William D. Sproul

Nanometer scale multilayered hard coatings

Abstract Multilayer coatings with layer thicknesses in the nanometer range have been shown to exhibit extremely high hardnesses, making them useful as abrasion-resistant coatings. Hardness values in excess of 5000xa0kg/mm2 have been achieved in multilayers composed of two nitride materials, such as TiN and VN, with bilayer periods of 5–10xa0nm. This article begins with a review of the deposition of multilayered coatings by reactive sputtering and the characterization of these coatings by transmission electron microscopy and X-ray diffraction. The hardness enhancements observed in both isostructural and non-isostructural nanometer-scale multilayers are then reviewed. Several explanations for this enhancement in hardness have been developed in order to understand the hardening process and to develop coatings with even higher hardness. Models based on dislocation motion within layers and across layer interfaces, as well as Hall–Petch-type models, have successfully been applied to multilayers to explain this hardness enhancement, and they are briefly outlined.

Analytical electron microscopy and Raman spectroscopy studies of carbon nitride thin films

Deposition of carbon nitride thin films on Si(100) and polycrystalline Zr substrates was performed by dc magnetron sputtering of a graphite target under a pure nitrogen ambient. The resulting carbon nitride films (CNx) are primarily amorphous with a small volume fraction of nanocrystallites. Both energy‐dispersive x‐ray analysis and electron energy loss spectroscopy measurements showed that the amorphous matrix has uniform nitrogen‐to‐carbon ratios ∼0.4–0.8 depending on deposition conditions. Carbon and nitrogen K edge structures obtained from electron energy loss spectroscopy studies suggest that the amorphous carbon nitride matrix is primarily sp2 bonded. Apart from the carbon–nitrogen stretching vibration, Raman spectra of CNx films closely resemble those of diamondlike carbon films. Intensity and peak width changes of Raman features in the 1300–1600 cm−1 range due to inorganic carbon (D and G peak) under different deposition conditions are explained in terms of the extent of structural disorder in the...

Nano‐indentation studies of ultrahigh strength carbon nitride thin films

Carbon nitride (CNx) thin films were prepared by dc magnetron sputtering of a graphite target in a nitrogen ambient onto Si(100) substrates held at ambient temperatures. The films are amorphous with a small volume fraction of nanocrystallites. All CNx coatings grown to a thickness of 1.5 μm are adherent and smooth. Nanoindentation studies showed clear dependence of hardness and effective modulus on deposition process parameters (nitrogen pressure, target power, and substrate bias). Most striking is the observation that CNx films can be synthesized with yield strength exceeding 5 GPa.

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.

Reactive unbalanced magnetron sputter deposition of polycrystalline TiN/NbN superlattice coatings☆

An opposed dual-cathode unbalanced magnetron sputtering system was used to deposit polycrystalline TiN/NbN superlattice coatings, with periods between 2.5 nm and 150 nm, on M2 tool steel substrates. Analytical techniques including X-ray diffraction. Auger spectroscopy and transmission electron microscopy were used to characterize the structure of the superlattice coatings. These showed that well-defined TiN and NbN layers were obtained, but the interfaces were not perfectly planar or abrupt. The mechanical properties of the superlattice coatings were characterized using a Vickers microhardness tester and scratch tester. Microhardness values for the 6 μm thick superlattice coatings ranged from 1800 to 5200 kgf mm−2 (Hr 0.05) and were strongly dependent on several deposition parameters such as superlattice period λ, nitrogen partial pressure, the ratio of gas flow to each target, and the negative substrate bias Vs. The highest hardness values were obtained for λ≈6 nm and Vs=150 V. The scratch adhesion critical load of superlattice coatings ranged from 1.8 kgf at Vs=200 V to 11 kfg at Vs=34 V.

Use of an externally applied axial magnetic field to control ion/neutral flux ratios incident at the substrate during magnetron sputter deposition

The development and characterization of an ultrahigh vacuum ‘‘unbalanced’’ dc magnetron sputter deposition system with a variable external axial magnetic field for controlling the ion‐to‐neutral flux ratio at the substrate during deposition with low negative substrate biases is reported. The target assembly is a planar‐magnetron (PM) with a toroidal magnetic‐field electron trap created using a set of permanent magnets. A pair of Helmholtz coils, located outside the vacuum chamber, produces an additional magnetic field Bext which is uniform along the axis orthogonal to both target and substrate surfaces. The value and sign of Bext has a strong effect on the plasma density near the substrate, and hence on the ion flux Ji incident at the substrate, with only a minor effect on the target‐atom flux. For a Ti target sputtered in pure Ar at 20 mTorr with a target‐substrate separation of 6.5 cm, changing Bext from −50 G (opposing the field of the outer PM pole) to +600 G (reinforcing the field of the outer PM pol...

Synthesis of superhard carbon nitride composite coatings

Crystalline carbon nitride/TiN composite coatings have been deposited using a dual‐cathode magnetron sputtering system onto polished silicon and M2 steel substrates held at room temperature. We propose that TiN provides a lattice‐matched structural template to seed the growth of carbon nitride crystallites. The resulting coatings are smooth, fully crystalline, with nanoindentation hardness in the range of 45–55 GPa. This hardness is in the low‐end range of diamond films. Suggestions for better seeding materials to further improve the hardness are proposed.

Physical vapor deposition tool coatings

Abstract Physical vapor deposition (PVD) of hard coatings such as titanium nitride have been an industrial reality since the beginning of the 1980s. Two PVD processes, low voltage electron beam and cathodic arc deposition, were responsible for the early commercial success of hard coatings on high speed steel tooling. Since that time, two other PVD processes have also been prosperous in the industrial world—high voltage triode electron beam and unbalanced magnetron sputtering. There are many similarities and differences between these four PVD hard coating processes, but not all of the commonly used PVD hard coatings can be deposited well in the four systems. Titanium nitride and titanium carbonitride are the two most widely used PVD tool coatings, and they can be deposited in all four PVD systems. Titanium aluminum nitride can be deposited easily with the unbalanced magnetron process and also with the cathodic arc processes as long as cast targets are used. Uniform composition cannot be maintained with the electron beam processes because of the different vapor pressures of titanium and aluminum. New PVD hard coatings are being developed and applied. Diamond-like carbon is now being tried for some non-ferrous cutting applications, and metal-carbon films are showing promise. Polycrystalline nitride superlattice coatings made of thin alternating layers of two hard coatings such as titanium nitride and niobium nitride are showing potential for abrasive cutting situations because of the very high hardness (up to 5200 HV) of these coatings. The search continues for potentially even better tool coatings. An intense effort is underway to produce the superhard crystalline carbon nitride, which is predicted to have a hardness as hard or even harder than diamond. Similarly, cubic boron nitride (CBN) coatings are still very much in the development stage. Thin CBN coatings can now be deposited up to approximately 2000 A in thickness, but stress in the films prevents thicker films from being made. Aluminum oxide, which has been very difficult to make by PVD techniques, except in the very slow r.f. sputtering mode, can now be deposited in the d.c. magnetron mode when pulsed power is used.

Structure and hardness studies of CNx/TiN nanocomposite coatings

Crystalline CNx/TiN multilayer composite coatings have been synthesized using an opposed cathode unbalanced magnetron sputtering system. Electron microscopy studies showed that the CNx/TiN coatings were fully crystalline and dense at small bilayer thicknesses. An amorphous phase was formed when the CNx layer thickness exceeded 4–5 nm. Two new d spacings extracted by Fourier transform of digitized images of the crystalline CNx region cannot be matched by known compounds formed by the detected elements. This provides limited evidence for the possible formation of a new carbon–nitrogen compound. Nanoindentation hardness about 45–55 GPa was reproducibly achieved for coatings produced under low nitrogen partial pressure and high substrate bias (−150 to −250 V). TiN (111) preferred orientation was strongly correlated to the high coating hardness.

Crystalline alumina deposited at low temperatures by ionized magnetron sputtering

Ionized magnetron sputtering based on the work of Rossnagel and Hopwood [J. Vac. Sci. Technol. B 12, 449 (1994)] has been used to deposit alumina films containing orthorhombic κ alumina and monoclinic θ alumina at substrate temperatures of 370 to 430u2009°C. An inductively coupled rf Ar/O2/Al discharge between the sputter source and the heated substrate table was used to effectively ionize not only Ar but more importantly Al and O2. Both ion energy as well as the ion flux to the substrate influence the structure and properties of the coatings. The ion energy was controlled by the substrate bias potential, and the ion flux by means of the rf power supplied to the coil. The effect of the degree of ionization and therefore the ion flux to the substrate was studied at a constant substrate bias potential of −70 V. It was found that as the ion flux to the substrate was increased, the film crystallinity increased (i.e. the Bragg diffraction peaks were sharper and had higher intensity). It was shown that the formatio...