Stock material or miscellaneous articles – Self-sustaining carbon mass or layer with impregnant or...
Reexamination Certificate
2002-06-18
2004-06-22
McNeil, Jennifer (Department: 1775)
Stock material or miscellaneous articles
Self-sustaining carbon mass or layer with impregnant or...
C428S332000, C428S699000, C428S701000, C428S702000, C428S698000
Reexamination Certificate
active
06753085
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heat-resistant coated member for use when sintering or heat-treating metals or ceramics in a vacuum, an inert atmosphere or a reducing atmosphere.
2. Prior Art
Powder metallurgy products are generally manufactured by mixing a binder phase-forming powder with the primary alloy, then kneading the mixture, followed by pressing, sintering and post-sintering treatment. The sintering step is carried out in a vacuum or an inert gas atmosphere, and at an elevated temperature of 1,000 to 1,600° C.
In a typical cemented carbide manufacturing process, a solid solution of tungsten carbide with cobalt or of titanium carbide or tantalum carbide is comminuted and mixed, then subjected to drying and granulation to produce a granulated powder. The powder is then pressed, following which such steps as dewaxing, pre-sintering, sintering and machining are carried out to give the final cemented carbide product.
Sintering is carried out at a temperature at or above the temperature at which the cemented carbide liquid phase appears. For example, the eutectic temperature for a ternary WC-Co system is 1,298° C. The sintering temperature is generally within a range of 1,350 to 1,550° C. In the sintering step, it is important to control the atmosphere so as to enable cemented carbide correctly containing the target amount of carbon to be stably sintered.
When cemented carbide is sintered at about 1,500° C., green specimens placed on a carbon tray often react with the tray. That is, a process known as cementation occurs, in which carbon from the tray impregnates the specimen, lowering the strength of the specimen. A number of attempts have been made to avoid this type of problem, either by choosing another type of tray material or by providing on the surface of the tray a barrier layer composed of a material that does not react with the green specimen. For example, ceramic powders such as zirconia, alumina and yttrium oxide are commonly used when sintering a cemented carbide material. One way of doing so is to scatter the ceramic powder over the tray and use it as a placing powder. Another way is to mix the ceramic powder with a solvent and spray-coat the mixture onto the tray or apply it thereto as a highly viscous slurry. Yet another way is to form a coat by using a thermal spraying or other suitable process to deposit a dense ceramic film onto the tray. These techniques are described in JP-A 2000-509102. Providing such an oxide layer as a barrier layer on the surface of the tray has sometimes helped to prevent reaction of the tray with the specimen.
However, reaction with the tray arises even with the formation of such a barrier layer. As a result, after use in one or two sintering operations, the barrier layer on the tray cracks and delaminates.
Delamination of the film allows the carbon tray to react more easily with the specimen. Moreover, given the risk that the film will delaminate, fragment into small pieces, and become incorporated into the green specimen during sintering, a new tray must be used each time sintering is carried out.
A need has thus been felt, particularly in regards to the use of such trays in sintering, for a way to prevent the specimen from reacting with the barrier layer and the barrier layer from reacting with the tray and delaminating. There exists in particular a desire for a tray material which has a long service life and which, regardless of how many times the tray is used in the sintering of powder metallurgy products, does not result in reaction of the specimen with the barrier layer or in separation of the barrier layer from the tray substrate.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a heat-resistant, corrosion-resistant, highly non-reactive, and inexpensive coated member for use when sintering or heat-treating metals or ceramics in a vacuum, an inert atmosphere or a reducing atmosphere.
We have discovered that heat-resistant coated members made of a substrate composed of a specific type of material and a layer which covers the substrate and is composed primarily of a rare earth-containing oxide have excellent heat resistance, corrosion resistance and non-reactivity when used in the sintering or heat treatment of metals or ceramics in a vacuum, an inert atmosphere or a reducing atmosphere.
Accordingly, the invention provides a heat-resistant coated member in which a substrate composed of a material selected from among molybdenum, tantalum, tungsten, zirconium, aluminum, titanium, carbon, and alloys, oxide ceramics, non-oxide ceramics and carbide materials thereof is coated with a layer composed primarily of a rare earth-containing oxide.
It is advantageous for the rare earth-containing oxide to be composed primarily of at least one element selected from among dysprosium, holmium, erbium, terbium, gadolinium, thulium, ytterbium, lutetium, europium and samarium, and preferably at least one element selected from among ytterbium, europium and samarium.
The layer composed primarily of a rare earth-containing oxide preferably includes ytterbium in an amount which accounts for at least 80 atom % of all the metal elements, including rare-earth elements, in the layer. Moreover, it is advantageous for the layer composed primarily of a rare earth-containing oxide to have a thickness of 0.02 to 0.4 mm and to be provided thereon with one or more layer of a compound of at least one element selected from among Group 3A to Group 8 elements in the short-form periodic table.
The substrate in the heat-resistant coated member of the invention is preferably made of carbon having a density of at least 1.5 g/cm
3
.
The heat-resistant coated member of the invention is typically used for sintering metals or ceramics in a vacuum, an inert atmosphere or a reducing atmosphere.
DETAILED DESCRIPTION OF THE INVENTION
As noted above, the heat-resistant coated member of the invention is intended for use particularly when sintering or heat-treating, in a vacuum, inert atmosphere or reducing atmosphere, the metal or ceramic from which a product is to be formed. The type of coating oxide, the type of substrate and the combination thereof must be varied and optimized in accordance with the product itself and the temperature and type of gas used in sintering and heat treatment.
The heat-resistant coated member of the invention is particularly effective as crucibles for melting metal or as parts for fabricating and sintering various types of complex oxides. Examples of such parts include setters, saggers, trays and molds.
In the invention, the substrate for forming such heat-resistant, corrosion-resistant members used in the sintering or heat treatment of metals and ceramics is selected from among molybdenum, tantalum, tungsten, zirconium, aluminum, titanium, carbon, and also alloys, oxide ceramics, non-oxide ceramics and carbide materials thereof.
When carbon is used as the substrate, the carbon substrate has a density of preferably at least 1.5 g/cm
3
, and especially 1.6 to 1.9 g/cm
3
. Carbon has a true density of 2.26 g/cm
3
. At a substrate density of less than 1.5 g/cm
3
, although the low density provides the substrate with good resistance to thermal shock, the porosity is high, which makes the substrate more likely to adsorb atmospheric moisture and carbon dioxide and sometimes results in the release of adsorbed moisture and carbon dioxide in a vacuum. Moreover, to enhance formation of the film on the substrate, it is preferable for the oxide layer to have a thermal expansion coefficient of not more than 4×10
6
to 7×10
6
.
When a transparent ceramic such as YAG is sintered, treatment within a temperature range of 1,500 to 1,800° C. in a vacuum, an inert atmosphere or a weakly reducing atmosphere tends to give rise to reactions between the substrate material and the film oxide and to reactions between the film oxide and the product on account of the elevated temperature. It is therefore important to select a substrate and film oxide combination that discourag
Hamaya Noriaki
Kaneyoshi Masami
Takai Yasushi
McNeil Jennifer
Shin-Etsu Chemical Co. , Ltd.
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