Introduction

High-efficiency materials operating at 1200 – 1800°C are required for use in modern technologies. Such materials are primarily technical ceramics, the main advantages of which are thermal stability, low density, inertness to many aggressive media, and high mechanical properties (elastic modulus, hardness, strength). However, the use of technical ceramics at significant thermal loads is limited because of their brittleness.

Composites reinforced with discrete and continuous fibers represent one of the main solutions to this problem. These composites exhibit nonlinear deformation up to the maximum load, retaining carrying capability with further loading, in contrast with unreinforced ceramics [1,2,3].

Reinforced composites are used in high-temperature protective structures and assemblies for spacecraft, gas turbine and engine parts, automobile braking systems, calibration plates in measurement technology, optical measuring systems, energy-intensive manufacturing structures, cutting tools, armor parts, sand-blasting nozzles, friction bearings, etc. [4,5,6,7,8,9]. However, several problems arise during their fabrication, the most important of which are distributing the reinforcement components uniformly in the matrix, orienting the reinforcement (anisotropic properties), creating the required component interface throughout the composition and structure, and reaching a high density [10].

Continuous or discrete fibers (with a small length-to-diameter ratio) are used to produce reinforced composites. Short fibers can be randomly oriented or oriented primarily in one direction, e.g., unidirectional fibers (packed in one direction) [10].

Matrix material comprises Al2O3, mullite, cordierite, Si3N4, AlN, SiC, C, etc. The reinforcement phase could be oxide, carbide, nitride, and boron fibers (Table 1) [11]. However, SiC and C fibers are of greatest interest to developers [12,13,14,15].

Table 1. Properties of Inorganic Fibers

Whisker crystals (WCs) are widely used for reinforcement and include aluminum-oxide, cordierite, and zircon matrices. However, aluminum-oxide matrices typically lose strength rapidly above 1000°C [16]. Fracture toughness K1c increased from 2.2 to 3.8 MPa·m1/2 if WCs SiCw were added to a cordierite matrix. Ceramic based on partially stabilized ZrO2 weakened after adding SiCw although its K1c increased. SiC and Si3N4 matrices are most promising for preparing composites and have the advantage of resistance to oxidation up to 1500 – 1800°C [17,18,19,20]. WCs SiCw and Si3N4w are widely used to reinforce Si3N4 matrices. Fibers Si3N4w become reinforcing only if coated with a barrier (interphase) [21,22,23,24,25]. Boron fibers are prepared by chemical decomposition of gaseous BCl3, after which B atoms deposit on a heated tungsten wire of diameter 30 – 40 μm [26,27,28,29,30,31]. Fibers of Al2O3 are used most often in composites based on metal matrices (Al and its alloys). The method used to produce them was similar to sol-gel technology followed by high-temperature treatment [32,33,34].

SiCf continuous fibers can be produced in two ways, i.e., decomposition of polycarbosilane with deposition of gaseous SiC onto a tungsten wire (cored wires) [11, 35, 36] (Fig. 1a ) and pyrolysis of polydimethylsilane in an autoclave at 100 atm followed by vacuum heat treatment (coreless wires) [11, 37] (Fig. 1b ).

Fig. 1.
figure 1

Photomicrographs of cored (a) and coreless SiC fibers (b ).

WCs SiCw and Si3N4w usually grew from a supersaturated high-purity gas phase at high temperatures (Fig. 2). Therefore, their composition was cleaner than that of SiCf. The WCs were single crystals and were practically free of impurities. Their properties could reach high values, e.g., tensile strength σten and elastic modulus Eel of 14 – 20 GPa and 700 GPa, respectively. WCs had several drawbacks, e.g., homogeneous materials were difficult to produce from them because they tended to form intertwined agglomerates and the crystals could be oriented as a result of which the material became anisotropic [38,39,40,41,42,43,44,45].

Fig. 2.
figure 2

Photomicrographs of whisker crystals SiCw (a) and Si3N4w (b ).

The main advantages of carbon fibers (Cf) were the low density ρ and high thermal stability in an inert medium (up to 2500°C). However, they require special protection if used in an oxidizing medium at temperatures above 400°C [46,47,48,49,50]. Organic fibers, most often cellulose (viscose) and synthetic fibers (polyacrylonitrile), act as raw material for producing Cf (Fig. 3). These fibers are produced by forcing highly viscous polymer through needles followed by thermal decomposition [51]. They are divided according to carbon content into carbonized (<90% C), coal (91 – 98% C), and graphitic (>98% C) [51,52,53]. Cf possessed high σten and Eel values [49, 50, 53]. Their composites were easily worked mechanically.

Fig. 3.
figure 3

Photomicrographs of carbon fibers: continuous Cf (a) and Cf ends (b ).

Destruction of reinforced composites includes several steps, i.e., the start of matrix microcracks, an increase in the number of matrix microcracks, loosening of fibers, and pulling out of fibers [46, 47]. The greatest energy during destruction of reinforced composites is expended to overcome friction as the fibers are extracted from the matrix. This indicates the importance of studying interactions at the fiber—matrix interface. For this, the fiber is coated (interphase) to increase the binding strength to the matrix [54].

The coating used most often for SiCw consists of a thin layer (<1 μm) of anisotropic pyrocarbon deposited on the SiCw surface by chemical vapor deposition [55,56,57,58,59,60,61]. One deficiency of pyrocarbon is its oxidation at 450 – 500°C. The oxidation resistance of SiC–SiCf composites increases if B is added to form B2O3, which can heal matrix microcracks at temperatures of 470 – 1100°C, like SiO2 does at higher temperatures (≥1400°C). Multilayered coatings are also effective.

Oxides, with the exceptions of HfO2 and ZrO2, react with Cf. Therefore, they are unacceptable for protecting it. SiO2 and B2O3 can heal microcracks so that such fibers can be used up to 1400 – 1500°C. Coatings of SiC and TiC, Si3N4 and AlN, and MoSi2 and TiSi2 or combinations of these materials are used at higher temperatures and can protect fibers up to 1700 – 1800°C. SiO2 films on fiber surfaces began to decompose at temperatures 1800°C so that they could not be used further [62, 63].

The most common methods for preparing composites were reviewed [64].

Stir Casting of Component Powders

Powder (≤1 μm, micron or submicron) of matrix component (Al2O3, SiC, Si3N4; Fig. 4a, b, c, respectively) was mixed with SiCw or Si3N4w, formed into blanks, and heat treated [65,66,67,68,69]. Low-melting oxide mixtures, usually eutectics, were used as the sintering additives. The materials with uniformly distributed WCs in the matrix had better mechanical properties than standard materials without WCs (Table 2). The advantages of these materials are the significant mechanical (bending strength σbend, Eel, K1c) and high-temperature properties (heat resistance, thermal and fire stability), chemical resistance, low density, and TCLE. The disadvantages are high brittleness, need to use micron and submicron starting powders, large shrinkage during sintering or hot pressing (up to 20 vol%), and complicated mechanical treatment [21,22,23,24,25].

Fig. 4.
figure 4

Microstructure of Al2O3 + 10 vol.% SiCw (a), SiC + 5 vol.% SiCw (b ), and Si3N4 + 20 vol.% Si3N4w (c).

Table 2. Physical and Mechanical Properties of Al2O3, SiC, and Si3N4 Composites

Polymer Impregnation and Pyrolysis (PIP)

The process for preparing composites reinforced by continuous C [70,71,72,73,74,75] or SiC fibers [76,77,78,79,80,81,82,83] appears as follows. A framework of continuous-intertwined fibers is impregnated with polymer melt and pyrolyzed several times at temperatures up to 1000°C. Table 3 presents the properties of the PIP materials. The advantages of PIP materials are the easy production technology, the ability to produce products of various geometries and configurations, and low density ρ and TCLE. The disadvantages are the low mechanical characteristics, the inability to be used at high temperatures (>400°C) without additional thermally stable coatings, and the long processing times to produce items because of the multiple impregnation – pyrolysis cycles.

Table 3. Physical and Mechanical Properties of PIP Composites

Liquid Silicon Infiltration (LSI)

SiC powder is mixed with technical soot and reinforced by adding SiCf (Cf). Blanks are prepared and impregnated with liquid Si (Fig. 5). The soot reacts with the Si melt to form secondary SiC between primary grains of SiC [84,85,86,87,88,89,90,91]. The obtained material, an analog of reaction-sintered silicon carbide (SiSiC), has several advantages, e.g., low ρ, high mechanical properties (Table 4), high thermal conductivity, ability to use large starting SiC powders, and low sintering temperature. The disadvantages of reinforced SiSiC materials are the high brittleness, low thermal stability (1200°C), and heat and chemical resistance [92,93,94,95].

Fig. 5.
figure 5

Microstructure of reinforced SiSiC materials with added SiCf: 4 (a) and 16 vol.% (b ).

Table 4. Physical and Mechanical Properties of SiSiC Reinforced with SiCf and Cf Fibers

Slurry Impregnation and Hot Pressing (SHIP) [96]

Fibrous carbon fabric is impregnated with a suspension (solution of SiC or Si3N4 powder with oxide additives), dried, and stacked as level sheets on each other. The composite blank is hot pressed (Fig. 6) [97,98,99,100,101,102,103,104,105,106]. The advantages of SHIP materials are the low ρ and TCLE, good workability, higher thermal stability (than PIP materials), increased mechanical properties (Table 5), and high tribological and corrosion properties. The disadvantages are the complicated production process, energy demand, low throughput of hot pressing, and limited shapes and sizes of items.

Fig. 6.
figure 6

Microstructure of hot-pressed Si3N4 + 20 vol.% Cf (a) and distribution of ceramic between Cf fibers (b ).

Table 5. Physical and Mechanical Properties of SHIP Materials

Reactive Melt Infiltration (RMI)

The method is based on free penetration of a porous blank by a metal melt. The penetration depth depends on the wetting angle of the matrix by the melt and the blank pore size. Matrix materials that have been used include Al2O3 [107], SiC [108, 109], TiC and TiB2 [109], ZrC [110,111,112,113,114,115], ZrB2 [116], SiBC [117], composites ZrB2–SiC [118,119,120], ZrB2–SiC–ZrC [121], MoSi2–SiC [122], etc. The matrix can contain WCs and fibers. The metal can be Al, Ti, Mo, and others including Si (Fig. 7) [107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122]. This process can produce items of complicated shapes practically without mechanical treatment. RMI materials have high mechanical (Table 6) and tribological properties. The disadvantages of these materials are the high ρ of several composites, poor corrosion properties, and low heat resistance and thermal stability (operating temperature less than 0.8 of the metal melting point).

Fig. 7.
figure 7

Microstructure of 80 vol.% SiC + 10 vol.% Ti + 10 vol.% Bf.

Table 6. Properties of Reinforced Composites Prepared by RMI [109]

Chemical Vapor Infiltration (CVI)

The CVI method is used to produce Bf, SiCf, SiCw, and Si3N4w [123, 124] and to coat fiber surfaces. However, it is also used to produce composites (the matrix is a framework of carbon-carbon material) [125] from biomorphic porous ceramics based on SiC [126, 127] including or based on carbides (B4C, SiC, TaC, and TiC) [128,129,130,131], nitrides (BN and Si3N4) [129, 132], borides (TiB2, ZrB2) [133, 134], or Al2O3 [135]. The advantages of CVI materials are the ability to produce items of various geometries and configurations, low ρ and TCLE, significant mechanical characteristics (Table 7), and broad range (up to 1000 – 1100°C) of operating temperatures (as compared with PIP materials). The disadvantages are the complicated and long production process, low heat resistance, thermal stability, and corrosion resistance.

Table 7. Properties of Composites Prepared by CVI

Self-Reinforcement During Sintering of Silicon Carbide and Nitride Ceramics

Sintered (SSN) and hot-pressed silicon nitride (HPSN). Ceramics are sintered by compacting particles as they rotate and slide relative to each other in a liquid phase of low viscosity that wets well the surface of Si3N4 particles. The oxide melt spreads as a thin layer over the Si3N4 particle surface owing to surface tension forces. Capillary forces ensure that all pore spaces between Si3N4 particles are filled. Mass transfer of material in the liquid phase facilitates its higher compaction. α-Si3N4 particles are transformed during high-temperature sintering into more stable β-Si3N4. This is associated with growth in the structure of elongated plate-like Si3N4 grains (Fig. 8) that reinforce the material. Micron Si3N4 powder (d0.5 ≤ 1 μm) consisting of a mixture of α- and β-Si3N4 and eutectic oxide compositions, which are used as the activating sintering additives, are the starting components. Tables 8 and 9 present the properties of SSN and HPSN materials [136,137,138,139,140,141].

Fig. 8.
figure 8

Microstructure (a) and diffraction image (b ) of sintered 85 mass% Si3N4 + 15 mass% YAG.

Table 8. Physical and Mechanical Properties of Sintered SSN Ceramics
Table 9. Physical and Mechanical Properties of Hot-Pressed HPSN Materials

The advantages of these materials are the increased mechanical characteristics, in particular K1c, uniform distribution of reinforcement grains throughout the material bulk, high thermal stability and heat resistance, and corrosion and wear resistance of the composites.

The disadvantages of these materials are the high sintering temperature, large shrinkage during liquid-phase sintering, low throughput, and inability to produce items with complicated shapes during hot pressing.

Reaction-bonded silicon nitride (RBSN). The starting components for RBSN are Si3N4 particles, sometimes an Si3N4–SiC composite, and Si powder. Prepared blanks are sintered in an N2 atmosphere that reacts with the Si in the materials to form elongated grains of secondary Si3N4 by analogy with WCs that crystallize in intergrain gaps between particles of primary Si3N4 (Fig. 9) and enhance the mechanical characteristics of the material. Primary α-Si3N4 grains transform into the stable β-Si3N4 phase during sintering. This is associated with grain growth [142,143,144,145,146,147]. Small amounts (up to 5 mass%) of oxide additives are often added to the composition, which is sintered again at 1800 – 1900°C, further compacting the material (Table 10). In the final step, RBSN material is impregnated with ethylsilicate solution to reduce the surface porosity. The advantages of these materials are the low shrinkage during sintering, ability to produce large items by cold and hot slip casting, small amounts of additives, relatively low sintering temperature, use of large-sized starting Si3N4 powders, uniform distribution of reinforcement grains throughout the material bulk, and high thermal stability and corrosion resistance. The disadvantages are the long sintering process, high P of the final items, and low mechanical characteristics (Table 10).

Fig. 9.
figure 9

Microstructure of reaction-bonded Si3N4.

Table 10. Physical and Mechanical Properties of Reaction-Bonded Si3N4 Materials [142]

Liquid-phase sintered silicon carbide (LPSSiC). The activating additives for LPSSiC materials are oxides uniformly distributed throughout the material bulk. β-SiC grains transform into thermodynamically stable α-SiC grains during prolonged sintering (>4 h) if modified α- and β-SiC powders are used. The β → α-SiC phase transition is associated with growth of needle-like grains (Fig. 10) primarily perpendicular to the formation axis of blanks [148] that reinforce the composite by increasing its mechanical characteristics (Table 11) [149,150,151,152,153,154]. The advantages of these materials are the increased K1c, uniform distribution of reinforcement grains throughout the material bulk, high thermal stability and heat resistance, and corrosion and wear resistance. The disadvantages are the high sintering temperature, lack of domestically produced cubic SiC powders (β-SiC), significant reduction of several mechanical characteristics (Eel, σbend) as the amount of starting β-SiC powders is increased (50 vol%), and prolonged sintering (Table 11).

Fig. 10.
figure 10

Microstructure of liquid-phase-sintered 85 mass% β-SiC + 15 mass% YAG with various high-temperature delays: 1 (a) and 8 h (b ).

Table 11. Mechanical Properties of Self-Reinforced Liquid-Phase Sintered Materials

Conclusion

Production methods, signature features, advantages, and disadvantages of composites based on transition-metal carbides, nitrides, and borides; covalent compounds (SiC, Si3N4); and Al2O3 and reinforced by Cf, SiCf, SiCw, and Si3N4w were reviewed. The properties of Cf, Bf, SiCf (cored and coreless), oxide fibers, and WCs were studied. Production methods of reinforced composites including powder metallurgy, polymer impregnation and pyrolysis, infiltration by a chemically active metal, impregnation by a suspension, gas-phase saturation, and self-reinforcement during sintering were analyzed.