The self-lubricating wear behaviour of a C-fibre reinforced borosilicate glass matrix composite in vacuum was investigated by using a rotating pump experimental facility. The vanes were made of the composite material and the stator of the pump was of cast iron. Glass composite wear was accompanied by material transfer onto the stator surface. The formation of isle-type and continuous graphitic films on the counter-body surface was observed. The continuous film provided adequate lubrication during friction, leading to a relatively low wear rate of the composite for the conditions investigated

Boccaccini, Aldo R.

Gevorkian, Gevork

English

Repositorium für Naturwissenschaften und Technik

echnical ReportCarbon-fibre reinforced glass matrix composi tes: self- lubricatingmaterials for wear applications in vacuumAldo R. BoccacciniDepartment of Materials, Imperial College of Scienee, Technology and Medicine, London (UK)Gevork GevorkianFachgebiet Maschinenelemente, Technische Universität Ilmenau, Ilmenau (Germany)The self-lubricating wear behaviour of a C-fibre reinforced borosilicate glass matrix composite in vacuum was investigated by usinga rotating pump experimental facility. The vanes were made of the composite material and the Stator of the pump was of cast iron.Glass composite wear was accompanied by material transfer onto the Stator surface. The formation of isle-type and continuousgraphitic films on the counter-body surface was observed. The continuous film provided adequate lubrication during friction, leadingto a relatively low wear rate of the composite for the conditions investigated.Glasmatrix-Verbundwerkstoffe mit Kohlenstoffaser-Verstärkung: selbstschmierende Werkstoffe für Reibungsanwendungenim VakuumDas selbstschmierende Verschleißverhalten von C-Faser-verstärkten Borosilicatglasmatrix-Verbundwerkstoffen im Vakuum wurdemittels einer Rotations-Vakuumpumpe untersucht. Die Schaufeln waren aus dem Verbundwerkstoff und der Stator aus Gußeisenhergestellt. Der Verschleiß des Glasmatrix-Verbundwerkstoffes wurde von einem Materialtransport auf die Statoroberfläche beglei-tet. Es wurde die Bildung einer graphitischen Schicht auf der Statoroberfläche beobachtet. Diese bleibende und kontinuierlicheSchicht sorgte für eine ausreichende Schmierung während der Reibungsbelastung, wodurch eine relativ niedrige Verschleißrate desVerbundwerkstoffes unter den untersuchten Bedingungen bewirkt wurde.1 . I n t r o d u c t i o nGlass and glass-ceramic matr ix composites containingfibres and particles as reinforcement are being increas-ingly considered for self-lubricating wear applicationsunder condit ions involving relative high temperaturesand in oxidative environments, including applications inthe automotive and aerospace sector [1 to 9].Carbon-fibre reinforced glass and glass-ceramic ma-trix composites present a low coefficient of friction andexhibit relatively low wear rates due to the solid lubri-cant properties of ca rbon fibres. It has been shown, forexample, that the coefficient of friction of C-fibre re-inforced borosilicate glass matr ix composi te against castiron in pin-on-disc experiments (under pressures of1 M P a and velocity of 1 m/s) is about 0 .1 , while the un-reinforced glass matr ix exhibited a friction coefficient of0.5 [1]. It has also been found that the friction coefficientof unreinforced borosilicate (Pyrex®) glass is about tenReceived 15 August 2000.t imes higher t h a n tha t of the c a r b o n reinforced mate r ia lwhen tested against steel [2].Despi te these encouraging results, the a u t h o r s a ren o t aware of previous research conduc ted to assess thesuitabili ty of glass mat r ix composi tes as wear- res i s tan tmater ia l in the v a c u u m technique, a l t hough glass, es-pecially of borosil icate compos i t ion , is a c o m m o n ma te -rial used in this appl icat ion area [10]. O the r mater ia ls ,no tably polymer-based composi tes , have been m o r e fre-quent ly considered for appl icat ions in v a c u u m p u m p s[11]. Thus , the p r imary purpose of the present w o r k wasthe investigation of the tr ibological behaviour of a car-bon-f ibre reinforced borosil icate glass mat r ix c o m p o s i t eas self-lubricating mater ia l for possible appl ica t ions inro ta ry vacuum pumps .2 . E x p e r i m e n t a l d e t a i l sT h e mater ia l investigated was a b idi rect ional C-fiber(Toray T800) reinforced borosil icate ( D U R A N ® ) glassAldo R. Boccaccini; Gevork Gevorkian:Table 1. Mechanical properties of the C-fibre reinforced glassmatrix composite investigated [13]propertybending strength in MPa 445Young's modulus in GPa 71work of fracture in kJ/m^ 21.9Figure 1. Schematic diagram showing the vane-stator vacuumpump configuration used for the tribological tests.mat r ix compos i te fabricated by Schot t Glas, Mainz(Germany) . The composi tes were p r epa red by the sol-gel-slurry me thod , as repor ted in the l i terature [12]. Thefibres were a r ranged in 0/90 ply layers. N o m i n a l proper-ties of the composi te are given in table 1 [13]. The fibrevo lume fraction was —0.4. T h e compos i t e exhibited a fairly h o m o g e n e o u s dis t r ibut ion of the fibres and ab-sence of porosi ty in the glass matr ix . T h e samples werereceived in the form of rec tangular tiles of nominal di-mens ions ((2.87 X 25 X 100) mm^) , from which thevanes for the tr ibological exper iments were cut.T h e exper iments were carr ied o u t on a vacuump u m p s tand , which is schematically depic ted in figure 1.T h e eccentrically located ro to r rotates in the cylindricalChamber of the p u m p . T h e vanes are located in the cut-t ings inside the rotor. D u r i n g the ro ta t ion , the vanesslide ou t of the rotor, pushed by centrifugal forces, untilthey m a k e contac t with the Stator a n d begin to slidea long it [14].T h e vanes were m a d e of the C-fibre reinforced glassmatr ix composi te . T h e size of the specimens was(2.87 X 24 X 25) m m ^ T h e con tac t ing surfaces of thevanes were cut at angles of abou t 65° . T h e p u m p Stator,serving as the counter-body, was m a d e of cast ironG G 2 5 (hardness : HB210 , surface roughness parameter:Ra = 0.87 pm) . It had a diameter of 70 m m and a lengthof 24 m m .The experiments were carried out at room tempera-ture. The pressure in the Chamber was measured to be700 mbar . The temperature was measured by means ofa thermocouple , connected to the Stator at a distance of0.5 m m from the surface of contact . The results wereregistered in a PC. After each experiment, the mass ofthe vane was determined through analytical weight. Theexperimental error in the determinat ion of the mass wasO.Ol g. The surface of the Stator was examined visuallyand the vane surfaces were observed by scanning elec-tron microscopy (SEM).Taking into consideration the tangential force on thevane, the load between the vane and the Stator was cal-culated to be in the ränge 3.8 to 5.8 N. The rotat ion ve-locity used was 1800 rpm, which resulted in a sliding ve-locity of 5.5 to 7.8 m/s, depending on the exact posit ionof the vane dur ing the rotation cycle.3 . R e s u l t s a n d d i s c u s s i o nThe tribological properties of C-fibre reinforced glassmatr ix composites have been investigated by several au-thors [1 to 9], but this is the first report dealing withwear behaviour in vacuum atmosphere. Dry sliding andabrasive wear behaviour, including the determinat ion ofwear rates and friction coefficients against different met-als (e.g. Steel, cast iron, a luminium alloys, etc.), ceramics(e.g. AI2O3, SiC), glass and abrasive counterpar ts (e.g.S i c paper) have been studied [1 to 9]. Generally, block-on-ring, r ing-on-ring or pin-on-disc testing configura-tions have been employed. The present work involves a more realistic testing condit ion relevant for applicationsin rotary vacuum pumps . Α rotary vane p u m p exper-imental Simulation was used here, where the vanes weremade of the C-fibre reinforced composite and the cast-iron Stator of the p u m p served as the counter-body.The dependence of the iron Stator surface tempera-ture u p o n dura t ion of sliding indicated that a significantincrease of the Stator temperature occurred. The tem-perature stabilized however after a given time period.Cont inuous temperature measurements indicated a tem-perature of 50 °C after 10 h test run. This value closelyagrees with those measured by McKitr ick et al. [3] dur-ing pin-on-disk wear tests of C-fibre reinforced glass ma-trix composi tes against cast iron.Figure 2 shows the time dependence of worn massloss of the C-fibre reinforced glass matr ix composi tevanes. After a 30 h run, which corresponds to 1182 kmof sliding distance, the absolute wear mass loss of thevanes was measured to be 0.10 g. This value of massloss, which is a measure of the wear resistance of thecomposite, is relatively low, if one relates it to the totalvolume of the vanes. This is a preliminary indication that10 2 0• T i m e in h-3 0 4 0>Figure 2. Time dependence of worn mass loss of C-fibre re-inforced glass matrix composite vanes during wear test in thevacuum pump configuration of figure 1.self-lubricating C-fibre reinforced glass matrix com-posites are candidate materials for wear-resistant appli-cations in vacuum pumps.The material microstructure plays an impor tan t rolein determining the active wear mechanism in fibre re-inforced composites, as pointed out in the literature[1 to 9]. In the present composites, on areas with highvolume fraction of fibres, a low rate of wear was de-tected and the surface was smooth . O n the contrary, inareas with lower fibre concentrat ion severe deteriorationof both fibre and matrix surface occurred.Α preliminary unders tanding of the wear mecha-nisms' operating was obtained by S E M Observation ofthe worn surfaces of the vanes (figures 3a and b). D a m -aged areas were clearly visible, which were the result offibre and matrix fractures caused by the abrasive wearmechanisms. Figures 3a and b show typical worn sur-faces of glass matrix composi te vanes after 30 h weartest durat ion. The typical feature is the accumulat ion ofwear debris in the form of a layer in some areas of theworn surface due to the effect of Protrusion of fibresoriented perpendicularly to the Stator surface, which im-pede the removal of matrix particles. This effect has alsobeen observed by Lu et al. in their study of the wearbehaviour of fibre reinforced glass matrix compositesagainst steel [8]. The observed inclined fractured edgesof the fibres, as seen for example in figure 3b, are causedby the fact that the vanes when sliding along the Statormake contact at a certain angle which depends upon thecontact Site geometry.The bending response of fibres oriented perpendicu-larly to the wear surface is critical for the wear behaviour- 3 p mb)Figures 3a and b. SEM micrographs showing typical worn sur-faces of vanes made from C-fibre reinforced glass matrix com-posite after 30 h run (1182 km friction distance). The fibreswere parallel (figure a) and perpendicular (figure b) to theStator surface.of these composi tes . This is because considerable shearloads are applied to the end of the fibres as a result ofthe ac t ing friction forces. Thus , fibre fracture m u s t occurfor wear to proceed. As pointed out by McK i t t r i ck et al.[3] fibre fracture occurs where there is an u n s u p p o r t e dlength. Loss of suppor t can result from the ch ipp ing ofthe mat r ix a round the fibre or by softening of the m a -trix, leaving the fibre as a cantilevered b e a m [3]. Thisfracture mechan i sm in fibres oriented perpendicular ly toAldo R. BoGcaccini; Gevork Gevorkian:20μιη^ original* surfaced) e)(— severe wear -) (mild wear)Figures 4a to f. Schematic illustration of the wear of graphite-fibre reinforced glass matrix composites, according to the modelof McKittrick et al. [3]; a) original surface, b) softening andflowing of glass, c) mild wear of smooth smeared film, d) chip-ping and breaking of smeared glass on fibres and interfilamentregions, e) breakage of unsupported critical fibre length, 0 softening and smearing of freshly exposed matrix.the Stator surface was clearly visible on the worn vanesurfaces (figure 3b). It is however absent in fibres lyingparallel to the Stator surface (figure 3a).T h e softening of the glass matr ix , which leads tosmear ing, affects the wear behaviour. A t high sUding ve-locities, higher interfacial t empera tures a re reached andgreater volume of glass can smear. T h e wear mecha-nisms of mater ial removal , as suggested by McKit t r icket al. [3], are illustrated schematically in figures 4a to f After the initial development of a s m o o t h surface (figure4a) , frictional heat ing causes softening a n d thermal ex-pans ion of the glass, which extends beyond the fibres asthese remain unexpanded (figure 4b) . Eventually theglass is smeared over the surface of the composi te (figure4c). D u r i n g this stage, small a m o u n t of glass is t rans-ferred to the counter-surface, result ing in mild wear. Forlonger sliding dura t ion , the glass layer may begin tobreak u p in some areas (figure 4d) leading to the forma-t ion of debris and severe wear damage . Glass fracturecan occur no t only at the original compos i t e surface butalso below it, leaving many graphi te fibres not sur-rounded by matr ix mater ia l . This is the stage docu-men ted in our own exper iments by figures 3a and b.Eventual ly these unsuppor t ed lengths of fibres arebroken off, leaving the surface relatively clean (figure 4e)a n d the process can start again (figure 4f).Thus , the wear of the glass mat r ix composi te wasaccompan ied by mater ia l transfer o n t o the Stator sur-face. This effect, which has been referred to as " thirdbody fo rmat ion" [9], has also been observed by othera u t h o r s in previous studies on the dry-wear behaviourof C-fibre reinforced glass mat r ix compos i tes [3, 8 and 9].Indeed the presence of a wear film of composi t ion in theSystem S i - C - O on the counterface is beneficial for a low wear rate and coefficient of friction.Figure 5. The counter-body (stator) after the wear experimentshowing the continuous shiny friction transfer film on the castiron surface.In our experiments, an isle-shaped discontinuousfilm was usually formed on the metal surface, which be-came cont inuous in some areas. The character of thismaterial transfer (i.e. isle or cont inuous film formation)generally depends upon the l oad -ve loc i t y (p-v) rängeat the contact . In the present composites, film formationshould be mainly associated with the extent of fragmen-tat ion of glass matrix and fibres. Moreover, a thermaleffect due to viscous deformation and smearing of thedetached glass particles should also be taken into ac-count [3]. Α significant feature determining the materialtransfer mechanism is the extent of adhesion of wearparticles to the metallic surface of the counter-body. Theextent of adhesion depends on ÜIQ p-v ränge, which isdifferent at different points of the stator in the eccentricrotor configuration employed (see figure 1). When theadhesion is poor, isle-type film formation occurs. On theother band , for s t reng adhesion, the transfer film be-comes continuous. As figure 5 shows, the cont inuousfriction film formed on the stator surface was shiny andthin. Visual inspection of the film led to the conclusionthat this is a graphite-rich film. Thus, dur ing the con-t inuous friction process, an elastic friction contact wasprovided, which led to self-lubricating condit ions and tothe relatively low wear of the composi te vanes. More-over, the formation of a continuous film is desired be-cause the wear particles are kept in the wear zone andTHE VACUUM CHAMBER REMAINS FREE OF WEAR DEBRIS. IT MUSTALSO BE POINTED OUT THAT THE ENVIRONMENT PLAYS AN I M -PORTANT ROLE IN THE PROCESS OF FILM TRANSFER FORMATION.IT IS KNOWN THAT VACUUM FAVOURS THE ADHESION OF WEARPARTICLES TO THE COUNTER-BODY [ 1 5 ] .4 . C O N C L U S I O N SWEAR TESTS OF C-FIBRE REINFORCED BOROSILICATE GLASS MATRIXCOMPOSITES WERE CARRIED OUT IN A VACUUM P U M P CONFIGU-RATION. FIBRE FRACTURE AND GLASS MATRIX PARTICLE DETACH-MENT WERE OBSERVED AS WEAR MECHANISMS, AND THE WEARBEHAVIOUR OF THE COMPOSITES COULD BE DESCRIBED QUAUTA-TIVELY BY THE MODEL OF MCKITTRICK ET AL. COMPOSITE WEARIS ACCOMPANIED BY MATERIAL TRANSFER ONTO THE STATOR SUR-FACE. Α GRAPHITIC FILM IS FORMED ON THE COUNTER-BODY SUR-FACE, WHICH PROVIDED ADEQUATE LUBRICATION DURING THEEXPERIMENT LEADING TO RELATIVELY LOW WEAR RATES. THESEPRELIMINARY EXPERIMENTS SHOW THAT THE SELF-LUBRICAÜNGGLASS MATRIX COMPOSITE MATERIAL INVESTIGATED MAY BE ANADEQUATE CANDIDATE FOR DRY WEAR APPLICATIONS IN VACUUMP U M P S . HOWEVER, FINAL CONCLUSIONS CAN BE DRAWN ONLYAFTER A THOROUGH EXAMINATION OF THE WEAR PROCESSES TAK-ING PLACE UNDER VARIOUS LOADS, VELOCITIES AND LEVELS OFVACUUM HAS BEEN UNDERTAKEN. T H I S IS THE FOCUS OF CUR-RENT RESEARCH.The authors are grateful to Professor W. Beier and Mr. R. Lie-bald, Schott Glas (Germany), for providing the compositesamples and for their interest in this work. Helpful discussionswith Professors H.-J. Schorcht and H. Kern, Technical Univer-sity Ilmenau (Germany), are acknowledged.5. References[1] Lu, Z.: Tribological properties of unidirectionally orientedcarbon fibre reinforced glass matrix composites. In: Fried-rich, K. (ed.): Advances in Composite Tribology. Amster-dam: Elsevier, 1993, p. 367-401.[2] Khanna, V. D.; Eliezer, Z.; McKittrick, J. et al.: Frictionand wear of glass-matrix/graphite-fibre composites. In:Hack, J.; Amateau, M. (eds.): Proc. Mech. Behavior ofMetal Matrix Composites, AIME, 1983, p. 227-234.[3] McKittrick, J. M.; Sridharan, N. S.; Amateau, M. F.: Wearbehaviour of graphite-fibre-reinforced glass. Wear 96, 1984,p. 285-299.[4] Bleay, S. M.; Scott, V. D.; Harris, B.: Wear studies of fibre-reinforced ceramic matrix composites. J. Mat. Sei. Lett. 13(1994) p. 663-667.[5] Völker, K.; Zum Gahr, K.-H.; Pannhorst, W. et al.: Unter-suchungen zur Haftreibung zwischen faserverstärktemBorosilikatglas und Kalk-Natron-Glas bei erhöhten Tem-peraturen. Matwiss. u. Werkstofftech. 30 (1999) p. 5 - 1 3 .[6] Reinicke, R.; Friedrich, K.; Beier, W. et al.: Tribologicalproperties of SiC and C-fibre reinforced glass matrix com-posites. Wear 225-229 (1999) p. 1315-1321.[7] Minford, E.; Prewo, K. M.: Friction and wear of graphite-fiber-reinforced glass matrix composites. Wear 102 (1985)p. 253-264.[8] Lu, Z.; Friedrich, K.; Pannhorst, W. et al.: Sliding wearof a unidirectional carbon fibre-reinforced glass compositeagainst steel. J. Mat. Sei. Lett. 12 (1993) p. 173-175.[9] Bianchi, V; Fournier, P; Piaton, F et al.: Carbon fibre-reinforced (YMAS) glass-ceramic matrix composites: dryfriction behaviour. J. Europ. Ceram. Soc. 19 (1999)p. 581-589.[10] Wutz, M.; Adam, H.; Walcher, W: Handbuch der Vaku-umtechnik. Braunschweig et al.: Vieweg, 1997.[11] Gevorgyan, G.; Heinritz, W; Schorcht, H.-J.: Experimentalstudy on wear of metal-polymer friction sites in vacuumcondition. In: Proc. of Int. Symp. on the Nature of SolidsFriction, Gomel (Belarus) 1999.[12] Pannhorst, W; Spallek, M.; Brückner, R. et al.: Fibre-re-inforced glasses and glass-ceramics fabricated by a novelprocess. Ceram. Eng. Sei. Proc. 11 (1990) nos. 7 - 8 ,p. 947-963.[13] Skopp, Α.; Woydt, M.; Habig, K.-H. et al.: Friction andwear behaviour of C- and SiC-fibre-reinforced glasscomposites against ceramic materials. Wear 169 (1993)p. 243-250.[14] Hablanian, M. H.: High-vacuum technology: a practicalguide. New York et al.: Dekker, 1990.[15] Miyoshi, K.: Considerations in vacuum tribology (ad-hesion, friction, wear, and solid lubrication in vacuum).Tribology Int. 32 (1999) p. 605-616.• 0 1 0 1 T 0 Ö 3Addresses of the authors:A. R. BoccacciniDepartment of MaterialsImperial College of Science,Technology and MedicinePrince Consort RoadLondon SW7 2 BP (UK)E-mail: a.boccaccini@ic.ac.ukG. GevorkianFachgebiet MaschinenelementeTechnische Universität IlmenauPostfach 100565D-98684 Ilmenau

Carbon-fibre reinforced glass matrix composites: self-lubricating materials for wear applications in vacuum

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Carbon-fibre reinforced glass matrix composites: self-lubricating materials for wear applications in vacuum

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Repositorium für Naturwissenschaften und Technik