Effects of fullerene C60 on the friction and wear characteristics of ceramics in ethanol

For the combinations of a pin of Si₃N₄ and five kinds of disk, the friction and wear test was carried out in ethanol, and in ethanol containing C₆₀ particles (1 wt%). A topographical analysis was also performed on the micro-asperities of the wear surfaces to estimate the behavior of C₆₀ particles, and the degree of surface damage. As a results, the following facts were found. (1) The addition of C₆₀ particles in ethanol decreased the wear rates of such ceramic disks as Al₂O₃, SiC and TiC and of the mating Si₃N₄ pins. (2) The addition of C₆₀ particles decreased the mean coefficient of friction for SiC, Si₃N₄, and TiC disks. (3) The wear rates of pin and disk depended on the topographies of worn surfaces such as the mean depth of micro-grooves, the mean tip radius of micro-asperities and so on.     

Wear properties of Saffil/Al,Saffil/Al2O3/Al and Saffil/SiC/Al hybrid metal matrix composites

The purpose of this study is to investigate the wear properties of Saffil/Al, Saffil/Al₂O₃/Al and Saffil/SiC/Al hybrid metal matrix composites (MMCs) fabricated by squeeze casting method. Wear tests were done on a pin-on-disk friction and wear tester under both dry and lubricated conditions. The wear properties of the three composites were evaluated in many respects. The effects of Saffil fibers, Al₂O₃ particles and SiC particles on the wear behavior of the composites were elucidated. Wear mechanisms were analyzed by observing the worn surfaces of the composites. The variation of coefficient of friction (COF) during the wear process was recorded by using a computer. Under dry sliding condition, Saffil/SiC/Al showed the best wear resistance under high temperature and high load, while the wear resistances of Saffil/Al and Saffil/Al₂O₃/Al were very similar. Under dry sliding condition, the dominant wear mechanism was abrasive wear under mild load and room temperature, and the dominant wear mechanism changed to adhesive wear as load or temperature increased. Molten wear occurred at high temperature. Compared with the dry sliding condition, all three composites showed excellent wear resistance when lubricated by liquid paraffin. Under lubricated condition, Saffil/Al showed the best wear resistance among them, and its COF value was the smallest. The dominant wear mechanism of the composites under lubricated condition was microploughing, but microcracking also occurred to them to different extents.     

Tribological behaviour of SiC?TiC?TiB2 composites under unlubricated sliding conditions

The friction and wear behaviour of eight different SiC TiC TiB₂ composite materials, with a practically constant SiC:TiC ratio of 1 and an increasing amount of TiB₂, was determined comparatively in oscillating sliding tests at room temperature under unlubricated conditions. The influence of the relative humidity (RH) of the surrounding air was investigated in tests in dry, normal, and moist air. All tests were performed against SiC balls and Al₂O₃ balls as counterbodies. The friction was affected by RH but barely at all by the composition of the composites. The wear resistance of the composites was found to be improved considerably by addition of TiB₂ in the range 20–60%. The highest wear resistance of the system was found when Al₂O₃ was used as the counterbody material.     

Tribological properties of self-lubricating CMC/Al2O3 pairs at high temperature in air

Three different self-lubricating ceramic matrix composites (CMCs) were fabricated by hot-pressed sintering. They are: Al₂O₃-50CaF₂, Al₂O₃-20Ag20CaF₂, and Al₂O₃-10Ag20CaF₂. Tribological tests were performed at temperatures ranging from 20°C to 800°C in air using a pin-on-disk tester. The experimental results show that the addition of the solid lubricants CaF₂ and Ag can evidently reduce the friction coefficients of alumina between 200°C and 650°C but not at room temperature and the wear rate of disks and pins at elevated temperature. The improvements in the friction and wear properties of CMC were due to the formation of a well-covered solid lubricating film. However, breakdown of the lubricating films at 800°C resulted in high friction and wear. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tribological Performance of Diamond and Diamondlike Carbon Films at Elevated Temperatures

In this study, the authors investigated the tribological performance of diamond and diamondlike carbon (DLC) films as a function of temperature. Both films were deposited on silicon carbide (SiC) by microwave plasma chemical vapor deposition and ion-beam deposition processes. Tribological tests were performed on a reciprocating wear machine in open air (20 to 30% relative humidity) and under a 10 N load using SiC pins. For the test conditions explored, the steady-state friction coefficients of test pairs without a diamond or DLC film were 0.7 to 0.9 and the average wear rates of pins were 10^?5 to 10^?7 mm³/N·m, depending on ambient temperature. DLC films reduced the steady-slate friction coefficients of the test pairs by factors of three to five and the wear rates of pins by two to three orders of magnitude. Low friction coefficients were also obtained with the diamond films, but wear rates of the counterface pins were high due to the very abrasive nature of these films. The wear of SiC disks coated with either diamond or DLC films was virtually unmeasurable while the wear of uncoated disks was substantial. Test results showed that the DLC films could afford low friction up to about 300° C. At higher temperatures, the DLC films graphitized and were removed from the surface. The diamond films could withstand much higher tempera-lures, but their tribological behavior degraded. Raman spectroscopy and scanning electron microscopy were used to elucidate the friction and wear mechanisms of both films at high temperatures.     

In situ formation of tribologically effective oxide interfaces in SiC‐based ceramics during dry oscillating sliding

The tribological behaviour of single‐phase SiC as well as both SiC–TiC and SiC–TiC–TiB₂ composite materials sliding against aluminium oxide has been investigated at room temperature with regard to the formation of wear‐reducing interfaces. The experiments were carried out in dry air for reasons of excluding the strong influence of water vapour. The introduction of the titanium phases into the SiC microstructure reduces the system wear by more than a factor of ten. The coefficient of friction is only slightly reduced but stabilised with time. The relatively abrasive oxide interface in the Al₂O₃/SiC pairing is altered to a soft and malleable oxide interface in the Al₂O₃/SiC–TiC–TiB₂ pairing. The wear reduction is mainly caused by a change of the wear mechanism so that the formed soft oxide wear debris is transferred to the counterbody, thus shifting the sliding plane further into the layer. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tribological behaviour of steel‐alumina sliding pairs under boundary lubrication conditions

The tribological behaviour of oil‐lubricated steel‐alumina sliding pairs was investigated using a ball‐on‐disc tribometer at room temperature. Commercial bearing balls of 10 mm diameter were mated to 99.7% Al₂O₃ discs, and additive‐free mineral oil was fed into the contact area. The sliding speed and the applied normal load were varied, and the initial surface roughness of the Al₂O₃ disc was altered using different polishing and grinding procedures. The results showed that the surface roughness of the ceramic discs dominated the tribological behaviour under the given experimental conditions. The sliding speed as well as the normal load showed less effect on the friction behaviour, but the amount of wear depended strongly on the normal load. From the results it was concluded that improvement of the surface roughness and optimised surface machining of the ceramic material can be essential for improving the tribological performance for boundary‐lubricated steel‐ceramic sliding pairs.     

Fabrication of copper–TiC–graphite hybrid metal matrix composites through microwave processing

The significant requirements such as wear resistance and better tribological properties in addition to good electrical conductivity necessitate the development of copper-based advanced metal matrix composites for electrical sliding contact applications. Though the addition of graphite to copper matrix induces self-lubricating property, the strength of the composite reduces. The improvement in the strength of the composite can be achieved by reinforcing harder ceramic particles such as SiC, TiC, and Al₂O₃. In this paper, the development of hybrid composite of copper metal matrix reinforced with TiC and graphite particles through microwave processing was investigated. The effects of TiC (5, 10, and 15 vol.%) and graphite (5 and 10 vol.%) reinforcements on physical and mechanical properties of microwave-sintered copper–TiC–graphite hybrid composites are discussed in detail. Micrographs show the uniform distribution of reinforcements in copper matrix. Microwave-sintered composites exhibited higher relative density, sintered density, and hardness compared with conventionally sintered ones.     

WEAR PERFORMANCE OF MULTILAYER-COATED CARBIDE TOOLS

Three multilayer-coated carbides [two trigon-shaped inserts: Ti(C,N)/TiC/Al₂O₃ (T1), Ti(C,N)/ Al₂O₃/TiN (T2) and one 80°-rhomboid shaped insert: TiC/Al₂O₃/TiN (T3)] were used to machine a martensitic stainless steel at various combinations of cutting speed and feed rate without coolant to assess their wear performance. Significant nose wear and chipping/fracture of the cutting edge were the predominant failure modes affecting tool performance at higher speed conditions. Plucking of tool materials was the main rake face wear phenomenon observed on T1 grade insert with alumina as the top-layer coating when machining at the lower speed conditions. Attrition and plastic flow were the main wear mechanisms observed on the ceramic coating layers, with dissolution-diffusion being the probable wear mechanism of the tool grades where tungsten carbide substrate had direct contact with the flowing chip. The fitted statistical wear models revealed T3 grade insert with 80°-rhomboid shape as having the highest speed-feed capability, resulting in the highest material removal rate relative to T1 and T2 grade inserts with trigon shapes.     

Microtribological studies of two-phase Al2O3–TiC ceramic at low contact pressure

The two-phase Al₂O₃–TiC ceramic (AlTiC) has many applications. One of the most common uses of AlTiC is for data recording heads where it is used as a bearing surface to support the magnetic sensing elements. This is one of the examples where the ceramic can be used in MEMS. Using Linear Tape Open (LTO) drive and metal particle (MP) tape media as the experimental platform; the wear of the AlTiC at very low loads and for very smooth surfaces has been studied.X-ray photoelectron spectroscopy (XPS), Auger electron Spectroscopy (AES) and Atomic Force Microscopy (AFM) were employed to analyse the AlTiC surface changes during wear at a variety of environmental conditions. Under all experimental conditions, the results showed the TiC phase of AlTiC to have been oxidized to form a surface layer. This gave rise to classical oxidational wear of that phase; with the delamination of the TiO₂ to form pullouts on the AlTiC surface and subsequent three-body abrasive wear particles were produced. The rate of oxidation of the TiC and hence the rate of production of the three-body wear particles increases with atmospheric water vapour content. In the experimental system chosen for this investigation, this results in an increase in differential wear, and hence pole tip recession of the magnetic metal poles of the recording heads. Pole tip recession was shown to correlate with increase in oxidation rate and also increase with atmospheric water vapour content.The wear of the Al₂O₃ phase was probably due to micro-adhesive wear with a wear rate much lower than that of the TiC phase.     

Abrasive wear resistance of textured steel rings

The fundamental aim of the present research is to study the effect of dimple shape and area density on abrasive wear in lubricated sliding. The other aims are to recommend a method of obtaining the local linear wear of a textured ring on the basis of profilometric measurement and to analyse the changes in the surface topography of this ring with selection of parameters that could monitor the “zero-wear” process.The experiments were conducted on a block-on ring tester. The stationary block made from cast iron of 50 HRC hardness was ground. The rotated ground ring was made from 42CrMO4 steel of 32 HRC hardness. The rings were modified by a burnishing technique in order to obtain surfaces with oil pockets. Oil pockets of spherical and of drop shape were tested. The pit-area ratios were in the range: 7.5–20%. The tested assembly was lubricated by oil L-AN 46. Because of the great hardness of the co-acting parts the wear resistance test was carried out under artificially increased dustiness conditions. The dust consists mainly of SiO₂ and Al₂O₃ particles. Measurement of local microscopic ring wear was made using a three-dimensional scanning instrument. The tendencies of ring surface topography changes during wear were analysed. Various methods of obtaining the local wear value during a low wear process were proposed and compared. We found that a spherical shape of dimples was superior to a drop shape with regard to wear resistance of steel rings.     

Surface Abuse Associated with Finish Turning of Gray Cast Iron with Ceramic Tools

Machining trials were carried out on a G-17 cast iron using round and square-shaped pure oxide (≥99 vol. % Al₂O₃ + ≤ 1 vol. % ZrO₂), mixed oxide (70 vol. % Al₂O₃ + 30 vol. % TiC) and silicon coarbide whisker-reinforced (75 vol. % Al₂O₃ + 25 vol. % SiC) ceramic tools in order to study the extent of damage on the machined surfaces. G-17 is a BS 1452 (1977) designation and equivalent to ANSI/ASTM A48-83 grade 40 designation. The test results show that the round-shaped ceramic tools produced better surface finish and less damage than square inserts under the cutting conditions investigated due to their increased nose radius, which increased the tool-workpiece contact area during machining. The surface finish deteriorates with prolonged machining and an increase in the depth of cut. The hardness values of the surface layer were generally above the average hardness value of the base material due to the hardening of the surface layer as a result of the high pressure, compressive stress and temperatures generated at the cutting edge during machining. A higher rate of hardening was observed when machining with the mixed oxide and SiC whisker-reinforced ceramic tools due, perhaps, to the relatively high temperatures generated at the higher cutting speed (500 m/min.) used.     

Review of oxidational wear: Part I: The origins of oxidational wear

This is the first part of a two-part in-depth review of the oxidational wear of metals. It discusses the parallelism between the formation of an oxide film for dry contact conditions and of other surface films for lubricated contacct. Wear modes are unified into two major classes of mild and severe wear, including both lubricated both dry and conditions. Oxidational wear is a mechanism of mild wear in which protective oxide films are formed at the real areas of contact (during the time of a contact) at the contact temperature, T_cc. When the oxide reaches a critical thickness ξ, usually 1 to 3 μm, the oxide breaks up and eventually appears as wear particles. These oxides are preferentially formed on plateaux, which alternately carry the load - as they reach their critical thickness - and are removed. Temperature is important in determining the structure of the oxide film present, which in turn affects the wear properties of the sliding interfaces. Hence, this part of the review concludes with a thorough treatment of the thermal aspects involved during the sliding of a typical laboratory simulation of the oxidational wear of steel specimens without lubrication. This treatment shows how the general surface temperature (T_s) and the division of heat (?)_at the interface can be calculated and used, in conjunction with the measured wear rate (w), to give information about a possible surface model consisting of N contracts on the (thermally expanded) operative plateau, the height of the plateau being identical to the critical oxide film thickness (ξ) mentioned above.Part II outlines recent research to determine the oxidational constant, ie the activation energy and the Arrhenius constant, relevant to oxidational wear. It is found that the Arrhenius constant for oxidational wear is different from that for static oxidation tests. Some typical values of N, ξ and T_c are calculated from oxidational wear experiments. A new oxidational wear theory designed to take into account the oxide growth which occurs at the general surface temperature, T_s (where T_s < T_c) , whilst operation plateau is out-of-contact. This theory is most relevant to weat at elevated temperatures, where it is not permissible to assume that out-of-contact oxidation is negligible. After a brief review of the small amount of work done on the effects of partial oxygen pressures on oxidational wear, Part II concludes with a discussion of the possible connection between the general oxidational wear theory for dry contacts and the wear which occurs at lubricated contacts.     

Wear of ceramic nozzles by dry sand blasting

Monolithic B₄C, Al₂O₃/(W,Ti)C and Al₂O₃/TiC/Mo/Ni ceramic composites, which provided a reasonably wide range of mechanical properties and microstructure, were produced to be used as nozzles materials. The erosion wear of the nozzle caused by abrasive particle impact was compared with dry sand blasting by determining the cumulative mass loss of the nozzles made from these materials. Results showed that the hardness of the nozzle material plays an important role with respect to its erosion wear. On the nozzle entry bore section, the B₄C nozzle appears to be entirely brittle in nature with the evidence of large scale-chipping, and exhibited a brittle fracture induced removal process. While the erosion mechanism of Al₂O₃/TiC/Mo/Ni nozzle appeared to be a preferential removal of the metal binder followed by pluck out of the undermined Al₂O₃ and TiC grains under the same test conditions. On the nozzle center bore zone, the B₄C nozzle fails in a highly brittle manner, and there are lots of obvious micro-cracks and small pits located on this area. While the primary wear mechanisms of Al₂O₃/TiC/Mo/Ni nozzle is plowing and micro-cutting by the abrasive particles. Both types of material removal model seem to be occurred for the Al₂O₃/(W,Ti)C nozzle.     

The effects of iron particles on the friction and wear mechanisms of ceramics in ethanol

For the combinations of an Si₃N₄ pin and five kinds of ceramic disk (SiC, Si₃N₄, Al₂O₃, ZrO₂, TiC), a friction and wear test was carried out in ethanol and in ethanol containing iron particles (1 wt.%, average diameter d = 200 nm, D = 12 μm under cohered condition) under a load in the range 5.88–11.50 N, at a sliding velocity of 0.138–0.196 m s⁻¹. A topographical analysis was also performed on the microasperities of the wear surfaces to estimate the behavior of the iron particles, and the degree of surface damage. As a result, the following facts were found. (1) The addition of iron particles in ethanol decreased both the wear rates of SiC and TiC disks and the mating pins, and also decreased the wear rate of the Al₂O₃ disk but increased that of the mating pin. The addition increased the wear rates of both ZrO₂ and Si₃N₄ disks and the mating pins. (2) The average coefficients of friction with the addition of iron particles were greater than those without iron particles. (3) The wear rates of pin and disk depended on the topographies of wear surfaces and the wear index Γ.     

Wear of Ceramic Cutting Tools in Ni-Based Superalloy Machining

The machining performance of monolithic and composite silicon nitride and Al₂O₃-based cutting tools in continuous turning of Inconel 718 was examined. The character of tool wear has been found to vary, depending on the feed rate and cutting speeds. At a lower cutting speed, of 120 m/min, tool life is restricted by depth-of-cut notching, while at high cutting speeds (300 m/min), tools fail due to nose wear and fracture. The sensitivity of monolithic Si₃N₄ and Al₂O₃ to depth-of-cut notching was found to he significantly reduced with the addition of SiC whiskers, and to a lesser extent with TiC particulates. The ceramic composites also exhibited resistance to nose and flank wear that was higher than that of the monoliths. The internal stress distribution for the cutting tool has been calculated using the finite element method and is the basis for explaining fracture beneath the rake face. Cutting tool wear results are discussed in terms of chemical and mechanical properties of the ceramic tool material, abrasive wear, thermal shock resistance, and metal cutting conditions.     

Friction–wear behavior and tribochemical reactions of different ceramics in HFC-134a gas

The friction and wear properties of the ionic ceramics Al₂O₃ and ZrO₂, and the covalent ceramics Si₃N₄ and SiC rubbing against an Al₂O₃ ball in vacuum (10⁻⁵ Pa) and in CF₃CH₂F (HFC-134a) gas at 10⁴ Pa were investigated using a ball-on-disk type tribometer. Without exposure to air, the surface composition and chemical state of the wear tracks and debris on the disks were determined with X-ray photoelectron spectroscopy (XPS). It is found that HFC-134a gas significantly reduces the friction and wear of all the ceramic couples, and that the ionic ceramic pairs show lower friction and wear. On the other hand, metal fluorides and/or fluorine-containing organic compounds are detected on the sliding surfaces. The differences in the friction–wear behavior of the ceramics rubbing in HFC-134a gas may be due to the products of tribochemical reactions, which are dependent on the bond type of the ceramics.     

Sliding-pin shapes and lubricant thickness change on a thin-film magnetic disk

In developing hard disk drives, it is necessary to keep lubricant as thick as possible during operations. For this purpose, we studied lubricant loss under different-shape contact-sliders on thin-film magnetic disks by using transparent-pin sliding tests with a built-in ellipsometer. We compared sliding pins with spherical, flat circle, flat square, and double-flat-rail surfaces.We found that lubricant loss was smaller under flat pins than under the spherical pin, and the smallest under the double-flat-rail pin among flat sliders. The results show that the horizontal and vertical shapes of sliders must carefully be selected for contact recording systems.     

Influence of surface preparation on roughness parameters,friction and wear

The aim of the present research was to investigate influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear. First the correlation between different surface preparation techniques and roughness parameters was investigated. For this purpose 100Cr6 steel plate samples were prepared in terms of different average surface roughness, using different grades of grinding, polishing, turning and milling. Different surface preparation techniques resulted in different R_a values from 0.02 to 7 μm. After this, correlation between surface roughness parameters and friction and wear was investigated. For this reason dry and lubricated pin-on-disc tests, using different contact conditions, were carried out, where Al₂O₃ ball was used as counter-body. It was observed that parameters R_ku, R_sk, R_pk and R_vk tend to have influence on coefficient of friction.     

Role of the strengthening phases in abrasive wear resistance of laser-clad NiCrBSi coatings

The influence of the strengthening phases on the tribological characteristics (wear intensity, specific work of wear, coefficient of friction) and the wear mechanisms in two-body abrasion tests with abrasives of different hardnesses (corundum Al₂O₃, ~2000 HV and silicon carbide SiC, ~3000 HV) has been investigated for PG-SR2 (Cr₂₃C₆, 1000–1150 HV), PG-10N-01 (Cr₇C₃, 1650–1800 HV; CrB, 1950–2400 HV), and 75% PG-SR2 + 25% TiC (TiC, 2500–2900 HV; (Cr,Ni)₂₃(C,B)₆ and (Ti,Cr)(C,B), ~2000 HV) coatings. The dominant role of the strengthening phases (compared with the role of the metal matrix) in the abrasive wear resistance of laser-clad NiCrBSi coatings has been estimated. Different wear mechanisms have been identified and, accordingly, different levels of coatings wear resistance have been achieved depending on the ratio between the hardness of the strengthening phases (carbides, borides, carboborides) and abrasive particles.