Patent GB-867833-A (GB)
Title: Improvements in or relating to ceramic articles of sintered alumina
Inventor: BURDEN HALDON
Filed: 1956-05-18
Abstract: Ceramic articles, e.g. cutting tools and wear-resisting surfaces, containing at least 90% by weight of sintered alumina are made by sintering alumina powder with an average grain size not greater than 4 microns while under substantial pressure, e.g. in a mould.  Up to about 10% by number of the grains may be between 4 and 10 microns in size.  Preferred pressures and temperatures during sintering are 2000-3000 p.s.i. and 1500-1800 DEG C.; these are applied for 5-30 minutes, conveniently in a graphite mould heated by the field of a high frequency generator or by an electric current.  Undesirable grain growth during sintering may be inhibited by the addition of up to 10% by weight of titanium or tungsten or either of their carbides or any combination thereof.  The carbides may be added as mixed crystals. To prepare the alumina powder for sintering a suitable fineness is obtained by ball-milling commercial calcined alumina for 240 to 360 hours using balls of at least 5 times the weight of alumina powder.  If steel balls are used treatment of the alumina with hydrochloric acid is necessary after milling to remove iron. Alternatively, milling may be effected with balls made of tungsten-titanium carbide mixed crystals, small amounts of which are worn off during milling and become admixed with the alumina. Cobalt or nickel may be incorporated in the carbide balls to harden them. Alumina ground with such balls may contain about 3% by weight of the carbide mixed crystals and about 0.3% by weight of cobalt or nickel.ALSO:Ceramic articles, e.g. cutting tools and wear-resisting surfaces, containing at least 90% by weight of sintered alumina are made by sintering alumina powder with an average grain size not greater than 4 microns while under substantial pressure, e.g. in a mould.  Up to about 10% by number of the grains may be between 4 and 10 microns in size.  Preferred pressures and temperatures during sintering are 2000-3000 p.s.i. and 1500-1800 DEG C.; these are applied for 5-30 minutes, conveniently in a graphite mould heated by the field of a high frequency generator or by an electric current. Undesirable grain growth during sintering may be inhibited by the addition up to 10% by weight of titanium or tungsten or either of their carbides or any combinations thereof.  The carbides may be added as mixed crystals.  To prepare the alumina powder for sintering, a suitable fineness is obtained by ballmilling commercial coloured alumina for 240 to 360 hours using balls of at least 5 times the weight of alumina powder.  If steel balls are used treatment of the alumina with hydrochloric acid is necessary after milling to remove iron. Alternatively, milling may be effected with balls made of tungsten-titanium carbide mixed crystals, small amounts of which are worn off during milling and become admixed with the alumina. Cobalt or nickel may be incorporated in the carbide balls to harden them.  Alumina ground with such balls may contain about 3% by weight of the carbide mixed crystals and about 0,3% by weight of cobalt or nickel.