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Graphene oxide doped YTZP nanocomposites obtained by spark plasma sintering

Name
Pavel
Surname
Peretyagin
Scientific organization
MSTU "STANKIN"
Academic degree
PhD student
Position
Vice-head of Laboratory of Electric Currents and Sintering Technology (LECAST)
Scientific discipline
New materials, Manufacturing technologies & Processes
Topic
Graphene oxide doped YTZP nanocomposites obtained by spark plasma sintering
Abstract
The use of graphene as a component for developing electroconductive ceramic is being profusely studied. It is a very promising additive as it has excellent mechanical properties, high electrical and thermal conductivities, it is lightweight and its aspect ratio allows reaching percolation with low contents.
In this work, 3Y-TZP/G composites were prepared by SPS of Zirconia-GO mixtures. Graphene content as low as 0,29 vol% allows obtaining nanostructured black zirconia but it has to be increased up to 1 vol% in order to reach electrical resistivity <100Ωcm, as it is required for EDM.
Keywords
Zirconia composites, graphene, Spark Plasma sintering, black zirconia
Summary

Introduction

Yttria stabilized zirconia is widely used as structural material in machinery due to its superior properties, such as high strength, high hardness, and high toughness (1-3). After sintering in oxidizing conditions pure dense zirconia has white colour. Therefore many studies have been performed up today for diversification of zirconia colour (4-9). In this regard the black colour has attracted many research works due to a large panoply of possible high added value applications, i.e. artificial jewellery, luxury watches as Apple Watch for example, knifes, bearing balls, optical devices, inductive charging system, fibre optics components, small precision tools, nozzles or PVD targets.  Most of the black zirconia commercial products are fabricated by adding 2 to 10 wt% black pigments as CoFe2O4 spinel to the starting 3YTZP tetragonal submicrometer powder and subsequent sintering in the temperature interval ranging from 1400oC to 1600oC.

In many of the above mentioned applications, the presence of metals like Co, Fe, is not appropriate due to technical requirements, i.e. PVD targets, inductive charging system, or health problem as allergy to Co, etc. Additionally these commercial materials are not electrical conductors and this is a handicap when facing machining of complex shapes as EDM can't be used for this purpose.

On the other hand, zirconia has a very low thermal conductivity (10-11). This fact has as a consequence a limitation in the dimension of the components to be fabricated by conventional processing routes. Advanced sintering technologies as HP also have limitations concerning the maximum thickness of the samples to be pressed, as heat has to be transferred from the mould to the sample. Spark plasma sintering has also limitations due to the high heating rates used during sintering and as a consequence important temperature gradients take place along the sample. In fact SPS open a new possibility for electric conductive materials as additionally to the heating process by the mould, conductive powders can be heated up by Joule effect during sintering (12-14). As a consequence, SPS seems to be only useful for the case of conductive materials and the most part of ceramics cannot take profit of the advantages of this interesting sintering technology.

The use of carbon nanostructures such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs) as additives for preparing electroconductive ceramics has been profusely studied. Nevertheless, the difficulty for getting good dispersions of CNTs and CNFs in ceramic matrices has limited the progress in this research line. Since the discovery of graphene, efforts have been focused on the use of this carbon nanostructure as reinforcement phase in ceramic matrices. In this sense, graphene oxide has several key advantages versus graphene regarding its processing such as, i) it can be obtained through a low cost method in high quantities (Kg.), ii) as any oxide it can be homogeneously dispersed in water, and consequently iii) mixtures of any ceramic oxide and graphene oxide can be processed following conventional ceramic's processing routes. On the top of this with a small volume fraction of graphene it is possible to reach the percolation threshold in the bulk sample to become a conductor material.

In the present work we fabricate theoretical dense electroconductive free of pores black zirconia compacts by adding a small fraction of graphene oxide. The obtained compacts were characterized mechanically and electrically. This new functionality (electrical-thermal conductivity) allows the possibility to use SPS avoiding the limitations derived from the low thermal conductivity of zirconia and also allows the machining of complex shaped components by electro discharge machining methods (EDM). The wear of the different composites has been also studied and compared to the behaviour of zirconia alone.

EXPERIMENTAL

Preparation of mixed powders

The starting materials used in this work were ZrO2 (3YTZP, Tosoh Corp., Japan) with average particle size of 180 nm and Graphene oxide synthesized from synthetic graphite by the modified Hummers method (23). Briefly, this method employs Hummers reagents with small amounts of NaNO3 and KMnO4. Concentrated H2SO4 was added to a mixture of synthetic graphite and NaNO3, and the mixture was cooled down using an ice bath. Afterwards, KMnO4 was slowly introduced in small doses to keep the reaction temperature below 20°C. The solution was heated to 35 °C and stirred for 3 h. At that point, a hydrogen peroxide (H2O2) 3% solution was slowly poured, giving rise to a pronounced exothermal effect up to 98 °C. The reaction mixture was stirred for 30 min and centrifuged (3700 rpm for 30 min) to discard the supernatant. The remaining solid material was then washed with water and centrifuged again; this process being repeated until the pH was neutral. A colloidal suspension of individual graphene oxide sheets in purified water (1 mg·mL−1) was prepared in 1 L batches and kept under ultrasound for 10 h. Afterwards, the suspension was centrifuged (3700 rpm for 30 min) to discard the filtered supernatant.

A colloidal method was used to prepare the GO/3YTZP powders. 40g of 3YTZP were added into 100 mL of water where the pH was previously fixed to 10 by adding NH4 OH. The 3YTZP powder was dispersed under stirring conditions for 30 min. Then, suspension of graphene oxide was dropwise added in order to prepare the desired composition. Mixed powders were kept stirred for 1h. Then, suspension was dried in a Lab Spray drier (Nano Spray Dryer B-90 Advanced, Buchi) at 110ºC exit temperature. The obtained powders were ready to press mixtures.

SPS sintering and characterization

The powder samples were placed into a graphite die with an inner diameter of 20 mm and cold uniaxially pressed at 20 MPa. Then, they were introduced in an SPS apparatus KCE-FCT-H-HP-D25-SD (FCT Systeme, Rauenstein, Germany) under low vacuum (10-1 mbar) and sintered at 1400ºC for 1 min under an applied pressure of 80MPa and a heating rate of 100ºC/min. Bulk density of the sintered bodies was measured by the Archimed’s method using water as solvent.

Vickers hardness was measured by using a durometer Micrometer 5103 (Buehler), loading 300 g for 10 seconds and making 30 measurements for sample.

The microstructure was studied using field emission scanning electron microscopy, FESEM (FEI: Quanta FEG 650).

The dielectric properties of the samples were studied by standard low-frequency impedance measurements (PSM1735-NumetriQ) in the 0.1Hz-1kHz frequency range.

The colour of the disks was also analysed using a spectrophotometer apparatus (Konica Minolta CM-700d model), which is based on the CIE standard light source of D65 as a light source, with a measurement area of 8 mm. The surface finish of all samples was polished down to 1μm with a sample thickness of 3 mm approximately. The final results were an average of four readings on each disk.

Results and Discussion

Microstructure

One of the well-known problems when reduced graphene oxide is directly used as the second phase in hydrophilic matrices, as it is the case of ceramic oxides, is the strong tendency to form aggregates due to its hydrophobic nature. As it was previously observed in the case of alumina+graphene oxide (22) in our case both components have polar surfaces favouring an electrostatic interaction between them consequently a perfect dispersions of both components (3YTZP, Graphene oxide) were reached. Additionally, these mixtures can be processed in water.

Addition of carbon second phases to ceramics have from a processing point of view several effects. Thus, during the powder packing step at the beginning of the SPS process, the graphene oxide sheets can acts as lubricant favouring the appropriate zirconia grains sliding to reach the optimum packing. Nevertheless, the incorporation of carbon second phases will difficult the composite densification. In this sense, Spark Plasma Sintering technique that applies pressure while heating is a very suitable technology for preparing this type of composites. Sintering temperature was selected by following the piston displacement during heating until the piston movement was stopped. In Figure 1 the microstructures of zirconia composites with 0, 0,29 and 0,39 wt% graphene oxide contents as examples are shown.

Figure 1 Microstructures of ZrO2-(0,29wt%)GO (left) and ZrO2-(0,39wt%)GO (right) composites

SPS process is carried out in reducing conditions, vacuum and graphite environment and the GO is reduced to pure graphene (22). The zirconia grain size on the microstructure of the zirconia + graphene SPS compacts is significantly affected by the presence of graphene. Graphene is not observed due to the low content added but its presence is inferred from its pinning effect on zirconia grain boundaries avoiding grain growth. Completely dense materials are obtained being the average grain size of zirconia very similar to the starting powder with biggest grains below 0,5 µm. Then, small additions of GO (<0,5 wt%) in combination with Spark Plasma Sintering allow obtaining dense materials without significant grain growth.

Electrical properties

In Figure 2 it is represented the electrical conductivity of ZrO2-graphene composites

Figure 2 Low frequency conductivity of ZrO2-graphene composites.

It is clearly shown that the percolation threshold takes place between 0.8 and 1 wt% graphene oxide. Samples with graphene contents below 1 wt% graphene (only 0.8 wt% graphene is shown for clarity) show conductivity values close to that of the pure YSZ whereas a gap of 8 orders of magnitude in conductivity is found on increasing the graphene content up to 1 %. The electrical behaviour of percolated samples is also different from un-percolated samples. Percolated samples show a conductivity plateau which corresponds to the dc conductivity of the percolated graphene network whereas the conductivity of the un-percolated samples increases with the frequency due to polarization between neighbouring conductive clusters. Over 1% graphene content, the conductivity of the samples gradually increases, but in much smaller gaps due to the increase of the vol. content of the conductive phase. Then, it has been determined that the minimum content for reaching the necessary low resistivity to be machined by using EDM technologies is around 1wt%. We will see now that the mechanical properties of the obtained zirconia + graphene SPS compacts with a so low content of GO addition in order to be electrical conductive are not significantly affected.

Physical and Mechanical properties

Table 1, shows the density and Vickers Hardness of ZrO2-GO composites.

Table 1. Density and hardness of ZrO2-graphene composites

 

GO content (wt%)

 

0

0.3

0.4

1.0

Density (g/cm3)

6.01

6.02

6.00

6.01

Hardness Hv10  (GPa)

12.97

12.44

12.56

12.50

ZrO2-Graphene composites show slightly lower hardness values than monolithic zirconia. This can be attributed to the presence of a carbon phase that is a comparatively a softer material. However, the difference can be considered in the range of the error bars of the measurement.

Colour properties

Finally, blackness measurements were carried out comparing three samples; pure 3Y-TZP zirconia (“white” zirconia), commercial “black” zirconia from Tosoh and 3Y-TZP-1wt%-GO composite. In Table 2 they are shown the values obtained for L* a* b* parameters.

Table 2. Whiteness index of zirconia–GO composite in comparison with commercial white and black zirconia

 

L

a

B

Commercial “White” zirconia

88,56

0,14

2,15

Commercial “Black” zirconia

44,39

0,01

-0,96

ZrO2-1wt%GO composite

36,44

0,23

0,57

 

The value of ΔL is increased in 18% when it is compared with the commercial black zirconia that is obtained by addition of Co3O4.

In summary these new family of zirconia-graphene nanocomposites are full dense materials, free of metals (Co, Fe), they reach a blackness value better than commercial black zirconia materials preserving zirconia hardness and they open the possibility to be machined by EDM thanks to their low electrical resistivity. Consequently they could be found a great number of new applications including wear resistant components as well as in the field of biomaterials were complex shaped components are required.

Conclusions

Black zirconia with blackness index higher than commercial products based on metals can be obtained by small addition of graphene. The low content in graphene allows maintaining mechanical properties of zirconia such as hardness.

Graphene-zirconia composites can be easily processed by starting form graphene oxide and performing reduction in-situ during Spark Plasma Sintering. When the graphene content is increased up to 1 wt%, electrical resistivity of composite is lower than 100 Ωcm that allows using electrodischarge machining for preparing complex shapes from these materials that are demanded for many applications fields.

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