Evaluation on frictional performance of three different oil-lubricated impregnated graphite seal rings for aircraft integrated drive generators

The frictional performance of the shaft-end graphite seal rings plays a critical role in determining the maintenance-free duration of aircraft integrated drive generators, and significantly depends on the types of graphite materials used. In this study, three types of graphite materials, including epoxy resin, metal antimony and furan resin impregnated ones, were selected for evaluation using a rotating-type tribometer. The frictional test configuration involved an actual graphite seal ring sliding against the surface of a 12Cr2Ni4A disc under the lubrication of 4050 high-temperature synthetic aerospace lubricating oil, which was highly consistent with the practical operational conditions. The frictional performance of the three different impregnated graphite seal rings, including coefficient of friction, two-dimensional (2D)/three-dimensional (3D) morphologies of wear marks and wear rate of ring mass, were quantitatively analyzed and compared. The experimental results indicated that the metal antimony-impregnated graphite seal ring behaved with the smallest coefficient of friction among the three seal rings. 2D and 3D surface roughness parameters of the metal antimony impregnated graphite seal are the smallest. The wear rate of the seal ring’s mass is only approximately 22% of that of furan resin impregnated seal ring. The corresponding investigations demonstrated that the currently employed epoxy resin-impregnated graphite seal rings could be replaced by new candidates with better frictional performance to improve the service life.


Introduction
Carbon-related materials, including graphene, graphite and their composites, have been widely used in various engineering fields owing to their excellent self-lubricating properties [1][2][3], and the most well-known application is acting as solid lubricants [4][5][6].The physical ability of self-lubrication is ascribed to the typical two-dimensional or layer-lattice structure inside the materials [7].Among them, graphite materials with high thermostability and chemical stability show competitive advantages in mechanical seal components running in extreme conditions, such as the shaft-end seal rings of liquid rocket turbopumps [8] and aircraft integrated drive generators [9,10].Graphite seal rings prevent the leakage of liquid or gas as much as possible with the cooperation of the mated metal parts, and mechanical contact friction between them is inevitable [11][12][13].Therefore, the service life of graphite seal rings significantly depends on the friction and wear characteristics of the used graphite materials.Conventional pure graphite can be regarded as the accumulation of twodimensional graphene layers.Two neighboring layers are weakly bonded by the interplanar van der Waals force and are prone to wear and peel off easily under friction [14,15].To overcome the limitations in mechanical strength and wear resistance, innovative graphite-based composites that incorporate metal or polymer materials, such as copper-MgP-graphite and copper/graphite composites [16,17], are proposed.Among them, impregnated graphite materials have received wide attention because of the relatively simple and stable technical processes.Through the filling of the impregnation materials, such as metal antimony, furan resin, and phenolic resin [18][19][20], the porosity inside the graphite matrix diminishes and the mechanical strength is enhanced accordingly.
Tribological behaviors of various impregnated graphite materials have been fully investigated under different lubrication regimes to explore the application feasibility in bearings and seals [21,22].Although the mechanical strength of the impregnated graphite materials is improved, the self-lubricating characteristics are still retained through the migration of graphite and the formation of graphite transfer film on the frictional interface [23].Zhang et al [24] tested the friction characteristics of resin-impregnated and non-impregnated graphite, and found that the dry friction coefficient of impregnated one was larger while the oil-lubricated and water-lubricated coefficients became much smaller.They attributed the tribological phenomenon to the decrease in open pores and the increase in hydrodynamic effect.The excellent tribological behaviors of impregnated graphite Moreover, the wear resistance of metal-impregnated graphite and resin-impregnated graphite materials under high temperatures is still prominent.Compared with non-impregnated graphite, the wear depth at 350 °C could be reduced by 60%-80% [25].The frictional performance of impregnated graphite improves within a certain range as the test temperature increases.Induced by the high temperature, a transferred carbon-bassed tribofilm with an ordered structure was formed on the interface [26].The influence of graphitization degree [27], test environment [28] and test configurations [29] on the tribological performance of various impregnated graphite materials has also been explored by researchers.Zhao et al [30] investigated the friction and wear mechanism of zinc-phosphate impregnated graphite against stainless steel by ring-on-ring configuration, and two different styles of worn taper angles on the worn surface could be observed.However, very few tribological studies are conducted using actual impregnated graphite components and the fundamental tests of graphite materials often fail to accurately replicate the operational state of these components.
In this study, in order to guide the material selection of impregnated graphite seal rings in aircraft integrated drive generators, the frictional performance of three different oil-lubricated impregnated graphite seal rings was evaluated using a ring-on-disc tribometer.Through a specially designed fixture, the adopted test configuration was an actual graphite seal ring sliding against the surface of a 12Cr2Ni4A disc under the lubrication of 4050 high-temperature synthetic aerospace lubricating oil, which was highly consistent with the practical operational conditions.The tested seal rings were made from epoxy resin, metal antimony and furan resin impregnated graphite materials, respectively.The coefficient of friction, two-dimensional (2D)/three-dimensional (3D) morphologies of wear marks and wear rate of ring mass, were analyzed and compared to find a better potential candidate for seal rings.

Experimental procedure
For a specific type of aircraft integrated drive generators, the currently employed epoxy resin impregnated graphite seal rings require frequent maintenance or replacement due to the limitation in frictional performance.To find a better candidate, three different impregnated graphite materials chosen from the commercially available products are evaluated in this study.According to the usage of impregnated graphite materials, the three seal rings are marked as M106H, M256D, and M298K, representing epoxy resin, metal antimony, and furan resin-impregnated graphite, respectively.Based on the hollow cylindrical workpieces supplied by graphite manufacturers, graphite seal rings with complicated features can be acquired through mechanical machining and high-precision polishing.The fundamental mechanical properties, provided by graphite manufacturers, are listed in table 1.The metal antimony-impregnated graphite can operate at a maximum temperature of 400 °C and has the best plasticity.
As displayed in figure 1(a), all the tribological tests are conducted on a rotating-type tribometer under the configuration of an actual graphite seal ring sliding against the surface of a metal disc.The disc is made of lowalloy structural steel 12Cr2Ni4A, which is the same as the metal material used in the seal system of the aircraft integrated drive generators and is suitable for alternating stress conditions [31].The diameter and thickness of the metal plate is 50 mm and 2 mm, respectively.The disc is fixed in the container of lubricant and lubricated by

Results and discussion
Regarding the working surface of the graphite seal rings, the initial surface flatness is about 0.09 μm and surface roughness Ra is 0.125 μm.Each graphite seal ring accumulates a total of 1 h of testing, including four tests of 15 min.The friction coefficients of M106H, M256D and M298K seal rings during the first test of 15 min are illustrated in figure 2. The curves present obvious fluctuations since they belong to the running-in stages.Among them, the friction curve of M298K exhibits the most pronounced fluctuations, while M256D is relatively stable.
In the running-in stage, the M256D seal ring behaves with the smallest friction coefficient among the three seal rings.New lubricating oil is used before each test, which can alleviate the influence of the residual wear debris on the frictional performance.Figure 3 displays the friction coefficients of the three seal rings during the fourth test of 15 min.The curves of friction coefficients present much slighter fluctuations and can be regarded as the stable stages.However, several obvious peaks still occur on the curve of the M298K seal ring, which indicates the poor stability of frictional behaviors.In the stable stage, the M256D seal ring also exhibits the best frictional performance among the three rings.The average friction coefficients with error bars of the three graphite seal     Without the infiltration of lubricating oil, the surfaces are somewhat shiny.Regarding the M106H seal ring, numerous serious wear grooves can be found on the surface at three different positions.There are also noticeable spalling pits.The wear grooves almost distribute on the whole area, which demonstrates the severity of wear.For the M256D seal ring, no evident wear grooves can be found at the first position.Some slight wear marks appear at the other two positions of the graphite surface.Compared with the other two seal rings, more wear grooves appear on the whole surface of the M298K seal ring.The optical micrographs of the seal rings demonstrate that the wear behaviors between the graphite seal rings and the metal disc mainly belong to the abrasive mode.The graphite debris produces three-body wear between the interfaces and causes the typical wear grooves.
Except for the typical wear grooves, some other types of wear marks can be also observed on the surfaces, as displayed in figure 6.The wear forms are closely related to the mechanical properties of the impregnated graphite materials.Regarding the M106H seal ring, the solidified epoxy resin aggregates within the pores of the graphite matrix are prone to detachment due to frictional rotation, resulting in both large-scale and small-scale spalling, as shown in figure 6(a).For the M256D seal ring, numerous small-size pits are formed on the surface due to spalling of impregnated metal antimony, which can trap wear debris and produce additional hydrodynamic effect under oil lubrication.However, the surface of the M298K seal ring features noticeable plow wear marks, which are caused by the cyclical shear force between the counterpart plate and the graphite seal ring.Due to so many wear grooves, the produced pits are mainly destroyed.Such plow wear marks influence the sealing reliability and easily result in oil leakage.Moreover, the wear debris cannot be stably trapped in the linear grooves and the free wear debris would deteriorate the tribological behaviors.Once substantial wear marks or pits occur on the graphite surface, the rotating stability will be significantly weakened.In reverse, it will further exacerbate the wear degree, influence the sealing reliability and cause sealing failure in final.
Moreover, the optical micrographs of the 12Cr2Ni4A metal discs for testing the three graphite seal rings are displayed in figure 7. The scratches on the metal surfaces demonstrate the appearance of abrasive wear.After the frictional tests, graphite transfer film is formed on the surfaces through the accumulation of graphite debris.Among the three metal discs, the integrity and continuity of the graphite transfer film on the metal disc for testing the M256D seal ring are the best.As listed in table 1, the plasticity of metal antimony impregnated graphite is the best, which is beneficial for decreasing the abrasive wear and promoting the formation of graphite transfer film.To quantitatively analyze the surface morphologies of wear marks, the three graphite seal rings are also observed by a 3D laser confocal profilometer microscope.The scale of the observation area is 0.99 mm × 0.99 mm.As shown in figure 8, the typical wear grooves caused by abrasive particles are very prominent in the 3D view.Several deep and wide wear grooves appear on the surfaces of the M106H and the M298K seal rings.The width of wear grooves can reach 58.2 and 110.6 μm, respectively.However, the wear grooves on the surface of the M256D seal ring are quite shallow and the largest width is only 26.0 μm.To analyze the surface roughness in  a more conservative way, the regions without prominent grooves are chosen for extracting the surface roughness profile.
Figure 9 displays the surface roughness profiles of the chosen regions.The surface roughness waveform of the M106H ring fluctuates significantly and gradually decreases from 1.5 μm to −2.8 μm.The surface roughness waveform of the M298K seal ring varies dramatically, featuring numerous peaks and valleys.Regarding the M256D seal ring, the overall surface roughness waveform is quite smooth except for a sharp dropping at the position of around 80 μm.Based on the horizontal line at the highest point of the curves, the sectional areas of the wear marks can be estimated for the M106H, M256D and M2898K seal rings.The respective areas are 526.472μm 2 , 306.126 μm 2 and 764.516 μm 2 , which demonstrate that the wear degree of the M256D ring is the slightest.By using the data of surface roughness profiles, the average roughness Ra can be calculated through the following formula: Where, n is the total count of data points, z i is the height of the point i, and z m is the average height of all points.
The original surface roughness of the seal rings before frictional tests is around 0.125 μm.The calculated Ra values for the three seal rings are 0.796 μm, 0.658 μm, and 0.859 μm, respectively, and the value of the M256D seal ring is the smallest.Furthermore, the 2D surface morphologies and the Gauss distribution curves of surface heights [33] are obtained from the 3D test data.As displayed in figure 10, the surface heights of the M106 seal ring predominantly fall within the range from 6 μm to 9 μm.The surface heights of the M256D seal ring mainly distribute in the range from 5 μm to 8 μm.However, the surface heights of the M298K seal ring are relatively large and fall within the range from 9 μm to 12 μm.
Several 3D surface roughness parameters, as defined in the standard of ISO 25178 [34,35], are calculated and listed in table 2, including root mean square height Sq, maximum height Sz, arithmetical mean height Sa, and developed interfacial area ratio Sdr.The root mean square heights of the M106H, M256D and M298K seal rings are 1.272 μm, 0.8766 μm and 2.354 μm, respectively.It can be clearly found that the performance of 3D surface roughness parameters of the M256D seal ring is the best.
The loss mass of the graphite seal ring after frictional tests can reflect the wear resistance at the macro level.As illustrated in figure 11, the initial masses of the M106H, M256D and M298K graphite seal rings, measured by a high-precision analytical balance, are 11.25243 g, 16.74315 g and 14.34268 g, respectively.As listed in table 1, the mass density of antimony impregnated graphite materials is the largest and that of epoxy resin impregnated graphite material is the smallest.In consideration of fabrication errors, the values of the initial mass are reliable.After frictional tests, the sealing rings are ultrasonic cleaned for 15 min and dried completely at a temperature of 110 °C.After undergoing the same frictional test procedures, their masses decrease to 11.17424 g, 16.70894 g and 14.21254 g, respectively.The corresponding wear loss masses are 0.07819 g, 0.03421 g and 0.13014 g, resulting in wear rates of 0.69%, 0.20% and 0.91%, respectively.The macro-scale results also demonstrate that the wear resistance of the M256D seal ring is the best.
In final, it is necessary to discuss the underlying mechanism about the well frictional performance of the M256D seal ring based on the experimental results.Firstly, the plasticity of the metal antimony is better than resin [36] and the produced wear debris is much less, which can alleviate the three-body wear degree significantly.Secondly, the graphite transfer film from the metal antimony impregnated graphite material is more continuous and complete, which can improve the frictional performance through the self-lubricity of the film.Thirdly, more pits appear on the surface of the M256D seal ring and they can act as micro-bearings to produce additional loading capacity [37].Therefore, the metal antimony impregnated graphite seal ring could be an ideal candidate to replace the currently employed epoxy resin-impregnated graphite seal rings within aircraft integrated drive generators.

Conclusions
With the application background of the seal components within aircraft integrated drive generators, the frictional performance of three different impregnated graphite seal rings, including the epoxy resin impregnated graphite seal ring M106H, metal antimony impregnated graphite seal ring M256D and furan resin impregnated graphite seal ring M298K, is evaluated in this study.The test configuration is highly consistent with the practical operational conditions.The main conclusions are as follows: (1) The curves of friction coefficients of the M256D graphite seal ring during the running-in and the stable stages are the smoothest.In the stable stage, the average friction coefficients of the M106H, M256D and M298K seal rings are 0.097, 0.054, 0.109, respectively.Compared to the other two seal rings, the average friction coefficient of the M256D seal ring is about 45% smaller.
(2) According to the optical micrographs of the graphite surfaces and metal surfaces, the wear behaviors mainly belong to the abrasive mode.Less wear grooves appear on the surface of the M256D seal ring and graphite transfer film with better integrity and continuity can be observed on the surface of the match metal disc.The pits on the surface could act as micro-bearings to produce additional loading capacity.
(3) The graphite surfaces are analyzed by 3D laser confocal profilometer microscope.For a chosen region, the sectional areas of the wear marks for the M106H, M256D and M2898K seal rings are 526.472μm 2 , 306.126 μm 2 and 764.516 μm 2 , respectively.The calculated Ra is 0.796 μm, 0.658 μm, and 0.859 μm, respectively.3D surface roughness parameters of the M256D seal ring are also the smallest.The root mean square heights of the M106H, M256D and M298K seal rings are 1.272 μm, 0.8766 μm and 2.354 μm, respectively.The wear degree of the M256D seal ring is the slightest overall.
(4) The wear rates of ring mass of the M106H, M256D and M298K seal rings are 0.69%, 0.20% and 0.91%, respectively.The macro-scale results also demonstrate that the wear resistance of the M256D seal ring is the best.The underlying mechanism about the well frictional performance of the M256D seal ring is the comprehensive effect of mechanical properties, formation of graphite transfer film and forms of wear marks.
[32], which is also consistent with the actual operating conditions.The lubricating oil follows the China standard of GJB-1263-1991 and can operate at a maximum temperature of 220 °C.The kinematic viscosity at 40 °C and 100 °C is 25.51 and 5.0 mm 2 •s −1 , respectively.The spherical bearing below the container is used to alleviate unbalanced loading between the graphite seal ring and the metal disc through the self-adjustment of parallelism.As shown in figure 1(d), the graphite seal ring is a thin-walled cylindrical structure with a flange in the middle.To prevent damage to such a thin-walled structure, a fixture is specially designed and makes full use of the middle flange.The structural drawing and practical photograph of the fixture are displayed in figures 1(b) and (c), respectively.The stepped inner cavity of the fixture matches the shape of the graphite seal ring.During the frictional tests, the contact region of the seal ring is annular shape with an inner diameter of 38.2 mm and an outer diameter of 41.7 mm.As the tribometer is a kind of low-speed heavyload device, the loading force is much larger but the rotational speed is much lower compared with the practical condition.However, the contact friction and wear of graphite seal rings mainly occur in the low-speed stage and hydrodynamic fluid film can be generated in the high-speed stages.Such a test scheme can well compare the tribological behaviors of the three types of graphite seal rings.Based on the practical PV (pressure × velocity) value of graphite seal rings, a rotational speed of 200 r•min −1 and a loading force of 200 N are adopted in the tests.The contact area is about 2.2×10 -4 m 2 and the calculated contact pressure is about 0.91 MPa.All the tests are conducted at room temperature.Each test will last 15 min, which can alleviate the in-depth changes by the accumulated frictional heat.

Figure 1 .
Figure 1.Frictional test configuration for impregnated graphite seals.(a) Rotating-type tribometer with a configuration of ring-ondisc, (b) structural drawing of the special-designed fixture for graphite seals, (c) practical photograph of the fixture, (d) practical photograph of the graphite seal ring.

Figure 2 .
Figure 2. Friction coefficients of (a) M106H seal ring, (b) M256D seal ring and (c) M298K seal ring during the first test of 15 min.

Figure 3 .
Figure 3. Friction coefficients of (a) M106H seal ring, (b) M256D seal ring and (c) M298K seal ring during the fourth test of 15 min.

Figure 4 .
Figure 4. Average friction coefficients of the three graphite seal rings during the running-in and the stable stages.

Figure 6 .
Figure 6.Other wear patterns of (a) the M106H, (b) the M256D and (c) the M298K graphite seal ring.

Figure 7 .
Figure 7. Optical micrographs of metal discs for testing (a) the M106H seal ring, (b) the M256D seal ring, (c) the M298K seal ring.

Figure 9 .
Figure 9. Surface roughness profiles of the three graphite seal rings.

Figure 8 .
Figure 8. 3D surface morphologies of (a) the M106H seal ring, (b) the M256D seal ring and (c) the M298K seal ring.

Figure 10 .
Figure 10.Two-dimensional surface morphologies and Gauss distribution curves of surface heights for (a) the M106H, (b) the M256D and (c) the M298K seal rings.

Figure 11 .
Figure 11.Variations in the mass of three graphite seal rings before and after the frictional tests.

Table 1 .
Mechanical properties of three types of impregnated graphite seal rings.
commercially available 4050 high-temperature synthetic aerospace lubricating oil from Sinopec Research Institute of Petroleum Processing

Table 2 .
3D surface roughness parameters of the three graphite seal rings.