More efficient lubrication is required to accomplish the highly demanding working conditions of new engines. The durability, emissions and fuel economy of all engines are firmly linked to lubrication efficiency. Low or ineffective lubrication can lead to large friction and wear losses, which could negatively influence fuel consumption and lifetime of engines. Moreover, high fuel use leads to more pollution, therefore, it is crucial to progress to new engines with lower emissions and a greater fuel economy.
All this can be obtained by using solid lubricants. Solid lubricants occupy an exceptional place in increasing wear resistance in circumstances where liquid lubricants are incompetent or impossible, like in space, vacuum or automotive.
During the past years, metal matrix nanocomposites have been of interest to many researchers as solid lubricants.
In the large field of nanotechnology, metal matrix-based nanocomposites have become a prominent area of research and development. Owing to their lubricious nature, nanocomposites containing transition metal carbides, nitrides, borides and oxides have attracted researchers worldwide.
Lightweight self-lubricating metal matrix composites with superior mechanical and tribological properties are appealing for several applications in the automotive and aerospace industries.
Interest of nanocomposites
By definition, composite materials are made of two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macro-, micro- and nanoscales within the finished structure .
One of the constituent materials, called the reinforcing phase, is in the form of fibers, sheets, or particles. It is embedded in the other material called the matrix phase. This is, though, a traditional definition that has nowadays become broader due to the existing sophisticated material architectures.
The reinforcing material and the matrix material can be metallic, ceramic, or polymeric. The overall properties of the composites are in many cases superior to those of the individual components due to the occurrence of synergistic effects .
Generally, the size of reinforcement influences mechanical properties such as strength, ductility and fracture of self-lubricating metal matrix composites (MMCs). By increasing the reinforcement size, tensile strength and ductility decrease simultaneously. MMCs reinforced by larger particles are susceptible to the formation of defects, such as cracking during mechanical testing, which results in premature failure of the composites.
Therefore, it is expected to have superior properties when the reinforcement size is in the nano range. If nanoparticles (NPs) are to be embedded in the metal matrix, the resulting material is often termed ‘nanocomposite’.
In general, it is desirable in terms of mechanical properties to have matrix grain size in the range of nanometer to achieve enhanced hardness, yield strength, and tribological properties such as wear-resistance and friction coefficient. Using nanosized particles as reinforcement also enhances both Young’s modulus and tensile strength of composites as well as improving tribological performance.
Tribology of self-lubricating nanocomposites
Tribology is the ‘science and engineering of interacting surfaces in relative motion’ . It comes from the Greek word ‘tribos’ (rubbing) and comprises the principles of lubrication, wear and friction . When under an external load, two materials are in contact with each other, the asperities of two surfaces come into close contact and during movement, and deterioration of the surfaces occurs, which is known as wear.
Usually, to avoid friction and consequently, deterioration of material under wear, liquid or solid lubricants are employed. However, in cases such as high vacuum environment, high-speed conditions, high applied loads and very low or high temperatures, liquid and grease type lubricants are undesirable.
In such a tribological system, the liquid and grease type lubricants are replaced with solid lubricant coatings, which are used to decrease the coefficient of friction (COF) and wear rate.
Solid lubricants are compound combinations containing additives that aim to reduce friction through the separation of contacting surfaces. High interest has been shown for the use of NPs as additives due to their nanoscale size, which allows them to penetrate contacts of diverse geometries, fill the gaps between contact asperities and form a protective boundary film, persistent under high pressure (Figure 1) [6,7].
Nanoparticles have been highly used as oil dispersions, solid lubricants, or coating inclusions. There are some benefits linked to the usage of NPs as lubricant additives. Due to their small size, they can cover the gaps between contact asperities .
They also have high strength and hardness due to their large surface area . However, this significant surface area leads to a larger surface energy, which makes them thermodynamically unstable, and their dispersions tend to agglomerate. To avoid this, nanoparticles need to be functionalized with a surfactant .
There are two forms in which NPs can exist:
- 2H – 2 layers per Hexagonal unit cell (Figure 2)
- IF – Inorganic Fullerene-like (Figure 3).
In the 2H- form, the layers are flat and have ‘dangling bonds’ 11 (edge effects that can deteriorate the NP by burnishing or oxidation). However, in the IF- structure the layers are rounded up forming ‘onion-like’ cages, which leads them to be more inert to chemicals [6,12,13].
It has been published that in relatively low PV (load x velocity), IF beat 2H NPs, although under high PV conditions, 2H are better .
Lubrication mechanism of IF- and 2H- MS2
The basal planes of MS2 platelets consist of sulphur atoms that are fully bonded and non- reactive (Figure 4). Therefore, the crystal has low reactivity towards the underlying surface leading to low absorption of water vapour. Furthermore, the surface energy of the terminal S atoms is rather low, allowing them to easily shear with respect to the metal surface.
The weak van der Waals S-S interaction between contiguous layers allows simple shearing of the layers with respect to each other, which contributes to a competent lubrication mechanism (Figure 2).
However, 2H-MS2 crystallites are extremely anisotropic and reactive. While the S atoms of the basal planes are fully bonded and consequently non-reactive, the M and S atoms on the other planes are not fully bonded and are consequently very reactive (Figure 2).
Therefore, the simple shear is disturbed and thus the competent lubrication. The platelets are then burnished and oxidised quite fast during friction between two mating surfaces. This explains the short lifetime of the lubricant because of the quick deterioration of the 2H-MS2 NPs.
The friction mechanism of IF-MS2 NPs is characterized by exfoliation of their outside layers to deliver single layers between the contact surfaces (see Figure 5) and reduce the coefficient of friction .
Schwarz et al. investigated the pressure and adhesion effect on fullerenes tribology . NPs attach to surfaces thanks to van der Waals forces, which are proportional to the particles’ radius and independent of the number of layers composing the IF- NP.
Fullerenes exfoliate because of the pressure applied to the NPs, which destabilizes the system. Under adhesion conditions, exfoliation of only the first sheets (1 or 2 first sheets) takes place. After that, sheets adhere to the surfaces forming a thin film which minimises the edge effects .
Because of their nanometre size, fullerenes can infiltrate pores in the contact surfaces easily, while 2H-MS2 NPs are considerably larger and cannot go through the pores. These pores act as reservoirs for IF-MS2 NPs and can frequently provide the contact area during friction. Some studies have already confirmed thanks to SEM images taken after friction that NPs can infiltrate impurities [14, 18] and be released and activated in the contact zone during friction.
NP-reinforced MMCs synthesis
NPs can be integrated into a tribological surface using different techniques. They can be simply dispersed in oil to reduce friction between surfaces . However, in some space applications where oil is not suitable, sprinkling, rubbing or burnishing methods are used . Numerous solid lubricants are mixed in aerosol and sprayed straight to the surfaces .
Others are electrodeposited with the help of a metal matrix . Moreover, powdery-like solid lubricants can be highly attached with suitable epoxy resins and adhesives to allow greater wear life to a surface .
Nevertheless, in state-of-the-art, thin films solid lubricants are chosen instead of bonded forms or powders. Thin films can be deposited on surfaces by advanced vacuum processes like magnetron sputtering [27,28] ion plating, ion beam assisted deposition (IBAD)  or plasma-enhanced chemical vapour deposition (PECVD) , in order to obtain strong bonding, uniform thickness, long wear life and dense microstructure.
This review provided a focused overview of solid lubrication with IF-NPs and 2H-NPs, starting with the fundamentals of their structure, followed by an analysis of their use as solid lubricants and corresponding tribological properties.
IF- and 2H- NPs have superior properties, including large aspect ratio, exceptional high Young’s modulus and strength.
These unique properties attract researchers to use them as reinforcement for metal matrix composites to enhance properties of composites and make them strong, lightweight and self-lubricating.
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