X. Chu

Model of superlattice yield stress and hardness enhancements

A model is presented that explains the yield stress and hardness enhancements that have been observed in superlattice thin films. The stress required for dislocations to glide across layers with different shear moduli was calculated using an expression that accounts for core effects and all interfaces in trapezoidal or sawtooth composition modulations. The predicted strength/hardness enhancement increased with increasing superlattice period Λ, before reaching a saturation value that depended on interface widths. A second mechanism, where dislocations glide within individual layers, was important at large Λ and gave a decrease in strength/hardness with increasing Λ. The combination of these two mechanisms gives a strength/hardness maximum versus Λ in good quantitative agreement with experimental results for nitride and metal superlattices. The results indicate that superlattice strength/hardness depends strongly on interface widths and the difference in shear moduli of the two components for Λ values below the maximum, and on the average shear modulus for larger Λ.

Deposition and properties of polycrystalline TiN/NbN superlattice coatings

Polycrystalline TiN/NbN superlattice coatings were deposited on M2 tool steel substrates using an opposed dual‐cathode unbalanced magnetron sputtering system. Superlattice deposition was achieved by placing the substrates on a cylindrical holder that rotated on an axis equidistant between, and parallel to, the faces of opposed Ti and Nb targets. Cross contamination of the targets and the alternating superlattice layers was minimized using a baffle or an extra‐large cylindrical substrate holder. The superlattice period was determined by the substrate holder rotation speed. Analytical techniques including x‐ray diffraction, energy‐dispersive spectroscopy and transmission electron microscopy were used to characterize the structure of the superlattice coatings. Microhardness values for the superlattice coatings as high as 5200 kg/mm2 Hv0.05 have been achieved, comparable to the reported highest hardness values of single crystal TiN/VN, TiN/V0.6Nb0.4N and TiN/NbN superlattice coatings. The results indicate that the hardness of the polycrystalline TiN/NbN superlattice coatings is affected not only by superlattice period, but also by nitrogen partial pressure and ion bombardment during deposition.

Mechanical properties and microstructures of polycrystalline ceramic/metal superlattices: TiN/Ni and TiN/Ni0.9Cr0.1

Abstract Polycrystalline superlattice coatings of TiN/Ni and TiN/Ni 0.9 Cr 0.1 of thickness 2–3 μm were deposited onto tool steel substrates using an opposed-cathode reactive unbalanced magnetron sputtering system. The TiN/Ni superlattices have repeated periods (Λ) from 1.8 to 62 nm with various Ni layer thickness (l Ni ) to Λ ratios, l Ni /Λ. TiN/Ni 0.9 Cr 0.1 superlattices have Λ =1.2–7.4 nm with l NiCr / Λ =0.3. The structures were analyzed by X-ray diffraction, as well as plan-view and cross-sectional transmission electron microscopy. The hardnesses and elastic moduli of the superlattices were measured using nanoindentation techniques. An increase in hardness with decreasing Λ and grain size was observed. A maximum hardness of 3500 kgf mm −2 for TiN/Ni, about 1.5 times of the rule-of-mixtures values, was found at l Ni / Λ =0.16 and Λ =2.2 nm. For TiN/Ni 0.9 Cr 0.1 , a maximum hardness of 3200 kgf mm −2 was found at l Ni / gL =0.3 and Λ =1.2 nm. No significant variations in TiN/Ni 0.9 Cr 0.1 film modulus were found as a function of Λ.

Deposition, structure, and hardness of polycrystalline transition-metal nitride superlattice films

Polycrystalline TiN/VN, NbN/VN, and TiN/NbN superlattices with periods {Lambda} between 2 and 160 nm were deposited onto steel substrates using an opposed-cathode reactive magnetron sputtering system. The nitrogen partial pressure and the substrate bias values were optimized in order to obtain dense stoichiometric films, which yielded the highest Vickers hardnesses H{sub V}. H{sub V} for TiN/VN and TiN/NbN superlattices reached maximum values of {approx}5000 kgf/mm{sup 2} at {Lambda}{approx}5{endash}10hnm, compared with {approx}2000 kgf/mm{sup 2} for homogeneous TiN, NbN, and VN films. In contrast, H{sub V}{approx}2000hkgf/mm{sup 2} was obtained for VN/NbN superlattices independent of {Lambda}. Model calculations in which the hardness enhancement was proportional to the difference in layer shear moduli gave good agreement with the data. The lack of hardness enhancement in VN/NbN indicates that any other hardening mechanisms, such as coherency strains and dislocation blocking by interfacial misfit dislocations, were not important. {copyright} {ital 1999 Materials Research Society.}

Stabilization of cubic CrN0.6 in CrN0.6/TiN superlattices

A transmission electron microscopy study of CrN0.6/TiN superlattices deposited by reactive magnetron sputtering is described. The stable structure of CrN0.60 is hexagonal, but high resolution transmission electron microscopy images of the superlattices showed that CrN0.6 layers ⩽10 nm thick were cubic, while 50 nm thick layers were hexagonal. That is, the cubic CrN structure was “epitaxially stabilized” by the cubic TiN, with which there is a 2.4% lattice mismatch. The superlattices with hexagonal CrN0.6 showed high strains and defect densities within ≈5 nm of each interface, presumably due to the 5.4% volume decrease associated with the cubic-to-hexagonal transformation. The effect of this strain on the transformation is discussed.

Reactive magnetron sputter deposition of niobium nitride films

The high‐rate reactive magnetron sputtering process by controlling the partial pressure of the nitrogen gas was used to deposit niobium nitride films. Despite the complexity shown in phase diagram of Nb–N, only three different crystalline phases, Nb metal, cubic δ‐NbN, and hexagonal δ’‐NbN, were observed in the parameter space explored by sputtering a niobium target in an atmosphere of argon and nitrogen. The effects of three deposition parameters including nitrogen partial pressure, target power, and substrate bias voltage were explored. All the deposition parameters affected the formation of different phases, the preferred orientation, and the relative amount of each phase formed, which, in turn, affected the properties of the coatings. The hardness of these reactively sputtered niobium nitrides ranges between 1700 and 4100 kgf/mm2 HV0.025. The highest hardness is significantly higher than the reported hardness value, 1400 kgf/mm2, for bulk niobium nitride, and the primary factor for the hardness increm...

Reactive magnetron sputter deposition of polycrystalline vanadium nitride films

Polycrystalline vanadium nitride films were deposited onto M2 steel substrates using a high‐rate reactive dc magnetron sputtering system by sputtering vanadium metal in an Ar+N2 atmosphere under N2 partial pressure control. The crystal structure, surface morphology, and properties of the films were affected by several process parameters such as nitrogen partial pressure, target power, and negative substrate bias. Analytical techniques including x‐ray diffraction and scanning electron microscopy were used to characterize the structure and morphology of the films, and the mechanical properties of the films were measured by a Vickers microhardness and a scratch adhesion testers. The nitrogen partial pressure was found to be the dominant deposition parameter for the formation of different phases which includes crystalline V metal, hexagonal β‐V2Nx, and cubic δ‐VNx, amorphous V–Nx solid solution, and their mixtures. The film hardness was affected by crystalline phase, and a maximum hardness of 3000 kgf/mm2 Hv0...

A Model of Superlattice Yield Stress and Hardness Enhancements

A model is presented that explains the yield stress and hardness enhancements that have been observed in superlattice thin films. The predicted strength/hardness enhancement increased with increasing superlattice period, Λ, before reaching a saturation value that depended on interface widths. The results indicate that superlattice strength/hardness depends strongly on interface widths and the difference in shear moduli of the two components for Λ values below the maximum, and on the average shear modulus for larger Λ.

Superhard Nanocomposite of Nitride Superlattices by Opposed-Cathode Unbalanced Magnetron Sputtering

Nanocomposite films of polycrystalline nitride superlattices with nanometer grain size were deposited onto tool steel substrates by a one-step process using an opposed-cathode high-rate reactive unbalanced magnetron sputtering system. The superlattices are composed of alternating thin layers of different nitrides such as TiN/NbN and TiN/VN. The thicknesses of two neighboring layers were between 3 and 150 nm and were determined by the rotating speed of the substrate holder and the sputtering rate of the individual layer material. The films exhibited exceptional hardness as high as 5200 kgf/mm 2 for TiN/NbN superlattice and 5 100 kgf/mm 2 for TiN/VN superlattice. The hardnesses of the superlattice coatings were strongly dependent on several deposition parameters such as the superlattice period, the nitrogen partial pressure, and the substrate bias voltage. One of the possible mechanisms for the hardness enhancement is the effect of nanophase materials, which were created mainly by the influence of the artificially layered-structure and the low energy ion bombardment during film growth.