Ceramic materials to be insert-cast and ceramic port liners

Abstract: Ceramic materials to be insert-cast contain not less than 65% of aluminum titanate as a crystalline phase, and have an average particle diameter of crystals thereof being not less than 10 µm, Young's modulus of 50 to 2,000 kgf/mm², compression strength of 5 to 40 kgf/mm², and porosity of 5 to 35%. These liners are free from cracking due to compression force during insert-casting, peeling-off during use, and facilitate insert-casting. In a port liner having a depressed portion at an outer peripheral surface, a reinforcement is filled into the depression which meets either one or both of requirements that differences in coefficient of thermal expansion at 800°C and Young's modulus between the reinforcement and the material of the liner body are in a range of ±0.1% and in a range of ±1,000 kgf/mm², respectively. Alternatively, the outer peripheral surface of the port liner body is worked to give an uneven surface where the liner is joined to a metal in insert-casting, or a stress-releasing slit may be formed in a wall of the port liner body. Further, the outer peripheral surface of the ceramic port liner body may be covered with a ceramic sheet which is irreveribly expandable by heating.

Description: The present invention relates to ceramic materials to be used when hollow tubular ceramic members are to be insert-cast with a molten metal such as aluminum or cast iron, and particularly, the invention relates to ceramic materials to be used for insert-casting when inner surfaces of exhaust ports in gasoline engines, diesel engines or the like are to be lined. Environmental pollution with exhaust gases from automobiles has become a serious social problem, and poisonous substances contained therein have mainly been removed by catalysts. From the standpoint of resources and costs, use amounts of noble metals such as Pt or Rh used as these catalysts have been demanded to be reduced. In addition, recently increasing 4 valve type engines have a problem in that purifying performance of catalysts is deteriorated due to reduction in temperatures of exhaust gases. As one of techniques for solving these problems, it has conventionally been proposed that temperatures of exhaust gases are raised by thermally insulating action of ceramic port liners which are lined to inner surfaces of engine exhaust ports. In general, in order to fit ceramic port liners to engine exhaust ports, the ceramic port liners are insert-cast with a metal such as aluminum simultaneously when engine cylinder heads are to be produced. However, great compression stress acts due to shrinkage through solidification of the molten metal. Thus, there is a problem in that when stress locally concentrate upon even a high strength ceramic material, the ceramic portion is broken. On the other hand, Japanese patent publication Nos. 46 26,330, 51-16,168, 60-5,544 and 56-7,996 propose ceramic materials used for hollow tubular members to be insert-cast with a metal. Japanese patent publication No. 46-26,330 relates to an invention in which a ceramic member made of high strength alumina-based or silicon carbide based ceramic material is made porous at its surface portion to withstand shrinkage of a metal during cooling. However, such a composite material is in a yielded state in which fine cracks are latently present. Consequently, if such a composite material is used at a portion undergoing vigorous vibration in such as an engine, it is feared that the cracks proceed to conspicuously deteriorate durability. Japanese patent publication No. 51-16,168 relates to an invention for producing heat insulating cast products aiming at low elastic modulus. The cast products are made of flexible ceramic semi-products consisting of a refractory filler and alumina cement, and their values of elastic modulus are 950 kg/mm² at the minimum. Thus, they may be broken due to compression stress during insert-casting. Japanese patent publication No. 60-5,544 is aimed at low thermal expansion (thermal shock resistance) and high strength by adding kaolin and magnesium silicate to aluminum titanate. However, the minimum elastic modulus is 8×10³ kgf/mm². Thus, breakage due to compression stress during insert-casting cannot completely be prevented by this method. Japanese patent publication No. 56-7,996 is to afford low thermal expansion and high strength to aluminum titanate by adding SiO₂ and ZrO₂ thereto. Growth of crystalline grains of aluminum titanate is restrained by these additives. However, this method also has the above-mentioned shortcoming possessed by the prior art techniques. In addition to the above-mentioned prior art methods, many attempts have been made to mitigate compression stress due to a molten metal during shrinkage in solidifying by coating the outer periphery of a ceramic member with a layer of asbesto, ceramic fibers made of alumina or the like to be interposed as a buffer member between the metal and the ceramic member by utilizing an elastic force possessed by the layer (for instance, Japanese patent application Laid-open Nos. 52-121,114 and 59-175,693). However, since the outer periphery of the ceramic member is coated with the ceramic fibers, a final product is more costy due to an increased number of steps. Furthermore, this method has a serious problem in that if elasticity of the ceramic fiber layer is lost due to high temperatures and vibrations during engine operation, the port liner is peeled off from the metal member. Further, Japanese patent application Laid-open No. 60-169,655 proposes hollow tubular ceramic members in which at least one high pressure-proof reinforcement is provided around the outer peripheral surface thereof. Since such ceramic members use tialite (aluminum titanate) and zirconia as ceramic bodies and the reinforcement, respectively, differences in thermal expansion and Young's modulus between them are so great that the ceramic members cannot withstand thermal shock and stress and are broken when being insert-cast with aluminum. Moreover, many attempts have also been proposed to prevent such breakage by making port liners themselves of a high strength material. However, since an extremely great stress is concentrated upon a depressed portion such as a central portion of a two barrel type port liner used in a four valve type engine, in which the outer surface is depressed inside, cracking cannot completely be prevented there. The present invention aims at least partly to obviate the above-mentioned problems encountered by the prior art techniques, and to provide ceramic materials to be insert-cast, which have excellent heat resistance and thermal shock resistance, and are free from cracking due to compression stress developed during insert-casting. Although previous researches have been made to increase strength of ceramic members by refining crystalline grains, the present inventors have found through repeated researches to accomplish the above objects that port liners can be prevented from cracking during insert-casting with a metal by affording rubbery elasticity upon the port liners through lowering Young's modulus instead of further increasing strength. The present invention has been accomplished based on this knowledge, and is characterized in that aluminum titanate is contained as a crystalline phase in an amount of not less than 65%, the average particle diameter of crystals thereof is not less than 10 µm, Young's modulus is in a range from 50 to 2,000 kgf/mm², compression strength is in a range from 5 to 40 kgf/mm², and porosity is in a range from 5 to 35%. These and other optional features, and advantages of the present invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attache d drawings, with the understanding that modifications, variations, and changes of the same may be made by the skilled person in the art to which the invention pertains. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a perspective view of a port liner for use in a four valve type engine; Fig. 2 is a sectional view of the port liner insert-cast into an engine head; Figs. 3a and 3b are electron microscope photographs of ceramic materials according to the present invention; First, one process for producing ceramic materials according to the present invention will be explained as follows: Raw ingredients are selected from the group comprising alumina, low soda alumina, calcined bauxite, purified rutile, crude rutile, anatase type titanium, ilmenite, ferrite, rouge, magnesia, magnesite, spinel, kaolin, quartz, and silica to prepare a raw material having a chemical composition essentially consisting of, in terms of "weight", from 40 to 65% of Al₂O₃, from 30 to 60% of TiO₂, and at least one kind of SiO₂, MgO, and Fe₂O₃ in a total amount of not more than 10%. To this raw material is added 0.1 to 1.0% of a deflocculant selected from water glass, ammonium polycarbonate, an amine, sodium pryrophosphate, etc. After 1.0 to 5.0% of a binder selected from PVA, MC, CMC, acrylate, etc. is added to the mixture, the resulting mixture is fully mixed and stirred together with 15 to 40% of water by means of a trommel, a ball mill or the like, thereby preparing a slurry having a viscosity of 200 to 1,000 cp. The thus obtained slurry is shaped in a cylindrical or port liner form by a casting method, which is dried and fired. Consequently, an aluminum titanate sintered body which contains not less than 65% of aluminum titanate as a crystalline phase and at least one kind of rutile, corundum, and mullite as other crystalline phase and has excellent heat resistance, thermal shock resistance, and heat insulation can be obtained. Contrary to the conventional common knowledge, according to the present invention, the crystals of aluminum titanate are fully grown up to the average particle diameter of not less than 10 µm by setting firing conditions to be, for instance, in a temperature range from 1,450 to 1,650°C, preferably in a range from 1,500 to 1,600°C, for around 1 to 16 hours. While coefficients of thermal expansion of the aluminum titanate crystals in a- and b-axes are positive, that of a c-axis is negative. Since difference in coefficient of thermal expansion between them are very great, grain boundaries and crystals themselves do not withstand differences in thermal expansion among the axes, so that a number of microcracks are formed between or inside the crystal grains. It was found that when the crystal grains of aluminum titanate are grown to the average diameter of not less than 10 µm, the frequency and size of occurrence of microcracks correspond to remarkable reduction in Young's modulus. Therefore, as a result of the growth of such crystal grains, a huge number of microcracks are formed between or inside the crystal grains. Thus, such aluminum titanate has the property that spaces inside microcracks approach together or separate from one another due to application of external forces, so that Young's modulus of 50 to 2,000 kgf/mm², compression strength of 5∼40 kgf/mm², and porosity of 5 to 35% can be afforded to the thus obtained ceramic material. Ceramic materials as having such a low Young's modulus can shrink together with an insert-casting metal simultaneously when the metal shrinks. Particularly in the case of such a complicated shape that breaks conventional ceramics of high strength and high Young's modulus due to concentration of stress, the ceramic material according to the present invention is free from cracking. Therefore, the ceramic materials according to the present invention are favorably used as, not to speak of cylindrical type port liners, a port liner 3 of a complicated shape shown in Figs. 1 and 2, which has two ports 2 on a cylinder side of a four valve type engine and a single exhaust opening 1 on an exhaust manifold side. Further, the ceramic material of the invention is also favorably used as liners in exhaust manifold. The coefficient of thermal expansion of the sintered body is lowered by microcracks contained therein, so that satisfactory heat insulation is exhibited even porosity is relatively small. Since aluminum titanate has a melting point of not less than 1,700°C, no particular limitation is posed upon an insert-casting metal. For instance, gray cast iron, nodular graphite cast iron, white cast iron, aluminum alloys, copper alloys, magnesium alloys, and zinc alloys may be used for insert-casting. Figs. 3(a) and 3(b) show electron microscope photographs of ceramic materials according to the present invention. In Fig. 3(b), white crystals are rutile, grey crystals are aluminum titanate, and black portions are pores. Reason for numerical limitations in the present invention are as follows: First, the reason why the rate of aluminum titanate in the crystalline phase is not less than 65% and the average particle diameter is not less than 10 µ is that if a ceramic material does not satisfy these requirements, its Young's modulus cannot fully be lowered and thus the object of the present invention cannot be attained. The reason why Young's modulus is set at from 50 to 2,000 kgf/mm² is that a ceramic material having Young's modulus of less than 50 kgf/mm² is difficult to produce, while that having Young's modulus of more than 2,000 kgf/mm² tends to be cracked like conventional ceramic materials. Particularly, it is preferable that Young's modulus is controlled to be in a range from 50 to 200 kgf/mm². The reason why compression strength is set at from 5 to 40 kgf/mm² is that if it is less than 5 kgf/mm², a ceramic material may be deformed during insert-casting and also has a handing problem, while if it is over 40 kg/mm² to the contrary, Young's modulus exceeds 2,000 kgf/mm². The reason why the porosity is set at from 5 to 30% is that if it is less than 5%, a satisfactory heat insulating effect cannot be obtained, while if it is more than 30%, both strength and Young's modulus fall outside the respective numerical limitations in the present invention. The reason why the ceramic material contains from 40 to 65% of Al₂O₃ and from 30 to 60% of TiO₂ in its composition is that if the composition fall outside these limitations, the rate of the crystals of aluminum titanate does not reach 65%. On the other hand, the reason why at least one of SiO₂, MgO, and Fe₂O₃ is limited to not more than 10% in a total amount is that if it is more than 10%, the rate of the aluminum titanate crystals do not reach 65% and/or the crystal particle diameter is less than 10 µm. The ceramic material according to the present invention has a coefficient of thermal expansion of not more than 2.0×10⁻⁶/°C (40∼800°C) and heat conductivity of 0.8 to 5.0×10⁻³ cal/cm·sec·°C. These physical properties are preferred with respect to port liners to be brought into direct contact with exhaust gases at high temperatures. Next, examples of the present invention will be given below. Raw materials were prepared to have a composition in any one of Run Nos. 1∼24 in Table 1 given later, and cast in a mold to prepare test pieces each having an elliptical section of 3 mm in thickness, 64 mm in a major axis and 36 mm in minor axis. Each of the test pieces thus obtained was fired in firing conditions given in Table 1, and physical properties of the obtained ceramic materials were measured. Next, a casting sand was charged into each of the test pieces, which was then insert-cast with aluminum. Thereby, metal-ceramic composite bodies each having an aluminum thickness of 7 mm were produced. After the casting sand was removed, the test pieces were checked as to whether they cracked or not. Results are shown in the lowermost row. As is clear in the above-mentioned explanation, the present invention provides ceramic materials which are suitable for forming in a hollow tubular shape to be insert-cast with a metal, having excellent heat resistance and thermal shock resistance, and free from occurrence of cracks due to deformation following compression stress developed during insert-casting. Thus, the ceramic materials according to the present invention are favorably employed as port liners or exhaust manifold liners which are brought into direct contact with exhaust gases at high temperatures. The ceramic materials according to the present invention are preferably associated with a buffer material such as ceramic fibers is wound around the ceramic materials to be insert-cast, because elasticity of the buffer material is then not lost. Thus, the present invention overcomes the problems possessed by the prior art techniques, and extremely greatly contributes industrial development. The invention is described here with particular reference to port liners, but is not limited to them and is applicable to other ceramic articles which are subjected to insert casting, e.g. manifold liners.

Claims: A ceramic material to be insert-cast, which contains not less than 65% of aluminum titanate as a crystalline phase, and has an average particle diameter of the crystals of said aluminum titanate of not less than 10 µm, Young's modulus of 50 to 2,000 kgf/mm², compression strength of 5 to 40 kgf/mm², and porosity of 5 to 35%, the material having been obtained at a firing temperature in the range 1550 to 1600°C. A ceramic material to be insert-cast according to claim 1, whose average coefficient of thermal expansion in the temperature range from 40°C to 800°C is not more than 2.0x10⁻⁶/°C. A ceramic material to be insert-cast according to claim 1 or 2, whose thermal conductivity is in the range from 0.8x10⁻³ to 5.0x10⁻³ cal/cm.sec°C. A ceramic material to be insert-cast according to any one of claims 1 to 3, whose Young's modulus is in a range from 50 to 200 kgf/mm². A ceramic material to be insert-cast according to any one of claims 1 to 4 wherein said ceramic material has a chemical composition of from 40 to 65% of A1₂0₃, from 30 to 60% of TiO₂, and at least one of SiO₂, MgO and Fe₂O₃ in a total amount of not more than 10%. A ceramic material to be insert-cast according to any one of claims 1 to 5 which contains at least one of rutile, corundum and mullite as a crystalline phase other than aluminum titanate. A ceramic material to be insert-cast according to any one of claims 1 to 6, which is for use as an engine port liner and/or manifold liner. An engine port liner or manifold liner made of a ceramic material suitable to be insert-cast, which material contains not less than 65% of aluminum titanate as a crystalline phase, and has an average particle diameter of the crystals of said aluminum titanate of not less than 10 µm, Young's modulus of 50 to 2,000 kgf/mm², compression strength of 5 to 40 kgf/mm², and porosity of 5 to 35%. An engine port liner or manifold liner according to claim 8 wherein said ceramic material has an average coefficient of thermal expansion in the temperature range from 40°C to 800°C of not more than 2.0x10⁻⁶/°C. An engine port liner or manifold liner according to claim 8 or claim 9 wherein said ceramic material has a thermal conductivity in the range from 0.8x10⁻³ to 5.0x10⁻³ cal/cm.sec°C. An engine port liner or manifold liner according to any one of claims 8 to 10 wherein said ceramic material has a Young's modulus in the range 50 to 200 kgf/mm². An engine port liner or manifold liner according to any one of claims 8 to 11 wherein said ceramic material has a chemical composition of from 40 to 65% of A1₂0₃, from 30 to 60% of TiO₂, and at least one of SiO₂, MgO and Fe₂O₃ in a total amount of not more than 10%. An engine port liner or manifold liner according to any one of claims 8 to 12 wherein said ceramic material contains at least one of rutile, corundum and mullite as a crystalline phase other than aluminum titanate.

Inventors: HAMANAKA TOSHIYUKI, HARADA TAKASHI NGK KITA-KAZOKU, HATTORI FUMIO, HAMANAKA, TOSHIYUKI, HARADA, TAKASHI 32, NGK KITA-KAZOKU APARTMENTS, HATTORI, FUMIO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1988
Main classification: C04B35
IPC codes: F02B77, C04B35, F01N7, F02B1, F02F1, F02F7, F02B3