This confirms that the diploma thesis entitled "Effect of TiO2 as an additive on the phase formation and densification of mullite-zirconium composites", submitted by Ms. Vini Bhagat, partially fulfilled the requirements for the degree of Bachelor of Technology in Ceramic Engineering. at National Institute of Technology, Rourkela is an authentic work done under my supervision and guidance. As far as I know, the matter covered in the thesis has not been submitted to anyone. Pratihar for his valuable advice, time and guidance in completing this project work.
My heartfelt thanks to all the faculty members for their suggestions during this project work. Technical students and the non-teaching staff for the help and cooperation extended to us. Mullite-Zirconia composite was prepared from reaction sintering of zircon sand and alumina with different percentages of titanium dioxide.
GENERAL INTRODUCTION
- Introduction
- Occurence
- Different polymophs of zirconia
- Phase transformations
- Unstabilized, stabilized and partially stabilized zirconia
- Mullite
- Mullite-Zirconia composites
- Effect of additives on the properties of Zirconia- Mullite composites
- Objective of the present study
- Procedure
- General Characterization
Mullite is an aluminosilicate widely used in traditional refractory applications and one of the most studied crystalline phases in Al2O3.SiO2 system. Sintered mullites, produced by heat treatment of the starting materials, mainly via solid-state reactions. The evolution of mullite microstructure is influenced not only by the chemical composition, but also by the nature of the starting raw materials (4).
One of the ways to improve the sintering and mechanical properties of mullite is by adding zirconium oxide. The radial decomposition of zirconium starts from the surface towards the center of the grain, leading to rapid crystallization of zirconium oxide and to the formation of amorphous silica [8]. The presence of a glassy phase at some of the triple junctions will occur at high temperatures, which is responsible for the decrease in the strength of zircon at high temperatures [6][9].
Manas K.Haldar, T.K.Pal and G.Banerjee (Ceramics International observed that the addition of yttria generally improves the physical, thermomechanical properties of the composites. It was shown that the addition of TiO2 results in an important effect on the mechanical properties of the composites because it is the controlling parameter for the zirconia grain growth (11) The bulk density of the sintered samples decreased with increase in temperature with varying amount of magnesium.
Lodha (Ceramics International synthesized the precursor powder for the composition of zirconia-mullite by co-precipitation in an aqueous medium by taking inorganic salts of aluminum, silicon and zirconium in the required amount of sand, studied the role of two additives, namely MgO and Cr2O3, in densification and compacted. The mechanical properties such as compressive strength, flexural strength and fracture toughness of the group containing MgO and Cr2O3 were also found to be better than the others. The bulk density and apparent porosity of the sintered product were measured by a conventional method of liquid displacement using Archimedes Principle in aqueous medium.
The bulk density of the sintered products was measured using conventional liquid displacement method using Archimedes principle in water medium. The dry weight of the sintered samples was determined (Wd), and then they were kept in the boiling water for approx. 2 hours to ensure complete filling of the open pores in the sintered samples. The temperature of the samples was then cooled to room temperature and the weight of the sample, suspended in water, was taken (Wsus).
The weight of the sample in a fully saturated state in air was then determined (Wsoa). To calculate the change in linear dimensions, the diameters of the sintered products were determined using digital calipers.
RESULTS AND DISCUSSIONS
- Densification Study
- Change in linear dimensions
- Biaxial flexural strength
- PhaseAnalysis
Fig.1: Variation in bulk density of zirconia-mullite composites with and without additives at different sintering temperatures. Variation of apparent porosity of ZM0, ZM1, ZM2 & ZM3 with sintering temperatures is shown in fig. Apparent porosity (A.P) initially decreases with temperature in all three compositions, whereas the decrease is more prominent in ZM1 &.
We can see that A.P values approach zero for the sample with 0.2 wt% TiO2 additive at 1600oC, the value is zero indicating a fully sintered phase; however, there may be some amount of closed pores. To be sure of the exact temperature for the completion of sintering, we need to carry out additional firing between the two temperatures of 1550oC and 1650oC. table 3, 4 & 5 and fig. 3; shows that at lower temperature the sample has undergone some expansion and very negligible shrinkage, but with the increase in temperature it has increased.
Fig.3 Variation in the dimensions of the sintered samples with the addition of TiO2 at different temperatures. Diameters and thickness of the sintered samples were calculated and the biaxial flexural strength was measured. Biaxial flexural strengths of the samples with different amounts of additives at different sintering temperatures are shown in Table 6 and shown in Fig.
But increase in addition of TiO2 resulted in decrease in biaxial flexural strength at 1500oC temperature and at 1550oC it increased for samples containing 0.1% TiO2 but again with further increase. While at a higher temperature of 1600oC, the maximum biaxial flexural strength is achieved with the composition containing 0.1% TiO2. In the first part of our research, the sintering of a mixture of zirconium sand and aluminum oxide with and without additives took place at 1500oC, 1550oC and 1600oC.
bulk density, apparent porosity), phase analysis by XRD, biaxial flexural strength and variation in dimensions. The densification study shows that the use of liquid phase forming additives in the composites causes densification at 16000oC with 0.2% TiO2. Measurements of biaxial flexural strengths confirm that the maximum strength at 1600oC is obtained by the sample containing 0.2% TiO2.
Skoog, R.E.Moore, Refractories of the past for the future: mullite and its applications as a bonding phase, Ceram. Claussen, Reaction-bonded mullite/zirconia composites, J. Leriche, Formation of microstructural defects in mullite-zirconia and mullite-alumina-zirconia composites obtained by reaction sintering of mixed powders, Trans.
