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.
Fig.1. Frictional curves of(a) worn volume loss vs. load and(b) friction coefficient vs. load on M2 steel, Al2O3 coating and composite coating S2.
An important toughening mechanism was presented by Chakraborty et al. The dissipation of energy originated from the plastic deformation of metal second phase was considered as the most essential toughening factor. It is imagined that dispersed metal particles can service as a lot of micro-absorbers. When loads were put on the coatings, the micro-absorbers would plastically deform in some degree to counter work microcrack appearance.Furthermore, it is believed that the metal phase can work as a bridge between the ceramic matrix to buffer and deflect microcrack propagation. as shown in Fig.2(b). Accordingly, the microcracks had to walk a farther route when they get across the metal phase. Unlike the monophase ceramic material, the in situ synthesized metal second phase is likely to lead to complex fracture behavior that mixed intergranular and transgranular fracture modes(Fig.2(b)) due to its composite microstructure mixed metal and ceramic phase. Additionally, the relatively higher fraction and homogeneous dispersion of metal phase in the composite coating also devote to toughness enhancement.
Fig. 2. Failure models of (a) ceramic monophase and (b) composite coatings.
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