E驅≈3S Journal of the European Ceramic Society 21(2001)251-261 elsevier. com/locate/jeurc Impact response and mechanical behavior of 3-D ceramic matrix composites Hsien-Kuang Liu", Chuan-Cheng Huang Department of Mechanical Engineering, Feng-Chia University, 100 Wenhua Road, 407 Taichung, Taiwan Received 29 September 1999: received in revised form 24 May 2000: accepted 4 June 2000 Abstract This paper examines the impact response, compressive strength, and flexural strength of three-dimensional carbon fiber rein- forced ceramics. The composite was fabricated by combination of the pressure infiltration method and sol-gel processing, using the mixture of silica sol and alumina particles. The effects of sol viscosity on impact response and flexural strength, and infiltration pressure on compressive strength are investigated. Impact tests were carried out using an Izod impact testing machine. It is found that impact energy of the specimens increases linearly with sol viscosity. The sol viscosity affects impact response by two factors, interfacial strength and volume fraction of silica. Higher sol viscosity leads to weaker interface and more residual silica, resulting in higher impact energy. Flexural strength decreases with sol viscosity according to an exponential decay function because higher viscosity leads to weak interface as well as lower flexural strength. Compressive strength increases with infiltration pressure, which follows a parabolic function. Higher infiltration pressure enhances infiltration of the mixture, resulting in dense matrix and strong interface. As a result, the composite shows a higher compressive strength due to less buckling of longitudinal fibers strongly con- fined by neighboring dense matrix and transverse fiber bundles. The compressive stress-strain history has been examined and related to how the specimens respond to the compression. c 2001 Elsevier Science Ltd. All rights reserved. Keywords: Alumino-silicate matrix; Carbon fibre; Composites; Mechanical properties; Sol-gel methods 1. Introduction process compared to that of a slurry. 4 The latter demonstrated that the slurry during consolidation must To fabricate ceramic matrix composites by combina- have a sufficiently high viscosity to prevent sedimenta ion of pressure infiltration method and sol-gel proces- tion and lower packing density. However, in our studies sing takes advantages of both methods. -3 The for the short fiber composite it was proved that high advantages include variety of reinforcement, low densi- viscosity of the mixture of sol and ceramic particles fication temperature, low shrinkage, and reduced drying leads to the reaction of sol with ceramic particles, and stresses. Authors have fabricated three-dimensional (3 herefore more sedimentation and low packing density D )green ceramic matrix composites' and short fiber Therefore, one of the goals of this work is to study the ceramic matrix composites by combination of both influence of sol viscosity on microstructure and mechan methods, using the mixture of sol and ceramic particles. ical properties of 3-D ceramic matrix composites In the former study, interparticle forces and particle size Recently, major progress has been made in the devel- distribution are the major factors that influence green pment of 3-D ceramic matrix composites. The driving density and drying stresses, while in the latter study, forces behind the development of these materials have sedimentation, infiltration rate, and viscosity of mixture been the need for high strength and high energy absorp- of sol and ceramic particles are the major factors that tion under multidirectional loadings, and conformity for influence mechanical properties of composites. Further- shape forming. The preform used in the composite cre- more, the mixture of sol and ceramic particles was found ates 3-D networks of reinforcing fibers that eliminate to have different behavior during the consolidation weak planes and prevent the material from planar type of failure. such as delamination. In the absence of weak planes, the impact energy could be dissipated in a more E-mail address: hkliu(@fcu.edu. tw(H.-K. Liu) localized area 6 Whether or not a 3-D ceramic matri 0955-2219/01/S. see front matter C 2001 Elsevier Science Ltd. All rights reserved PII:S0955-2219(00)00181-3
Impact response and mechanical behavior of 3-D ceramic matrix composites Hsien-Kuang Liu *, Chuan-Cheng Huang Department of Mechanical Engineering, Feng-Chia University, 100 Wenhwa Road, 407 Taichung, Taiwan Received 29 September 1999; received in revised form 24 May 2000; accepted 4 June 2000 Abstract This paper examines the impact response, compressive strength, and ¯exural strength of three-dimensional carbon ®ber reinforced ceramics. The composite was fabricated by combination of the pressure in®ltration method and sol±gel processing, using the mixture of silica sol and alumina particles. The eects of sol viscosity on impact response and ¯exural strength, and in®ltration pressure on compressive strength are investigated. Impact tests were carried out using an Izod impact testing machine. It is found that impact energy of the specimens increases linearly with sol viscosity. The sol viscosity aects impact response by two factors, interfacial strength and volume fraction of silica. Higher sol viscosity leads to weaker interface and more residual silica, resulting in higher impact energy. Flexural strength decreases with sol viscosity according to an exponential decay function because higher viscosity leads to weak interface as well as lower ¯exural strength. Compressive strength increases with in®ltration pressure, which follows a parabolic function. Higher in®ltration pressure enhances in®ltration of the mixture, resulting in dense matrix and strong interface. As a result, the composite shows a higher compressive strength due to less buckling of longitudinal ®bers strongly con- ®ned by neighboring dense matrix and transverse ®ber bundles. The compressive stress-strain history has been examined and related to how the specimens respond to the compression. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Alumino-silicate matrix; Carbon ®bre; Composites; Mechanical properties; Sol±gel methods 1. Introduction To fabricate ceramic matrix composites by combination of pressure in®ltration method and sol±gel processing takes advantages of both methods.1ÿ3 The advantages include variety of reinforcement, low densi- ®cation temperature, low shrinkage, and reduced drying stresses. Authors have fabricated three-dimensional (3- D) green ceramic matrix composites1 and short ®ber ceramic matrix composites2 by combination of both methods, using the mixture of sol and ceramic particles. In the former study, interparticle forces and particle size distribution are the major factors that in¯uence green density and drying stresses; while in the latter study, sedimentation, in®ltration rate, and viscosity of mixture of sol and ceramic particles are the major factors that in¯uence mechanical properties of composites. Furthermore, the mixture of sol and ceramic particles was found to have dierent behavior during the consolidation process3 compared to that of a slurry.4 The latter demonstrated that the slurry during consolidation must have a suciently high viscosity to prevent sedimentation and lower packing density. However, in our studies for the short ®ber composite it was proved that high viscosity of the mixture of sol and ceramic particles leads to the reaction of sol with ceramic particles, and therefore more sedimentation and low packing density. Therefore, one of the goals of this work is to study the in¯uence of sol viscosity on microstructure and mechanical properties of 3-D ceramic matrix composites. Recently, major progress has been made in the development of 3-D ceramic matrix composites. The driving forces behind the development of these materials have been the need for high strength and high energy absorption under multidirectional loadings, and conformity for shape forming.5 The preform used in the composite creates 3-D networks of reinforcing ®bers that eliminate weak planes and prevent the material from planar type of failure, such as delamination. In the absence of weak planes, the impact energy could be dissipated in a more localized area.6 Whether or not a 3-D ceramic matrix 0955-2219/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00181-3 Journal of the European Ceramic Society 21 (2001) 251±261 www.elsevier.com/locate/jeurceramsoc * Corresponding author. E-mail address: hkliu@fcu.edu.tw (H.-K. Liu)
H.K. Liu, C -C. Huang/Journal of the European Ceramic Society 21(2001)251-261 composite is more impact resistant depends not only on pressure infiltration and sol-gel methods. The effect of the constituents used but, more important, on how the sol viscosity and infiltrate n pressure on mechanical network is constructed and how the impact load is properties of composites is studied. The correlation applied. It is therefore the goal of this research to char- between mechanical properties and damage modes of acterize the impact behavior of the materials. composites is investigated A review of literature indicates that most previous research was dedicated to the impact behavior of uni directional or 2-D ceramic composites, but very little 2. Experimental procedure as been done on that of 3-D ceramic composites. The major reason is the difficulty in preparing specimens 2. 1. Materials uitable for impact tests. For 3-D ceramic composites under Charpy impact testing, several interesting con The silica sol was prepared by the following recipe in clusions were proposed. First, the matrix is divided by which tetraethyl-orthosilicate (TEOS), ethanol, deio- the 3-D network structure and increases dynamic tough- nized water and 7 wt. of HNO3 were mixed and stir ness of the composite. Second, the compliant fiber net- red at a volume ratio of 1: 1: 1. 6: 0.06 to obtain 175 ml work raises the resistance to crack propagation. Third, sol. In order to study the effect of sol viscosity on the coating of fiber improves impact resistance. Fourth, mechanical properties of 3-D ceramic matrix compo- fiber pull-out increases impact resistance. Damaged by sites, four kinds of viscosity are selected. The sol with delamination, 2-D C/C composite has higher impact viscosity 3.78 cP was obtained according to the proce rupture work than that of 3-D C/C composite For cross- dure above, while sols with viscosities 4.17 and 8.5 cP ply ceramic composites under drop-weight impact, high are prepared by evaporating 250 and 350 ml sols, temperature environment and applied tensile loading respectively, to 175 ml. The lowest viscosity 3.02 cP was make the damage of the composite more severe. This prepared by changing volume ratio of TEOS from 1.0 to indicates that different testing conditions significantly 0. 25. 17 g of alumina particles(a-Al2O3, average dia- influence the impact response of the materials meter d=0.3 um, purity =99.99%, p=3.965 g/cm To deeply understand impact response of the materi- ProChem Co, USA)were added to the sol to prepare ls, static tests are necessary. For ceramic matrix com the infiltrate. The infiltrate was further mixed both in an posites under compression, o dilatational fracture within ultrasonic bath and electromagnetic blender in turns the matrix dominates composite failure at low confine until the particles in the infiltrate were dispersed un ment pressures, while fiber kinking caused by high con- formly and well wetted. The carbon fiber used in the 3- finement pressure indicates shear dominated mechanisms. D preform is Toho T-300 fiber. The properties of fiber The processing and mechanical properties of 3-D car- are as follows: tensile strength 3920 MPa, tensile mod bon/SiC and carbon/Si N4 at high temperature under ulus 235 GPa, diameter(d)=7 um, p=1.77 g/cm3, Sic were reviewed. A brocesses of cross-weave carbon/ 12000 filaments in a fiber tow. The 3-D preform is a vacuum and fracture xural strengths of 3-D carbon/ three-axis orthogonal structure. Fiber volume fraction SiC at room and high temperatures are much lower of the 3-D preform is 38.2%. A one-shuttle weaving than those unidirectional carbon/SiC composites. The scheme is adopted to fabricate the 3-D fabric. 6 high-temperature strength is higher than that at room In order to predict volume fraction of silica in the temperature for 3-D carbon/SiC, while the trend is silica/alumina matrix, solid content in the silica sol was reverse for 3-D carbon/ Si3N4. The fracture process of measured. A drying and sintering experiment was con cross-weave carbon/SiC does not involve cracking by a ducted on the silica sol with four kinds of viscosity. The single dominant crack but occurs by the development of silica sol was dried at 80C to obtain dried gel. There- multiple transverse fractures of groups of four to eight after the dried gel was sintered at 1300C for 2 h to yield fibers followed by longitudinal cracking at the interface. silica. The volume of silica sol was controlled as 175 cm3 The cracks were temporary arrested by the internal(ml), and the weight of dried gel and silica were mea- voids until specimens fail and there is extensive fiber sured. Therefore volume of silica can be obtained with its debonding and pull-out. Although ceramic matrix com- density of 2.2 g/cm. These data are shown in Table posites reinforced by carbon fibers are vulnerable to degradation in an oxidizing environment at relatively 2. 2. Fabrication low temp hey provide valuable information for he material used in oxygen free environment as well as A pressure infiltration apparatus was used to perform for other material systems the processing of 3-D ceramic matrix composites. The Based on the knowledge we have gained, the prime apparatus consists of a cylinder with an inner diameter objective of this work is aimed at evaluating impact of 50 mm, a base cavity for collecting the liquid, a response, flexural strength, and compressive strength of plunger, and a filter assembly. The filter assembly con- 3-D woven ceramic matrix composites fabricated by sists of nitrate cellulose membrane filter paper with pore
composite is more impact resistant depends not only on the constituents used but, more important, on how the network is constructed and how the impact load is applied. It is therefore the goal of this research to characterize the impact behavior of the materials. A review of literature indicates that most previous research was dedicated to the impact behavior of unidirectional or 2-D ceramic composites,7ÿ9 but very little has been done on that of 3-D ceramic composites. The major reason is the diculty in preparing specimens suitable for impact tests. For 3-D ceramic composites under Charpy impact testing,9 several interesting conclusions were proposed. First, the matrix is divided by the 3-D network structure and increases dynamic toughness of the composite. Second, the compliant ®ber network raises the resistance to crack propagation. Third, the coating of ®ber improves impact resistance. Fourth, ®ber pull-out increases impact resistance. Damaged by delamination, 2-D C/C composite has higher impact rupture work than that of 3-D C/C composite. For crossply ceramic composites under drop-weight impact,7 high temperature environment and applied tensile loading make the damage of the composite more severe. This indicates that dierent testing conditions signi®cantly in¯uence the impact response of the materials. To deeply understand impact response of the materials, static tests are necessary. For ceramic matrix composites under compression,10 dilatational fracture within the matrix dominates composite failure at low con®nement pressures, while ®ber kinking caused by high con- ®nement pressure indicates shear dominated mechanisms. The processing and mechanical properties of 3-D carbon/SiC and carbon/Si3N4 at high temperature under vacuum11 and fracture processes of cross-weave carbon/ SiC were reviewed.12 Flexural strengths of 3-D carbon/ SiC at room and high temperatures are much lower than those unidirectional carbon/SiC composites. The high-temperature strength is higher than that at room temperature for 3-D carbon/SiC, while the trend is reverse for 3-D carbon/Si3N4. The fracture process of cross-weave carbon/SiC does not involve cracking by a single dominant crack but occurs by the development of multiple transverse fractures of groups of four to eight ®bers followed by longitudinal cracking at the interface. The cracks were temporary arrested by the internal voids until specimens fail and there is extensive ®ber debonding and pull-out. Although ceramic matrix composites reinforced by carbon ®bers are vulnerable to degradation in an oxidizing environment at relatively low temperatures, they provide valuable information for the material used in oxygen free environment as well as for other material systems. Based on the knowledge we have gained, the prime objective of this work is aimed at evaluating impact response, ¯exural strength, and compressive strength of 3-D woven ceramic matrix composites fabricated by pressure in®ltration and sol±gel methods. The eect of sol viscosity and in®ltration pressure on mechanical properties of composites is studied. The correlation between mechanical properties and damage modes of composites is investigated. 2. Experimental procedure 2.1. Materials The silica sol was prepared by the following recipe in which tetraethyl-orthosilicate (TEOS), ethanol, deionized water and 7 wt.% of HNO3 were mixed and stirred at a volume ratio of 1:1:1.6:0.06 to obtain 175 ml sol. In order to study the eect of sol viscosity on mechanical properties of 3-D ceramic matrix composites, four kinds of viscosity are selected. The sol with viscosity 3.78 cP was obtained according to the procedure above, while sols with viscosities 4.17 and 8.5 cP are prepared by evaporating 250 and 350 ml sols, respectively, to 175 ml. The lowest viscosity 3.02 cP was prepared by changing volume ratio of TEOS from 1.0 to 0.25. 17 g of alumina particles (a-Al2O3, average diameter d 0:3 mm, purity=99.99%, =3.965 g/cm3 , ProChem Co., USA) were added to the sol to prepare the in®ltrate. The in®ltrate was further mixed both in an ultrasonic bath and electromagnetic blender in turns until the particles in the in®ltrate were dispersed uniformly and well wetted. The carbon ®ber used in the 3- D preform is Toho T-300 ®ber. The properties of ®ber are as follows: tensile strength 3920 MPa, tensile modulus 235 GPa, diameter (d)=7 mm, =1.77 g/cm3 , 12000 ®laments in a ®ber tow. The 3-D preform is a three-axis orthogonal structure. Fiber volume fraction of the 3-D preform is 38.2%. A one-shuttle weaving scheme is adopted to fabricate the 3-D fabric.6 In order to predict volume fraction of silica in the silica/alumina matrix, solid content in the silica sol was measured. A drying and sintering experiment was conducted on the silica sol with four kinds of viscosity. The silica sol was dried at 80C to obtain dried gel. Thereafter the dried gel was sintered at 1300C for 2 h to yield silica. The volume of silica sol was controlled as 175 cm3 (ml), and the weight of dried gel and silica were measured. Therefore volume of silica can be obtained with its density of 2.2 g/cm3 . These data are shown in Table 1. 2.2. Fabrication A pressure in®ltration apparatus was used to perform the processing of 3-D ceramic matrix composites. The apparatus consists of a cylinder with an inner diameter of 50 mm, a base cavity for collecting the liquid, a plunger, and a ®lter assembly. The ®lter assembly consists of nitrate cellulose membrane ®lter paper with pore 252 H.-K. Liu, C.-C. Huang / Journal of the European Ceramic Society 21 (2001) 251±261
H.K. Liu, C -C Huang /Journal of the European Ceramic Society 21(2001)251-261 Table I Summary of mechanical properties of 3-D ceramic matrix Specimen Sol Infiltration Green Impact energy Impact Compression Compression Compression vIscosIty pressure composite energy/area strain (cP)a density (% (%) (MPa) 1.25 MPa 25MPa74.1 19. 8702870 401 455.59 0.775 1059 0.50MPa271.9 021.00MPa76.8 enti-Poise. unit of sol viscosity Mega Pascal, unit of infiltration pressure size of 0.I um, a stainless steel wire cloth, and a perfo- mm in width(b), and 7 mm in thickness (h1). The speci rated solid steel disk. The filter assembly was placed on mens were clamped perpendicularly at one end, and left the top of the base cavity. The stainless steel wire cloth 30 mm free for the impact. The mass of the impactor, a was used to keep the filter paper from rupturing Cylin- pendulum, was 4.54 kg. After the pendulum impacted der, filter assembly, and base were tightly sealed by he specimen at top 10 mm, a pointer can read the thread and O-ring. The 3-D fabric was cut as a rectangle impact energy on the machine based on the difference of with dimension 3.8 x25x0.85 cm, and then inserted the height of the pendulum before and after the impact into a rectangular hole within a circular acrylic disk Four different specimens(IA, IB, IC, and ID)were with 5.0 cm in diameter and 0.85 cm thick. The fabric impacted, which were fabricated by sol viscosity of 3.02, and disk were put in the cylinder and attached to the fil- 3.78, 4.17, and 8.50 cP, respectively. First letter"I ter assembly closely. The infiltrate was then poured into the specimen number means impact the cylinder and an mts testing machine set at constant pressure mode was used to provide the infiltration pres- 2.3.2. Compressive test sure on the plunger. Four kinds of infiltration pressure The dimension of specimens for compression is were used: 0.5, 0.75, 1.0 and 1.25 MPa. The infiltration 37. 5x24x7 mm(L x bx h). The compression loading is procedure stops when the plunger reaches the upper sur- applied in the length direction, and both ends are grip- face of the fabric Using pressure infiltration, the thick 3- ped by the fixture for 15 the through-thickness D fabric can be efficiently infiltrated and consolidated direction. The test is conducted by Instron 4468 testing via silica sol/alumina particle route in a single step. machine and crosshead speed is set at 0. 1 mm/s. Com After infiltration, the green composite is dried in an pressive strength o and compressive modulus E are cal oven at 60C and humidity 95% for 24 h. In order to culated by the following formula raise solid content, the dried composite is soaked into silica sol under vacuum for 3 h and then dried for 24 h P (1) Densification of the composite was conducted by hot pressing furnace(FCPHP-R-5, FRET-20, High Multi 5000, Multi-purpose High Temperature Furnace, E PL Japan). Conditions for hot pressing are temperature 1600C and pressure 10 MPa for 1f h by flowing nitro- gen gas. After hot pressing, mechanical properties and where P is maximum load, P/8 is the slope on the com microstructure of composites were evaluated pressive load-displacement curve at final stage. Four different specimens PA, PB, PC, and PD were com 23. Characterization pressed, which were fabricated by infiltration pressure of 0.50, 0.75, 1.0, and 1. 25 MPa, respectively 2.3.1. Impact test The impact tests were conducted using an Izod test 2.3.3. Flexural test machine(Model: TMI-43-01, Test Machine Co, USA) The dimension of specimens for three-point bending The dimension of specimens is 40 mm in length(L), 12 test is 40x 12x7 mm(Lx). The span is 30 mm
size of 0.1 mm, a stainless steel wire cloth, and a perforated solid steel disk. The ®lter assembly was placed on the top of the base cavity. The stainless steel wire cloth was used to keep the ®lter paper from rupturing. Cylinder, ®lter assembly, and base were tightly sealed by thread and O-ring. The 3-D fabric was cut as a rectangle with dimension 3.82.50.85 cm, and then inserted into a rectangular hole within a circular acrylic disk with 5.0 cm in diameter and 0.85 cm thick. The fabric and disk were put in the cylinder and attached to the ®lter assembly closely. The in®ltrate was then poured into the cylinder and an MTS testing machine set at constant pressure mode was used to provide the in®ltration pressure on the plunger. Four kinds of in®ltration pressure were used: 0.5, 0.75, 1.0 and 1.25 MPa. The in®ltration procedure stops when the plunger reaches the upper surface of the fabric. Using pressure in®ltration, the thick 3- D fabric can be eciently in®ltrated and consolidated via silica sol/alumina particle route in a single step. After in®ltration, the green composite is dried in an oven at 60C and humidity 95% for 24 h. In order to raise solid content, the dried composite is soaked into silica sol under vacuum for 3 h and then dried for 24 h. Densi®cation of the composite was conducted by a hot pressing furnace (FCPHP-R-5, FRET-20, High Multi 5000, Multi-purpose High Temperature Furnace, Japan). Conditions for hot pressing are temperature 1600C and pressure 10 MPa for 1 1 2 h by ¯owing nitrogen gas. After hot pressing, mechanical properties and microstructure of composites were evaluated. 2.3. Characterization 2.3.1. Impact test The impact tests were conducted using an Izod test machine (Model: TMI-43-01, Test Machine Co., USA). The dimension of specimens is 40 mm in length (L), 12 mm in width (b), and 7 mm in thickness (h). The specimens were clamped perpendicularly at one end, and left 30 mm free for the impact. The mass of the impactor, a pendulum, was 4.54 kg. After the pendulum impacted the specimen at top 10 mm, a pointer can read the impact energy on the machine based on the dierence of the height of the pendulum before and after the impact. Four dierent specimens (IA, IB, IC, and ID) were impacted, which were fabricated by sol viscosity of 3.02, 3.78, 4.17, and 8.50 cP, respectively. First letter ``I'' in the specimen number means impact. 2.3.2. Compressive test The dimension of specimens for compression is 37.5247 mm (L b h). The compression loading is applied in the length direction, and both ends are gripped by the ®xture for 15 mm in the through-thickness direction. The test is conducted by Instron 4468 testing machine and crosshead speed is set at 0.1 mm/s. Compressive strength s and compressive modulus E are calculated by the following formula P bh 1 E P L bh 2 where P is maximum load, P= is the slope on the compressive load±displacement curve at ®nal stage. Four dierent specimens PA, PB, PC, and PD were compressed, which were fabricated by in®ltration pressure of 0.50, 0.75, 1.0, and 1.25 MPa, respectively. 2.3.3. Flexural test The dimension of specimens for three-point bending test is 40127 mm (L b h). The span is 30 mm. Table 1 Summary of mechanical properties of 3-D ceramic matrix composites Specimen type Sol viscosity (cP)a In®ltration pressure Green composite density (%) Flexural strength (MPa)b Flexural modulus (GPa) Impact energy (J) Impact energy/area (J/m2 ) Compression strain (%) Compression strength (MPa) Compression modulus (MPa) BA 3.02 1.25 MPa 78.1 23.92 1.483 BB 3.78 1.25 MPa 74.1 19.82 0.840 BC 4.17 1.25 MPa 73.4 19.03 0.793 BD 8.50 1.25 MPa 70.0 18.27 0.694 IA 3.02 1.25 MPa 78.1 0.598 8813.81 IB 3.78 1.25 MPa 74.1 0.721 9455.59 IC 4.17 1.25 MPa 73.4 0.733 9502.57 ID 8.50 1.25 MPa 70.0 0.775 10590.16 PA 3.02 0.50 MPa2 71.9 3.51 24.97 1.392 PB 3.02 0.75 MPa 73.7 2.35 27.32 1.496 PC 3.02 1.00 MPa 76.8 2.43 31.07 3.042 PD 3.02 1.25 MPa 78.1 3.53 37.69 4.065 a cp, centi-Poise, unit of sol viscosity. b MPa, Mega Pascal, unit of in®ltration pressure. H.-K. Liu, C.-C. Huang / Journal of the European Ceramic Society 21 (2001) 251±261 253
H.K. Liu, C-C. Huang /Journal of the European Ceramic Society 21(2001)251-261 The test is conducted by Instron 4468 testing machine and crosshead speed is set at 0.008 mm/s. Flexural strength o and flexural modulus E are calculated by the 40 following formula e 3 PL E=84bh3 where p is maximum load, P/8 is the initial slope on the load-displacement curve. Four different specimens BA BB, BC, and Bd were tested, which were fabricated by sol viscosity of 3.02, 3.78, 4.17, and 8.50 cP, respec Infiltration pressure(MPa) tively Fig. 1. Compressive strength as a function of infiltration pressure for 3-D ceramic matrix composites. 3. Results and discussion Table I summarizes the effect of infiltration pressure on compressive strength, and sol viscosity on flexural strength and impact response of 3-D ceramic matrix composites (CMC). Green composite density of each pecimen is also listed. The correlation among two pro- cessing parameters and three mechanical properties are discussed as follows 3.l.C As shown in Fig. 1, compressive strength of PA, PB, and PD specimens increases with infiltration pres- sure according to a parabolic function. Compressive stress-strain curves for four specimens are shown in Fig 0.00 2. In the curve for the PD specimen fabricated by the highest infiltration pressure of 1. 25 MPa, the peak value and slope near the peak value lead to its highest com- pressive strength and modulus, respectively. Based on Fig. 2. Stress-strain curves for compression of PA, PB, PC, and PD our observation, the damage of a compressed 3-D CMC is complicated because the dominated damage mode is unclear. The damage may include three interactive expands perpendicular to the direction of compression modes: expansion mode, shear mode, and buckling loading due to Poissons effect and leads to debonding mode. Although the expansion mode was concluded as Both debonding and compressive loading cause buck ne dominated damage mode for most brittle materi- ling of the fiber bundle. As shown in Fig. 2, the gradual ls, o the 3-D fiber network may resist the expansion slope in the curve for PA specimen indicates its lowest and play an important role in compression of the 3-d modulus of 1. 392 GPa among four compressed speci- MC. Therefore the effect of infiltration pressure on mens. Low infiltration pressure prevents infiltration of impressive strength and the related damage modes are alumina particles into fiber bundles. As a result, low investigated y her porosity The lowest infiltration pressure, 0.5 MPa, for PA obtained, resulting in lower modulus and weak inter pecimens leads to its lowest compressive strength of face. Therefore, under compression the crack propa 24.97 MPa. Serious buckling of the fiber bundle in the gates through weak interface and causes debonding compressed PA specimen is shown in Fig. 3a. At the Besides, transverse fiber bundles become compliant due onset of compression, the matrix crack initiates from to poor infiltration. Without strong confinement force surface ceramic layer and propagates into the fiber/ from transverse fiber bundles, serious buckling of the matrix interface. As the crack extends, the matrix longitudinal fiber bundle occurs
The test is conducted by Instron 4468 testing machine and crosshead speed is set at 0.008 mm/s. Flexural strength and ¯exural modulus E are calculated by the following formula 3 2 PL bh2 3 E P L3 4bh3 4 where p is maximum load, P= is the initial slope on the load±displacement curve. Four dierent specimens BA, BB, BC, and BD were tested, which were fabricated by sol viscosity of 3.02, 3.78, 4.17, and 8.50 cP, respectively. 3. Results and discussion Table 1 summarizes the eect of in®ltration pressure on compressive strength, and sol viscosity on ¯exural strength and impact response of 3-D ceramic matrix composites (CMC). Green composite density of each specimen is also listed. The correlation among two processing parameters and three mechanical properties are discussed as follows. 3.1. Compressive strength and damage As shown in Fig. 1, compressive strength of PA, PB, PC, and PD specimens increases with in®ltration pressure according to a parabolic function. Compressive stress±strain curves for four specimens are shown in Fig. 2. In the curve for the PD specimen fabricated by the highest in®ltration pressure of 1.25 MPa, the peak value and slope near the peak value lead to its highest compressive strength and modulus, respectively. Based on our observation, the damage of a compressed 3-D CMC is complicated because the dominated damage mode is unclear. The damage may include three interactive modes: expansion mode, shear mode, and buckling mode. Although the expansion mode was concluded as the dominated damage mode for most brittle materials,10 the 3-D ®ber network may resist the expansion and play an important role in compression of the 3-D CMC. Therefore the eect of in®ltration pressure on compressive strength and the related damage modes are investigated. The lowest in®ltration pressure, 0.5 MPa, for PA specimens leads to its lowest compressive strength of 24.97 MPa. Serious buckling of the ®ber bundle in the compressed PA specimen is shown in Fig. 3a. At the onset of compression, the matrix crack initiates from surface ceramic layer and propagates into the ®ber/ matrix interface. As the crack extends, the matrix expands perpendicular to the direction of compression loading due to Poisson's eect and leads to debonding. Both debonding and compressive loading cause buckling of the ®ber bundle. As shown in Fig. 2, the gradual slope in the curve for PA specimen indicates its lowest modulus of 1.392 GPa among four compressed specimens. Low in®ltration pressure prevents in®ltration of alumina particles into ®ber bundles. As a result, low green density (Table 1) as well as higher porosity is obtained, resulting in lower modulus and weak interface. Therefore, under compression the crack propagates through weak interface and causes debonding. Besides, transverse ®ber bundles become compliant due to poor in®ltration. Without strong con®nement force from transverse ®ber bundles, serious buckling of the longitudinal ®ber bundle occurs. Fig. 1. Compressive strength as a function of in®ltration pressure for 3-D ceramic matrix composites. Fig. 2. Stress-strain curves for compression of PA, PB, PC, and PD specimens. 254 H.-K. Liu, C.-C. Huang / Journal of the European Ceramic Society 21 (2001) 251±261
H.K. Liu, C -C Huang /Journal of the European Ceramic Society 21(2001)251-261 compression Fig 3.(a) Serious buckling of the fiber bundle in the PA specimen compression, (b) intermediate buckling of the fiber bundle in the PB Fig 4.(a)Moderate buckling of the fiber bundle in the PC specimen The infiltration pressure for PB specimens is 0.75 in compression,(b)slight buckling of the fiber bundle in the PD spe. MPa, which is higher than that of PA specimens and results in higher compressive strength of 27.32 MPa and The curve implies the microstructure evolution in the higher modulus of 1.496 GPa. Higher infiltration pres- compressed 3-D ceramic composite. At the beginning of sure results in better infiltration of sol/particle mixture, compression, shear force causes matrix microcracks leading to better packing density of the particles. This inside the fiber bundle. Later, compression loading enhances interfacial strength and increases compressive tends to close the microcracks and leads to gradual strength. As shown in Fig. 3b, intermediate buckling of increase of modulus of the composite indicated by the two fiber bundles in the specimen is observed, which is slope of stress-strain curve before strain of 2.8%. As the less serious than that in the PA specimen. The buckling strain is larger than 2.8%, the matrix begins to expand also causes cracks in inter-bundle regions among long- due to Poissons effect, but at the same time is con- itudinal and transverse fiber bundles, and deformation strained by the transverse fiber bundles, leading to rapid of transverse fiber bundles originally in a rectangular increase of the slope(modulus). Finally, transverse fiber bundles can not provide sufficient confinement force Similarly, the higher infiltration pressure of 1.0 MPa and therefore compression causes failure of the compo- for PC specimens than for PB specimens leads to higher site by the buckling mode. As shown in Fig. 4b, the compressive strength of 31.07 MPa and modulus of longitudinal fiber bundle only slightly buckles because 3.042 GPa. As shown in Fig. 4a, compression causes better confinement force is provided through stronger moderate buckling of the fiber bundle and leads to fiber interface, denser matrix, and 3-D fiber network breakage at the convex side. Few cracks occur in the transverse fiber bundle due to buckling of longitudinal 3. 2. Flexural strength and damage bundle For PD specimens, the infiltration pressure is 1.25 As shown in Fig. 5, flexural strength of BA, BB, BC, MPa, which is the highest among four compressed spe- and BD specimens decreases with silica sol viscosity cimens and results in the highest compressive strength according to an exponential decay function. Flexural of 37.69 MPa and modulus of 4.065 GPa. This result is stress strain curves for BA, BB, BC, and BD specimens due to better matrix strength caused by higher compo- are depicted in Fig. 6. Four curves indicate similar trend site green density. As shown in Fig. 2, the slope for PD except for load history after maximum load, affected by promptly increases when the strain is larger than 2.8%. sol viscosity. a typical flexural damage configuration is
The in®ltration pressure for PB specimens is 0.75 MPa, which is higher than that of PA specimens and results in higher compressive strength of 27.32 MPa and higher modulus of 1.496 GPa. Higher in®ltration pressure results in better in®ltration of sol/particle mixture, leading to better packing density of the particles. This enhances interfacial strength and increases compressive strength. As shown in Fig. 3b, intermediate buckling of two ®ber bundles in the specimen is observed, which is less serious than that in the PA specimen. The buckling also causes cracks in inter-bundle regions among longitudinal and transverse ®ber bundles, and deformation of transverse ®ber bundles originally in a rectangular shape. Similarly, the higher in®ltration pressure of 1.0 MPa for PC specimens than for PB specimens leads to higher compressive strength of 31.07 MPa and modulus of 3.042 GPa. As shown in Fig. 4a, compression causes moderate buckling of the ®ber bundle and leads to ®ber breakage at the convex side. Few cracks occur in the transverse ®ber bundle due to buckling of longitudinal bundle. For PD specimens, the in®ltration pressure is 1.25 MPa, which is the highest among four compressed specimens and results in the highest compressive strength of 37.69 MPa and modulus of 4.065 GPa. This result is due to better matrix strength caused by higher composite green density. As shown in Fig. 2, the slope for PD promptly increases when the strain is larger than 2.8%. The curve implies the microstructure evolution in the compressed 3-D ceramic composite. At the beginning of compression, shear force causes matrix microcracks inside the ®ber bundle. Later, compression loading tends to close the microcracks and leads to gradual increase of modulus of the composite indicated by the slope of stress-strain curve before strain of 2.8%. As the strain is larger than 2.8%, the matrix begins to expand due to Poisson's eect, but at the same time is constrained by the transverse ®ber bundles, leading to rapid increase of the slope (modulus). Finally, transverse ®ber bundles can not provide sucient con®nement force, and therefore compression causes failure of the composite by the buckling mode. As shown in Fig. 4b, the longitudinal ®ber bundle only slightly buckles because better con®nement force is provided through stronger interface, denser matrix, and 3-D ®ber network. 3.2. Flexural strength and damage As shown in Fig. 5, ¯exural strength of BA, BB, BC, and BD specimens decreases with silica sol viscosity according to an exponential decay function. Flexural stress strain curves for BA, BB, BC, and BD specimens are depicted in Fig. 6. Four curves indicate similar trend except for load history after maximum load, aected by sol viscosity. A typical ¯exural damage con®guration is Fig. 3. (a) Serious buckling of the ®ber bundle in the PA specimen in compression, (b) intermediate buckling of the ®ber bundle in the PB specimen in compression. Fig. 4. (a) Moderate buckling of the ®ber bundle in the PC specimen in compression, (b) slight buckling of the ®ber bundle in the PD specimen in compression. H.-K. Liu, C.-C. Huang / Journal of the European Ceramic Society 21 (2001) 251±261 255