The phase composition and evolution of the twin structure of PSZ crystals, depending of the concentration
stabilizing impurities have been investigated by X-ray diffraction and transmission electron microscopy.
The measurements of hardness and fracture toughness by microindentation have been carried out. In
work is shown that the high mechanical characteristics of PSZ crystals are related to features of their
phase composition and structure, namely the degree of tetragonality the phase and the presence of domains
ranging in size from tens to hundreds of nanometers.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3507

Borik, M.A.

Bublik, V.T.

Kulebyakin, A.V.

Lomonova, E.E.

Milovich, F.O.

Myzina, V.A.

Osiko, V.V.

Tabachkova, N.Y.

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 1 No 3, 03CNN04(4pp) (2012)   2304-1862/2012/1(1)03CNN04(4) 03CNN04-1  2012 Sumy State University The Influence of Stabilizing Impurities and Annealing on the Structure and Mechanical Properties of Partially Stabilized Zirconia  F.O.Milovich1,*, N.Y.Tabachkova1, V.T.Bublik1, E.E.Lomonova2,  M.A.Borik2, A.V.Kulebyakin2, V.A.Myzina2, V.V.Osiko2  1 National University of Science and Technology "MISIS",  117936 Moscow, Leninsky pr., 4 2 Prokhorov General Physics Institute, Russian Academy of Sciences,  119991, Moscow, Vavilov Str., 38  (Received 4 July 2012; published online 18 August 2012)  The phase composition and evolution of the twin structure of PSZ crystals, depending of the concentra-tion stabilizing impurities have been investigated by X-ray diffraction and transmission electron microsco-py. The measurements of hardness and fracture toughness by microindentation have been carried out. In work is shown that the high mechanical characteristics of PSZ crystals are related to features of their phase composition and structure, namely the degree of tetragonality the phase and the presence of do-mains ranging in size from tens to hundreds of nanometers.   Keywords: Partially stabilized zirconia, transmission electron microscopy, X-ray diffraction, materials of high strength and durability, skull melting technique in a cold crucible.   PACS numbers: 61.10.Nz, 62.20. – x, 68.37.Lp ______________ * filippmilovich@mail.ru   1. INTRODUCTION  The partially stabilized zirconia crystals (PSZ), at the expense of its unique structure, have high tribologi-cal characteristics. Their properties allow using of the material where strength is required when sliding and friction in conjunction with high temperatures, aggres-sive environments and extreme loads. The combination of properties such as fracture toughness, hardness, wear resistance, low friction coefficients, chemical and biological inertness, makes PSZ crystals are promising materials for the manufacture of durable and super acute medical tool. The high strength of PSZ crystals allows producing tools with sharpness cutting edge to 100 nm for cardiology and neurosurgery, vascular sur-gery, ophthalmology, etc. [1]. Such properties of PSZ crystals are related to fea-tures of their phase composition and structure, namely the presence of nano and micro twin structure [2]. Tech-nological change in the growth conditions and annealing of PSZ crystals affects on the structure and therefore on the mechanical properties of the material. The subject of the investigation is evolution of phase composition and the twin structure, depending on the impurity concen-tration Y2O3, as well as the identification of influence of the dopant on the microhardness and fracture toughness of PSZ crystals, i.e. establishing connection with the me-chanical properties of the structure.  2. EXPERIMENTAL  2.1 Materials  PSZ crystals were grown by directional solidifica-tion in a cold crucible (induction skull melting tech-nique). The growth of PSZ crystals was carried out in the range of compositions ZrO2 - Y2O3 (2.8, 3.0, 3.2, 3.7, 4.0 mol. %). The dimensions of grown crystals were: a cross-section of 10 to 50 mm, height from 70 to 100 mm. The wafers were cut from the crystals, thickness about 300 µm, theirs plane was oriented perpendicular to the 100  and 111  axis of the crystal. 2.2 Instruments  Investigation of phase composition was performed by means of x-ray diffraction Bruker D8. The structure was investigated by transmission electron microscopy using a microscope JEM 2100 at an accelerating voltage of 200 kV. The samples were polished to a thickness of 200 µm for preparation to TEM studies. To cut a disc of 3 mm thick was used ultrasonic cutting, and then the hole was formed in the central part of the disc. For fur-ther thinning of the samples was used ion etching. The measurements of microhardness and fracture toughness were carried out on Vickers microhardness 402MVD Instron Wolpert Wilson Instruments with a maximum load of up to 2 kg, and Wolpert Hardness Tester 930 with a minimum load of 5 kg on the wafer with the planes, which was cut parallel to the planes (001).  3. RESULTS AND DISCUSSION   3.1 Phase analysis  In the diffraction pattern was obtained from a sam-ple, cut out from the crystal along the plane (100).  There were reflections - second, fourth and sixth orders from this plane. A more detailed consideration area diffraction pattern of large-angles 2   showed that this material has two tetragonal phases belonging to space group symmetry R42/mnc, as seen in Fig. 1a. The ap-pearance of the diffraction reflections at the same time (006) and (600) is due to twinning, as will be shown below in the study by transmission electron microscopy. The imposition of the splitting of reflections due to the tetragonality and the splitting of the Kα doublet pre-vented the analysis of the profile of the diffraction lines. Therefore, an experiment was conducted with the Kβ radiation, which confirmed the splitting of the lines and the presence of these two tetragonal phases. The phase composition of zirconia doped with differ-ent concentrations Y2O3 by X-ray diffraction showed that all studied samples, independently of the content  F.O. MILOVICH, N.Y. TABACHKOVA, V.T. BUBLIK, ET AL. PROC. NAP 1, 03CNN04 (2012)   03CNN04-2 of stabilizing impurities, have two phases of tetragonal zirconia with varying degrees of tetragonality. For a tetragonal phase t with the ratio c/a was 1,014-1,015, for the other tetragonal phase t' ratio c/a little differ-ent from the 1 and amounted to 1,004-1,005. Yttrium-enriched phase of t' is "nontransformable" in contrast to the t phase, with a lower content of yttrium, which under the influence of mechanical stress undergoes martensitic transition to the monoclinic form. Increas-ing the concentration of the stabilizing impurity from 2.8 to 4.0 mol. % Y2O3, leads to a monotonic increase in the quantity weak-tetragonal phase and a decrease in the quantity of "transformed" phase.      Fig. 1 – a: X-ray diffraction patterns of the YSZ 2.8 mol. % Y2O3; b: X-ray diffraction patterns of the YSZ 4.0 mol. % Y2O3  3.2 Research of structure partially stabilized zirconia crystals   Investigation of the structure of zirconium dioxide in the TEM showed that the appearance electron diffrac-tion patterns, all of the samples were single crystalline. Fig. 2 presents a typical electron diffraction pattern of samples cut out across a 〈111〉 axis of a crystal. On the electron diffraction, there are reflections of the type (110) are forbidden in the cubic lattice and allowed in the te-tragonal, that confirms the results of x-ray analysis. TEM method is not able to separate the two tetragonal phases, due to their close lattice parameters. All of the samples had a twin domain structure. Fig. 3 is an elec-tron micrograph of microtwins in the [100]-oriented ZrO2 2.8 mol %Y2O3 sample. Most of the domains are elongated. The twinning plane of the domains is {110}. Primary twin plates are also twinned, forming a herringbone structure of twin   Fig. 2 – Selected area diffraction pattern of a sample in a 〈111〉 direction with reflections from (110), which correspond-ing to the tetragonal phase    Fig. 3 – Bright-field image of twin domains in the ZrO2 2.8 mol. % Y2O3 sample  domains. Twinning occurs on planes inclined to the four fold axis С. Twinning is possible on the (101) and (011) planes and impossible on the (110) plane, parallel to the С axis. High resolution TEM examination re-vealed twins on the order of 15 nm in size (Fig. 4). It is reasonable to assume that the twinning process contin-ues as long as there is elastic stress sufficient for twin-ning. Study of PSZ crystals with different concentra-tions of the stabilizing impurity showed that increasing in the concentration of Y2O3 affect the form and disper-sion of the twin domains. At a concentration of Y2O3 from 2.8 to 3.2 mol. % twinning first goes in the larger domains, which in turn also twinned. Observation of twin domains in the high resolution showed that nano-sized twins are found only in sam-ples with concentrations up to 3.2 mol. % Y2O3. At high concentrations, the stabilizing impurity (3.7 –  4 mol. % Y2O3) hierarchy of twinning is not visible, traces of atomic planes within the domains are not violated. It allows assuming that the twinning occurs at the same time and is localized in small volumes. At the cooling  THE INFLUENCE OF STABILIZING IMPURITIES AND ANNEALING ON THE… PROC. NAP 1, 03CNN04 (2012)   03CNN04-3   Fig. 4 – High-resolution TEM image of twin domains in the ZrO2 2.8 mol. % Y2O3 sample  the crystal phase transition from cubic to two-phase region, in accordance with the phase diagram, occurs at lower temperatures, therefore the character of the twinning structure was changing. The relative position of domains in a sample of 3.7 mol. % was different from the other concentrations. Generally domains were ar-ranged perpendicular to each other, as shown in Fig. 5.    Fig. 5 – Bright-field image of twin domains in the ZrO2 3.7 mol. %  With increasing impurity concentration to 4.0 mol. % the morphology of the twins changed, as shown in Fig.  6.  3.3 Mechanical properties  For detection of anisotropy in the PSZ crystals, measurements of microhardness and fracture tough-ness were carried out in different directions, on orient-ed samples. For this purpose, each sample was oriented on a diffractometer and glued on the plane-parallel substrate, which indicated the direction of the axes. For measuring the microhardness there were chosen loads   Fig. 6 – Bright-field images of twin domains in the ZrO2 4.0 mol. % Y2O3 sample   at which the microhardness independent of the load. As microhardness anisotropy was not observed, further measurements for a set of statistics were carried out in one direction. Fracture toughness was determined by the formula:  12/15/21 )/()/(035,02/1CHHCELK C , (1.1)  where: K1c – intensity factor stresses, L – length of ra-dial cracks, a – half-width of the printout, C – Poisson's ratio, E – Young's modulus, H – microhardness. For obtaining dependence fracture toughness on the percentage of Y2O3 content, was chosen the total load for all concentrations. For the measurements was cho-sen load of 10 kg. Fracture toughness, as the micro-hardness was measured in all directions. In all samples the cracks arose on ends of the diagonals of the inden-tation, as shown in Fig. 7.     Fig. 7 – Vickers indentation on the (001) plane of PSZ with cracks are clearly visible  After comparing fracture toughness in all direc-tions, it turned out that the difference lies in the range of values error. Thus, further measurements were per-formed for a set of statistics in one direction only. The resulting average values of fracture toughness and mi-crohardness with error measurement are presented in Table 1.  F.O. MILOVICH, N.Y. TABACHKOVA, V.T. BUBLIK, ET AL. PROC. NAP 1, 03CNN04 (2012)   03CNN04-4 Table 1 – Dependence of microhardness and fracture toughness from the Y2O3 concentration   The table shows that the correlation between the composition and microhardness, within error, is not observed, but it is clear that with increasing concentra-tion of Y2O3, fracture toughness decreases. The sample with the lowest concentration of the stabilizing impuri-ty 2.8 mol. % showed the most resistant to cracking. Good mechanical properties can be associated with a complex twin structure, while fracture toughness de-crease agrees with decrease in the quantity of t phase. The decrease t phase, affects the "transformational" hardening, when the moving microcrack induces mar-tensitic t-m transition, which absorbs the energy of stresses, and eventually stops the microcrack.  REFERENCE  1. M.A. Borik, E.E. Lomonova, V.V. Osiko, V.A. Myzina, J. Cryst. Growth 275, 2173 (2005). 2. N.Y. Tabachkova, E.E. Lomonova, M.A. Borik, F.O. Milovich, V.T. Bublik, A.V. Kulebyakin, V.A. Myzinа, V.V. Osiko, J. Inorg. Mater. 48 №2, 158 (2012). Compositions Y2O3, % 2,8 3,0 3,2 3,7 4,0 The values of microhardness, HV (300 g) 1410±40 1420±30 1400±20 1420±25 1450±40 The values fracture toughness, МPа∙m-1/2 (10  kg) 12±0,6 10±0,6 9±0,6 7±0,6 6±0,6 

The Influence of Stabilizing Impurities and Annealing on the Structure and Mechanical Properties of Partially Stabilized Zirconia

The Influence of Stabilizing Impurities and Annealing on the Structure and Mechanical Properties of Partially Stabilized Zirconia

Abstract

Similar works

Full text

Available Versions

Electronic Sumy State University Institutional Repository