Friction materials performance characteristics

Carbonaceous lubricants are used in most friction materials as a cost-effective way to control friction material performance, wear, and NVH characteristics. Commonly used carbonaceous lubricants are natural and synthetic graphites and metallurgical and petroleum cokes. 

Organic surfaces are encountered in a wide range of situations where interfacial properties impact a material s performance characteristics. For example, a polymer s interfacial characteristics determine chemical and physical properties such as permeability, wettability, adhesion, friction, wear, and biocompatibility. " However, polymers frequently lack the optimum surface properties for these applications. Consequently, surface modification techniques have become increasingly desirable in technological applications of polymers. - These processes are capable of tuning the properties of... 

Wear is an economic consideration. Wear resistance generally, but not always, is inversely related to friction level and other desirable performance characteristics within any class of friction material. The objective is to provide the highest level of wear resistance in the normal use temperature range, a controlled moderate increase at elevated temperatures, and a return to the original lower wear rate when temperatures again return to normal. Contrary to common belief, maximum wear life does not require maximum physical and mechanical properties. 

Apart from the frictional effects, two other performance characteristics need to be considered, namely load-carrying capacity and wear. Most of the available information on these subjects has been obtained from the practical use of commercial materials and will be discussed later, but Groszek and Witheridge found that molybdenum disulphide ground conventionally in air gave a small increase in Mean Hertz Load and Weld Load in a Four-Ball Test as 5% dispersions in a mineral oil. However, oleophilic molybdenum disulphide, ground in n-heptane, gave a 170% increase in Mean Hertz Load and a 260% increase in Weld Load. 

Carbon fibers are also of interest as replacements for asbestos fibers in friction materials, such as brake shoes (221. This move was initially driven in order to avoid the health hazards associated with asbestos. In comparison to asbestos reinforced brake formulations, isotropic carbon fibers have been shown to provide much higher friction performance and superior wear resistance, together with many advantages in operating characteristics. These advantages translate to other friction materials, such as clutch plates, and to a much broader range of applications in which carbon fibers are used as reinforcing or filler materials in various matrices. Carbon fibers in the forms of mats, felts, and paper insulation also present viable replacements for asbestos fibers. 

Table 1 Comparison of the performance characteristics of various classes of friction materials...

Friction materials are often characterized in terms of friction, wear, and noise performance. However, as explained earlier, these are characteristics of the complete brake system and not of the friction couple alone. Friction material characterization techniques... 

The endurance benefits and performance characteristics of polyamide-imides qualify them to be an excellent replacement for metal in aerospace components. Inherent nonflammability, chemical resistance, and superb strength at elevated temperatures allow polyamide-imide compounds to be used in metal applications once unheard of for plastic materials. Neat resin, friction and wear, along with glass- and carbon-reinforced polyamide-imide compounds are incorporated in a variety of parts and locations in aerospace vehicles. 

The objective in this study was to compare the long-term performance of these oils. To do this, changes in chemical and physical properties of the used oils were monitored using American Society of Testing Materials (ASTM) standard test methods [2]. Typically, used engine oil analyses include measurements of viscosity, acid and base numbers, water, glycol, soot, and metals content. In addition to the standard tests, fuel economy, deposit-forming tendencies, and friction and wear characteristics were determined on new and used oil samples in this study. 

Typical static friction coefficients are given in Table 1. These data demonstrate that the absolute traction values for synthetic surfaces are satisfactory in comparison with natural turf, provided that shoes with the appropriate surfaces are employed. Synthetic surfaces by virtue of their constmction are to a degree directional, a characteristic which, when substantial, can significantly affect both player performance and ball roU. This effect is evident in a measurement of shoe traction in various directions with respect to the turf—pile angle. Some traction characteristics are directiy affected by the materials. 

Let us examine some examples of improved service characteristics of metal-polymer friction joints with the help of electrical fields. A metal-polymer joint (MPJ) is a combination of metal and polymer parts operating in coordination [66]. The durability of an MPJ depends on its design, the properties of constituent materials and the operation conditions, including temperature, pressure, mutual displacement velocity, ambient media, physical fields, radiation etc. During operation, an MPJ undergoes certain changes in its material structure, wearing, etc. that impair its performances and life of the joint as a whole (Fig. 4.18, solid arrows). 

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