A METHOD OF PRODUCING A FUNCTIONALLY GRADIENT MATERIAL

Abstract: The present invention relates to a method of producing a functionally gradient material whose components and struc­ ture are continuously adjusted by the coating, plating and powder metallurgy techniques to change its function in turn and provides a method of producing a functionally gradient material superior in heat-resistance, corrosion-reistance and resistance to thermal fatigue by distributing a third compo­ nent having a lower Young's modulus or formed of high-­ strength material sufficiently durable to the fracture strength among ceramics as a first component and metals or other ceramics as a second component to change the function.

Description: The present invention relates to a method of producing a mono-layer or multi-layer composite material formed of ceramic and metallic layers or a plurality of ceramic layers, and in particular to a method of producing a functionally gradient material whose components and structure are continuously adjusted by coating, plating and/or powder metallurgy techniques so as in turn to adjust its function. One of key-technologies for the development of aeronautics, space engineering and nuclear fusion reactors is the development of a super heat-resistant material superior in heat-blocking property and the development of a heat-blocking material aiming at a light weight aircraft. It is known to coat ceramics or the like on metal or alloy surface is to produce such a heat-blocking material. For example, MCrAlY (wherein M is a metal) is coated as a relaxation layer on a surface of a Ni-base super alloy by a plasma-coating method and then ZrO₂Y₂O₃ is coated on said relaxation layer formed of MCrAlY also by a plasma-coating method. Also known are the ion-plating method, in which a protective layer is impact-coated by vaporization on a substrate under a vacuum of 10⁻³ to l0⁻³ Torr 1.3 to 0.13 Pa, the plasma-CVD method, in which a heat-blocking coat is formed by gas-phase synthesis, and the ion beam method all of which have been frequently used. It is important for the production of a mono-layer, two-layer or graduated multi-layer type coating layer with superior heat-blocking properties, whose insulating properties and strength are continuously changed, by such insulative coating methods to obtain a composite layer of superior thermal stress-resistance. However, the composite material obtained by the conventional coating method, plating method and the like is not supported by any calculation of a relaxation functional structure of thermal stress and the composition is changed merely stepwise rather than continuously, so that it has been difficult to control the density-form distribution of components continuously to obtain a minimum thermal stress distribution in sufficient correspondence to a temperature potential of each part of a material, whereby obtaining a material of superior adhesion. Accordingly, problems such as exfoliation and deterioration of corrosion-resistance due to the initiation of cracks have occurred with thermal fatigue and/or with aging. Kaczmarek et al in an article entitled 'Microprocessor Control of the Spraying of Graded Coatings' (Mat. Res. Soc. Symp. Proc. Vol. 30 [1984] pp207-215) disclose the use of a microprocessor-controlled powder feeder in a spray-coating process used to form a coating of MgOZrO₂ on a stainless steel substrate with graded intermediate layers comprising NiCrAl and MgOZrO₂. Pores are liable to arise in the intermediate layer, depending on the coating technique used. The present invention consists in a method of producing a functionally gradient composite material comprising a ceramic material as a first component, a metal or another ceramic material as a second component and an intermediate layer between said first and second components, the intermediate layer having a continuously changing ratio of the first component to the second component across its thickness and including micropores and/or a third component comprising a high strength material and/or a material of lower Young's modulus than said first and second components, characterized in that said third component and/or said micropores are selected and distributed in correspondence with the combination of said first and second components such that the specific stress distribution R(X) is equalized and reduced in average value compared with the specific stress distribution of the combination of the first and second components alone. The method of the invention can be used to produce a functionally gradient material superior in heat-resistance, corrosion-resistance and thermal-stress fracture resistance by continuously controlling the structure and components and distributing micropores or a material having a relatively low Young's modulus and a high strength as a third component in the intermediate layer. The present inventors have determined the boundary conditions for the concentration-distribution function, distribution-form parameters and heat conduction equation of the first and second components and determined the specific stress distribution (R) from the heat conductivity (λ), the Young's modulus (E) and the like, and have found that the addition and distribution of micropores or of a component having a lower Young's modulus and a high strength as a third component is remarkably effective for the production of ceramic/metal- or ceramic/ceramic type functionally gradient material of superior adhesion and thermal stress fracture-resistance, since the stress-distribution level is reduced and equalized. (R=thermal stress/mixed average compression fracture stress). The present invention will be followingly described in more detail with reference to the accompanying drawings, in which:- Fig. 1 is a diagram showing an infinite flat plate having a dimensionless thickness of the following magnitude X: 0 ≦ X ≦ 1 Fig. 2 is a schematic diagram showing a concentration distribution of a component A and a component B; Fig. 3 is a component distribution diagram in the case where a distribution form parameter n is equal to 4, the component A being SiC, and the component B being C; FIG. 4 is a graph showing values of physical properties in the continuous layer; FIG. 5 is a graph showing values of a temperature T, a stress σ and a specific stress σ/σ y in the continuous layer; FIG. 6 is a diagram showing an apparatus for coating TiC, SiC and C by the CVD method; FIG. 7 is a schematic diagram showing a functionally gradient material comprising SiC, C and TiC in which a density form is continuously controlled; FIG. 8 is a component distribution diagram of a functionally gradient material comprising ZrO₂· 9Y₂O₃ and NiCrAlY; FIG. 9 is an outline showing one example of a powder-conveying device for arranging particles; and FIG. 10 is a component distribution diagram of a functionally gradient material comprising PSZ and a functionally gradient material comprising PSZ and nickel. At first, as to the infinite flat plate, as shown in FIG. 1, having a dimensionless thickness X of the following magnitude: 0 ≦ X ≦ 1 the heat conduct ion equation, the concentration distribution function g A (X) of the component A, the concentration distribution function g B (X) and the physical property value function f(X) are defined by the following equation (1) to (4): (d/dx)λ(X)(dT/dx) = 0 (1) wherein the boundary conditions: T(0) = 1,500°K, T(1) = 300°K g A (X) = X n (2) g B (X) = 1 - X n (3) In this case, the physical property value distribution function is expressed in the form of the following equation (4): f(X) = P A X n + P B (1 - x n ) (4) (wherein P A , P B is each physical property-heat conductivity λ, Young's modulus E and linear expansion coefficient α - of the component A and the component B, respectively.) n is a distribution form parameter and the cases of (I) 0 < n < 1, (II) n = 1 and (III) 1 < n are shown in FIG. 2. Next, solving the equation (1) and the equation (2) under the following four assumptions, the temperature distribution T(X), the thermal stress distribution σ (X) and the specific stress distribution R(X) are expressed by the following equation (5), (6) and (7), respectively: Said four assumptions include (i) a stationary condition; (ii) an elastic deformation; (iii) the consideration of a dependency of physical properties of materials on temperatures; and (iv) the dependency of the physical properties of a functionally gradient material upon a mixture average rule. wherein σ(X) = -E(X)α(X) { T(X) - 300} (6) wherein E(X) = E A X n + E B (1 - X n ) σ (X) =α A X n + α B (1 - X n ) R(X) = σ(X)/σ y (X) (7) wherein σ y is a mixture average compression fracture stress. It has been made obvious from the calculation results by these fundamental equations that it is important, for the production of a material superior in bonding strength, to suppress the value of R(X) max in the functionally gradient material to as low as value as possible and to make the distribution of that value uniform. The present invention is characterized in that in order to flatten the specific stress distribution R(X) and reduce the stress level, micropores or a third component comprising1 ○ a high-strength component and/or2 ○ a component having a lower Young's modulus are incorporated in the intermediate layer in which the structure and components are continuously changed. Said high-strength component1 ○ and said component having a lower Young's modulus2 ○ may be used individually or in combination. Since the distribution R(X) and the highest value thereof R(X) max are changed in dependence upon the combination of the first component and the second component, it is necessary to select and distribute the third component in accordance with said combination. It has been found from the investigation of the relation between n and R(X) max in the case of a typical ceramics/metal that n ≧ 1 is optimum for the achievement of the minimum thermal stress level in two components composite and in addition, a range of 1 ≦ n ≦ 10 is desirable where X = 1. Since the addition and distribution of the third component in correspondence to the value of the distribution R(X) as above described leads to the reduction of the thermal stress level and the flattening of the distribution of thermal stress level, the addition and distribution of the third component positively affects the falling-out separation and the breakage of the coated layer. A specific embodiment of the present invention, in which the first component is SiC and the second component is C, will be described in detail. Fig. 3 is a component-distribution diagram in the case where the first component is SiC, the second is C and the distribution parameter n is 4. In addition, the physical properties of the intermediate layer are shown in Fig. 4. The temperature-distribution, stress-distribution and specific stress distribution R(X) are obtained by the use of these values with the results as shown in Fig. 5. In order to make this R(X) uniform and reduce the average value of R(X), TiC is distributed in correspondence to the distribution of R(X), for example in a sine-curved shape, and simultaneously micropores formed due to the thermal shrinkage by the cooling are distributed. Thus, the specific stress distribution (σ/σ y )can be flattened and minimized to obtain a superior bonded boundary surface and increased strength. A foamed metal, graphite powder, ceramic fibers and the like are effective as the third component having a low Young's modulus. In addition, in the production of such a functionally gradient material a micro-scale controlling method, such as the CVD method or the PVD method, is suitable for forming a thin film having a thickness of several tens µm while a large-scale controlling method, such as the powder arranging method, the plasma-torch method and the spray-forming method, is suitable for producing a functionally gradient material having a large film thickness of several mm. Examples The present invention will be below described in detail with reference to Examples. Example 1 As shown in Fig. 6, a pair of water-cooled copper electrodes 2 are inserted into a vessel 1 made of stainless steel which can be evacuated, a graphite substrate (4 cm x 1.2 cm x 0.2 cm) 3 being put between said pair of water-cooled copper electrodes 2, and said graphite substrate 3 being electrified through said electrodes to heat the graphite substrate 3 up to 1,500°C. Measurement of the temperature is carried out by means of a two-color pyrometer and the temperature of the substrate is controlled within ± 10°C. In addition, the vacuum within the furnace is held at 10⁻³ Torr (0.13Pa) by means of a hydraulic rotary pump. Reference numeral 4 designates a gas inlet nozzle. (A) At first H₂ gas is introduced into the furnace at a flow rate of 700 cm³/min and then C₃H₈ gas at a flow rate of 30 cm³/min. The pressure of the gas within the furnace is held at 10 Torr (1.3x10³Pa) by means of a vacuum valve. This operation leads to the generation of a reaction C₃H₈ →C ↓+H₂, leading to the segregation of C on the graphite substrate. Synthesized segregated C is plate-shaped, having a density of 1.3 g/cm³, and showing a columnar structure and an amorphous structure. The segregation rate of plate-shaped C is 0.6 mm/hr. Thus, plate-shaped segregated C of 0.6 mm thick is formed on the graphite substrate for 1 hour. (B) SiCl₄ vapour is added in these raw material gases by the use of H₂ gas as a carrier from another vessel (the vapour pressure is adjusted by changing the temperature) with holding the temperature, the flow rate of C₃H₈, the flow rate of H₂ and the pressure of the gas within the furnace constant. The flow rate of SiCl₄ is increased from 0 to 170 cm³/min linearly within two hours. The deposited substance obtained in this process is a functionally gradient material in which SiC fine particles are dispersed in a matrix formed of C as a result of the following reaction: C₃H₈ + SiCl₄ + H₂ → C ↓ + SiC ↓ + HCl + H₂ The deposited substance obtained in the case, where the flow rate of SiCl₄ is 30 cm³/min, has a Si-content of 0.5 wt %, a density of the matrix formed of C being 2.0 g/cm³, SiC flakes being dispersed in the form of β -type, and the size being (50nm) 500 Å in diameter and (10nm) 100 Å in thickness. With an increase of the flow rate of SiCl₄ the density of the matrix formed of C, the Si-content and the size of SiC dispersed are all increased. On the other hand, the segregation speed is reduced with an increase of the flow rate of SiCl₄. These parameters are continuously changed for two hours, as above described. The segregated substance of 0.8 mm thick is obtained in this period of time. The dispersion condition of C and SiC in the segregated substance is changed from the condition, in which SiC is dispersed in C, to the condition, in which C is dispersed in SiC, with a lapse of time. In said synthetic step of SiC + C simultaneously TiCl₄ vapour is introduced into the furnace from another system so as to achieve the change in flow rate from 0 to 10 cm³ /min within 15 minutes. The quantity of Ti contained in the composite material is continuously increased up to the maximum value of 0.2 wt % after 25 minutes. Subsequently, upon changing the flow rate from 10 to 0 cm³/min within 15 minutes, the content of Ti becomes 0 after 30 minutes since the state (B) started. Thus, TiC as the third component is distributed in a sine function manner, whereby reducing the stress level. The dispersed TiC is fine particles (0.1 µm or less) and the thermal expansion coefficient of TiC is larger than that of the matrix formed of C, so that micropores are formed at several atom intervals in the circumference of the fine particles of TiC at a point of time when they were cooled from the synthesis temperature. In a ternary (SiC-C-TiC) functionally gradient material the Young's effect due to these micropores and the increased strength effect due to the addition of TiC are overlapped, whereby achieving the flattening of the specific stress distribution. (C) Subsequently, the segregation is carried out for one hour from three hours after since the synthesis was started under the conditions that the flow rate of SiCl₄ is maintained at 170 cm³/ min and the temperature, the flow rate of C₃H₈, the flow rate of H₂ and the pressure of the gas within the furnace are same as those initially set. It is β -type Sic that is segregated in this period of time. This β -type SiC has a density of 3.2 g/cm³ and a columnar structure. The segregation speed of SiC is 0.2 mm/hr. A functionally gradient material having a continuously changed density and a controlled form as shwon in FIG. 7 is obtained by the above described experimental synthesis. C can not be bonded with SiC by the conventional methods but according to the method of the present invention, C can be bonded with SiC through the continuously controlled stage (B) as shown in the drawing. A lower-density C material, which is stable at higher temperatures but soft and easily oxidized, is formed on an X-side while a higher-density SiC material, which has a higher hardness and the superior oxidation-resistance, is formed on a Y-side. The central portion has both porperties. Although SiC has been tried to be coated on the C material, the SiC film is broken due to the generation of a strain resulting from a difference between C and SiC in linear expansion coefficient, so that it has been necessary that the SiC film is remarkably thin (several µm) but according to the method of the present invention, the strain can be let escape by the existence of the micropores, whereby giving SiC in the portion (C) a thickness necessary and sufficient for using as a structural material. A functionally gradient material, in which the structure, density and function are continuously changed, is disclosed in this Example. Such a functionally gradient material can not be obtained by a well-knwon solid reaction of C with SiC. Example 2 A nickel-base superalloy IN 100 (8 cm x 10 cm x 0.2 cm) polished with waterproof emery No. 600 and scrubbed with trichloroethylene as a substrate is set in a water-cooled chamber. Then, a composite coating layer with a continuously controlled structure is formed by the plasma spraying method with holding and adjusting the vacuousity within the chamber, whose inside is replaced with Ar gas, at about 80 to 200 Torr (10.6x10³ to 2.7x10⁴Pa). This layer has a thickness of about 240 µm. In order to improve the splicing strength of the substrate with the coating layer, a travelling arc is generated between the substrate and a plasma gun electrode to heat the substrate up to about 700 to 800°C. Ar and H₂ gas are used as a plasma gas. ZrO₂· 9Y₂O₃ powders and Ni16Cr6A10.6Y powders are used as spray powders. These powders are supplied to a plasma jet with continuously changing a ratio thereof and coated to obtain a continuous structural layer having a component distribution as shown in FIG. 8 and Table 1. At this time, a void ratio is increased by reducing the vacuousity in an atmosphere and increasing a particle diameter of powders supplied for use in the plasma and reduced by increasing the vacuousity and decreasing the particle diameter of powders supplied. A lower-Young's modulus layer is formed in the continuous structural layer by adjusting the vacuousity and the particle diameter in such a manner to distribute micropores in a sine-curved shape in a range of t from 0.6 to 1.0. This layer shows a function of relaxing a thermal stress. The resulting Ni-base superalloy plate with the composite layer is subjected to an annealing treatment for 5 hours at 1,100°C under vacuum to improve the bonding strength due to the diffusion in the boundary surface (i), make an alloy density of about 100% in a range of t from 0 to 0.6 (ii),and make the dispersed micropores spherical in shape in a range of t from 0.6 to 1.0 (iii). In the production of a ZrO₂·9Y₂O₃-Ni16Cr6A10.6Y-coated Ni-base alloy according to this Example a ratio of ZrO₂·9Y₂O₃ to NiCrAlY is continuously changed by the plasma spray under vacuum and a micropore-distributed layer having a lower Young's modulus as the third layer is formed in the ZrO₂·9Y₂O₃-Ni16Cr6A10.6Y-coated Ni-base alloy, so that a superior splicability can be given and a thermal stress-relaxing layer is formed, whereby the separation of the ceramic layer and the generation of cracks can be reduced remarkably in comparison with the conventional one. Accordingly, the resulting functionally gradient material can be used stably for a long time as a highly heat-resistant and heat-blocking composite material. Example 3 Ni powders having an average particle diameter of 3 µm and 30 µm and PSZ powders having an average particle diameter of 0.05 µm and 0.1 µm are used as raw material powders. But, the PSZ powders (partially stabilized ZrO₂ powders) are used in the form of agglomerated powders having the adjusted secondary particle diameter of 50 to 100 µm. Ni powders having a particle diameter of 3 µm and PSZ powders having a particle diameter of 0.05 µm are mxied for 10 minutes in accordance with the component-distribution rule of n = 5. Referring now to FIG. 9, reference numeral 5 designates a blower for use in the conveyance, 6 designating a feeder of Ni powders having a particle diameter of 3 µm, 7 designating a feeder of Ni powders having a particle diameter of 30 µm, 8 designating a feeder of PSZ powders having a particle diameter of 0.05 µm, 9 designating a feeder of PSZ powders having a particle diameter of 0.1 µm, 10 designating a powder-mixing and stirring mixer, and 11 designating a powder-collecting portion. A flow rate of the carrier gas is 10 to 20 m/s and a powder-settling and collecting speed is about 1 to 1.6 cm/s at that time. After a lapse of 10 minutes a quantity of Ni powders having a particle diameter of 30 µm or PSZ powders having a particle diameter of 0.1 µm fed is gradually increased so that a ratio of rough powders (PSZ powders having a particle diameter of 0.1 µm) may reach about 20 % at a point of time of 15 to 20 minutes (corresponding to X = 0.8) and then the quantity of the rough powders fed is gradually decreased so that the ratio of the rough powders may reach 0 % at X = 1. At this time, a relative density of the structure-controlled sediment is 30 to 40 %. Nextly, the resulting structure-controlled sediment is preliminarily sintered in an atmosphere of hydrogen for 10 minutes at T = 900°C and successively, subjected to the CIP molding process at P = 450MPa to obtain a molded product having a relative density of 60 to 70%. The relative structural distribution in the molded product shows no change. This molded product is sintered for 3 hours at 1,250°C in an atmosphere having a vacuousity of 10⁻³ to 10⁻⁵ Torr (0.13 to 1.3x10⁻³Pa). The structural distribution is controlled by utilizing difference in sintering speed due to the magnitude of the diameter of the raw material powders so that the micropore-distribution may reach the maximum value at the relative density of 80 to 100 % and X = 0.8. The component-distribution is shown in Table 2 and FIG. 10. A functionally gradient material superior in adhesion and sufficiently endurable to the thermal stress fracture is obtained by distributing stress dispersion type micropores in the continuously-changed structural layer by the particle-arranging method under the above described conditions. Effects of the Invention As above described, according to the present invention, the density- and form distribution of the third component is controlled continuously so that the thermal stress distribution corresponding to the temperature potential may be equalized and the average stress value may be reduced, so that the separation of the heat-blocking ceramic layer and the like and the generation of cracks can be prevented, whereby obtaining a composite material superior in toughness, adhesion and insulating characteristic. In addition, the function of the material can be controlled for each portion by continuously changing the structure, construction and components, whereby giving a specialized function (gradient function) to the material. Thus, also a functionally gradient material superior in ultrahigh temperature-resistance/ultralow temperature-resistance, ultrahigh temperature-resistance/radiation damage-resistance, insulation-resistance/thermal deterioration-resistance, abrasion-resistance/toughness, optical character/magnetic permeability or the like can be obtained.

Claims: A method of producing a functionally gradient composite material comprising a ceramic material as a first component, a metal or another ceramic material as a second component and an intermediate layer between said first and second components, the intermediate layer having a continuously changing ratio of the first component to the second component across its thickness and including micropores and/or a third component comprising a high strength material and/or a material of lower Young's modulus than said first and second components, characterized in that said third component and/or said micropores are selected and distributed in correspondence with the combination of said first and second components such that the specific stress distribution R(X) is equalized and reduced in average value compared with the specific stress distribution of the combination of the first and second components alone. A method as claimed in claim 1, characterized in that said first component is C, said second component is SiC, and said intermediate layer includes micropores and/or a third component comprising TiC. A method as claimed in claim 1 or claim 2, characterized in that said first component is ZrO₂.Y₂O₃, said second component is MCrAlY, wherein M is a metal and the intermediate layer includes micropores. A method as claimed in claim 1, characterized in that ceramic fibers are distributed in the intermediate layer as said third component.

Inventors: HIRANO, TOHRU, KUROISHI, NOBUHITO, NIINO, MASAYUKI, NIRAI, TOSHIO, SUZUKI, AKIO, WATANABE, RYUZO
Applicants: DAIKIN INDUSTRIES, LIMITED, HIRAI, TOSHIO, NATIONAL AEROSPACE LABORATORIES OF SCIENCE & TECHNOLOGY AGENCY, SUMITOMO ELECTRIC INDUSTRIES LIMITED, WATANABE, RYUZO
Application year: 1986
Main classification: C23C4
IPC codes: C23C14, B22F7, C23C16, C23C4, B32B7, B32B18, B32B15, B22F3