Study of grain boundary deformation mechanisms in cemented carbides using a model potential for the W-C-Co
Martin Gren (Chalmers University of Technology, Gothenburg, Sweden), Martin Petisme, and Göran Wahnström
Cemented carbide is a composite material consisting of tungsten carbide grains cemented in a metallic binder. Cobalt is usually the metallic binder. A cemented carbide is produced by means of powder metallurgy in which WC and Co powders are mixed, pressed, and then sintered together. The material inherits significant hardness from the continuous skeleton of hard WC grains while the ductile binder phase serves to increase the toughness. These properties make cemented carbide ubiquitous in a multitude of applications, such as high-speed machining of steels, rock drilling, and for wear parts.
As coatings have increased the material's resistance to abrasive wear, the plastic deformation of the bulk material is often what limits the tool life in practice at high cutting speeds. Grain boundary sliding (GBS) is thought to be the dominating deformation mechanism in WC–Co based cemented carbides at temperatures approaching the Co binder phase melting point.
In order to study the effect of Co infiltration of WC/WC grain boundaries on the sliding, we have performed GBS simulations for bicrystal systems using molecular dynamics. To get good statistical properties and for geometrical reasons the number of atoms needed to model realistic systems lie in the thousands to millions. Also, the number of time steps needed to simulate sliding for relevant sliding distances lie in the millions. This makes first-principles methods such as density functional theory currently impractical. We have therefore used a classical inter-atomic potential for the W-C-Co system. The potential is of the Tersoff form. The interactions for the W-C system are taken from a W-C-H potential found in literature. We have fitted the remaining interactions involving Co using force matching to describe dissolved W and C in Co, and WC/Co interfaces. The Large-scale Atomic/Molecular Massively Parallel Simulator (lammps) software was used to perform the molecular dynamics simulations.
It is experimentally known that a majority of the grain boundaries involve low-energy prismatic or basal planes. For this reason we chose to study sliding of two model grain boundary systems, one with a grain boundary with coincidence index 2 including a basal plane, and one with a grain boundary with coincidence index 4 including a prismatic plane.
We have studied the effect on sliding of segregated Co atoms and thin Co films of varying thickness at 500 K, 1000 K, 1500 K, and 2000 K, where 2000K is above the melting point for Cobalt. The shear stresses for constant sliding rates have been compared and it is concluded that GBS is significantly facilitated by nanometer-thick Co films. Further, we observe that submonolayer segregation of Co generally increases the resistance towards sliding. However, for the grain boundary with coincidence index 2 at temperatures of 1000 K and below this effect is not observed. We found this to be due to the fact that the 0.5 monolayer of Co film in theses cases remains ordered during the sliding process. At higher temperatures for this grain boundary and at all studied temperatures for the other grain boundary the film instead becomes disordered, which hinders the sliding.