Apparently, there is always a paradox between hardness and toughness. However, the high hardness does not predicate good properties. The ceramic coating is vulnerable to fatigue and delamination under mechanical load effect due to its intrinsic brittleness.For example, the undesirable brittleness of Al2O3 coating considerably degrades its wear resistance(see Fig.1(a)) which can be assumed to local high stress resulting in microcrack formation and propagation, though it possesses higher microhardness compared with the composite coating. Usually, the ceramic coating is inclined to take place intergranular fracture failure under load effect, as is shown in Fig.2(a). In contrast, the preferable toughness of the composite coating remarkably improves its wear resistance, because the in situ synthesized metal second phase particles dispersed within the coating can efficiently prevent the stress concentrtion and absorb the fracture energy. Rosso introduced a fact that when metal particulate-reinforced ceramic suffered from frature failure, it had to absorb more fracture energy than monophase ceramic by observing their stress-strain curves.
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