We can all imagine a metallic element as a piece of hard, shiny material. When we reduce the size of a metal down to the nanoscale and we focus on their constituent structure of atoms, we observe that minimal changes at this tiny scale can produce surprising behaviours of the material at the human scale. For example, if a grain size of a bulk metal is made smaller than 100 nm, the material becomes stronger. These tiny metallic pieces are called nanocrystals. This change on the macroscopic properties happens because the plastic deformation mechanism that determines the force that the material is able to withstand before it loses its original shape, changes dramatically when reducing the size. This effect is related to the inter-defects distance of crystals, which is the distance between the imperfections (like grain boundaries) appearing in the atomic structures of metals and which disturb locally the regular arrangement of the atoms. This little distance between defects is indeed of key importance for everyday life as it defines properties of the materials at the macroscopic level, such as strength, wear resistance, or ductility (the ability to sustain deformation without failure). Thus, the reliability of the metals and alloys composing, for example, cars and aircrafts we are traveling on, depends on this tiny and apparently insignificant space. Understanding and developing techniques to shape this distance (which is of the order of a few hundred nanometres in metals) will give us control over commercially important characteristics of materials.
Producing nanometals, which are industrially relevant macroscopic pieces of materials with the properties of nanocrystals, is a challenge yet to be achieved. The race to upscale nanometals covers both the search of new materials and the feasibility to be profitable in our society in a practical way. This race started in the first half of the 20th century by two scientific milestones. In 1938, two independent investigations by Guinier1 and Preston2 discovered a fine-scale chemical phenomenon in metals leading to the precipitations of certain compounds. Such nanometric precipitations are nowadays known as Guinier-Preston zones, and they are the responsible of the strengthening of the aluminium alloys, massively used in aeronautics. These Guinier-Preston zones when finely distributed stop the mechanisms behind plastic deformation of metal. Thus, the effect of these pseudo-precipitates makes metals react stronger against deformation.
The second milestone was achieved in 1950s with the discovery of the influence of the size of crystallites on the fracture of metals. In two individual scientific contributions, Hall3 and Petch4 found that the smaller the grain size of a metal, the larger the resistance against plastic deformation. Now we know that this is due to the opacity of the grain boundaries to dislocation movement. Thus, again, the larger quantity and finer distribution of grain boundaries define stronger resistance of the metal against deformation.
After the discovery of the Guinier-Preston and Hall-Petch effects, it has been clear that smaller is stronger. During the second half of the last century, a generous research effort was devoted to the development of techniques to control the growth of the precipitates and to reduce the grain size, both chemically and mechanically. The chemical path was demonstrated to be extremely cost-efficient. Micro-alloyant elements in modern alloys nucleate smaller grains in large quantities or decorate the grain boundaries in such a way they are thermally more stable. They can also promote new phases (and phase boundaries) inside the grain, adding more barriers to hamper the strain or gaining ductility without losing the strength. Other approaches utilize meta-stable structures to prevent the dissolution of impurity elements during cooling, like nano-bainite5, a strong structure in steels forming by a controlled cooling after heating. Fortunately, in the last few years, the combination of chemical and thermal production techniques with mechanical strain has demonstrated unprecedented potential to bring the properties of nanocrystals to larger pieces. In the last few years researchers have found different methods to apply severe plastic deformations (SPD) on metals, what reduces the grain size by orders of magnitude. In these SPD techniques the material is deformed so much that the crystals are successively broken down into smaller crystals. This process is carried out in extreme conditions, under compressive stress induced by high pressure and low temperature. Experiments carried out so far have shown promising results, achieving grains sizes in the order of tens of nanometres. Nonetheless, right now, the amount of material producible is very small and the cost makes the material only of interest in very specific applications (such as resistance against impacts). Still, the fact that the strength values close to the theoretical maximum1 in iron alloys have been obtained, suggests that we are getting closer to the right size of nanocrystals from an application perspective.
Mechanically, surface and bulk have different functions in structural pieces. For instance, the hardness of a material is a property related to the surfaces, while its toughness depends mainly on the bulk matter. Hence, while nanograined bulk metals is still the central goal, it might be the case that the properties of nanocrystals are needed only on the surfaces. But how this can be done and will the thin surface nanocrystalline layer possess beneficial properties of bulk nanometals?
Previous studies have suggested that this could be possible. By intensively imposing friction between two surfaces, the softer one develops very high deformation near the surface, to such an extent that the size of the crystals is reduced to below 100 nm. It has already been shown that friction conditions in the process of machining of metals at particular cutting parameters tend to reduce the size of the crystals on the surface of the material in the area known as the white-layer2. This type of layers, though been known for many decades, have not been deeply characterised because of their small thickness. Such research could be especially valuable for common construction materials like steels, for which such layers have not been obtained so far by purpose. So, it is still a question if this surface layer really has the properties of bulk nanometals - e.g. high strength?
This question has been addressed in a research project in which it has been demonstrated that in high-speed machining of steel under certain working conditions a material with very high strength can be generated in the surface3. This occurs when a deformation mechanism called rotational dynamic recrystallisation is activated. In this mechanism, groups of atoms within the bigger crystal rotate generating new crystals on place of the original one, which in successive steps breaks down the original structure into nanograins. Samples from this kind of experiments have been used to study the mechanical properties of the layer of nano-crystalline material. The outcome of these tests was a desired one, yet surprisingly exceeding the expectations: the strength of the nano-crystalline layer came out to be 3 times higher than that of the bulk steel! Such increase could not be explained solely by small grain size, and an intensive study of the structure and composition of the layer has been performed to understand the origin of that strong improvement of the properties.
For this purpose, a set of advanced electron microscopy techniques has been used. First, it was found that there is a low texture in the recrystallised zone, i.e., the crystals are roundish and are randomly oriented. Furthermore, it was found that this nanocrystalline material is very relaxed, i.e. there is low level of internal strain and low the defect (dislocations). That comes from the increase in the temperature that accompanies the deformation - the friction increases the temperature at to the point the diffusion (atoms rearranging to lower energy positions) "repairs" the lattice. This explains why the material remains ductile after the friction treatment.
Figure 1: Left: diagram of a hard tool fractioning against an industrial steel - Centre: micrograph of a nanocrystallised surface after friction - Right: plot the strength of non-treated steel (red) and the same material after friction (blue).
Rearrangement of atoms also changes the chemical structure of the material, how the elements are distributed. The initial steel contains carbon mostly in the form of cementite [Fe3C] layers, interspersed with pure iron forming a very characteristic and well-studied perlitic structure (see Figure 1). X-ray chemical analysis has shown that these cementite structures no longer exist in the friction affected layer. Moreover, excess carbon does not segregate in the grain boundaries (which is a typical scenario in this kind of materials), but is rather completely dissolved in iron forming an oversaturated solid solution. Theory tells, that such process is by fact possible, if the mechanical forces are high enough to degrade the cementite layer by layer, and high mobility of the defects at elevated temperature takes care of transporting carbon into the ferrite lattice4. Provided a high cooling rate after friction, the carbon would remain in the iron lattice as a supersaturated solid solution. This solution is known to increase the elasticity and thus to delay the onset of irreversible plastic deformation of the material, in other words - its strength. Thus, friction treatment of perlitic steel promote all the conditions required for extreme strength increase in the surface layer: enormous deformation values breaking down large crystal grains, high temperature initiating nanostructure relaxation and carbon dissolution, high cooling rate stabilizing this supersaturated nanocrystalline material.
In summary, it has been found that the friction can dramatically change the surface properties of a perlitic steel and increase the ultimate strength. More importantly, it has been found that, under certain working conditions, machining has the potential to treat the surface of the parts being manufactured, with little added industrial cost.
Friction treatment of surfaces has the potential to become an industrially viable treatment to improve the properties of manufactured surfaces. Adequate friction can generate an elastic and highly resistant layer up to 15 micrometres thick. A proper understanding of how they are produced and how to exploit them can represent a competitive advantage for innovative companies.
Doctor Bentejui Medina-Clavijo is currently a Senior Fellow at the European Organization for Nuclear Research (CERN), Switzerland. Granted doctor at the University of Mondragon in the field of manufacturing and materials science, B. Medina-Clavijo focussed on metals and micro-mechanics after moving to CIC nanoGUNE, Spain. Now at CERN, B. Medina-Clavijo studies superconducting compounds and magnets, for future upgrades of particle accelerators.
Professor Andrey Chuvilin is currently the head of the Electron Microscopy Laboratory at nanoGUNE in San Sebastian, Spain. Granted doctor and candidate of science in physics and math 1998 from the Institute of Inorganics Chemistry SB RAS, Novosibirsk. Afterwards, at the University of Ulm, Germany, Prof. Chuvilin worked on low voltage high resolution TEM of nanocarbon materials, convergent beam electron diffraction, image simulations and processing. Then, at CIC nanoGUNE, Spain, A. Chuvilin combined those fundamental lines with applied research in chemistry and manufacturing.
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