Safety and comfort have become the important indexes to value modern autos. The application of airbags and ABS has improved the reliability, safety and multifunction of vehicle brake system.
To achieve the brake capacity of brake system and vehicle driving safety, new friction materials need to be developed. The basic requirement for friction materials is to have an enough friction coefficient to ensure steady braking and long service life.
The vehicle braking process is the relative movement between the friction materials and the brake drum (disc) which can absorb the kinetic energy to decelerate.
As the vehicle speeds up, its surface temperature will reach 300-500℃, therefore the friction materials must be temperature resistant.
One hundred years ago the brake products were mainly cotton threads. Thanks to the speedy technology development today, the friction pads in brake and gearing have formed four types: asbestos-based friction pads, semimetal friction brake pads, asbestos-free friction brake pads and metal-based sinter friction brake pads.
As asbestos is the recognized strong carcinogen, seeking for the substitute materials has never stopped since 1970s.
When vehicle manufacturers and brake material suppliers determined to develop new and safer asbestos substitutes, various new friction materials, the “semimetal” and semimetal mixed-type friction materials, showed up.
The semimetal brake pads use coarse steel wool as reinforcement fiber which brings the friction material pocking and magnetism.
The intensity and thermal conduction of the steel fiber make the semimetal brake pad different from the asbestos one. The former can endure high braking temperature and emit the heat from its surface while braking; the friction heat warms up the brake fluid and quickens the aging of piston seal ring of the brake caliper and return spring.
One of the greatest shortages of the semimetal brake pad is its high metal content damaging the surface of the brake disc (drum) and making noise. The large amount of metal fiber and metal particle in this material cause abrasion and plough wear to the brake drum (disc).
Tests have proved that this kind of brake pad is not suitable for the steel brake drum (disc) of large heavy-load equipment or transportation equipment for its wear resistance and brake effect shorten the service life of the brake drum (disc).
The asbestos-free brake material is applicable to drum-type or disc-type brakes. In terms of performance, the asbestos-free brake pad, like the asbestos one rather than the semimetal one, is poor in heat conduction and temperature resistance.
But in anti-friction and noise reduction, the asbestos-free brake pad excels asbestos one.
The substitutes for asbestos brake pad are mainly glass fiber, aromatic fiber, carbon fiber, ceramic fiber and other organic synthetic fibers, the performance of which lies on the fiber type and other additives.
The asbestos-free brake pad has basically reached the technical indexes of relevant standards and its application is extending. At present there are a variety of asbestos-free brake pads such as “environmentally friendly sisal fiber reinforced auto brake pad” and carbon composite friction material which is successfully applied to spacecraft and aircraft brake equipment but can hardly be widely used owing to its high price.
The brake friction material is the mixture of adhesives and many kinds of additives, and the brake performance, service life and temperature resistance of the brake pad lies on the composing and relative proportion of the ingredients.
The principal ingredients of the asbestos friction material are asbestos, adhesives (resin or various modified resins), regulators (china clay, barite, barium sulfate, chromite powder, glass fiber, PNBR, zinc oxide, grammite, graphite, etc.), colorants (iron red, iron black, iron yellow, carbon black, etc.) and so on.
Besides the above-mentioned materials, the asbestos-free friction material has large amount of metal fiber and reduced iron powder particles. For the high-grade friction materials there are some carbon fiber, aromatic fiber, PU fiber, supramoly, copper fiber, tin powder, aluminum powder, antimony sulfide, etc.
The press temperature of the current friction material is generally at 150-160℃, the press pressure 18 MPA, and the post treatment temperature 150-180℃. The time of the post treatment differs from different sizes, usually 4-12 hours. |
