7)179-184 C 1997 Elsevier Sciene PII:s0272-8842(96)00021 0272884219751700+00 Preparation of High Strength Ceramic Fibre Reinforced silicon Nitride Composites by a Preceramic polymer Impregnation Method H Morozumi, K. sato, A. Tezuka, H. Kaya &T Isoda Corporate Research and Development Laboratory, TONEN Corporation, 1-3-1 Nishiturugaoka, Ohimachi, Saitama 356, Japan Received 11 September 1995; accepted 20 December 1995) Abstract: High strength ceramic composites, reinforced with Si-N and Si-B-0-N ceramic fibres, were prepared by the preceramic polymer impregnation inethod Low-viscosity polysilazanes with a 70-90 wt% ceramic yield, were used as a matrix precursor. To obtain dense composites, the impregnation, curing and firing processes were repeated. Three-point flexural strength of the UD and 2D (cross-ply) ceramic fibre reinforced ceramic composites were shown to be as high as 1000 and 600 MPa, respectively, This resulted from the dense silicon nitride matrix and controlled fibre/matrix interface. c 1997 Elsevier Science Limite 1 INTRODUCTION pressure.(2)There are no aids in firing, so high purity ceramics are readily obtained. (3)Can be Ceramic fibre reinforced ceramic matrix compo- applied to complex and near-net shaped parts. (4) sites(CMCs)are being developed as excellent Dense and homogeneous matrices are readily materials with high strength and oxidation resis- obtained. Accordingly, with high expectation of tance at high temperature. Several processes, PCPl, our development activity has continued. In ncluding preceramic polymer impregnation addition, TONEN's preceramic polymer technolo (PCPD), 3chemical vapour infiltration(CVI), 4, gies have provided stoichiometric, homogeneous reaction bonding and hot-pressing, have been and high purity silicon nitride fibre(SNF). -9The employed for the fabrication of continuous fibre Snf has excellent properties, including high reinforced composites. The strength of the compo- strength and oxidation resistance at high tempera site that is obtained by the cvi process is relatively low, because the density of the matrix is as low as boron nitride fibre(SNBF)that has good potential 80%0 of the theoretical value. The reaction at elevated temperatures was described by bonding method provides residual metal in the Funayama et al. o We have reported on unidirec product, so the strength decreases at elevated tcm- tional(UD) carbon fibre reinforced composites peratures The hot-pressing method will degrade and UD SNF reinforced composites, prepared by the fibre reinforcement through treatment at high PCPI 3.1 Schwab et al. have reported on 2D temperature and pressure, and is not amenable to (plain weave laminate) silicon carbide fibre rein the production of complex shapes forced composites by a similar process. 12An PCPI is the fabrication process of CMCs derived initial attempt is made in this paper to apply SNF trom organometallic or inorganic precursors. The and SNBF in the fabrication of 2D(cross-ply) advantages of PCPI are as follows: (1) There is no composites. Furthermore, this paper describes the damage to the reinforcement because the firing influence of polysilazanes on the properties of process is at low temperature and atmospheric resultant products
PII: SO272-8842(96)00021-l Ceramics International 23 (1997) 179-I 84 0 1997 Elsevier Science Limited and Techna S.r.1. Printed in Great Britain. All rights reserved 0272~8842/97 $17.00+ .OO Preparation of High Strength Ceramic Fibre Reinforced Silicon Nitride Composites by a Preceramic Polymer Impregnation Method H. Morozumi, K. Sato, A. Tezuka, H. Kaya &T. lsoda Corporate Research and Development Laboratory, TONEN Corporation, 1-3-1 Nishiturugaoka, Ohimachi, Saitama 356, Japan (Received 11 September 1995; accepted 20 December 1995) Abstract: High strength ceramic composites, reinforced with Si-N and Si-B-O-N ceramic fibres, were prepared by the preceramic polymer impregnation method. Low-viscosity polysilazanes with a 70-90 wt% ceramic yield, were used as a matrix precursor. To obtain dense composites, the impregnation, curing and firing processes were repeated. Three-point flexural strength of the UD and 2D (cross-ply) ceramic fibre reinforced ceramic composites were shown to be as high as 1000 and 600 MPa, respectively. This resulted from the dense silicon nitride matrix and controlled fibre/matrix interface. 0 1997 Elsevier Science Limited and Techna S.r.1. 1 INTRODUCTION Ceramic fibre reinforced ceramic matrix composites (CMCs) are being developed as excellent materials with high strength and oxidation resistance at high temperature. Several processes, including preceramic polymer impregnation (PCPI),ip3 chemical vapour infiltration (CVI),4,5 reaction bonding and hot-pressing,6 have been employed for the fabrication of continuous fibre reinforced composites. The strength of the composite that is obtained by the CVI process is relatively low, because the density of the matrix is as low as 70-80% of the theoretical value. The reaction bonding method provides residual metal in the product, so the strength decreases at elevated temperatures. The hot-pressing method will degrade the fibre reinforcement through treatment at high temperature and pressure, and is not amenable to the production of complex shapes. PCPI is the fabrication process of CMCs derived from organometallic or inorganic precursors. The advantages of PCPI are as follows: (1) There is no damage to the reinforcement because the firing process is at low temperature and atmospheric pressure. (2) There are no aids in firing, so high purity ceramics are readily obtained. (3) Can be applied to complex and near-net shaped parts. (4) Dense and homogeneous matrices are readily obtained. Accordingly, with high expectation of PCPI, our development activity has continued. In addition, TONEN’s preceramic polymer technologies have provided stoichiometric, homogeneous and high purity silicon nitride fibre (SNF).7-9 The SNF has excellent properties, including high strength and oxidation resistance at high temperature. Recently, progress in continuous silicon boron nitride fibre (SNBF) that has good potential at elevated temperatures was described by Funayama et al. lo We have reported on unidirectional (UD) carbon fibre reinforced composites and UD SNF reinforced composites, prepared by PCPI.3,‘t Schwab et al. have reported on 2D (plain weave laminate) silicon carbide fibre reinforced composites by a similar process.i2 An initial attempt is made in this paper to apply SNF and SNBF in the fabrication of 2D (cross-ply) composites. Furthermore, this paper describes the influence of polysilazanes on the properties of resultant products. 179
H. Morozumi et al 2 EXPERIMENT Table 1. The properties of the ToNEN poysilazanes Molecula Density 2. 1 Matrix precursor (mPas)at30°c Perhydropolysilazane(PHPS) and methylhydro PHPS 800 1.1-1.2 1050 SNC 11-1.2 4080 oligosilazane (SNC) were used as matrix pre- cursors. PHPs was synthesized by ammonolysis of a dichlorosilane pyridine adduct. SNC was Table 2. The properties of the toNEN fibres synthesized by coammonolysis of a dichlorosilane- SNF pyridine adduct and a monomethyldichlorosilane- pyridine adduct. 3 The properties of PHPs and Tensile strength(GPa) SNC are shown in Table 1. These polymers are Tensile modulus(GPa) 200 200 Density transparent liquids with low viscosity, and have a Typical composition si 596 thermosetting property. By heating to 100-300 C. ( % a thermal crosslink BNcO 37.1 328 oceeds, and glass- like hard 4 solids are obtained By firing to 800-1500oC in 2.7 7.6 nitrogen or inert gases, the cured products convert to amorphous silicon nitride based ceramics with 70-90 wt% yield. The yield is extremely high in nation, curing and firing processes were performed preceramic polymers(e.g. repeatedly polycarbosilane lastylene, etc. ) that have 50-60 wt% yield. The differences between PHPS 2.3.2 2D(Cross-ply composites) and SNC are shown in the composition. The SNC was used as a matrix precursor, because the H NMR spectrum of SNC exhibits a peak cor- UD composite, SNF/SNC, showed higher flexural responding to SiMe due to the addition of a strength than the SNF/PHPS. SNF and SNBF monomethyldichlorosilane. SNC consists mainly of were used as reinforcement. 2D(cross-ply)compo- structured units of SiH,NH and SihMeNH sites were fabricated as follows. A unidirectional fibre aligned preform w d by windi 2.2 Ceramic fibre strand. which was infiltrated with snc. on to a drum. To obtain a UD prepreg, the preform was Two kinds of ceramic fibres, produced by pressed to a thickness of 0. 2 mm ToNEN, were used as reinforcements. SNF The prepreg was then cut and stacked in the derived from perhydropolysilazane SNBF is derived structure of [0/90/90/012. The stacked prepregs from polyborosilazane. 10 The properties of both were cured in a n2 atmosphere with a pressure of fibres are shown in Table 2. The surfaces of these 0.05-0. 1 mPa at 100-300 C for 1-3 h. The cured fibres were coated with a C-rich layer by chemical body was then fired in a n2 atmosphere to 1350.C vapour deposition(SiClg-CHa gases system). The To densify the products the same repeating process purpose of this fibre coating is not only to prevent as in the Ud composites was performed reaction between fibre and matrix, but also to give adequate interface shear strength. The thickness of 2. 4 Characterization the coated layer was about 0. I um. The densification of the fired material in the repeat- 2.3 Fabrication procedure ing proccss was monitored by mcasuring bulk dcn sities. The products were characterized by SEM 23. I Ud(Unidirectional fibre reinforced AES, X-ray diffraction analysis, and a three-point composites bending test at room temperature.( For cross-ply PHPS and SNC were used as matrix precursors. composites, the high temperature strength was UD composites, SNF/PHPS and SNF/SNC, were measured at 1250%C in a n2 atmosphere. The three fabricated as shown in Fig. 1. A unidirectional point bending test was performed in accordance SNF aligned preform was prepared by winding a with JIs R1601 except for thickness of sample strand, which was infiltrated with precursor, on to Samples were cut into 1.0-1. 2 x 4 x 40 mm bars a mandrel. The preform was pressed to a thickness and tested with a span length of 30 mm at a cross of 5 mm, and cured in a n2 atmosphere with a head speed of 0.5 mm/min JIS R1601 is not always pressure of 0.050. 1 MPa at 100-300C for 1-3 h. advantageous to continuous fibre reinforced com- The cured body was then fired in a n2 atmosphere posites which show non-brittle failure, since this to 1350C. To densify the product, the impreg- method shows a complex fracture mixed with tensile
180 H. Morozumi et al. 2 EXPERIMENT Table 1. The properties of the TONEN poysilazanes 2.7 Matrix precursor Molecular weight (A&) Density Viscosity (mPa+) at 30°C Perhydropolysilazane (PHPS) and methylhydrooligosilazane (SNC) were used as matrix precursors. PHPS was synthesized by ammonolysis of a dichlorosilane-pyridine adduct. SNC was synthesized by coammonolysis of a dichlorosilanepyridine adduct and a monomethyldichlorosilanepyridine adduct. l3 The properties of PHPS and SNC are shown in Table 1. These polymers are transparent liquids with low viscosity, and have a thermosetting property. By heating to 10&3OO”C, a thermal crosslinking proceeds, and glass-like hard solids are obtained. By firing to 800-1500°C in nitrogen or inert gases, the cured products convert to amorphous silicon nitride based ceramics with 70-90 wt% yield. The yield is extremely high in comparison to other preceramic polymers (e.g. polycarbosilane, polysilastylene, etc.) that have 50-60 wt% yield. The differences between PHPS and SNC are shown in the composition. The ‘H NMR spectrum of SNC exhibits a peak corresponding to SiMe due to the addition of a monomethyldichlorosilane. SNC consists mainly of structured units of SiHzNH and SiHMeNH. 2.2 Ceramic fibre Two kinds of ceramic fibres, produced by TONEN, were used as reinforcements. SNF is derived from perhydropolysilazane. SNBF is derived from polyborosilazane. lo The properties of both fibres are shown in Table 2. The surfaces of these fibres were coated with a C-rich layer by chemical vapour deposition (SiC14-CH4 gases system). The purpose of this fibre coating is not only to prevent reaction between fibre and matrix, but also to give adequate interface shear strength. The thickness of the coated layer was about 0.1 pm. 2.3 Fabrication procedure 2.3.1 UD (Unidirectionaljibre reinforced composites) PHPS and SNC were used as matrix precursors. UD composites, SNF/PHPS and SNF/SNC, were fabricated as shown in Fig. 1. A unidirectional SNF aligned preform was prepared by winding a strand, which was infiltrated with precursor, on to a mandrel. The preform was pressed to a thickness of ~5 mm, and cured in a N2 atmosphere with a pressure of 0.05-0.1 MPa at 10&3Oo”C for l-3 h. The cured body was then fired in a N2 atmosphere to 1350°C. To densify the product, the impregPHPS 800-I 000 1.1-1.2 1 O-50 SNC 800-I 000 1.1-1.2 40-80 Table 2. The properties of the TONEN fibres SNF SNBF Tensile strength (GPa) 2.2 Tensile modulus (GPa) 200 Density 2.5 Typical composition Si 59.6 (%) B - N 37.1 : 0.4 27 2.2 200 2.5 48.0 3528 5:4 7.6 nation, curing and firing processes were performed repeatedly. 2.3.2 20 (Cross-ply composites) SNC was used as a matrix precursor, because the UD composite, SNF/SNC, showed higher flexural strength than the SNF/PHPS. SNF and SNBF were used as reinforcement. 2D (cross-ply) composites were fabricated as follows. A unidirectional fibre aligned preform was prepared by winding a strand, which was infiltrated with SNC, on to a drum. To obtain a UD prepreg, the preform was pressed to a thickness of ~0.2 mm. The prepreg was then cut and stacked in the structure of [O/90/90/O]2. The stacked prepregs were cured in a N2 atmosphere with a pressure of 0.05-0.1 MPa at IOO-300°C for l-3 h. The cured body was then fired in a N2 atmosphere to 1350°C. To densify the products, the same repeating process as in the UD composites was performed. 2.4 Characterization The densification of the fired material in the repeating process was monitored by measuring bulk densities. The products were characterized by SEM, AES, X-ray diffraction analysis, and a three-point bending test at room temperature. (For cross-ply composites, the high temperature strength was measured at 1250°C in a N2 atmosphere.) The threepoint bending test was performed in accordance with JIS R1601 except for thickness of sample. Samples were cut into 1.0-1.2 x 4 x 40 mm bars and tested with a span length of 30 mm at a crosshead speed of 0.5 mm/min. JIS R1601 is not always advantageous to continuous fibre reinforced composites which show non-brittle failure, since this method shows a complex fracture mixed with tensile
Preparation of fibre reinforced Si3 N, composites 2.6 24 2.2 Drum Winding Cutting and Curing Firing N2100300cN2,1350°C 18 1.4 Impregnation and Curing Firing Product 12 N2,1350° npregnation and Firing Times Fig. 1. Fabrication process of continuous fibre reinforced Si-N composite Fig. 2. Densification curve of UD composite and shear failure. In order to fracture samples by a As is obvious from Fig. 2, the steps of PCPI have tensile stress, the ratios of span to thickness were theoretically proceeded et to 25-30. In addition, for UD composites, an Mechanical properties of UD composites are interlaminar shear strength(ILSS)test was also shown in Table 3. The stress-deflection curve performed in accordance with JIS K7057 For the shown in Fig 3. The SNF/SNC composite demon 2D samples the direction of the fibre on a surface strates a flexural strength of over I GPa at room layer was aligned to the orientation of the tensile temperature. We have reported the effect of coating stress. a true density was measured with a pycno- on the fibre, 4 in the carbon coated case, 2D (plain meter by using n-decane as the medium. The total weave laminate)silicon carbide fibre reinforced porosity was calculated by the true density for 2D composite has demonstrated a non-brittle failure with fibre pull-out and a high flexural strength of 294 MPa. On the contrary, in the non-coated case, the composite has shown catastrophic failure and a 3 RESULTS AND DISCUSSION relatively low flexural strength of 60 MPa. In this study, the SNF reinforced composites have also 3.1 UD (Unidirectional fibre reinforced composites) shown a non-brittle failure, similar to the one in the carbon coated case. Therefore, this suggests that The densification curves are shown in Fig. 2. the coating on the fibre is essential for an adequate Because of the shrinkage of precursors in firing, interface shear strength in CMCs reimpregnation of polymers is required to densify The auger depth profiles of coated SNF surface the product. The curve for the product agrees well show that a carbon-rich layer, which is con with the theoretical curve calculated from eqn(1). aminated with silicon, is deposited(Fig. 4). In an (The impregnation efficiency is assumed to be 0.9. attempt to perform auger analysis on the fractu Equation(1)is defined as follows: Pc(n)=Pm(1 -Vr)+prVr-pmPe Table 3. The properties of CMcs prepared by the PCPI method Firing condition 1350C: 8 times 1350.C: 7 times Pm)=Pn-1)(1-ICYPp/pm) Fibre SNF SNF SNF SNBF P(0)=(1-H) Matrix precursor SNC SNC where n is the number of firing times, pc, pr, Pm and Total porosity(%) Flexural strength R.T. 649 1049 618 627 Pp are the densities of composites, fibres, matrix (fired polymers)and polymers, respectively, Vr and ILSS (MI P are the volume fraction of fibres and porosities, A(%a) 808595.5945 y is the char yield of polymers, I is the impregna- R: apparent effectiveness =0c/(o,V,A): A is assumed to be tion efficiency and C is the polymer concentration 10UD)and0.5(2D)
Preparation of jibre reinforced S&N4 composites 181 Drum Winding Cutting and Curing Firing N2.100-300°C N2,13!X”C L+, + ;: :. ;; : t Polysilazane T’ Impregnation and Curing \ fifing Nz, loo-300°C N2,13WC i83ma f .“zAhr Product Fig. 1. Fabrication process of continuous fibre reinforced Si-N composite. and shear failure. In order to fracture samples by a tensile stress, the ratios of span to thickness were set to 25-30. In addition, for UD composites, an interlaminar shear strength (ILSS) test was also performed in accordance with JIS K7057. For the 2D samples, the direction of the fibre on a surface layer was aligned to the orientation of the tensile stress. A true density was measured with a pycnometer by using n-decane as the medium. The total porosity was calculated by the true density for 2D composites. 3 RESULTS AND DISCUSSION 3.1 UD (Unidirectional fibre reinforced composites) The densification curves are shown in Fig. 2. Because of the shrinkage of precursors in firing, reimpregnation of polymers is required to densify the product. The curve for the product agrees well with the theoretical curve calculated from eqn (1). (The impregnation efficiency is assumed to be 0.9.) Equation (1) is defined as follows: Pc(n) = Pnl(l - Vf) + PfVf - PnJ(n-I) (1) with P(n) = P(n-I)(1 - ~CY&hn) P(0) = (1 - Vf) (24 (2b) where n is the number of firing times, pc, pr, pm and pi, are the densities of composites, fibres, matrix (fired polymers) and polymers, respectively, Vr and P are the volume fraction of fibres and porosities, Y is the char yield of polymers, I is the impregnation efficiency and C is the polymer concentration. 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 Polycarbosilant I 1 3 5 7 Impregnation and Firing Times Fig. 2. Densification curve of UD composite. As is obvious from Fig. 2, the steps of PCPI have theoretically proceeded. Mechanical properties of UD composites are shown in Table 3. The stress-deflection curve is shown in Fig. 3. The SNFjSNC composite demonstrates a flexural strength of over 1 GPa at room temperature. We have reported the effect of coating on the fibre,i4 in the carbon coated case, 2D (plain weave laminate) silicon carbide fibre reinforced composite has demonstrated a non-brittle failure with flbre pull-out and a high flexural strength of 294 MPa. On the contrary, in the non-coated case, the composite has shown catastrophic failure and a relatively low flexural strength of 60 MPa. In this study, the SNF reinforced composites have also shown a non-brittle failure, similar to the one in the carbon coated case. Therefore, this suggests that the coating on the fibre is essential for an adequate interface shear strength in CMCs. The auger depth profiles of coated SNF surface show that a carbon-rich layer, which is contaminated with silicon, is deposited (Fig. 4). In an attempt to perform auger analysis on the fracture Table 3. The properties of CMCs prepared by the PCPI method Type UD 2D Firing condition 1350°C : 8 times 1350°C : 7 times Fibre SNF SNF SNF Matrix precursor PHPS SNC SNC Bulk density 2.54 2.35 2.36 Vf @I 43 54 57 Total porosity (%) - 7.2 Flexural strength R.T. 649 1049 618 (MPa) 1250°C 546 ILSS (MPa) 55.1 45.4 - Ra (%) 80 85 95.5 SNBF SNC 2.38 58 5.8 627 595 94.5 “I?: apparent effectiveness = o,/(ofV,A); A is assumed to be 1 .O (UD) and 0.5 (2D)
1200 3.0 乏1000 2.8 FPHPS 日800 C-SNC 巴2.6 60 SNF/SNC SNF/PHPS 200 22 0040.8121.62.0 Displacement (mm) 2.0 Fig 3. Stress-deflection curves of UD composites 100011001200130014001500 Firing Temperature('C PHPS Si c SNC tter Times(min) Fig. 4. AES depth profiles of coated SNF. 5. The variation of polysilazanes in pyrolysis. (a) truc density as a function of firing temperature.(b) XRD patterns of polysilazanes pyrolysed at 1350C surface of the composite, the fibre pull-out has would inherently w are similar in Si based com- an increase in density. provided similar profiles as above. These results Since both matrices are would suggest that the pulled out face is the inter- ponent and structure, densification(an increase in face between the coated surface and the matrix. It atomic density) would provide an increase in elas- is reasonable to speculate that coating the fibre tic modulus. The true density of fired SNC and would give adequate interface shear strength by PHPS are 2.5 and 2. 8 g/cm, respectively. there controlling the interactie fore, it is presumed that the elastic modulus(em) The properties of composites are different of pyrolysed PHPS was much larger than that of according to the kinds of matrix precursor. The pyrolysed SNC. If Er> Em, the stress in the fibre is flexural strength at room temperature of SNF/ greater than in the matrix, because the fibre bears PHPS and SNF/SNC are 649 MPa and the major part of the applied load. This effect 1049 MPa, respectively. This difference is due to results in an increase in the effectiveness of fibre the crystallization behaviour of the precursors. strcngth to compositc strength. The flexural Though PHPS is selected as a high impregnation strength of samples using SNC matrix has become precursor with low viscosity, this PHPS contains high as noted above. To neglect the influence of excess Si compared with the stoichiometric com- the amount of scatter in the Vr on the composite position of silicon nitride. The deviation from the strength, the apparent effectiveness of fibre heoretical Si/N ratio results in low crystallization strength to composite strength(R)is defined as temperature ( 1200C). On the other hand, follows SNCs crystallization temperature is above R=Oe/(orVrA) 1.c, because snc does not contained excess Si, and its pyrolysis product has a stoichiometric where oc and or are the strength of composites and composition of a complex silicon nitride and sili- fibres, and a is the ratio of the fibre which is con carbide. The true density and X-ray diffraction aligned to the orientation of the tensile stress. For atterns of pyrolysed polymers are presented in UD and 2D composites, A is assumed to be 1.0 and Fig. 5. In preceramic poly le cry 0.5, ively
182 H. Morozumi et al. 0 0.4 0.8 1.2 1.6 2.0 Displacement (mm) Fig. 3. Stresswleflection curves of UD composites Sputter Times (min) Fig. 4. AES depth profiles of coated SNF. surface of the composite, the fibre pull-out has provided similar profiles as above. These results would suggest that the pulled out face is the interface between the coated surface and the matrix. It is reasonable to speculate that coating the fibre would give adequate interface shear strength by controlling the interaction. The properties of composites are different according to the kinds of matrix precursor. The flexural strength at room temperature of SNF/ PHPS and SNF/SNC are 649 MPa and 1049 MPa, respectively. This difference is due to the crystallization behaviour of the precursors. Though PHPS is selected as a high impregnation precursor with low viscosity, this PHPS contains excess Si compared with the stoichiometric composition of silicon nitride. The deviation from the theoretical Si/N ratio results in low crystallization temperature (M 1200°C). On the other hand, SNC’s crystallization temperature is above 15OO”C, because SNC does not contained excess Si, and its pyrolysis product has a stoichiometric composition of a complex silicon nitride and silicon carbide. The true density and X-ray diffraction patterns of pyrolysed polymers are presented in Fig. 5. In preceramic polymer, the crystallization 3.0 2.01 * I , , , I ! , , 1 1000 1100 1200 1300 1400 1500 Firing Temperature (“C) 10 20 30 40 50 60 70 26 (deg.) Fig. 5. The variation of polysilazanes in pyrolysis. (a) Plot of true density as a function of firing temperature. (b) XRD patterns of polysilazanes pyrolysed at 1350°C. would inherently cause an increase in density. Since both matrices are similar in Si based component and structure, densification (an increase in atomic density) would provide an increase in elastic modulus. The true density of fired SNC and PHPS are 2.5 and 2.8 g/cm3, respectively. Therefore, it is presumed that the elastic modulus (E,) of pyrolysed PHPS was much larger than that of pyrolysed SNC. If Er> E,,.,, the stress in the fibre is greater than in the matrix, because the fibre bears the major part of the applied load. This effect results in an increase in the effectiveness of fibre strength to composite strength. The flexural strength of samples using SNC matrix has become high as noted above. To neglect the influence of the amount of scatter in the Vf on the composite strength, the apparent effectiveness of fibre strength to composite strength (R) is defined as follows: R = a,l(arvfA) (3) where o, and of are the strength of composites and fibres, and A is the ratio of the fibre which is aligned to the orientation of the tensile stress. For UD and 2D composites, A is assumed to be 1 .O and 0.5, respectively
Preparation of fibre reinforced Si 800 (I)The products with high volume fraction of fibre can be obtained easily. Since both the polymer rheology and the thermosetting nature are similar to 600 organic thermosetting resin(c g. epoxide phenolic 兰旨400 resin), the FRP's fabrication process can be applied to the shaping process of PCPI.(2) To densify the matrix, shrinkage of the whole material, including 200 /SNC /SNC reinforcement, is not necessary in PCPI. While in the hot-pressing method, UD composites could be densified easily because the degree of freedom of shrinkage is two. However, 2D composites could not 0.4 0. 8 1.2 be well-densified because the degree of freedom of Displacement(mm) shrinkage is only one(the direction of thickness) Fig. 6. Stress-deflection curves of 2D composites In high temperature flexural tests, the strengths at room temperature are maintained to 1250C Fig. 8). There is no obvious difference in strength Thus, the high strengths of UD composites are between the SNF composite and SNBF composite, due to improved interaction between coating on because both fibres(SnF, SnBF) do not degrade the fibre and adequate matrix at the present manufacturing temperature and pressure. In the fibre evaluation, SNBF has better 3. 2 2D (Cross-ply composites) properties at elevated temperatures than SNF. Therefore, SNBF is expected to be applied as the Polished cross-sections of cross-ply composites are reinforcement of CMCs(target temperature shown in Fig. 7. The contrast of the fibre/matrix 1500C or above) interface and microcracks. attributed to the back scattered-electron intensity, are observed. From repetition of the impregnation and firing process, 4 CONCLUSION dense products are obtained. The properties of the samples are shown in Table 3. The excellent flexural By the PCPI method, dense SNF and SnbF rein strength(= 600 MPa) is obtained in samples of both forced silicon nitride composites were obtaincd SNF composite and SNBF composite. The stress- Three-point flexural strengths of the UD and 2D deflection curve is shown in Fig. 6. The fracture sur- (cross-ply) composites were shown to be as high as face shows a non-brittle failure with fibre pull-out, as 1000 and 600 MPa, respectively. This resulted from in the UD composites. The apparent effectiveness of improved interaction between the coating on the fibre strength in 2D composites is also extremely fibre and an adequate matrix. Using SNC as the high. This would be due to the following reasons: matrix precursor to fabricate these composites has 10m (b) ig. 7. Backscatterd electron image(composition)of polished cross-section SNBF reinforced composite, (a)Low magnifi ation,(b) high magnificatic
Preparation of jibre reinforced Si3N4 composites 183 0 0.4 0.8 1.2 Displacement (mm) Fig. 6. Stress-deflection curves of 2D composites. Thus, the high strengths of UD composites are due to improved interaction between coating on the fibre and adequate matrix. 3.2 20 (Cross-ply composites) Polished cross-sections of cross-ply composites are shown in Fig. 7. The contrast of the fibrelmatrix interface and microcracks, attributed to the backscattered-electron intensity, are observed. From repetition of the impregnation and firing process, dense products are obtained. The properties of the samples are shown in Table 3. The excellent flexural strength (zz 600 MPa) is obtained in samples of both SNF composite and SNBF composite. The stressdeflection curve is shown in Fig. 6. The fracture surface shows a non-brittle failure with fibre pull-out, as in the UD composites. The apparent effectiveness of fibre strength in 2D composites is also extremely high. This would be due to the following reasons: (I) The products with high volume fraction of fibre can be obtained easily. Since both the polymer rheology and the thermosetting nature are similar to organic thermosetting resin (e.g. epoxide phenolic resin), the FRP’s fabrication process can be applied to the shaping process of PCPI. (2) To densify the matrix, shrinkage of the whole material, including reinforcement, is not necessary in PCPI. While in the hot-pressing method, UD composites could be densified easily because the degree of freedom of shrinkage is two. However, 2D composites could not be well-densified because the degree of freedom of shrinkage is only one (the direction of thickness). In high temperature flexural tests, the strengths at room temperature are maintained to 1250°C (Fig. 8). There is no obvious difference in strength between the SNF composite and SNBF composite, because both fibres (SNF, SNBF) do not degrade at the present manufacturing temperature and pressure. In the fibre evaluation, SNBF has better properties at elevated temperatures than SNF. Therefore, SNBF is expected to be applied as the reinforcement of CMCs (target temperature is 1500°C or above). 4 CONCLUSION By the PCPI method, dense SNF and SNBF reinforced silicon nitride composites were obtained. Three-point flexural strengths of the UD and 2D (cross-ply) composites were shown to be as high as 1000 and 600 MPa, respectively. This resulted from improved interaction between the coating on the fibre and an adequate matrix. Using SNC as the matrix precursor to fabricate these composites has (4 - 10pm P) - lpm Fig. 7. Backscatterd electron image (composition) of polished cross-section of 2D SNBF reinforced composite. (a) Low magnification, (b) high magnification
H. Morozumi et al 800 REFERENCES 1. FITZER, E& GADOw, R, Am. Ceram Soc. Bull., 65 (1986)326-3 600 2. ZIEGLER. G. In Proc. Ceramics: Toward the 21st Cen- tury, Yokohama, Japan, 16-18 October, The Ceramic Society of Japan, 1991, pp. 448-64 3. SATO, K. SUZUKI.T. FUNAYAMA O& ISoDa 400 T.,J. Ceram Soc. Jpn, 100(1992)444-7 4. VELTRI, R. D, CONDIT, D. A& Galasso, F. J. Am. Ceram. Soc., 72 (1989)478-80 5. StINtoN, D. P, CaPUto.A.3. loWden.r. a Am Ceram. Soc. Bull., 65(1986)347-50 200 ■ SNBF/SNC 6. STRIFE, J. R, brennaN, JJ. prew, K. M O SNF/SNC Ceram, Eng. Sci. Pruc., 11 (1990)871-919 7. ISODA, T, KAYA, H, NISHIL, H, FUNAYAMA, O SUZUKI, T& TASHiro. Y, J Inorg. Orgc 0 Poym,2(1992)151-110 500 1000 1500 8. FUNAYAMA, O, ARAL, M, TASHIRO, Y, AOKL, H Testing Temperature(c) SUZUKI T. TAMURA K. KAYA.H. NISHIL H. ISODA, T, J. Ceram. Soc. Jpn, 98 (1990)104-7 Fig 8. High-temperature strength of 2D composites under ar 9 ISODA.T. In 3rd (1CC1-111), Controlled Interface Structure, ed. H Elsevier Science Publishing Co, New York, I m脑 55-65 10. FUNAYAMAO. NAKAHARA TEZUKA. A SHIl, T& ISoDA, T, J, Mater. Sci., 29(1994)2238-44. I1. ISODA, T, In Proc. of euro-Japanese Colloquium on taken an important role in the relation between er Ceramic Fibres in The 6th European Conf. on Composite nd Em, because SNC maintained a relatively low Materials(ECCM-6). The European Association for density under the firing conditions site Materials. 1993 12. SCHWAB. S. T. GRAEF, R. C. PaN. Yi-M. DAVIDSON, D L, In Proc. 16th Conf on Composites Materials and Structures. Cocoa Beach. FL. 12-15 1992, The American Ceramic Society ACKNOWLEDGEMENTS 13 T. FUNAYAMAO. SHIMIZU The Society of Polymer Science, Japan. The Society of Portions of this work were conducted by the Polymer Science, Japan, 1993. pp. 78-9 Petroleum Energy Centre with financial support 14. MOROZUMI,H- SATO ZUKA, A, KAYA, H. from the Ministry of International Trade and ISODA, T In Proc. of fall Meeting of The Cerami Society of Japan. The Ceramic Society of Japan, 1993, Industry
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