Elsevier

Ceramics International

Volume 46, Issue 7, May 2020, Pages 9002-9010
Ceramics International

Microwave hybrid sintering of Al2O3 and Al2O3–ZrO2 composites, and effects of ZrO2 crystal structure on mechanical properties, thermal properties, and sintering kinetics

https://doi.org/10.1016/j.ceramint.2019.12.147Get rights and content

Abstract

Structural ceramics such as Al2O3 and Al2O3–ZrO2 composites are widely used in harsh environment applications. The conventional sintering process for fabrication of these ceramics is time-consuming method that requires large amount of energy. Microwave sintering is a novel way to resolve this problem. However, to date, very limited research has been carried out to study the effects of different ZrO2 crystal structures on Al2O3–ZrO2 composites, especially on the sintering kinetics, when fabricated by microwave sintering.

The microwave hybrid sintering of Al2O3 and Al2O3–ZrO2 composites was performed in this study. Tetragonal zirconia and cubic zirconia were used as two different reinforcements for an α–alumina matrix, and the mechanical and thermal properties were studied. It was found that Al2O3 experienced a remarkable increase in fracture toughness of up to 42% when t-ZrO2 was added. Al2O3–c-ZrO2 also showed increased fracture toughness. The sintering kinetics were also thoroughly investigated, and the average activation energy values for the intermediate stage of sintering were estimated to be 246 ± 11 kJ/mol for pure Al2O3, 319 ± 71 kJ/mol for Al2O3–c-ZrO2, and 342 ± 77 kJ/mol for Al2O3–t-ZrO2. These values indicated that the activation energy was increased by the addition of either type of ZrO2, with the highest value shown by Al2O3–t-ZrO2.

Introduction

Structural ceramics are ceramic materials that exhibit enhanced mechanical and physical properties under very harsh conditions. These materials demonstrate advantageous properties such as high-temperature stability, strength, wear resistance and corrosion resistance. They are generally fabricated from powders via different manufacturing processes that allow them to be tailored for specific applications [1].

Alumina is one of the most widely used, studied, and cost-effective materials in the group of structural ceramics, with outstanding properties such as a high melting point, high temperature stability, high wear resistance, corrosion resistance, hardness, strength, electrical conductivity, and high bio-compatibility. It is used as a material for combustion engine components, pump components, gears, electrical insulators, electronic devices substrates, refractories, and hip and knee joint prostheses [[2], [3], [4]].

Investigating and controlling the microstructure of structural ceramics has led to ceramic–ceramic composites that are suitable for a wide range of engineering applications. Ceramic composites composed of structural ceramics exhibit significantly superior characteristics at both room temperature and high temperatures compared to monolithic ceramics in a variety of material systems. This indicates the presence of a structural synergy between the two phases [5]. Among ceramics, zirconia–added alumina is extensively used in a variety of applications, including cutting tools, dies, prosthetic parts, oxygen sensors, and structural parts, all because of the superior mechanical properties of Al2O3–ZrO2 composites compared to Al2O3 and ZrO2 alone [4,6,7].

Generally, Al2O3–ZrO2 composites are synthesized using conventional sintering process. However, conventional sintering consumes large amounts of energy and requires very long processing times. Gafur et al. [8] conventionally sintered Al2O3–t-ZrO2 composites at 1580 °C in a heating cycle of 7.5 h. Tuan et al. [9] sintered Al2O3–t-ZrO2 composites conventionally at 1600 °C in a heating cycle of 6 h.

Microwave sintering is a sintering process with energy and time saving benefits. The heating rates are very high and the processing temperatures are lower than those in conventional sintering. The microstructures obtained are usually finer than those obtained from conventional sintering, and hence, the properties of the material are better that lead to quality product. Microwave sintering utilizes microwave electromagnetic radiation, which is absorbed volumetrically by the sample. The sample starts volumetric bulk self-heating from the core at a very rapid heating rate. Microwave hybrid sintering also utilizes a radiant heat source to ensure uniform heating across the surface and core, and to help bring a sample with low loss to a temperature where the sample starts coupling with the microwaves sufficiently [[10], [11], [12], [13]].

Given these advantages, microwave hybrid sintering has previously been proposed as a solution for sintering Al2O3–ZrO2 composites with lower energy and less time consumed, and to provide a better microstructure and properties [6]. However, to date, very limited research has been conducted on how the different crystal structures of ZrO2 (t-ZrO2 and c-ZrO2) affect, for instance, the mechanical and thermal properties of Al2O3–ZrO2 composites, when sintered by microwave hybrid sintering under same conditions. Some mechanisms for enhancing the mechanical properties of Al2O3–ZrO2 composites, such as stress-induced transformation toughening, also need to be thoroughly demonstrated for the composites sintered via microwave hybrid sintering. There is also a need for investigating the sintering kinetics of microwave hybrid sintered Al2O3–ZrO2 composites, when using zirconia with two different crystal structures (t-ZrO2 and c-ZrO2) under the same conditions.

The activation energy for sintering is one of the most important thermodynamic parameters, and can shed light on the different densification parameters involved in the sintering process. Generally, two methods are employed for measuring the activation energy of sintering, the isothermal method and non-isothermal method. Two types of non-isothermal methods can be used: the constant heating rate (CHR) method and the master sintering (MS) curves formation method [[14], [15], [16], [17]]. The general equation for the CHR method is as follows [18]:ln (T dρ/dT a) = -Q/RT + ln[f(ρ)] + ln A – n ln d.where T is the temperature (K), ρ is the relative density, a is the heating rate (K/h), Q is the activation energy (kJ/mol), R is the ideal gas constant (J/mol), d is the grain size, and n is the grain size exponent that depends on the dominant diffusion mechanism. A is a material parameter that is insensitive to T, ρ, and d, while f(ρ) is a function that depends on only relative density. The activation energy can be calculated if the density and grain size remain constant. Since there is negligible grain growth at intermediate stage of sintering [19,20], d is considered as constant. Moreover, because the experiments using the CHR method are performed at different heating rates, giving several measurements at a certain density, the term ln[f(ρ)] is also considered to be constant [18,21]. Samples are sintered at different constant heating rates, and the activation energy is calculated by plotting the Arrhenius-type plot between ln (T dρ/dT a) and ln 1/T.

In this research work, alumina–zirconia composites were sintered via microwave hybrid sintering. Tetragonal ZrO2 and cubic ZrO2 were used as two different additives in an α-Al2O3 matrix. The objective was to determine how the two different structural types of zirconia affected the microwave hybrid sintering of α-Al2O3, with a focus on the mechanical properties, thermal properties, and activation energy calculations for the intermediate stage of sintering using the CHR method. To perform the sintering kinetics analyses, the microwave furnace was modified, and a dilatometry system was built in-house.

Section snippets

Experimental

The materials used were α-Al2O3 powder (Sumitomo Chemical, Japan, AKP 50 High Purity Alumina, with an average particle size of 0.7 μm, and a purity greater than or equal to 99.99%), 3 mol% yttria-stabilized tetragonal zirconia powder (Tosoh Corporation, Japan, TZ-3YS-E, with an average particle size of 1 μm), and 8 mol% yttria-stabilized cubic zirconia powder (Tosoh Corporation, Japan, TZ8YS, with an average particle size of 1 μm). Three powder combinations were made: pure alumina powder, Al2O3

Results and discussion

Table 1 lists the nomenclature for the composite samples that will be considered next. Fig. 2 shows the results of the XRD analyses of the ball-milled powders. The powders were uniformly mixed. The A + tZ powder also contained a small amount of monoclinic zirconia due to the presence of a small amount of monoclinic phase in the as-received t-ZrO2 powder.

The XRD analyses of the sintered samples (Fig. 3) showed that no new phase originated during sintering, and the small amount of the metastable

Conclusions

This work involved a comparative study of the microwave hybrid sintering of three different samples, i.e., pure Al2O3, Al2O3–t-ZrO2 and Al2O3–c-ZrO2. The samples were sintered in a significantly short time, resulting in very high relative densities, without any of the normally used pre-sintering processes (e.g., the use of cold isostatic pressing to enhance the green density) or post-sintering treatments (e.g., heat treatment to enhance the densities of the sintered samples).

The activation

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the National Research Foundation of Korea (NRF), Ministry of Education, Ministry of Science and ICT (MSIT), Republic of Korea, for the financial support under project number NRF-2018R1D1A1B07043025.

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