Nano-cBN

Most of the reports on h-BN deposition during the last years are in combination with the low-pressure nano-cBN deposition by PVD methods because a h-BN interlayer is formed before the c-BN is able to nucleate. 

Deposition of nano-cBN films is possible with ion-assisted CVD and ion-assisted PVD techniques. Since the c-BN crystal size in such layers is in the range of nanometers, such layers should be called nano-cBN. The amount of grain boundaries in such materials is rather high and this fact should not be negated by using the notation pure c-BN . 

In most cases nano-cBN deposition is supported by the generation of ions. Parameters for the substrate bias (ion energy, ion mass, etc. [187]) are similar to that in PVD as well as in CVD methods. Therefore a equal growth mechanism in both methods can be considered. When starting the deposition process, commonly an oriented h-BN layer is deposited on the substrate. On... 

To describe the nano-cBN deposition four models have been proposed the compressive stress model [190, 191], the sub-plantation model [192, 193], the selective sputter model [194], and the momentum transfer model [195],... 

Another possibility to explain the ion assisted c-BN deposition is the subplantation model. The nucleation of c-BN crystals takes place under the surface of the substrate caused by sub-plantation of the ions and stress. The sub-plantation and high nucleation rates result in the nano-cBN coatings. 

The basic concept of the c-BN nucleation by ion-beam-deposition has been described by Weissmantel et al. in 1980 [197, 198]. Adhesion problems and difficulties in analytical characterization have caused large problems. A breakthrough of nano-cBN coatings and their various applications seems to be possible. 

The methods mostly used for nano-cBN deposition are ion-beam-assisted deposition (IBAD) [199] mass selected ion beam deposition (IBD) [200] ion plating [201] RF- or magnetron sputtering [202] and laser deposition [203] (Fig. 13). 

Fig. 13. Dependence of the nano-cBN deposition on deposition parameters by IBAD experiments. The results of literature data are summarized for the parameters ion energy and substrate temperature...

This method uses a laser for evaporating the boron compounds from a target (e.g., BN target). To regulate the flux of BN to the substrate the laser can be pulsed. Additionally, a nitrogen/argon ion-beam is generated and directed onto the substrate s surface. Various layers have been deposited, and the BN transition from substrate to the h-BN interlayer and finally to the nano-cBN has been studied in detail [203],... 

Using conventional thermal CVD various BN modifications but no c-BN or nano-cBN are formed. Therefore, to synthesize c-BN a plasma is applied to... 

Mendez et al. [231] showed in deposition experiments with RF plasma and without substrate bias that there is a coherence of the nano-cBN amount with the substrate temperature and the plasma power. Additionally a dependence on the substrate material used (e.g. Si or NaCl) could be found. Further... 

Matsumoto et al. [233] tried a bias assisted plasma jet with the reactive gas mixture Ar-N2-BF3-H2 for selective etching of h-BN [234]. Using a d.c. bias voltage, the nano-cBN grain size in the layers could be increased from 7 nm (-150 V) to 12 nm (-80 V) [235], By optimizing the process, nano-cBN coatings thicker than 20 pm with c-BN grain size up to 100 nm could be deposited [230],... 

Matsumoto and co-workers showed that the deposition of thick nano-cBN layers is possible and the adhesion on Si substrates is of acceptable quality [230]. The one major question is whether this process is also suitable for hardmetal (WC-Co) substrates or not, and whether there might be any problems with the Co binder phase and the BF3 in the gas phase ... 

The stress in c-BN layers caused by the high energetic ions is a problem, because many of the nano-cBN coatings delaminate during or immediately after deposition. Several investigations have showed ways to reduce the stress in the layers (e.g. buffer layers [205, 217, 236], regulation of the ion energy [207], or ion-induced stress relaxation [206]). 

The outlook for industrial applications of nano-cBN coatings looks quite good, but some problems with the deposition process and the substrate materials still have to be solved. 

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