Comparison of two test methods for evaluation of forming tool materials

Two test methods often used to simulate the tribological performance of forming tool materials are compared in this investigation through an evaluation of the friction and wear properties of four tool steels in dry sliding. One test (slider‐on‐flat‐surface (SOFS)) utilises a vertical disc sliding on a horizontal flat test surface, and the other (load scanner (LS) tests) utilises two crossed cylindrical rods. The test conditions were selected as equal as possible for the two tests, and the following conclusions are made. Somewhat unexpectedly, the friction and wear results differed substantially between the two tests. The SOFS test showed a better potential to evaluate wear resistance since one test sample is in continuous contact with the other. The LS test can generate higher contact pressures since the two rods contact each other in an unworn condition throughout the whole test stroke. LS indicate that two hard and smooth tool steels tested against each other generally give low friction and good galling resistance. The two hard couples tested sustained the highest loads without any detectable surface damage. For the same combinations of hard steels, SOFS gave a higher friction due to the wear of the disc. The carbides in the disc material resist wear better than the matrix and will consequently wear the disc by abrasion, which adds to the friction. The above conclusions are drawn from a rather limited examination using only one set of test parameters. In fact, the two tests are both very flexible as to the way they can be used, and they both have advantages and limitations in tribological studies. They should rather be considered complementary than competitive. Copyright © 2008 John Wiley & Sons, Ltd.     

Near-Edge X-ray Absorption Fine Structure Imaging of Spherical and Flat Counterfaces of Ultrananocrystalline Diamond Tribological Contacts: A Correlation of Surface Chemistry and Friction

A recently installed synchrotron radiation near-edge X-ray absorption fine structure (NEXAFS) full field imaging electron spectrometer was used to spatially resolve the chemical changes of both counterfaces from an ultra- nanocrystalline diamond (UNCD) tribological contact. A silicon flat and Si₃N₄ sphere were both coated with UNCD, and employed to form two wear tracks on the flat in a linear reciprocating tribometer. The first wear track was produced using a new, unconditioned sphere whose surface was thus conditioned during this first experiment. This led to faster run-in and lower friction when producing a second wear track using the conditioned sphere. The large depth of field of the magnetically guided NEXAFS imaging detector enabled rapid, large area spectromicroscopic imaging of both the spherical and flat surfaces. Laterally resolved NEXAFS data from the tribological contact area revealed that both substrates had an as-grown surface layer that contained a higher fraction of sp ²-bonded carbon and oxygen which was mechanically removed. Unlike the flat, the film on the sphere showed evidence of having graphitic character, both before and after sliding. These results show that the graphitic character of the sphere is not solely responsible for low friction and short run-in. Rather, conditioning the sphere, likely by removing asperities and passivating dangling bonds, leads to lower friction with less chemical modification of the substrate in subsequent tests. The new NEXAFS imaging spectroscopy detector enabled a more complete understanding of the tribological phenomena by imaging, for the first time, the surface chemistry of the spherical counterface which had been in continual contact during wear track formation.     

Wear transition diagram for silicon carbide

To obtain information on the tribological behaviour of silicon carbide at elevated temperatures, unlubricated ball-on-flat wear tests were conducted on sintered silicon carbide in self-mated sliding in air. The contact load was varied from 3.2 to 98.0 N, and a temperature range of 23°C to 1000°C was used. Scanning electron microscopy, Fourier transform infrared spectroscopy and energy-dispersive spectroscopy were used to elucidate the wear mechanisms. The results of the tests and observations were employed to construct a wear transition diagram, which provides a summary of tribological information including friction coefficient, wear coefficient and wear mechanisms as a function of temperature and load. The wear transition diagram for the sintered silicon carbide studied is divided into four regions plus one transition zone. At room temperature, under high loads and high environmental humidity, the tribological behaviour is controlled by tribochemical reactions between the silicon carbide surface and water vapour in the environment. Under low loads and at temperatures below 250°C, wear occurs by ploughing and polishing. At temperatures about 250°C and under low loads, tribooxidation and formation of cylindrical wear particles control the tribological behaviour. Wear occurs by microfracture when the load is increased above a critical value; and both the friction coefficient and the wear coefficient increase.     

Investigations on the friction force anisotropy of the silicon lattice

State of the art thin film technologies allow silicon, the material of choice in semiconductor applications, to be used for micro-electro-mechanical systems (MEMS) type microactuators. To investigate the suitability of silicon for these applications, friction force tests for a silicon-silicon interface were performed. For microactuators using friction bearings there is a great need for a general understanding of friction and wear phenomena. Since silicon wafers in general exhibit a single crystalline structure, the investigations included activities regarding the influence of the single crystal silicon’s orientation. The test result shows a periodic change in the coefficient of friction depending on the slider’s rotational position. For instance, a single crystal silicon disk with a {1 1 1} surface crystal orientation exhibits six recurring maxima of the friction force per rotation when tested against a specimen with the same crystal orientation. The contact between wafer and specimen results in a coefficient of friction μ reaching its maximal value of 0.5 every 60° of rotation. To find the root cause for this repetitive behavior, the sliding directions for maximal friction values were compared to the wafer’s respective crystal orientation. For a {1 1 1} silicon wafer, the atoms at the surface are arranged in equilateral triangles. The angle of 60° between the atoms in these triangles corresponds with the periodicity of the friction force. It therefore may be concluded that the coefficient of friction follows the crystal structure. Depending on the lattice orientation, the friction force varies by more than 50%. This information is crucial for designing a micro-slide bearing as well as choosing a combination of lattice orientations that yield minimal friction.     

Tribological characterization of alumina and silicon carbide under lubricated sliding

The aim of this study was to investigate the tribological properties of four different advanced ceramics - monolithic alumina, SiC whisker-reinforced alumina, monolithic silicon carbide and SiSiC-during lubricated sliding. Advanced techniques of electron microscopy and spectroscopy have been used to characterize the materials before and after testing. Tests have been performed where two flat discs were rotated against each other under closed contact in an environment of oil and water. The main wear mechanisms of the four ceramics are identified and discussed. What clearly emerges from these studies is the much more reliable performance of the silicon carbides than the aluminas. The silicon carbides have a low wear rate where microfracture and oxidation are the main deteriorating mechanisms. The capability to maintain smooth surfaces and thus also a high degree of hydrodynamic lubrication is largely due to the potential of the water to dissolve formed wear debris. The main wear mechanism of the aluminas is surface fracture. The rough fracture surfaces and the fact that the wear fragments form a discontinuous surface film will reduce the effect of the lubrication, thus accelerating the wear. Furthermore, a deformation layer with microcracks develops in the contact which decreases wear resistance.     

Evaluation of Hot Pressed Silicon Nitride as a Rolling Bearing Material

A series of tests has been conducted to evaluate the suitability of silicon nitride as a bearing material for rolling contact applications. The ability of silicon nitride to be lubricated by some conventional lubricants was found to be satisfactory. This was determined by wettability studies, lubricant film thickness and traction coefficient measurements on the optical EHD rig and friction coefficient measurements by the pin-on-disk method. The abrasive wear coefficient, measured on a lopping machine using 600 grit SiC abrasive, was found to be high compared to other ceramics. It was also dependent on the composition of the silicon nitride. Comparative rolling contact fatigue tests on steel and silicon nitride flat washers were conducted using steel rollers and balls. A high wear rate leading to grooving in the rolling track on silicon nitride was observed. The spalling resistance of silicon nitride was found to be higher than that of steel under the test conditions used. Surface interactions in the Si₃N₄-M50 steel contacts, detrimental to the life of the steel rolling elements, were recognized. Attempts were made to reduce the severity of these interactions and prolong the life of bearings containing ceramic elements.     

Stick–Slip Friction of Stainless Steel in Sodium Dodecyl Sulfate Aqueous Solution in the Boundary Lubrication Regime

The service life of a diesel fuel injector is highly affected by the tribological properties of the fuel. This study aims to investigate the friction and wear behaviors of emulsified bio-oil (EBO), which is a very promising alternative fuel for engines. Sliding wear tests were performed with a ball-on-disk tribometer using a GCr15 steel ball and a flat specimen as a counterpart. In these tests, the total sliding distance was 500 m, the load ranged from 10 to 20 N, and the oscillation frequency ranged from 30 to 50 Hz. Experimental results showed that EBO had better tribological properties than diesel oil and crude bio-oil. Contact load and oscillation frequency significantly influenced friction coefficient, wear volume, and wear damage pattern. The friction coefficient decreased with an increase in load and increased with an increase in oscillation frequency. Furthermore, the wear volume slightly increased with an increase in load or oscillation frequency. The damage mechanism is attributed to adhesive wear under low load and to abrasive wear under high load. The transition in the wear mechanism is related to the adsorption of the molecules in the EBO, the microstructural contact surface, and the mechanical actions.     

Ultra-low friction coefficient in alumina–silicon nitride pair lubricated with water

In a ball-on-disc wear test, an alumina ceramic body sliding against a silicon nitride ceramic body in water achieved an ultra-low friction coefficient (ULFC) of 0.004. The profilometer and EDX measurements indicated that the ULFC regime in this unmated Al₂O₃–Si₃N₄ pair was achieved because of the formation of a flat and smooth interface of nanometric roughness, which favored the hydrodynamic lubrication. The triboreactions formed silicon and aluminum hydroxides which contributed to decrease roughness and shear stress at the contact interface. This behavior enables the development of low energy loss water-based tribological systems using oxide ceramics.     

Influence of oxygen on the wear of silicon

The results of a series of wear tests on semiconductor silicon chips are presented. In these tests, hemispherical anodized aluminum sliders rubbed against flat silicon specimens with a small-amplitude oscillatory motion. The influence of several parameters, including the load, the slider roughness, the type of anodization, the contact geometry, the environment and the initial condition of the silicon surface, on the wear of the silicon was studied. Of these the influences of the environment and the initial condition of the silicon surface were most pronounced. It was concluded that oxide formation and growth on the silicon were important factors in the wear of the silicon and tended to increase the wear. It was also concluded that the primary wear mechanism of the silicon for low load is probably delamination or surface fatigue.     

Effects of Vapor Environment and Counter-Surface Chemistry on Tribochemical Wear of Silicon Wafers

The effects of counter-surface chemistry and adsorption of water and alcohol from the environment on the tribological responses of silicon surfaces were investigated using atomic force microscopy. When scratching with SiO₂ tips at contact pressures below the hardness of the materials, changing the environment yielded drastically different wear behaviors. In humid air, the adsorbed water molecules facilitated wear of the surface and material removal. In N₂ environment, there was subsurface deformation but no wear, so the surface protruded outward in the rubbing region. In the ethanol vapor condition, the adsorbed alcohol molecules acted as a lubricant and prevented any discernible changes to the surface even at contact pressures above 1 GPa. These results extend upon previous studies of vapor-phase alcohol lubrication using even more protective longer-chain alcohols where failure was observed at much lower contact pressures in macroscale tests, probably due to high-pressure asperity contacts. Thus, the chemical environment can govern the response of silicon to mechanical rubbing. Rubbing with a diamond tip, however, yielded protrusions in all three environments, showing that the chemistry of the counter-surface also contributes to the tribological response; in this case, diamond is not tribochemically reactive toward Si surface. The protrusion formed by the diamond tip in ethanol vapor was only ~20 % the height of the one in humid air, even though the measured friction coefficients (and so the applied shear forces) were similar. These results clearly show that the surface chemistry at the tribological interface can substantially alter both the wear and subsurface damage processes.     

Friction and wear of titanium alloys sliding against metal, polymer, and ceramic counterfaces

Recent advances in lower-cost processing of titanium, coupled with its potential use as a light weight material in engines and brakes has renewed interest in the tribological behavior of titanium alloys. To help establish a baseline for further studies on the tribology of titanium against various classes of counterface materials, pin-on-disk sliding friction and wear experiments were conducted on two different titanium alloys (Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo). Disks of these alloys were slid against fixed bearing balls composed of 440C stainless steel, silicon nitride, alumina, and polytetrafluoroethylene (PTFE) at two speeds: 0.3 and 1.0 m/s. The friction coefficient and wear rate were lower at the higher sliding speed. Ceramic sliders suffered unexpectedly higher wear than the steel slider. The wear rates, ranked from the highest to the lowest, were alumina, silicon nitride, and steel, respectively. This trend is inversely related to their hardness, but corresponds to their relative fracture toughness. Comparative tests on a Type 304 stainless steel disk supported the fracture toughness dependency. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses confirmed the tendency of Ti alloys to transfer material to their counterfaces and suggested possible tribochemical reactions between the ceramic sliders and Ti alloy disks. These reaction products, which adhere to the ceramic sliders, may degrade the mechanical properties of the contact areas and result in high wear. The tribochemical reactions along with the fracture toughness dependency helped explain the high wear on the ceramic sliders.     

Influence of surface texture on boundary lubricated sliding contacts

The friction and wear behaviour of boundary lubricated sliding surfaces is influenced by the surface texture. By introducing controlled depressions and undulations in an otherwise flat surface, the tribological properties can be improved. Lubricant can then be supplied even inside the contact by the small reservoirs, resulting in a reduced friction and a prolonged lifetime of the tribological contact.In the present paper, well-defined surface textures were produced by lithography and anisotropic etching of silicon wafers. The wafers were subsequently PVD coated with thin wear resistant TiN or DLC coatings, retaining the substrate texture. The size and shape of the depressions were varied and evaluated in reciprocating sliding under dry and boundary lubricated conditions.     

An investigation of the abrasive wear of mineral-cutting tools

The paper describes an investigation of wear of tools in cutting abrasive rock and is relevant to the problems encountered in the coal mining industry. The emphasis of the paper is on tribological aspects rather than tool performance aspects and the treatment is at a macroscopic level. A range of tool geometries and materials were included in the investigation and the effects of length of cut and depth bf cut studied.It is concluded that the wear of a cutting tool differs from the wear observed in conventional sliding contact situations with respect to the relationship between wear and applied load. This is attributed to the displacement control, rather than load control, of the cutting process and the consequent degradation of the abrasive under high contact pressures. The rate of removal of material by wear at the cutting edge of the tool, within the range of variables examined, (a) is independent of the rake and clearance angles of the tool and the width of the wear flat, (b) is proportional to the distance cut (excluding an initial high rate) and the width of the tool, and (c) decreases with increasing depth of cut and tip hardness.     

Advanced Friction–Wear Behavior of Organic Brake Pads Using a Newly Developed System

The objective of this study was to investigate the influence of an advanced performance system on the tribological behavior of brake pad material using a specially designed brake pad tester system following standard SAE J-661. The tribological behavior and friction and wear characteristics of the organic brake pad samples were evaluated. During braking tests, the samples, in contact with a cast iron disk, were studied at different disc speeds, temperatures, and braking cycles under a constant pressure. In order to understand the friction and wear behavior, the unworn surfaces, worn surfaces, and wear debris were characterized by means of scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX). Furthermore, the surface characteristics and differences in the wear modes of the brake pad samples were examined. Wear debris was permitted to deform the brake pad surfaces, leading to friction layers and enabling the estimation of the friction behavior of the brake pads. The results showed that the best friction–wear behavior was obtained with lower braking cycles at low speeds and temperature. Thus, the newly developed brake pad tester system proved very effective in evaluating the performance of the brake pad samples.     

Tribological behaviour of carbon and low alloy steels: effect of mechanical properties and test conditions

Abstract

In this paper, the effects of mechanical properties and test conditions on the tribological behaviour of ISO C45 carbon steel and ISO 42CrMo4 low alloy steel were studied. The tribological tests were carried out, without lubrication, on a reciprocating friction tester. Cylinder on flat contact configuration was adopted. The results showed that there is no obvious relationship between the mechanical properties and the friction ones. However, the variation in the coefficient of friction depends on the test conditions. In contrary to normal load, the effect of sliding speed on the coefficient of friction is not the same for the two steel nuances. The tribological properties are dependent, however, on the nature of wear debris.     

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.     

The lubricated wear of ceramics : Part II: The wear and friction of silicon nitride and steel in the presence of mineral oil or an ester based lubricant

The friction and wear characteristics of combinations of silicon nitride, alumina and AISI 52100 steel in the presence of mineral oil containing anti-wear, dispersant and detergent additives have been investigated in a tri-pin-on-disc machine. The tests were carried out at a nominal temperature of 100°C for a range of sliding speeds, loads and total sliding distances. In Part II of this two-part paper a comparison will be made between the tribological performance of these sliding pairs of materials in mineral oil and ester based lubricant environments. The results of the investigation showed that the alumina performed relatively poorly under these test conditions, whereas silicon nitride showed good potential as an improved wear resisting material compared with 52100 steel. Wear factors of the order of 10⁻¹⁰ mm³/Nm were deduced for the alumina, while values as low as 10⁻¹¹ mm³/Nm were typical of the silicon nitride sliding against 52100 steel discs. The alumina pins wore by a process of brittle fracture at the surface, whereas the silicon nitride pins wore primarily by a tribochemical polishing mechanism. The rate of tribo-chemical wear was found to be proportional to the nominal contact area.     

Wear of Silicon Carbide in Sliding Contact with Lubricated Thin-Film Rigid Disk

Silicon carbide (SiC) is a potential ceramic material for recording heads, yet its tribological performance against lubricated thin-film rigid disks is not fully known. Square pins with a 100 mm radius spherical surface were made from hot pressed SiC, chemical vapor deposited (CVD) SiC, and Al₂O₃TiC, and tested with lubricated thin-film disks. The pin-on-disk tests showed that the region of contact on the spherical surface of the SiC and CVD-SiC pins wears away to form a circular wear plateau with smears in and around the plateau. The wear plateau is formed rapidly in the first 1000 drag revolutions and then very gradually grows in size with further revolutions. Analysis of the smears showed that a large fraction of the smears contained SiO₂ which had been oxidized from SiC due to high temperatures generated at the pin surface in contact with the disk. In contrast, tests with Al₂O₃. TiC pins did not show any formation of a wear plateau on the pins.     

Tribological Characterization of Aromatic Thermosetting Copolyester–PTFE Blends in Air Conditioning Compressor Environment

The tribological behavior of a wide range of compositions using blends of aromatic thermosetting polyester (ATSP) with polytetrafluoroethylene (PTFE) has been investigated. PTFE was chosen as the blending material because of its low coefficient of friction and good performance at high temperatures and resistance to chemicals. ATSP blends were used to specifically combat some of the shortcomings of PTFE like its extremely low wear resistance and poor mechanical properties, and special processing requirements due to its high melt viscosity. Controlled tribological experiments simulating an air conditioning compressor operating with R134a refrigerant under realistic operating conditions were carried out with different ATSP/PTFE compositions, as well as four different state-of-the-art commercially available composites containing carbon fibers, graphite and PTFE. It was found that the newly synthesized composites exhibited superb tribological characteristics as far as low friction and low wear were concerned. The wear performance of PTFE was greatly improved, while it was shown that greater amounts of ATSP used in the blend lead to lower wear and the amount of ATSP did not significantly alter the friction coefficient. Material transfer and development of a weak film on the disk surface was observed, especially for the blends with higher PTFE content.     

Abrasive wear resistance of starch consolidated and sintered high speed steel

The abrasive wear resistance of starch consolidated (SC) and super solidus liquid phase sintered (SLPS) M3/2 high speed steel (HSS) samples have been evaluated by a two-body micro-abrasion test (low stress abrasion), using 6 μm diamond abrasive particles, and a three-body abrasion test (high stress abrasion), using significantly larger abrasive particles of blast furnace slag (600 HV) and silicon carbide (2400 HV), respectively. In the tests a commercial powder metallurgical (PM) HSS was used as a reference material.The results show that the microstructure of the SC and SLPS HSS samples is strongly dependent on the sintering temperature used. With increasing temperature the microstructure ranges from a porous (5% porosity) relatively fine grained low temperature sintered microstructure to a fully dense relatively coarse grained high temperature sintered microstructure with eutectic carbides/carbide networks. However, despite the pronounced microstructural differences displayed by the as-sintered HSS microstructures these show a relatively high abrasive wear resistance, comparable with that of a HIPed HSS reference, both under low and high stress abrasion contact conditions. The characteristic features of the low and high temperature sintered microstructures, i.e. the pores and coarse eutectic carbides/carbide networks, only show a limited impact on the wear rate and the wear mode (dominant wear mechanism). The results obtained imply that near net shaped components manufactured by starch consolidation and super solidus liquid phase sintering might be of interest in tribological applications.