Influence of Sliding Speed on the Tribological Characteristics of Si3N4-hBN Ceramic Materials

The influence of sliding speed on the unlubricated tribological behaviors of silicon nitride–boron nitride (Si₃N₄-hBN) composites was investigated with two modes in air by a pin-on-disc tribometer. Using the upper disc–on–bottom pin test mode, as the sliding speed increased, the friction coefficient of the sliding pairs showed an upward trend; for example, from 0.18 at the sliding speed of 0.40 m/s to 0.54 at the sliding speed of 1.31 m/s for the Si₃N₄/Si₃N₄–20% hBN pair. The surface analysis indicated that a tribochemical film consisting of SiO₂ and H₃BO₃ formed on the wear surfaces of the Si3N4/Si3N4–20% hBN sliding pair at sliding speeds of 0.40 and 0.66 m/s. Moreover, the formation of this film lubricated the wear surfaces. At the sliding speed of 1.31 m/s, no tribochemical film formed on the wear surfaces, most likely due to the increase in surface temperature. In the upper pin–on–bottom disc test mode, the wear mechanism was dominated by abrasive wear, and no tribochemical products could be detected on the wear surfaces. The increase in sliding speed weakened the degree of abrasive wear, leading to a decrease in the friction coefficients.     

Tribology characteristics of magnesium alloy AZ31B and its composites

In this paper, wear characteristics of magnesium alloy, AZ31B, and its nano-composites, AZ31B/nano-Al₂O_3, processed by the disintegrated melt deposition technique are investigated. The experiments were carried out using a pin-on-disk configuration against a steel disk counterface under different sliding speeds of 1, 3, 5, 7 and 10 m/s for 10 N normal load, and 1, 3 and 5 m/s for 30 N normal load. The worn samples and wear debris were then examined under a field emission scanning electron microscopy equipped with an energy dispersive spectrometer to reveal its wear features. The wear test results show that the wear rates of the composites are gradually reduced over the sliding speed range for both normal loads. The composite wear rates are higher than that of the alloy at low speeds and lower when sliding speed further increased. The coefficient of friction results of both the alloy and composites are in the range of 0.25–0.45 and reaches minimums at 5 m/s under 10 N and 3 m/s under 30 N load. Microstructural characterization results established different dominant mechanisms at different sliding speeds, namely, abrasion, delamination, oxidation, adhesion and thermal softening and melting. An experimental wear map was then constructed.     

Delamination wear of metal injection moulded 316L stainless steel

The wear behavior of metal injection moulded (MIM) stainless steels was studied using a pin-on-disc apparatus under dry sliding conditions. Pin specimens were MIM 316L stainless steel, while disc specimens were wrought 316L stainless steel. At low sliding speeds (0.2–0.6 m/s), the wear rates gradually decreased with increasing sliding speed, but then increased at high sliding speeds (0.6–2 m/s). The adhesive-induced delamination wear dominated at low sliding speeds, while abrasive-induced delamination wear dominated at high sliding speeds. At low sliding speeds, the surface densification occurred on the worn surface of pin specimens, hence no difference was found between the wear resistances of MIM pins containing 2% and 6% porosity. In contrast, the abrasive-induced delamination wear at high sliding speeds was enhanced by porosity; therefore the wear rates of MIM pins containing 6% porosity were higher than those of MIM pins containing 2% porosity.     

Friction and wear behaviour of plasma-sprayed Cr2O3 coatings against steel in a wide range of sliding velocities and normal loads

This study investigates the influence of sliding speed and normal load on the friction and wear of plasma-sprayed Cr₂O₃ coatings, in dry and lubricated sliding against AISI D2 steel. Friction and wear tests were performed in a wide speed range of 0.125–8 m/s under different normal loads using a block-on-ring tribometer. SEM, EDS and XPS were employed to identify the mechanical and chemical changes on the worn surfaces. A tangential impact wear model was proposed to explain the steep rising of wear from the minimum wear to the maximum wear. The results show that the wear of Cr₂O₃ coatings increases with increasing load. Secondly, there exist a minimum-wear sliding speed (0.5 m/s) and a maximum-wear sliding speed (3 m/s) for a Cr₂O₃ coating in dry sliding. With the increase of speed, the wear of a Cr₂O₃ coating decreases in the range 0.125–0.5 m/s, then rises steeply from 0.5 m/s to 3 m/s, followed by a decrease thereafter. The large variation of wear with respect to speed can be explained by stick-slip at low speeds, the tangential impact effect at median speeds and the softening effect of flash temperature at high speeds. Thirdly, the chemical compositions of the transfer film are a-Fe₂O₃ in the speed range 0.25–2 m/s, and FeO at 7 m/s. In addition, the wear mechanisms of a Cr₂O₃ coating in dry sliding versus AISI D2 steel are adhesion at low speeds, brittle fracture at median speeds and a mixture of abrasion and brittle fracture at high speeds. Finally the lubricated wear of Cr₂O₃ coating increases sharply from 1 to 2.8 m/s.     

Wear Behavior of Magnesium Alloy AZ91 Hybrid Composite Materials

The influence of hybrid reinforcements including silicon carbide and graphite particles with a size 37–50 μm on the wear characteristics of AZ91 magnesium alloy was studied. The dry sliding wear test was conducted using a pin-on-disc wear testing machine in the load range of 20 to 80 N at different sliding velocities in the range of 1.047 to 2.618 m/s. The results show that the wear resistance of composites was much better than that of the base matrix material under the test conditions. At a speed of 1.047 m/s and load of 40 N, the wear rate (mm³/km) of the unreinforced alloy was 6.3, which reduced to 3.8 in the case of 3% reinforced composite. The antiwear ability of magnesium alloy composite was found to improve substantially with the increase in silicon carbide and graphite content from 1 to 3% by weight and the wear rate was found to decrease considerably. At a speed of 1.047 m/s and load of 80 N, the wear rate (mm³/km) reduced from 11.8 to 9.1 when the reinforcement content increased from 1 to 3%. However in both the unreinforced alloy and reinforced composite, the wear rate increased with the increase in load and sliding velocity. An increase in the applied load increases the wear severity by changing the wear mechanism from abrasion to particle cracking-induced delamination. The worn surface morphologies of the composite containing 3% reinforcement by weight for the sliding velocity of 1.047 m/s were examined using scanning electron microscopy. Different wear mechanisms, namely, abrasion, oxidation, and delamination, have been observed.     

Relation Between Tribolayers and Wear Behavior During Dry Sliding of Fe-Al Hot-Dipped Coating

An Fe-Al coating consisting of FeAl and Fe₃Al was prepared on AISI 1045 steel by hot-dip aluminizing and subsequent high-temperature diffusion. Dry sliding wear tests were performed for Fe-Al coating against AISI 52100 steel under various sliding speeds and loads. During sliding, thin tribolayers formed on the worn surfaces of the Fe-Al coating. After wear, they were observed to be a nonoxidized mechanically mixed layer (MML) at 0.5 m/s, an oxide-containing MML at 0.75–2.68 m/s, and an in situ oxide layer at 4 m/s. The tribolayers presented a close relation with the wear behavior. Because of their different ingredients, structures, and types, the tribolayers resulted in significant changes in the wear behavior. At 0.75–2.68 m/s (except for 2.68 m/s, 40 N), the compact tribooxide layers exerted a protective function for Fe-Al coating to reduce the wear rate. However, for the tribolayers containing no or trace tribooxides at 0.5 m/s or the unstable ones formed at 2.68 m/s, 40 N and 4 m/s, no protection was presented. In these cases, the Fe-Al coating would be partly or totally ground off, thus presenting poor wear resistance at high wear rates.     

Frictional seizure of aluminium observed by X-ray imaging

Friction and wear of non-ferrous metals and coatings are of considerable interest in the light of use of the above in tribo-contacts. As most of the mechanisms governing the seizure of materials are concentrated at the center of the contact and are hidden, it is difficult to observe any precipitate changes in the behavior of such materials using conventional tools. In this work, we have used an X-ray microscope for in-situ observation of frictional seizure, wear and interfacial features during the testing of aluminium (AA1100) specimens sliding against Al 6061 disk. This technique enables the observation of interfacial features of the hidden contact despite the small density differences that exist between the sliding pair of materials. Seizure tests were conducted at different sliding speeds of 2 and 4 m/s. Aluminium was found to undergo incipient melting during sliding at a sliding speed of 2 m/s whereas the specimen tested at 4 m/s was found to undergo a severe plastic distortion thus restricting further testing of the same. Frictional heating of concentrated spots resulted in melting wear and diffusion assisted bonding of surface layers. This was later found to develop into a conformal contact following a lumpy transfer of material. The mechanisms of seizure and wear were affected by the sliding speeds during testing. At lower sliding speed (2 m/s) the transfer and bonding of deposits occurred due to direct contact of nascent sliding surfaces.     

Dry Sliding Friction and Wear Characteristics of Cu-Sn Alloy Containing Molybdenum Disulfide

The wear and sliding friction response of a hybrid copper metal matrix composite reinforced with 10 wt% of tin (Sn) and soft solid lubricant (1, 5, and 7 wt% of MoS₂) fabricated by a powder metallurgy route was investigated. The influence of the percentages of reinforcement, load, sliding speed, and sliding distance on both the wear and friction coefficient were studied. The wear test with an experimental plan of six loads (5–30 N) and five sliding speeds (0.5–2.5 m/s) was conducted on a pin-on-disc machine to record loss in mass due to wear for two total sliding distances of 1,000 and 2,000 m. The results showed that the specific wear rate of the composites increased at room temperature with sliding distance and decreased with load. The wear resistance of the hybrid composite containing 7 wt% MoS₂ was superior to that of the other composites. It was also observed that the specific wear rates of the composites decreased with the addition of MoS₂. The 7 wt% MoS₂ composites exhibited a very low coefficient of friction of 0.35. The hardness of the composite increased as the weight percentage of MoS₂ increased. The wear and friction coefficient were mainly influenced by both the percentage of reinforcement and the load applied. Wear morphology was also studied using scanning electron microscopy and energy-dispersive X-ray analysis.     

Layer-Graded Cu/B4C/Graphite Hybrid Composites: Processing,Characterization, and Evaluation of Their Mechanical and Wear Behavior

In this article, we synthesized and studied functionally graded multilayered Cu/B₄C/graphite hybrid composites. Two classes of layer-graded composites were considered: pure Cu layer with two layers consisting of different particle sizes and uniform particle volume and a pure Cu layer with a single additional layer. The properties of the layer-graded composites were compared to those of single layer composites of two different particle sizes (1–20 µm and 60–90 µm). The composites were tested for compression strength, flexural strength, hardness, density, and wear and braking performance at a range of sliding speed conditions (5, 10, 30, and 35 m/s). The microstructure of the interfaces in the layer-graded composites was characterized to determine the quality of bonding. We found that the layer-graded composites possess improved compression and flexural strength due to lower porosity and residual compressive stress in the composite layer aided by the work-hardening of the Cu layer. The presence of the ductile Cu layer improves the toughness and crack resistance properties of layer-graded composites by macrostructure toughening mechanism. The layer-graded composites possess improved wear resistance and braking performance at both low and high sliding speed conditions due to reduced third-body wear, oxidation, and softening of composites, aided by effective heat conduction through the Cu layer. Finally, the wear mechanisms operating at various speeds were discussed with the help of microscopic and X ray diffraction studies.     

Sliding speed-temperature wear transition maps for Si3N4/iron alloy couples

The superior high temperature resistance of silicon nitride (Si₃N₄) based ceramics makes them suitable for tribological applications above room temperature or in high speed unlubricated sliding. There are some published works on the wear behaviour of Si₃N₄/metal alloys. However, experimental data are shown in a form that is not of direct use for engineers involved in materials selection. In the present work, Si₃N₄ pins were tested against tool steel and grey cast iron on a pin-on-disc tribometer. Ceramics were produced by hot-pressing and tested without lubrication at variable temperature and sliding speed. SEM/EDS and XRD analysis were used for chemical and microstructural characterisation of worn surfaces and wear debris. At low speeds (0.05–0.5 m s⁻¹) and room temperature, Si₃N₄ surfaces are polished-like due to a combination of humidity-assisted tribo-oxidation and abrasive action of very fine wear debris. At high sliding speeds (2–3.5 m s⁻¹), as well as for temperatures in the range 400–600°C, an extensive coherent tribolayer mainly composed by iron oxides spreads over the ceramic surfaces. Polishing and protection by adherent tribolayers are the mechanisms responsible for observed severe and mild wear regimes, respectively. Wear maps are constructed showing the transition of wear regimes in Si₃N₄/iron alloys contacts determined by constant flash temperature curves. Equations for calculation of bulk and flash contact temperatures in tribocontacts between dissimilar materials are deduced.     

Lubrication of Ceramics by Tribopolymerization: A Designed Experiment to Determine Effects of Monomer Structure,Load, and Speed on Wear

An experimental study of ceramic lubrication by tribopolymerization at high loads and high speeds, using a pin-on-disk (fixed ball-on-flat) machine with alumina-on-alumina, is presented. In order to extend the range of applied loads and sliding velocities beyond those used in previous studies, a three-factor, two-level designed experiment was carried out to determine the effects of monomer structure, load, and speed on wear. Five monomers of widely varying chemical structure were used at one percent concentration in a hydrocarbon carrier fluid, hexadecane. They consisted of (a) one condensation-type monomer, a partial glycol ester of a longchain dimer acid, and (b) four vinyl-type addition monomers. Two levels of load (40 and 160 N) and speed (0.25 and 1.0 m/s) - each varying by a factor of four were used; thus the range of frictional heat generation was 16 to 1.

The results of this study were rather surprising and changed our thinking on the mechanism(s) by which monomers can act to reduce ceramic wear. For example, at low speeds — regardless of load — the monomers used were very effective in reducing wear, with reductions ranging from 44 to 98 percent depending on the monomer and load. However, at high speeds, the monomers were generally ineffective; in some cases, increases in wear were observed. This was unexpected.

Possible explanations for this behavior — including surface temperature effects and tribochemical reactions — are discussed. Results presented on Fourier Transform Infrared (FTIR) Spectroscopy of worn ceramic surfaces and wear debris show that the film-formation from the monomer solutions is complex, involving a combination of aluminum soap formation as well as evidence of oligomer/polymer formation in some cases, notably diallyl phthalate.     

Tribological Behavior of Aluminum Hybrid Composites Studied by Application of Factorial Techniques

This article analyzes the influence of graphite reinforcement, load, sliding speed, and sliding distance on tribological behavior of A356 aluminum matrix composites reinforced with silicon carbide and graphite using the full-factorial design. The wear rates of A356/10SiC composite material and A356/10SiC/1Gr and A356/10SiC/3Gr hybrid composites have been analyzed. The composites were obtained by a modified compocasting procedure. Tribological tests were performed on a block-on-disc tribometer without lubrication. The testing included sliding speeds of 0.25 and 1.0 m/s, normal loads of 10 and 20 N, and sliding distances of 300 and 900 m. The analysis of the obtained results was performed using the full-factorial method based on the signal-to-noise (S/N) ratio. The effects of load, sliding speed, weight percentage of graphite reinforcement, and sliding distance on the wear rate are 38.99, 17.87, 13.95, and 11.25%, respectively. The best tribological characteristics were exhibited by the A356/10SiC/1Gr hybrid aluminum composite.     

Wear characteristics of a diffusion bonded sintered steel with short term surface treatments

A pin on disc machine was used to investigate the tribological behavior of a diffusion bonded sintered steel, with and without surface treatments of steam oxidation and manganese phosphating, over a wide range of speed (0.2–4 m/s) and applied load (4–500 N) in conditions of dry sliding and starved lubrication by oil impregnation of the porous structure of the materials. Besides the calculated wear rates, the wear mechanisms were determined by examination of the components of the rubbing system (sintered pin, disc and generated debris). A transition from a mild to a severe wear regime was identified, denoted by sharp changes of the wear rate. A transient wear regime, interposed between the mild and severe wear regimes, was detected. The rubbing surface quality degradation was in terms of material displacement around the pin circumference due to a delamination wear mechanism. Such regime was detected for the base sintered steel in dry sliding at 1 m/s for the load range 60–80 N and for both surface treatments in oil impregnated sliding at 0.5 m/s for the load range 200–300 N. Oil impregnation of the base sintered steel expanded the mild wear regime towards higher loads throughout the whole sliding speed range compared to dry sliding. For the lower speeds of 0.2 and 0.5 m/s, manganese phosphated samples in dry sliding exhibited higher transition loads compared to the base sintered steel. The lower oil impregnability of the surface treated samples, due to the sealing of porosity by steam oxidation, led to slightly lower transition loads in oil impregnated sliding, compared to the base sintered steel.     

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.     

Dry sliding wear properties of unhybrid and hybrid Al alloy based nanocomposites

Abstract

Nanosize B₄C and/or MoS₂ particles reinforced AA2219 alloy composites were prepared using the stir casting process. The wear properties were evaluated for several speed (3.14–5.65 m s^?1), load (10–50 N) and distance (0–2500 m) conditions. The nanoparticles dispersion, density, wear resistance, morphology of the worn surface and loose wear debris were discussed in detail. The wear resistance improvement results by nanoparticle addition correspond well with the hardness. Between the nanocomposites, hybrid composites show significantly higher wear resistance for all load, speed and sliding distance conditions. The better wear resistance is attributed to the matrix strengthening by nanoparticles and the lubricant-rich tribolayer controlled wear in the hybrid composites. The intensity of abrasive, oxidation and delamination wear mechanisms decide the wear rate at any particular wear testing condition.     

Dry sliding wear characteristics of some industrial polymers against steel counterface

In this investigation, the influence of test speed and applied pressure values on the friction and wear behaviour of polyamide 66 (PA 66), polyoxymethylene (POM), ultrahigh molecular weight polyethylene (UHMWPE), 30% glass fibre reinforced polyphenylene-sulfide (PPS+30%GFR) and aliphatic polyketone (APK) polymers were studied. Friction and wear tests of PA 66, POM, UHMWPE, PPS+30%GFR and APK versus AISI D2 steel were carried out at dry condition on a pin-on-disc arrangement. Tribological tests were performed at room temperature at different pressures (0.35–1.05 MPa) and sliding speeds (0.5–2.0 m/s). The results showed that, for all polymers used in this investigation, the coefficient of friction decreases linearly with the increase in pressure. The specific wear rate for UHMWPE, PPS+30%GFR and APK were in the order of 10⁻⁵ mm³/N m, while the wear rate value for PA 66 was in the order of 10⁻⁶ mm³/N m. In addition to this, the wear rate value for POM was in the order of 10⁻³ mm³/N m. Furthermore, as the results of this investigation, the wear rate showed very little sensitivity to the applied pressures and test speed.     

Comparative Study on Wear Behaviors of Metal-Impregnated Carbon Material and C/C Composite Under Electrical Sliding

In this article, the wear behavior of metal-impregnated carbon materials (MIC) and carbon–carbon composites(C-C) was investigated using a self-made current-carrying wear tester producing an electrical current of 40–160 A and a contact speed of 10–50 m/s. The worn surfaces were observed by means of scanning electron microscopy (SEM), and a new parameter for current-carrying stability that describes the stability of the current as a function of wear was proposed. The results indicate that the wear rate of both materials tested increased with either an increase in electrical current or contact sliding speed. Compared to the metal-impregnated carbon material, the C-C composite material not only displayed superior wear resistance but superior current-carrying stability as well. With increasing electrical current, the current-carrying stability of the two materials changed within a narrow range at a speed of 20 m/s and decreased at a speed of 50 m/s. Wear failure was mainly due to electrical erosion occurring at high speed and high current.     

Monitoring of Wear Characteristics of TiN and TiAlN Coatings at Long Sliding Distances

TiN and TiAlN thin hard coatings have been widely applied on machine components and cutting tools to increase their wear resistance. These coatings have different wear behaviors, and determination of their wear characteristics in high-temperature and high-speed applications has great importance in the selection of suitable coating material to application. In this article, the wear behavior of single-layer TiN and TiAlN coatings was investigated at higher sliding speed and higher sliding distances than those in the literature. The coatings were deposited on AISI D2 cold-worked tool steel substrates using a magnetron sputtering system. The wear tests were performed at a sliding speed of 45 cm/s using a ball-on-disc method, and the wear area was investigated at seven different sliding distances (36–1,416 m). An Al₂O₃ ball was used as the counterpart material. The wear evolution was monitored using a confocal optical microscope and surface profilometer after each sliding test. The coefficient of friction and coefficient of wear were recorded with increasing sliding distance. It was found that the wear rate of the TiAlN coating decreases with sliding distance and it is much lower than that of TiN coating at longer sliding distance. This is due to the Al₂O₃ film formation at high temperature in the contact zone. Both coatings give similar coefficient of friction data during sliding with a slight increase in that of the TiAlN coating at high sliding distances due to the increasing alumina formation. When considering all results, the TiAlN coating is more suitable for hard machining applications.     

纳米级铜粉改善润滑油抗磨性能的研究

在ＭＨＫ－５００型环块摩擦磨损试验机上,研究了纳米级金属铜粉（直径在１０￣５０ｎｍ）加入到矿物油中的润滑性能。结果表明,在低中滴动速度下（滑动速度分别为１．２８５ｍ／ｓ和２．５７ｍ／ｓ）,加有纳米级铜粉的润滑油表现出优良的抗磨性能。通过ＳＥＭ扫描电镜对磨痕形貌进行分析,提出了低滑动速度和高滑动速度下,纳米级铜粉分别以“垫片”和“滚珠”形态参与润滑。     

Tribological Behavior of NiAl–1.5 wt% Graphene Composite Under Different Velocities

The progress in aerospace field requires a new NiAl matrix composite that can stand against wear and decrease the energy dissipation through decreasing friction. In this study, the tribological behavior of NiAl–1.5 wt% graphene composite is investigated at room temperature under a constant load of 12 N and different sliding velocities. The results show that the friction coefficient and wear rate increase with increasing sliding velocity from 0.2 to 0.4 m/s due to the adhesion between the sliding bodies and tearing of the graphene layer. The friction coefficient and wear rate tend to decrease at a sliding velocity of 0.6 m/s as a result of severe plastic deformation and grain refinement of the worn surface. However, at 0.8 m/s the friction coefficient reaches a minimum value and the wear rate increases and changes the wear mechanism to fatigue wear. It can be concluded that various wear mechanisms lead to different tribological performance of NiAl–1.5 wt% graphene composite.