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Focus on the concept of pressure-velocity-time (pVt) limits for boundary lubricated scuffing
Wear 402–403 (2018) 179–186
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Focus on the concept of pressure-velocity-time (pVt) limits for boundary lubricated scuffing
T
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Ł. Wojciechowskia, , T.G. Mathiab a b
Institute of Machines and Motor Vehicles (IMRiPS), Poznan University of Technology, Poland Laboratoire de Tribologie et Dynamique des Systèmes (LTDS) - C.N.R.S., École Centrale de Lyon, France
A R T I C L E I N F O
A B S T R A C T
Keywords: Scuffing Boundary lubrication PVt limits
The mechanism of scuffing has been the object of tribological investigations for many years, but the factors that cause its activation are still poorly recognised by engineers and scientists. An industrial approach to this problem requires the determination of the maximum values of operational parameters that secure against the occurrence of scuffing if they are not exceeded. The example most commonly used in practice (especially by bearing, polymers or gears producers) is the pV (pressure x velocity) criterion, which offers the possibility of calculating the limits for friction pairs that are working under different kinetic configurations of materials and tribological conditions. However, such an approach is a significant simplification, since it does not take into account the unequivocal parameters of the tribosystem, such as the duration or frequency of its operation close to its mechanical limitations. These parameters can have a significant impact on the stability of the properties of lubricants, particularly in the context of an elastohydrodynamic film or boundary layer forming. This is why the concept of pVt (pressure x velocity x time) limits with regard to scuffing performance is postulated and elucidated in this paper. Taking into consideration the multi-parametric approach to the technological creation of the surface layer features, investigations regarding the dependence of scuffing performance on morphological, rheological and physical-chemical properties were performed. In order to better understand the fundamentals of the scuffing process, a series of systematic tribological double-blind trials were carried out with an active lubricant (olefin sulphide) on poorly lubricated cylinder/plane interfaces. The scuffing process was analysed at the final phase under boundary lubricated conditions, specifically for ground and burnished steel cylinders (AISI 4130) and polished planes of cast iron (EN-GJL-300).
1. Introduction
recovery [10] or adiabatic shear instability [11]. Unfortunately, most of these theories are mutually incompatible, despite the fact that they have all been experimentally confirmed. This state of affairs emphasizes the complexity of the SP mechanism and the multitude of factors that may affect its initiation. Factors activating the SP can be grouped into a few sets involving different features and parameters of the tribosystem, and typically, they are divided into three groups based on the material configuration, surface finish and operating cycles of the machinery (including lubrication aspects), as Ludema [4] proposed in his phenomenological description of scuffing. Irrespective of the identified, specific mechanisms of scuffing, the problem of its initiation particularly concerns friction pairs operating under conditions of high loads and slow speeds. In such tribosystems, which are characteristic for boundary lubrication (BL), the conditions generate contact pressures that can increase beyond the level specific for elastohydrodynamic lubrication, and can lead to the plastic deformation of asperities. Due to the fact that BL cannot secure long-term
The scuffing process (SP) is a catastrophic form of wear, and due to its sudden onset and capacity to cause disastrous wear, it may lead to the functional failure or definitive destruction of the frictional operating parts of machines and devices. As it is macroscopic in nature, parts damaged by scuffing cause emergency stoppage and call for costly and time-consuming replacement. Although many crucial friction pairs (e.g. manual and automatic gear boxes, piston rings/cylinder liners, camshafts/cam followers, bearings, etc.) are exposed to scuffing, there is still a lack of well-established knowledge indicating universal factors responsible for the activation of this process. The well-known theories focused on the SP connect it with the achievement of some critical temperature at the asperities [1,2], the speed of debris accumulation in the contact area [3,4], the energetic activation of cooperating surfaces [5,6], the interaction between the polar constituents of lubricants and surfaces [7–9], the rate of the protective metal oxides’ destruction and
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Corresponding author. E-mail address: [email protected] (Ł. Wojciechowski).
https://doi.org/10.1016/j.wear.2018.02.019 Received 18 November 2016; Received in revised form 21 December 2017; Accepted 16 February 2018 Available online 19 February 2018 0043-1648/ © 2018 Elsevier B.V. All rights reserved.
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Nomenclature γI+/σI σS σE
A FN MI MS ME p tSC
Acid/base component of surface free energy in the initial state [mJ/m2] Residual stresses in the initial state [MPa] Residual stresses in the scuffed state [MPa] Evolution of residual stresses [MPa]
Size of the measured wear area [mm2] Load [N] Number of motifs in the initial state Number of motifs in the scuffed state Evolution of the number of motifs Pressure [MPa] Time to scuffing [s]
configurations of materials and tribological conditions. Another practical use of pV limits is the selection of polymers for tribological applications. Thanks to this, it is possible to calculate conditions where the rapid, catastrophic wear or overheating of polymers can occur. Besides the polymer and bearing industry, the pV limit is a useful parameter to determine the maximum performance of seals. It is typically assumed that during the SP, the maximum value of pV is present along the gear contact path (as the product of the normal Hertzian pressure and the sliding speed) and remains constant. This simplification does not take into account some additional parameters, such as the transitional regime, the type of lubricant, its viscosity and temperature, the materials and geometry of the gears, etc. This is why, in the case of gear boxes, their performance determination can require some lubricant criteria (e.g. the lubricant film thickness, the pressureviscosity coefficient, the dynamic viscosity of oil etc.) [16] or even the bulk (mass) temperature (assumed as the temperature of the tooth flank before teeth meshing occurs [17]). The development of the pV criteria is the friction power concept suggested that SP occurs when the heat of the sliding contact reaches some critical value. The friction power is usually determined as the product of three factors: nominal contact load, sliding speed and coefficient of friction. Horng presented the most advanced form of this concept [18]. According to it, frictional heating and the lubricant layer breakdown are limited to peaks of the single surface asperities. Therefore, the value of the classic pressure-velocity criteria should be referred to the real contact area (so-called True Friction Power Intensity concept [18,19]). Fundamentally, it can be stated (Kennedy [20]) that during friction the energy is dissipated as heat on the sliding surfaces in the contact area, fluctuating due to morphology of rubbing asperity's and residual stresses. Thence, the rate of heat generated per unit area of contact, is the product of the local COF, pressure and velocity. In the specific case of BL, the classic approach to pV limits can turn out to be insufficient, because time plays a key role in the durability and stability of boundary layers (the time factor is successfully applied to calculate frictional energy in gears, also in scuffing aspect [17,21]). Too long operation near the upper limit of the acceptable pV can cause accelerated “consumption” of the oil and the premature breakdown of the boundary layer. Points exceptionally exposed to this phenomena are asperities’ peaks, potentially plastic deformed and/or overheated due to working under BL conditions. Energetic activated surfaces of so formed asperities are the perfect paths of the SP initiation. This is why the authors propose an extension of the typical pressure-velocity criterion by the time factor, paying attention to the duration of the influence of operational parameters on the boundary layer. In fact, the newly formulated criterion of pVt limits (pressure x velocity x time [J/m2]) gives information about the level of energy which must be delivered to the tribosystem to activate the SP. Of course, the SP identification applying methods based on determining the friction power or energy is just one of prediction options for this catastrophic damage. Alternative methodologies can be found in scuffing resistance tests, e.g. the heat power intensity and bulk temperature as elements of the gears scuffing parameter [17], the socalled shock test for identification of the coated gears scuffing [22] or the limiting pressure of seizure [23]. Taking into account above considerations and the aspect of the
protection against the SP, it is important to recognise the breaking point of the boundary layer, which activates the scuffing wear. Detailed analysis of the identified factors activating scuffing [1–11] results in the conclusion that all factors are real in individual, specific cases of the various analysed tribosystems, but none of them solve the issue in a universal way. This is why the authors believe that those examining the problem of the general conditions of scuffing activation should not only consider the aspect of the proper configuration of materials and lubricants, but also the aspect of some precursors or initial, technological parameters characterizing the surface state directly before the SP commences. The previous work of the authors [8,9,12–14] points to the necessity of a complex, multi-parametric approach to this question. The multi-parametric methodology of the creation of surface properties consists of the achievement of a positive compromise between its rheological, morphological and physical-chemical features. It is possible to distinguish the quantities or parameters that describe surface properties particularly well with regard to scuffing resistance. Residual stresses should be considered as the rheological characteristic most strongly influencing scuffing resistance [9,12]. The physical-chemical group of properties should be represented by the wettability (in the case of inactive lubricants) and/or the surface polarity (in the case of lubricants with AW/EP additives) [8,9,14]. Surface morphology offers the greatest number of parameters that can be applied to the determination of the surface's scuffing performance. However, according to previous papers [12,14], the mean number of the ENISO 12085's roughness motifs is a surface property best related to scuffing performance. A very important aspect of SP is the ability to identify its beginning. From the tribometrical point of view, observing values and potential fluctuations of the coefficient of friction (COF) seems to be the best way to do this. In accordance with the classical definition of the COF (the ratio of friction force and normal force), it can be stated that all factors potentially determining the friction behaviour of the friction pair are taken into consideration. In spite of appearances, the COF is quite a complex quantity and its value is sensitive to many factors. Blau [15] classified these factors into eight categories based on: contact geometry, flow and properties of fluids, lubricant chemistry, relative motion, applied forces, presence of third bodies, temperature in the contact zone, and stiffness and vibrations. All of these factors have more or less influence on the shape, texture and size of the real contact area between rubbing parts. Moreover, the contact area can evolve during friction (as a consequence of the synergistic or antagonistic actions of the above factors), so that its morphological, rheological and physical-chemical characteristics can essentially change, effectively disturbing initial operating conditions. For this reason, it seems to be crucial to analyse the state of the surface morphology both before and after SP initiation. The information obtained from this analysis could identify geometric and lubrication conditions favourable or unfavourable for scuffing. On the other hand, a practical engineering approach to this problem requires characterisation of the maximum values of some operational parameters that when not exceeded should secure against the SP. The parameter most commonly used in industry (e.g. by bearing producers) is the pV criterion. The pV (pressure x velocity) limits of contacting parts have tremendous importance, offering the possibility of calculating the limitations in friction nodes working in different kinetic 180
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(Fig. 2a). The cylinders were internally and externally axially ground to reduce the dangers of cylindricity error. The materials for the surfaces submitted to tribological investigations were carefully selected in order to reduce the risks of metallurgical situations favouring scuffing, and therefore cylinders (of 45 mm external diameter and 12 mm width) were manufactured from AISI 4130 steel. The cylinders were ground, consequently offering anisotropic surface morphology (Sa = approx. 0.5 µm). The cylindrical surfaces were then submitted to burnishing with five values of pressure, using two symmetrical spherical sectorshaped rolls of 50 mm diameter, described in detail elsewhere [9]. The selected load conditions gave the following values of burnishing pressure: 1st: 1.3 GPa, 2nd:1.64 GPa, 3rd: 1.87 GPa, 4th: 2.06 GPa and 5th: 2.22 GPa. Systematic areal morphological analyses were performed using an optical interferometer on millimetric regions relevant to the contact surfaces during experimental tribological investigations. Five areas of 1.2 mm × 0.9 mm every 72° on each cylindrical surface (in the initial and scuffed state) were examined. Metrological analyses were carried out very carefully and conscientiously, taking into consideration the calibration, as well as the transfer function and measurement limitations, of the selected topometric device. Analysis of the mean number of the EN-ISO 12085's roughness motifs (recognised as scuffing invariants [12]) was applied to study the relationship between the activation of the SP and the morphological state of initial and worn surfaces. According to the EN-ISO 12085 standard, a roughness motif can be defined as a portion of the primary profile between the highest points of two local peaks of the profile, which are not necessarily adjacent. However, for the purpose of the 3D analysis and due to the developed method of identifying a roughness motif using watershed segmentation, the motif should be considered as a specific catchment basin surrounded by watershed lines. The procedure of determining motifs and their characteristics (number, height and area) involves three stages: segmenting the surface using the watershed algorithm, building a special change tree describing the relationship between peaks and valleys, and simplifying this tree using the Wolf pruning process. This procedure is described in detail elsewhere [25,26]. The residual stresses were measured by the X-ray diffraction method. In order to satisfy the statistical requirements, measurements were conducted at three points every 120° on the cylindrical surfaces (in the initial and scuffed state) of the AISI 4130 specimens. The acid/base component of the surface free energy (SFE) was used for the quantitative determination of the surface polarity. The SFE of the rubbing bodies was established on the basis of the measurements of
correct configuration of materials of the friction pair, the multi-parametric model of the tribosystem is proposed in Fig. 1. The energetic method for the determination of the scuffing activation period was presented for the first time in paper [13]. There, scuffing energy meant the work needed in order for scuffing to occur for a friction pair under selected tribological conditions (the measuring/ calculating procedure used to identify SP initiation by the scuffing energy has been characterised in detail in [13]). However, apart from the unquestionable benefits from the application of this methodology, there is some limitation to its practical use. This limitation is principally associated with a certain complexity with regard to the quantity of scuffing energy, which is not necessarily easy to use in industrial practice. Therefore, the authors believe that the pVt concept could be a much easier solution to determine the scuffing performance of frictional operating parts exposed to this type of wear. This approach also has a strong economic foundation. According to statistics from the International Organization of Motor Vehicle Manufacturers (OICA), the worldwide production of passenger cars (PC) and commercial vehicles (CV) in 2015 amounted to approx. 90.7 million. In turn, the motorization rate in 2014 was equal to approx. 661 PC and CV per 1000 inhabitants in NAFTA (North American Free Trade Agreement) countries and 569 PC and CV per 1000 inhabitants in EU (European Union) countries. Considering the fact (as Holmberg et al. estimated in [24]) that the energy needed for PC to overcome friction in the piston assembly comprises nearly 45% of all engine friction energy losses, and approx. 10% of this energy arises in the friction nodes operating in BL conditions, it can be stated that in the automotive industry alone, the risk of scuffing affects a gigantic number of objects. In order to experimentally confirm this concept of SP activation, a series of systematic tribological investigations were carried out with an active lubricant (olefin sulphide) boundary lubricated cylinder/plane interface. The SP was analysed at the final phase, specifically for finally ground and burnished steel cylinders (AISI 4130) and polished planes made from cast iron (EN-GJL-300). The part of the investigations presented in this paper consisted of the determination of the relationship between the pVt value activating scuffing and the initial and scuffed states of the surface, characterised by prior identification of its representative rheological (residual stresses), physical-chemical (surface polarity) and morphological (number of motifs) properties.
2. Methodology A geometrical configuration of a flat block in contact with a rotating cylinder was selected, principally for contact mechanical reasons
Fig. 1. Model of the tribosystem using the multi-parametric approach to the creation of its anti-scuffing properties and durability.
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Fig. 2. Geometry of friction pair (a), procedure of pVt limits calculations (b), method of load application in the scuffing test (c) and the coefficient of friction vs. time to scuffing (d).
Table 1 Residual stresses, a number of roughness motifs and an acid/base component of SFE in depending on the machining treatment. Machining and surface treatment
Be (1.30 GPa) B (1.64 GPa) B (1.87 GPa) B (2.06 GPa) B (2.22 GPa)
Residual stressesa [MPa] Initial state σI
Scuffed state σS
Evolutionc σE
Initial state MI
Scuffed state MS
Evolutiond ME
Initial state γI+/−
−141 −229 −279 −297 −304
−251 −303 −428 −444 −348
110 74 149 147 44
285 239 165 152 154
51 57 57 51 51
234 182 108 101 103
10.6 10.3 15.5 20.3 17.5
G G G G G
+ + + + +
a
[9]. [8]. Evolution due to scuffing: σE = | σS- σI|. Evolution due to scuffing: ME = | MS- MI|. G + B: grinding + burnishing (the value of the burnishing tools’ pressure).
b c d e
static contact angle on the surfaces of the cylindrical specimens. The measurement procedure has been described in detail in [8]. In order to satisfy the statistical requirements, the angle measurement procedure was repeated 10 times, and the average value was used to calculate the SFE and its components using an acid/base method. Due to problems with the measurement of the contact angle on scuffed surfaces, the procedure for the SFE determination was performed only for the initial state of the cylindrical surfaces of the AISI 4130 specimens. Only the acid/base component of the SFE is responsible for the polar interactions of metallic surfaces (e.g. with AW/EP lubricants’ additives), therefore only this part of the total value of the SFE was taken into consideration in the analysis. The criterion for the determination of scuffing was an increase in the friction coefficient under a constant load (Fig. 2d). The experiments were performed under single drop lubrication using gear oil containing
Table 2 Results of scuffing investigations and pVt limits calculations. Machining and surface treatment
Pressure of SP activation [MPa]a
Sliding velocity (Fig. 2) [m/s]
Time to scuffing [s]
pVt × 103 [MJ/ m2]
G G G G G
91.8 98.7 99.9 109 123
0.5
536 571 640 951 796
24.61 28.18 31.99 51.81 48.98
+ + + + +
B B B B B
(1.30 GPa) (1.64 GPa) (1.87 GPa) (2.06 GPa) (2.22 GPa)
Acid/base component of SFEb [mJ/m2]
Number of roughness motifs [pcs.]
a The final load in the scuffing test (Fig. 2) divided by the size of the measured wear area.
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for all analysed cylindrical surfaces. Table 2 presents the results of scuffing investigations and calculations of the pVt limits. Taking into consideration the results of the residual stresses investigations (Table 1) and the pVt limits calculations (Table 2), some observations concerning their relationship can be made. Additionally, the graphical interpretation of this issue is shown in Fig. 3. One can observe a clear, statistically confirmed (by the r-Pearson coefficient) dependence between pVt limits and residual stresses in the initial, technological state of the surface. It is evident that the increase in compressive residual stresses causes an increase in scuffing resistance (Fig. 3a). The growth of compressive stresses results in an enhancement of the cohesive forces responsible for maintaining the entire structure of the metallic surfaces during wear. As a result of the wear process (and finally scuffing activation), the growth in the value of compressive stresses can be recognised. However, it does not translate into a statistically confirmed influence on pVt limits. The absolute values of the evolution of residual stresses (the difference between stresses in the initial and scuffed state) point to the cold work hardening of surfaces caused by the wear process. Nonetheless, this process does not have a direct influence on the scuffing performance (Fig. 3c). For comparison, the values of the correlation (r-Pearson) coefficient between residual stresses (in all analysed states) and classical pV (pressure x velocity, Table 2) limits were calculated. The results were as following: r = −0.794 (for σI vs. pV), r = −0.395 (for σS vs. pV) and r = −0.465 (for σE vs. pV). Taking into account the assumed analysis conditions (the sample size n = 5 and statistical significance α = 0.1), it cannot be found statistically confirmed correlation between residual stresses and pV limits (in all cases │r│ < rCR = 0.805). Fig. 4 presents the evolution of the surface morphology during the wear process, represented by the mean number of roughness motifs. The analysis of their values in the initial and scuffed state allows us to determine the relationship between the motifs and the pVt limits (Fig. 5). Our prior investigations [12] have shown that roughness motifs can be recognised as invariants in scuffing initiation. The key role here is an unfavourable, critical relationship between the shaping of the surface asperities (pressures are enough to cause a breakdown of the boundary layer) and the empty volume of valleys (insufficient for a safe amount of oil and its distribution to the real contact zone). The analysis of the data presented in Fig. 5 points to the fact that the mean number of motifs is the parameter with the statistically strongest influence on the scuffing performance. As one might have expected, the decrease in the number of motifs causes an increase in their area, and as a consequence a decrease in the pressures between elements of the friction pair. This is why values of pressures characteristic for the activation of catastrophic wear are higher for the surfaces with the lowest number of motifs (when the peaks of the asperities are “flatter and milder”). A similar trend can be observed for the difference in the number of motifs in the initial and scuffed states and the value of the pVt limits (Fig. 5c). It can be conjectured that the area of asperities, and consequently the pressures on them, can evolve in frictional conditions. This evolution occurs very often at the beginning and end of the operating of friction pairs exposed to the SP. In the initial stage, asperities are subject to abrasive and plastic ‘running-in’. That is when the highest peaks are cut off or flattened, and the area of asperities increases. As a result, the frictional conditions are stabilised, and the intensity of the wear can be limited. Additionally, time may turn out to be an important factor influencing the evolution of the morphology. In general, a longer working time can cause an increase in the area of the asperities, and as a consequence the pressures needed for scuffing initiation. In all probability, the reason here is a slow, but systematic, frictional cold work hardening of the surface's highest peaks. For comparison, the values of the correlation (r-Pearson) coefficient between motifs (in all analysed states) and pV limits were calculated.
Fig. 3. pVt limits vs. residual stresses in the initial state σI (a),the scuffed state σS (b) and their evolution due to scuffing σE= | σI – σS| (c) [r–Pearson correlation coefficient, rCR–critical value for Pearson correlation, α–statistical significance].
ca. 5% olefin sulphide as an extreme pressure additive. The scuffing activation period was then measured as the time it took for scuffing to occur, which is equivalent to the situation of poor lubrication in the friction pair. The scuffing kinetics were performed at a sliding speed of 0.5 m/s between the rotating AISI 4130 cylinder and the stationary, flat block (EN-GJL-300 cast iron with flake graphite). The load incrementally applied to the friction pair is presented in Fig. 2c. In order to satisfy the statistical requirements, the scuffing tests were performed four times for each pair of cylinders and flat blocks. After the scuffing tests, the areas of wear traces were measured on the scuffed surfaces of the EN-GJL-300 cast iron flat blocks. The final load in the scuffing test divided by the size of the measured wear area allowed us to calculate the pressure between the elements of the friction pair at the moment of SP activation. The value of this pressure multiplied by the sliding velocity (between AISI 4130 cylinders and EN-GJL300 flat blocks – Fig. 2b) and the time to the commencement of scuffing determined the value of the pVt quantity for the analysed friction pairs.
3. Selected results and discussion Table 1 presents the results of the residual stresses (in both the initial and scuffed state), roughness motifs (in both the initial and scuffed state) and the acid/base components of the SFE (only in the initial state) 183
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Fig. 4. The 3D isometric views and SP generated evolution of the mean number of motifs of AISI 4130 cylinders burnished at different pressures: 1.3 GPa (a), 1.64 GPa (b), 1.87 GPa (c), 2.06 GPa (d), 2.22 GPa (e). MI-a number of motifs in the initial state, MS-a number of motifs in the scuffed state, ME = | MS- MI|.
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activation is connected with a change in the reactivity of this surface, and the concentration of oxygen on it. For example, the increase in the acid/base component of the SFE (which is answerable for the polar interactions of a surface) can cause the increase in the surface concentration of oxygen. The presence of oxygen is a necessary condition for the activation of AW/EP sulphur-based additives, and its increased concentration on the surface caused the growth of the sulphur (present here in the olefin sulphide) concentration in the contact area. Thus, in the case of the lubrication by oils containing surface-active sulphur compounds (olefin sulphide here), the increase in the acid/base component of SFE can favourably influence scuffing resistance. This effect can be used for the preparation of metallic surfaces using lubricants enriched by tribological additives. However, it must be confirmed for the additives based on non-sulphur compounds. For comparison, the value of the correlation (r-Pearson) coefficient between the acid/base component of SFE in the initial state and pV limits was calculated. The value of this coefficient is equal to −0.720 and is lower than the critical value (rCR = 0.805). Therefore, it cannot be found statistically confirmed correlation between the acid base component of SFE and pV limits. 4. Conclusions On the basis of the experimental observations presented in this investigation (limited to only one configuration of the tribosystem), the following conclusions can be formulated:
• The pVt limits can be considered to the prediction of SP activation in • • Fig. 5. pVt limits vs. the mean number of motifs in the initial state MI (a),the scuffed state MS (b) and their evolution due to scuffing ME = |MI – MS| (c).
•
•
Fig. 6. pVt limits vs. the acid/base component of SFE.
The results were as following: r = −0.767 (for MI vs. pV), r = −0.393 (for MS vs. pV) and r = −0.746 (for ME vs. pV). Taking into account the assumed statistical conditions, it cannot be found statistically confirmed correlation between motifs and pV limits (in all cases │r│ < rCR = 0.805). An analysis of the data shown in Fig. 6 allows us to recognise a certain trend in the value of pVt limits as a function of the acid/base component of the SFE. The influence of the surface polarity on scuffing
the case of friction pairs operating under BL conditions. There is a clear, statistically confirmed dependence between pVt limits and previously identified invariants [12] and surface features [8,9,12] strongly affecting scuffing performance. The increase in the compressive residual stresses on the initial surface state causes an increase in scuffing resistance. This effect is connected with the enhancement of cohesive forces responsible for maintaining the entire structure of the metallic surfaces during wear. The mean number of roughness motifs is the morphological parameter with the strongest influence on scuffing performance. The decrease in the number of motifs translates into an increase in their area, and as a consequence a decrease in the pressures between elements of the friction pair indispensable to SP activation. The operating time can be a factor influencing the morphological evolution, whether favourable or not, in the context of scuffing. A longer duration of work can cause an increase in the area of the asperities, and as a consequence in the pressures necessary for scuffing initiation. This effect is caused by the running-in process and the systematic frictional cold work hardening of the surface's highest peaks. It was confirmed by the dependence between the evolution of the number of motifs and pVt limits (Fig. 5c). There is a clear relationship between pVt limits and the acid/base component of SFE. The increase of this responsible for polar interactions component can improve the scuffing performance due to the increased surface concentration of oxygen. Its presence is necessary for the activation of AW/EP sulphur-based additives and its increased concentration can be conducive to the formation of sulphurbased boundary layers.
5. Perspectives Further investigations regarding this problem should include the following areas:
• Verification of the precursory behaviour of the pVt limits for dissimilar configurations of the geometry, materials and type of lubricants of a friction pair.
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• Topological analysis of the surface in terms of the synergism and
initiation by adiabatic shear instability, Wear 258 (10) (2005) 1471–1478. [12] Ł. Wojciechowski, T.G. Mathia, Proposal of invariant precursors for boundary lubricated scuffing, Wear 340–341 (2015) 53–62. [13] Ł. Wojciechowski, S. Eymard, Z. Ignaszak, T.G. Mathia, Fundamentals of ductile cast iron scuffing at the boundary lubrication regime, Tribol. Int. 90 (2015) 445–454. [14] Ł. Wojciechowski, K.J. Kubiak, T.G. Mathia, Roughness and wettability of surfaces in boundary lubricated scuffing wear, Tribol. Int. 93 (Part B) (2016) 593–601. [15] P.J. Blau, The significance and use of the friction coefficient, Tribol. Int. 34 (2001) 585–591. [16] J. Castro, J. Seabra, Scuffing and lubricant film breakdown in FZG gears. Part II: new PV scuffing criteria, lubricant and temperature dependent, Wear 215 (1998) 114–122. [17] J. Castro, J. Seabra, Influence of mass temperature on gear scuffing, Tribol. Int. 119 (2018) 27–37. [18] J.H. Horng, True friction power intensity and scuffing in sliding contacts, J. Tribol. 120 (1998) 829–834. [19] J.H. Horng, M.L. Len, J.S. Lee, The contact characteristics of rough surfaces in line contact during running-in process, Wear 253 (2002) 899–913. [20] F.E. Kennedy, Frictional heating and contact temperatures, in: B. Bushan (Ed.), Modern Tribology Handbook, 1 CRC Press, 2001, pp. 235–272. [21] A. Ziegltrum, T. Lohner, K. Stahl, TEHL simulation on the influence of lubricants on load-dependent, Tribol. Int. 113 (2017) 252–261. [22] W. Tuszyński, R. Michalczewski, M. Szczerek, M. Kalbarczyk, A new scuffing shock test method for the determination of the resistance to scuffing of coated gears, Arch. Civil. Mech. Eng. 12 (2012) 436–445. [23] W. Piekoszewski, M. Szczerek, W. Tuszyński, The action of lubricants under extreme pressure conditions in a modified four-ball tester, Wear 249 (2001) 188–193. [24] K. Holmberg, P. Andersson, A. Erdemir, Global energy consumption due to friction in passenger cars, Tribol. Int. 47 (2012) 221–234. [25] S. Mezghani, H. Zahouani, Characterisation of the 3D waviness and roughness motifs, Wear 257 (2004) 1250–1256. [26] F. Blayteron, The areal features parameters, in: R. Leach (Ed.), Characterization of Areal Surface Texture, Springer-Verlag, Berlin-Heidelberg, 2013, pp. 45–67.
antagonism of its features and their influence on SP activation.
Acknowledgements The authors thank Dr. R. Majchrowski (Poznan University of Technology) for the topography measurements and F. Blateyron (Digital Surf) for help in use of Mountains® Software. References [1] Yu.N. Drozdov, Thermal aspects of scoring in simultaneous rolling and sliding contact, Wear 20 (2) (1972) 201–209. [2] S.C. Lee, H.S. Cheng, Experimental validation of critical temperature-pressure theory of scuffing, Tribol. Trans. 38 (3) (1995) 738–742. [3] J. Enthoven, H.A. Spikes, Infrared and visual study of the mechanisms of scuffing, Tribol. Trans. 39 (2) (1996) 441–447. [4] K. Ludema, A review of scuffing and running-in of lubricated surfaces, with asperities and oxides in perspective, Wear 100 (1984) 315–331. [5] A.P. Semenov, The phenomenon of seizure and its investigations, Wear 4 (1) (1961) 1–9. [6] R.M. Matveevsky, Friction power as a criterion of seizure with sliding lubricated contact, Wear 155 (1) (1992) 1–5. [7] H.A. Spikes, A. Cameron, Scuffing as a desorption process – an explanation of the Borshoff effect, ASLE Trans. 17 (2) (1974) 92–96. [8] Ł. Wojciechowski, T.G. Mathia, The polarity of metallic surfaces in the context of the corrosive and scuffing wear control, Tribol. Int. 90 (2015) 473–480. [9] Ł. Wojciechowski, T.G. Mathia, Conjecture and paradigm on limits of boundary lubrication, Tribol. Int. 82 (Part B) (2015) 577–585. [10] A.W. Batchelor, G.W. Stachowiak, Some kinetic aspects of extreme pressure lubrication, Wear 108 (2) (1986) 185–199. [11] J. Hershberger, O.O. Ajayi, J. Zhang, H. Yoon, G.R. Fenske, Evidence of scuffing
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Focus on the concept of pressure-velocity-time (pVt) limits for boundary lubricated scuffing
T
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Ł. Wojciechowskia, , T.G. Mathiab a b
Institute of Machines and Motor Vehicles (IMRiPS), Poznan University of Technology, Poland Laboratoire de Tribologie et Dynamique des Systèmes (LTDS) - C.N.R.S., École Centrale de Lyon, France
A R T I C L E I N F O
A B S T R A C T
Keywords: Scuffing Boundary lubrication PVt limits
The mechanism of scuffing has been the object of tribological investigations for many years, but the factors that cause its activation are still poorly recognised by engineers and scientists. An industrial approach to this problem requires the determination of the maximum values of operational parameters that secure against the occurrence of scuffing if they are not exceeded. The example most commonly used in practice (especially by bearing, polymers or gears producers) is the pV (pressure x velocity) criterion, which offers the possibility of calculating the limits for friction pairs that are working under different kinetic configurations of materials and tribological conditions. However, such an approach is a significant simplification, since it does not take into account the unequivocal parameters of the tribosystem, such as the duration or frequency of its operation close to its mechanical limitations. These parameters can have a significant impact on the stability of the properties of lubricants, particularly in the context of an elastohydrodynamic film or boundary layer forming. This is why the concept of pVt (pressure x velocity x time) limits with regard to scuffing performance is postulated and elucidated in this paper. Taking into consideration the multi-parametric approach to the technological creation of the surface layer features, investigations regarding the dependence of scuffing performance on morphological, rheological and physical-chemical properties were performed. In order to better understand the fundamentals of the scuffing process, a series of systematic tribological double-blind trials were carried out with an active lubricant (olefin sulphide) on poorly lubricated cylinder/plane interfaces. The scuffing process was analysed at the final phase under boundary lubricated conditions, specifically for ground and burnished steel cylinders (AISI 4130) and polished planes of cast iron (EN-GJL-300).
1. Introduction
recovery [10] or adiabatic shear instability [11]. Unfortunately, most of these theories are mutually incompatible, despite the fact that they have all been experimentally confirmed. This state of affairs emphasizes the complexity of the SP mechanism and the multitude of factors that may affect its initiation. Factors activating the SP can be grouped into a few sets involving different features and parameters of the tribosystem, and typically, they are divided into three groups based on the material configuration, surface finish and operating cycles of the machinery (including lubrication aspects), as Ludema [4] proposed in his phenomenological description of scuffing. Irrespective of the identified, specific mechanisms of scuffing, the problem of its initiation particularly concerns friction pairs operating under conditions of high loads and slow speeds. In such tribosystems, which are characteristic for boundary lubrication (BL), the conditions generate contact pressures that can increase beyond the level specific for elastohydrodynamic lubrication, and can lead to the plastic deformation of asperities. Due to the fact that BL cannot secure long-term
The scuffing process (SP) is a catastrophic form of wear, and due to its sudden onset and capacity to cause disastrous wear, it may lead to the functional failure or definitive destruction of the frictional operating parts of machines and devices. As it is macroscopic in nature, parts damaged by scuffing cause emergency stoppage and call for costly and time-consuming replacement. Although many crucial friction pairs (e.g. manual and automatic gear boxes, piston rings/cylinder liners, camshafts/cam followers, bearings, etc.) are exposed to scuffing, there is still a lack of well-established knowledge indicating universal factors responsible for the activation of this process. The well-known theories focused on the SP connect it with the achievement of some critical temperature at the asperities [1,2], the speed of debris accumulation in the contact area [3,4], the energetic activation of cooperating surfaces [5,6], the interaction between the polar constituents of lubricants and surfaces [7–9], the rate of the protective metal oxides’ destruction and
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Corresponding author. E-mail address: [email protected] (Ł. Wojciechowski).
https://doi.org/10.1016/j.wear.2018.02.019 Received 18 November 2016; Received in revised form 21 December 2017; Accepted 16 February 2018 Available online 19 February 2018 0043-1648/ © 2018 Elsevier B.V. All rights reserved.
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Nomenclature γI+/σI σS σE
A FN MI MS ME p tSC
Acid/base component of surface free energy in the initial state [mJ/m2] Residual stresses in the initial state [MPa] Residual stresses in the scuffed state [MPa] Evolution of residual stresses [MPa]
Size of the measured wear area [mm2] Load [N] Number of motifs in the initial state Number of motifs in the scuffed state Evolution of the number of motifs Pressure [MPa] Time to scuffing [s]
configurations of materials and tribological conditions. Another practical use of pV limits is the selection of polymers for tribological applications. Thanks to this, it is possible to calculate conditions where the rapid, catastrophic wear or overheating of polymers can occur. Besides the polymer and bearing industry, the pV limit is a useful parameter to determine the maximum performance of seals. It is typically assumed that during the SP, the maximum value of pV is present along the gear contact path (as the product of the normal Hertzian pressure and the sliding speed) and remains constant. This simplification does not take into account some additional parameters, such as the transitional regime, the type of lubricant, its viscosity and temperature, the materials and geometry of the gears, etc. This is why, in the case of gear boxes, their performance determination can require some lubricant criteria (e.g. the lubricant film thickness, the pressureviscosity coefficient, the dynamic viscosity of oil etc.) [16] or even the bulk (mass) temperature (assumed as the temperature of the tooth flank before teeth meshing occurs [17]). The development of the pV criteria is the friction power concept suggested that SP occurs when the heat of the sliding contact reaches some critical value. The friction power is usually determined as the product of three factors: nominal contact load, sliding speed and coefficient of friction. Horng presented the most advanced form of this concept [18]. According to it, frictional heating and the lubricant layer breakdown are limited to peaks of the single surface asperities. Therefore, the value of the classic pressure-velocity criteria should be referred to the real contact area (so-called True Friction Power Intensity concept [18,19]). Fundamentally, it can be stated (Kennedy [20]) that during friction the energy is dissipated as heat on the sliding surfaces in the contact area, fluctuating due to morphology of rubbing asperity's and residual stresses. Thence, the rate of heat generated per unit area of contact, is the product of the local COF, pressure and velocity. In the specific case of BL, the classic approach to pV limits can turn out to be insufficient, because time plays a key role in the durability and stability of boundary layers (the time factor is successfully applied to calculate frictional energy in gears, also in scuffing aspect [17,21]). Too long operation near the upper limit of the acceptable pV can cause accelerated “consumption” of the oil and the premature breakdown of the boundary layer. Points exceptionally exposed to this phenomena are asperities’ peaks, potentially plastic deformed and/or overheated due to working under BL conditions. Energetic activated surfaces of so formed asperities are the perfect paths of the SP initiation. This is why the authors propose an extension of the typical pressure-velocity criterion by the time factor, paying attention to the duration of the influence of operational parameters on the boundary layer. In fact, the newly formulated criterion of pVt limits (pressure x velocity x time [J/m2]) gives information about the level of energy which must be delivered to the tribosystem to activate the SP. Of course, the SP identification applying methods based on determining the friction power or energy is just one of prediction options for this catastrophic damage. Alternative methodologies can be found in scuffing resistance tests, e.g. the heat power intensity and bulk temperature as elements of the gears scuffing parameter [17], the socalled shock test for identification of the coated gears scuffing [22] or the limiting pressure of seizure [23]. Taking into account above considerations and the aspect of the
protection against the SP, it is important to recognise the breaking point of the boundary layer, which activates the scuffing wear. Detailed analysis of the identified factors activating scuffing [1–11] results in the conclusion that all factors are real in individual, specific cases of the various analysed tribosystems, but none of them solve the issue in a universal way. This is why the authors believe that those examining the problem of the general conditions of scuffing activation should not only consider the aspect of the proper configuration of materials and lubricants, but also the aspect of some precursors or initial, technological parameters characterizing the surface state directly before the SP commences. The previous work of the authors [8,9,12–14] points to the necessity of a complex, multi-parametric approach to this question. The multi-parametric methodology of the creation of surface properties consists of the achievement of a positive compromise between its rheological, morphological and physical-chemical features. It is possible to distinguish the quantities or parameters that describe surface properties particularly well with regard to scuffing resistance. Residual stresses should be considered as the rheological characteristic most strongly influencing scuffing resistance [9,12]. The physical-chemical group of properties should be represented by the wettability (in the case of inactive lubricants) and/or the surface polarity (in the case of lubricants with AW/EP additives) [8,9,14]. Surface morphology offers the greatest number of parameters that can be applied to the determination of the surface's scuffing performance. However, according to previous papers [12,14], the mean number of the ENISO 12085's roughness motifs is a surface property best related to scuffing performance. A very important aspect of SP is the ability to identify its beginning. From the tribometrical point of view, observing values and potential fluctuations of the coefficient of friction (COF) seems to be the best way to do this. In accordance with the classical definition of the COF (the ratio of friction force and normal force), it can be stated that all factors potentially determining the friction behaviour of the friction pair are taken into consideration. In spite of appearances, the COF is quite a complex quantity and its value is sensitive to many factors. Blau [15] classified these factors into eight categories based on: contact geometry, flow and properties of fluids, lubricant chemistry, relative motion, applied forces, presence of third bodies, temperature in the contact zone, and stiffness and vibrations. All of these factors have more or less influence on the shape, texture and size of the real contact area between rubbing parts. Moreover, the contact area can evolve during friction (as a consequence of the synergistic or antagonistic actions of the above factors), so that its morphological, rheological and physical-chemical characteristics can essentially change, effectively disturbing initial operating conditions. For this reason, it seems to be crucial to analyse the state of the surface morphology both before and after SP initiation. The information obtained from this analysis could identify geometric and lubrication conditions favourable or unfavourable for scuffing. On the other hand, a practical engineering approach to this problem requires characterisation of the maximum values of some operational parameters that when not exceeded should secure against the SP. The parameter most commonly used in industry (e.g. by bearing producers) is the pV criterion. The pV (pressure x velocity) limits of contacting parts have tremendous importance, offering the possibility of calculating the limitations in friction nodes working in different kinetic 180
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(Fig. 2a). The cylinders were internally and externally axially ground to reduce the dangers of cylindricity error. The materials for the surfaces submitted to tribological investigations were carefully selected in order to reduce the risks of metallurgical situations favouring scuffing, and therefore cylinders (of 45 mm external diameter and 12 mm width) were manufactured from AISI 4130 steel. The cylinders were ground, consequently offering anisotropic surface morphology (Sa = approx. 0.5 µm). The cylindrical surfaces were then submitted to burnishing with five values of pressure, using two symmetrical spherical sectorshaped rolls of 50 mm diameter, described in detail elsewhere [9]. The selected load conditions gave the following values of burnishing pressure: 1st: 1.3 GPa, 2nd:1.64 GPa, 3rd: 1.87 GPa, 4th: 2.06 GPa and 5th: 2.22 GPa. Systematic areal morphological analyses were performed using an optical interferometer on millimetric regions relevant to the contact surfaces during experimental tribological investigations. Five areas of 1.2 mm × 0.9 mm every 72° on each cylindrical surface (in the initial and scuffed state) were examined. Metrological analyses were carried out very carefully and conscientiously, taking into consideration the calibration, as well as the transfer function and measurement limitations, of the selected topometric device. Analysis of the mean number of the EN-ISO 12085's roughness motifs (recognised as scuffing invariants [12]) was applied to study the relationship between the activation of the SP and the morphological state of initial and worn surfaces. According to the EN-ISO 12085 standard, a roughness motif can be defined as a portion of the primary profile between the highest points of two local peaks of the profile, which are not necessarily adjacent. However, for the purpose of the 3D analysis and due to the developed method of identifying a roughness motif using watershed segmentation, the motif should be considered as a specific catchment basin surrounded by watershed lines. The procedure of determining motifs and their characteristics (number, height and area) involves three stages: segmenting the surface using the watershed algorithm, building a special change tree describing the relationship between peaks and valleys, and simplifying this tree using the Wolf pruning process. This procedure is described in detail elsewhere [25,26]. The residual stresses were measured by the X-ray diffraction method. In order to satisfy the statistical requirements, measurements were conducted at three points every 120° on the cylindrical surfaces (in the initial and scuffed state) of the AISI 4130 specimens. The acid/base component of the surface free energy (SFE) was used for the quantitative determination of the surface polarity. The SFE of the rubbing bodies was established on the basis of the measurements of
correct configuration of materials of the friction pair, the multi-parametric model of the tribosystem is proposed in Fig. 1. The energetic method for the determination of the scuffing activation period was presented for the first time in paper [13]. There, scuffing energy meant the work needed in order for scuffing to occur for a friction pair under selected tribological conditions (the measuring/ calculating procedure used to identify SP initiation by the scuffing energy has been characterised in detail in [13]). However, apart from the unquestionable benefits from the application of this methodology, there is some limitation to its practical use. This limitation is principally associated with a certain complexity with regard to the quantity of scuffing energy, which is not necessarily easy to use in industrial practice. Therefore, the authors believe that the pVt concept could be a much easier solution to determine the scuffing performance of frictional operating parts exposed to this type of wear. This approach also has a strong economic foundation. According to statistics from the International Organization of Motor Vehicle Manufacturers (OICA), the worldwide production of passenger cars (PC) and commercial vehicles (CV) in 2015 amounted to approx. 90.7 million. In turn, the motorization rate in 2014 was equal to approx. 661 PC and CV per 1000 inhabitants in NAFTA (North American Free Trade Agreement) countries and 569 PC and CV per 1000 inhabitants in EU (European Union) countries. Considering the fact (as Holmberg et al. estimated in [24]) that the energy needed for PC to overcome friction in the piston assembly comprises nearly 45% of all engine friction energy losses, and approx. 10% of this energy arises in the friction nodes operating in BL conditions, it can be stated that in the automotive industry alone, the risk of scuffing affects a gigantic number of objects. In order to experimentally confirm this concept of SP activation, a series of systematic tribological investigations were carried out with an active lubricant (olefin sulphide) boundary lubricated cylinder/plane interface. The SP was analysed at the final phase, specifically for finally ground and burnished steel cylinders (AISI 4130) and polished planes made from cast iron (EN-GJL-300). The part of the investigations presented in this paper consisted of the determination of the relationship between the pVt value activating scuffing and the initial and scuffed states of the surface, characterised by prior identification of its representative rheological (residual stresses), physical-chemical (surface polarity) and morphological (number of motifs) properties.
2. Methodology A geometrical configuration of a flat block in contact with a rotating cylinder was selected, principally for contact mechanical reasons
Fig. 1. Model of the tribosystem using the multi-parametric approach to the creation of its anti-scuffing properties and durability.
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Fig. 2. Geometry of friction pair (a), procedure of pVt limits calculations (b), method of load application in the scuffing test (c) and the coefficient of friction vs. time to scuffing (d).
Table 1 Residual stresses, a number of roughness motifs and an acid/base component of SFE in depending on the machining treatment. Machining and surface treatment
Be (1.30 GPa) B (1.64 GPa) B (1.87 GPa) B (2.06 GPa) B (2.22 GPa)
Residual stressesa [MPa] Initial state σI
Scuffed state σS
Evolutionc σE
Initial state MI
Scuffed state MS
Evolutiond ME
Initial state γI+/−
−141 −229 −279 −297 −304
−251 −303 −428 −444 −348
110 74 149 147 44
285 239 165 152 154
51 57 57 51 51
234 182 108 101 103
10.6 10.3 15.5 20.3 17.5
G G G G G
+ + + + +
a
[9]. [8]. Evolution due to scuffing: σE = | σS- σI|. Evolution due to scuffing: ME = | MS- MI|. G + B: grinding + burnishing (the value of the burnishing tools’ pressure).
b c d e
static contact angle on the surfaces of the cylindrical specimens. The measurement procedure has been described in detail in [8]. In order to satisfy the statistical requirements, the angle measurement procedure was repeated 10 times, and the average value was used to calculate the SFE and its components using an acid/base method. Due to problems with the measurement of the contact angle on scuffed surfaces, the procedure for the SFE determination was performed only for the initial state of the cylindrical surfaces of the AISI 4130 specimens. Only the acid/base component of the SFE is responsible for the polar interactions of metallic surfaces (e.g. with AW/EP lubricants’ additives), therefore only this part of the total value of the SFE was taken into consideration in the analysis. The criterion for the determination of scuffing was an increase in the friction coefficient under a constant load (Fig. 2d). The experiments were performed under single drop lubrication using gear oil containing
Table 2 Results of scuffing investigations and pVt limits calculations. Machining and surface treatment
Pressure of SP activation [MPa]a
Sliding velocity (Fig. 2) [m/s]
Time to scuffing [s]
pVt × 103 [MJ/ m2]
G G G G G
91.8 98.7 99.9 109 123
0.5
536 571 640 951 796
24.61 28.18 31.99 51.81 48.98
+ + + + +
B B B B B
(1.30 GPa) (1.64 GPa) (1.87 GPa) (2.06 GPa) (2.22 GPa)
Acid/base component of SFEb [mJ/m2]
Number of roughness motifs [pcs.]
a The final load in the scuffing test (Fig. 2) divided by the size of the measured wear area.
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for all analysed cylindrical surfaces. Table 2 presents the results of scuffing investigations and calculations of the pVt limits. Taking into consideration the results of the residual stresses investigations (Table 1) and the pVt limits calculations (Table 2), some observations concerning their relationship can be made. Additionally, the graphical interpretation of this issue is shown in Fig. 3. One can observe a clear, statistically confirmed (by the r-Pearson coefficient) dependence between pVt limits and residual stresses in the initial, technological state of the surface. It is evident that the increase in compressive residual stresses causes an increase in scuffing resistance (Fig. 3a). The growth of compressive stresses results in an enhancement of the cohesive forces responsible for maintaining the entire structure of the metallic surfaces during wear. As a result of the wear process (and finally scuffing activation), the growth in the value of compressive stresses can be recognised. However, it does not translate into a statistically confirmed influence on pVt limits. The absolute values of the evolution of residual stresses (the difference between stresses in the initial and scuffed state) point to the cold work hardening of surfaces caused by the wear process. Nonetheless, this process does not have a direct influence on the scuffing performance (Fig. 3c). For comparison, the values of the correlation (r-Pearson) coefficient between residual stresses (in all analysed states) and classical pV (pressure x velocity, Table 2) limits were calculated. The results were as following: r = −0.794 (for σI vs. pV), r = −0.395 (for σS vs. pV) and r = −0.465 (for σE vs. pV). Taking into account the assumed analysis conditions (the sample size n = 5 and statistical significance α = 0.1), it cannot be found statistically confirmed correlation between residual stresses and pV limits (in all cases │r│ < rCR = 0.805). Fig. 4 presents the evolution of the surface morphology during the wear process, represented by the mean number of roughness motifs. The analysis of their values in the initial and scuffed state allows us to determine the relationship between the motifs and the pVt limits (Fig. 5). Our prior investigations [12] have shown that roughness motifs can be recognised as invariants in scuffing initiation. The key role here is an unfavourable, critical relationship between the shaping of the surface asperities (pressures are enough to cause a breakdown of the boundary layer) and the empty volume of valleys (insufficient for a safe amount of oil and its distribution to the real contact zone). The analysis of the data presented in Fig. 5 points to the fact that the mean number of motifs is the parameter with the statistically strongest influence on the scuffing performance. As one might have expected, the decrease in the number of motifs causes an increase in their area, and as a consequence a decrease in the pressures between elements of the friction pair. This is why values of pressures characteristic for the activation of catastrophic wear are higher for the surfaces with the lowest number of motifs (when the peaks of the asperities are “flatter and milder”). A similar trend can be observed for the difference in the number of motifs in the initial and scuffed states and the value of the pVt limits (Fig. 5c). It can be conjectured that the area of asperities, and consequently the pressures on them, can evolve in frictional conditions. This evolution occurs very often at the beginning and end of the operating of friction pairs exposed to the SP. In the initial stage, asperities are subject to abrasive and plastic ‘running-in’. That is when the highest peaks are cut off or flattened, and the area of asperities increases. As a result, the frictional conditions are stabilised, and the intensity of the wear can be limited. Additionally, time may turn out to be an important factor influencing the evolution of the morphology. In general, a longer working time can cause an increase in the area of the asperities, and as a consequence the pressures needed for scuffing initiation. In all probability, the reason here is a slow, but systematic, frictional cold work hardening of the surface's highest peaks. For comparison, the values of the correlation (r-Pearson) coefficient between motifs (in all analysed states) and pV limits were calculated.
Fig. 3. pVt limits vs. residual stresses in the initial state σI (a),the scuffed state σS (b) and their evolution due to scuffing σE= | σI – σS| (c) [r–Pearson correlation coefficient, rCR–critical value for Pearson correlation, α–statistical significance].
ca. 5% olefin sulphide as an extreme pressure additive. The scuffing activation period was then measured as the time it took for scuffing to occur, which is equivalent to the situation of poor lubrication in the friction pair. The scuffing kinetics were performed at a sliding speed of 0.5 m/s between the rotating AISI 4130 cylinder and the stationary, flat block (EN-GJL-300 cast iron with flake graphite). The load incrementally applied to the friction pair is presented in Fig. 2c. In order to satisfy the statistical requirements, the scuffing tests were performed four times for each pair of cylinders and flat blocks. After the scuffing tests, the areas of wear traces were measured on the scuffed surfaces of the EN-GJL-300 cast iron flat blocks. The final load in the scuffing test divided by the size of the measured wear area allowed us to calculate the pressure between the elements of the friction pair at the moment of SP activation. The value of this pressure multiplied by the sliding velocity (between AISI 4130 cylinders and EN-GJL300 flat blocks – Fig. 2b) and the time to the commencement of scuffing determined the value of the pVt quantity for the analysed friction pairs.
3. Selected results and discussion Table 1 presents the results of the residual stresses (in both the initial and scuffed state), roughness motifs (in both the initial and scuffed state) and the acid/base components of the SFE (only in the initial state) 183
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Fig. 4. The 3D isometric views and SP generated evolution of the mean number of motifs of AISI 4130 cylinders burnished at different pressures: 1.3 GPa (a), 1.64 GPa (b), 1.87 GPa (c), 2.06 GPa (d), 2.22 GPa (e). MI-a number of motifs in the initial state, MS-a number of motifs in the scuffed state, ME = | MS- MI|.
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activation is connected with a change in the reactivity of this surface, and the concentration of oxygen on it. For example, the increase in the acid/base component of the SFE (which is answerable for the polar interactions of a surface) can cause the increase in the surface concentration of oxygen. The presence of oxygen is a necessary condition for the activation of AW/EP sulphur-based additives, and its increased concentration on the surface caused the growth of the sulphur (present here in the olefin sulphide) concentration in the contact area. Thus, in the case of the lubrication by oils containing surface-active sulphur compounds (olefin sulphide here), the increase in the acid/base component of SFE can favourably influence scuffing resistance. This effect can be used for the preparation of metallic surfaces using lubricants enriched by tribological additives. However, it must be confirmed for the additives based on non-sulphur compounds. For comparison, the value of the correlation (r-Pearson) coefficient between the acid/base component of SFE in the initial state and pV limits was calculated. The value of this coefficient is equal to −0.720 and is lower than the critical value (rCR = 0.805). Therefore, it cannot be found statistically confirmed correlation between the acid base component of SFE and pV limits. 4. Conclusions On the basis of the experimental observations presented in this investigation (limited to only one configuration of the tribosystem), the following conclusions can be formulated:
• The pVt limits can be considered to the prediction of SP activation in • • Fig. 5. pVt limits vs. the mean number of motifs in the initial state MI (a),the scuffed state MS (b) and their evolution due to scuffing ME = |MI – MS| (c).
•
•
Fig. 6. pVt limits vs. the acid/base component of SFE.
The results were as following: r = −0.767 (for MI vs. pV), r = −0.393 (for MS vs. pV) and r = −0.746 (for ME vs. pV). Taking into account the assumed statistical conditions, it cannot be found statistically confirmed correlation between motifs and pV limits (in all cases │r│ < rCR = 0.805). An analysis of the data shown in Fig. 6 allows us to recognise a certain trend in the value of pVt limits as a function of the acid/base component of the SFE. The influence of the surface polarity on scuffing
the case of friction pairs operating under BL conditions. There is a clear, statistically confirmed dependence between pVt limits and previously identified invariants [12] and surface features [8,9,12] strongly affecting scuffing performance. The increase in the compressive residual stresses on the initial surface state causes an increase in scuffing resistance. This effect is connected with the enhancement of cohesive forces responsible for maintaining the entire structure of the metallic surfaces during wear. The mean number of roughness motifs is the morphological parameter with the strongest influence on scuffing performance. The decrease in the number of motifs translates into an increase in their area, and as a consequence a decrease in the pressures between elements of the friction pair indispensable to SP activation. The operating time can be a factor influencing the morphological evolution, whether favourable or not, in the context of scuffing. A longer duration of work can cause an increase in the area of the asperities, and as a consequence in the pressures necessary for scuffing initiation. This effect is caused by the running-in process and the systematic frictional cold work hardening of the surface's highest peaks. It was confirmed by the dependence between the evolution of the number of motifs and pVt limits (Fig. 5c). There is a clear relationship between pVt limits and the acid/base component of SFE. The increase of this responsible for polar interactions component can improve the scuffing performance due to the increased surface concentration of oxygen. Its presence is necessary for the activation of AW/EP sulphur-based additives and its increased concentration can be conducive to the formation of sulphurbased boundary layers.
5. Perspectives Further investigations regarding this problem should include the following areas:
• Verification of the precursory behaviour of the pVt limits for dissimilar configurations of the geometry, materials and type of lubricants of a friction pair.
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• Topological analysis of the surface in terms of the synergism and
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