Composites in Vehicles Brake Systems-Selected Issues and Areas of Development

Modern composite materials, thanks to their excellent properties, are widely used [...].

over by metal matrixes. However, they have their drawbacks-they can cause loud noise or vibrations, and significantly increase the value of the coefficient of friction, mainly due to the tendency to scuffing and adhesion [36,37].
Various types of fibrous materials are excellent as reinforcement. As mentioned, asbestos worked perfectly in this role. Its properties turned out to be difficult to replace [38]. In less loaded systems, this task is easier. For example, plant fibers, shells (of bananas or coconuts), animal hair or cellulose can be used there. The problem increases when more performance is required from the friction composite. Manufacturers then use carbon and glass fibers as reinforcement as well as aramids (including Kevlar). Unfortunately, the problem is that these materials require aggressive chemicals in the production process or can be harmful or irritating themselves [39,40]. The studies in which it was proposed to entrust this role to copper, which must then be in a fibrous form, look promising [41].
One of the most important ingredients are friction modifiers. Their selection and proportions are crucial for an appropriate, equivalent compromise between the value of the coefficient of friction and the coefficient of abrasive wear rate. Hard components, such as steel or cast iron, significantly increase the value of the coefficient of friction. This is because in cooperation with the cast-iron brake disc, adhesion occurs, and as a result, small fragments are "pulled out" [42][43][44]. Unfortunately, this accelerates wear and may cause undesirable braking noises. To reduce these unfavorable properties, the materials are admixed with so-called solid lubricants. They reduce the value of COF and form a thin film on the contact surface, reducing dry friction. Copper works best for this. In addition to lubrication, it perfectly conducts heat, which ensures better heat dissipation from the contact zone [45]. Unfortunately, copper is harmful to both terrestrial and aquatic organisms [46,47]. For this reason, significant limits on its content have been introduced, which are to apply from 2025. Unfortunately, so far no substitute has been found to match the properties of copper. Numerous studies show that graphite is the closest [48].
The last group are fillers. Their role is to fill the empty spaces between the other components [49]. Therefore, materials of this type usually have low price and a finegrained geometry. Fly ash is the most popular here [50]. As a by-product of combustion, it is a cheap material. Important fact is, that ash is indifferent to the environment. It also does not negatively affect the tribological properties of brake linings, some researchers even show that its high content reduces the maximum temperature achieved during braking.
In recent years, a significant development of composite materials has been noticeable. This gives hope that the composition of the materials for friction linings will be developed, which will meet the more and more restrictive regulations related to ecological aspects, while meeting the increasingly difficult working conditions resulting from the increasing power of internal combustion engines, and the related to it higher accelerations, both at speeding up and braking [51][52][53][54].