What Are the Advantages of High-temperature friction powders？

Powder coating&#;s durability and affordability have made our high temperature powder coating a favorite amongst our clients. With its ability to adhere strongly to most metals, it is a great option to consider for your upcoming project. And, if you&#;re wondering whether or not it&#;s heat resistant, the short answer is, &#;yes&#;.

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The long answer is a bit more detailed, so please continue below to learn about some of our high-temperature powder coating options.

What is Powder Coating?

Powder coating is one of the most common coating methods that professionals turn to when they require fast, even, and reliable industrial finishing. Applied as a free-flowing dry powder, the coating provides unparalleled versatility thanks to its easy application. Unlike conventional liquid paints, which require an evaporating solvent for application, powder coating uses electrostatic application methods before being cured under high heat. Most powders require baking at around 350-400 degrees Fahrenheit for 10-20 minutes to fully cure.

Here at Crest Coating Inc., we have a variety of different high temperature coatings available, and bring our long history of experience and expertise to help our clients find the right products for their specific needs. Thanks to the high temperatures at which it is cured, powder coating is viable in conditions that may not be suitable for other coating techniques. From exhaust and mufflers to heat shields and industrial equipment, high-temperature uses are completely viable. But just how far does that heat resistance go? The truth is, a powder coating&#;s heat resistance varies with the materials that are used. An epoxy coating will be very different from a nylon, fluoropolymer, or ceramic coating, for example.

There are four heat resistance categories that powders will generally fall under:

• Ambient to 200°F
• Up to 500°F
• Up to 800°F
• Up to 1,800°F

So, depending on the temperatures and environment you expect to see, we have a range of coatings that we can recommend. Read below to see a few examples:

General Powder Coatings

The most common powder coatings can withstand temperatures between ambient to 200°F, although most are on the lower end of the spectrum. These general powders will do the trick for most typical applications, but may be susceptible to significant color change and other issues once you start pushing that 200°F+ range. As a rule of thumb, general powders can be used on most external components without a problem, but should be avoided when temperatures are expected to exceed 150°F, like engine components or exhaust parts.

>>> Related Article: When to Choose Nylon Powder Coating

Fluoropolymer Powder Coating

The next level up with regard to heat resistance is the use of Fluoropolymer coatings. Fluoropolymers are a family of plastic resins which are based on fluorine/carbon bonds. This coating is characterized by its multiple carbon&#;fluorine bonds, and is often most commonly seen on Teflon coated frying pans. These are usually rated between 300°F to 500°F and are a great choice if additional properties are desired like non-stick performance, chemical, or abrasion resistance.  Some fluoropolymer powder coatings are also commonly used on exterior applications of commercial and residential high-rise buildings.

Specialty High Temperature Coating

Need even more heat resistance? That&#;s going to push you into specialty high temperature powder territory. The specialty high temperature powder coatings we have available are rated between 600-800°F, and withstand these higher temperatures while still offering good corrosion resistance and some unique cosmetic options. These are ideal for protecting the exhaust components we mentioned earlier on, as well as heater shrouds and other commercial products from high-temperatures.  Despite the high temperature rating, high-temperature powder coating is not recommended for direct flame exposure.

High Temperature Ceramic Coating

Ceramic coatings are the highest heat resistance category readily available to be applied by coating. Rated to temperatures up to 1,800°F, this is the ideal material to use when your project involves extreme heat environments.  High temperature ceramic coatings can also perform as a thermal insulator, reducing the amount of heat that radiates from a surface &#; making them ideal for the most demanding automotive, aerospace, and industrial applications.  Furthermore, many of these coating options only require an ambient cure, and possess excellent wear properties, allowing them to be applied to a wide range of substrates.  Their versatility and strength are why ceramic coatings are often found in sub-sea applications, in your engine compartment, in aircraft, and basically anywhere that you&#;ll find extreme levels of heat, pressure, and friction.

Trust Us With Your Heat Resistant Powder Coating Needs

Crest Coating, Inc., established in , has unsurpassed experience and knowledge in industrial powder coating services and exotic liquid coatings. For over 40 years we have retained the status of Chemours Licensed Industrial Applicator, as well as Approved Applicator for Whitford Xylan®, Halar®, BCS Technologies, and other exotic coatings. We have experience working on complex projects and provide expertise and support for any of our clients&#; needs.

At Crest Coating, we value the extensive coating and application knowledge of our staff, who have decades of experience.  To find the right coating for your next project, fill out this quick form today, or contact one of our experts by calling 714-635-.

Understanding the complex nature of wear behavior of materials at high-temperature is of fundamental importance for several engineering applications, including metal processing (cutting, forming, forging), internal combustion engines, etc. At high temperatures (up to °C), the material removal is majorly governed by the changes in surface reactivity and wear mechanisms. The use of lubricants to minimize friction, wear and flash temperature to prevent seizing is a common approach in engine tribology. However, the degradation of conventional liquid-based lubricants at temperatures beyond 300 °C, in addition to its harmful effects on human and environmental health, is deeply concerning. Solid lubricants are a group of compounds exploiting the benefit of wear diminishing mechanisms over a wide range of operating temperatures. The materials incorporated with solid lubricants are herein called &#;self-lubricating&#; materials. Moreover, the possibility to omit the use of conventional liquid-based lubricants is perceived. The objective of the present paper is to review the current state-of-the-art in solid-lubricating materials operating under dry wear conditions. By opening with a brief summary of the understanding of solid lubrication at a high temperature, the article initially describes the recent developments in the field. The mechanisms of formation and the nature of tribo-films (or layers) during high-temperature wear are discussed in detail. The trends and ways of further development of the solid-lubricating materials and their future evolutions are identified.

This paper is an effort to review the notable SLs (and their based composites) used in tribological applications to impart reduced friction and wear at HT in dry sliding conditions. The mechanism behind their lubricity, chemistry and friction reduction is discussed in detail. A summarized graph showing the range of working temperature for various SLs and their demonstrated CoF during dry sliding is also presented. The idea of a futuristic &#;smart&#; tribo-material is introduced.

In the framework of industrial applications, such as HT forming, forging, stamping, cutting, including areas of relative motion in engines, etc., the foremost importance for a solid-lubricating material is to minimize friction and wear, and also deliver chemical, corrosion, thermal, and mechanical stability. Irrespective of the chosen manufacturing process for material fabrication, low and steady friction in addition to low wear rates must be demonstrated at a wider range of temperatures since the work piece employed during operations can easily extend to or beyond ~ °C [ 7 ]. This demands the synergetic effect of several solid lubricants in order to achieve a low friction and wear at a large scale of temperatures; as few SLs responsible for minimizing friction at low temperatures are also seen to chemically react and generate a lubricious glazed layer at HT and further enlarge the solid-lubrication range [ 8 ]. The key characteristics of an HT solid lubricant (self-lubricating material) are shown in . represents the classes of HT solid-lubricants based on their chemistry (structure) and their general mechanism of friction reduction, which will be discussed in detail in the subchapters.

Minimizing the wear of tribo-bodies through the application of a conventional liquid lubricant is a common phenomenon. However, the oils and greases limitation are their decomposing at temperatures beyond 300 °C and their harmful effects on environmental and human health [ 3 ]. Volatilization, mitigation, and condensation of oil- and grease-based lubricating mediums at extreme conditions (temperature, pressure, altitude) such as in aircrafts, piston&#;cylinder arrangements, optical or thermal control surfaces, etc. are widely accepted. Considering these limitations of liquid lubricants, if applied in the scenario of extreme working conditions, the durability of the mechanical system as a whole may be limited. The use of solid-based lubricants (SL), such as MoS 2 , WS 2 , graphite, PTFE, Ag, hBN, etc., is a viable solution to minimize friction and wear over a wide range of temperatures from room up to ~ °C. Solid lubricants are usually incorporated into the materials (or at the interface of two mating surfaces) in a relative motion, which then is believed to in situ form a lubricious phase or compound due to tribo-chemical reactions at HT [ 3 , 4 ] and to provide a constant transfer of lubricant at the tribo-interface. It is reported that under precise conditions of temperature, humidity, and material composition, they tend to form a &#;glazed&#; self-lubricating layer on the material surface under sliding wear [ 4 , 5 ], which offers a significant reduction in coefficient of friction (CoF) and wear. The main advantages of SL over the liquids are better lubricity, good thermal and chemical stability, improved dimensional stability to achieve finishing with high precision, etc.; however, limitations include its inability to carry away heat and provide damping effects during operation [ 6 ]. Few works on near-dry or minimum quantity lubrication (MQL) or minimum quantity cooling (MQC) utilizing cutting fluids or vegetable oils in combination with solid lubricants (such as PTFE, hBN, CaF 2 , WS 2 , boron oxide, etc.) during machining of difficult-to-cut materials (Ni superalloy) have come into the picture [ 6 ]. However, a lack of promising HT tribological studies in combination with a poor environmental outcome still exists and limits their widespread usage. b shows the percentage of research works published concerning notable solid lubricants in HT tribology since &#;.

A significant increase in the number of operations performed at high temperatures (HT~upto °C) has led to an exponential growth of interest in the field of hot tribology ( a). Wear at HT is a serious concern in a wide variety of technological processes and working systems, including but not limited to material processing, bearings, automotive, metal cutting, hot forging, stamping, forming, etc. In particular, many components function beyond a normal temperature range, unfolding numerous tribological complications, pose a substantial uncertainty in material reliability and performance due to enhanced friction and wear. Changes at tribo-contacts of the interacting bodies and possible new phase formation are common attributes of the HT wear process [ 1 ]. The tribo-bodies are highly influenced by a complex transformation of physical, mechanical, and surface reactivity due to simultaneous synergy of oxidation, diffusion, and tribological stress [ 2 ]. However, some materials such as steel and its alloys are reported to benefit from the protective nature of the tribo-oxide layer generated over its surface at HT sliding [ 3 ]. Nonetheless, easy spallation of the generated tribo-oxide layer owing to its non-adherent nature, ineffective Pilling&#;Bedworth ratio, or lattice mismatch with a host surface is largely conveyed [ 4 ].

2. Potential High-Temperature Solid-Lubricants

2.1. Soft Metals

Soft metals categories an array of materials with relatively low hardness (2.5&#;4 Mohs), such as gold, silver, lead, bismuth, indium, and platinum. The responsible mechanism of lubrication in soft metals is their greater ductility and low shearing strength [5]. The ease to plastically deform during sliding results in the formation of tribo-surfaces allowing a low coefficient of friction (CoF) and wear. Usually, the dynamic hardness for soft metals is higher as compared to static hardness; therefore, a larger force is required to cause the plastic deformation in a dynamic state [9]. However, an increased softness at HT may result in surface extrusion or failure and, thus, in inefficient lubrication [10]. In general, silver (Ag) and gold (Au) are of great interest in the field of solid lubrication due to a good thermal conductivity in combination with a low shear strength, especially in the areas of a high frictional heat development at the wear interface.

2.1.1. Silver (Ag)

Due to its high thermal conductivity (430 W/mK), non-toxicity, and relatively low cost, silver is the most commonly used noble metal as a solid lubricant. However, silver, upon a high inclusion (or coating thicker than 1 µm) in the matrix, can cause high friction and wear rate (in comparison to a virgin substrate). An increased plastic deformation, cutting, plowing, and material transfer to the counter body is commonly reported in such cases [11,12]. Commonly, it is considered that soft metals as reinforcements in matrix tend to be more durable and provide a long-lasting lubricity as compared to the coatings. Quick exhaustion of Ag and its limited lubricity at temperatures above 300 °C, resulting in the coating lapse and increased porosity, is reported in [13]. demonstrates a scheme of lubrication via the diffusion mechanism in a soft metal-based SL.

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It is seen that a silver content of ~15 wt% or more in a host material is beneficial in a decrease in CoF and wear with an increase in temperature [14,15]. At temperature < 400 °C, sufficient reduction in CoF and wear of Ag deposited on Al2O3 substrate is demonstrated [16,17]. However, upon an increase in temperature, the wear rate is accelerated due to expulsion and an increased softening of Ag layers [17]. A reduced and stable CoF at both 400 and 600 °C was reported in Ni-Ag composite [18]. In another study, a significant decrease in CoF and wear was noticed at 200 °C for Ni-Cr alloy-based coating with 10 wt% of each Mo and Ag [19]. However, at a temperature of 400 °C, a high Ag expulsion from the coating results in increased wear. A significant decrease in friction and wear of TiAl alloys incorporated with Ag was demonstrated from room temperature up to 400 °C as compared to a neat alloy in [20]. A five-fold drop in the wear for the Ag-containing films was demonstrated at 600 °C owing to a lubricious tribo-film formation [21]. In most of the works, a 10&#;15 wt% Ag inclusion was found to be the optimum concentration for wear reduction. shows the CoF and relative wear rates for various Ag-based solid-lubricating materials on a wide range of sliding temperatures, as reported in recent publications [10,13,14,15,16,17,18,19,20,21]. The relative wear rate values are calculated after dividing the wear rate value at the reported temperature by room temperature (RT) value. An efficient lubrication range is shaded in b.

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A sizeable level of variance in the range of temperature of effective lubrication has been noted. In addition, a disparity in the range of friction and wear values is possible due to the design in the experiment, test methods, and external factors (operator, etc.) in play. Certainly, the microstructure and pre-existing defects (vacancy, lattice mismatch, voids, pores, etc.) can also greatly affect the diffusion of Ag into the surface, resulting in variation in lubricating capacity. However, pores or cracks may also improve the efficiency of lubrication due to the storage of lubricant in the existing defects [22], demonstrating a self-adaptive behavior due to squeezing out or the storing back of the lubricant so as to accommodate the lubricating film for better lubrication ( ).

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2.1.2. Bismuth (Bi)

Bismuth has been fairly little recognized and mostly confused with soft metals such as Pb and Ti, which share similar physical properties. In recent years, the combinational use of Bi with Pb/Graphite or Cu as a solid lubricant to improve wear property in the material has come into the picture [23,24]. The restricted use of Pb due to its toxicity has raised the interest in Bi as a &#;green and ecologically clean&#; solid lubricant [25].

Bismuth has a low hardness (2&#;2.5 Mohs) and a low melting point (~270 °C), which results in its easy dispersal under asperity contacts, during which local flash temperatures are high enough for Bi melting [25]. Under tribo-conditions, smearing of generated Bi tribolayer protects the direct contact between tribo-bodies. The bismuth tin bronze with 10% Bi exhibited lower friction in comparison to 5% Bi but underwent shrinkage porosity, and bismuth precipitation on the grain boundaries of the matrix has been specified in [26]. Limited shrinkage porosity was shown by solid-lubricating Cu-Sn bearings produced by powder metallurgy [27]. Bismuth is susceptible to forming grain boundary phases that are unfavorable to the mechanical properties of Cu-Bi alloys. However, Sn is shown to be the best alloying element for preventing Bi precipitation on the grain boundaries [28]. The optimal Bi content for bimetal bronze bearings operating under the boundary lubrication condition is 3 wt% of Bi [29]. The mechanical performance of bismuth bronze alloys, CuSn10Bi4 and CuSn6Bi6 in the thrust bearing tests concluded that Bi is not as good dry-lubricant as the lead in the tested alloys due to its poor bearing performance having both low load capacity and a high coefficient of friction (CoF) [30]. Lead (Pb), in addition to Bi and/or graphite, as an inclusion to synergically obtain the best tribo-property, is shown to have the best results.

2.1.3. Other Soft Metals (Au, Cu, In)

The use of electroplated Au-based coating as a solid lubricant in micromechanical and electronics industries is quite common. Gold high ductility and malleability results in easy distribution of frictional stresses during sliding wear conditions [31]. The use of Au in the yttria-stabilized zirconia (YSZ) matrix is shown to improve the sliding wear of YSZ ceramics [32]. Ball-on-disk tribotest results showed that in comparison to reference YSZ ceramic, YSZ/Au coatings demonstrated a significant decrease in CoF and generated less wear debris with limited smearing of Au from the surface. The decrease in CoF (up to 0.2) was owed to the microstructure adaptive changes at elevated temperatures in addition to the formation of lubricious Au transfer films. At least 20 wt% of Au inclusion was stated to cause a diminution of abrasive wear mechanism and impart gold-based lubricity. A considerable decrease in CoF in the range of 0.36&#;0.5 was recorded from RT-800 °C upon 30 wt.% of CaF2 and Au inclusion in a ZrO2(Y2O3) matrix composites [33]. Plastic deformation and material flow were encountered for both CaF2 and Au while sliding. Extrusion or transfer of Au from subsurface to the surface resulted in its dispersion and provided enough lubrication during low-temperature sliding. At temperatures of 400 °C or above, the sliding surface showed the existence of both CaF2 and Au, pointing to a synergetic solid-lubrication phenomenon. The use of Au in conjunction with other solid lubricants to enlarge the lubrication temperature range has also been reported in [34,35]. However, due to Au&#;s high cost, its use is limited in large-scale fields.

Copper (Cu) is another member of the soft metal group, which is widely accepted as a solid lubricant. Its high thermal conductivity (398 W/mK) helps to maintain a low temperature at the tribo-pair contact zone. A decrease in CoF in partially stabilized zirconia (PSZ) from 0.40 to 0.20, 0.17, and 0.14 upon copper powder, copper films, and CuO films inclusions/formations, respectively, is reported in [36]. The effect of Cu in brake friction materials has been studied in [37], and the existence of Cu particles within a definite concentration has been stated resulting in the stabilization of sliding by forming a granular layer of the mechanically mixed layer (MML), which are showing main contact sites of pad and disc and further, decreasing the average CoF and fluctuation peaks during sliding. The recrystallized Cu nanoparticles might act as lubricant in the tribolayer formed during sliding at 650 °C [37]. An addition of 40 vol.% graphite to the copper&#;tin composites showed a low coefficient of friction of 0.15 [38].

Indium (In) based solid lubricants are still scarcely reported. For example, in [39], PVD TiN coatings with indium demonstrated a superior performance up to 450 °C.

2.3. Alkaline-Earth Fluorides

Alkaline-earth fluorides such as LiF, CaF2, and BaF2 are well-known to provide solid lubrication at HT of 500&#;900 °C [3]. This is due to the reason that the material (CaF2) exists at a slip plane (compacting Ca atomic plane), and at HT conditions, the atomic force in the phase decreases, resulting in an easy shearing. However, alkaline-earth fluorides demonstrate poor tribological behavior at low-to-moderate temperature ranges. The responsible mechanism of friction and wear reduction in fluorides of alkaline-earth metals are reported to be their &#;softening&#; around 500 °C, termed as the &#;transition point&#; from a brittle to plastic or ductile state [93]. At low-to-moderate temperatures, they tend to be brittle, resulting in amplified wear (mainly abrasion) due to the third body effect [94]. Though the introduction of both mixtures of CaF2 and BaF2 to decrease the lubrication temperature to 400 °C as a result of lowering the melting point of composites is also reported in ref. [95]. Incorporation of rare earth trifluorides such as LaF3, NdF3 to reduce friction and wear at HT are also conveyed in ref. [96].

An improved friction and wear property of the SPSed ZrO2(Y2O3) matrix composites with an inclusion of 31 wt% BaF2 and 19 wt% CaF2.was demonstrated at temperatures beyond 400 °C [97]. The CoF of the composite stabilized around 0.4, while it escalated for the reference material up to 1 at 800 °C. At RT sliding, the composites demonstrate poor behavior with signs of significant plastic deformation and delamination. A considerable decrease in friction and wear of Al2O3-50 wt% CaF2 composite at 400 °C was reported in [98], while a further decrease by two orders of magnitude at 650 °C, in comparison to the reference Al2O3 was noted. The formation of a Ca-rich lubricious layer on the surface of composites was held responsible for it. However, delamination of the formed lubricious tribolayer was seen at 800 °C, resulting in unstable friction. Similar to the previous works by ref. [96,97] the composites performed poorly at RT.

Cura et al. reported a synergetic effect of Ag and CaF2, resulting in enlarging the lubrication range from 200 to 650 °C [99]. Additionally, widening of lubrication range through the use of CaF2 and Au lubricants in the ZrO2(Y2O3) matrix was studied in [33]. At 400 °C and beyond, the composites demonstrated the formation of a smooth CaF2 lubricating layer, including Au lubricants. A similar effect of synergism was shown by others [100,101,102]. demonstrates the effect of sliding temperature on CoF and relative wear rate of several fluoride-based composites and coatings as reported in recent literature [10,33,97,98,99]. The wear rates are relative to the material&#;s corresponding room temperature values (material&#;s wear rate at a particular HT dived by their RT value).

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In general, the mechanism of synergic lubrication can be divided into three steps: (1) tribo-chemistry&#;at HT, the generation of lubricious compounds from the reaction between fluorides and matrix material occurs, resulting in low friction. (2) Transfer-layer&#;at moderate temperatures, the formation of tribo layer or transfer layer (rich in solid lubricants) protects the direct contact between tribo-bodies and thus minimizing friction and wear. (3) Glaze layer&#;at extreme HT, the formation of an oxide-rich glaze layer (with lubricating compounds) is seen. The glazed layer is reported to carry a wear-resistant and friction-reducing property [1,4]. A schematic of the synergetic effect at HT is presented in .

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2.5. Challenges, Opportunities, and Concluding Remarks

Production of solid-lubricating materials via powder sintering is widely reported. Limitations such as grain growth, poor mechanical properties due to a long holding time during sintering, etc., have surfaced to a quicker sintering technique, i.e., spark plasma sintering (SPS). However, a more efficient, faster, and low-energy consuming technique known as microwave sintering (MS) is equally heightening [128]. The quick heating incurred during MS due to the energy transformation rather than energy transfer (as in SPS) results in volumetric heating, further giving rise to a much finer and uniform microstructure. Apart from powder metallurgy (sintering), PVD techniques to fabricate solid-lubricating coatings have been widely observed. On the other hand, laser claddings to produce thick solid-lubricating coatings have been greatly undervalued despite their effectiveness. Deposition of a single layer using laser cladding technique is studied by several. However, very few exist on multilayer deposition due to limitations in mechanical property of sub-layer during re-melting [129].

The potential of additive manufacturing (3D printing) to fabricate bulks or coatings through layer-by-layer deposition is not yet reported. The possibility to generate complex geometries of solid-lubricating materials can be of high importance. Alternatively, the production of &#;smart&#; solid-lubricating materials [3] demonstrating variations in their mechanical and chemical behavior upon an applied external stimuli using an approach of 4D printing is also foreseen [130].

and demonstrate a graphical approximation of effective temperature range and their corresponding CoF for various groups of solid-lubricating materials (solid lubricants), respectively. In general, no single material exists that can cope with the complete tribological demands of working from room-to-extremely high temperatures (~ °C). However, the combination of various solid lubricants (such as soft metals, fluorides, etc.) to widen the lubrication temperature range (up to °C) is perceived (described in former sections). In order to accomplish the extreme temperature tribological needs, an HT solid-lubricating material should be designed as per the following considerations: (a) a CoF value below 0.2, (b) wear rate below10&#;6 mm3/Nm, and to work on a wide-ranging temperature from cryogenics to HT. There is a need for a more detailed study to understand the synergism of solid lubricants to provide lubrication under the aforementioned considerations over a wide temperature range.

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The promising study of new age &#;diamond like carbon&#; (DLC) coatings to minimize friction (up to <0.01) and wear is reported in few studies [131,132]. Their ability to regulate surface chemistry and structure under sliding conditions has conferred it to be &#;adaptive&#; in nature. In addition to this, the combination of DLC with other solid lubricants such as TMDs, soft metals, etc., is stated to increase the lubrication range [133,134]. A low CoF and long endurance (operation) in dry/humid environmental sliding conditions under humid air, vacuum, and dry nitrogen atmosphere was reported for DLC-based nanocomposite coatings of WC/DLC/WS2 phases [133]. The CoF was 0.1 in humid air, 0.03 in a vacuum, and 0.007 in dry nitrogen. However, more research is needed for DLC-based coatings to understand the phenomena of surface adaptation and wear mechanism, especially during synergetic effects with other solid lubricants.

The current study brings into consideration the dependence of several factors during the self-lubrication of solid lubricants such as environment, operating conditions, the preciseness of testing methods, etc. Most of the work is based on a sliding test, which is incapable of a detailed comment on the behavior of solid lubricants in a dynamic mode of operations [135]. In addition, there exists a major lack of atomistic- and nano-level analysis of the evolving physical and chemical properties of surfaces and/or sub-surfaces, which is expected to broaden the understanding behind discussed mechanisms of operation. There still remains an unclarity in the details of feedstock/precursors composition and their methodical study, raising the concern for the correct experimental inputs. In this regard, the approach of simulation possibly will open the doors for better understanding about the effect of inclusions, their concentration and morphology, chemistry and evolution of buried sliding surface, predicting new inclusions, their reactions with the host matrix, the effect of the environment (cryogenics, vacuum), etc. on the lubrication behavior of solid-lubricating materials.

With an increasing demand for materials to perform at extreme temperature applications to reduce friction and wear in the present industrial revolution, there is a parallel approach to save the environment, energy, and incurred life cycle cost. This exponential rise in material developments has not only propelled us towards environment-friendly footsteps but also towards designing a &#;smart&#; tribo-material, which can be perceived as more efficient and multifunctional in approach ( ). In addition to being adaptive and re-structurable, the new generation of tribo-materials is expected to show properties such as bio-mimicking (inspired from nature such as human skin, snake skin, fish scales, etc. to minimize friction, erosion) [16] and the ability to self-diagnose (such as, in fiber-reinforced plastics, useful in fast damage diagnosis, etc.).

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Multidisciplinary studies in the designing of solid-lubricating materials are also foreseen. Cross-connection of tribology with other areas of physics, materials, mechanical engineering, and biomedical might help to strengthen the investigation in the design of a multifunctional and smart tribo-material. Clubbing with other areas such as information technology is believed to make advancements in &#;self-diagnosis&#; and &#;repair&#; through the use of artificial intelligence [3,136,137]. It is certain that under the canopy of a multidisciplinary approach, the tribology of HT solid/self-lubricating materials will take a leap from &#;self-adaptive&#; to &#;smart&#; to &#;intelligent&#; lubricating material.

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