Self-Lubricating Properties of Laser Claddings for High Temperature Forming Processes

Self-Lubricating Properties of Laser

Claddings for High Temperature Forming Processes

Tugce Caykara

Mechanical Engineering, master's level (120 credits) 2018

Luleå University of Technology

Department of Engineering Sciences and Mathematics

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Acknowledgements

I would first like to thank the TRIBOS Consortium for giving me this wonderful opportunity to be a part of 4

generation of Erasmus Mundus TRIBOS Program. It was a great experience to study and learn tribology at these leading institutions. I am so grateful to all of the members of the consortium for their support through my education and help in overcoming numerous obstacles.

I would like to thank my supervisors Professor Braham Prakash, Associate Professor Jens Hardell and Hector Torres. I would not be able to complete my thesis without help of my supervisors. Their guidance meant a lot to me through the project. I would also like to thank my co-supervisor Dr. Manel Rodriguez Ripoll from Austrian Excellence Centre for Tribology (AC2T Research GmbH, Austria). This work was partly funded by the Austrian COMET Programme (Project K2 XTribology, Grant No. 849109) and has been carried out within the “Austrian Excellence Center for Tribology” (AC2T research GmbH).

Therefore, I would also like to acknowledge the Austrian Excellence Center for their support as well as sample preparation and characterization.

I would also like to thank the staff in Tribolab, machine elements division and material science department for their support and assistance with equipment.

Last but not the least, I would like to thank my family and my friends for their lifetime support and encouragement through my years of study and my life in general. This accomplishment would not have been possible without them.

Thanks for all your encouragement!

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Abstract

This thesis summarizes the work done on tribological characterization of multifunctional hardfacing

coatings with self-lubricating properties, intended for use in mechanical components operating in high

temperature applications for which conventional lubricants are no longer effective. Deposition

techniques like laser cladding have a great potential in reworking/repair of high value industrial

components in order to extend their lifetime. It is expected that the use of self-lubricating laser claddings

could be useful in high temperature applications like metal forming, leading to decreased friction and

wear. In this study, the tribological behavior of self-lubricating claddings has been studied against steel

and aluminum counter surfaces, using ASI52100 bearing steel in addition to AA6082 and AA2007

aluminum flat pins as the counter bodies. Nickel- and iron-based powders have been chosen for the

preparation of claddings. Self-lubricating properties of Ag/MoS

have been compared to an untreated

reference cladding and grade 1.2367 tool steel. For steel counter surfaces, tribological properties in the

temperature range between RT and 600⁰C have been investigated and at 300°C for aluminum counter

surfaces. Tribological tests were done by a high frequency linear oscillation (SRV) test machine under

reciprocating conditions. The wear scar and volume of coatings were measured by using a 3D optical

profilometer. SEM/EDS analysis were additionally performed for the characterization of microstructure

and wear scar. The results indicated that MoS

reduced friction and wear of the Fe-based cladding

material when tested against steel at room temperature compared to the reference alloy and grade 1.2367

tool steel, and that the addition of silver further decreased wear in addition to early stage friction. It was

also observed that the tribolayer, which was formed during the sliding of Ni-based - 5 Ag - 10 MoS

and

against aluminium under lubricated conditions, was protective and provided low and steady friction.

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List of Figures

Figure 1 : Number of publications per year dealing with self-lubricating materials at high temperature (11)... 2

Figure 2: The structure of molybdenum disulfide (18) ... 3

Figure 3: The lamellar structure under a shearing load (18) ... 4

Figure 4 : Ratio of deposition methods of TMD's for high temperature applications (11) ... 4

Figure 5: the coefficient of friction depending on film thickness and sliding speed for a steel ball sliding on silver coated steel disc (32) ... 6

Figure 6: Layered structure of Ag2MoO4 where blue, green and red represent Ag, Mo and O respective (41) ... 8

Figure 7: Single step (a), (b), (c) and two step (d) laser cladding processes (54) ... 10

Figure 8: Pin-on-disk configuration ... 14

Figure 9: Optical microscopy imaging of the as-deposited claddings: a) a detail of the etched microstructure of Metco 42C, b) Metco 42C - 10 MoS2 and c) Metco 42C - 5 Ag - 10 MoS2 ... 15

Figure 10: SEM imaging of a) Metco 42C - 10 MoS2 and of b) an aggregate found in Metco 42C - 5 Ag - 10 MoS2 ... 16

Figure 11: Optical microscopy of Ni-based – 5 Ag - 10 MoS2 ... 17

Figure 12: The friction coefficient results of Metco42C-based samples at different temperatures a) RT, b) 450°C c) 600°C ... 18

Figure 13: The variation in friction for Metco42C reference samples at 600°C ... 19

Figure 14: The variation in friction for Metco42C – 10 MoS2 samples at 600°C ... 20

Figure 15: The variation in friction for Metco42C – 5 Ag – 10 MoS2 samples at 600°C ... 20

Figure 16: Specific wear rates of claddings and the reference tool steel ... 21

Figure 17: Specific wear rates for steel-based counter bodies ... 22

Figure 18: a) Overview and b) a detail of the smearing of silver observed by SEM in Metco 42C - 5 Ag - 10 MoS2 at RT ... 22

Figure 19: SEM results for 450°C for a) Metco 42C and b) Metco 42C - 10 MoS2 ... 23

Figure 20: The observation of the transfer layer under SEM for 30 seconds at 450°C tests for Metco 42C - 5 Ag - 10 MoS2 ... 23

Figure 21: SEM image of Metco 42C - 10 MoS2 1.trial in Figure 14 ... 24

Figure 22: SEM image of Metco 42C - 10 MoS2 3.trial in Figure 14 ... 24

Figure 23: SEM image of Metco 42C - 5 Ag - 10 MoS2 trial 3 in Figure 15 ... 25

Figure 24: SEM image of Metco 42C - 5 Ag - 10 MoS2 trial 1 in Figure 15 ... 25

Figure 25: Friction coefficient for Metco 42C - 5 Ag - 5 MoS2 dry tests against AA6082 ... 26

Figure 26: Friction coefficient for Ni-based 5 Ag 10 MoS2 dry tests against AA6082... 26

Figure 27: Friction coefficient for 1.2367 tool steel dry tests against AA6082 ... 27

Figure 28: Friction coefficient for Ni-based - 5 Ag - 10 MoS2 dry tests against AA2007 ... 28

Figure 29: Friction coefficient for 1.2367 tool steel dry tests against AA2007 ... 28

Figure 30: Friction results against AA6082 pins at 300°C for lubricated contacts ... 29

Figure 31: Specific wear rate of AA6082 counter bodies ... 30

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Figure 32: SEM image of wear scars on Molykote-lubricated samples a,d) 1.2367 tool steel, b,e) Metco 42C - 5

Ag - 5 MoS2 and c,f) Ni-based - 5 Ag -10 MoS2 ... 30

Figure 33: SEM/EDS mapping on AA6082 pin tested against Ni-based - 5 Ag - 10 MoS2 at 300°C ... 31

Figure 34: SEM/EDS mapping on AA6082 pin tested against Metco 42C - 5 Ag - 5 MoS2 at 300°C ... 32

Figure 35: SEM/EDS mapping on AA6082 pin tested against 1.2367 tool steel at 300°C ... 32

Figure 36: the cross section of AA6082 pins tested at 300°C against a) Ni-based - 5 Ag - 10 MoS2 b) Metco 42C - 5 Ag - 5 MoS2 c) 1.2367 tool steel. The reciprocating sliding direction is shown. ... 32

Figure 37: XPS spectrum of the pin surface after sliding against Ni-based - 5 Ag - 10 MoS2 ... 33

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List of Tables

Table 1: Chemical composition of the coating base alloy Metco 42C (wt. %) ... 12

Table 2: Chemical Composition of the Ni-based coating (wt. %) ... 12

Table 3: Chemical composition of the grade 1.2367 tool steel commercial material (wt. %) ... 12

Table 4: Composition and surface roughness of the resulting claddings used for tests with steel ... 13

Table 5: Composition and surface roughness of the resulting claddings for tests with aluminum ... 13

Table 6: The chemical composition (weight%) of AA6082 and AA2007 Al alloys ... 14

Table 7: Test parameters for SRV tribotest ... 14

Table 8: Composition as measured by EDS of the spots marked in Figure 10b ... 16

Table 9: Chemical composition of the cladding matrix as measured in the as-deposited samples ... 17

Table 10: Composition as measured by EDS of the spot marked in Figure 20 ... 23

Table 11: Chemical composition as seen by EDS of the spots marked in Figure 32 ... 31

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1. Introduction

In recent years, there has been an increased interest in high temperature industrial processes like hot stamping in the automotive industry due to the possibility of producing lightweight steel components with high mechanical strength. The hot stamping method involves heating up of the blank above the austenitisation temperature, forming the hot steel blank and subsequently cooling it down rapidly to achieve a fully martensitic microstructure. The martensite provides a significant strength increase. The process allows for the production of components with high mechanical strength while using thinner sheets, leading to weight reduction, lower fuel consumption and decreased harmful emissions in automobiles. However, there are some problems with formability and with tool tribology at high temperatures, as these forming processes take place under severe contact conditions (1). Moreover, hot stamping is not the only process or application of industrial interest taking place under high temperatures (HT), as the list also includes gas turbine seals, bearings and variable stator vane bushings, valve stems or even furnace components (2). It is expected that such applications would benefit greatly from low friction and wear solutions effective at high temperature.

Moreover, in recent years there has been an increased interest in aluminum alloys for transportation vehicles, due to their potential for decreased vehicle weight and CO

emissions (3). Aluminum alloys have good mechanical and chemical properties like high strength, stiffness to weight ratio, good formability, good corrosion resistance and recyclability compared to heavier materials (4). However, some high performance aluminium alloys experience poor formability at room temperature and their manufacture has to be performed at elevated temperatures (5). Under such conditions, the tool experiences severe adhesive wear which shortens its lifetime, while high friction increases energy needs and is detrimental for the quality of the finished part. Moreover, there is a limited amount of studies on tribological behavior of aluminum alloys at elevated temperatures (6).

One of the possible solutions for improved tribological performance in high temperature applications can be the use of solid lubricants. As conventional lubricant greases and oils degrade above 350⁰C, they cannot be used under severe conditions like high temperature (7). The present study focusses on the high temperature tribological behavior of iron- and nickel-based claddings with the incorporation of different solid lubricants as a possible low-cost alternative for applications such as hot stamping.

1.1 State of the Art

Materials with self-lubricating properties for high temperature applications have gained an increasing

interest since the end of the 1990s. The rising number of literature dealing with self-lubricating materials

are illustrated in Figure 1. One of the first works was done at NASA Lewis Research Center by Sliney

and co-workers for aerospace applications in 1974. Thermal spray coatings with the addition of materials

like silver, calcium fluoride, barium fluoride were studied. While alkaline earth fluorides provided

effective lubricating at high temperatures, silver was used for low temperature lubrications. Good

tribological properties of these coatings were observed as a result of lubricant diffusing to the contact

and reducing friction and wear (8) (9) (10).

2

Figure 1 : Number of publications per year dealing with self-lubricating materials at high temperature (11)

Aouadi et al. studied Mo

N/MoS

/Ag nanocomposite coatings deposited by PVD technique in a nitrogen environment. It was observed that the addition of Ag and S was beneficial for reducing the coefficient of friction at high temperatures as a consequence of the diffusion of Ag to the surface at 350°C and the formation of lubricious silver molybdates at 600°. Ag content > 16% and S content between the range of 5 – 14% were found to provide the lowest friction coefficient. The composite coating Mo

N/MoS

/Ag also reduced the wear rate up to two orders of magnitude (12).

In a study conducted by Chen et al., the tribological properties of NiCrAlY-Ag-Mo composite coating deposited by atmospheric plasma spraying under dry contacts were investigated at the temperatures between 20 and 800°C. The friction coefficients of composite coatings were lower and close to 0.3 at the temperatures tested with a peak of 0.45 friction coefficient at 400°C. A similar trend was also observed for wear rate results. It was found that lubrication was provided by silver at the temperatures lower than 400°C while silver molybdate and molybdenum oxide showed lubrication at the temperatures higher than 400°C, with the amount of silver molybdate and molybdenum oxide increased with temperature (13).

Nickel-based composite coatings with the addition of Ag or Ag-Mo deposited by plasma spraying were evaluated by Zhang et al. under ball-on-disk configuration from room temperatures to 800°C in air. It was observed that the addition of Ag or Ag-Mo in NiCoCrAlY-Cr

Co

coatings reduced friction and wear rate. The friction was reduced by the diffusion and formation of Ag films on the surface of NiCoCrAlY-Cr

O

-Ag coating at 600°C. For the NiCoCrAlY-Cr

O

-15AgMo coating, the friction was reduced by low shear strength of silver at temperatures under 400°C and due to the synergistic effect of both silver molybdate and nickel molybdate above 400°C (14).

1.2 Solid Lubricants

Solid lubricants have the potential to replace oils and greases in high temperture applications and in many cases, they can be incorporated to coatings and bulk materials in order to prepare self-lubricating materials, as previously described. Unlike oils, they can be effectively used under severe conditions as the ones listed below:

▪ Extreme pressure and temperature conditions, like in metal forming

▪ In the presence of radiation, as they have a higher resistance to deterioration than oils

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▪ In highly dusty places due to higher resistance to abrasive wear

▪ Lightweight applications, as solid lubricants do not require bulky recirculation systems

▪ Applications involving intermitted loading conditions and corrosive environments.

Furthermore, an additional advantage of solid lubricants is that they can be used at places where availability for maintenance is low, like in space applications. However, there are some disadvantages like their higher friction and wear compared to oil-based hydrodynamic lubrication, in addition to poor self-healing properties (15).

Some of the most relevant solid lubricant classes will be discussed below.

1.2.1 Transition Metal Dichalcogenides (TMDs)

This class of solid lubricants is one of the most widely used, sharing the formulation MX

, where M is a transition metal like Mo or W and X is a chalcogen atom like S, Se or Te (16). MoS

and WS

are widely known transition metal dichalcogenides (17). TMDs have a layered structure where the transition metal is sandwiched between two chalcogen atoms. The sandwiched structure of MoS

is illustrated in Figure 2. While the bonding between transition metals and chalcogens is covalent, chalcogens are hold together by weak Van der Waals forces. Due to this, the layers can easily slide over each other under a shearing force, leading to low shear strength between the layers and thus to decreased friction. Figure 3 visualizes the shearing in the case of a sliding contact between two surfaces.

Figure 2: The structure of molybdenum disulfide (18)

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Figure 3: The lamellar structure under a shearing load (18)

The most widely used deposition methods reported in literature for TMDs in high temperature applications are shown in Figure 4. This figure shows that the most widely used deposition method is powder metallurgy closely followed by PVD, with laser cladding and thermal spraying far less popular (11). It is important to note that the deposition method might lead to unwanted changes in the chemical composition of the solid lubricants. For instance, it has been observed that during laser cladding processes sulphur compounds can thermally degrade, reacting with chromium and forming chromium sulfides (19) during sample preparation.

Figure 4 : Ratio of deposition methods of TMD's for high temperature applications (11)

In general, MoS

is regarded to be more suitable than WS

due to lower costs, although the oxidation resistance of WS

is higher than that of MoS

, as it has been reported that molybdenum disulfide oxidizes at the temperatures between 370 and 480°C (20) while WS

oxidizes at around 540°C (21) (11). In any case, MoS

has significant advantages as it is very effective in vacuum and dry air, making it well suited for space applications. However, it is known that MoS

doesn’t perform well in moist environments due to the detrimental role of water vapor. A higher coefficient of friction for WS

was similarly noticed when sliding in humid air. In particular, the coefficient of friction has been reported to rise from around 0.05 in dry inert gas/vacuum to 0.15-0.2 in humid air for MoS

and WS

. This decrease is due to the formation of WO

or MoO

which are the reaction product of unsaturated bonds with oxygen and moisture (17).

As for the use of TMDs in self-lubricating materials, Muratore and Voevodin (22) studied YSZ-Ag-Mo

coatings deposited by PVD with the addition of MoS

at different temperatures ranging from 25°C to

700°C. It was found that a pure reference MoS

coating showed significantly lower coefficient of friction

compared to other samples. However, it was ineffective at temperatures higher than 300°C. The addition

of MoS

in YSZ-Ag-Mo coatings contributed to reducing the coefficient of friction up to temperatures

5

of 700°C but this high temperature lubrication was obtained due to the formation of lubricous oxides. It was also found that the optimum MoS

composition in the coating was 8 wt. %.

In a research done by Hu et al. to extend MoS

lubrication with the addition of tellurium by using PVD technique, a Mo-S-Te coating was tested against MoS

, MoSe

, MoTe

layers at two different temperatures including 300°C and 450°C in ambient air. While the coefficient of friction for MoS

was around 0.15, Mo-S-Te showed better results with a value close to 0.05 at 300°C and at 450°C, the coefficient of friction increased to 0.5 for MoS

and 0.1 for Mo-S-Te. Raman spectroscopy of the Mo- S-Te samples tested at 450°C showed a strong peak for MoS

and a weaker peak for MoO

, therefore it was considered that a tellurium-based barrier formed on the surface of the coating slowing down the oxidation of MoS

up to 450°C. Pure MoSe

showed similar initial friction values at 300°C compared to Mo-S-Te but friction increased afterwards (23).

Li and Xiong (24) studied the interaction of MoS

and graphite at high temperatures and they observed that the coefficient of friction was lower when both lubricants were used due to a synergistic effect of graphite and MoS

.

Yang et al. (25) studied NiCr-Cr

C

coatings with the addition of 30 wt. % WS

at three different temperatures, RT, 300°C and 600°C. The coatings were deposited by laser cladding technique. It was reported that the coefficient of friction values at all three temperatures decreased with the addition of WS

. However, wear rates were only lower at 300°C for NiCr-Cr

C

-WS

. Interestingly, it was noted that WS

had decomposed, forming lubricous chromium sulfides during laser cladding deposition due to high temperatures.

1.2.2 Soft Metals

Soft metals such as gold, silver, indium, tin and lead are effective solid lubricants and provide low coefficient of friction values potentially close to 0.1 (17). The low friction is a result of their low shear strength, rapid recovery and also recrystallization (26). The main mechanism for low shear strength is stated to be plastic deformation due to low ductility which allows them to shear plastically (8).

One of the most used soft metal is silver due to its outstanding properties such as oxidation resistance as well as electrical and thermal conductivities, in addition to a comparatively high melting point making it suitable for applications with high levels of frictional heating (27). Silver also shows a better oxidation resistance than other soft metals like Pb, In, Sn (11) therefore it is preferred for high temperature applications. However, it must be noted that Pb, In and Sn can provide better lubrication at room temperature applications compared to Ag, Au and Pt (27). Although lead performs better than silver and gold at RT, its high toxicity has prevented its widespread use, making necessary the use of alternative solid lubricants (27), (11). Gold has also a high thermal conductivity, in addition to low shear strength and chemical inertness (27). In a study by Ouyang et al. dealing with ZrO

(Y

O

)-30CaF

-30Au composites at temperatures up to 800°C, it was found that the addition of gold improved the friction and wear properties at low temperatures due to plastic deformation (28). Copper can be another example of a soft metal which also provides high thermal conductivity, thus dissipating frictional heat. However further experimental work is needed to find out its potential, as its use as a solid lubricant is sparsely addressed in the literature (11).

In any case, silver is the most commonly used soft metal due to lower costs and being more

environmentally friendly than its counterparts, as reported in the available literature. Sliney (8) showed

that its addition to coatings was useful for reducing friction at low and moderate temperatures. Erdemir

et al. (29) also presented positive results on the friction coefficient and wear rates of Al

O

ceramics

with the incorporation of silver and gold. Aouadi et al. (12) studied composite coatings of

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Mo

N/MoS

/Ag thin films deposited by unbalanced magnetron sputtering with different compositions at three different temperature -RT, 350°C, 600°C , reporting that the friction coefficient was the lowest for samples with an Ag content above 16 wt.%. This was attributed to the migration of silver to the surface at high temperature. In this case, the addition of silver did not contribute to better wear results at RT and 350°C when compared to the undoped Mo

N coating. The addition of both Ag and MoS

was found beneficial for reducing coefficient of friction. While Ag migrates to the surface at 350°C, it forms lubricious silver molybdate phases inside wear track at 600°C.

Particularly in high vacuum applications, thin films of soft metals like silver can be beneficial (18). It was assumed that lubrication was provided by the softening of those layers during sliding (30). However, it was also shown the shear stresses have a detrimental role when the coating is thicker than some threshold value. If the coating is thicker than this value, wear by delamination might occur due to dislocation accumulation and strain hardening (31). This critical thickness is generally less than 1 µm for coated materials like Cd, Ag, Au and Ni on steel substrates. While thin layers can protect the harder substrate from plastic deformation and crack nucleation, layers thicker than a critical value can lead to ploughing and further plastic deformation, being the source of a worsened tribological behavior (31).

However, it is also important to note that too thin coating layers are prone to wear out quickly (27).

Arnell et al. showed that the critical film thickness for silver was around 1 µm during sliding against steel under ultrahigh vacuum with an applied load of 1 N, with the friction coefficient increasing for lower and higher layer thicknesses as shown in Figure 5 (32).

Figure 5: the coefficient of friction depending on film thickness and sliding speed for a steel ball sliding on silver coated steel disc (32)

Sliney et al. studied the different concentrations of silver up to 50 wt. % added to fluorides in plasma- sprayed coatings, reporting that the addition of 35 wt. % silver showed the lowest coefficient of friction in the temperature range between RT and 400°C (8).

According to the literature, the uncontrolled diffusion of silver to the sample surface is a concern in

silver-based coatings operating at high temperature, as this behavior leads to higher wear rate and

coating failure due to high porosity (33). However, the soft metal depletion can be prevented as reported

in the literature: diffusion barriers can be used in multilayer coatings, although with increasing costs

(34) (35). Torres et al. have also shown that the use of sulphur-based compounds is beneficial as it can

slow down silver depletion in coatings operating at high temperatures (36).

7 1.2.3 Lubricious Oxides

Lubrication by means of oxides can be very beneficial for high temperature applications due to the elevated oxidation rates involved. Some metal oxides can provide lubrication at high temperatures by becoming very ductile, this reduction in friction is not observed for all metal oxides (18). While some oxides deform plastically and lower the coefficient of friction (37) others may break and lead to high abrasive wear. It was also observed that even for lubricous oxides the coefficient of friction is highly affected by temperature, with significant changes in tribological behavior being observed for some oxides at different temperatures. This suggests that there is some characteristic threshold temperature responsible for a brittle-ductile transition (38). One of the most effective known metal oxide is lead oxide (PbO), it has been reported to be beneficial at high temperature applications (8) but its toxicity has prevented its widespread use (18).

The oxides of transition metals like W, Mo and V form oxygen-deficient compounds with missing crystallographic planes, known as Magneli Phases. This microstructure in Magneli phases results in crystallographic planes with reduced shear strength which can slide easily against each other and provide lubrication in a similar way to TMDs (11) (39).

Erdemir (37) presented a crystal-chemistry model to determine the lubrication performance of oxides formed on sliding surfaces at high temperatures. The model made extensive use of the ionic potential φ

= Z/r, defined as the ratio between cationic charge Z and the radius of the cation r. The higher the ionic potential, the greater the interaction between the oxygen anions and their cations so the oxygen anions screen their cation more effectively and they are not prone to have interaction with other cations. The oxides with high cationic charges like V

O

, WO

, Re

O

are generally soft materials and shear easily, consequently they provide low coefficient of friction. In contrast, the oxides with lower ionic potential like Al

O

, ZrO

, MgO, ThO

are expected to be ineffective as lubricants. These oxides are very stiff and hard to shear even at high temperatures, since their cations interact with each other and form covalent or ionic bonds. According to this model, the lubrication performance can be predicted for double or mixed oxides. While the oxide systems, Cs

O-SiO

, with big difference in ionic potential can provide low friction, the oxide systems, TiO

-Al

O

, which have slight differences in ionic potential might lead to high coefficient of friction values (37) (40).

The tribological properties of molybdenum oxide on MoN-coatings deposited by PVD for high temperature applications have been studied by Suszko et al. (38), with increased wear resistance observed at 230°C due to formation of MoO

, although at 400°C the wear rates were found to be higher.

This was attributed to the thermal degradation of MoO

limiting its lubricating role at higher temperatures.

A trend observed in the literature in recent years involves the combination of soft metals like Ag and transition metal dichalcogenides, as it has been reported to provide effective lubrication in a broad temperature range by the formation of new solid lubricant phases like double oxides at high temperature.

Aouadi et al. (41) prepared Ag/MoS

-containing PVD coatings and tested them at room temperature,

300°C and 600°C. Friction coefficient as low as 0.1 were observed at 600°C and the good tribological

behavior was attributed to the formation of lubricous silver molybdate phases with a layered structure,

as shown in Figure 6. This is interesting as the in-situ formation of new lubricants at high temperature

like silver molybdates can extend the effectiveness of the Ag/MoS

combination beyond 600°C. Chen

et al. have also reported the formation of silver molybdates for plasma sprayed coatings with the addition

of silver and pure molybdenum (42) but the reported beneficial role of sulfur controlling the diffusion

of silver gives further justification for the choice of Ag/MoS

as solid lubricants for high temperature

use.

8

Figure 6: Layered structure of Ag2MoO4 where blue, green and red represent Ag, Mo and O respective (41)

1.2.4 Graphite

Another example of lamellar solid lubricants is graphite which is a soft material with good electrical and thermal conductivity compared to diamond. It is also known to provide low friction and high wear resistance. Due to its good lubricity, abundance and low cost, it is used in a broad range of applications such as molds and dies in metal forming, as well as flange faces of rail and railcar wheels. It can be found in nature and produced synthetically. It can be used in different forms such as powder, colloidal dispersion, solid and composite, furthermore it can be dispersed in water, oils and greases. Graphite is one of the polymorph of carbon which has a layered crystal structure held together by weak van der Waals forces and thus being easy to shear (27).

It was observed that the friction coefficient of graphite varies between 0.07 and 0.15 depending on test conditions, form of graphite, contact configuration and testing rig. In open air, it can show effective lubrication properties up to temperatures of 400°C. The coefficient of friction tends to increase with temperatures. At higher temperatures, oxidation leads to high friction and the loss of effective lubrication. In dry air, inert atmospheres, or vacuum, the friction coefficient is higher initially and it decreases to 0.2 at 1300°C. It was found out that the lubricity of graphite is not due to only layered crystal structure but it also highly depends on the presence of certain condensable vapors like water (27), although (43) showed that n-heptane and isopropanol are more effective for improving the lubricity of graphite compared to water. This has been considered to be due to saturation of π electrons which cause difficulties on layers sliding. The formation of transfer layers which is thought to be beneficial at protecting against wear has also been observed in many experiments (27).

Cai et al. conducted tribological experiments on Ni-based graphite/CaF

/TiC composite coatings prepared by plasma spray. Tribological tests were done against steel balls with an observed decrease in friction of 25.9%-53% and reduced wear by 18.6%-70.1% compared to pure unlubricated Ni-base alloy coatings. The reduced friction and wear was attributed to the formation of transfer films composed of ferric oxides, graphite and CaF

. The main mechanism for wear was found to be delamination of transferred layer (44).

Zhen et al. performed tribological tests of nickel-based composites containing Ag, BaF

/CaF

with

different amounts of graphite against Si

N

ball at different temperatures from room temperature to

800°C. While the composite with 0.5 wt. % addition of graphite showed the lowest friction coefficient

except at the maximum temperature of 800°C, the composite with 2.0 wt. % graphite showed the lowest

wear rates at temperatures above 200°C (45).

9

Sharma et al. found that the addition of graphite in to the aluminum alloy 6101 provided beneficial effect and it has reduced friction and wear. It was also observed that the minimum friction and wear rate was obtained for the addition of 4 wt. % graphite (46).

1.2.5 Alkaline Earth Fluorides

The fluorides of calcium and barium can provide effective solid lubrication at high temperatures between about 500°C and 900°C, however they do not perform well at lower temperatures (8). The limitation for low temperature lubrication is due to their brittle structure leading to three-body abrasion however, they become good solid lubricant above 500°C due to their brittle-ductile transition (47). Torres et al.

reviewed the current literature and reported that in general, the brittle-ductile transition has occurred above 400°C for fluorides such as CaF

(11)

Ouyang et al. studied the tribological behavior of ZrO

/Al

O

ceramic composites with the addition of solid lubricants such as BaF

, CaF

synthesizing by spark plasma sintering. As expected, it was observed that the ceramic composites with the addition of CaF

and BaF

performed better at temperatures above than 400°C and they performed worse at room temperature (48).

Jin et al. conducted experiments on self-lubricating ceramic matrix composites and observed their tribological behavior against Al

O

counter face material at temperatures from 20°C to 800°C in air using a pin-on-disk configuration. The matrix with 50% CaF

led to a decrease in the coefficient of friction in the temperature range of 200°C and 650°C. The samples at 650°C were investigated under SEM/EDS and the formation of a protective tribolayer could be observed. The increase in friction at 800°C was due to the instability of this tribolayer. It was also observed that CaF

did not perform well at room temperature (49).

1.3 Laser Cladding

The use of coatings is one of the most effective methods to modify the tribological behavior of a system, since the surface properties of the material can be improved with acceptable costs. The thickness of the coating can be an important parameter when choosing the deposition technique. Thin coatings like those which are produced by methods like physical vapor deposition (PVD), chemical vapor deposition (CVD) and electroplating cannot withstand intense mechanical wear. Therefore, laser cladding can be chosen to produce thicker coatings being able to perform longer under severe conditions. Laser cladding also shows important advantages over other coating methods like high efficiency, strong bonding to the substrate and low dilution (50). Furthermore, this technique can be used to re-work and repair high- value components, increasing the product life and preventing high investments to replace a complete part (51). Dubourg and Archambeault (52) reviewed the available scientific publications and patent dataset on this topic for the 22 years period from 1985 to 2007, studying the implementation of laser cladding method and they observed an exponential increase in scientific publications on this technique due to the need for high wear-resistant hard coatings.

Laser cladding is based on the melting of the precursor powder using a high-powered laser beam. The

thickness of the as-deposited coating typically varies between 50 µm and 2 mm. The molten powder

solidifies rapidly to form a coating on the substrate, with good bonding with the substrate due to

localized dilution. Two different approaches can be used for laser claddings; single step which involves

adding material to the melt pool as injected powder or wire. Or two-step, by which the coating material

is applied onto the substrate usually in paste form to be subsequently melted with the laser beam although

in this case it might cause pore formation due to the evaporation of organic binders. Two-step laser

cladding is also time-consuming because of additional process steps and the difficulty of achieving a

10

uniform layer on complex-shaped components, but it is an easier process when the base powder has to be mixed with additional phases like solid lubricants, for instance.

In general, laser cladding is highly sensitive to process parameters. While insufficient energy input causes poor adherence, too high energy might cause the coating to excessively mix with the substrate, causing high dilution. Additionally, the feed rate of the coating material for single step processes is crucial to ensure a uniform coating over the substrate but the process is difficult to control. For instance, the end product might have a rough surface (50). However, using scanned beam instead of static optics can control the beam area and provide flexibility (53). The process allows reproducibility, localized cladding, application on complex-shaped substrates, control of coating adherence, dilution and coating thickness. With those advantages, the technique becomes superior to other methods (50).

Figure 7: Single step (a), (b), (c) and two step (d) laser cladding processes (54)

In Figure 7, the single step process (a), (b), (c) and two-step process (d) are shown. For pre-placed cladding, inert gas (e.g. argon or CO

) is used to transfer the energy to clad and substrate. (55).

As for the use of laser claddings in high temperature tribological applications, several references have been found in the available literature (see for instance (25) and (56)), although this technique is considered to be largely underrepresented in comparison to PVD coatings or powder sintering (11), likely due to the thermal degradation of solid lubricants even under protective atmospheres.

Additionally, many papers deal with laser cladding of Ni-based alloys like self-fluxing nickel as they are easier to melt ( (57) and (58)), neglecting other iron-based alternatives.

1.4 Research Gaps

Taking into account the findings of the previous literature review, it is clear that although a significant

amount of research has been done on self-lubricating materials at high temperature, there are still

relevant fields which have not been properly addressed so far. In particular, the following perceived

research gaps will need to be bridged throughout the present study.

11

▪ Fe-based self-lubricating materials for high temperature applications are underrepresented in the available literature. Iron-based alloys are less costly and could be good replacements for nickel- or titanium-based materials which currently hold the trend for the most researched laser cladding materials according to (52).

▪ In this study, flat pins were used as the counter body. It must be noted here that the use of spherical counter bodies (Hertzian contact) is prevalent in the available literature, leading to unrealistically high contact pressures during tribotesting. However, the choice of a flat-on-flat contact geometry is expected to be closer to metal forming applications.

▪ The tests were performed against metallic (bearing steel and aluminium) pins in order to be close to metal forming, although the use of ceramic-based counter bodies is widely reported in the literature.

▪ No published literature on aluminium alloys sliding against laser claddings at high temperatures has been found. This research could give interesting results for the potential application of hot stamping of aluminium alloys, which is gaining interest in recent years.

▪ Wear/damage to the counter body is not well studied or even reported in many publications.

1.5 Aim and Objectives

The aim of the project is to improve the tribological behaviour in metal forming applications by evaluating the effect of solid lubricants incorporated into iron- and nickel-based laser claddings by testing them against steel and aluminium counter bodies. The desired tribological properties can be defined as low wear of the material in addition to low and stable coefficient of friction combined to decreased damage of the counter body, under conditions as close as possible to those found in metal forming.

The objective of the project involves evaluating iron-based hardfacing materials with the addition of different solid lubricants sliding against steel counter bodies at temperatures including room temperature (RT), 450°C and 600°C. In addition, nickel- and iron-based hardfacing materials with the addition of different solid lubricants sliding against aluminium at 300°C will also be evaluated, within the range of temperatures expected on the tool’s surface during processes such as hot stamping. In this case, the combination of silver and MoS

as solid lubricants will be evaluated at high temperature as it is considered in the available literature to be highly effective. To this end, friction data during tribotesting will be evaluated in addition to measuring the resulting wear in the samples. Moreover, SEM/EDS characterization will be performed on both hardfacing and counter body materials to investigate the microstructure and the worn surfaces.

2. Experimental Procedure

2.1 Materials

In this study, laser cladding was chosen for the preparation of the self-lubricating coatings, due to the

good quality of the resulting claddings and their excellent bonding to the substrate. An iron-based alloy

coating was selected for the tests against steel counter bodies, due to lower costs than previously studied

nickel-based claddings. Moreover, nickel- and iron-based claddings were also tested against

representative aluminium alloys. The claddings were prepared using a single-pass diode laser on a pre-

placed paste formed by a powder mixture with ethanol as the binder. Argon gas was also used as a

protective atmosphere to prevent the oxidation of the coatings during laser cladding. AISI 304 stainless

steel plates were used as the substrate to prevent oxide scale formation during preparation or tribotesting.

12

After deposition, the claddings were machined to a flat surface with a thickness of the resulting coatings between 0.5 mm and 1 mm.

Prior to testing against steel, the resulting coatings were further manually ground with grit #360 and

#600 abrasive paper to achieve a consistent surface roughness with no preferential orientation. The surface roughness was measured using a Zygo New ViewTM 7300 3D optical profiler. Finally, the materials were cleaned in an ultrasonic bath using heptane and rinsed with acetone prior to testing.

The chosen Fe-based powder for coating was the commercially available Oerlikon Metco 42C, whose chemical composition is detailed in Error! Reference source not found.. The high Cr-content was e xpected to improve oxidation resistance at high temperatures.

Table 1: Chemical composition of the coating base alloy Metco 42C (wt. %)

Cr Ni C Fe

17 2 0.18 Balance

The chosen Ni-based powder used for testing against aluminium alloys contains Cr which improves the corrosion and oxidation resistance, in addition to B and Si which reduce the melting temperature of the powder mixture. The composition of the Ni-based alloy used for tests against aluminium is shown in Table 2.

Table 2: Chemical Composition of the Ni-based coating (wt. %)

Cr Si B C Ni

4 2.5 1 0.2 Balance

The hot work tool steel 1.2367 (Böhler W303) was chosen as a reference to compare with the claddings with a commercial benchmark material in both steel and aluminium contact tests. The as-delivered martensitic steel was heat treated with two tempering steps (1 hour at 530°C and 1.5 hours at 490°C), both followed by air cooling. The composition of the tool steel is presented in Table 3.

Table 3: Chemical composition of the grade 1.2367 tool steel commercial material (wt. %)

C Si Mn Cr Mo V Fe

0.38 0.4 0.4 5 2.8 0.55 Balance

2.1.1 Specimens for tests against steel

Samples of unmodified Metco 42C were deposited to be used as the reference material during testing.

Additionally, MoS

and Ag were chosen as solid lubricants and mixed with the base powder to prepare self-lubricating claddings, whose tribological behavior at high temperature will be compared to that of the reference alloy. Moreover, the grade 1.2367 tool steel was chosen to compare with the claddings.

The three different chemical compositions of the coatings and the tool steel to be studied are detailed in

Table 4. Counter body pins were manufactured from AISI52100 cylinder rollers, machined to a diameter

of 2 mm and with an as-delivered hardness of 62 – 66 HRC. Grit #600 abrasive paper was also used to

grind the edges to reduce the edge effect during testing.

13

Table 4: Composition and surface roughness of the resulting claddings used for tests with steel

Type of cladding Addition of solid lubricant [wt. %]

Ra roughness [µm] Hardness [HV1]

Metco 42C Unmodified. 0.07 ± 0.02 535 ± 6

Metco 42C - 10 MoS

10% MoS

0.11 ± 0.03 538 ± 15

Metco 42C - 5 Ag - 10 MoS

5% Ag, 10% MoS

0.18 ± 0.06 542 ± 30 Grade 1.2367 tool steel Unmodified 0.09 ± 0.02 684 ± 8

2.1.2 Specimens for tests against aluminium

The two best performing self-lubricating claddings were chosen for aluminium contact tests. In this case, tool steel samples were used again to compare cladding performances with a commercial product used for the manufacture of high temperature tools. The two different chemical compositions and tool steel to be studied are shown in Table 5. In this case, an iron-based with a slightly decreased MoS

-content was used this time, as it had been found to lead to an optimised microstructure.

Table 5: Composition and surface roughness of the resulting claddings for tests with aluminum

Type of cladding Addition of solid lubricant [wt. %]

Ra roughness [µm] dry contact

Ra roughness [µm] lubricated contact

Hardness [HV1]

Metco 42C -5 Ag - 5 MoS

5% Ag, 5% MoS

0.14 ± 0.02 0.14 ± 0.02 500 ± 16 Ni-Based - 5 Ag - 10

MoS

5% Ag, 10% MoS

0.15 ± 0.00 0.23 ± 0.05 386 ± 37 1.2367 tool steel Unmodified 0.06 ± 0.02 0.09 ± 0.02 684 ± 8

For the dry reciprocating tests, the samples were grinded as in the previous case with grit #360 and #600 abrasive paper rotating them in the latter step to achieve no preferential orientation.

Lubricated tests were also performed due to the severity of the contact under dry condition. To increase the adhesion of the commercial solid lubricant, the samples were grinded on one direction with grit #360 abrasive paper, with resulting Ra roughnesses of 0.14 ± 0.02 , 0.23 ± 0.05 and 0.09 ± 0.02 µm for Metco 42C - 5 Ag - 5 MoS

, Ni-based - 5 Ag - 10 MoS

and the tool steel respectively. Molykote D – 55, a commercial solid lubricant containing graphite and CaF

, was used for lubrication purposes. Molykote was mixed with a binder and acetone and applied by spraying on to the surface. Commercial epoxy glue was used as a binder. The lubrication binder was prepared by first adding 10 wt.% binder in 80 wt. % acetone, then mixing it with 10% wt. Molykote lubricant. The resulting layer was cured in a furnace at 180°C for 15 minutes prior to testing. The measured average thickness of the lubricant layer was (80 ± 20) µm for Ni-based - 5Ag - 10 MoS

, (60 ± 30) µm for Metco 42C - 5 Ag - 5 MoS

and (70 ± 20) µm for the reference hot work tool steel. The Ra roughness of the lubricant layer was measured to be (1.01

± 0.08) µm. The SRV tests were performed perpendicular grinding direction in order to observe the worst-case scenario.

The counter body pins were manufactured from AA6082 and AA2007 aluminium alloys, due to their

interest for automotive and aerospace applications. The chemical composition of these chemicals are

presented in Table 6.

14

Table 6: The chemical composition (weight%) of AA6082 and AA2007 Al alloys

Alloy Mn Fe Mg Si Cu Zn Cr Ti Pb Al

AA6082 0.4-1 0-0.5 0.6- 1.2

0.7-

1.3 0-0.1 0-0.2 0-0.25 0-0.1 - Balance AA2007 0.5-1 0-0.8 0.4-

1.8 0-0.8 3.3-

4.6 0-0.8 0-0.1 0-0.2 0.8-1 Balance

2.2 Procedure

The tribological tests in the present study were performed by using an Optimol SRV high temperature reciprocating friction and wear tribometer. The temperature of the samples was measured on the surface using a thermocouple in order to obtain accurate values. At least three repetitions were done for each material combination. The tests were conducted under dry condition against steel and both dry and lubricated condition in case of aluminium. The flat pin-on-disk configuration was used to provide contact pressures closer to metal forming processes. The diameter of the contact with the coatings was 2 mm. The testing configuration is shown in Figure 8. The other test parameters were the same for both set of tests and are summarised in Table 7, including the calculated contact pressures of 16 MPa for the steel tests and 6 MPa for the aluminium, close to the parameters reported in the literature. Sliding speeds were chosen to be close to the values found in the literature for hot stamping (59) (5).

Table 7: Test parameters for SRV tribotest

Contact Steel Aluminium

Normal Load 50 N (16 MPa) 20 N (6 MPa)

Sliding Frequency 12.5 Hz (0.1 m/s for 4 mm stroke) 5 Hz (0.04 m/s for 4 mm stroke)

Test Duration 900 s 15 s (dry) 1800 s (lubricated)

Temperatures RT, 450°C, 600°C 300°C

Figure 8: Pin-on-disk configuration

After testing, the same cleaning procedure was followed, with ultrasonic bath with heptane and rinsing with acetone except for those samples used in lubricated contact tests. The wear scar and wear volume of coatings were measured by using an Alicona InfiniteFocus 3D optical profilometer, as weight loss measurements can be inaccurate at high temperatures due to sample oxidation. The resulting wear rates were subsequently calculated using Archard’s equation:

𝑤 =

𝐹∙𝑠

where w is the wear rate in mm

/Nm, V is the measured wear volume in mm

, F is the normal load in N

and S is the sliding distance in meters. The estimation for pin wear, on the other hand, was done by

15

measuring the height of pin with a caliper. SEM and EDS measurements were also performed on selected wear scars using a JEOL JSM IT-300 and JEOL JCM 6000 scanning electron microscopes coupled with an Oxford Instruments EDS for elemental analysis. The pin surface was further investigated with Thermo Scientific Theta Probe XPS spectrometer with a monochromated, micro- focused Al K-alpha X-ray source to gain information about the tribofilm which was created during in case of tests against aluminium.

3. Results and Discussion

3.1 Microstructure prior to testing

The microstructure of the as-deposited claddings both for Fe-based and Ni-based turned out to be crack and pore-free, showing a good bonding and low dilution with the substrate as commonly observed for laser-cladded alloys and could be confirmed in this case by means of optical microscopy on cross sections of the as-deposited coatings. The microstructures of the as-deposited claddings are shown in Figure 9, Figure 10 and Figure 11, as it will be further detailed.

Figure 9: Optical microscopy imaging of the as-deposited claddings: a) a detail of the etched microstructure of Metco 42C, b) Metco 42C - 10 MoS2 and c) Metco 42C - 5 Ag - 10 MoS2

16

Figure 10: SEM imaging of a) Metco 42C - 10 MoS2 and of b) an aggregate found in Metco 42C - 5 Ag - 10 MoS2

In particular, the microstructure of the reference Metco 42C samples is shown in Figure 9a, as seen from an etched sample. The dark grey regions consisted of martensite, which explains the high hardness of all of the deposited claddings (Table 4), although some retained austenite remained after sample preparation (bright regions). As for the self-lubricating claddings, Figure 9b shows the microstructure of Metco 42C - 10 MoS

, with abundant darker second phases spread across the cladding, related to the addition of MoS

solid lubricant. Figure 9c corresponds to Metco 42C - 5 Ag - 10 MoS

. In this case, brighter phases considered to be silver-based were observed encapsulated in the darker, MoS

-based agglomerates. This mechanism was expected to prevent silver from floating to the surface of the claddings during laser melting, ensuring a uniform distribution of the solid lubricants (60)

Further characterization of the as-deposited self-lubricating claddings was done by means of SEM/EDS.

Chemical analysis performed by EDS on a large aggregate found in Metco 42 – 5 Ag - 10 MoS

(Figure 10b) is summarized in Table 8. Spot A shows a high Cr and S content, attributed to the formation of chromium sulfides from the thermal decomposition of MoS

during cladding. The resulting Cr

S

phases were considered to be effective high temperature solid lubricants (60). Spot B corresponded to a silver- rich particle.

Table 8: Composition as measured by EDS of the spots marked in Figure 10b

Spot Composition (wt. %)

Fe Cr S Ag

A 41.8 23.4 18.4

B 10.9 24.6 23.5 13.2

Additional chemical characterization was performed by means of EDS on the cladding matrix (not in

the second phases) for all of the three coatings. The results are given in Table 9. Interestingly, a decrease

in the chromium content in the matrix was found for both self-lubricating claddings, from values close

to 18 wt. % for the reference material to 8 wt. % in Metco 42C – 5 Ag – 10 MoS

. This was attributed

to the depletion of chromium during the formation of Cr

S

phases from MoS

during laser cladding and

could affect the tribological behavior of the coatings during tribotesting.

17

Table 9: Chemical composition of the cladding matrix as measured in the as-deposited samples

Material Composition (wt. %)

Fe Cr S Ni

Metco 42C 68.6 18.7 6.9

Metco 42C - 10 MoS

83.8 8.8 1.6

Metco 42C - 5 Ag - 10 MoS

85.6 8.2 1.8

Figure 11: Optical microscopy of Ni-based – 5 Ag - 10 MoS2

Similarly to Fe-based cladding with 10% MoS

and 5% Ag, the Ni-based cladding sample showed a similar microstructure featuring the formation of chromium sulphides from MoS

during sample preparation in addition to the encapsulation of silver, as seen in Figure 11. This microstructure has been discussed in further details in a previous master thesis (61).

3.2 Tribological tests of Fe-based claddings against steel 3.2.1 Friction Results

The measured coefficient of friction values for all four material pairs are shown in Figure 12. It was observed that the addition of MoS

to the Fe-based coatings reduced the friction coefficient and the addition of silver was helpful in reducing initial friction peaks at room temperature up to 200 seconds.

The reference alloy was observed to stabilize at high friction above 1.1, with a similar tribological

behavior at RT compared to the tool steel samples.

18

Figure 12: The friction coefficient results of Metco42C-based samples at different temperatures a) RT, b) 450°C c) 600°C

a)

b)

c)

19

At 450°C, the addition of MoS

or Ag did not affect the friction coefficient results, with very similar friction for all claddings (~0.5), and the only noticeable difference being an initial friction spike for the unmodified alloy. At the maximum temperature of 600°C, friction behavior was more unstable and less repeatable for all three iron-based claddings, although the self-lubricating Metco 42C – 10 MoS

and Metco 42C – 5 Ag – 10 MoS

were observed to reduce the friction coefficient at 600°C compared to the reference material. The unmodified alloy showed unstable friction as high as 1.25. Moreover, a high variation in friction coefficient results could be observed for the repetitions several Fe-based claddings at 600°C, as it can be observed in Figure 13, Figure 14 and Figure 15. The commercial benchmark tool steel showed surprisingly better results in terms of friction at this temperature compared to the iron- based claddings.

Figure 13: The variation in friction for Metco42C reference samples at 600°C

20

Figure 14: The variation in friction for Metco42C – 10 MoS2 samples at 600°C

Figure 15: The variation in friction for Metco42C – 5 Ag – 10 MoS2 samples at 600°C

21 3.2.2 Wear Behaviour

The measured wear is given in Figure 16 for the claddings and in Figure 17 for the counter bodies. In general, wear of both the self-lubricating claddings and the counter bodies was lower at RT compared to what was observed when testing the unmodified reference material. The combined addition of Ag and MoS

showed better results than Metco 42C - 10 MoS

at room temperature, more than halving the wear rates calculated for Metco 42C - 10 MoS

. It was also observed that the tool steel sample was better in terms of wear compared to other claddings at RT.

Increasing the temperature to 450°C led to a completely different wear behavior. Both self-lubricating claddings showed negligible wear, while the reference alloy experienced low material loss. On the other hand, counter body wear was much larger than at RT for all of the tested claddings. Interestingly, the unmodified reference alloy led to lower pin wear than the self-lubricating coatings, about one third lower (1.2 10

mm³/Nm, compared to 2.0 10

mm³/Nm for Metco 42C - 10 MoS

). The commercial tool steel material showed similar pin wear to that of Metco 42C – 10 MoS

.

At 600°C, while Metco 42C – 10 MoS

and Metco 42C – 5 Ag – 10 MoS

showed low cladding wear results, the wear of the counter body was still high, in line with what had been observed at 450°C although with a larger scatter. The reference alloy, on the other hand, showed an opposite behavior at 600°C: large cladding wear, more than one order of magnitude larger than its self-lubricating counterparts (above 2.0 10

mm³/Nm), and comparatively low pin wear close to 3.0 10

mm³/Nm.

Although in this case, the addition of solid lubricants was considered to protect the cladding, it could not prevent the pin from wearing off. Lastly, the tool steel had the lowest sample wear although the pin wear was the highest for this material (close to 2.5 10

mm³/Nm).

Taking all the previous into account, it can be said that some mechanism took place which decreased cladding wear at the expense of the pin, a process which might be related to the softening of bearing steel above 500°C (62). It is also counterintuitive that the material with the highest chromium content in the matrix (the reference Metco 42C alloy, as shown in Table 4) would experience such high wear at 600°C as in general chromium is expected to reduce the onset of oxidational wear at high temperature.

Figure 16: Specific wear rates of claddings and the reference tool steel

22

Figure 17: Specific wear rates for steel-based counter bodies

3.2.3 SEM/EDS characterization

In order to cast further light into the different tribological behavior observed for the chosen materials, further characterization of the wear scars was performed by means of SEM/EDS. The lower friction and wear observed for Metco 42C – 5 Ag - 10 MoS

at RT have been attributed to the observed smearing of silver along the wear scar, as shown in Figure 18. This mechanism was considered to be beneficial for the tribological behavior of the cladding at RT, as silver can accommodate between the sliding surfaces and reduce friction.

Figure 18: a) Overview and b) a detail of the smearing of silver observed by SEM in Metco 42C - 5 Ag - 10 MoS2 at RT

As previously described, at 450°C the friction coefficient was almost the same for all different materials

tested. Therefore, the wear scars of the reference alloy Metco 42C and Metco 42C - 10 MoS

were

observed under SEM to cast further light into the mechanisms involved. Figure 19 shows the surfaces

for those two materials under SEM. For those samples, the wear in the cladding was low while it was

high for the pin.

23

The SEM results showed transfer layers made of iron oxides from the counter body (spot A in Figure 20, Table 10), as the low Cr and Si contents suggests this layer was made of oxidised bearing steel. In this case, tribolayer formation resulted in approximately the same friction coefficient for all of the claddings as those oxides controlled the frictional behavior during testing, regardless of solid lubricant addition. With this fact in mind, additional tests with a shorter duration of 30 seconds were performed with the same parameters (including a temperature of 450°C) to see the cladding performance with decreased oxide layer formation. Although the oxide transfer was clearly reduced (Figure 20), the test duration was enough to allow for significant tribolayer formation and no changes in tribological behaviour were observed for the three different iron-based claddings.

Figure 19: SEM results for 450°C for a) Metco 42C and b) Metco 42C - 10 MoS2

Figure 20: The observation of the transfer layer under SEM for 30 seconds at 450°C tests for Metco 42C - 5 Ag - 10 MoS2

Table 10: Composition as measured by EDS of the spot marked in Figure 20

Spot Composition (wt. %)

Fe Cr Si O

A 69.5 1.8 0.2 23.1

24

At 600°C, high variation in friction and in wear was observed. SEM/EDS characterization of self- lubricating Metco 42C - 10 MoS

and Metco 42 - 5 Ag - 10 MoS

samples was performed to gain further insight. It was observed that the main wear mechanism differed among samples of the same cladding.

This might be due to the fact that adhesion and the formation of oxides is a random process with low repeatability, leading to different wear scar morphologies.

Figure 21: SEM image of Metco 42C - 10 MoS2 1.trial in Figure 14

Figure 22: SEM image of Metco 42C - 10 MoS2 3.trial in Figure 14

Although the samples in Figure 21 and in Figure 22 were both Metco 42C - 10 MoS

they showed different mechanism, which might be the reason for the observed high scatter in friction and wear data.

The SEM image in Figure 21 indicates a significant contribution from abrasive wear and ploughing while in Figure 22 corresponds mostly to a random adhesion mechanism. The friction and wear results indicate that higher friction and wear was observed where abrasion played a main role: while the friction coefficient was above 0.75 and the wear rate was 2.67 10

mm

/Nm for the sample in Figure 21, the friction coefficient was close to 0.5 and wear 3.89 10

mm

/Nm for the sample in Figure 22. A similar behaviour could also be observed for Metco 42C - 5 Ag - 10 MoS

samples after testing at 600°C. While the friction coefficient was above 0.75 for the abraded sample in Figure 23, the sample in Figure 24 shows higher friction coefficient in the first 200 seconds and then the friction decreases and stabilizes around 0.5 with significant adhesive wear. The specific wear rate for trial 3 Metco 42C - 5 Ag - 10 MoS

sample was 2.67 10

mm

/Nm, while it decreased to 7.53 10

mm

/Nm for the trial 1 Metco 42C - 5 Ag

- 10 MoS

sample.