Over the past 20 years, numerous studies have been devoted to the development and understanding of enhanced hardness of coatings in the form of nanolayers and nanocomposites. In many cases, hardness in excess of 40 GPa can be reached. For all hard coatings reported in the literature, at least one component of the coating is a ceramic material. As is well known, ceramic materials have high hardness, which improves their resistance against abrasive wear. However, hardness does not always correlate with wear resistance. Other parameters such as H/E and H3/E2 have been used to correlate with wear resistance, and some correlations appear to be good. However, under conditions in which wear is determined by chemical factors, e.g., interactions with air or lubricant and grain boundaries embrittled by segregation of impurities, we cannot rely on purely mechanical parameters to predict wear resistance. When tested under conditions in which ceramic materials in coatings are susceptible to flaws and fracture because of their brittleness, fracture toughness is an important attribute for such coatings with unavoidable intrinsic flaws or when such flaws are created under loading.
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