MECHANICAL AND WEAR PROPERTIES OF COPPER-LEAD ALLOY PREPARED BY POWDER METALLURGY PROCESSING TECHNIQUE

Transcription

1 Journal Journal of Chemical of Chemical Technology and and Metallurgy, 51, 6, 51, 2016, MECHANICAL AND WEAR PROPERTIES OF COPPER-LEAD ALLOY PREPARED BY POWDER METALLURGY PROCESSING TECHNIQUE Muthuchamy Ayyapan, Narendra Kumar Uttamchand, Raja Annamalai Arunjunai Rajan Department of Manufacturing Engineering School of Mechanical Engineering VIT University Vellore Tamil Nadu, India Received 30 June 2015 Accepted 03 June 2016 ABSTRACT Cu-12Pb alloy powder samples were prepared and sintered at 500ºC, 700ºC and 900ºC. Their mechanical and tribological properties were identified. The sintering temperature effect on the dimensional shrinkage was studied. It was found that shrinkage in the axial direction of sintering was generally higher than that in the radial one. A significant reduction of the wear rate was achieved upon addition of Pb. The wear tests conducted at room temperature revealed that the Pb film obtained initially served as a lubricating layer but ruptured because of plastic deformation on further Pb load increase thus changing the wear mode form oxidative to abrasive. Keywords: shrinkage, self- lubricating materials, liquid phase sintering, wear volume, oxidative wear. INTRODUCTION Considerable efforts have been put to develop materials that can conform to the requirements of tribological systems. Cu-Pb alloys are widely used as antifriction materials in the form of bearings, gears, seals, brakes and cutting tools in mechanical assemblies. Resistance towards wear, scoring, corrosion and fatigue are the primary requirements of ball bearing materials. The latter should also have a low coefficient of friction (COF) and adequate plasticity, toughness and compressive strength. Lead has good scoring resistance, while copper has fair to poor one while sliding against steel [1]. Pb being a soft metal adheres well to these properties, but due to its poor yield strength and low hardness it needs to be alloyed with Cu and similar hard metals for application purposes. It not only increases the real contact area during sliding but also possesses properties of anti-seizure, self- lubrication and deformability. Cu-Pb alloys are generally used in applications which involve higher operating speed and lower mechanical loads. However, the main constraint arises due to limited solubility of Pb in Cu [2]. Pb mixes well in a liquid state but its solubility decreases with temperature decrease during solidification. It is well recognized that pore volume reduction results in shrinkage during compact materials sintering. There can be various types of shrinkages such as volume shrinkage, closely linked with densification during sintering, axial shrinkage along the direction of pressing and radial shrinkage observed at a right angle to the direction of pressing. It is not correct to assume isotropic conditions of powder compacts after pressing unlike the case of casting or an equality of linear shrinkages in the radial and axial directions. Furthermore, the magnitudes of the shrinkages considered and the R/A ratios vary within wide limits depending on a number of factors which govern the properties of the powders being compacted and the technological parameters of compacting and sintering. Pathak et al. [2] have shown 726

2 Muthuchamy Ayyapan, Narendra Kumar Uttamchand, Raja Annamalai Arunjunai Rajan that Cu-Pb alloys mechanical properties deteriorate with Pb content increase. They observed that the wear rate decreases initially but then starts to increase with sliding velocity increase under constant load conditions. The tangential force stays unchanged during sliding but the pressure varies depending on the area of contact. Furthermore friction is considered as a combined effect of adhesion, ploughing and asperity deformation, whose individual contributions depend on factors like environment, topography and material property at the sliding interface. The tribomechanical responses of these alloys have been extensively studied, especially in case of changing loads and compositions [3, 4]. Buchanan et al. [1] have claimed that an increase in pressure causes a transition from high to low friction values. Hence, the ability to squeeze out lead onto the surface will significantly influence the friction values. The mechanisms of wear have been studied for decades and it has been confirmed that oxidation, adhesion and abrasion affect significantly the process of wear. However the sintering temperature effect on the wear rate has not been extensively studied. It is reported that the antiseizure property is degraded when Pb is less than 8 %, while the alloy s strength is deteriorated when it exceeds 30 %. That is why Pb content is limited within the 8 %- 30 % range [5]. The aim of the present study was to investigate the nature of the wear track and the variation of the wear rate of Cu and Cu-12Pb alloy at various sintering temperatures. EXPERIMENTAL Premixed Cu-12Pb powder was used for the preparation of the specimens. The powder size distribution is listed in Table 1. The average particle size was ca µm. The powders were compacted in a 50 ton compaction press at a pressure of 250 MPa. They were further sintered at 500ºC, 700ºC and 900ºC with a heating rate of 5ºC min -1 in H 2 atmosphere with a holding time of 1 h. All the compacts were weighed and all the dimensions were calculated prior to and after sintering. The density and porosity of the specimens were calculated from these values. The particle size distribution of the powders was determined by using laser scattering unit, Mastersizer The cylindrical samples were polished and etched using standard metallographic techniques for microscopic examination. A Leica optical microscope and an image analyzer were used to examine the microstructures of the samples. A tensile test was carried out at room temperature using Instron testing machine. The tensile strength was measured at a crosshead speed of 1 mm min -1 under a full scale load of 500 N using a gauge length of 26 mm and width of 5.8 mm. The 0.2 % tensile strength and percentage total elongation were evaluated from this test. A Brinell hardness test was performed using a 10mm diameter indenter under a load of 500 kgf. A pin on disc wear testing machine was used to investigate the wear properties. Rectangular test pins of 1.5 mm 12.7 mm dimensions were tightly held in the holder of the wear testing unit. Further a load of 1 kg and 5 kg was applied on the pin through a pulley and a belt system. The flat surface of the pin in contact with the disc was polished by 0.01 m alumina, while the surface of the disc was cleaned by acetone and subsequently polished by 1/0, 2/0, 3/0 and 4/0 grit emery papers prior to each test to ensure smooth sliding. Provisions for inserting a thermocouple sensor for measuring the temperature at the interface of the pin and disc were also made. The pin was weighed prior to and after the test to record the weight loss. The diameter of the wear track was set at 68 mm by moving the load pan backward and forward. The speed of rotation was set at 0.25 m s -1 by adjusting the RPM at 70. The duration of the test was 100 min. The wear volume was calculated by using the data of linear displacement recorded by calibrated transducer attached to the lever linkage system. The transducer was connected to the device which displayed the output as a direct measure of the linear displacement due to wear. In order to study the mode of wear FEI Quanta 200 scanning electron microscopy was used. All the tests were carried out at room temperature. Table 1. Powder size distribution of premixed Cu-Pb alloy. D10 D50 D

3 Journal of Chemical Technology and Metallurgy, 51, 6, 2016 RESULTS AND DISCUSSION Physical properties Fig. 1 shows the micrograph of the premixed Cu- Pb powder. The immiscible phase of Pb has segregated on the dendritic cell boundaries of the Cu matrix. After compaction the density of the green compacts were found to vary in the range of 2 % - 3 % (Fig. 2). Fig. 3 indicates clearly that shrinkage is better expressed along the direction of compaction when compared to the other two axes. However, the shrinkage along the length and the width were found similar in magnitude. Maximum densification was observed at 900ºC as seen in Fig. 4. This is attributed to a liquid formation at this temperature value, which promotes densification by enhancing diffusion kinetics. It can be also attributed to the higher pressure difference at 900ºC (Eq. 1) causing grains attraction [6]. P = 2νLV Cos φ / d (1) The total number of pores of a pore shape factor approaching unity increases also with increase of the sintering temperature (Fig. 5). The porosity manifests mainly in the form of small voids formed between Cu and Pb interfaces due to poor wetting and mutual in- Fig. 1. SEM micrographs of premixed Cu-Pb alloy powder. Fig. 3. Sintering temperature effect on the shrinkage along the length, width and height of a TRS compact. Fig. 2. Sintered density and green density as a function of the theoretical density at various sintering temperature values. Fig. 4. Sintering temperature effect on the densification parameter. 728

4 Muthuchamy Ayyapan, Narendra Kumar Uttamchand, Raja Annamalai Arunjunai Rajan Fig. 5. Sintering temperature effect on the total number of rounded pores in a compact. solubility of Cu and Pb. German [6] has reported that the grain boundary and pore configuration control the sintering rate after the initial stage. Furthermore, pore motion is possible by surface diffusion and evaporationcondensation. Hence, temperature proves to be the major factor determining the microstructure evolution in sintering. Recently German et al. [7] have quantitatively studied the sintering shrinkage and dimensional un-uniformity and stated that gravity provided another fundamental stress that acted on the powder compacts during sintering. As gravity is not isotropic, it induces anisotropy and non-uniform shrinkage. Metallographic observations Fig. 6 shows the microstructures of Cu and Cu- 12Pb alloy sintered at 500ºC, 700ºC and 900ºC. The micrographs show random distribution of Pb in the Fig. 7. Variation of the yield strength with sintering temperature change. Cu matrix sintered at 500ºC and 700ºC; whereas they appear as discrete colonies at the Cu grain boundaries, primarily at the Cu-Cu triple junctions, at 900ºC. The brighter area refers to the Cu rich matrix phase of the primary and monotectic constituents, while the darker area refers to the Pb phase which exists as a liquid above and below the monotectic temperature. It is observed that the sintering temperature increase brings about a progressive decrease in the mean free path of Pb phase formed at Cu phase triple junctions. Mechanical properties Figs. 7 and 8 illustrate the sintering temperature effect on the yield strength and ultimate tensile strength of Cu-12Pb alloy, respectively. It is found that the yield strength decreases with increase of the sintering temperature. A decrease of 38 % is observed on sintering Fig. 6. Optical micrographs of Cu-Pb alloy sintered at various temperatures. 729

5 Journal of Chemical Technology and Metallurgy, 51, 6, 2016 Fig. 8. Variation of UTS with sintering temperature change. Fig. 9. Variation of the percentage of the total elongation to fracture with sintering temperature variation for Cu-Pb alloy. temperature increase from 500ºC to 700 ºC. A further decrease by 53 % is found in case of sintering temperature increase from 700ºC to 900ºC. Ductility measurements, obtained by tensile elongation over a gauge length of 26 mm, reveal that Cu-Pb alloys sintered at 900ºC possess high ductility. The improvement of total elongation to fracture for Cu-Pb alloy sintered at 500ºC and 900ºC is about 289 %. The dihedral angle decreases with sintering temperature increase in accordance with Eq. 2 [6]: Cos φ/2 = ν ss /ν sl (2) where φ is the dihedral angle, while ν ss and ν sl are the solid-solid and solid-liquid interfacial energies. Pb has higher solid-liquid energy with Cu, which inhibits the penetration between the Cu particles at lower temperatures. However it penetrates the Cu-Cu grain boundaries with sintering temperature increase. This results in φ decrease. As φ decreases more liquid penetrates the grain boundary as observed at 900ºC. Pb being a soft and weak constituent decreases the yield strength but improves the percentage elongation (Fig. 9) when alloyed with Cu. It is so because some of the Cu-Cu interfaces are replaced by Cu-Pb interfaces at higher temperatures. Fig. 10 shows the high magnification micrograph of the fractured surface of the alloy sintered at 900ºC. The change of the bulk hardness value with temperature increase is given in Table 2. Fig. 10. SEM micrographs of the fractured surface of Cu-Pb alloy sintered at 900 C. 730

6 Muthuchamy Ayyapan, Narendra Kumar Uttamchand, Raja Annamalai Arunjunai Rajan Table 2. Hardness of Cu-Pb alloy sintered at various temperatures. Sintering temperature, C Vickers Hardness (DPH), HV Tribology studies Figs. 11 and 12 show the variation of the wear volume of Cu-Pb alloy pin as a function of the sliding distance under loads of 1 kg and 5 kg, respectively. There is a significant reduction in the wear volume of Cu after being alloyed with Pb. However, at higher loads the wear rate increases. During the application of load, Pb gets smeared over the surface of the pin and forms a tribological film which reduces the friction, but at higher loads the film gets worn off. The fractured film surface appears as metallic debris. The worn surfaces are shown in Fig. 13. Salak et al. [8] have showed that when three grains of a single phase are in contact with another phase of a lower melting point and forms a dihedral angle of less than 60º, the low melting point phase spreads over the faces. At still smaller dihedral angles, it covers the grain surfaces of the phase of the higher melting point. Thus a network of the low melting point Fig. 11. Variation of the wear volume with the sliding distance at different sintering temperatures for an alloy pin tested at a load of 1 kg. Fig. 12. Variation of the wear volume with the sliding distance at different sintering temperatures for an alloy pin tested at a load of 5 kg. Fig. 13. SEM micrographs of worn surfaces of Cu-Pb pins tested at 1 kg and 5 kg loads. 731

7 Journal of Chemical Technology and Metallurgy, 51, 6, 2016 Table 3. Variation of oxygen on the worn surfaces of Cu-Pb alloy pin tested at 1 kg & 5 kg load. Sintering Temperature ( C) 1 kg (%) 5 kg (%) phase forms in the structure. This provides to conclude that a continuous Pb network may be expected to form at 900ºC because Pb is forming lower dihedral angles with Cu. Energy dispersive spectroscopic investigations reveal that Pb is randomly distributed across the matrix at 500ºC and 700ºC. In fact Pb is absent at the Cu grain boundaries at 700ºC. It is expected that gravity played an important role in determining the Pb distribution in the Cu matrix. The ruptured Pb film can repair itself to some extent by forming a fresh layer and thereby keeping the surface damage low until the entire Pb film is worn out. This may not be possible for Cu-Pb alloy sintered at 700ºC which can explain their high wear volumes. The tribolayer of Pb serving as a solid lubricant gets flattened or plastically deformed in the course of application of a higher load, which increases subsequently the wear volume. Porosity plays a significant role in determining the wear rate. The pores act as lubricant reservoir and/ or lubricating channels [9]. The difference in values of the wear rates in case of Cu-12Pb alloy sintered at 700ºC and 900ºC can also be due to the fact that the porosity in the compact sintered at 900ºC is higher than the compact sintered at 700ºC, so the wear debris generated during the process of wear can also get entrapped by the pores. Wear increase at 5 kg can be also due to increase of the frictional heating or to breakage of the Fig. 14. X-Ray diffraction diagrams of the wear track of alloy pins tested at 1kg and 5kg loads. 732

8 Muthuchamy Ayyapan, Narendra Kumar Uttamchand, Raja Annamalai Arunjunai Rajan tribolayer. With load increase the surface being heated by the frictional process becomes significant. The investigation of the surface of the worn particles using SEM and EDS reveal various mechanisms involved in the wear of the material. At a low sliding speed and a low load the wear process is oxidation determined, which is confirmed by the EDS analysis of the wear track (Table 3, Fig. 14). In case of oxidation determined wear, the formation and wear off-oxides on the rubbing surfaces play a significant role in the friction and wear process. The presence of oxide films does not refer to low friction is low, but to less damage [1]. According to Bowden and Tabor [10], the COF is low at low pressure because of the large interfacial electrical resistance caused by an oxide film formation. However, at high stresses the oxide film is likely to be destroyed, which reduces the electrical resistance and causes adhesion at that site. They state that complete mutual insolubility between metal pairs is required for reducing friction and wear. Furthermore, while solid mutual insolubility of pairs of metals prevents scoring, at least one of the metal pairs in a sliding contact has to be from the B-subgroup of the periodic table. The flowing on the surface due to plastic deformation can be seen at high magnifications. In this case the debris become coarser and sometimes they act as strain hardened material at the interface of the pin and the disc prohibiting their contact and thereby reducing the wear volume. The accumulation of the debris at the interface can be confirmed by the negative wear volume observed in the graphs (Fig. 14). CONCLUSIONS The study on the sintering temperature effect on the microstructure, as well as the mechanical and tribological behavior of the Cu-Pb alloy leads to the following conclusions: The lead globules are randomly distributed in the matrix of Cu-Pb alloy sintered at 500ºC and 700ºC, whereas the sintering at 900ºC results in a liquid phase formation and Pb uniform appearance at Cu grain boundaries, at the triple junctions in particular. The strength of the alloy is dependent on the sintering temperature. The increase of the latter results in strength decrease. There is a decrease in the interface temperature with Pb addition. It may be stated on the basis of the results obtained from this investigation that Cu matrix is pushed deeper during the beginning of the wear process and a layer of Pb is formed. It serves as a lubricating film. Cracking and ploughing of the latter starts with load increase which finally results in a high wear rate due to its removal. The wear is mild and mainly oxidative in nature in presence of 1 kg load. An onset of severe wear is observed when the applied load is increased to 5 kg. The wear mechanisms identified by this investigation refer to oxidation, abrasion and surface plastic deformation causing metallic debris formation. Acknowledgements The authors would like to acknowledge RGEMS seed fund from VIT University, Vellore for support of this research work. They acknowledge also the DST-FIST facilities at the Department of Manufacturing Engineering, VIT University, Vellore. REFERENCES 1. V.E. Buchanan, P.A. Molian, T.S. Sudarshan, A. Akers, Frictional behavior of non-equilibrium Cu-Pb alloys, Wear, v. 146, 1991, pp J. P. Pathak, S.N. Tiwari, On the mechanical and wear properties of copper-lead bearing alloys, Wear, v. 155, 1992, pp Yuko Tsuya, Riitsu Takagi, Lubricating properties of lead films on Copper, Wear, v. 7, 1964, pp V.E. Buchanan, P.A. Molian, T.S. Sudarshan, A. Akers, Friction behavior of non-equilibrium Cu-Pb alloys, Wear, v. 146, 1991, pp T. Tanaka, M. Sakamoto, K. Yamamoto, T. Higuchi, US Patent No , R.M. German, Liquid Phase Sintering, 1985, Plenum Press, New York, USA. 7. E.A. Olevsky, R.M. German, Effect of gravity on dimensional change during sintering. І. Shrinkage anisotropy, Acta Materialia, v. 48, 2000, pp

9 Journal of Chemical Technology and Metallurgy, 51, 6, A. Salak, V. Prochazka, E. Navara, Effect of some additions on the microstructure and mechanical, fatigue, and friction properties of sintered lead bronze, Poroshkovaya Metallurgiya, v. 7, 1971, pp B. Dubrujeaud, M. Vardavoulias, M. Jeandin, The role of porosity in the dry sliding wear of a sintered ferrous alloy, Wear, v. 174, 1994, pp F.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, 1956, Wiley, New York. 734