On the processing of starch consolidated high speed steel - Microstructure and tribological properties

This thesis focuses on processing of high speed steel (HSS) powder into components using starch consolidation (SC) and supersolidus liquid phase sintering (SLPS). The work consists of four parts. First, the possibilities and constrains of the SC process are investigated and the role of starch in the green body fabrication clarified. Secondly, the possibility to sinter green bodies to full density using SLPS is investigated and the influence of sintering temperature on the as-sintered microstructures evaluated. Further, the mechanical and tribological properties of the as-sintered microstructures were evaluated using hardness indentation, scratch testing and three different wear tests. Finally, the use of physical vapour deposited (PVD) coatings, TiN and WC/C, for improved tribological performance of HSS components in sliding contacts was investigated and the importance of pre- and post-surface treatment illuminated. Also, the mechanisms of material transfer and tribo film formation and how these influences the tribo contact was investigated. The results show that it is possible to combine SC and SLPS in order to produce HSS components with fully dense microstructure and retained green body geometry from coarser HSS powder. The strength of the green bodies is mainly controlled by the morphology and distribution of starch and green bodies easy to handle can be obtained. The sintering temperature has a strong impact on the resulting microstructures, i.e. microstructures sintered at low temperatures show a porous microstructure while microstructures sintered at high temperatures show a coarser more inhomogeneous microstructure. The mechanical and tribological properties of SC HSS are in different extent controlled by the microstructure. The presence of a residual porosity will result in lower hardness but also in increased cracking tendency if porous areas are loaded. In contrast, microstructures sintered at high temperatures result in lower hardness and a more plastic behaviour. The abrasive wear resistance is, as compared with HIPed HSS, surprisingly good both under low and high stress abrasion conditions and the influence of any residual porosity is relatively small. Under sliding wear conditions a porous microstructure should be avoided since it will increase the wear of a softer counter surface. However, the deposition of a PVD coating will significantly improve the performance also of a porous microstructure. Evaluation of the sliding wear characteristics of PVD coated HSS shows that, besides a correct choice of the coating material, the control of surface structure topography is of outmost importance in order to obtain low friction and stable contact conditions. In summary, the obtained results are positive regarding the near net shape manufacturing (based on SC and SLPS) of PVD-coated HSS components for tribological applications