Air Foil Thrust and Journal Bearing Coatings: A Review

Air foil thrust and journal bearings are used in a variety of high-speed turbomachinery applications, such as in cryogenic turbo-expanders, turbochargers, high-temperature blowers, micro-power generation gas turbines, and aircraft air cycling machines [1]. The bearing design provides for an oil-free, high-speed low-friction axial and radial load support during the operation of turbo-compressors and expanders. Oil-free bearing system design is critically important in applications such as hydrogen compression, where contaminants should be kept to a minimum.

Air foil bearings operate through entrainment of air into a thin converging gap created between the foil and a thrust runner or journal. The fluid-structure elasto-aerodynamic interactions of the entrained air and thin flexible foil create a lubricating film of air, separating the contiguous surfaces. Air foil journal bearings can support loads of up to approximately 10 kN radially and operate at speeds upwards of 150,000 rpm. Axial loads and speeds for foil thrust bearings are shown in Fig. 1. During startup and shutdown operations, the entrainment speed is quite low and the generated hydrodynamic (aerodynamic) load-carrying capacity is insufficient to fully separate the contiguous surfaces, resulting in their direct contact. The surfaces of the foil and the thrust runner may also come into direct contact during destabilizing impact events or because of external excitations. To protect against such instances, surface coatings are employed to enhance the foil, the journal, and runner's durability, robustness, and frictional behavior.

A number of relatively recent reviews have documented the development and the state-of-the-art in air foil-bearing design [1–4]. Despite numerous advantages, foil, journal, and thrust bearings have comparatively low load-carrying capacities compared with their liquid phase counterparts. As already noted, air foil bearings suffer direct surface contact during startup and shutdown. Therefore, surface coatings are critical in preventing premature wear of top foil journal and top foil runner interfaces [5,6]. The key properties of coatings, substrates, and coating—substrate combinations for air bearings are described by Bhushan et al. [7] as

• good mechanical properties: ductility, hardness, and yield strength

• dimensional stability

• corrosion and oxidation resistance

• contiguous surface coating comparability

• thermal and mechanical shock resistance

• thermal conductivity to transfer heat from the interface

• formation of soft protective oxide films

• low surface friction and wear

• good antigalling property if the coating is penetrated

• substrate-coating compatibility

• thermal properties comparable between coating, adhesion layer, and substrate

This paper reviews the application of specific thermo-mechanical conditions at the interface of air foil bearings' sliding surfaces. The state-of-the-art in air foil thrust and journal bearings' coating technologies is highlighted. Trends in coating design are identified and the direction for future air foil-bearing coatings is discussed.

Foil bearings are often required to operate in environments with elevated temperatures due to the nature of their application, for example in micro-gas turbines. Additionally, the air film temperature increases as a result of parasitic viscous shear and compressive heating during entrainment [15]. The primary mechanism of heat transfer within the air film is conduction across the film thickness [16]. Convective heat transfer is limited due to the low density of air. Lehn [16] showed that for air foil thrust bearings approximately 80% of the heat is dissipated away from the contact through convective heat transfer from the reverse side of the rotor disc and through the top foil-to-bump foil and base plates (Fig. 2). Secondary cooling effects include radial convective heat transfer to the housing and forced convection via the air supplied through the bump foils. Salehi et al. [17] also showed similar results for air foil journal bearings, where 80% of the heat transfer took place by conduction through the journal and foils. The remaining 20% was transferred through side leakage.

During machine startup and shutdown direct contact of the top foil and rotor leads to the generation of coulombic frictional heating. On these occasions, machine performance becomes most critical and challenging in nature. The following thermal analysis considers the interfacial thermal problem in the absence of a mediating film of air and in the first instance considers the contacting solid bodies as semiinfinite.

Thrust runners experience radially increasing velocities, often leading to a convex runner surface as a result of viscous heat dissipation and thermal expansion. In addition, centrifugal stress distortions in the runner can occur. To account for these distortions, structural tailoring and compliant foundations are used which are key for effective coating performance [31].

Coating–substrate adhesion is critical for preventing delamination of the coating at their interface [34]. The propensity for delamination and spalling is dependent on the interfacial fracture energy and varies with substrate and coating elasticity, hardness, and ductility [32,34]. With specific regard to the foil coatings, the flexural strength of the coating is the key attribute for its adhesive performance.

The selection of coating pairs with complementary hardness is important to ensure effective running-in and long-term performance of the bearing. The importance of hardness compatibility in the foil-bearing coating system selection is shown in Fig. 5. The results shown in Fig. 5 are taken from an investigation of a foil journal bearing with various coatings [35]. When the foil coating is harder than the journal coating (positive abscissa values), the journal roughness reduces during operation. However, these tests were rated as unsuccessful, in the majority of cases, due to significant reported foil coating wear. In the cases when the much softer coatings are used on the foil, the journal roughness is largely unaffected and all the successful tests are found in this region (within the thermal limits of the coating). Other combinations were also tested in Ref. [35]; however, the hardness data were not available.

The ratio $E′/H$ in Eq. (10) and a variant of it in Eq. (12) relate to the coating and substrate system to accumulate elastic strain before failure and is the key parameter in determining tribological performance and elastic resilience of the interface in an impact [41].

The flexural strength of the coating is also of importance due to contact deformation of the top foil and thermo-mechanical distortion of the thrust rotor. It has been shown that multilayer coatings, comprising inter-layers of low elastic moduli reduce the bending stresses in comparison to the hard coatings alone [41]. In addition, the use of thin hard coatings is effective at reducing bending stress and improving fatigue life [42].

Much more attention should be paid to the failure of coatings, often caused by generated sub-surface stresses as the result of combined direct normal loading and shear [43,44]. However, most contact mechanics analyses are developed for nonconforming contacts, mainly based on Hertzian or neo-Hertzian conditions, where the contacting surfaces may be considered semiinfinite in nature. This is not usually the case for thin hard coatings, regarded as thin bonded layered solids with various degrees of conformity [45,46]. Solutions for coated surfaces of hard materials with various degrees of conformity now exist, opening the way for detailed contact mechanics analysis of air foil and journal bearings.

In summary, a variety of thermo-mechanical coating—substrate and coating—coating properties have been identified so far. These properties will be used later for the critical review of the current state-of-the-art air foil-bearing coatings.

A variety of coating techniques are available for foil, journal, and thrust runner surfaces. Before the coating is applied, the substrate condition may determine the appropriate method for surface preparation. For example, the use of undercoats may be necessary to prevent the oxidation of ferrous substrates, and surface cleaning procedures or surface roughening.

A variety of techniques are prevalent for foil coating. These include physical vapor deposition (PVD) techniques such as sputtering [47,38], radio frequency (RF) plasma-assisted chemical vapor deposition [8], electroplating [8], fusion process [49], and other proprietary processes [6]. Thermal spray processes, including the use of a detonation gun, high-velocity oxygen fuel spray, plasma spray, and flame spray are considered too aggressive for thin foil substrates. Plasma spray coatings may require surface roughening pretreatments, such as grit blasting which have been shown to adversely damage the foil [6]. Although detonation gun coating does not usually require any pretreatment, foils tend to wrinkle and distort due to mechanical and thermal stresses even when continuously cooled [6].

A key element of the method of coating is the bonding strength, and the coating's ability to handle high centrifugal and thermal stresses. For example, PS304 HVOF with small nickel-chromium (Ni-Cr) and Cr₂O₃ particle sizes have been used to reduce PS304 coating roughness after finishing with polishing papers and compounds. Although initial roughening during operation provides little benefit in terms of roughness characteristics [50], improvements in bond strength in comparison to plasma spray methods have been noted. For thick (∼100 μm) hard face coatings by plasma and detonation gun methods, further efforts have been made to improve PS304 dimensional stability through heat treatment cycles [51]. Composition grading to reduce the generated thermal stresses is also proposed in Ref. [51]. Alternatively, electroplating can produce thinner (∼1-μm-thin dense chromium coatings). Reduced coating thickness lowers stresses due to centrifugal and thermal sources [50].

Broadly three categories may be used for applications of coatings in foil bearings: (i) low-temperature applications <300 °C, (ii) mid-range temperature applications: 300–700 °C, and (iii) high-temperature applications >700 °C. The journal substrate material is typically air-hardened AISI A2 tool steel [52], or A-286 age-hardened, or iron-nickel-chromium superalloy or similar materials [53]. Foil substrates are commonly made of nickel-based superalloys such as Inconel X-750, Inconel 625, or Inconel 718. They are primarily chosen for their high-temperature mechanical properties.

Polyimide, polyamide, and polytetrafluoroethylene (PTFE) foil coatings are generally limited to a maximum temperature of 250 °C and are frequently paired with a hard chromium or thin dense chromium-coated surface. This action produces favorable antigalling characteristics in case the polymer foil coating is worn through. These coated films are often doped with a solid lubricant such as graphite, molybdenum disulfide (MoS₂), and boron nitride. Polymer coatings provide low cost and acceptable life performance when not subjected to excessive temperatures [54]. Zywica et al. [5,55] showed that a commercially available fluoropolymeric coating (AS20) paired with a Cr₂O₃-coated journal could present an acceptable wear behavior for 10,000 start–stop cycles at room temperature with minimal wear. The maximum operating temperature of the coating was given as 280 °C.

WS₂ and MoS₂ overlays have been shown to provide promising results by Bagiński and Żywica [56], who used a foil-bearing test rig with a chromium oxide Cr₂O₃-coated journal to show that MoS₂ provided the lowest friction. However, the MoS₂/C (carbon-doped molybdenum disulfide) on a hard titanium aluminum nitride (TiAlN) provided the best trade-off between friction and wear, notably outperforming a WS₂ overlay. A comparison of the coatings is highlighted in Fig. 6. Critically, all the experiments were conducted at cold startup conditions and therefore, the results do not indicate high-temperature performance. Interestingly, WS₂ and the carbon-doped MoS₂/C are reported to have the same maximum operating temperature of 500 °C [39,56].

Korolon™ developed by Mohawk Innovative Technology Inc has been used in ramjet and gas turbine engines. Tungsten disulfide WS₂ (Korolon™ 900 and Korolon™ 800) overlays have been shown to be highly effective for both friction and wear performance [8,9,57]. WS₂ top foil coatings have been shown to perform well against chromium-coated journals [8,9,57]. Furthermore, Heshmat et al. [57] showed excellent wear and friction behavior for dense chromium-coated journals against WS₂ with a sacrificial solid lubricant overcoat (Korolon™ 800). A comparison of coating performance under initial testing at 650 °C is shown in Fig. 7. The coating was demonstrated on a 240-lb thrust turbojet engine, conducting 70 stop–start cycles (up to 54,000 rpm) over a running period of 14 h.

Bhushan and Gray [58] investigated the thermal stability of a number of candidate foil and journal coatings using extended duration tests (300 h) and thermal cycling up to temperatures of 650 °C. It was shown that sputtered foil coatings of TiC, Cr₂O₃, and Si₃N₄ and CdO-graphite and chemically adherent chromium oxide were statically thermally stable. In the case of the journal: plasma-sprayed Ni-Cr-bonded CrB₂, Co-Mo-Cr-Si with Ni-aluminide undercoat, Cr₃C₂ with Ni-Cr binder and NASA PS106 (Ni-Cr-Ag-CaF₂) alongside sputtered Cr₂O₃ and chemically adherent chrome-oxide had most promising static thermal stability. Bhushan [59,60] optimized and characterized RF sputtering parameters for depositing hard refractory chromium oxide coating onto Inconel X-750 foils. Bhushan [61] developed and tested a CdO-graphite-Ag coating using extended start–stop air foil journal bearing tests at temperatures up to 427 °C and 14 kPa contact pressure. The coating endurance was improved by introducing ultrafine silver, performing 9000 start–stop cycles at 288 °C and 28 kPa and 3000 cycles at 35 kPa.

In an exhaustive experimental study, Bhushan and Gray [35] investigated 15 combinations of foil and journal coatings using a bearing test rig. The most promising combinations for <370 °C were a foil coated with CdO and graphite and a chrome carbide-coated journal. While for temperatures <540 °C, a foil coated with NASA PS-120 (Tribaloy 400, silver, and CaF₂) against an uncoated foil, and for temperatures <650 °C, both the foil and journal should be coated by a chemically adherent Cr₂O₃. The chemically adherent Cr₂O₃ coating system was successfully tested with a load of 35 kPa for 2000 start–stop cycles. Bhushan [53] showed that chromium oxide coating, sputtered onto an InconelX-750 foil, running against a detonation gun applied chromium carbide on an A286 journal performed very well during a variety of start–stop, impact, and static oven tests. The coating successfully completed 3000 cycles at a temperature of 650 °C and was subjected to a normal pressure of 14 kPa in a partial arc bearing test. This included 9000 start–stop cycles at a maximum test temperature of 427 °C, although the coating could have continued for much longer under these conditions. During start–stop cycles at a higher pressure of 35 kPa the coatings survived for 3000 start–stop cycles. The coatings also survived 100 g impact accelerations with the journal running at 30,000 rpm. The roughness of contiguous surfaces was 0.05 µm and the coating thickness was ∼1 µm.

Chromium oxide (Cr₂O₃) was used in the work of Zywica et al. [5], covering a steel journal using a plasma-spraying technique. For this application, the bearing foil was made of Inconel Alloy 625. A thin polymer coating (AS20) was also applied on one side of the top foil to prevent any damage to the foil during startup and shutdown. The coating was chosen for reduction of friction, based on experimental work investigating a range of commercially available coatings including (AS18, AS20, AS48, AS783, and AS785) and composite and nanocomposites such as MoS₂TiW and nc-Wc/a-C. They also highlighted the importance of shortening the duration of startup/shutdown phases, noting that polymer coatings produced a rapid temperature rise during the running-in period which subsequently decreased. Bhushan [53] found that Cr₂O₃ was effective as a foil coating.

The use of vanadium nitride (VN) and TiSiN (applied to both front and back surfaces of the foil) and MoS₂ on the front surface in gas foil bearings was investigated by San Andrés and Jung [62] under a pressure of 25.6 kPa at 50,000 rpm with applied excitation frequencies in the range of 200–400 Hz. The use of VN and TiSiN-coated top foils was shown to reduce the energy losses during startup and shutdown. However, all tests were conducted at room temperature.

For high temperatures, Bhushan [63] developed a coating combining hard-wearing Cr₂O₃ and the ductility and thermal expansion coefficient of a nickel chromium combination (Ni 80%: Cr 20%) binder. The coating was applied to journal foils and compared with a sputtered Cr₂O₃ coating using start–stop and high-speed rubbing tests. A chrome carbide-coated journal surface was used for both. The C_r2O₃ completed the test (9000 cycles) while the Cr₂O₃ with Ni-Cr completed 3000 cycles. Interestingly the Cr₂O₃ with metallic binder had significantly lower friction at 650 °C compared with room temperature [43]. Later PS304, a composite solid lubricant coating, was developed. The coating comprised silver (10% wt), eutectic barium fluoride (BaF₂) (5% wt), and calcium fluoride (CaF₂) (5% wt), combined with chromium oxide (Cr₂O₃) as a hardener (20% wt) within a Ni-Cr matrix, acting as a binder (60% wt) [64]. The roughness of the coating is dependent on the deposition and finishing technique used. Using HVOF instead of plasma spray and small particle sizes and 1500 grit SiC paper for polishing, roughness values of 0.05 µm can be achieved. PS304 acts as a solid lubricant coating originally developed to address the limitations of chromium carbide coating in terms of reliability and high processing costs [50]. In addition, PS304 has been shown to initially roughen during running-in before polishing to a desirable roughness of <0.1 µm [50]. PS400 improved the performance by reducing the need for postprocessing to achieve an acceptable surface finish. DellaCorte and Edmonds [65] showed high temperatures (e.g., 800 °C) are required initially to create a lubricious glaze. After the glaze was formed during an initial high-temperature run, the low-temperature frictional performance was much improved. Heshmat et al. [50] concluded that directly applied thin dense chrome on the foil and run against the same, or alternative high-temperature coating on the shaft, provides an excellent life at temperatures of up to 820 °C in comparison to PS304. Stanford and DellaCorte [66] used a novel ion diffusion technique to apply CU-4Al coatings to foils. They tested the coating against a PS304-coated journal using a bearing test rig. The experiments indicated the foil coating assisted with break in and gave more stable friction across a range of temperatures and reduced top foil wear.

As with the case of journal bearings, the review of thrust air foil bearings is divided into low-temperature, mid-range temperature, and high-temperature applications. The thrust runner is typically steel, while the foils (like in the case of foil journal bearings) are commonly made of Inconel X-750, Inconel 625, or Inconel 718.

Kim et al. [67] showed the effective use of a PTFE thrust bearing-coated top foil. However, they did not mention the runner material or its coating. Balducchi et al. [68] also used a PTFE-coated top foil against a hard-coated titanium runner. Walker et al. [69] investigated a wide range of foil coatings using a scratch test machine to investigate their cohesive and adhesive behavior as well as thermal stability using a high-temperature oven. The investigated PTFE coating was not favored for further testing. However, Ni-P and Si-O (13–23 µm), MoS₂, WS₂, and PS400 were recommended. Interestingly, MoS₂ showed a wide standard deviation for measured roughness values after heating when compared with WS₂. WS₂ is known to have a higher maximum operating temperature, reported as 500 °C [18], although some have reported even higher values [60]. An organically bonded (Dow-Corning Molykote 88) MoS₂-coated thrust air-bearing top foil and a flame-plated Cr₂O₃-coated runner were shown to provide acceptable performance during adverse operating conditions [61].

Heshmat et al. [57] compared a variety of Korolon™ coatings on an Inconel X-750 top foil, including a polymer-based coating containing solid lubricants. A WS₂ coating with solid lubricants and a nickel chromium coating (also containing solid lubricants) (Korolon™ 1350A) with and without a WS₂ overcoat (Korolon™ 800) have also been investigated. These combinations were paired with a variety of coatings on the journal surface, including a PS304 plasma-sprayed hard chromium coating. Heshmat et al. [57] used a thrust bearing pad tribometer for a series of ramp-up and shutdown tests at 30–810 °C for 100- to 500-s cycles. They found a dense chromium coating against a WS₂ with a solid lubricant overcoat of a nickel-chrome provides the best tribological performance. The maximum service temperature of the WS₂ with a solid lubricant overcoat is 385 °C and for the case of nickel-chromium coating is 800 °C. The coating was demonstrated on a 240-lb thrust turbojet engine, conducting 70 start–stop cycles (up to 54,000 rpm) over a running period of 14 h. Jahanmir et al. [8,9] conducted room temperature tests for WS₂ coating (Korolon™ 900) against counter-faces of DLC, chromium, or hydrogenated DLC. It was found that at room temperature the performance of the DLC and chromium surfaces were quite similar. At high temperature (500 °C), the H-DLC provided the lowest coefficient of friction in the boundary regime of lubrication. However, reduced hydrodynamic lift at high temperature was noted. The DLC coating was found to induce a higher wear-rate on the pad coating. The increase in WS₂ wear with DLC was attributed to its hardness but also PEVD DLC films have been shown to have high nanoscale roughness [70,71] (see Eqs. (10) and (11)). For all surfaces, the benefits were observed when the WS₂ film was on the foil and the hard coating was applied to the runner.

Fanning and Blanchet [10] investigated coatings for air foil thrust bearings using a test rig comprising a single top foil against a rotating disc. It was noted that Inconel X-750 top foil and a PS304-coated runner disc provide the best low-speed frictional performance. The authors also showed that the roughest counter-face surfaces (Korolon™ 1350A coated foil and PS304-coated runner disc) provided the lowest speed hydrodynamic lift-off and hydrodynamic friction. Korolon™ 1350A is a 25-µm-thick nickel-chromium coating with an overcoat of 50-µm-thick WS₂ including solid lubricants. The resulting surfaces from the reported tests are shown in Fig. 8.

Heshmat et al. [57] reported a breakthrough in the performance of foil thrust bearings in terms of load-carrying capacity, speed, and operating temperature through the use of Korolon™ 1350A coating, which allowed for bearing operation at temperatures of up to 815 °C with PS304 as a composite solid lubricant. PS304 contains silver (10% wt), eutectic barium fluoride (5% wt), and calcium fluoride (5% wt) combined with a chromium oxide hardener (20% wt) within a nickel-chromium matrix (60% wt). While silver is used for low-temperature solid lubrication, eutectic barium and calcium fluoride are used as solid lubricants operating from ambient to 900 °C. The nickel chromium-to-chromium oxide ratio of three to one provides a comparable thermal expansion coefficient (12.4 × 10⁻⁶ m/°C) to that of common substrate materials such as Inconel X-750 (14 × 10⁻⁶ m/°C) [72]. Low-temperature lubricating characteristics of silver complement the high-temperature characteristics of the eutectic barium fluoride and calcium fluoride to provide self-lubrication from ambient to 900 °C [73]. Dykas and Tellier [31] also chose a PS304-coated runner against an Inconel X750 top foil to investigate early component life wear and reported similar findings.

Fanning and Blanchet [10] noted that HVOF-deposited PS304 disc coatings (using a hydrogen-fueled system) provided enhanced lift-off and touch-down speeds. It also required fewer running-in cycles to reach steady-state conditions.

Blanchet et al. [73] investigated the use of PS304 in thrust-washer tests, running against Inconel X-750 at low contact pressures of 40 kPa, sliding speeds of 5.4 m/s and both ambient and 500°C temperatures to simulate conditions in air foil bearings during startup/shutdown conditions. In all cases, the coefficient of friction was around 0.5, while the wear-rate ranged from 1 to 3 × 10⁻⁴ mm³/Nm. They noted that running under continuous sliding resulted in the roughening of surfaces (⁠$Rq$ > 2 µm) and large recesses with (>100 µm) were filled with fine debris, observed on the wear track. This was in contrast with the observation of surface polishing (smoothening) high-temperature foil-bearing operations experienced in cyclic startup and shutdowns.

According to Heshmat et al. [50], running of PS304 against Inconel X750 foils at elevated temperatures, upward of 650 °C, in start–stop cycles led to target roughness of $Rq$ = 0.1 µm, but the generated glossy surface was achieved after consuming many foil bearings. Furthermore, the load-carrying capacity when the same foil was used against PS304 was around 10% of the case where a thin dense chromium coating was applied on the runner surface. The results of tests carried out by Radil and DellaCorte [74] confirm that without the presence of such low-friction glossy surfaces, high starting torques and reduced load-carrying capacities are inevitable.

A summary of key coatings presented in the preceding sections is provided in Tables 1 and 2 of the Appendix, for foil journal and thrust bearings, respectively. In low-temperature applications, the use of soft coatings such as MoS₂ and polymer composite coatings has been effective. These coatings are generally limited to relatively low thermal conductivities and diffusivity, high thermal expansion coefficients, and limited maximum operation temperatures. However, it must be noted that the specific properties can vary depending on the exact composition and structure of coating. These coatings are commonly applied to the foil as at high Peclet numbers (⁠$Pe$⁠), resulting from appreciable sliding speeds before lift-off or shutdown, the contact temperature rise is controlled primarily by the journal and rotor coating thermal properties as shown in Eq. (7). The low-friction soft coatings help to minimize the generated heat in the contact (Eq. (5)). These coatings are paired with conductive journal and rotor coatings such as nickel-chromium or Cr₂O₃ based coatings.

At higher temperatures, hard wear-resistant ceramic coatings and metal matrix composites are more commonplace. The oxidation resistance of these coatings allows for higher upper operating temperatures experienced in these applications. To maintain low abrasive wear high surface hardness of the two surfaces is key. Coatings such as nickel-chromium also can minimize differences in thermal expansion coefficients of coating and substrate (depending on the substrate material's thermal properties) (Eq. (8)) while high thermal conductivity reduces temperature differences between the coating and substrate (Eqs. (7) and (9)). To minimize frictional heat generation (Eq. (5)), composite coatings are used such as PS400, containing friction-mitigating silver, eutectic barium fluoride, and calcium fluoride.

When coatings such as hydrogenated DLCs are used, considerations of the thermal implications are key, among other important properties. For example, the foil-on-disc test rig used by Jahanmir et al. [9] showed the importance of rotor coating thermal properties. Jahanmir et al. [9] investigated the difference in the performance of DLC and chromium-coated rotors. At higher temperatures, the DLC-coated rotor led to accelerated wear of the WS₂-coated foils. This in part is due to the rise in interfacial temperature in the contact caused by the thermal conductivity of the rotor DLC coating (Eq. (7)). The nascent gas film formed during lift-off can also be reduced due to elevated gas film temperatures and interfacial slip (Eq. (2)).

At a high-performance level, air foil bearings incorporating smart materials have been proposed by Martowicz et al. [75]. The implication for thermal monitoring and structural and thermal control, integrated into coating design, is quite promising. Designing the tribological coating system, thermal management system, and machine operating performance together through advance monitoring techniques provides a promising route to advanced machine performance. There is however a significant demand from emerging industries for high volume lower cost air foil bearings requiring advancements in low-cost mass production manufacturing for air foil-bearing coatings and this poses a significant challenge.

The effective performance of air foil bearings in high-speed applications relies heavily on advanced coatings for both journal and thrust bearings. The current study emphasizes the crucial role of coatings in overcoming challenges during startup, shutdown, and high-temperature operations. It categorizes coating technologies according to temperature ranges of applications, highlighting specific composite coatings, for breakthrough improvements in load-carrying capacity and reduction of friction. The insights provided offer a roadmap for enhancing the efficiency and reliability of air foil bearings in various industrial applications, particularly in turbomachinery.

There are no conflicts of interest.

No data, models, or code were generated or used for this article.

$c$ =

empirical wear constant

$d$ =

thickness of coating

$h$ =

film thickness

$m$ =

work hardening exponent

$p$ =

pressure

$q$ =

coulombic frictional heat

$A$ =

apparent interfacial contact area

$D$ =

depth of heat penetration

$H$ =

hardness

$K$ =

thermal conductivity

$L$ =

contact length

$N$ =

contact load

$T$ =

temperature

$W$ =

wear volume

$hm$ =

minimum film thickness

$kb$ =

Boltzmann's constant

$ko$ =

wear coefficient

$q2$ =

heat transferred to the foil

$Ec$ =

Young's modulus of elasticity

$Klc$ =

plane strain fracture toughness

$Re$ =

effective radius

$Tf$ =

friction torque

$Tm$ =

maximum flash temperature rise

$E′$ =

composite (effective) modulus of elasticity

$Kn$ =

Knudsen number

$Pe$ =

Peclet number

$ΔT$ =

temperature difference between coating and substrate

$α$ =

coefficient of thermal expansion

$β$ =

average asperity tip contact radius

$γ$ =

frictional heat partition ratio

$δ$ =

molecular diameter

$ΔσT$ =

change in thermally induced interfacial stress

$κ$ =

thermal diffusivity

$λ$ =

Stribeck gas film ratio

$μ$ =

coefficient of boundary friction

$vc$ =

Poisson's ratio of coatings

$σ$ =

root mean square surface roughness

$σy$ =

yield stress

$τ$ =

ratio of coated and uncoated flash temperature rise

$Ψ$ =

plasticity index

$ω$ =

angular velocity

Tables 1 and 2