Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review

*Divyansh Mittal, Daljeet Singh and Sandan Kumar Sharma*

#### **Abstract**

Solid lubricants separate two moving surfaces and reduce wear. Materials' ability to act as solid lubricants depends on their characteristics relative to contact surfaces. Chemically stable fluorides (BaF2, CaF2), boron nitride (h-BN), transition metallic sulphides (MoS2, WS2), soft metals (Au, Ag), binary and multi-component oxides, such as silver-containing sulphates, chromates, and oxides, and MXenes are effective solid lubricants. Solid lubrication depends on the material's structure. Structure, mechanical properties, chemical reactivity, and kind of substance characterise these materials (refractories, ceramics, glass, etc.). High temperatures (>300°C) are obtained at asperities due to frictional heat produced when two surfaces rub. High temperatures can breakdown lubricants, but the resulting compounds must be lubricants; otherwise, corrosive vapours or abrasive solids can occur. High thermal conductivity helps lubricants remove heat generated by rubbing. Lubricants must not be melted, as the solid will lose strength and distort or be removed like liquid. Tensile strength, compressibility, and hardness are significant mechanical qualities for solid lubricants in extreme conditions. This chapter discusses solid lubricants and their structure. Also discussed are solid lubricants' mechanical and thermal properties. The lubricating mechanism and conclusion are also conferred.

**Keywords:** solid lubricants, tribology, friction, wear

### **1. Introduction**

Solid lubricants are defined as solid materials that are consciously added to or naturally generated on contact surfaces while they are in motion to decrease wear and friction and offer protection from damage. These are primarily used in harsh conditions, i.e., high temperatures, abrupt changes between vacuum and moist air atmosphere, heavy loads, high speeds, chemically reactive environments, and severe thermal shock circumstances [1–5]. Severe wear and oxidation, high friction, and premature failure are unavoidable in the absence of a lubrication mechanism under the aforementioned operating circumstances. The self-lubricating parts are necessary for a variety of industries, including hot metal processing, power generation, automotive, aviation, and aerospace to operate consistently and dependably. The relative motion between rolling/sliding surfaces makes sense for solid lubrication research to focus on minimising and eliminating material and energy losses. When exposed to harsh environments, effective solid lubrication will help to decrease the tendency to failure of components, to enhance the material and energy utilisation by increasing the efficacy and overall performance. There is a sizable demand for lubricating ceramic, metallic, and polymeric parts in rolling and sliding contacts, including, cylinder wall/ piston rings, variable stator vane bushings, bearings for space satellites [1, 2, 6–8]. The tribological design primarily concerns about minimization of unfavourable wear and in and contemporary engines, seals, and bearings for next-generation propulsion systems is the. To further improve engine efficiency and lower NOx and CO2 emissions, these components' friction coefficients and wear rates should be lower than 0.2 and less than 10−6 mm3 N−1 m−1, respectively, and independent of ambient temperature, applied load, and sliding velocity. It has been challenging to develop low friction and wear exhibiting materials over a broad temperature range [9]. Currently used solid lubricants face significant challenges, particularly in terms of chemical stability and high-temperature structural or intermittent but reliable lubricity of surface. Ballistic missiles, hypersonic transportation, and other motion systems also need an atmosphere with exceptionally high working temperatures [10]. Multicomponent oxides, binary oxides (V2O5, B2O3, and PbO), layered materials (transition metal dichalcogenides, hexagonal boron nitride, and graphite), soft metals, polymer composites, and alkaline earth fluorides are all taken into consideration as solid lubricants and self-lubricating materials/coatings in this chapter (Ti3SiC2, Ti2AlC, etc.). It is feasible to directly apply high-temperature solid lubricants to the surfaces of machinery parts by using straightforward techniques like painting and burnishing. Examples of methods for creating self-lubricating materials or coatings at hightemperature include additive manufacturing, hot isostatic pressing, laser cladding, chemical vapour deposition, electrodeposition, thermal/plasma spraying, magnetron sputtering, spark plasma sintering, pulsed laser deposition, hot pressing, and pressureless sintering [2, 8, 11–14].

The purpose of this chapter is to deliver a theoretical framework for the tribological properties of solid lubricants. Tribological properties of different types of solid lubricants, such as chemically stable fluorides (BaF2, CaF2), boron nitride (h-BN), transition metallic sulphides (MoS2, WS2), soft metals (Au, Ag), binary and multicomponent oxides, such as silver-containing sulphates, chromates, and oxides, mixtures of various solid lubricants and MXenes has been discussed in the following section.

### **2. Tribological behaviour of different solid lubricants**

The most often used solid lubricants are molybdenum disulfide, graphite, soft metals, and polytetrafluoroethylene (PTFE) [1–5, 10, 15]. Despite the fact that each of these lubricants has some limits, they have all been widely utilised, either individually or in various combinations. New self-lubricating compounds and composites/ coatings have been developed as a result of the shortcomings of conventional lubricating solids. Numerous polymers are often used at cryogenic temperatures or in vacuum because of high chemical stability, excellent machinability, and low densities and friction coefficient [10]. The following subsections discuss the tribolological properties of each solid lubricant in brief.

*Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

#### **2.1 Polytetrafluoroethylene (PTFE) and polyimides**

Currently, polytetrafluoroethylene (PTFE) and Polyimides are used as solid lubricants because of their thermal stability in broad range of environment change and superior tribological properties. However, these have some limitations such as poor radiation and dimensional stability, low strength, high thermal expansion, and poor thermal conductivity, which limit their applications [10]. For these molecules, the hexagonal structure with lattice constant 0.562 nm is laterally packed [15]. The ease of gliding parallel to the c axis of these rod-like molecules results in PTFE's low friction feature. Since PTFE, including graphite and MoS2, exhibit the lowest coefficient of friction, can be used as a bearing material and retains its lubricity and mechanical properties up to 260°C [10]. PTFE typically has a friction coefficient of 0.04 against steel, but under exceptionally demanding conditions, it can reach 0.016 [10]. The PTFE coating results in decreasing the wear rate by 39% of finger seals [16].

PTFE is generally used with additives such as graphite, fluoride, MoS2, or powdered graphite in order to minimise the cold flow under high speed and heavy load circumstances. These powdered additions typically decrease a material's transport capacity even if they might improve its tribological qualities. The application of fibre reinforcement in this situation satisfies the requirement for maximum load capacity [17]. In aviation bearings and those bearings that are subject to heavy loads, woven fabric fibres are typically utilised to increase the bonded PTFE liners' creep resistance. Non-metallic plain cylindrical bearings with a load capacity of more than 207 MPa and minimal wear and friction up to 121°C were tested. The bearings are constrained, though, by creep deformation at higher temperatures [1].

A collection of polymeric synthetic resins with the imide group that are resistant to high temperatures, corrosion, and abrasion are known as polyimides. It is generally used in films and coatings. The polyimide varnish shows reduction of wear and friction of coating up to 500°C by addition MoS2 or CFx solid lubricants. Polyimide-bonded CFx films have been shown to be efficient gas bearing lubricants up to 350°C. Alkalies quickly degrade polyimides, despite their resistance to the bulk of conventional solvents and chemicals. Polyimides were frequently used in mechanical components, seals, gears, bearings, and at Rolls Royce, Pratt & Whitney, GE Aircraft Engine, etc. One such illustration is the DoPontTMVespel CP-8000 material utilised for the stator bushings in the compressor of the BR710 engine. Fibercomp has a compressive strength of 172 MPa at 260°C and a friction coefficient of 0.1 to 0.2. Surface brittle fracture from polyimides' susceptibility to brittleness always results in wear damage [10]. It's interesting to note that the UV-assisted direct ink writing (DIW) technique can create polyimide 3D architectures with only about 6% volume shrinkage [18]. The superior tribological behaviour showing self-lubricating devices using digital light processing and post-heat treatment was printed using PTFE-filled photosensitive polyimide (PSPI). The PSPI-7 weight percent PTFE composites that were 3D printed have strong mechanical characteristics, such as improved interlayer bonding, thermal stability up to 390°C, and tensile strengths more than 90 MPa. Furthermore, wear rates are lowered by 98%, while friction coefficients are reduced by 88%. For instance, at 20N, the surface lubrication and alternate lubrication friction coefficients are each lowered to 0.09 and 0.04, respectively. A significant targetregion lubricating bearing made in 3D printing was successfully demonstrated.

It can be summarised that PTFE is solid lubricant with numerous of characteristics that includes self-cleaning capability, low friction, high electrical resistance, heat resistance to a wide range of temperatures, corrosion resistance, durability, and non-flammability. This type of film lubricant lowers friction between two surfaces without the use of oil or grease. Low-friction, corrosion resistance, and dry lubrication are some of the applications for PTFE lubricant.

#### **2.2 Soft metals**

Hard Metals Pure metals like Pt, Au, Zn, Pb, Sn, Ag, and In, some of which function as solid lubricants due to their sufficient softness. The formation of a shearsimple tribo-layer and increased ductility are the primary mechanisms of lubrication of soft metals. The low melting point having soft metal films of Zn, Pb, Sn, and In related alloys feature multi-slip systems that can effectively correct for microstructural faults through frictional heat during sliding when operating at low temperatures and lightly loaded conditions. However, the Mohs hardness of Ag, Au, and Pt is low, and their melting temperatures are high. **Table 1** displays the tribological behaviour of materials that self-lubricate and contain soft metals [19–23].

The coatings of binary Sn-Co alloy are frequently utilised in engineering applications to replace hard chromium coatings. Ion-plated Pb coatings outlast sputtered MoS2 coatings in rolling contact bearing applications due to the production of lubricious PbO inside the coatings. These coatings were developed for use on rolling elements that rotate slowly, such as those used in space mechanics. Pb-Sn-Cu coating on steel is used for decades to prevent rust. Lubricious Ag-Cu-Pb-S and Pb-Sn-Ag additives were equally dispersed throughout the TiC-reinforced high-speed steel pre-forms to establish an interpenetrating network microstructure for minimising friction and wear at extreme temperatures [24]. At 300–700°C, the wear rate can even be decreased by two orders of magnitude. Ag, Au, and Pt are examples of soft metallic coatings that have been produced utilising electrodeposition and physical vapour deposition methods. These coatings are especially helpful in hostile conditions and at extremely high temperatures, like in spacecraft. It is feasible to avoid undesirable subsurface cracking, extreme wear, and friction by utilising oxygen-ion assisted screen cage ion plating to bond Au and Ag lubricating coatings. In this case, low and constant friction coefficients and little wear were achieved principally due to the high adhesion of lubricious Ag and Au films to alumina [25]. Self-lubricating Au and Ag films can be used in advanced jet engines, spaceships, and fast-moving equipment with light loads. An Au-Co alloy coating was created to efficiently lubricate the roll ring assembly on the space station. Examples include silver [22] or MoS2 [26] which have hard surface texture to reduce wear and friction. Additionally, a number of metals, such as Cr, Mo, Ni, Cu, and Fe, have high friction at room temperature, but as they slide above their oxidational temperatures, their coefficients of friction significantly improve and are reduced to just about half of their initial values. The improvement in sliding friction is related to the development of oxidised products in the wear track. For tribological applications, like turbomachinery parts of fretting interfaces, seals, and bearings, up to 580°C, films made by ion bombardment assisted deposition (IBAD), magnetron sputtering, plasma spraying, pulsed laser deposition, and AlCuFeCr, Cu/Mo, Zn/W, Ni/Ti, and Au/Cr multi-layered techniques have been created recently.

It can be concluded that soft metal such as silver, tin, and lead acts as self-lubricating film on a hard substrate because of their low melting temperature and shear strength. Tin and lead have been coated on the piston skirt surface as these can be used as the self-lubricant overlay. As aforementioned, lead has been widely applied in internal combustion engines as an overlay for journal bearings. However, polymer


**Table 1.**

*Tribological behaviour of soft metals.*

*Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

film having solid lubricants like PTFE, graphite and MoS2perform lower friction properties and has been applied as the overlay substituting the soft metals.
