Dry sliding wear of A356-Al-SiCp composites

Aluminium alloy (A356)-SiC composites containing 15 and 25 wt.% silicon carbide particles (average size 43 μm) were tested for sliding wear at different loads using a pin on disc machine. Composites exhibited better wear resistance compared with unreinforced alloy up to a pressure of 26 MPa. Scanning electron microscopy examination of worn surfaces and subsurfaces show that the presence of dispersed SiC particles help in reducing the propensity of material flow at the surface, at the same time leading to the formation of an iron-rich layer on the surface.

Omistajuus Maailmanpankin diskurssissa : muuttuvatko avun toimijoiden valtasuhteet? Tapaustutkimuksena avun uusi arkkitehtuuri ja yhteiskunnalliset voimasuhteet Mosambikissa

Samanaikaisesti, kun tämän päivän kehitysdiskurssissa painotetaan omistajuutta ja kumppanuutta, monien mielestä avunantajien asema suhteessa avunsaajiin on jopa vahvistunut entisestään. Jos vahvistuneen avunsaajan omistajuuden ajatellaan tarkoittavan vallan siirtoa avunantajilta avunsaajille, vaikuttaisi diskurssin ja käytännön välillä olevan ristiriita. Työssäni yritän selvittää eri tekijät ja historiallisen taustan, joiden seurauksena liberaali käsitteistö, etenkin omistajuus, on noussut Maailmanpankin diskurssiin. Tätä kautta pyrin selvittämään, mitä omistajuus tarkoittaa Maailmanpankille, ja millaisiin valta-asetelmiin uusi diskurssi johtaa. Työni tapaustutkimuksessa pohdin omistajuuskysymyksiä tämän päivän Mosambikissa: edesauttaako nykyinen liberaali kehitysdiskurssi vallansiirtoa Mosambikin kontekstissa a) avunantajilta avunsaajamaalle b) perimmäiselle edunsaajalle, eli kansalle. Tutkimukseni on luonteeltaan laadullinen. Historiallisen lähestymistavan kautta pyrin selvittämään taustaa ja syitä Maailmanpankin diskurssin muutokseen, ja sitä kautta selvittämään nykyään vallitsevia avunantajien ja avunsaajien välisiä valtasuhteita. Tähän käytän lähteinäni ensimmäisessä osiossa Maailmanpankin omia julkaisuja ja toisessa osiossa muita kirjallisia lähteitä. Mosambikin tapaustutkimukseni lähdeaineisto koostuu paikallisten järjestöjen ja tutkimusinstituuttien edustajille tekemistäni avoimista teemahaastatteluista sekä muista kirjallisista, Mosambikin avun kontekstia käsittelevistä lähteistä. Omistajuuskäsite Maailmanpankin diskurssissa on laajentunut huomattavasti: kun alussa omistajuudella tarkoitettiin lähinnä sitoutumista, niin nykyään se tulisi ymmärtää pyrkimyksenä lisätä avunsaajamaiden hallintaa kehitysprosesseissa, mikä puolestaan lisäisi huomattavasti avunsaajamaiden päätösvaltaa avunkäytön suhteen. Toisessa osiossa kuitenkin osoitan, että vaikka Maailmanpankki antaa omistajuusdiskurssin myötä ymmärtää, että aloitteen kehitysinterventiosta on tultava avunsaajamaalta itseltään, niin todellisuudessa asetelma on erilainen. Maailmanpankin ja muiden avunantajien keinot vaikuttaa ovat muuttuneet, mutta vaikutus on pysynyt yhä vahvana. Omistajuus- ja kumppanuusdiskurssi toimii merkittävässä asemassa nykyisessä avun kontekstissa, jossa avunantajat vaikuttavat avunsaajamaihin sisältä päin: vallankäytön mekanismit ovat enemmän piilossa. Näin ollen aloite tulee yhä vieläkin avunantajilta, ja omistajuus tarkoittaa lähinnä avunsaajamaan sitoutumista ohjelmiin. Kansalaisyhteiskunta puolestaan on otettu mukaan tukemaan avunantajien agendaa ja varmistamaan kannatus muutoksilie myös yhteiskunnan sisältä päin. Mosambikin tapaustutkimuksessa huomiot ovat jokseenkin samankaltaisia. Omistajuusdiskurssin myötä avunantajat vaikuttavat mahdollisesti yhä tehokkaammin Mosambikin hallituksen sisällä, etenkin avun käyttöön liittyvissä kysymyksissä. Mosambikin hallitus ei kuitenkaan vaikuta edes haluavan vahvempaa omistajuutta, koska nykyinen toteutettava malli palvelee sen etuja loistavasti: antamalla periksi tietyillä alueilla avunantajille hallitus vahvistaa asemiansa muiden kotimaisten toimijoiden edessä. Avunantajilla voi tahtoessaan olla tärkeä rooli avunsaajamaiden sisäisiin valtasuhteisiin vaikuttamisessa. Mosambikin esimerkin perusteella vaikuttaa kuitenkin siltä, että merkittävää muutosta kansan ja hallituksen välisissä valtasuhteissa ei ole tapahtunut.

Influence of Inclination Angle and Machining Direction on Friction and Transfer Layer Formation

In the present investigation, unidirectional grinding marks were created on a set of steel plates. Sliding experiments were then conducted with the prepared steel plates using Al-Mg alloy pins and an inclined pin-on-plate sliding tester. The goals of the experiments were to ascertain the influence of inclination angle and grinding mark direction on friction and transfer layer formation during sliding contact. The inclination angle of the plate was held at 0.2 deg, 0.6 deg, 1 deg, 1.4 deg, 1.8 deg, 2.2 deg, and 2.6 deg in the tests. The pins were slid both perpendicular and parallel to the grinding marks direction. The experiments were conducted under both dry and lubricated conditions on each plate in an ambient environment. Results showed that the coefficient of friction and the formation of transfer layer depend on the grinding marks direction and inclination angle of the hard surfaces. For a given inclination angle, under both dry and lubricated conditions, the coefficient of friction and transfer layer formation were found to be greater when the pins slid perpendicular to the unidirectional grinding marks than when the pins slid parallel to the grinding marks. In addition, a stick-slip phenomenon was observed under lubricated conditions at the highest inclination angle for sliding perpendicular to the grinding marks direction. This phenomenon could be attributed to the extent of plane strain conditions taking place at the asperity level during sliding. DOI: 10.1115/1.4002604]

Enhancement of Surface Flashover Performance of High Voltage Ceramic Disc Insulators

In this paper, an effort is made to study accurately the field distribution for various types of ceramic insulators used for high-voltage transmission. The surface charge simulation method (SCSM) is employed for the field computation. With the help of SCSM program, a Novel field reduction electrode is designed and developed to reduce the maximum field around the pin region. In order to experimentally analyze the performance of discs with field reduction electrode, special artificial pollution test facility was built and utilized. The experimental results show better surface flashover performance of ceramic insulators used in high-voltage transmission and distribution systems.

Role of Surface Texture, Roughness, and Hardness on Friction During Unidirectional Sliding

In the present investigation, experiments were conducted by unidirectional sliding of pins made of FCC metals (Pb, Al, and Cu) with significantly different hardness values against the steel plates of various surface textures and roughness using an inclined pin-on-plate sliding apparatus in ambient conditions under both the dry and lubricated conditions. For a given material pair, it was observed that transfer layer formation and the coefficient of friction along with its two components, namely adhesion and plowing, are controlled by the surface texture of the harder mating surfaces and are less dependent of surface roughness (R (a)) of the harder mating surfaces. The effect of surface texture on the friction was attributed to the variation of the plowing component of friction for different surfaces. It was also observed that the variation of plowing friction as a function of hardness depends on surface textures. More specifically, the plowing friction varies with hardness of the soft materials for a given type of surface texture and it is independent of hardness of soft materials for other type of surface texture. These variations could be attributed to the extent of plane strain conditions taking place at the asperity level during sliding. It was also observed that among the surface roughness parameters, the mean slope of the profile, Delta (a), correlated best with the friction. Furthermore, dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing taking place at the asperity level.

Friction and wear behaviour of continuous fibre as cast Kevlar-phenolic resin composite

A compression moulded Kevlar-phenolic resin composite consisting of 30 wt% continuous fibres was slid against a steel disc such that the fibre axis was normal to the sliding plane. The sliding experiments were conducted in a normal pressure range of 0.47–4.27 MPa and at a sliding speed of 0.5 ms–1. The initial sliding interaction is abrasive. With further sliding, as patches of polymer transfer film develop on the polymer pin and counterface, the interaction becomes adhesive and steady-state friction is established. The wear resistance of the polymer was found to be related to the stability of this film.

A study of microstructure and wear behaviour of TiB2/Al metal matrix composites

The present paper deals with the study of microstructure and wear characteristics of TiB2 reinforced aluminium metal matrix composites (MMCs). Matrix alloys with 5, 10 and 15% of TiB2 were made using stir casting technique. Effect of sliding velocity on the wear behaviour and tribo-chemistry of the worn surfaces of both matrix and composites sliding against a EN24 steel disc has been investigated under dry conditions. A pin-on-disc wear testing machine was used to find the wear rate, in which EN24 steel disc was used as the counter face, loads of 10-60N in steps of 10N and speeds of 100, 200, 300, 400 and 500 rpm were employed. The results showed that the wear rate was increased with an increase in load and sliding speed for both the materials. However, a lower wear rate was obtained for MMCs when compared to the matrix alloys. The wear transition from slight to severe was presented at the critical applied loads. The transition loads for the MMCs were much higher than that of the matrix alloy. The transition loads were increased with increase in TiB2 and the same was decreased with the increase of sliding speeds. The SEM and EDS analyses were undertaken to demonstrate the effect of TiB2 particles on the wear mechanism for each conditions.

Elasto-plastic contact stress analysis of joints subjected to cyclic loading

Rigorous elastic-plastic finite element analysis of joints subjected to cyclic loading is carried out. An incremental-iterative algorithm is developed in a modular form combining elasto-plastic material behaviour and contact stress analysis. For the case of the interference fit, the analysis sequentially carries out insertion of the pin and application of the load on the joint, covering possible initiation of separation (and/or yielding) and progressively the receding/advancing contact at the pin-plate interface. Deformations of both the plate and the pin are considered in the analysis. Numerical examples are presented for the case of an interference fit pin in a large plate under remote cyclic tension, and for an interference fit pin lug joint subjected to cyclic loading. A detailed study is carried out for the latter problem considering the effect of change in contact/separation at the pin-plate interface on local stresses, strains and redistribution of these stresses with the spread of a plastic zone. The results of the study are a useful input for the estimation of the fatigue life of joints. Copyright (C) 1996 Elsevier Science Ltd

Microstructure control and wear of Al2O3-SiC-(Al,Si) composites made by melt oxidation

Ceramic matrix composites of Al2O3-SiC-(Al,Si) have been fabricated by directed melt oxidation of aluminum alloys into SiC particulate preforms. The proportions of Al2O3, alloy, and porosity in the composite can be controlled by proper selection of SLC particle size and the processing temperature. The wear resistance of composites was evaluated in pin-on-disk experiments against a hard steel substrate. Minimum wear rate comparable to conventional ceramics such as ZTA is recorded for the composition containing the highest fraction of alloy, owing to the development of a thin and adherent tribofilm with a low coefficient of friction.

Observations on the role of the steel disc counterface during the sliding of various structural ceramics

Sliding wear characteristics and mechanisms of structural ceramics, namely Al2O3, zirconia-toughened alumina, tetragonal zirconia polycrystals (TZP) and Si3N4 against a steel counterface are influenced by mechanical and tribochemical interactions, specific to the combinations studied. The present paper studies the role of the disc in the sliding wear process of the above ceramics. Experiments were conducted at a pressure of 15.5 MPa between 0.1 and 12.0 m s(-1) with ceramic pins sliding against an EN-24 steel disc. Except in the case of TZP, the disc morphology is sensitive to variations in speed rather than to the pin material. The disc track is (i) mildly abraded at low speeds (about 0.1-0.75 m s(-1)), (ii) severely abraded at intermediate speeds (about 1.0-3.0 m s(-1)), (iii) covered with black patches at high speeds (about 4.0-6.0 m s(-1)) and (iv) completely black at very high speeds (about 7.0-12.0 m s(-1)). In the case of TZP, although black patches appear, transfer of TZP onto the disc surface and high wear of TZP occurs at 4.0 m s(-1). The order of the wear of the disc estimated from profilometric measurements is the same for all the ceramics. Except for Si3N4, the onset of wear of the ceramics is associated with the appearance of deep 'V' grooves on either side of the profile of the disc track. This can be explained on the basis of the thermal and hardness variations. Although other interaction products specific to the ceramic pin are present, the formation of iron oxides dominates the wear of the disc.

Sliding wear of YTZP ceramic against steel: observations on ceramic transfer and wear transition

Sliding tests were conducted, in air, of YTZP ceramic pins against steel discs at an applied pressure of 15.5 MPa over a speed range of 0.3 to 4.0 ms(-1). Pin wear was not detectable until 2.0 m s(-1), after which a finite but small wear rate was observed at 3.0 m s(-1), accompanied by a red glow at the contacting surface. A transition in wear behaviour and friction (mu) occurred at 4.0 ms(-1), increasing the former by over two orders of magnitude. Both mu and wear behaviour changed with time at 4.0 m s(-1). During initial periods mu was high and wear rate increased steadily with time accompanied by ceramic transfer onto the disc, which increased with time. When disc coverage exceeds a certain threshold value, mu decreased rapidly and the wear rate stabilized at a very high value. Metal transfer was not observed at any speed. High surface temperatures brought about significant adhesion between TZP and steel and this together with enhanced plastic deformation brought about a transition in wear behaviour.

Synthesis of path generating flexible-link mechanisms

Flexible-link mechanisms are those linkage mechanisms (or structures) which are capable of motion by virtue of elastic deformation of one or more;links. In such mechanisms a single flexible link; can replace several rigid links and joints resulting in fewer links, fewer pin joints, reduced overall weight and reduced mechanical error. In spite of such clear advantages, contributions toward flexible-link mechanisms remain very scarce. The area of flexible-link mechanisms offers much scope for further exploration. This paper attempts to show the potential of flexible-link mechanisms in accomplishing a kinematic task like path generation. Synthesis of a four-bar mechanism with a flexible rocker for circular and straight line path generation is carried out. Displacement analysis of the structure is carried out using finite element method (FEM) and synthesis is formulated and solved as an optimization problem. Several numerical examples are presented for illustration. Based on the results obtained with these examples, the flexible-link mechanism considered shows good promise for-path generation.

Dry sliding wear of Al alloy 2024-Al203 particle metal matrix composites

In the present investigation, Al 2024-15vol.%Al2O3 particulate (average size, 18 mu m) composites were fabricated using the liquid metallurgy route. The wear and friction characteristics of Al alloy 2024 and Al 2024-15vol.%Al2O3p, composite in the as-extruded and peak-aged conditions were studied using a pin-on-disc machine (with a steel disc as the counterface material). The worn surfaces, subsurfaces and the debris were analysed in a scanning electron microscope.The performance of the composite in the as-extruded condition is slightly inferior to that of the unreinforced alloy. However, in the T6 condition, although the wear rates of two materials are initially comparable, the unreinforced alloy seizes while the composite does not within the tested range employed. In the as-extruded condition, the presence of Al2O3 particles is not particularly beneficial as they fracture and result in extensive localized cracking and removal of material from the surface. In the peak-aged condition, however, while the unreinforced alloy exhibits severe plastic deformation and undergoes seizure, there is no significant change in the mechanism in the case of the composite. Except in the case of the peak-aged unreinforced alloy, worn surfaces of all other materials show the presence of an iron-rich layer.

Mechanism of seizure of aluminium-silicon alloys dry sliding against steel

A steel ball was slid on aluminium-silicon alloys at different temperatures. After the coefficient of friction had been measured, the surface shear stress was deconvoluted using a two-term model of friction. The ratio of surface shear stress to bulk hardness was calculated as a function of temperature, silicon content and alloying additions. These results are qualitatively similar to those recorded for pre-seizure specimens slid against an En24 disc in a pin-on-disc machine. This similarity, when viewed in the context of the phenomenon of bulk shear, provides a model for seizure of these alloys.

Sliding wear of Al2O3-SiC-(Al,Si) composites against a steel counterface

The sliding-wear behavior of Al2O3-SiC-Al composites prepared by melt oxidation against a steel counterface has been recorded in a pin-on-disk machine. At high speeds and pressures (10 m/s, 20 MPa), friction and wear appear to be principally controlled by the in-situ formation of an interfacial film that consists of a layer of Fe3O4. The formation of this him is examined as a function of sliding speed, lubrication, and composite microstructure. A model is proposed in which high surface temperatures cause the preferential extrusion of aluminum from the composite onto the pin/disk interface. This promotes the adhesive pickup of iron and its oxidation to form a stable tribologically beneficial layer of Fe3O4.