Effect of sintering atmosphere on the property of ceria doped tetragonal zirconia polycrystals (Ce-TZP)

I would also like to thank all the professors of the Department of Metallurgy and Materials Engineering for their help in my work. Samples sintered in N2 atmosphere show a lower relative density as well as a smaller proportion of the tetragonal phase compared to air sintered at all temperatures studied. A maximum fracture toughness of 13 MPa.m-1/2 was observed for 12 mol % Ce-TZP sintered in an air atmosphere at 1600 °C.

Fig.4.5 (d) XRD pattern of zirconium samples doped with Ceria 12mol% sintered in N2 atmosphere at different temperatures. SEM of 10 mol% CeO2-ZrO2 sintered in nitrogen atmosphere SEM of 12 mol% CeO2-ZrO2 sintered in nitrogen atmosphere SEM of fracture surface.

Fig. 4.3 Agglomerate size distribution of ceria doped zirconia powder calcined at 850°C

Chapter 1

INTRODUCTION

THE IMPORTANCE OF ZrO 2 AS STRUCTURAL MATERIAL
Knives and Scissors
Seals, Valves and Pump Impellers
Orthopedics implants
Refractory Applications

STRUCTURE
STABILIZATION

Partially stabilized Zirconia (PSZ)
Fully stabilized zirconia

ZrO2–MgO System
ZrO 2 – CaO System
ZrO2-Y 2 O 3 System
ZrO 2 -CeO 2 SYSTEM
TRANSFORMATION TOUGHENING MECHANISM

Chapter 2

The chemical inertness of the material to the physiological environment reduces the risk of infection. This is said to degrade the mechanical properties of the ceramic, making it unsuitable for use in the temperature range C. Calcium oxide is one of the most common oxides used to form a solid solution with ZrO2.

The nature and composition of tetragonal, non-transformable tetragonal (t') and the transformation of these phases from cubic are discussed in detail by Anderson et al [7]. The slope of the field separation of the transformable tetragonal phase and tetragonal plus cubic (T + F in fig. 1.6) is important. The sensitivity of the material to high temperature aqueous environments is a cause for concern and has led to the development of the ZrO2–CeO2 alloy system.

There is no suppression of vapor decomposition processes and the vapor pressure of ZrO and Zr increases.

Fig. 1.1: (a)The orientation of oxygen ions together with angle of the ZrO 2 seven fold co ordination about the Zr +4 (b)Monoclinic zirconia [2]

LITERATURE REVIEW

Chapter 3

SAMPLE PREPARATION

Estimation of ZrO 2 from zirconium oxychloride and Ceria from Ammonium Ceric Nitrate

Powder Synthesis
DSC/TG 2. Density

Green Compaction
Sintering
Characterization

The solution was mixed and filtered to ensure complete mixing and to remove any dirt or other impurities. 3 ml of ZrOCl2 was taken in a weighed crucible and excess NH4OH was added to ensure complete precipitation of Zr(OH)4. The precipitate was dried and calcined for 2 hours at 1100 °C. We took the burnt powder in a mortar and, depending on the weight of the powder, 4 wt. It was then tapped slowly to allow the powder to settle evenly and fill density better.

Similarly, rectangular bars of dimensions 60 mm x 5 mm x 5 mm were pressed into a rectangular die of given dimensions. A laser diffraction method with a multiple scattering technique was used to determine the particle size distribution of the ground powder. The bulk density and apparent porosity of the sintered sample were measured according to Archimedes' principle.

From the X-ray diffraction data of the calcined samples of different compositions, the lattice parameter value was calculated. The speed of the rotating disc and the load with which the grinding was performed varied according to the desired status. At high loads, such as several kilograms, the measured hardness of ceramics decreases due to breakage of the material during indentation.

The hardness of the materials was calculated based on the size of the impression produced under load by a pyramid-shaped diamond indenter. The size of the print (diagonals) was measured using a calibrated optical microscope (ZEISS brand). In this method, a local tensile stress is induced in the transverse direction of the applied compressive stress.

The edges of the rectangular bars were ground on 600 grit grinding wheels using polisher, Automet-3 and Ecomet-3 (made by Buehler) to give a parallel and smooth surface. The edges of the rectangular bars were ground on 600 grit grinding wheels using polisher, Automet-3 and Ecomet-3 (made by Buehler) to give a parallel and smooth surface.

Fig. 3.1: Flow Chart of Estimation of Zirconia from Zirconia Oxy-Chloride The same procedure was applied for estimation of Ammonium Ceric Nitrate

Chapter 4

RESULTS AND DISCUSSION

TG-DSC Analysis

The TG and DSC curves of the raw powder prepared by the co-precipitation method are shown in Fig.4.1 (a) and Fig.4.1 (b). Ceria doped Zirconia and about 20.54% in the case of 12 mol% Ceria doped Zirconia It was proposed earlier [4,14] that while coprecipitating Zr4+ and Ce3+ cations, cerium enters the solid solution in a trivalent state. The persistent changes observed in the color of the precipitate during further processing immediately after precipitation suggest that both Ce4+ and Ce3+ may be present in the precipitated hydroxides.

The weight loss observed is attributed to the thermal decomposition of the precipitated hydroxides and to the remaining water.

Temperature (°C)

Phase analysis in calcined powder by XRD

2 θ (degree)

12CeO 2

Particle Size Analysis
Dilotometry Analysis
Phase analysis in sintered samples

The powder shows a small size of Ce-TZP crystallites even after calcination at 650 °C, as can be seen from the broadening peaks. Calcining at higher temperatures did not change the peak position, but the intensity of the diffraction peaks of the tetragonal phase gradually increased as the calcination temperature increased to 9500C. From Scherer's formula [24] (Equation 4 as mentioned in Section 3.4.3.1), the average crystallite size was calculated from the peak broadening.

The crystallite size was found to vary from 28 nm for the lowest calcination temperature to 38 nm for the highest calcination temperature for 10 mol% Ceria doped zirconia. For 12 mol% Ceria doped zirconia powder, the crystallite size was found to vary from 27 nm for the lowest calcination temperature to 37 nm for the highest calcination temperature. The measured crystallite size for samples calcined at different temperatures is given in Table 4.1. The crystallite size was found to increase with increasing calcination temperature.

The distribution of particle sizes after calcinations of the precipitate at 850°C for 10 mol% and 12 mol% ceria-doped zirconia, respectively, is shown in fig.4.3. The average particle/agglomerate size obtained at 50% cumulative volume is 19.4 µm for 10 mol% Ceria-doped Zirconia and 5.84 µm for 12 mol% Ceria-doped Zirconia powder. Although the results of laser scattering and scanning electron microscopy provide information on the distribution and shape of particles/agglomerates, they are not sufficient to assess the strength of these agglomerates.

The total shrinkage of the compact is about 22-23% and the maximum shrinkage temperature is 1150 °C as determined by the derived curve. The sharp, well-defined peaks indicate the highly crystalline nature of the synthesized phase without any impurity phase. The phases thus obtained remain stable even during high-temperature sintering. This was possible due to the very nature of the synthesis technique adopted in this investigation.

Table 4.1: Crystallite Size at different temperatures

10CeO

Densification Study
Mechanical Properties

Hardness Test (Vickers hardness)

The main phases observed after air sintering were those of tetragonal and monoclinic. Only the tetragonal phase appeared in both 10 and 12 Ce-TZP samples, but some monoclinic phase is also present in both samples at 16,000 °C. This is due to the fact that at high temperatures tetragonal to monoclinic transformation takes place due to the increase in grain size. At 14,000 °C, 10Ce-TZP shows tetragonal and few monoclinic phases, but 12Ce-TZP shows pure tetragonal phases. At high temperature, in addition to t-ZrO2 and monoclinic phases, additional phases were found in the nitrogen-sintered sample.

The additional phase was found to be (Zr,Ce)O2 solid solution [5,22]. The solid solution with tetragonal crystal structure as shown by JCPDS cards (82-1398 and 38-1437). It was found that the sample cracked after sintering in a nitrogen atmosphere. For sintering in air, the density in 10 mol% ceria-stabilized zirconia increases with temperature increases up to 1550°C, beyond which the density gradually decreases. However, in the case of 12 mol%, ceria-stabilized zirconium dioxide density increases slowly up to 1500°C with higher increases up to 1550°C, beyond which the increase is less.

The same samples were sintered in a nitrogen atmosphere. It was observed that the density increased at every temperature up to 1600°C. The density was higher for 12 mol% cerium oxide-stabilized zirconia than 10 mol% cerium oxide-stabilized zirconia. The theoretical density of the sample has also been calculated based on the percentage of tetragonal and monoclinic phase present in the sintered sample. The results are shown in Table 4.2 and Table 4.3 for air and nitrogen atmosphere respectively.

It has been shown that the density is better in the case of samples sintered in an air-oxidized atmosphere than in a nitrogen atmosphere. One of these is the intrinsic property of the phases present and the microstructure of the material. In this chapter, the hardness, compressive strength and flexural strength tests were carried out.

Fig 4.5 (c) and Fig.4.5 (d) show the XRD pattern of the samples sintered in Nitrogen atmosphere

Temperature °C

Diametrical Compression Test / Brazilian Disk Test
Compressive Strenth of ZrO 2 -10 and 12 mol% CeO 2 sintered in (a) air and (b) Nitrogen atmosphere
Flexural Strength
Flexural Strength of Ce-TZP Sintered in (a) air and (b) nitrogen atmosphere
Fracture Toughness
Microstructure Analysis

The fracture toughness was calculated using the SENB method by INSTRON Model- Hounsfield H10KS, U.K. Fig.4.7.4 (a) and fig.4.7.4 (b) show the fracture toughness of 10 and 12 Ce-TZP sintered in air and nitrogen atmosphere respectively. It was found that for the air-sintered sample, the fracture resistance is much higher than the nitrogen-sintered sample. A maximum strength of 13 MPam-1/2 was observed for the air-sintered sample for 12 mol% CeO2-ZrO2, but for the nitrogen-sintered sample a maximum of 8.7 MPa.m-1/2 was achieved for the same sample.

The fracture toughness of sintered samples in nitrogen atmosphere is less, which can be explained in terms of microstructure, crystallite size and phase composition in the sintered sample. The microstructure of the nitrogen-sintered sample is found to be porous and the XRD result revealed that there is a solid solution phase (Zr,Ce)O2 present in the nitrogen-sintered sample, which deteriorates the mechanical properties. Ce-TZP (10 and 12 mol%) powder was prepared by precipitation from aqueous precursors ZrOCl2 (for ZrO2) and ammonium cerium nitrate (for CeO2).

The precipitated powder was washed, followed by calcination at 850 °C. DSC/TG study of the precipitated dried powder showed an endothermic peak in the low temperature range (~100 °C) due to the removal of absorbed water and an exothermic peak at a higher temperature (~400 °C) due to the crystallization of ZrO2.XRD of the calcined powder of both compositions (10 and 12 mol%) shows tetragonal zirconium up to 950 °C. The air-sintered samples show a high sintering density and a high volume fraction of the tetragonal phase, while the sample sintered with nitrogen shows a low density and the retention of the tetragonal phase is also very low. The microstructure of the N2 sintered sample shows a porous structure compared to the air sintered sample which is dense.

Sakai, “Effect of reduction on the mechanical properties of CeO2 doped tetragonal zirconia ceramics”, Acta Mater. Fantozzi, “Static and Cyclic Crack Propagation in Ce-TZP Ceramics with Different Amounts of Transformation Hardening”, J. Lathabai, “Effect of Grain Size on Slurry Wear of Ce-Tzp Ceramics”, Scripta Mater.