Etude de l'effet des parametres de mise en oeuvre et des constituants sur les proprietes mecaniques et tribologiques des garnitures de freins

In the automotive brake friction industry, once a formulation of brake lining is approved by a car manufacturer, any variation of the molding cycle, composition and concentration of the ingredients is not tolerated, unless subjected to the whole very expensive certification process, from mixing ingredients to instrumented City Traffic Tests. Indeed, experience gained over more than one century of use of the brake pads shows that such small variations, can have considerable effects on the stability of the coefficient of friction and on the loss of effectiveness of braking (FADE) and thus; on the safety of the user. The objective of this thesis is to scientifically rationalize this established empiric fact by a systematic study. Consequently, this work deals with the effect of the processing parameters and ingredients type and concentration on the physical, mechanical and tribological properties of low metallic and ceramic friction materials.;These materials are molded by the process of positive molding generally used to manufacture original brake linings. The tribological behavior is studied through one of the most important standard tests in the friction industry (SAE J661) commonly called the CHASE Test. Chase tests require cutting up a brake lining into test blocks and running them on the inside circumference of a cast iron, simulated brake drum. The Chase Test is composed of seven successive stages during which the sample is subjected to braking under various conditions of temperature and loading. The various graphs and tables resulting from this test are analyzed in terms of fade, wear, recovery, effect of the temperature, etc.;In the first part of the thesis, brake pads were manufactured under various molding conditions (pressure, temperature and cure cycle). It is found that the increase in the temperature and the pressure of molding improve the mechanical properties in compression and flexure but decrease the dimensional stability. However, dimensional stability improves when the time of molding increases. With regard to the tribological behavior, the results of the Chase tests showed that the increase in the molding temperature increase the coefficient of friction and improves its stability under repeated braking and high temperature braking. However, the overall behavior remains strongly dependent on the cycle time, which; brake industry wish shortest. Combined increase of molding pressure and curing time improves the stability of the coefficient of friction under repeated braking. The same effect is observed for the recovery behavior. However, the effect on braking at high temperature is negligible.;Thereafter, the effect of the type of resin, its concentration as well as post-curing on the physical, mechanical and tribological properties is thoroughly studied. For this purpose, four formulations containing each one a different phenolic resin were prepared. These are a non-modified phenolic resin, a phenolic resin modified with rubber acrylonitrile-butadiene, a phenolic resin modified with epoxy, and finally, a phenolic resin modified with phosphorus. For each one of these resins, three concentrations were examined. The increase in the resin concentration improves the mechanical behavior. However, the dimensional stability decreases. The Chase tests show that the increase in the resin concentration improves the braking behavior and wear resistance. The best performance is obtained with the modified phosphorus phenolic resin. It is also shown that post-curing has a beneficial effect on the behavior of friction materials. It was established that even if post-curing slightly decreases the coefficient of friction; it improves the stability of these coefficients on a broad range of temperature and improves wear resistance.;The last part of this thesis is devoted to the study of the effect of fillers on the physical, mechanical and tribological properties of friction materials. In this context, six various types of mineral fillers are incorporated in the formulations: barium sulphate, calcium carbonate, pulverized calcium carbonate, mica, expanded vermiculite and talc. The results of the mechanical and physical tests show that barium sulphate addition, calcium carbonate and of pulverized calcium carbonate improves the mechanical properties and dimensional stability. However, the addition of vermiculite, mica and talc decrease the mechanical properties and the dimensional stability of these materials. The results of the tribological tests show that the addition of calcium carbonate and pulverized calcium carbonate improves the friction behavior and wear resistance. On the other hand, the barium sulphate addition reduces the braking performance at high temperature and decreases the wear resistance of these materials. It is shown that the additions of mica, expanded vermiculite and talc fillers, decrease the wear resistance and induce instability when braking at high temperature. However, the expanded vermiculite addition improves recovery behavior. It is also noted that wear resistance of expanded vermiculite filled materials can be improved by adding calcium carbonate fillers.