Chapter No 4 - (for Hamza) .docx

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Abstract: Nano-crystalline tungsten nitride thin films are synthesized on AISI-304 steel at room temperature using Mathertype plasma focus. The surface properties of exposed substrate against different deposition shots are examined for crystal structure, surface morphology and mechanical properties using X-ray diffraction (XRD), atomic force microscope (AFM), field emission scanning electron microscope (FESEM) and nano-indenter. The XRD results show the growth of WN and WN2 phases and the development of strain/stress in the deposited films by varying the number of deposition shots. Morphology of deposited films shows the significant change in the surface structural with the change in ion energy flux (no. of deposition shots). The change in the ion energy flux results in the development of strain/stress in the deposited films that leads to improvement of hardness of deposited films.

2|Page 3. X-Ray Diffraction results Fig. 4.1 shows the XRD spectra of the thin coatings synthesized on the stainless steel substrate for 10, 20 and 30 focus shots along with the unexposed sample as reference. The distance between the anode tip and substrate sample was kept constant at 8 cm, moreover the substrate sample is exposed for the various focus shots and placed the substrate at room temperature for deposition purpose. Therefore, room temperature deposition is employed which is suitable for many substrates to be used in the plasma focus chamber for deposition purposes. The XRD results of the unexposed sample indicate the presence of three peaks that are consistent with the plane reflections (111), (200) and (220). The XRD results indicate the presence of crystalline phases of WN2 (104), (009), (113) and WN (111) for different number of focus shots. These results are corresponds to the reference card numbers [01-075-0998, 01-0251256]. Evolution of the WN phases on the substrate surface confirms the presence of tungsten nitride thin film on the substrate surface. The substrate sample after deposition shots shows the new peak related to WN2 (113) plane and the intensity of the peak varies with the change in the number of focus shots with a very prominent peak shift as already present in the untreated samples towards the lower angle shift. XRD results represent the new peaks of WN2 (104), WN2 (009), WN (111) plane reflections. There is a shift towards the low angle which might be due to the incorporation of ions into the lattice sites as a result a large expansion is observed. The energetic ions when interact with the sample surface it can deeply penetrate into the surface layer which leads to the increase in the local temperature that enhances the chance to adatom mobility or increase in the strain energy that is suitable for the crystal growth. As compare with the peaks present in the unexposed samples the new peaks at the same place are much broader. The XRD results show the intensity of WN2 (113) peak enhances with the increase in the number of focus shots. It is known that the intensity of the peak is directly related with the crystal phase that increases with the increase in the WN2 (113) phase with the increase in the number of shots. The effect of various deposition shots on tungsten nitride phases, their intensities, position and d-spacing are presented in Table 1. The incorporation of the energetic ions of nitrogen in to the lattice places is responsible for the enhancement of the d-spacing and broadening of the peak. The peak broadening shows the crystalline size of the tungsten nitride thin films is small. And at higher number of focus shots the intensities of the peaks related to the tungsten nitride phases increases and the XRD results shows the decrease in the substrate peaks. The increase in the peak intensity of the nitrides and decrease in the stainless steel-AISI 304 reflection planes indicates the deposition of tungsten nitride thin films that increases with the increase in the number of shots. The XRD results shows the shifting with caparison along with the reference data that confirms the existence stress and strain in the thin films deposited at the substrate surface. The presence of the residual stress or strain in the films is may be due to the lattice distortion by energetic ions of nitrogen that are implanted into the interstitial position and create the lattice defects as well as thermal shocks to the surface of the exposed substrate. Ion implantation in the substrate surface may be responsible to the compressive stresses while the thermal shocks on the thin film may be responsible to the tensile stresses. The change in the d∆𝑑

spacing of the deposited films may leads to the formation of strain ( ) in the deposited film which is calculated by 𝑑

using expression (23). 𝑑𝑜𝑏𝑠 − 𝑑𝑟𝑒𝑓 ∆𝑑 = 𝑑 𝑑𝑟𝑒𝑓 Fig. 4.2 indicates that the exposure of substrate sample against 10, 20 and 30 shots generates the residual strain in the surface of exposed samples. For 10 focus shots, the sample indicates the existence of residual strain in the thin film deposited on the substrate surface. While the samples exposed against 2o shots indicates the presence of little tensile strain is in WN (111) and WN2 (009) phases. The strain developed in the deposited films is relaxed at higher number of focus shots moreover at 30 shots the tensile strain reduces in both phases discussed above. A compressive strain is observed in WN2 (113) phase when substrate sample is exposed for 10 shots and it relaxes at higher number of focus shots. When sample exposed for 10 focus shots then WN 2 (104) phase indicates the presence of compressive strain and on at 20 shots it changes to the tensile strain while on further increasing the focus shots it transfer again into compressive strain.

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1000 800

WN2(116)

WN2(101)

Intensity (a.u.)

1200

WN (220)

WN2(009)

1400

600

30 Shots

400

20 Shots

200

10 Shots

0

Un Exposed 30

35

40

45

50

55

60

65

70

75

80

85

Position (2 Theta)

Figure 3. XRD spectra of unexposed substrate along with exposed for 10, 20 and 30 deposition shots

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Table 1. Intensities, d-spacing position and hkl values of various tungsten nitride phases observed at 10, 20 and 30 deposition shots.

Deposition Parameter Untreated

phase S. Steel AISI-304

10 shots

WN2

20 shots

WN WN2

30 shots

WN WN2

WN

(hkl) 111 200 220 104 009 113 111 104 009 113 111 104 009 113 111

2θ 42.865 50.204 74.150 42.467 49.842 67.036 73.297 41.919 49.309 66.971 72.520 42.500 49.810 66.923 73.460

dobs (Å) 2.10982 1.81725 1.27775 2.12869 1.82959 1.39442 1.29156 2.15523 1.84812 1.39562 1.30239 2.12709 1.83070 1.39650 1.28803

Intensities (cps) 461.269 450.313 528.816 235.563 196.105 104.64 101.009 334.56 181.579 109.872 101.01 200.922 269.738 148.495 95.068

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0.016

WN2 (104) WN2 (009) WN2 (113) WN (111)

0.014 0.012

Residual Strain

0.010 0.008 0.006 0.004 0.002

Tensile

0.000 -0.002

Compressive

-0.004 -0.006 10

20

30

Number of Deposition shots

Figure 4. Residual strain of exposed for 10, 20 and 30 deposition shots

4. Surface Morphology Results 4.1 FESEM graphs The surface morphology of nano-structured tungsten nitride thin films deposited with different 10, 20 and 30 number of deposition shots along with unexposed substrate is shown in Fig. 5. Some structures are visible on unexposed substrate surface which may be developed during polishing process. The FESEM results of substrate surface exposed for 10 shots, show nearly round shape and well ordered grains. The grains having distinct boundaries with sharp edges are smoothly deposited on the substrate surface. The grains are well compact and uniformly distributed exhibiting smooth deposition of thin film. The grains of various sizes can be observed on surface of deposited films. The grains of average size are homogeneously spread over the whole film. The substrate exposed for 20 focus shots shows that the grains start agglomerating on the surface. The agglomerates of various shapes can be seen in graph with the development of some cracks and voids on the surface of deposited film. The grains present in these agglomerates have no well-defined boundary and cannot easily be distinguished from one and other. The surface morphology of substrate exposed at 30 shots shows that the surface of under laying layer is smooth and uniformly distributed over the whole surface with agglomeration on the surface. These round shape agglomerates are well developed having distinct, sharp boundaries. Such structures of the films are expected by the film deposited by plasma focus device. Energetic nitrogen ions from plasma focus can penetrate deeper into the substrate surface and increase the local temperature that leads to increase in adatom mobility or high strain energy. These agglomerated grains various size while average size of these agglomerated grains is around 170±10 nm while the increase in the agglomeration may increase the surface roughness.

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Figure 5.

FESEM graphs of substrate exposed for 10, 20 and 30 deposition shots

5. Mechanical Properties Improved hardness/elastic modulus is significant characteristic of hard deposited films. Nano Indenter is employed to analyze the hardness (GPa) of exposed substrate. Figure 8 shows the hardness of exposed substrate for various (10, 20 and 30) deposition shots along with unexposed substrate. The substrate sample exposed for 10 deposition shots shows significant increase in the hardness as compared to the unexposed while the hardness of the exposed sample for 20 shots shows much greater hardness value at the surface of exposed samples and it decreases with the increase in the depth. The hardness of exposed sample for 30 shots shows almost the same depth profile throughout with little variation. Figure 9 shows the hardness of exposed sample for various (10, 20 and 30) shots at different depths of deposited film. Graph shows the maximum hardness of exposed samples at the top surface. The hardness of sample exposed for 10 and 20 shots increases at the penetration depth (25 nm) and maximum hardness is

7|Page observed for 20 focus deposition shots and on further increase in the number of deposition shots (i.e. 30 shots) it decreases. The graph shows the hardness of exposed substrate at different (25, 50, 75, 100 and 800 nm) penetration depths. In general the hardness of exposed substrate decreases with the increase in the penetration depth and the maximum value is observed at the top surface of exposed samples. The modulus of exposed substrate is present in Fig. 10. It shows that the modulus of the deposited film increases with the increase in the number of deposition. The elastic modulus qualitatively follows the enhancement of the hardness of the hard coatings. With the increase in the number of deposition shots, the elastic modulus increases.

22 20 18

Hardness (GPa)

16 20 Shots

14

30 Shots

12 10 8 10 Shots

6 4

Un exposed

2 0

100

200

300

400

500

600

700

800

Displacement into Surface of Deposited Film (nm)

Figure 8.

Hardness of samples treated with various deposition shots

900

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18 16

Hardness (GPa)

14

25 nm 50 nm 75 nm 100 nm 800 nm

12 10 8 6 4 2 0

10

20

30

Number of Deposition Shots

Figure 9.

Hardness of samples at different depths of exposed samples for different shots

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600

un-exposed 10 shots 20 shots 30 shots

550

Elastic Modulus (GPa)

500 450 400

30 Shots 20 Shots

350

10 Shots

300 250

Un-exposed

200 0

100

200

300

400

500

600

700

800

Displacement into Surface of Deposited Film (nm) Figure 10.

Modulus of samples exposed for 10, 20 and 30 deposition shots

900

10 | P a g e 6. Conclusion

The combination of tungsten nitride nano composite films steel was accomplished using energetic N2 ions of plasma focus device. The samples were exposed to the different deposition shots and then analyzed structural, surface morphological and mechanical properties. The XRD spectra show the development of WN phases (WN2 and WN) on the Steel substrate exposed for 10, 20and 30 deposition shots. SEM results show that nano structured grains are similarly spread over the substrate surface. The SEM graph of substrate showing for 10 shots shows nanostructure grains which are uniformly spread over the whole surface. The average grain size experiential in SEM graph is 45±10 nm. In SEM graphs of exposed samples for 20 and 30 shots, the result of grain agglomeration can be seen by increasing the nitrogen ion energy dose (number of deposition shots). Maximum agglomeration is observed for 30 deposition shots with the size of agglomerated species around 180±20 nm. The surface topography of sample showing for 10 deposition shots was studied using SEM which shows rough and uneven topology the roughness of the exposed substrate increases with the increase in the number of focus shots and maximum roughness was observed as ~52.2 nm at 30shots. Nano indentation results of exposed sample shows the enhancement in the hardness of substrate with the increase in the ion energy dose. The hardness and the surface roughness values of exposed samples for different deposition shots is higher than the unexposed sample which confirms the deposition of tungsten nitride films on the substrate surface. The phase analysis, grain size and roughness SEM value of exposed sample shows the formation of nano-structural tungsten nitride thin film on the substrate using plasma focus device.

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