powder in cold uniaxial double-action pressing, cold isostatic pressing (60 and 120 MPa), and sintering. Five starting powders are produced by processing a ...
Powder Metallurgy and Metal Ceramics, Vol. 46, Nos. 7-8, 2007
REFRACTORY AND CERAMIC MATERIALS CONSOLIDATION OF NANOCRYSTALLINE ZrO2–CeO2–Y2O3 POWDERS DEPENDING ON THEIR PRODUCTION CONDITIONS E. V. Dudnik, A. V. Shevchenko, A. K. Ruban, Z. A. Zaitseva, V. M. Vereshchaka, V. P. Red’ko, and A. A. Chekhovskii UDC 621.762:546-31 The paper examines the consolidation of 95 mole% ZrO2–2 mole% CeO2–3 mole% Y2O3 nanocrystalline powder in cold uniaxial double-action pressing, cold isostatic pressing (60 and 120 MPa), and sintering. Five starting powders are produced by processing a suspension after hydrothermal decomposition in different conditions. It is established that a homogeneous microstructure forms only in a material from the powder subjected to two homogenizing grindings. After cold uniaxial pressing and cold isostatic pressing, the sintered samples reach a relative density of 0.96 to 0.94. The bending strength is 600 to 660 MPa. The efficient consolidation of ceramics requires comprehensive processing of starting nanocrystalline powders to modify their morphology. Keywords: zirconia, nanocrystalline powder, uniaxial double-action pressing, cold isostatic pressing, consolidation In order to produce ZrO2-based bioimplants, materials with a stable structure and high bending strength and critical fracture toughness are needed [1, 2]. It is believed that ZrO2-based materials, ZrO2–Y2O3 system, possess excellent mechanical properties but are susceptible to low-temperature aging in humid environment. Therefore, they have limited application in developing new types of bioimplants [1, 3]. These materials become more resistant to lowtemperature aging if Y2O3 is partly replaced by CeO2 [4, 5]. Transformation hardening is the main contributor to the desired properties. The main requirement in producing transformation-hardened ceramics is to form a porousless fine structure consisting of tetragonal zirconia grains (T-ZrO2), which can transform into another phase under applied stress [6]. Nanocrystalline ZrO2-based powders used as starting materials for bioimplants cause problems associated with the size effect and in-service microstructural stability. As structural components become smaller, the number of interfaces, whose properties may differ from those in larger-grained ceramics, increases [7, 8]. The objective of this paper is to establish features peculiar to the consolidation of nanocrystalline 95 ZrO2– 2 CeO2–3 Y2O3 powder∗ 95 ZrO2–2 CeO2–3 Y2O3 depending on processing conditions.
∗ The powder composition is in mole%.
Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 7–8 (456), pp. 45–58, 2007. Original article submitted May 25, 2006. 1068-1302/07/0708-0345 ©2007 Springer Science+Business Media, Inc.
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Decanted suspension
Drying ( t = 70 °C, τ = 8 h) (t = 70 °C, τ = 8 h) Drying
Powder P1
Powder P2
Grinding in IPA (τ = 6 h )
Grinding of wet product in IPA (τ = 6 h) Drying (t = 60 °C, τ = 8 h)
Drying (t = 60 °C, τ = 8 h)
Annealing (t = 600 °C, τ = 1.5 h)
Annealing (t = 600 °C, τ = 1.5 h)
Grinding in IPA (τ = 3.5 h)
Grinding in IPA (τ = 3.5 h)
Drying (t = 60 °C, τ = 8 h)
Drying (t = 60 °C, τ = 8 h)
Powder P3-1
Powder P3
Powder P2-1
Fig. 1. Processing the decanted suspension resulting from hydrothermal decomposition in the synthesis of the 95 ZrO2–2 CeO2–3 Y2O3 powder
Hydrothermal synthesis of the starting powder is detailed in [9]. A suspension after decantation was processed in drying, grinding in isopropyl alcohol (IPA), and annealing in different sequences (Fig. 1). The powders were compacted by cold uniaxial pressing (CP) and cold pressing with subsequent cold isostatic pressing (CIP) [10–14]. Before pressing, a 10% solution of polyvinyl alcohol in distilled water was introduced into all powders. The powders were mechanically mixed with a plasticizer, the introduction of the plasticizer and the granulation of the powder being combined. The powders were pressed in a die mold 12 mm in diameter (PSU-10 press, 40 MPa pressing force). Cylindrical compacts 14 mm in height were formed. Five samples were produced from each of the powders P1, P2-1, P3, and P3-1 (Fig. 1). The sizes and weight of the compacts were used to determine changes in their density. All powders had satisfactory formability. To remove the plasticizer, the samples were smoothly heated to the temperature of isothermal holding (700°C) for 5 h. After the isothermal holding, the samples were cooled down in the furnace for 2 h. A high-pressure pump station (NSVD-2000) and a hydrostat were used for cold isostatic pressing. Glycerin was the operating medium. Prior to pressing, the samples were placed into elastic covers and vacuumized. CIP was conducted under 60 ± 2 MPa and 120 ± 7 MPa. Holding under pressure lasted for 5 min. After loading, the samples were held for 5 min to reach equilibrium, and then timing started. Following the cold uniaxial pressing and combined molding, the samples were sintered in open air at 1300°C. They were heated for 8 h to reach the temperature of isothermal holding. After the isothermal holding, the samples were cooled down in the furnace (1.5 h). An x-ray phase analysis (XPA), electron microscopy, and petrography were employed to examine the properties of the powders and compacts. A DRON-1.5 diffraction meter (Cu-Kα-radiation, scanning rate 1 to 4 °C/min) was used for the XPA. The sizes of the primary particles were determined using coherent scattering regions. Electron microscopy was based on a Camebax SX-50 electron microprobe. Microstructural and phase analyses were carried out with petrography using a MIN-8 microscope and a standard set of immersion liquids (60–620 magnification). After different processing conditions, the specific surface area of nanocrystalline powders was determined with the BET method and bottle density with the Archimedes method in distilled water. All powders contain a colorless isotropic phase whose refractive indices vary from 1.91< n < 2.04 in the powder P1 to n = 2.04 in the powders P2 and P3; in addition, the powders P2-1 and P3-1 contain tiny anisotropic crystals.
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10 μm
10 μm
a
b
10 μm
10 μm
c
d
10 μm
e
Fig. 2. Morphology of the nanocrystalline 95 ZrO2–2 CeO2–3 Y2O3 powders resulting from different suspension processing conditions: a) P1; b) P2; c) P2-1; d) P3; e) P3-1 (here and in Figs. 3–8)
According to XPA (Table 1), not only ZrO2-based cubic solid solutions (F-ZrO2) but also traces of a ZrO2based monoclinic solid solution (M-ZrO2) have been identified in these powders. M-ZrO2 is an anisotropic substance whose refractive index is lower than that of F-ZrO2. Therefore, the total refractive index in the powders P2-1 and P3-1 is lower as well. TABLE 1. Properties of Nanocrystalline 95 ZrO2–2 CeO2–3 Y2O3 Powders Powder P1 P2 P2-1 P3 P3-1
Size of initial particles, nm
Specific surface area, m2/g
Bottle density, g/cm3
10 18 19 18 18
101.6 73.13 63.9 84.4 75.7
4.65 5.67 5.50 5.33 5.27
Phase composition F-ZrO2 F-ZrO2 F-ZrO2, M-ZrO2 traces F-ZrO2 F-ZrO2, M-ZrO2 traces
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The primary particles in the powder P1 are almost half as large as in the powders P2–P3-1. The primary particles of these powders are practically equal in size. The powders are soft-agglomerated; i.e., the agglomerates formed under van der Waals forces and can easily broke down under an external load [15]. The agglomeration factor (the ratio between the mean sizes of aggregates and particles [16]) varies from 1000 in the powder P1 to ≈360 in the powders P2 and P2-1 and ≈260 in the powders P3 and P3-1. Several types of agglomerates form in nanocrystalline ZrO2-based powders [17]. First-order agglomerates (up to 100 nm) are formed from the primary particles in calcination synthesis and second-order agglomerates in subsequent processing of the previous agglomerates (they reach 20 µm, while the mean size is 4.4 µm). Quite large (to 10 µm) irregular angular second-order agglomerates formed in the powder P1 when dried (Fig. 2a). Similar but smaller agglomerates (to 5 µm, some to 10 µm) formed in the powder P3 (Fig. 2d). After additional grinding in isopropyl alcohol (IPA), agglomerates larger than 10 µm were not revealed (Fig. 2e). Round agglomerates up to 5 µm were observed in the powder P2 (Fig. 2b). They became smaller after additional grinding in IPA though some agglomerates of 7 µm remained (Fig. 2c). According to an analysis of morphologic changes in the powders P1– P3-1, the powders P3 and P3-1 have inherited [18] the aggregate state of P1, despite the additional processing. It is believed that two systems of agglomerated powders formed before pressing: one contained irregular agglomerates (P1, P3, P3-1) and the other roundish ones (P2 and P2-1). Table 2 shows how the relative density of the samples changed after different processing options. The theoretical density of a sintered ZrO2-based material is assumed to be 6.03 g/cm3. After CP, the relative density of compacts of the powders P1, P2-1, and P3-1 was practically the same (0.40 to 0.41) and that of compacts of the powders P2 and P3 was somewhat lower (0.34 and 0.37, respectively). After the plasticizer had been removed, the relative density of the P1 sample slightly increased (≈5%) and that of the samples of the powders P2–P3-1 decreased and was equal to 0.33–0.32. When the plasticizer was removed, all the samples had the same phase composition: F-ZrO2. After CIP under 60 MPa, the relative density of the compacts increases by 5% and reaches 0.39 to 0.49; after CIP under 120 MPa, it increases by 6 to 8% and reaches 0.4 to 0.5 depending on processing of the starting powders (Table 2). The maximum relative density (0.49 and 0.5) is reached by the compacts of the powder P1, which retained the activity acquired in its production. During CP, the major structural modifications (regrouping, void filling, breakdown of some constituents) seem to occur at the level of second-order agglomerates. After CP, structural modifications proceed to the level of the remaining first-order agglomerates. This determines the structural development of the samples in CIP, plasticizer removal, and sintering. The large second-order agglomerates formed in the powder P1 after drying seemingly represent homogenous blocks with uniform porosity. In CP, these blocks are partially deformed and create a kind of framework, the vacant sections of which are filled with finer and more active powder particles (first-order agglomerates) (Fig. 3a). In annealing at 1300°C, these blocks are sintered to the maximum density as they have no abnormally large pores, cracks, or defects that would inhibit sintering. However, the sections between these blocks are sintered to form a loose and seemingly foamed structure (Fig. 4a) since they are filled with softer agglomerates that consist of individual, nonconnected aggregates. Note that the powder P1 was not additionally processed and retained the activity acquired in its production to the extent maximum. Therefore, sintering processes are most intensive in this sample and lead to zonal isolation. As a result, sections with directional crystallization (Fig. 4a) are formed, and the relative density of the sample remains low (Table 2). Petrography confirms that processes occurring in this sample are not unique. There are colorless and optically transparent sections of two types consisting of anisotropic grains smaller than 1 or 2 µm and directionally crystallized grains whose size is beyond the resolving power of the optical microscope (
