SiC interlayer

Fig. 9 Coefficients of friction versus wear cycles for2.5 jim DLC/0.02 fim SiC interlayer coating on T1-6AI-4V, showing reduction in i with increased load. Sliding material, WC-Co sliding speed, 2 mm/s (0.08 in7s).

For typical sample of shFRC, alumina fiber/ alumina composite with SiC interlayer was prepared in this study. The used alumina fiber (Nitivy Co. Ltd., S-640D) is 3000 filaments bundle with diameter of 0.3 mm. Fine alumina (Sumitomo Chemical Co. Ltd., AKP-50, rjO.23 Gm) and SiC (Ibiden Co. Ltd., Ultrafine, 00.35 Dm) powders were used as matrix and interlayer, which plays role in healing agent. 

A candidate interlayer consisting of dual coatings of Cu and Nb has been identified successfully for the SiC-Ti3Al-I-Nb composite system. The predicted residual thermal stresses resulting from a stress free temperature to room temperature (with AT = —774°C) for the composites with and without the interlayers are illustrated in Fig. 7.23. The thermo-mechanical properties of the composite constituents used for the calculation are given in Table 7.5. A number of observations can be made about the benefits gained due to the presence of the interlayer. Reductions in both the radial, and circumferential, o-p, stress components within the fiber and matrix are significant, whereas a moderate increase in the axial stress component, chemical compatibility of Cu with the fiber and matrix materials has been closely examined by Misra (1991). 

These problems concerning a micron-scale deposit and an in volume substrate have rarely been studied in the past. However, from a survey of literature on cases other than that of SiC on steel, it was found that one or several interlayers may change the stress levels in the ceramic. Moreover, once a critical thickness has been reached, these interlayers have a beneficial effect. Some work has been carried out on TiN/ SiC layers on a variety of substrates. However, in all these studies, information on mechanical stability is fairly limited. Concerning the PECVD of SiC on cutting tools coated with TiN, adherence variations in relation to pretreatment processes were reported. ... 

Recently, the introduction of a metallic interlayer between a coating deposit and its substrate and its effects on mechanical properties were studied. The authors experimented with a multilayer approach a metallic layer sandwiched between two highly reactive layers. For the SiC system, no previous reference on the interposition of a ductile layer between the deposit and the substrate was found, and that motivate the study reports here. 

Subsequent preliminary comparative studies of the behavior of an SiC based layer on Ta, Mo, Ti and steel substrates showed that better mechanical stability was obtained with a coating deposited on tantalum. This element was consequently considered to make PFCVD deposit/interlayer/steel stacks. Tantalum can be produced by physical vapor deposition (PVD), at variable thickness, with reproducible morphology. Note that preparation by chemical vapor deposition with or without plasma assistance (CVD or PECVD) is possible at low temperature but would require an optimization study in order to be compatible with the deposition conditions of the silicon carbide layer, the aim being to increase the mechanical stability. 

Fig. 18 Schematic representations and micrographs of damage case of the SiC / steel reference system and SiC/Ta/steel triple layers (x corresponds to the Ta interlayer thickness in microns).

Fig. 20 Critical scratching load L, macro-indentation failure load F and residual stresses SiC in the triple layer system, e. corresponds to the thickness of the Ta interlayer.

The methods applied to assess mechanical stability give proof of the beneficial role of the interposition of a PVD tantalum layer between the PECVD deposited ceramic layer and the substrate a factor of 2 gain in strain and in critical load with respect to the SiC/steel pair. However, it is nonetheless clear that the optimum thickness of the interlayer could not be established with certainty. At the present time, there does not appear to be any experimental explanation for this. The only factor that might be stressed is... 

In Refs. [303-305], the interface structure was also investigated by cross-sectional HRTEM. The diamond films were grown by the three-step process, and the conditions are listed in Table H.3. Consequently, an HOD film was grown in the center of the Si(lOO) substrate. In the carburization step, there was an a-C film of 250-nm thickness on Si, in which p-SiC, diamond, and graphite were embedded. A closer examination indicated that there existed an interlayer of 1.5- to 2-pm thickness between Si and the a-C layer, which was identified as a-SiC [305]. Since the bias voltage is not usually applied uniformly across the Si wafer, the distribution of these materials depended on the location on the Si substrate. Near the edge of the... 

Another interesting aspect of the simulation results concerns the location of the counterions in the interlayer space. The (001) perspective of simulated Cs+-mont-morillonite (Fig. 8- 4) shows that, at a water content [ 150 g H20 kg-1] approximately corresponding to a monolayer, the cations always adopted locations such that they were aligned with the center of a siloxane ditrigonal cavity on the basal surface of one clay layer and with the base of an SiC>4 tetrahedron on the basal surface of the opposing clay layer. At lower water contents this configuration was not stable, and the interlayer counterions tended to align themselves with the centers of siloxane cavities on the basal surfaces of both clay layers. 

Vadali. V. S. S. Srikanth, H. A. Samra, T. Staedler and X. Jiang, Nanocrystalline diamond/p-SiC composite interlayers for the deposition of continuous diamond films on W and Mo substrate materials, Surf. Coat. Technol, 201(22-23), 8981-85 (2007). 

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