ICMCTF2001 Session B2-1: CVD Hard Coatings and Technologies

Tetrakis(dimethylamino)titanium, TDMAT, is an important precursor for the metal-organic chemical vapor deposition (MOCVD) of TiN and TiSiN thin films, both of which are used as hard coatings. Understanding the kinetics and mechanism of the gas phase reactions in these processes will lead to a better understanding of the CVD process, and an improvement in material properties. As a basis for understanding the CVD of TiN and TiSiN, the focus of this research is on the thermal decomposition of TDMAT alone.

The experiments were performed in a hot-wall LPCVD reactor coupled to a molecular beam sampling system for the quadrupole mass spectrometer (MBMS). The gases are injected into the flow reactor through a temperature controlled, moveable injector. At the end of the reactor a fraction of the gases are sampled and formed into a molecular beam, which passes along the axis of the system to the ion source of the mass spectrometer.

The decomposition follows first order kinetics in the temperature range investigated (333-593K). The activation energy changes from 16 kJ mol^-1 at low temperatures to 166 kJ mol^-1 at high temperatures, which is indicative of a change in reaction mechanism from a heterogeneous mechanism at lower temperatures to a homogeneous one at higher temperatures. An increase in surface-to-volume ratio (S/V) increases the rate constants for each regime, but does not change the activation energies. The increase in rate constants in the high temperature regime with increased S/V indicates the presence of a surface component at high temperature in addition to the gas phase reaction.

Product species of the decomposition were also monitored. Several possible mechanisms for the gas-phase decomposition reaction have been postulated. Results of comparisons between species present in mass spectral data and those predicted by the mechanisms will be presented.

The use of fossil fuels containing small sulphur, vanadium, etc. may lead to degradation of the boilers and tubes commonly employed in power plants due to the well-known hot corrosion attack. It has been shown that the use of aluminide and chromide coatings are able to extend the service life of the materials employed but whereas the formers do it up to a certain extent, the latters have been shown to protect a further attack of the alloy by forming typically a CrS scale that interferes diffusion of the active species. Austenitic AISI 304 stainless steel may be the material of choice under certain conditions due to its ratio properties/cost. However, the amount of Cr of about 18 wt% may be unsufficient to form, grow and regenerate the protective scales formed upon exposure to the aggresive environment because of the presence of a relative high Ni amount. The use of higher alloyed steels on the contrary makes the prices rise due to the need of a higher Ni content so as to maintain the austenitic structure. Therefore, it could be of interest to increase the amount of Cr on the surface of relatively cheap materials by Surface Engineering.

Either single Cr deposition or Al/Cr or Si/Cr co-deposition has been the goal of different studies mainly on Armco iron, low alloy steels or in Ni-base alloys employing different coating techniques. However, to our belief, very little or no work has been performed on investigating the formation of pure chromide coatings on austenitic stainless using the Chemical Vapour Deposition in Fluidised Bed Reactors (CVD-FBR). Thus, in this work, we will be presenting the results obtained by applying this technique to a commercial austenitic 18Cr-8Ni stainless steel at temperatures ranging from 825 to 900°C using different H₂/HCl ratios so as to obtain the best coating quality (i.e. composition and morphology). Attack from HCl to the alloy will be hindered to some extent by increasing the H2 amount in the coating medium. The deposition velocity will be shown to increase with temperature but the quality of the coatings will be inferior to a certain extent. A solution of compromise is therefore found to fulfil all the requirements.