Sintering of cemented carbides : experiments and modeling

Abstract: During sintering of cemented carbides large volumetricshrinkage takes place. About half the volume of the originalgreen body is lost during this process. Due to lotal densitydifferentes within the green body undesired shape changes mayalso occur during sintering. In order to be able to predict theshape changes during sintering a detailed understanding of thesintering processes is needed.The work presented in this thesis is divided in two parts,experiments and modeling. In tbc experimental part a largedilatometry study is presented. Both ordinary dilatometry anddilatometry where and axial load of up to 1 MPa is applied tothe sample has been used. The main powder was aWC9.87 wt% Co powder mixture but other mixtures havebeen considered as well. It was found that most of thedensification takes place in the solid state even thoughcemented carbides usually are considered as to sinter in thepresence of a liquid phase. Densification was also found totake place during heating of the sample and as soon asisothermal conditions were reached the densification ratedecreases exponentially with time. A surface study by ScanningTunneling Microscope (SIM) was also carried out in order toinvestigate how wetting of Co on WC surfaces takes place.In the modeling part a semi-empirital model describing thesintering is presented. It was found in the experimental partthat the basic mechanisms of sintering are not yet fullyunderstood so a more exact model could not be presented.However, the model represents the experimental information verywell. After implementation of the model in a Finite Element(FEM) program a simulation of both pressing and sinteringresulted in a very good prediction of shape changes duringsintering as compared with experimental data.Keywords:cemented carbides, WC-Co, sintering,dilatometer, nonisothermal heating, SIM, wetting, modeling,simulation.