Manufacturing technologies for nanocomposite ceramic structural materials and coatings

Abstract

The new material class of ceramic nanocomposites, containing at least one phase in nanometric dimension, has achieved special interest in previous years. While earlier research was focused on materials science and microstructural details in laboratory scale the subject of developing suitable manufacturing technologies in technical scale is the challenge for the manufacturing engineer. The same high-performance features which make the nanocomposite materials so interesting in their properties are absolutely detrimental if it comes to production of these materials. Extreme hardness, toughness and abrasion resistance make the state of the art cutting-and-machining operations extremely cost intensive so that, from a manufacturing point of view, true near-net-shape manufacturing is mandatory to accomplish reasonable cost targets. Ceramic feedstocks with both, high solid content to reduce shrinkage and warping and stable processing conditions are required to accomplish this aim of near-net-shape processing. Stable and reproducible processing conditions, e.g. favourable rheological properties for injection moulding are essentials for the manufacturing engineer. These prerequisites of ceramic production technologies cannot be reached with pure nanopowders in the 10–20 nm range but materials with a micro-nano architecture can fulfill these requirements, using a mixture of a submicron-sized matrix in the 100–200 nm range and smaller nanosized additives in <20% content which contribute the desired functionality. By using these micro-nanocomposites near-net-shape ceramic forming technologies such as injection moulding, gel casting and slip casting have been developed which lead to high-performance materials at affordable production cost. Advanced surface technologies include nanoceramic coatings made by thermokinetic deposition processes. Modern ceramic processing, i.e. spray drying leads to fine granulated nanopowders with appropriate flowability for subsequent APS plasma or HVOF supersonic flame spraying projection. Developing high turnover processes for pure nanopowders is a difficult and expensive task especially if safe workplace protection measures have to be applied. Powders of this kind have recently become available but at extremely high price. A novel process named HVSFS, high-velocity suspension flame spraying, has been developed to omit the granulation step and to perform direct spraying of liquid nanoparticle dispersions in a high-velocity oxygen fuel spraying torch with robot controlled kinematics. This process (HVSFS) is well suited to produce dense and, if desired, very thin coatings of various oxide-based nanocomposites and cermets for tribological and further applications in automotive, aerospace and mechanical engineering.