l liquid p particle (debris) x, y, z coordinates 1.3 Abbreviations A-F Analytical Ferrograph B-M Bichromatic Microscope DLC Diamond-like carbon DM Diamagnetic DR-F Direct Reading Ferrograph e.g. exempli gratia, for example eqn. equation, formula fb for brevity F-G Ferrogram FG Ferrograph FM Ferromagnetic F/ T Foxboro/ Trans-Sonic,Inc. H-B Halogen Bulb of B-M Hg-B Mercury Burner of B-M Mag. Magnification, Magn. Magnetic M-M Magnetomotoric O-I Oblique Illumination PMG Photomicrograph PM Paramagnetic REB Rolling-Element-Bearing Ref. [No.] Reference No. R-L Reflected Light in B-M Sf Spheroid, Spherical debris T-L Transmitted Light in B-M WPC Wear Particle Concentration WSI Wear Severity Index Aus Wissenschaft und Forschung 16 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 1. Nomenclature 1.1 Symbols B [T], Magn. field induction [T]= [kg.s -2 A -1 ] B x , B y x,y components of B dz/ dt [m.s -1 ], velocity FS [N.m -3 ], Ferrograph Strength, FS = (1/ 2μ o ) ∂/ ∂z (B x2 ) F z [N], magnetomotoric force, F z = [(χ p χ l )/ 2μ 0 ]V p B 2 H [A.m -1 ], Intensity of Magn. field H = B/ μ 0 H x ,H z x, z components of H, L Large Particles Count M [A.m -1 ], magnetization, M p = (χ p / μ 0 )V p .B R [m], radius of a sphere S Small Particles Count S f [N], Stokes‘ viscous friction S f = 6 π.η.R.dz/ dt, [N] t [s], time V p [m 3 ], volume of a particle operator of the gradient = grad =(∂/ ∂ x ,∂/ ∂ y ,∂/ ∂ z ) η [Pa.s], dynamic viscosity ν [cSt], ν = η/ ρ, oil kinematic viscosity, 1 cSt = 1.10 -6 m 2 .s -1 μ 0 permeability of vacuum, μ 0 = 4 π.10 -7 [kg.m.s -2 A -2 ]=[N.A -2 ] ρ [kg.m -3 ], oil density Ø diameter of a sphere, Ø = 2R χ p , χ l [1], susceptibility of the particle & liquid (bulk) 1.2 Subscripts 0 vacuum f friction, flow of oil Aircraft turbine engines and helicopter gearboxes wear debris morphology via analytical ferrograph S. Slacik* Eingereicht: 1. 3. 2017 Nach Begutachtung angenommen: 20. 8. 2017 Since the early 1970, both Analytical and Direct Reading Ferrograph (A-F,DR-F) belong to most powerful diagnostic tools of aircraft engines and airframe systems (hydraulic & rotorcraft gearboxes). Though ferrographic lab procedures are rather time consuming and demand highly trained staff, benefits of their use outweigh costs. The article recalls briefly main principles of A-F & DR-F. Photomicrographs of wear debris circulating in engine (gearbox) with oil are presented and mechanisms of their formation, e.g. adhesive, abrasive & cutting wear, rolling friction fatigue & fatigue fracturing, high-speed rubbing are commented. Keywords Analytical Ferrograph, Direct Reading Ferrograph, wear debris, aircraft engines Abstract T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 16 2. Introduction In the early seventies (of the past century) quite new and surprisingly simple technique has been developed which permits separation of ferromagnetic (FM for brevity, fb) wear debris (and, to some degree, paramagnetic (PM, fb) as well) from the sample of aircraft engine lubricating oil and collection of these wear particles onto a glass substrate in such a way that coarse, larger particles (of a more ferromagnetic nature, being of higher susceptibility) appear at one end and the smaller debris (being of less FM nature) at the other one. This technique, called Ferrograph (FG, fb), was developed primarily to needs of air force aircraft jet engines diagnostics. Lately, FG has been sophisticated and involved into widespread use thanks to effort of E.R. Bowen, J.P. Bowen, V.C. Westcott, R.L. Wright from F/ T, W.W. Seifert from Massachusetts Institute of Technology and others, last but no least D.P. Anderson. Author would like to pay a tribute to all of them, but to late Mr. Westcott, who is credited with inventing the Ferrograph, first. 3 Analytical Ferrograph “Ferrograph” comes from latin word ferrum, which translates to English “iron”. FG is the method of ferrous metal wear particles separation from the oil as well as this debris study. Separation of particles from oil is performed in strong (and non-homogeneous, “leaking”) magnetic field. For explanation, see Figure 1, depicting Czech A-F REO1, developed by company REO Amos (REO Trade), Opava. The sample of lubricant, taken from the “representative point” of lubrication system, (having distribution of wear debris typical for whole engine) being homogenized via vivid shaking and diluted with proper solvent, is expelled from the syringe by a piston, slowly proceeding oil forward thr plastic pipe and poured onto plastic substrate, elastically U-bent and put into mount of a slight forward slope. Plastic substrate is a big advantage, as other makes of FG prepare F-Gs on more expensive (and fragile) glass substrate of a non-wet barrier, bounding and directing the liquid flow down the flat substrate. Diluted oil sample is let to flow slowly down the substrate, in a strong cross-oriented high-gradient (“leaking”) magnetic field. The close-up of REO1 F-G substrate mount is shown in Figure 2. Typically, 3 cm 3 of oil sample is diluted with 1 cm 3 of organic solvent to increase debris mobility within the oil. The flowing suspension of debris and liquid is influenced by highly divergent magnetic field. Magnetomotoric (M-M, fb) force attracts primarily FM debris down the substrate accto their size and magnetic properties. Hence, the larger particle and debris of higher susceptibility settle first, smaller at a longer distance from the oil entry. This way A-F sorts debris in oil sample according to their size and material. Only very large PM a/ o even diamagnetic (DM, fb) particles are trapped on F-G a/ o those of a compound FM/ PM a/ o FM/ DM structure, e.g. Figure 3 and 7. Having the sample pumped through and debris deposited on the substrate, the F-G is then fixed with 4 cm 3 of the same organic solvent, pumped in situ through the F-G, washing out remnant of the oil and locking particles where they have settled. Now, giving the thinner time to evaporate, the F-G is prepared and ready for microscopic examination. Today´s AF microscopes are still called “bichromatic” (B-M, fb) as a relic of Foxboro/ Trans- Sonic (F/ T, fb) trademark of 70‘s. In principle, B-M is medicinal microscope of transmitted light with additional horizontal Kőhler illuminator of reflected light. Two independent light sources illuminate F-G. While the beam of transmitted light passes through F-G substrate (and green filter in F/ T B-M) to a high-aperture condenser and eyepiece, the beam of reflected light (in F/ T B- M passed through red filter) is projected down through condenser and objective on the substrate, from which is reflected back to eyepiece. Hence, two-colour, bichromatic, for transparent debris looking green, opaque particles red. In this article, all PMGs were taken via Olympus BX-60 optical B-M, using halogen bulb (H-B, fb) and/ or mercury burner (Hg-B, fb) of some 3-4 domina- Aus Wissenschaft und Forschung 17 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 * Ing. Stanislav Slacik, CSc., ALS Czech Republic, s.r.o., Praha, the Czech Republic F - $ + * - & : $ * Figure 1: Analytical Ferrograph REO1 Figure 2: Close-up of FG REO1 F-G substrate mount T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 17 ∂ FS = (1/ 2μ 0 ) -- (B x2 ), [N.m -3 ] (8). ∂z Common FGs‘ magn. systems manifest their maximum value of FS being typically of the order of circa 250.10 7 N.m -3 next to poles, where the field is most distorted. But, higher above the poles, in situ of the F-G substrate, the M-M force is less. Author had measured the FS (8) in the height of substrate position of REO1 A-F, being of 0,5 .10 7 N.m -3 , Ref.[3], what seems to be not much, but it is still hundred times the gravitation force, acting on a particle. M-M force F z (with negligible help of gravitation) accelerates debris down, against the viscous drag force S f which in the case of Sf can be expressed as: Aus Wissenschaft und Forschung 18 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 ting spectral lines. Light of the Hg-B is brilliant, giving some length of coherence, resulting in high contrast view. Figure 4 depicts two spheroids (Sf, fb) A and B, Ø of circa 0,75 μm. Very minute details can be resolved. Couple of PMGs A1, A2 depict the same Sf and so does the couple B1, B2. The resolution is further enhanced by simultaneous use of iris and field diaphragms, what results in oblique illumination (O-I, fb), note shadows in Figure 4. Former B-M had objectives of low aperture, so all Sfs looked glossy in them. This is seen in Figure 4, as left column (A1, B1) was focused intently below the Sf perimeter. Direct Reading Ferrograph A-F enabled real qualitative insight into wear mode of oil-wetted parts, but at a price of significant time spent by F-G preparation and analysis. Direct Reading Ferrograph (DR-F, fb) as the quantitative wear measuring instrument, emerged several years after A-F. Like in A-F, the sample of lubricating oil is in DR-F diluted with solvent and pumped through transparent (glass) capillary tube in a high-gradient magn. field. M-M force F, acting on a small (one domain) magnet (particle) having magn. moment M due to external magn. field of the induction B can be expressed as: F = M B (1), ref.[1]. Magn. moment: M = (χ/ μ 0 )V p B (2), so the vertical force attracting debris down the substrate, F z , can be written as: F z = (χ/ μ 0 )BV p B = (χ/ 2μ 0 )V p B 2 (3). Considering, that magn. field attracts the mass of oil as well: F z = [(χ p χ l )/ 2μ 0 ]V p B 2 (4). The field is more or less homogenous in the horizontal plane (x,y) and for FM alloys (Fe, Co, Ni, steel) we may put: (χ p χ l ) ≈ χ p (5), so the eqn. (4) can be rewritten into (6): ∂ F z = (χ p / 2μ 0 )V p -- (B x2 ) (6) ∂z It can be shown that for FG performance the value of field induction is as important as its divergence, because: ∂ ∂ -- (B x2 ) = 2B x -- (B x ) (7) ∂z ∂z Let denote the function below as “Ferrograph Strength” (FS, fb): Figure 3: Large non-FM particle sitting on sub-micron FM adhesive wear debris. Two levels of focusing. Mag.500x, R-L: Hg-B, T-L: H-B,O-I. Ref. [2] 2 3 32 Figure 4: Spheroids. Mag.1000x, R-L: Hg-B, T-L off, O-I. Ref. [2] T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 18 S f = 6π.η.R.dz/ dt (9). (Stoke’s law), so the downward velocity of a Sf against the oil can be found, as the particle is accelerated by M-M force until the equilibrium with the growing drag is reached: ∂ dz/ dt = (χ p / 12μ 0 πRη)V p. -- (B x2 ) = χ p. V p. (FS) = ∂z = 2χ p. R 2 .(FS)/ 9η (10). Here we used the eqn. of spherical volume, V p = 4πR 3 / 3 (11). From (10) it is clear that for the value of FS, the oil sample viscosity (η), Sf debris size (R) and its material (χ p ), speed of the debris fall is given and so is the distance travelled by debris through capillary before sedimentation on its wall. The more debris precipitated in a tube section, the higher is optical density. Two optical sensors are set, one at the capillary entrance, the other one slightly downstream to get the readings of particles density, settled at each of the two points. First position is of large (L) debris (Ø above circa 5 μm), second of small (S), typically submicron size. Sum of (L+S) is a scale of Wear Particle Concentration (WPC), value of Wear Severity Index, WSI = (L+S).(L-S), a/ o WSI = L 2 - S 2 (12) is the alert signal of any significant wear change. In the case of REO1 and standard NATO Code O-156 oil sample diluted to circa 15 cSt, theoretical falling speed of Ø 5 μm FeO iron oxide Sf is circa 0,1 10 -3 m.s -1 , aluminium Sf of Ø 5 μm falls down by just 0,3 μm per second. That is why PM/ DM debris sediment through all the F-G trace, see Figure 5, marks “ + and x”. REO1 is unique in another feature. Standard F-G, prepared for A-F reasons, may be in DR-F REO21 analysed quantitatively later on. Scanning sensor of REO21 reads optical density of all the F-G sediment trace since its entry to drain exit, see Figure 5. Instead of just two figures (WPC, WSI), sedimentation portrait of the oil sample is obtained. Flow velocity profiles across the channel call for integral averaging, see the picture above (Figure 5). Lower picture - 2 F-Gs of oil sample from helicopter gearbox were made (marks □ and ■), oil from FG drain captured and put for repeated F-G make, now the oil bears no remaining ferrum debris (marks + and x). All FM particles were removed during the first pass through FG. Ref. [4]. Whenever the condition of engine and/ or gearbox deteriorates, both WPC and WSI tend to increase, but WSI often first. Adhesive (rubbing) wear Mild adhesive wear is the most beneficial wear mode of oil-wet parts. After Beilby layer formation during break-in, only adhesive wear debris is produced by engine in a good condition. Adhesive wear debris form on F-G strings parallel to magn. field, see Figure 6 and Figure 3. FM (steel) debris is of 5-20 μm size, clean surface of no oxides and/ or traces of cutting, they are leaves of pure metal, prevailing length to thickness circa 3: 1 or less for small debris and 10: 1 for large. When the engine is overloaded heavily, the size of adhesive debris may shift to 50-200 μm, giving alert of an imminent failure. Adhesive wear is a problem of engine/ gearbox starts & stops when boundary lubrication and/ or even dry rubbing take place. Ingestion of fine sand particles into lubricating system (what is common to turboshafts and/ or turboprops operation, Figure 7) can lead not only to detection of abrasive wear particles, but to growing concentration of adhesive debris as well. As a rule, imme- Aus Wissenschaft und Forschung 19 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 Q &(E G Figure 5: REO21 optical density reading. < Strings of FM debris settled accto B x vector Figure 6: Adhesive wear debris. B x Mag.100x, Ref. [2]. Q &(E G T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 19 ling-Element-Bearing (REB, fb) (accompanied by oil starvation) and/ or titanium fire, gross spherical particles may appear in oil. E.g. in Figure 8. For more details, see Ref. [5]. Cutting Wear Cutting wear takes place if one (hard) surface penetrates the other (softer). Such a harder part of the frictional pair appears as result of the oil system pollution by abrasive particles (third-body-problem, discussed above) and/ or via misalignment and/ or fracture somewhere inside the engine/ gearbox structure, see Figure 9. F-G was prepared from the oil sample of a turboprop engine reported to shorten the switch-off run time till stop. A-F confirmed heavy cutting distress inside an engine transmission and recommendation to remove the engine from operations was given. Subsequently the misalignment of the cage against outer ring inside one of transmission REB was revealed in the course of engine overhaul. Cutting wear can evolve as a result of operator´s turbine oil misuse. Figure 10 depicts particle of heavy cutting wear. A-F was applied on oil from foreign air force turbofan REBs lubricated erroneously by oil NATO code O-133 instead of proper O-135. Worsening by hot climate, this resulted in very low oil viscosity (poor lubrication) and engine high wear rate reported as “high iron content in oil”. Other debris of severe sliding wear is in Figure 10. As a rule, such debris is larger than 50 μm, sometimes their surface bears parallel striation imprinted by other surface asperities, e.g. bottom PMG of Figure 11. Fine wires can appear on F-G as result of break-in process when so called Beilby (amorphous shear-mixed) Aus Wissenschaft und Forschung 20 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 diate oil fill change can revert increased wear rate back to normal, mild.If a high-speed adhesive sliding takes place inside an engine, like cage misalignment in Rol- Figure 7: SiO 2 crystal found on F-G PMG Left: Mag.100x, R-L off, T-L: H-B, Right: Mag.500x, Ref.[2]. Figure 8: Large (Ø 20 µm) compound Sf of Fe and Fe-Cu composition, probably steel-bronze. Remnants of oil after turbofan REB destroyed in an oil-starvation. Mag.1000x, (left) & 200x (right) R-L: Hg-B, O-I, T-L off. Ref. [2] and [5]. Figure 9: Cutting wear particles (note the twice curled wire-like particle over 100 µm long, in the center of PMG). Oil from turboprop engine. Mag.1000x, R-L off, T-L: H-B, Ref.[2]. Figure 10: Heavy cutting wear particle, turbine oil misuse. Turbofan engine. Mag.500x, R-L: Hg-B,T-L: H-B, Ref.[2]. T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 20 layer rises in a process resembling fine abrasion, see in Figure 12. Detection of cutting wear debris on F-G is always alerting issue (with exception for fine wires in the course of break-in) and further operation of the engine/ gearbox shall be carefully monitored. Wear via fatigue, Rolling contact fatigue Fatigue, (ftg, fb) which causes pitting, flaking and cavities on a surface of ball and/ or roller bearing (REB, fb), is a subsurface, later surface located phenomenon. Rolling of a rigid body over elastic race induces normal and shear stresses in it, see the scheme at Figure 13. Tensile, compressive and shear stresses can be high enough to leave microplastic strains there. The circa 45° upgoing shear stress τ reaches its maximum in certain depth below the surface of the race. Having collected sufficient number of cycles it is right here, where the fatigue fractures initiates. Microcracks propagate in circa 45° upward. Subsequent detachment of particles from Beilby layer in the front of a crack is the base of Sf generation. Repeated passage of the rolling body over the microcrack forces it to “breathe”, close and open cyclically. Once oil penetrates the crack, high elasto-hydrodynamic pressures slips and shifts walls and tonque of the propagating crack. Debris detached from the naked Beilby layer leaves crack in a form of near perfect Sf, as the sphere is the body of minimum surface-to-volume ratio. The size of observed Sf ranges from a fraction of 1 μm to Ø 15 - 20 μm, but most frequently Sfs of 2 - 3 and/ or 3 - 4 μm in Ø can be found. The Ø of a Sf is related closely to width of fatigue crack, generating them. See Figure 14. Aus Wissenschaft und Forschung 21 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 Figure 11: Heavy cutting wear particles, helicopter gearbox of degraded gear oil. Mag.500x, R-L: Hg-B, T-L: H-B, Ref.[2]. Figure 12: Fine wires generated by break-in process. Turboprop engine shortly after overhaul. Mag.1000x, R-L: Hg-B, T-L: H-B, Ref.[2]. Figure 14: Size of Sfs found on F-G. Figure 13: Rolling over elastic plane. T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 21 Finally, “fatigue chunks” appear as fatigue cracks intersect and surface of races and/ or bodies exfoliates. This debris is extremely large (up to 150 μm plus, see Figure 15, 16), connected to last stage of fatigue spalling. Importantly, fatigue chunks are never covered with oxide layers, their surface is metallic, shiny and clear instead, see Figure 16, 17. So is the surface of coarse Sfs, generated during that final stage of fatigue failure, Figure 18. If oxide appears, it is a result of subsequent corrosion induced by traces of water in the oil, e.g. Figure 19. “Laminar particles”, which are very large (150 - 250 μm), arise from elder particle, being passed through the rolling contact once and/ or several times again. They have a form of very thin leaves of clear metal with length-to-thickness ratio up to 50: 1 plus. Many holes and pits on them are the result of “micro-forging” in the rolling contact, e.g. see Figure 19. As fatigue failure of REB proceeds further, the misalignment and mutual rubbing of REB races and cage ge- Aus Wissenschaft und Forschung 22 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 An appearance of Sfs on F-Gs in multiplicity must be taken seriously and oil samples taken more frequently, as pitting (and/ or other fatigue phenomenon) already started. Any rapid increase in a quantity of Sfs shall be understood then as a warning of imminent REB fatigue failure. Figure 15: Fatigue chunks crumbling. Figure 16: Typical ftg chunk of a REB failure. Sf at right corner. + Figure 17: Surface of fatigue chunk due to exfoliation of teeth in turboprop gears. Bright lines are traces of grinding. Mag.200x, R-L: Hg-B polarized, O-I. Figure 18: Sf of Ø 4,0 µm. Mag.1000x, R-L: Hg-B, O-I,Left: Focused on the Sf´s top, Right: Focused by 0,7 µm lower Traces of FeO.Fe2O3 oxide on some facets, most facets clear, shiny. Fe 2 O 3 formation on large Sf Ø 20 m coming from REB races exfoliation, Figure 19: Fatigue chunk debris Mag.400x R-L: H-B polarized, T-L: H-B. T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 22 nerate mixed population of ftg Sfs and cutting-wear debris. E.g. PMG Figure 20, 21. Besides, excessive heat and high shear stress put on a lubricant during highspeed rubbing causes oil matrix polymerization and organic, translucent Sfs creation, e.g. see PMGs at Figure 22, 23. Again, tripolymeric Sf are stable due to minimum surface to their volume. An appearance of tribopolymeric particles is always clear sign of oil distress and need of its change. Some tribopolymeric Sfs are of dark colour and a size close to fatigue metallic Sfs. In such a case the phenomenon of autofluorescence gives the chance to determine whether the Sf is of organic and/ or metallic nature. The F-G is irradiated by ultraviolet excitation light from mercury burner, reflected thr dichroic mirror. According to Stoke´s law, an object on F-G emits visible light of a longer wavelength, specific to its material. This helps to distinguish. E.g. Figure 23. It is possible to distinguish between crystal of silica (SiO 2 ) and a “cuboid”, which is other form of polymeric particles in lubricant. Polymeric cuboids manifest strong fluorescence effect, while silica not, e.g. Figure 24. High-speed sliding frictional contact during rubbing a/ o Titanium fire (Ti-fire, fb) is a mode of large composite Sfs formation, see explaining scheme on Figure 25. Repeated contact and separation at high sliding speed is accompanied by microwelding of surface asperities, followed by braking them so that created Sfs have got a Aus Wissenschaft und Forschung 23 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 3 Figure 23: Autofluorescence effect, 2 polymeric Sfs in plane field of oxides and silica. What seems to be empty spaces at PMG A is 2 polymeric Sfs, Ø 1 and 2µm seen clearly in near-violet spectrum of Hb-B, PMG B. Mag.500x, R-L: Hg-B, through dichroic mirror T-L: H-B. Spheroid Cutting wear debris Tribopolymeric Sf Figure 20: Common appearance of Sf and cuttingwear debris on F-A. Mag.1000x R-L: Hg-B, O-I. 2 Sfs above large cutting wear debris Figure 21: Simultaneous appearance of Sfs and cuttingwear debris. Sample from turbofan with destroyed compressor REB. Upper PMG: Mag.500x, R-L: Hg-B, O-I, Lower PMG: Mag.1000x. Figure 22: Tribopolymeric particles Sf, Ø 3,0 µm left, cylindrical, right, Mag.500x Figure 24: Comparison of a crystal of silica SiO 2 (left) and cuboid in visible (centre) and near-violet light. Mag.500x, R-L: Hg-B thr dichroic mirror. T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 23 labyrinth seal into lubricant. Although “dry” part of the engine failed, evidence of it was clearly imprinted into the oil. Their Ti-Fe chemical composition was confirmed by X-ray fluorescence spectral analysis subsequently. Ti-Fe Sfs have proven that Ti-fire destroyed the engine indeed and their occurrence in the oil of “on-wing” engines gives a red light, prompting to stop their operations. Typical size of Ti-Fe compound Sfs found on F-Gs is 10 to 14 μm, but in some cases lesser, 2 - 6 μm, e.g. given on Figure 26 - 29. The glossy appearance of Sfs on the right PMG of Figure 27 and left of Figure 28 is due to focusing below Sf’s top. Hence, the polished look of Sfs in early B-Ms of rather low objective aperture. First type of a fatique-related particles are pure spheroids, Sfs, alerting of an impending pitting a/ o spallation failure of REB, e.g. in Figure 4. If not halted in time, further operation of an engine/ gearbox can result in a complete and sudden failure of REB via ring spallation, e.g. see in Figure 16. Note that not all Sfs found on F-G are of fatigue wear origin. Many very gross Sfs may come from welding, grinding and other operations applied in engine production and maintenance course. Spallation is result of rolling contact fatique (ftg, fb) fracturing of REB outer/ inner ring which starts with REB races pitting. First warning gives detection of a big number of Sfs on a F-G. So, fatigue spallation debris (as another fatigue-related particle) are debris of flat and glossy appearance with prevailing length to thickness ration of 10 and/ or 20 to 1. On the surface of them defects and pits may be seen (REB race). Their circumfe- Aus Wissenschaft und Forschung 24 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 compound structure, e.g. Fe-Cu, Fe-Zn-Cu, Fe-Ti and so on. See Figure 8, 26, 27, 28, 29. In the Czech skies of 90‘s several cases of Ti-fire had occurred in a fleet of USSR-made turbofan. Rubs between high-pressure spool and internal casings happened in an area of Ti-alloy-made labyrinth sealing, pressurized from compressor. Fires happened on take-off rated engines during aerobatic manoeuvres due to age-related loss of flexural stiffness. All engines burned-out completely in a few seconds, but rub-released Sfs spread to remains of oil system nevertheless, being aspirated through failed (/ '6 . (+ F ( $ + Figure 25: Ti-fire, Oil Starvation high-speed rubbing origin of Sfs. 3 Figure 28: Sf of Ti-fire, Ø 1,5 µm. Two levels of focusing, Hg-B, O-I, Mag.1000x. B is focused just on the Sf´s top. Figure 29: Another Sf of a Ti-fire. PMGs are focused on the top (left), 0.7 (centre) and 1,4 µm (right) below. < Figure 26: Strings of Sfs (laying in accordance with B x ) on F-G. Mag.400x,R-L: H-B Figure 27: Compound Ti-Fe Sf Ø 14 µm born in Ti-fire. Focused on the top (left), below (centre) and on the perimeter (right) Mag.1000x, R-L: Hg-B, O-I, T-L: H-B T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 24 rential shape is random, see in Figure 16, 19. They are released to oil in result of final REB races crumbling in consequence of fatigue pitting. Other typical debris of the end of REB fatigue life are laminar particles, see in Figure 19, 30. These are flat and extremely thin (up to 50 : 1) particles of random perimeter shape typically having through holes and rough surface, as they are formed via cold working (rolling and pressing) through REB passage, see in Figure 16. Although their origin must not be contact fatigue but other wear mechanisms evenly, their number on F-Gs increases sharply with the REB spallation onset. Gearboxes of turboprops & rotorcraft suffer from fatigue exfoliation of gears on the pitch line, e.g. in Figure 17. As rolling/ sliding of gear teeth is of rather complicated kinematics they generate broad band of debris type - from adhesive, sliding and cutting wear particle (as slip between teeth grows with distance from the pitch line) to fatigue fracturing debris (Sfs, fatigue spalls & laminar particles). Teeth come to pure rolling contact just on a pitch line. So after some millions of meshing cycles fatigue takes place finally. Sfs start to appear on F-Gs and as subsurface fractures intersect, the carburized surface starts to exfoliate and crumble into oil as spallation particles in the end. For detailed explanation of fatigue fracturing, see Ref. [5]. References [1] Ilkovic,D: Fyzika, SVTL, Bratislava, 1962. [2] Slacik,S.: Atlas oterovych castic pro ferografii (Ferrography Wear Particle Atlas) REO Trade, Opava, 2010. [3] Slacik,S., Hala,A.: Indukce a intenzita magnetickeho pole v pracovni sterbine ferografu REO1, research report, TLL ACR, Praha, 1995. [4] Slacik,S: Author´s archive. [5] Slacik,S: Spherical Particles Observed in Lubricant of Aircraft Engines by Analytical Ferrography. XVII. ISA- BE, paper No. ISABE-2005-1243, Munich, 2005. Aus Wissenschaft und Forschung 25 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 Figure 30: Laminar particle. Note signs of cold working and perforation (in circles) on fatigue spalls as debris pass thr REB repeatedly. See pits & holes. Heligearbox. Mag.1000x, R-L: Hg-B, T-L: H-B, Ref. [2]. Aktuelle Informationen über die Fachbücher zum Thema „Tribologie“ und über das Gesamtprogramm des expert verlags finden Sie im Internet unter www.expertverlag.de Anzeige T+S_2_2018_.qxp_T+S_2018 08.02.18 15: 35 Seite 25