Functionally graded hardmetals with a continuously graded symmetrical profile

The processing of continuously graded hardmetals with a symmetrical gradient by the electrophoretic deposition process is studied. Electrophoretic deposition (EPD) is a fairly rapid low cost process, capable of producing FGMs with continuous gradients. This paper reports on the processing of WC-Co-Ti(C,N) plates with a gradient in cobalt and Ti(C,N) by means of electrophoresis and subsequent cold isostatic pressing and liquid phase sintering. In order to create a symmetric gradient profile, a model for the EPD process was developed. The EPD model allows predicting the composition and slope of the gradient in the green as well as sintered material from the starting composition of the suspensions, the EPD operating parameters and the powder-specific EPD characteristics. The EPD model was applied to engineer WC-Co-Ti(C,N) FGM plates in order to create the optimal gradient profile. The resulting graded materials show a continuous variation in composition, microstructure and mechanical properties. In the specific illustrated examples, the Ti(C,N) content decreases from 5 wt% on the outer hard edge to almost 0 wt% in the tough bulk of the material whereas the cobalt content after sintering increases along the same direction from about 5-7 wt% to 10-15 wt%. The Vickers hardness HV10 is found to increase continuously from about 16.1 GPa in the bulk of the material to 18.1 GPa on the edge, whereas the fracture toughness decreases along the same 2 direction from 8.8 to 6.6 MPa.m(1/2).