Materials and Design 28 (2007) Short communication. A study on tribological behaviour of tin-based bearing material in dry sliding

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1 Materials and Design 28 (2007) Short communication A study on tribological behaviour of tin-based bearing material in dry sliding A. Zeren *, E. Feyzullahoglu, M. Zeren University of Kocaeli, Faculty of Engineering, Vinsan Campus, _Izmit, Turkey Received 13 January 2005; accepted 26 May 2005 Available online 26 July 2005 Materials & Design Abstract In this study, we investigated the tribological behaviour of two different tin-based bearing materials in dry sliding conditions. One of these alloys with low Sb content (7%) is known as SAE 12 and is widely used in the automotive industry and the other with high Sb content (20%) is a Sn Sb Cu alloy. Wear and friction characteristics were determined with respect to sliding distance, sliding speed and bearing load, using a Tecquipment HFN type 5 journal bearing test equipment. Hardness measurements were carried out to determine the effect of the increase in Sb content and its impact on tribological properties. Light microscopy is used to understand the tribological events in these two different bearing materials. Thus, the purpose of this study is to investigate the tribological properties of tin-based bearing alloys with different compositions, used especially in heavy industrial service conditions. Tests were carried out in dry sliding conditions, since despite the presence of lubricant film, under heavy service conditions dry sliding may occur from time to time, causing local wear. As a result of local wear, bearing materials and bearing may be out of their tolerance limits in their early life time. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Tin-based materials; Tribology; Dry sliding 1. Introduction Many materials have been tried as bearing components. In 1839, Babbitt patented a Sn Sb Cu alloy for use in journal bearings. White metal is now widely used as a material for sliding bearings operating under oil lubrication, for example, bearing for general industrial use, marine use and automotive use. One of the most heavy duty applications of thrust bearings is in hydroelectric power stations for support of the shaft, carrying hydraulic turbine and electric generator. White metal can be fundamentally classified into two types. One has lead as its main component, the other tin. A bearing works in * Corresponding author. Tel.: /51168; fax: address: zeren@kou.edu.tr (A. Zeren). a stabilised manner when a proper film thickness is formed and maintained between shaft and bearing. However, under unacceptably high loads and shaft revolution speeds, or improper lubricating conditions, a bearing is often damaged when a sufficient thickness of the oil film is not formed between shaft and bearing. Under these conditions, shaft and white metal make partial contact with each other during the sliding wear process. This condition is called boundary lubrication [1 4]. Lead and tin white metal alloys are commonly the first choice for bearing materials in offering superior compatibility with steel shafts, their ability (due to their softness) to embed foreign particles, and their unique ability to adapt to misalignment by mild wiping on initial run-in as enabled by their low melting points. Table 1 covers representative physical properties of typical Babbitt compositions [2] /$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi: /j.matdes

2 A. Zeren et al. / Materials and Design 28 (2007) Table 1 Physical properties of babbitte alloys (tin-based white metals) Tin-base Designation ASTM B SAE Nominal composition (%) Tin Lead Antimony Copper Arsenic Specific gravity Melting point ( C) Complete liquefaction ( C) Brinell hardness Ultimate tensile strength (MPa) Compressive yield strength (MPa) Approximate strength retained at 100 C C C Despite their higher cost, tin babbitts are often used in preference to lead babbitts for their excellent corrosion resistance, easy bonding, and less tendency towards segregation. SAE 12 (ASTM Grade 2) is widely used in industrial and automotive bearings. SAE 11 and ASTM Grade 3 also find extensive industrial use [2 5]. Table 2 shows the characteristics of widely used bearing materials [5]. The babbitts are among the most widely used materials for hydrodynamically lubricated bearings. Babbitts are either tin or lead-based alloys having excellent embeddability and conformability characteristics. They are unsurpassed in compatibility and thus prevent shaft scoring [6 9]. Tin Babbitt alloys commonly contain about 3 8% copper and 5 8% antimony. Within a soft solution matrix antimony in tin, small, hard Cu 6 Sn 5 copper tin intermetallic particles are dispersed. Increasing copper increases the proportion of Cu 6 Sn 5 needles or stars in the microstructure. An increase in antimony above 7.5% results in antimony tin cubes. Hardness and tensile strength increase with greater copper and antimony content, while ductility decreases. Low antimony (3 7%) and low copper content (2 4%) provide maximum resistance to fatigue cracking. Since these low-alloy compositions are relatively soft and weak, a compromise is often made with fatigue resistance and compressive strength [1,2]. Table 2 Characteristics of widely used bearing materials Physical property Significance of property in service Characteristics of widely used materials White metals Copper-base alloys Aluminium-base alloys Fatigue strength Compressive strength Embeddability Conformability Compatibility Corrosion resistance To sustain imposed dynamic loadings at operating temperature To support uni-directional loading without extrusion or dimensional change To tolerate and embed foreign matter in lubricant, so minimising journal wear To tolerate some misalignment or journal deflection under load To tolerate momentary boundary lubrication or metal-to-metal contact without seizure To resist attack by acidic oil oxidation products or water or coolant in lubricant Adequate for many applications, but falls rapidly with rise of temperature Wide range of strength available by selection of composition As above As above As above Excellent-unequalled by any other bearing materials Tin-base white metals excellent in the absence of sea water. Leadbase white metals attacked by acidic products Inferior to white metals. Softer weaker alloys with low melting point constituent, e.g., lead; superior to harder stronger alloys in this category. These properties can be enhanced by provision of overlay, e.g., lead tin or lead indium, on bearing surface where appropriate Lead constituent, if present, susceptible to attack. Resistance enhanced by lead tin or lead tin copper overlay Similar to copper-base alloys by appropriate selection of composition Inferior to white metals. Alloys with high content of low melting point constituent, e.g., tin or cadmium; superior in these properties to copper-base alloys of equivalent strength. Overlays may be provided in appropriate cases to enhance these properties Good. No evidence of attack of aluminium-rich matrix even by alkaline high-additive oils

3 320 A. Zeren et al. / Materials and Design 28 (2007) Experimental 2.1. Experimental apparatus The apparatus used for tribological studies is illustrated in Fig. 1. Different pressure values were achieved by using different loads and different velocities were obtained by a speed regulator which allows speeds up to 1900 rpm. The temperatures of the friction surfaces were measured by the thermometer placed at 3.5-mm beneath the surface of the bearing housing Aim of the experiments The following aspects were investigated in the experiments, for moderate and very slow speeds: The effect of sliding distance consequently sliding time on tribological behaviour. The effect of load on tribological behaviour. The effect of velocity on tribological behaviour. The effect of temperature on tribological behaviour. Fig. 1. Journal bearing system used in the experimental study. (1) Control box with speed regulator. (2) D.C. shunt wound motor. (3) Bearing housing. (4) Torque hanger. (5) Load hanger. Table 3 Chemical compositions of specimens Sb Pb Cu Ag As Bi Cd Cr Zn S Al Fe In Tl Te Ni Sn < <

4 A. Zeren et al. / Materials and Design 28 (2007) Materials and experimental conditions In the experimental set up used, the shaft was made of AISI 440C stainless steel and bearings were made of tin-based white metal. The chemical analysis of bearing materials is given in Table 3. The relative bearing clearance was as shown in Fig. 2. Hardness of specimens is represented in Table Results of the experiments In Fig. 3, variation of friction torque with sliding distance is presented for the tin-based bearing alloys used AA SECTION Thermometer hole Dept: 3.5 mm in the experiments. For high sliding distances (above 50,000 m), friction torque appears to be almost constant and this constant value is higher for both of the alloys higher for. The amount of wear was determined using the weight loss measured by a Scaltec balance. Fig. 4 shows which wear increases linearly with sliding distance for both alloys. As given in Table 4, is harder than and consequently wear is 50% lower in this alloy. This indicates that service life of will be doubled with respect to the conventional. Thus, one can conclude that, use of will be more advantageous since labour and material cost and also machine down time will be lowered. Fig. 5 shows the relationship between friction coefficient and sliding distance for two different bearing alloys. It can be seen from the figure, that friction coefficient is approximately constant for sliding distances 10 8 Fig. 2. Technical drawing of bearing used in the tests. We ig ht l os s (g) r Table 4 Hardness and wear in dry sliding of tested materials Materials Hardness (HB) Wear in dry friction (g) Sliding Distance (m) Bearing Load : 115 N, Shaft Speed : 750 rpm Fig. 4. Relationship between weight loss and sliding distance Frict ion Toq r u e (Nm m) Fr iction Coe fficien t Sliding Distance (m) (Bearing Load : 115 N, Shaft Speed : 750 rpm) Sliding Distance (m) Bearing Load : 115 N. Shaft Speed 750 rpm Fig. 3. Relationship between friction torque and sliding distance. Fig. 5. Relationship between friction coefficient and sliding distance.

5 322 A. Zeren et al. / Materials and Design 28 (2007) over 50,000 m, and this constant value is higher for. Fig. 6 illustrates the relationship between friction coefficient and shaft speed. The friction coefficient remains constant up to 1400 rpm and then sharply decreases at this speed. This result indicates that both of these materials can be used at high shaft speeds for a long time without a significant wear and change in bearing tolerances. This is an important property for engines used in automotive and hydro-electric industry. Fig. 7 represents relationship between friction coefficient and bearing load. As expected, friction coefficient increased with increasing bearing load for both materials. The microstructure of the alloys that were used in the experimental study was investigated by light microscope to understand the tribological behaviour better. The alloys were prepared by metallographical techniques and etched with Nital after polishing. The micrographs shown in Figs. 8 and 9 belong to the alloys and, respectively. Fig. 8. Microstructure of, 500, etched with Nital Fr ic tion C oefici f en t Shaft Speed (rpm) Fig. 9. Microstructure of, 500, etched with Nital. Fr ic tion C oefici f en t Fig. 6. Relationship between friction coefficient and shaft speed Bearing Load (N) Fig. 7. Relationship between friction coefficient and bearing load. As seen in Fig. 8, the Cu 6 Sn 5 intermettallics are distributed in the Sn matrix and have characteristic large star shapes that can be easily identified [1]. The SbSn intermetallics are small white precipitate, dispersed in the solid solution. In Fig. 9, the SbSn cubics are clearly identified in the Sn matrix. These cubics are not observed in alloys that have Sb contents lower than 7.5% [1]. The higher hardness and better tribological properties of, compared to, are due to the presence and homogeneous dispersion of these hard, sharp cornered cubics, in the matrix. 3. Conclusion 1. and alloys can be used in dry sliding conditions.

6 A. Zeren et al. / Materials and Design 28 (2007) It is shown that performance of under heavy service conditions is better than due to its alloying elements. 3. The amount of wear in is lower than under similar tribological loading conditions. Thus, service life of is expected to be longer than that of. 4. Increasing Sb content from 7.5% to 20% provided an increase in hardness of. 5. Wear with respect to sliding distance is lower in WM- 5 alloy that has higher Sb content. 6. Friction coefficient with respect to sliding distance is lower in alloy that has higher Sb content. This result indicates lower metal metal friction for bearing and shaft materials and thus lower thermal effects in the bearing. In the literature, seizure failure type of Tin Babbitt bearings is Babbitt failure at critical temperatures [10]. Thus, materials having lower friction coefficient are preferred. 7. As shown in Fig. 6, for both alloys friction coefficient sharply decreases after shaft speed of 1400 rpm. So both of these materials can be used at high shaft speeds and and are suitable materials for automotive industry. 8. WM group alloys are not new materials. These materials are known since 1800s. These bearing materials, that are basically Sb Sn Cu alloys, can be safely used in a large variety of industrial applications, by adding different alloying elements and providing the microstructural characterization. References [1] Lepper K, James M, Chashechkina J, Rigney DA. Sliding behaviour of selected aluminum alloys. Wear 1997; : [2] Booser E, Khonsari M. Applied tribology bearing design and lubrication. NY: Wiley; [3] Iliev H. Failure analysis of hydro-generator thrust bearing. Wear 1999; : [4] Tamura K, Ishihara S, Goshima T, Tachi Y. Effect of cyclic load and sliding speed on sliding wear characteristics of a bearing lined with WJ7 white metal. Proc Inst Mech Eng Part J J Eng Tribol 2004;218: [5] Neale MJ. Bearings: a tribology handbook. Scotland: Butterworth/Heinemann; [6] Hamrock BJ. Fundamentals of fluid film lubrication. McGraw- Hill; [7] Shigley JE, Mischke CR. Mechanical engineering design. Singapore: McGraw-Hill; [8] Rigney DA. Comments on the sliding wear of metals. Tribol Int 1997;30(5): [9] Jacobson B. The Stribeck memorial lecture. Tribol Int 2003;36: [10] Wang Q. Seizure failure of journal bearing conformal contacts. Wear 1997;210:8 16.