The Effect of a Small Amount SiO2 on Sintering Kinetics of Tetragonal Zirconia Nanopowders

Lakusta et al. Nanoscale Research Letters (2017) 12:398 DOI 10.1186/s11671-017-2178-6 NANO EXPRESS Open Access The Effect of a Small Amount SiO2 on Sintering Kinetics of Tetragonal Zirconia Nanopowders Marharyta Lakusta*, Igor Danilenko, Tetyana Konstantinova, Galina Volkova, Igor Nosolev, Oksana Gorban, Susanna Syniakina and Valery Burkhovetskiy Abstract In the present paper the sintering behavior of 3 mol% yttria-stabilized zirconia (3Y-TZP) with and without small amount (0.2 wt %) of SiO2 additive was investigated. It has been studied the silica impact which was added in two ways (co-precipitation and mechanical mixing) on sintering kinetics of 3Y-TZP nanopowders at the initial sintering stage. It was found the silica additive leads to the changing in the predominant sintering mechanism at the initial sintering stage from volume (VD) to the grain boundary diffusion (GBD) in nanopowders obtained by co-precipitation. It was shown that the way of silica addition also significantly influence the sintering kinetics of 3Y-TZP. In case of nanopowders with silica additive obtained by mixing method, sintering process occurred due to the predominance of VD mechanism. It was found that the silica additive and the mechanical activation leads to the acceleration of the sintering process. Keywords: Zirconia nanopowders, Sintering kinetics, Silica additive, Sintering mechanisms, Initial sintering stage Background It is a well-known fact that zirconia is really remarkable for a wide range and combination of physical and mechanical properties, such as high fracture toughness; high strength and hardness; biocompatibility; ionic conductivity; radiation and chemical resistance [1]. A lot of properties combined in one material, zirconia. It is possible due to the ability of zirconia to phase transformations [2]. Zirconia can exist in three states: a monoclinic state, a tetragonal state, and a cubic state. These states can be stabilized by adding such additives as Y2O3, MgO, CaO [3]. Yttriastabilized tetragonal zirconia (Y-TZP) has been known as an important structural ceramic and is used for products of grinding media, the optical fiber connector and precision parts. In all cases of using zirconia nanopowders, the producer of ceramics parts need to know the optimal pressure-temperature-time regimes for obtaining dense or porous nanostructure ceramics. The advantage of nanopowders is the possibility of low temperature sintering * Correspondence: Material Science Department, Donetsk Institute for Physics and Engineering (DIPE) named after O.O. Galkin of the NAS of Ukraine, Nauky av., 46, Kiev 03028, Ukraine and as a result the ceramic structure homogeneity. It is known that granulometric (size and shape of particles and the size of aggregates and agglomerates), phase and chemical composition of starting powders as well as the same characteristic of agglomeration as agglomerate “hardness” determine the compactions and sintering regime. The agglomeration is conditioned by van der Waals forces between particles. If these forces are weak the agglomerates are referred to as “soft” agglomerates. These agglomerates can be easily broken in a liquid medium by ultrasonic, or/ and dispersants additions. In contrast, strong forces between particles due to high temperature calcinations or incorrect chemical additions result in “hard” agglomerates. In this case, it is too high to realize the benefits of the nanosized primary crystallites. The high sintering temperature leads to bimodal grain size distribution and phase separation in zirconia ceramics. The prevention of hard agglomeration is a one of the basic aims in nanopowders synthesis process as well as uniform particles shape and narrow size distribution [2]. As for the additives that influence the zirconia nanopowders structure (Al2O3, NiO, Cr2O3, SiO2, GeO2) it has become possible to obtain new ceramics with © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Lakusta et al. Nanoscale Research Letters (2017) 12:398 specific properties. The impact of various additives on the sintering kinetics has been investigated by many researchers [3–7]. One of the well-known researchers in the field of research of the effect of different additives on the tetragonal zirconia, Matsui has reported that the silica additive accelerates the sintering process because the sintering mechanism is changed from grain boundary to volume diffusion by silica addition [3, 4]. In the present paper the impact of small amount of slightly soluble SiO2 additive on the kinetics of the initial sintering stage of ceramics based on 3Y-TZP has been studied. In our previously investigation of the sintering kinetics of 3Y-TZP nanopowders we have got contradictory results using nanopowders which were obtained in DIPE laboratory with the same chemical composition. Our results did not agree with the results of studies conducted over the TZ-3Y nanopowders of Tosoh company production. The reason for the difference of predominant sintering mechanisms at the initial stage has been identified in our study [8]. It was concluded that this result is due to the influence of mechanical activation on powders structure, phase composition and the sintering kinetics of 3Y-TZP. Methods For the investigation it has been chosen 3Y-TZP nanopowder (3 mol% Y2O3-stabilized tetragonal zirconia) obtained in the DIPE of the NASU (Ukraine) by coprecipitation method. It was used a chloride technology and the addition of 0.2 wt% SiO2 to produce these nanopowders. The preparation technique has been described in detail in paper [9]. The silica additive was added in two ways: 1) by co-precipitation method was obtained the nanopowders: with and without the addition of the silica 3Y-TZP- 0,2 wt % SiO2; 3Y-TZP, respectively; 2) by the mixing method was obtained nanopowders with silica and with mechanical activation for 4 and 8-h milling PMM4-3Y-TZP-0,2 wt % SiO2 and PMM8-3Y-TZP-0,2 wt % SiO2 (PMM4 and PMM8 abbreviations were marked for powders name mixing and milling for 4 and 8 h). To separate the impact of the silica and the effect of mechanical activation were obtained 3Y-TZP with the same milling time 4 and 8 h PM4-3Y-TZP and PM8-3YTZP (the PM4 and PM8 abbreviation means milling for 4 and 8 h). All the obtained nanopowders were calcinated at 1000 °C within 2 h. Then in the last two cases nanopowders were mechanically milled in a planetary mill. Thereafter, all nanopowders were pressed at 300 MPa and sintered to the temperature of 1500 0 C with different Page 2 of 9 heating rates of 2.5, 5, 10, and 20 °C/min in the dilatometer (NETZSCH (...truncated)