Soft Ceramics for High Temperature Lubrication
Prof. dr. ir. J.E. ten Elshof
The main research focus of this thesis is on the development of the next generation of solid lubricants for high temperature forming of steel. These lubricants are based on ceramic nanoparticles which are more resistant to temperature and oxidation than traditional lubricants. Nowadays, the most common products are aqueous suspensions of graphite, molybdenum disulfide or polytetrafluoroethylene (TeflonTM). Nevertheless, some problems are associated to their use, for instance pitting corrosion on the steel surface caused by partial decomposition of the lubricants at high temperature, and, more in general, a dirty working environment.
Advanced inorganic solid lubricants were developed to overcome these problems. The performance of solid lubricants in forging processes is correlated with their plastic behavior. This means that when a force is applied, needed to shape the metal piece, the lubricants should absorb part of the force, transforming it into a physical deformation or a chemical change. The role of the lubricant is to decrease the necessary forces to form the metal pieces, protection of the metal surface, and/or participation in the process of releasing the piece from the mold.
It is straightforward to think that the plasticity (softness) of solid lubricants originates from naturally soft networks. At first glance, a common lubrication mechanism can be inferred from the mechanical behavior of the different materials. The plastic deformability of the networks seems to be connected to the ease of dislocation propagation in the network. In this thesis, this property was explored by studying three different classes of materials: (1) soft structured networks, i.e. layered oxides; (2) soft atomic networks, i.e. bismuth metal and (3) soft amorphous networks, i.e. organosilicas. Control over the synthesis and chemical modification is an important factor to obtain materials that can be part of the new generation of solid lubricants. Some of the tested materials were able to outperform graphite in terms of a decrease of friction force. The results of this research are the input for the achievement of the next generation of solid lubricants for high temperature applications.