March 6-11, 2016
The Field Assisted Sintering Technique/Spark Plasma Sintering (FAST/SPS) is a new method for fast processing of variety metallic and ceramic materials with unique microstructure frequently far from equilibrium. Despite an essential progress in fundamental understanding and in upscaling of this technique from laboratory to industrial scale, some challenges are still remaining. One of them is application of a tool made from material other then traditionally used graphite. The advantages of graphite are high electrical conductivity, low temperature dependence of strength and good machinability. At the same time, the strength of graphite is relatively low. This property restricts the pressure used during FAST/SPS to 50-100 MPa resulting in a reduced sintering rate, residual porosity, enhanced sintering temperature and time and, correspondingly, in increased grain growth. Besides, graphite trends to react with many materials at sintering temperatures building carbides, carbonates, reducing oxides and others. This can lead to an essential change in phase composition particularly in chemically instable materials, loss in functional properties, embrittlement, chemical expansion and disintegration of the material during sintering or subsequent cooling. Therefore, investigation of new tool materials replacing the graphite is an actual working task in FAST/SPS.
Graphite tool can be potentially replaced with a tool manufactured from hot working steel, nickel or molybdenum based alloy, hard metal, conductive ceramics or composites. The inserts manufactured from these materials can be used in the tool design as well. Additionally, coating of contacting surfaces can be applied to eliminate or to reduce chemical interaction between tool elements (die and punch) and between tool and sintered preform. Obviously, suitable choice of an alternative tool material depends on sintering temperature, used pressure, chemical affinity to the sintered material and economical consideration. The experience in the application of alternative tool materials for FAST/SPS is nowadays limited to only several case studies. Therefore, the aim of the present work was to extend the knowledge by applying a hot working steel Böhler W360-Isobloc as a tool material at low-temperature field assisted sintering. Two types of experiments were carried out. Firstly, the risk of diffusion bonding or even partial melting between two steel discs during current assisted sintering at various temperatures was studied. The coating of discs with TiN by PVD was found to be a very promising measure to reduce this risk. Another effective approach is the insertion of a graphite foil between the contacting surfaces. This option can be used if there is no chemical interaction between graphite foil and sintered material. Finally, the coating by BN-spray was investigated. This coating reduces the interaction between steel discs to some extent, but was found to be quite inhomogeneous and frequently instable. Moreover, BN-spray coating can be easily removed from contacting surfaces during densification when the punch moves into the die. Second type of experiments was related to the FAST/SPS of ZnO in an alternative steel tool. In this case, a special adjustment of the PID controller was strictly required to follow the prescribed temperature profile. The influence of sintering temperature, pressure and holding time on sintering kinetics, density and microstructure of samples was investigated. Our experience in application of a steel tool for FAST/SPS is summarized in form of concluding remarks.
Alexander Laptev, "Application of steel as an alternative tool material for field assisted sintering in SPA" in "Electric Field Assisted Sintering and Related Phenomena Far From Equilibrium", Rishi Raj (University of Colorado at Boulder, USA) Thomas Tsakalakos (Rutgers University, USA) Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/efa_sintering/61