M.C. Carpintero

Aluminizing and chromizing bed treatment by CVD in a fluidized bed reactor on austenitic stainless steels

The deposition of Al and Cr on steel coupons by chemical vapor deposition in fluidized bed reactors (CVD-FBR) has been studied. Before performing the experiments, thermochemical calculations were run using the free-energy minimization method afforded by the HSC Chemistry software. Metal sub-halide chemistry was used to produce the gas precursors of deposition. With this method we have achieved 10 μm thick Al coatings at low deposition temperatures and short deposition times. However, we have not yet succeeded in the deposition of Cr and further work is needed.

Kinetic studies of Cr and Al deposition using CVD-FBR on different metallic substrates

Abstract Thermochemical calculations for the deposition of aluminium and chromium using the metal–subhalide chemistry have been performed by the HSC chemistry software. Before depositing, the flow regimes that lead to a fluidized bed without particle elution have been measured at room and deposition temperatures. Aluminium diffusion coatings have been obtained on austenitic AISI 304 stainless steel and IN-100 Ni-base alloy; the deposition being much faster on the steel substrates. However, the chromium diffusion coatings are only achieved on the IN-100 Ni-base superalloy since in the steel, volatilization of Fe occurs at the first stages of deposition.

Effect of fluidized bed CVD aluminide coatings on the cyclic oxidation of austenitic AISI 304 stainless steel

Abstract Austenitic AISI 304 stainless steel finds a vast variety of applications due to its good performance in different environments and its relatively low price. However, its high temperature operation limit is restricted to approximately 950°C due to the formation of volatile CrO3 under oxygen environments. Thus, the application of aluminide coatings may increase the upper temperature LIMIT of this steel. Chemical vapour deposition in fluidised bed reactors (CVD-FBR) has been used to coat AISI 304 steel at temperatures of 525°C for 1.5 h followed by a heat treatment up to 900°C. Cyclic oxidation experiments of both coated and uncoated specimens have been conducted under atmospheric pressure air at 950°C. The results are very promising for the application of the CVD-FBR process as a surface modification technology since the behaviour of the coated specimens is much better than that of the uncoated ones.

Silicon/silicon oxide coating on AISI 304 stainless steel by CVD in FBR: analysis of silicides and adherence of coating

Abstract Silicon can be deposited and diffused on AISI 304 surfaces to increase corrosion resistance in aqueous acidic environments, high temperature oxidation and erosion resistance. This coating has been obtained by chemical vapor deposition in a fluidized bed reactor (CVD-FBR) at relatively low temperatures (450–550 °C). The deposition of silicon on AISI 304 steel can lead to the formation of interfacial silicide compounds (Fe 3 Si, Fe 5 Si 3 , FeSi and FeSi 2 ). However, the adherence of these coatings is not good when the amount of Si is increased. We studied the best conditions to deposit Si on AISI 304 to minimize the formed corrugated and non-adherent silicide layer. We have optimized the silicon coatings on the stainless steels at temperatures of 450 °C for 60 min. Optical microscopy, XRD and scanning electron microscopy/energy dispersive X-ray spectroscopy have been employed to characterize the obtained products. The results will show that some CVD-FBR silicon coating types are expected to provide.

High Temperature Al/Si Diffusion Coatings Deposited by Chemical Vapor Deposition in Fluidized Bed Reactors (CVD-FBR)

The ceramic coatings are excellent candidates to protect metallic structures that work at high temperature. Inside these ceramic coatings, the mullite is a good option, since it presents very good mechanical properties, great corrosion resistance, high thermal resistance and high durability. The CVD-FBR (Chemical Vapor Deposition by Fluidized Bed Reactor) is an interesting technique to create adherent films on metallic surfaces to protect them. Furthermore, this method is cheap and easy to apply, and the most important characteristic is that could be applied at much lower temperatures comparing for example to pack cementation. The first step to obtain mullite coatings would be the co-deposition of aluminium and silicon coatings by CVD-FBR to decrease the thermal mismatch between substrate and coating. Thermodynamic calculations were made before experiments to study the oxidation system and optimize the working conditions using Thermo-Calc code. These depositions take place in a fluidized bed reactor. The base material used was an AISI 304 stainless steel. This technique is based upon the reaction among aluminium chloride (AlCl3 (g)) and silicon chloride (SiCl4(g)), among other precursor species. The optimization of the conditions (deposition temperature, time, fluxes, etc) is discussed in the present work. The coatings design is completed with diffusion simulations during the CVD process and subsequent heat treatment. The analysis of results is carried out by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersion Spectroscopy (EDS) methods. In addition further oxidation of these precursor coatings is made in order to obtain the definitive system of protective ceramic layer. The oxidation of coating samples is made at different temperatures and time conditions to obtain the best mullite structure, based on predicted thermodynamic calculations by Thermo-Calc.

Towards high temperature materials performance through ion implantation

The present contribution intends to highlight the role of ion implantation as an advanced surface modification technology. In order to show the role of this technique, a comparative study of the high temperature performance of some commercial steel grades after having been ion implanted with different elements is presented. The implanted and non-implanted steels were tested at high temperatures, depending on the steel grade under isothermal as well as cyclic oxidation experiments. It is concluded that ion implantation may -play a beneficial effect depending on the nature of the implanted ion as well as the oxidation conditions.