Strength and machining of gelcast SIC ceramics

Abstract

The aqueous gelcasting technique was employed to form silicon carbide green samples for mechanical strength measurements and machining tests. The monomers used in this work were acrylamide (AM) and methylenebisacrylamide (MBAM). Polymerisation reaction was promoted by the addition of catalyst (N, N, N′, N′-tetramethyl ethylenediamine) and initiator (ammonium persulfate). Characterisation of the green, dried gelcast samples included measurements of density and bend strength (flexural strength). It was found that the bend strength of dry, green, gelcast samples depends critically on the slurry solid loading, the amount of monomers and the ratio of monomers contents (AM/MBAM). The highest strength value of 29 MPa was found with samples produced from the slurry with 25 vol% solid loading, the monomers content of ∼10 wt% and the ratio AM/MBAM of ∼16. An increase in solid loading from 25 to 35 vol% caused a decrease in the green sample strength to 20 MPa. Successful machining was achieved with all samples having bend strengths of over ∼6 MPa using standard machining equipment. In one set of samples, machining was done with cemented carbide (W-Co) tools and in the other set with ceramic cutting tools (Al2O3-TiC) using the same cutting speeds, feed rate and depths of cut. Ceramic cutting tools exhibited approximately three times longer life than the cemented carbide tools. The effect of solid loading and the concentration of monomers on the sintered density were also investigated.