ZIRCONIA-CONTAINING CERAMIC COMPOUND AND METHOD OF MAKING SAME

Abstract: A ceramic material having good thermal shock resistance consists of a matrix of zircon within which there is a dispersion of zirconia particles having a bimodal particle size distribution. The zirconia phases have mean particle sizes of (a) less than 4 micrometres, and (b) from 4 to 25 micrometres, each being 2 to 20 per cent (preferably 10 per cent) by weight of the material. The zirconia particles are admixed with particulate zircon; the mixture is then moulded and fired at 1400 to 1700<o>C. Alternatively, the finer particles of the mixture may be produced in situ by solid state reaction of alumina and dissociated zircon at firing.

Description: TITLE; ZIRCONIA-CONTAINING CERAMIC COMPOUND AND METHOD OF MAKING SAME TECHNICAL FIELD This invention concerns ceramic materials. M particularly it concerns refractory ceramic materials w good thermal shock resistance. BACKGROUND ART Over the last decade, considerable work has b undertaken to improve the thermal shock resistance ceramic materials. For example, a magnesia partia stabilised zirconia material having good thermal sh resistance has been described in the specification of Patent No 4,279,655. In addition, in his paper entitl "Improved thermal shock resistant refractories fr plasma-dissociated zircon", published in the Journal Materials Science, volume 14, pages 817 to 822, 197 R C Garvie has shown that a ceramic material formed fr dissociated zircon (DZ) can have its thermal sho resistance properties improved substantially if it contai about 10 per cent (by weight) of monoclinic zircon particles having a mean diameter of about 13 micrometres. DISCLOSURE OF THE PRESENT INVENTION The present inventor has now found that superi thermal shock resistance properties can be acquired zircon-based ceramic materials if the ceramic materia contain zirconia particles of two different sizes, so th the ceramic material consists of a matrix of the zirc material within which zirconia particles of a first si and also zirconia particles of a second size are locate Furthermore, this improvement in thermal shock resistan properties by the inclusion of a bimodal size distributi of dispersed zirconia is exhibited for a wide range of t concentrations of the dispersed zirconia. Thus, according to the present invention, a ceram material having good thermal shock resistance properties characterised by a) a matrix of zircon; b) a dispersion, within the zircon matrix, of zirconia particles having a first mean particle size; and c). a dispersion, within the zircon matrix, of zirconia particles having a second mean particle size, said second mean particle size being greater than said first mean particle size. Typically, the smaller zirconia particles comprise fr 2 to about 20 per cent by weight of the product materi and the larger zirconia particles comprise from 2 to abo 20 per cent by weight of the product material. The matrix material may be zircon sand or dissociat zircon (DZ). Dissociated zircon is usually made by droppi zircon sand through a plasma furnace to form spheres reactive silica in which crystals of zirconia are embedde When the material of the present invention is made (s below) the silica recombines with the zirconia during t firing step to form zircon. The preferred distributions of zirconia particles a about 10 per cent by weight of the fine fraction (that i zirconia particles which have a mean particle size which less than 4 micrometres and preferably in the range fr 0.5 to about 2.0 micrometres) and about 10 per cent weight of the coarser zirconia fraction (zirconia particl which have a mean particle size in the range from 4 about 25 micrometres, and preferably about 13 micrometres The present invention also encompasses the method manufacture of the new ceramic materials. According to this aspect of the present invention, method of making the improved ceramic material defin above comprises the steps of a) milling a powder of the matrix zircon material; b) dispersing within the milled powder a predetermin proportion of zirconia particles which have a fir mean particle size; c) dispersing within the milled powder a predetermi proportion of zirconia particles which have second mean particle size, said second parti size being greater than said first particle size d) moulding the mixture formed by steps (b) and (c) a desired shape; and e) firing the moulded shape at a temperature in range from about 1400°C to about 1700°C. The firing time for step (e) may be up to five hou but the preferred firing time is about 1 hour. Preferably, the milling step (a) is performed until mean particle size is less than about 2 micrometr Optionally, a wax binder or the like may be added to t mixture formed by steps (b) and (c) to facilitate t moulding step (d). It has been found that the larger zirconia particl (having a particle diameter in the range from 4 to micrometres) are available commercially - for example, the MEL-S grade of zirconia powder produced and sold Magnesium Elektron Co. The smaller zirconia particles c be made by separating the fine particles from commercial available zirconia powder or by milling a commercial available zirconia powder, preferably until the me particle diameter has been reduced to below 2.0 micrometr (this milling may be effected with the particles admix with the zircon particles, thus combining steps (a) a (b)). Alternative methods of producing the mixture of fi particles of zirconia and zircon exist. One such method to mill the zircon grains for a prolonged period usi magnesia (or calcia or yttria) partially stabilis zirconia as the grinding medium. During this milling, t grinding media wears by shedding fine particles.. Thus fi zirconia particles, from the grinding medium, are add automatically to the DZ particles during the milling st (a), and control over the quantity of fine zirco particles in the zircon is achieved by controlling the t of the milling. Another of the alternative ways of producing a mixt of zircon and fine zirconia particles is by a solid st reaction between zircon and alumina. Pure alumina powder added to the batch of zircon. The alumina may be fin divided before its addition to the zircon or it may milled to a suitable state of sub-division together w the grains of zircon. After the coarser zirconia partic have been added to the mixture of zircon and alumina, the firing step is effected, when the firing temperat exceeds about 1450 C, the following reaction takes place: 2Zr Si 04. + 3A12-03, = 2ZrO2_ + 3A1-203...2SiO2_. Only the amount of alumina that is needed to produce predetermined concentration of fine zirconia particl should be added to the zircon. It will be appreciated t this technique results in the production of a minor mulli (3Al 2 0 3 .2Si0 2 ) phase in the product ceramic material. Yet another technique for producing the admixture zircon and fine zirconia particles, which can be used the zircon is dissociated zircon (DZ), involves parti leaching of the DZ grains with hot, concentrated sodi hydroxide solution. If the sodium hydroxide soluti contains about 50 per cent by weight of NaOH and is used a temperature of 300 C, it will leach silica fr dissociated zircon to leave zirconia. The leaching of DZ is thus performed until the zircon contains an excess zirconia in the range from 2 to about 20 per cent by weig (preferably about 10 per cent by weight). This partial leached material is then milled until the average partic size is less than about 2 micrometres, to produce t preferred admixture of steps <a) and (b) . (Note that ho concentrated sodium hydroxide solution will not lea silica from zircon sand. ) OM DETAILED DESCRIPTION OF AN EXAMPLE WHICH ILLUSTRATES THE PRESENT INVENTION A first batch of ceramic bars was made by the follow technique. 438 gm of dissociated zircon and 62 gm of "Li A" alumina were wet-milled with 750 ml of isopropyl alco and 4 kg of magnesia partially stabilised zirconia for hours. After drying the milled batch and separating powder from the grinding medium, 55.6 gm of MEL-S grade monoclinic zirconia particles were added to the separa powder, together with 24 zirconia grinding balls. T mixture was tumbled for half an hour, after which grinding balls were removed and 4 per cent by weight o binder was blended into the mixture. The binder compri 80 per cent glycerol and 20 per cent of "Versikol K (Trade Mark). Eight rectangular bars of the mixture, e bar measuring 6 mm x 6 mm x 50 mm, were pressed in a d then isopressed at 207 MPa. The pressed bars were t heated at the rate of 80 C per hour until a temperature 1600 C had been attained. The bars were held at t temperature for one hour, then the furnace was switch off. When the bars had cooled to room temperature, th were ground until their cross-section was 3 mm x 3 mm. The initial modulus of rupture (MOR.) of four of t ground bars was measured. The remaining four bars we heated to 950 C, then quenched into water at ro temperature. After this thermal shock treatment, t retained modulus of rupture (MOR ) was measured. A second batch of eight bars was prepared in the sa manner as the first batch except that the coarse zircon particle fraction was omitted. A third batch of eight ba was prepared in the same way as the ' first batch, exce that no alumina (that is, no fine zirconia particle phas was included in the batch. A fourth batch of eight bars w prepared in the same way as the first batch, except neith alumina nor the coarse zirconia particles was included the batch. The bars of each of the second, third and fou batches were tested in the same way as the bars of first batch. The zirconia phase composition and the measu properties of the bars of each of the batches are lis below: Wt % of t % of MOR. MOR Batch fine coarse (in 1 (in r Density No. Zr0 2 Zr0 2 MPa) MPa) (gm/cc) 1 10 10 172 79 4.46 2 10 - 248 30 4.38 3 - 10 152 68 4.21 4 ' _ __ 138 0 4.20 Inspection of this data shows that the bars made f the first batch, which contained both fine and coa zirconia particles had the best combination of MOR. ( MPa) and MOR (79 MPa). Thermally shocked bars made f the fourth batch (neither coarse nor fine zirco particles present) were so weak that they broke dur handling. Bars made from the second batch, which had o fine zirconia particles present, were very strong (MOR. 248 MPa) but the thermal shock resistance was only a mod value (MOR of 30 MPa). The ars made from the third bat which contained only coarse zirconia particles, acceptable values of MOR. (152 MPa) and MOR (68 MPa) these values were still 25% less than those of the b made from the first batch. It has been found that the characteristic features materials prepared according to the present invent include: 1. Values of MOR. are greater than about 160 MPa. 2. Values of MOR (obtained after the thermal sh test described above) are at least 45 per cent the values of MOR.; (that is, values of MOR greater than about 72 MPa). 3. The bulk density is greater than 4.25 gm/cc. 4. The open porosity is less than about 5%. 5. The matrix phase consists either of pure zircon a mixture of pure zircon (major matrix phase) mullite (minor matrix phase), with the matrix ph being at least 70 per cent of the ceramic mater and having a mean grain size which is less than micrometres. 6. The mean particle size of the fine zirco particles is less than 3 micrometres and content of fine zirconia particles in the prod material is in the range from 2 to 20 per cent weight, with a preferred value of 10 per cent weight. 7. The mean particle size of the coarse zirco particle is in the range from 4 to 20 micrometre with a preferred mean size of about 13 micrometre the coarse fraction of zirconia particles being the range from 2 to 20 per cent by weight, with preferred value of 10 per cent by weight. INDUSTRIAL APPLICATION The material of the present invention may be used most areas where conventional ceramics and refractories a used. Some examples of suitable uses of the material are: thermocouple protection tubes; furnace tubes; laboratory and foundry crucibles; flow control systems for molten metals (including tundish pouring nozzles and sliding gates for the continuous casting of steel); pump linings; pump components; die casting machine components; and nozzles for handling molten non-ferrous metals such a aluminium, aluminium alloys, zinc and zinc alloys. The material of the present invention is especial suitable as a replacement for refractory grade partial stabilised zirconia materials, which are from three to t times more expensive than the materials of the present invention, and which have lower values of MOR. (from 21 49 MPa), and MOR (from 11 to 25 MPa) and an open porosi in the range from 15 to 22 per cent. OM

Claims: CLAIMS 1. A ceramic material having good thermal sh resistance properties, said material comprisin matrix of zircon and being characterised .by a) a dispersion, within the matrix, of zirco particles having a first mean particle si and b) a dispersion, within the matrix, of zirco particles having a second mean particle si said second mean particle size being grea than said first mean particle size. 2. A ceramic material as defined in claim 1, furt characterised in that the first mean particle s is less than 4 micrometres and the second m particle size is in the range from 4 to micrometres. 3. A ceramic material as defined in claim 1, furt characterised in that the first mean particle si is in the range from 0.5 to 2.0 micrometres and t second mean particle size is about 13 micrometres 4. A ceramic material as defined in claim 1, clai or claim 3, further characterised in that t zirconia particles having a first mean partic size comprise from 2 to 20 per cent by weight the material, and the zirconia particles having second mean particle size comprise from 2 to 20 p cent by weight of the material. 5. A ceramic material as defined in claim 1, claim or claim 3, further characterised in that t zirconia particles having a first mean partic size comprise about 10 per cent by weight of t • V ceramic material, and the zirconia particles havi a second mean particle size comprise about 10 p cent by weight of the ceramic material. 6. A ceramic material as defined in any precedi claim, in which said zircon matrix is formed fr zircon sand, or from dissociated zircon. 7. A ' ceramic material as defined in any precedi claim, in which said zircon matrix comprises major phase of zircon and a minor phase of mullit 8. A method of making a ceramic material having go thermal shock resistance properties, said meth being characterised by the steps of a) milling a powder of zircon material; b) dispersing within the milled powder predetermined proportion of zirconia particl which have a first mean particle size; c) dispersing within the - milled powder predetermined proportion of zirconia particl which have a second mean particle size, sa second mean particle size being greater th said first mean particle size; d) moulding the mixture formed by steps (b) a (c) to a desired shape; and e) firing the moulded shape at a temperature the range from about 1400°C to about 1700°C. 9. A method as defined in claim 8, furth characterised in that steps (a) . and (b) a performed by milling the zircon material with grinding medium selected from the group consisti of magnesia partially stabilised zirconia, calc partially stabilised zirconia and yttria partial stabilised zirconia, whereby fine , zircon particles are . shed from the grinding medium provide said zirconia particles having a first m particle size. 10. A method as defined in claim 8, furt characterised in that steps (a) and (b) performed by admixing coarse zircon grains coarse zirconia particles and milling this mixt of coarse grains and particles. 11. A method as defined in claim 8, furt characterised in that steps (a) and (b) performed by. partially leaching grains dissociated zircon with hot, concentrated sod hydroxide solution to remove a proportion of silica in the grains of dissociated zircon, t milling the partially leached dissociated zirc grains. 12. A method as defined in any one- of claims 8, 9 a 10, in which the zircon is either zircon sand dissociated zircon. 13. A method as defined in any one of claims 8 to 1 further characterised in that the first me particle size is less than 4 micrometres and t second mean particle size is in the range from 4 25 micrometres. 14. A method as defined in any one of claims 8 to 1 further characterised in that the first me particle size is in the range from 0.5 to 2 micrometres and the second mean particle size about 13 micrometres. 15. A method as defined in any one of claims 8 to 1 further characterised in that the zircon particles having a first mean particle si comprise from 2 to 20 per cent by weight of t material, and the zirconia particles having second mean particle size comprise from 2 to 20 p cent by weight of the material. 16. A method as defined in any one of claims 8 to 1 further characterised in that the zircon particles having a first mean particle si comprise about 10 per cent by weight of the ceram material, and the zirconia particles having second mean particle size comprise about 10 p cent by weight of the ceramic material. 17. A method of making a ceramic material having go thermal shock resistance properties, "said meth being characterised by the steps of a) mixing together particulate zircon a particulate alumina to obtain a predetermin proportion of alumina in the zircon, sa particles of zircon and alumina having a me particle size in the range 0.5 to 4 micrometres; b) adding to said admixture a predetermin proportion of zirconia particles having a me particle size in the range from 4 to micrometres; c) moulding the admixture of step (b) to a desir shape; and d) firing the moulded shape at a temperature the range from 1400°C to 1700°C. 18. A method as defined in claim 17, furth characterised in that the predetermined proportio of alumina and zirconia are such that the fir ceramic material contains from 2 to 20 per cent weight of zirconia particles having a mean parti size in the range from 4 to 25 micrometres and f 2 to 20 per cent by weight of zirconia. partic having a mean particle size in the range from 4 25 micrometres. 19. A method as defined in claim 17, furt characterised in that the predetermined proporti of alumina and zirconia are such that the fi ceramic material contains about 10 per cent weight of zirconia particles having a mean parti size in the range from 4 to 25 micrometres about 10 per cent by weight of zirconia partic having a mean particle size in the range from 4 25 micrometres. 20. A method as defined in claim 17, claim 18 or cl 19, in which the particles of zircon and alumi have a mean particle size in the range from 0.5 2.0 micrometres, and the particles of zircon added in step (b) have a mean particle size about 13 micrometres. 21. A method as defined in any one of claims 8 to 2 including the addition of a binder to the admix materials before effecting the moulding and firi steps. 22. A method as defined in any one of claims 8 to 2 in which said firing step is performed for up to hours. 23. A method as defined in claim 22, in which sa firing step is performed for about one hour. *

Inventors: GARVIE RONALD CHARLES, GARVIE, RONALD, CHARLES
Applicants: COMMW SCIENT IND RES ORG, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR
Application year: 1983
Main classification: C04B35
IPC codes: C04B35