L Ceseracciu et al International Journal of Refractory Metals& Hard Materials 23(2005)375-381 increasing, thus, the apparent fracture toughness of the produced by cyclic Hertzian indentation was either cone material. This can be achieved by several ways, such cracking or quasi-plasticity (due to microcracking as ion tempering, temperature quenching, variation of depending on the material type and the size of the the microstructure at the surface(functionally graded microstructure materials)or by the use of laminated composites [2, 3]. The objective of this study is to characterize the con- The residual stress at the surface of laminated com- tact resistance of an alumina-based laminated compos posites is usually achieved by alternating materials with ite, with compressive residual stresses at the surface, different thermal expansion behaviour, so, when joined subjected to cyclic loading, in comparison to a mono- at high temperature and later cooled to room tempera- lithic stress-free alumina. This is done because ceramic ture, they develop residual stresses due to the different surfaces, specially when employed as hard materials, hrinkage behaviour of the different materials. It is also are more susceptible to cracking by contact loading than possible to use other mechanisms of expansion mis- by external remote loading. Moreover, contact loading match for achieving residual stresses, such as martensitic applied during a relatively long time can degrade the transformation of zirconia, which implies a 4% volume materials even if the load is below the threshold for change [4]. These multilayers normally have a symmet inducing damage cal stacking in order to avoid unbalanced stresses, which In a previous work[15], we studied, for the same lam would result in bending of the sample inated composites, the resistance to the development of Among the ceramic laminated composites that can be ring cracking(considered as the first stage of damage) produced, one of the most preferred material combina- under both static and cyclic loading. This research tions is alumina and zirconia. Usually, at least one of showed that the laminate material presents a better he layers of the material is made of an alumina/zirconia behaviour under both types of loading than the mono- composite, in order to tailor the coefficient of thermal lithic alumina, and that both materials degrade more expansion(CTE)and, consequently, the residual stres- rapidly under cyclic loading due to the existence of ses. In this way, the channel cracking produced during fatigue sintering can be avoided The reason for choosing alu The objective of this work is to present the surface mina and zirconia as the constituent materials of cera- morphology of an alumina/zirconia laminated compos- mic laminates is normally because of the excellent ite, in comparison with a monolithic alumina, when sub bonding between the layers in the absence of excessive jected to cyclic loading with a number of cycles larger diffusion between components, their good thermo- than the critical one to produce the first damage. That mechanical properties and their relatively ease of is, while in a previous study the objective was to quan- processing tify the appearance of damage as a function of number Moreover, layered ceramics also present the of cycles and applied load, in this study we evaluate the ge that, under certain conditions, they have different surface fractographies and sub-surface damage proved long-crack resistance with respect to produced as the different materials are subjected, under monolithic counterparts [5]. This long-crack toughness a constant load, to different number of cycles improvement can be achieved by several mechanisms, such as weak interfaces [6], containment of martensitic transformation [7], existence of porous layers [8] or 2. Experimental crack deflection due to the elastic mismatch at the inter- face. In addition, the existence of residual compressive 2. 1. Sample preparation stresses also reduces the effective stress intensity factor consequently, the apparent fracture toughness / 8, of cracks located at the compressive layers, increasing, To obtain the ceramic sheets suitable for the prepara- tion of laminated composites, two powders were used One of the preferred ways of studying the contact high purity(99.7%)alumina(Alcoa A16-SG, Alcoa Alu damage is by Hertzian indentation. Hertzian indenta- minum Co., New York, USA) with an average particle tion presents the advantage over sharp indentations in size of 0. 3 um, and tetragonal zirconia polycrystal the fact that damage in the material can be produced (TZ3Y-S, Tosho Corp. Japan) containing 94.7% of in most cases without appreciable plastic deformation, ZrO2 and 3 mol% of Y2O3(usually referred to as 3Y which simplifies the analysis because fracture can occur TZP)with an average particle size of 0.3 um within an elastic field. Moreover. Hertzian indentations The different powders were mixed with organic bind- resemble more closely the real contact that the material rs, dispersant, plasticizers and solvents to obtain suit is expected to suffer in service. In recent years, there have able slips for tape casting. After mixing with organic been several studies on Hertzian contact fatigue of components, the slurry containing the ceramic powders ceramics, where the degradation of the material under was tape casted onto a mylar sheet moved at a constant clic loading has been evaluated by measuring the de- speed of 200 mm/min. Detail on this technique can be crease of strength of the material [12-14]. The damage found elsewhere [16].increasing, thus, the apparent fracture toughness of the material. This can be achieved by several ways, such as ion tempering, temperature quenching, variation of the microstructure at the surface (functionally graded materials) or by the use of laminated composites [2,3]. The residual stress at the surface of laminated com￾posites is usually achieved by alternating materials with different thermal expansion behaviour, so, when joined at high temperature and later cooled to room tempera￾ture, they develop residual stresses due to the different shrinkage behaviour of the different materials. It is also possible to use other mechanisms of expansion mis￾match for achieving residual stresses, such as martensitic transformation of zirconia, which implies a 4% volume change [4]. These multilayers normally have a symmetri￾cal stacking in order to avoid unbalanced stresses, which would result in bending of the sample. Among the ceramic laminated composites that can be produced, one of the most preferred material combina￾tions is alumina and zirconia. Usually, at least one of the layers of the material is made of an alumina/zirconia composite, in order to tailor the coefficient of thermal expansion (CTE) and, consequently, the residual stres￾ses. In this way, the channel cracking produced during sintering can be avoided. The reason for choosing alu￾mina and zirconia as the constituent materials of cera￾mic laminates is normally because of the excellent bonding between the layers in the absence of excessive diffusion between components, their good thermo￾mechanical properties and their relatively ease of processing. Moreover, layered ceramics also present the advan￾tage that, under certain conditions, they have an im￾proved long-crack resistance with respect to their monolithic counterparts [5]. This long-crack toughness improvement can be achieved by several mechanisms, such as weak interfaces [6], containment of martensitic transformation [7], existence of porous layers [8] or crack deflection due to the elastic mismatch at the inter￾face. In addition, the existence of residual compressive stresses also reduces the effective stress intensity factor of cracks located at the compressive layers, increasing, consequently, the apparent fracture toughness [9–11]. One of the preferred ways of studying the contact damage is by Hertzian indentation. Hertzian indenta￾tion presents the advantage over sharp indentations in the fact that damage in the material can be produced in most cases without appreciable plastic deformation, which simplifies the analysis because fracture can occur within an elastic field. Moreover, Hertzian indentations resemble more closely the real contact that the material is expected to suffer in service. In recent years, there have been several studies on Hertzian contact fatigue of ceramics, where the degradation of the material under cyclic loading has been evaluated by measuring the de￾crease of strength of the material [12–14]. The damage produced by cyclic Hertzian indentation was either cone cracking or quasi-plasticity (due to microcracking), depending on the material type and the size of the microstructure. The objective of this study is to characterize the con￾tact resistance of an alumina-based laminated compos￾ite, with compressive residual stresses at the surface, subjected to cyclic loading, in comparison to a mono￾lithic stress-free alumina. This is done because ceramic surfaces, specially when employed as hard materials, are more susceptible to cracking by contact loading than by external remote loading. Moreover, contact loading applied during a relatively long time can degrade the materials even if the load is below the threshold for inducing damage. In a previous work [15], we studied, for the same lam￾inated composites, the resistance to the development of ring cracking (considered as the first stage of damage) under both static and cyclic loading. This research showed that the laminate material presents a better behaviour under both types of loading than the mono￾lithic alumina, and that both materials degrade more rapidly under cyclic loading due to the existence of fatigue. The objective of this work is to present the surface morphology of an alumina/zirconia laminated compos￾ite, in comparison with a monolithic alumina, when sub￾jected to cyclic loading with a number of cycles larger than the critical one to produce the first damage. That is, while in a previous study the objective was to quan￾tify the appearance of damage as a function of number of cycles and applied load, in this study we evaluate the different surface fractographies and sub-surface damage produced as the different materials are subjected, under a constant load, to different number of cycles. 2. Experimental 2.1. Sample preparation To obtain the ceramic sheets suitable for the prepara￾tion of laminated composites, two powders were used: high purity (99.7%) alumina (Alcoa A16-SG, Alcoa Alu￾minum Co., New York, USA) with an average particle size of 0.3 lm, and tetragonal zirconia polycrystals (TZ3Y-S, Tosho Corp. Japan) containing 94.7% of ZrO2 and 3 mol% of Y2O3 (usually referred to as 3Y￾TZP) with an average particle size of 0.3 lm. The different powders were mixed with organic bind￾ers, dispersant, plasticizers and solvents to obtain suit￾able slips for tape casting. After mixing with organic components, the slurry containing the ceramic powders was tape casted onto a mylar sheet moved at a constant speed of 200 mm/min. Detail on this technique can be found elsewhere [16]. 376 L. Ceseracciu et al. / International Journal of Refractory Metals & Hard Materials 23 (2005) 375–381