《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Al2O3-SiC-28.pdf

Journal of the European Ceramic Society 15(1995)191-199 Printed in Great Brita Fractography of Fatigued and Fractured Regions in a Silicon Carbide Whisker Reinforced Alumina Composite Ashok Kr. Ray, Swapan Kr. Das, Prabir Kr. Roy National Metallurgical Laboratory, Jamshedpur 831007, Bihar, India S Banerjee Research and Developmerit Centre for Iron and Steeh SAIL, Ranchi 834002, Bihar, India (Received 18 May 1994; revised version received 19 July 1994; accepted 1 August 1994) Abstract whe h applications, these ceramic uld often encounter fatigue cracked and fast fractured regions in four and cyclic loading which produce crack extension point bend specimens prepared from 25 wt% silicon Therefore, the fractographic features of the fatigue carbide whisker reinforced alumina composites were failed samples need to be examined to identify the examined by Scanning Electron Microscopy. In the likely micromechanism of crack advance under fatigue cracked region, the alumina matrix failed monotonic and cyclic loading in this composite mainly in a transgranular mode and the whiskers Recently, Dauskardt et al. have made an exten failed mainly with a fat fracture surface but with- sive fractography of fatigue failed regions in a 15 out pullout. On the other hand, in the fast fractured vol% SiC whisker-reinforced alumina composite region, the whiskers failed predominantly by pullout The identification of the fractographic features at and the alumina matrix failed in a mixed mode with the low, medium and high Stress Intensity Range about half in transgranular and the other half in (AK) fatigue region as well as in the fast fracture intergranular fracture. Thus, to improve the fracture region can give us a clue to the likely mechanisms toughness of the material, the grain boundar of fracture in our material strength of alumina and the matrix whisker inter facial bonding should be improved. To increase the resistance to fatigue, the fracture strength of the 2 Experimental Procedure alumina grains should be improved by using finer a-alumina particles and the fatigue strength of theThe four point bend specimens were sliced, pre- whisker has to be increased by improving the uni- pared, surface finished and randomised from a 15 formity in distribution of B-Sic whiskers during hot mm X 50 mm x 12 5 mm preformed billet of 25 wt% pressing silicon carbide whisker reinforced alumina com posite material(Fig. 1(a). The details of fabrication of the billet are given elsewhere. 1 Introduction The alumina powder used was of a-Lype. The Sic whiskers had been produced by a carburize Fatigue crack growth rate(FCGr), 2and fracture tion process at 1600 C where the sources of silicon toughness-3of 25 wt% silicon carbide whisker and carbon were rice husk ash and rice husk reinforced alumina ceramic composite have been hydrocarbons. studied and reported. -Such studies are impor- The particle size of the alumina powder was less tant since these materials have potential applica- than 1 um. The average whisker diameter was tion in the production of structural components 0-1-025 um as revealed in the lt plane of the used at elevated temperatures, in high efficiency montage of the ceramic composite investigated heat engines and heat recovery systems and for here( fig. 1 (b)). The length of the whiskers varied making cutting tools to rnachine special materials. between 10 and 30 um

Journal of Ihe European Ceramic Sociery 15 (1995) 191-199 Elsevier Science Limited Printed in Great Britain 0955-2219/95/$9.50 Fractography of Fatigued and Fractured Regions in a Silicon Carbide Whisker Reinforced Alumina Composite Ashok Kr. Ray, Svvapan Kr. Das, Prabir Kr. Roy National Metallurgical Laboratory, Jamshedpur 83 1007, Bihar, India & S. Banerjee Research and Development Centre for Iron and Steel-SAIL, Ranchi 834002, Bihar, India (Received 18 May 1994; revised version received 19 July 1994; accepted 1 August 1994) Abstract Fatigue cracked and fast fractured regions in four point bend specimens prepared from 2.5 wt% silicon carbide whisker reinforced alumina composites were examined by Scanning Electron Microscopy. In the fatigue cracked region, the alumina matrix failed mainly in a transgranular mode and the whiskers failed mainly with a flat fracture surface but without pullout. On the other hand, in the fast fractured region, the whiskers failed predominantly by pullout and the alumina matrix j&led in a mixed mode with about half in transgranular and the other half in intergranular fracture. Titus, to improve the fracture toughness of the material, the grain boundary strength of alumina and the matrix whisker interfacial bonding should be improved. To increase the resistance to fatigue, the fracture strength of the alumina grains should be improved by using finer a-alumina particles and #the fatigue strength of the whisker has to be increased by improving the uniformity in distribution of P-Sic whiskers during hot pressing. 1 Introduction Fatigue crack growth rate (FCGR)‘,2 and fracture toughness1-3 of 25 wt% silicon carbide whisker reinforced alumina ceramic’ composite have been studied and reported.‘-3 Such studies are important since these materials have potential application in the production of structural components used at elevated temperatures, in high efficiency heat engines and heat recovery systems and for making cutting tools to machine special materials. 191 When used in such applications, these ceramic components would often encounter monotonic and cyclic loading which produce crack extension. Therefore, the fractographic features of the fatigue failed samples need to be examined to identify the likely micromechanism of crack advance under monotonic and cyclic loading in this composite. Recently, Dauskardt et ~1.~ have made an extensive fractography of fatigue failed regions in a 15 ~01% Sic whisker-reinforced alumina composite. The identification of the fractographic features at the low, medium and high Stress Intensity Range (AK) fatigue region as well as in the fast fracture region can give us a clue to the likely mechanisms of fracture in our material. 2 Experimental Procedure The four point bend specimens were sliced, prepared, surface finished and randomised from a 15 mm X 50 mm X 12.5 mm preformed billet of 25 wt% silicon carbide whisker reinforced alumina composite material (Fig. l(a)). The details of fabrication of the billet are given elsewhere.3 The alumina powder used was of a-type.’ The Sic whiskers had been produced by a carburization process at 1600°C where the sources of silicon and carbon were rice husk ash and rice husk hydrocarbons.3s5 The particle size of the alumina powder was less than 1 pm. The average whisker diameter was 0.1-0.25 pm as revealed in the LT plane of the montage of the ceramic composite investigated here (Fig. l(b)). The length of the whiskers varied between 10 and 30 pm.3

fractography of fatigued and fractured regions 193 Wei had concluded in their work6 that whisker orient during processing of hot R=0.1.f=1Hz pressed Sic-whisker reinforced alumina leads to anisotropy in both fracture toughness and fracture x- with v notch 8 strength of the composites. In other words, their fracture strengths are limited by the nonunifor mity of the distribution of the whiskers, i.e by the ability to disperse the sic whiskers. They also n=15.5 found that the dispersion of the whiskers improved by using finer alumina powder and hence an increase in the fracture strength of the compos- ite was observed. Nevertheless, they have clearly stress Intensity Rang,△K(MPa「) observed6, , that similar to our composite under Fig. 4. Fatigue crack growth data of 25 wt% SiC reinforced investigation(Fig. 1(b)), the whiskers were prefer- Al,O3 composite entially aligned perpendicular to the hot pressing distribution of whiskers sug- four point bend specimens. On the remaining half, gested that a great deal of rearrangement of notches were inserted exactly at the centre of the whiskers and powder occurred in the initial stage 3 mm x 50 mm surface of the specimens with a of densification of the composites and/or the carborundum wheel to a depth of 0. 1 mm. There- matrix material underwent considerable deformation after, the specimens were subjected to fatigue loading in a servohydraulic test machine MTS-880 The dimensions of the billet and the orientation (100 kn capacity) to precrack the specimen in an of the four point bend specimens(3 mm x 4 mm articulated bridge fixture, 2 till the ratio of crack X 50 mm)in the billet is given in Fig. 1(a). The length to width was 005 on the (4 mm X 50 mm) 4 mm X 50 mm faces, were normal to the hot-press- surface of the specimen. Precracking was con ing direction so that both the direction of crack ducted at a force of 4-5 kN, load ratio, R=0-1 propagation( Fig 3)and the crack plane were par- and frequency, f= 20 Hz. After this, the fatigue allel to the LT plane(Fig. l(a)) and normal to the crack growth rate in the specimen was determined hot-pressing direction. The 3 mm X 50 mm faces at increasing values of Stress Intensity Range AK of the specimen were parallel to the st plane (Fig. 4). under normal four point bend loading A Vickers indentation produced at 0-8 kN load(see Fig 3)until the ratio of crack length to width at the centre of the 3 mm x 50 mm face of the was about 0.45-0.5. on the 4 mm x 50 mm sur- sample acted as the crack starter on half of the face of the specimen. Thereafter, the test to deter- mine the fatigue crack growth rate(FCGr) was P/2 terminated and the specimen was subjected to External span monotonic loading to determine the fracture toughness of the material as per ASTM STP 410. For both FCGR as well as fracture toug ness(Kic tests, a l kn load range of the MTS 880 test machine was used. The test frequency was I Hz and the loading rate was 0.25N s". Tests were conducted in laboratory atmosphere and at ambient temperature. The fracture toughness testing produced the fast fracture. the details of fracture toughness and FCgr tests are given elsewhere. 1,2 P/2 The fracture surfaces were coated with a thin film of gold(thickness 0-02 um) prior to SEM (Scanning Electron Microscopic examination, in a JEOL JSM 840A microscope) The three regions, namely low AK correspond ing to 0-8-1-8 MPa vm; high AK corresponding to 28-3 MPa vm; and, the fast fracture region corre- ALL DIMENSIONS ARE IN MM sponding to a Kic value of 5.9 MPa vm Fig. 3. Indented and precrad.a sf pe four corners of cation of 30 X under SEM(Fig. 5)-using the imen for four point first, identified on the fracture surface at a bend loading. Cracks are located a indentation details of the crack growth rate data generated on

Fractography of fatigued and fractured regions 193 Becher and Wei had concluded in their work6 that whisker orientation during processing of hot pressed Sic-whisker reinforced alumina leads to anisotropy in both fracture toughness and fracture strength of the composites. In other words, their fracture strengths are limited by the nonuniformity of the distribution of the whiskers, i.e by the ability to disperse the SIC whiskers. They also found that the dispersion of the whiskers improved by using finer alumina powder and hence an increase in the fracture strength of the composite was observed. Nevertheless, they have clearly observed6,’ that similar to our composite under investigation (Fig. l(b)), the whiskers were preferentially aligned perpendicular to the hot pressing axis. This type of distribution of whiskers suggested that a great deal of rearrangement of whiskers and powder occurred in the initial stage of densification of the composites and/or the matrix material underwent considerable deformation or creep during hot pressing. The dimensions of thle billet and the orientation of the four point bend #specimens (3 mm X 4 mm X 50 mm) in the billet is given in Fig. l(a). The 4 mm X 50 mm faces, were normal to the hot-pressing direction so that both the direction of crack propagation (Fig. 3) and the crack plane were parallel to the LT plane (Fig. l(a)) and normal to the hot-pressing direction. 3 The 3 mm X 50 mm faces of the specimen were parallel to the ST plane. A Vickers indentation produced at 0.8 kN load at the centre of the 3 mm X 50 mm face of the sample acted as the crack starter on half of the ip” Extcrn;D span 1 p’2 ,&---25 -4 * 2. L-501 ALL DIMENSIONS ARE IN MM. Fig. 3. Indented and precracked specimen for four point bend loading. Cracks are located at the four comers of indentation. 3 $5 ; 2 R= 0.1, f q 1Hz z P k- with V notch & % precrack pi ,g l with xi indentation & a precrack 5 : [ n I 16.1 g nz15.5 x 8 ,g 3 4 5 Stress Intensity Range, AK (MP afi) Fig. 4. Fatigue crack growth data of 25 wt% Sic reinforced A1,03 composite. four point bend specimens. On the remaining half, notches were inserted exactly at the centre of the 3 mm X 50 mm surface of the specimens with a Carborundum wheel to a depth of 0.1 mm. Thereafter, the specimens were subjected to fatigue loading in a servohydraulic test machine MTS-880 (100 kN capacity) to precrack the specimen in an articulated bridge fixture,2 till the ratio of crack length to width was 0.05 on the (4 mm X 50 mm) surface of the specimen. Precracking was conducted at a force of 4-5 kN, load ratio, R = 0.1 and frequency, f = 20 Hz. After this, the fatigue crack growth rate in the specimen was determined at increasing values of Stress Intensity Range AK (Fig. 4) under normal four point bend loading (see Fig. 3) until the ratio of crack length to width was about 0.45-0.5, on the 4 mm X 50 mm surface of the specimen. Thereafter, the test to determine the fatigue crack growth rate (FCGR) was terminated and the specimen was subjected to monotonic loading to determine the fracture toughness of the material’ as per ASTM STP 410.* For both FCGR as well as fracture toughness (K,,) tests, a 1 kN load range of the MTS- 880 test machine was used. The test frequency was I Hz and the loading rate was 0.25 N ss’. Tests were conducted in laboratory atmosphere and at ambient temperature. The fracture toughness testing produced the fast fracture. The details of fracture toughness and FCGR tests are given elsewhere.‘,2 The fracture surfaces were coated with a thin film of gold (thickness 0.02 pm) prior to SEM (Scanning Electron Microscopic examination, in a JEOL JSM 840A microscope). The three regions, namely low AK corresponding to 0.8-l .8 MPa &; high AK corresponding to 2.8-3 MPa 6; and, the fast fracture region corresponding to a K,, value of 5.9 MPa G, were, at first, identified on the fracture surface at a magnification of 30 X under SEM (Fig. 5) - using the details of the crack growth rate data generated on

A.K. Ray, S. K. Das, P. K. Roy, S. Banerjee re surface. Right-hand side--low AK region left-hand side-fast fracture Fig. 6. At low AK (0-8-1.8 MPa Vm)region, the majority of region(FF) the whiskers failed with a square fracture without evidence the specimen as a guideline the difference between the fatigue and fast fracture could, however, be discerned through observation even with the naked eye. The distinction between the low and the high AK fatigue regions was made from the crack length versus the number of cycles data generated during the fatigue loading of the specimen Each region was, at first, carefully scanned at low magnification, to identify the general and uni- formly distributed features. Thereafter, it was examined at two different magnifications 4500 X and 7500 X, in order to identify the fractographic features and the mechanism of fracture in the low AK and the fast fracture regions Fig. 7. At high AK(28-3 MPa vm) region the whiskers 3 Fractographic Observations failed in a mixed mode, i.e. both with a square fracture and also by pullo A summary of the results of the fractographic observations is reported in Table 1 vm), the whisker failed in two different ways: that Table 1 states that, at low AK(08-1. 8 MPa is about 60% of the whiskers failed with a flat Nm), the whiskers failed predominantly by shear- fracture and the balance failed by pullout(Fig. 7) ing with a fat or square fracture without any visi- On the other hand, in the fast fracture region, the ble evidence of necking; this was typical of fatigue whiskers failed predominantly by pullout(Fig 8) fracture(Fig. 6). However, at high AK(2- 8-3 MPa Table 1 also reports that the matrix alumina grains failed predominantly through transgranular fracture( Fig 9)at low AK(0-8-1-8 MPav m).In gions in a 25 wt% silicon carbide whisker reinforced alumina failed in a mixed mode wherein about 45% of the Monotonic alumina grains failed by intergranular and the bal- Mechanisi 2345 ance by the transgranular mode as shown in Fig MPaym MPaym MPavm 10 During monotonic loading, the alumina grains Whisker Pullout ( in the fast fracture region also failed in a mixed mode with a slightly increased percentage of inter- granular (55%)and the balance by transgranular S intergranular (% mode-as shown in Fig. 11 It is noteworthy that in the low AK region, the Crack o River pattern 5PbP A crack had frequently branched(Fig. 12)and deflected(Fig. 13)while it propagated. River pattern

194 A. K. Ray, S. K. Das, P. K. Roy, S. Banerjee Fig. 5. Entire fracture surface. Right-hand side-low AK region; middle-high AK region; left-hand side - fast fracture region (FF). the specimen as a guideline. The difference between the fatigue and fast fracture could, however, be discerned through observation even with the naked eye. The distinction between the low and the high AK fatigue regions was made from the crack length versus the number of cycles data generated during the fatigue loading of the specimen. Each region was, at first, carefully scanned at low magnification, to identify the general and uniformly distributed features. Thereafter, it was examined at two different magnifications 4500 X and 7500 X, in order to identify the fractographic features and the mechanism of fracture in the low AK and the fast fracture regions. 3 Fractographic Observations A summary of the results of the fractographic observations is reported in Table 1. Table 1 states that, at low AK (0.8-1.8 MPa &), the whiskers failed predominantly by shearing with a flat or square fracture without any visible evidence of necking; this was typical of fatigue fracture (Fig. 6). However, at high AK (2.8-3 MPa Table 1. Fractographic features in the fatigue and fracture regions in a 25 wt% silicon carbide whisker reinforced alumina Mechanism of failure Fatigue Monotonic Low AK High AK Fracture 0.8-1.8 2.8-3.0 5.96 MPad6 MPavSi MPa fi Whisker PntloQt T/&) 5 40 85 vs. shear (%) 95 60 15 Matrix Transgranular (x) 95 55 45 vs. intergranular (%) 5 45 55 o Branching P A A o Deflection P A A Crack Q River pattern P A A P = present; A = absent. Fig. 6. At low AK (0.8-1.8 MPa 6) region, the majority of the whiskers failed with a square fracture without evidence of large scale pullout. Fig. 7. At high AK (2.8-3 MPa 6) region the whiskers failed in a mixed mode, i.e. both with a square fracture and also by pullout. &), the whisker failed in two different ways: that is about 60% of the whiskers failed with a flat fracture and the balance failed by pullout (Fig. 7). On the other hand, in the fast fracture region, the whiskers failed predominantly by pullout (Fig. 8). Table 1 also reports that the matrix alumina grains failed predominantly through transgranular fracture (Fig. 9) at low AK (0.8-l .8 MPa&). In the high AK (2.8-3 MPa&) region, the matrix failed in a mixed mode wherein about 45% of the alumina grains failed by intergranular and the balance by the transgranular mode, as shown in Fig. 10. During monotonic loading, the alumina grains in the fast fracture region also failed in a mixed mode with a slightly increased percentage of intergranular (-55%) and the balance by transgranular mode-as shown in Fig. 11. It is noteworthy that in the low AK region, the crack had frequently branched (Fig. 12) and deflected (Fig. 13) while it propagated. River pattern

《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Al2O3-SiC-28

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Al2O3-SiC-28