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.     

Evaporated vanadium nitride as a friction material in dry sliding against stainless steel

High friction coefficients and severe galling have always hampered machining of stainless steel. Lubricants and special procedures in cutting and forming operations are common practice. Coatings deposited on tools in order to alter the adhesive nature of the stainless steel is an appealing idea and it has long been proposed as a solution to the problem. However, to date no coating, which is also capable of enduring the temperatures encountered during machining has successfully fulfilled that prophecy.Despite being one of the first PVD tool coatings commercially produced, titanium nitride (TiN) is still also one of the most widely used. A vast number of coatings have indeed joined TiN for tooling applications but none of these has excelled in contact with stainless steel. Using quantum calculations vanadium nitride (VN), a ceramic compound closely related to TiN, has recently been suggested to be much less prone to adhesive cladding to iron alloys than TiN. Experimental verification of those results is the focus of this work.Reactive electron beam evaporation is regarded as the premier method for producing TiN. Using that very method, VN and TiN coatings were produced for this work and tested in sliding in contact with stainless steel and other selected materials. In these tests VN is shown to be less prone to galling as compared to TiN, especially against stainless steel.     

Dry sliding friction and casing wear behavior of PCD reinforced WC matrix composites

The friction and casing wear properties of PCD reinforced WC matrix composites were investigated using a cylinder-on-ring wear-testing machine against N80 casing steel counterface under dry sliding conditions. The results indicate that the friction and casing wear rate of PCD reinforced WC matrix composites are the lowest among the materials. As the applied load and sliding speed steadily increase, the friction coefficients of PCD reinforced WC matrix composites decrease. In addition, the casing wear rates increase with increasing load, but decline with sliding velocity. The dominant wear mechanism of the PCD composite is the micro-cutting wear, accompanied by adhesive wear.     

High-temperature sliding wear of metals

Temperature can have a considerable effect on the extent of wear damage to metallic components. During reciprocating sliding, under conditions where frictional heating has little impact on surface temperatures, there is generally a transition from severe wear to mild wear after a time of sliding that decreases with increase in ambient temperature. This is due to the generation and retention of oxide and partially-oxidized metal debris particles on the contacting load-bearing surfaces; these are compacted and agglomerated by the sliding action, giving protective layers on such surfaces. At low temperatures, from 20 to 200°C, the layers generally consist of loosely-compacted particles; at higher temperatures, there is an increase in the rates of generation and retention of particles while compaction, sintering and oxidation of the particles in the layers are facilitated, leading to development of hard, very protective oxide ‘glaze’ surfaces. This paper reviews some of the main findings of extensive research programmes into the development of such wear-protective layers, including a model that accounts closely for the observed effects of temperature on wear rates during like-on-like sliding.     

Effect of geometrical parameters in micro-grooved crosshatch pattern under lubricated sliding friction

Tribological test was carried out using a pin-on-disc geometry with textured SKD11 pin on bearing steel disc, under sliding in paraffin oil. Micro-grooved crosshatch pattern has been fabricated with various angles and widths. The effects of geometrical parameters on friction were mainly examined in mixed and elastohydrodynamic lubrication. The results show that friction control can be achieved by fabricating the micro-grooved crosshatch pattern on a contact surface. It is observed that each geometrical parameter of texture influence on friction, especially decrease of groove aspect ratio and increases of groove sliding length show friction reduction performance. Crucial parameter G_l was proposed for micro-grooved crosshatch texture. The friction mechanism is explained by micro fluid flow with limited theoretical approach.     

Effect of applied voltage on friction and wear characteristics in mixed lubrication

The effect of an electric field applied between rubbing surfaces on friction and wear characteristics was examined using a ballon-disc testing apparatus under different lubrication conditions. The lubrication conditions were changed by changing the viscosity of the lubricants. By applying an electric field between the rubbing surfaces, the oxidation of the rubbing surface at the anode side is enhanced, and suppressed on the cathode side surface. The oxide film formed on the anode surface being harder than the bulk steel, the rubbing surface at the anode side was little worn, but that at the cathode side was abrasively worn. This wear characteristic could be utilised to attain favourable running-in. The application of an electric field, however, is considered to promote the breakdown of any EHL film formed. Therefore the effect of the application of an electric field between rubbing surfaces is influenced by the lubrication condition that is being tested.     

Effect of copper content on the mechanical and sliding wear properties of monotectoid-based zinc-aluminium-copper alloys

One binary zinc-aluminium monotectoid and five ternary zinc-aluminium-copper alloys were produced by permanent mould casting. Their wear properties were examined using a block-on-ring test machine. Hardness, tensile strength and percentage elongation of the alloys were also determined and microhardness of aluminium-rich α phase was measured.It was observed that the hardness of the alloys increased continuously with increasing copper content up to 5%. Their tensile strength also increased with increasing copper content up to 2%, but above this level the strength decreased as the copper content increased further. Microhardness of the aluminium-rich α phase was also affected by the copper content in a manner similar to that of the tensile strength. It was found that the wear loss of the alloys decreased with increasing copper content and reached a minimum at 2% Cu for a sliding distance of 700 km. However, the coefficient of friction and temperature due to frictional heating were found to be generally less for the copper containing alloys than the one without the element. The effect of copper on the wear behaviour of the alloys was explained in terms of their microstructure, hardness, tensile strength, percentage elongation and microhardness of the α phase.     

Effect of wear on EHD film thickness in sliding contacts

A theoretical solution to the elastohydrodynamic (EHD) lubrication problem in sliding contacts, which takes into consideration the effect of the change in shape of the gap due to wear on the load‐carrying capacity, is presented. The model of such a contact is based on assumptions of Grubin and Ertel (von Mohrenstein). The resultant dimensionless Reynolds and film profile equations have been solved numerically for a number of cases with several values of thickness of the worn layer. Iteration of the EHD film thickness is performed by means of the secant method. Values of the calculated dimensionless film thickness are presented as a function of dimensionless wear. The conclusions concern the influence of the linear wear on the film thickness in heavily loaded sliding contacts. Copyright © 2006 John Wiley & Sons, Ltd.     

Unlubricated sliding and rolling wear of thermoplastic dynamic vulcanizates (Santoprene) against steel

The friction, sliding and rolling wear characteristics of thermoplastic dynamic vulcanizates (TPV; Santoprene^® grades), composed of polypropylene (PP), ethylene/propylene/diene rubber (EPDM) and extender oil, were studied against steel counterparts in dry condition. The composition and basic mechanical properties of the TPV of various hardness (Shore A = 60°, 70° and 80°) were evaluated. The wear performance of the TPVs was investigated in different tribotests, viz. pin-on-plate (POP), cylinder-on-plate (fretting) and rolling ball-on-plate (RBOP), whereby “plate” was always the rubber. From the above tests the coefficient of friction (COF) and specific wear rate were determined. It was established that with increasing hardness usually both COF and the specific wear rate were reduced. Values of the COF and wear rate depended strongly on the configuration and testing parameters of the related tribotests. The wear mechanisms were concluded by inspecting the worn surfaces by white light profilometry and scanning electron microscopy (SEM), respectively, and discussed.     

Fretting wear and friction behaviour of engineering ceramics

Friction and wear characteristics of partially stabilized zirconia (PSZ) fretted against itself and against high carbon steel were investigated. The results for the transformation toughened PSZ ceramics are compared with the behaviour of more brittle alumina ceramic under the same test conditions. Fretting tests in air were carried out on a high frequency wear test rig at room temperature using a cross-cylinder configuration. It was found that both the oxide ceramics were more resistant to fretting wear than the steel. Surface cracking was observed on the alumina wear scars while microfracture and delamination dominated on the PSZ wear scars. When metallic samples were fretted against ceramics, metallic film transfer to the ceramic surfaces occurred.     

The influence of alumina particle dispersion and test parameters on dry sliding wear behaviour of zinc-based alloy

The present investigation deals with dry sliding wear characteristics of a zinc-based alloy (ZA 37) with and without Al₂O₃ particle dispersion over a range of sliding speeds and applied pressures. The matrix alloy has been examined under identical test conditions in order to examine the role played by the second phase alumina particles on wear behaviour. The observed wear behaviour of the samples has been explained in terms of specific characteristics like cracking tendency, lubricating, load bearing and deformability characteristics, and thermal stability of various microconstituents. The nature of predominance of one set of parameters (causing higher wear rate) over the other (producing a reverse effect) was thought to actually control the wear behaviour. Examinations of the characteristic of wear surfaces and subsurface regions also enabled to understand the operating wear mechanism and to substantiate the wear behaviour.At low sliding speed, significantly lower wear rate of the matrix alloy over that of the composite was noticed. This has been attributed to increased microcracking tendency of the composite than the matrix alloy. Reduced wear rate and higher seizure pressure experienced by the composite over that of the matrix alloy at the higher sliding speeds could be explained to be due to enhanced compatibility of matrix alloy with dispersoid phase and greater thermal stability of the composite in view of the presence of the dispersoid. The maximum temperature rise due to frictional heating has been observed to be low in the case of matrix alloy than composite at low speed while the trend reversed at higher speeds. In general, the wear rate and temperature increased with applied pressure and speed. Seizure pressure reduced with increasing speed while the seizure resistance (pressure) of the matrix alloy was more adversely affected by speed than that of the composite.     

An investigation on sliding wear behavior of PVD coatings

Ti-6Al-4V alloy rubbing against aluminum-bronze 630 was evaluated in this work. High velocity oxygen fuel (HVOF) WC-10%Co-4%Cr thermal sprayed and TiN, CrN and DLC physical vapor deposition (PVD) coatings were applied to increase titanium substrate wear resistance. Pin-on-disk tests were performed with a normal force of 5 N and at a speed of 0.5 m/s, with a quantitative comparison between the five conditions studied. Results showed higher wear resistance for Ti-6Al-4V alloy DLC coated and aluminum-bronze 630 tribological pair and that the presence of graphite carbon structure acting as solid lubricant was the main wear preventing mechanism.     

Microstructure and abrasive wear in silicon nitride ceramics

It is well known that abrasive wear resistance is not strictly a materials property, but also depends upon the specific conditions of the wear environment. Nonetheless, characteristics of the ceramic microstructure do influence its hardness and fracture toughness and must, therefore, play an active role in determining how a ceramic will respond to the specific stress states imposed upon it by the wear environment. In this study, the ways in which composition and microstructure influence the abrasive wear behavior of six commercially-produced silicon nitride based ceramics are examined. Results indicate that microstructural parameters, such as matrix grain size and orientation, porosity, and grain boundary microstructure, and thermal expansion mismatch stresses created as the result of second phase formation, influence the wear rate through their effect on wear sheet formation and subsurface fracture. It is also noted that the potential impact of these variables on the wear rate may not be reflected in conventional fracture toughness measurements.     

A comparison of the relative friction and wear responses of PTFE and a PTFE-based composite when tested using three different types of sliding wear machines

In this work, the tribological behavior of a commercially available polytetrafluoroethylene-based composite material filled with polyimide microparticles has been thoroughly investigated using different approaches. Specifically, two standard tribological tests, i.e., pin-on-disc and thrust washer measurements, have been compared with sliding tests performed on real components using a specially designed lab bench. The obtained results demonstrated that, despite the different testing methods (continuous rotation vs. reciprocating linear movement for the pin-on-disc/thrust washer and sliding tests, respectively), the different techniques provide highly comparable data within the adopted experimental conditions and can be successfully combined to assess the overall tribological features of this PTFE-based polymer composite.     

Effect of temperature on friction and wear of prehardened tool steel during sliding against 22MnB5 steel

Abstract

Mechanical components in tribological systems exposed to elevated temperatures are gaining increased attention since more and more systems are designed to operate under extreme conditions. In hot metal forming, the effect of temperature on friction and wear is especially important since it is directly related to process economy (tool wear) and quality of the produced parts (friction between tool and workpiece). This study is therefore focused on fundamental understanding pertaining to the tribological characteristics of prehardened hot work tool steel during sliding against 22MnB5 boron steel. The tribological tests were carried out using a high temperature reciprocating sliding friction and wear tester under a normal load of 31 N (corresponding to a contact pressure of 10 MPa), a sliding speed of 0·2 m s^?1 and temperatures ranging from 40°C to 800°C. It was found that friction coefficient and specific wear rate decreased at elevated temperature because of formation of compacted wear debris layers on the surfaces.     

Effect of niobium interlayer on high-temperature sliding friction and wear of silver films on alumina

In this study, we investigated the effect of a thin Nb bonding layer (15–20 nm thick) on the high-temperature sliding friction and wear performance of Ag films ( 1.5 m thick) produced on -alumina (Al₂O₃) substrates by ion-beam assisted deposition (IBAD). The friction coefficients of Al₂O₃ balls against the Ag-coated Al₂O₃ flats were 0.32 to 0.5 as opposed to 0.8 to 1.1 against the uncoated flats. Furthermore, these Ag films reduced the wear rates of Al₂O₃ balls by factors of 25 to 2000, depending on test temperature. Wear of Ag-coated Al₂O₃ flats was hard to measure after tests at temperatures up to 400°C. At much higher temperatures (e.g., 600°C), these Ag films (without a Nb layer) were removed from the sliding surfaces and lost their effectiveness; however, Ag films with the Nb bonding layer remained intact on the sliding surfaces of the Al₂O₃ substrates even at 600°C and continued to impart low friction and low wear.     

Surface roughness evolutions in sliding wear process

Wear debris analysis is a technique for machine condition monitoring and fault diagnosis. One key issue that affects the application of wear debris analysis for machine condition monitoring is whether the morphology of the wear particles accurately depicts their original states and the surface morphology of the components from which the particles separate. This study aimed to investigate the evolution of the surface morphology of wear debris in relation to change in the surface morphology of wear components in sliding wear process. Sliding wear tests were conducted using a ball-on-disc tester under proper lubrication and improper lubrication conditions. The study of the particle size distribution and the surfaces of both the wear debris and the tested samples in relation to the wear condition and the wear rates of the wear components were carried out in this study. The evolutions of the surface topographies of both the wear debris and the wear components as wear progressed were investigated. This study has provided insight to the progress of material degradation through the study of wear debris. The results of this research have clearly demonstrated that: (a) there is a good correlation of the surface morphology of wear debris and that of the wear components, and (b) the surface morphology of wear debris contains valuable information for machine condition monitoring.     

Effect of grain refinement and modification on the dry sliding wear behaviour of eutectic Al-Si alloys

The present study deals with an investigation of dry sliding wear behaviour of grain refined and or modified eutectic (Al-12Si) Al-Si alloy by using a Pin-On-Disc machine. The indigenously developed Al-1Ti-3B and Al-10Sr master alloys were used as grain refiner and modifier for the grain refinement of α-Al dendrites and modification of eutectic Si, respectively. Various parameters have been studied such as alloy composition, sliding speed, sliding distance and normal pressure. The cast alloys, master alloys and worn surfaces were characterized by SEM/EDX microanalysis. Results suggest that, the wear resistance of eutectic Al-Si alloys increases with the addition of grain refiner (Al-1Ti-3B) and or modifier (Al-10Sr). Further, the worn surface studies show that adhesive wear was observed in Al-12Si alloy in the absence of grain refiner and modifier. However, an abrasive and oxidative wear was observed when the grain refiner and modifier are added to the same alloy. Commercially available LM-6 (12.5%Si) alloy was used for comparison.     

Effect of friction cycles on wear particle generation of carbon nitride coating against a spherical diamond

The in-situ observations of wear particle generation of carbon nitride coating on silicon repeatedly sliding against a spherical diamond have been studied in terms of the critical friction cycles and normal loads. An environmental scanning electron microscope (E-SEM), in which a pin-on-disk tribotester was installed, has in-situ provided direct evidence of when and how the wear particle generation do occur during the repeated sliding of carbon nitride coating against a spherical diamond. The in-situ observations of non-conductive carbon nitride coating are therefore available free from surface charging with controllable relative humidity. The repeated sliding tests at a sliding speed of 50 μm/s have been carried out with the purpose of observing the ‘No wear particle generation’ region when varying normal load from 10 to 250 mN. It appears that until 20 friction cycles, the maximum Hertzian contact pressure P_max for ‘No wear particle generation’ can be improved from 1.39 Y to 1.53 Y if silicon is coated by carbon nitride with a thickness of 10 nm, where Y is defined as the yield strength of silicon. The applicable enlargement of the ‘No wear particle generation’ region of carbon nitride coating has therefore been comparatively discussed with the silicon substrate from the view points of the friction coefficient and the specific wear rate. The mode transition maps have also been summarized for the repeated sliding of carbon nitride coating in terms of ‘No wear particle generation’, ‘Wear particle generation by microcutting’ and ‘Wear particle generation by microcutting and microfracturing’ three typical modes.     

The tribochemistry of silicon nitride: effects of friction, temperature and sliding velocity

In order to obtain information on the mechanisms of tribochemistry in silicon nitride, we studied the effects of chemical parameters (temperature and concentration of reagent) and tribological parameters (load and sliding speed) on the kinetics of the reaction, i.e. the rate of material removal. The temperature dependence of the wear rate of silicon nitride has been studied in several solutions. In CrO₃ and in KOH, the removal rate increases with temperature; the apparent activation energy is 20 kJ/mole in CrO₃ and 22 kJ/mole in KOH. In water, material removal is temperature independent, in KMnO₄, its rate decreases with increasing temperature. These changes are accompanied by parallel variations in the coefficient of friction. The reaction rate presents a complex dependence on the concentration of CrO₃ solutions. In water and CrO₃ solutions, we observed a strong dependence of friction and material removal rate with the load. With the changes in temperature, concentration and load, it is found that the reaction rate (in mm³/(N·m)) is linear with the coefficient of friction above a threshold value μ_th≈0.2. The velocity dependence is complicated by the phenomena of mixed lubrication. In all cases, the lack of solid wear particles and the production of ammonia have verified the tribochemical nature of the material removal. The mechanism of stimulation of the chemical reaction by friction is a quasi-static stretching of the bonds at the interface and a high local vibration energy of the atoms at the sliding contact.