Due to the growing environmental awareness worldwide, containment provisions for CO2 emissions in mobility systems and increasing performance requirements the demands on mechanical systems and their materials continuously rise. These high demands require the implementation of new technical approaches, for example of light-weight strategies in automotive powertrains, and directly raise questions about the suitability of the most promising technical solution. Two basic parameters, the surface hardness of the tooth flanks and the core fatigue strength of the tooth root, illustrate exemplarily increasing demands on material grades used for gear wheels in automotive powertrains.
In addition to light-weight strategies, a reduction in friction and an increase of the fatigue lifetime are two other major development directions to strive the mentioned targets. It is clear that any kind of solution must show an equal application profile, preferably an improvement, compared to the state-of-the-art solutions. For tribological systems, the following paths may offer lower friction and higher load carrying capabilities:
1. Alternative base oils and additives (such as esters, polyglycols),
2. Thin film coatings (e.g. DLC) and/or
3. Novel steel metallurgies.
In previous investigations on the slip-rolling resistance of thin film coatings (a-C, ta-C,

Acknowledgments IX

Abstract XI

Table of contents XIII

Nomenclature XVII

1 Introduction and aim of this work 1

2 State of the art 5

2.1 Bearing and gear steels for advanced applications 5

      2.1.1 Review on bearing materials 5

      2.1.2 Review on gear materials 7

2.2 Rolling contact fatigue (RCF) 10

      2.2.1 Evolution of residual stresses in rolling contact mechanism 10

      2.2.2 White Etching and Dark Etching Areas 12

2.3 Fe-C-Si system 16

      2.3.1 Effect of silicon alloying in martensitic and bainitic steels 19

2.4 Slip-rolling friction 21

2.5 Contact configuration (Hertzian contact) 22

2.6 Lubrication 25

      2.6.1 Lubrication regimes 26

      2.6.2 Tribological interaction of lubricant additives with metal surfaces 27

2.7 High performance thin film coatings 30

      2.7.1 Vapor deposition 30

      2.7.2 Temperature influence on formation and growth of thin film coatings 32

      2.7.3 Low friction carbon-based coatings 33

      2.7.4 Zr-based coatings 34

3 Experimental details 36

3.1 Characterization of testing materials 36

      3.1.1 Chemical analysis 38

      3.1.2 Heat treatment 39

      3.1.3 Structure and hardness 40

3.2 Investigated coating systems 42

3.3 Twin disc testing machines 46

      3.3.1 Amsler tribometer (A135) 46

      3.3.2 Optimol 2Disc tribometer 47

      3.3.3 Lubricant properties 48

      3.3.4 Failure criterion, test abort criterion 49

4 Experimental results 51

4.1 Slip-rolling tests 51

      4.1.1 Slip-rolling reference tests of uncoated steels 51

      4.1.2 Slip-rolling resistance of coated systems 57

      4.1.3 Tribological behavior of self-mated DLC coatings 64

4.2 Surface analysis at test end 66

      4.2.1 SEM/ EDX observations 66

      4.2.2 Chemical analysis of detected tribo-reactive layers (EDXelement mapping) 68

      4.2.3 Further considerations on the chemical structure (XPS and AES) 71

4.3 Structural XRD results 75

      4.3.1 X-ray diffraction profile of V300, NC310YW and CSS-42L 75

      4.3.2 Residual stresses in manufacturing state 77

      4.3.3 Residual stresses after tribological tests at P0mean = 1.94 GPa 79

      4.3.4 Residual stresses after tribological tests at P0mean = 2.5 GPa 82

4.4 Structural analysis of cross sections after tests 84

      4.4.1 Microscopic overview of steel structures and hardness 84

      4.4.2 Further structural investigations (SEM, FIB/ TEM) 86

      4.4.3 cm/am ratio and soluted carbon content in V300 and NC310YW 96

5 Discussion and tribological considerations 100

5.1 Influence of alternative steels 102

5.2 Influence of high performance thin film coatings 104

5.3 Influence of lubricant additives 105

5.4 Global functional profile of coated and uncoated steels 107

6 Summary and Outlook 109

7 References 112

8 List of figures and tables 127

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