Preparation and characterisation of submicron/nano structured powders from tungsten carbide –cobalt/alternative binders hardmetals



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PREPARATION AND CHARACTERISATION OF SUBMICRON/NANO STRUCTURED POWDERS FROM TUNGSTEN

CARBIDE –COBALT/ALTERNATIVE BINDERS HARDMETALS


Peter ADRIAENSEN en Raf MOORS

Afstudeerwerk ingediend tot het behalen van het diploma van

industrieel ingenieur in elektromechanica optie automatisering

master in de industriële wetenschappen: elektromechanica

Promotoren: dr. ir. T. Laoui (University of Wolverhampton)

dr. ir. A. Van Bael (XIOS Hogeschool Limburg)



XIOS HOGESCHOOL LIMBURG

DEPARTEMENT INDUSTRIELE WETENSCHAPPEN EN TECHNOLOGIE

  1. Academiejaar 2004 - 2005


Abstract

Cobalt has been the most suitable and most commonly used binder for tungsten carbide based hardmetals. The most important factor in favour of cobalt (Co) is its excellent wetting behaviour for tungsten-carbide (WC).


Due to the poor corrosion resistance of Co, its high cost and environmental toxicity, substantial research has been devoted to find suitable alternative binders for WC systems. The aim is to reduce the amount of Co, or possibly, to completely replace Co binder. Two promising alternatives are described and utilised in this project, the first one is a mixture of iron (Fe), nickel (Ni) and cobalt (Co) and the second alternative is composed of iron (Fe) and manganese (Mn). Compared to cobalt, Fe and Mn are very cheap and non toxic.
A literature review was performed on different relevant aspects covering the field of hardmetals, powder preparation methods, powder metallurgy and nanomaterials. The submicron/nano-structured composite powders were prepared by the mechanical alloying method using both planetary ball and high-energy ball milling processes.
A series of experiments were performed with the planetary ball mill by varying milling time (2.5, 5, 10 hrs) and rotation speed (250, 400rpm) parameters to process WC-10wt%Co, WC-10%FeNiCo and WC-10%FeMn. It was noticed that as the milling time increased (above 2.5 hours for 150rpm) the amount of elements (Fe, Cr) picked up from the stainless steel vial inner wall increased. The contamination level increased further at a rotation speed of 400rpm. This indicates that both speed and time should be kept low to minimise contamination or a hard steel vial should be utilised. For that, additional powders were prepared using the high-energy ball mill.
The grain size of WC phase was calculated using the Scherer equation and the corresponding X-ray diffraction peaks while the WC particle size was evaluated using scanning electron microscopy images. Composite powders were successfully made in which fine WC particles (submicron down to about 200nm size) were distributed within the matrix (Co, FeNiCo or FeMn).

The next step would be to compact such powder for a subsequent sintering process. For that appropriate compaction dies were designed using Inventor CAD software. A die to produce cylindrical samples for microstructural and hardness analyses was designed as well as another die to produce samples for 3-point bending tests. Both dies were designed according to ASTM standards.



Acknowledgements

The aim of our final thesis project was to prepare submicron/nano-structured powders from WC-Co system and replace Co with suitable alternatives. This project was accomplished at the University of Wolverhampton (UK) in line with our Master Degree Industrial Sciences.


First of all, we would like to thank everybody who helped to bring our final thesis to a good end. A special word of thanks goes to our supervisor Dr. ir. T. Laoui and to S. Hewitt, of the University of Wolverhampton, for enriching us with the knowledge they have and the daily good care for us.
Further, we would like to thank Dr. ir. A. Van Bael, of the XIOS Hogeschool Limburg, for allowing us the opportunity to accomplish our training in Wolverhampton and for reading our final thesis project and Ms. Bauwens for helping us arrange the paperwork involving our stay in Wolverhampton.
We would also like to thank our parents, for giving us the opportunity to do our thesis project abroad.
Last word of thanks to everybody, especially our parents and girlfriends, for supporting us in the difficult times we sometimes had.
Our stay at the UK was part of a project in the Erasmus framework.

Table of contents





Abstract III

Acknowledgements V

Table of contents 1

List of figures 7

List of tables 11

List of symbols 11

1Introduction and project objectives 12

1.1Introduction 12

1.2Objectives of the thesis 14

1.2.1Looking for alternative binders to Co for WC particles 14

1.2.2Powder Processing by Mechanical Alloying (MA) 15

1.2.2.1Searching for best fit parameters for planetary ball milling 15

1.2.2.2Horizontal high energy simoloyer 16

1.2.3Powder characterization 16

2Literature review 17

2.1Hard metals 17

2.1.1Introduction 17

2.1.2Powder production 17

2.1.3Powder production techniques 18

2.1.3.1Atomization 18

2.1.3.2Gas- and water atomization 18

2.1.3.3Centrifugal process 20

2.1.3.4Chemical processes 21

2.1.3.5Electrolysis 21

2.1.4WC-Co 22

2.1.5Alternative binders to Co for WC 22

2.1.5.1Fe-Mn as alternative binder to Co for WC 22

2.1.5.2Fe/Ni/Co as alternative binder to Co for WC 23

2.1.6Grain growth 24

2.1.6.1Grain growth inhibitor 24

2.1.6.1.1The effect of V8C7 and Cr2C2 additives on the sintering of WC-Co 26

2.1.6.1.2Effect of V8C7 and Cr3C2 additions on WC-Co grain growth and mechanical properties 26

2.2Powder metallurgy 28

2.2.1The process 28

2.2.1.1Mix the powder with a suitable lubricant 28

2.2.1.2Powder compaction 28

2.2.1.3Sintering 29

2.2.2Reasons for using PM 29

2.2.3Applications of PM 31

2.2.3.1Self-lubricating bearings 31

2.2.3.2Hard metals 31

2.2.3.3Friction materials 32

2.2.4The future of PM 33

2.3Nanostructural materials 33

2.3.1What are nanostructured materials 33

2.3.2Synthesis 34

2.3.2.1Mechanical alloying 35

2.3.2.1.1Mechanism of alloying 37

2.3.2.1.2Types of mills 42

2.3.2.1.2.1Planetary ball mills 43

2.3.2.1.2.2High energy ball milling 45

2.3.2.1.2.3Other types of mills 47

2.3.2.1.3Process variables 49

2.3.2.1.3.1Milling container 50

2.3.2.1.3.2Milling speed 50

2.3.2.1.3.3Milling time 51

2.3.2.1.3.4Grinding medium 51

2.3.2.1.3.5Ball-to-powder weight ratio 53

2.3.2.1.3.6Extent of filling the vial 53

2.3.2.2Liquid phase techniques 54

2.3.2.3Vapour phase techniques 54

2.3.2.4Plasma heating 55

2.3.2.5Solid phase techniques 56

2.3.2.6Equal channel Angular Extrusion 56

2.3.2.6.1Simple shear concept 58

2.3.2.6.2Inhomogeneous deformation 59

2.3.3Properties 61

2.3.4WC-Co particles 61

2.4Crystal structures and Point Defects 62

2.4.1The Body-Centered-Cubic (BCC) structure 62

2.4.2The Hexagonal-Close-Packed (HCP) structure 63

2.4.3Miller indices – Cubic Crystals 64

2.4.4Close Packed planes 66

2.5Grain measurement of WC 67

2.5.1BET Surface Area 67

2.5.2X-ray sedigraph 68

2.5.3Laser Diffraction 68

2.5.4Ultracentrifuge 69

2.5.5Photon correlation spectrography 70

2.5.6Microscopical image analysis, SEM, TEM 70

2.5.7X-ray line broadening 71

2.5.8Chemical reaction 71

3Experimental procedure 73

3.1Description of the powders 73

3.1.1Tungsten carbide (WC) 73

3.1.1.1Tungsten carbide < 20µm 73

3.1.1.2Tungsten carbide < 4.3µm 74

3.1.2Cobalt (Co) 74

3.1.2.1Cobalt < 20µm 74

3.1.2.2Cobalt < 4,3µm 75

3.1.3Iron (Fe) 75

3.1.4Nickel (Ni) 75

3.1.5Manganese (Mn) 75

3.1.6Vanadium Carbide (VC) 75

3.2Preparation of the powders 76

3.3Milling process 76

3.3.1Planetary ball mill 76

3.3.2Horizontally high energy mill 77

3.3.3Development of dies for compaction 78

3.3.3.1Compaction die 78

3.3.3.1.1The die for compaction 79

3.3.3.1.2The upper punch 79

3.3.3.1.3The lower punch 80

3.3.3.1.4The die for the Charpy test 80

3.3.3.1.5Die for 3 point bending test 81

3.4Analysis methods 81

3.4.1Sieving 81

3.4.2X-ray diffraction 82

3.4.2.1Formulae used in the X-ray diffraction 83

3.4.2.2Example: unit cell size from Diffraction data 84

3.4.2.3Instrumentation 85

3.4.3XRF 86

3.4.3.1Description of the machine 86

3.4.3.2Preparation of the samples 87

3.4.4Scanning electron microscopy 87

3.4.4.1Description of the machine 87

3.4.4.2Preparation of the samples 88

3.4.5Optical microscopy 88

4Results and discussion 90

4.1Results from planetary ball mill 90

4.1.1Getting started 90

4.1.2Reference sample 90

4.1.2.1XRD 91

4.1.2.2XRF 92

4.1.2.3Calculated grain size of the starting WC particles 93

4.1.3XRD results of the planetary ball milled samples at 250 rpm 94

4.1.3.1The 2,5 hours milled sample 95

4.1.3.2The 5 hours milled sample 96

4.1.3.3The 10 hours milled sample 97

4.1.4The SEM pictures of the 250 rpm samples 98

4.1.4.1.1The 2,5h milled sample 98

4.1.5XRD results of the planetary ball milled samples at 400 rpm 100

4.1.5.1The 5 hours milled sample 100

4.1.5.2The 10 hours milled sample 101

4.1.6The SEM pictures of the 400 rpm samples 102

4.1.6.1.1The 10h milled sample 102

4.1.7Contamination level 103

4.1.8The grain size calculation 104

4.1.8.1The Scherer equation 105

4.1.8.1.1The planetary ball milled samples at 250 rpm 105

4.1.8.1.2The planetary ball milled samples at 400 rpm 106

4.1.8.1.3Conclusions of the Scherer equation 106

4.1.9Strain evaluation from peak shift 110

4.2Results from the high energy mill 110

4.2.1The estimation of the contamination level of the milled powder 111

4.2.2SEM pictures from samples milled with the high energy mill 112

4.2.2.1SEM pictures from Fe/Mn as alternative binder 113

4.2.2.2SEM pictures from Fe/Ni/Co as alternative binder 114

5Conclusions and suggestions for further work 116

5.1Introduction 116

5.2Conclusions 117

5.2.1The planetary ball mill 117

5.2.2The horizontally high energy mill 117

5.2.3The alternative binders 118

5.3Further work 118

6References 120

Appendix A (Website for the project) 1

Appendix B (Videoconferencing facilities at UoW) 1

Appendix C (Technical drawings) 1

Appendix D (Dutch Summary) 1




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