Method of producing a multilayer material having a gradually changing composition

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing monolayered or multilayered composite materials consisting of ceramic and metal, ceramic and ceramic, or the like, and more particularly, to a method of producing functionally gradient materials with properties varying continuously in the direction of the thickness by adjusting the distribution of components and the structure thereof through the synthesis effected by the self-propagating reaction of the mixed fine particles including constitutive elements of these ceramics and metals, etc.

As one of the recent important technological themes in the fields of aeronautics, space engineering, and nuclear fusion reactor, etc., are given the development of a super heat-resistant material with superior heat-blocking properties, and that of a heat-blocking material aiming at a light-weight aircraft. As the producing method of such a heat-blocking material, there has been conventionally known the method of coating the surface of a metal and an alloy with ceramics or the like; for example, the surface of a Ni-­base super alloy is coated with MCrAlY (wherein, M is a metal) as a relaxation layer and ZrO₂.Y₂O₃ in order according to a plasma-coating method. Furthermore, an ion-­plating method wherein a heat-blocking material is fixed with inpact on a substrate by vaporization under vacuum of 10⁻² to 10⁻³ Torr; a plasma-CVD (Chemical Vapor Deposition) method wherein a heat-blocking material is fixed by vapor-­phase synthesis; and an ion-beam method have been adopted frequently.

However, any of said conventional methods such as the plasma coating method, ion-plating method, plasma-CVD method and ion-beam method has a defect of low efficiency, since it takes much time to form a thick coating because a coating layer obtained per an unit time is very thin. Such defect is revealed more remarkably as the surface area of the substrate to be coated becomes larger. Furthermore, there is another defect that the ion-plating method and the plasma-CVD method require a large-scale chamber and an additional equipment, and that the plasma-coating method requires large energy to heat and fuse a coating material.

Though the monolayered or the multilayered coating obtained by the above methods should naturally be superior in adhesion and against a thermal stress, each of composite materials provided by said conventional coating or plating method has not been proved to have a stress relaxing structure by a theoretical calculation. The structure of such a material has been obtained by merely varying the composition thereof stepwise and does not have superior adhesion, since the compositional distribution and temperature gradient in such a material are not controlled, in the producing process, to be continuous so that its structure may obtain the minimum thermal stress distribution corresponding to a temperature potential of each portion of the material under the working conditions.

Accordingly, said composite material has problems such as exfoliation of the coating layer due to the thermal stress caused repeatedly in the operation and the variation by the passage of time, and the deterioration of corrosion resistance of the composite material due to the generation of cracks.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of producing materials superior in heat resistance, corrosion resistance, adhesion, and thermal-stress fracture resistance readily at low cost and in a short time.

In order to form an intermediate layer between ceramic as a first component and a metal or other ceramic as a second component, in which the ratio of both components varies continuously, the present inventors adopted what is called a self-propagating reaction, wherein the synthetic reaction of the components is carried out only by self-­heating of the mixed fine particles of metallic and nonmetallic constitutive elements of the ceramics after the ignition, and repeated experiments and studies. In the experimental process, we have found that such a material can be possessed of a residual stress which cancels a thermal stress occurring under the working condition of high temperature, by the following process: 1) determining a concentration-distribution function, a distribution parameter, and a boundary condition of heat conduction equation of each of said both components; 2) calculating specific stress R (thermal stress σ divided by mixture average compression fracture stress σy) of each portion of such a material under working condition using heat conductivity (λ), the Young's modulus (E), etc.; and 3) adjusting the mixture ratio of said both components so as to flatten and minimize this specific stress distribution.

The method of a first embodiment according to the present invention is characterized in producing a material having a layer of ceramic as a first component, a layer of a metal as a second component and an intermediate layer lying between said layers and including said first and second components in continuously gradient ratios so that the properties of the material may change continuously; including a step to form said intermediate layer by igniting the mixture of powders of metallic and nonmetallic constitutive elements of said ceramic and powder of said metal and causing synthetic reaction of the powder mixture.

The method of a second embodiment according to the present invention is characterized in producing a material having a layer of ceramic as a first component, a layer of other ceramic as a second component and an intermediate layer lying between said layers and including said first and second components in continuously gradient ratios so that the properties of the material may change continuously; including a step to form said intermediate layer by igniting the mixture of powders of metallic and nonmetallic constitutive elements of the ceramic as the first component and powder of the other ceramic as the second component and causing synthetic reaction of the powder mixture.

The method of a third embodiment according to the present invention is characterized in producing a material having a layer of ceramic as a first component, a layer of other ceramic as a second component and an intermediate layer lying between said layers and including said first and second components in continuously gradient ratios so that the properties of the material may change continuously; including a step to form said intermediate layer by igniting the mixture of powders of metallic and nonmetallic constitutive elements of the ceramic as the first component and those of metallic and nonmetallic constitutive elements of the other ceramic as the second component and causing synthetic reaction of the powder mixture.

The composition of the present invention will be described in more detail according to the appended drawings.

Fig. 1 illustrates a infinite flat plate of the dimensionless thickness of 0 ≦ × ≦ 1. In said infinite flat plate, the heat conductivity equation, the concentration distribution functions g_A(x), g_B(x) of two components A and B, and the physical property value function f(x) are defined by equations (1) to (4), respectively. The boundary condition in using the material is:

    T(0) = 1500 K

    T(1) = 300 K

If the concentration distribution function is assumed to be as follows:

    g_A(x) = xⁿ      (2)

    g_B(x) = 1 - xⁿ      (3)

the physical property value function is represented as follows:

    f(x) = P_Axⁿ + P_B(1 - xⁿ)      (4)

    (where P_A and P_B represent the following physical property values of components A and B, respectively: heat conductivity λ, the Young's modulus E, thermal expansion coefficient α).

In the above equations, '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, which illustrates the equations (2) and (3), assuming the position of x = L to be an origin, and a range of (1 - L) to be a full scale of the abscissa axis.

Next, the equations (1) and (2) are worked out based on the following four assumptions:

• (i) a stationary condition is held
• (ii) there exists an elastic deformation
• (iii) the dependency of the physical properties of the material on the temperature is taken into account
• (iv) the physical property value of the material is determined based on the mixture average rule of each component.

Then, a temperature distribution T(x), a thermal stress distribution σ(x), and a specific stress distribution R(x) are represented by equations (5), (6), and (7).

    T(x) = K ·∫ dτ/{(λ_A - λ_B) τⁿ + λ_B} + 1500      (5)

wherein,

    K ≡ 1200/∫ dτ/{(λ_A - λ_B)τⁿ + λ_B}

    σ (x) = -E(x)α(x){T(x) - 300}      (6)

here, the equation (4) is adopted:

    E(x) = E_Axⁿ + E_B(1 - xⁿ)

    α(x) = α_Axⁿ + α_B(1 - xⁿ)

where E_A and E_B are the functions of σ(x), when taking an elastic deformation into account.

    R(x) = σs(x)/σ_y(x)      (7)

wherein, σ_y is a mixture average compression fracture stress.

From the calculation results of these fundamental equations, it has become obvious that making the value of R(x)_max as small as possible and uniforming the distribution thereof is essential in obtaining a material with excellent bonding strength. The present invention is characterized by adjusting the mixture ratio of two components A and B or by adding a third component in such an optimum way as to reduce the stress level and to flatten said specific stress distribution R(x) of the intermediate layer in which the structure and the content of each component thereof are varied continuously. If the specific stress distribution R is calculated numerically by each of said equations after determining two components A and B and varying the distribution-form parameter n and the origin position L variously, n and L which minimize the specific stress distribution R can be obtained individually.

A result of obtaining the relationship between n and R(x)_max concerning the typical combination of ceramics/metal has revealed that, in order to minimize the thermal stress with the group of two components, it is preferable that 'n' is 0.5 and more (n≧0.5), and it is more adequate that 'n' is positioned within a range of 0.5≦n≦5 when taking into account the bonding strength in the case of x=1.

Next, the case that TiB₂ is determined as the first component (component B) of a material, and that Cu is determined as the second component (component B) according to the present invention will be described concretely. 'n' and 'L' which minimize the specific stress distribution R are obtained as n=0.8, and L=0.1 by said numerical calculation. Fig. 3 shows the component distribution obtained by equations (2) and (3) concerning TiB₂ and Cu included in the material at that time. In Fig. 3, the range of x=0.0 to 0.1 corresponds to the top portion of the coating layer of TiB₂ content 100 %; the range of x=0.1 to 1.0 corresponds to an intermediate layer in which TiB₂ reduces gradually and Cu increases gradually; and the position of x=1.0 corresponds to the surface of a substrate (copper) of Cu content 100 % respectively. Then, the heat conductivity λ, the Young's modulus E, and the thermal expansion coefficient α , obtained by the equation (4), of the top portion of the coating layer and of the intermediate layer are shown in Fig. 4. The temperature distribution T, thermal stress distribution σ, and specific stress distribution R which are obtained by substituting the values shown in Fig. 4 into the equations (5), (6) and (7) in accordance with a realistic thermal analysis on elasticity and plasticity are as shown in Fig. 5. As for T of Fig. 5 as same as that of Fig. 1, the temperature of the top portion of the coating layer of x= 0.0 to 0.1 which is exposed to high temperature circumstances reaches 1500 K, and it reduces gradually in the intermediate layer to 300 K at the surface of the substrate of x=1.0. Accordingly, the material wherein both of TiB₂ and Cu components are adjusted in such a mixture ratio can be possessed of a superior bonding interface and strength even under the working circumstances of high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and characteristics of the present invention will become apparent from the following description in conjunction with the preferred embodiments thereof referring to the appended drawings, in which:

• Fig. 1 is a diagram of an infinite flat plate of a dimensionless thickness of 0≦x≦l;
• Fig. 2 is a schematic diagram showing concentration distributions of components A and B;
• Fig. 3 is a diagram showing a component distribution on the conditions of the distribution form parameter of n=0.8, and L=0.1, A=Cu, B=TiB₂;
• Fig. 4 is a diagram showing physical property values of the continuous layers;
• Fig. 5 is a diagram showing values of temperature T, stress σ , and specific stress σ / σ_y of the continuous layers;
• Fig. 6 illustrates an example of the producing method according to the present invention;
• Fig. 7 is a cross section view of a produced material;
• Fig. 8 shows a mixture ratio of the thickness direction of raw material powders;
• Fig. 9 shows a component distribution of the produced material;
• Fig. 10 shows a temperature distribution of the material immediately after synthesizing and forming thereof;
• Fig. 11 shows a residual stress distribution of the material after being produced;
• Fig. 12 shows a distribution of stress which occurs in the material (during working);
• Figs. 13 (a) and (b) to 16 (a) and (b) show schematic views of materials, showing the respective compositional distributions before the reaction in (a)s, those after the reaction in (b)s; and
• Fig. 17 (a) and (b) show the characteristics of the producing method according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the description of the present invention proceeds, it is to be noted that the same parts are designated by the same reference numerals throughout the appended drawings.

As shown in Fig. 6, copper alloy 1 to be coated is enclosed with a heat-resisting frame 2, and said frame 2 is filled with the powders of Ti and B which are the constitutive elements of TiB₂ as the first component and those of Cu as the second component, varying the mixture ratio in the thickness direction D continuously as shown in Fig. 8. Said mixture ratio is so determined that the component distribution (n=0.8, L=0.1) as shown in Fig. 3 can be obtained, and actually the mixed fine particles of different mixture compositions are laminally filled in sequence to lead to layers of a regular thickness as shown in Fig. 6, since it is difficult to regulate the mixture ratio so as to vary precisely in a continuous way. A pressure of more than 200 kg/cm² is applied to every layer to vertically so as to compress the fine particles thereof. After completing the laminated filling, the whole is put in a container and treated by vacuum degassing. Then, during further compression in the direction shown by arrows with a pressure of more than 200 kg/cm², an ignition coil 4 put over the upper surface of the compressed powder 3 is ignited, thereby, said compressed powder 3 is also ignited. Then, the fine particles Ti and B start a synthetic reaction into TiB₂, said synthetic reaction proceeding rapidly in a sheet state toward the surface of the copper alloy 1 only by the enormous self-generating reaction heat. In this synthetic process of TiB₂, the Cu powders are also fused by said reaction heat, and in the intermediate layer, a matrix of the double phase structure containing TiB₂ and Cu is formed, thereby the coating layer of high density can be obtained due to the effect of said applied pressure. In such obtained coating layer 5, as its composition, the content of TiB₂ is 100 % on the surface layer portion, and that of Cu is gradually increased inside and reaches 100 % on the surface of copper alloy 1. In such a way, the synthesis and formation of a material wherein the first and the second components are ceramic TiB₂ and metal Cu respectively is settled in an instant; thus, the production is completed after letting the pressure out and cooling.

As the effective igniting method to initiate self-­propagating reaction, is selected the method wherein metallic wires are stretched around the surface or the inside of the powder mixture, and are fed with electricity for an instant, thereby the powder mixture is ignited. The metallic wires are preferably of the mixture composing metals such as Ti, Zr, and the like, in order to protect the coating layer from contamination. It is possible to form the surface of the coating layer flatly or in an optional configuration by initiating said reaction from the adequate position inside said mixture. Moreover, a synthetic layer of high density can be formed by correcting in order the volume shrinkage due to the reaction under the pressure of a compression spring, hydraulic power, gasses, etc. Said synthesis is carried out by applying pressure to the mixture perpendicularly to the coating surface, and at the same time, igniting said mixture on the plane intersecting perpendicularly to the pressure application direction, and advancing the synthesis in said direction. Preferably, said pressure is applied in parallel to the advancing direction of heating reaction, though said reaction may be advanced along the coating plane by igniting the edge in the case of forming a relatively thin synthesized coating layer in a wide area. Furthermore, as an alternative of the coating method of carrying out the reaction on a substrate metal to unite the substrate metal and a material with a predetermined component distribution, alloy powders may be distributed in a layer style, and fused by the reaction heat of synthesis, and thereby these fused alloy powders may be substituted for a substrate metal.

Fig. 9 is a schematic view of the component distri­bution of a produced material; in which the bottom line represents X coordinate corresponding to that of Fig. 3, and the boundary line F between TiB₂ and Cu in the region (B) corresponds precisely to the curve line of n=0.8 of Fig. 3. Fig. 10 shows an estimated temperature distribution (refer to a broken line) of the material just after the synthesis and formation; Fig. 11 shows the result of calculating according to the thermal analysis a stress residual in the material which has been cooled rapidly from the estimated temperatures to a room temperature, said residual stress showing a tensile stress as shown in this figure. Fig. 12 shows by a solid line a stress which occurs in said material under a working temperature circumstance, that is, an imaginary circumstance of a rocket engine in operation. From this figure, the following can be understood. Though a compression thermal stress similar to α of Fig. 5 occurs in said material due to the same temperature distribution, expressed by a broken line, as those of Figs. 1 and 5, this compression thermal stress is counterbalanced by said residual tensile stress, whose curve corresponds to that of Fig. 11 and is expressed by a single-­dotted chain line, thereby the total stress generated in the operation can be largely relaxed as shown by arrow marks. In other words, the mixed powders of constitutive elements Ti and B included in the first component TiB₂ is mixed with the powder of Cu included in the second component Cu so as to be an optimum composition distribution, and the above mixture is ignited, thereby the synthetic reaction is carried out only by self-heating, and the synthesis and formation of a material whose properties change continuously is completed in an instance, and at the same time, such a residual stress as cancels a thermal stress which will occur in the working is added to said material, thereby remarkable adhesion and thermal-stress fracture resistance are obtained.

The production method according to the present invention can be applied generally to various kinds of materials consisting of the combination of ceramic and metal or that of ceramic and ceramic without the necessity of restricting to the above embodiments. Though materials which can be produced are described in detail in the literature ("Energy-Saving Manufacture of Inorganic compounds with High Melting Temperature Sunshine Journal No. 4, (1985) 6, Japan"), some of them require preheating at high temperature in the producing process, and some of them must be reacted in a high-pressure container, and others deposit defective compound in the product. Then, available materials without said defects are: for example, TiB₂ - ­Ti(TiB), ZrB₂ -Zr(ZrB₂), ZrB₂ - Cu(ZrB₂), NbB₂ - Cu(NbB₂), Ta₃B₄ Cu(Ta₃B₄), TiB₂ -Al(TiB₂) in a group of boride-­metal; Tic - Ti(TiC), ZrC - Zr(ZrC), TiC - Cu(TiC), ZrC - ­Cu(ZrC) in a group of carbides-metal; TiC - ZrC, TiB - SiC, TiB - ZrB, and so on in a group of ceramics-ceramics. The compounds in the parentheses represent substances which are distributed continuously with metal. With these materials, as well as said embodiments, it is possible to determine distribution parameters such as (n, L), etc. of each component so as to flatten and minimize a specific stress distribution R after predetermining the temperature distribution T(x) of the materials in the operation (See Fig.1). In the case of boride and carbides with high heat-­conductivity, it is possible to feed oxide and nitride with low heat-conductivity into the ceramic layer in advance or to preheat one side of the ceramic layer so as to increase the thermal gradient during production. It is also possible to feed partially ceramic powders or metal powders into the intermediate layer, so as to flatten and minimize the specific stress distribution.

Figs. 13 to 16 illustrate respective examples of said materials in binary and ternary systems, representing schematic compositional distributions before and after reactions in (a)s and (b)s, respectively. More partic­ularly, Figs. 13(a) and (b) illustrate an example of the ceramic-metal group; Figs. 14(a) and (b) the ceramic-metal (ceramic) group; Figs. 15(a) and (b) the ceramic-ceramic (ceramic or metal) group; and Figs. 16(a) and (b) the ceramic-ceramic (metal) group. Though the fine powder of ceramic as the second component is made from the mixture of metallic element Zr and nonmetallic element C composing ceramic ZrC as its raw materials, the fine particles of ceramic ZrC itself may be available. Figs. 17(a) and (b) illustrate schematic composition distributions of a metal of another type before and after reaction, respectively. From these figures, it is found that since the synthesis and the formation of ceramic is carried out by the self-propagating reaction of raw materials instantaneously according to the producing method of the present invention, a staircase-­shaped compositional distribution obtained on pressing the raw material powders describes a smooth curve line after the reaction, because of the short range dispersion and substance movement due to the reaction. As described above, the producing method according to the present invention has an advantage that a continuous composition gradient can be readily obtained.

As has been apparent from the above description, the material-producing method according to the present invention has many advantages and will contribute much to the production of such a composite material as has a continuous properties gradient, i.e. a functionally gradient material, due to its construction that the intermediate layer of the composite material wherein the ratio of a first component and a second component thereof varies continuously is formed by means of an instantaneous synthetic reaction caused only by self-heating after igniting the mixture of powder of metallic and nonmetallic constitutive elements of ceramic as the first component and that of metal or other ceramic as the second component. Such advantages are as follows: the thick material of layers can be readily formed on a large area at low cost and in a short time without requiring other heat energy than the self-heat of the component powder. Furthermore, even though a compositional distribution of raw material powders is like a staircase shape to some degree before the reaction, a gradient distribution can be obtained due to the synthetic reaction, and at the same time, the regulation of the mixture ratio of the components allows said intermediate layer to gain such a residual stress as cancels a thermal stress which generates during the operation. Furthermore, a substrate metal can be coated with the composite material at ordinary temperature, thereby a range of materials design can be expanded.

Although the present invention has been described in connection with preferred embodiments thereof, many variations and modifications will now become apparent to those skilled in the art. It is preferred, therefore, that the present invention is limited not to the specific disclosure herein, but only to the appended claims.