JournulofrheEuropem CeramicSociely 16 (1996)601-612 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0955-2219(9S)OOl81-5 0952219/961$15.00 Enhanced Mechanical Properties by Grain Boundary Strengthening in Ultra-Fine-Grained TZP Ceramics Y. J. He, A. J. A. Winnubst, C. D. Sagel-Ransijn, A. J. Burggraaf & H. Verweij University of Twente, Faculty of Chemical Technology, Laboratory for Inorganic Materials Science, PO Box 217, 7500 AE, Enschede, The Netherlands (Received 10 March 1995; revised version received 18 July 1995; accepted 12 October 1995) Abstract For fine-grained ceramics, grain boundaries have a considerable effect on the properties and perfor- The mechanical properties of ceramics with the mance of the materials.‘s2 An example of this is compositions of 5 mol% YO,.s-TZP (ZY5) and the role of grain boundary morphology on frac- 4 mol% YOl.i -4 mol% CeO, -TZP (ZY4Ce4) ture toughness. Watanabe et al.“3 introduced the were investigated. Nanocrystalline powders of both concept of grain boundary character distribution TZPs were synthesized by a gel precipitation tech- (GBCD) in alloy polycrystals. This distribution nique. Powder compacts were made by cold iso- gives the frequency of specific grain boundary static pressing, and were then pressureless sintered configurations and is an important tool in control- or sinter-forged to dense TZP ceramics (density: ling intergranular fracture by grain boundary 9699%) with grain sizes of 180 nm. Shear design. They pointed out that low-energy bound- deformation during sinter-forging was found to aries are resistant to fracture while high-energy favourably afleet densification and microstructure. (incoherent) boundaries are preferential sites for The mechanical properties of both types of ultra- crack nucleation and propagation. The toughness fine-grained TZP ceramics were examined up to of polycrystals increases with the relative fraction 760°C. The results point towards an improved grain of low energy boundaries. A similar approach to boundary structure and the elimination of residual grain boundary design for polycrystalline ceramics flaws obtained by the sinter-forging technique. The is also indicated by Krell et a1.4 They suggested sinter-forged samples exhibited a higher Vicker that the concept of grain boundary strength can hardness compared to that of pressureless sintered be specified in three ways: specimens. After sin ter-forging the fracture energy (1) As the critical stress intensity klCsb required value was found to be 325 J/m2 and the fracture to propagate a crack along a grain boundary. toughness to be IO MPa ml”. A Weibull modulus of (2) As the specific fracture energy ygb. 21 was obtained for the sinter-forged samples which (3) As the local, microscopic (tensile) fracture was sigm$cantly higher than the value of 8 for the stress afpb. pressureless sintered specimens. The enhancement The enhancement of the grain boundary strength of reliability is obviously connected to a decrease in for structural ceramics in the case of fracture con- the average size and concentration of flaws caused trolled mainly by an intergranular mode will result by inhomogeneous powder particle packing due to in an improvement of mechanical properties, espe- the presence of irregular agglomerates. cially fracture toughness. High macroscopic strength is achieved, for instance, by reducing the concentration and size of flaws in the bulk’ or by 1 Introduction limiting the possibility of, normally occurring, subcritical growth of extrinsic flaws.6 Typical requirements for structural ceramics are Recently the possibilities of using ultra-fine-grained high strength, ductility, fracture resistance, chemi- ceramics or even nanocrystalline materials7** were cal inertness and a stable microstructure. For a explored. As suggested by Burggraaf et a1.7,8the char- specific ceramic material, better properties can acteristic features of nano-structured materials are: generally be obtained by improving microstruc- l A significant fraction of atoms is situated in tural characteristics such as grain size, homogene- the grain boundary region and/or associated ity and distribution of porosity, and process flaws. with interfacial effects. 601 602 Y. J. He et al. l An interaction exists between the constituent cursors. The materials studied were ZY5 (5 mol% domains. YO,.,-TZP) and ZY4Ce4 (4 mol% Y0,.54 mol% It is expected that ultra-fine-grained ceramics may CeO,-TZP). The precursor solution was dropped possess special properties, related to the presence slowly into an excess of a 25 wt% ammonia solu- of a large grain boundary surface area. An exam- tion with pH >l 1. The formed gel was washed ple of this is surprisingly high superplastic defor- subsequently with a water/ammonia mixture solu- mation rates, obviously connected with short tion to remove Cl-. Washing with ethanol was intergrain diffusion distances, observed by Boutz then used to remove the free water. A more et aL9 Very interesting high temperature mechani- detailed process has been given by Groot Zevert et cal properties are reported by Theunissen et al. lo al. I6 The dried gel was calcined subsequently at They found that Y-TZP ceramics with a smaller 500°C for 2 h followed by milling procedure using grain size exhibit a higher fracture toughness and ZrO, balls. Compacts were made by cold isostati- bending strength at high temperature than those tally pressing at 50 MPa and crushed again in an with a coarser grain size. Practical applications of alumina mortar to obtain better powder flowabil- nano-structured zirconia were limited up to now ity. This powder was uniaxially pressed at 80 MPa because it is difficult to produce them repro- in a die to form a rectangular shape which was ducibly with a high density (295%). This problem compacted further by cold isostatical pressing can be solved by the application of sinter-forging (CIP) at 400 MPa. The compacts were pressure- which is useful for the elimination of flaws and less sintered in air at 1150°C for 10 h with heating grain boundary reinforcement.2%“,‘2 Hence, sinter- and cooling rates of 2”C/min. forging can be used to shift the full densification temperature for ceramics to a lower tempera- 2.2 Sinter-forging ture,r3,14 so that grain growth can be limited fur- The samples for sinter-forging experiments were ther. This makes it possible to prepare dense pre-sintered by a heating rate of 2”C/min to ceramics with grain sizes in the nano-scale region. 1000°C for 15 min to improve compact strength. For sinter-forged tetragonal zirconia materials The density changed from 46% for the initial with grain sizes of 100-200 nm, densities > 97% green body after CIP to 64%. These samples were are reported by Boutz et al. I5 It has also been subsequently machined to 26 X 8 X 6 mm3 with reported that ionic conduction properties are plane-parallel 26 X 6 mm* sides. Sinter-forging significantly influenced by sinter-forging due to experiments were performed in air at 1150°C the change of grain boundary properties.’ To our under a constant load corresponding to an initial knowledge, effects of sinter-forging on mechanical stress of 90 MPa. The initial stress is defined with strength and Weibull modulus (reliability) as well respect to dimensions just before loading. The sin- as on high temperature mechanical properties have ter-forging heating schedule was: heating from not yet been reported before for tetragonal zirco- ambient temperature to 950°C at 600”C/h and nia polycrystal (TZP) with ultra-fine structure. then to 1150°C at 300”C/h. The load was imposed The major objective of this study was to investi- on the 26 X 6 mm2 plane of the samples and gate the effects of sinter-forging and the influence raised linearly at 1150°C from zero to its final of grain boundary properties and process flaws on value in 5 min and then kept constant for 25 min. fracture toughness, bending strength and Weibull Details of the sinter-forging setup are given in modulus at temperatures up to 760°C. The sinter- Refs. 13, 14 and 17. forging technique was used to strengthen grain boundaries and consequently increase the mechan- 2.3 Characteristics ical properties. The effect of flaws on reliability of The densities of the green and sintered compacts ceramics is discussed. Transmission electron micro- were measured by the Archimedes technique in scopy (TEM), scanning electron microscopy (SEM), Hg. The bending strength was measured by the X-ray photoelectron spectroscopy (XPS) and X-ray 4-point bending technique with an inner and outer diffraction (XRD) were used to analyze the fracture span of 10 and 20 mm, respectively, using a surface, flaw size and grain boundary properties. crosshead speed of 0.7 mm/min. The specimens for bending strength measurement were cut into 26 X 5 X 2 mm3 bars and polished on the 26 X 5 2 Experimental Procedure mm* side opposite to the loading piston (hence subjected to tensile stress). After machining and 2.1 Powder preparation polishing, all specimens were annealed for 10 min Nanocrystalline tetragonal zirconia powders with at 1000°C with heating and cooling at 2_5”C/min. high sinter reactivity were synthesized by a gel The fracture toughness, Kit, was measured by the precipitation technique using metal chloride pre- 3-point single-edge notched beam (SENB) method Mechanical properties of TZP ceramics 603 with a span of 12 mm, a crosshead speed of 0.3 mum load of 2 N with a loading speed of 14.4 X mm/min and 15 X 3 X 1 mm3 specimens. A notch 10e2 N/s and a hold time of 5 s at the maximum of 450 Frn depth and 50 pm width was cut with- load.