ICMCTF2005 Session B3: CVD Coatings and Technologies

Nowadays metallurgical factories must combine productivity gain and production costs. Most metallurgical coatings are deposited using batch processes. For instance vacuum-processing requires stop periods, entailing servicing costs and productivity drop. That is why atmospheric processes are promising to answer industrial metallurgist needs. Atmospheric pressure chemical vapor deposition (APCVD) in an attractive process for on-line coating of metal pieces. Moreover the use of metal organic compounds as precursors allows deposition at low temperatures. Given these two points, atmospheric pressure MOCVD is an attractive technique for the growth of metallurgical coatings.

Cr-based coatings were grown under atmospheric pressure in different gaseous atmospheres (N₂, H₂, NH₃) from the thermal decomposition of Cr(CO)₆at 300°C in a cold wall CVD reactor. The use of Cr(CO)₆as precursor is interesting because of its low cost and its high volatility, which permits high flow rates.

Original chromium oxycarbide and oxynitride coatings were deposited on steel. These phases were previously obtained by plasma assisted CVD and low pressure CVD, i.e. under non-equilibrium conditions. In the present work, the low deposition temperature likely accounts for their formation. We have investigated correlations between the growth conditions and their main chemical, structural and physical features. Preliminary results on the mechanical and corrosion behaviour of these Cr-based coatings are also reported.

The two main ways to improve the efficiency of turboengines are to decrease part weight and to increase operating temperature. Allowing light titanium alloys to work at higher temperature through appropriate protection against oxidation would extend their use in helicopter turbines. Within this frame, the present contribution reports on an original chemical vapor deposition (CVD) process to coat Ti6242 alloy with Al and Pt.

Depositions were performed in a vertical cold wall reactor on all surfaces of 15 mm Ti alloy coupons to prepare for subsequent oxidation treatments. Pt and Al were deposited either sequentially or simultaneously using Me3(MeCp)Pt (VI) and TIBA or DMEAA for Pt and Al precursors, respectively. Deposition temperatures were up to 623K and reactor pressure was in the range of 1 to 10 kPa.

The morphology of the Al film deposited without the process improvements discussed in this work is disordered, and this behaviour, which is deleterious to the protective role of the coating against oxygen ingress, is further degraded in the presence of Pt due to the catalytic effect of the latter. In order to obtain compact and smooth coatings, deposition temperature and pressure, and input gas composition (namely presence of surfactants, hydrogen content and nature of the Al precursor) have been successfully investigated.

Thermogravimetric measurements on coated samples in dry air up to 873K for 100h reveal a significant improvement of weight gain showing self-limiting oxidation. These results were associated with the evolution of the morphology, the microstructure and the composition gradients of the coatings before and after oxidation, and these changes were investigated by SEM, TEM, SIMS and XRD. It is concluded that coatings produced by this process show promise for use as effective protection against oxidation of Ti6242 alloys and consequently may raise the maximum operating temperature tolerated by corresponding parts.

Owing to its high electrical breakdown field, high thermal conductivity and excellent chemical inertness, the wide bang-gap silicon carbide (SiC) semiconductor presents outstanding properties for power, microwave and sensor electronic applications. Because monocrystalline SiC can, under practical conditions, only be grown from the vapour phase at high temperatures, the growth of high quality SiC crystals is a challenging and exciting field.

In this talk, we review the development of the High Temperature CVD (HTCVD) SiC crystal growth technique. Simply described, the technique is based on the continuous feeding of Si- and C-bearing gas precursors to a seed crystal maintained at a sufficiently high temperature (1900-2300 °C) to enable homoepitaxial growth rates of interest for bulk growth applications. The HTCVD process can, however, differ substantially from conventional SiC epitaxy owing to the possible presence of homogeneous gas phase nucleation and the need to accommodate the length of the growing crystal. Examples of factors affecting the growth rate and the quality of the grown material are presented. The properties of 2 and 3-inch diameter wafers sliced from HTCVD grown crystals are discussed. In high purity semi-insulating wafers, the growth conditions can for example affect the incorporation of intrinsic deep levels such as vacancies and antisites. Issues relevant to the growth of conducting n- and p-type wafers are presented, such as the capability of the technique to produce low micropipe density p-type wafers by the use of a metal-organic source for Al doping.

Titanium nitride (TiN) coatings have been extensively used for tribological applications, corrosion protection, decoration, biomaterials, etc. However, for certain applications the properties of single TiN coatings must be improved; generally the hardness and the adhesion to the substrate need to be increased and the coating porosity reduced. Among other strategies, this can be done by the synthesis of multilayered coatings in which the TiN layers alternate with metal layers, or by the formation of nanocomposite coatings where a phase of TiN is combined with other nitride phases (for example, AlN or Si₃N₄).

In this work we have studied the synthesis of TiN/W and TiN/Ta multilayered coatings by Chemical Vapor Deposition in a Fluidized Bed Reactor at Atmospheric Pressure (AP/FBR-CVD). It is well known that the good heat and mass transfer of the fluidized bed system yield very homogeneous coatings. Now, our results show for the first time that AP/FBR-CVD can be used for the deposition of multilayered coatings with periodicities below 100 nm.

First, the conditions for the deposition of every single layer were investigated. TiN coatings were deposited from titanium tetrachloride and ammonia by AP/FBR CVD at temperatures in the range of 750 °C to 850 °C. Interestingly, we found that an activation of the bed of particles through alternating reaction steps lead to an increase of the TiN deposition rate, due to the in situ formation of titanium subchlorides. The metal layers were obtained by reduction of tungstene chloride or tantalum chloride with hydrogen. The mechanical and tribological properties of the TiN/W and TiN/Ta multilayered coatings are presented.