Structure and mechanical properties of polycrystalline CrN/TiN superlattices

Polycrystalline CrN/TiN superlattice films were deposited on M1 tool steel using unbalanced reactive magnetron sputtering with opposed cathodes. The Cr and Ti targets were sputtered in Ar-N(2) mixtures with partial pressure control of the N(2) As the N(2) partial pressure was increased from 0.1 to 1.1 mTorr, TiN(x) films went from stoichiometric B1-cubic TiN to slightly overstoichiometric TiN, while CrN(x) films went from cubic Cr-N solid solutions to hexagonal Cr(2)N to B1-cubic CrN. Since the N(2) partial pressure required to form stoichiometric CrN was approximate to 10 times that required to form stoichiometric TIN, nitrogen was inlet at the Cr target position to maximize the difference in N(2) partial pressures. Two series of CrN/TiN superlattices, with TIN fractions of 0.4 and 0.6, were deposited with periods ranging from 2 to 60 nm. X-ray diffraction showed a very strong (111) texture with first-order satellite peaks around the (111) Bragg peak. Kinematical diffraction simulations of the superlattice x-ray patterns indicated a strong composition modulation and a significant fluctuation in d-spacing that was related to ion bombardment defects. Cross-sectional transmission electron microscope images showed a columnar film structure with well-defined superlattice layers. Nanoindentation of 2-mu m-thick CrN/TiN samples showed a maximum hardness of 35 GPa at a period of 2.3 nm, compared to 25 GPa for TiN and 14 GPa for CrN films. The maximum superlattice hardness was thus approximate to 75% larger than the rule-of-mixtures value. The hardness enhancement mechanisms are discussed. (C) 1998 American Vacuum Society. [S0734-2101(98)01705-9].