THE SUPERFICIAL LAYER AS BASICS FOR MODELLING THE TRIBOLOGICAL PROCESSES

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1 48 THE SUPERFICIAL LAYER AS BASICS FOR MODELLING THE TRIBOLOGICAL PROCESSES Ion Crudu University Dunărea de Jos of Galaţi, Romania ABSTRACT This paper presents a sintethysis of the researches done during a period of more than 20 years, the results being presented in many doctorate theses, scientific papers in international conferences. The wear process may be studied taking into account the modifications within the superficial layer. Their evolution depend on the tribosystem type *sliding tribosystem TA, rolling tribosystem TR, abrasive tribosystem TZ, cavitation tribosystem TV). The proposed model is also named tribological model for a mechanical functioning system and it is named tribomodel when studying it in laboratory conditions. KEYWORDS: tribosystem, tribomodel, superficial layer. 1. INTRODUCTION A very rapid development of the technical systems is related to the continuous increase of the performances associated to the mechanical systems, characterised by a demand for allowing higher speeds, heavier loads, all these associated to a greater reliability. On the other hand, the requests upon the quality and the volume of many products become more numerous. There are manufactured by the help of many mechanical technologies that imply movements of heavy loads, manipulations of different materials *solids, liquids and gases). Everywhere and everytime during these technological processes, there are developed complex tribological processes that include mechanical but also thermal, chemical and electrical aspects. Improvement of the mechanical systems is achieved by increasing reliability and this could be realised by directioning the tribological processes in order to diminish wear, friction. Usually the results obtained on already existing industrial systems may be extrapolated but the diversity of the materials, design solutions and exploitation conditions require tests on specially equipped stands in laboratory, especially for complex systems (building and agricultural machines). 2. THE SUPERFICIAL LAYER AND THE LABORATORY TEST In order to introduce new materials and to improve the properties of the existing ones, specialists recommend tests on laboratory samples, using special rigs. The results are compared to those obtained for the already knew materials. Laboratory tests start with mathematical modelling [B2, B8] that could establish one or several relationships among the basic parameters of the studied tribological process: friction coefficient, wear intensity, temperature evolution. The rich documentation on the particular aspects of the moving contacts allows including the tribological aspects in the model [B1, B3, B6, B12, B13, B16, B17, B21]. The main factor that governs the reliability of a mechanical system is the wear intensity. This is determined especially by phenomena and processes that take place within the superficial layers of the contacting bodies. The parameters of the superficial layer were the subject of many and accurate researches [B6, B9, B10, B12, B13, B15]. I try to direction the research of our team towards studying the influence of the superficial layer parameters on wear. Generally, The study of a contact between two bodies is a complex problem for three reasons:

2 49 a) the characterisation at the atomic scale of the nature and structure of the surfaces in contact demands accuracy and very modern and expensive measuring equipment; b) the properties of the intermediate bodies (either liquid, gas, particles or mixture of these ones) are difficult to be known when the system is working under different conditions of loading, speed and environment, c) the experiment are sometimes complex and the tribological parameters are difficult to be measured during the process, they being recorded as values for initial and final state. The complexity of the phenomena and processes that take place within the superficial layers is related to the movement type between the contact bodies (sliding, rolling or combinations of those), the material bodies but also the nature of the intermediate body (lubricants, abrasive material etc.). During more than 20 years, the evolution of the superficial layer parameters has being the aim of many specialists at our Department of Machine Design but also specialists in different fields (heavy industry like metallurgy, mining, building activities, food industry, transports, manufacturing etc.) [P1,, P10, J3, P16, B22]. These researches have as fundamental the concept of tribosytem, as defined in DIN (fig. 1). Having as criteria the relative movement and the intermediate body there were defined four types of tribosystems [B4, B5, P3]: a) sliding tribosystems (for antifriction and friction) TA and sliding-rolling tribosystems (especially antifriction ones) TAR; b) rolling tribosystems TR (for antifriction state) and rolling-sliding tribosystems (antifriction and friction) TRA; c) abrasive tribosystems (with abrasive intermediate body, in fluid or with abrasive particles) TZ; d) cavitation tribosystems (in guided current, in free current and in fluid submitted to vibrations). Fig. 1 Tribosystem structure. Figure 2 presents the model of a tribosystem including as input the characteristics of the superficial layers (Ss(to), Ss(t1) and those of the tribosystems [CT(to), CT(t1)] and as commanding parameters, the design and the exploitation parameters. The tribosystem evolution is appreciated by parameters as wear or other resulting ones as gaps between elements, noise, vibration. Fig. 2 The model of a tribosystem.

3 50 Fig. 3 Factors. Fig. 4 Tribomodel TAm. Figure 3 gives details the input and output parameters of the superficial layer (X1.X10), the commanding parameters (U1 U6). For laboratory tests, it was proposed the notion of tribomodel, a system obtained by particularising the dates in figure 2 and figure 3. Figure 4 presents the tribomodel for the sliding tribosystem, characterised by a running in stage. This stage is influenced by the quality of the intermediate body. The running in stage for the friction tribosystems as brakes [B20] is related to contact accommodation. Figure 5 presents the tribomodel for rolling tribosystem [B19], characterised by an incubation phase of the damaging processes. The study of these processes and the variation of some superficial layer parameters were done also by X-ray diffractometry, on a DRON 3 system [X4, X5, X6].

4 51 Fig. 5 Tribomodel for rolling tribosystems. As concerning the abrasive tribosystems and those with cavitation, the model presented in figure 2 may be applied. CONCLUSIONS 1) The application of the presented tribosystematics allows studying the wear and friction processes taking into account the parameters modifications and the tribosystem type. 2). Based on tribosystematics, the activity related to the tribological aspects may be organised in plants, especially in metallurgical ones, chemical installations, food industry. 3) The studies on the modifications of the superficial layers are presented in many recent researches [B7, B11, B14, B18, B19, B20, B22] REFERENCES Books, PhD thesys B1. Atopov V.V., et al., 1988, Modelirovanie Kontaknih napriajenii, Iz. Masinostroenie,. Moskva. B2. Braun E.D., et al., 1989, Modelirovanie tribologiceskih protessov. Spravocinik po tribotehnike, tom.1. Iz Masinostroenie Moskva Warsawa. B3. Creţu S. S., 2002, Mecanica contactului, vol.1. Ed. Gh.Asachi, Iaşi. B4. Crudu I., 1983, Încercarea materialelor, (Coord. D.R.Mocanu. Ed. Tehnică, Bucureşti. B5. Crudu, I, 2000, Bazele proiectării în organe de maşini, Editura ALMA, Galaţi, B6. Czicos H., 1978, Tribology, Elsevier Sc.Publ. Comp. Amsterdam Oxford- New York, B7. Diaconu N., 2002, Studiul influenţei mediului de lucru asupra comportării unor unsori şi al proceselor din stratul superficial la tribosisteme de rostogolire şi rostogolire cu alunecare, PhD Thesis, University Dunărea de Jos Galaţi. B8. Evdokimov L. A., 1980, Planisovanie i analiz experimentov pri resenii zadaci trenia i iznos. Iz. Nauka Moskva. B9. Enache St., 1994, La qualite des surfaces metalliques, Ed. Tehnică, Bucureşti. B10. Zum Gahr H. K., 1987, Microstructure and Wear of materials, Elsevier - Amsterdam Oxford New York Tokio. B11. Ionescu M. P., 2001, PhD Thesis, University Dunărea de Jos Galaţi. B12. Iliuc I., 1974, Tribologia straturilor subţiri. Ed. Academiei RSR Bucureşti. B13. Kersenbaum V. I., 1987, Mehanotermiceskoe formirovanie poverhnostei treniia, Iz. Masinostroenie, Moskva. B14. Nicolau M., 2001, PhD Thesis, University Dunărea de Jos Galaţi. B15. Olaru D. N., 1995, Tribologie. Ed. Univ. Tehnică, Iaşi. B16. Popinceanu N. et al., 1985, Probleme fundamentale ale contactului de rostogolire, Ed. Tehnică, Bucureşti. B17. Quinn F. J., Terence, 1991, Physical Analysis for Tribology, Ed. Cambridge Univ. Press Cambridge-New York,. B18. Rîpă M., 2002, Studiu pe tribomodel al tribostratului în procesul de uzură specific roţilor dinţate. PhD Thesis, University Dunărea de Jos Galaţi. B19. Spânu C., 2002, Studii şi cercetări pe tribomodel privind deformaţiile plastice în stratul superficial la rostogolire şi alunecare, PhD Thesis, University Dunărea de Jos Galaţi. B20. Tomescu L., 1999, Contribuţii la studiul stratului superficial al compozitelor cu matrice PTF pe tribomodele de alunecare, teză de doctorat, Univ. Dunărea de Jos Galaţi. B21. Tudor A., 2002, Frecarea şi uzarea materialelor. Ed. 122 BRWEN Bucureşti. B22. Zaharia C., 2003, Studii şi cercetări privind influenţă regimului de răcire asupra unor caracteristici mecanice ale benzilor laminate la cald, PhD Thesis, University. Dunărea de Jos Galaţi. B23. *** (1994) Disspative processes in tribology. Editor D. Dowson (20 th Leeds Lyon Simposion), Amsterdam.

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