INSERT AND CUTTING TOOL

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An insert made of a sintered silicon nitride which comprises β-Si3N4 as the main component and contains Mg and rare earth elements Re (Y, La, Ce, Er, Dy, Yb) in amounts of 1.0 to 7.0%% by mole in terms of MgO and 0.4 to 1.0%% by mole in terms of oxide respectively with the total amount being 1.7%% by mole or above and below 7.5%% by mole. The insert has a gradient structure wherein the oxygen content increases from the surface toward the inside, namely, the oxygen content in the area of less than 0.5mm in depth is 0.8 to 1.5%% by mass and that in the area of 0.5mm or above in depth is 1.1 to 2.3%% by mass, with the oxygen content difference between both being 0.1 to 1.0%% by mass.COPYRIGHT KIPO & WIPO 2010