TRIBOLOGICAL INDEX FOR AN ALUMINIUM MATRIX COMPOSITES

Transcription

1 97 TRIBOLOGICAL INDEX FOR AN ALUMINIUM MATRIX COMPOSITES Ovidiu BRATCU, Constantin SPÂNU 1 University Dunărea de Jos Galaţi ovidiu.bratcu@ugal.ro ABSTRACT Using a special device, the tribological index (friction coefficient and wear) for a composite based on an aluminium alloy, reinforced with graphite particles and in lubricated conditions was investigated. Two kinds of investigations were done: with constant sliding speed and variable pressure, and with constant pressure and variable sliding speed. It was observed that in both cases the friction coefficients have low values and decrease with the increase of the sliding distance. Relating to wear, it was noticed that it increases with the pressure but surprisingly, it decreases for the considered values once the average sliding speed increases. The last fact needs further researches. KEYWORDS: friction coefficient, wear, aluminium alloy composite. 1. INTRODUCTION Aluminium alloys are usually used, among other metallic matrix for making composites. They contain silicon carbide (SiC), alumina (Al 2 O 3 ) or graphite (Gr) as reinforcement particles. The graphitic aluminium matrix composites displays antifriction and low wear properties, depending very much on particular working conditions. It was established, for example, that for sliding condition the composites exhibit different wear mechanisms, which can coexist, and that insignificant modification of the testing conditions can generate unexpected changes in tribological behaviour of the investigated material ([1], quoted in [2]). That is why detailed laboratory tests are necessary The paper present the experimental study concerning the same tribological index (the friction coefficient and the wear) in sliding condition for an aluminium matrix composite, reinforced with graphite particles. The variable parameters were pressure, sliding speed and sliding distances. 2. THE TESTING DEVICE. SAMPLES AND METHODOLOGY The configuration of the testing device for sliding condition is described in [3] and is illustrated in figure 1. It is made up of fixed triboelements (samples) and a cast iron grade Fc 200 disk of 200mm diameter and 10mm thickness, under rotating movement (mobile triboelement). The disk is in permanent contact (under a controlled pressure) with the samples that can be arranged at different radius from the axis of the support. In figure 1 they are arranged at the same radius, as requested by this research. For this device in [4, 5], it is defined a parameter called the reciprocal coverage coefficient, using the formulae: A1 A k 2 r = (1) 2 Ac where: - A 1 : the area of the small size triboelement; - A 2 : the area covered by the small size triboelement on the area of the large size one, even if sometimes they coincide; - A c : the surface described by the rotation movement performed by the largest size triboelement around its own axis. The testing device construction makes possible the achieving of different values for the reciprocal coverage coefficient, by locating the fixed triboelements onto different mean radius, as that the value of the area A 2 from k r relation can be varied. The value of the reciprocal coefficient varies from to

2 98 Fig. 1. Construction of the testing device and placement of the samples 1- driving axis, 2- specific testing environment enclosure, 3- mobile triboelement, 4- fixed triboelement (sample), 5- base board of testing environment enclosure, 6- torsion measuring device. Placing the fixed triboelements on the grey cast iron disk on the maximum diameter of φ 200 was avoided in order to prevent the peripheral heat phenomena. For practical reasons the minimum radius is fixed at 40 mm. In our research the samples were positioned at constant radius of 66 mm. This way the reciprocal coverage coefficient was equal to was avoided The tested material is a composite with a matrix of the ATCu4MgTi alloy (4.3%Al; 0.3%Cu; 0.27%Mg, 0.2%Ti; 0.29%Fe) manufactured by casting using the vortex method and reinforced with 3% graphite particles of 70 µm average size. The specimens made from this material were prismatically (20x30x6) (see figure1), with roughness parameter R a =3.2 µm and 400 MPa Brinell hardness. The contact between the disk and the samples was lubricated with Romanian oil KA RESULTS AND DISCUSSIONS At the beginning, it was analysed the evolution of the friction coefficient, conditions, for a 5000 m sliding distance with an average constant sliding speed (v = 1.5 m/s). The contact pressure between samples and rotating disk was stated, successively, at the following values: 0.22, 0.77, 1.3, 2, 2.5, 3.2 and 3.7Mpa, respectively. The results are printed in table 1 and represented in figure 2. It was observed that for the most pressure values the friction coefficient decreases as the sliding distance increases. The maximum value is approximately 0.08, and the minimum is Thus, the tribosystem was considered to be in the so-called antifriction state [3, 6, 7]. Table 1 Friction coefficient for variable pressure. Pressure [MPa] p=0.22 p=0.77 p=1.3 p=2 p=2.5 p=3.2 p=3.72 Sliding distance[m] Friction coefficient µ

3 99 Friction coefficient Friction coefficient vs.pressure p=0.22 MPa p=0.77mpa p=1.3mpa p=2 MPa p=2.5mpa p=3.2mpa p=3.72mpa Sliding distance [m] Fig. 2. The influence of pressure on the friction coefficient depending on the sliding distance.. Table 2 Wear for variable pressure Pressure [MPa] Wear [g] Wear vs. pressure. Wear [g] Pressure [MPa] Fig. 3 The influence of pressure on wear at the end of the 5000 m sliding distance. Table 3 Friction coefficient for variable sliding speed. Sliding speeed [m/s] v=2 v=2.5 v=3 v=3.5 v=4 Sliding distance[m] Friction coefficient µ For the same experimental conditions the loss of mass (the wear in grams) it was measured at the end of the sliding distance. The results are printed in table 2 and represented in figure 3. From the graphic it results that the wear increases with the pressure, but the values are very small for the friction distance. In another series of tests, with the same reciprocal coverage coefficient Kr, but at a constant level of the contact pressure (p = 2 MPa), it was studied the influence of the average sliding speed (v = 2m/s, v = 2.5m/s, v = 3m/s, v = 3.5m/s and v = 4m/s) on the friction coefficient. The results are printed in table 3 and represented in figure 4.

4 100 Table 3 Friction coefficient for variable sliding speed (continuation from above) Friction coefficient vs. average sliding speed v=2 m/s v=2.5 m /s v=3 m /s v=3.5 m /s v=4 m /s friction coefficient Sliding distance [m] Fig. 4. The influence of the average sliding speed on friction coefficient depending on sliding distance. It can be observed that the friction coefficient decreases as the sliding distance increases. The maximum value is about 0.07 and the minimum is approximately The greatest values are found for a sliding speed equal to 2 m/s. Again, the tribosystem was found to be in an antifriction state For the experimental conditions specified in table 3 and for the same pressure (p=2 MPa), the wear (in grams) measured at the end of the 5000 m sliding distance is represented in table 4, and graphically in figure 5. Surprisingly, for the considered values of pressure, at the end of the sliding distance, the wear decreases. The possible causes are the reduction of the asperity highest, modification of the lubrication conditions and a steady- state temperature. Pressure [MPa] Wear [g] Table 4 Wear for variable pressure Wear [g] Wear vs.pressure Average sliding speed [m/s] Fig. 5. The influence of average sliding speed on wear at the end of the 5000 m sliding distance. 4. CONCLUSIONS The composite aluminium-based alloy, reinforced with graphite particles, in lubricated conditions, is recommended to be used for an antifriction state, because of its friction coefficient has low value. The relatively low wear for the stated conditions (pressures, average sliding speeds, sliding distance) indicates that this composite is acceptable for kinematical transmissions.

5 101 We consider that the surprising decrease of the wear when the average sliding speed is variable, needs further researches. The dependence of wear upon sliding distance must be investigated for longer distances and it is expected to obtain increased values, caused by the new modifications of the superficial layer. REFERENCES 1. Hutchings, I.M., 1994, Tribological properties of metal matrix composites, in Material Science and Technology, Vol. 10, p I7. 2. Crudu I., Bratcu O., Mareş M., 1996, Tribological investigations of sae aluminium matrix composites, in Buletinul Institutului Politehnic Işi, Tom XLII (XLVI), Fasc. 1-2, Section IX, Ştiinţa şi ingineria materialelor. 3. Bratcu O., 1997, Contribuţii la studiul tribosistemelor de alunecare de fricţiune şi antifricţiune, Teză de doctorat, Universitatea Dunărea de Jos Galaţi. 4. Bratcu O., Spânu C., Rîpă M., 1997, Considerations on the reciprocal coverage of a tribosystem influence on some tribological state indexes for sliding tribosystems, The Annals of Dunărea de Jos Univ. of Galaţi, Fascicle VIII, Tribology, vol. I, p Bratcu O., Spânu, C., 1999, The influence of reciprocal coverage coefficient of a tribosystem on friction coefficient for a particular sliding tribosystem, Proc. Conf. of the 3-rd International Conference of Tribology-BALKANTRIB 99, Romania. 6. Bratcu O., Spânu C., Rîpă M., 2000, Characterization of same sliding tribosystems using state concept, Proc. Conf. of the 8-th Symposium on Mechanism and Mechanical Transmissions, with international participation, Timisoara, Romania. 7. Bratcu O., Spânu C., et al., 1998, Unele aspecte termice în tribosistemele de alunecare bazalt sinterizatfontă cenuşie, în condiţii uscate şi de lubrifiere, Lucrările celei de a 9-a Conferinţă de Lubrificaţie şi Tracţiune Elastohidrodinamică, VAREHD, Suceava, Romania.