Preparation and characterisation of 60mol% Al2O3 & 40mol%ZrO2 microcomposite by SOL-GEL precipitation route

Abstract

A material having two or more distinct constituent materials or phases such that the integrated material has properties noticeably different from constituents is a composite material. For example in alumina-zirconia ceramic composite system, the incorporation of - Al2O3 into stabilized zirconia changes the properties drastically. Ceramic materials traditionally have properties of high compressive strength, high wear resistance, high abrasion resistance and the ability to withstand high temperatures. But one major drawback with ceramic materials is their poor toughness. Zirconia, as a ceramic material, possess remarkable properties such as high melting point, inertness to chemicals, high hardness etc. It has an added advantage that when retained in the tetragonal form in the product during use it gets transformed to monoclinic zirconia and during this process of conversion it increases the toughness of the material- the process known as transformation toughening. This unique property of zirconia can also be used to improve the properties of other ceramic materials like alumina spinel, mullite etc. Transformation toughening is one of the best methods of improving the fracture toughness and strength of brittle ceramic materials. The stability of tetragonal zirconia phase (t-ZrO2) is the main concern in these systems as it determines the stress required for the transformation and thus the resulting toughening achieved. Various combinations of constituent ceramic materials are taken such as Zirconia alumina, Zirconia spinel, Zirconia silicon carbide, Zirconia mullite etc. Objective of our work is to synthesize composite having 60mol% Al2O3 and 40mol% ZrO2 by sol-gel precipitation route. We are adopting this process because the distribution of the ZrO2 particles must be uniform and the size should be slightly lower than the critical diameter at which spontaneous transformation takes place. Before starting this work, a thorough study has been made on the theory, mechanism and the research done by various authors. We have discussed the theory and mechanism of transformation toughening. If the crystal can be restrained from undergoing martensitic transformation during cooling from high temperature by the constraint imposed by the matrix, it can be considered in a metastable state. Should the elastic constraint be removed, as would be the case ahead of the tip in a propagating crack, then the crystal could transform, the shear strain developed would oppose the growth of the crack; it is this effect which is used to advantage in transformation toughening. In our project work we have prepared Alumina powder samples. For this we have adopted sol-gel precipitation route. In this process we have Zirconium oxy chloride and ammonium nitride salt for preparation of powder. From the salts clear solution is prepared. Then gel is formed by addition of ammonia solution which is 50% dilute. This gel is dried in an oven drier. After drying the sample is ground to obtain powder. This dried powder is subjected to calcination at different temperatures starting from 300°C to 1000°C. Then the calcined powders are taken for phase analysis by XRD. We have observed presence of cubic phase in the temperature range of 600°C to 800°C. The disadvantage of cubic phase is that it makes the material prone to fracture. The strength of the sample increases but the toughness is very less. The cubic phase of zirconia is retained in the refractory material where very high strength is required. The tetragonal phase is observed in the temperature range of 800C to 1000°C. This tetragonal phase present in sample, while use transforms into monoclinic phase so the toughness of the material is increased. This process is known as “Transformation Toughening”. The monoclinic phase is generally unstable hence stabilizers a re required. It improves the toughness of the sample.