A ceramic material includes of -sialon (Si.sub.(6-z)Al.sub.zO.sub.zN.sub.(8-z)), polytype 15R, an intergranular phase, and yttrium. The polytype 15R includes twin grains.

A surface coated cutting tool comprises: a tool substrate and a coating layer on a surface of the tool substrate; wherein the coating layer comprises a lower layer, an intermediate layer, and an upper layer, in sequence from the tool substrate toward the surface of the tool. The lower layer comprises an A layer having an average composition represented by formula: (Al.sub.1-xCr.sub.x)N, where x is 0.20 to 0.60; the intermediate layer comprises a B layer having an average composition represented by formula: (Al.sub.1-a-bCr.sub.aSi.sub.b)N, where a is 0.20 to 0.60 and b is 0.01 to 0.20; and the upper layer comprises a C layer having an average composition represented by formula: (Ti.sub.1--Si.sub.W.sub.)N where is 0.01 to 0.20 and is 0.01 to 0.10; and the upper layer has a repeated variation in W level with an average interval of 1 nm to 100 nm between adjacent local maxima and minima.

A hard material layer system with a multilayer structure, comprising alternating layers A and B, with A layers having the composition Me.sub.ApAO.sub.nAN.sub.mA in atomic percent and B layers having the composition Me.sub.BpBO.sub.nBN.sub.mB in atomic percent, where the thermal conductivity of the A layers is greater than the thermal conductivity of the B layers. Me.sub.A and Me.sub.B each comprise at least one metal of the group Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and Al, p.sub.A indicates the atomic percentage of Me.sub.A and p.sub.B indicates the atomic percentage of Me.sub.B and the following is true: P.sub.A=P.sub.B, n.sub.A indicates the oxygen concentration in the A layers in atomic percent and n.sub.B indicates the oxygen concentration in the B layers in atomic percent and the following is true: n.sub.A<n.sub.B, and m.sub.A indicates the nitrogen concentration in the A layers in atomic percent and m.sub.B indicates the nitrogen concentration in the B layers in atomic percent and the following is true: p.sub.A/(n.sub.A+m.sub.A)=p.sub.B/(n.sub.B+m.sub.B).