Effect of Y3Al5O12 Addition on The Microstructural Evaluation And Mechanical Properties of Spark Plasma Sintered ZrB2-SiC Composites

The addition of Y 3 Al 5 O 12 (YAG) into the ZrB 2 -SiC composites was investigated in the present research. Composites were densified by SPS method at 1850ºC under a uniaxial pressure of 50 MPa for 20 min. Microstructural evaluation and mechanical properties were evaluated with a various content of YAG (1-5 wt%). The microstructural and phase analysis showed that incorporation of YAG promoted the densification process from the solid-state sintering to liquid phase sintering. The highest density (99.81% RD) and fracture toughness (6,44 ± 0.23 MPa.m 1/2 ) were obtained for the composite containing 5 wt % YAG after the SPS process. Although hardness and elastic modulus of samples were decreased with the increasing of YAG amount, measured values were comparable with the literature.


INTRODUCTION
Belong the family of Ultra High Temperature Ceramics (UHTCs), transition metal borides like ZrB2 and HfB2 have unique combination of thermophysical properties, including high melting temperature, high strength and hardness, high thermal and electrical conductivity and high chemical stability [1][2][3]. Due to these unique properties, these materials are potential candidates for high temperature applications like propulsion systems, rocket nozzles, re-entry and hypersonic vehicles involving sharp leading edges and nose cones for re-entry and hypersonic vehicles. In order to be manoeuvrable at hypersonic velocities, sharp leading edges and nose cones are required control surfaces. If low radius leading edges are designed, manoeuvrability will be higher. However, formation of much greater aerothermal heating, leading edges part may reach higher temperatures (> 2000°C) during re-entry [4,5]. The currently used materials will not resist such extreme temperatures and more stable materials are required for use in this type of high temperature applications. In the earlier studies [6][7][8][9][10][11][12] ZrB2-SiC and HfB2-SiC composites have come to the forefront under these extreme conditions due to their high strength and high stability. Recently, yttrium and aluminium based additives including Al2O3, Y2O3 have been successfully used to improve the sinterability of ZrB2, forming a liquid phase or removal of surface oxides. In addition to sinterability, it has been stated that these additions played an important role for improving mechanical properties and oxidation resistance [13][14][15][16][17][18][19][20]. YAG gel coated ZrB2-SiC composites were prepared by pressureless sintering by He et all [15]. Obtained results showed that coated YAG have led to high relative density (about 97%) at the sintering temperature of 1950°C due to the changing of the sintering mechanism from solid state to liquid phase sintering.
Yttrium aluminium garnet (YAG or Y3Al5O12) is the only unambiguously stable phase in Y2O3-Al2O3 system which has a great interest as a high temperature application material as a matrix due to its high temperature strength. In addition to matrix phase, it is used as a sintering agent for some ceramic matrices. To our best knowledge, YAG has been formed during the sintering of ZrB2 based ceramics using Y2O3 and Al2O3 addition in the system [16,21,22]. However, formation of a single phase YAG with Y2O3-Al2O3 system is difficult in the high pressure and temperature assisted sintering techniques like SPS, HP. In this research, therefore, commercial YAG powder were added to the ZrB2-SiC composites consolidated by the spark plasma sintering. The influence of different YAG contents were investigated on the microstructure evaluation and mechanical properties of composites.

Powder Preparation and SPS of powders
Commercially available ZrB2 (ABCR GmbH, Grade A), SiC, (HC Starck-UF-05) powders and YAG droplets (ABCR GmbH) were used as starting powders. YAG droplets were first crushed in a ring mill (WC-Co media) and the crushed powders were milled in a planetary ball mill (Pulverisette, P6, Fritsch) for size reduction in a ZrO2 milling media (10 mm ball diameter and 80 ml jar capacity). Ball to powder ratio was selected as 10:1 and the powders were milled about 120 min at a rotational speed of 450 rpm. In order to minimize cold welding, 0.5 % wt. stearic acid (Merck,Germany) was added as a process control agent. After the milling process, 1, 3 and 5% wt. YAG mixed with ZrB2-25 vol %SiC composite powders were prepared using the planetary ball mill in a Si3N4 media (10mm ball diameter, 250 ml jar capacity) with 2propanol for 90 min at a rotational speed of 450 rpm. The slurry was then dried using a rotary evaporator (WB2000, Heidolph) at 50 rpm and 55°C for 1 h. Dried powders were sieved under 100 μm to break up agglomerates.
Sintering of composites was carried out in an SPS furnace (HPD-50, FCT GmbH, Germany) at 1850°C under a uniaxially pressure of 50 MPa for 20 min. The heating rate was kept constant as 100°C min -1 . In every sintering cycle 6 gr of powder was placed into the graphite die with an inner diameter of 20 mm. Graphite foil with a thickness of 1 mm was incorporated inside the die to prevent reaction between the die and powders. The temperature was increased with a controlled electric current and measured on the graphite die surface with an optical pyrometer.

Characterization of sintered samples
Bulk densities were measured by using Archimedes method after removing the graphite foil layer formed on the sintered sample surfaces. In order to determine relative density values, all theoretical values for ZrB2-SiC-YAG composites were calculated from volume-based rules of mixtures. The bulk densities of ZrB2, SiC and YAG were accepted as 6,1 g/cm 3 , 3.2 g/cm 3  test, the lengths of radial cracks were measured which had formed in the corners of the Vickers indentation mark on the surface. Next, the most used empirical equation proposed by Evans [25] was used to calculate fracture toughness values, which is following: where KIC is fracture toughness (MPa.m 1/2 ), H is Vickers hardness (MPa), c is the average length of the cracks (μm), and α is the half average length of the diagonal (μm).

Phase and Microstructural Analysis of Sintered Samples
The morphology and XRD patterns of the milled YAG powder were given in Fig.1. According to the Fig.1.b the XRD patterns of the YAG powder indicated the existence of Y3Al5O12 (PDF-JCPDS 33-0040) peaks and YAlO3(PDF-JCPDS 33-0041), the perovskite-like form of yttrium aluminium oxide phases. SE-SEM micrographs of YAG powders exhibited spherical form with a mean particle size of < 2 µm (Fig.1.a).
The XRD patterns of the all the sintered samples were given in The BSE-SEM images for the polished surfaces of all sintered samples were presented in Fig.3.
In all figures, bright contrasted areas were related to ZrB2, the dark areas attributed to SiC, and the grey areas at the interface of ZrB2-SiC belong to YAG phases as confirmed with EDX. SiC and YAG phases were homogeneously distributed around the ZrB2 grains and no agglomeration was detected. Due to the internal residual stresses formed during SPS, pull out defects (indicated as yellow arrows on the image) were observed in the microstructures. The measured relative densities of the samples densified at 1850ºC (Table 1) were in the range of 98-100% of the true densities, and nearly full densification was obtained in sample ZrB2-SiC-5YAG. The SEM images of fracture surfaces of ZrB2-SiC and ZrB2-SiC-5YAG were given in Fig.4.  4.b also showed that ZrB2-SiC-5YAG sample exhibited the intercrystalline cracking characteristic which promoted the higher fracture toughness.

Mechanical properties of sintered samples
Hardness, fracture toughness and elastic modulus of sintered samples as a function of YAG content were indicated in Fig.5. A fracture toughness of 4.87 ± 0.21 MPa.m 1/2 was measured for ZrB2-SiC composite, increased after the addition of YAG. The highest fracture toughness was obtained for the composite containing 5 wt% YAG with a value of 6,44 ± 0.23 MPa.m 1/2 .
The toughness values were higher to the result of He et all [15] who reported toughness value of 4.13 ± 0.45 MPa.m 1/2 for presureless sintered ZrB2-SiC-YAG composite. Cracks propagate in the material predominantly by an intercrystalline mechanism (Fig. 4.b), with separate grains fractured by a transcrystalline mechanism for ZrB2-SiC-5YAG sample. The crack path created by the Vickers indentation tests for the ZrB2-SiC-5YAG sample was illustrated in Fig.6.
Inspection of the crack propagation revealed crack deflection and crack bridging in ZrB2-SiC-5YAG sample. These toughening mechanisms increased energy dissipation during the crack propagation and thereby raising fracture toughness. Also, mismatch of the thermal expansion coefficient between SiC, YAG and ZrB2 could also produce tensile stress on grain boundaries in the ZrB2, leading to the formation of microcracks as shown with red arrows in Fig.6. The microcracks dissipated the deformation energy and decreased the stress intensity of the main crack and thereby improving the fracture toughness of the material.
As it was well known that fine grained materials have high hardness, which was supported by much research [28][29][30]. With the addition of YAG, the hardness of samples decreased from 18.74 ± 0.43 GPa to 16.71 ± 0.55 GPa (Fig.5.a) because of the increase of volume fraction of YAG, which is lower hardness than do ZrB2 and SiC grains. As for the lowest values of hardness in ZrB2-SiC-5YAG, it was also due to larger grain size, clearly observed in Fig. 3.d.
The elastic modulus E of the samples with YAG additives ranged from 512.13 GPa to 458.97 GPa which was lower than ZrB2-SiC without YAG (528,13 GPa). As it was well known that, density and grain boundary phases were the two major factors that influence E value of the materials. As all samples with the YAG added had a relative density greater than 98% RD, thus, the value of E has been dominated by the secondary phase. Similar to hardness results, elastic modulus of YAG is lower than ZrB2-SiC matrix, which resulted in a decrease of those of composites with increasing of YAG content. Also, it was thought that the decline in the elastic modulus was due to the formation of microcracks caused by the mismatch of thermal expansion of ZrB2-SiC and YAG phases.      The SEM images of fracture surfaces of a) ZrB2-SiC and b) ZrB2-SiC-5YAG samples The SEM images of fracture surfaces of a) ZrB2-SiC and b) ZrB2-SiC-5YAG samples   The crack path created by the Vickers indentation tests for the ZrB2-SiC-5YAG sample.

Figure 6
The crack path created by the Vickers indentation tests for the ZrB2-SiC-5YAG sample.