Inorganic fibrous material as reinforcement for composite materials and process for production thereof

An inorganic fibrous material for reinforcing composite materials, said fibrous material composed of a central layer and a surface layer, wherein PA0 the surface layer is formed of an inorganic material composed of PA1 (i) an amorphous material consisting substantially of Si, M, C and O, wherein M is Ti or Zr, or PA1 (ii) an aggregate consisting substantially of ultrafine crystalline particles of beta-SiC, MC, a solid solution of beta-SiC and MC, and MC.sub.1-x having a particle diameter of not more than 500 .ANG. wherein M is as defined above and x is a number represented by 0<x<1, and optionally containing amorphous SiO.sub.2 and MO.sub.2, or PA1 (iii) a mixture of the amorphous material (i) and the aggregate (2), and PA0 the central layer is formed of an inorganic material other than said inorganic material; and a process for production thereof.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an inorganic fibrous material which is useful as 
a reinforcement for composite materials and has improved wetting property 
with respect to metals, plastics or ceramics as a matrix of the composite 
materials, and to a process for producing the inorganic fibrous material. 
2. Description of the Prior Art 
Metals, plastics and ceramics reinforced with inorganic fibers such as 
carbon fibers and alumina fibers as a reinforcing material, known 
respectively as FRM, FRP and FRC, have attracted wide attention as 
structural materials having high mechanical strength. 
Inorganic fibers used for production of composite materials are required to 
have high mechanical strengths such as tensile strength and modulus of 
elasticity and good wetting property with respect to metals, plastics and 
ceramics as a matrix, and to undergo little degradation by reaction with 
the matrix. 
Carbon fibers have excellent strength and modulus of elasticity, but very 
bad wetting property with resepct to a matrix such as molten aluminum. 
Furthermore, carbon fibers tend to react with molten aluminum at high 
temperatures and cause a drastic reduction in the strength of the 
resulting composite material. Hence, if the carbon fibers are directly 
used to reinforce composite materials, the resulting products cannot have 
the desired mechanical strength. 
Some methods have been proposed in an attempt to eliminate the above 
inconveniences. For example, Japanese Laid-Open patent publication No. 
85,644/1980 discloses a method which comprises coating the surface of 
carbon fibers with a polymeric organosilicon compound such as 
polycarbosilane, polysilane or polycarbosiloxane, and rendering the 
polymeric organosilicon compound inorganic thereby to form a ceramic 
composed mainly of silicon carbide as a surface layer. Japanese Laid-Open 
patent publication No. 53,728/1984 discloses a method which comprises 
providing a vitreous layer on the surface of carbon fibers, coating it 
with a polymeric organosilicon compound such as polycarbosilane, 
polysilane or polycarbosiloxane, and rendering the polymeric compound 
inorganic to form a surface layer. 
The method of Japanese Laid-Open patent publication No. 85,644/1980 is not 
satisfactory because the adhesion of the central carbon fiber layer to the 
surface layer is low and strains remain between the two layers with the 
result that the fibers themselves have reduced tensile strength and 
flexibility. 
According to the method disclosed in Japanese Laid-Open patent publication 
No. 53,728/1984, the resulting reinforcing fibers themselves have good 
mechanical strength, but the wetting property of the carbide forming the 
surface layer with respect to a plastic or molten metal is not sufficient. 
Furthermoe, since the carbide has high reactivity with a molten metal such 
as molten aluminum, a composite material having satisfactory mechanical 
strength cannot be obtained by using the resulting reinforcing fibers. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an inorganic fibrous material 
composed of a central layer and a surface layer thereon, which is useful 
for reinforcing a composite material, and in which there is little 
residual strain between the central layer and the surface layer and the 
adhesion strength between the two layers is high. 
Another object of this invention is to provide an inorganic fibrous 
material which is useful for reinforcing a composite material and has very 
good wetting property with respect to plastics and molten metals. 
Still another object of this invention is to provide a process for 
producing an inorganic fibrous material which is useful for reinforcing a 
composite material and has such wetting property as mentioned above. 
According to this invention, there is provided an inorganic fibrous 
material for reinforcing composite materials, said fibrous material 
composed of a central layer and a surface layer, wherein 
the surface layer is formed of an inorganic material composed of 
(i) an amorphous material consisting substantially of Si, M, C and O 
wherein M is Ti or Zr, or 
(ii) an aggregate consisting substantially of ultrafine crystalline 
particles of beta-SiC, MC, a solid solution of beta-SiC and MC, and 
MC.sub.1-x having a particle diameter of not more than 500 .ANG. wherein M 
is as defined above and x is a number represented by 0&lt;x&lt;1, and optionally 
containing amorphous SiO.sub.2 and MO.sub.2, or 
(iii) a mixture of the amorphous material (i) and the aggregate (2), and 
the central layer is formed of an inorganic material other than said 
inorganic material. 
According to this invention, there is also provided a process for producing 
an inorganic fibrous material for reinforcing composite materials, which 
comprises a first step of applying polytitanocarbosilane or 
polyzirconocarbosilane to an inorganic fibrous material forming a central 
layer, a second step of heating the inorganic fibrous material in an 
oxygen-containing gaseous atmosphere to render the polytitanocarbosilane 
or polyzirconocarbosilane infusible, and a third step of heating the 
infusible inorganic fibrous material in a non-oxidizing atmosphere to form 
an inorganic surface layer. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The inorganic fibrous material forming the central layer in this invention 
includes, for example, inorganic fibers such as carbon fibers, alumina 
fibers, silicon carbide fibers and boron fibers, and ceramic whiskers such 
as silicon carbide whiskers and silicon nitride whiskers. The carbon 
fibers which are porous and have a small fiber diameter are particularly 
preferred. The carbon fibers may be obtained from polyacrylonitrile or 
pitch. 
The diameter of the inorganic fibrous material forming the central layer 
varies depending upon its type. Usually, the fibers have a diameter of 1 
to 200 micrometers, and the whiskers have a diameter of 0.1 to 2 
micrometers. 
The surface layer bonded to the central layer is formed of an inorganic 
layer composed of 
(i) an amorphous material consisting substantially of Si, M, C and O 
wherein M is Ti or Zr, or 
(ii) an aggregate consisting substantially of ultrafine crystalline 
particles of beta-SiC, MC, a solid solution of beta-SiC and MC, and 
MC.sub.1-x having a particle diameter of not more than 500 .ANG. wherein M 
is as defined above and x is a number represented by 0&lt;x&lt;1, and optionally 
containing amorphous SiO.sub.2 and MO.sub.2, or 
(iii) a mixture of the amorphous material (i) and the aggregate (2). 
The inorganic layer forming the surface layer has a thickness of usually 
0.1 to 50 micrometers, preferably 0.5 to 10 micrometers. 
The polytitanocarbosilane or polyzirconocarbosilane (to be sometimes 
referred to simply as "precursors") used as a precursor for forming the 
inorganic surface layer in this invention may be produced, for example, by 
the methods described in U.S. Pat. Nos. 4,347,347 and 4,359,559. 
One example of the method of its production comprises adding 
polycarbosilane having a number average molecular weight of 200 to 10,000 
and mainly containing a main-chain skeleton represented by the formula 
##STR1## 
wherein R represents a hydrogen atom, a lower 
alkyl group or a phenyl group, and an organometallic compound of the 
formula 
EQU MX.sub.2 
wherein M represents Ti or Zr, and X represents 
an alkoxy, phenoxy or acetylacetoxy group, so that the ratio of the total 
number of the structural units of --Si--CH.sub.2 -- of the polycarbosilane 
to that of the structural units of --M--O-- of the organometallic compound 
is in the range of from 2:1 to 200:1, and reacting them under heat in an 
atmosphere inert to the reaction to bond at least part of the silicon 
atoms of the polycarbosilane to the metal atoms of the organometallic 
compound through oxygen atoms to produce a precursor of the inorganic 
layer. 
The process of this invention for producing the reinforcing inorganic 
fibrous material will now be described. 
In the first step, polytitanocarbosilane or polyzirconocarbosilane is 
applied to an inorganic fibrous material forming the central layer. 
There is no limitation on the method of applying the precursor to the 
inorganic fibrous material. For example, it can be achieved by coating or 
immersing the inorganic fibrous material with or in the precursor in the 
molten state, or by immersing or coating the inorganic fibrous material in 
or with a solution of the precursor in an organic solvent such as hexane, 
heptane, benzene, toluene or xylene. The latter method is preferred 
because it can form a precursor layer of a uniform thickness efficiently 
on the surface of the inorganic fibrous material. 
Preferably, the thickness of the precursor layer is 0.1 to 65 micrometers, 
especially 0.5 to 15 micrometers. A precursor layer of such a thickness 
can be obtained by adjusting the concentration of the precursor in the 
organic solvent to 1 to 15% by weight, and immersing the inorganic fibrous 
material in this solution. The concentration of the precursor in the 
organic solvent may be further increased, but increased concentrations are 
liable to cause thickness unevenness in the precursor layer. 
In the second step of the process of this invention, the inorganic fibrous 
material to which the precursor has been applied is heated in an 
oxygen-containing gaseous atmosphere to render the precursor infusible. 
Air is conveniently used as the oxygen-containing gas. The heating 
temperature is generally 100.degree. to 300.degree. C. 
Heating gives rise to a three-dimensional structure in which silicon atoms 
in different chains are interrupted by oxygen atoms. This renders the 
precursor infusible. When fibers susceptible to oxidation such as carbon 
fibers are used as the inorganic fibrous material forming the central 
layer, heating at excessively high temperatures should desirably be 
avoided. 
In the third step, the inorganic fibrous material having the infusible 
precursor as a surface layer is heated in a non-oxidizing atmosphere to 
form the aforesaid inorganic layer. 
The heating temperature is usually 800.degree. to 2,000.degree. C. 
The reinforcing inorganic fibrous material of this invention may be applied 
to such matrices as metals, ceramics and plastics. 
Examples of the metals are aluminum, magnesium and alloys of these. 
Examples of the ceramics are carbide ceramics such as silicon carbide, 
titanium carbide, zirconium carbide, vanadium carbide, niobium carbide, 
tantalum carbide, boron carbide, chromium carbide, tungsten carbide, and 
molybdenum carbide; nitride ceramics such as silicon nitride, titanium 
nitride, zirconium nitride, vanadium nitride, niobium nitride, tantalum 
nitride, boron nitride, aluminum nitride and hafnium, nitride; nitride 
ceramics such as silicon nitride, titanium nitride, zirconium nitride, 
vanadium nitride, niobium nitride, tantalum, nitride, boron nitride, 
aluminum nitride and hafnium nitride; and oxide ceramics such as alumina, 
silica, magnesia, mullite and cordierite. 
Specific examples of the plastics include epoxy resins, modified epoxy 
resins, polyester resin, polyimide resins, phenolic resins, polyurethane 
resins, polyamide resins, polycarbonate resin, silicone resins, phenoxy 
resins, polyphenylene sulfide resins, fluorine resins, hydrocarbon resins, 
halogen-containing resins, acrylic resins, ABS resin, 
ultrahigh-molecular-weight polyethylene, modified polyphenylene oxide and 
polystyrene. 
The reinforcing fibrous material of this invention has a high adhesion 
strength between the central layer and the surface layer and good wetting 
property with respect to matrices of composites, for example molten 
metals, plastics and ceramics. Hence, the bond strength between the 
reinforcing inorganic fibrous material and the matrix is high, and the 
resulting composite material has excellent mechanical strength even in a 
direction at right angles to the aligning direction of the inorganic 
fibers. For example, it has high flexural strength measured in a direction 
at right angles to the aligning direction of the fibers and high shear 
strength. Furthermore, since the reinforcing inorganic fibrous material of 
this invention undergoes little degradation by reaction with molten 
metals, a composite material produced by using it has excellent mechanical 
strength, and can withstand use for an extended period of time. 
The various mechanical properties of a composite material produced by using 
the reinforcing fibrous material of this invention are measured by the 
following methods. 
(A) Interlayer shear strength 
A sample of a composite material having inorganic fibers 
(100.times.12.times.2 mm) aligned monoaxially is placed on two pins each 
having a length of 20 mm and a radius of curvature of 6 mm, and compressed 
by a presser having a radius of curvature of 3.5 mmR at its end by the 
so-called three-point bending method. The interlayer shear stress 
kg/mm.sup.2) is measured and defined as the interlayer shear strength. 
(B) Flexural strength in a direction perpendicular to the fibers 
A fiber reinforced composite material having a thickness of 2 mm in which 
the fibers are aligned monoaxially is produced. A test piece 12.7.times.85 
mm) is taken from it so that the axial direction of the test piece crossed 
the direction of the fiber alignment at right angles. The thickness of the 
test piece is 2 mm. A curvature of 125 mmR is provided in the central part 
in the thickness direction and the central part is finished in a thickness 
of about 1 mm. The test piece is subjected to the 3-point bending method. 
The flexural strength is expressed in kg/mm.sup.2. 
The interlayer shear strength and the flexural strength in a direction 
perpendicular to the fibers are measures of the strength of bonding 
between the matrix and the fibers. 
(C) Tensile strength 
A fiber reinforced composite material having a thickness of 2 mm in which 
the fibers are aligned monoaxially is produced. A test piece 
(12.7.times.230 mm) is taken from it so that the axial direction of the 
test piece is the same as the direction of the fiber alignment. The 
thickness of the test piece is 2 mm. A curvature of 125 mmR is provided in 
the central part in the thickness direction and the central part is 
finished in a thickness of about 1 mm. The tensile test is carried out at 
a stretching speed of 1 mm/min. The tensile strength is expressed in 
kg/mm.sup.2. 
The following examples illustrate the present invention more specifically.

REFERENTIAL EXAMPLE 1 
Production of polytitanocarbosilane 
Three parts by weight of polyborosiloxane was added to 100 parts by weight 
of polydimethylsilane synthesized by dechlorinating condensation of 
dimethyldichlorosilane with metallic sodium, and the mixture was subjected 
to thermal condensation at 350.degree. C. in nitrogen. Titanium alkoxide 
was added to the resulting polycarbosilane having a main-chain skeleton 
composed mainly carbosilane units of the formula (Si--CH.sub.2) and 
containing a hydrogen atom and a methyl group at the silicon atom in the 
carbosilane units, and the mixture was subjected to crosslinking 
polymerization at 340.degree. C. in nitrogen to give polytitanocarbosilane 
composed of 100 parts of the carbosilane units and 10 parts of titanoxane 
units of the formula --Ti--O--. The resulting polytitanocarbosilane had a 
number average molecular weight of about 2,500. 
REFERENTIAL EXAMPLE 2 
Production of polyzirconocarbosilane 
Three parts by weight of polyborosiloxane was added to 100 parts by weight 
of polydimethylsilane synthesized by dechlorinating condensation of 
dimethyldichlorosilane with metallic sodium, and the mixture was subjected 
to thermal condensation at 350.degree. C. in nitrogen. Zirconium alkoxide 
was added to the resulting polycarbosilane having a main-chain skeleton 
composed mainly of carbosilane units of the formula (Si--CH.sub.2) and 
containing a hydrogen atom and a methyl group at the silicon atom in the 
carbosilane units, and the mixture was subjected to crosslinking 
polymerization at 340.degree. C. in nitrogen to give 
polyzirconocarbosilane composed of 100 parts of the carbosilane units and 
9 parts of zirconoxane units of the formula --Zr--O--. The resulting 
polytitanocarbosilane had a number average molecular weight of about 
2,800. 
EXAMPLE 1 
Commercial carbon fibers having a diameter of about 8.5 micrometers and a 
tensile strength of 250 kg/mm.sup.2 were immersed in a 10% by weight 
n-hexane solution of the polytitanocarbosilane prepared in Referential 
Example 1, and heated in air at 150.degree. C. for 3 hours and further in 
nitrogen gas at 1300.degree. C. for 1 hour to form inorganic fibrous 
materials composed of a central layer of the carbon fibers and an 
inorganic surface layer. The inorganic fibrous materials had a tensile 
strength of 240 kg/mm.sup.2. 
The inorganic fibrous materials were monoaxially aligned, and aluminum 
foils (1070) were individually laid over them. By using hot rolls at a 
temperature of 670.degree. C., the inorganic fibrous materials and the 
aluminum were consolidated to produce composite foils. Twenty seven such 
composite foils were stacked, left to stand in vacuum at 670.degree. C. 
for 10 minutes, and then hot-pressed at 600.degree. C. to produce an 
inorganic fiber-reinforced aluminum composite material. The composite 
material contained 30% by volume of the fibers and had a tensile strength, 
in the fiber direction, of 60 kg/mm.sup.2, a flexural strength, in a 
direction at right angles to the fibers, of 28 kg/mm.sup.2 and an 
interlayer shear strength of 7 kg/mm.sup.2. 
COMATIVE EXAMPLE 1 
Commercial carbon fibers having a diameter of about 8.5 microns and a 
tensile strength of 250 kg/mm.sup.2 were immersed in a 1.5% by weight 
n-hexane solution of polycarbosilane having a number average molecular 
weight of about 2,000 and then heated in air at 120.degree. C. for 30 
minutes to form a silicon oxide substrate layer on the surface of the 
carbon fibers. These carbon fibers were immersed in a 15% by weight 
n-hexane solution of the above polycarbosilane, and then heated at 
1100.degree. C. in a nitrogen gas atmosphere for 30 minutes to give 
fibrous materials having a silicon carbide ceramic as a surface layer. The 
resulting fibrous mterials had a tensile strength of 238 kg/mm.sup.2. 
An aluminum composite material was produced in the same way as in Example 1 
except that the resulting fibrous materials were used instead of the 
fibrous materials used in Example 1. The composite material had a tensile 
strength, in the fiber direction, of 21 kg/mm.sup.2, a flexural strength, 
in a direction at right angles to the fibers, of 18 kg/mm.sup.2, and an 
interlayer shear strength of 3.9 kg/mm.sup.2. 
EXAMPLE 2 
The reinforcing inorganic fibrous materials obtained in Example 1 were 
monoaxially aligned on aluminum alloy foils (6061) having a thickness of 
0.5 mm, and aluminum alloy foils were laid over the fibrous materials. By 
using a hot roll at a temperature of 670.degree. C., the inorganic fibrous 
materials and the aluminum alloy were consolidated to produce composite 
foils. Twenty seven such composite foils were stacked, left to stand in 
vacuum at 670.degree. C. for 10 minutes, and then hot-pressed at 
600.degree. C. to produce a inorganic fiber-reinforced aluminum composite 
material. The composite material contained 30% by volume of the fibers and 
had a tensile strength, in the fiber direction, of 86 kg/mm.sup.2, a 
flexural strength, in a direction at right angles to the fibers, of 38 
kg/mm.sup.2 and an interlayer shear strength of 10 kg/mm.sup.2. 
COMATIVE EXAMPLE 2 
An aluminum composite material was produced in the same way as in Example 2 
except that the polycarbosilane-clad carbon fibers obtained in Comparative 
Example 1 were used instead of the reinforcing inorganic fibrous materials 
used in Example 2. The composite material had a tensile strength, in the 
fiber direction, of 34 kg/mm.sup.2, a flexural strength, in a direction at 
right angles to the fibers, of 21 kg/mm.sup.2 and an interlayer shear 
strength of 6.7 kg/mm.sup.2. 
EXAMPLE 3 
Example 1 was repeated except that alumina fibers (diameter 14 micrometers, 
tensile strength 160 kg/mm.sup.2) were used instead of the inorganic 
fibers. Inorganic fibrous materials were obtained which were composed of a 
central layer of alumina fibers and an inorganic surface layer and had a 
tensile strength of 152 kg/mm.sup.2. 
The inorganic fibrous materials were monoaxially aligned, and aluminum 
foils (1070) were individually laid over them. By using hot rolls at a 
temperature of 670.degree. C., the inorganic fibrous materials and the 
aluminum were consolidated to produce composite foils. Twenty seven such 
composite foils were stacked, left to stand in vacuum at 670.degree. C. 
for 10 minutes, and then hot-pressed at 600.degree. C. to produce an 
inorganic fiber-reinforced aluminum composite material. The composite 
material contained 30% by volume of the fibers and had a tensile strength, 
in the fiber direction, of 50 kg/mm.sup.2, a flexural strength, in a 
direction at right angles to the fibers, of 23 kg/mm.sup.2 and an 
interlayer shear strength of 5.2 kg/mm.sup.2. 
COMATIVE EXAMPLE 3 
An inorganic fiber-reinforced aluminum composite material was produced in 
the same way as in Example 3 except that the same commercial alumina 
fibers as in Example 3 were used without treatment as reinforcing fibers. 
The resulting composite material contained 30% by volume of the fibers and 
had a tensile strength, in the fiber direction, of 40 kg/mm.sup.2, a 
flexural strength, in a direction at right angles to the fibers, of 16 
kg/mm.sup.2 and an interlayer shear strength of 2.8 kg/mm.sup.2.