Functionally Graded Materials (FGMs)
A functionally graded transition between two materials with different material properties like thermal expansion increases the reliability of the connection between the two materials and provides the ability to control deformation, dynamic response, wear, corrosion, etc. FGMs can be used to connect many different materials such as metal/ceramic, alumina/zirconia, alumina/steel, tungsten-carbide/steel, tungsten/copper, polymer/concrete, bones/metal, and aluminium/polyethylene.
In the fusion domain tungsten layers are required as toppings for ferritic/martensitic steel material on the blanket or the divertor in a fusion reactor and they have to be reliably connected to the carrier material. This is a challenge as the two materials differ in their thermal expansion. A continuous transition between the two materials (Functionally Graded Materials; FGM’s) is able to drastically reduce thermal induced stresses. The joining of dissimilar materials, with the opportunity to benefit from the properties of two material systems, provides both the ability to control deformation, dynamic response, corrosion, and wear, and the ability to apply FGMs to different complex environments. FGM’s have been successfully produced by a variety of techniques, such as electro-deposition, infiltration, sintering, explosive welding, physical and chemical vapor deposition, laser sintering and vacuum plasma spraying. The vacuum plasma spraying technique has been developed at Research Center Juelich.
Fig. 2: Deformation through the cooling down after joining of tungsten/steel tubes at high temperature (deformations are amplified in this representation)An evaluation of these fabrication methods against the four categories layer thickness, gradation range, vacuum and feasibility is given in Fig. 1. The ability to produce thick layers and to realize layers with the full gradation range are regarded as positive process features, because these aspects are important for the successful reduction of thermal induced stresses by a smooth gradation. The vacuum conditions are important for the avoidance of oxides, which otherwise would reduce the joining strength. VPS seems to be a very promising process fulfilling all four criteria. It provides the full gradation range from 0% to 100 %, the deposition takes place at a rather low pressure and sufficient thick layers of up to 1-2 mm are technically feasible.
Innovations & Advantages
Tests at the FZJ have shown that the functionally graded transition between two materials with different thermal expansion increases the reliability of the connection between the two materials and provides the ability to control deformation, dynamic response, wear, and corrosion. FGMs can be used to connect many different materials such as metal/ceramic, alumina/zirconia, alumina/steel, tungsten-carbide/steel, polymer/concrete, bones/metal, and aluminium/polyethylene.
Current and Potential Domains of Application
This technology has been identified in the framework of the FUTTA (Fusion Technology Transfer Action) project, funded by the European Commission, and with the collaboration of Eurofusion. It constitutes an example for a promising technology that is expected to provide a substantial transfer potential from the Nuclear Fusion domain towards other domains including Space.