GB2200317A

GB2200317A - Isostatic moulding

Info

Publication number: GB2200317A
Application number: GB08800823A
Authority: GB
Inventors: Edwin Granville Butler
Original assignee: AE PLC
Current assignee: AE PLC
Priority date: 1987-01-27
Filing date: 1988-01-14
Publication date: 1988-08-03
Anticipated expiration: 2008-01-14
Also published as: GB8800823D0; GB2200317B; GB8701761D0

Abstract

A method for the hot isostatic pressing of green bodies comprises the encapsulation of the green body with paste formed from a mixture of boron oxide powder, glass-forming powder and water. The paste is dried to form a hard, protective cement-like shell around the green body. The coated body is then hot isostatically pressed in the vessel of a hot isostatic press after raising the vessel interior to a temperature where the coating forms a viscous impermeable membrane. The coated green body may be further supported in a powder bed within a container within the pressure vessel. An example using a silicon nitride green body is given.

Description

A METHOD FOR THE PRODUCTION OF ENGINEERING MATERIALS The present invention relates to a method for the encapsulation and compaction by hot isostatic pressing of ceramic materials and the subsequent removal of the encapsulating material.

The technique of hot isostatic pressing (HIP) requires that the body to be pressed be separated from the pressurising medium by an impermeable membrane. The methods used heretofore are not entirely suitable for complex shapes having relatively delicate features when in the green state. The sealing of a green body inside an impermeable membrane is generally known as encapsulation and that of removing the membrane as decapsulation.

One method of encapsulation is to place the green body inside a rigid glass envelope which is then evacuated, sealed and then HIPped. This method is only s-uitable for simple shapes used for materials development as the collapsing glass envelope can easily damage delicate features of green bodies having complex shapes.

A second method is to cover the green body in glass powder in a heat-resistant container. The glass powder is then melted under vacuum in the HIP vessel and then HIPped, the molten glass providing the impermeable membrane. The glass may be only a layer on the surface of the green body or the green body may be submerged beneath a substantial quantity of glass in the container. Such a method is described by Adlerborn et al in US Patent No. 4,446,100. After HIPping the excess glass may be poured off and the remaining adherent layer of glass removed by, for example, grit blasting. A practical difficulty with this method is that less dense ceramic materials tend to float to the surface of the molten glass rendering the HIPping operation unsuccessful.Various methods have been adopted tu maintain the green body submerged but these methods have disadvantages in that they add to the difficulty of use of the apparatus.

A second disadvantage with this method is that the removal of the adherent glass layer is very labourintensive and may itself result in damage to features of complex shaped parts.

A third method which is described by Adlerborn et al in European Patent Application No. 0118702 is to carry out the HIPping stage with the green body immersed in fused boron oxide or a mixture of glass and boron oxide. The residual layer of boron oxide remaining after HIPping may be relatively easily removed by water or steam in an autoclave. A disadvantage with this method, however, is that pure fused boron oxide is difficult to handle. The fused oxide is prone to frothing and tends to creep out of the container thus- contaminating the HIPping vessel.

The disadvantage that a less dense green body may tend to float is also present.

It is an object of the present invention to provide a method for the reliable HIPping of green bodies and which method does not have the attendant disadvantages of the prior art described above.

According to the present invention a method for the hot isostatic pressing of green bodies comprises the steps of mixing boron oxide powder with glass-forming powder, adding water to the mixed powders to form a paste, coating the green body with the paste, drying the coated green body, placing the coated green body in the vessel of a hot isostatic press, raising the temperature within the vessel to cause the coating on the green body to form a viscous impermeable membrane and t hen hot isostatically pressing the green body.

After HIPping the coating may be easily removed by the same techniques used for the removal of substantially pure boron oxide notwithstanding the glass content.

The paste formed from the mixture of water, boron oxid-e and glass-forming powder has been found to set to a cement-like consistency when dried.

Consequently, handling of the coated green bodies has been greatly facilitated and the dried coating even providing protection for delicate features of the green bodies during handling.

It has been found that the molten coating used in the method of the present invention does not exhibit the frothing tendency of molten boron oxide but forms a stable coherent mass.

In a modification of the method of the present invention the coated green bo-dy may be further supported within a powder bed in a container in the HIPping vessel. An essential requirement of such a powder bed is that the material of the bed is not. wetted by the molten encapsulating material. Failure to satisfy this condition may result in the encapsulating material being drawn away from the green body by capilliary action within the powder bed. Support within a non-wettable powder bed helps to ensure that the coating is maintained intact around the green body.

The term "glass-forming powder" is intended to encompass any known materials which are considered to form glasses in the molten state and which are compatible with boron oxide. Examples of such materials include silica, lead silicates, aluminium silicates and mixtures of glass-forming oxides, etc.

The content of boron oxide may lie in the range from 40 wt% to 95 wt%. Preferably the content of boron oxide may lie in the range from 60 to 85 wt%.

The boron oxide content may vary depending upon the type of glass-forming powder employed. The objective is to prevent a continuous or recticular matrix of the non-soluble glass phase being formed. Sufficient boron oxide is necessary to fulfil this condition to ensure easy removal of the envelope after HIPping.

In order that the present invention may be more fully understood it will now be described by way of illustration only with reference to the following example and the accompanying drawing which shows a cross-section through a green body coated in accordance with the method of the present invention and supported in a powder bed.

Referring now to the drawing and where a green body is denoted at 10. The green body has a coating 11 comprising-a mixture of 80 wt% boron oxide and 20 wt% silica. The mixture was prepared by mixing the appropriate weights of the two powders together and then adding sufficient water and mixing to constitute a viscous paste. The paste was then applied to the green body to form a reasonably uniform coating of about 5 mm thickness. The coating was then dried slowly in air to avoid cracking. When dry the coating 11 had a hard, easily handleable cement-like quality.

The coated and dried body was then placed in a graphite container 12 and a powder bed 13 of boron nitride provided around the body.

The container 12 together with its contents was then placed inside the vessel (not shown) of a hot isostatic press. The vessel was then evacuated 0 and the temperature increased to 700 C. The pressure end temperature inside the vessel were increased to 200 MPa and 1750 0C using a nitrogen atmosphere and maintained for 60 minutes. On release of the pressure and removal of the container after cooling the coated body 10 was easily removed from the powder bed 13. The pressed body was subjected to a treatment comprising leaching in boiling water to remove the coating 11. The body 10 was completely undamaged and exhibited a uniform density throughout.

In the example given above the green body was silicon nitride. Any other ceramic material may be hot isostatically pressed using the method as may many metallic materials such as, for example, those known as iron-, nickel- or cobalt-based superalloys.

Claims

1. A method for the hot isostatic pressing of green bodies, the method comprising the steps of mixing boron oxide powder with glass-forming powder, adding water to the mixed powders to form a paste, coating the green body with the paste, drying the coated green body, placing the coated green body in the vessel of a hot isostatic press, raising the temperature within the vessel to cause the coating on the green body to form a viscous impermeable membrane and then hot isostatically pressing the green body.

2. A method according to Claim 1 wherein the impermeable membrance is removed by water after hot isostatic pressing.

3. A method according to either Claim 1 or Claim 2 wherein between 40 and 95 wt% of boron oxide is mixed with between 60 and 5 wtZ of glass-forming powder.

4. A method according to Claim 3 wherein between 60 and 85 wt% of boron oxide is mixed with glass-forming powder.

5. A method according Claim 3 wherein substantially 80 wt% of boron oxide is mixed with glass-forming powder.

6. A method according to any one preceding claim wherein the glass-forming powder includes silica.

7. A method according to any one preceding claim wherein substantially 80 wt% of boron oxide is mixed with substantially 20 wt% silica.

8. A method according to any one preceding claim wherein the coated green body is supported by a powder bed within the isostatic press vessel.

9. A method according to Claim 8 wherein the material of the powder bed is not wetted by the molten coating membrane material.

10. A method according to either Claim 8 or Claim 9 wherein the material of the powder bed comprises boron nitride.

11. A method according to any one preceding claim wherein the green body comprises silicon nitride.

12. A method according to any one preceding claim from 1 to 10 wherein the green body comprises a metallic body.

13. A method according to any one preceding claim wherein the green body is coated with a thickness of about 5mm prior to hot isostatic pressing.

14. A method substantially as hereinbefore described with reference to the accompanying specification and drawing.