ICMCTF 2023 Session B2-MoA: CVD Coatings and Technologies

Thermal chemical vapor deposition (CVD) is well suited to produce high-purity, high-crystallinity films compatible with high temperature structural applications. A good example of such structural coatings is the silicon carbide matrix of ceramic matrix composites (CMCs). However, in SiC/SiC composites, the role of the matrix in the overall mechanical behavior appears to be minor compared to that of the fibers or the interphase.Yet, intrinsically, the CVD coating can exhibit all these qualities. The key is to adjust the process parameters to obtain an appropriate composition and microstructure. The evaluation of the mechanical properties of the coating itself must also be performed using specific tests. These two aspects will be developed in the presentation through two examples.

The first one is a high-strength, gas-tight CVD SiC sheath, to complete the composite structure of a nuclear fuel claddings. Long, free standing SiC tubes were prepared at atmospheric pressure using CH₃SiHCl₂/C₃H₆/H₂ mixtures, in a fast, original, and near net shape process. Sliding the heating system along the tubular substrate allowed the continuous deposition of long and thick CVD SiC sheaths. Their composition, microstructure and surface morphology were analyzed in details. C-ring specimens were cut from the tubes and submitted to high temperature compressive tests. The deposition rate, Si/C atomic ratio, crystalline state and surface roughness of the CVD-SiC tubes are related to the gas phase reactions. Their thermomechanical properties can be improved by adjusting the through-thickness composition gradient using a proper precursor mixture.

The second example is a CVD silicon coating used as the bond coat (BC) of an environmental barrier coating on a CMC. In use, the BC may oxidize, which can cause local stresses and eventually damages. Centrifugal loads can also generate some creep deformation of the rotating parts. It is thus crucial to control the creep behavior of the BC, and hence its microstructure, through the CVD process. The deposition of polycrystalline silicon from SiHCl₃/H₂ was explored and a selection of coatings were prepared on fine carbon fiber substrates for testing. The morphology and microstructure of the deposits were investigated by SEM and EBSD, and the creep properties by 3-point bending. Several microstructures were obtained by various combinations of CVD conditions and thermal annealing. Different responses of the silicon coatings to mechanical stresses have been measured, illustrating different deformation mechanisms.

Although the use of protective Ti(C,N)/Al₂O₃ coatings produced by chemical vapour deposition (CVD) on cemented carbide (WC-Co) substrates is the state of the art in the high-speed metal cutting, the effect of the substrate treatment on the microstructure and properties of such coatings is not fully understood yet. In this contribution, the role of the size of the tungsten carbide grains (1.9 µm and 0.7 µm, respectively) and the substrate treatment (as-sintered, wet blasted, ground or polished) was systematically investigated for a constant amount of the cobalt binder (10 wt.%) in the cemented carbide. The hard coatings with different architectures, i.e., the TiC_0.6N_0.4 monolayers and complete TiC_0.6N_0.4/Al₂O₃ stacks, were deposited using the same parameters of the CVD process.

The microstructure analyses carried out using scanning electron microscopy with electron backscatter diffraction, transmission electron microscopy, laboratory X-ray diffraction and nanobeam synchrotron diffraction revealed that the kind of the substrate treatment strongly influences the preferred orientation and the lateral size of the grains in the coatings, as well as the residual stress in both phases (fcc-Ti(C,N) and α-Al₂O₃). Finally, the adhesion of the coatings to the substrate was quantified using scratch tests. The strongest preferred orientation of crystallites, which was {211} in fcc-Ti(C,N) and {001} in α-Al₂O₃, developed in coatings with consistently narrow grains. The texture was always weakened, when coarse Ti(C,N) and/or Al₂O₃ grains grew in the coatings in addition to the narrow grains. The dependence of the texture degree and the grain size on the substrate treatment was also observed for the α-Al₂O₃ layer, although the bonding layer between fcc-Ti(C,N) and α-Al₂O₃ consisted of very small crystallites having no pronounced preferred orientation. The comparison of the single-layer Ti(C,N) coatings with the Ti(C,N)/Al₂O₃ stacks helped us to understand the effect of the additional thermal exposure of the samples during the Al₂O₃ deposition on the microstructure of the Ti(C,N) layer.

The results of this study emphasize that the surface condition of cemented carbide substrates needs to be considered as it has a clear impact on the microstructure formation of the Ti(C,N)/Al₂O₃ coatings produced in CVD processes.

The synthesis of metal diboride thin films is recently attracting large interest. Boron forms binary compounds with most metals.These materials in general are high-melting, extremely hard solids with high degrees of thermal stability and chemical inertness.

In this work the preparation of metal diboride coatings of ZrB₂ and HfB₂ by CVD is described. A LPCVD process using MeCl₄ (Me = Zr or Hf), BCl₃, H₂ and Ar is applied. At deposition temperatures between 800°C and 1000°C diboride layers were prepared. The coatings were characterized with respect to phase composition, crystal structure, hardness and wear behaviour. The deposited diboride layers show well defined crystallites with a high hardness up to 32 GPa for ZrB₂ and 38 GPa for HfB₂. In dependence of substrate temperature and precursor ratio layers with different textured crystalline structure were obtained with different deposition rates. Phase composition and structure were examined using SEM and EDX-analysis. The measured tensile stress in the obtained coatings depends on the deposition conditions and varies between 350 MPa and 600 MPa.

A strong adherence on hardmetal inserts is achieved by using a thin TiN bonding layer prior the diboride deposition. Scratch test measurements showed critical loads of about 90 N. In wear tests a high performance of the CVD diboride coatings was observed. HfB₂ coated inserts showed a higher lifetime in comparison with state-of-the-art CVD- and PVD-TiB₂ coatings in face-milling TiAl6V4.