T he general objective of the project is the application of low temperature CVD processes to the deposition of protective coatings against wear and corrosion. The low temperature process aims to avoid deterioration of the substrate characteristics. Two major techniques will be applied for this purpose: rf plasma enhanced CVD (at SCK/CEN) and laser activated CVD (at TU Delft). Both techniques are developed in parallel research on the same types of materials as far as coatings and substrates are concerned. The deposition of boron nitrides is the main goal of the programme because of its high hardness, corresponding wear resistance and compatibility with most metals. At the end of the programme the coating quality will be tested under industrial conditions by aluminium extrusion tests (at SIDAL nv) using prototype dies coated with boron nitride by both low temperature techniques
The influence of the deposition parameters on the properties of boron nitride (BN) films grown by plasma enhanced chemical vapour deposition (PECVD) has been studied. Diborane and nitrogen (or ammonia) diluted in hydrogen are used as reactant gases. Stoichiometric films could only be obtained for films grown with a growth of c-BN. The coatings are very hard but a rather high coefficient of friction (0.4) against hardened steel.
For laser activated chemical vapour deposition (LACVD) a new reactor has been designed. Assembly of the system has been completed. Different metalloorganic precursors have been tested by PECVD for later use in LACVD. Dimethyl amino borane was found to give trans boron nitride layer at low temperatures and presumably cis boron nitride at a very small boron/nitrogen (B/N) ration in the gas phase.
Both plasma enhanced chemical vapour deposition (PECVD) and laser induced chemical vapour deposition (LICVD) have been used for the low temperature (starting from 300 C) deposition of boron nitride coatings. In the parallel plate PECVD configuration films could be grown on the cathode and the anode. Both ammonia and nitrogen gas were tested successfully as the nitrogen source while boron hydride was used as the boron source. On the grounded plate, stoichiometric boron nitride could only be obtained when a high excess of the nitrogen containing precursor was used. For films grown at the radio frequency (RF) plate the mixing of boron and nitrogen is less critical. The films have a hexagnoal short rangeorder (turbostatc stucture). An argon plasma pretreatment influences the structure of the film and very hard films are produced. The mechanical quality of the films (adhesion, friction coefficient) was not sufficient to perform industrial tests.
Borane dimethyle amine can successfully be use din a PECVD process to produce boron nitride layers if additional ammonia is provided. The resulting layers are stable in air although they contain an excess of boron. Generally the layers possess the turbostratic structure. Only when a large excess of ammonia is present in the gas phase does the cubic structure develop.
A reactor for LICVD was designed and constructed. The layers formed do not consist of stoichiometric boron nitride since they are not stable in air. The design of the reactor can be improved by inverting it to exploit natural convection for the transport of reactive species to the surface.
THE 4 MAIN PHASES OF THE PROJECT ARE :
- PLASMA ACTIVATED CVD :
COATINGS OF CUBIC BORON NITRIDE AND TUNGSTEN CARBIDES WILL BE DEPOSITED STARTING FROM THE CORRESPONDING HALIDES (WF6, BF3) OR HYBRIDES (B2H6).
- LASER AND LIGHT-ACTIVATED CVD :
LOW TEMPERATURE DEPOSITION OF SUITABLE COMBINATIONS OF MATERIALS LIKE CUBIC BORON NITRIDE, TITANIUM AND ZIRCONIUM BORON NITRIDES, ALUMINA AND SILICON NITRIDE WILL BE REALISED BY OPTICALLY EXCITED AND LASER ACTIVATED CVD, STARTING FROM VOLATILE ORGANO-METALLIC AND HALIDE METALLOPHORES. ADHESION OF LAMINAR COMPOSITES WILL BE OPTIMISED BY CONTROLLED NUCLEATION AND GROWTH.
- TECHNOLOGICAL EVALUATION OF COATINGS :
IN THE INVESTIGATIONS MENTIONED ABOVE, STRUCTURES WILL BE DETERMINED BY X-RAY DIFFRACTION, SCANNING ELECTRON MICROSCOPY AND TRANSMISSION ELECTRON MICROSCOPY. COMPOSITION AND VALENCE STATES WILL BE INVESTIGATED BY X-RAY PHOTOELECTRON SPECTROSCOPY, SECONDARY ION MASS SPECTROSCOPY AND AUGER DEPHT PROFILING. THE QUALITY OF THE COATING WILL BE EVALUATED BY A VARIETY OF DESTRUCTIVE AND NON-DESTRUCTIVE METHODS.
- EVALUATION UNDER INDUSTRIAL CONDITIONS :
THIS WILL BE CARRIED OUT ON PROTOTYPE EXTRUSION DIES.
Funding SchemeCSC - Cost-sharing contracts