... Graphite is carbon crystal and nonmetal material with the natures of silver gray, soft texture, metal luster, Mohs hardness 1~2, specific ... also used in piston ring, sealing ring and bearing of the equipment of corrosive medium delivery and does not require any lubricant ... in graphite must not be in excess of some dozens of PPM. Especially the content of boron should be less than 0.5PPM ...