Thermal stability of superhard nanocomposite coatings consisting of immiscible nitrides

We discuss the possible origin of the high thermal stability of our superhard nanocomposites, prepared according to the generic concept for their design published earlier. It will be shown that in systems which show a strong thermodynamically driven segregation (i.e. they are immiscible), the nanostructure and resulting superhardness (measured at room temperature) remain stable up to high temperatures of greater than or equal to 1100 degreesC. Spinodal decomposition which occurs during deposition leads to spontaneous formation of such nanostructures. The latter is fulfilled for plasma CVD of binary nanocomposites, such as nc-TiN/a-Si3N4, W2N/a-Si3N4, nc-VN/a-Si3N4 and others deposited at a relatively high pressure of greater than or equal to 1 mbar and high plasma density. In the case of plasma PVD at much lower pressure and temperature, formation of the nanostructure is not fully completed during deposition. Upon annealing, such nanocomposites show a complex structural relaxation, accompanied by an increase of the hardness, after which they remain stable up to a high temperature of greater than or equal to 1000 degreesC. This will be demonstrated by (Ti1-xAlx)N/a-Si3N4 nanocomposite coatings. Finally, we discuss the complex issue of the stability of ternary and quaternary nc-TiN/a-Si3N4/a- and nc-TiSix nanocomposites. (C) 2001 Elsevier Science B.V. All rights reserved.