Processing

Elsevier Editorial System(tm) for Journal of the European Ceramic Society Manuscript Draft Manuscript Number: Title: Processing of Zirconia Nanoceramics from a Coarse Powder Article Type: Full Length Article Keywords: Zirconia; Nanoceramic; Porosity; Sintering; Microstructure Corresponding Author: Dr. Andraž Kocjan, Corresponding Author's Institution: Jozef Stefan Institute First Author: Andraž Kocjan Order of Authors: Andraž Kocjan; Vaclav Pouchly; Zhijian Shen Abstract: We have merged the benefits of coarse-grained and nanocrystalline powders by the consolidation and sintering of coarse, yet nanostructured, mesoporous Y-TZP powder. The powder was composed of loosely aggregated, nanoscale crystallites, which can be seen as secondary particles or agglomerates. Their homogeneous, defect-free packing proved to be a viable pathway for the processing of zirconia nanoceramics. The powder consolidation yielded homogeneous green bodies with hierarchical heterogeneities in terms of intra- and inter-particle pore packing. The hierarchical heterogeneities had a pronounced effect on the densification and grain growth. The intra-particle pore coalescence along with a "frozen" inter-particle porosity, prolonged the pore-pinning effect, separating the densification and grain-growth mechanisms. Increased heating rates promoted the grain growth and densification via a competitive mechanism of primary crystallite ordered coalescence, while by applying pressure, the crystallite growth was completely prevented, making such a coarse powder suitable for the fabrication of zirconia nanoceramics. Suggested Reviewers: Wei-Hsing Tuan [email protected] Suk-Joong Kang [email protected] Vladimir Srdic [email protected]

Cover Letter

Andraž Kocjan Engineering ceramics departmentArrhenius Laboratory Jožef Stefan Institute Ljubljana, Slovenia

August 20th, 2014

Dear Editor, I would like to submit a manuscript entitled Processing of Zirconia Nanoceramics from a Coarse Powder. The manuscript summarizes part of the results from postdoctoral project entitled Improved Reliability of Translucent Dental Zirconia and Alumina Ceramics, which was awarded and funded by the JECS Trust foundation through the 2011 call “Frontiers of Research“. It is my pleasure to be submitting another manuscript to your top-ranked journal related to ceramics and material science, since I am also obliged to hand it over into consideration by the contract signed with JECS Trust. The manuscript is, in a way, a follow up paper published last year, entitled Colloidal processing and partial sintering of high-performance porous zirconia nanoceramics with hierarchical heterogeneities (33 (2013) 3165-3176). The already published one deals with porous zirconia, while the submitted one about dense counterparts. Both studies take advantage of the “atypical” mesoporous Y-TZP powder. In the present manuscript we want to show that it is possible to process a dense, zirconia nanoceramic using a coarse powder. The powder consists of secondary particles composed of loosely aggregated, nanoscale crystallites (which can be seen as agglomerates or aggregated domains). Thus, the benefits of a coarse-grained powder, i.e., the minimizing of problems related to agglomeration in order to increase the homogeneity and decrease the packing defects within the ceramic green body, and those of nanocrystalline powders, i.e., having an increased sintering activity, were merged. The consolidation yielded green bodies with hierarchical heterogeneities (HH) in terms of intra- and interparticle pores on account of the spherical mesoporous particle packing. HH had a pronounced effect on both densification and grain growth, increasing the overall sintering activity. The intra-particle pore coalescence and a “frozen” inter-particle porosity prolonged the pore-pinning effect, separating the densification and grain growth mechanisms. Consequently, a zirconia nanoceramic could be processed from such coarse powder. The effect of sintering approach on the grain growth (apparently via primary-crystallite ordered coalescence) and grain size distribution are also discussed. The authors confirm that the manuscript, or its contents in some other form, is based on original work and has not been published previously by any of the authors and/or is not under consideration for publication in another journal at the time of submission.

I look forward to hearing from you.

Sincerely, Andraž Kocjan

*Summary of Novel Conclusions

Summary of Novel Conclusions We have merged the benefits of coarse-grained and nanocrystalline powders by the processing of coarse, yet nanostructured, mesoporous Y-TZP powder, thus, providing an original route for attaining (zirconia) nanoceramics.

*Manuscript Click here to view linked References

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Processing of Zirconia Nanoceramics from a Coarse Powder Andraž Kocjan1,*, Vaclav Pouchly2, Zhijian Shen3 1

Engineering Ceramics Department, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

2

CEITEC BUT, Brno University of Technology, Technicka 10, 616 00 Brno, Czech Republic

3

Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, S106 91 Stockholm, Sweden

Abstract We have merged the benefits of coarse-grained and nanocrystalline powders by the consolidation and sintering of coarse, yet nanostructured, mesoporous Y-TZP powder. The powder was composed of loosely aggregated, nanoscale crystallites, which can be seen as secondary particles or agglomerates. Their homogeneous, defect-free packing proved to be a viable pathway for the processing of zirconia nanoceramics. The powder consolidation yielded homogeneous green bodies with hierarchical heterogeneities in terms of intra- and inter-particle pore packing. The hierarchical heterogeneities had a pronounced effect on the densification and grain growth. The intra-particle pore coalescence along with a “frozen” inter-particle porosity, prolonged the pore-pinning effect, separating the densification and grain-growth mechanisms. Increased heating rates promoted the grain growth and densification via a competitive mechanism of primary crystallite ordered coalescence, while by applying pressure, the crystallite growth was completely prevented, making such a coarse powder suitable for the fabrication of zirconia nanoceramics.

Keywords: Zirconia, Nanoceramic, Porosity, Sintering, Microstructure

*Corresponding author: email: [email protected], tel.: +38614773895, fax.: +38614773171

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1. Introduction In the past 20 years, ultrafine, nanocrystalline oxide powders of high purity, possessing characteristic length scales below 100 nm, have become available in sufficient quantities for them to be used for processing. When consolidated they exhibit an increased sintering activity, due to the high density of the grain boundaries and interfaces, where a large fraction of the atoms reside on the particle surfaces as a result of the increased surface-to-volume ratio. If these ultrafine powders are shaped and sintered to form dense bodies, while retaining a final grain size below 100 nm, they are referred to as nanocrystalline ceramics or nanoceramics. Unfortunately, the processing of such powders remains a challenge, but the resulting properties of nanoceramics can be significantly different from those of their traditional counterparts, i.e., coarse-grained ceramics, as they frequently exhibit superior mechanical, electrical, and thermal properties.[1] [2][3] In the case of yttria-stabilized zirconia (YSZ), in nanocrystalline form, it can deform superplastically by grain-boundary sliding.[4] On the other hand, its inherent thermal conductivity is significantly reduced due to the grain-size-dependent, interfacial, thermal resistance.[5] Combined with improved mechanical properties, this makes the YSZ nanoceramic an excellent candidate for thermal barrier coatings.[6] Furthermore, nanocrystalline YSZ is a well-established electrolyte material for solid-oxide fuel cells and gas-sensor applications, since it exhibits orders-of-magnitude-higher ionic conductivities than its coarse-grained counterpart.[7] More recently, yttria-stabilized tetragonal zirconia (Y-TZP) has become a popular bioceramic that can be used in implants, dental posts, abutments and fixed partial dentures (overlaid with veneering porcelain).[8][9] In the latter case, the increased number of clinical failures in terms of chipping and delamination of the veneer, due to the build-up of stresses during the preparation,[10] resulted in the emergence of full-contoured, monolithic, Y-TZP dental restorations. Thus, the Y-TZP dental ceramic also tends to be manufactured with smaller grains, preferably nanocrystals, in order to obtain a higher optical transmittance to increase the translucency, which was formerly provided by means of a porcelain veneer[11]. Moreover, the nanocrystalline nature of such ceramics increases their resistance to low-temperature degradation (LTD).[12] There are two interchanging pathways for increasing the likelihood of successfully processing YTZP nanoceramics: i) to synthesize or purchase nanocrystalline powder that is as fine as possible with no, or a controlled level of, aggregates/agglomerates[13][14][15][16][17] and/or ii) to employ pressure-assisted sintering techniques [18]. By using very fine nanocrystalline powders the sintering activity is increased, i.e., the finer the powder, the lower the temperature that is required for complete densification, consequently resulting in a finer final microstructure. However, fine ceramic nanopowders come with various agglomeration states, since agglomeration is a natural result of the dominant effect of interparticle forces. The nanoparticles reside in ordered or disordered assemblies, held together by weak physical, but usually by strong chemical bonds, depending on the (post-)synthesis treatments.[3] If the agglomerates are not removed (and agglomeration avoided) during shaping, the resultant green-body microstructures can possess two types of inhomogeneously distributed pores, i.e., inter- and intra-“agglomerate” pores.[19] During sintering, the elimination of the inter-agglomerate pores needs high temperatures, encouraging grain growth, which is detrimental to keeping the grain size on a nanometre scale.[20] Handling and shaping is an additional drawback of fine nanopowders. Shaping via dry-pressing requires granulation[21] and/or high consolidation

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pressures in order to obtain relatively homogeneous green bodies. However, when wetconsolidation routes are employed the suspension’s solids loadings are lower and the particle surface larger than in coarse-grained powder suspensions, making the shaping step more challenging. Additives have to be carefully removed during the calcination/sintering step, but before that, drying is another crucial step, during which, if care is not taken, green bodies consisting of nanoparticles are prone to cracking due to the increased capillary forces.[22] In general, using pressureless sintering it is difficult to obtain a dense microstructure with grains smaller than the initial agglomerates. On the other hand, uniaxial pressure-assisted sintering methods are very costly and can only be used to produce bodies with very simple geometrical shapes. Complex shapes can be sintered by using hot isostatic pressing, but then the costs increase substantially. Therefore, much effort has gone into the use of pressureless sintering techniques. It was shown that when isothermal sintering is employed for several hours to several tens of hours at temperatures with the highest densification rate (or slightly below), fully dense Y-TZP nanoceramics were prepared with only a five times increase in the primary crystallite or aggregate size.[23][24] However, the as-prepared nanoceramics still possessed some residual porosity, which, for example, can be detrimental for obtaining a translucent ceramic, which is preferred in dentistry. Another drawback of such an approach is the lengthy sintering time. During the pressureless sintering of nanoceramic green bodies, the pores are usually being coalesced together, increasing the overall pore size, which is detrimental for retaining the grain growth in the intermediate/final stage of pressureless sintering on account of the pore-pinning effect.[25] Moreover, the prerequisite for green bodies is a high packing density and, more importantly, microstructural homogeneity over the whole body, to ensure sintering temperatures low enough to minimize the grain growth. This is a more straightforward process and more easily attained when dealing with coarse-grained powders; however, with both the initial and final microstructures being on an order-of-magnitude larger scale, with a lower overall sintering activity. The goal of the present work was to merge the benefits of having a coarse-grained powder, i.e., to minimize problems related to agglomeration, in order to increase the homogeneity and decrease the packing defects within the ceramic green body, while retaining the benefits of the nanocrystalline powders, i.e., having nanometre-scale primary crystallites and therefore an increased sintering activity, and thus obtain a dense nanoceramic at moderate sintering temperatures. This was achieved by the processing of a nanostructured mesoporous Y-TZP (Meso-TZ) starting powder. It consisted of loosely aggregated primary nanoscale crystallites, composing secondary particles (which can be seen as agglomerates or aggregated domains). The dry- or wet-processing consolidation route of the powder yielded green bodies with hierarchical heterogeneities (HH) in terms of intra- and inter-particle pores on account of the spherical mesoporous particle packing. The apparent activation energies of the sintering were evaluated using the master sintering curve (MSC) approach for different stages of sintering in order to follow the sintering behaviour of the ceramics with HH. Finally, different sintering approaches were adopted along with conventional sintering, i.e., microwave sintering and post hot isostatic pressing (HIP), in order to check its influence on the final microstructures of sintered, consolidated, Meso-TZ powder in terms mechanical properties, grain growth, grain size distribution and their suitability/feasibility for dental applications.

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2. Material and methods In the present study a custom-made Y-TZP mesoporous (labelled as Meso-TZ) powder, with 2.94 mol% of yttria and a small content of alumina (0.2–0.4 wt%), was used, having a BET surface area of 18 m2/g. The powder is a semi-product with the potential to be commercialized in the near future, and so the producer cannot be disclosed here. The preparation of green bodies via the centrifugation slip casting of the weakly flocculated Meso-TZ suspensions is described elsewhere.[26] Conventional pressureless sintering in air was performed in a resistance chamber oven (Nabertherm GmbH, Lilienthal, Germany) with the following heating regime: 2°C/min to 600°C with a dwell time of 2 h in order to remove the organics; then 5 °C/min to the desired temperature with a dwell time of 2 h; and cooling down at a rate of 10°C/min. After obtaining the closed porosity conventionally, the samples were post-HIPed in a hot isostatic press (ABRA Shirp, Switzerland) for 2 h at 1200°C and 200 MPa i n an argon atmosphere. As an additional reference, microwave sintering was performed in a lab-built microwave chamber supplied with a multi-module microwave of 2.45GHz. Silicon carbide was used as the microwave absorber. The temperature was measured using a digital infrared pyrometer. The optimized sintering conditions, yielding a fully dense body, were a heating rate of 50°C/min and a dwell time of 30 minutes at 1300°C. Master sintering curves (MSCs) were constructed from the dilatometric analyses of green bodies recorded at four heating rates, i.e., 2, 5, 10 and 20°C/min. The values for the activation energy of the sintering were found by minimizing the mean-perpendicular-curve-distance criterion, which together with the used software is described elsewhere.[27] The pore volume and the pore size distribution of the green bodies were determined using a mercury-intrusion porosimeter (AutoPore III 9410, Micromeritics, Norcross, GA, USA) within the pore-diameter interval 10 nm to 360 µm. The surface tension and the contact angle of the mercury were set to 0.485 mN/m and 130°, respectively. The densities of the specimens were estimated either by using mercury-intrusion porosimetry or by Archimedes’ method, with water as the immersing medium and using a value of 6.08 g/cm3 for the theoretical density (TD). The hardness and indentation toughness of the samples were determined using a Vickers indentation hardness tester (Zwick GmbH & Co., Ulm, Germany). The indents were made using a Vickers-type indenter by applying a 10 kg load for 15 seconds and the indentation toughness was then calculated from the equation of Anstis et al., [18] which was later modified by Kaliszewski et al. [19] to account for the effect of the compressive stresses due to the surrounding transformation zone. X-ray diffraction (XRD) patterns from the as-sintered samples were recorded using a diffractometer (X‘pert PRO MPD, PANalytical, Almelo, Netherlands) equipped with a PIXcel detector and using a Cu-Kα1 radiation source. The measurements were carried out using fixed divergence (1°) and anti-scattering slits, a 10-mm mask, a continuous scan mode in the 2θ range 20–100°, with a step size of 0.00656° and a scan speed of 0.01113°/s. The crystallite size analyses from the X-ray powder diffraction peak broadenings were performed with the Rietveld method using the FullProof software package (FullProof Suite).

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A microstructural analysis of the green bodies was performed with a SEM (Jeol JSM-7000F, Jeol, Tokyo, Japan) at an accelerating voltage of 5 kV on ion-beam polished surfaces prepared using an SM09010-CP ion-beam polisher (Jeol, Tokyo, Japan), where argon was used as the source of ions at 5 kV. The grain size was evaluated using the linear-intercept method with no correction factor employed.[28] The intercept length of the grains was determined on SEM micrographs of polished and thermally etched samples. The grain size distributions were obtained from planimetric analyses of at least 3 SEM micrographs (taking into account ≥1600 grains) using image-analysis software (ImageJ). The etching temperatures were 50-100°C lower than the sintering temperatures with an etching time of 5 min.

3. Results and Discussion 3.1. Green-body characteristics – hierarchical heterogeneities (HH) The consolidation of the Meso-TZ powder was conducted via the centrifugal slip casting of weakly flocculated suspensions (the suspension preparation and its characterization are described elsewhere [26]). The packing of the about 150-nm-sized spherical secondary particles, composed of loosely aggregated, primary, nanoscale crystallites (aggregated domains with an estimated average size of 27 nm [26]), yielded HH in terms of pore packing, as shown in Figure 1a. The consolidation of such coarse-grained, aggregated particles resulted in a highly homogeneous, defect-free green body (inset of Fig. 1a) packed with nanoparticles. Two types of pores are present; inter- and intra-(secondary)-particle pores, both distributed homogeneously throughout the whole green body. A secondary aggregated coarse particle contains from 2 to 5 pores, depending on its size or the number of primary crystallites. The as-obtained green bodies possessed relative density of 51.0% of theoretical (TD). In parallel with the wet approach, freeze granulation was applied in order to produce granules from the Meso-TZ, which were dry pressed at various pressures. The resulting densities obtained are summarized in Table 1. After the uni-axial pre-pressing of the freeze-dried Meso-TZ granules, having a granule size (d50) of 58.5 µm, at 70 MPa, a green density of 47.9% TD was obtained. Subsequent post-cold-isostatic pressing (CIP) at 200 MPa, which is the most usual industrially applied pressure, resulted in a slight increase of the green density to 49.7% TD. If the CIP pressure was increased to 800 MPa, a value of the green density comparable to centrifugal slip casting (51.0% TD) was obtained. This illustrates the number of possible choices for the processing of such coarse-grained, yet nanostructured, powder, making such powder highly versatile in terms of processing. For comparison, when commercial benchmark granulated powder from Tosoh (TZ-3YSB-E) was cold isostatically pressed (CIP) at 200 MPa a relative density of 49.5% TD was obtained.[29]. The pore size distribution in the Meso-TZ green body with HH was analysed using mercuryintrusion porosimetry (Figure 1b). Plotting the cumulative and incremental pore-volume intrusion as a function of the mean pore diameter of the centrifuged and dry-pressed green bodies confirmed the presence of HH, i.e., the bimodal pore size distribution, originating from intra- and interparticle pores as a consequence of the mesoporous particle packing. The intraparticle porosity was estimated to represent around 10% of the total porosity.[26] According to Figure 1b, the intraparticle pores are 4–12 nm in size. To illustrate the absence of larger agglomerates and the superior dry-consolidation ability of the granulated powder the pore size distribution of

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the uni-axial pre-pressed Meso-TZ granulated powder was compared to the centrifuged, slipcasted green body. The only difference between them is in the higher pore volume and in the increased median pore size, i.e., from 42 nm to 48 nm. These differences were solely attributed to the enhanced packing of the secondary particles, resulting in an increase of the green density from 47.9 to 51% TD (Table 1). Both green bodies exhibited a monomodal and relatively narrow interparticle pore size distribution.

3.2. Sintering Sintering curves for the green bodies prepared by uni-axial dry pressing (70 MPa) and centrifuge slip casting are shown in Figure 2a. Both curves exhibit a typical sigmoidal shape. The one prepared from the uni-axially dry pressed Meso-TZ granulate underwent less pronounced sintering compared to the centrifugal slip-casted specimen. This was expected, due to the lower initial relative density. However, both samples were fully dense (≥99% TD) after just 2 h at 1300°C, i.e., relative densities of 99.7 and 9 9.4% TD were obtained for the uni-axially drypressed and centrifugal slip-casted samples, respectively. A temperature increase to 1350°C resulted in an increase in the relative density to 99.85% TD for both samples. From these results it can be concluded that a higher degree of packing is obtained when centrifuge slip casting (or when additional CIP at 800 MPa) is used for the shaping, yielding the lowest conventional sintering temperature of 1300°C. A linear shrinkage and the densification rate versus temperature for the centrifugal-casted Meso-TZ green body are shown in Figure 2b. The material starts to shrink soon after 850°C, having the highest densification rate at a temperature slightly higher than 1200°C. As already observed from the sintering curves (Figure 2a; having 2-hour dwell), the material becomes fully dense with temperatures higher than 1300°C. As compared to the low-calcined, high-BET powder from Tosoh (TZ-3YB-E), the slightly earlier (~50°C) onset of shrinking[30] can be attributed to the higher specific surface area, i.e., 18 versus 14 m2/g, as well as the slightly higher maximum densification rate (~25°C). However, a n almost 100–150°C lower sintering temperature was needed to fully densify the MesoTZ sample (Figure 2) as compared to the Tosoh benchmark pressed body.[30] Liu studied the influence of grain size on the temperature required to densify zirconia to full density[17] by relating the experimental results to the theoretical predictions derived by Hansen.[31] It was shown that a zirconia compact of (aggregated) grains having an average diameter of 310 nm required a temperature of 1450°C for full densification. This coincides well with the Tosoh TZ-3YB-E material, having aggregates (with primary crystallites of 50–80 nm in size[14]) in the size range 200–250 nm, which need a temperature of about 1400°C for full densification.[3 0] On the other hand, 150-nanometre-sized, loosely aggregated primary crystallites (27 nm) from Meso-TZ powder sinter more like 85nanometre-sized zirconia consolidated particles, reaching full density between 1300 and 1350°C, rather than like 170-nanometre-sized particle s, achieving full densification at 1400°C, according to Liu`s work.[17] This enhanced sintering activity could be related to the mesoporous structure of the Meso-TZ green body (Figure 1a). One would expect that a microstructure containing HH would sinter differentially, i.e., intraparticle versus interparticle sintering, where the intraparticle sintering would dominate due to the higher density.[1][32][33] However, this was not the case, obviously due to, on the one hand, homogeneous packing of the secondary particles throughout the green body and, on the other, on account of the loose aggregation of primary crystallites forming mesopores in contrast to the dense aggregates usually encountered in synthesized zirconia powders.[13] [14]

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The pronounced effect of HH in Y-TZP on the initial and intermediate stages of sintering was shown by Kocjan and Shen.[26] During the initial stage of sintering, the inter-particle pores remained intact, as was expected. However, intra-particle pore coalescence took place, decreasing the number, but increasing the size of the pores within the secondary particles (Figure 3a). This unique observation opposes the theory that was, however, developed for micron-sized particles, rather than nanosized ones and was attributed to the surface-energy minimization. With pronounced densification as part of the intermediate stage of sintering, both types of pores were slowly being annihilated (Figure 3b).[26] Another benefit to the overall densification (and final grain size distribution) of these phenomena taking place during the initial to intermediate stages of sintering is in the fact that the Meso-TZ green bodies had a “frozen” pore size distribution until 1150°C (81.7% TD),[26] p rolonging the pore-pinning effect, which separates the densification and grain growth mechanisms. On the other hand, during the sintering of conventional, non-aggregated (loosely), (nano)powder, the pore size distribution tends to increase during the initial-intermediate stage of sintering, due to the particle rearrangements under capillary forces and necking, prior to the turning point where the pore size is decreased.[34][18][33][15] 3.3. Master sintering curves (MSC) concept – Apparent activation energy of sintering The MSC concept was employed to check the possible impact of the intra-particle pore coalescence during the initial/intermediate stage of sintering on the whole sintering process. For the evaluation of the apparent activation energy of sintering, the green samples produced via the centrifugal slip casting of weakly flocculated Meso-TZ suspensions were used. The analysed samples were extracted from two directions: the longitudinal (perpendicular to centrifuge force) and the transversal (along the centrifuge force) in order to check the possible influence of centrifugal force on the as-cast sample’s homogeneity. The MSC curve and the sintering activation-energy evaluation for the longitudinal direction are shown in Figure 4b, while a summary of the obtained apparent activation energies in both directions is presented in Table 2. The fact that the overall sintering activation energy (from 51 to 100% TD) was practically the same for both shrinkage directions, i.e., for the longitudinal (600 kJ/mol) as well as for the transversal direction (590 kJ/mol), is indicative of a high degree of ordering and homogeneity in the centrifuged casted samples not influenced by the direction of the centrifugal force. Recently, Song et al.[35] and Pouchly et al.[36] exploited the concept of two-stage MSC. By simply cutting the MSC it is possible to distinguish the contribution of the desired sintering stage, or density range, to the partial apparent activation energy, since different densifying mechanisms are competing during the sintering. In the present case, to check the influence of the intra-particle pore coalescence the MSC was split at 81% TD (Figure 4b). If the cut is made appropriately, separating two or more densifying mechanisms at their end/start, the separated MSC curves should yield a good fit. The cut at 81% TD represents a boundary where the observed Meso-TZ microstructures during sintering significantly altered, i.e., at the point where the hierarchical heterogeneous nature in the microstructure was almost lost (Figure 3b), meaning at higher densities the intra-particle pore coalescence (and their annihilation) was not contributing to the overall (classical) densification mechanisms any longer. The outcome of the MSC cutting was the obtained apparent activation energies of sintering obtained for separate density regions. From the results in Figure 4b and Table 2 it can be observed that for the lower density range the apparent activation energy of the sintering was significantly higher than for the higher one, i.e., 620 vs 460 kJ/mol. Similar decreases in the apparent activation energies of

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sintering for Y-TZP were already observed by Granger et al.[37] and Pouchy et al.[36]. The former observed a decrease from 935 (up to 73% TD) to 310 kJ/mol (above 90% TD), while the latter reported from 1270 to 630 kJ/mol separating them at 93% TD. Apparently, the increased apparent activation energy for sintering in the lower-density region is a consequence of the surface diffusion acting at low temperatures and hindering the densification, combined with the limited appearance of point defects (pore-pinning effect).[37][36] However, Granger et al.[37] and Pouchy et al.[36], used zirconia powders without an alumina addition, which is well known for lower the overall apparent activation energy of the sintering, but not to a significant extent [38]. In the case of the meso-TZ samples, in comparison with the mentioned authors, the apparent activation energy of sintering as low as 630 kJ/mol shows the enhanced sinterability of this coarse, yet nanostructured, powder, on account of the intra-particle pore coalescence. Moreover, the additional decrease the of sintering activation energy points to a change of the densification mechanism during the sintering. This can probably be related to the pore-pinning effect, where intra-particle pores are already removed and thus the densification can proceed in a more pronounced manner.

3.4. The effect of non-conventional sintering approaches on the microstructural evolution of Y-TZP green bodies with hierarchical heterogeneities In comparison to conventional sintering, centrifuged casted Meso-TZ samples were also sintered in a microwave furnace and using the post hot isostatic pressing (HIP) treatment to check their impact on the microstructural features as well as the mechanical properties. Microwave sintering with its possible increased heating rates (up to 50°C/min) is gaining popularity, since rapid sintering approaches are becoming important, especially in the field of dentistry, due to the market demand for speeding up production outputs. On the other hand, by using HIP we wanted to check how close to a nanoceramic Y-TZP we can get, since the porous secondary particles in the Meso-TZ powder consist of nanosized crystallites, while it is known that HIPing hinders the grain growth in the final stage of sintering, when the minimized/lowest sintering temperature can be applied.[39] 3.4.1. Final relative densities and mechanical properties Microwave sintering yielded fully dense samples with a relative density of 99.5% TD (Table 3). Interestingly, the higher heating rate obviously promoted densification and reduced the dwell time by four times to reach full density, as compared to conventional sintering in air (where the sample exhibited a slightly higher relative density of 99.7% TD after 2 hours at 1300°C (Table 3). Here, one has to take into account the difficulty of a precise determination and control of the temperature in a microwave oven. The enhanced densification can also be attributed to a genuine microwave effect, which for Y-TZP should exist, although it is not fully understood.[40] Post HIPing of the conventionally pre-sintered sample (1200°C/2h, ρrel = 94.7% TD; Table 3) with a pressure of 200 MPa resulted in the highest relative density of 99.8 % (Table 3). In spite of the various sintering regimes used, the mechanical properties of all the fully dense samples were comparable, with Hv being 13.7–14.1 GPa and Kifr 4.4–5.2 MPa m1/2. 3.4.2. Microstructural analyses: Linear intercept vs. planimetric grain size analysis SEM micrographs of the thermally etched, polished surfaces are shown in Figure 5. The corresponding estimated grain are listed in Table 4. The conventionally sintered Meso-TZ sample (Figure 5a; Cnv-1300) possesses a fine microstructure with no defects or inclusions,

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consisting of monomodally distributed equiaxed grains with an average size of 142 nm. However, when the sample was sintered in a microwave furnace at a 10-times higher heating rate, but with a 4-times shorter dwell time, the grain size increased to 165 nm at a comparable relative density (Figure 5b, Table 3 and 4). Compared to the conventionally sintered sample (Figure 5a) the microstructure contains some larger grains in a matrix of finer ones, indicating that the distribution tends to be broader or even multimodal. Some residual porosity could still be observed. It is interesting to observe such an increase in the grain size and the appearance of a fraction of larger grains, compared to conventional sintering, yet with a much shorter dwell time. Therefore, a genuine microwave effect (or a significantly higher temperature) on the grain size should not be excluded.[40] In addition, it was surmised that with increased heating rates the ordered/oriented coalescence of primary crystallites could take place, resulting in the formation of the larger grains in a matrix formed of initial secondary particles (Figure 1a). A similar growth mechanism was already observed and postulated by Hu and Shen for the SrTiO3and Si3N4-based systems sintered using the spark plasma sintering (SPS) technique.[41],[42] Post HIPing of a conventionally pre-sintered sample (Figure 5c) resulted in an very fine microstructure with an average grain size just above the limit for a nanoceramic, i.e., 108 nm, (as was also the case for the conventionally sintered sample at 1200°C, being not yet fully dense; Table 3). The grain growth during post-HIPing was limited, and the average grain size was practically unchanged (Table 4). Additionally, planimetric grain size analyses were performed from SEM micrographs (Figure 5) in order to evaluate the respective grain size distributions of differently sintered Meso-TZ specimens. The probability density functions of grain size distributions are presented in Figure 6. The conventionally sintered sample exhibits a monomodal and relatively narrow grain size distribution. Assuming the grains have the geometry of spheres, calculating the diameters derived from calculated areas of grain intersections, dSph, an average value of 129 nm (174 nm if Feret`s diameter, dFer, is taken) was obtained. The grain size distribution of the microwavesintered sample is broader than the conventionally sintered, yet still of the monomodal type. However, it is clear that the probability function has an outlying tail towards larger grain sizes. If the conventionally sintered sample contains an insignificant number of grains greater than 275 nm, the number of grains from 275 to 325 nm is significant for the microwave-sintered sample. Moreover, the tail is even expanded from 325 to 400 nm, indicating the fraction of unproportionally larger grains that can be seen in the microstructure (Figure 5b). The calculated average diameter for the microwave-sintered sample from the distribution was 143 nm (dFer = 190 nm). As expected, the finest, narrowest grain size distribution was achieved with post HIPing. The average grain size was 109 nm (dFer = 149 nm), similar to that obtained from the linear interception process, while practically all the grains in the microstructure were smaller than 250 nm. 3.4.3. XRD analyses XRD patterns were obtained from the surfaces of differently sintered samples. Diffractograms with intersections from 28–36 2theta are shown in Figure 7. All three samples were composed only of tetragonal zirconia. They were free of the monoclinic phase and, obviously, they were also free of the cubic phase (or at least the number and/or the size of the crystallites was under the detection limit of the present set-up). The diffractograms of the conventional- and microwave-sintered samples were comparable, while the diffractogram from the HIPed sample exhibited a broader t(101) peak positioned at 30.25° 2theta, indicating smaller crystallite sizes.

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Comparing the intensity ratios of the tetragonal doublet peaks, positioned around 35° 2theta, It(002)/It(110), the HIPed sample had the highest ratio, i.e., 0.82, as compared to the conventionally and microwave sintered samples, having 0.75 and 0.7, respectively. The ratio of the as-received powder was 0.5 (not shown). The increase in the ratio for the sintered samples compared to the as-received powder could be connected to a small amount of tetragonal zirconia in the matrix that had undergone ferroelastic domain switching, which is known to occur as a result of an externally applied stress.[43] As follows, the introduced stress during sintering was the largest when employing the HIP and the least when microwave sintering was used. The HIPing was performed under an isostatic pressure of 200 MPa at temperature, where the grain growth was limited, which makes it plausible to assume that, during this type of densification (from 94.7 to 99.8 % TD; Table 2), some stresses were introduced, particularly in grain junctions via grain sliding. It is interesting to note that the microwave-sintered sample possessed lower intensity ratios for the tetragonal doublet peaks compared to the conventionally sintered sample. This indirectly supports the idea of the ordered/oriented coalescence of primary crystallites (in a secondary particle; Figure 1a) under increased heating rates in the microwave-sintered sample. We assume that if this happens, it does so in a smoother fashion, like the preferential heating of grain boundaries [44]. In that case there should not be any frictional forces present or they should be minimal, as compared to classic densification mechanisms, where the matrix is subjected to some degree of stress/constraint during various diffusional mechanisms and shrinkage. In order to obtain more insight into the microstructures of the differently sintered samples, the average crystallite sizes were calculated using a Rietveld refinement of the X-ray diffraction patterns. The results are listed in Table 4. The crystallite sizes of 125, 162 and 110 nm were obtained for the conventional, microwave and HIP samples, respectively. As expected, HIPing yielded the smallest crystallite size, while microwave-sintered sample possessed the highest, which is in line with the results from the grain sizes obtained using a different analytical method (Table 4). However, when the crystallite sizes were compared to the grain sizes obtained with the linear-intercept method, it can be seen that apart from the microwave sample, the average crystallite sizes for both the conventional and HIPed samples fall in line with the grain sizes. This is indicative of the fact that the grains are not composed of sub-domains or of smaller crystallites, as is the case in the as-received powder (Figure 1a). The slight deviation is in the conventional sample. Namely, the crystallite size is about 12 % smaller than the grain size. The same percentage is obtained when considering the conventionally sintered sample at 1200°C (Table 2). The percentage is not significant, but is still indicative of the differences originating during densification and grain growth for the different heating rates and/or pressures. It appears that when higher heating rates or pressures are employed the subdomains in the grains or crystallites within the secondary particles are pinched off or coalesced into single-crystalline units, i.e., grains.

3.5. Potential application of Meso-TZ powder The consolidation of the Meso-TZ coarse powder, composed of loosely aggregated, nanoscale crystallites, produced HH. It was previously shown by Kocjan and Shen that, on one hand, a zirconia green body with HH represents a superior starting point for the preparation of porous zirconia with enhanced mechanical and thermal properties.[26] On the other hand, such a microstructure also proved to be beneficial for sintering to full density, having a pronounced sintering activity, since a completely dense sample (99.7% TD; Table 3) was achieved after 2

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hours of sintering at 1300°C, yielding a fine average grain size of 142 nm, being slightly above the size limit for a nanocrystalline ceramic. Nonetheless, the as-sintered specimens possessed reasonably good translucency, typical mechanical properties (Table 3) and were completely resistant to low-temperature degradation, since 80-hour ageing in water at 134°C resulted in only traces of monoclinic phase (Xm