Tribological investigations with near eutectic AlSi alloys found in engine vane pumps – Characterization of the material tribo-functionalities

Journal Pre-proof Tribological investigations with near eutectic AlSi alloys found in engine vane pumps – Characterization of the material tribo-functionalities Florian Summer, Michael Pusterhofer, Florian Grün, István Gódor PII:

S0301-679X(20)30079-7

DOI:

https://doi.org/10.1016/j.triboint.2020.106236

Reference:

JTRI 106236

To appear in:

Tribology International

Received Date: 18 November 2019 Revised Date:

9 January 2020

Accepted Date: 30 January 2020

Please cite this article as: Summer F, Pusterhofer M, Grün F, Gódor Istvá, Tribological investigations with near eutectic AlSi alloys found in engine vane pumps – Characterization of the material tribofunctionalities, Tribology International (2020), doi: https://doi.org/10.1016/j.triboint.2020.106236. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Florian Summer: Conceptualization, Methodology, Visualization, Investigation, Writing - Original Draft, Writing - Review & Editing Michael Pusterhofer: Investigation, Florian Grün: Conceptualization, Supervision István Gódor: Methodology, Investigation

Tribological investigations with near eutectic AlSi alloys found in engine vane pumps – Characterization of the material tribofunctionalities Florian Summera,*, Michael Pusterhofera, Florian Grüna , István Gódora a

Montanuniversität Leoben, Chair of Mechanical Engineering, 8700 Leoben, Austria.

Abstract This paper gives attention to wear and friction processes of sliding contacts with cover parts of variable displacement engine vane pumps. Two near eutectic aluminium-silicon alloys for different casting processes were characterized by tribological investigations. This was done with the aid of specifically designed test methods on model scale as well as by damage analysis of real life parts from engine tests. The latter was additionally used as a validation measure for the model tests, which proved transferability of damage events from model tests to the engine parts. The results emphasize significance of the material structure of near eutectic AlSi materials on the wear performance. A pronounced load bearing structure of eutectic Si is able to provide high wear resistance. Keywords: Engine vane pump; wear; AlSi; tribometer.

1

Introduction

In recent years, research and development in mobility and powertrain has increasingly focused on further optimizing the efficiency and performance of conventional internal combustion engines (ICEs) to meet increased environmental demands [1,2]. This has led to continuous redesign of ICEs and their components [3–5]. In this context, also the tribological performance, thus friction losses and wear resistance, is of upmost importance targeting fuel economy goals [1,3,4]. When looking at the performance of various tribological contacts in an ICE, for example journal bearings or the piston group, the lubrication is the most critical parameter. In this regard, the engine oil pump transports the lubricant to the various sliding contacts preventing dry friction. In novel engines variable displacement vane (VDV) pumps are widely used in the automotive sector targeting to improve engine efficiency [6]. This is realized by adjustment of the oil flow rate depending on the engine demand. The main components of a VDV pump are a rotor with several radially disposed vanes generating the vane chambers which transport the oil, an inner centering ring, an outer pressure ring, a slide ring with a circular inner bore, a shaft, a spring and last but not least the pump housing and covers. Figure 1a depiction of a VDV pump with the main sliding components highlighted is shown. Within a VDV pump several tribological contacts take place, viz. between vanes and outer ring, vanes and centering ring, vane sides and rotor, see Figure 1a, but also between rotor front end and housing/cover, vane front end and housing/cover as well as centering and outer ring front end and housing/cover, see Figure 1b.

1

Figure 1 Depiction of a VDV pump: (a) schematic assembling structure (selected moving contact parts are highlighted), (b) exploded view of pump housing including vane assembly and pump cover

Owing to this fact friction and corresponding wear events occur, which – in a worst case scenario – may lead to oil pump failures and subsequently to an engine break down. Based on this aspect several studies regarding wear phenomena and lubrication conditions of VDV pumps have emerged using different test configurations [6–10]. In these studies, mainly the contacts of vanes with the outer and inner ring as well as the rotor have been investigated. It is emphasized that for vane/outer ring contacts various lubrication regimes occur and a different wear performance among various outer ring materials exists – with the absolute wear heights of the outer rings documented in the range of 1-2 µm [7]; that no significant wear processes between the inner ring and vanes take place [6]; that in the rotor vane contact soot acts wear promoting based on chemical effects [8] and that the wear amount of pump components depend on the duty cycles performed [9]. The contacts with the housing parts are recognized as tribological systems within the vane pump [9], but referring studies to friction and wear of these contacts are rare. Pump housing and covers are typically manufactured with the aid of a casting process [11]. In this regard, AlSi alloys are prevalent. Morphology and structure as well as the resulting properties of AlSi casting alloys are very sensitive to many factors of alloying elements, manufacturing process itself (e.g. sand cast or pressure die cast) as well as specific manufacturing process parameters (e.g. local thermal conditions, feeding capacity, applied pressure, …). Near eutectic AlSi alloys are favoured due to the excellent casting properties particularly needed at complex geometries [12] such as in the case of pump housing and pump covers. Sliding functionality and wear resistance of AlSi alloys in tribological contacts is mainly based on its heterogeneous nature and hard phase mechanism of sliding materials [13]. Especially hypereutectic AlSi alloys have been studied and linked with outstanding wear resistance based on the load bearing function of primary silicon phases owing to higher hardness and stiffness compared to Al base material [14–16]. In that relation, the wear resistance of selected eutectic AlSi-11/12% alloys, without primary silicon phases, is lower compared to that of hypereutectic representatives [17,18]. Under intensive mixed friction sliding conditions, additionally, the formation of modified boundary layers consisting of fractured particles and oil additive elements has been documented [16,19]. For such sliding contacts the lubricant anti-wear additive system interacts with AlSi surfaces particularly with Si phases forming anti wear phosphate films on them similar to that known from steel surfaces [16,20–22]. The activities of the present work cover investigations of tribological contacts of VDV pumps occurring with AlSi based pump covers. Primarily, friction and wear performance of two near eutectic AlSi cast alloys and the effect of the microstructure is investigated and discussed with the aid of microstructural analysis, analysis of engine test components and a developed test methodology on model scale. Besides, a validation of the 2

designed model test method based on comparison of model test results and engine part analysis is carried out.

2

Materials of pump covers - microstructural analysis

For this study two different Aluminium cast materials of the pump cover were selected and made parts of. The elemental alloy composition is listed in Table 1. The first material is EN AC 43300, chemically named as EN AC-AlSi9Mg, which can be described as a hypoeutectic AlSi alloy processed by sand cast procedure [23]. The second material, EN AC 46000, also referred to as EN AC-AlSi9Cu3(Fe), represents also a hypoeutectic AlSi alloy but processed by pressure die cast method. In addition to the elemental composition, Table 1 also defines short names (A1 and A2) for the used alloys, which shall be used in the paper hereafter. Table 1 Elemental composition of employed pump cover materials according to [23] Alloy name

Short name

Si

Fe

Cu

Mn

Mg

Cr

Ni

Zn

Pb

Sn

Ti

Al

EN AC 43300

A1

9.0-10.0

0.19

0.05

0.1

0.25-0.45

-

-

0.07

-

-

0.15

rest

EN AC 46000

A2

8.0-11.0

1.3

2-4

0.55

0.05-0.55

0.15

0.55

1.2

0.35

0.15

0.25

rest

Figure 2 gives an overview about the microstructure of components made of the alloy A2 produced by die casting process. The depiction shows large area pictures taken with both light microscopy (LIMI) and scanning electron microscopy (SEM). A fine microstructure consisting of α-mixed crystal, eutectic Si and intermetallic phases is seen. For such alloys precipitated phases are Al5FeSi, Al15(CuFe)3Si2 and Al15(Mn,Fe)3Si2, Al2Cu phases and complex intermetallic phases (with Si, Fe, Mg, Cu, Zn) [24,25].

Figure 2 A2 microstructure overview in SEM/LIMI

More detailed insight regarding the individual phases of A2 material is provided by Figure 3a and b as well as by the elemental analysis in Figure 3b and Table 2. A variety of different phases can be seen. These are embedded in a matrix of α-mixed crystal (Figure 3b and Table 2, Spectrum 1). Along the grain boundaries a needle structured Si eutectic (the needle width is close to the sub-micrometer level) can be found. Polyhedral globulites (beige colour in Figure 3a and light grey appearance in Figure 3b) are conspicuous to be the largest phases present with maximum equivalent diameter size of approximately 13 µm. These are Fe, Mn, Si and Cr rich phases (probably some Alx(Fe,Mn)xSix phases) based on the elemental characterization by energy dispersive x-ray spectroscopy (EDX) technique, see Figure 3b and Table 2, Spectrum 2. Locally, also Al2Cu phases are present in the eutectic as well as complex intermetallic phases with Si, Fe, Cu and Zn elements (see Spectrum 3 in Figure 3b and Table 2). The individual phases formed do also provide different 3

mechanical properties in terms of hardness and modulus. Nano-mechanical properties of different phases forming for Al-Si alloys have been characterized comprehensively in literature and therefore can be derived thereof. Si is known to be around 10 times harder than Al and Si provides an up to 2 times higher elastic modulus [26]. At elevated temperatures these differences are even more pronounced [27]. The corresponding properties of AlFeMnSi phases are close to that of Si in terms of hardness and for the modulus values are even higher to that of Si [26,27]. Al2Cu is in the middle with 5 times higher hardness and 1.3 higher modulus of Al [27].

Figure 3 A2 microstructure analysis: (a) LIMI, (b) SEM/EDX elemental analysis Table 2 Element list of EDX point analysis in Figure 3b (measured with 20 kV and values given in at.%) Spectrum

Al

Si

1

98.3

1.7

2

69.9

12.6

3

72.8

12.2

4

93.1

5.5

Cr

Mn

Fe

2.2

4.1

11.2 3.1

Cu

Ni

10.5

0.7

0.9

Zn

0.7 0.5

In Figure 4 the microstructure of the alloy A1 produced by sand casting process is seen. The structure of this alloy is of dendritic nature and is composed of Al α-mixed crystal and the eutectic phase. The formation of the individual areas is more coarsely distributed compared to the die cast material. Besides the dendritic Al α-mixed crystal with dissolved Si (Figure 5b and Table 3, Spectrum 1) the eutectic Si (Figure 5b and Table 3, Spectrum 4) can be recognized clearly. The Si phases are of spheroidal shape with a size over the micrometer level. Furthermore, intermetallic phases are documented (Figure 5a and Figure 5b). Firstly, phases of AlSiMgFe(Mn) elemental composition can be identified according to EDX point analysis of Figure 5b and Table 3, Spectrum 3. As exemplarily seen in Figure 5b, these phases are shaped Chinese script like. Secondly, needle shaped phases are found locally. EDX analysis, as measured in Figure 5b and Table 3, Spectrum 2, proves this phases to be of AlSiFe(Mn) elemental composition.

4

Figure 4 A1 microstructure overview in SEM/LIMI

Figure 5 A1 microstructure analysis: (a) LIMI, (b) SEM/EDX elemental analysis performed with 20 kV

Table 3 Element list of EDX point analysis in Figure 5b (measured with 20 kV and values given in at.%) Spectrum

Mg

1 2 3 4

15.7

Al

Si

98.6

1.4

Mn

Fe

74.6

15.2

0.6

9.6

51.2

28.3

0.3

4.5

20.6

79.4

Hence, the components produced by the two materials provide significant different microstructures deriving from the material composition and manufacturing process. In this regard, the different size and distribution of eutectic Si can be highlighted. In Figure 6 the difference is emphasized by comparison of similar optical pictures of A2 (a) and A1 (b) microstructures. As seen in case of A1 Si particles are large, roughly distributed and partly spherically shaped. In contrast, for A2 Si phases are small, finely distributed and needle like shaped. Larger particles in case of A2 are mainly limited to AlFeMnSi phases (beige coloured phases in Figure 3a and Figure 6a), which appear only seldomly.

5

Figure 6 LIMI pictures of pump cover microstructures: (a) A2, (b) A1

3

Damage analysis of VDV pumps from engine tests

Of both materials, A1 respectively A2, components have been tested in full scale engine tests. Thereafter, surface and damage analysis has been carried out in order to identify the sliding and wear processes of the application. The component parts have been investigated by LIMI and SEM technique aided by EDX elemental analysis. For A2 material, characteristic surfaces are documented in Figure 7 and Figure 8. Figure 7 shows an overview picture of one characteristic pump cover part, whereas Figure 8 highlights the corresponding characteristic local details of sliding tracks but taken from several pump cover parts. The overview picture (Figure 7) shows that different sliding areas and contact intensities occur. On the one hand, contact regions with severe tribological damages exist. On these surface areas, deep sliding grooves and local scuffing marks of the Al matrix material can be seen, whereas hard phases (Si or intermetallic phases) are absent – see Figure 8a to Figure 8c. These areas are located on certain areas of the larger ring of the pump cover (contact with vanes slides) as well as on the upper bridge between larger and smaller ring of the pump cover as highlighted in the overview picture of one representative pump cover. On the other hand, contact regions being still particle structured as shown in Figure 8d and Figure 8e can be found. The larger particles, forming the obvious load bearing structure, can be referred to as Al(FeMn)Si phases. Locally, these larger particles are ripped out, see Figure 8e. These surface conditions are striking for vane contacts with the larger ring of the pump cover left and right sided in Figure 7, but also for the contact pump cover (larger pump cover ring) with the outer ring of the pump (outer ring is highlighted in Figure 1a). Furthermore, regions with increased deposits (darker layer seen in LIMI depiction of Figure 8d) on the Al surface are documented. These are located on the brink of the contacts potentially being transported there by centrifugal forces.

6

Figure 7 Overview picture of A2 pump cover after engine test run

Figure 8 Damage analysis of A2 pump cover parts: (a) LIMI image of wear grooves, (b) SEM image of particle depleted contact surface (c) SEM image of wear grooves, (d) low loaded areas with particle structured surface in LIMI depiction, (e) SEM picture of surface areas with torn out intermetallic phases

Component parts being made of A1 show less intensively harmed sliding surfaces. Similar to A2, documentation is emphasized with the aid of one overview picture of a representative pump cover part (Figure 9) and detail pictures taken from several characteristic parts (Figure 10). As seen in the overview depiction in Figure 9 the sliding marks are less pronounced over the whole contact surface. Looking into details most of the contact surfaces provide a strongly particle structured surface pattern. For selected regions the surface even appears polished revealing a microstructure that is only known from micro-section preparation as can be seen in Figure 10a. As load bearing particles all hard phases including well distributed Si phases of sufficient size are present on the sliding surface. At the outer regions deposits can be found to a larger amount, but are not restricted to – see Figure 10c, which shows a transition region from the smaller ring of the pump cover to the bridge towards larger ring of the pump cover part. The deposits regions have been analysed in detail revealing the elements and the underlying phases being present – see Figure 10d. 7

Overall the surface is mixed up based on various phases being present forming a modified boundary layer. Firstly, C rich deposits (black in material contrast depiction) can be found, see Spectrum 1 and 2, obviously being soot or combustion deposits. Moreover, elements P, O, Zn and S, known to derive from degradation products of anti-wear additive agents of the engine oil, such as zincdialkyldithiophosphate compound (ZDDP) [22,28,29], are present in the modified boundary layer possibly being deposited (Spectrum 3 and 4). In addition, as Spectrum 6 - 8 show, Si particles are found well distributed and partly covered with Ca/Mg pads, known to origin from detergent agents of the lubricant additives system. Furthermore, small particles containing Fe are measured on the modified surface, which may be wear products of the counterpart surfaces within the oil pump or origin from other tribosystems in the engine. Regions with pronounced wear pattern or intensive tribological damages are missing for parts of A1.

Figure 9 Overview picture of A1 pump cover after engine test run

Figure 10 Damage analysis of A1 pump cover parts: (a) LIMI picture of outer contact area with particle structured surface, (b) LIMI picture of load bearing particle structure, (c) LIMI image of deposit regions, (d) SEM/EDX point analysis of deposit regions

8

Table 4 Element list of EDX point analysis in Figure 10d (measured with 7.5 kV and values given in at.%) Spectrum 1 2 3 4 5 6 7 8 9

85.2 97.2 19.4 23.8 16.7 30.3 31.8 26.5 25.4

C 2.5 2.7 53.8 50.8 55.4 47.4 1.8 7.9 32.7

O

Mg

Al

Si

P

S

0.8 0.4 1.3 1.1

0.6 0.3 4.1 3.4 0.6 16.4 23.9

Ca

Ti

Fe

Cu

Zn

11.5

0.5 17.1 14.2 65.8 49.2 0.7

11.5 8.5

0.1 7.0

7.0 15.2 5.4 3.5

0.4

1.0

14.7

1.1

Regarding the surface conditions of the counterparts no significant differences between A2 and A1 sliding partners are recognized. Overview pictures of the selected counter-bodies (inner and outer ring of the pump, vanes) are documented in Figure 11. The centering ring (Figure 11a) shows no signs of sliding tracks. The surface profile seen in Figure 11a is generated by the manufacturing process. On the outer ring and on the vane lamellae (Figure 11b and Figure 11c) sliding areas can be seen. Generally, the surfaces of the mating bodies appear rough deriving from the initial manufacturing condition. Especially, the surface of the outer ring (Figure 11) is covered with deep grooves non-directional to the sliding direction. These conditions can be regarded as wear-promoting for abrasive processes. On the vane lamellae the largest sliding areas are seen, thus hypothesizing to be the main sliding contact partner.

Figure 11 LIMI evaluation of VDV pump components: (a) centering ring, (b) outer ring, (c) vanes

A closer look on the vane surfaces is shown in Figure 12. The vane surfaces appear smoothened from the sliding contact with the AlSi material, but no signs of damage processes. The sliding area in the steel vanes is covered with brownish and blue tribofilm layers deriving from ZDDP anti-wear additive layers from the engine oil. These layers, forming for tribological contacts on steel as reported and investigated comprehensively in literature, are known to actively from in sliding contacts and mainly consist of a phosphate network with a gradient in hardness to protect the substrate from shear stressed induced damage [22,30,31]. The anti-wear and anti-seizure function of such tribofilms bases on the gradient nature in hardness and stiffness coupled with reformation mechanism. Hence, the area of lowest resistance to wear processes is shifted from the material to the interface and the tribofilm. The phosphates appear blue in the light microscope and seem to grow pad like for the current sliding situation, which is characteristic for large area sliding contacts [32]. The brown base layer is formed with S, Zn and O elements (see exemplary element point analysis S1 in Figure 12) according to the extreme pressure functionality of ZDDP additive and seems to represent a sort of base layer for the phosphate to grow on (S2 point analysis in in Figure 12). Similar findings have been confirmed in literature [22,32]

9

Figure 12 LIMI images and elemental point analysis of vane surfaces covered with anti-wear tribopads (seen in blue colour on a brownish base layer)

10

4

Model testing

In the following sub-sections, the activities concerning the development and appliance of a proper model test method characterizing the tribological performance of the two AlSi materials in consideration of the real life application are presented.

4.1 Derivation of a model test method Tribological testing always needs certain simplifications if the contact to be considered if not being tested in the application itself [33,34]. In this regard, a model test configuration should be realized as close as possible to the application conditions under the inevitable simplifications. In order to ensure similar sliding conditions and damage processes taking place in the application a damage equivalent transfer from component to model tester is needed. According to Figure 13 transfer of the main tribological parameters, viz. structural parameters, operational parameters and interaction parameters, as defined in [35], has been carried out from the application to model testing as follows: Interaction parameters: The contact and lubrication conditions of the application are of rotational mode of an axial plane contact of one rotating and one still-standing tribopartner without a narrowing oil gap, thus operation in mixed friction sliding regime is most likely to settle. Considering these aspects ring on disc test configuration, where a disc specimen is axially rotating against a still-standing ring in an oil bath under mixed friction conditions, as schematically depicted in Figure 14, has been selected. The test set up is implemented on a TE92 rotatory tribometer from Plint Phoenix Tribology. The tribological performance can be documented by a wide range of system characteristics (and visualized schematically in Figure 14): − − − − − −

−

−

Defined normal loading FN – an air bellows applies the normal load to the contact. Defined sliding speed v – a motor drives a shaft on which the disc specimen is directly mounted. Defined ring specimen temperature T1 – measured by a thermocouple in the ring specimen close to the ground level of the oil bath and used for temperature control by heaters beneath the oil bath. Contact near temperature T2 – measured by a thermocouple closely beneath the sliding contact in the ring specimen. T2 > 150°C has been defined as one test stop criteria indicating system fail. Coefficient of friction (COF) µ – carried out by measuring of the resulting friction torque Mµ . µ > 0.15 has been defined as one test stop criteria indicating system fail and sliding collapse. Wear height – measured by a capacitive proximity sensor placed in the ring middle responding the relative distance to the disc specimen holder. Wear height > 320 µm has been defined as one test stop criteria indicating system fail. Contact potential CP – between the mating specimen partners an electrical potential difference has been applied and recorded by using a Lunn-Furey circuit [36] responding the electrical conductivity of the contact ranging from isolation (50mV) to fully conductive (0mV). In addition, the gravimetric wear of the disc specimen is determined.

Further details of the ring on disc test set up and the implementation within the tribometer are described in [34,37]. Structural parameters: The mating surfaces of the model test set up are made of the pump cover materials (disc specimen) and a 34CrNiMo6 steel counterpart (ring specimen). In this regard, the disc specimens are directly manufactured out from the AlSi cover parts (the sliding surface is not harmed by the manufacturing process), thus providing the real life material structure and surface condition of the component. The 34CrNiMo6 material substitutes the vane material of the real life application. The ring specimen are polished 11

prior to the testing targeting a surface roughness of Ra = 0.03 µm based on the smoothed condition of the vane parts after engine test runs. Furthermore, the same engine oil – Castrol BOT960 – of the real life application has been used. In contrast, the model environment differs from the real life environment due to missing of the internal combustion and related effects.

Figure 13 Transfer to a tribological model test system

Figure 14 Used test configuration: ring on disc (schematically shown with measurement parameters)

Operational parameters: Figure 15 and Figure 16 depict the applied test strategies to study the tribological performance considering the operation conditions of the application. On the one hand, a load step test program has been carried out in order to investigate the emergency running properties by stepwise increase of the normal loading until the system fails. On the other hand, a constant long term test has been designed in order to study the long-run operation condition, which shall be in the time range of the sliding duration of the components, under moderate loading condition. Both test strategies are performed at elevated engine temperatures, e.g. 130 °C ring specimen temperature, which can be considered also to be present in the real life application. An average constant sliding speed of 1.4 m/s is set as simplification based on the component situation. Details regarding the run of the model test programs can be read off from Figure 15 (load step test) and Figure 16 (constant long term test).

12

Figure 15 Load step test strategy

Figure 16 Constant long term test strategy

4.2 Results of the model tests In the current section the tribometric results of the model tests are presented. The tribometric data presented hereafter is obtained with using either A1 or A2 as disc specimen for the load step test program or long term test program respectively. Figure 17 depicts a characteristic test plot for A2 material tested according to the load step test strategy. During the running in and heating period the COF undergoes significant changes. At low temperatures, immediately after test start, the COF decreases. Simultaneously, the contact potential settles to its maximum value of 50 mV. This process indicates running in events and a potential formation of an isolating load bearing structure known for AlSi alloys with sufficient amount of Si under sliding condition. During heating up the COF starts to re-increase, settling at a level of 0.075 while the CP remains on highest position. Due to the high COF friction heat is generated and the contact near temperature rises accordingly. Selected tests have been stopped manually at this stage to study the sliding surface for this state of sliding. Figure 18a-c shows disc and ring surfaces for this condition. On the steel ring specimen ZDDP anti-wear tribofilms consisting mainly of a phosphate network have been formed with a pad like structure, see Figure 18a. The phosphates are seen in blue colour for the used LIMI. Beneath the P-pads a brownish S-Zn-O layer is formed on the Fe based substrate. As seen, the formation intensity of the tribofilms is dense for the model test condition taking into account that the image shown is representative for the entire contact area on the steel surfaces. On the AlSi material the top sliding surface consist of few large exposed particles embedded within a modified boundary layer (Figure 18b) which appears dark/black in the LIMI shots. Selectively, local areas of blank Al matrix (bright spots) can be found. The large particles can be identified by EDX technique as Al(FeMn)Si phases partly covered with ZDDP tribofilms – see Figure 18c and Table 5 Spectrum 1 and 2. In this regard, Fe, Al and Si elements derive from the intermetallic phase representing the substrate for the tribofilms, whereas P, O, Zn, S and Ca elements are part of the tribofilm network. The dark modified surface layer consists of Al and Si, resulting from the fine microstructure of α-mixed crystal and eutectic deformed 13

under shear loads, and ZDDP tribofilm elements as listed beforehand apparently in deposited form mixed up with the boundary layer (elemental analysis according to Figure 18c and Table 5 Spectrum 3 and 4).The beforehand described surface conditions result in high friction losses, which has also been emphasized in previous studies [16]. Up to a load level between 10 and 11 MPa nominal loading the A2 material enables stable sliding conditions. Upon further load increase the system is overloaded and severe wear processes occur. This transition to instable sliding conditions in the load step test program is reported by the break of the CP signal and the rise of the wear graph, see Figure 17. During this period the COF drops slightly and becomes hectic. Corresponding surface conditions are marked by wear grooves, material transfer and local scuffing damage similar to that of shown in Figure 18d. The total gravimetric wear loss after test stop can be taken from Table 6 proving the wear induced damage to the Al pump cover material after reaching the load limits.

Figure 17 Load step test - characteristic result for A2 material (2 similar reruns)

14

Figure 18 Damage analysis of model tests with A2 material: (a) LIMI image of ring specimen after completing the heating period for load step test strategy, (b) LIMI picture of the corresponding A2 disc counterpart surface to (a), (c) SEM/EDX analysis of A2 surface similar to that of (b), (d) LIMI image of the worn A2 disc surface after long term testing Table 5 Element list of EDX point analysis in Figure 18c (measured with 7.5 kV and values given in at.%) Spectrum

C

O

Al

Si

P

S

Ca

Fe

Zn

1 2 3 4

11.7 25.9

12.6 11.4

39.6 22.8

7.3 4.8

2.6 2.3

5.6 10.3

3.1 2.0

11.2 7.4

6.4 13.1

18.4 16.1

19.6 19.2

28.7 21.3

19.6 29.7

2.9 3.7

3.7 2.5

2.3 3.8

4.7 3.6

Table 6 List of gravimetric wear for A2 at model tests (calculated from weight loss considering a density of 2,75 g/cm³) Average gravimetric wear for

µm

Load step tests Constant tests

309,6 396,7

Figure 19 depicts a constant long term test of A2. The test is performed at a load level of 5 MPa, which is half of the transition load level reached in the load step test program, after an identical running in and heating period. At the beginning of the test, conditions are similar to that of the load step tests in terms of set values during run in and heating, friction and sliding response characteristic (see test plots of Figure 17 and Figure 19) and surface conditions as shown in Figure 18a-c. Upon reaching the constant phase instead of further loading, the COF changes from high values to a moderate lower COF level of 0.025. Apparently, the initially formed tribofilm contact gets removed by mild wear. The resulting surface alignment implies lack of reformation due to lower specific loadings and less need for anti-wear tribofilm protection under constant 15

sliding conditions. Up to a time of five hours of stable sliding conditions are prevalent with only minor wear processes taking place during mixed friction sliding. However, thereafter the systems changes to instable sliding conditions with a high wear rate and hectic COF characteristics. During this sliding state surface conditions of the disc specimen are similar to that of those presented in Figure 18d. Obviously a time induced failure, potentially a slow loss of local hard phases on the top sliding layer, takes place for this material under the given test conditions. Potentially, at some point when losing the load bearing particles the system may suddenly turns to instable sliding suffering from high wear. In Table 6 average gravimetric wear values obtained of all tests for A2 are summarized highlighting the wear afflicted performance of this material during all tests performed.

Figure 19 Constant long term test – characteristic result for A2 material (2 similar reruns)

Tribo-systems using disc specimen made of A1 material produced by sand cast process show similar friction values but different performance limits under comparable test conditions to A2. During the load step test (Figure 20) the COF also increases during the initial heating period and thereafter settles at 0.075-0.08 accompanied by a high CP value, which has been also noted for A2. For A1, despite a singular short time break down and reformation of the CP, which goes along with a slight change of the COF, these sliding conditions are of steady state nature until test end. In contrast to the performance of A2, no system fail indicated by severe wear processes occur for the whole test duration. This is emphasized by the missing of a capacitive wear signal (after correction of the initial displacement), stable CP and gravimetric wear evaluation, of which total average values for all load step tests performed are listed in Table 7. Test stop is caused by temperature controlled test rig shut down owing to high friction heat development in the oil bath based on the higher level of friction with ongoing test duration. For long term test conditions (Figure 21) also outstanding wear resistance has been observed for A1 material. In first instance one has to notice that with A1 material the whole test duration has been successfully completed without reaching any test stop criteria, such as high friction, high temperature or wear limit. During the whole test duration, a high CP and no significant wear height have been measured. This indicates that wear resistant isolating contact conditions prevail. This is also confirmed by gravimetric wear evaluation of the disc specimen of the long term tests performed. After more than 50 hours of mixed friction sliding at elevated temperatures and friction only a minor amount of disc wear, listed in Table 7, is generated. 16

Figure 20 Load step test - characteristic result for A1 material (2 similar reruns)

Figure 21 Constant long term test - characteristic result for A1 material (2 similar reruns) Table 7 List of gravimetric wear for A1 at model tests (calculated from weight loss considering a density of 2,75 g/cm³) Average gravimetric wear for

µm

Load step tests Constant tests

3,1 10,9

Figure 22a-d depicts characteristic surfaces of both disc and steel counterpart specimen of load step tests as well as constant long term tests of systems tested with A1 material. In this regard, surface conditions do not 17

differ between load step tests and long term tests. On the steel specimen ZDDP tribofilm formation with a common structure of S-Zn-O base layer and blue coloured phosphate pads on top is seen, see Figure 22a. On the Al material a pronounced load bearing particles structure forms on the surface area in contact. As shown in Figure 22b and c the whole contact areas is strewn with particles, apparently being elevated from the Al base material. In case of A2, the load carrying particles are limited to few AlFeMnSi phases, whereas, as seen in case of A1 load bearing particles occur all over the surface and are well distributed. Most of the matrix material between the particles is covered by a dark boundary layer, similar to A2 material. Figure 22b additionally highlights that the load bearing particles are frequently covered with blue pads. Detailed SEM/EDX analysis confirms the load bearing phases in case of A1 to be eutectic Si (Figure 22d, Table 8, Spectrum 3). The pads on top of the Si phases do have similar elemental composition known from anti-wear degradation products forming on steel, viz. P, S, Zn, Ca, and O (see Figure 22d, Table 8, Spectrum 1 and 2). Hence, the formed Si particle structure, in case of A1, results in a much better load carrying structure with a higher level to resist wear and instable sliding conditions compared to the boundary layers forming at A2 material.

Figure 22 Damage analysis of model tests with A1 material: (a) LIMI image of characteristic steel counterpart surface, (b) LIMI picture and (c) SEM picture of the load bearing particle on the Al disc specimen, (d) detailed SEM/EDX analysis of load bearing particles of A1 Table 8 Element list of EDX point analysis in Figure 22d (measured with 7.5 kV and values given in at.%)

18

Spectrum

C

O

Al

Si

P

S

Ca

Fe

Zn

1 2 3

11.4 8.0 9.0

40.5 37.5 1.5

0.4 0.5

23.9 30.4 89.1

8.9 8.1

1.2 1.1

10.0 9.6

1.4

3.7 3.5 0.4

5

Comparison of model test results with damage analysis of engine tests

The comparison of model test results with damage analysis of pump cover parts from engine tests enables the validation and verification of the model test method. Figure 23 highlights the main findings for the different tribosystems obtained for engine parts and model tests. The analysis of the various pump cover parts of the engine tests shows a clear and different damage/wear amount between sand cast material and die cast material. In case of the sand cast material A1, the pump cover parts of the engine tests have not suffered under significant damage/wear, thus loss of material. In contrast, the die cast (A2) pump cover parts from engine tests showed high damage (pronounced wear scars) implying a change to the initial component geometry. The model tests demonstrated the same rating. On the one hand, the sand cast material A1 shows high load bearing capacity in the mixed friction sliding regime. In this regard a load level limit could not been reached because of temperature depended test rig limits around 15 MPa. Furthermore, during long term testing an insignificant wear rate has been measured, which once again fits with the component findings. On the other hand, model tests of die cast systems (A2) resulted in loading limits of 10-11 MPa, after which severe failure takes place, as well as high wear rates for the long term test provoking a wear depended test shut down after only 10-15 % of the total test duration. On the mating surfaces these similarities between model tests and damage analysis results of engine test parts have been noticed too. This ensures damage and surface process equality between reduced and full scale system. In both cases on the sand cast surfaces (A1 material) load bearing structures of eutectic Si phases do form. On the Si particles in contact ZDDP anti-wear tribopads consisting of phosphate structures grow up, while deeper Al matrix between the particles is locally covered with tribopad obviously deposits respectively mixed up in a modified boundary layer. In this regard, tribochemical processes are more pronounced on the model test system, which may can be traced back to the fact that the model tests are carried out with fresh oil not affected by the engine stressing of the oil. Moreover, the heat development during the tribotests encourages the formation of ZDDP degradation products. The same goes for the surface processes documented on the counterparts. While in case of engine parts tribofilms form more in a local nature, a wide area (massive) formation on the whole contact track is noticed for the model tests. For the die cast surfaces (A2 material) wear grooves are evident both on real life parts of engine tests and model test specimen. Closely looking at these areas reveal that the intermetallic phases and the Si phases get unhinged upon the tribological contact being present thus not being able to form a wear resistant boundary surface.

Figure 23 Comparison of performance/surface processes between engine validation and model tests

19

6

Summary and outlook

The current study fathoms tribological processes taking place for sliding contacts within variable displacement vane pumps. In particular, the contact of the moving parts such as vane slides with the pump cover, made of near eutectic AlSi materials, has been investigated comprehensively by conducting analysis of engine parts, tribometric model tests under defined conditions with accompanying damage analysis of specimen and comparison thereof. In this regard, a proper model test configuration has been derivated and validated. The obtained results yield new knowledge regarding the friction and wear mechanism of near eutectic AlSi materials under mixed friction condition and the specific impact of the microstructure based on manufacturing procedure. Specific findings are highlighted below: • • •

•

•

• •

The derivated test configuration and test conditions are able to visualize sliding and wear phenomena of VDV pump elements with the pump cover. Wear performance of near eutectic AlSi materials differ significantly depending on the formed microstructure. For EN AC 43300 eutectic Si formed a load bearing particle structure thus providing a pronounced wear protection effect. Based on the low occurrence of larger intermetallic phases with sufficient size no significant effects on sliding and wear performance of those was noticed for this alloy. In case of EN AC 46000 the smaller Si phases were not able to provide enhanced wear protection. Only a few larger intermetallic phases provided a load bearing functionality but based on the low occurrence only to a limited extend. In comparison, EN AC 43300 alloy showed higher wear resistance than EN AC 46000 under similar test condition. This goes along with performance in engine tests. The higher wear resistance can be attributed to the load bearing functionality of the larger eutectic Si phases in case of EN AC 43300. Thus it can be concluded, that for AlSi sliding materials hard phases with sufficient size and distribution are a crucial parameter for higher sliding wear resistance. In contrast, friction performance was found to be similar between the two materials tested under the given test conditions. Tribofilms composing of elements such as O, S, Zn, P and Ca/Mg are present on Si phases, steel counterpart and the Al matrix under the given model test conditions of this study. These layers are likely to be anti-wear films deriving from oil additive compounds such as ZDDP . In this regard, the tribopads appear to actively form on the Si phases, the steel counterpart. On the Al matrix potentially the antiwear elements get mixed up in the modified boundary layer in deposit form. Furthermore, it has been shown that such anti-wear layers are also found on corresponding engine parts both for the Al material and the steel counterpart, thus being not only a model test phenomenon.

In future, the designed model test method can be used to carry out screening activities and further research with regard to material optimization and understanding of tribological contacts of VDV pump covers.

References [1] [2]

20

Schommers J, Scheib H, Hartweg M, Bosler A. Reibungsminimierung bei Verbrennungsmotoren. MTZ Motortech Z 2013;74(7-8):566–73. https://doi.org/10.1007/s35146-013-0170-y. BMW. Die Ottomotorenfamilie des Next-Generation-Baukastens von BMW.

[3] [4] [5]

[6]

[7] [8] [9]

[10] [11] [12] [13]

[14] [15]

[16] [17] [18] [19]

[20]

[21]

[22] [23] [24] [25] [26]

[27]

21

Jückstock R. Perspektiven zur Reibungsreduzierung im Grundmotor. MTZ Motortech Z 2014;75(15):114–9. https://doi.org/10.1007/s35146-014-0421-6. Hoen T, Schmid J, Stumpf W. Weniger Verschleiß und Ölverbrauch durch Spiralgleithonung bei Deutz-Motoren. MTZ - Motortechnische Zeitschrift 2009;70(4):324–9. https://doi.org/10.1007/BF03225485. Summer F, Grün F, Offenbecher M, Taylor S. Challenges of friction reduction of engine plain bearings – Tackling the problem with novel bearing materials. Tribology International 2018;131:238–50. https://doi.org/10.1016/j.triboint.2018.10.042. Agostini L, Tramaglia F, Marchetti L, Fauda A. Entwicklung von Ölpumpen mit variablem Verdrängungsvolumen mittels simulierter Verschleißprognose. MTZ Motortech Z 2018;79(12):32–41. https://doi.org/10.1007/s35146-018-0119-2. Mucchi E, Agazzi A, D'Elia G, Dalpiaz G. On the wear and lubrication regime in variable displacement vane pumps. Wear 2013;306(1-2):36–46. https://doi.org/10.1016/j.wear.2013.06.025. Motamen Salehi F, Morina A, Neville A. The effect of soot and diesel contamination on wear and friction of engine oil pump. Tribology International 2017;115:285–96. https://doi.org/10.1016/j.triboint.2017.05.041. Doikin A, Zadeh EH, Campean F, Priest M, Brown A, Sherratt A. Impact of Duty Cycle on Wear Progression in Variable-displacement Vane Oil Pumps. Procedia Manufacturing 2018;16:115–22. https://doi.org/10.1016/j.promfg.2018.10.170. Georgiou EP, Drees D, Bilde M de, Anderson M. Pre-screening of hydraulic fluids for vane pumps: An alternative to Vickers vane pump tests. Wear 2018;404-405:31–7. https://doi.org/10.1016/j.wear.2018.02.022. van Basshuysen R, Schäfer F. Handbuch Verbrennungsmotor: Grundlagen, Komponenten, Systeme, Perspektiven. 7th ed. Wiesbaden: Springer Vieweg; 2015. Mbuya TO, O Odera B, P Ng 'ang S, Oduori M. Effective recycling of cast aluminium alloys for small foundries. Journal of Agriculture, Science and Technology 2010;12. Briscoe BJ, Sinha SK. Chapter 1 - Tribological applications of polymers and their composites – past, present and future prospects. In: Friedrich K, Schlarb AK, editors. Tribology of Polymeric Nanocomposites (Second Edition). Oxford: Butterworth-Heinemann; 2013, p. 1–22. Dey SK, Perry TA, Alpas AT. Micromechanisms of low load wear in an Al–18.5% Si alloy. Wear 2009;267(14):515–24. https://doi.org/10.1016/j.wear.2008.11.011. Lozano DE, Mercado-Solis RD, Perez AJ, Talamantes J, Morales F, Hernandez-Rodriguez MAL. Tribological behaviour of cast hypereutectic Al–Si–Cu alloy subjected to sliding wear. Wear 2009;267(1-4):545–9. https://doi.org/10.1016/j.wear.2008.12.112. Grün F, Summer F, Pondicherry KS, Gódor I, Offenbecher M, Lainé E. Tribological functionality of aluminium sliding materials with hard phases under lubricated conditions. Wear 2013;298–299(0):127–34. Chen M, Alpas AT. Ultra-mild wear of a hypereutectic Al–18.5wt.% Si alloy. Wear 2008;265(1-2):186–95. https://doi.org/10.1016/j.wear.2007.10.002. Chen M, Meng-Burany X, Perry TA, Alpas AT. Micromechanisms and mechanics of ultra-mild wear in Al–Si alloys. Acta Materialia 2008;56(19):5605–16. https://doi.org/10.1016/j.actamat.2008.07.043. Dienwiebel M, Pöhlmann K, Scherge M. Origins of the wear resistance of AlSi cylinder bore surfaces studies by surface analytical tools. Tribology International 2007;40(10–12):1597–602. https://doi.org/10.1016/j.triboint.2007.01.015. Pereira G, Lachenwitzer A, Nicholls MA, Kasrai M, Norton PR, Stasio GD. Chemical characterization and nanomechanical properties of antiwear films fabricated from ZDDP on a near hypereutectic Al–Si alloy. Tribol Lett 2005;18(4):411–27. https://doi.org/10.1007/s11249-004-3592-3. Nicholls MA, Norton PR, Bancroft GM, Kasrai M, Stasio GD, Wiese LM. Spatially resolved nanoscale chemical and mechanical characterization of ZDDP antiwear films on aluminum?: Silicon alloys under cylinder/bore wear conditions. Tribol Lett 2005;18(3):261–78. https://doi.org/10.1007/s11249-004-2752-9. Spikes H. The History and Mechanisms of ZDDP. Tribol Lett 2004;17(3):469–89. https://doi.org/10.1023/B:TRIL.0000044495.26882.b5. DIN EN 1706:2013-12: Aluminium und Aluminiumlegierungen – Gussstücke – Chemische Zusammensetzung und mechanische Eigenschaften. Backerud L, Chai G, Tamminen J. Solidification characteristics of aluminum alloys. Vol. 2. Foundry alloys. American Foundrymen's Society, Inc., 1990 1990:266. Taylor JA. Iron-Containing Intermetallic Phases in Al-Si Based Casting Alloys. Procedia Materials Science 2012;1:19–33. https://doi.org/10.1016/j.mspro.2012.06.004. Tupaj M, Orłowicz AW, Mróz M, Trytek A, Markowska O. The Effect of Cooling Rate on Properties of Intermetallic Phase in a Complex Al-Si Alloy. Archives of Foundry Engineering 2016;16(3):125–8. https://doi.org/10.1515/afe-2016-0063. Chen CL, West GD, Thomson RC. Characterisation of Intermetallic Phases in Multicomponent Al-Si Casting Alloys for Engineering Applications. MSF 2006;519-521:359–64. https://doi.org/10.4028/www.scientific.net/MSF.519-521.359.

[28] Martin J. Antiwear mechanisms of zinc dithiophosphate: a chemical hardness approach. Tribol Lett 1999;6(1):1– 8. https://doi.org/10.1023/a:1019191019134. [29] Nicholls MA, Do T, Norton PR, Kasrai M, Bancroft GM. Review of the lubrication of metallic surfaces by zinc dialkyl-dithiophosphates. Tribology International 2005;38(1):15–39. https://doi.org/10.1016/j.triboint.2004.05.009. [30] Bec S, Tonck A, Georges JM, Coy RC, Bell JC, Roper GW. Relationship between mechanical properties and structures of zinc dithiophosphate anti–wear films 1999;455(1992):4181–203. https://doi.org/10.1098/rspa.1999.0497. [31] Nano-Mechanical and Chemical Characterization of Tribofilms Formed Under Conformal Contact Conditions; 2013. [32] Pondicherry K, Grün F, Gódor I, Bertram R, Offenbecher M. Applicability of ring-on-disc and pin-on-plate test methods for Cu–steel and Al–steel systems for large area conformal contacts. Lubrication Science 2013;25(3):231–47. https://doi.org/10.1002/ls.1187. [33] Czichos H. Tribologie-Handbuch: Tribometrie, Tribomaterialien, Tribotechnik. 3rd ed. Wiesbaden: Springer Fachmedien; 2010. [34] Summer F, Bergmann P, Grün F. Damage Equivalent Test Methodologies as Design Elements for Journal Bearing Systems. Lubricants 2017;5(4):47. https://doi.org/10.3390/lubricants5040047. [35] Totten GE. Friction, lubrication, and wear technology. Materials Park, Ohio: ASM International; 2017. [36] Furey MJ. Metallic Contact and Friction between Sliding Surfaces. A S L E Transactions 1961;4(1):1–11. https://doi.org/10.1080/05698196108972414. [37] Summer F, Grün F, Schiffer J, Gódor I, Papadimitriou I. Tribological study of crankshaft bearing systems: Comparison of forged steel and cast iron counterparts under start–stop operation. Wear 2015;338-339:232–41. https://doi.org/10.1016/j.wear.2015.06.022.

22

Research Highlights • • • •

Friction and wear characterization of engine vane pump components is presented. Wear performance of similar near eutectic AlSi materials differs significantly depending on manufacture conditions and microstructures formed. Wear rankings of engine tests and ring on disc model test are similar. Protective tribofilms (rich in P, O, Zn, S) do form on Si phases.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

None