A method for manufacturing a diamond composite, includes: a) mixing diamonds with additives, the mixture comprising at least 50 wt % and less than 95 wt % of diamonds and more than 5 wt % additives; b) forming a work piece from the mixture using a pressure of at least 100 Mpa; c) heating the formed work piece to at least 300° C. for removing possible water and wholly or partially removing additives; d) heating the work piece and controlling the heating temperature and heating time so that a certain desired amount of graphite is created by graphitization of diamonds, wherein the amount of graphite created by graphitization is 3-50 wt % of the amount of diamond; e) infiltrating silicon or silicon alloy into the work piece.