Structure–performance relations in nanocomposite coatings

Properties of hard nanocomposite coatings, especially hardness, can be explained by their nanostructure. Hardness maxima are found for different nanocrystalline/amorphous materials deposited by different techniques at typically 20% of the amorphous phase. A zone model is proposed which correlates the hardness to the relative phase content. The hardness of nanocomposite coatings peaks at the common minimum of the grain size of the crystalline phase and the grain separation. For an adequate description of the performance of a coating, the thermal stability, oxidation behavior and frictional behavior should be included in addition to hardness. In a friction situation involving at least two friction partners, the overall behavior of the system is determined by many-body interactions. While thermal stability and oxidation properties as inherent material properties can be directly linked to the nanostructure of the coating, the frictional behavior of a coating cannot be generalized independent of the friction conditions.