urna Am, cern,s,%61547-53(303 Influence of Residual Stresses on the Wear Behavior of Alumina/Alumina-Zirconia Laminated Composites Francesco Toschi. Cesare Melandri, Paola Pinasco. Edoardo Roncari, Stefano Guicciardi, and Goffredo de Portu National Research Council (CNR). Institute of Science and Technology for Ceramics (ISTEC). 48018 Faenza(RA). Italy Symmetric structures of laminated ceramic composites were uch cases, the ineffectiveness of increased surface toughness in AlyOyLrOz composite prepared by tape casting. The compos- exaggerated phase transformation in zirconia-based materials and side. This configuration caused residual compressive stresses high contact temperatures reached by the material pairs under very to be induced on the surface due to the different thermal severe operating conditions. The aim of this study is to determine whether laminated pansion coefficients of the various layers, leading to an ceramics with improved apparent surface toughness, conferred by residual stress was determined using the indentation tech nique. The tribological behavior of these laminated structures metrical lamination of Al, O, and AL,O /ZrO, layers, can have a was evaluated using the pin-on-disk method for different loads within a defined range of applied loads and sliding speed terial higher wear resistance than the corresponding stress-free ma and sliding speeds. Comparison with the results obtained from stress-free alumina showed that, within the range of these experimental conditions, the improvement in surface tough ness leads to a reduced friction coefficient and increased wear Il. Experimental Procedure resistance of the composites. Possible wear mechanisms are (1) Preparation of Materials The selected materials were Al, O, and an Al,O /ZrO, compos te(60/40 vol%), which have different shrinkages during sintering 1. Introduction and different thermal expansion coefficients (a 9.0x 10 100×10°℃fom25°to1400c. respectively). These differ- D UE to their high hardness, low specific weight, chemical ences are sufficient to induce residual stresses in the laminated ceramics are widely considered to be excellent candidates for cracking under the experimental conditions described below wear-resistant materials, However, their inherent brittleness causes This study entailed the use of a high-purity (99.7%) alumina ne concern. Because the performances of wear-resistant mmy powder(Alcoa Al6-SG. Alcoa Aluminum Co. New York. NY) rials are mainly related to the properties of thin surface layers. 1.2 with an average particle size of 0.3 um, and a zirconia powde an improvement in surface toughness appears to be an interesting (TZ3Y-S, Tosho Corp, Tokyo, Japan) doped with 3 mol%Y,O way to improve the tribological behavior of ceramics. In fact, since (usually referred to as 3Y-TZP)with an average particle size of 0.3 removal of material in engineering ceramics under sliding condi tions is generally caused by the propagation of surface cracks On the basis of previous experience es. 4.15 the different powders resulting from tensile stresses in the wake of rubbing contact, an were mixed with organic binders, dispersant, plasticizers, and ncrease in apparent surface toughness should lead to an improve solvents to obtain suitable slips for tape casting. Slurry composi ment in wear resistance. It has been observed that when toughness tions were the same for both Al-O, and Al-O / ZrO, composite increases, wear resistance increases by approximately 3 orders of powders. magnitude, measured in air at room temperature at low load and Sheets of pure alumina(hereinafter designated"A")and of the very slow sliding speed. To achieve this goal. efforts have beer mposite alumina/zirconia with a volume ratio of 60/40(herein made to modify the surface properties using zirconia phase after designated"AZ")were uced. A description of the transformation. Another method of improving the surface tough processing procedures can be found elsewhere. Samples with a ness of a material while avoiding the problems related to phase hybrid laminated structure of alumina and alumina/zirconia com transformation" is to produce laminated structures designed to posite (hereinafter referred to as"A/AZ, with seven A and four induce compressive residual stresses at the surface by combining AZ layers)and laminated structures containing I I layers of pure the different thermophysical characteristics (i.e, thermal ex alumina(hereinafter referred to as"AA")were prepared by on and shrinkage on sintering) of the different materials ed,It has been shown,. that although these techniques can stacking eets at 75 C at a pressure of 30 MPa for 30 min increase resistance to static contact damage (i.e. apparent surface The latter batch of samples was prepared to obtain a material toughness), they do not necessarily improve wear resistance. In with the same surface porosity as the A/AZ material, but with zero (or very low) residual stresses In the hybrid samples, the structure was designed to leave the layers of alumina (A)on the two surfaces( Fig. 1)to stimulate compressive surface stresses. Due to butine editor lower thermal expansion coefficient and shrinkage during sinter. ing, the external alumina layers underwent residual compressive To obtain a perfectly symmetrical structure, two A layers were Manuscript No. 187121. Received February 25, 2002: approved April 3. 2003 mber, American Ceramie Society used on each side. This allowed one layer to be removed from each ide