Chemical vapour deposition of coatings

Chemical vapor deposition (CVD) is an important technique for surface modification of powders through either grafting or deposition of films and coatings. The efficiency of this complex process primarily depends on appropriate contact between the reactive gas phase and the solid particles to be treated. Based on this requirement, the first part of this review focuses on the ways to ensure such contact and particularly on the formation of fluidized beds. Combination of constraints due to both fluidization and chemical vapor deposition leads to the definition of different types of reactors as an alternative to classical fluidized beds, such as spouted beds, circulating beds operating in turbulent and fast-transport regimes or vibro-fluidized beds. They operate under thermal but also plasma activation of the reactive gas and their design mainly depends on the type of powders to be treated. Modeling of both reactors and operating conditions is a valuable tool for understanding and optimizing these complex processes and materials. In the second part of the review, the state of the art on materials produced by fluidized bed chemical vapor deposition is presented. Beyond pioneering applications in the nuclear power industry, application domains, such as heterogeneous catalysis, microelectronics, photovoltaics and protection against wear, oxidation and heat are potentially concerned by processes involving chemical vapor deposition on powders. Moreover, simple and reduced cost FBCVD processes where the material to coat is immersed in the FB, allow the production of coatings for metals with different wear, oxidation and corrosion resistance. Finally, large-scale production of advanced nanomaterials is a promising area for the future extension and development of this technique.

https://oatao.univ-toulouse.fr/649/1/Vahlas_649.pdf

Principles and applications of CVD powder technology

Realising the potential of duplex surface engineering

Gear tooth contact analysis and its application in the reduction of fatigue wear

The feasibility of Al deposition on Mg and Mg-alloys by means of Fluidized Bed CVD (FBCVD) or Pack Bed CVD (PBCVD) processes is evaluated by means of thermochemical computations and experimentation. For the thermodynamic analysis, a closed system consisting of Al2O3 as filler, Al as donor, halide or halide compound as process activator and Ar as inert gas was considered. Different activators were evaluated in the temperature range from 280 to 420 °C. Among them NH4Cl, and iodine are the best candidate compounds due to the relatively high partial pressures of the formed aluminium halides. The analysis indicates that the deposition of aluminium on magnesium takes place through a displacement reaction mechanism on the surface followed by limited inward diffusion of Al. Pack bed experiments have shown that by use of NH4Cl activator, aluminium is deposited locally. Iodine activator results in more even but porous coatings. The process is simple and promising for the formation of corrosion and wear resistant coatings on magnesium. However, in order to provide a long-term protection, the coating should be deposited under conditions that eliminate through-thickness pores. Further developments are required in order to achieve dense, pore-free coatings.

Deposition of aluminium on magnesium by a CVD process

The use of a fluidized bed process for the production of hard wear- and corrosion-resistant coatings of carbides, nitrides and carbonitrides of cementing metals is promising since it can lead to a flexible surface treatment technology combining adaptability with relatively low capital and operational costs. In this paper the feasibility of a fluidized bed process for the formation of titanium carbide coatings using chemical-vapour-deposition-based reactions is discussed. The method involves a treating agent consisting of a powder mixture of Al 2 O 3 with FeTi which is fluidized at elevated temperature with Ar. As an activator of the process NH 4 Cl was used. The use of explosive hydrogen is avoided by the formation of the vapours of the materials to be coated in situ in the reactor. With this method, adherent and dense coatings of TiC were applied on various steel grades.

Formation of TiC coatings on steels by a fluidized bed chemical vapour deposition process

Alloy steel nitriding is currently employed to improve corrosion and wear resistance, to increase fatigue strength and to increase superficial hardness. The corrosion and wear properties of nitrided parts can be further improved by adding processing steps to the nitriding program. An oxidation step is added to produce a black Fe 3 O 4 layer to improve salt-spray corrosion resistance. Corrosion resistance is improved without surface roughening and this process does not require additional machining or polishing steps before completion. A CVD step is added, to deposit TiN hard-coatings on hardened and tempered alloy steels, for increased wear resistance. Nitriding provides enhanced support for this overlay coating. Deposition of these layers can be completed during a single cycle within the nitriding retort, by simply extending the process program. One pulsed plasma nitriding technology (ELTROPULS) offers a controlled plasma power duty-cycle that permits these combination layers to be added at low temperatures, that precludes geometrical distortion and dimensional change and does not require rehardening and tempering to restore core properties. Precursor reacting only within the plasma over the substrate surface permit mechanical masking and make for coating uniformity, independent of line-of-sight considerations.

Single cycle, combination layers with plasma assistance

A thermodynamic approach of a fluidized bed chemical vapour deposition process for the formation of Ti-bearing coatings on steels is attempted. For the thermochemical analyses, a system consisting of Al 2 O 3 or SiO 2 , Fe 2 Ti, Fe and C as well as NH 4 Cl or NH 4 I as activators of the process was considered. The partial pressures of the equilibrium gaseous species and the condensed phases of the above closed system have been calculated for amounts of the above constituents analogous to those used in the experimental practice and for different amounts of NH 4 Cl activator. The analyses indicate, that in all cases, the Ti-halide vapours, which are necessary for the coating formation, exist in the examined temperature range 773–1473 K. Different modes for the coating deposition are discussed. With NH 4 Cl as activator higher deposition rates of the coatings can be expected, in comparison to the NH 4 I. SiO 2 is not a good candidate as filler oxide as it reacts with the fluidizing medium. Although the process is feasible without hydrogen, addition of 10% of the latter is beneficial for the process as it increases the partial pressures of the Ti-chlorides.

S. Kinkel

Papers

Formation of TiC coatings on steels by a fluidized bed chemical vapour deposition process

Single cycle, combination layers with plasma assistance

Thermochemical aspects on the formation of Ti-bearing coatings in a fluidized bed CVD process