448 BELL SYSTEM TECHNICAL JOURNAL 



may be rolled or otherwise worked to the desired thickness^'''^^. An advan- 

 tage of this type of bimetal fabrication is the use of alloy bonding at the 

 interface instead of a solder which might limit the operating temperature 

 of the material". 



13. Metallic Friction M aterials^^'^ 



The ordinary type of friction material for brake linings, clutch facings, 

 and similar uses is generally composed of asbestos with an organic t\^e 

 of binder. Under normal operating conditions, this type of material is 

 quite satisfactory, but where severe conditions of operation are encountered, 

 the heat generated at the braking surfaces may be sufiScient to decompose 

 the binder and cause rapid wearing of the friction facing. 



By powder methods, however, a metallic matrix can be formed with 

 admixtures of friction producing ingredients to give a facing that is capable 

 of withstanding the high temperatures generated under severe operating 

 conditions. The exact composition of the facing is determined by the 

 requirements, and a number of different metallic and non-metallic mate- 

 rials are used. Generally, however, the basic ingredient of the matrix 

 is copper to which may be added such modifying metals as tin, lead, zinc 

 or iron. The friction-producing powder is generally an abrasive such as 

 silica or emery which is varied in amount according to the coefficient of 

 friction that is desired. 



The metallic elements may constitute only about 50 per cent of the part 

 by volume with the other 50 per cent represented by non-metallic ingredi- 

 ents and pores. In consequence, therefore, the facing is weak and brittle 

 and is usually bonded to a strong backing plate. 



The friction materials are prepared in the normal manner by mixing 

 suitable powders, compressing in suitable form, (usually as thin annular 

 rings) and sintering. The sintering operation is generally performed so 

 that the part is bonded to the backing plate at the same time. Finishing 

 operations are then performed to adjust the facing to size and proper shape 

 for use. 



14. Cores Jor Inductance Coils for Telephone and Radio^^^'' 



Although the manufacture of cores for induction coils for telephone and 

 radio use does not tit into the field of powder metallurgy as more strictly 

 defined in the Introduction, the procedure is in many ways so similar to 

 the processes described above, and the product of such interest, that a 

 brief description is included in this review. 



The coils in communication circuits may operate over a wide range of 

 frequencies from voice frequencies up to millions of cycles per second. 



