426 BELL SYSTEM TECHNICAL JOURNAL 



a bronze having finely divided graphite uniformly distributed throughout 

 the mass. It was prepared by mixing the oxides of tin and copper with 

 graphite, compressing the mass and heating. There was reduction of the 

 oxides by the graphite and partial diffusion of the copper and tin-to give 

 a porous bronze structure in which excess graphite was uniformly distributed 

 in amounts as high as 40 per cent by volume. In addition, there was 

 sufficient porosity for the introduction of 2 to 3 per cent of oil. Later 

 developments utilized the metal powders rather than the oxides^^, and porous 

 bearings in a variety of compositions and forms have constituted a large 

 part of the total production of powder metallurgy products over the years. 

 Of considerable influence on the design and utilization of this type of bearing 

 has been the demand by the automotive industry for large quantities of 

 small bearing parts. Many of these parts are at inaccessible places, and 

 the value of a self- lubricating surface is apparent. As suggested previously, 

 these bearings are not all of the simple pressed porous alloy structure 

 described, but many arc complicated such as those having a steel backing 

 coated with a porous sponge alley ?f copper-nickel in which the voids are 

 impregnated with Babbitt metaP^. 



A later development, and one which has had tremendous industrial signif- 

 icance, was the production of cemented carbides^^-^^'^^ and their use in 

 cutting tools, dies, and hard surfaced parts of many types. Essentially 

 these consist of finely divided tungsten carbide particles bonded by cobalt, 

 or in some few instances, nickel or iron. Other carbides such as those of 

 tantalum, titanium, or columbium may be added to impart special prop- 

 erties. 



Powder metallurgy is admittedly an art that has progressed more rapidly 

 than the science, but the gap is being closed by investigations of a funda- 

 mental nature. Much of the lack of correlated information in the field 

 has been due, in part, to an understandable reluctance of the manufacturers 

 to divulge information on their processes to competitors, and largely, as well, 

 to the narrow specialized uses that apparently discouraged a general syste- 

 matic investigation of the problems involved. Within the past ten or fifteen 

 years, mainly through the efforts of producers of metal powders, research 

 of a fundamental nature has been stimulated. Another factor has been 

 the large scale adoption of the powder metallurgy process by the automobile 

 industry for use in the preparation of many different parts. The field 

 is still narrow and speciaUzed, but the art has progressed to the point where 

 powder metal parts are competing, in some instances, with parts made 

 by the standard melting, casting, and machining procedures. 



As in many similar situations where rapid expansion has occurred, there 

 has been a tendency, not as yet based on actual performance, to oversell 

 the product. This is a sign of healthy activity on the part of the exploiters 



