WO2004035506A1

WO2004035506A1 - Hardened/toughened freeze cast ceramics

Info

Publication number: WO2004035506A1
Application number: PCT/GB2003/004465
Authority: WO
Inventors: Ronald Jones
Original assignee: The Morgan Crucible Company Plc
Priority date: 2002-10-14
Filing date: 2003-10-14
Publication date: 2004-04-29
Also published as: AU2003271961A1; GB2394221B; GB0223902D0; GB2394221A

Abstract

A method for producing a ceramic article, the method comprising the steps of: a) freeze casting a green body; b) firing the green body to form a sintered body having residual porosity; c) impregnating the sintered body with a first impregnant material to form a barrier layer beneath the surface of the sintered body; d) impregnating remaining surface porosity of the sintered body with a second impregnant comprising a ceramic precursor other than a precursor for chromium oxide; and e) firing the sintered body to decompose the ceramic precursor to form ceramic.

Description

HARDENED/TOUGHENED FREEZE CAST CERAMICS

This invention relates to a method of hardening and/or toughening freeze cast ceramics, and the products resulting there from.

In conventional ceramic casting processes, a mixture of powders, binders and optionally lubricants, often in water, is introduced into a mould, compacted under high pressure in a press and removed from the mould. The pressing is then fired to a high temperature to bind the ceramic powders together. The process of pressing results in a variable compaction density in the mould. Consequently unpredictable shrinkage occurs during firing, which often leads to distortion. For a given product such shrinkage and/or distortion can be dealt with by having an enlarged and distorted mould so that of firing the consequent shrinkage and distortion results in a product close to desired shape and size. Frequently a lot of post-firing machining is required to produce the finished article. Additionally, as ceramic powders are abrasive, considerable wear is caused to the mould.

One form of precision casting is slip casting and involves the pouring of a water based slurry into an absorbent mould which draws out the water leaving a ceramic component in its 'green state' which can be removed from the mould, dried and fired. The slip cast component undergoes considerable shrinkage during firing and subsequent cooling but can be closer to finished form than in casting.

In both these processes firing is a necessary process to bond the powders together and give the ceramic its full strength, but very precise dimensional accuracy cannot be maintained. Because of the excellent mechanical properties of some ceramics, the use of this material in an engineering context is becoming more important and consequently precision products are required.

Freeze casting is a process by which high precision forming can be achieved. In conventional freeze casting, a particulate material is mixed with an aqueous sol, which consists of 'nano- phase' (10^"9m) particles which can bond together when the water is removed from the sol by freezing. During ice formation the particles of the sol are expelled from the growing ice particles, and in the process forced together to the extent that the sol becomes unstable, the van der Waals attractive forces between the sol particles overcoming the electrostatic repulsive forces between the sol particles. The sol particles can then agglomerate and bind the remaining particles in the mixture. The material is usually mixed as a slurry, which can be vibration cast, poured, or pressure fed, into a mould.

In order to achieve the required component geometry the slurry has to be frozen in the mould to convert the sol into solid form. The casting is then removed from the mould. The freezing causes an irreversible chemical reaction gelling the ingredients so that, when returned to normal or elevated temperatures, the casting possesses enough mechanical strength to be removed from the mould and for handling and the further operations of drying and firing.

Typical of such processes are DE 4037258, US 4428895, US 4552800, US 4569920, US 5647432, US 5716559, US 5954121, US 6024259, US 6199836, US 6322729, US 2001000633- A, and US 2001042929- A.

A problem with the materials produced by this route is that they tend to be porous as a natural consequence of the lack of shrinkage. As is generally known, the amount of porosity is determined by the amount of water present in the initial mixture, and the size of porosity is determined by the size of the ice crystals formed. For many applications, porous surfaces are not good. For example, in the use of ceramics for molten metal resistant tooling such as in zinc and aluminium die-casting. Accordingly the inventor has developed a process for closing the porosity of a freeze cast ceramic by infiltration of the ceramic with ceramic forming materials.

Impregnation of ceramics by materials that form chromium oxide is well known (see for example GB 1466074). However such processes leave a residual surface porosity that is not suited to such processes as zinc and aluminium die-casting. By sealing the surface with a second impregnant material the porosity can be closed and a toughened and hardened surface can be produced. The second impregnant material comprises a ceramic precursor and may also comprise nanometric and micrometric sized ceramic particles. The ceramic precursor is preferably a silicon containing material that decomposes on heating to form a silicon based ceramic such as silicon carbide, silicon oxide, silicon nitride, or mixtures thereof. For example the chemical precursor may comprise a silane, siloxane, silazine, or mixture thereof to produce respectively silicon carbide, silicon oxide, silicon nitride or mixtures thereof. Other ceramic precursors may also be used additionally or in place of silicon based precursors to produce other ceramic materials that have a surface lubricating effect (e.g. aminoborazines to produce graphite-like boron nitride). In addition to the ceramic precursor other materials may be used in the impregnated material to modify properties or to act as nucleating sites for the decomposition of the precursor. Use of silicon carbide or diamond-like boron nitride as a hard filler are examples of property modifying materials. The silicon carbide can also act as a nucleating site for the decomposition of polysilanes or polysiloxanes so that formation of crystalline impregnant is encouraged.

Use of polysiloxanes and like materials to form silicon carbide for use in joining ceramics has been disclosed in a publication of the Oak Ridge National Laboratory under the title "Microwave Joining of SiC" by R.Silberglitt, G.A. Danko, and P.Colombo at pages 205-211 (viewable at http://www^ns.oriil.gov/progi^,anis/energveff/aim/aiiniial/.98sec3-4.pdf). This discloses the use of:-

SR-350, silicone resin available from GE; • SR-355, silicone resin with excess C available from GE;

PCS, polycarbosilane available from Nippon Chemicals; D-PPC, polycarbosilane with excess C available from Solvay; AHPCS, allylhydridopolycarbosilane available from Starfire Systems; AHPCS, with addition in the laboratory of 1 wt % nanosize (<20 nm) SiC powder; and

• Ceraset SN TM , a polysilazine available from Allied Signal Composites, as ceramic precursors. These materials may be used in the present invention.

Polysilazines have been used to impregnate composite materials, for example in "A Process for C_f /SiC Composites Using Liquid Polymer Infiltration" (J. Am. Ceram. Soc, 84 [10] 2235-39 (2001) http://me-www.colorado.cdu/~rajr/ultratemp/J ACS_^ Special%20Issue/15 Rak.pdf) a fibre preform was pressure impregnated with silicon carbide and then the polysilazine was infiltrated into this body and pyrolysed.

The applicant has found that impregnating simply with a ceramic precursor such as a silane or silazine does not completely close the surface porosity. Accordingly the present invention provides a method for producing a ceramic article, the method comprising the steps of:- a) freeze casting a green body; b) firing the green body to form a sintered body having residual porosity; c) impregnating the sintered body with a first impregnant material to form a barrier layer beneath the surface of the sintered body; d) impregnating remaining surface porosity of the sintered body with a second impregnant comprising a ceramic precursor other than a precursor for chromium oxide; and e) firing the sintered body to decompose the ceramic precursor to form ceramic.

The barrier layer need not form an impermeable layer within the body, but can merely impede impregnation of the ceramic precursor below the surface porosity. Typically, the ceramic precursor is impregnated to a depth of 0.05-10mm, e.g. 1mm. Further features of the invention are as set out in the claims in the light of the description.

Substrate materials and impregnant materials that have successfully been made to date by the inventor are set out in Table 1

SUBSTRATES

Typical compositions for the substrate include freeze cast ceramics formed from the following materials:-

Substrate Example 1 - 94% Alumina substrate

The composition of Table 2 can be freeze cast and fired at ~ 1200°C to form an alumina having a Nickers hardness (H_v) of ~ 800H_V, a surface porosity of -15%, and a thermal conductivity of 2-5 W m/ °K.

Substrate Example 2 - Alumina bonded silicon carbide

The composition of Table 3 can be freeze cast and fired at ~ 1250-1350°C to form an alumina bonded silicon carbide.

Substrate Example 3 - Fused silica

The composition of Table 4 can be freeze cast and fired at ~ 1100-1200°C to form a fused silica body.

1st IMPREGNATION

A first impregnation is made to form a barrier layer beneath the surface of the ceramic article. Typically a chromium oxide impregnation is used but the inventions is not restricted to such a material.

For a chromium impregnation, components of any of the above materials are immersed in an aqueous solution of chromic acid (specific gravity 1.7). The time of immersion depends upon the volume of geometry of the component. It is then removed and drained and allowed to dry in air. The component is then placed in an oven and the temperature increased to 80°C and held for a period of time to start to dry the component. Gradually the temperature is increased to 120°C and the component is held at this temperature until fully dry through to the interior. The temperature is further gradually increased to 500°C and held at this temperature for between 0.5 and 3 hours depending on the mass of the component. The temperature is then lowered at the natural rate of the furnace.

The chromium oxide thus formed in the pores of the ceramic increases hardness, toughness and strength but does not fully close off surface porosity. This can be repeated one or more times to achieve a higher degree of toughness and hardness.

2ND IMPREGNATION The chromium impregnated substrates are further impregnated to close off the surface porosity. The second stage in treatment of the tool or component surface is to fully seal the surface and as with the chromium impregnation, this should not cause significant dimensional change. This is achieved by pyro lysis of polymeric ceramic precursors, which have been applied to the surface porosity. Some typical examples of the impregnating systems include those that on pyrolysis form silicon carbide, silicon oxide, silicon nitride, or mixtures thereof. This impregnation can be repeated, depending on how good a surface seal is required.

One general family of polymers found useful in pyrolysis are the polysiloxanes. These can be methyl, phenyl, methyl phenyl, vinyl, vinyl phenyl and/or propyl substituted. Impregnation Example 1 - Crystalline Silicon Carbide

An impregnating mixture may be made using 80g of a polysiloxane polymer (SR350 from GE (USA)) added to 260g toluene and xylene mixed in 50:50 ratio and dissolved using a high speed stirrer. 50g of fine silicon carbide powder (SiC Microgrit 0.59um particle size - Grade HSC059 from Superior Graphite (USA)) are added to the polymer solution using high shear dispersion in the presence of a dispersing agent (e.g. KN9021 or KN9027, available from Zschimmer & Schwarz (Germany).). This solution is then ready for use. The component is dipped into the solution and fully immersed. This is done in a vessel capable of withstanding pressure and vacuum. Firstly the vessel is evacuated to a pressure below ~267Pa (2mm Hg) then held for between 10 and 30 minutes. Air pressure of ~0.4Mpa (4 bar) is then gradually applied. The component is then dried in an oven at 80°C then the temperature is increased to 175°C and held for 2 hours to bring about polymerisation of the system and drive off solvent. The temperature is then gradually increased to 920°C and a nitrogen atmosphere introduced into the furnace. The component is held at 920°C for between 30 mins and 3 hours depending upon the thermal mass of the component. It is then cooled to room temperature and removed from the furnace. This operation is repeated until the surface is satisfactorily sealed for use

Impregnation Example 2 - Silicon Carbide (Crystalline) This uses the same procedure as example 1 but follows the following components in the same proportions as example 1.

Impregnation example 3 - Silicon Carbide/ Boron Nitride

An impregnation mixture may be made using 35g of Boron Nitride Powder (1-5 μm - Norton Ceramics (USA)) with 260g of degassed Allylhydridopolycarbosilane (AHPCS - Starfire Systems (USA)). This system is then used for infiltration and then undergoes a curing cycle. This curing cycle takes place in the presence of nitrogen at 150°C and ~5.5MPa (800 p.s.i.) nitrogen.

The system is then pyrolysed by gradually heating to 1000°C under argon and holding for an adequate period of time so that the surface of the component experiences 1000°C for 2 hours. It is then cooled to room temperature in the furnace. PROPERTIES OF IMPREGNATED MATERIALS

Typical impregnated article properties and applications are set out in Table 5.

As can be seen from the example of impregnating alumina with silicon carbide, substantial increases in hardness and thermal conductivity are achieved. In this process the first impregnant material not only forms a barrier layer but also toughens the material. The second impregnant material seals, toughens, and hardens the surface of the material. Since the freeze cast component can be made to near net shape (having a very small shrinkage in drying and firing - typically <1%) this route enable hard tough articles to be made with very little (if any) post casting machining.

Applications for such infiltrated freeze cast ceramics include:-

• wear resistant parts for textile machinery (e.g. threadguides)

• wear resistant parts for the paper industry (e.g. dewatering blades) • wear resistant parts for metallurgy (e.g. wire drawing cones, metal casting moulds)

• wear resistant parts for the food and pharmaceutical industries (e.g. food forming rollers and moulds, cutting blades, mixer parts, mill parts)

• wear resistant parts for machinery in general (e.g. parts of pumps, valves, wear resistant linings) • wear resistant parts for tooling (e.g. moulds, dies, drawing tools, can closing tools)

The process can also be used to improve the surface properties of articles such as crucibles, heat sinks, and brake parts by improving the wear resistance, thermal conductivity, and/or oxidation resistance of the articles.

Claims

1. A method for producing a ceramic article, the method comprising the steps of:- a) freeze casting a green body; b) firing the green body to form a sintered body having residual porosity; c) impregnating the sintered body with a first impregnant material to form a barrier layer beneath the surface of the sintered body; d) impregnating remaining surface porosity of the sintered body with a second impregnant comprising a ceramic precursor other than a precursor for chromium oxide; and e) firing the sintered body to decompose the ceramic precursor to form ceramic.

2. A method, as claimed in Claim 1, in which the first impregnant material comprises a chromium compound that decomposes on oxidation to form chromium oxide

3. A method, as claimed in Claim 1 or Claim 2, in which the second impregnant material comprises a ceramic precursor selected from the group silanes, siloxanes, silazines or mixtures thereof.

4. A method, as claimed in Claim 3, in which the silanes, siloxanes, silazines or mixtures thereof include ceramic precursors selected from the group polysilanes, polysiloxanes, and polysilazines.

5. A method, as claimed in any one of Claims 1 to 4, in which the second impregnant material comprises a nucleating agent for the ceramic formed from the ceramic precursor.

6. An article made by the method of any preceding claim.

7. An article, as claimed in Claim 6, in which the article is a wear resistant part for use in industry.