Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
2000-07-19
2002-02-12
Pelham, Joseph (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S385000, C051S293000, C051S307000
Reexamination Certificate
active
06346689
ABSTRACT:
This invention relates to a composite hard material and to a cell for forming a composite hard material and to methods for forming the composite hard material as well as articles formed from hard materials.
Our earlier international patent application Nos. PCT/AU88/00058, PCT/AU85/00271 and PCT/AU92/00127 disclose methods of forming polycrystalline diamond composite hard material (PCD) and polycrystalline cubic boron nitride hard material (PCBN). International application PCT/AU88/00058 also discloses a cell for forming the hard material.
Current commercial production of PCD's and PCBN hard materials is performed in triaxial, belt and girdle or similar apparatus at pressures of at least 50 Kb. These apparatus yield products with the geometric configuration of wafers or discs that are optionally bonded to carbide substrates during the fabrication process. These shapes are not optimum for certain hard-rock and mining operations and further machining and cutting to shape of the discs or bonded composites can be needed. This adds to final costs.
A first aspect of the invention concerns forming shaped pieces of hard material which can be used without major EDM finishing. Conventional techniques for forming composite hard material generally yield a disc or cylinder shaped piece of material. The disc or cylinder is not perfectly symmetrical and generally the finished piece is machined from the disc or cylinder by laser cutting or an EDM process. For example, if it is desired to form tips for a mining machine, a cylinder of hard material is produced and the cylinder is then subject to EDM processing to form the dome or conical shaped tip. The EDM processing ensures that the hard material is formed into the required shape. EDM finishing adds significantly to the time taken to produce the finished product and also the cost of the finished product.
This first aspect of the invention may be said to reside in a method for producing a hard composite material, including:
locating a charge of material in a mould having at least one shaped cavity having the shape of a finished product;
subjecting the mould and the charge of material to high temperature and pressure to form a composite hard material; and
maintaining the charge subject to essentially quasi-hydrostatic pressure during the application of pressure and high temperature.
By maintaining the charge under quasi-hydrostatic pressure and at low temperature gradient, the final shape of the product is determined by a predicted mould form which results in the required product size and shape and therefore product of the required shape can be produced in the forming process without the need for substantial EDM finishing. This therefore reduces costs and time taken to produce a finished article from the composite hard material.
Preferably the method further includes maintaining a low temperature gradient radially and axially across the charge during formation of the composite hard material to reduce internal stresses that might result in delamination.
Preferably the charge is maintained at a substantially low temperature gradient during formation of the hard material.
Preferably the mould having the shaped cavity is formed from graphite or similar material and a plurality of shaped cavities are formed in the mould.
Preferably the shaped cavities are dome shaped or cone shaped for the formation of tips for a mining machine.
Preferably the shape composite hard material is formed in a cell as described below.
The second aspect of the invention may be said to reside in a cell for forming a composite hard material, including:
a cell wall defining a central region for receiving the composite hard material;
the cell wall having;
(a) a ductile wall member;
(b) a heater for providing heat energy when electrical current is applied to the heater;
(c) means for maintaining a substantially low temperature gradient throughout the central region in which composite hard material is formed;
(d) a water barrier wall arranged inwardly of the ductile wall member and heater; and
(e) a barrier layer arranged inwardly of the water barrier wall for preventing intrusion of material from which the water barrier wall is formed into the composite hard material during formation of the composite hard material.
By maintaining a substantially low temperature gradient within the central region of the cell the material produced has lower residual stress that resits delamination and fracture.
Preferably the means for maintaining a substantially low temperature gradient comprises a metal reflector formed from high melting point metal material arranged between the ductile wall member and the water barrier wall for reflecting radiant energy from the heater and also conducting heat energy axially towards ends of the cell.
Preferably the means for maintaining a substantially low temperature gradient may also includ graphite spacers within the central region in which the hard material is located to alter the current density and improve thermal conductivity axially from the central region thereby aiding in the reduction of the thermal gradient caused by heater damage and by thermal conductivity.
Preferably the water barrier wall comprises a glass wall which also serves to enhance even pressure distribution and create a quasi-hydrostatic environment within the cell.
Preferably the heater comprises a high melting point material such as graphite or molybdenum.
Preferably the barrier layer comprises titantium foil or similar material.
Preferably the ductile wall is formed from talc or other material which provides good thermal and electrical insulation. The ductile wall also assists in creation of the hydrostatic environment within the cell to ensure even pressure distribution over the hard material during formation of the composite material.
This aspect of the invention also provides a method of forming a composite constituents into a cell as described above and applying electric current to the heater to heat the charge and applying pressure to pressurise the charge to thereby form the composite hard material.
A third aspect of the invention may be said to reside in a method for forming a composite hard material, including:
locating a charge of constituents of the composite material in a cell for the formation of the composite hard material;
increasing the temperature of the charge and subjecting the charge to pressure to form the composite hard material; and
reducing the pressure quickly after formation of the composite hard material whilst maintaining a high temperature.
Preferably the pressure is elevated to about 25 Kb during the formation of the composite hard material and the pressure is pressure is reduced rapidly from about 25 Kb to ambient pressure in about four minutes.
Preferably the temperature of the charge is raised to approximately 600° C. and pressure is increased to 6 Kb. The temperature and pressure are then increased simultaneously to a temperature of 1050° C. and a pressure of 10 Kb. The temperature is then held and pressure is raised to a maximum pressure of about 25 Kb. At 25 Kb pressure, the temperature is slowly raised to the sintering temperature and held for a predetermined period after which the temperature is again ramped slowly to 800° C. Once this temperature is stabilised, the pressure is then reduced to the ambient pressure over a period of about 4 minutes. At about 0.5 to 1 Kb, the pressure release is slowed and the temperature ramped to ambient slowly over about 8 minutes.
A third aspect of the invention concerns the structure of the hard composite material articles.
This aspect of the invention provides a composite hard material article, including:
a body having a variable microstructure, said microstructure being represented in an outer surface and an interior core, the body being formed from composite hard material including hard particles; and
the body having a surface layer at the outer surface which has a higher hard particle content than the interior core thus forming a more closely packed hard particle network in the surface layer than in th
Clark Donald Lindsay
Major Alan
Willis Paul Edwin
Fish & Richardson P.C.
Pelham Joseph
The Australian National University
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