Effect of densification and TiB2 reinforcement on the wear of molybdenum disilicide

Wear tests were done in a pin‐on‐disc machine by sliding MoSi₂ pins against hard‐steel discs in a normal load range of 5–140 N and a speed of 0.5 m/s under nominally dry conditions in the ambient. The specific wear rate of the pin undergoes two transitions: severe to mild at low load and mild to severe at high load. The mild‐wear domain is distinguished by the formation of a protective mechanically mixed layer of steel and its oxides, transferred from the counterface in particulate form. Increasing the hardness by densification and TiB₂ reinforcement lowers the specific wear rate and expands the mild‐wear load domain. However, even when the volume wear rate is normalised with respect to the real contact area (load/hardness) the non‐dimensional wear factor is still seen to decrease with densification and reinforcement. This indicates that fracture toughness may also play an important role in determining the wear‐resistance of these materials. The surface coverage on the pin by the mechanically mixed layer increases with densification and reinforcement. This revised version was published online in July 2006 with corrections to the Cover Date.     

The wear characteristics of ultrahigh molecular weight polyethylene against a high density alumina ceramic under wet (distilled water) and dry conditions

In recent years there has been growing interest in the use of high density alumina ceramic material for the femoral ball in association with ultrahigh molecular weight polyethylene (UHMWPE) for the acetabular component in total replacement hip joints.The wear characteristics of UHMWPE pins sliding against a high density alumina ceramic disc in the presence of distilled water in a tri-pin-on-disc machine have been revealed in very long-term experiments reported in this paper. A total sliding distance in excess of 6000 km was achieved and very low mean wear coefficients of the order of 10^?8 mm³ N^?1 m^?1 were recorded.Experiments were also carried out over a shorter sliding distance under dry conditions and the average wear coefficient of 2 × 10^?7mm³N^?1m^?1 was consistent with earlier findings. In these dry tests, comet-like streaks of polyethylene were transferred to the ceramic counterface, but no such transfer was noted during the wet tests. When distilled water was added to the test chamber after a considerable period of dry sliding, the wear coefficient rapidly decreased to about 10^?8 mm³ N^?1 m^?1 and the streaky transfer film disappeared from the ceramic counterface.The possibility of hydrodynamic action between the wear face on the pins and the counterface was investigated by reversing the direction of sliding. Surface topography changes on both the pins and the discs and friction and bulk temperatures of the pins were recorded throughout the tests.It is concluded that the excellent dry wear coefficients of UHMWPE sliding on alumina ceramic counterfaces are about twenty times greater than those experienced by the same materials in the presence of distilled water. The tribological advantage of the ceramic with respect to stainless steel having a similar surface roughness has been confirmed in dry sliding involving UHMWPE, but further work is required to determine whether or not the same advantage can be achieved under wet conditions.     

The role of specimen stiffness in sliding and impact wear

The results of pin-on-disc sliding tests and of impact wear tests are presented. Titanium alloy specimens were used for the sliding tests and high strength steel specimens were used for the impact tests: in both types of test stainless steel was the counterface material. The test duration, the nominal contact stress and the effective “stiffness” of the pins were varied; the effective stiffness was varied by changing the unsupported length of the pins while all other experimental conditions were maintained invariant. Experimental data on the wear track depth and roughness, obtained by profilometry, and on the specimen and counterface surfaces, which were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) techniques, are presented: the data include morphological features and material transport observations. Subsurface sections of specimens were analyzed by SEM, EDX techniques and transmission electron microscopy. Characteristic subsurface zones are identified and described with respect to both morphology and composition in the near-surface microstructure. The data clearly indicate that the stiffness is an important factor in experimental work in sliding as well as in impact wear.     

Effect of microstructure on the sliding wear performance of a Zn–Al–Ni alloy

Some observations pertaining to the sliding wear characteristics of a zinc–aluminium alloy containing nickel under varying material and test conditions have been reported in this investigation.

Dry sliding wear tests were conducted on as-cast and heat-treated zinc-based alloy pins using a pin-on-disc machine. A steel disc was employed as the counterface. Sliding speeds adopted were 0.42, 2.68 and 4.60 m/s while the traversal distance was fixed at 500 m. Wear tests were conducted at different pressures using separate pins in each case. Seizure pressure of the pins (prior to traversing the sliding distance of 500 m) was determined at each speed.

Wear rate and the extent of frictional heating increased with pressure and speed whereas seizure pressure practically followed a reverse trend. The wear rate versus pressure plot of the as-cast alloy pins assumed two slopes at the lowest speed wherein low slope (indicating the occurrence of mild wear situation) was noticed initially. This was followed by the attainment of a higher slope suggesting severe wear condition at increased pressures. At higher speeds, one slope only (identical to the higher slope at the minimum speed) was noted. Wear rate versus pressure plots of the heat-treated alloy pins followed a trend similar to the as-cast ones except that two slopes were noted up to the intermediate speed in the former case.

Heat treatment changed the as-cast dendritic structure of the zinc-based alloy into the one with an improved uniformity of the distribution of various microconstituents, the nickel containing phase remaining practically unaffected. Softening of the (as-cast) alloy was also observed as a result of the heat treatment. However, in spite of reduced hardness, the heat-treated alloy pins attained improved wear behaviour (i.e. reduced frictional heating and low wear rate) over the as-cast ones irrespective of the test conditions. This was attributed to a more uniform distribution of microconstituents and reduced cracking tendency of the alloy as a result of the heat treatment. The alloy pins also attained better seizure pressure in heat-treated condition comparing with the as-cast ones at all the speeds except the maximum for the same reasons. A reversal in the trend at the maximum speed was thought to be due to the over-softening of the already softened (heat-treated) alloy pins under the influence of large frictional heat generated at the (maximum) speed. Under the circumstances, the heat-treated alloy pins tended to adhere/fuse with the disc extensively while this tendency was relatively less for the as-cast ones in view of their higher hardness. Further, the extent of the negative influence of cracking tendency reduced allowing thermal stability to predominate the wear behaviour of the as-cast alloy pins in this case. The factor led to somewhat higher seizure pressure of the (as-cast) alloy pins at the maximum speed comparing with the heat-treated ones.

Low wear rates correlated with less damage to the worn surfaces and to the regions below the worn surfaces and finer debris formation. Seizure led to severe damage to the worn surfaces and to the regions below the worn surfaces while the debris formed was quite bulky and coarser.     

Dry sliding wear behavior of nano-sized SiC pins against SiC and Si3N4 discs

Micro- and nano-sized hot-pressed silicon carbide pins have been characterized by room-temperature unlubricated disk-on-pin tribological tests on hot-pressed silicon carbide and silicon nitride discs. The mean grain size was shown not to influence the steady state friction coefficient. The mean grain size clearly affected the disc wear rate: the finer was the grain size the lower was the disc wear rate. No impact of the grain size was observed on the pin wear rate. The basic wear mechanisms were grain fracture and fine abrasion. By depth-sensing indentation, it was shown that a possible explanation of the different wear behaviour between micro- and nano-sized silicon carbide are the values of mechanical properties, especially hardness, when they are measured on volumes scaling with the material microstructure.     

A disc-on-flat wear test under starved lubrication

The ‘disc-on-flat’ system has been used for testing wear under starved lubrication conditions. The rotating discs were made of a carbon steel, the surface finish of the discs being ground. Some of the discs were treated by ‘vibrogrooving’, to form regular microrelief. The flat counterface was made of a ground copper alloy. An industrial oil was used as a lubricant in a one-drop test procedure. Microreliefs of a non-touching design with a relative groove area of 34% caused less wear than microreliefs of a touching type with a relative groove area of 50%, whereas the wear caused by the ground disc was high, with seizure after 200–250 m of sliding distance. A one-drop disc-on-cylinder test also showed that the wear caused by discs with regular microrelief was considerably less than that caused by discs treated by conventional finishing (grinding, polishing).     

Wear of spheroidal graphite cast irons for tractor drive train components

This study was prompted by a desire to improve the wear resistance of power transmission components in rear axle drives on commercial farm tractors. Reciprocating wear tests were conducted under lubricated and non-lubricated conditions on three spheroidal cast irons which varied in strength and hardness (designated GGG450, GGG600, and GGG700). Hemispherically tipped steel pins (designated 42CrMoS4/41CrS4) were used as the sliders. Except for the selection of the test duration, test procedures were similar to those described in ASTM Standard Test Method G133 for linearly reciprocating sliding. Among the three cast irons tested, the harder and stronger the alloy, the lower was its wear rate. Wear factors were approximately four orders of magnitude lower for experiments lubricated with fresh, fully formulated oil. There was a linear relationship between the Brinell hardness of the alloys and the negative logarithm of the wear factors that were expressed in mm³/N-m. Wear of lubricated test pins was not measurable due to the presence of deposits; however under non-lubricated sliding, the ratio of the wear of the flat specimen to that of the pin decreased as the hardness of the flat specimens approached that of the pin specimen.     

The effect of phosphating on the friction and wear properties of grey cast iron

The friction and wear resistance of two commercial manganese phosphate coatings have been evaluated. Grey cast iron wear pins were treated by the two processes and were tested by sliding against a steel disc, under both lubricated and dry sliding wear conditions.Phosphating increases the sliding distance to scuffing as well as the scuffing load, whilst marginally reducing the coefficient of friction. No advantage was found in phosphating dry sliding surfaces.Phosphating reduces the likelihood of adhesive wear in marginal or poorly lubricated sliding couples. The choice of phosphate coating is primarily dependent on the surface finish of the sliding counterface; thin coatings are suitable if the counterface is smooth but thicker coatings are superior against rougher surfaces.     

Wear behavior of Ni–P and Ni–P–Al2O3 electroless coatings

In this research, hardness and wear resistance of two types of electroless coating have been investigated including Ni–P and Ni–P–Al₂O₃ coatings. These coatings were applied on AISI 1045 steel discs by electroless deposition process and then they were heat treated at 200, 400 and 600 °C for 1 h. Wear resistance of deposits was measured by the pin on disc method and wear surfaces and debris were studied by scanning electron microscopy (SEM). Also, microstructural changes were evaluated by X-ray diffraction (XRD) analysis.The results showed that the existence of alumina particles in Ni–P coating matrix led to an increase in the hardness and wear resistance of the deposits. It was also found that heat treated coatings at about 400 °C have the maximum hardness and wear resistance.     

On the feasibility of using cryo‐treatment as a tool to enhance the abrasive wear behaviour of solid‐lubricated polymeric composites

Cryo‐treatment, a bulk modification technique, is fast emerging as a way with which to improve the wear resistance of metals. This technique has also shown the ability to enhance significantly the abrasive wear performance of some polymers and their short glass‐fibre reinforced composites. In this work, short carbon‐fibre reinforced composites of some heat resistant polymers, such as polyetherimide, polyethersulphone, polyamide 6,6, polyetheretherketone, and polytetrafluoroethylene, were selected to explore the potential of cryo‐treatment. The selected materials were cryogenically treated by cooling them to the temperature of liquid nitrogen. The abrasive wear tests were carried out at ambient temperature in single pass conditions at various loads, on a pin‐on‐disc machine, using silicon carbide paper as a counterface. The investigations revealed that this technique has definite potential to increase the wear performance of carbon‐fibre reinforced composites. An increase in hardness due to cryo‐treatment was thought to be responsible for an observed improvement in wear performance. However, the extent of improvement in the wear performance was not matched by an increase in the hardness value. Scanning electron microscopy proved useful in examining the morphological changes in the composites due to cryo‐treatment.     

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.     

Effect of heat treatment on wear behavior of HVOF thermally sprayed WC-Co coatings

A WC-17Co coating was deposited onto ST37 mild steel substrate using HVOF spray technique and then heat treated at different temperatures in a vacuum chamber. The coatings were then evaluated in the as sprayed and heat treated conditions. Inspections by SEM and phase analysis by XRD indicated that some brittle eta (η) phases were produced at high temperature heat treatments. Generation of these phases increased the coating's hardness and decreased fracture toughness of the coating. Tribological properties were studied under dry condition by using pin on disc machine and diamond metal matrix composite disc as counterface. Wear test results showed that as sprayed deposit had the best wear resistance and its wear mechanism was sharp cutting abrasion. The weight loss in heat treated samples increased by increasing heat treatment temperature and the wear mechanisms gradually changed from cutting to gouging abrasion.     

Counterface effects on the wear of a composite dry-bearing liner

Plastics-based dry-bearing liners used for flight control bearings in aircraft are usually mated against counterfaces of 440C stainless steel hardened to about 700 HV and finished to R_a ≈ 0.05 μm. In this paper experiments to examine the possibility of reducing liner wear by modifications to the counterface are described. Accelerated (pin-on-disc) tests were made against 440C stainless steel of varying hardness and roughness, electroplated with copper and cadmium, ion implanted with nitrogen, copper and cadmium, vacuum deposited with TiN and TiC, diffusion treated with nitrogen, boron, sulphur, Sn-Cu and Sn-Sb and coated with ceramics-cermets (Al₂O₃, Cr₂O₃, (Cr₂C₃)-Ni-Cr and WC-Co). The most important counterface properties influencing liner wear are the hardness and surface roughness, and for ceramic and cermet coatings, the harder and smoother the surface, the lower is the liner wear. No evidence was found to indicate that the chemical nature of the counterface has a major affect on the liner wear.     

A hip wear simulator with 100 test stations

A novel high-capacity hip wear simulator of the pin-on-disc type was designed, built, and validated. This so-called Super-CTPOD (circularly translating pin-on-disc) device has as many as 100 separate test stations, being an advanced version of the previously validated 12-station CTPOD. A validity test was done so that in all stations the specimens and the test conditions were as similar as possible. Hence, for the first time in this field, an adequate number of similar tests was done for a proper statistical analysis of wear data. The pins were conventional, gamma-sterilized ultra-high molecular weight polyethylene, and the discs were polished CoCr. The lubricant was diluted calf serum and the test length 3 million cycles. In the course of the test, the pins became highly polished, whereas the discs remained practically unchanged. The majority of the polyethylene wear particles were rounded, with a mean diameter of 0.25 microm. The 100 wear factor values computed from the 100 steady state wear rate values of the pins were normally distributed, the mean +/- 95 per cent confidence interval being 1.63 +/- 0.017 x 10(-6) mm3 /N m. The standard deviation was 5.4 per cent of the mean. There were no outliers. The wear mechanisms and the wear factor agreed well with clinical findings. Altogether, the Super-CTPOD test system was shown to be a unique combination of validity, low variation, capacity, efficiency, reliability, productivity, economy, ease of operation, and compact size.     

Microstructure and Wear Behavior of a Manganese Bronze Bearing Material under Unlubricated Conditions

In an attempt to collect information about the tribological performance of copper-based bearings, the friction and wear behaviors of C86300 manganese bronze were investigated. The characteristics of the base material were determined by structural and mechanical investigations. Then, dry sliding pin-on-disc wear tests were performed against an AISI 52100 steel counterface. After the wear tests, the worn surfaces of the pins and wear debris were studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. In addition, light optical microscopy and microhardness measurements were performed for examination of the steel counterfaces and worn pin subsurface layers. With increasing normal load, the wear rate of commercial C86300 alloy (containing 0.6 wt% Si) decreased initially and then began to increase. After reaching a maximum wear rate at the load of about 60 N, the wear rate decreased again with a further increase in the normal load. However, the wear rate of this C86300 alloy mainly decreased with increasing sliding speed. Adhesive and abrasive wear were the dominant wear mechanisms under the designed conditions.     

The role of microstructure in the impact wear of two aluminum alloys

The impact wear resistance of two aluminum alloys was investigated using flat-ended aluminum specimens impacted upon a stainless steel counterface. The counterface itself was held stationary in some tests (pure normal impact) and moved transverse to the normal impact direction in other tests (compound impact).The alloys investigated were aluminum-copper: 2011-T3 which was formulated for free-machining applications and 2124 which possessed very high fracture toughness. Thus, one alloy favors crack nucleation and growth, while the other suppresses these. A variety of tests were conducted with both alloys in compound impact loading. The peak impulsive stress was found to influence wear rates significantly; the relative sliding velocity is also an important parameter.Surface and subsurface microscopy were used to define operative wear mechanisms. With the 2011-T3 alloy, the characteristic subsurface features support the delamination theory of wear. With the 2124 alloys, subsurface features differ significantly. These features are discussed in the light of microstructural variations in the alloys.     

Wear mechanisms in electroless nickel coatings

Electroless nickel coatings are susceptible to severe adhesive wear against plain carbon steel under dry sliding contact, because the low interfacial and high surface free energies of this material combination produce highly compatible surfaces. Controlled heat treatment eliminates severe wear by inducing crystallization of amorphous nickel to Ni₃P, which is incompatible with steel. Electroplating the counterface with chromium also eliminates severe adhesive wear owing to the efficient barrier properties of its surface oxide but a stainless steel counterface exacerbates the problem because of its single-phase microstructure and the low durability of its oxide film. The reciprocating diamond scratch test gives a different ranking order of wear rate from the Falex, pin-on-flat and Taber tests, because of its dependence on fracture toughness as the dominant material parameter rather than adhesive transfer. Wear mechanisms observed in the investigation include adhesive transfer, oxidation, three-body abrasion, micromachining, plastic deformation and delamination. In general, no correlation is found between wear rate and hardness; this is primarily because of changes in the mechanism of wear effected by processing, counterface materials or sliding conditions.     

基于标准销—盘试验的石墨/硬质合金副滑动摩擦磨损特性研究

通过标准销—盘摩擦磨损试验对石墨与硬质合金副的滑动摩擦磨损特性进行深入研究。结果表明,硬质合金销表面有石墨转移膜形成,并发生了轻微的"抛光"磨粒磨损;提高法向载荷和滑动速度可增加石墨转移膜形成,从而降低摩擦因数;与石墨高速铣削试验的刀具磨损结果进行对比发现,标准销—盘摩擦磨损试验未能复制硬质合金刀具的严重磨损现象,说明该摩擦试验方法对于石墨切削摩擦行为的研究具有一定的局限性,需要进一步改进。     

Some observations on the wear of aluminium rubbing on steel

Wear tests on high pressure counterformal contacts between an aluminium alloy and a low carbon steel produced different regimes of wear depending on specimen geometry and whether the aluminium specimen was disposed as the stationary specimen or the counterface. Oxidative wear was maintained at high levels of unit load (200 MN m^?2) provided that the surfaces were not subjected to large-scale plastic flow. Wear rates were greatly increased when the amount of aluminium transfer onto the steel surface was substantial, and under these circumstances evidence of back transfer was observed.     

Effect of transfer film structure, composition and bonding on the tribological behavior of polyphenylene sulfide filled with nano particles of TiO2, ZnO, CuO and SiC

The tribological behavior of polyphenylene sulfide (PPS) filled with inorganic nano particles was studied. The fillers investigated were TiO₂, ZnO, CuO and SiC whose sizes varied from 30 to 50 nm. The polymer composites were compression molded with varying proportions of these fillers. Wear and friction tests were performed in a pin-on-disk configuration at a sliding speed of 1.0 m/s, nominal pressure of 0.65 MPa, and counterface roughness of 0.10 μm Ra. The polymer composite pins slid against hardened tool steel counterfaces. The transfer films of the composite materials formed on the counterfaces during sliding were studied by optical microscopy and X-ray photoelectron spectroscopy (XPS) and the adhesion between the transfer film and counterface was measured in terms of the peel strength. It was found that the wear rate of PPS decreased when TiO₂ and CuO were used as the fillers but increased with ZnO and SiC fillers. The optimum wear resistance was obtained with 2 vol.% CuO or TiO₂. These filled composites had the coefficients of friction lower than that of the unfilled PPS. The wear behavior of the composites is explained in terms of the topography of transfer film and adhesion of transfer film to the counterface as observed from peel strength studies. There is a good correlation observed between the transfer film-counterface bond strength and wear resistance.