coatings, for examples, Al2O3 [20] and hydroxyapatite (HA) [21]. The
results showed that Vickers hardness of Al2O3/CNT nanocomposite
coating was 906±6 HV0.2 while the hardness value of a pure Al2O3
coating was about 100 HV0.2 lower. In addition, slightly enhanced
fracture toughness from 4.14 to 4.62 MPa m−1/2 due to CNT addition
was also observed. The fracture toughness of HA coating was found to be
improved over 50% with an addition of only 4 wt.% CNTs. There was also
a study which used CNTs as reinforcement in thermal sprayed Al–Si
alloy coating. The hardness of this Al–Si/CNT nanocomposite coating
was found to be 146 HV0.3 which was much higher than that of
unmodified Al–Si coating (i.e. 85 HV0.3) [22]. In addition, tensile
properties of CNT reinforced Al–Si nanocomposite was studied and it
was found that the elastic modulus of plasma sprayed Al–Si/CNTs was
improved by ~78% and strain to failure was reduced by ~46% compared
to those of the plasma sprayed Al–Si sample [23]. Structural transformation
in CNTs during thermal spray processing such as plasma spray
(PS), high velocity oxy-fuel spray (HVOF), cold spray (CS) and plasma
spraying of liquid precursor (PSLP) was also investigated by Keshri et al.
[24]. It was found that some of these processes produced kinked and/or
fractured CNTs in the coatings due to high impact of molten particles on
the substrate during coating formation. The relatively high flame
temperature involved in plasma spray was also an induced reaction
between CNTs and molten Al–Si particles, resulting in a formation of SiC
layer in Al–Si/CNT coatings [24,25].