Ceramic materials for use in insert-casting and processes for producing the same

Abstract: A ceramic material for use in insert-casting, which contains not less than 65% by volume of aluminum titanate as a crystalline phase. The average particle diameter of crystals of the aluminum titanate is not less than 10 µm, and not more than 20% by volume of a glass phase containing a rare earth element in an amount of 0.5 to 16% by weight when calculated in the form of an oxide is contained. Young's modulus is 50 to 2,000 kgf/mm². A process for producing the same is also disclosed, which comprises the steps of preparing a powder by mixing a material, as an Aℓ₂O₃ source material, containing not more than 96% by weight of Aℓ₂O₃ and having an average particle size of not less than 3 µm, a material having an average particle diameter of not more than 3 µm as a TiO₂ source material, and a given rare earth element in an amount of 1.8% by weight when calculated in the form of an oxide, shaping the powder, and drying and firing the shaped body. The ceramic material may have an actual beak strain of not less than 6 × 10⁻³. The process for producing such a ceramic material includes a step of cooling the shaped body at a rate of at least not less than 500°C/hr in a temperature range not less than 200°C during and/or after the firing.

Description: The present invention relates to ceramic materials for use in enclosing a hollow tubular ceramic material by insert-casting with a melt of a metal such as aluminum or cast iron. Particularly, the invention relates to heat-insulating ceramic materials for use in insert-casting as port liners, exhaust manifold liners, piston cavities, cylinder top plates, etc. in gasoline engines, diesel engines and the like. The invention also relates to processes for producing such ceramic materials. Recently, environmental pollution by automobile exhaust gases has become a serious social problem, and methods for removing poisonous materials in automobile exhaust gases with use of catalysts are mainly employed. Reduction in amounts of noble metals such as Pt, Rh, etc. used as catalysts therefor comes desirable from the stand point of natural resources and costs. Further, four valve engines, which have recently increasingly been employed, pose a problem in that a catalytic purification performance is reduced by reduction in temperatures of exhaust gases. As one of methods for solving the above problems, Japanese Patent Application Laid-open No. 63-236,759 proposes that the inner surface of an exhaust port or an exhaust port liner is lined with a port liner made of a ceramic material having a low Young's modulus so that the temperature of exhaust gases may be increased by its heat insulating property. Further, Japanese Patent Publication No. 57-3,629 and Japanese Patent Application Laid-open No. 56-92,164 disclose addition of a rare earth element to aluminum titanate as also intended in the present invention. However, there are problems that the materials disclosed in Japanese Patent Application Laid-open No. 63-236,759 suffer deterioration in strength due to cooling-heating cycle owing to changes in temperatures of the exhaust gases when in use, and that the materials are broken. In the inventions of both Japanese Patent Publication No. 57-3,629 and Japanese Patent Application Laid-open No. 56-92,164, dense sintered bodies having a low thermal expansion are obtained by adding a given amount of an oxide of Y, La and/or Ce to aluminum titanate. However, Young's modulus of all these sintered bodies are high, and they cannot be used for materials in insert-casting. As the ceramic materials used for increasing the temperature by their heat insulating property, those having lower Young's modulus are preferred, because they can mitigate great compression stresses occurred through shrinkage of a molten metal during insert-casting of liners. NGK Insulators, Ltd. have formerly developed ceramic materials for use in insert-casting, in which not less than 65% by volume of aluminum titanate is contained as a crystalline phase, and proposed them in Japanese Patent Application Laid-open No. 63-236,759. However, succeeding researches revealed that although the above materials had the Young's modulus E falling in the range from 50 to 2,000 kgf/mm², some of them were broken on insert-casting, while others were not. Therefore, stable insert-castability cannot be assured, which poses a practical problem. The present invention is to solve or at least mitigate the conventional problems mentioned above, and has been accomplished to provide practical ceramic materials suitable for insert-casting, which have excellent heat resistance, thermal shock resistance and heat insulation, are free from occurrence of cracks resulting from compression forces generated on insert-casting, and have durability in cooling-heating cycles during use. The invention also provides processes for producing such ceramic materials. The ceramic material for use in insert-casting according to the present invention is set out in claim 1. Further, the process for producing the ceramic materials for use in insert casting according to the present invention is set out in claim 6. The coefficients of thermal expansion of a- and b-axes of the aluminum titanate crystal are positive, while that of the c-axis is negative. Further, differences among them are great. Thus, strain occurs in the grain boundaries owing to changes in temperatures such as cooling-heating cycles. In conventional materials constituted by fine crystals, such a strain does not become a great problem. On the other hand, particularly in the case of reduced Young's modulus ceramic materials for use in insert-casting, in which crystals of aluminum titanate are grown to not less than 10 µm on the average, it was found that the above strain becomes so large that the gap between the grains widens, reducing strength. In contrast, the present inventors have found that when a very small amount of at least one rare earth element such as Y, Ce or La is added, reduction in strength by the cooling-heating cycles can be prevented. This is considered that the added rare earth element collects at the intergranular glass phase to increase strength of the glass phase and bonding forces between the glass phase and the aluminum titanate crystals. The present invention has been accomplished on the basis of the above-mentioned recognition. The present inventors have found that the ceramic materials having the structure which has aluminum titanate having a given average crystalline particle diameter as a main crystalline phase and in which the glass phase containing a given amount of the rare earth element is present in the grain boundaries of the aluminum titanate crystals can attain low Young's modulus aimed at by the present invention and durability in cooling-heating cycles. As a method for producing the ceramic material having the above crystalline structure, the ceramic material for use in insert-casting can be obtained, by using a coarse Al₂O₃ source material having a low purity,a fine TiO₂ source material, and a given amount of a rare earth element, in which not less than 65% by volume of aluminum titanate is contained as a favorable crystalline phase of the above material, the average particle diameter of the crystals is not less than 10 µm, Young's modulus is 50 to 2,000 kgf/mm², compression strength is 5 to 40 kgf/mm², and porosity is 5 to 35%. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a section view of a port liner for use in a four valve engine; Fig. 2 is a perspective view of a port liner inserted in an engine head by casting; Fig. 3 is a sectional view of an exhaust manifold liner after insert-casting; Fig. 4 is a graph illustrating a cooling-heating cycle for evaluation in the present invention; Figs. 5a and 5b are SEM photographs showing the structures of crystals in a fired body according to the present invention before and after the cooling-heating cycle, respectively; and Figs. 6a and 6b are SEM photographs showing the structures of a fired body in Comparative Examples before and after the cooling-heating cycle, respectively. Reasons for the limitations in the ceramic materials according to the present invention and their preferable ranges are as follows: The reasons why the amount of aluminum titanate and the average particle diameter of the crystals of the aluminum titanate are limited to not less than 65% by volume and not less than 10 µm, respectively, are that the amount or the average particle diameter are less than 65% by volume or less than 10 µm, respectively, Young's modulus exceeds 2,000 kgf/mm² so that cracks occur on insert-casting. Further, if the amount of the glass phase is more than 20% by volume, Young's modulus exceeds 2,000 kgf/mm². Thus, the amount of the glass phase is limited to not more than 20%. In addition, if the amount of the rare earth element in the glass phase is less than 0.5% by weight when calculated in the form of the oxide, the reduction rate in the four point bending strength in the cooling-heating cycle is more than 10%. If the rare earth element exceeds 16% by weight, Young's modulus becomes more than 2,000 kgf/mm². Thus, the content of the rare earth element in the glass phase is limited to 0.5 to 16% by weight when calculated in the form of the oxide. Reason for the preferred ranges in the present invention are as follows: If the compression strength is less than 5 kgf/mm² and/or if the four point bending strength is less than 0.2 kgf/mm², the ceramic material may be deformed on insert-casting, so that handling problems rise. If the compression strength is more than 40 kgf/mm² or if the four point bending strength is more than 2.0 kgf/mm², Young's modulus may exceed 2,000 kgf/mm². Further, if the porosity is less than 5%, a sufficient heat insulating effect cannot be obtained. If the porosity is more than 35%, Young's modulus may fall outside the numerical range in the present invention. It is most preferable that the Young's modulus is set particularly at 50 to 200 kgf/mm². If the rare earth element in the glass phase exceeds 10% by weight when calculated in the form of the oxide, Young's modulus becomes greater than 200 kgf/mm². In addition, it is preferable that the composition of the ceramic material consists essentially of 40 to 65% by weight of Aℓ₂O₃ and 30 to 60% by weight of TiO₂. The reason for this is that if the composition falls outside the above composition, the crystal amount of the aluminum titanate is difficult to exceed 65% by volume. The reason why at least one kind of SiO₂, MgO and Fe₂O₃ is preferably in a total amount of not more than 10% is that if it exceeds 10% by weight, the crystal amount of the aluminum titanate is also difficult to exceed not less than 65% by volume, and the crystalline particle diameter may not be 10 µm or more. Furthermore, when the ceramic material according to the present invention possesses the coefficient of thermal expansion of not more than 2.0 × 10⁻⁶ (40 to 800°C) and a heat conductivity of 0.8 × 10⁻³ to 5.0 × 10⁻³ cal/cm·sec·°C, the material is suitable for port liners to be brought into direct contact with exhaust gases at high temperatures. Next, reasons for the limitations in the Aℓ₂O₃ source material and the TiO₂ source material in the producing process according to the present invention are as follows: If the Al₂O₃ source material has the purity of more than 96% and the average particle diameter of more than 3 µm, the amount of the unreacted portion increases due to low reactivity of the Al₂O₃ source material so that Young's modulus becomes higher than 2,000 kgf/mm² because of insufficient growth of microcracks and that cracking of the ceramic material is feared on insert-casting with aluminum. Furthermore, if the average particle diameter of the Al₂O₃ source material is less than 3 µm, the average particle diameter of aluminum titanate is reduced to less than 10 µm irrespective of the purity of the Al₂O₃, so that Young's modulus becomes higher than 2,000 kgf/mm². Moreover, if the content of impurities contained in the Al₂O₃ source material is more than 20% by weight of SiO₂, more than 20% by weight of Fe₂O₃, or more than 20% by weight of SiO₂ + Fe₂O₃ in total, it may be that the amount of the aluminum titanate is less than 65% by volume and that the average crystal particle diameter is less than 10 µm. From the above, the Al₂O₃ source material must have the purity of not more than 96% by weight and the average particle diameter of not less than 3 µm. The amount of the impurities is preferably 0 to 20% by weight of SiO₂, 0 to 20% by weight of Fe₂O₃, or 0 to 20% by weight of SiO₂ + Fe₂O₃ in total. If the average particle diameter of the TiO₂ source material is more than 3 µm, the amount of the unreacted portion increases, so that Young's modulus exceeds 2,000 kgf/mm². Thus, it is feared that the material are cracked on insert-casting with aluminum. Therefore, the TiO₂ source material has the average particle diameter of not more than 3 µm. When the rare earth element such as Y, La or Ce is added in an amount of not more than 1.8% by weight, Young's modulus can be controlled to not more than 2,000 kgf/mm². If it is added in an amount less than 0.3% by weight, Young's modulus can be controlled to not more than 200 kgf/mm². Examples of processes for producing the ceramic materials according to the present invention will now be described. Materials are selected from alumina, calcined bauxite, purified rutile, crude rutile, anatase type titanium, ilmenite, ferrite red iron oxide, electrofused magnesium, magnesite, electrofused spinel, kaoline, quartz, electrofused silica, specific rare earth elements, etc. to give a chemical composition consisting essentially of 40 to 65% by weight of Al₂O₃, 30 to 60% by weight of TiO₂, and not more than 10% by weight of at least one kind of SiO₂, MgO and Fe₂O₃ in total. At that time, it is necessary that a starting material containing not more than 96% by weight of Aℓ₂O₃ and the average particle diameter of not less than 3 µm as the Aℓ₂O₃ source material is mixed with a material having the average particle diameter of not more than 3 µm as the TiO₂ source material. Further, the rare earth element may be added in the form of a nitrate, an oxide or any other compound. In order to obtain an intended effect with a very small addition amount of the rare earth element, it is preferable to uniformly disperse fine particles of the material thereof. 0.1 to 1.0% by weight of a deflocculant selected from water glass, ammonium polycarbonates, amines, sodium pyrophosphates, or the like is added to the mixture, to which 1.0 to 5.0% by weight of a binder selected from PVA, MC, CMC, acrylates or the like is added. The resulting mixture is sufficiently mixed and stirred by means of a trommel, a pot mill or the like together with 15 to 40% by weight of water to prepare a slurry having a viscosity of 200 to 1,000 cp. The slurry is shaped in a cylindrical form or in the form of a port liner by casting, which is dried and fired. As a result, the aluminum titanate sintered body can be obtained, which has excellent heat resistance, thermal shock resistance, heat insulation and durability in cooling-heating cycles and which contains not less than 65% by weight of aluminum titanate as a crystalline phase and not more than 20% by weight of the glass phase involving 0.5 to 16% by weight of the rare earth element when calculated in the form of the oxide. One or more of rutile, corundum and mullite may be present as other crystalline phases. The crystals of aluminum titanate are sufficiently grown to not less than 10 µm by setting the firing conditions to, for instance, 1,450 to 1,650°C, preferably 1,500 to 1,600°C, for 1 to 16 hours. By so doing, the ceramic material can acquire physical properties of Young's modulus of 50 to 2,000 kgf/mm², preferably compression strength of 5 to 40 kgf/mm², four point bending strength of 0.2 to 2.0 kgf/mm², and porosity of 5 to 35%. Since the ceramic materials having such a low Young's modulus can shrink together with a metallic material when the cast metallic material shrinks, no cracks occur due to concentration of stress particularly in the case of such a complicated shape. A conventional high strength, high Young's modulus ceramics is liable to break due to concentration of stresses. Therefore, the ceramic materials according to the present invention can favorably be applied to, as a matter of course, conventional cylindrical port liners as well as a port liner 3 having a complicated shape shown in Fig. 1 or 2, and a port liner for an exhaust manifold as shown in Fig. 3. In these figures, two ports 2 are provided on a cylinder side of a four valve engine, and a single exhaust opening 1 is provided in an exhaust manifold side. Heat conductivity is lowered by the cracks in the sintered body, so that a sufficient heat insulating effect can be obtained even when the porosity is relatively small. Since aluminum titanate has a melting point of not less than 1,700°C, no particular limitation is posed upon metals to be cast. Gray cast iron, nodular graphite cast iron, white cast iron, aluminum alloy, copper alloy, magnesium alloy, and zinc alloy may be used for insert-casting. Further, the intergranular glass phase is firmly bonded to aluminum titanate by including a very small amount of a rare earth element in the glass phase, so that a ceramic material having durability in the cooling-heating cycle can be obtained without deteriorating low Young's modulus favorable for insert-casting. Next, Examples of the present invention will be given. Experiment 1 : By using an Al₂O₃ source material shown in Table 1 and a TiO₂ source material shown in Table 2 as main components, a raw material was formulated to give each of compositions in Test Run Nos. 1-35 shown in Table 3, and cast into a mold, thereby preparing elliptical section test pieces having a thickness of 3 mm, a main axis of 64 mm and a minor axis of 36 mm. Each test piece was fired under conditions given in Table 3, and various physical properties were measured with respect to thus obtained fired bodies. Next, after molding sand is charged into each fired body, it was enclosed with aluminum by casting to prepare a metal-ceramic composite body having an aluminum thickness of 7 mm. After the molding sand was removed, it was checked whether cracks occurred in the test piece or not. Results are shown in Table 3. In a cooling-heating cycle test of Table 1, the test piece was repeatedly subjected to a cooling-heating cycle of keeping it at room temperature for 10 minutes and 900°C for 20 minutes 50 times as shown in Fig. 4. Fig. 4 gives a schedule and the temperature of the cooling-heating cycle. A reduction rate of the four point bending strength of the test piece between before and after the cooling-heating cycle test was measured. Amounts of aluminum titanate and a glass phase were determined as % by volume based on their area-occupying rates in an SEM photograph. Further, an amount of a rare earth element in the glass phase was measured by an EDX analysis by means of a transmission electron microscope. In order to facilitate observation of changes in the cooling-heating cycle with respect to the materials according to the present invention and as Comparative Examples, Figs. 5a, 5b, 6a and 6b are microphotographs of the body as sintered, and the sintered body having undergone the above cooling-heating cycle 600 times. That is, Figs. 5a and 5b show the crystalline structures of the fired body according to the present invention before and after the cooling-heating cycles, respectively. Figs. 6a and 6b show the crystalline structures of the fired body of Comparative Example before and after the cooling-heating cycle, respectively. From Figs. 5a, 5b, 6a and 6b, it is seen that the gap is widened by the cooling-heating cycle in the ceramic material of Comparative Example, but that no conspicuous difference is recognized between before and after the cooling-heating cycle in the ceramic materials according to the present invention. A-1 A-2 A-3 A-4 A-5 Chemical analysis value (wt%) SiO₂ 0.9 2.1 2.7 2.7 2.4 Aℓ₂O₃ 98.1 96.0 94.8 95.1 95.3 Fe₂O₃ 0.1 0.4 0.5 0.4 0.4 TiO₂ 0.6 1.3 1.8 1.6 1.7 Others 0.3 0.2 0.2 0.2 0.2 Average particle diameter (µm) 12.9 13.1 9.8 3.0 1.4 T-1 T-2 T-3 Chemical analysis value (wt%) SiO₂ 0.0 0.0 0.0 Aℓ₂O₃ 0.0 0.0 0.0 Fe₂O₃ 0.0 0.0 0.0 TiO₂ 99.6 99.4 99.5 Others 0.4 0.6 0.5 Average particle diameter (µm) 5.8 3.0 0.7 From results in Table 3, it is recognized that the ceramic materials according to the present invention are free from occurrence of cracks on insert-casting, and that they suffer fewer deterioration in the cooling-heating cycle test as compared with Comparative Examples. As is clear from the foregoing explanation, according to the ceramic materials for use in insert-casting in the present invention and the production process thereof, by using coarse Al₂O₃ material having a low purity, fine TiO₂ source material and a rare earth element, it is possible to obtained the crystalline structure in which not less than 65% by volume is aluminum titanate crystal phase, the average particle size of the crystals is not less than 10 µm, and not more than 20% by volume of a glass phase containing the rare earth element in an amount of 0.5 to 16% by weight when calculated as an oxide is present. As a result, the ceramic materials for use in insert-casting can be obtained, which have low Young's modulus, excellent heat resistance, thermal shock resistance and heat insulation and durability in the cooling-heating cycles, and are free from occurrence of cracks due to compression forces generated on the insert-casting.

Claims: A ceramic material for use in insert-casting, which contains not less than 65% by volume of aluminum titanate as a crystalline phase, wherein the average particle diameter of the crystals of the aluminum titanate is not less than 10 µm, a glass phase containing a rare earth element in an amount of 0.5 to 16% by weight when calculated in the form of an oxide is present in an amount of not more than 20% by volume, and the Young's modulus is in the range 50 to 2,000 kgf/mm². The ceramic material according to claim 1, which has at least one physical property selected from compression strength of 5 to 40 kgf/mm², four point bending strength of 0.2 to 2.0 kgf/mm², and porosity of 5 to 35%. The ceramic material according to claim 1 or claim 2, which has a composition consisting essentially of 40 to 65% by weight of Al₂O₃ and 30 to 65% by weight of TiO₂ as main ingredients. The ceramic material according to any one of claims 1 to 3, which contains not more than 10% by weight of at least one of SiO₂, MgO and Fe₂O₃ in total. The ceramic material according to any one of claims 1 to 4 wherein the amount of the rare earth element is not more than 1.8% by weight. A process for producing a ceramic material for use in insert-casting, comprising the steps of preparing a powder by mixing a material, as an Al₂O₃ source material, containing not more than 96% by weight of Al₂O₃ and having an average particle size of not less than 3 µm, a material having an average particle diameter of not more than 3 µm as a TiO₂ source material, and a given rare earth element in an amount of not more than 1.8% by weight when calculated in the form of an oxide, shaping the powder, and drying and firing the shaped body.

Inventors: FUKAO KANAME, HAMANAKA TOSHIYUKI, HARADA TAKASHI, KONDO NOBORU, FUKAO, KANAME, HAMANAKA, TOSHIYUKI, HARADA, TAKASHI, KONDO, NOBORU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1989
Main classification: C04B35
IPC codes: C04B35