GB2394221A

GB2394221A - Impregnating the surface of a freeze-cast ceramic

Info

Publication number: GB2394221A
Application number: GB0223902A
Authority: GB
Inventors: Ronald Jones
Original assignee: Morgan Crucible Co PLC
Current assignee: Morgan Advanced Materials PLC
Priority date: 2002-10-14
Filing date: 2002-10-14
Publication date: 2004-04-21
Anticipated expiration: 2022-10-14
Also published as: AU2003271961A1; WO2004035506A1; GB0223902D0; GB2394221B

Abstract

A method for producing a ceramic article, the method comprising the steps of: <SL> <LI>a) freeze casting a green body; <LI>b) firing the green body to form a sintered body having residual porosity; <LI>c) impregnating the sintered body with a first impregnant material to form a barrier layer beneath the surface of the sintered body; <LI>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 <LI>e) firing the sintered body to decompose the ceramic precursor to form ceramic. </SL> The first impregnant is preferably a chromium compound that decomposes an oxidation to form chromium oxide. The second impregnant is preferably a ceramic precursor selected from silanes, siloxanes, silazines or mixtures thereof.

Description

( _ARDENED/TOUGHENED FREEZE CA_T 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 0 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 5 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 20 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 2s 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 funning can be achieved. In conventional 30 freeze casting, a particulate material is mixed with an aqueous sol, which consists of 'nano-

phase' (109m) 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 I causes an irreversible chemical reaction gelling the ingredients so that, when resumed to normal or elevated temperatures, the casting possesses enough mechanical strength to be removed from 0 the would 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 6()24259, 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 20 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 2s 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 30 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 (c.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 s examples of property modifying materials. The silicon carbide can also act as a nucleating site for the decomposition of polysilancs or polysiloxanes so that formation of crystalline impregnant is encouraged.

Use of polysilaxancs and like materials to form silicon carbide for use in joining ceramics has 0 been disclosed in a publication of the Oak Ridge National Laboratory under the title "Microwave Joining of SiC" by R. Silherglitt, G.A. Danko, and P.Colombo at pages 2()5-211 (viewable at http:!!\wwnns.oml.,ov/procrams/eners vcf'f/aim/ann oat /9Sscc3-4.pdt). This I discloses the use of: À SR-350, silicone resin available from GE; 5 À SR-355, silicone resin with excess C available from GE; À PCS, polycarbosilane available from Nippon Chemicals; À D-PPC, polycarbosilane with excess C available from Solvay; À AHPCS, allylhydridopolycarbosilane available from Starfirc Systems; À AHPCS, with addition in the laboratory of I wt TO nanosizc (<20 nm) SiC powder; 20 and À Ceraset SN IM, 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 25 Cf /SIC Composites Using Liquid Polymer Infiltration" (J. Am. Ceram. Soc., 84 [1()] 2235-39 (2001) http:iime-www. colorado.edu/rajilultratempiJA(::S_Special /02()1ssuc/15_Ral(.pdf) a fibre preform was pressure impregnated with silicon carbide and then the polysilazine was infiltraecd into this body and pyrolysed.

30 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 sintercd body having residual porosity;

( c) impregnating the sintered body with a first impregnant material to form a harrier 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 s oxide; and c) firing the sintered body to decompose the ceramic precursor to form ceramic.

The barrier layer need not forth an impermeable layer within the body, but can merely impede in impregnation of the ceramic precursor below the surface porosity. Typically, the ceramic precursor is impregnated to a depth of 0.05-1 Omm, e.g. I mm.

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 5 inventor are set out in Table I Table 1

Substrate Ceramic Impregnated Ceramic Alumina Chromium Oxide Silicon Carbide Silicon Carbide (Crystalline) Fused Silica Silicon Oxy Carbide (Amorphous) Alumino-Silicate Silicon Oxy Carbide/Boron Nitride Silicon Nitride (Crystalline) Silicon Oxy Nitride (Amorphous) Silica Alumina SUBSTRATES

Typical compositions for the substrate include freeze cast ceramics formed from the following 20 materials: Substrate Example 1 - 94% Alumina substrate The composition of Table 2 can be freeze cast and fired at 1 20()"C to form an alumina having a Vickers hardness (Hv) of 850()Hv' a surface porosity ofl5'l/0, and a thermal conductivity of 25 2-5 W m/ t>K.

( Tat ale 2 Weight% 48-20() mesh tabular alumina 40 -325 mesh tabular alumina 7.5 Alcoa CT3000 Alumina Fine ground alumina 7.5 d5(, 0.71lm. Surface area/ BET 7.0 Alcoa CT9FG Alumina do = 5.01lm. Surface 45 area/ BET o.8; Drv powder blend = 100% Sol added as excess to I ooze powder blend Morisol AS 2()/40 (The Morissons Group, 13 England) 20 nm SiO2 particles 40% wt Dispex A40 (Ciba Specialty Chemicals, 0.()1 Switzerland) Glyeerin 2 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 5 bonded silicon carbide.

Table 3

Weight% Silicon Carbide 100 mesh 40 Silicon Carbde 325 mesh 20 -325 mesh Tabular Alumina 17 -325 mesh Fused Silica 3.3 Alcoa CT3000 Alumina 7.7 Alcoa CT9FG Alumina 12.0 Drv nowder blend = 10()% . Sol added as excess to 1 ()0% powder blend Morisol AS 20/40 1 Dispex A40 0.01 0 Substrate Example 3 - Fused silica The composition of Table 4 can be freeze cast and fired at 1100- 12()0"C to form a fused silica body.

( Table 4

Weight% -325 mesh silica 9() Fumed Silica Elkem Grade 983 (Elkem ASA, 10 Norway) Dry powder blend = 100% Sol + water added as excess to 100% powder blend Morisol AS 20/40 26.6 Water 3.1 Dispex A40 0.1 I st IMPREGNATION 5 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 0 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 1 20"C and the component is held at this temperature until fully dry through to the interior. The 5 temperature is further gradually increased to 500 C and held at this temperature t'or 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 20 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

2s 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 pyrolysis of polymeric ceramic precursors, which have been applied to the surface porosity. Some typical examples of the impregnating systems include those that on pyrolysis 30 form silicon carbide, silicon oxide, silicon nitride, or mixtures thereof. This impregnation can he 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 I - Crystalline Silicon Carbide An impregnating mixture may be made using 8()g of a polysiloxanc polymer (SR350 from GE (USA)) added to 260g toluene and xylene mixed in 50:50 ratio and dissolved using a high s speed stirrer. 50g of fine silicon carbide powder (SIC Microgrit 0.59um particle size - Grade HSC059 from Superior Graphite (USA)) arc added to the polymer solution using high shear dispersion in the presence of a dispersing agent (e.g. KV9021 or KV9027, available from Zschimmer & Schwarz (Germany).). This solution is then ready for use.

lo 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 polymerization of the system and drive 5 off solvent. The temperature is then gradually increased to 920"C and a nitrogen atmosphere introduced into the furnace. The component is held at 990"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.

20 This operation is repeated until the surface is satisfactorily sealed for use Impregnation Example 2 - Silicon Carbide (Crystalline) 25 This uses the same procedure as example I but follows the following components in the same proportions as example 1.

Material Type Supplier Wacker Silicon Resin ME Polymer Wactcr Chemie GmbH (Germany) Nano Phase SiC SiC Powder Stark (US & Germany) 40-90nm Surface area/BET 2()-50 Starck 13 SiC Impregnation example 3 - Silicon Carbide/ Boron Nitride An impregnation mixture may be made using 35g of Boron Nitride Powder ( I -5 Em - Norton Ceramics (USA)) with 260g of degassed Allylhydridopolycarhosilane (AHPCS - Starfire 35 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 40 adequate period of time so that the surface of the component experiences 100() 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.

Table 5

System Properties Application Alumina impregnated with Hardness 2000 Hv Ceramic Injection Moulds /Chrome/Silicon Carbide Thermal conductivity Metal Injection moulds 30 W.m' Ki OF Nylon Injection Moulds Silicon Carbide Hardness 3000 Hv Die Casting moulds for zinc impregnated with Thermal conductivity and aluminium /Chrome/Silicon Oxy 50 W.m'K Carbide As can be seen from the example of impregnating alumina with silicon carbide, substantial s increases in hardness and thermal conductivity are achieved. In this process the first impregnant material not only fonms 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 0 casting machining.

Applications for such infiltrated freeze cast ceramics include: À wear resistant parts for textile machinery (e.g. threadguides) À wear resistant parts for the paper industry (e.g. dewatering blades) 15 À wear resistant parts for metallurgy (e.g. wire drawing cones, metal casting moulds) À wear resistant parts for the food and phannacoutical industries (e.g. food forming rollers and moulds, cutting blades, mixer parts, mill parts) À wear resistant parts for machinery in general (e.g. parts of pumps, valves, wear resistant linings) 20 À 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.

X

Claims

( CLAIMS

1. A method for producing a ceramic article, the method comprising the steps of: a) freeze casting a green body; 5 b) firing the green body to form a sintcrcd 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 I o 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

5 chromium compound that decomposes on oxidation to form chromium oxide 3. A method, as claimed in Claim I 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 polysilaz.ines.

25 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.