There is wide use of sintered iron-based friction materials intended to operate both under dry friction and liquid lubrication conditions. Sintered (porous) friction materials consist of metallic and non-metallic components. Metallic components endow the material with strength, whereas non-metallic components enhance the coefficient of friction and decrease the tendency to jamming. Characteristic of sintered friction materials is a thin surface layer formed in the friction process, which plasticity and viscosity are determined by the components constituting the material.
The surface layer, more plastic when at room temperature, and especially at elevated temperatures, as compared with the bulk of the friction material, provides a positive gradient of mechanical properties in depth of the material and prevents effectively overdeformation. The plastic surface layer aids in reducing local unit pressures and surface temperatures and contributes to running-in. By running-in is meant the property of a friction material to increase, by means of wear or plastic deformation, the actual surface of mating. The surface layer of sintered friction materials should feature a heterogeneous structure, i.e. constitute a mixture of the base components and fine hard inclusions. The hard particles present in the surface layer contridute to resistance of the material to wear, since such particles being arranged favourably with respect to the mated surface take the major portion of the load under the force action. With a poor adhesion of the hard particles and the base material, the former are crambled out at high sliding speeds and, when ingressed in the friction zone, can result in an increased wear.
There is known a sintered iron-based friction material (cf., for example, a book by Ignatov L. I. "Proizvodstvo frictsionykh materialov na zhelznoi osnove", Metallurgya Publishers, Moscow, 1968) having the following chemical composition expressed in percent by mass:
copper: 15 PA1 graphite: 9 PA1 silicon oxide: 3 PA1 barium: 6 PA1 asbestos: 3 PA1 the balance being iron. PA1 copper: 9 to 25 PA1 manganese: 6.5 to 10 PA1 boron nitride: 6 to 12 PA1 boron carbide: 3 to 15 PA1 silicon carbide: 1 to 6 PA1 molybdenum disulphide: 2 to 5 PA1 the balance being iron. PA1 copper: 1 to 3 PA1 tin: 0.5 to 2 PA1 barium sulphate: 3 to 5 PA1 graphite: 4 to 10 PA1 molybdenum disulphide: 2 to 6 PA1 pyroceramic: 1 to 3 PA1 lead: 0.1 to 4 PA1 the balance being iron. PA1 copper: 4 to 15 PA1 nickel sulphate: 2 to 8 PA1 graphite: 4 to 10 PA1 pyroceramic: 2 to 10 PA1 lead: 2 to 8 PA1 the balance being iron. PA1 copper: 1.5 to 3 PA1 tin: 1.0 to 2.0 PA1 zinc sulphide: 2.0 to 4.0 PA1 graphite: 6.0 to 10.0 PA1 pyroceramic: 2.0 to 5.0 PA1 lead: 10.0 to 20.0 PA1 the balance being iron. PA1 copper: 3 PA1 tin: 2 PA1 zinc sulphide: 3 PA1 graphite: 8 PA1 pyroceramic: 3 PA1 lead: 20 PA1 iron: 61. PA1 copper: 1.5 to 3 PA1 tin: 1 to 2 PA1 zinc sulphide: 2 to 4 PA1 graphite: 6 to 10 PA1 pyroceramic: 2 to 5 PA1 lead: 10 to 20 PA1 the balance being iron.
Another sintered iron-based friction material (cf., USSR Inventor's Certificate No. 358,401) is known to have the following chemical composition expressed in percent by mass:
Still another sintered iron-based friction material (cf., USSR Inventor's Certificate No. 379.665) is known to have the following chemical composition, in percent by mass:
Hard particles of silicon oxide, asbestos, boron carbide, silicon carbide, iron carbide, and iron oxide present in the above sintered iron-based friction materials as abrasive additions to increase the coefficient of friction are responsible for high temperatures (up to 900.degree. C.) brought about on the surface of the mated materials under friction, which results in changing the structure of the surface layers of the material. The structural changes in turn result in decreased strength characteristics.
Thus, said sintered iron-based friction materials are known to exhibit inadequate strength characteristics and fail to provide the appropriate durability of friction devices wherein they are used.
There is also known a sintered iron-based friction material (cf., USSR Inventor's Certificate No. 503,927) comprising, in percent by mass:
Pyroceramic contained in said sintered iron-based friction material is loosely bound with the base, thus making the entire material weakened. Moreover, due to rather high content of abrasive particles in said friction material it is impossible to obtain perfect contact between the material and the body to be mated with, which affects adversely the running-in.