e 1996 Elsevier Science Printed in Great Britain. All rights 0955-2219(95)00163-8 955-22199631500 Fatigue Crack Growth Rate and Fracture Toughness of 25 wt% Silicon Carbide Whisker Reinforced Alumina Composite with Residual porosity AK.Ray, E.R. Fuller&S. Banerjeec "National Metallurgical Laboratory, Jamshedpur-831007, Bihar, India National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Research and Development Centre for Iron and Steel, SAIL, Ranchi-834002, Bihar, India (Received 8 December 1994; revised version received 20 July 1995; accepted 28 August 1995) Abstract and cyclic loading which produces crack extension Accordingly, in the present investigation, the fatigue The main purpose of this study was to determine the crack growth bchaviour(FCGR) and the fracture fracture toughness and the fatigue crack growth toughness(K,c) of a high density, 25 wt% silicon rate behaviour of 25 wt%o silicon carbide whisker carbide whisker reinforced alumina composite reinforced alumina ceramic composite. The fracture have been studied toughness values determined using the indentation In general, the determination of FCGR and kic technique depended significantly on the crack length of such ceramic materials is difficult since the produced at the corners of the indentation which, in specimens are small, Young's modulus of such turn, depended on the load used for the indentation materials is rather high and the material is brittle and anisotropy in orientation of whiskers in the Consequently, the load, displacement and crack matrix. However, the fracture toughness values length which are all required to be measured determined using the precracked four-point bend for the determination of Kic and FCGR--are specimens were in general higher than that obtained very small and their precise measurement poses by the indentation technique and the value was some problems. In addition, the permissible 596+0. 15 MPa m. The fatigue crack growth dimcnsional tolerance of the specimens and those behaviour in this material was similar to that in of the grips and fixtures used to test the speci the case of metals. However, the exponent for the mens, have to be very close. Particularly difficult is fatigue crack growth rate was 15.5, significantly the precracking of ceramic specimens since these higher than that usually observed in metals. The materials have very low toughness. Moreover, the likely micromechanism of crack growth under crack initiation in such materials often requires a monotonic and cyclic loading in this composite has load which is higher than that required for crack been identified from fractography of fatigue failed extension. Therefore specimens fail before crack samples. growth is achieved in a controlled manner, because he precision and dimensional tolerance of the fixture used to precrack the specimens are not 1 Introduction adequate to avoid spurious loading, that is load ing in modes other than in mode I Silicon carbide whiskers have been incorporated in such ceramic materials as alumina to improve the general mechanical propertiesand the resis- 2 Material and Specimen Orientation ce to catastrophic failure in particular. These ceramic composite materials have potential appli- The ceramic composite material was prepared by cation in the production of structural components mixing a-alumina powder of particle size l um used at elevated temperatures, 0-12i e in high effi- with 25 wt% B-silicon carbide whiskers the aver- cy heat engines and heat recovery systems, age whisker diameter was 0.45-065 um and the and for making cutting tools to machine special length ranged from 10 to 80 um. This mixture materials. When used in such applications, these was hot-pressed at 1700 to 1850 C under a pressure ceramic components often encounter monotonic of 25 MPa for 30 min to produce a preformed billetJournul (I/ the Europcw~ Ckrwi~ SocictJ: 16 ( 1996) 503-5 13 0 1996 Elsevier Science Limited 0955-2219(95)00163-8 Printed in Great Britain. All rights reserved 0955-2219/96/$15.00 Fatigue Crack Growth Rate and Fracture Toughness of 25 wt% S:ilicon Carbide Whisker Reinforced Alumina Composite with Residual Porosity A. K. Ray,” E. R. Fullerb & S. Banerjee” “National Metallurgical Laboratory, Jamshedpur-831007, Bihar, India ‘National Institute of Standards and Technology, Gaithersburg, MD 20899, USA “Research and Development Centre for Iron and Steel, SAIL, Ranchi-834002, Bihar, India (Received 8 December 1994; revised version received 20 July 1995; accepted 28 August 1995) Abstract The main purpose of this study was to determine the fracture toughness and the fatigue crack growth rate behaviour of 25 wt% silicon carbide whisker reinforced alumina ceramic composite. The fracture toughness values determined using the indentation technique depended sign$cantly on the crack length produced at the corners of the indentation which, in turn, depended on the load used for the indentation and anisotropy in orientation of whiskers in the matrix. However, the fracture toughness values determined using the precracked four-point bend specimens were in general higher than that obtained by the indentation technique and the value was 5.96 f 0.15 MPa m ‘I2 . The fatigue crack growth behaviour in this material was similar to that in the case of metals. How(ever, the exponent for the fatigue crack growth rate was 15.5, sigru~cantly higher than that usually observed in metals. The likely micromechanism of crack growth under monotonic and cyclic loading in this composite has been identtjied from fractography of fatigue failed samples. 1 Introduction Silicon carbide whiskers, have been incorporated in such ceramic materials as alumina to improve the general mechanical properties’-’ and the resis￾tance to catastrophic failure in particular. These ceramic composite materials have potential appli￾cation in the production of structural components used at elevated tempera.tures,‘@i2 i.e. in high effi￾ciency heat engines and heat recovery systems, and for making cutting tools to machine special materials. When used in such applications, these ceramic components 0fl:en encounter monotonic and cyclic loading which produces crack extension. Accordingly, in the present investigation, the fatigue crack growth behaviour (FCGR) and the fracture toughness (K,,) of a high density, 25 wt% silicon carbide whisker reinforced alumina composite have been studied. In general, the determination of FCGR and K,, of such ceramic materials is difficult since the specimens are small, Young’s modulus of such materials is rather high and the material is brittle. Consequently, the load, displacement and crack length - which are all required to be measured for the determination of K,, and FCGR - are very small and their precise measurement poses some problems. In addition, the permissible dimensional tolerance of the specimens and those of the grips and fixtures used to test the speci￾mens, have to be very close. Particularly difficult is the precracking of ceramic specimens since these materials have very low toughness. Moreover, the crack initiation in such materials often requires a load which is higher than that required for crack extension. Therefore specimens fail before crack growth is achieved in a controlled manner, because the precision and dimensional tolerance of the fixture used to precrack the specimens are not adequate to avoid spurious loading, that is load￾ing in modes other than in mode I. 2 Material and Specimen Orientation The ceramic composite material was prepared by mixing a-alumina powder of particle size < 1 pm with 25 wt% p-silicon carbide whiskers. The aver￾age whisker diameter was 0~450.65 pm and the length ranged from 10 to 80 pm.8 This mixture was hot-pressed at 1700 to 1850°C under a pressure of 25 MPa for 30 min to produce a preformed billet. 503