Aluminum oxide grain.Cotton fiber abrasive material will not smear on titanium and stainless steel.SAIT-LOK-R??????????iscs are made of multiple layers of cotton fiber abrasive impregnated material. These quick change discs produce a smooth finish in one step. Nylon screw threaded hub for easy mounting to the backing pad.., Multi-layer disc configuration provides longer life than coated abrasive discs.Cotton fiber abrasive material will not smear on titanium and stainless steel.Outlasts costed discs 10 to 1 for less downtime and increased productivity.See Sait-Lok-R??????????Backing Pads for Cotton Fiber Discs.