Descending fracture toughness of tungsten sintered carbide

[Crazytony]

Hi,
I measure Vickers hardness on the specimens from tungsten sintered carbide and from the cracks on the edges of the indentation I measure the lenghts of the cracks. I count the fracture toughness over Palmqvist empirical formula from the lenghts of the cracks. I measure every month and values are every measurement lower, because the cracks are longer. I measure values still on the same Vickers indentor on the same specimen. It´s possible, that the sintered carbide can loose his toughness. The measured sintered carbide is not loaded. Can the sintered carbide naturally change properties? Something like aluminium alloy natural aging?
Thanks for answers.
Tony

 

[MikeHalloran]

AFAIK 'sintered carbide' is a composite, comprising WC powder in a metallic binder, commonly cobalt, but possibly others.

Also AFAIK, neither the WC itself nor the composite material has any useful ductility or toughness. I have been involved in manufacture and installation of highly stressed carbide swaging dies, which either lasted indefinitely in service or shattered like glass within their first cycle.

In your experiment, consider the possibility that the binder is corroding, or, more likely I think, that your measurement is what is extending the cracks.



Mike Halloran
Pembroke Pines, FL, USA