Acta mater.Vol.47,No.18.pp.4711-4725,1999 On behalf of Acta metallurgica In PI:S13596454(99)00246-3 1359-645499520.00+0.00 PERGAMON CYCLIC FATIGUE OF INTRINSICALLY BRITTLE CERAMICS IN CONTACT WITH SPHERES D. K KIMT, Y.G. JUNGH, L M. PETERSON and B R. LAWN Materials Science and Engineering Laboratory, National Institute of Standards and Technology Gaithersburg. MD 20899, U.S.A. Received 24 March 1999; accepted 28 July 1999) Abstract--Contact damage modes in cyclic loading with spheres are investigated in three nominally brittle mall numbers of cycles and low loads consists of tensile-driven macroscopic cone cracks ("brittle"mode) dary damage at lar loads consists of shear-driven distributed micro damage (quasi-plastic"mode), with attendant radial cracks and a new form of deeply penetrating subsidi- first mode, based on time-integration of slow growth of cone cracks, is presented. This model provides relations for the remaining strength in terms of number of cycles and naterials design. Extrapolations of these relation ative, highlighting the need for further underst mics. Comparison with static contact data indicates a strong mechanical (as opposed to chemical "y mponent in the cyclic fatigue in the quasi-plastic region. Published by Elsevier Science Lid on behalf of Keywords: Structural ceramics; Fracture; Fatigue; Yeild phenomena; Microstructure 1 INTRODUCTION with spheres, where the stress field is largely com- Fatigue properties are important in any long-term, pressive and of uncommonly high intensity (i.e. in large-scale structural applications where cyclic loads excess of GPa)[16-21]. Earlier studies on damage are experienced. Such properties have been accumulation and fracture around notches in com- measured in several ceramics using conventional pressive loading constitute a precedent for such pre-cracked tensile test specimens or bend bars[I- effects [22-24. Contact fatigue is relevant to bear- 91. A key element of the fatigue response is micro- ings and engine components [21] dental restor structure:fine, homogeneous ceramics, character- ations [25, 26] and analogous biomechanical ized by crack-size-invariant toughness values replacements (hip joints, heart valves), and other undergo cyclic fatigue by time-integrated, chemi- applications where loads are concentrated. two dis- cally-enhanced slow crack growth [10] coarse, het- tinct contact damage modes have been identified [18]:(i) in homogeneous ceramics, well-defined cone erogeneous ceramics, characterized by R-curves, cracks (brittle"mode);(i)in heterogeneous cer- undergo fatigue mainly by time- independent mech- anical degradation of grain bridging or other crack- amics, microdamage within diffuse but well-defined yield zones (quasi-plastic" mode). Again, in the tip shielding mechanism [11-15. The competitive brittle materials the fatigue is attributable to chemi- roles of chemical and mechanical processes in the fatigue responses of ceramics is an issue of continu. cally-driven, time-independent slow crack growth, ing debate. whereas in the quasi-plastic materials it appea be governed primarily by a mechanical component tigue effects in ceramics can be even more dependent on number of cycles rather than on time ly evident in Hertzian-type contact loading [271. Both damage modes are deleterious to the remaining strength of the material. However, the gUest Scientist from: Department of Materials Science mechanics of the associated fatigue processes and Engineering, Korea Advance Science and remain obscure Technology, Yusong, Taejon 305-701, Korea. Present address: Department of Ceramic Science and n this paper we present results of a fatigue study Engineering. Changwon National University, Chang with spherical indenters on nominally brittle cer- S, i.e. relatively homogeneous ceCYCLIC FATIGUE OF INTRINSICALLY BRITTLE CERAMICS IN CONTACT WITH SPHERES D. K. KIM{, Y.-G. JUNG{, I. M. PETERSON and B. R. LAWN} Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, U.S.A. (Received 24 March 1999; accepted 28 July 1999) AbstractÐContact damage modes in cyclic loading with spheres are investigated in three nominally brittle ceramics, soda-lime glass, porcelain and ®ne-grain silicon nitride, in moist environments. Initial damage at small numbers of cycles and low loads consists of tensile-driven macroscopic cone cracks (``brittle'' mode). Secondary damage at large numbers of cycles and high loads consists of shear-driven distributed micro￾damage (``quasi-plastic'' mode), with attendant radial cracks and a new form of deeply penetrating subsidi￾ary cone cracks. Strength tests on indented specimens are used to quantify the degree of damage. Both damage modes degrade the strength: the ®rst, immediately after cone crack initiation, relatively slowly; the second, after development of radial cracks, much more rapidly. A fracture mechanics model describing the ®rst mode, based on time-integration of slow growth of cone cracks, is presented. This model provides simple power-law relations for the remaining strength in terms of number of cycles and contact load for materials design. Extrapolations of these relations into the quasi-plastic region are shown to be non-conser￾vative, highlighting the need for further understanding of the deleterious quasi-plastic mode in tougher cer￾amics. Comparison with static contact data indicates a strong mechanical (as opposed to chemical) component in the cyclic fatigue in the quasi-plastic region. Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. Keywords: Structural ceramics; Fracture; Fatigue; Yeild phenomena; Microstructure 1. INTRODUCTION Fatigue properties are important in any long-term, large-scale structural applications where cyclic loads are experienced. Such properties have been measured in several ceramics using conventional pre-cracked tensile test specimens or bend bars [1± 9]. A key element of the fatigue response is micro￾structure: ®ne, homogeneous ceramics, character￾ized by crack-size-invariant toughness values, undergo cyclic fatigue by time-integrated, chemi￾cally-enhanced slow crack growth [10]; coarse, het￾erogeneous ceramics, characterized by R-curves, undergo fatigue mainly by time-independent mech￾anical degradation of grain bridging or other crack￾tip shielding mechanism [11±15]. The competitive roles of chemical and mechanical processes in the fatigue responses of ceramics is an issue of continu￾ing debate. Fatigue e€ects in ceramics can be even more strongly evident in Hertzian-type contact loading with spheres, where the stress ®eld is largely com￾pressive and of uncommonly high intensity (i.e. in excess of GPa) [16±21]. Earlier studies on damage accumulation and fracture around notches in com￾pressive loading constitute a precedent for such e€ects [22±24]. Contact fatigue is relevant to bear￾ings and engine components [21], dental restor￾ations [25, 26] and analogous biomechanical replacements (hip joints, heart valves), and other applications where loads are concentrated. Two dis￾tinct contact damage modes have been identi®ed [18]: (i) in homogeneous ceramics, well-de®ned cone cracks (``brittle'' mode); (ii) in heterogeneous cer￾amics, microdamage within di€use but well-de®ned yield zones (``quasi-plastic'' mode). Again, in the brittle materials the fatigue is attributable to chemi￾cally-driven, time-independent slow crack growth, whereas in the quasi-plastic materials it appears to be governed primarily by a mechanical component, dependent on number of cycles rather than on time [27]. Both damage modes are deleterious to the remaining strength of the material. However, the mechanics of the associated fatigue processes remain obscure. In this paper we present results of a fatigue study with spherical indenters on nominally brittle cer￾amics, i.e. relatively homogeneous ceramics with no signi®cant R-curve, using an indentation±strength Acta mater. Vol. 47, No. 18, pp. 4711±4725, 1999 Published by Elsevier Science Ltd On behalf of Acta Metallurgica Inc. Printed in Great Britain PII: S1359-6454(99)00246-3 1359-6454/99 $20.00 + 0.00 {Guest Scientist from: Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Yusong, Taejon 305-701, Korea. {Present address: Department of Ceramic Science and Engineering, Changwon National University, Changwon, Kyungnam 641-773, Korea. }To whom all correspondence should be addressed. 4711