Strength and Stability of Interfaces in Cemented Carbides

Doktorsavhandling, 2004

Cemented carbides are hard composite materials of great technological importance. They are widely used as tool materials in a large variety of applications where the demands on hardness and toughness are high, including e.g.≈mining, turning, cutting, and milling. They are powder metallurgical products produced by liquid phase sintering, with a hard carbide phase as main component embedded in a softer metal binder phase. This thesis is a theoretical study of structures and energetics of interfaces in WC-Co cemented carbides. Many properties of the material are controlled by interfacial energies, which are sensitive to chemical modifications on an atomic level. The theoretical investigations are based on the results of first-principles density-functional theory calculations for a broad selection of model interface systems, using the plane-wave pseudopotential method. The microstructural development is directly dependent on interface energies during sintering. WC grains in WC-Co develop a characteristic truncated triangular shape. The grain morphology has been reproduced by calculations in good agreement with quantitative experimental data. The known fact of better wetting in the WC-Co system than in the TiC-Co system is confirmed and explained in terms of a larger contribution of metal-metal W-Co bonding at Co/WC interfaces. It is demonstrated that Co easily infiltrate most WC/WC grain boundaries, but some special grain boundaries can remain intact. These special grain boundaries are characterized by a high density of coincidence sites. They are significantly stronger and more stable than generic boundaries, and should withstand higher load. The results are compared to, and shown consistent with, experimental observations. The results show that cobalt readily segregates to free carbide surfaces as well as to carbide-carbide grain boundaries. Interface segregation is predicted to take place in small amounts, less than a monolayer is formed in the grain boundaries, which is in agreement with experimental observations. The presence of intergranular Co atoms has a significant effect on the interface energetics. The resistance to grain decohesion and to metal infiltration is increased. It is found that there is a general trend along the 3d-series of increasing propensity for segregation, both to grain boundaries and to free carbide surfaces. The calculations show that V, Cr and Mn have the largest beneficial effect on grain boundary strength.