![Sample our Engineering & Technology journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5933/TOC-Subject-Sample-2021-Engineering-Tech.jpg"})
Abstract
The tungsten carbide-cobalt system is one in which the carbide matrix phase is to some extent soluble in the cobalt binder and which readily sinters to give 100% density. The characteristics of this system have been examined by determination of densification curves and metallographic examination and compared with a previous investigation on the behaviour of the insoluble tungsten carbide-copper system.
It is found that the good densification characteristics of tungsten carbide-cobalt are due to the initial solution of the carbide by the cobalt. The cobalt diffuses in the solid state into interfaces between carbide particles and as the temperature increases dissolves carbide from the adjacent particle surfaces. Forces arising from minimization of surface energy then act to bring about close packing of the carbide. By this means 100% density is achieved with 8 wt.-%, or more, of cobalt. With between 5 and 8 wt.-% cobalt 100% density is obtained by initial solution followed by a few minutes further sintering during which a solution/reprecipitation mechanism is operative.
If sintering is prolonged, a rigid carbide network is formed and this then determines the contraction of the compact on cooling. Even when 100% density is attained at sintering temperature the cobalt contracts on solidification and cannot fill the intervening space between the carbide particles, so that shrinkage-pipe-porosity forms.
Porosity seals off from the surrounding atmosphere when ∼90% density has been reached. If the gas entrapped is insoluble, then the pores shrink until the balance is attained between the surface energy and the excess pressure in the pores and this results in a porosity level of the order of 0·2%
Notes
* Manuscript received 30 March 1972.