An improved gear pump having members with different hardnesses includes a housing defining an internal bore with inlet and outlet passageways. A head member includes a wall between the inlet and outlet passageways and extends toward a toothed rotor. A toothed idler gear meshes with the rotor. The head member and idler gear are made of materials having a hardness greater than that of the rotor material. In one embodiment the idler gear is made of ceramic material whereas the head member and rotor are made of cast iron, with the head member treated to have a hardness greater than that of the rotor.