Self-Lubricating Composites

Solid lubricants technology is a flourish field that deserves the attention of the designer of machines and devices that will operate in ordinary as well as in extreme environments. This chapter describes solid lubrication processes, the mechanisms by which solid lubricants function, the properties of solid lubricants, and the materials involved in solid lubrication and techniques for their application. Reliability of solid lubrication and wear life of solid-film lubricants are being improved by designing machine elements specifically to employ solid lubricants and by careful matching of the solid lubricant with the substrate bearing material. Solid lubricants are applied either as surface coatings or as fillers in self-lubricating composites. Tribological (friction and wear) contacts with solid lubricant coatings typically result in transfer of a thin layer of material from the surface of the coating to the counterface, commonly known as a transfer film or tribofilm. The wear surfaces can exhibit different chemistry, microstructure, and crystallographic texture from those of the bulk coating due to surface chemical reactions with the surrounding environment. As a result, solid lubricant coatings that give extremely low friction and long wear life in one environment can fail to do so in a different environment. Most solid lubricants exhibit non-Amontonian friction behavior with friction coefficients decreasing with increasing contact stress. The main mechanism responsible for low friction is typically governed by interfacial sliding between the worn coating and the transfer film. Strategies are discussed for the design of novel coating architectures to adapt to varying environments.

Self-lubricating metal matrix composites (SLMMCs) are a class of materials that have potential to help engineers meet the demands of global initiatives for green manufacturing and sustainability. While SLMMCs have existed for many decades with traditional lubricant materials like graphite and other lamellar solids, such as MoS₂, WS₂, h-BN and CaF₂, BaF₂, scientists have recently incorporated nanostructured versions of the materials (e.g., carbon nanotubes and graphene). At the same time, new manufacturing and processing techniques have come online, such as additive manufacturing techniques that may provide significant innovation for SLMMCs. In this chapter, the current state of SLMMC research is reviewed, including materials, processing methods, and tribological performance. Processing and property relationships are described, such as influence of testing parameters and content of solid lubricants on friction and wear. Improvements in tribological behavior, as much as possible, are interpreted through third-body approach, which emphasizes materials phenomena at the sliding interface – including mechanical, structural, and chemical changes to the parent materials. Based on the review of SLMMCs and their tribology, recommendations for future research are made that emphasize the use of new materials, new processing routes, and research approaches that seek to reveal more completely the mechanisms by which these materials form tribofilms that are effective at lowering friction and reducing wear.

Friction and wear during sliding or rolling of solid surfaces are universal phenomena, and they reflect the tendencies of energy to dissipate material to deteriorate. In general, solid surfaces in relative motion require lubrication, which dramatically reduces the extent of friction and wear. The situation when no external lubrication is required is called self-lubrication. Self-lubricating polymer composite materials are two-phase system that contain soft second-phase particles in the polymer matrix. The soft phase exposed to the surface during sliding. So the properties of both the hard matrix and the soft second-phase particles, as well as the shape and size of the particles, control the processes of deformation and flow of the soft phase. This chapter details polymer matrix structures, self-lubricating polymer composites, mechanisms of polymer composite lubrication, transfer film mechanisms, factor affecting polymer composites on friction, and wear and application of polymer composites.

In the last few years, polymeric materials filled with different kinds of nanomaterials have attracted particular attention as useful alternatives in structural components subjected to severe friction and wear loading conditions. The intention of this chapter is to give a comprehensive picture of these nanofillers and to show their ability to improve friction and wear behavior of polymer composites. The aim is to organize the current state-of-the-art knowledge on these nanomaterials and point out on the key mechanisms governing their reinforcing effects. Despite the existing differences between literature results, there is a general agreement on the crucial role played by size, shape, concentration, and distribution of these fillers within the polymer matrix. The compatibility/interaction between filler and matrix is another important aspect in determining good filler dispersion and effective load transfer between the phases. As a consequence, the development of polymer nanocomposites showing high tribological features requires a deep selection of the nanofiller type and dimension along with its possible surface modification. Fortunately, modern technologies allow the design and the preparation of complex hybrid nanostructures able to put together the benefit of several structural factors. Although the state of the art demonstrates the potential of these materials, further researches are, however, necessary in order to definitely reach all possible improvements attainable for future high-demanding tribological applications.

Structural ceramic composites have received increasing attention over the past few decades for their potential applications in various fields. Lubrication is usually required for moving ceramic parts because of their high coefficient of friction under dry sliding conditions. Self-lubricating ceramic composites have been applied in severe operating conditions where conventional lubrication method, such as liquid lubrication, is unavailable. The solid lubricants added in self-lubricating ceramic composites can reduce the coefficient of friction. However, they decrease mechanical properties and then weaken antiwear property of the ceramic composites, which consequently restricts self-lubricating ceramic composites’ application scope. Therefore, there is a contradiction between the antifriction and antiwear properties of self-lubricating ceramic composites and many efforts from researchers have been devoted to resolve it. In this chapter, two new types of self-lubricating ceramic composites were elaborated. Graded self-lubricating ceramic composites were developed by adopting the design concept of functionally graded materials (FGMs). Their characteristics are that the solid lubricant content decreases with a gradient from the surface to the center and thermal residual compressive stresses exist in the surface after the sintering process. The gradient distribution of solid lubricant and the thermal residual compressive stresses are used to improve the mechanical properties of the ceramic composites. Another new type of self-lubricating ceramic composites is those with the addition of coated solid lubricants. The solid lubricant powders are firstly coated by metal or metallic oxide, etc., to form core-shell structured composite powders and then mixed with the ceramic matrix powders to prepare self-lubricating ceramic composites by sintering. The shell substance is used to protect the solid lubricant core from reacting with the ceramic matrix during the sintering process and promote the relative density of the ceramic composites. The two new types of self-lubricating ceramic composites showed superior mechanical properties and tribological properties to the traditional self-lubricating ceramic composites.

Polymers and polymer composites are often described as solid lubricants because they provide relatively low friction coefficients and wear rates in unlubricated and other extreme tribological conditions. However, few, if any, are inherently lubricious or wear resistant. These materials achieve useful tribological properties by depositing a layer of debris onto the mating counterface; this sacrificial layer is called a transfer film and it shields the polymer from damage by the harder counterface. The friction and wear performance of these systems depend as much on the formation, evolution, and stability of the transfer film as they do on the structure and composition of the solid lubricant itself. Although it is understood that transfer films are essential for high tribological performance of polymers and polymer composites, the causal relationship between transfer film qualities and wear resistance remains uncertain due largely to the difficulty in quantitatively measuring their properties. There have been increased efforts, particularly in the last 10 years, to develop quantitative methods to assess the topographical, adhesive, mechanical, and chemical properties of polymer transfer films. This chapter reviews the latest efforts to measure transfer film qualities and quantitatively relate them to the tribological performance of solid lubricant polymers.

The production of self-lubricating composites containing second phase particles is one of the most promising choices for controlling friction and wear in energy efficient modern systems. Initially, we present a new microstructural model/processing route able to produce a homogeneous dispersion of in situ generated, discrete, solid lubricant particles in the volume of sintered composites. The high mechanical and tribological performances of the composites are a result of the combination of matrix mechanical properties and structural parameters, such as the degree of continuity of the metallic matrix, the nature, the amount, and the lubricant particle size and shape which determine the mean free path between solid lubricant particles and the active area covered by each lubricant particles. This new route was achieved by in situ formation of graphite nodules due to the dissociation of a precursor (SiC particles) mixed with metallic matrix powders during the feedstock preparation. Thermal debinding and sintering were performed in a single thermal cycle using a plasma-assisted debinding and sintering (PADS) process. Nodules of graphite (size ≤20 μm) presenting a nanostructured stacking of graphite foils with thickness of a few nanometers were obtained. Micro-Raman spectroscopy indicated that the graphite nodules are composed of a so-called turbostratic 2D graphite which has highly misaligned graphene planes separated by large interlamellae distance. The large interplanar distance and misalignment among the graphene foils has been confirmed by transmission electron microscopy and is, probably, the origin of the remarkably low dry friction coefficient (0.06). The effects of precursor content (0 to 5 wt% SiC) and of sintering temperature (1100 °C, 1150 °C and 1200 °C) on tribolayer durability and average friction coefficient in the lubricious regime (μ < 0.2) are presented and discussed. In addition, the effect of the metallic matrix composition (Fe-C; Fe-C-Ni; Fe-C-Ni-Mo) is presented. Friction coefficient decreased and durability drastically increased with the amount of graphite formed during sintering, whereas friction coefficient was little affected by sintering temperature. However, the durability of the tribolayer was greatly increased when lower sintering temperatures were used. The addition of alloying elements considerably reduced wear rate and friction of specimens and counter-bodies. Friction coefficient values as low as 0.04 were obtained for the Fe-C-Ni-Mo composites. We also analyzed the effect of precursor content and of sintering temperature on the tribological behavior under constant normal load sliding tests. Again, the presence of graphite nodules significantly reduced the friction coefficients and wear rates, whereas the sintering temperature hardly affected these parameters. The results were compared with those caused by other forms of graphite (nodules in nodular cast iron and powder graphite) and were discussed in terms of the crystalline structure of the analyzed graphite using micro-Raman spectroscopy. Chemical analyses of the wear scars using scanning electron microscopy (SEM – EDX) and Auger electron spectroscopy (AES) showed a tribolayer that was composed predominantly of carbon and oxygen. This tribolayer is removed and restored during sliding and is continuously replenished with graphite. Finally, the strong effect of surface finishing is presented and discussed.

The three-dimensional lubricating layer on a ceramic surface realizes the integration of the structure and lubricating function in ceramic materials, which can achieve outstanding lubricating properties and maintain the excellent mechanical properties of ceramics, solving the special lubrication and wear failure in mechanical systems under extreme conditions (e.g., corrosive environment and wide-temperature range condition). In this chapter, two kinds of surface-lubricating structural-laminated ceramics with high reliability were designed and prepared based on experiment research and theoretical simulation. These ceramics can achieve stable and effective lubrication in a water environment and wide-temperature range condition. These materials are Al₂O₃/Ni- and Al₂O₃/Mo-laminated composites suitable for use in a water environment and in wide-temperature range conditions, respectively. The relation between the surface microstructure of the prepared materials and their properties (mechanical and tribological) was investigated. Results indicated that the integration of the structure and lubricating function of the ceramic composites is realized through the bionic, surface microstructure, and three-dimensional self-lubricating design of the materials, further improving their lubricating and practical properties. Factors that can influence the tribological behavior and wear failure of the above materials were proposed through the systematic study of the tribological behavior under different environments and test conditions, as well as the relation among the structure, composition, and properties of these two kinds of materials. In addition, theoretical models of the relation between the structural parameters and performance of the materials were built. These methods provided theories and technologies for the preparation and application of high performance lubricating materials that can be used in corrosive and wide-temperature range environments.

In this chapter, an overview of theories and investigated computational models is presented. Among all available theoretical models, Quantum Mechanics (QM), Molecular Mechanics (MM), Monte Carlo (MC), and Molecular Dynamics (MD) are the most used models. MD was selected as the focus of this chapter, because of its high accuracy in predicting the molecular level motions while keeping the computational costs relatively low as well as availability of well-established modeling softwares (i.e., LAMMPS). MD models have been used to investigate mechanical and chemical behaviors of different phenomena, including friction and self-lubrication. The authors further reviewed available MD models in previous literatures with focus on self-lubricating materials. These models direct the contribution of different self-lubricating agents including graphite, graphene, MoS₂, and poly tetra-fluoro ethylene (PTFE) on the friction behavior of different composites. This review was conducted in order to show the power of computational modeling to predict the molecular level behaviors of different physical models.

Human activities have affected the balance in the eco-system and put the habitant surrounding us in a catastrophic danger. Most of these wrecking effects which had been strengthened or emerged recently would also last for a long period of time. Taking immediate thoughtful actions are necessary to prevent high risk human activities. Based on the data collected for global surface mean temperature and CO₂ emission, it has been shown that a huge discrepancy in climate is already occurring. There are various reasons which are causing climate change, specifically global warming, which most of those would be discussed in this study. The negative effects of different materials on the eco-system have one of the biggest shares in environmental concerns. Emerging Eco-tribology science helped design products and processes for the environment. The design of products has been significantly modified to be more environmental friendly and sustainable. A brief review of tribology and different definitions, which had been changed during the time, will be discussed in this study. Furthermore, eco-tribology has a positive effect in reducing energy consumption and increasing the lifetime of the products. Lubricant is an example of the materials which produce environment destructive waste. In the recent decades, researchers improved the lubricant materials to be less destructive for the environment. Self-lubricant composites, for instance, significantly reduced the drawbacks of lubricant materials. The advantage is not only improving wear resistance and reduced COF, but also the elimination of the need for external lubricants. Aluminum/graphite (Al/Gr) composites have been used as self-lubricating materials due to the superior lubricating effect of graphite during sliding process. This study reviews the environmental concerns and advances of eco-tribological, lubricant and self-lubricant composites.