The main driving force for solid state sintering is an excess of surface free energy. The process of solid-state sintering is complex and dependent on the material and furnace (temperature and gas) conditions. There are six main stages that sintering processes can be grouped in which may overlap with one another: 1) initial bonding among particles, 2) neck growth, 3) pore channel closure, 4) pore rounding, 5) densification or pore shrinkage, and 6) pore coarsening. The main mechanisms present in these stages are evaporation, condensation, grain boundaries, volume diffusion, and plastic deformation.