Article Planar Contact Test Method Applied to Solid Lubricants

Henrik Buse 1,2,* , Fabian Schueler 3 and Erika Hodúlová 4,*

1 Institute of Production Technologies, Faculty of Materials Science and Technology, Slovak University of Technology, J. Bottu 23, 917-24 Trnava, Slovakia 2 Hochschule Mannheim—Kompetenzzentrum Tribologie, Paul-Wittsack-Straße 10, 68163 Mannheim, Germany 3 Materiales GmbH, Offakamp 9f, 22529 Hamburg, Germany; [email protected] 4 Faculty of Materials Science and Technology, Slovak University of Technology in Bratislava, Paulínska 16, 917-24 Trnava, Slovakia * Correspondence: [email protected] (H.B.); [email protected] (E.H.); Tel.: +49-621-292-6530 (H.B.)

Abstract: A new method of material and testing is demonstrated with a planar contact fretting tribometer under typical fretting wear conditions. The usual abstraction of contact geometries with an easy-to-align point or line contacts is deliberately dispensed to do justice to the frequently flat contacts of machine elements (shaft-hub connection, seats, etc.). For the study, a new method of targeted observation of the contact surfaces during the test is used, which allows a time-lapse animation of the fretting wear progress of solid lubricant mixtures. Thus, the formation of possible transfer film build-up and the type of wear mechanism occurring can be visualized. This technique represents, in conjunction with additional analytical methods such as microscopy and SEM/EDX, a powerful tool to provide a better insight into the mechanisms of solid lubricant action under fretting conditions. To demonstrate the potential of this approach, a time to damage study is performed on commercial and self-prepared pastes from solid lubricants and white oil, where  calcium hydroxide is a commonly employed solid lubricant for the avoidance of fretting wear is  compared to other materials. Citation: Buse, H.; Schueler, F.; Hodúlová, E. Planar Contact Fretting Keywords: fretting wear; time lapse; fretting corrosion; tribometry; fretting test; lubrication test; Test Method Applied to Solid calcium fluorite; calcium hydroxide Lubricants. Lubricants 2021, 9, 58. https://doi.org/10.3390/ lubricants9060058 1. Introduction Received: 8 March 2021 Accepted: 17 May 2021 Fretting wear occurs in many technical and everyday applications. Examples of Published: 21 May 2021 fretting from earlier times [1–3] are still of relevance today [4], as shown in Figure1. In particular, the tooth shaft hub coupling of Figure1b is a classic example topic of early

Publisher’s Note: MDPI stays neutral functional failure or machine breakdown. Fretting wear on the bearing seat on the shaft of with regard to jurisdictional claims in Figure1d can always occur due to shaft bending with inappropriate tolerances and can published maps and institutional affil- lead to complete failures, such as the fretting damage depicted in Figure1c. The iations. pin shown in Figure1e is a widely used connection element in consumer products, and the wear on these parts is typical for fretting but mostly uncritical. The only outdated example is the metal Oldham coupling seen in Figure1a, but it clearly demonstrates how small motion compensation can lead to fretting wear. Figure1g, on the other hand, presents

Copyright: © 2021 by the authors. a modern shaft hub coupling where a change in machine harmonics led to fretting wear Licensee MDPI, Basel, Switzerland. problems. The shape of a new tooth is approximately given by the red lines. This article is an open access article Often the collective load would not be described as critical. Low loads, for example, distributed under the terms and may lead to less fit and more displacement. Large contacting surfaces are exposed to conditions of the Creative Commons self or externally induced vibrations, and fretting conditions can be met if the displacement Attribution (CC BY) license (https:// does not exceed the contact length. creativecommons.org/licenses/by/ 4.0/).

Lubricants 2021, 9, 58. https://doi.org/10.3390/lubricants9060058 https://www.mdpi.com/journal/lubricants Lubricants 2021, 9, x FOR PEER REVIEW 2 of 14

to self or externally induced vibrations, and fretting conditions can be met if the displace- ment does not exceed the contact length. It should be the objective to avoid or mitigate the reason for the vibration or relative motion, but this is often not possible by economical design or use of a machine. Some best practices define the necessary precision of mating parts (e.g., bearing seats) to avoid rela- tive motion; most elements, however, require a clearance to allow a certain freedom of motion or for assembly reasons. Hence, for the avoidance or mitigation of fretting wear as a symptom of small oscillating relative motion in large contacts, there is a need for rea- sonable treatment. In this article, we report and focus on the use of a new model test bench, which was developed for the simulation of planar contacts. The usual abstraction of the contact ge- ometries with an easily aligned point or line contacts is deliberately omitted to meet the usually flat contacts in applied tribosystems (shaft-hub connection, bearing seats, mating surfaces, etc.). This also gives lubricants a tiny clearance and volume in between asperities of the mating surfaces. If the lubricant is not able to stay in contact by its properties, the chance to get back into the contact is minimal. Solid lubricants mixed in oil or in a coating matrix (so-called antifriction coatings) can provide protection against typical fretting wear with symptoms of adhesion and/or metal debris forming [5,6]. Their main function is to provide low, constant and controlled friction by building up a continuous film and be chemically stable under the given condi- tions. Solid lubricants are known to be a solution of last resort when lubrication is required under extreme conditions. The lubricity of some of the best-known solid lubricants, such as graphite, hexagonal boron nitride (h-BN) or molybdenum disulfide (MoS2), is at- Lubricants 2021, 9, 58tributed to their layered lattice structure. Due to weak chemical interactions, the plate-like 2 of 14 particles of these materials can align themselves towards a sliding motion and do thus lead to low friction values [7,8].

FigureFigure 1. 1. (a(–af–)f )Fretting Fretting wear wear of of differen differentt machine machine elements elements [1] [1 ]and and (g (g) )loss loss due due to to fretting fretting wear. wear.

In contrast, certainIt should other be inorganic the objective comp toounds avoid can or mitigate provide thelubrication reason for under the vibrationsome or relative specific conditions,motion, such but as this higher is often temperat not possibleures, where by economicalthey become design softer orand use build of a up machine. Some transfer films onbest the practices surfaces define of friction the necessary pairs but precisiondo resist oxidation of mating reactions. parts (e.g., bearing seats) to avoid As an introductionrelative motion; to the planar most elements, contact test however, method require with a anew clearance test bench, to allow the article a certain freedom of compares andmotion evaluates or fordifferent assembly mixtures reasons. of white Hence, oil/solid for the lubricant avoidance each or mitigationat 50 weight of fretting wear percent (wt.%),as see a symptom Table 1. Few of small are known oscillating for relativetheir performance motion in largeof avoiding contacts, fretting there is a need for wear. The impactreasonable of oscillating treatment. motion on the surfaces was monitored by a camera setup, and also the frictionIn forces this article,and surface we reporttemperatures and focus were on recorded the use for of the a new whole model duration test bench, which of the test. Observationwas developed of the forcontact the simulation surface during of planar the test contacts. run is Thea new usual key abstraction method to of the contact examine reactiongeometries layers or with signs an of easily wear aligned and its pointmechanisms. or line contacts After the is deliberatelycompletion of omitted the to meet the experiment, wearusually patterns flat contacts and chemical in applied composition tribosystems of particular (shaft-hub surface connection, areas were bearing ex- seats, mating amined by lightsurfaces, microscopy etc.). Thisand sca alsonning gives electron lubricants microscopy a tiny clearance (SEM). and volume in between asperities of the mating surfaces. If the lubricant is not able to stay in contact by its properties, the chance to get back into the contact is minimal. Solid lubricants mixed in oil or in a coating matrix (so-called antifriction coatings) can provide protection against typical fretting wear with symptoms of adhesion and/or metal debris forming [5,6]. Their main function is to provide low, constant and controlled friction by building up a continuous film and be chemically stable under the given conditions. Solid lubricants are known to be a solution of last resort when lubrication is required under extreme conditions. The lubricity of some of the best-known solid lubricants, such as graphite, hexagonal boron nitride (h-BN) or molybdenum disulfide (MoS2), is attributed to their layered lattice structure. Due to weak chemical interactions, the plate-like particles of these materials can align themselves towards a sliding motion and do thus lead to low friction values [7,8]. In contrast, certain other inorganic compounds can provide lubrication under some specific conditions, such as higher temperatures, where they become softer and build up transfer films on the surfaces of friction pairs but do resist oxidation reactions. As an introduction to the planar contact test method with a new test bench, the article compares and evaluates different mixtures of white oil/solid lubricant each at 50 weight percent (wt.%), see Table1. Few are known for their performance of avoiding fretting wear. The impact of oscillating motion on the surfaces was monitored by a camera setup, and also the friction forces and surface temperatures were recorded for the whole duration of the test. Observation of the contact surface during the test run is a new key method to examine reaction layers or signs of wear and its mechanisms. After the completion of the experiment, wear patterns and chemical composition of particular surface areas were examined by light microscopy and scanning electron microscopy (SEM). Lubricants 2021, 9, x FOR PEER REVIEW 3 of 14

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Table 1. Test series data.

Table 1. Test seriesMohs data. No. of Sam- Solid Lubricant Mix [wt.%] Hard. ples Solid Lubricant Mohs Hard.50 Solid Mix Lubricant, [wt.%] No. of Samples Calcium fluorite/CaF2 of different coarseness * 4 4 Calcium fluorite/CaF of different 5050 Solidwhite Lubricant,oil 2 4 4 coarseness *Calcium hydroxide/Ca(OH)2 2 50 SL, 50 white white oil oil 1 Calcium hydroxide/Ca(OH)Ca(OH)2–commercial2 product 2 2 15 to 25 50 SL SL, acc. 50 to white SDS ** oil 3 1 Ca(OH)2–commercialAluminum product hydroxide, Gibbsite/Al(OH)3 2 15 50 toSL, 25 50 SL white acc. oil to SDS ** 1 3 Aluminum hydroxide, Gibbsite/Al(OH)Natrium hydroxide/NaOH3 50 SL,50 SL,50 white 50 white oil oil 1 1 Natrium hydroxide/NaOHHexagonal boron nitride/h-BN 30 SL, 50 SL,70 white 50 white oil oil 1 1 Hexagonal boron nitride/h-BNPTFE 50 SL, 30 SL,50 white 70 white oil oil 1 1 PTFEGraphite 30 SL, 50 SL,70 white 50 white oil oil 1 1 GraphiteCalcium sulphate CaSO4 50 SL, 30 SL,50 white 70 white oil oil 1 1 CalciumSimilar sulphate for all CaSO tests:4 Pair of lapped 100Cr6/1.3505 Steel specimens 50with SL, 60–62 50 white HRC, oil 64 mm² contact 1 Similar for all tests:area, Pair of 15.6 lapped MPa/1 100Cr6/1.3505 kN normal Steel load, specimens 200 µm with stroke 60–62 at HRC,49.5 Hz 64 mm frequency2 contact area,and the 15.6 temperature MPa/1 kN normal of the load, 200 µm stroke at ◦ 49.5 Hz frequency andspecimens the temperature of 80 °C; of * the Natural specimens CaF of2 (Sachtleben 80 C; * Natural Minerals), CaF2 (Sachtleben ** Safety Minerals), Data Sheet. ** Safety Data Sheet.

2. Materials and2. MaterialsMethods and Methods The test setup Theconsists test setup of two consists flat specimens of two flat with specimens an ideal with contact an ideal area contact of 64 areamm². of 64 mm2. The The test benchtest has bencha rigid has swivel a rigid joint swivel alignm jointent alignmentfor the specimens for the specimensto allow a well-distrib- to allow a well-distributed uted plane contact,plane which contact, plane which is also plane parallel is also to parallel the axis to of the motion. axis of Pressure-sensitive motion. Pressure-sensitive foil foil is used as ais visual used asalignment a visual check. alignment The foil check. used The in the foil experiment used in the has experiment its maximum has its maximum intensity colorintensity at or above color 2 MPa. at or above 2 MPa. Figure 2 showsFigure the test2 shows bench, the information test bench, on information the specimen on setup the specimen and contact setup align- and contact align- ment procedure.ment The procedure. bench-top Theapparatus bench-top is shown apparatus in Figure is shown 2a, and in Figureall preparation2a, and all and preparation and handling activitieshandling can be activities performed can beon performedthe table. The on thenormal table. force The drivetrain normal force extends drivetrain to extends to below the table-top,below where the table-top, the power where and the measurement power and measurementelectronics are electronics also placed. are The also placed. The power and measurementpower and measurementelectronics are electronicsin separate are cabinets. in separate A display cabinets. is placed A display nearby is placed nearby with a mouse withand akeyboard mouse and to keyboardoperate the to operatemachine. the Figure machine. 2b shows Figure 2theb shows ideal thecontact ideal contact area for specimens. The upper and lower specimens are 8 mm wide, 10 mm long and have a area for specimens. The upper and lower specimens are 8 mm wide, 10 mm long◦ and have a thickness of thickness3 mm. Because of 3 mm. the specimens Because the are specimens rotated by are 90° rotated on their by 10–8 90 onmm their surface 10–8 mm surface normal, as shownnormal, in Figure as shown 2d, the in Figureideal contact2d, the is ideal a square contact of 8 is mm a square side length. of 8 mm The side extra length. The extra 2 mm length offers2 mm space length for offers the motion space for of the motionupper specimen of the upper and specimena clamping and position a clamping position tolerance for the lower specimen. If the and roughness orientation are tolerance for the lower specimen. If the surface roughness and roughness orientation are critical for an experiment, the specimen orientation must be considered in the setup. critical for an experiment, the specimen orientation must be considered in the setup.

FigureFigure 2. 2.(a ()a The) The test test bench, bench, (b ()b sample) sample contact contact size, size, (c )(c alignment) alignment test test and and (d ()d sample) sample relative relative motion. mo- tion. An automated opening of the contact in defined intervals is intended in order to An automatedobserve opening the wearing of the surfacescontact byin defined acquiring intervals images duringis intended a test. in The order setup to ob- uses an industrial serve the wearingcamera surfaces and aby telecentric acquiring lensimages with during an image a test. circle The fittingsetup uses the specimenan industrial dimensions. The schematic of this procedure is shown in Figure3.

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camera and a telecentric lens with an image circle fitting the specimen dimensions. The schematic of this procedure is shown in Figure 3. Opening of the contact is to be considered as relevant in the tribological system anal- ysis [8]. The opening could provide an escape for trapped particles or a new flow of a lubricant to more central contact areas. A different approach is closed contact with a trans- parent specimen [9], but this limits the abstraction of the tribological system as one ele- ment has to be replaced by a transparent body with different properties. Figure 3a shows the schematic of the opening procedure, where the normal force drive can also unload and move the upper specimen to a defined position, where the up- per specimen holder completely clears the sight of an angled camera. The camera is mounted on the orange casing parts shown in Figure 2a. The process is automated via the axis positions and regular checks for plausible normal forces while the contact-opening sequence is being performed. Additionally, a movable mirror as the actual setup on the machine is shown in the schematic of Figure 3a. The mirror can swing in between the specimens while the contact is open to allow the recording of images of the upper speci- men. The mirror was added subsequently to the tests performed for this study. The other images of Figure 3 show: (b) to (d) the iterative procedure of contact alignment; (e) the initial condition of the lower specimen; (f) lubrication application and amount; (g) lubri- cation squeeze out after a single force application; (h)–(k) test images while running until failure. The drivetrain for the oscillating motion is an adjustable eccentric drive with a servo motor as the main power source and a phase shift mechanism to control the displacement eccentricity. The crankshaft drive is much stiffer than voice coil drive solutions and does not require hydraulic power. A PID-control (proportional, integral, derivative) of the phase shift is sufficient to sustain the desired displacement amplitude. The displacement measurement system consists of a commercial laser triangulation sensor mounted on a passive mass path near the fixed specimen, the resolution of the device is not better than 0.4 µm and the linearity not better than 0.1% at a sample cycle of 110 µs. The quasi-static normal force measurement is performed via a strain measurement load cell (~2 mV/V, 0.5% linearity) in the force path on the motion axis, which is also used for opening and closing the contact. The axis supplies up to 2 kN of the normal force. The Lubricants 2021, 9, 58 axis control system for the normal force and contact-opening axis are based on “GRBL”4 of 14 open-source software, which uses g-code for axis commands such as CNC machines (com- puterized numerical control).

Figure 3. Contact-opening schematic (a) and intermediate images (b–k) of the CaSO4 Test. Figure 3. Contact-opening schematic (a) and intermediate images (b–k) of the CaSO4 Test.

Opening of the contact is to be considered as relevant in the tribological system analysis [8]. The opening could provide an escape for trapped particles or a new flow of a lubricant to more central contact areas. A different approach is closed contact with a transparent specimen [9], but this limits the abstraction of the tribological system as one element has to be replaced by a transparent body with different properties. Figure3a shows the schematic of the opening procedure, where the normal force drive can also unload and move the upper specimen to a defined position, where the upper specimen holder completely clears the sight of an angled camera. The camera is mounted on the orange casing parts shown in Figure2a. The process is automated via the axis positions and regular checks for plausible normal forces while the contact-opening sequence is being performed. Additionally, a movable mirror as the actual setup on the machine is shown in the schematic of Figure3a. The mirror can swing in between the specimens while the contact is open to allow the recording of images of the upper specimen. The mirror was added subsequently to the tests performed for this study. The other images of Figure3 show: (b) to (d) the iterative procedure of contact alignment; (e) the initial condition of the lower specimen; (f) lubrication application and amount; (g) lubrication squeeze out after a single force application; (h)–(k) test images while running until failure. The drivetrain for the oscillating motion is an adjustable eccentric drive with a servo motor as the main power source and a phase shift mechanism to control the displacement eccentricity. The crankshaft drive is much stiffer than voice coil drive solutions and does not require hydraulic power. A PID-control (proportional, integral, derivative) of the phase shift is sufficient to sustain the desired displacement amplitude. The displacement measurement system consists of a commercial laser triangulation sensor mounted on a passive mass path near the fixed specimen, the resolution of the device is not better than 0.4 µm and the linearity not better than 0.1% at a sample cycle of 110 µs. The quasi-static normal force measurement is performed via a strain measurement load cell (~2 mV/V, 0.5% linearity) in the force path on the motion axis, which is also used for opening and closing the contact. The axis supplies up to 2 kN of the normal force. The axis control system for the normal force and contact-opening axis are based on “GRBL” open-source software, which uses g-code for axis commands such as CNC machines (computerized numerical control). The friction measurement is performed on the structural path from the fixed specimen holder to the drivetrain via two piezo-electric strain sensors in a structure symmetric configuration for the cancelation of bending strain charges. This allows only the resulting Lubricants 2021, 9, x FOR PEER REVIEW 5 of 14

Lubricants 2021, 9, 58 The friction measurement is performed on the structural path from the fixed speci-5 of 14 men holder to the drivetrain via two piezo-electric strain sensors in a structure symmetric configuration for the cancelation of bending strain charges. This allows only the resulting push/pull strain strain charge charge to to be amplified amplified by the charge amplifier.amplifier. TheThe resultingresulting calibrationcalibration charge perper forceforce isis quitequite low,low, withwith ~0.5~0.5 pC/pC/N,N, andand requires requires a sophisticated amplifier. amplifier. Data acquisition hardware uses the typicaltypical 16-16- toto 24-bit24-bit analogueanalogue interfacesinterfaces forfor thethe measurements. The observation setup uses an industrial camera with aa telecentrictelecentric lenslens fittedfitted to the the specimen specimen dimensions dimensions for for the the object object size. size. The The control control software software is custom is custom pro- programmedgrammed in LabVIEW in LabVIEW for forthe thetest test bench bench measurements measurements and and functionalities. functionalities. The The user user in- interfaceterface displays displays every every important important measure measure and and th thee control control data data as as well well as the live fretting loopsloops andand thethe last last acquired acquired images. images. Additionally, Additionally, the the contact contact alignment alignment is supported is supported in the in softwarethe software with with a special a special set ofset positions of positions for thefor normalthe normal force force drive. drive. The acquired data data are are displayed displayed and and saved saved in indifferent different ways. ways. The The data data from from the fric- the frictiontion and and displacement displacement measurement measurement are areusable usable to show to show fretting fretting loops loops [4. With [4]. With these these hys- hysteresisteresis loops, loops, the contact the contact condition condition or fretting or fretting regime regime can be can distinguished be distinguished between between partial partialslip and slip gross and slip. gross Additionally slip. Additionally,, the mixed the mixedslip is possible slip is possible if low and if low high and friction high friction with a withloss of a the loss effective of the effectivesliding stroke sliding through stroke the through elastic thedeformation elastic deformation occurs over occurstime. Figure over time.4 shows Figure data4 acquiredshows data from acquired the test from bench. the Both test bench.examples Both were examples taken from were tests taken with from a 2 testskN load. with The a 2 kNdeformation load. The of deformation the contact ofas the well contact as the astest well bench as the structure test bench (mostly structure Alu- (mostlyminum Aluminumstructure, around structure, 12.5 around µm/kN) 12.5 is partµm/kN) of the is measurement part of the measurement since there sinceis only there one isdisplacement only one displacement sensor. A partial sensor. slip A with partial very slip low with strokes very lowcan be strokes achieved can beby achievedmotion con- by motiontrol and control could be and a couldtopic of be future a topic studies. of future The studies. coefficient The coefficient of frictionof (COF) friction for(COF) time dia- for timegram diagram data is also data calculated is also calculated with peak-to-peak with peak-to-peak values, root values, mean root square mean (RMS) square values (RMS) or valuesother methods or other from methods the raw from force the loop raw data. force Th loope peak data. values The are peak used values for later are usedinterpre- for latertation. interpretation.

Figure 4.4. Exemplary fretting loop forfor highhigh frictionfriction withwith lowlow strokestroke ((aa)) andand lowlow frictionfriction withwith highhigh strokestroke ((bb).).

Testing SolidSolid LubricantsLubricants An overview of of the the current current test test series series is sh isown shown in Table in Table 1. Most1. Most solid solid lubricants lubricants could 2 ◦ couldbe mixed be mixed1:1 with 1:1 medical with medical white oil white with oil a withviscosity a viscosity of 70 mm²/s of 70 mmat 40 /s°C. at The 40 mixtureC. The mixtureconsistency consistency varies with varies the with particle the particlesize and size surface and surfacearea; the area; consideration the consideration of such of effects such effectswould wouldbe an important be an important objective objective for further, for further, more moredetailed detailed studies studies with promising with promising solid solidlubricants. lubricants. For the For use the of use hexagonal of hexagonal boron boron nitride nitride and graphite, and graphite, the amount the amount of solid of solid lub- lubricantricant was was reduced reduced to achieve to achieve a paste-like a paste-like cons consistency.istency. Since Since the white the white oil itself oil itselfcan miti- can mitigate fretting for a short period of time, it is expected that results do mainly, but not gate fretting for a short period of time, it is expected that results do mainly, but not exclu- exclusively, reflect the behavior of the solid lubricants. Additionally, the differences in the sively, reflect the behavior of the solid lubricants. Additionally, the differences in the mis- miscibility behavior of solid lubricants and white oil must be considered as a limitation of cibility behavior of solid lubricants and white oil must be considered as a limitation of this this study. study. Studies and data on the fretting wear behavior of solid lubricants in a coating matrix are rather common than in oil-based mixtures [10,11], but these are mostly limited to the investigation of polytetrafluoroethylene (PTFE) or MoS2.

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In general, at the boundary or mixed friction regimes, more types of solid lubricants are common [12] and known. For calcium fluoride (CaF2), the behavior on fretting wear seems not to be published, and for calcium hydroxide (Ca(OH)2), only a few but promising Lubricants 2021, 9results, x FOR PEER are available REVIEW [12]. 6 of 14

Besides Ca(OH)2, which also finds commercial applications, other metal hydroxides, e.g., Al(OH)3 or NaOH, could behave similarly if a reaction with iron occurs at the metallic surface. According toStudies the literature, and data they on havethe fretting not been wear tested behavior on fretting of solid wear lubricants avoidance. in a coating matrix 3. Results are rather common than in oil-based mixtures [10,11], but these are mostly limited to the investigation of polytetrafluoroethylene (PTFE) or MoS2. 3.1. Results from Online and Image Data In general, at the boundary or mixed friction regimes, more types of solid lubricants The online and image data of Figures5 and6 show the initial damage type on the are common [12] and known. For calcium fluoride (CaF2), the behavior on fretting wear lubricated test specimen and the initial and progressive damage mechanisms. The different seems not to be published, and for calcium hydroxide (Ca(OH)2), only a few but promising lubricants can beresults clearly are divided available into a[12]. fretting wear avoidant category and a non-effective category. This can be seen in Figure7, with bar graphs of the corresponding time to Besides Ca(OH)2, which also finds commercial applications, other metal hydroxides, damage (TtD), mean coefficient of friction (COF) and the stroke stability as a measure e.g., Al(OH)3 or NaOH, could behave similarly if a reaction with iron occurs at the metallic of any noticeable stick slip. This is the standard deviation of all measured stroke values. surface. According to the literature, they have not been tested on fretting wear avoidance. Stroke stabilities smaller than 5 µm indicate a stable run without stick slip. The TtD is a concept where the test is stopped after the rise to a specific friction force 3. Results or COF level, and the time from the start until this happens is called TtD. As it can be seen in Figure5, a COF3.1. level Results of 0.2 from is often Online stable and forImage several Data hours of the runtime. Within such a stable phase, a frictionThe forceonline limit and is image set on data the testof Figures bench software5 and 6 show to define the theinitial end damage of type on the the test. As a resultlubricated of a TtD test shutdown, specimen the and specimen the initial can and be progressive analyzed for damage the wear mechanisms. type The differ- leading to high forces,ent lubricants and the TtDcan valuebe clearly itself divided can be used into fora fretting comparison wear of avoidant the different category and a non- solid lubricants. effective category. This can be seen in Figure 7, with bar graphs of the corresponding time PTFE, NaOH,to damage Graphite (TtD), and h-BN mean belong coefficient to the of category friction of(COF) non-effective and the stroke solid lubri-stability as a measure cants. All showof short any run noticeable times, andstick some slip. evenThis is show the standard excessive deviation fretting wear of all and measured high stroke values. COF values afterStroke a few cycles.stabilities smaller than 5 µm indicate a stable run without stick slip.

Figure 5. COF-Data. Figure 5. COF-Data.

The TtD is a concept where the test is stopped after the rise to a specific friction force or COF level, and the time from the start until this happens is called TtD. As it can be seen in Figure 5, a COF level of 0.2 is often stable for several hours of the runtime. Within such a stable phase, a friction force limit is set on the test bench software to define the end of the test. As a result of a TtD shutdown, the specimen can be analyzed for the wear type leading to high forces, and the TtD value itself can be used for comparison of the different solid lubricants.

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intends that Al(OH)3 may be suited to achieve high friction fit capabilities, which can de- crease the likeliness of fretting wear. Figure 8 also shows that the tests are within the gross Figure 6. Lower slip specimen fretting microscopy regime–elastic after the deformation test. is around 5%–10% of the stroke value of 200 µm.

PTFE, NaOH, Graphite and h-BN belong to the category of non-effective solid lubri- cants. All show short run times, and some even show excessive fretting wear and high COF values after a few cycles. As a benchmark and a comparison to the raw white oil and solid lubricant mix, a commercial Ca(OH)2-containing paste, labeled Ca(OH)2 Com. S1–S3, was tested in three samples, which achieved runtimes reaching 30–50 h until the first major damage with very low COF values. In nearly all cases, the major damage occurred due to an adhesive cold weld, as shown in the lower right image of Figure 6, which led to high friction values. The Ca(OH)2 white oil mix was therefore expected to reach a good performance and lasted 100 h until the test was stopped for surface analysis. A 5 wt.% amount of PTFE in a one-off sample test decreased the performance, but a clear result misses further statistical proof. CaF2 of different grades is tested (average particle diameters: L25 with 2.5 µm, L55 with 5.5 µm, L90 with 9.0 µm), the finer L25 particles showed a better time to damage trend in comparison to the coarser grades. L90 was also tested after the addition of 5 wt.% PTFE, and also here, a lower TtD was observed. FigureFigureCaSO 7.7. The4 lasted time to to 180 damage, damage, h with mean mean an coefficientexceptional coefficient of of fricti long frictionon steady and and stroke strokestate stability of stability the (standard observed (standard deviation). surface deviation). and friction level. Figure 6 shows this state with the additional cold weld on the lower left, whichAs formed a benchmark in the last and minutes a comparison of this test. to the raw white oil and solid lubricant mix, a commercialIn Figure Ca(OH) 5, graphs2-containing of the raw paste, Ca(OH) labeled2, CaF Ca(OH)2 and 2CaSOCom.4 pastes S1–S3, also was show tested a in signifi- three samples,cantly higher which COF achieved level at runtimes the start, reaching which in 30–50 most h cases until thestabilized first major over damage the course with of very the low COF values. In nearly all cases, the major damage occurred due to an adhesive cold experiment. This could last for up to 50% of the TtD. The commercial Ca(OH)2 paste ap- weld,peared as with shown the inmost the stable lower performance, right image of wh Figureich is6 ,preferable which led for to higha defined friction friction values. behav- The Ca(OH)ior in an2 application.white oil mix was therefore expected to reach a good performance and lasted 100 h until the test was stopped for surface analysis. A 5 wt.% amount of PTFE in a one-off Al(OH)3 depicted the most unusual behavior in this test series. The COF levels and samplethe stroke test stability decreased as a the measure performance, of the stick but aslip clear were result very misses high and further stayed statistical at those proof. levels CaF2 of different grades is tested (average particle diameters: L25 with 2.5 µm, L55 for around 40 h until the test was stopped for analysis. Furthermore, the Al(OH)3 image with 5.5 µm, L90 with 9.0 µm), the finer L25 particles showed a better time to damage trend in Figure 6 indicates no major wear. The fretting loop data of Figure 8 shows also a very in comparison to the coarser grades. L90 was also tested after the addition of 5 wt.% PTFE, dynamic behavior for Al(OH)3 compared to the similar Ca(OH)2 variants. This first result and also here, a lower TtD was observed.

Figure 8. Fretting loops for variants of Ca(OH)2, CaF2 and Al(OH)3.

3.2. Analysis of Solid Lubricants Effects of under Fretting Conditions In general, fretting conditions are known to be very harsh for lubrication access, and different wear mechanisms can occur that can include the formation of microwelds, oxi- dation of the boundary layer and formation of abrasive particles and surface disruption. It is also well-known that fretting can lead to extreme local temperature on the sliding

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intends that Al(OH)3 may be suited to achieve high friction fit capabilities, which can de- Lubricants 2021, 9, 58 8 of 14 crease the likeliness of fretting wear. Figure 8 also shows that the tests are within the gross slip fretting regime–elastic deformation is around 5%–10% of the stroke value of 200 µm.

CaSO4 lasted 180 h with an exceptional long steady state of the observed surface and friction level. Figure6 shows this state with the additional cold weld on the lower left, which formed in the last minutes of this test. In Figure5, graphs of the raw Ca(OH) 2, CaF2 and CaSO4 pastes also show a signif- icantly higher COF level at the start, which in most cases stabilized over the course of the experiment. This could last for up to 50% of the TtD. The commercial Ca(OH)2 paste appeared with the most stable performance, which is preferable for a defined friction behavior in an application. Al(OH)3 depicted the most unusual behavior in this test series. The COF levels and the stroke stability as a measure of the stick slip were very high and stayed at those levels for around 40 h until the test was stopped for analysis. Furthermore, the Al(OH)3 image in Figure6 indicates no major wear. The fretting loop data of Figure8 shows also a very dynamic behavior for Al(OH)3 compared to the similar Ca(OH)2 variants. This first result intends that Al(OH)3 may be suited to achieve high friction fit capabilities, which can decrease the likeliness of fretting wear. Figure8 also shows that the tests are within the gross slip fretting regime–elastic deformation is around 5%–10% of the stroke value of Figure 7. The time200 to damage,µm. mean coefficient of friction and stroke stability (standard deviation).

Figure 8. Fretting loops for variants of Ca(OH)2, CaF2 and Al(OH)3. Figure 8. Fretting loops for variants of Ca(OH)2, CaF2 and Al(OH)3.

3.2.3.2. Analysis Analysis of of Solid Solid Lubricants Lubricants E Effectsffects of of under under Fretting Fretting Conditions Conditions InIn general, general, fretting fretting conditio conditionsns are are known known to be to very be very hars harshh for lubrication for lubrication access, access, and differentand different wear wearmechanisms mechanisms can occur can occur that ca thatn include can include the formation the formation of microwelds, of microwelds, oxi- dationoxidation of the of theboundary boundary layer layer and and formation formation of ofabrasive abrasive particles particles and and surface surface disruption. disruption. ItIt is is also also well-known well-known that that fretting fretting can can lead lead to to extreme extreme local local temperature temperature on on the the sliding sliding surfaces, known as “flash temperatures.” These temperature rises, which can increase up to >1000 ◦C and occur for every rough surface but also for surfaces that are separated by small lubricant particles because the applied load is carried over a few small contact points [13]. Lubricants 2021, 9, x FOR PEER REVIEW 9 of 14

surfaces, known as “flash temperatures.” These temperature rises, which can increase up Lubricants 2021, 9, 58to > 1000 °C and occur for every rough surface but also for surfaces that are separated by 9 of 14 small lubricant particles because the applied load is carried over a few small contact points [13]. Besides the parameters describing the tribological contact, such as contact area, load, frequency and temperature,Besides the fretting parameters wear describingis also strongly the tribological impacted by contact, various such other ascontact pa- area, load, rameters [14], frequencye.g., environmental and temperature, factors such fretting as water wear or isoxygen also strongly content. impactedHowever, bythe various other latter aspects areparameters themselves [14 ],strongly e.g., environmental influenced by factors the geometry such as of water the tribological or oxygencontent. con- However, tact [15]—an “open”the latter geometry, aspects aresimilar themselves to a ball strongly contact, influencedwhich allows by penetration the geometry of ofsur- the tribological rounding air muchcontact better [15]—an than “open”a planar geometry, contact over similar a large to aarea. ball contact, which allows penetration of Figure 9 surroundingshows an overview air much of better light thanmicroscopy a planar images contact of over every a large promising area. solid lubricant with a redFigure rectangle9 shows marking an overview the SEM/EDX of light (energy microscopy dispersive images X-Ray of every spectros- promising solid lu- copy) areas. Thebricant areas with depicting a red rectangle different markingphenomena the SEM/EDXhave been (energyselected dispersive for electron X-ray mi- spectroscopy) croscopy analysisareas. in Theorder areas to demonstrate depicting different the potential phenomena and the have versatility, been selected which for the electron fret- microscopy ting test offersanalysis together in with order additional to demonstrate analysis the techniques. potential In and the the following, versatility, a brief which over- the fretting test view of the testedoffers solid together lubricants with additionalis given, and analysis the potential techniques. lubrication In the following,effects are ainitially brief overview of the discussed. tested solid lubricants is given, and the potential lubrication effects are initially discussed.

FigureFigure 9. 9.Microscopy Microscopy overview overview of of layers layers from from solid solid lubricants lubricants and and area area designation designation for for SEM/EDX. SEM/EDX.

3.2.1. h-BN, Graphite,3.2.1. h-BN, PTFE Graphite, PTFE The lubricity ofThe some lubricity of the of best-known some of the solid best-known lubricants solid (such lubricants as graphite, (such h-BN as graphite, or h-BN or MoS2) is attributedMoS2 )to is their attributed layered to lattice their layered structure. lattice Due structure. to weak chemical Due to weak interactions, chemical interactions, the plate-like theparticles plate-like of these particles materials of these can materialsalign themselves can align towards themselves a sliding towards motion a sliding motion and do thus leadand to do low thus friction lead to values. low friction While values.graphite While is known graphite to provide is known lubrication to provide to lubrication to temperatures temperaturesup to 400 °C but up then to 400 tends◦C but to oxidize then tends and to lose oxidize lubricity, and loseh-BN lubricity, is known h-BN to is known to be chemically beinert chemically and stable inert up andto 1000 stable °C up[16]. to 1000 ◦C[16]. Mixtures fromMixtures graphite from and graphitehexagonal and boron hexagonal nitride boron(h-BN) nitride were analyzed (h-BN) were in the analyzed in the fretting wear testfretting rig. In wear both test cases, rig. Instrong both wear cases, occurred strong wear already occurred after less already than after one hour less than one hour of testing. Frictionof testing. levels Friction were extremely levels were high extremely from the highbeginning from the of the beginning test. It is of known the test. It is known that both lubricants,that both however, lubricants, display however, high fric displaytion coefficients high friction and coefficients production and of productionwear in of wear in dry air, inert atmospheresdry air, inert or atmospheres vacuum and or lead vacuum to early and failure. lead to In early this failure. case, the In absence this case, of the absence of oxygen and water,oxygen due and to water,the characteristics due to the characteristics of the tribological of the contact tribological situation, contact might situation, be might be a a feasible explanationfeasible explanationfor the observed for the behavior. observed behavior. Additionally, Additionally,tests with PTFE tests as witha solid PTFE organic as a solidlubricant organic failed lubricant early. After failed 0.5 early. h, After 0.5 h, friction and temperaturefriction and increased temperature continuously, increased which continuously, indicates which a proceeding indicates loss a proceedingof lu- loss of brication. Sincelubrication. the polymer Since is expected the polymer to be is chemically expected to indifferent be chemically towards indifferent the existing towards the existing conditions, the root cause for the early failure can be attributed to the limitations of the load capacity.

3.2.2. Calcium Hydroxide, Ca(OH)2 In commercial antiseizing pastes, calcium hydroxide is commonly used as a lubricant to efficiently separate surfaces under extreme conditions. While most available literature Lubricants 2021, 9, x FOR PEER REVIEW 10 of 14

conditions, the root cause for the early failure can be attributed to the limitations of the load capacity.

Lubricants 2021, 9, 58 10 of 14 3.2.2. Calcium Hydroxide, Ca(OH)2 In commercial antiseizing pastes, calcium hydroxide is commonly used as a lubricant to efficiently separate surfaces under extreme conditions. While most available literature on calcium hydroxideon calcium is patent-based, hydroxide is patent-based,technical literature technical also literaturedescribes alsothe remarkable describes the remarkable lubrication propertieslubrication of its properties lubricant of pastes its lubricant [5,17,18]. pastes Tests [ 5performed,17,18]. Tests in this performed study clearly in this study clearly displayed the displayedbeneficial theproperties beneficial of the properties solid lubricant of the solid for fretting lubricant conditions. for fretting After conditions. a After a short period ofshort unstable period and of unstablehigh friction, and high during friction, which during no signs which of no wear signs could of wear be couldob- be observed, served, the testthe continued test continued at constantly at constantly low friction low friction until it until was itstopped was stopped after 100 after h 100dura- h duration. The tion. The microscopicmicroscopic analysis analysis showed showed the fo thermation formation of a oftribolayer, a tribolayer, which which was wasfurther further explored explored by SEM/EDX.by SEM/EDX. The chemical analysisThe chemical clearly analysis indicates clearly calcium indicates as being calcium a constituent. as being The a constituent. main ratio, The main ratio, however, seemshowever, to be iron seems oxide to be(see iron Figure oxide 10). (see Literature Figure 10). reported Literature that reported the presence that the of presence of cal- cium hydroxide supports the formation of more mechanically stable Fe O modification [7]; calcium hydroxide supports the formation of more mechanically stable Fe3O4 modifica-3 4 tion [7]; however,however, it was itnot was possible not possible to anal toyze analyze to what to extent what extentthat might that mighthave happened. have happened. Further analysis with RAMAN spectroscopy could lead to further insight into the composition of Further analysis with RAMAN spectroscopy could lead to further insight into the compo- tribolayers [19,20]. sition of tribolayers [19,20].

Figure 10. EDX of transfer films visible on backscatter electron images. Figure 10. EDX of transfer films visible on backscatter electron images.

3.2.3. Calcium Fluorite, CaF2 3.2.3. Calcium Fluorite, CaF2 Calcium fluoride,Calcium also known fluoride, as also fluorspar, known has as fluorspar,been known has as been a high-temperature known as a high-temperature lubricant for decadeslubricant [21,22]. for decades Similar [ 21to, 22calcium]. Similar hydroxide, to calcium its beneficial hydroxide, properties its beneficial evolve properties evolve only at higheronly temperatures at higher temperaturesand pressures, and and pressures, hence, it and currently hence, itfinds currently application finds application in in ma- material compositesterial composites for grinding, for for grinding, dry running for dry materials running or materials in antiseizing or in antiseizing pastes. How- pastes. However, ever, its use asits an use additive as an additive for greases for greases[23] or [polymers23] or polymers [24] has [24 also] has been also investigated. been investigated. Similar Similar to calciumto calcium hydroxide, hydroxide, fluorspar fluorspar showed showed an impressive an impressive tribolayer tribolayer built-up built-up perfor- performance mance during duringthe fretting the fretting tests, as tests, can be as canseen be in seenthe time-lapse in the time-lapse extracts extracts of Figure of Figure11. 11. Three types ofThree natural types CaF of2 that natural differed CaF 2inthat particle differed size in were particle tested. size The were running-in tested. The running-in period with highperiod and withunsteady high andfriction unsteady lasted friction40–55 h, lasted and only 40–55 small h, and signs only of smallwear could signs of wear could be found. Afterwards,be found. the Afterwards, test continued the testto run continued at low friction to run atlevels low frictionuntil it was levels stopped until it was stopped without significantwithout damage significant after 140–180 damage h. after 140–180 h. The time-lapseThe images time-lapse indicate images that the indicate change that in thefriction change level in frictiongoes along level with goes along with changes in the changessteel surface in the structure. steel surface For th structure.e method of For contact-opening the method of and contact-opening intermediate and interme- image acquisition,diate Figure image 11 acquisition, gives good Figure feedbac 11k gives on initial good conditions feedback onon initialthe first conditions cycles on the first cycles depicted in the first row as well as the tribolayer formation in the following rows. Additionally, the tribolayer intensity (dark surface) under visible light conditions varies clearly with the used CaF2 particle size. The images shown give a high-level overview of tribolayer formation. Image details have more information to be viewed and interpreted in

the future. Lubricants 2021, 9, x FOR PEER REVIEW 11 of 14

depicted in the first row as well as the tribolayer formation in the following rows. Addi- tionally, the tribolayer intensity (dark surface) under visible light conditions varies clearly Lubricants 2021, 9, 58 with the used CaF2 particle size. The images shown give a high-level overview of tribo- 11 of 14 layer formation. Image details have more information to be viewed and interpreted in the future.

Figure 11. SelectedFigure contact-opening 11. Selected contact-opening interval images interval of CaF2 images tests, lower of CaF2 specimen. tests, lower Every specimen. column isEvery a single column test and every is a single test and every row indicates approx. equal runtime until failure. row indicates approx. equal runtime until failure.

Further microscopyFurther microscopypictures clearly pictures showed clearly the showedformation the of formation a tribolayer, of a and tribolayer, and SEM/EDX confirmedSEM/EDX larger confirmed amounts larger of Ca, amounts Fe, F, O of and Ca, C Fe, as F,constituents. O and C as The constituents. presence The pres- of carbon canence likely of carbonbe attributed can likely to remainders be attributed of the to remainderswhite oil. The of theinvestigation white oil. Theof the investigation exact compositionof the exactof the composition tribolayer was of thenot tribolayerpart of this was work’s not partscope; of however, this work’s the scope; litera- however, the ture describesliterature the formation describes of theFeF formation3 species as of part FeF 3ofspecies the films as part that of form the filmswith thatCaF2 form on with CaF2 iron surfaceson [25]. iron surfaces [25]. The presenceThe of Ca presence as part of of Ca the as film part suggests of the film the suggestspresence the of CaF presence2, but also of CaF an2 oxide-, but also an oxide- containing Cacontaining compound Ca might compound have formed. might have Wang formed. et al. explain Wang et the al. strong explain tribological the strong tribological performanceperformance of fluorspar ofat elevated fluorspar temperatur at elevatedes temperatures with a transition with of a the