A comparative examination of the friction coefficient of two different sliding bearing

Transcription

1 Materials Science Forum Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland A comparative examination of the friction coefficient of two different sliding bearing M. Svéda 1, A. Roósz 1-2, G. Buza 3, L. Kuzsella 2 1 Hungarian Academic of Science-University of Miskolc Research Group of Materials Science, University of Miskolc, H-3515 Miskolc-Egyetemváros, Hungary 2 University of Miskolc, Materials Science Institute, Physical Metallurgy Department, H-3515 Miskolc-Egyetemváros, Hungary 3 Bay Zoltán Institute of Materials Science and Technology femmaria@gold.uni-miskolc.hu Keywords: Monotectic alloys, Sliding Bearings, Scanning electron microscopy Abstract: The aim of this research work is to investigate the sliding properties of the monotectic surface layers developed by a laser surface-treatment technology. The coatings-remelting technology has been chosen from the laser surface treatment methods. The surface of Al-Si alloy was coated with a lead layer by galvanizing, then the basic material and the surface layer were remelted together by using laser beam produced a monotectic Al- Pb surface layer. The structure of monotectic surface layer has been determined by means of a light microscope and scanning electron microscope. The sliding properties of the basic material (as cast, nearly eutectic Al-Si alloy), the Al-Si-Pb monotectic surface layer as well as the Al-Cu-Sn sliding bearing (Al-Cu matrix and Sn sliding layer) used in the gas industry have been investigated. Introduction The monotectic alloy-systems are often used for the material of sliding bearings. These sliding bearings can be made by powder metallurgy [1-3], by a strip casting method [4-7], by a planner flow casting method [8] and a rheocasting method [9-11]. Most part of the bearings are built into the combustion engines. It is necessary to make structures having small sizes and masses with high thermal- and mechanical efficiency in order to increase the power of engines. The two most important functions of a bearing are to take up force and to decrease the force of friction developing under the influence of the relative movement to the minimum. Owing to the complicated character of load a sliding bearing shall be of several layers and these layers shall often have different properties. The most common materials of the sliding bearings used nowadays are lead-tin-bronze and aluminum-copper-tin alloys. However these materials have already reached their highest limit of load, therefore it has become necessary to develop sliding bearings having higher load ability. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-19/09/16,08:45:30)

2 472 Materials Science, Testing and Informatics II Experiments The monotectic surface layers were developed by a CO 2 laser equipment having a power of 5 kw in the Bay Zoltan Institute for Materials Science and Materials Technology. As the first step of the new technology, the surface of the Al-Si sample was coated by a lead layer having a thickness of about 0.1 mm by galvanizing. After that the lead layer and the surface of the sample having a thickness of about 1 mm were remelted together using different laser power (2; 2.5; 3; 3.5 kw) and a feed rate of 600 mm/min, produced a monotectic alloy layer. Because the high cooling rate at the resolidification the lead droplets can not separate from the aluminium matrix, and these soft droplets serve for solid lubrication material. The essence of the laser alloying that only the properties of the surface layer change, the properties of the whole mass of work piece remain unchanged. During the laser treatment, 8 bands were remelted on each sample with an overlap of 50%. The focus diameter of laser was 2 mm. The samples were coated with a graphite spray owing to the low absorption of Al-alloys. The Al-Cu-Mg basic material was coated with the tin layer by using centrifugal casting. The surface layers and gliding properties of basic materials (cast, nearly eutectic Al-Si alloy), monotectic Al-Si-Pb alloys developed by using laser remelting as well as Al-Cu-Mg-Sn sliding bearings have been investigated by tribological investigations of pin on disk system. The surface layer damaging in the course of tribological investigations has been investigated by using scanning electron microscope. Sample preparation and investigation methods After bedding the samples into a conducting synthetic resin, they were ground, polished and etched in the 0.5% water solution of HF. The surface layer prepared such a way was investigated by using a scanning electron microscope. In case of an pin-on disc type tribological investigation a pin having a ball form end was pressed perpendicular to the specimen having a disc-shape by a given force (F s ). The disc was rotateted with a constant angle-velocity, at a given distance (r) from the rotation axis of disc. The distance and the force of normal direction were adjusted before starting the measurement and it is constant during the measurement. The measurements were made by a THT-S-AX-0000 type tribometer (Fig. 1). The equipment determines the friction force (Fs) of two displacement-measuring units, and this force can be registered continuously depending on the frequency of data collection. So the value of coefficient of friction can be calculated as the quotient of F s force and F n force coming from the constant load. F s µ = (1) F n The investigation parameters were the following: temperature: 25 o C, pin-material: Cr6 ball with a diameter of 6 mm, surface velocity: 10 cm/s, abrasion path: 2500 m. The abrasion surfaces were investigated by using a scanning electron microscope in order to determine the damage of surface layers.

3 Materials Science Forum Vols Fig. 1, THT-S-AX-0000 type tribometer Results Fig. 2, Pin on disk type tribometer schematic diagram Figs. 3 and 4 are the scanning electron microscope images taken of the original surface layers of Al-Si-Pb and Al-Cu-Mg-Sn alloys. No sharp boundary can be observed between the basic material and the surface layer in case of the Al-Si-Pb alloy, the surface layer grows directly from the basic material. A metallic bond develops between the two layers. In case of the Al- Cu-Mg-Sn alloy there is an adhesion bonding between the basic material and the Sn surface layer. Fig. 3, Al-Si-Pb monotectic surface layer Fig. 4, Surface layer of Al-Cu-Mg-Sn alloy SEM,BSE image, 25x SEM,BSE image, 25x Fig. 5 shows the scanning electron microscope image of the surface layer of Al-Si-Pb alloy, the areas having a light round shape are the Pb precipitations that serve as solid lubricants and ensure suitable sliding properties. The light grey area is the Si-phase of eutectic. In case of the Al-Cu-Mg-Sn sliding bearing alloy the suitable sliding properties are ensured by the surface layer having an Sn-content of 95 mass% and not by any precipitations (Fig. 6). Fig. 5, Lead drops in remelted band SEM, BSE image, 1000x Fig. 6, Sn surface layer SEM, BSE image, 1000x

4 474 Materials Science, Testing and Informatics II The values of friction coefficient determined by the measurements are contained in Table 1. Table 1 The values of coefficient of friction for three different materials Materials Friction coefficient Cast Al-Si alloy 0.62 Laser remelted Al-Si-Pb monotectic surface layer 0.53 Al-Cu-Mg-Sn alloy surface layer 0.54 The value of the measured friction coefficient is approximately in a good agreement with the values of the friction coefficient of AlPb10 and Al-Cu-Mg-Sn alloys made by a different technology, described in the references [12-17]. The value of the friction coefficient of Al-Si eutectic alloy serving as a basic material is higher than the values of the friction coefficient of Al-Pb-Si and Al-Cu-Mg-Sn. The value of the friction coefficient of monotectic surface layer developed by them has suitable properties for using them as a material for sliding bearing. Figs 7-8 are the secondary electron scanning electron microscope images (SE SEM) of the worn surfaces of Al-Si-Pb and Al-Cu-Mg-Sn alloys. The surface layer was wiped during the wearing. Fig. 7, The worn surfaces of Al-Si-Pb SEM,SE image, 50x Fig. 8, The worn surfaces of Al-Cu-Mg-Sn SEM,SE image, 50x Fig. 9, The worn surfaces of Al-Si-Pb SEM, BSE image, 500x Fig. 10, The worn surfaces of Al-Cu-Mg-Sn SEM, BSE image, 500x

5 Materials Science Forum Vols Figs show the back-scattered scanning electron microscope images (BSE SEM) of the worn surface of Al-Si-Pb alloy remelted by laser. In Fig. 9 the light lead drops can well be seen. These lead drops can be wiped during wearing so they can ensure the suitable gliding properties. Fig. 10 shows the wiped lead drops in a higher magnification (500x). Figs show the BSE SEM image of the worn surface of Al-Cu-Mg-Sn alloy. It can well be seen that during wearing the surface layer tears up and in some places the Al-Cu-Mg basic matrix appears (Fig. 12). The reason of it is that the thickness of surface layer of specimen taken from the curved surface for the purpose of tribological investigations was not uniform. Al-Cu Sn Fig. 11, The worn surfaces of Al-Cu-Sn alloy SEM, BSE image, 50x Fig. 12, The worn surfaces of Al-Si-Pb alloy SEM, BSE image, 50x In case of sliding bearings besides the sliding properties - the wearing properties of the investigated surface layers are also very important characteristics from the point of view of life. The investigations of sliding properties were completed by profilometric investigations by which the worn volume can be determined by the topografic scanning of the worn surface. The wearing investigations can also be characterized by the value of coefficient of wearing; it can be determined from the area under the peak. Figs. 13 and 14 show the results of profilometric investigations of the worn, laser remelted Al-Si-Pb alloy and worn Al-Cu-Mg-Sn alloy. The shapes of the profilometric curves of the two alloys differ from each other, but the worn volumes are the nearly same in both cases. Fig. 13, Results of profilometric investigations of the worn Al-Si-Pb layer

6 476 Materials Science, Testing and Informatics II Fig. 14, Results of profilometric investigations of the worn Al-Cu-Mg-Sn layer Conclusions The aim of investigations is to compare the structures and sliding properties of monotectic surface layer of Al-Si-Pb alloy remelted by laser, made by us and the Al-Cu-Mg-Sn alloy used in the gas industry. It can be stated that the values of friction coefficient of laser-remelted Al-Si-Pb alloy and Al- Cu-Mg-Sn alloy agree with the values of friction coefficient of similar alloys made by different technologies described in the references. The value of friction coefficient of Al-Si eutectic alloy serving as basic material is higher than the values of friction coefficient of surface layers of Al-Pb-Si and Al-Cu-Mg-Sn alloys. The values of coefficient of friction of surface layers are in agreement, so the monotectic surface layer developed by them has suitable properties for using it as a material for making sliding bearings. References [1] K. Nishiyama et al: Japan Soc. Power Metall. (1993) p. 645 [2] D. P. Howe, A. A. Torrance, J. D. Wialliams; Mat. Sci Tech. Vol.7 (1991) pp [3] M. Zhu et. al; Wear, 242 (2000) pp [4] S. Mohan, V. Agarwala, S. Ray; Mater. Trans. JIM 33 (1992) p [5] L. Ratke, S. Diefenbach; Mat. Sci. Eng. Vol. R15 (1995) [6] S. K. Srivastava, S. Mohan, V. Agarwala, R. C. Agarwala; Metall. Mater. Trans. A 25A (1994) pp [7] J. Z. Zhao, S. Drees, L. Ratke; Mat. Sci. Eng. A 282 (2000) pp [8] T. Berrenger, P. R. Sahm; Z. Metallk. 87 (1996) 3 p. 188 [9] R. Gray, S. Mohan, V. Agarwala, R.C. Agarwala; Z. Metallk. 84 (1993) p. 723 [10] S. Mohan, V. Agarwala, S. Ray; Mater. Trans. JIM 33 (1992) pp [11] V. Agarwala et al.; Mat. Sci. Eng. A 327 (2002) pp [12] J. An et al.: Tribology International 36 (2003) pp [13] J. An, at al:,mat. Characterization 48 (2002) pp [14] V. Agarwala, et al: Mat. Sci. and Eng. A327 (2002) pp [15] M. Zhu et al.: Wear 9198 (2002) pp 1-7 [16] M. Zhu et al.: Wear 242 (2000) pp [17] J. An, at al: Mat. Characterization 47 (2001) pp