JPH0711360A - Carbon-aluminum composite material - Google Patents

Abstract

PURPOSE:To provide a carbon-aluminum composite material in which the coefficient of linear thermal expansion is controlled equally to that of a ferrous material as a piston for a reciprocating internal combustion engine and a seal material used in an environment of high temps. or that in which the temp. changes, light in weight and having self-lubricity. CONSTITUTION:The carbon-aluminum composite material in which, in a composite material in which the phases of carbon and aluminum or their alloy are all formed of continuous ones, the volume content of carbon occupied in the composite material is regulated to 50 to 85vol%, pores are present at the inside of the carbon by 1 to 14vol% to the carbon, the content of pores other than those in the carbon is regulated to <=10vol% to the whole body of the composite material, and the balance aluminum or the alloy thereof, and the coefficient of linear thermal expansion is regulated to 7X10<-6>/K to 15X10<-6>/K is produced.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention provides a carbon-aluminum composite material having a coefficient of thermal expansion controlled to be equal to that of an iron-based material and having a light weight and self-lubricating property. The material is a piston or a piston for a reciprocating internal combustion engine. It is suitable as a sealing material used in high temperatures or environments where the temperature changes.

[0002]

2. Description of the Related Art Aluminum is lightweight and is used for pistons of internal combustion engines. However, aluminum or its alloy has a large thermal expansion coefficient of 21 × 10 ⁻⁶ / K. The thermal expansion coefficient of general iron-based materials is 10-12 x 1
It is 0 ^-6 / K, which is about half that of aluminum. In a reciprocating internal combustion engine, the narrower the gap between the piston and the cylinder, the better the fuel consumption efficiency. This gap can be made smaller as the coefficient of thermal expansion of the material is smaller. Alternatively, the closer the coefficient of thermal expansion between the cylinder and the piston, the narrower the gap. At present, iron-based materials are widely used for cylinders, and it would be a great advantage if the lightweight aluminum piston could have the same coefficient of thermal expansion as iron-based materials. Furthermore, if the material itself has lubricity, the amount of engine oil can be reduced.

Since carbon has a self-lubricating property,
It is used as a sealant and solid bearing. Carbonaceous
The general carbon material obtained by baking and hardening graphite particles is tough
Is low and lacks reliability when used as a structural material
It Coefficient of thermal expansion is 3-5 × 10 ^-6/ K compared to iron-based materials
Is quite small. Also, with carbon material reinforced with carbon fiber
A C / C composite material has high strength and toughness as a structural material.
It is high enough to be used, but its coefficient of thermal expansion is carbon fiber.
0-1x10 controlled by the thermal expansion coefficient of fiber^-6/ K and very low
Yes. Since it is a fiber-reinforced composite material, its physical properties are anisotropic.
However, there are many cases where the shape of a part is restricted.
It is difficult to use as a structural material. Contact with sealing materials and bearing materials
Iron-based materials are widely used for the shaft of the person to be touched. Shi
However, general carbon material or C / C composite material and iron material
The difference in the coefficient of thermal expansion between
It is not suitable for use in an environment where

Materials made of aluminum or carbon are rarely used alone, and in many cases, they are used as a part in combination with other materials, especially iron-based materials.
Especially when used in an environment where the temperature is high or the temperature changes, it is important that the coefficient of thermal expansion be close to that of an iron-based material.

[0005]

A composite material of aluminum or its alloy and carbon or graphite is known as a material having a light weight and a self-lubricating property. However, regarding the thermal expansion behavior, the existing materials are as follows. There is a problem. The thermal expansion coefficient α _c of the carbon fiber reinforced aluminum composite material is v
Is represented by the following formula (I), where is the aluminum volume content. Where α ₁ is the coefficient of thermal expansion of aluminum, α ₂ is the coefficient of thermal expansion of carbon fiber, E ₁ is the elastic modulus of aluminum,
E ₂ is the elastic modulus of the carbon fiber. 7 for E ₁ as a typical value
0 GPa, E ₂ 250 GPa, α ₁ 21 × 10 ^-6 /
Curves 1 in FIG. 1 show changes in the coefficient of thermal expansion of the composite material with respect to the concentration of aluminum, using 0 × 10 ⁻⁶ / K for K and α ₂ , respectively.

[0006]

[Equation 1]

Charcoal obtained by baking carbonaceous or graphite particles
In the composite material in which the base material is impregnated with aluminum, carbon fiber
Carbon phase and aluminum as in reinforced aluminum composites
Since each of the phases is a continuous phase, according to formula (I)
The coefficient of thermal expansion is expressed. E as a representative value₁70 GP
a, E₂10 GPa, α₁3.5 × 10^-6/ K, α
₂21 × 10^-6/ K, respectively, the heat of the composite material
The change in expansion coefficient with respect to the concentration of aluminum is shown in the curve of Fig. 1.
Shown in line 2.

Since the composite material in which carbon or graphite particles are dispersed in aluminum does not have a continuous phase of carbon, the influence of aluminum is more exerted than the composite material obtained by impregnating aluminum in a carbon material obtained by baking and hardening carbonaceous or graphite particles. Receive strongly. Therefore, the coefficient of thermal expansion of the composite material in which carbon or graphite particles are dispersed in aluminum is generally larger than that of the curve 2 in FIG. 1, and shows a change represented by the curve 3, for example.

That is, in the existing technology, at a composition ratio (aluminum volume content is 15 to 85 vol% and the balance is carbon) in which the effect of combining aluminum and carbon is sufficiently exerted, the thermal expansion coefficient of the iron-based material is 10 ~ 12 × 10 ^-6 /
I can't get K. Therefore, the present inventor has conducted intensive studies to solve these problems, and found that the above problems can be solved by a composite material in which carbon and aluminum are both continuous phases and carbon contains an appropriate amount of pores. The present invention has been completed. That is, an object of the present invention is to provide a carbon-aluminum composite material which has a thermal expansion coefficient close to that of an iron-based material, is lightweight, and has a self-lubricating property.

[0010]

[Means for Solving the Problems]
In a composite material in which carbon and aluminum or their alloys are both continuous phases, the volume content of carbon in the composite material is 50 to 85 vol%, and 3 to 10 vo with respect to the carbon.
1% of the pores exist inside the carbon, the pores outside the carbon are 6 vol% or less with respect to the whole composite material, and the rest are aluminum or its alloy, and the coefficient of linear thermal expansion is 15 × 10
^-6 / K or less is readily accomplished by a carbon-aluminum composite material, characterized in that it is 7 × 10 ^-6 / K or more.

The present invention will be described in detail below. The carbon of the present invention is substantially composed of carbon atoms and is three-dimensionally continuous. The form of carbon is not particularly limited. For example, it is possible to use a graphite material in which a graphite crystal is developed, a carbon material in which a graphite crystal is not well developed, amorphous carbon or glassy carbon which hardly shows X-ray diffraction, or a mixture thereof. More specifically, natural graphite, artificial graphite, steel making coke, foundry coke,
Pitch coke, needle coke, carbon black, activated carbon, etc. can be used as a raw material. A carbonaceous substance produced directly from a gas phase by thermally differentiating a gas such as methane can be used as a raw material. Further, as the carbon precursor, an organic material such as pitch or thermosetting resin can be used. When carbon is used as carbon particles, it is generally 0.1
It is preferably about 5,000 μm.

If the carbon phase is not continuous, it is more affected by the large thermal expansion of aluminum, and the thermal expansion coefficient of the composite material becomes large as shown by the curve 3 in FIG. 1, which is not preferable. Further, when only carbon fiber is used for carbon, the pores inside the carbon which will be described later are out of the scope of the present invention, and the elastic modulus of carbon fiber is about one digit higher than that of aluminum, and as shown by curve 1 in FIG. The coefficient of thermal expansion of (1) is dominant and unsuitable, but a small amount of carbon fiber may be added to the carbon phase. The carbon concentration in the composite is 50-85 vol%. When the carbon concentration is higher than this, the thermal expansion coefficient becomes too small, and the aluminum content decreases, so that the toughness of the composite material decreases, which is not preferable. Further, when aluminum is excessive, the coefficient of thermal expansion becomes too large as compared with the iron-based material, and the self-lubricating property of the composite material is lost.

On the other hand, the aluminum of the present invention is pure aluminum or its alloy. Since the coefficient of thermal expansion of an aluminum alloy does not change much depending on its composition, the composition of the alloy can be selected according to other physical properties such as the elastic modulus of the composite material. The only requirement is that the aluminum be three-dimensionally continuous in the composite. The discontinuity of aluminum reduces the toughness of the composite material. As is well known, carbon itself is a brittle material, and the high toughness of composite materials depends on aluminum. If this aluminum is discontinuous, the composite becomes brittle and is not suitable as a structural material.

The pores in carbon are pores surrounded by carbon. The coefficient of thermal expansion of carbon is smaller than that of aluminum. The carbon surrounding the pores is susceptible to deformation due to the presence of the pores, absorbs the large thermal expansion of aluminum and achieves a coefficient of thermal expansion smaller than that expected from equation (I). The pores in this carbon are the most important requirement in the present invention. Porosity is 1 to 14 vol% with respect to carbon, preferably 3 to 10
vol% is good. If there are too many pores in the carbon, the carbon phase will not exhibit its original properties and is unsuitable. On the other hand, if it is too small, the ability to control the coefficient of thermal expansion is lost, which is unsuitable. A suitable upper limit of the pore size is a size that is contained in a continuous carbon phase and the carbon phase functions as one phase, that is, a size that is about one order of magnitude smaller than the typical size of the carbon phase. If the pores have the same size as the carbon atoms, it becomes unsuitable because it becomes an atomic defect and no longer functions as pores. Therefore, the size of pores in carbon is preferably 0.01 to 100 μm. The shape of the pores is not particularly limited.

The pores outside the carbon are the pores existing at the interface between the carbon and aluminum or inside the aluminum. The strength of the composite material depends mainly on aluminum, and the pores outside the carbon serve as the starting point of aluminum fracture, which reduces the strength of the composite material. Therefore, the amount of pores other than carbon is 10 vol% or less, preferably 6 vol% or less, based on the whole composite material.

As a method for producing the composite material of the present invention, for example, carbonaceous particles having appropriate pores inside are baked and solidified in an inert atmosphere to form a porous body, and the porous body is impregnated with a molten aluminum. There is a way. In the obtained carbon / aluminum composite material, both carbon and aluminum or an alloy thereof form a continuous phase, and the volume content of carbon in the composite material is 50 to 85 vol%, which is 1% of the carbon.
Porosity of up to 14 vol% is present inside the carbon, pores outside the carbon are 10 vol% or less with respect to the entire composite material, and the rest is aluminum or its alloy, and the linear thermal expansion coefficient is 15 × 10 ⁻⁶ / K or less and 7 × 10 ⁻⁶ / K or more.
This composite material is molded by a conventional method and is suitably used as a piston or various sealing materials for a reciprocating internal combustion engine.

[0017]

EXAMPLES The present invention will be described in more detail with reference to the following examples. Example 1 Carbonaceous particles (pitch coke, average 100 μm) and a binder (phenolic resin) were mixed, pressure was applied to the mixture, and the mixture was baked at about 3000 ° C. to obtain a bulk density of 1.17 g / c.
c, average linear thermal expansion coefficient from room temperature to 125 ° C. 4.4 ×
A porous body of 10 ⁻⁶ / K was obtained. The porosity in the carbon of this porous body was measured by the mercury porosimetry, and it was about 8
It was vol%. JIS heated to 800 ℃
A composite material was prepared by impregnating an aluminum alloy of Class 8A (AC8A) specified in H2117 (1975) at a pressure of about 800 MPa.

The average linear thermal expansion coefficient of the obtained carbon / aluminum composite material from room temperature to 125 ° C. is 14 × 10 ^-6 /
It was K. When the cross section of the composite material was observed with a scanning electron microscope, all the pores were present in carbon. The size of the pores was about 50 μm or less. Carbon density is 2.2 g / cc, aluminum alloy density is 2.64 g
/ Cc, the content rate of each constituent element calculated from the weight change in each step is 53 vol% carbon, 43 vol% aluminum alloy, 8 vol% in carbon (relative to carbon), 0 vol outside the carbon. %Met.

Example 2 A bulk density of 1.31 g / cc and an average coefficient of linear thermal expansion from room temperature to 125 ° C. of 4.5 × 10 ^-6 / were obtained in the same manner as in Example 1 except that coal tar pitch was used as the binder. K
To obtain a porous body of. When the pores in carbon of this porous body were measured by the mercury porosimetry, it was about 4 vol% with respect to carbon. Thereafter, a composite material was prepared in the same manner as in Example 1. The average linear thermal expansion coefficient of the obtained carbon / aluminum composite material from room temperature to 125 ° C. was 14 × 10 ⁻⁶ / K. When the cross section of the composite material was observed with a scanning electron microscope, all the pores were present in carbon. The size of the pores was about 25 μm or less. The content of each constituent element calculated by the same method as in Example 1 is 60 vol% of carbon, 38 vol% of aluminum alloy, and 4 vol% of pores in carbon.
(Relative to carbon), the amount of pores outside the carbon was 0 vol%.

Example 3 Performed except that a pitch coke having an average particle size of 50 μm was used.
Bulk density of 1.54 g / cc from room temperature
Average linear thermal expansion coefficient up to 125 ℃ 5.0 × 10 ^-6/ K's
A porous body was obtained. The porosity in the carbon of this porous body is determined by the mercury pressure.
It was about 5 vol% with respect to carbon as measured by the in-house method.
It was. Thereafter, a composite material was prepared in the same manner as in Example 1. Got
Carbon-aluminum composites from room temperature to 125 ° C
Has an average linear thermal expansion coefficient of 11.3 × 10^-6Was / K.
The content rate of each component calculated by the same method as in Example 1 is
70 vol% carbon and 28 vo aluminum alloy
1%, 5 vol% of pores in carbon (relative to carbon), carbon
The outside pores were 0 vol%.

Example 4 Needle coke (average 50 μm) and binder (coal tar pitch) were mixed and extruded, and then about 3000
Calcined at ℃, bulk density 1.66g / cc, room temperature to 12
A porous body having an average linear thermal expansion coefficient of 2.4 × 10 ⁻⁶ / K up to 5 ° C. was obtained. When the pores in carbon of this porous body were measured by the mercury porosimetry, it was about 5 vol% with respect to carbon. Thereafter, a composite material was prepared in the same manner as in Example 1. The average linear thermal expansion coefficient of the obtained carbon / aluminum composite material from room temperature to 125 ° C. was 7.8 × 10 ⁻⁶ / K. When the cross section of the composite material was observed with a scanning electron microscope, some pores were present in carbon and the remaining pores were mainly present at the interface between carbon and the aluminum alloy. The size of pores in carbon was about 10 μm or less. The content rate of each constituent element calculated by the same method as in Example 1 is 75 vol% of carbon,
Aluminum alloy 18vol%, pores in carbon 5vol
% (Relative to carbon), and pores outside the carbon were 4.5 vol%. The linear thermal expansion coefficients of the carbon / aluminum composite materials obtained in the above examples are shown by circles in FIG. A material having a thermal expansion coefficient of 10 to 12 × 10 ⁻⁶ / K which is the same as or close to that of the iron-based material can be obtained.

Comparative Example Comparative Example 1 Pitch-based carbon fiber having a tensile elastic modulus of 250 GPa and JIS
H2117 (1975) defined 8 type A aluminum alloy (AC8A) is compounded by high pressure solidification casting method,
A unidirectionally reinforced aluminum composite material having a fiber volume fraction of 35 vol% and an aluminum alloy volume fraction of 75 vol% was prepared. The average linear thermal expansion coefficient from room temperature to 125 ° C. measured in the fiber direction was 7.5 × 10 ⁻⁶ / K.

Comparative Example 2 Pitch-based carbon fiber having tensile elasticity of 250 GPa and JIS
H2117 (1975) defined 8 type A aluminum alloy (AC8A) is compounded by high pressure solidification casting method,
A unidirectionally reinforced aluminum composite material having a fiber volume fraction of 50 vol% and an aluminum alloy volume fraction of 50 vol% was prepared. The average linear thermal expansion coefficient from room temperature to 125 ° C. measured in the fiber direction was 5.1 × 10 ⁻⁶ / K.

[0024]

INDUSTRIAL APPLICABILITY The present invention is suitable as a piston for a reciprocating internal combustion engine and a sealing material used in a high temperature or an environment where the temperature changes. Provides a self-lubricating carbon-aluminum composite material.

[Brief description of drawings]

FIG. 1 Existing carbon / aluminum composites (curves 1 to
3) and the relationship between the thermal expansion coefficient of the composite material according to the present invention and the volume content of aluminum will be described.

Claims (2)

[Claims] 1. A composite material in which both carbon and aluminum or an alloy thereof are in a continuous phase, the volume content of carbon in the composite material is 50 to 85 vol%, and the pores of 1 to 14 vol% with respect to the carbon. It exists inside carbon, the pores outside the carbon are 10 vol% or less with respect to the whole composite material, and the rest is aluminum or its alloy, and the linear thermal expansion coefficient is 15 × 10 ⁻⁶ / K or less, 7 × 10 Carbon / aluminum composite material characterized by ^-6 / K or more. 2. A piston for a reciprocating internal combustion engine comprising the carbon-aluminum composite material according to claim 1.