FABRICATION OF NOVEL ZrO2(Y2O3)-Al2O3 CERAMICS HAVING HIGH STRENGTH AND TOUGHNESS BY PULSED ELECTRIC-CURRENT PRESSURE SINTERING (PECPS) OF SOL-GEL DERIVED SOLID SOLUTION POWDERS

Ken Hirota*1, Kengo Shibaya1, Masaki Kato*1, and Hideki Taguchi2

*1 Faculty of Science and Engineering, Doshisha University, Kyo-Tanabe Kyoto 610-0321, Japan

2 The Graduate School of Natural Science and Technology (Science), Okayama University, Okayama 700-8530, Japan

Keywords: Zirconium oxide; Aluminum oxide; Yttrium oxide; Pulsed electric-current pressure sintering (PECPS); Mechanical properties

ABSTRACT

ZrO2 based ceramics containing 25 mol% Al2O3 and 0.90~1.125 mol% Y2O3, i.e., ZrO2(1.2~1.5 mol%Y2O3)-25mol%Al2O3 have been fabricated at 1523 to 1623 K (1250~1350°C) for 10 min under 60 MPa in Ar by pulsed electric-current pressure sintering (PECPS) of sol-gel derived cubic ZrO2 solid solution (ss) powders. Dense tetragonal-ZrO2 (t-ZrO2) phase composite ceramics (=99.5%) sintered at 1623 K (1350°C), being composed of =~?200 nm grains, revealed high bending strength sb =1.5 GPa and high fracture toughness KIC =15.5 MPa·m1/2 simultaneously. Precise investigation has been performed on the relationship between their microstructures and mechanical properties, the former of which depend on the content of Y2O3 and calcining temperatures. SEM/TEM observations cleared that these improved mechanical properties might be originated from homogeneous distribution of a-Al2O3 particles around the dense t-ZrO2 grain matrix; the precipitation of a-Al2O3 could be achieved by adopting the (ss) powders and PECPS. The Y2O3 content in fine ZrO2 grains has much effect of controlling the stress-induced transformation toughening of tetragonal to monoclinic ZrO2.

INTRODUCTION

Since the discovery of ZrO2-toughening mechanism based on the stress-induced transformation from tetragonal to monoclinic phases by Garvi [1], partially stabilized zirconia (PSZ) with a small amount of Y2O3 addition has been much focused, and many studies have been performed on the fabrication of other stabilizer added dense PSZ. In addition to these, ZrO2(Y2O3) based and ZrO2(Y2O3)/Al2O3 composite ceramics fabricated using hot pressing (HP) and hot isostatic pressing (HIP) have been developed [2-6]. On the other hand, the solid solution (ss) in the ZrO2-Al2O3 system has not been paid attention; because it was believed that the ZrO2-Al2O3 system did not form the ss even at higher temperatures. However, since the report by Alper [7] on the formation of ZrO2 (ss) containing 7mol% Al2O3, the sol-gel derived ZrO2(ss) powders were prepared and then 75mol%ZrO2-25mol%Al2O3(ss) powders were HIP sintered at 1373 K (1100°C) under 196 MPa for 1 h [8]. Their mechanical properties were evaluated; fracture toughness KIC of 23 MPa·m1/2 was achieved, however, their three-point bending strength sb was remained as low as 570 MPa. After that, there has been no report on the fabrication of dense monolithic or composite ceramics that show high sb =GPa and high KIC =20 MPa·m1/2 at the same time. If bulk ceramics having both high sb and high KIC simultaneously are developed, they can cast aside the concept of "Ceramics are brittle" and spread their application fields widely.

In the present study, we have prepared ZrO2(Y2O3)-Al2O3(ss) nanometer-sized powders by the sol-gel method and densified them with a pulsed electric-current pressure sintering (PECPS) [9], which method is suitable for the fabrication of high-strength dense ceramics consisting of small grain matrix. In addition, to achieve high fracture toughness, we took into account of the transformation toughening of ZrO2. Based on these concepts, we have considered as follows; 1) the sintering method has been changed from the conventional electric furnace, HP and HIP to PECPS with an extreme high heating rate under strong electric pulse field, which means PECPS would make it possible to fabricate dense ceramics composed of fine grains; 2) we have already achieved high KIC ceramics in the ZrO2-Al2O3 systems [8]; 3) in the ZrO2(Y2O3)-Al2O3(ss) powders, Al2O3 also would act as partially stabilizer in the ZrO2 based ceramics; and 4) it has been reported that 25mo% Al2O3 addition improves the bending strength of ZrO2(Y2O3) ceramics [10,11]. Then, we have selected the composition of ZrO2 (1.2~1.5mol%Y2O3)-25mol% Al2O3 from the conventional ZrO2(2.0 ~3.0mol%Y2O3) which has been used as the high-toughness ceramics.

EXPERIMENTAL PROCEDURE

Preparation of ZrO2(Y2O3)- 25mol%Al2O3 ceramics

The preparation of ZrO2(Y2O3)-25mol%Al2O3 solid solution (ss) powders and the fabrication of dense ceramics using these powders are described in previous our paper [8]. The (ss) powders with the composition of 75mol%ZrO2 (1.2~1.5mol%Y2O3)-25mol%Al2O3 [ZrO2:Y2O3:Al2O3=74.10~73.875:0.9~1.125:25.0 mol%] were prepared using Zr(OC3H7)4 (~99.9% pure), Y(OC3H7)3 (~99.9% pure), and Al(OC3H7)3 (~99.9% pure), as starting materials [8,12]. The as-prepared powder (precursor) was calcined at 1093K (820°C) for 75mol%ZrO2(1.5mol%Y2O3)-25mol%Al2O3 (henceforth, abbreviate as ZrO2(1.5Y)-25 mol% Al2O3 and denote as [1.5Y]) and 1138 K (865°C), 1153 K (880°C) and 1168 K (895°C) for ZrO2(1.2Y)-25mol%Al2O3 ([1.2Y]) composition powders for 1 h in air. As will be described latter, these temperatures were determined based on the crystallization temperatures about 1088 K(815°C) and 1133 K(860°C) for 1.5Y and 1.2Y powders, respectively, from the results of XRD and DTA/TG analyses.

Calcined powder compacts after CIPing at 245 MPa for 3 min were sintered with a pulsed electric-current pressure sintering (PECPS: SPS-5104A, SPS SYNTEX INC., Tokyo, Japan) (on-off interval=12:2) with a heating rate of 100 K·min-1 (1.667 K/s), at 1523 to 1623 K (1250~1350°C) under 60 MPa in Ar using a carbon mold (?40-?16-30h mm) and plunger (?39.9-40h mm).

EVALUATION OF SAMPLES

Microstructures

Thermal analysis of precursors was conducted using a differential thermal analysis and thermal gravimetry (DT-TG 60H, Shimadzu, Kyoto, Japan) in air with a heating rate of 10 K·min-1 (0.1667 K·s-1). Crystalline phases were identified by X-ray diffraction (XRD) analysis (CuKa radiation, Rint 2000, Rigaku, Osaka, Japan). The volume fraction of the monoclinic ZrO2 (m- ZrO2) phase for the test samples was determined from the intensity ratio of the monoclinic (111) and (11-1) diffraction lines to the tetragonal (111) line by XRD analysis [13]. Bulk densities (Dobs) of sintered ceramics after polishing with a diamond paste (nominal size ?1~3 µm) were evaluated by Archimedes method. Theoretical densities (Dx) of ceramics were calculated as follows; the lattice parameters of t-ZrO2 phase were estimated to be a=0.360520~0.360657 and c=0.518758~0.518948 nm, and those of m-ZrO2 phase also were a=0.519003~0.518014, b=0.514807~0.5169561, c=0.535040~0.535023 nm, and =98.6960~98.8594° using Rietveld analysis [14], Then the values of Dx(t-ZrO2(1.5Y))=6.0510 and Dx(m-ZrO2(1.5Y))=5.7725 Mg·m-3 were obtained. From the t/m-ZrO2 volume ratios and the values of Dx(a-Al2O3)=3.987 Mg·m-3 (JCPDS: #10-0173), the Dx values of composite ceramics were calculated. Both Dx(t-ZrO2(1.2Y)) and Dx(m-ZrO2(1.2Y)) were assumed to be the same as Dx(t-ZrO2(1.5Y)) and Dx(m-ZrO2(1.5Y)), respectively, because of a small difference in Y2O3 addition.

Microstructural observation on the as-prepared and calcined powders, and the fractured or polished surfaces of ceramics were conducted using a field emission-type transmission electron microscope (FE-TEM, JEM-2100F, JEOL Ltd., Tokyo, Japan) and a scanning electron microscope (FE-SEM, JSM-7001FD, JEOL Ltd.) equipped with an energy dispersive spectroscopy (EDS, JED-2300/T and JED-2300/F, JEOL Ltd., respectively). Before TEM observation, the specimens were processed into thinner using a focused ion beam (FIB, FB-2200, Hitachi High-Tech Fielding Co. Ltd., Tokyo, Japan). The grain sizes were determined by an intercept method [15].

Mechanical properties

After crystalline phase identification, test bars (~3 × 3.5 × 11 mm3) for mechanical-property measurements were cut from the ceramics with a diamond cutting-blade and then their four sides were polished to mirror surface with a diamond paste (nominal particle size ?1-3 µm). Three-point bending strength (sb) was evaluated with a cross-head speed of 0.5 mm · min-1 and an 8 mm-span length using WC jigs. Vickers hardness (Hv) and fracture toughness (KIC) were evaluated using a Vickers hardness tester (HMV, Shimadzu) with an applying load of 19.6 N and a duration time of 15 s for the former, and the indentation fracture method (IF) with Niihara's equation using a Vickers hardness tester (VMT-7, Matsuzawa, Osaka, Japan) with applying load of 196 N and a duration time of 15 s for the latter [16,17].

RESULTS AND DISCUSSION

Characterization of powders and ceramics

Fig. 1...