specimens tested with the fibers oriented perpendicular to the sliding surface exhibit better frictional behavior than those with fibers parallel to the sliding surface. Both friction coefficient and wear rate reach a minimum value at a velocity between about 120 and 180 m s^?1. Large graphite grain size and poor liquid metal infiltration within the fibers have a detrimental effect on wear.     

Influence of Polyfluo-Wax on the Friction and Wear Behavior of Polyimide/Epoxy Resin–Molybdenum Disulfide Bonded Solid Lubricant Coating

Polyimide/Epoxy resin–molybdenum disulfide bonded solid lubricant coatings (denoted as PI/EP-MoS₂) were prepared. The influence of polyfluo-wax (denoted as PFW) on the microhardness and friction and wear behavior of as-prepared PI/EP-MoS₂ lubricant coating was measured using a microhardness tester and a reciprocating ball-on-disc tribometer, respectively. The worn surfaces of the lubricant coatings were observed with a scanning electron microscope, and their wear rate was determined with a Micro XAM surface mapping microscope. Moreover, the transfer films formed on the counterpart steel ball surfaces were analyzed by X-ray photoelectron spectroscopy. Results indicate that the incorporation of a proper content of PFW filler is effective at improving the antifriction performance of the PI/EP-MoS₂ lubricant coating while maintaining better wear resistance. Moreover, the friction coefficient of the lubricant coating decreases with increasing content of PFW from 2 to 10%, and the one with a filler content over 6% PFW has a steady friction coefficient of 0.07. The improvement in the antifriction performance of the lubricant coating with the incorporation of the PFW filler is attributed to the excellent lubricity of homogeneously distributed PFW.     

Etched-pocket, dry-bearing materials

Novel, dry-bearing materials have been fabricated by photo-etching an array of very small, blind holes in metal surfaces and filling them with solid lubricant formulations. Friction and wear were assessed against a tool-steel counterface in reciprocating line-contact conditions. Preliminary experiments demonstrated feasibility with a range of substrates (bronze, steel, cobalt alloy) and lubricants (PTFE + Pb, MoS₂ + polyimide). Detailed studies were made with different copper alloys to examine the effects of substrate hardness and the distribution, size, spacing and depth of the etched pockets. Be---Cu containing PTFE + Pb exhibits wear lives over a range of stresses (Hertzian) to 375 MPa which are appreciably longer than those of a porous bronze composite containing the same lubricant. Wear rates of etched Be----Cu are also much lower than those of a typical, woven PTFE fibre-type of dry-bearing composite. Wear life is not, in general, limited by the depth of the etched pockets and several failure mechanisms have been identified.