MATERIAL CHEM ELSEVIER Materials Chemistry and Physics 53(1998)155-164 Thermal aging behavior of an Sic-fiber reinforced glass matrix composite in a non-oxidizing atmosphere A R Boccaccini, J. Janczak-Rusch, I Dlouhy d Institute for Mechanics <tnd Materials, University of California, Sun Diego, La olla. CA 92093-0404, USA EMPA-Thuen, CH-3602 Thun Switzerlan Institute of Physics of Materials, CZ-61662 Brno, Cech Republi eccived 9 September 1997: accepted 24 November 199 Abstract The thermal aging in argon of a commercially available sic- fber reinforced glass matrix composite was investigated at temperatures in the range 500-700'C for exposure duration of up to 1000 h. An inert atmosphere sed to study the effects of temperature alone, thus minimizing and neglecting the effects of oxidation. The mechanical properties of aged samples were evaluated at room-temperature by using four-point flexure strength and three-point flexure chevron-notch techniques. The interfacial properties were determined by push-out inden tation measurem ing an in-situ SEM indentation apparatus. The fracture toughness values determined by the chevron-notch tests were liule affected b ging conditions and were in the range 19-26 MPay'm. The frictional interfacial shear stress was not affected by the aging condition For the most severe aging conditions investigated( 1000 h at 600oC and 100 h at 700 C), a significant loss of flexure strength and stiffness of the samples was detected, which has been ascribed to microstructural changes that occurred in the material during aging as a consequence of the softening of the glass matrix. At these aging conditions, a lower interfacial shear stress for fiber-matrix debonding initiation was measured, which may be explained also by the occurrence of matrix softening and void formation. @1998 Elsevier Science S.A. All rights reserved Keywords. Glass mats ix tuIlipusiles, Aging: Fracture brtluvior: SiC-liber 1. Introduction through matrix microcracks pre-existing in the material as a result of the fabrication process, or through cracks formed The development of faw-tolerant fiber-reinforced glass, under mechanical or thermomechanical thermal shock glass-ceramic and ceramic matrix composites in the last 20 loading. Oxygen penetration from the free- ends of the fibers years has resulted in a new family of structural materials exposed to the environment is also possible and in this case exhibiting quasi-ductile fracture behavior. oxidation resis- degradation of the structural integrity of the material may nce and high temperature capability [1]. A fundamental occur as a result of long-term exposure at high-temperature requisite for these materials to be useful at high temperatures The thermal stability of silicate matrix composites in oxidiz- is the retention of a weak fiber/matrix interface, which is ing environments has been investigated quite extensively in responsible for their naw-Lolerant behavior 1.2]. In silicate the past by conducting thermal aging experiments overa wide matrix composites containing silicon oxycarbide fibers such range of temperatures [9. 11-20]. A common result of inves s Nicalon and Tyranno as reinforcement [3-9]. the weak tigations conducted at temperatures in the range 500-700C interface is provided by a carbon-rich layer around the fibers is that there is a decrease of tensile and flexural strength of which is grown in-situ during composite fabrication [10] the composites as a result of oxidation of the carbon-rich This carbon-rich interlayer can be severely degraded when interfacial layer. Moreover, push-out tests on the fibers of the material is used in oxidizing environments at temperatures aged samples have revealed al increase in the interfacial shear as low as =400C[9.11. 12]. Oxygen penetration can occur stress which has been attributed to the formation of a strong SiOz interfacial bond [ 11. 12, 21, 22]. Thus in the tentioned Technische Universitat Ilmenau, PF 100565, D-98684 Ilmenau, Germany, temperature range(500-700C), a loss of the 'pseudo-duc- Tel :+493677694401: fax:+49 36/7691597: e-mail: aldo, boccaccini(@ tile composite behavior has been frequently observed, with maschinenbau, tu-ilmenau dc reduced fiber pull-out effects [11, 12, 17, 19, 22 0254-0584/98/$19.000 1998 Elsevier Science S.A. All rights reserved PHIS02540584(97102075-0
MATERIALS CHEM;fR\RtdD ELSEVIER Materials Chemistry and Physics 53 ( 1998) 155-164 Thermal aging behavior of an Sic-fiber reinforced glass matrix composite in a non-oxidizing atmosphere A.R. Boccaccini a.*, J. Janczak-Rusch b, I. Dlouhy ’ Abstract The thermal aging in argon of a commercially available Sic-fiber reinforced glass matrix composite was investigated at temperatures in the range 500-700°C for exposure duration of up to 1000 h. An inert atmosphere was used to study the effects of temperature alone, thus minimizing and neglecting the effects of oxidation. The mrchanical properties of aged samples were evaluated at room-temperature by using four-point flexure strength and three-point flexure chevron-notch techniques. The interfacial properties were determined by push-out indentation measurements using an in-situ SEM indentation apparatus. The fracture toughness values determined by the chevron-notch tests were little affected by the aging conditions and were in the range 19-26 MPaJm The frictional interfacial shear stress was not affected by the aging conditions either. For the most bevere aging conditions invest&ated ( 1000 h at 600°C and 100 h at 7OO”C), a significant loss of flexure strength and stiffness of the samples was detected, which has been ascribed to microstructural changes that occurred in the material during aging as a consequence ofthe softening of the glass matrix. At these aging conditions, a lower interfacial shear stress for fiber-matrixdebonding initiation was measured, which may be explained also by the occurrence of matrix softening and void formation. 0 1998 Elsevier Science S.A. All rights reserved E;e~~r~r-d.r: Glass matrix compobites: A:mg: Fracture behavior; Sic-fiber 1. Introduction The development of flaw-tolerant fiber-reinforced glass, glass-ceramic and ceramic matrix composites in the last 20 years has resulted in a new family of structural materials exhibiting quasi-ductile fracture behavior, oxidation resistance and high temperature capability [ I]. A fundamental requisite for these materials to be useful at high temperatures is the retention of a weak fiber/matrix interface. which is responsible for their flaw-tolerant behavior [ 1.21. In silicate matrix composites containing silicon oxycarbide fibers such as Nicalon and Tyranno as reinforcement [ 3-91, the weak interface is provided by a carbon-rich layer around the fibers which is grown in-situ during composite fabrication [IO], This carbon-rich interlayer can be severely degraded when the material is used in oxidizing environments at temperatures as low as = 400°C [ 9,11,12]. Oxygen penetration can occur a * Corresponding author. Praent ad&s: Fachgebirt Wrrkstofftftrchnik, Technische Universitgt Ilmenau. PF 100565. D-98684 Ilmenau. Germany. Tel.: +49 3677 694401: fax: +19 3677 691597; e-mail: aldo.boccaccini@ maschinenb~u..tu-illncnau.de 0154-0584/98/$19.00 ‘C 1998 Elsevier Science S.A. All righta resewed Plls0254-0584(97)02075-0 through matrix microcracks pre-existing in the material as a result of the fabrication process, or through cracks formed under mechanical or thermomechanical (thermal shock) loading. Oxygen penetration from the free-ends of the fibers exposed to the environment is also possible and in this case degradation of the structural integrity of the material may occur as a result of long-term exposure at high-temperature. The thermal stability of silicate matrix composites in oxidizing environments has been investigated quite extensively in the past by conducting thermal aging experiments over a wide range of temperatures [ 9.1 l-201. A common result of investigations conducted at temperatures in the range 500-700°C is that there is a decrease of tensile and flexural strength of the composites as a result of oxidation of the carbon-rich interfacial layer. Moreover, push-out tests on the fibers of aged samples have revealed an increase in the interfacial shear stress, which has been attributed to the formation of a strong SiO-, interfacial bond [ 11.12,2 1,221. Thus. in the mentioned temperature range (500-700°C)) a loss of the ‘pseudo-ductile’ composite behavior has been frequently observed, with reduced fiber pull-out effects [ 11,12,17,19,22]
A R. Boccaccini er al. /Materals chemistr and Physics 33 (1998)155-10 Experimental research has been conducted also on the silicate DURAN) glass matrix composite fabricated by mechanical properties of these composites at high tempera- SchoLl Glaswerke(Mainz. Germany ) Information on th tures in inert atmospheres. The first work on this subject was composite constituents is given in Table 1. The composites probably that carried out as early as in 1969 by Crivelli- were prepared by the sol-gel-slurry method. Details on the Visconti and Cooper [23] un C-fiber reinforced silica Tested processing technique are found in the literature [5,6].The at 800C. A more detailed investigation was conducted by samples were received in the form of rectangular test bars of Prewo et al. [24], showing that the ultimate strength of Sic nominal dimensions (4.5 Im X 3. 8 ImIn X 100 IlIm ).The Nicalon) fiber-lithium aluminosilicate matrix composites density of the composites was 2.4 g cm-3and their fiber did not change significantly from the valuc at room-temper- volume fraction =0.4. An SEM image showing the micros- ature when tested in argon at temperatures as high as 1300C. tructure of an as-received sample is shown in Fig. 1. The However, limited data exist related to the thermal aging Youngs modulus of as-received samples(E=122+2 GPa) behavior in inert atmospheres of this kind of composites, i.e. was determined using a forced resonance frequency tech for unstressed long-tcrm exposures at high temperatures in,. nique, as shown elsewhere [29] for example, vacuum or argon. Certainly, the lack of interest Thermal aging involved encapsulating the as-received bars in the performance of composites in non oxidizing environ- in silica tubes after these had been evacuated and filled with ments has been motivated by the apparent absence of signif- argon. Argon of technical purity was used, the oxygen content icant envisaged potential applications for these materials in in the capsules being of a level lower than 0.001 Lorr(. 133 such environments. Besides the fact that some application Nm). The capsules were introduced in a furnace at given possibilities in non-oxidizing environments do exist [25], aging temperatures and maintained for 100 h. Three the assessment of thermal aging behavior in inert atmospheres temperatures were tried: 500%C, 600oC and 700C presents a convenient way to separate'pure'thermaleffects temperatures were chosen to correspond with those used in from oxidation effects, both of which are likely to occur for separate studies on thermal shock and thermal cycling behav exposures in air. Moreover, knowledge of the materials ior of similar composites [27, 29]. They cover the upper response at high-temperatures in the absence of oxygen will temperature range at which these composites are likely to find aid in assessing the relative e fficacy of strategies developed for inhibiting the rate of oxidation embrittlement (e.g. fiber Table I coatings and use of oxygen getters in the matrix [26]).On operties of the composite constituents [5.61 this basis. the on-going research program, focused on inves- Property Matrix: DURAN Fibre: SiC Nicalon tigating the thermomechanical performance of commercially (borosilicate glass) (NL 202 available SiC-Nicalon fiber reinforced silicate matrix com- posites, has contemplated the study of aging effects in inert Density(g cm - atmospheres in addition to the experiments in oxidizing envi- Young's modulus( GPa) 63 ronments. In this article, the results of the investigations con- Poissons ratio 020 ducted employing non-oxidizing atmospheres are presented Thermal expan 3.25×10 coefticient (K A separate article deals with the results in oxidizing environ Tensile strength(MPa) 2750 ments[27] Fibre volume fraction: 0.4 A commercial borosilicate glass matrix composite rein- Fibre radius: 5-9 um forced with Sic (Nicalon) fibers was investigated. Aging was conducted at temperatures between 500C and 700C in argon. Several techniques were used to detect the possible degradation of the structural integrity and mechanical prop- erties of the materials after the heat-treatments, including standard four-point flexure strength tests, three-point flexor chevron-notch tests. and fiber push-out indentation tests nother objective of this article is to examine the applicability of the chevron-notched specimen technique to monitor changes in the mechanical properties in this class of compos- ite materials. In the literature reviewed by the authors, only one study was found which has considered in the past the u of chevron-notched specimens to evaluate fracture properties in fiber reinforced ceramic matrix composite 2. Experimental The material investigated was a commercially available Fig. 1 SEM image of a polished section of an as-received sample. showing unidirectional SiC Nicalon(NL202)fiber reinforced boro- the fairly regular fiber distribution and absence of porosity
Experimental research has been conducted also on the mechanical properties of these composites at high temperatures in inert atmospheres. The lirst work on this subject was probably that carried out as early as in 1969 by CrivelliVisconti and Cooper [ 231 on C-fiber reinforced silica tested at 800°C. A more detailed investigation was conducted by Prewo et al. 1241, showing that the ultimate strength of Sic (Nicalon) fiber-lithium aluminosilicate matrix composites did not change significantly from the value at room-temperature when tested in argon at temperatures as high as 1300°C. However, limited data exist related to the thermal aging behavior in inert atmospheres of this kind of composites, i.e. for unstressed long-term exposures at high temperatures in, for example, vacuum or argon. Certainly, the lack of interest in the performance of composites in non-oxidizing environments has been motivated by the apparent absence of significant envisaged potential applications for these materials in such environments. Besides the fact that some application possibilities in non-oxidizing environments do exist [ 2.51, the assessment of thermal aging behaviorin inertatmospheres represents a convenient way to separate ‘pure’ thermal effects from oxidation effects, both of which are likely to occur for exposures in air. Moreover, knowledge of the materials response at high-temperatures in the absence of oxygen will aid in assessing the relative efficacy of strategies developed for inhibiting the rate of oxidation embrittlement (e.g. fiber coatings and use of oxygen getters in the matrix [ 261). On this basis. the on-going research program, focused on investigating the thermomechanical performance of commercially available Sic-Nicalon fiber reinforced silicate matrix composites, has contemplated the study of aging effects in inert atmospheres in addition to the experiments in oxidizing environments. In this article, the results of the investigations conducted employing non-oxidizing atmospheres are presented. A separate article deals with the results in oxidizing environments [ 271, A commercial borosilicate glass matrix composite reinforced with Sic (Nicalon) fibers was investigated. Aging was conducted at temperatures between 500°C and 700°C in argon. Several techniques were used to detect the possible degradation of the structural integrity and mechanical properties of the materials after the heat-treatments, including standard four-point Rexure strength tests, three-point flexure chevron-notch tests, and fiber push-out indentation tests. Another objective of this article is to examine the applicability of the chevron-notched specimen technique to monitor changes in the mechanical properties in this class of composite materials. In the literature reviewed by the authors, only one study was found which has considered in the past the use of chevron-notched specimens to evaluate fracture properties in fiber reinforced ceramic matrix composites 128 1. 2. Experimental The material investigated was a commercially available unidirectional Sic Nicalon (NL202) fiber reinforced borosilicate (DURAN) glass matrix composite fabricated by Schott Glaswerke (Mainz, Germany). Information on @e composite constituents is given in Table 1. The composites were prepared by the sol-gel-slurry method. Details on the processing technique are found in the literature [5,6]. The samples were received in the form of rectangular test bars of nominal dimensions (4.5 mm X 3.8 mm X 100 mm). De density of the composites was 2.4 g crnm3 and their fiber volume fraction = 0.4. An SEM image showing the microstructure of an as-received sample is shown in Fig. 1. The Young’s modulus of as-received samples ( E = 122 f 2 GPa) was determined using a forced resonance frequency technique, as shown elsewhere [ 291. Thermal aging involved encapsulatingthe as:received bars in silica tubes after these had been evacuated and filled with argon. Argon of technical purity was used, the oxygen content in the capsules being of a level lower than 0.001 torr [O. 13; Nm-‘). The capsules were introduced in a furnace at given aging temperatures and maintained for 100 h. Three aging temperatures were tried: 500°C. 600°C and 700°C. Thxe temperatures were chosen to correspond with those used in separate studies on thermal shock and thermal cycling behavior of similar composites [ 27,291. They cover the upper temperature range at which these composites are likely to find Table 1 Properties of the composite conbtituentz. [5.6] Property Matrix: DURAN Fibrc: Sic Nicalon (borosilicate glaw) (NL 203) ~ Density ig cm-‘) 2.23 3.55 Young’s modulus ( GPa) 63 196 Poisson‘5 ratio 0.22 0.20 Thermal expansion 3,2s x lo-” 3X10-h ~~ coeflicient I K- I ) Tensile strength (MPa) =60 2750 Fibre volume fraction: O.-l Fibre radius: S-9 pm Fig. 1. SEM image of a polished section of an as-received sample. showing the fairly regular fiber distribution and absence of porosily
A R, Boc'caccint et al. /Materials Chemistr and Physics 53(/998)155-104 applications. For the 600"C tests. aging experiments of 250 and 1000 h were conducted also. After thermal treatment. the mass and dimensions of the samples were measured, and the outer surfaces inspected carefully for the appearance of any macroscopic damage, such as change of color, delamination, fiber debonding/protrusion, etc. X-ray diffraction (XRD) analyses were performed on polished sections of the sample after aging in order to detect any crystallization in the matrix Aged samples together with samples in the as-received con- dition were characterized using several mechanical testing The flexural strength( modulus of rupture)and Youngs modulus were determined in four-point bending using 32 mm outer span and 16 mm inner span. A cross-head speed of 0. 1 mm s was used. The chevron-notched specimen tech nique was employed for fracture toughness determination Fig. 2. Indentor acting on an SiC-Nicalon fiber in an as-received sample during a push -out test Chevron notches with angles of 90 were cut carefully using a thin (0. 15 mm) diamond wheel. A three point bending sure. Similar measurements on as-received samples have configuration at stant cross-head speed of 0. 1 mm been conducted previously [29,35]. The measurement ofthe min"was employed and the test bars were 60 mm in length. interfacial characteristics was conducted using an in-situ Graphs of load versus time were recorded and the maximu SEM indentation apparatus(Touchstone Ltd, West Virginia, force was determined from each trace. The fracture toughness USA). The thickness of the specimens( two samples foreach value was calculated from the maximum load( Fmax )and the condition) was in the range 340-362 um. A diamond inden orresponding minimum value of geometrical compliance tor was used to apply the load to the fibers at a displacement function(Ymin" )according to the equation 30] rate of 0. 18 um s-. The apparatus offers high accuracy for Fm y the indentor positioning, which allows the push-out (1) to be carried out without fiber and /or matrix damage. The load and displacement data are digitally recorded and the where B and w are the thickness and height of the specimens, push-out events are observed in-situ. As an example of the respectively. The calculation of the geometric function Yn technique. Fig. 2 shows the indentor acting on an Sic Nicalon for chevron-notched bend bars was based on the use of fiber in an as-received sample. Between 15 and 45 fibers were Bluhm's slice model [31]. To overcome the associated com- indented at each condition and the average interfacial fric putational complexities of this approach, Shang-Xian [321 tional shear stress(r). as well as the interfacial shear stress and Withey et al. [ 33] have suggested a simplified solution at the debonding initiation( Tinit )and the debonding shear to determine normalized stress intensity factors for chevron- strength (Tb) were determined. A complete description of notched specimens using trigonometric functions. The cal- the indentation push-out technique and details concerning the ulation procedure used for the purposes of this investigation calculation of the stresses are presented elsewhere [351 has been described in detail elsewhere [34]. The chevron notch depth ag was measured after testing from SEM micro- graphs of fractured specimens. Additionally, the acoustic 3. Results emission technique was used during the test. Traces of cumu- lative number of counts(acoustic emission(AE) events 3 1. Mechanical tests were obtained in the same time scale as the load versus time plots. This technique allows for an accurate detection of the Table 2 summarizes the results obtained from the mechan- onset of microcracking at the chevron notch tip which occurs ical tests performed. The data for the Youngs modulus and when a sharp increase in the number of AE events is observed. the ultimate flexural strength for the as-received and heat Valid measurements for computing Kle are those in which treated material correspond closely to values reported in the this increase of AE events coincides with the end of the linear literature for similar composites [5,6], except for the two part of the force versus time trace, as explained below, The more extreme aging conditions of 600oC-1000 h and 700C work of fracture( woF) was evaluated from the area under 00 h. For these two conditions, there is an abrupt decrease the load-displacement curve. At least five measurements for of the flexural strength and a(less pronounced decrease of rere pe d and the results ar he young s modulus. the fracture toughness values deter fracture surfaces of broken samples were observed by SEM. mined by the chevron-notched specimen technique seem to Push-out indentation tests on aged samples were performed be little dependent on the aging condition. The material exhib in order to assess if any change had occurred in the interfacial its average Ki values in the range 19-26 MPay'm, which are properties as a consequence of the high-temperature expo- similar to data quoted in the literature for similar fiber rein
applications. For the 600°C tests, aging experiments of 2.50 and 1000 h were conducted also. After thermal treatment, the mass and dimensions of the samples were measured, and the outer surfaces inspected carefully for the appearance of any macroscopic damage, such as change of color, delamination, fiber debonding/protrusion, etc. X-ray diffraction (XRD) analyses were performed on polished sections of the samples after aging in order to detect any crystallization in the matrix. Aged samples together with samples in the as-received condition were characterized using several mechanical testing techniques. The Aexural strength ( modulus of rupture) and Young’s modulus were determined in four-point bending using 32 ~-II outer span and 16 mm inner span. A cross-head speed of 0.1 mm s- ’ was used. The chevron-notched specimen technique was employed for fracture toughness determination. Chevron notches with angles of 90” were cut carefully using a thin (0.15 mm) diamond wheel. A three point bending configuration at a constant cross-head speed of 0.1 mm min-’ was employed and the test bars were 60 mm in length. Graphs of load versus time were recorded and the maximum force was determined from each trace. The fracture toughness value was calculated from the maximum load (F,,,,,) and the corresponding minimum value of geometrical compliance function ( Ymln+) according to the equation [ 30 ] : where B and Ware the thickness and height of the specimens, respectively. The calculation of the geometric function Y,,,,“* for chevron-notched bend bars was based on the use of Bluhm’s slice model [ 3 I 1. To overcome the associated computational complexities of this approach, Shang-Xian [ 32 ] and Withey et al. [ 331 have suggested a simplified solution to determine normalized stress intensity factors for chevronnotched specimens using trigonometric functions. The calculation procedure used for the purposes of this investigation has been described in &tail elsewhere [ 341. The chevronnotch depth cl0 was measured after testing from SEM micrographs of fractured specimens. Additionally. the acoustic emission technique was used during the test. Traces of cumulative number of counts (acoustic emission ( AE) events) were obtained in the same time scale as the load versus time plots. This technique allows for an accurate detection of the onset of microcracking at the chevron notch tip. which occurs when a sharp increase in the number of AE events is observed. Valid measurements for computing K,, are those in which this increase of AE events coincides with the end of the linear part of the force versus time trace, as explained below. The work of fracture ( WOF) was evaluated from the area under the load-displacement curve. At least five measurements for each condition were performed and the results averaged. The fracture surfaces of broken samples were observed by SEM. Push-out indentation tests on aged samples were performed in order to assess if any change had occurred in the interfacial properties as a consequence of the high-temperature expoFig 2. lndentor acting on an Sic-Nicalon fiber in an as-received si during a push-out test. sure. Similar measurements on as-received samples have been conducted previously [ 29,351. The measurement of the interfacial characteristics was conducted using an in-situ SEM indentation apparatus (Touchstone Ltd., West Virginia, USA). The thickness of the specimens (two samples foreach condition) was in the range 340-362 km. A diamond indentor was used to apply the load to the fibers at a displacement rate of 0.18 km s-‘. The apparatus offers high accuracy for the indentor positioning, which allows the push-out process to be carried out without fiber and/or matrix damage. The load and displacement data are digitally recorded and the push-out events are observed in-situ. As an example of the technique, Fig. 2 shows the indentor acting on an SIC Nicalon fiber in an as-received sample. Between 1.5 and 45 fibers were indented at each condition and the average interfacial frictional shear stress ( Tag), as well as the interfacial shear stress at the debonding initiation (.i;,,,,) and the debonding shear strength ( 7db) were determined. A complete description of the indentation push-out technique and details concerning the calculation of the stresses are presented elsewhere [ 351. 3. Results 3.1. Mec~hnizic~al tests Table 2 summarizes the results obtained from the mechanical tests performed. The data for the Young’s modulus and the ultimate flexural strength for the as-received and heattreated material correspond closely to values reported in the literature for similar composites [ 5,6]. except for the two more extreme aging conditions of 6OO”C-1000 h and 7OO”C- 100 h. For these two conditions, there is an abrupt decrease of the flexural strength and a (less pronounced) decrease of the Young’s modulus. The fracture toughness values determined by the chevron-notched specimen technique seem to be little dependent on the aging condition. The material exhibits average K,, values in the range 19-26 MPaJm, which are similar to data quoted in the literature for similar fiber rein-
158 A.R. Boccaceint er al/ Marericis Chemistr and Piivsics 53(7998)155-1o4 Table 2 Mechanical properties of SiC-fiber reinforced DURAN-glass matrix composites after agings at different durations and temperatures in argon Aging condition Y Flexure strength Fracture touches Work of fracture K, (st, dev, Jm-2) Duration(h) (MPa) f MPam) 500:100 240(0.9) 600:100 7550 600:250 25.7!.3 600:1000 0.8(0.7) 700:100 00 9.2(1.1 638 not measured not measured Determined in four-point bending. etermined using three N aged 600"C/100h CE+O 03D ig. 3. Typical load-displacement plots obtained in chevron-notch tests for samples aged for 100 h at different temperatures: (a)500 C, (b)600 C, (c)700C forced glasses and glass-ceramics 16] and for SiC/SiC MPay/m)[16]. The lower work of fracture of the samples itcs [36]. These Kle values are also similar to the aged at 600 C for 1000 h and at 700C for 100 h can be fracture toughness of aluminum alloys( 22 MPa/m)an correlated with the lower flexure strength of these materials 30% lower than those of graphite epoxy composites
Table 2 Mechanical properties of Sic-fiber reinforced DURAN-glass matrix composites after agings at different durations and temperatures in argon Aging condition Young’s modulus * Flexure strength <’ Fracture toughness ’ Temp. (“C); E CT If,, (st. dev.) Duration (h) IGPd) (!VfPd) (IMPa\im) Work of fracture h (Jm-‘1 500: 100 128 603 600; roe 123 691 600: 250 126 722 600: 1000 107 253 700; 100 100 184 as-received ’ 122 688 ~’ Determined in four-point bending. h Determined using three-point bending chevron-notched specimens. ’ From a previous study [ 291. 24.0 (0.9) 23.8 (1.4) 25.7 (1.3) 203 (0.71 19.2 r i.1) not measured 6990 75.50 8310 ~ 6450 1420 not measured .~~ &%O’C/100h 1200 - 800 - ml 600 - -AE 400 - - 4E+4 0 00 0.10 0 20 0.30 040 deflection 1600 I a 1 I I I , , I , I < , * force (b) . WI 1400 - a@6COT/iCOh - lx+4 - AE . events - 4E+4 , 0.10 0.20 0.30 deflection [mm] OiJ, ! s 3 > IOE,O 000 0 10 0.20 0.30 0.40 deflection Fig. 3. Typical load-displacement plots obtained in chevron-notch tests for samples aged for 100 h at different temperatures: (a) ~OO@C, (b) 6oo”c, cc) 7~7~. forced glasses and glass-ceramics [ 161 and for Sic/Sic MPaJm) [ 161. The lower work of fracture of the samples composites [ 361. These K,, values are also similar to the aged at 600°C for 1000 h and at 700°C fo? 100 h can-be fracture toughness of aluminum alloys (22 MPaJm) and only correlated with the lower flexure strength of these materials. = 30% lower than those of graphite-epoxy composites (34 Typical load-displacement plots obtained in chevron-notch
R Boccaccini er u. / Materials Chemistry and PhysIcs 53(1998)155-104 3.2. Push-out indentation tests Fig. 5 shows typical stress-displacement curves during aged 600"C/100 h 600/250h oush-out indentation for samples aged for 100 h at 500C 600C and 700C, The interfacial parameters, frictional shear 600e/1000h stress( Tr), debonding initiation shear stress(Tinit )ane debonding shear strength(Tdb), were determined from the data of the push-out experiments using the linear model of Marshall [37]. The calculated values are listed in Table 3 Similar measurements on as-received samples have been con ducted previously [29,35], and the results are also listed in Table 3. These data confirm the existence of a weak fiber/ matrix interfacial bond, which results from the presence of a thin carbon-rich laver at the interface. a characteristic feature of these composite materials [38]. It is observed that for the aged samples, Tr remains nearly constant(Tr=13-18 MPa) and close to the value obtained for the as-received material Thus. these results indicate that the carbon-rich interfacial 8x 0.00 0. 10 deflection (mm) 0.40 0.5 layer at the matrix/fiber interface, which provides thecom- Fig4. Load-displacement curves obtained in chevron-notch tests for three posite'properties to the material, i.e. the pseudo-ductile frac samples aged at 600C for different durations ture behavior, has not been degraded significantly under the heat-treatments in inert environments. The average debond tests are shown in Fig. 3 for samples aged for 100 h at dif- ing shear stress( Tab) was also little influenced by the aging ferent temperatures. Also the acoustic emission events conditions, being also in close agreement with the values recorded during the tests are plotted. Fig 4 shows the load- determined in the as-received material(Tab=29+3 MPa or displacement curves obtained in chevron-notch tests for three 37+4 MPa for sample thicknesses of 340 and 355 um samples aged at 600C for different durations. The poorer respectively). Table 3 also reveals that for agings at 500C mechanical response of the samples aged at the most severe all three characteristic stresses are very close to the conditions, 100 h at 700C and 1000 h at 600C. hecomes received values and in both cases the standard deviation of evident by observing the plots the data are similar. However, starting with the aging at 10 Fig. 5. Typical stress-displacement curves during push-out indentation for samples aged for 100 h at 500C. 600C and 700C Table 3 Interfacial property data measured by push-out indentation technique, Aging of the samples was conducted for 100 h at the temperatures indicated Thickness Average diameter of tcsted fibre shear stress hear stress debonded fues ( MPa Tab(MPa) Tr(MPa) s receive 17.55÷1.23 16.8±3.7 4±3.3 15.6±2.0 As received 18.00±I.9 172±4.5 375±4,4 175±3.6 Aging 500C 362 16.17±.0 0+5,6 17.7±2.6 aging600°C 16.48±97 8.1±1.7 94±8.5 4.6±3.1 A 1625±1.30 8.5±4.4 11.2±3.1 0.0+4. 324±82 13.0±2.6
L59 1600 1111,,~1,,11~,,1111,1111 force PI 1400 - aged 600 “C / 100 h 6OO”C/ 250 h - 1200 - 600”C/1000h - 1000 - 800 - 600 - 0.00 0 10 0.20 0.30 deflection [mm] 0 40 0.3 Fig. 4. Load-displacement curves obtained in chevron-notch tests for three samples aged at 600°C for different durations. tests are shown in Fig. 3 for samples aged for 100 h at different temperatures. Also the acoustic emission events recorded during the tests are plotted. Fig. 4 shows the loaddisplacement curves obtained in chevron-notch tests for three samples aged at 600°C for different durations. The poorer mechanical response of the samples aged at the most severe conditions, 100 h at 700°C and 1000 h at 6OO”C, becomes evident by observing the plots. 3.2. Push-out indentation tests Fig. 5 shows typical stress-displacement curves during push-out indentation for samples aged for 100 h at 5OO”C, 600°C and 700°C. The inter-facial parameters, frictional shear stress (rr,), debonding initiation shear stress (Ti”i,) and debonding shear strength ( TV,,), were determined from the data of the push-out experiments using the linear model of Marshall [ 371. The calculated values are listed in Table 3. Similar measurements on as-received samples have been conducted previously [ 29,351, and the results are also listed in Table 3. These data confirm the existence of a weak fiber/ matrix interfacial bond, which results from the presence of a thin carbon-rich layer at the interface, a characteristic feature of these composite materials [ 381. It is observed that for the aged samples, rfTfr remains nearly constant ( 7rr = 13-l 8 MPa) and close to the value obtained for the as-received material. Thus, these results indicate that the carbon-rich interfacial layer at the matrix/fiber interface, which provides the ‘composite’ properties to the material, i.e. the pseudo-ductile fracture behavior, has not been degraded significantly under the heat-treatments in inert environments. The average debonding shear stress (Tag) was also little influenced by the aging conditions, being also in close agreement with the values determined in the as-received material ( rdb = 29 f 3 MPa or 37 i4 MPa for sample thicknesses of 340 and 355 p,m, respectively). Table 3 also reveals that for agings at 500°C all three characteristic stresses are very close to the asreceived values and in both cases the standard deviation of the data are similar. However, starting with the aging at 0 1 2 3 4 5 6 7 8 9 IO Displacement (microns) Fig. 5. Typical stress-displacement curves during push-out indentation for samples aged for 100 h at 500°C 600°C and 700°C. Table 3 Interfacial property data measured by push-out indentation technique. Aging of the samples was conducted for 100 h at the temperatures indicated Specimen Thickness (pm) Average diameter of Debonding initiation Debonding shear Frictional Percentage of teared tibre shear stress strength shear stress debonded fibrea (pm) L,,, ( MPa) 7db (MPa) G (MPa) (S’o) As received 310 17.55 + 1.23 16.5 i 3.7 29.4 + 3.3 15.6k2.0 0 As received 355 IS.00 i I .97 17.2k4.5 37.5 14.4 17.5 f 3.6 0 Aging 500°C 362 16.17+ 1.06 17.3 i2.7 35.Ok5.6 17.7 & 2.6 0 Aging 600°C 318 16.48 k 1.97 8.1 + 1.7 39.4 i 8.5 14.6i3.1 13 Aging 700°C 360 16.25 i 1.30 8.5 * 4.4 27.3 i 10.9 11.2+3.1 25 without debonded libres: 10.0~4.1 32.4 k 8.2 13.012.6 0