Abstract: The invention provides a novel machining tool suitable for machining, e.g., lathing, of a workpiece made from a difficult-to-machine material such as fully sintered ceramics and quench-hardened high-chromium cast irons. The machining tool has a conically pointed cutting chip of a cemented metal carbide and provided with a round-bottom groove of a 0.2-0.5 mm radius of curvature running with an inclination angle of 15-60° relative to the cutting face of the chip. The invention also provides a machining method for a workpiece of a difficult-to-machine material by using the above-mentioned machining tool, in which ultrasonic vibrations are applied to the tool in the cutting direction so that abrasive wearing of the flank surface can be decreased.

Abstract: A high density metal-boride based ceramic sintered body consists essentially of component (A) at least one of TiB.sub.2, ZrB.sub.2, CrB.sub.2, HfB.sub.2, VB.sub.2, TaB.sub.2, NbB.sub.2, MnB.sub.2, MoB.sub.2, YB.sub.2, AlB.sub.2, MgB.sub.2, CrB, VB, TaB, NbB, MoB, HfB, YB, ZrB, HfB, TiB, MnB, W.sub.2 B.sub.5 and Mo.sub.2 B.sub.5, componet (B) 0.1 wt. %-10 wt. % based on the total amount of a metal binder comprising at least one of cobalt boride, nickel boride and iron boride, and component (C) 0.1 wt. %-10 wt. % based on the total amount of at least one of a double carbide comprising Ti, Zr, W and C, ZrCN, HfCN, or a double carbo-nitride comprising Ti, Zr, Hf and C, N.The sintered body is not easily oxidized and has extremely few pores and high shock resistance even using a fine grain raw material powder because component (C) has an oxygen removing effect during sintering.

Abstract: New ceramics have excellent mechanical properties such as high density, strength, hardness and toughness. The new ceramics are prepared by sintering a mixture prepared by adding 1 to 90 percent by weight of chromium carbide to titanium carbo-nitride together with some elements selected from the group of transition metals of groups IV, V, VI and VII, ferrous metals and semi-metals. For some types of new ceramics, boron carbide is added, too.