UHTCs possess outstanding physical and engineering properties, such as high hardness and strength, low electrical resistivity and good chemical inertness. The extremely high melting point and the low self-diffusion coefficient make these materials very difficult to sinter to full density: temperatures above 2000°C and the application of pressure are necessary conditions. However these processing parameters lead to coarse microstructures with trapped porosity, all features that prevent the full UHTC potential to be achieved. The composite approach via introduction of secondary phases, such as metals, nitrides, carbides or silicides, has become a successful strategy to improve densification and thermo - mechanical properties of UHTCs. In general the addition of these secondary phases does decrease the sintering temperature, but may generate residual secondary phases that are detrimental for high temperature applications. The most diffused methods to densify UHTCs are pressure-assisted techniques such as hot pressing (HP) or spark plasma sintering (SPS), but appropriate choice of sintering aids also allows densification to occur without applied pressure (PLS). Each sintering method presents advantages and drawbacks and its use depends on the application and requirements of the final component. Mechanical and physical properties of the UHTCs are closely linked with starting powder, densification process, microstructure and intergranular secondary phases. Therefore, understanding the relationship between processing, microstructure and properties is fundamental in order to produce UHTC composites with superior performance. In this talk, an overview of the most important additives for UHTCs will be provided and possible densification mechanisms will be illustrated. For each class of composite a comparison between different sintering techniques will be presented (HP, SPS, PLS). Relevant mechanical properties, such as hardness, flexural strength at room and high temperature, fracture toughness and Young's modulus will be reported.
Sintering and densification of UHTCs
Diletta Sciti;Laura Silvestroni;Stefano Guicciardi;
2012
Abstract
UHTCs possess outstanding physical and engineering properties, such as high hardness and strength, low electrical resistivity and good chemical inertness. The extremely high melting point and the low self-diffusion coefficient make these materials very difficult to sinter to full density: temperatures above 2000°C and the application of pressure are necessary conditions. However these processing parameters lead to coarse microstructures with trapped porosity, all features that prevent the full UHTC potential to be achieved. The composite approach via introduction of secondary phases, such as metals, nitrides, carbides or silicides, has become a successful strategy to improve densification and thermo - mechanical properties of UHTCs. In general the addition of these secondary phases does decrease the sintering temperature, but may generate residual secondary phases that are detrimental for high temperature applications. The most diffused methods to densify UHTCs are pressure-assisted techniques such as hot pressing (HP) or spark plasma sintering (SPS), but appropriate choice of sintering aids also allows densification to occur without applied pressure (PLS). Each sintering method presents advantages and drawbacks and its use depends on the application and requirements of the final component. Mechanical and physical properties of the UHTCs are closely linked with starting powder, densification process, microstructure and intergranular secondary phases. Therefore, understanding the relationship between processing, microstructure and properties is fundamental in order to produce UHTC composites with superior performance. In this talk, an overview of the most important additives for UHTCs will be provided and possible densification mechanisms will be illustrated. For each class of composite a comparison between different sintering techniques will be presented (HP, SPS, PLS). Relevant mechanical properties, such as hardness, flexural strength at room and high temperature, fracture toughness and Young's modulus will be reported.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
