question: How is it possible that car brakes don't lose their fricition after braking for a few times?
comment: Brake pads and rotors are designed with a lot of material, so that some can wear down as the brakes are used without requiring immediate replacement
passages: The reduction of friction termed brake fade is caused when the temperature reaches the "kneepoint" on the temperature-friction curve and gas builds up between disc and pad. All brake linings are cured under mechanical pressure following a heating and cooling curve backstroke, heating the friction material up to to "cure" (cross-link) the phenolic resin thermoset polymers: There is no melting of the binding resins, because phenolic resins are thermoset, not thermoplastic. In this form of fade, the brake pedal feels firm but there is reduced stopping ability. Fade can also be caused by the brake fluid boiling, with attendant release of compressible gases. In this type of fade, the brake pedal feels "spongy". This condition is worsened when there are contaminants in the fluid, such as water, which most types of brake fluids are prone to absorbing to varying degrees. For this reason brake fluid replacement is standard maintenance.
   This glazing can be easily removed by either gently using emery paper on them, or by driving the vehicle carefully whilst implementing light use of the brakes for several miles.
   After cooling, faded brakes usually perform as well as before, with no visible change to the brake shoes and/or pads. However if the brakes have been excessively hot for a prolonged period of time, glazing can occur on both of the friction linings of the shoes and pads.
  
  When this happens, the contacting surfaces of the linings will have a smooth, shiny appearance, and will not perform as efficiently to slow the vehicle under braking.
   Brake fade typically occurs during heavy or sustained braking. Many high-speed vehicles use disc brakes, and many European heavy vehicles use disc brakes. Many U.S. and third-world heavy vehicles use drum brakes due to their lower purchase price. On heavy vehicles, air drag is often small compared to the weight, so the brakes dissipate proportionally more energy than on a typical car or motorcycle. Thus, heavy vehicles often need to use engine compression braking, and slow down so braking energy is dissipated over a longer interval. Recent studies have been performed in the United States to test the stopping distances of both drum brakes and disc brakes using a North American Standard called FMVSS-121. The results showed that when newer compounding of friction materials typically used in disc brakes is applied to drum brakes that there is virtually no difference in stopping distance or brake fade. As the United States changed its FMVSS-121 rules for Class Eight trucks built in 2012 to reduce stopping distances by about 1/3rd there was no recommendation to use either drum or disc brakes in the current law. 
   Section::::Factors contributing to fade.
   BULLET::::- Durability – Friction brakes have wear surfaces that must be renewed periodically. Wear surfaces include the brake shoes or pads, and also the brake disc or drum. There may be tradeoffs, for example a wear surface that generates high peak force may also wear quickly.
  
  BULLET::::- Weight – Brakes are often "added weight" in that they serve no other function. Further, brakes are often mounted on wheels, and unsprung weight can significantly hurt traction in some circumstances. "Weight" may mean the brake itself, or may include additional support structure.
   The issue involves a friction device in the pedal designed to provide the proper “feel” by adding resistance and making the pedal steady and stable. This friction device includes a “shoe” that rubs against an adjoining surface during normal pedal operation. Due to the materials used, wear and environmental conditions, these surfaces may, over time, begin to stick and release instead of operating smoothly. In some cases, friction could increase to a point that the pedal is slow to return to the idle position or, in rare cases, the pedal sticks, leaving the throttle partially open.
   Brake fade occurs most often during high performance driving or when going down a long, steep hill. It is more prevalent in drum brakes due to their configuration. Disc brakes are much more resistant to brake fade because the heat can be vented away from the rotor and pads more easily, and have come to be a standard feature in front brakes for most vehicles.
  
  Section::::Causes of brake fade.
   Friction-based braking systems are susceptible to "brake fade" when used extensively for continuous periods, which can be dangerous if braking performance drops below what is required to stop the vehicle – for instance if a truck or bus is descending a long decline. For this reason, such heavy vehicles are frequently fitted with a supplementary system that is not friction-based.
   Long dual-tire skid marks on highways, made by trucks with drum brakes, are visible examples of non-linearity between brake response and pedal pressure. Large trucks still use drum brakes because they are economical and fit easily where an equivalent disc brake does not. More recently disc brakes for trucks have been promoted listing features such as no fade, possible because they have no self-assist (self-servo).
  
  Section::::Railroads.
   The self-assist mechanism affects the water pump and the amount of fade. For example, the Ausco Lambert and Murphy brakes have self-assist roughly proportional to pad friction, so total braking is reduced as roughly the square of the loss in friction. Many other self-assist designs, such as band brakes and many common drum brakes, have exponential self-assist, described by formula_1, where formula_2 is the natural logarithm base, formula_3 is the coefficient of friction between shoes and drum, and formula_4 is the angle of engagement between shoes and drum. A small change in friction causes an exponential change in self assist. In many common brakes, a slight increase in friction can lead to wheel lockup with even light application. For example, on damp mornings, drum brakes can lock on first application, skidding to a stop even after the brake pedal has been released. Conversely, a slight decrease in friction can lead to severe brake fade.
   In August 2010, the Wall St. Journal reported that experts at the National Highway Traffic Safety Administration had examined the "black boxes" of 58 vehicles involved in sudden-acceleration reports. The study found that in 35 of the cases, the brakes weren't applied at the time of the crash. In nine other cases in the same study, the brakes were used only at the last moment before impact.
  
  Section::::Other possible causes of unintended acceleration.:Driver error.:Factors leading to pedal misapplication.
   Newer drum technologies and turbine cooling devices inside of these drums has also eliminated any edge disc brakes may have had in heavy duty applications. By installing brake turbines inside of a specially configured drum, temperatures are many times cut in half and brake fade is nearly eliminated. 
   Some operators of heavy goods trains have begun to use brake shoes (and pads) using various kinds of plastic (like kevlar). One of those is DB Cargo, and cites less wear and especially less noise, which is important when driving in build-up areas.
  
  Section::::Bicycle rim brake.
  
  This comprises a pair of rectangular open boxes which are mounted on the brake calipers of a bicycle and that hold the brake blocks which rub on the rim of a bicycle wheel to slow the bicycle down or stop it.
  
  Section::::Cataloguing.
   BULLET::::- The regenerative braking effect drops off at lower speeds, and cannot bring a vehicle to a complete halt reasonably quickly with current technology, although some cars like the Chevrolet Bolt can bring the vehicle to a complete stop on even surfaces when the driver knows the vehicle's regenerative braking distance. This is referred to as One Pedal Driving.
  
  BULLET::::- Current regenerative brakes do not immobilize a stationary vehicle; physical locking is required, for example to prevent vehicles from rolling down hills.
   Drum brakes can be grabby if the drum surface gets light rust or if the brake is cold and damp, giving the pad material greater friction. Grabbing can be so severe that the tires skid and continue to skid even when the pedal is released. Grab is the opposite of fade: when the pad friction goes up, the self-assisting nature of the brakes causes application force to go up. If the pad friction and self-amplification are high enough, the brake stay engaged due to self-application, even when the external application force is released.
   Brake failure is also caused by brake drum thermal expansion in which brake shoe clearance becomes excessive from wear. This was largely remedied in the 1950s by self-adjusting brakes. Maladjustment with wear is still a factor in trucks with drum air brakes. A Canadian survey of randomly stopped heavy trucks found over 10% of trucks using self-adjusting brakes had at least one brake out of adjustment, due either to failure of the self-adjust mechanism or wear beyond the capacity of the self adjuster. Newer brake pistons ("cans") extend stroke from about 65 mm to about 75 mm; since about 30 mm of stroke is used just putting the pads in contact with the drum, the added 10 mm of stroke is over 25% increase in useful stroke. Longer stroke reduces especially wear-related fade, but drum brakes are still fundamentally prone to fade when hot.
   Either type could potentially fixed by ensuring a clean mounting surface on either side of the brake disc between the wheel hub and brake disc hub before usage and paying attention to imprinting after extended usage by leaving the brake pedal heavily depressed at the end of heavy usage. Sometimes a bed in procedure can clean and minimize DTV and lay a new even transfer layer between the pad and brake disc. However it will not eliminate hot spots or excessive run out.
  
  Section::::Common problems.:Dust.
   Brake fade failures can cascade. For example, a typical 5-axle truck/trailer combination has 10 brakes. If one brake fades, brake load is transferred to the remaining 9 brakes, causing them to work harder, get hotter, and thus fade more. Where fade is non-uniform, fade may cause a vehicle to swerve. Because of this, heavy vehicles often use disproportionately weak brakes on steered wheels, which hurts the stopping distance and causes brakes on non-steered wheels to work harder, worsening fade. An advantage of low-fade brakes such as disc brakes is steered wheels can do more braking without causing brake steer.
   Drum brake fade can be reduced and overall performance enhanced somewhat by an old "hot rodder" technique of drum drilling. A carefully chosen pattern of holes is drilled through the drum working section; drum rotation centrifugally pumps a small amount air through the shoe to drum gap, removing heat; fade caused by water-wet brakes is reduced since the water is centrifugally driven out; and some brake-material dust exits the holes. Brake drum drilling requires careful detailed knowledge of brake drum physics and is an advanced technique probably best left to professionals. There are performance-brake shops that will make the necessary modifications safely.
   "Brake fade" is a condition caused by overheating in which braking effectiveness reduces, and may be lost. It may occur for many reasons. The pads which engage the rotating part may become overheated and "glaze over", becoming so smooth and hard that they cannot grip sufficiently to slow the vehicle. Also, vaporization of the hydraulic fluid under temperature extremes or thermal distortion may cause the linings to change their shape and engage less surface area of the rotating part. Thermal distortion may also cause permanent changes in the shape of the metal components, resulting in a reduction in braking capability that requires replacement of the affected parts.
   Maximum braking force is obtained when there is approximately 10–20% slippage between the braked wheel's rotational speed and the road surface. Beyond this point, rolling grip diminishes rapidly and sliding friction provides a greater proportion of the force that slows the vehicle. Due to local heating and melting of the tyres, the sliding friction can be very low. When braking at, or beyond, the peak braking force the steering is ineffective since the grip of the tyre is entirely consumed in braking the vehicle.
   Brake fade
  
  Vehicle braking system fade, or brake fade, is the reduction in stopping power that can occur after repeated or sustained application of the brakes, especially in high load or high speed conditions. Brake fade can be a factor in any vehicle that utilizes a friction braking system including automobiles, trucks, motorcycles, airplanes, and bicycles.
   The discs are commonly made from cast iron and a certain amount of surface rust is normal. The disc contact area for the brake pads will be kept clean by regular use, but a vehicle that is stored for an extended period can develop significant rust in the contact area that may reduce braking power for a time until the rusted layer is worn off again. Rusting can also lead to disc warping when brakes are re-activated after storage because of differential heating between unrusted areas left covered by pads and rust around the majority of the disc area surface. Over time, vented brake discs may develop severe rust corrosion inside the ventilation slots, compromising the strength of the structure and needing replacement.
   The hydraulic braking system is designed as a closed system: unless there is a leak in the system, none of the brake fluid enters or leaves it, nor does the fluid get consumed through use. Leakage may happen, however, from cracks in the O-rings or from a puncture in the brake line. Cracks can form if two types of brake fluid are mixed or if the brake fluid becomes contaminated with water, alcohol, antifreeze, or any number of other liquids.
  
  Section::::An example of a hydraulic brake system.
  