Beside CNT redistribution, some CNTs could be partially damaged
during spraying due to high deposition temperature of flame
~3000–3350 °C [29]. During flame spray, in-flight SS/CNT composite
powder was subjected to heat only for a very short period of time
considering the short distance used in this experiment. Nevertheless,
partial loss of CNTs due to oxidation could occur during this spraying
process as supported by the SEM micrographs that CNTs in the coating
were much shorter than those on original SS particles. From the SEM
fracture surface, the remaining CNTs seemed to be embedded within
the splat as well as form a linkage between splats. Considering the
much higher melting point of CNTs (i.e. N2300 °C [30]) compared to
that of SS (~1400 °C) coupled with the unusually high thermal
conductivity (~3000 W/mK [31]) of CNTs compared to the value of
~19 W/mK of SS, it was likely that stainless steel melted and engulfed
the CNTs within its liquid volume before hitting the substrate. There
might be also some structural damage to CNTs during coating
formation but considering the high strength of CNTs and lower in-
flight particle velocity during flame-spraying compared to those in the
case of cold-spray or plasma-spray processes [24], this type of
mechanical damage should be minimized in this case.
The results of phase analysis of coatings by X-ray diffraction
method shown in Fig. 11 were found to be similar for both coatings.
The pattern comprised mainly of Fe as observed for feedstock
powders before spraying (see Fig. 6). Results of EDS analysis shown
in Fig. 12 indicated that carbon content detected in the composite
coating was ~6 wt.% while only 2.5 wt.% was obtained for stainless
steel coating. This seemed to confirm an incorporation of CNTs in the
composite coating, which also corresponded to the observed SEM
micrographs. Although the carbide formation at SS/CNT interface was
possible, based on our available data, the amount could be very small
and the carbide might only exist as thin layer in the order of nanometer
size at the interface similar to SiC layer formed due to reaction
between Si and CNT in the Al–Si/CNT composite investigated by Laha
et al. [25]. However, considering the much lower temperature
involved in the flame spray system compared to that of the plasma
spray (PS) and high velocity oxy-fuel spray (HVOF) system used by