In recent years, near-nano (submicron) and nanostructured materials have attracted increasingly more attention from the materials community. Nanocrystalline materials are characterized by a microstructural length or grain size of up to about 100 nm. Materials having grain size of ∼0.1 to 0.3 μm are classified as submicron materials. Nanocrystalline materials exhibit various shapes or forms, and possess unique chemical, physical or mechanical properties. When the grain size is below a critical value (∼10–20 nm), more than 50 vol.% of atoms is associated with grain boundaries or interfacial boundaries. In this respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid. Therefore, grain boundaries play a major role in the deformation of nanocrystalline materials. Nanocrystalline materials exhibit creep and super plasticity at lower temperatures than conventional micro-grained counterparts. Similarly, plastic deformation of nanocrystalline coatings is considered to be associated with grain boundary sliding assisted by grain boundary diffusion or rotation. In this review paper, current developments in fabrication, microstructure, physical and mechanical properties of nanocrystalline materials and coatings will be addressed. Particular attention is paid to the properties of transition metal nitride nanocrystalline films formed by ion beam assisted deposition process.

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Nanocrystalline materials and coatings

One of the simplest ways to measure the mechanical properties of a thin film is to deform it on a very small scale. Because indentation testing with a sharp indenter is one convenient means to accomplish this, nanoindentation, or indentation testing at the nanometer scale, has become one of the most widely used techniques for measuring the mechanical properties of thin films. Other reasons for the popularity of nanoindentation stem from the ease with which a wide variety of mechanical properties can be measured without removing the film from its substrate and the ability to probe a surface at numerous points and spatially map its mechanical properties. The utility of the mapping capability is illustrated in Figure 1, which shows several small indentations made at selected points in a microelectronic device. The hardness and modulus of the device were determined at each point. In addition to microelectronics, nanoindentation has also proved useful in the study of optical coatings, hard coatings, and materials with surfaces modified by ion implantation and laser treatment.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400041634

Measurement of thin film mechanical properties using nanoindentation

The paper discusses processing and property aspects of oxide films formed on a Ti–6Al–4V alloy by AC plasma electrolytic oxidation (PEO) in aqueous solutions containing aluminate, phosphate, silicate and sulfate anions and some of their combinations. Structure, composition, mechanical tribological and corrosion resistant characteristics of the films formed are studied by SEM, XRD and microhardness analyses, and by scratch, impact, pin-on-disc friction and potentiodynamic corrosion testing. It is found that the films produced from the aluminate–phosphate electrolyte are dense and uniform and are composed mainly of Al 2 TiO 5 and TiO 2 phases of the rutile form. The films possess a beneficial combination of 50–60 μm thickness, 575 kg/mm 2 hardness and high adhesion and provide a low wear rate (3.4×10 −8 mm 3 /Nm) but a relatively high friction coefficient of μ=0.6–0.7 against steel, caused by material transfer from the counterface. A minimum friction coefficient of μ=0.18 is recorded during the testing of softer rutile–anatase films, 7 μm thick, produced from a phosphate electrolyte. Both of these types of film show good corrosion resistance in NaCl and physiological solutions, where the corrosion current is approximately 1.5 orders of magnitude lower than that of the uncoated substrate. SiO 2 /TiO 2 -based films with 70–90 μm thickness and high bulk porosity produced from silicate and silicate–aluminate electrolytes demonstrate better corrosion behaviour in H 2 SO 4 solution, due to the greater chemical stability of the film phase components in this environment.

Characterisation of oxide films produced by plasma electrolytic oxidation of a Ti–6Al–4V alloy

Nanoparticles have been shown to influence mechanical properties; however, transport properties such as viscosity have not been adequately studied. This might be due to the common observation that particle addition to liquids produces an increase in viscosity, even in polymeric liquids, as predicted by Einstein nearly a century ago. But confinement and surface effects provided by nanoparticles have been shown to produce conformational changes to polymer molecules, so it is expected that nanoparticles will affect the macroscopic viscosity. To minimize extraneous enthalpic or other effects, we blended organic nanoparticles, synthesized by intramolecular crosslinking of single polystyrene chains, with linear polystyrene macromolecules. Remarkably, the blend viscosity was found to decrease and scale with the change in free volume introduced by the nanoparticles and not with the decrease in entanglement. Indeed, the entanglements did not seem to be affected at all, suggesting unusual polymer dynamics.

Nanoscale effects leading to non-Einstein-like decrease in viscosity

Using a variety of depth-sensing indentation techniques, the creep response of high-purity indium, from room temperature to 75 °C, was measured. The dependence of the hardness on the variables of indentation strain rate (stress exponent for creep (n)) and temperature (apparent activation energy for creep (Q)) and the existence of a steady-state behavior in an indentation test with a Berkovich indenter were investigated. It was shown for the first time that the indentation strain rate (-este-/h) could be held constant during an experiment using a Berkovich indenter, by maintaining the loading rate divided by the load (-este-/P) constant. The apparent activation energy for indentation creep was found to be 78 kJ/mol, in accord with the activation energy for self-diffusion in the material. Finally, by performing -este-/P change experiments, it was shown that a steady-state path independent of hardness could be reached in an indentation test with a geometrically similar indenter.

Indentation power-law creep of high-purity indium

Nanoindenter techniques have been used to determine the hardness. Young’s modulus, and strain rate sensitivity of nanophase TiO2, which is currently available only in very small quantities and which cannot be tested by most conventional techniques. Hardness and Young’s modulus both increase linearly with sintering temperature over the range 25–900°C but come to within only 50–70% of the single crystal values. Strain rate sensitivity, on the other hand, is measurably greater for this material than for single crystal rutile, and the value of strain rate sensitivity increases as the grain size and the sintering temperature are decreased. In its as-compacted form, the strain rate sensitivity of nanophase TiO2 is approximately a quarter that of lead at room temperature, indicating a potential for significant ductility in these ceramic materials. Finally, a significant scatter in hardness values has been detected within individual nanophase samples. This is interpreted as arising from microstructural inhomogeneity in these materials.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400009602

Mechanical properties of nanophase TiO 2 as determined by nanoindentation

Elastic moduli of both as-hot-pressed and oxygenated YBa{sub 2}Cu{sub 3}O{sub {ital x}} were measured, under isostatic pressures up to 1 GPa, using a combination of strain gage and ultrasonic techniques. Pressurization of the as-hot-pressed samples caused no significant change in moduli; however, pressurization of the oxygenated sample produced a 5-fold increase in bulk modulus. The results show that the significant decreases in moduli between the as-hot-pressed and the oxygenated state were due to extensive microcracking, of an extremely fine nature, that occurred during the oxygenating sequence.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400011444

M. J. Mayo

Papers

Mechanical properties of nanophase TiO 2 as determined by nanoindentation

Effect of microcracking on the measured moduli of bulk YBa 2 Cu 3 O x