Sintering is a process used to modify or produce materials made of metal or ceramics. The base material is heated, i.e. thermal energy is applied to a powder compact. The compact is compacted by the sintering process and the average grain size increases.
In literature, two terms are used for the heat treatment of ceramics, "firing" and "sintering". Generally, the term "firing" is used when the processes running during heating are quite complex. This is the case, for example, with many ceramics traditionally made from clay, but also with processes in which many undefined parameters influence the final product properties. Less complex cases, on the other hand, are described using the term "sintering". This includes many modern processes with clearly defined process conditions and controllable process parameters.
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M.N. Rahaman, "Ceramic Processing and Sintering" 2nd edition, CRC Press, 2003, New York
Sintering is classified as one of the four basic elements of materials science and engineering. With progress in the synthesis and processing of advanced materials, the importance of sintering as a technology for material processing is increasing. There are many different sintering techniques and dimensions, which can lead to different microstructures and properties of the sintered product. It is possible to produce sintered products with reproducible and adjusted microstructures. The grain size, the sintered density and size and the distribution of other phases – including pores – can be controlled by a microstructure test.1
The widespread use of sintering processes has led to a situation today where there are a large number of different approaches to explaining the process of sintering. One approach to developing an understanding of sintering is to consider the behaviour or behavioural changes during sintering in relation to controllable variables and processes. This can be done empirically by observing the sintering behaviour under certain controlled process conditions or theoretically by modelling the process. The theoretical analyses and experimental investigations carried out over the last 50 years have led to a comprehensive qualitative knowledge of the sintering driving force, the sintering mechanism and the influence of the main process parameters such as grain size, temperature and applied pressure. However, the databases and models are less successful in providing a qualitative description of sintering for most systems.
In general, the process and material parameters provide meaningful sets of values for theoretical and experimental studies. Some parameters such as sintering temperature, applied pressure, average grain size and gaseous atmosphere can be controlled with sufficient precision. Other characteristic properties of the powder and particle packing are more difficult to control, but also have a significant influence on sintering.2
Basically, sintering processes can be divided into two types: solid phase sintering and liquid phase sintering. Solid phase sintering occurs when the powder compact is completely compacted to a solid state at sintering temperature. In contrast, liquid phase sintering is a process in which a liquid phase is present.
In addition to solid phase and liquid phase sintering, other types of sintering can be distinguished. These include, for example, temporary liquid phase sintering and viscous flow sintering.
Viscous flow sintering occurs when the volume share of the liquid phase is high enough that complete densification can be achieved simply by the flowing of the grain-liquid mixture without any change in grain shape during densification.
Temporary liquid phase sintering, in contrast, is a combination of liquid phase sintering and solid phase sintering. In this sintering technique, a liquid phase is formed at the beginning of the sintering process, which disappears again in the course of the process and thus completes the compression into the solid state.
In general, the degree of compaction can be increased in any process by using finer powders and increasing the temperature. However, this can become critical if the amount of liquid is reduced. Exactly which process is to be used is determined by the properties expected of the product and by the parameters that limit the processes.
Viscose flow sintering is recommended due to its convenience and the easy availability of raw materials. If the expectations are very specific, e.g. if the product is to carry high loads at high temperatures, the avoidance of a liquid phase becomes particularly critical. In this example, solid phase sintering is recommended. Liquid phase sintering can be regarded as a compromise solution given that the requirements for powder quality and high temperatures are less stringent than those for solid phase sintering. Careful control of the distribution, quantity and potentially the uniformity of the liquid phase during crystallisation when cooling offers further possibilities for structural improvements.
Usually, the driving force for fusion is considered in terms of surface energy reduction. An additional driving force can be exerted by applying pressure to the powder during heat treatment. This technique of hot pressing can be very helpful for materials that are otherwise difficult to sinter.
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[1] M.N. Rahaman, "Ceramic Processing and Sintering" 2nd edition, CRC Press, 2003, New York
[2] R.J. Brook (edt.) "Concise Encyclopedia of Advanced Ceramic Materials", Pergamon, 1991, Oxford
