Sintering

This introductory chapter addresses an overview of the main processes taking place during the so-called Sintering phenomenon. The driving forces and mechanisms for the competitive processes, coalescence and grain growth, are also briefly discussed. The goal is not to provide and exhaustive review of the field, but to give the reader a concise perspective of the commonly accepted concepts of sintering, pathing the way for the next chapters of this book to address the specifics of nanosintering and field assisted processes.

Materials at the nanoscale typically show distinct behavior from their microsized counter parts. This is not different in sintering, and this chapter collects some previous studies on nano effects on this important phenomenon. It is constantly argued by researchers that classical continuum sintering model requires re-examination in the context of nanosintering. A possible way of doing this is by incorporating multiple mechanisms which may cause nano effects into the modeling. In this chapter we introduce both analytical and computational approaches. Analytical method is used to provide a guidance and first insight into nanosintering. Various computational models are then constructed using techniques such as Finite Difference (FD), Finite Element (FE), and Molecular Dynamics (MD), in order to capture the coupled features.

One of the main goals in sintering is to obtain dense compacts with retained grain sizes. Therefore, a detailed understanding of the phenomenon of grain growth and the parameters affecting it is of prime importance for a successful processing. In this chapter, two kinds of models (thermodynamic and kinetic ones) are constructed to explain the role of segregation playing in controlling the thermal stability and the grain growth of nanocrystalline materials. Thermodynamically, analogous to the Gibbs adsorption theorem, a concept that solute atoms segregating in GBs could decrease the GB energy is proposed, i.e. once the GB energy reduces to almost zero with solute segregation, the system approaches metastable equilibrium state. Kinetically, the difference of solute concentrations between the GBs and the bulk produces a drag force on the GB migration. Incorporating a constant drag term into the parabolic equation, a model describing the nanocrystalline growth kinetics subjected to the GB segregation is presented. Considering that the thermodynamic method cannot describe the process of grain growth and that the kinetic one could do nothing for predicting the metastable stability, a coupling of thermodynamic and kinetic ways seems particularly important to provide an insight for understanding the thermal stability and grain growth of nanocrystalline materials. On this basis, from Borisov’s empirical relationship, the rationality of coupling thermodynamic and kinetic ways has been demonstrated. Assuming a linear relationship between the GB energy and the average grain size, an analytical thermo-kinetic model with incorporating the reduced GB energy into the drag equation of grain growth is proposed.

If all other things are equal, nanopowders will sinter faster and at lower temperatures than larger powders. However, the increased surface area to volume ratio of these materials presents additional processing challenges that correspond to greater difficulty in achieving the goal of sintering for finer powders. This is not related to the “nano” effects as described in previous chapters, but to powder characteristics that can strongly influence the sintering behavior. These characteristics can be seen as the “real life” parameters, such as agglomeration state and contaminations that if not addressed properly can confuse sintering tendencies and complicate sintering effects at the nanoscale. This chapter presents the effects that may contribute to nanosintering and the importance of adequate processing of nanopowders for achieving optimum sintering behavior.

The sintering models described in previous chapters consider only a limited number of particles or analytical analyses of sintering, without a close connection to the sintering of large parts. While the previous chapter addressed the serious issues that have to be considered in the “real world” of sintering, great advances have been made on the simulations of sintering of real complex shapes. This chapter addresses simulations based on the Discrete Element Method (DEM), a promising new technique to model sintering by directly taking the microstructure into account. Each grain is modeled as an individual particle which interacts with its neighbors by given force laws that depend on the diffusion mechanisms. In this chapter the grain-scale simulation model is explained in detail and it is shown how it can be used to model sintering of both standard and nanoparticle powders. By directly considering effects like rearrangement, crack formation and anisotropy development new insights into many sintering processes are gained. Especially the influence of grain rearrangement, which cannot directly be considered with traditional continuum mechanical approaches, is studied in detail. As a demonstration example constrained sintering of inkjet printed silver powders is given. Since the method is still relatively new, not all sintering mechanisms are fully considered yet. It is shown how grain coarsening models could be improved by considering the growth process on the grain scale as well.

This chapter focuses on the sintering of nanoparticle-based porous materials. First, nanoparticle-based porous materials are defined. Then, the fundamentals of sintering are briefly revisited but from a porous point of view, describing effects of parameters on porous structure and shrinkage, and the unique characteristics and challenges for the sintering of nanoparticle-based porous materials are addressed. The discussion on the sintering is divided into single composition systems and composites. For single composition systems, porous materials can be obtained by partial sintering, template-based sintering, or reaction sintering. For porous composite materials, the microstructures can be realized by direct sintering of composite systems, using nanoparticles as a second phase in the sintering of larger-sized particle matrix, or unique reaction processes. Finally, the current state of nanoparticle-based porous material sintering is summarized and future directions are analyzed.

The goal of consolidating powders to achieve high densities at lower temperatures and with a small grain size has motivated considerable efforts in the search for methods to activate the sintering process. Enhancement of the consolidation process has been attempted through various approaches including mechanical activation of the powders, the addition of sintering aids, and the use of electromagnetic fields. The latter approach has received considerable attention in recent years, largely due to the widespread use of devices utilizing current and pressure to consolidate powders. The Spark Plasma Sintering method (also known by other names) has seen a remarkable increase in its utilization over the past two decades. This was largely due to the many significant, and in some cases, unique accomplishments. In this chapter we will focus then on the role of the electric field in sintering with emphasis on recent observations, particularly those pertaining to the consolidation of nanostructured materials.

Field assisted sintering studies have produced a wealth of data about the densification behaviors of many powder systems. However, the sheer volume of work has not met with sufficient mechanistic descriptions of the processes in metal or ceramic systems. This fact has, and is, limiting the acceptance and widespread use of this promising technique in larger than laboratory scale manufacturing. We describe here the nature of the influences of electric fields and/or currents, changes in heating rate, and the effects of applied pressures upon ceramic and metal systems in context of the commonly accepted stages of sintering. As many of the specific mechanisms discussed have not been directly characterized within field assisted sintering studies we focus on the established theoretical underpinnings to better understand their influence and to enable definitive future experimentation on this area of research.

In the previous chapters we have mainly described the effects of the electrical field on the densification behavior when using Field Assisted Sintering. In addition to those effects, Field Assisted Sintering also offers the possibility of applying a mechanical pressure as well as high heating rates during sintering. The result is a significant change in the sintering process. That is, we have gone through the description of conventional sintering in the initial chapters of this book, and there is a great temptation to use that theory directly to understand SPS. But comparing free sintering and SPS is like comparing the performances of two ways of transportation such as bicycle and motorcycle. The first one relies on a natural driving force (the rider’s muscles), the second one gets further energy from the attached engine. The translation movement obtained through rotation of wheels is the same but kinetics and possibly final destination are not! Therefore, the present chapter aims at reviewing the separate contributions of applied stress and heating rate on the densification behavior and microstructure development of oxide ceramics. Carefully-designed experiments coupled with physical understanding enable us to figure out the real advantages and drawbacks of this technique as well as to extend its possibilities.

The microstructural characterization of materials is a critical step to understand structure--property relationships in sintered materials. Altering the processing parameters during sintering can lead to variations of the materials microstructure and, hence, their macroscopic properties. This chapter reviews experimental techniques for the atomic resolution characterization of microstructural defects. Emphasis is given to a variety of electron microscopy techniques and how these can be used to gain a more fundamental understanding of sintering behavior, such as defect segregation and grain growth. The recent advent of novel in situ electron microscopy techniques has enabled the atomic-scale investigation of densification mechanisms and their kinetics that occur during sintering. A review of available techniques is presented and first experimental results are discussed.