To my family, who I love dearly…

List of included papers

I D.H.E. Persson, S. Jacobson and S. Hogmark, Antigalling and low friction properties of a laser processed Co-based material. Journal of Laser Applications, 2003. 15(2): p. 115-9.

II D.H.E. Persson, S. Jacobson and S. Hogmark, The influence of phase transformations and oxidation on the galling resistance and low friction behaviour of a laser processed Co-based alloy. Wear, 2003. 254(11-12): p. 1134-40.

III D.H.E. Persson, S. Jacobson and S. Hogmark, Effect of tempera-ture on friction and galling of laser processed Norem 02 and Stellite 21. Wear, 2003. 255(1-6): p. 498-503.

IV D.H.E. Persson, E. Coronel, S. Jacobson and S. Hogmark, Sur-face analysis of laser cladded Stellite exposed to self-mated high load dry sliding. Wear, in press.

V D.H.E. Persson, L. Hammerström, S. Jacobson and S. Hogmark, The Micro Marker Technique - a new method to study deforma-tion of tribological surfaces. In manuscript.

VI D.H.E. Persson, S. Jacobson and S. Hogmark, A physical model for the superior tribological performance of Stellites in highly loaded sliding contacts. In manuscript.

Papers I-IV are reproduced with permission from the publishers.

The author’s contribution

The author has been the principal writer of all the included papers and per-formed all experiments and analyses, excluding the TEM work in papers IV and VI.

Contents

1 Introduction .........................................................................................111.1 The science of tribology and its economical impact ..................121.2 Metallic materials suitable for sliding contact............................12

Sliding contact ............................................................................12Abrasive contact..........................................................................13“Trouble makers”........................................................................13Surface damage by galling..........................................................14

1.3 How to avoid surface damage ....................................................141.4 The optimum low friction system...............................................141.5 Aim of this thesis........................................................................15

2 Properties of Stellites...........................................................................162.1 Phase composition and crystallographic orientation ..................172.2 Stress induced phase transformations.........................................172.3 Wear resistance...........................................................................172.4 Oxides.........................................................................................182.5 Laser processed Stellites ............................................................182.6 The problem of Stellites in nuclear environments......................21

3 Tribological testing..............................................................................233.1 Test motivation – simulation of gate valves...............................233.2 The Load-Scanner ......................................................................243.3 Test setup for low nominal contact pressures.............................263.4 Evaluation techniques.................................................................26

4 Tribological behaviour of Stellites (Papers I, II and IV-VI)................274.1 Macroscale deformation and friction..........................................274.2 Shear stress induced fcc to hcp phase transformations...............294.3 Formation of the easy shear layer – self-aligning of hcp basal

planes..........................................................................................314.4 Accumulated deformation and wear...........................................324.5 Friction at low nominal contact pressures ..................................364.6 Influence of oxide layer formation .............................................364.7 Stellite in sliding contact with other materials ...........................37

5 Candidates for Stellite replacement in nuclear power plants (Paper III) . .............................................................................................................38

6 Conclusive summary ...........................................................................40

7 Speculations regarding materials for Stellite replacement...................42

8 Sammanfattning (Summary in Swedish) .............................................45

9 Acknowledgements..............................................................................47

10 References ...........................................................................................49

Abbreviations

bcc Body-centered cubic BWR Boiling water reactor CVD Chemical vapor deposition EDS Energy dispersive x-ray spectroscopy fcc Face-centered cubic FIB Focused ion beam hcp Hexagonal closed-packed HV Vickers hardness number LOM Light optical microscopy RT Room temperature SEM Scanning electron microscopy TEM Transmission electron microscopy XPS X-ray photoelectron spectroscopy XRD X-ray diffraction

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1 Introduction

This thesis reveals the tribological mechanisms behind the intrinsic low fric-tion potential of the Co-based family of alloys called Stellites1. These are widely used as hardfacing materials for components active in severe tri-bological environments, e.g. turbine blades, bearings, spindles and valves.

The selection of Stellites is promoted by properties such as abrasion and corrosion resistance, high-temperature strength, and last but not least, an excellent low-friction performance during unlubricated self-mated sliding under high nominal contact pressures. For instance, Stellite is commonly used as the sealing surface material of gate valves in nuclear power plants. The material exhibits an excellent tribological function under the conditions of high temperature, pressure and humidity prevailing during an emergency situation, thus securing the function of the valve. However, the Stellites have one large drawback in this application. The main part of the background radiation from a boiling water reactor (BWR) primary circuit originates from the Co60 isotope. This isotope is a result of activation of Co59 in wear and corrosion fragments from Stellite. Consequently, the development of a Co-free alloy replacing the Stellite on critical components in nuclear power plants is desired.

This was the ultimate challenge when the research of this thesis was planned. A Co-free material would significantly lower the background radia-tion in the vicinity of the primary circuit, leading to an improved working environment, e.g. for maintenance.

Numerous investigations of Stellites with respect to their tribological properties in the areas of abrasion [1-3], adhesion [2, 4, 5] and erosion [2, 6] have previously been conducted. However, though being an established and important group of materials, a satisfactory explanation to why they exhibit low-friction properties under severe sliding conditions was not found in the literature.

The main part of this thesis is dedicated to clarify the complex mecha-nisms defining the tribological performance of Stellites in sliding contact. This was performed by combining information from a series of tribological tests and modern high-resolution analysis, with classical low friction theories [7, 8]. Investigations of a potential Co-free material for Stellite replacement were also performed. All specimens were produced by laser cladding, result-

1 Registered trademark of Deloro Stellite.

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ing in a finer microstructure and more homogenous properties of the materi-als compared with conventional welding. The commonly used Stellite 6 and Stellite 21 were chosen as primary test materials.

1.1 The science of tribology and its economical impact Originating from the Greek word tribos, which means to rub, the science of tribology involves the physical, chemical and mechanical processes triggered in the interface between two surfaces in mechanical contact. Further, it in-cludes the understanding and optimisation of the mechanisms behind friction and wear between surfaces in sliding contact.

From an industrial point of view, the economic impact of understanding tribology is significant. About 15 years ago, it was estimated that 1.5 % of the gross national product would be saved by paying proper attention to tri-bology in education and research, and then applying the knowledge in indus-trial applications [9].

Looking into a more everyday example: If the friction losses in a combus-tion engine would be reduced by 10 %, the fuel consumption would corre-spondingly decrease with 1.5 % [10]. For a vehicle driving a distance of 20 000 km in one year and consuming an average of 0.1 l/km this would consequently save 30 l of petrol per year. A further consequence is less con-sumption of natural resources and reduced emissions. The economical and environmental gains of applying tribology knowledge on a worldwide scale are evident.

1.2 Metallic materials suitable for sliding contact Almost all metallic materials find use in tribological applications. However, through industrial development and research, different categories of materi-als have been optimised for various applications [11, 12].

Sliding contact Bearing alloys based on Cu are examples of metallic materials with the po-tential to give low friction in sliding contact. They also have a reasonably high loading capacity combined with necessary compliance. Cu-based alloys make use of their high thermal conductivity. They are typically alloyed with small quantities of “solid lubricants” in the form of Sn, Pb or Sb. Grey cast irons are another type of metallic materials with similar properties. They are the most commonly used materials in cylinders of automotive engines. Flake like graphite inclusions serve to give low friction.

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Any type of steel can also be a candidate for sliding contact. If high con-tact pressures are expected the surface should be nitrided to reduce the fric-tion and increase the resistance against adhesive wear.

Another class of materials often used in severely loaded sliding contacts are Stellites. Their advantage is a potential for low friction and a high resis-tance to adhesive and abrasive wear in combination with high thermal resis-tance. The intrinsic tribological properties of Stellites are the main theme of this thesis.

Abrasive contact In situations where the sliding contact involves hard particles, or when slid-ing against a harder material with a rough surface, a material with high abra-sive resistance is desired. The abrasive wear resistance typically increases with increasing hardness. Consequently, hardened steel is a very common choice in applications such as excavator and road construction equipment. Severe abrasive attack may require a material strengthened with hard parti-cles (e.g. carbides). Examples of metallic materials that make use of large volume contents of carbides are tool steels and white cast irons. If the latter are alloyed with Cr to give a network of Cr-carbides, their abrasion resis-tance is further improved. High carbon Stellites have a similar Cr-carbide network, but in a Co-based matrix. They are often used when a combination of heat resistance and abrasive resistance is needed.

Manganese-alloyed steels (Mn steel or Hadfield steel) are another exam-ple of abrasion resistant materials. Thanks to their potential to transform from a relatively ductile austenitic to a hard martensitic structure during surface deformation, they are used in heavy abrasive situations such as jaws of rock crushers. Thus, the ideal compound of a ductile bulk with a hard surface layer is self-generated.

“Trouble makers” Finally, it should be mentioned that some metallic materials are almost al-ways giving trouble in the form of high irregular friction and severe wear in sliding contacts. Common examples are austenitic stainless steels, Al-, Ti-, and Ni-based alloys. For the stainless steels, Ti- and Ni-alloys the problem arises due to a relatively thin oxide layer (friction is almost always reduced by the presence of oxides in the interface [13]) in combination with a strong work hardening ability, high fracture elongation, and low thermal conductiv-ity.

The problem of Al-alloys is that the oxide (Al2O3) is much harder than the Al-alloy itself. The oxide layer fractures easily and spalls off. Oxide frag-ments entrapped in the counter surface will severely abrade the soft Al-alloy during sliding.

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Surface damage by galling Common examples of materials showing gross surface damage when ex-posed to high loads in sliding contact are the austenitic stainless steels. For instance, during sheet metal forming they have a tendency to adhere to the tool surface in the form of deformation hardened patches. These will act as abrasives in consecutive forming operations, generating indents and scratches in the metal sheet. This sequence is usually named galling.

Which are the mechanisms behind this process? When the lubricant fails to separate the surfaces, the sheet metal will adhere to the tool surface [14]. As sliding continues, an increase in shear stress will affect both materials. Shear will deformation harden the surface layer of the austenitic stainless steel, primarily by mechanical twinning and dislocation interactions. Finally the level of shear stress will be so high that shear fracture will be initiated between the work hardened surface layer and the softer bulk material. The hard patches generated may adhere to the tool surface. However, the shear stress will not be high enough to induce any significant plastic deformation of the harder tool surface.

1.3 How to avoid surface damage To avoid surface damage in sliding contact shear must be restricted to the interface between the mating materials, i.e. the shear resistance in the inter-face must be lower than that of the adjacent material in the mating surfaces.

The most commonly used solution is to add a lubricant to the system. This will restrict the shear to the lubricant, since it has the lowest shear resis-tance in the system. As a consequence, the shear stress affecting the materi-als will be significantly reduced, and thus, the friction is reduced and surface damage is avoided.

However, for many reasons, lubrication may partially fail, or it may not even be allowed for the system. Therefore, in an optimum system the mate-rials should have intrinsic properties that spontaneously create a contact that restricts the shearing to the interface, even without lubricants.

1.4 The optimum low friction system A system that minimises the shear stresses that affect the surface will also exhibit low-friction properties. This thesis will reveal that the beneficial tribological properties of Stellite follow the classical low friction theories proposed more than 50 years ago by Bowden and Tabor [7]. They concluded that an optimum low friction system has two basic properties:

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A hard bulk, providing a small area of real contact, and a thin and easily sheared surface layer.

With this combination, the real contact area will be both small and easily sheared, and consequently the system will give low friction. A common way to design such a system is to apply a soft coating or a solid lubricant on a hard bulk material.

1.5 Aim of this thesis The aim of this thesis is to explain the mechanisms behind the tribological performance of Stellites in sliding contact. The physical model presented should assist in the development of new Stellites as well as Co-free alterna-tives.

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2 Properties of Stellites

The original Stellite was developed in the beginning of the 20th century by Elwood Haynes and consisted of Co with Cr as the single alloying element [2]. Later, additions of W, Mo and C improved the wear characteristics, and further modifications by adding Ni, Fe, C, Si and B developed other Stellites for use in high temperature and high-impact wear situations. The nominal compositions of two commonly used Stellites are given in Table 1 and their mechanical and physical properties compared to those of pure Co in Table 2.

Table 1. Nominal alloying content (wt %) of Stellite 6 and 21.(1)

Co Cr C W Mo Si Mn Ni Fe

Stellite 6 Bal. 28 1.2 4.5 1.1 1 <3 <3 Stellite 21 Bal. 27 0.25 5.5 1.5 1 2.5 <3 (1) Stellite 6 and Stellite 21 were chosen as primary test materials in this thesis.

Table 2. Typical values of mechanical and physical properties of Stellite 6 and 21.

Co Stellite 6 Stellite 21

Hardness (1) [HV] 125 400 (2)

550 (3)320 (2)

450 (3)

Modulus of elasticity [GPa] 210 210 210 Yield strength [MPa] 550 500 Ultimate tensile strength [MPa] 250 900 700 Fracture elongation [%] 1 9 Thermal conductivity [W/m·K] 69 14.8 14.5 Density [kg/m3] 8900 8460 8340 Melting point [°C] 1495 1350 1280 (1) As a rule of thumb an increasing level of C in Stellites will form more carbides, and conse-quently increase the hardness. The hardening of Stellites involves both solid-solution harden-ing and carbide precipitation [15]. (2) Conventionally welded. (3) Laser cladded.

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2.1 Phase composition and crystallographic orientation A dendritic structure is obtained when Stellites are cast or deposited by con-ventional welding

Stellite 6 mainly consists of Cr-rich M7C3 carbides in a solid solution dominated by Co (M represents W as well as residual elements). The inter-metallic phases Co7W6 and Co3W and M23C6 carbides have also been de-tected [2, 4, 16, 17].

Stellite 21 has a significantly lower C content than Stellite 6 and conse-quently forms less carbides, cp. Table 1. Further, it contains Mo instead of W, and therefore consists of a Co-rich solid solution mainly containing the intermetallic phases Co3Mo and Co7Mo6 and smaller amounts of the carbide M23C6 [18].

2.2 Stress induced phase transformations In contrast to pure Co, which is transformed from a face-centered cubic (fcc) to a hexagonal closed-packed (hcp) structure when cooled below 417 ºC [19], the Co-rich solid solution of the Stellites retains its fcc phase even at room temperature (RT), due to its alloying elements. However, if subjected to sufficiently high stresses, the metastable fcc phase transforms to hcp [2, 6]. The transformation is considered as martensitic and occurs by shear [2]. The stacking fault energy should be minimised if the fcc to hcp phase transformation is to be facilitated. Additions of Mo, W and Cr stabilise the hcp phase, while Ni, Fe, Mg and C increase the stacking fault energy in Stel-lites [2, 15]. As will be discussed later, the formation of hcp is essential for the beneficial tribological behaviour of Stellites. Therefore, substrate inter-mixing should be kept to a minimum if the substrate contains Fe, Ni or any other element [4, 15, 20].

2.3 Wear resistance When comparing different Stellites, it was found that an increase in hardness and carbide content is beneficial for the abrasive wear resistance [1, 3, 21].

Attempts to lower the Co content in Stellite 6 have been performed. The properties of the modified materials have been satisfactory in all but one area, namely galling resistance [22]. This implies that Co plays an important role in the tribological behaviour of Stellites in sliding contact.

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2.4 OxidesCr and Co, which are the main elements of Stellites, can form a number of oxides of which Cr2O3 and CoO dominates. The oxides are very tough and strongly bonded to the matrix [23, 24].

2.5 Laser processed Stellites All materials evaluated in this thesis were produced by laser cladding. The laser process utilises gas-atomised powder that, with the assistance of a laser beam, is locally melted onto a substrate material in a controlled atmosphere, see Fig. 1. To create a metallically bonded clad layer the substrate was melted to a depth about 50 µm. The shallow melt depth and the efficient cooling provided by the substrate material combine to create a clad layer with desired composition and a fine microstructure [25]. Multiple clad layers can be deposited. Comprehensive research has been performed in the area of laser cladding of Stellites (mainly Stellite 6) [25-30].

Laser cladding is performed on a variety of industrial components, e.g. valves and shafts. Competing processes as thermal spraying and conven-tional welding are usually outperformed by the laser cladding process with respect to adhesion of the deposited layer, homogeneity of the deposited material and stability of the component geometry. In addition, laser cladding uses a low, well-controlled heat input and hence low level of substrate in-termixing compared to conventional welding [4].

Stellites solidify in a fine scaled dendritic structure when laser cladded, see Figs. 2-4. The fine structure is a result of a rapid solidification process due to the small amount of material to be solidified (about 10 mm3), and the effective thermal conduction of the substrate. The hardness of laser cladded Stellites is, consequently, higher than for that of conventionally welded [4], see Table 2.

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a) b) Figure 1. Principles of the laser cladding process. The laser beam is focused to melt the metal powder added onto the substrate. A flow of inert gas (e.g. Ar or He) carries the metal powder and simultaneously protects the melt pool from detrimental oxida-tion. A single clad bead is created by moving the substrate relative to the laser beam and by a slight overlap of consecutive parallel beads a clad layer is formed. a) Schematic illustration. b) Live image of laser cladding.

a) b) Figure 2. Polished and etched cross-sections of laser cladded Stellite 6 (a) and Stel-lite 21 (b). LOM.

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a) b) Figure 3. Carbide network of Stellite 6 as exposed by different degrees of etching. Note the fine structure within the carbide dendrites in a) and the sponge-like three dimensional structure revealed in b).

a) b) Figure 4. The structure of Stellite 21. The whiter areas are enriched with Mo (as revealed by EDS), indicating the presence of intermetallic phases. a) Dendritic struc-ture of the intermetallic phases and carbides. b) <111> texture of the Cobalt-rich matrix revealed by extensive etching.

X-ray diffraction (XRD) measurements show that the Co-rich matrix of both Stellites solidify in an fcc phase. As also observed by others [31, 32], both materials become strongly textured with the <100> direction perpendicular to the substrate surface. Further, XRD pole figure investigations on a laser cladded low C version of Stellite 21 reveal no preference for the lateral fcc <100> directions [33].

The toughness of Stellites is primarily determined by the carbide volume fraction and morphology. An increased level of carbides will lower the duc-tility. Consequently, the hardness and ductility of Stellites have to be opti-

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mised to suit the application. However, if only toughness is desired, alloying elements that stabilise the fcc phase should be added [2].

2.6 The problem of Stellites in nuclear environments In contrast to the often excellent technical function of Stellites, a setback has been observed in the nuclear power industry, when used on components situated in the primary circuit. Stellites contribute to the background radia-tion level, due to the naturally occurring isotope Co59, which may be released from Co containing surfaces in the form of wear and corrosion products. The Co59 isotope is transported to the reactor vessel where it is activated to the radioactive isotope Co60 by neutron capture in the fission process. Following the steam, the Co60 isotope is transported throughout the primary circuit where it sticks onto e.g. pipe walls. With a half-life of 5.27 years the isotope decays under emission of 1.17 and 1.33 MeV gamma radiation. The piping system of the primary circuits is mainly made of austenitic stainless steel, which typically also contains some small amount of Co. Yet, in spite of the relatively small areas of Stellites in the primary circuit compared to the pip-ing system, the contribution to the background radiation from Co particles from Stellites is believed to be much larger than that of the piping [34]. Cor-rosion studies support this by revealing high Co release rates from fresh Stel-lite surfaces. The release rate is then decreased by the growth of a protective oxide layer, see Fig. 5 [35]. However, as the valve sealing surfaces recur-rently must be ground to make a close fit, fresh Stellite surfaces are regularly introduced into the primary circuit, giving rise to the higher release rate.

The background radiation is a significant problem for the nuclear industry as it obstructs the accessibility to the BWR primary circuit, which for safety reasons must be checked regularly [36]. As a precaution, service technicians are only allowed to be near the primary circuit for a restricted time and must use protective gear. However, the function of the component can never be jeopardised, and consequently the Stellites, despite the Co60 issue, are chosen for critical components, such as the sealing surface on gate valves.

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Figure 5. Average Co release rate as a function of time after insertion of non-corroded Co base alloys in 250 ºC water. Initially the release rate is high, but stabi-lises at lower levels after about a month [35].

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3 Tribological testing

3.1 Test motivation – simulation of gate valves In the late 1990’s, before the author’s work commenced in this area, the Tribomaterials Group at Uppsala University was requested by the Swedish nuclear power plant company OKG Aktiebolag to test and rank high per-formance Co-free hardfacing materials for nuclear power plant gate valves. The test materials were produced by a number of companies, all competing to deliver the most suitable material [37]. Since no existing tribological test equipment could simulate the desired contact situation, a new tribometer had to be designed, the question was how?

In an emergency situation, the reactor vessel has to be closed immediately to hinder the steam from leaving the primary system. This is done by closing gate valves, see Fig. 6, which are situated close to the reactor vessel.

The gate valve is opened and closed by moving the disk in the vertical di-rection, manoeuvred by e.g. an electric motor. With guides leading the disk into its desired position, the manoeuvring appears to be straightforward. However, in an emergency situation, the high-pressure steam flowing against the disk might tilt it slightly, which would disturb the alignment between the sealing surfaces of the disk and seat. The largest degree of disk tilting occurs when the valve is almost closed, as shown in Fig. 6. As a consequence of the tilting, the contact situation between the disk and seat can be compared to a “scissoring motion”, as the edges of the sealing surfaces come into contact.

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a) b) Figure 6. Outline of a gate valve. a) The disk can be manoeuvred in the vertical direction to close or open the valve and thereby control the flow of steam from the reactor vessel. b) A closing disk seen in the direction of the pipe. Darker areas of the seat indicate positions where the tilting of the disk is most severe, creating a scissor-ing motion between the disk and seat.

Having a relatively small edge-radius (approximately 2 mm), the edges of the sealing surfaces are exposed to high contact pressures, much higher than the intended regular flat-on-flat contact between the valve disc and seats. Consequently, the edges of the sealing surfaces might be plastically de-formed in the case of disk tilting. Showing an excellent tribological perform-ance under these severe conditions, the Stellites are frequently used on seal-ing surfaces.

3.2 The Load-Scanner A new tribometer was designed to simulate the scissoring motion of gate valves. It utilises the configuration of two crossed cylindrical surfaces slid-ing against each other under a continuously increasing load. The tribometer is named the Load-Scanner since it is suitable for analysis of a wide range of loads during a single test run, see Fig. 7 [38, 39].

The test may either be performed in one direction (single stroke) to assess friction and galling properties, or as a reciprocating multiple stroke test, also including wear and friction evaluation. During testing, the horizontal posi-tion of the moving specimen is recorded and force gauges measure both spring and friction forces. These data allow the user to calculate the friction coefficient as a function of load or position at any unique point along the sliding track. A sketch of the Load-Scanner is shown in Fig. 7.

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Figure 7. Sketch of the Load-Scanner. As the movable specimen holder travels to the left, the loading spring is stretched and thereby increases the load on the speci-mens.

a) b) Figure 8. Geometry of the test rods. The rods are mated in such a way that the rounded edges slide against each other. The edges have a cylindrical shape of radius 2 mm. This conformal contact leads to plastic flattening deformation of the cylindri-cal surface as indicated in b). a) The overall rod shape and positioning of the laser cladded layer. b) Detail showing a part of the cylindrical edge. The arrow indicates the sliding direction and the ellipse the size of the contact area. The plastically deformed vol-ume is schematically indicated on the cross section surface.

The movable specimen holder is driven by an electric motor and the normal load is created as the loading spring force pulls the lever down. As the two crossed cylindrical test surfaces (see Fig. 8a) are pressed together they de-form plastically until the contact pressure reaches the load carrying capacity of the softer material.

Due to the non-conformal contact geometry, the loads are high enough to cause gross plastic deformation (flattening of the cylindrical sliding path) throughout the load interval, see Fig. 8b. The sliding distance is roughly 1 mm per passage, due to the crossed-cylinder configuration.

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3.3 Test setup for low nominal contact pressures As a part of the thesis, specimens and holders facilitating testing under lower nominal contact pressures were also designed, see Fig. 9. To ensure a well distributed pressure over the flat specimen surfaces, both specimens were mounted in self-adjusting semi cylindrical sliding bearings. The specimen holders are mounted on the Load-Scanner in the same position as the speci-mens in Fig. 7. If testing is to be performed under a constant nominal contact pressure, the normal force is set by using dead weights.

Figure 9. Side views of the sample holders and specimens for the low contact pres-sure configuration. The smaller specimen (white, 4×6×4 mm3), which is clamped to the upper specimen holder, slides against the larger specimen (black, 8×30×8 mm3),which is clamped to the lower specimen holder. Both specimen holders can rotate, thus allowing self-aligning of the specimens. The sliding direction is indicated by the arrow.

3.4 Evaluation techniques To identify the materials and their tribological behaviour light optical and scanning electron microscopy (LOM and SEM) was employed to investigate the surface appearance, chemical analysis was performed by energy disper-sive x-ray spectroscopy and x-ray photoelectron spectroscopy (EDS and XPS) and phase analysis of the microstructure was performed by x-ray dif-fraction and transmission electron microscopy (XRD and TEM).

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4 Tribological behaviour of Stellites (Papers I, II and IV-VI)

4.1 Macroscale deformation and friction For both Stellite 6 and 21, the coefficient of friction in the load scanner test is high during the first stroke, see Fig. 10. The sliding track generated along the rounded edge is macroscopically flattened and remarkably smooth for both materials, as exemplified in Fig. 11a. During the following stroke, µ is about 0.05 lower than during the first. In the case of Stellite 21, the friction stabilises at this level for well over the 100 strokes indicated here. For Stel-lite 6 the friction starts to climb immediately and reaches the final high and rather erratic level after some 25 strokes, see Fig. 11b. The underlying rea-sons between the differences in their tribological performance will be dis-cussed later.

a) b)Figure 10. Friction of unlubricated Stellite 21 (a) and 6 (b) during the first 100 strokes (50 double strokes) in the load scanner test. Note that the first stroke, which involves the initial macroscopic flattening of the wear track, shows a µ level that is roughly 0.05 higher than during the following stroke.

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a) b)Figure 11. a) Flattened and smooth wear track of Stellite 21 exposed for one stroke at a normal load 1000 N. b) Stellite 6 surface displaying a rough wear track after 100 double strokes (at 1800 N).

The higher friction during the first stroke is explained by the substantial plastic deformation involved only in this stroke. The previously unloaded materials yield to a large depth, much as in hardness indentation. This de-formation results in a widened distribution of the load and substantial defor-mation hardening down to a depth of several hundreds of micrometers, see Fig. 12. Subsequent passages will cause much less hardening and will not cause any apparent deformation below the most superficial layers.

Figure 12. Hardness profiles below the surface of self mated Stellite 6 and Stel-lite 21 exposed to one single stroke at a normal load of 1500 N. The much shallower hardness profiles below ground surfaces of the Stellites are shown for comparison. (Microhardness HV0.025)

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As a part of this thesis, a new method was developed for high-resolution characterisation of friction induced deformation during sliding. The basic principle is to introduce an artificial vertical marker in the near surface re-gion of the material to be deformed. The markers are produced by high-precision etching and following metal filling by a chemical vapour deposi-tion (CVD) process in a combined focused ion beam (FIB) and SEM instru-ment. Knowing the initial shape of the marker, the deformation characteris-tics of Stellite 6 was revealed by the marker deformation, see Fig. 13a. The main part of the plastic deformation occurs during the first stroke as illus-trated in Fig. 13b. Due to the increased hardness and the reduction of nomi-nal stresses caused by the widened contact track, each following stroke will only cause very superficial plastic deformation. Similar observations have been made on Stellite 21.

a) b)Figure 13. a) Principle of the deformation analysis technique, showing schematic cross-sections of a marker prior to and after testing. The geometrical change of the marker reveals the type and extent of surface deformation. b) Cross-section of a Stellite 6 surface exposed to a normal load of 1200 N during one forward stroke (arrow indicates the direction of sliding of the counter surface). The major part of the surface shearing is confined to a superficial region of a few µm, as revealed by the marker. Note also the stiffening effect of the carbide dendrite revealed about 1 µm from the surface.

4.2 Shear stress induced fcc to hcp phase transformations

In addition to the macroscopic deformation and strain hardening described, the Stellites also undergo phase transformations when subjected to suffi-

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ciently high shear stresses. The cobalt rich phase transforms from the origi-nal, as solidified, fcc phase to a hexagonal closed-packed (hcp) structure. This was found by XRD directly on the wear track (see Fig. 14) and by ana-lysing the phase distribution and texture in the TEM in a 5 µm deep cross section of the wear track, see Fig. 15. The information depth of the XRD analysis is a few µm. Both techniques showed a complete transformation from an fcc to an hcp phase, within the depths analysed.

Figure 14. X-ray diffraction of as solidified Stellite 21 (fine polished, marked with +) and after 500 double strokes at 2000 N (marked with ). The bars indicate expected positions and relative intensities of XRD peaks from polycrystalline, non textured fcc and hcp Co-rich solid solution in Stellite 21, respectively (cp. [31]).

Figure 15. Bright field TEM micrograph of a Stellite 21 cross-sectional foil from the centre of a wear track exposed to 100 double strokes at 1800 N. Phase analysis re-veal a complete transformation to hcp in the entire foil. A strong texture is revealed in individual grains, but no principal general orientation is present.

Surface

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4.3 Formation of the easy shear layer – self-aligning of hcp basal planes

On top of the hardened and phase transformed layers, an additional very thin layer is present. Within this layer the Co hcp basal planes have been tilted and aligned parallel to the surface during the sliding, as exemplified in Fig. 16. The maximum layer thickness observed was 30 nm. The reorienta-tion of the hcp crystal structure caused by the sliding is shear driven, and the key to the low-friction properties of Stellites. The layer is easily sheared since the basal planes, that are the only easily activated slip planes in the hcp phase, now are oriented exactly parallel to the direction of shear, see Fig. 17. The tribological mechanism is reminiscent of layered solid lubricants such as graphite. This observation correlates well to XRD analysis on the wear track (Fig. 14).

Figure 16. Imaging in the TEM (top left) of the near surface area in Fig. 15. An approximately 30 nm thick superficial hcp layer, consisting mainly of basal planes parallel to the worn surface as detected by convergent beam diffraction analysis (top right). The five lowest figures show element maps of the selected area (see top left figure). They reveal a Cr, Mo, C-rich region (on the left side) enclosed by surround-ing bulk material and the Co-enriched surface layer.

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Figure 17. The key to low-friction behaviour of Stellites is the formation of the thin, easily sheared layer within which the hcp basal planes have become oriented parallel to the worn surface. The layer is visible in the upper part of this TEM high-resolution lattice cross section image close to the area in Fig. 16. A protective plati-num based layer was deposited on top of the surface before preparation of the cross section (lighter layer on top).

4.4 Accumulated deformation and wear The wear and surface damage of the tested rods can be divided into different stages. Firstly, the originally cylindrical surface is flattened, primarily during the first stroke. This flattening involves plastic deformation down to large depths. Similarly to a hardness indent by a spherical indenter, the depth of the plastic zone can be expected to be roughly the same as the width of the deformed track, as indicated in Fig. 8b. However, no wear fragments are created during the first stroke.

During subsequent strokes the additional plastic deformation will be very much shallower. The superficial shear will gradually transport material to-wards the sides of the track, see Fig. 18. This transport will create tongues of heavily deformed material along the wear track. These tongues will eventu-ally break loose, as exemplified in Fig. 19, forming flake like fragments of the type shown in Fig. 20. The amount of wear debris created and collected along the edges of the wear track continuously increases.

There is also a lesser amount of very large (maximum observed is 400 µm) and thick fragments that seem to be the result of bulk shear and fracture. These fragments exhibit an almost intact structure of the base mate-rial.

Surface

Aligned hcp basal planes

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a) b)Figure 18. Gradual transport of surface material towards the edges of the sliding track, caused by the repeated shear deformation of Stellite 21 after 100 double strokes at 1500 N. a) Overview of the flattened track on the cylindrical surface. b) Detail (white box in a) showing a shear tongue that is about to break off.

a) b)Figure 19. Wear particle formation along the edge of the sliding track. a) Top view of Stellite 21 showing the site where a wear fragment has been lost (area within black box in Fig. 18a). b) Cross section parallel to the sliding direction of Stellite 6 showing the edge of the flattened wear track (to the left) and a shear tongue about to flake off, after 10 double strokes at 1800 N.

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a) b)Figure 20. Flake like wear fragments from Stellite 6 collected alongside the sliding track. a) Overview. b) Detail.

The wear debris produced along the wear track may become trapped be-tween the moving surfaces and reintroduced into the track during the recip-rocating movement. These fragments become squeezed or rolled down into the surface, causing additional plastic deformation of the materials. During repeated sliding the wear tracks of Stellite 21 typically continue to stay mac-roscopically smooth, but are scored by fragments adhering to the counter surface. Contrastingly, the Stellite 6 surface develops an increasing rough-ness, with large debris formed and substantial transfer of material between the surfaces, see Fig. 21.

Figure 21. Tilted view of the wear track profile of Stellite 6 exposed to 100 double strokes at a normal load of 1800 N, cp. Fig. 11b. A large number of wear fragments have been reintroduced from the track borders, now covering large parts of the sur-face. The wavy appearance of the wear track is caused by local variations in the process of wear of one part and transfer of the wear debris to the other.

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The heavily deformed material and reintroduced fragments may form exten-sive layers or tribofilms, see Fig. 22. Within these layers, small carbide frag-ments indicate that the carbide structure is broken down into small fragments by the shear deformation. These layers also contain higher levels of oxygen than the bulk material throughout their thickness (as confirmed by EDS), indicating that they to a large extent are formed by addition of agglomerated wear fragments. Note that also under these conditions of rather coarse sur-faces the coefficient of friction stays on a modest level around 0.3 (Fig. 10b).

a) b)

c)Figure 22. a) Formation of a wear debris layer on top of a Stellite 6 specimen after 100 double strokes at a normal load of 1800 N. Note the layered, almost turbulent, heavily deformed structure in b) and the numerous small carbide fragments (light contrast) in c).

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4.5 Friction at low nominal contact pressures Interestingly, the beneficial tribological behaviour of the Stellites seems to require a high normal pressure. For comparison, the two Stellite materials have also been tested in unlubricated, multi-cycle flat-on-flat sliding under nominal contact pressures of 20 MPa (less than 1% of the nominal pressures in the load scanner). In this test the phase distribution in the surface layers (as measured by XRD) was only slightly modified compared to the ground Stellite 6 and 21. A small fraction of the fcc phase is transformed to hcp (primarily in the 1110 direction perpendicular to the wear track sur-face), but no aligning of hcp basal planes parallel to the surface could be detected.

The corresponding coefficient of friction is also higher (averaging about 0.4) compared to testing under high nominal contact pressures (approxi-mately 0.25).

4.6 Influence of oxide layer formation The thickness of the oxide containing layer (notably Cr2O3 and CoO) is of the same order of magnitude on the ground and the sliding contact surfaces of Stellite 6 and 21, as revealed by XPS. From TEM-analysis the thickness of the oxygen enriched layer of as-ground and worn Stellite 21 is approxi-mately only 10 nm thick, that is even thinner than the zone of aligned Co basal planes, see Fig. 16. This thickness refers to the areas not covered by adhered fragments. The fragments exhibit a high oxygen content throughout their thickness.

The absence of oxygen in the surrounding atmosphere has only a mar-ginal effect on the coefficient of friction, but leads to a significant increase in surface damage and wear as seen for Stellite 21 in Fig. 23. Note the similari-ties of the wear track appearance of Stellite 6 after 100 double strokes, see Fig. 21. As Stellite 21 shows only limited wear when tested in air (Fig. 18a), it can be concluded that the presence of a protective oxide layer is essential for reducing the wear rate. However, the detection of similar amounts of aligned hcp basal planes parallel to the worn surface for both tests, indicates that the deciding factor for the level of friction is the formation and shear of the textured hcp surface layer.

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a) b)Figure 23. Wavy wear track of Stellite 21 after 100 double strokes at 1800 N in a low oxygen containing environment. The arrows indicate the sliding direction. a) Overview. b) Tilted view of the area marked in a).

4.7 Stellite in sliding contact with other materials In related unpublished work, the present authors have noted (using XRD and EDS) that Stellites may readily transfer to the counter surface when sliding against various other engineering materials under high contact pressures. In this manner a Stellite against Stellite contact with beneficial properties may form.

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5 Candidates for Stellite replacement in nuclear power plants (Paper III)

Alternative materials completely without or with significantly smaller amounts of Co, and other unsuitable elements rendering an increase in back-ground radiation, are sought for. Since other parts of the existing valve sys-tems and the motors operating the valves are designed for the relatively low friction obtained with Stellites, alternative valve sealing materials must also display a low-friction behaviour. If not, expensive redesigns or replacements of valve systems have to be made. This leaves us with demands on candidate materials, such as follow:

They must have the potential to give low friction and wear, they must not contain any significant amount of Co or any other element that could give isotopes detrimental in terms of background radiation level,they must display adequate corrosion properties and be applicable to the sealing surfaces of existing valves.

Numerous investigations in the area of possible Stellite replacement alloys have been performed with varying results, but often negative with respect to low-friction properties and galling resistance [5, 22, 40-44].

A promising group of Co-free alloys for Stellite 6 replacement was pre-sented by the Electric Power Research Institute (EPRI). The alloys, recog-nised by the trademark Norem, are Fe-based with a structure consisting of eutectic (M7C3) and non-eutectic (M6C and M3C) carbides in an austenitic matrix [41]. The nominal chemical composition and hardness of Norem 02 is given in Table 3.

Table 3. Nominal alloying content (wt %) and hardness of Norem 02 [41].

Fe Cr C Mn Si Ni Mo N Hardness [HV]

Norem 02 Bal. 25 1.2 4.5 3.3 4.0 2.0 0.2 350 (conv. welded) 400 (laser cladded)

Early results showed excellent galling resistance of a conventionally welded Norem 02 at RT [41] due to favourable strain-induced phase transformation from austenite to martensite. However, when tested at temperatures above

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190 ºC, which is required for the nuclear application, no such transformation occurred and an inferior sliding wear performance was observed [43].

Nominally, a strain induced transformation from austenite to martensite in Norem 02 does not occur at temperatures above 180 °C. To investigate if laser cladding could increase this critical temperature, sliding tests were performed at different temperatures under high nominal contact pressure. The results were similar to those of conventional welding, i.e. they indicated that Norem 02 is not suitable as a replacement of Stellite in nuclear power plant circuits at elevated temperatures, see Fig. 24a. However, when used at RT, laser processed Norem 02 showed excellent friction and wear character-istics, equal to those of Stellites, see Fig. 24b.

a) b) Figure 24. Examples of surfaces of self-mated Norem 02 after one stroke at a load of 1000 N. Tests performed at a) 250 °C and at b) RT. Note the rough surface topog-raphy in a) indicative of severe plastic deformation and high friction (µ~0.6) com-pared to the smooth wear track in b) displaying low friction (µ~0.25).

Further investigations of Norem 02 are needed to explore if the critical tem-perature of its beneficial phase transformation can be extended to tempera-ture levels typical of critical nuclear reactor components, i.e. from RT to 350 °C.

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6 Conclusive summary

The results and discussions in the previous sections can be summarised into a physical model of the tribologically induced material modifications behind the intrinsic low-friction and anti-galling properties of the Stellites, as illus-trated in Fig. 25.

MechanismsForms a thick deformation hardened layer Forms a thin, superficial easily sheared layer The easy-shear layer is continually regenerated, replacing worn off mate-rial

Technical benefits Offers low-friction properties Avoids severe galling by restricting shear and adhesive transfer to very thin superficial layers Offers self-generated protective layers by re-deposition of wear fragments (in closed sliding contacts)

Uniqueness Parallel transformations to harder and softer (more easily sheared) phases (at the tribologically preferred locations) Relatively ductile and soft base material (compared to hardened steels) Relatively strong deformation hardening Very thin shear layer The same advantages are obtained also when Stellites are used against other engineering materials Not dependent on boundary lubricants or oxide layers (presence of oxy-gen) to avoid seizure

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RestrictionsRequires a sufficiently high normal pressure to operate (high nominal pressures or low pressure situations with some form of misalignment or shape deviation that causes a stress concentration) Only two Stellites (Stellite 6 and 21) have been investigated Only two test set-ups were included in these investigations

Figure 25. Schematic view of the self-generating low-friction system of Stellites typical of highly loaded sliding contacts.

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7 Speculations regarding materials for Stellite replacement

This thesis has fulfilled the primary objective by revealing the mechanisms behind the tribological performance of Stellites in highly loaded sliding con-tacts. Both Stellites investigated are dependent on two major features. Firstly, they form a deformation hardened layer, and secondly, they form a thin, superficial easily sheared layer.

So far, no Co-free alternative has fully matched the low-friction proper-ties of Stellite throughout the entire temperature region relevant for the nu-clear applications described earlier. With guidance of our new results, how should another metallic system with similar properties be designed? To an-swer this question, we start with remembering that the fcc to hcp transforma-tion occurs by shear and involves a change in stacking sequence of the closed-packed planes in the fcc structure (ABCABC…) to the hcp stacking sequence (ABABAB…). The basal {0001} planes of the hcp phase will re-tain the orientation of the fcc close packed {111} planes [15, 45]. As the hcp phase contains only one easily activated slip plane unlike the fcc phase that contains four, the hcp phase is harder and generally less ductile [46].

The significant influence of crystallographic texture with respect to fric-tion, was noted almost 40 years ago by Buckley and Johnson [46]. They stated that hcp metals with preferred slip on the basal planes (e.g. Co), will exhibit much lower coefficients of friction than fcc metals (e.g. Cu) and non-basal slip hcp metals (e.g. Ti).

Some 30 years ago, Huppmann and Clegg [47] found a preferred orienta-tion of polycrystalline Co after sliding, having the hcp basal planes parallel to the worn surface. The present observations of the Co-rich solid solution, dominating the structure of Stellites, coincide with these observations. Fur-ther, Huppmann and Clegg [47] intentionally stabilised the polycrystalline Co in the fcc phase by alloying with 8 wt % Fe. This lead to a doubled fric-tion and a significant increase in wear rate compared to the initial experi-ment with hcp Co. These results are in line those of Buckley and Johnson [46]. In addition, Wheeler and Buckley [48] stated that a high stress should be expected to facilitate texturing in hexagonal systems, due to the limited number of slip systems in the hcp phase. This is in line with the results from self-mated Stellite sliding under low contact pressures. To summarize, the

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tribological properties of pure elements correlate well to the observations made on the Stellites, both for high and low nominal contact pressures.

With this in mind, we return to the work of Buckley and Johnson [46]. They performed sliding tests of 18 different hcp metals in vacuum, and a strong correlation between the lattice parameters and the friction perform-ance of the materials was observed. The lattice parameters influence the number of operating slip systems in the hcp system. Co exhibits an almost ideal atomic stacking ratio2 in the hcp system [45], thus preferably generat-ing slip in the basal plane system [46]. However, hcp structure is far from a guarantee for low friction. Ti has an atomic stacking ratio of 1.586 [45]. The deviation from the ideal the atomic stacking ratio will also activate two more slip planes [46]. This results both in an increased ductility and higher fric-tion, similar to fcc metals such as Cu, which have four operative slip planes.

Unfortunately, by selecting various hcp metals with beneficial stacking ratios and checking them with respect to demands on thermal and phase sta-bility, weldability, and so forth, no competitive candidates to Co emerges. This also correlates well to the observations made by Buckley and Johnson [46].

Another possibility could be to use a Co material fully in the hcp phase. However, even if properly oriented (basal planes along the sliding surface), it would still not show the nice behaviour of Stellites. The risk of shear along planes far below the surface would be overwhelming, and the resulting wear rate probably much higher than in the case of only a few atomic layers of aligned hcp structure (as in the case of Stellites), that prevents shear further down in the structure. The continuous, thin superficial formation of the tex-tured hcp surface layers in Stellites can consequently be regarded as the key for a successful combination of limited wear and low-friction properties in severe sliding contacts.

Having problems to find a suitable candidate within the hcp family, we must turn to the fcc and body-centered cubic (bcc) alloys. Of these two, fcc seems to be the most attractive, since it also has closed-packed planes that are easy to shear. The problem is, as mentioned previously, that fcc has four close packed plane directions (in contrast to the single one in hcp), making it susceptible for shear deformation in various directions, with the risk of gall-ing as a consequence (cp. with the behaviour of austenitic stainless steels). Something else must subsequently be added to avoid the shear deformation to propagate in the material, but what? Actually, the answer might be close at hand. Comparing Stellite 6, chromium alloyed cast irons (similar structure as Stellites) and Norem 02, they have two main properties in common. Firstly, they display good tribological properties in sliding contact (at RT). Secondly, they contain a large amount of carbides that reinforces the matrix.

2 The stacking ratio obtained by dividing the hcp lattice parameters, i.e. c/a. The ideal ratio for the hcp stacking sequence is 1.633. Co has a stacking ratio of 1.622.

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The original reason for adding carbides to the metallic structure might just have been a desire to obtain a harder material, while other beneficial proper-ties may have been more or less accidental. If the carbide network is ho-mogenous and strong, i.e. resilient to fracture during sliding, it could possi-bly act as physical barrier preventing the shear deformation of the metal phase to propagate into the bulk material. Furthermore, the superficial fcc phase is expected to align in a preferred orientation during sliding [49]. Con-sequently, the metal enclosed within the carbide “walls” in the near surface region have the possibility to become textured in the sliding direction, while the carbide network assists in restricting the lateral shear. The mechanical properties of the carbide network are thus essential if the shear is to be con-fined to the interface between the mating materials. A fine-meshed carbide network should be beneficial, as it limits the amount of metal confined inside each “cell” (defined by the surrounding carbide walls), thus restricting the lateral shear deformation of the metal. In addition, the fine-meshed network will have thin walls that, if fractured, potentially could create small wear fragments, causing minimum additional damage to the mating surfaces. Fur-ther, if the carbide/metal composite is hard, it will limit the area of contact, adding to the low-friction potential of the suggested system. Finally, the presence of a though and well bonded oxide is of significant importance to minimise the risk of strong adhesion forces, caused by metal-to-metal con-tact.

If we have an open mind, and continue to create an improved understand-ing of the tribological behaviour of materials exposed to sliding contact, a successful Stellite replacement alloy for high load sliding applications might eventually be born. With respect to other demands such as weldability and corrosion properties, combined with the speculations above, an alloy based on Fe or Ni with a significant amount of C and carbide forming elements such as Cr (also included for corrosion resistance), might be the solution. To start with, additional high-resolution analysis on Norem 02 wear tracks (e.g. as in Papers IV-VI), aiming at fully clarifying the mechanisms behind its excellent tribological behaviour at RT, in contrast to the massive galling occurring at elevated temperatures, see Fig. 24, would be of great impor-tance for further expansion of this theory. The search continues…

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8 Sammanfattning (Summary in Swedish)

Målet med detta arbete har varit att förklara de utmärkta tribologiska egen-skaper som material i den koboltbaserade familjen vid namn Stellite uppvi-sar. Trots att dessa legeringar ofta används i industriella applikationer, samt att omfattande forskning bedrivits, har dess tribologiska egenskaper vid gli-dande kontakt hittills inte kunnat beskrivas på ett tillfredsställande sätt.

Ett starkt motiv till att reda ut anledningen bakom Stelliternas goda tribo-logiska egenskaper är att de används i många viktiga nukleära tillämpningar, t ex på skalventilers tätningsytor. De tribologiska egenskaperna hos leger-ingen ger en utmärkt teknisk funktion, men korrosions- och nötningsfrag-ment från Stelliterna ger upphov till radioaktiv bakgrundstrålning i primär-kretsen via bildandet av isotopen Co60. Detta utgör ett allvarligt problem, t ex vid underhållsarbete nära primärkretsen. Således finns en stark drivkraft att ersätta Stelliterna med koboltfria alternativ. Dessa nya material bör ha lik-värdiga eller helst ännu bättre tribologiska egenskaper som Stelliterna, men inte bidra till bakgrundsstrålningen. Med en ökad förståelse om hur Stelliter-na uppnår sina goda tribologiska egenskaper ges förbättrade förutsättningar att utveckla ett koboltfritt ersättningsmaterial.

Arbetet visar att Stelliternas tribologiska egenskaper följer de teorier som presenterades av två av tribologins mest kända forskare, Bowden och Tabor, redan för mer än 50 år sedan. Lågfriktionssystemet som de beskrev består av två centrala komponenter:

Ett hårt material som ger en liten kontaktyta, samt ovanpå detta ett tunt och lättskjuvat ytlager.

För att nå dessa egenskaper används oftast någon form av fast eller flytande smörjmedel på ett hårt substrat. I de nukleära tillämpningarna som nämns ovan är smörjmedel inte tillämpbart. Anledning till att Stelliterna trots detta uppvisar goda tribologiska egenskaper under glidande kontakt vid höga las-ter är dess inre förmåga att spontant både deformationshårdna (skapar ett hårt material) och samtidigt generera ett lättskjuvat ytskikt. Den schematiska bilden på avhandlingens framsida illustrerar ett tvådimensionellt tvärsnitt av denna kombination. Ett nanometertunt ytskikt av lättskjuvade basplan i den hexagonala kristallstrukturen är den huvudsakliga nyckeln till den låga frik-tionen (de fyra översta atomlagren på bilden). Detta texturerade ytskikt stöds av det underliggande deformationshärdade området som även det består av

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en hexagonal fas, men dock inte lika välordnad som vid ytan. Trots att kon-takttrycken varit så höga att Stelliterna utsatts för massiv plastisk deforma-tion så är tjockleken på det lättskjuvade ytskiktet endast ett tiotal nanometer.

Vid lägre kontaktryck genereras inte det lättskjuvade ytskiktet. Det krävs således en viss grad av skjuvspänning för att initiera den upplinjering av kristallina basplan som är nödvändig för att Stelliterna ska erhålla sin opti-malt låga friktion.

Insikten om hur Stelliter fungerar i hårt belastade glidande tribologiska kontakter har erhållits genom omfattande laboratorietest av laserdeponerade Stelliteskikt. Proverna har efter test utvärderats med hjälp av de modernaste elektronmikroskopen vid Ångströmlaboratoriet vid Uppsala universitet.

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9 Acknowledgements

This work has been performed at the Materials Science Division, The Ång-ström Laboratory, Uppsala University, Sweden. Further, it has been a part of the national tribology research program “High Performance Mechanical Components (HiMeC)”.

The financial support by Duroc Energy AB, The Swedish Research Council, The Swedish Foundation for Strategic Research and the Swedish nuclear power plants, above all Ringhals AB, is highly appreciated.

Special recognition to Göran Embring at Ringhals AB for being an in-valuable resource of knowledge in the area and a good mentor. Together with Christer Gunnarsson (also at Ringhals), who also shown a great interest and engagement in this research, they are deeply thanked for giving me the huge opportunity to share the industrial experience of valve and materials technology accumulated at Ringhals.

My gratitude to Paavo Rahkola at Duroc Energy for introducing me to the area of laser and materials processing as well as sample preparation. You have been of excellent help when the questions arise! Recognition and thankfulness also goes to all my other friends at Duroc Energy AB and Du-roc Tooling AB for providing all test samples and a lot of good memories during this work.

My deepest appreciation to my industrial supervisors Johan Kinell, and his predecessors Göran Wahlström and Carl Bergström and the founder of Duroc AB, Lennart Olofsson, both for giving me the opportunity to perform this work, but also for all the nice times we had during this journey.

Words are hard to find for thanking my excellent supervisors, Prof. Sture Hogmark and Prof. Staffan Jacobson. You combine scientific knowledge and the art of communication in an astonishing manner, but most important, you are really fun to spend time with!

My other co-authors, Ernesto Coronel and Lars Hammerström, who to-gether with Nils-Olov Ersson at the Ångström Laboratory, are recognised for great collaboration and stimulating conversations.

A big hug to my little sister Sofia Persson, who truly captured the essence of Stellites low-friction behaviour on the cover of this thesis, which is dedi-cated to her and the rest of my family, including the best grandma in the world. Tack mamma, pappa, Charlotta med familj, Sofia och mormor för att ni givit mig ett sådant härligt stöd under denna resa!

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Last, but certainly not least, warm thanks to the rest of my colleagues at the department for supporting this research in a remarkable way. Without your help, support and friendship, neither the first, nor this last line of this thesis would ever have been written…

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10 References

1. Silence, W.L., Effect of structure on wear resistance of Co-, Fe-, and Ni-base alloys, Transactions of the ASME. Journal of Lubrication Technology 1978 100(2) p. 428-35.

2. Antony, K.C., Wear-resistant cobalt-base alloys, Journal of Metals 1983 35(2) p. 52-60.

3. de Mol van Otterloo, J.L. and de Hosson, J.T.M., Microstructure and abrasive wear of cobalt-based laser coatings, Scripta Materialia 1997 36(2) p. 239-45.

4. Frenk, A. and Kurz, W., Microstructural effects on the sliding wear resistance of a cobalt-based alloy, Wear 1994 174(1-2) p. 81-91.

5. Vikstrom, J., Galling resistance of hardfacing alloys replacing Stellite, Wear 1994 179(1-2) p. 143-6.

6. Heathcock, C.J., Ball, A. and Protheroe, B.E., Cavitation erosion of cobalt-based Stellite alloys, cemented carbides and surface-treated low alloy steels, Wear 1981 74(1) p. 11-26.

7. Bowden, F.P. and Tabor, D., The Friction and Lubrication of Solids, Part I, Ch. V, 1950 Oxford, Clarendon Press.

8. Rabinowicz, E., Friction and wear of materials, 1965 New York, John Wiley. 9. Jost, P.H., Tribology. Origin and future, Wear 1990 136(1) p. 1-17. 10. Taylor, C.M., Automobile engine tribology - design considerations for effi-

ciency and durability, Wear 1998 221 p. 1-8. 11. Wear control handbook, ed. M.B. Peterson and W.O. Winer, 1980 New York,

American society of mechanical engineers. 12. Jacobson, S. and Hogmark, S., Tribologi Friktion, smörjning och nötning,

1996, Liber Utbildning AB. 13. Rabinowicz, E., Lubrication of metal surfaces by oxide films, A.S.L.E. - Trans-

actions 1967 10(4) p. 400-7. 14. Bowden, F.P. and Tabor, D., Seizure of metals, Institution of Mechanical En-

gineers - Proceedings 1949 160(3) p. 380-3. 15. Beltran, A.M., Cobalt-base alloys, in Superalloys II, C.T. Sims, N.F. Stoloff,

and W.C. Hagel, Editors 1987 Wiley, New York p. 135-63. 16. Frenk, A. and Kurz, W., High speed laser cladding: solidification conditions

and microstructure of a cobalt-based alloy, Materials Science & Engineering A 1993 A173(1-2) p. 339-42.

17. Osma, A., Kayali, E.S. and Ovecoglu, M.L., The effect of elevated temperature and silicon addition on a cobalt-based wear resistant superalloy, Journal of Ma-terials Science 1996 31(17) p. 4603-8.

18. Radu, I., Li, D.Y. and Llewellyn, R., Tribological behavior of Stellite 21 modi-fied with yttrium, Wear 2004 257 p. 1154-66.

19. Metals handbook, ed. J.R. Davis, 1980 Ohio, American society for metals. 20. Bhansali, K.J. and Miller, A.E., Role of stacking fault energy on the galling

and wear behavior of a cobalt base alloy, Wear 1982 75(2) p. 241-52.

50

21. de Hosson, J.T.M. and de Mol van Otterloo, L., Surface engineering with lasers of Co-base materials, Surface Treatment. Computer Methods and Experimental Measurements. Comput. Mech. Publications, Southampton, UK 1997 p. 341-59.

22. Hickl, A.J., An alternate to cobalt-base hardfacing alloys, Journal of Metals 1980 32(3) p. 6-12.

23. Dunckley, P.M., Quinn, T.F.J. and Salter, J., Studies of the unlubricated wear of a commercial cobalt-base alloy at temperatures up to about 400 C, ASLE Transactions 1976 19(3) p. 221-31.

24. So, H., Chen, C.T. and Chen, Y.A., Wear behaviours of laser-clad stellite al-loy 6, Wear 1996 192(1-2) p. 78-84.

25. de Hosson, J.T.M. and de Mol Van Otterloo, L., Surface engineering with lasers: Application to Co based materials, Surface Engineering 1997 13(6) p. 471-81.

26. Liu, C.A., Humphries, M.J. and Mason, D.W., Effect of laser-processing pa-rameters on the formation and properties of a Stellite hardfacing coating, Thin Solid Films 1983 107(3) p. 251-7.

27. Kathuria, Y.P. and Tsuboi, A., Laser cladding of stellite #6: A detailed analy-sis, Proceedings of the SPIE The International Society for Optical Engineering 1996 2789 p. 86-92.

28. Frenk, A., Henchoz, N. and Kurz, W., Laser cladding of a cobalt-based alloy: Processing parameters and microstructure, Zeitschrift fuer Metallkunde 1993 84(12) p. 886-92.

29. Colaco, R., Carvalho, T. and Vilar, R., Laser cladding of stellite 6 on steel substrates, High Temp. Chem. Processes 1994 3 p. 21-9.

30. Frenk, A., et al., Analysis of the laser-cladding process for stellite on steel, Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science 1997 28(3) p. 501-8.

31. Capp, M.L. and Rigsbee, J.M., Laser processing of plasma-sprayed coatings, Material Science and Engineering 1984 62(1) p. 49-56.

32. Pizurova, N., et al., Structure and phase composition of the surface of laser-alloyed steel, Metallic Materials 1992 30(2) p. 59-64.

33. Farooq, M.U., Microstructural investigation of different materials by use of EBSD technique, in Materials Science and Engineering 2004 Chalmers Univ. of Tech., Gothenburg.

34. Hermansson, H.P. and Falk, I., Release rates of cobalt from construction mate-rials in oxidizing and reducing nuclear reactor water environments, Scandina-vian Journal of Metallurgy 1984 13(2) p. 107-11.

35. Taylor, N.K. and Armson, I., Corrosion product release from stellites and stainless steel in high pressure, high temperature lithiated water, in Water Chemistry of Nuclear Reactor Systems 3 1983, Thomas Telford.

36. Lundgren, K., Elkert, J. and Ingemansson, T., Project DORIS - Dose reduction in Swedish BWRs, 1993 ABB Atom, Västerås.

37. Wänstrand, O., Hogmark, S. and Jacobson, S., Friction and Seizure Test Rig Results (OKG Report), 1998 Ångstöm Laboratory.

38. Hogmark, S., Jacobson, S. and Wänstrand, O., A new universal test for tri-bological evaluation, in Proceedings of the 21 st IRG-OECD Meeting 1999 Amsterdam, Netherlands.

39. Hogmark, S., Jacobson, S. and Wänstrand, O., The Uppsala load-scanner - an update, in Proceedings of the 22 nd IRG-OECD Meeting 2000 Cambridge, United Kingdom.

51

40. Burdett, W.B., Development of cobalt free wear resistant alloys for nuclear applications, Surface Engineering 1992 8(2) p. 131-5.

41. Ocken, H., The galling wear resistance of new iron-base hardfacing alloys: a comparison with established cobalt- and nickel-base alloys, Surface and Coat-ings Technology 1995 77(1-3) p. 456-61.

42. Cachon, L., Denape, J., Sudreau, F. and Lelait, L., Tribological qualification of cobalt-free coatings for pressurized water reactor primary-circuit gate valve applications, Surface and Coatings Technology 1996 85(3) p. 163-9.

43. Kim, J.-K. and Kim, S.-J., The temperature dependence of the wear resistance of iron-base Norem 02 hardfacing alloy, Wear 2000 237(2) p. 217-22.

44. Bahn, C.B., et al., Wear performance and activity reduction effect of Co-free valves in PWR environment, Wear 2004 231(1) p. 51-65.

45. Hook, J.R. and Hall, H.E., Solid state physics, 2 ed, 1991 Chichester, Wiley. 46. Buckley, D.H. and Johnson, R.L., Influence of crystal structure and some prop-

erties of hexagonal metals on friction and adhesion, Wear 1968 11(6) p. 405-19.

47. Huppmann, W.J. and Clegg, M.A., Tribological behaviour of polycrystalline cobalt as related to crystallographic texture and structure, ASLE Transactions 1973 16(2) p. 107-14.

48. Wheeler, D.R. and Buckley, D.H., Texturing in metals as a result of sliding, Wear 1975 33(1) p. 65-83.

49. Goddard, J., Harker, H.J. and Wilman, H., The surface reorientation caused by unidirectional abrasion on face-centred cubic metals, Proc. Phys. Soc. 1962 80 p. 771-82.