Fundamentals of Spark Plasma Sintering (SPS): An Ideal Processing Technique for Fabrication of Metal Matrix Nanocomposites

Abstract

The pioneering work related to the spark plasma sintering (SPS) technique, also known as pulsed electric current sintering (PECS), started in around 1906 when the first direct current (DC) resistance sintering (RS) apparatus was developed. Later, a similar process was developed and patented in the 1960s. The present-day SPS which is now widely used for sintering metals and ceramics was introduced by the Sumitomo Coal Mining Co., Ltd. of Japan in 1990. Since then this technique which is based on the idea of using plasma generated by an electric discharge machine for sintering has attracted immense attention in the area of powder technology and development of the composites. The SPS process can be considered as a modified hot pressing process where a pulsed electric current is passed through a graphite die and the specimen is heated by the Joule heat transferred from the pressing die. In SPS, the compaction and sintering stages are combined in a single operation. Due to the pulsed electric current and the spark plasma effect, the SPS process is capable of introducing simultaneous rapid heating and cooling rates; accompanied with high pressure, this process can realize nearly full densification at a relatively lower temperature, within a very short time. In the past, materials were confined only to the monolithic form, but for better physical, chemical and tribological properties, composite materials have evolved. Metal matrix nanocomposites (MMNCs) have evoked keen interest in recent times because of their excellent structural and functional properties and have the potential to replace the existing materials in a wide range of applications. MMNCs refer to materials where rigid nanosized reinforcements, typically having size <100 nm, are embedded in ductile metals or alloys which act as the matrices providing attractive physical and mechanical properties such as high specific modulus, strength-to-weight ratio, fatigue strength, temperature stability and wear resistance. The properties of the MMNCs can be designed and custom-made as per the requirement of the application. MMNCs received much attraction as compared to the metal matrix composites (MMCs) due to the size and strength of the nanometric reinforcements. Apart from the nanofillers, a fine grain size of the matrix could also contribute to the improvement in the properties of the composites. As conventional processing techniques require a long holding time at high sintering temperatures, which can damage the structure of the nanofillers, processes like the SPS are the ideal routes for development of such composites. Also during conventional sintering, abnormal grain coarsening becomes particularly severe which in turn implies that achieving a very high level of densification and maintaining a fine grain size is very difficult. Attaining a uniform dispersion of the nanofillers in the composites is not easy using liquid-processing techniques due to the difference in the densities of the nanofiller, and the matrix and the non-wettability at the interface between them lead to a heterogeneous structure that affects the overall properties of the nanocomposites. By SPS, prevention or reduction in grain growth, to maintain the nanostructure of the matrix, is possible through careful control of consolidation parameters, particularly heating rate, sintering temperature and time. Due to the short period of sintering, grain growth can be restricted, and materials having submicron-sized or nanosized microstructures having enhanced properties can be developed. As the SPS technique requires very short sintering time, it is ideally suited for the development of nanocomposites reinforced with carbonaceous nanofillers like graphene or its derivatives and carbon nanotubes (CNTs), as short sintering time is essential for preserving their structural integrity and intrinsic properties. A wide variety of materials like metals and alloys, ceramics, composites, cermets etc. can be successfully developed by the SPS process. However, it should be noted that although SPS presents many advantages as compared to other conventional sintering techniques, it also has a few limitations. One of the major drawbacks of the SPS process is the heterogeneity of temperature field during the temperature cycle, resulting in heterogeneous microstructures in the sintered samples. Also, SPS allows only simple symmetrical shapes and is expensive as it requires a pulsed DC generator. The focus of this chapter will be on the fundamentals of the SPS technique, the kinetics of densification and grain growth during SPS and how the SPS technique could be effectively used to develop metal matrix nanocomposites (MMNCs). The chapter provides a detailed overview of the SPS process, the current state of research in the area of MMNCs developed by the SPS and its future.