Cermet and process for producing it

A process for forming a cermet comprising the steps of:

• (a) blending, grinding, granulating and compressing an initial mixture for forming a cermet core comprising:
• a binder phase of 5% to 25% by mass of at most 2% by mass Al, and a group consisting of metallic W, Ti, Mo, V and Cr, and
• a hard material phase of 95% to 75% by mass including carbonitrides of a cubic B1 crystalline structure and containing 30 to 60% by mass Ti, 5 to 25% by mass W, 5 to 15% by mass Ta, of which up to 70% by mass can be replaced by Nb, 0 to 12% by mass Mo, 0 to 15% by mass V, 0 to 2% by mass Cr, 0 to 1% by mass Hf and Zr, the (C+N) content in excess of 80 mol % and the nitrogen content N/(C+N) ratio ranging between 0.15 and 0.7
• (b) thereafter sintering the cermet core in a sintering furnace with graphite heat conductors by:
• (b.sub.1) heating the cermet core up to the melting point of the binder phase under vacuum with a pressure of .ltoreq.10.sup.-1 mbar,
• (b.sub.2) thereafter heating the cermet core up to the sintering temperature and holding the sintering temperature for 0.2-2 hours, and
• (b.sub.3) subsequently cooling down to 1200.degree. C.;
• (c) repeating steps (b.sub.1)-(b.sub.3) in a gas mixture of N.sub.2 and CO with a N.sub.2 /(N.sub.2 +CO) ratio between 0.1 and 0.9 under an average pressure alternating by 10% to 80% about the mean pressure in a period of time between 40 and 240 seconds and determined by the linear relation y=(7/10.times.+2.5).+-.10%, whereby y=the average pressure in (mbar) and x=the binder content in % by mass, and a N.sub.2 /(N.sub.2 +CO) ratio y, which is determined by y=(16/11x-0.12)+10% with x=N/(C+N) in the cermet;
• and
• (d) thereafter cooling under inert gas from a group consisting of argon and nitrogen or under vacuum.

A cermet comprising:

• a cermet core zone formed with:
• at least one hard material phase of 95% to 75% by mass of a cubic B1 crystalline structure of 30 to 60% by mass Ti, 5 to 25% by mass W, 5 to 15% by mass Ta of which up to 70% by mass can be replaced by Nb, 0 to 12% by mass Mo, 0 to 5% by mass V , 0 to 2% by mass Cr, 0 to 1% by mass Hf and Zr, the (C+N) content being excess of 80 mol % and the nitrogen content N/(C+N) ratio ranging between 0.15 and 0.7, and
• a binder phase of 5% to 25% by mass of at least one metal selected from the group consisting of Co and Ni and containing at most 2% by mass Al, and at least one member of the group consisting of metallic W, Ti, Mo, V and Cr dissolved in the metal;
• and
• a surface layer above the core zone defined by a penetration depth of 0.01 to 3 .mu.m and measurable through an energy dispersive microanalysis on a measuring surface >(0.5.times.0.5)mm.sup.2, said surface layer being formed with:
• a surface binder from the group consisting of Co and Ni and amounting to <90% by mass in relation to the underlying cermet core zone, and
• a Ti content amounting to 100 to 120% in relation to the Ti content of the underlying cermet core zone, and wherein the cermet core and in the surface layer, respectively, there are even distributions of the binder, and
• the sum of the contents of W, Ta, Mo, Nb, V and Cr in the surface layer over a thickness of 0.01 to 3 .mu.m amounts to 80 to 100% by mass in relation to the sum of said contents of the underlying cermet core zone.