The development of laser synthesized nonoscale ceramic powders is restricted by the high cost of the chemical precursors and the lack of industrial application to justify such high quality materials
The goal of this project is to develop a European industry for nanoscale powders.
The laser synthesis of silicon containing nanopowders was realised from inexpensive liquid precursors; namely, hexamethyldisilazane for (Si, C, N) and hexametyldisiloxane for (Si, C, O). Their low viscosity made them able to be injected by nebulisation into the laser irradiation cell. However, it was demonstrated that more viscous polysilazane can lead as well to nanopowders as far as an adapted dropwise injection is conceived.
A tunable laser in the range 9-11 microns was used for (Si, C, O) powders synthesis.
Tungsten carbide nanopowder was prepared for cutting tools applications. The precursors of tungsten carbide were tungsten hexafluoride and acetylene. Small amounts of sulfurhexfluoride were added for sake of photoabsorption. But attempts to get rid of the remaining sulfur did not succeed in lowering the sulfur level below 20 ppm, which is compulsory to sinter such hard metalcarbides.
Erosion resistants coatings have been elaborated in the form of silicon carbide dispersion in a metal matrix composite. Dispersion of SiC particules in the Ni or Co metal matrix is close from 100 nm level. A two step process was worked out: electrophoretic deposition of SiC consolidated by electrochemical deposition of the metal. The mechanical stability of the deposit dictates to limit its thickness below ten microns. Three layers must be deposited in order to reach an apropriate thickness of 20 microns. A significant resistance to silica dust impact has been demonstrated.
Dealing with sintering of nanopowders, a considerable knowledge has been accumulated on how the nanopowders of SiC and (Si, C, N) must be processed, namely: powder conditionning (degazing, pre-cristallisation), dispersion of sintering aids, samples fabrication, sintering in various conditions (temperature, pressure, atmosphere). Densification over 95% has been obtained with the help of aluminium and ytrium oxide additives. The overall problem lies in promoting densification while preserving a fine cristalline structure. Practical answers related to process conditions are now available.
The project is based on innovative answers to these restrictions :
* firstly in using cost effective precursors, a new concept of synthesis reactor and an advanced laser technology
* secondly in developing two original application fields :
- tools made from sintered cermets for wear and mechanical properties
- electrochemical composite coatings for which nanoscale SiCN and TiB2 ceramic reinforcement enhance erosion and wear resistance.
Having the advantage of the availability of large quantities of such powders, their sintering behaviour will be with a view to the conventional applications in the field of ceramics.