《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Progress in Silicon-Based Non-Oxide Structural Ceramics

Int J. of Refractory Metals Hard Materials 15(1997)13-4 Printed in Great Britain. All rights reser ELSEVIER PII:S0263·4368(96)00046-7 Progress in Silicon-Based Non-Oxide Structural Ceramics Wolfgang Dressler*& ralf riedel Fachgebiet Disperse Feststoffe, Fachbereich Materialwissenschaft, Technische Hochschule Darmstadt, Petersenstr 23 A, D-64287 Darmstadt, Germany (Received 1 August 1996; accepted 16 August 1996) Abstract: The progress in monolithic Si,N, and SiC as well as in Si3N4/SiC composites for structural applications is reviewed. The conventional processing including the powder synthesis, densification and microstructural design is dis- cussed. The mechanical properties of the resulting silicon based non-oxide cera- mics and their industrial applications as structural components are summarized As an alternative route to fabricate Si,N /SiC composites the hybrid processing utilizing the thermal conversion or organosilicon precursors to amorphous and polycrystalline multicomponent materials is described. The hybrid processed cera- mics exhibit ultra-high temperature stability with respect to crystallization, oxida- tion and decomposition. o 1997 Elsevier Science Limited 1 INTRODUCTION oxidizing environments only up to temperatures in the range of 1200-13000C. At higher tem- high hardness and strength, excellent creep, oxi- dized. even in the bulk. This behavior is dation and corrosion resistance as well as their related to the presence of sintering promoting low density, silicon nitride(Si3N4)and silicon compounds like Mgo, Al2O3 or Y2O3 which nitride/carbide(Si, N,/SiC) based ceramics and react with SiOz formed during the oxidation ceramic composites are promising candidate reaction to give low viscous silicates, (iii)th materials for high temperature applications in use of conventionally processed secondary motor and turbine devices and are frequently phase free SiC in the high temperature field is ed as cutting tools, a typical applio limited to low strength applications, ,> du hard materials. However, these materials still the reduced flaw tolerance caused by the low mechanical performance at temperatures above uid phase sintered Sic reveals a higher sensitiv- 1200oC is required. These limitations are due to ity towards oxidation owing to the reaction of the intrinsic properties of ceramics or result the used sintering aids with the oxide product from the used processing process and basically formed on the SiC surface,(v)the conven- an be attributed to the following points: (i) tional fabrication of dense Si3N,/SiC-composites ceramics are difficult to apply with high relia- is difficult due to the distinct sintering behavior bility owing to their intrinsically brittle behavior. of the Si3 na and Sic powder particles used as In contrast to this, metals are limited by corro- the starting materials. .8 However, significant sion problems and by reduced performance at improvements of the room and high-tempera temperatures approaching their melting point, ture properties of Si3 Na- and Sic-based cera- (ii) commercial Si3N, parts can be applied in mics like strength, fracture toughness, creep and oxidation resistance have been achieved by *Present address: Robert Bosch GmbH, FVIFLW, Post- tailoring of microstructure, -I7 by generation of fach 106050, D-70049, Stuttgar Si,N,/SiC micro/micro-., 18-20 or micro/nano- 13

ELSEVIER Int. J. of Refractory Metals & Hard Materials 15 (1997) 13-47 © 1997 Elsevier Science Limited Printed in Great Britain. All rights reserved 0263-4368/97/$17.00 PII: S0263-4368(96)00046-7 Progress in Silicon-Based Non-Oxide Structural Ceramics Wolfgang Dressier* & Ralf Riedel Fachgebiet Disperse Feststoffe, Fachbereich Materialwissenschaft, Technische Hochschule Darmstadt, Petersenstr. 23 A, D-64287 Darmstadt, Germany (Received 1 August 1996; accepted 16 August 1996) Abstract: The progress in monolithic Si3N4 and SiC as well as in Si3Nn/SiC￾composites for structural applications is reviewed. The conventional processing including the powder synthesis, densification and microstructural design is dis￾cussed. The mechanical properties of the resulting silicon based non-oxide cera￾mics and their industrial applications as structural components are summarized. As an alternative route to fabricate Si3N4/SiC composites the hybrid processing utilizing the thermal conversion or organosilicon precursors to amorphous and polycrystalline multicomponent materials is described. The hybrid processed cera￾mics exhibit ultra-high temperature stability with respect to crystallization, oxida￾tion and decomposition. © 1997 Elsevier Science Limited 1 INTRODUCTION Owing to their advantageous properties, like high hardness and strength, excellent creep, oxi￾dation and corrosion resistance as well as their low density, silicon nitride (Si3N4) and silicon nitride/carbide (Si3N4/SiC) based ceramics and ceramic composites are promising candidate materials for high temperature applications in motor and turbine devices and are frequently used as cutting tools, a typical application for hard materials. However, these materials still have application limits where reliability or mechanical performance at temperatures above 1200°C is required. These limitations are due to the intrinsic properties of ceramics or result from the used processing process and basically can be attributed to the following points: (i) ceramics are difficult to apply with high relia￾bility owing to their intrinsically brittle behavior. In contrast to this, metals are limited by corro￾sion problems and by reduced performance at temperatures approaching their melting point, (ii) commercial Si3N4 parts can be applied in *Present address: Robert Bosch GmbH, FVIFLW, Post￾fach 106050, D-70049, Stuttgart. 13 oxidizing environments only up to temperatures in the range of 1200-1300°C. At higher tem￾peratures, the material creeps' and is oxi￾dized 2"3 even in the bulk. This behavior is related to the presence of sintering promoting compounds like MgO, A1203 or Y203 which react with SiO2 formed during the oxidation reaction to give low viscous silicates, (iii) the use of conventionally processed secondary phase free SiC in the high temperature field is limited to low strength applications, 4"5 due to the reduced flaw tolerance caused by the low fracture toughness of additive free SiC, (iv) liq￾uid phase sintered SiC reveals a higher sensitiv￾ity towards oxidation owing to the reaction of the used sintering aids with the oxide product formed on the SiC surface, 6 (v) the conven￾tional fabrication of dense Si3Nn/SiC-composites is difficult due to the distinct sintering behavior of the Si3N 4 and SiC powder particles used as the starting materials. 7"8 However, significant improvements of the room and high-tempera￾ture properties of Si3N4- and SiC-based cera￾mics like strength, fracture toughness, creep and oxidation resistance have been achieved by tailoring of microstructure, 9-'7 by generation of Si3N4/SiC micro/micro -7'8'18-26 or micro/nano-

D R. Riede y H Fig. 1. Crystal structure of trigonal a-Si, Na(P31c). View nearly along z-axis. Silicon atoms in red and nitrogen atoms in Fig. 2. Crystal structure of hexagonal B-Si, N,(P63). View nearly along z-axis. Silicon atoms in red and nitrogen atoms In green

14 W. Dressier, R. Riedel H ! ..J Z Fig. 1. Fig. 2. Crystal structure of trigonal ~-Si,N4 (P31c). View nearly along z-axis. Silicon atoms in red and nitrogen atoms in green. Crystal structure of hexagonal/~-Si3N4 (P6~). View nearly along z-axis. Silicon atoms in red and nitrogen atoms in green

Silicon-based non-oxide structural ce H 3 Fig. 3. Crystal structure of hexagonal a-SiC polytype 6H(P6, mc). View nearly along y-axis. Silicon atoms in red and Fig. 4. Crystal structure of cubic B-SiC polytype 6H 3C. View nearly along [101]-direction. Silicon atoms in red and carbon atoms in blue

Silicon-based non-Qxide,~ructural ceramics 15 + , , . YT" Z 3 4 Fig. 3. Fig. 4. Crystal structure of hexagonal a-SiC polytype 6H (P63 mc). View nearly along y-axis. Silicon atoms in red and carbon atoms in blue. Crystal structure of cubic fl-SiC polytype 6H 3C. View nearly along [101]-direction. Silicon atoms in red and carbon atoms in blue

16 W. Dressler, R. riedel composites, by using high melting sintering AB except that it is rotated by 180% on the c- additives, 4 by reduction of additive content* axis. s Consequently, the channels present in and by devitrification of grain boundary pha- the B-modification are closed off into two inter ses.as reviewed in this article. Additionally, stices and the c-dimension of a-si3N an alternative route for the fabrication of silicon (c=0. 5617 nm) is approximately twice that of nitride, silicon carbide ceramics and Si3N,/SIC- B-Si3N4(c=0. 29107 nm). The lattice parameters composites, the hybrid processing, 4-0, 43,44 in the a directions are similar: a(a-Si3N4) shows exceptional possibilities to meet the =0.7818 nm, a(B-Si3 NA)=0-7595 nm aforementioned requirements. Here, organosili The technical synthesis routes lead mainly to con polymers(hybrids)are converted to silicon- a-Si,N, which converts to B-Si3 Na during liquid based inorganic materials this processing phase sintering. Grun" calculated the free technique not only the phase composition and enthalpy of a-to B-transformation at 10 Pa and microstructure but also the thermo-mechanical 298 K to be -30 kJ mol. Hampshire and properties of the final ceramics can be tailored Jack*/ reported that the activation energy for by the choice of organosilicon system, the the a-to B-transformation during liquid phase design of the intermediate processing steps(to sintering is similar to the dissociation energy of convert the system to an inorganic amorphous the Si-n bond, 435+38 kJ mol Thus the intermediate) and the use of annealing treat- mechanism of the reconstructive transformation ments(to transform the inorganic intermediate seems to be the breaking of Si-N bonds, the into the desired final crystalline ceramics) solution of the less stable a-phase in the formed This article in general reviews the progress in liquid phase and finally the reprecipitation of Si3N4 and Sic based materials achieved in the less soluble, more stable b-modification past decade. In particular, the fabrication and The technical production of Si,n4 powder properties of Si,N,/SiC composites derived predominantly performed on four different from (i) conventional processing and (ii) from routes: (i) the direct nitridation of silicon pow advanced techniques are comparatively dis- der,(ii) the carbothermal reduction and subse quent nitriding of Sio2,(ii the diimide process and, (iv) the gas phase reaction of silanes with ammonia 2 SILICON NITRIDE (Si,N,)-INTRINSIC The direct nitridation of elemental silicon to STRUCTURAL PROPERTIES AND achieve stoichiometric Si, Na was developed SYNTHESIS from Weiss and Engelhardt in 1910 and is still the most common industrial processing Si3 N, is a highly covalent compound(70% cova-route lence)having a density of 3 19g cm-3and 3Si+2N2-100140c occurring in two different crystal structures the a-and B-modification. The a-structure possesses AH=-750 kJ mol the space group P3lc the B-modification is hex The resulting SiN, powder is subsequently agonal(P63/m)as depicted in Figs 1 and 2. The milled and consists mainly of the a-modifica- polymorphs consist of slightly distorted tetra tion. In this connection, the choice of starting hedral SiN,(sp hybridization of Si)and planar silicon powder quality (grain size and purity) NSi,(sp- hybridization of f N units The SiN determines on the one hand the price of the tetrahedrons are joined by sharing nitrogen cor- resulting Si,N, and on the other hand the purity ners so that each nitrogen is common to three of the product. Especially the use of semicon tetrahedra. The unit cells of a-Si3n4 and ductor silicon leads to extremely pure Si, NA- B-Si3NA are represented by Si12N16 and Sions, powders. 4 The carbothermal reduction method respectively, as shown in Figs 1 and 2. The starts from a mixture of fine Sio, and carbon B-structure is composed of puckered rings of powder. This mixture is converted into Si N4 at alternating Si andn atoms having a stacking 1500 C in flowing N2 quence of ABAB+ and forming channels 1500°C ( diameter about 0. 15 nm) along the c-direction 3SiO2+6C+2N2 Si3N4+6Co(2) The a-modification contains the same AB layer In order to convert the initial SiOz completel and an additional layer CD which is similar to into Si3n4 excess carbon is necessary. .>3 By

16 IV.. Dressier, R. Riedel composites, 27-39 by using high melting sintering additives, 4°'4~ by reduction of additive content 43 and by devitrification of grain boundary pha￾ses 3"42 as reviewed in this article. Additionally, an alternative route for the fabrication of silicon nitride, silicon carbide ceramics and Si3N4/SiC￾composites, the hybrid processing, 34-36"43"44 shows exceptional possibilities to meet the aforementioned requirements. Here, organosili￾con polymers (hybrids) are converted to silicon￾based inorganic materials. Using this processing technique not only the phase composition and microstructure but also the thermo-mechanical properties of the final ceramics can be tailored by the choice of organosilicon system, the design of the intermediate processing steps (to convert the system to an inorganic amorphous intermediate) and the use of annealing treat￾ments (to transform the inorganic intermediate into the desired final crystalline ceramics). This article in general reviews the progress in Si3N 4 and SiC based materials achieved in the past decade. In particular, the fabrication and properties of Si3N4/SiC composites derived from (i) conventional processing and (ii) from advanced techniques are comparatively dis￾cussed. 2 SILICON NITRIDE (Si3N4) -- INTRINSIC STRUCTURAL PROPERTIES AND SYNTHESIS Si3N4 is a highly covalent compound (70% cova￾lence) having a density of 3.19g cm -3 and occurring in two different crystal structures the a- and fl-modification. The a-structure possesses the space group P31c the fl-modification is hex￾agonal (P63/m) as depicted in Figs 1 and 2. The polymorphs consist of slightly distorted tetra￾hedral SiN4 (sp 3 hybridization of Si) and planar NSi3 (sp 2 hybridization of N) units. The SiN4 tetrahedrons are joined by sharing nitrogen cor￾ners so that each nitrogen is common to three tetrahedra. The unit cells of a-Si3N4 and fl-Si3N 4 are represented by Si12Ni6 and Si6Ns, respectively, as shown in Figs 1 and 2. The fl-structure is composed of puckered rings of alternating Si and N atoms having a stacking sequence of ABAB 45 and forming channels (diameter about 0.15 nm) along the c-direction. The a-modification contains the same AB layer and an additional layer CD which is similar to AB except that it is rotated by 180 ° on the c￾axis. 45 Consequently, the channels present in the fl-modification are closed off into two inter￾stices and the c-dimension of a-Si3N 4 (c=0.5617nm) is approximately twice that of fl-Si3N4 (c=0.29107 nm). The lattice parameters in the a directions are similar: a(a-Si3N4) =0.7818 nm, a(fl-Si3N4)=0.7595 nm. 46 The technical synthesis routes lead mainly to a-Si3N4 which converts to fl-Si3N4 during liquid phase sintering. Grfin 46 calculated the free enthalpy of a- to fl-transformation at 105 Pa and 298 K to be -30kJ mol-'. Hampshire and Jack 47 reported that the activation energy for the a- to fi-transformation during liquid phase sintering is similar to the dissociation energy of the Si-N bond, 435+38kJ mol -~. Thus the mechanism of the reconstructive transformation seems to be the breaking of Si-N bonds, the solution of the less stable a-phase in the formed liquid phase and finally the reprecipitation of less soluble, more stable fi-modification. The technical production of Si3N 4 powder is predominantly performed on four different routes: (i) the direct nitridation of silicon pow￾der, (ii) the carbothermal reduction and subse￾quent nitriding of SiO2, (iii) the diimide process and, (iv) the gas phase reaction of silanes with ammonia. The direct nitridation of elemental silicon to achieve stoichiometric Si3N4 was developed from Weiss and Engelhardt in 191048 and is still the most common industrial processing route 5,.52 3Si + 2N2 1 lOO-14oo°c ~" Si3N4 AH= -750 kJ mol-1. (1) The resulting Si3N4 powder is subsequently milled and consists mainly of the a-modifica￾tion. In this connection, the choice of starting silicon powder quality (grain size and purity) determines on the one hand the price of the resulting Si3N 4 and on the other hand the purity of the product. Especially the use of semicon￾ductor silicon leads to extremely pure Si3N n￾powders? 4 The carbothermal reduction method starts from a mixture of fine SiO2 and carbon powder. This mixture is converted into Si3N 4 at 1500°C in flowing N~. 3Si02+6C+2Nz 15oooc > Si3N4+6C0 (2) In order to convert the initial SiOz completely into Si3N4 excess carbon is necessary. 5~'53 By

Silicon-based non-oride structural ceramics sing quartz sand and clay minerals low-priced pure and fine Si,N4 powder. The diimide pro oroducts are achievable 4, 55 Highly pure pow- cess is industrially used to process Sian, por ders can be produced by applying synthetic ders for advanced applications tarting materials derived from pyrolytic or Owing to the predominantly homogeneous ol- gel reactions nucleation, the reaction of SiCa or SiH4 with The liquid phase reaction of SiCl4 and NH3 NH3 in the gas phase provides fine and pure forming Si(NH)2 has been firstly performed by Si, powders with large specific surfaces Persoz in 1830 and has been investigated in (2-20 m" g)and high sintering activities detail by Blix and Wirbelauer, 7 Glemser and Similar to the liquid phase reaction of Sicl4 Naumann and Mazdiyasni and Cooke 9 later. with NH,(see eqn(3))amorphous Si3n4 pre- Billy showed that the ammonolysis of Sicla cursors are formed which have to be thermally under these conditions results in a polymerized converted to crystalline Si, N4. The use of the polysilicon diimide low priced Sicla requires the extraction of the n SiCl+6n nh C-RT [Si(NH)2I by-product NHCl(see eqn (3)). In contrast to this only H2 is evolved during the ammonolysis +anNeCI (3) of the expensive and spontaneous inflammable During the subsequent calcination the gener- (in air)SiHa. The synthesized amorphous pre- ated silicium diimide transforms into an amor- ceramic compounds are crystallized to mainly phous Si,N, accompanied by the evolution of -SiaNa at temperatures between 1200 and NH, or N,/Hz. At temperatures above 1200 C 1500C under nitrogen. Both methods are used the diffusion controlled crystallization to in industry to process commercially availab a-Si, n takes place possessing an activation Si,, qualities to produce engine and turbine energy of 306 kJ mol parts. In Table 1 the properties of some com mercially available Si3N4 powders are summa rized [SI(NH)]. 1200-1400°C a-Si3N +N2+3H 3 SILICON CARBIDE (SiC)-INTRINSIC The particle morphology, particle size and STRUCTURAL PROPERTIES AND phase composition is determined by the pro- SYNTHESIS cessing parameters such as temperature, reac tion time and impurities. 2. 0. The liquid phase The fundamental structural elements of the reaction of SiCl, with NH, gives extraordinary various polytypes of Sic are covalently(88% Table 1. Characteristics of commercially available Si, N,-powders derived from diffe measurements by the authors Powder type SN-E10 SN-ESP Grade GP LC 12-SX A200 Production process Liquid phas Liquid pha Gas phase Direct diimide nitridation Manufacturer Ube Industries, Ube Industries, H C Starck, H.C. Starck, Toshiba Ceramics, Tokyo Tokyo Berlin Tokyo Impurities(wt%) 18-21,(15) 0 <0001 <001 <0005 <0002 <0002 pecinc surtace are (98) Mean particle size 05,(055) 05,(079) 06,(048) 阝-SiN4(wt%) 5,(41) <10,(64)

Silicon-based non-oxide structural ceramics 17 using quartz sand and clay minerals low-priced products are achievable? 4"55 Highly pure pow￾ders can be produced by applying synthetic starting materials derived from pyrolytic 53 or sol-gel 52 reactions. The liquid phase reaction of SIC14 and NH3 forming Si(NH)2 has been firstly performed by Persoz 56 in 1830 and has been investigated in detail by Blix and Wirbelauer, 57 Glemser and Naumann 58 and Mazdiyasni and Cooke 59 later. Billy" showed that the ammonolysis of SiCI4 under these conditions results in a polymerized polysilicon diimide. nSiC14 + 6nNH30°C-- RT) [Si(NH)2], +4nNH4C1. (3) During the subsequent calcination the gener￾ated silicium diimide transforms into an amor￾phous Si3N 4 accompanied by the evolution of NH3 or NJH2. At temperatures above 1200°C the diffusion controlled crystallization to ~-Si3N461 takes place possessing an activation energy of 306 kJ moli 1200-- 14OO°C -- [Si(Nn)2], , ) ~-Si3N4 n pure and fine Si3N4 powder. The diimide pro￾cess is industrially used to process Si3N4 pow￾ders for advanced applications. Owing to the predominantly homogeneous nucleation, the reaction of SiCl4 or SiH4 with NH3 in the gas phase provides fine and pure Si3N4 powders with large specific surfaces 64"65 (2-20m 2 g-') and high sintering activities. Similar to the liquid phase reaction of SiCl4 with NH~ (see eqn (3)) amorphous Si3N4 pre￾cursors are formed which have to be thermally converted to crystalline Si3N 4. The use of the low priced SiCl4 requires the extraction of the by-product NH4C1 (see eqn (3)). In contrast to this only H2 is evolved during the ammonolysis of the expensive and spontaneous inflammable (in air) Sill4. The synthesized amorphous pre￾ceramic compounds are crystallized to mainly ~-Si3N 4 at temperatures between 1200 and 1500°C under nitrogen. Both methods are used in industry to process commercially available Si3N 4 qualities to produce engine and turbine parts. In Table 1 the properties of some com￾mercially available Si3N4 powders are summa￾rized. +N2+3H2 (4) The particle morphology, particle size and phase composition is determined by the pro￾cessing parameters such as temperature, reac￾tion time and impurities. 62"63 The liquid phase reaction of SiCl4 with NH3 gives extraordinary 3 SILICON CARBIDE (SIC) -- INTRINSIC STRUCTURAL PROPERTIES AND SYNTHESIS The fundamental structural elements of the various polytypes of SiC are covalently (88% Table 1. Characteristics of commercially available Si3N4-powders derived from different production processes. () Means measurements by the authors Powder type SN-E I O SN-ESP Grade G P LC 12-SX A 200 Production process Liquid phase Liquid phase Gas phase Direct Carbothermal diimide diimide nitridation reduction Manufacturer Ube Industries, Ube Industries, H.C. Starck, H.C. Starck, Toshiba Ceramics, Tokyo Tokyo Berlin Berlin Tokyo Impurities (wt%) O <2 (1.1) (1.0) 1.1-1.6 1.8-2.1, (1.5) 2.0 C <0-2 <0.2 <0.05 <0.2 0.9 CI < 0-01 0.01 < 0.1 < 0.001 -- Fe < 0.01 0.01 < 0.01 < 0.008 0.007 A1 < 0-005 -- < 0.004 < 0.005 0.2 Ca < 0-005 < 0.002 < 0.002 < 0.002 0.01 Specific surface area (9.8) (7.5) (12.2) (21.4) -- (m 2 g ') Mean particle size ds,, (#m) fl-Si3N4 (wt%) 0.5, (0.55) (0.64) 0.5, (0.79) 0.6, (0.48) 0.9 <5, (4.1) (<3) <10, (6.4) s8, (5-6) 2