The micro-hardness (HV0.2) along the depth direction of the coating
was measured with a load of 1.96 Nand dwell time of 15 s using a
HXD-1000B micro hardness tester. Before micro-hardness testing,
the standard sample was used for calibration, and each value of
the micro-hardness was the average of three measurements. The
fracture toughness of the coatings was measured by the Vickers
indentation method. Vickers indentations were prepared on the
cross section of the coating with a load of 30 kg and dwell time of
15 s using a HBRV-187.5 Vickers-hardness, and morphologies
of indentations were observed using S-3400 scanning electron
microscope equipped (SEM). Base on fracture mechanics theories
of indentation, the driving force for crack propagation was caused
by the load pressure, so fracture toughness of the coatings was evaluated
by observing the length of the indentation crack which acted
as an indirect evaluation [31–34]. To ensure the reliability of the
results, the standard sample was used for the calibration before testing,
and fracture toughness of the coating was evaluated by using
multiple measurements way to accomplish