Unique mechanical properties of nano-grained YAG transparent ceramics compared with coarse-grained partners

    Unique mechanical properties of nano-grained YAG transparent ceramics compared with coarse-grained partners H.M. Wang, Z.Y. Huang, J.S. Jiang, K. Liu, M.Y. Duan, Z.W. Lu, J. Cedelle, Z.W. Guan, T.C. Lu, Q.Y. Wang PII: DOI: Reference:

S0264-1275(16)30578-0 doi: 10.1016/j.matdes.2016.04.094 JMADE 1738

To appear in: Received date: Revised date: Accepted date:

19 January 2016 28 April 2016 30 April 2016

Please cite this article as: H.M. Wang, Z.Y. Huang, J.S. Jiang, K. Liu, M.Y. Duan, Z.W. Lu, J. Cedelle, Z.W. Guan, T.C. Lu, Q.Y. Wang, Unique mechanical properties of nano-grained YAG transparent ceramics compared with coarse-grained partners, (2016), doi: 10.1016/j.matdes.2016.04.094

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ACCEPTED MANUSCRIPT Unique mechanical properties of nano-grained YAG transparent ceramics compared with coarse-grained

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partners

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College of Aeronautics and Astronautics, Sichuan University, Chengdu 610064,

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China b

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HM. Wang a, ZY. Huang a, JS. Jiang b, K. Liu c, MY. Duan c, ZW. Lu d, J. Cedelle e, ZW. Guan f, TC. Lu d, *, QY. Wang a, f, *

College of Architecture and Civil Engineering, XHU University, Chengdu 611930,

College of Physics and Electronic Engineering, Sichuan Normal University,

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c

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China

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Chengdu 610101, China

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China

Université Paris Ouest Nanterre La Défense, 50 Rue de Sèvres, 92410 Ville d’Avray,

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e

College of Physical Science and Technology, Sichuan University, Chengdu 610064,

France

School of Mechanical Engineering, Chengdu University, Chengdu 610106, China

Abstract: In order to improve the inherent brittleness of the conventional coarse-grained YAG transparent ceramics, nano-grained YAG transparent ceramics were fabricated at 5 GPa /450 °C using the super high pressure sintering technique. Their mechanical properties were investigated by nanoindentation and the Vickers indentation measuring methods. It was found that the nano-grained YAG transparent ceramics 1 * Corresponding author. E-mail address: [email protected] (Tiecheng. Lu), [email protected] (Qingyuan. Wang)

ACCEPTED MANUSCRIPT exhibited much better ductility compared with the coarse-grained counterparts. The grain-boundary sliding provides the main contribution to plastic deformations of

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nano-grained YAG transparent ceramics.

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Keywords: nano ceramics; YAG, nanoindentation; mechanical property; toughness 1. Introduction

Yttrium Aluminium Garnet (Y3Al5O12, YAG) transparent ceramics are

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well-known optical materials. The low manufacturing cost and the absence of severe

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limitation in size and geometry make the YAG transparent ceramics more attractive in comparison to YAG single crystals [1]. At present, the YAG transparent ceramics can

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be produced using vacuum sintering or hot pressing sintering techniques [2]. These

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sintering techniques are performed at rather high temperatures (~0.8 Tmelt), resulting

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in the YAG transparent ceramics with the micro-sized individual grains, as the high temperature can accelerate atomic motion and promote the growth of grains.

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It is well known that ceramic materials' intrinsic brittleness (premature failure due to brittle fracture) limits their wide applications. However, as for the YAG transparent ceramics, the traditional toughening methods (i.e. phase transformation toughening technique, whisker toughening technique, particles toughening technique and so on ) cannot be used since they decrease the transparency of YAG ceramics [3]. In particular, it was believed that nano-grained ceramics may greatly improve the mechanical properties when compared with their conventional coarse-grained counterparts [4]. Recently, some researchers have fabricated the nano-grained Mg2Al2O4 and YAG transparent ceramics using super high pressure sintering

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ACCEPTED MANUSCRIPT equipment (diamond anvil cell) [5-6]. Despite these achievements, obstacles remain on the pathway towards the characterization of mechanical properties nano-grained

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YAG transparent ceramics. The small volume of samples fabricated by diamond anvil

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cell obstructed attempts to study the mechanical properties of nano-grained ceramics. Additionally, a very high pressure (~8 GPa) required by the diamond anvil cell technique led to cracks within specimens. Therefore, in the past the studies were

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confined at the sintering mechanisms and optical properties of the nano-grained

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transparent ceramics. In our previous work [7], we employed cubic-type multi-anvil high-pressure apparatus to fabricate the crackless nano-grained YAG transparent

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ceramics at a modest pressure (5 GPa). Moreover, the sintering mechanism and the

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compressive yield at different pressures were analyzed.

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Indentation tests involving hard indenters have been the basis for measurement of the size of a residual plastic impression in the specimen as a function

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of the indenter load. In particular, it has been used recently to study the plastic deformation behaviour in small volumes [8-9]. Especially, nanoindentation is a powerful technique widely employed to determine the mechanical properties of nanostructured materials [10]. It may be more likely to be used to investigate the plastic behavior of ceramic materials. In the current work, the nano-grained YAG transparent ceramics (NGC) were fabricated by the super high pressure sintering method. As a comparison, the coarse-grained YAG transparent ceramics (CGC) were sintered by vacuum sintering method. The nanoindentation and Vickers indentation techniques were also used to

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ACCEPTED MANUSCRIPT study the mechanical properties of NGC and CGC. 2. Experimental Section

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2.1. Sample preparation

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Yttrium nitrate (Y(NO3)3·6H2O, 99.99%) and ammonium aluminum sulfate (NH4Al(SO4)2·12H2O, 99.99%) were mixed according to the stoichiometric ratio of YAG (Y3Al5O12) in deionized water. The precipitant solution was prepared by

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dissolving ammonium hydrogen carbonate (NH4HCO3, analytical grade) in a mixed

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solvent of alcohol and distilled water. The mixed solution was dripped into the precipitant solution at a dripping speed of 3 ml/min under stirring at 20 °C. The

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suspension aged for 20 h, was filtered and washed with distilled water and alcohol

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and finally the precipitate was obtained. Then precursors were produced after the

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precipitate was dried at 80 °C for 24 h in an oven. The obtained precursors were calcined at 1100 °C in order to produce the YAG nanopowders. The average grain size

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of YAG nanopowders were about 30 nm determined by X-ray line broadening and calculated using Scherrer's equation. The YAG nanopowders were uniaxially pressed into pellets at a pressure of 20 MPa and then isostatically cold pressed at 200 MPa to obtain green bodies. The green bodies were sintered at 1780°C for 20 h in a high temperature vacuum sintering furnace under a vacuum condition of 10-3 Pa. The heating and cooling rates were both set to 10 °C/min. Subsequently sintered pellets were annealed at 1450°C for 20 h in air. Then the coarse-grained YAG transparent ceramics were fabricated. High-pressure sintering experiments were also carried out using 6×800MN

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ACCEPTED MANUSCRIPT large volume cubic-type multi-anvil high-pressure apparatus. The nano-grained YAG

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fabrication details are similar to that in our previous works [7].

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transparent ceramics were obtained at 450°C and under a pressure of 5 GPa. The

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2.2. Characterization of materials

The phase and crystallinity were analyzed via X-ray diffraction (XRD, Cu-Kα radiation 1.54 Å, Model D/max2000PC, Rigaku, Japan). The grain size and

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microstructures of the nano-grained YAG transparent ceramics and coarse-grained

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partners were investigated by Scanning Electron Microscope (SEM, Model S-4800, Hitachi, Japan) and Transmission Electron Microscope (TEM, Model JEM 1200EX,

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2.3. Nanoindentation

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Japan).

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Nanoindentation technique using a nanoindentation system (Hysitron Triboscope, Hysitron, USA) equipped with SPM for in situ imaging was employed to

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investigate the mechanical properties of the specimen. In all tests, nanoindentations were carried out on the samples with a sharp diamond Berkovich indenter with a radius of 150 nm. The nanoindentation system has load and depth sensing resolutions of 1 nN and 0.0002 nm, respectively. Careful calibration of the projected contact area between the indenter tip and the specimen is a prerequisite condition to any nanoindentation test. The instrument calibration was conducted using a standard fused silica prior to testing. The loading and unloading rate was maintained at 0.5 mN/s for each peak load (2 mN, 4 mN, 6 mN, 8 mN and 10 mN). At least 5 indentations were performed at each load level. For each test, the indented areas were in situ scanned to

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ACCEPTED MANUSCRIPT obtain the surface and indentation patterns using SPM, prior to and after indenting. Indented imprints were also analyzed using a depth cross-section analysis. During

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nanoindentation processes, the maximum thermal drift rate was controlled to be 0.05

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nm/s. 2.4. Vicker's indentation

Five Vicker’s indentations were introduced onto the surface of individual

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samples using a Vicker's diamond indenter (AKASHI, AVK-A, Japan). All the

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samples were indented with the Vicker’s hardness apparatus with a load of 49 N and a holding time of 15 s. The length of the crack was defined as the longest diagonal of

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3. Results and Discussion

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the indentation and measured using SEM.

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Figure 1 shows the X-ray diffraction patterns of the YAG nanopowders, nano-grained and crose-grained YAG transparent ceramic specimens. It shows that all

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the diffraction peaks could be well indexed as the cubic garnet structure of YAG. The photographs (A1 and B1) and SEM images (A2 and B2) are shown in Figure 2. It can be seen that the grain sizes of the CGC and NGC are about 5 μm and 60 nm, respectively. Remarkably, the color of the NGC (A1) is light yellow. Distinguishing the colorless CGC (B1) suggested that the absorption spectrum of the NGC is different from that of CGC, due to color center effect [11]. Additionally, the NGC photograph shows that there are no cracks. Relative density ρr (ρr=ρ/ρtheo, ρtheo=4.56 g cm-3) of the NGC and CGC measured by Archimedes method is 99.68% and 99.89%, respectively. In order to further investigate the microstructures of NGC and CGC, the

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ACCEPTED MANUSCRIPT transmission electron microscopy (TEM) and high-resolution analytical transmission electron microscopy (HRTEM) were employed. Figure 3 shows the TEM (A1 and B1)

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and HRTEM (A2 and B2) micrographs. These data suggest that the grain sizes of

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NGC and CGC are the same as the SEM results. The pores (in the white circles) of nanometer size, which are not the basic centers of the light scatting [12], are observed in the NGC. However, the pores are in fact entirely absent in CGC. This is the main

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factor why the relative density of NGC is less than that of CGC. There are no

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dislocations, twinnings or other defects in the HRTEM images, while it also shows the clear grain boundary with no secondary phase.

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The nanoindentation technique has been developed into an established

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method for investigating the mechanical properties of nano materials at nanoscale

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valumes[13-14]. During nanoindentation testing, forces and displacements were recorded to generate load-depth curves. The Oliver-Pharr method was applied to

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determine the hardness (H) and Young’s modulus (E). The H and E were calculated using the following relationships [15]: (1) (2) where Pmax is the maximum load applied and A is the projected contact area between the indenter tip and the specimen at the maximum load. hc is the contact indentation depth determined from load-depth curves. S is the initial unloading stiffness determined by the slope of the unloading curve dP/dh at the maximum load and

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the correction factor that depends on the geometry of the indenter (for the Berkovich

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is 1.034). Young’s modulus E and the hardness H of the NGC and conventional CGC

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calculated are shown in Table 1. The parameters of the nanoindentations are also

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summarized in this table. hmax is the depth of nanoindentation at the maximum load. hr is the depth of nanoindentation after unloading. There is a simple relationship, i.e. hs=hmax﹣hc, while a is the half length of the nanoindentation at the maximum load.

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The lower E and H of NGC in relation to CGC are observed. Generally, the lower

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Young's modulus in nano materials is attributed to the lower density. However, the difference in density is just 0.21%, whereas the difference in Young's modulus is

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about 22%. The same phenomenon was also observed by Chaim [16] in

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nanocrystalline ZrO2-3 wt% Y2O3 ceramics. It was attributed to the relative density

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and the grain size. The percolative composite model explained these changes in terms of the percolation of the elastic wave through the different intercrytalline

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microstructural components. The difference in hardness between NGC and CGC indicates different elastic-plastic deformation characteristics. Under the same testing conditions, the NGC has a greater depth (hmax, hr, hc) of the nanoindentation. It is implied that the NGC produced more plastic deformation during the nanoindentation testing. Figure 4 shows the typical load-displacement curves of the nano-grained and coarse-grained YAG transparent ceramics at a peak load of 10 mN. The visible pop-in was observed on the loading curves of the CGC. But the NGC reveals the continuous and smooth loading and unloading curves without pop-in observation. It indicates that

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ACCEPTED MANUSCRIPT the NGC underwent the elastic-plastic deformation without fracturing while the load was applied. On the contrary, around the indentation the fracture was produced in the

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CGC when the load was about 2.3 mN. It is evident that NGC can exhibit significant

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ductility before failure.

In this study, nanoindentation together with an in situ SPM imaging technique was employed to investigate the mechanical properties of the nano-grained

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and coarse-grained YAG transparent ceramics. The SPM micrographs of

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nanoindentations and their cross-sectional profiles at 10 mN peak load are shown in Figure 5. The cross-sectional profiles were plotted via a line trace of SPM images

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along the diagonal section of the selected indent (blue arrowed line in the SPM

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images). On the one hand, one can estimate by looking at the SPM images that the

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average arithmetic surface roughness (R) of the nanoindentation areas is approximately 3nm. As for ceramic materials, the surface roughness does not

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influence the nanoindentation measurement at any applied load [17]. On the other hand, pile-ups around the indentation imprints can be observed in the cross-section of the nanoindentation profiles. Similar pile-ups around the indent were observed in the nanocrystallization during the nanoindentation of a bulk amorphous metal alloy at room temperature, indicating the severity of plastic flow around this region during indentation [17]. Therefore, we inferred that the pile-ups produced by plastic deformation of NGC were with 22 nm height. This is ten times more than that of CGC which demonstrates that the toughness of NGC is better than that of the coarse counterpart.

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ACCEPTED MANUSCRIPT While the conventional micro-indentation technique relates to an observation scale of millimeters, the nanoindentation technique measures a representative material

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volume at the scale of nanometers. Hence, in order to contrast the results of the

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nanoindentation technique, the different mechanical properties of the NGC and the CGC were also investigated by using the conventional indentation technique. The Vickers indentation test was employed to determine the K1C of nano-grained and

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coarse-grained YAG transparent ceramics from empirical relationships[18-19]. Due to

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simplicity and ease of use, equation (3) derived by Ponton and Rawlings was adopted for this investigation.

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(3)

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where P is the load; C is the crack length; HV is Vicker’s hardness value; E is Young’s

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modulus (obtained from the nanoindentation test) and 0.016 is an empirical value. The micro-hardness tester was used to produce at least 5 indentations on all surfaces

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examined. An indentation load of 49 N was used. The average Vicker's valves of NGC and CGC are 10 GPa and 13 GPa, respectively. The indented surface and the resulting crack lengths were measured using scanning electron microscope. According to the SEM images as shown in Figure 6, which revealed the Vicker’s indentation microstructures of the NGC and CGC, the average crack length C is 87 μm and 137 μm, respectively. Substituting the data into equation (3), it is calculated that K1C= 4.45 MPa m1/2 for NGC and K1C= 2.23MPa m1/2 for CGC. There is a good agreement between the currently measured fracture toughness value and that obtained by Jiang Li [20] for CGC. Importantly, the fracture toughness value of NGC is approximately

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ACCEPTED MANUSCRIPT two times that of CGC. This indicates that the NGC exhibits more significant ductility before failure. This conclusion corresponds to the results from the nanoindentation

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testing.

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The biggest difference between the nano-grained materials and the coarse-grained materials is that nano-grained materials have many more atoms within grain boundaries. Jiang and Weng [21] considered the nano-grained ceramics as a

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multi-phase system, i.e. nanocrysallines, grain boundaries and porous. A

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micromechainc-based composite model was developed to elucidate and predict the compressive yield strength. Moreover, computer simulations have highlighted the

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grain boundary as the key role of plastic deformation in nano-grained materials,

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especially at very high stress and strain rates [22]. In order to investigate the plastic

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deformation mechanism of NGC, the plastic deformation zone beneath the indenter was calculated by the continuum model, which is based on Johnson's spherical cavity

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model [23]. This model allows the radius of the plastic deformation zone in the sub-surface, r, to be predicted from the indentation load, P, and the yield strength, σ', as follows: σ

(4)

Experimental observations by many researchers [24], after the hardness values of several ceramics and their corresponding yield stress values, show the law of σ'=0.33H, where both values derived from polycrystalline ceramics. Therefore, using Eq. (4) with the rule σ'=0.33H, the plastic deformation zone radius as a function of load can be obtained, as shown in Figure 7. It shows that such the radius is linearly

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ACCEPTED MANUSCRIPT increased with the load for NGC, while it is nonlinearly increased with the load for CGC.

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Generally, the plastic deformation mechanism of polycrystalline materials is

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associated with the nucleation and movement of dislocations in materials. The dislocations begin moving along the slip systems and plastic deformation initiates when the shear stress reaches its critical value. However, there are partial crystals

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which produce plastic deformations due to the polycrystalline materials consisting of

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many grains with random orientation. By increasing the applied load, more and more slip systems are activated and large plastic deformations are exhibited. During the

, where c is a strength coefficient and n is the strain hardening exponent.

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law,

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plastic deformation, the relationship of stress and strain can be described by a power

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Thus we can infer that the relationship between the plastic deformation zone radius and the load is nonlinear for CGC. It coincides with the results from the

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nanoindentation test.

Raj and Ashby [25] dealt with grain boundary sliding, by assuming that the

plastic deformation between adjacent grains occurs by diffusing alone and the grain boundary sliding path is a sinusoidal shape. The deformation model during the nanoindentation test is shown in Figure 8. According to the grain boundary sliding model, the sliding rate,

can be estimated by the following relation: (5)

where the parameters

and h are the wavelength and amplitude of grain boundary

sliding path, respectively.

is the atomic volume;

is the thickness of the grain

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ACCEPTED MANUSCRIPT boundary; Dv and DB are the volume and boundary diffusion coefficients, respectively; k is a material parameter; T is the temperature. If it is assumed that each grain

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boundary layer will generate an identical sliding rate, a strain rate can be defined as

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(6)

For a constant amplitude, grain size and loading rate, a number of parameters can be grouped and strain can be expressed as

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(7)

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where ηe is an effective viscosity. This result agrees with the dependence of the plastic deformation zone radius on the load. It indicates that the grain boundary sliding

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4. Conclusions

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transparent ceramics.

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provides the main contribution to plastic deformation of nano-grained YAG

In summary, the super high pressure technique was employed to fabricate the

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nano-gained YAG transparent ceramics. The nano-grained YAG transparent ceramics reveal the unique mechanical properties relative to their coarse-grained counterparts. Young’s modulus and hardness of nano-grained YAG transparent ceramics is less than the coarse-grained YAG transparent ceramics due to the grain size and the pores in the nano-grained YAG transparent ceramics. During the nanoindentation testing, on the load-displacement curve of coarse-grained YAG transparent ceramic there is an apparent pop-in. However, the load-displacement curve of nano-grained YAG transparent ceramic shows continuous and smooth characteristics. After investigating the maximum penetration depth (hmax) value and the depth (hr) of residual indentation,

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ACCEPTED MANUSCRIPT the values of nano-grained YAG transparent ceramics are larger than the coarse-grained YAG transparent ceramics. Meanwhile, in the Vickers indentation test

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the fracture toughness value (K1C) of NGC was approximately two times that of CGC.

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These characteristics show that the nano-grained YAG transparent ceramics have a greater fracture toughness than the coarse-grained counterparts. Furthermore, the plastic deformation of nano-grained YAG transparent ceramic is controlled by the

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grain-boundary sliding which differs from coarse-grained YAG transparent ceramics.

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Acknowledge

This work is supported by National Natural Science Foundation of China (Grant

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No. 11027405, No. 10976018 and No. 50872083).

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Figure 1. X-ray diffraction patterns of nano YAG powders, coarse-grained YAG transparent ceramics and nano-grained YAG transparent ceramics

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Figure 2. Photographs and SEM micrographs of the nano-grained YAG transparent ceramics (A1, A2) and coarse-grained YAG transparent ceramics (B1, B2)

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Figure 3. The TEM and HRTEM micrographs of nano-grained (A1, A2) and coarse-grained (B1, B2) YAG transparent ceramics

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Figure 4. load-displacement curves of the nano-grained and coarse-grained YAG transparent ceramics in nanoindentation at peak load of 10mN

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Figure 5. SPM image and corresponding cross-sectional profile (the top corresponding cross-sectional profile along the blue arrowed line in the bottom SPM image)

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Figure 6. The SEM micrographs of Vicker indentation of nano-grained (a) and coarse-grained (b) YAG transparent ceramics

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Figure 7. The plastic deformation zone radius as a function of load for nano-grained and coarse-grained YAG transparent ceramics

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Figure 8. Grain boundary sliding model during the nanoindentation test

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ACCEPTED MANUSCRIPT Table 1. the Young’s modulus, hardness and parameters of nanoindentation of nano-grained and coarse-grained YAG transparent ceramics

Nano-grained

Pmax (mN)

hmax (nm)

hc (nm)

hr (nm)

A (nm2)

E (GPa)

H (GPa)

0.9968

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181

143

83

6×105

212

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271

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0.9989

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117

77

43

4×10

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Coarse-grained

ρ/ρtheo

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specimen

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ACCEPTED MANUSCRIPT Graphical abstract Figure abstract

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Figure 6. SPM image and corresponding cross-sectional profile (the top corresponding cross-sectional profile along the blue arrowed line in the bottom SPM image)

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ACCEPTED MANUSCRIPT Highlights 1. The nano-grained Yttrium Aluminium Garnet transparent ceramics were prepared by using high pressure technique.

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2. Nano-grained Yttrium Aluminium Garnet transparent ceramics were studied by nanoindentation.

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3. Nano-grained Yttrium Aluminium Garnet transparent ceramics have a greater fracture toughness than the coarse-grained partners.

4. The grain-boundary sliding provides the main contribution to plastic deformation of

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nano-grained Yttrium Aluminium Garnet transparent ceramics.

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