The microhardness values under 100g load corresponding to the coatings S1,S2,S3 and S4 are 792 HV100g,877 HV100g,777 HV100g and 769 HV100g, respectively. The coating S2 exhibits better microhardness than the others, which can be attributed to more uniformly dispersed Fe-rich phase and dense microstructure.However, its microhardness value at 100 g load is a little lower than that of Al203 monophase coating (1100 HV1oog), which is easily explained by the formation of low hardness metal second phase.Therefore, the composite coating S2 was selected to further perform indentation observation and friction test due to its higher microhardness.
A load dependence of microhardness of the coating S2 is shown in Fig.1. It is clearly found that the Vickers microhardness of the coating S2 is slowly dropped down from 877 HV100g to 719 HV500g with the increase of applied loads from 100 to 500g. This phenomenon can be explained by the indentation size effect.
Fig.1.Dependence of Vickers microhardness values on different loads for the composite coating S2.
The elastic deformation was primarily made as loading, followed by producing a plastic deformation as stress exceeded elastic limit.It is noted that the elastic deformation recovery always occurred in the vicinity of the remaining indentation impression , thus,the indentation size would be reduced in some extent after removing the indenter. The plastic deformation degree corresponded to different applied loads, so a higher load had to produce larger indentation and plastic deformation, resulting in a relatively lower microhardness value.
Usually, the indentation morphology can reveal the coating toughness from another angle. It is clearly seen that the indentation of Al2O3 coating appears a brim warp, and some microcracks are also formed, as shown in Fig.2(a), which suggests that the coating took place the brittle fracture failure under the effect of load(100g). However, the distinct indentation shape in Fig.2(b) illustrates that the composite coating displays better toughness, and neither fracture nor microcracks appear.
Fig.2. Indentation morphologies of (a) Al2O3 coating and(b) the composite coating S2 under 100g.
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