Process for producing ceramic fiber-reinforced composite material

Abstract

To provide a process for producing a ceramic fiber-reinforced composite material, which suppresses the deterioration of an interface layer, improves mechanistic properties and has excellent durability even under a high temperature, even ceramic fibers formed of silicon carbide fibers are used, without complicating the production steps. To obtain a ceramic fiber-reinforced composite material, by melt-infiltrating a composite material substrate obtained by forming ceramic fibers, formed of silicon carbide fibers and having an amorphous structure, into a composite with a matrix formed of an inorganic substance, with an alloy having a composition that is constituted by a disilicide of at least one or more transition metal among transition metals selected from scandium, yttrium, titanium, zirconium, hafnium, and tantalum, and silicon as the remainder, and having the silicon content ratio of 66.7 at % or more and less than 90.0 at %.

Claims

1. A process for producing a ceramic fiber-reinforced composite material that is formed by infiltrating the entirety or a part of pores that are present in a composite material substrate obtained by forming ceramic fibers into a composite with a matrix formed of an inorganic substance, with an infiltrating material, wherein the composite material substrate consists of silicon carbide fibers having an amorphous structure, and free carbon that is present in the entirety or a part of pores that are present in the amorphous structure, and the infiltrating material is an alloy having a composition that is constituted by a disilicide of at least one or more transition metal among transition metals selected from scandium, yttrium, titanium, zirconium, hafnium, and tantalum, and silicon as the remainder, has a silicon content ratio, including the silicon in the transition metal disilicide, of 66.7 at % or more and less than 90.0 at % and gives a melting point that is lower than that of a single body of silicon, and the process consists of melt-infiltrating the pores that are present in the composite material substrate with the infiltrating material at a temperature that is equal to or more than the melting point of the alloy as the infiltrating material and that is less than 1,400 C., wherein the alloy as the infiltrating material reacts with the free carbon in the pores during the melt infiltration to generate silicon carbide and a carbide of the transition metal.

2. The process for producing a ceramic fiber-reinforced composite material according to claim 1, wherein the melting point of the residual alloy that is solidified in the pores after the melt infiltration is higher than the melting point of the alloy as the infiltrating material prior to the melt infiltration.

3. The process for producing a ceramic fiber-reinforced composite material according to claim 1, wherein the silicon carbide fibers of the amorphous structure are woven into an orthogonal three-dimensional woven fabric, wherein fiber volume fractions of the woven fabric are 20%, 20% and 0.3%, respectively, in the X, Y and Z directions.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is the phase diagram (schematic view) of an exemplary embodiment of the alloy as the infiltrating material used in the present invention; and

(2) FIG. 2 is the micrograph of the cross-sectional surface of the ceramic fiber-reinforced composite material of the present invention.

DETAILED DESCRIPTION

(3) The specific embodiment of the process for producing a ceramic fiber-reinforced composite material of the present invention may be any one as long as it is a process for producing a ceramic fiber-reinforced composite material that is formed by infiltrating the entirety or a part of pores that are present in a composite material substrate obtained by forming ceramic fibers into a composite with a matrix formed of an inorganic substance, with an infiltrating material, wherein the ceramic fibers are formed of silicon carbide fibers and have an amorphous structure, the infiltrating material is an alloy having a composition that is constituted by a disilicide of at least one or more transition metal among transition metals selected from scandium, yttrium, titanium, zirconium, hafnium, vanadium, niobium and tantalum, and silicon as the remainder, has a silicon content ratio (including the silicon in the transition metal disilicide) of 66.7 at % or more and 95.0 at % or less, and gives a melting point that is lower than that of a single body of silicon, and the process includes melt-infiltrating the pores that are present in the composite material substrate with the infiltrating material at a temperature that is equal to or more than the melting point of the alloy as the infiltrating material and that is 1,400 C. or less.

(4) Namely, the ceramic fiber-reinforced composite material according to the present invention is obtained by pre-forming a composite material base material obtained by forming ceramic fibers, which are formed of silicon carbide fibers and have an amorphous structure, as reinforcing fibers into a composite with an oxide or an inorganic substance such as carbon as a matrix (base material), and melt-infiltrating pores or voids that remain in the composite material substrate with a transition metal disilicide/silicon alloy as the infiltrating material.

(5) The composite material substrate is preformed by forming a preform of ceramic fibers into a composite with an inorganic material as a matrix (base material) by a chemical vapor deposition process (CVD), a chemical vapor infiltration process (CVI), a ceramic precursor infiltration-pyrolysis process (PIP), a reactive sintering process (RS) or the like.

(6) As the inorganic substance that forms the matrix, carbon, a nitride, a carbide, an oxide, a phosphate, a boride, crystallized glass or the like can be applied.

(7) The ceramic fibers can be applied to either long fibers or short fibers, and as reinforced forms of the preforms thereof, various states of arrangement such as unidirectional reinforcement, woven fabric lamination, three-dimensional woven fabric and woven fabric lamination/suturation can be applied.

(8) In order to react the melt-infiltrated transition metal disilicide/silicon alloy and the free carbon in the pores to generate carbides, as processes for providing free carbon to the inside of the pores in the composite material substrate prior to the infiltration, there are many techniques such as a chemical vapor deposition process (CVD), a chemical vapor infiltration process (CVI), an infiltration-pyrolysis process (PIP) for a carbon precursor resin and a process for slurry infiltration of a carbon powder, and the free carbon as formed can also have various forms such as amorphous carbon, crystalline carbon, graphite, carbon nanotube and graphene.

(9) A schematic drawing of a representative phase diagram of the transition metal disilicide/silicon alloy as the infiltrating material is shown in FIG. 1.

(10) Here, T.sub.mp1 is the temperature at which the melting point of the silicon alloy is the lowest, and T.sub.mp2 is the melting point of the transition metal disilicide, and a predetermined component ratio within the composition range A is used as the composition of the transition metal disilicide/silicon alloy in the present invention.

(11) These transition metal disilicide/silicon alloys have significant features that the transition metal disilicides have melting points (T.sub.mp2) that are higher than the melting point of silicon (1,414 C.), the transition metal disilicide/silicon alloys have melting points that are lower than the melting point of silicon (1,414 C.), and the like.

(12) The features of the transition metal disilicide/silicon alloys in a binary system, which are summarized for every transition metal, are shown in the following Table 1.

(13) TABLE-US-00001 TABLE 1 Amount of Specific Lowest silicon (at %) Melting gravity of melting (at lowest point of transition point ( C.) melting point) disilicate ( C.) metal (g/cc) None 1414 Scandium 1000 84-86 (1230) 3.0 Yttrium 1260 85-87 1850 4.5 Titanium 1330 84-86 1478 4.5 Zirconium 1370 87-89 1620 6.5 Hafnium 1330 91-93 1543 13.3 Vanadium 1400 95-97 1677 6.0 Niobium 1400 96-98 1940 8.6 Tantalum 1375 92-94 2200 16.7

(14) In general, an alloy having a low lowest melting point and containing a transition metal disilicide having a high melting point is desirable, and it is found that titanium, zirconium and hafnium are preferable.

(15) In vanadium, niobium and tantalum, the transition metal disilicide has a high melting point, whereas the transition metal disilicide/silicon alloy has a slightly high melting point. Conversely, in the cases of scandium and yttrium, the transition metal disilicide/silicon alloy has a low melting point, whereas the transition metal disilicide tends to show a slightly low melting point.

(16) Among these, hafnium, zirconium and yttrium can be preferably used as the transition metal disilicide/silicon alloy for infiltration since the transition metal disilicide/silicon alloy has a low melting point, the transition metal disilicide has a high melting point, and the transition metal disilicide has relatively excellent oxidation resistance.

(17) For example, in the case of a hafnium disilicide/silicon alloy containing 8 to 9 at % of hafnium, the melting point is decreased to about 1,330 C. at the lowest, and thus it becomes possible to decrease the melt infiltration temperature to about 1,380 C.

(18) At this temperature, for example, even general silicon carbide fibers having an amorphous structure (Tyranno ZMI fibers and Lox-M fibers (trade names: Ube Industries, Ltd.)) or the like are applied as ceramic fibers, it becomes possible to significantly suppress the decrease in the strength of the fibers in the melt infiltration step.

(19) By providing the entirety or a part of the pores that are present in the composite material substrate with free carbon prior to the melt infiltration, for example, when the pores are melt-infiltrated with the hafnium disilicide/silicon alloy containing 8 to 9 at % of hafnium, the silicon in the alloy reacts with the free carbon present in the pores to generate silicon carbide, whereas the reaction amounts of the hafnium and free carbon in the alloy is relatively small, and thus the amount of the hafnium in the hafnium disilicide/silicon alloy changes little.

(20) Therefore, the silicon on the hafnium disilicide/silicon alloy decreases, and the melting point in the hafnium disilicide/silicon alloy consequently increases according to the phase diagram shown in FIG. 1.

(21) It is also possible to bring the melting point of the residual hafnium disilicide/silicon alloy after the infiltration close to the melting point (T.sub.mp2) of hafnium disilicide by suitably adjusting the amount of the free carbon, the infiltration temperature and the infiltration time, and the melting point of the residual hafnium disilicide/silicon alloy can be increased to about 1,400 C. that is the melting point of silicon.

EXAMPLES

(22) Next, the process for the production of a ceramic fiber-reinforced composite material and the ceramic fiber-reinforced composite material produced according to the present process will be explained in more detail.

(23) As the ceramic fibers, amorphous silicon carbide fibers formed of a chemical composition of SiZrCO (Tyrrano ZMI fiber (trade name: Ube Industries, Ltd.) which had been woven into an orthogonal three-dimensional woven fabric having a shape of about 120 mm120 mm4 mm was used as a preform.

(24) The fiber volume fractions of the woven fabric are 20%, 20% and 0.3%, respectively, in the X, Y and Z directions.

(25) A carbon layer having a thickness of about 0.1 to 0.3 m was first formed on the fiber surface on the preform of the amorphous silicon carbide fibers by a chemical vapor deposition process (CVI process) using propane (C.sub.3H.sub.8).

(26) Furthermore, SiC having a thickness of 5 to 10 m was deposited on the fiber surface by a CVI process using silicon tetrachloride (SiCl.sub.4) and propane (C.sub.3H.sub.8) to form a matrix, to thereby form a composite material substrate that is a premolded product. The composite material substrate has a bulk density of about 1.8 g/cc and a pore rate of about 25%.

(27) The composite material substrate that had been pre-formed in this way was cut into about a width of 30 mma thickness of 4 mma length 50 mm and used as a sample, and the powders of the transition metal disilicide/silicon alloys having the respective composition shown in Examples 1 to 4 in the following Table 2 were each applied thereto by using a spray glue (Spray Glue 77 (trade name: manufactured by 3M)).

(28) The powder was applied five times to every surface of the sample, and the sample to which the transition metal disilicide/silicon alloy had been applied was put into a carbon crucible and heated in vacuum by using a carbon heater furnace to infiltrate the pores in the composite material substrate with the transition metal disilicide/silicon alloy to give a ceramic fiber-reinforced composite material (Examples 1 to 4).

(29) The temperatures for the infiltration treatment were each preset to a temperature that is about 50 C. higher than the melting point, as shown in the following Table 2, and the heat treatment time was 1 hour.

(30) Furthermore, an example in which the infiltration treatment with the infiltrating material was not conducted (Comparative Example 1), an example in which silicon was melt-infiltrated in vacuum at 1, 470 C. (Comparative Example 2), and an example in which silicon was melt-infiltrated in vacuum at 1, 430 C. (Comparative Example 3) were defined as comparative examples.

(31) TABLE-US-00002 TABLE 2 Matrix composition Free Content ratio of Impregnation after carbon infiltrating tempreature infiltration in pores material (at %) ( C.) (pore parts) Comparative None (Impregnation Example 1 material was absent) Comparative None Si (100) 1470 Si Example 2 Comparative None Si (100) 1430 Si Example 3 Example 1 None Si (92)Hf (8) 1380 HfSi.sub.2 + Si Example 2 None Si (88)Hf (12) 1380 HfSi.sub.2 + Si Example 3 None Si (88)Zr (12) 1420 ZrSi.sub.2 + Si Example 4 None Si (86)Y(14) 1310 YSi.sub.2 + Si Example 5 None Si (85)Ti (15) 1380 TiSi.sub.2 + Si Example 6 None Si (93)Ta (7) 1420 TaSi.sub.2 + Si Example 7 Present Si (88)Hf (12) 1420 HfSi.sub.2 + (carbon Si + SiC black) Example 8 Present Si (88)Hf (12) 1420 HfSi.sub.2 + (phenol Si + SiC resin char)

(32) A bending test piece of 44length 50 mm was processed from each ceramic fiber-reinforced composite material, and bending tests and measurements of pore rates at 1,200 C. and 1,300 C. under room temperature in argon were conducted.

(33) Furthermore, a sample of 444 mm was cut out, and the change in the weight by oxidation was measured by a thermal gravimetry in the air.

(34) The measurement conditions were an airflow amount of 100 mL/min and a temperature raising velocity of 10 C./min, and the temperature was retained at 1,200 C. for 5 hours and the change in the weight during that time (increase in amount by oxidation) was measured.

(35) The obtained results are shown in the following Table 3.

(36) TABLE-US-00003 TABLE 3 Increase in Bending strength (MPa) amount by Pore Room oxidation Density rate temper- 1200 1300 (mg/mm.sup.2) (g/cc) (vol. %) ature C. C. Comparative 1.80 24.1 253 172 124 0.06 Example 1 Comparative 2.45 2.9 85 45 38 0.06 Example 2 Comparative 2.10 16.7 140 65 35 0.06 Example 3 Example 1 2.60 3.4 250 188 162 0.02 Example 2 2.75 2.0 255 197 175 0.02 Example 3 2.55 1.8 180 160 142 0.05 Example 4 2.50 2.6 245 185 162 0.01 Example 5 2.45 3.1 249 175 151 0.06 Example 6 2.62 2.2 170 146 121 0.07 Example 7 2.65 2.0 242 206 181 0.02 Example 8 2.60 3.9 248 210 175 0.02

(37) In Comparative Example 1 in which the infiltration treatment with the infiltrating material was not conducted, the pore rate was high as 24.1%, whereas the bending strength was a tolerable value of about 253 MPa.

(38) In Comparative Example 2 in which the sample was melt-infiltrated with silicon (Si: 100%) under vacuum at 1, 470 C., the bending strength was decreased to 85 MPa in accordance with the decrease in the strength due to thermal decomposition of the ceramic fibers during the infiltration treatment.

(39) In the case of Comparative Example 3 in which the infiltration temperature was set to 1,430 C. that is slightly higher than the melting point of silicon (1,414 C.) so as to suppress the decrease in the strength of the ceramic fibers, the pore rate was slightly high as about 16.7% due to the high viscosity of the molten silicon, i.e., infiltration could not be sufficiently conducted.

(40) Furthermore, even the heat treatment temperature was decreased to 1,430 C., slight decrease in the strength due to thermal decomposition of the ceramic fibers was also observed.

(41) On the other hand, in either of Examples 1 to 4 of the present invention, since the melt infiltration temperature could be suppressed to 1,400 C. or less, thermal decomposition of the ceramic fibers was significantly suppressed, and thus a ceramic fiber-reinforced composite material having a high bending strength could be obtained.

(42) Furthermore, the pore rate was 4% or less at the most and thus the infiltration property was extremely fine. Specifically, it is understood that, in the cases of the transition metal disilicide/silicon alloys containing hafnium and yttrium, respectively, the increase in amount by oxidation at 1,200 C. is also decreased, and thus materials also having excellent oxidation resistance can be obtained.

(43) As Example 5 in the present invention, a pre-formed composite material substrate as in the above-mentioned Examples 1 to 4 was subjected to a vacuum infiltration treatment and a dry-curing treatment at 120 C., by using an aqueous solution containing 19.2 wt % of carbon black had been dispersed therein (Aqua-Black 162 (trade name: Tokai Carbon Co., Ltd.)) to which 0.5 wt % of an acrylic resin-based binder (Merposol (trade name: Matsumoto Yushi-Seiyaku Co., Ltd.) had been added.

(44) These vacuum infiltration/drying/curing treatments were repeatedly conducted five times to thereby disperse carbon black in the pores to give a composite material substrate, and the composite material substrate was melt-infiltrated with a hafnium disilicide/silicon alloy having a composition of Si (88 at %)-Hf (12 at %) in vacuum at 1,380 C. to give a ceramic fiber-reinforced composite material.

(45) FIG. 2 is the micrograph of the cross-sectional surface of the obtained ceramic fiber-reinforced composite material.

(46) Most of the pores became SiC that was generated by the reaction between the filled carbon and the silicon in the melt-infiltrated hafnium disilicide/silicon alloy, and the gaps thereof were filled with the residual hafnium disilicide/silicon alloy.

(47) As a result of an analysis of the crystal phase of this by an X-ray diffractometry, the peak of HfSi.sub.2 was not changed whereas the peak of metallic Si was extremely small.

(48) Furthermore, the peak of crystalline SiC was significantly increased, whereas the peak of HfC was observed little.

(49) Accordingly, it was found that a considerable part of Si in the hafnium disilicide/silicon alloy became SiC by the reaction with the free carbon in the pores, and most of the hafnium disilicide/silicon alloy phase remaining in the material was HfSi.sub.2.

(50) Furthermore, no decrease in the bending strength due to metal infiltration was observed, and thus a fine composite material could be obtained.

(51) As Example 6 in the present invention, a pre-formed composite material substrate as in the above-mentioned Examples 1 to 4 was soaked in a solution obtained by diluting a novolak-type phenol resin (J-325 (trade name: DIC Corporation)) with a solvent (methyl alcohol) at a ratio of 1:1, vacuum deaeration for about 24 hours was conducted to infiltrate the pores with the phenol resin, the solvent was removed in a vacuum drier at 100 C. for 5 hours, and the phenol resin was cured in the air at 160 C.

(52) These vacuum infiltration/drying/curing treatments were repeatedly conducted four times, and a heat treatment at 800 C. for 1 hour in an argon atmosphere was conducted to thereby carbonize the phenol resin to give a composite material substrate having pores containing carbon, and the composite material substrate was melt-infiltrated with a hafnium disilicide/silicon alloy having a composition of Si (88 at %)-Hf (12 at %) in vacuum at 1,380 C. to give a ceramic fiber-reinforced composite material.

(53) Also in this Example 6, a ceramic fiber-reinforced composite material having an excellent bending strength and contains a small amount of the residual hafnium disilicide/silicon alloy could be obtained as in Example 5.

(54) The ceramic fiber-reinforced composite material of the present invention can prevent the alloy as the infiltrating material, and the like from melting at a high temperature and partially scattering to thereby inhibit the oxidation resistance, and thus is preferable for, for example, movable parts that are used under high temperatures such as moving blades in gas turbines, and also exerts excellent performances in any use such as improvement of bending strength and improvement of anticorrosive property.