Effect of PMMA pore former on microstructure and mechanical properties of vitrified bond CBN grinding wheels

Vitrified bond cubic boron nitride (CBN) grinding wheels with various porosities (36.5–43.5%) were fabricated by adding polymethylmethacrylate (PMMA) and activated carbon pore former. The effects of the type and content of pore formers as well as the size of PMMA on final porosity, microstructure and mechanical properties were investigated. PMMA was confirmed to be a more appropriate pore former for vitrified bond CBN grinding wheels than activated carbon. The porous specimens prepared with PMMA demonstrated quasi-spherical pores with more uniform pore size distribution, and higher bending strength and hardness than those prepared with activated carbon. The higher content of pore former led to increased porosity of sintered specimens, resulting in a decrease in the bending strength and hardness. Furthermore, as the size of PMMA increased, the total porosity remained almost unchanged, while the bending strength and hardness decreased firstly and then increased. Observations carried out by scanning electron microscope (SEM) and optical microscope showed that the size and shape of pores produced by PMMA were related to those of the initial PMMA microspheres, so that good control of pore size and microstructure could be obtained in vitrified bond CBN grinding wheels.