1. Influence of pulse parameters and cathodic cage size on plasma nitriding DEPARTMENT OF PHYSICS QUAID-I-AZAM UNIVERSITY 45320 ISLAMABAD, PAKISTAN Prof. Dr. M. Zakaullah

2.  Introduction  Advantages of nitriding  Fundamental mechanism  Nitriding methodologies  Plasma characterization  Experimental setup  Results  Conclusions Presentation layout 2

3.  Nitriding is a thermochemical treatment in which atomic nitrogen is introduced in to the surface of the specimens.  It offers a cost effective and an efficient surface modification technique, without altering the bulk properties of the substrates.  However, the optimization of process parameters is necessary for obtaining reproducible results. Introduction 3

4.  The effectiveness of surface processing occurring in the plasma depends on the plasma parameters such as electron temperature, electron number density, vibrational temperature, rotational temperature and density of nitrogen active species.  In low temperature plasma, excitation, ionization and dissociation are generally triggered by electrons collisions, and therefore electron temperature which gives average kinetic energy, is of significant importance. Introduction 4

5.  The gas temperature which is equivalent to rotational temperature affects the discharge chemistry because it controls the reaction rates of active species generation (dissociation, excitation or ionization mechanism).  To attain the unperturbed and more reliable information about plasma parameters and active species density, optical emission spectroscopy is an effective approach. Introduction 5

6.  Enhanced surface hardness  Reduced wear rates  Reduced corrosion rates  High dimensional stability  Retaining bulk features Advantages of nitriding 6

7.  Any substance is constituted of tiny atoms, may be assumed like spherical balls.  During the surface hardening process, empty spaces between the atomic sites is filled by some elements’ atoms having smaller size.  Essential requirement of surface hardening is: o Elements to be filled in atomic states, o Surface to be treated can adsorb atoms, o Atoms can be dissolved in the atomic matrix of the substrate. Fundamental mechanism 7

8. Fe N 8

9. Nitriding methodologies  Gas nitriding  Liquid nitriding/ Salt bath nitriding  Conventional plasma nitriding  Cathodic cage plasma nitriding 9

10. Nitriding methodologies  Gas (ammonia) nitriding o Environmental hazards, o Long processing time (more than 20 hours).  Liquid nitriding/ Salt bath nitriding o Need to replace toxic salts in 3-4 months; dispose off issues, o Any leakage may cause serious health issues, o Difficult to manage in large size. 10

11.  Conventional plasma nitriding 11 Nitriding methodologies

12.  Conventional plasma nitriding o Advantageous over gas and liquid nitriding because it is environmental friendly, o Lower power and gases consumption, reduced processing time (~4-6 hours). o But it also exhibits some disadvantages due to high voltage appearing to the specimens such as: border effect, hollow cathode effect, and non uniform temperature distribution. 12

13. Experimental apparatus 13

14.  Cathodic cage plasma nitriding (CCPN) 14

15.  In Cathodic Cage Plasma Nitriding system o Specimens are covered with a mesh type cathode, o The discharge is terminated at the cathode; not the specimens, o Specimens are at the floating potential/ relatively lower than the cathodic potential, o The temperature distribution around the specimens is uniform, o Uniform nitrided layer with removal of edge effect o Generates secondary electrons o Confine existing electrons by electrostatic force 15

16. Plasma characterization 16

17. Optical spectrum Argon lines used for measurement of excitation temperature 17

18. Optical spectrum 18

19. The electrons gain more energy from the electric field at a higher input power. 19

20. Electron and Excitation temperature The decrease in temperature is caused by the decrease in the mean free path. 20

21. The ionization cross section of hydrogen atoms is lower than that of nitrogen, therefore temperature increases with hydrogen admixture. 21 Electron and Excitation temperature

22. Electron number density The ionization of plasma species is mainly caused by electron impact. At a higher power, electrons gain higher energy from the electric field, and electron impact ionization increases 22

23. Electron number density The electrons mean free path decrease with pressure, and therefore electron impact ionization probability decreases. 23

24. Electron number density The electron number density increases with hydrogen admixture due to lower ionization energy. (Hydrogen 13.6 eV; Nitrogen 14.53 eV) 24

25. Nitrogen atomic species density [N] increases due to increase in electron impact dissociation with power. 25

26. Nitrogen atomic species density [N] increases due to increased collisional probability of electron with nitrogen molecules at higher pressure. 26

27. Nitrogen atomic species density Electrons have high kinetic energy (electron temperature) with increased hydrogen fraction, and dissociation probability increases. However, beyond 40 %, nitrogen molecules to be dissociated are small in concentration. 27

28. Excitation temperature Excitation temperature is maximum at lowest duty cycle. 28

29. Vibrational and rotational temperatures Vibrational and rotational temperatures are maximum at the lowest duty cycle. 29

30. Emission intensities and dissociation Higher dissociation rates and maximum emission intensities at lowest duty cycle 30

31. Experimental conditions  Treatment time: 4 hours  Current 1.2 A  Gases admixture N2: H2: Ar= 58:40:2  Pulsed frequency 40 kHz  Pulsed duty cycle 15-75 %  Cathodic cage diameter 13-21 cm 31

32. Microhardness Hardness is maximum at the lowest duty cycle Sharp decrease with load, thin layer deposition 32

33. XRD analysis Expanded austinite phase is formed, and it is independent of duty cycle Base material peaks are suppressed at low duty cycle (better nitrided layer deposition) 33

34. SEM analysis Scratches are significantly suppressed at low duty cycle More thick nitrided layer is formed at low duty cycle Base 15 % 30 % 34 15 % 30 %

35. Microhardness Hardness is maximum for small cathodic cage Significant enhancement even at higher loads 35

36. XRD analysis Expanded austinite phase formed independent of cathodic cage size. Additionally iron nitrides are developed for smaller cathodic cages. 36

37. SEM analysis Scratches in base material are suppressed by uniform nitride layer Layer thickness is maximum for smallest cathodic cage. Base 13 cm 15 cm 37 15 cm 13 cm

38. Wear analysis (ball on disc tester) Base 15 cm 13 cm Base material exhibits sever wear with wide and in-depth wear track, and deep plow. Processed specimen’s shows shallow as well as minor wear tracks 38

39. Friction coefficient Processed specimens have smoother and lower friction coefficients. This smoothness is caused by enhanced surface hardness as well as higher compressive strains. Higher surface hardness resists the plastic deformation, whereas strain is supportive to close or avoid the micro-cracks Base 13 cm15 cm 39

40. Wear (quantitative analysis) 40

41. Emission intensities and dissociation 41

42. Active species density 42

43. Conclusions  Plasma nitriding in the presence of a cathodic cage (CCPN) helps to remove the edge effects associated with conventional plasma nitriding.  In pulsed dc CCPN, surface properties and plasma reactivity can be controlled effectively by varying the pulsed duty cycle.  At low duty cycle, hardness is significantly enhanced, and thick nitride layers are deposited. However, the deposited phases are independent of pulsed duty cycle. 43

44. Conclusions  The excitation temperature, rotational temperature and vibrational temperature are recorded the maximum for the lowest duty cycle.  The dissociation rate of nitrogen molecules is the maximum at the lowest duty cycle.  This experiment represents enhanced plasma reactivity as well as better surface characteristics with the decrease in the duty cycle. 44

45. Conclusions  Hardness is significantly enhanced with reducing the cathodic cage diameter.  The expanded austenite phase is found to be decomposed to iron nitrides while using the smallest cathodic cage.  The thickness of nitride layer is the maximum for the smallest cathodic cage diameter. 45

46. Conclusions  Nitrogen dissociation rate and active species density are maximum for the smallest cathodic cage diameter.  From this study, it is recommended that in CCPN reactor, the minimum duty cycle and the smallest possible smallest cathodic cage are favorable for improved results. 46

47. 47 Thank you for your attention!