ICMCTF2008 Session B3-1: CVD Coatings and Technologies

Carbon foams are attractive potential materials for shock insulation in extremely high temperatures ; but the mechanical properties of the as-prepared foams are too low. Chemical Vapor Infiltration (CVI) of carbon or refractory ceramics is an interesting solution to overcome this drawback. However CVI itself contains several issues in terms of material quality. The thickness and nanostructure of the reinforcing deposit has to be uniform throughout large samples ; achieving this goal through the control of processing parameters requires a sound knowledge of the process physico-chemistry. In that context, process modeling may bring some useful guidelines.

The presented work focuses on the deposition of pyrocarbon from pure propane in carbon foam samples with ~ 96% initial porosity, the treatment being stopped when porosity reaches ~85%. Depending on the chosen nanotexture of pyrocarbon, some infiltration gradients may appear in the samples. The modeling is aimed at determining the importance of gas diffusion and deposition kinetics during deposition.

One of the elements of the model is the evolution of the internal surface area and average pore diameter with the infiltration. This has been determined by X-ray computed microtomography (CMT) and 3D image analysis featuring a simulation of structure growth. Previous studies on pyrocarbon deposition have allowed to build a simplified chemical mechanism featuring maturation, i.e. gas-phase hydrocarbon pyrolysis leading to reaction intermediates, and deposition of the latter. Combining this chemical model with the resolution of balance equations at reactor-scale and sample scale, and inserting the previously determined structure evolution model brings a complete modeling frame, that compares favorably with experimental results. A discussion of the interplay between transport and maturation/deposition kinetics is given, as a guideline for the choice of optimal infiltration parameters.

The quality of Ceramic-Matrix Composites (CMC) depends partly on the matrix infiltration step, performed by the Chemical Vapor Infiltration (CVI) process, because it is decisive on the residual porosity and the matrix quality. These two elements contribute to confer adequate thermo-mechanical characteristics to the composite. During infiltration, various phenomena are governed by properties which depend on the morphological evolution of the preform. Experimental determinations of optimal operating conditions prove to be time-consuming and expensive. Thus, it seems interesting to assess this morphological evolution by densification modeling.

The proposed simulation is based on the use of random walkers. The physical phenomena: binary, Knudsen and transition-regime gas diffusion and deposition reactions are taken into account by the parameters of the random walks. The walkers are injected in binarized 3D blocks of raw preforms. By observing their global behavior and thanks to an instant control of fluid/solid interfaces, it becomes possible to account for the morphological evolution and to display the influence of various parameters on the infiltrability, i.e. the ability of a given preform to receive large amounts of filling matrix.

Two kinds of binarized blocks are used. First, synthetic blocks can be built in order to control the validity of the code. Second, the simulation is performed in 3D images of real composite materials acquired by tomography. Algorithms have been developed to segment the blocks into the initial substrate, the matrix and the porous space. The actual infiltrated fiber arrangements allow validating the densification simulation algorithm by comparing real and simulated matrix deposits. The simulations help to understand the influence of the physicochemical parameters and of the preform geometry on the infiltrability.

Nanostructured multilayer coatings exhibit enhanced properties. Most of them are constituted of transition metal carbides and nitrides and even if Ti-based coatings play a major role there is an increasing interest for Cr-based ones, especially for their better resistance to corrosion and oxidation. They are generally deposited under very low pressure by Plasma and PVD processes. These vacuum techniques permit sharp interfaces and the growth of nanometric thick layers. Both for economical reasons and to develop continuous deposition processes, atmospheric pressure CVD exhibits advantages compare to vacuum techniques. We have previously demonstrated that Cr carbides and nitrides coatings can be grown at low temperature by low pressure CVD using metalorganic precursors. However, these Cr sources exhibit a relatively low vapor pressure, which does not allow their use in reliable atmospheric CVD processes.

Pulsed direct liquid injection is a well adapted technology to feed the CVD reactors with precursors exhibiting a poor volatility. As a result, DLI-CVD can operate under atmospheric pressure and is suitable to develop new industrial CVD processes. Many works deal with DLI-CVD of functional oxide thin films but the growth of non oxide ceramic coatings is almost unexplored. In this paper, we describe novel atmospheric pressure DLI-CVD processes for the deposition of Cr-based coatings including carbides, nitrides and even metal chromium. Single coatings and nanostructured multilayer coatings, e.g. CrN-CrC_x with a bilayer period as low as 50 nm have been grown successfully. Structural characterizations and preliminary mechanical properties are presented.