Effects of Nitrogen Flow Ratio on Structures, Bonding Characteristics, and Mechanical Properties of ZrN_x Films

Reviewer 1 Report

1. I would suggest authors rewrite some parts of the conclusion more quantitative manner. The conclusion sounds like hypothesis paragraph of Introduction. 

Thanks for the suggestion! The ”Conclusions” was rewritten.

2. Add some more sentences in introduction.

Thanks for the suggestion! The “Introduction” was improved. New sentences were added in lines 27–28 and 39–44. New references 11–14 and 28 were added.

Reviewer 2 Report

The authors investigated structure, bonding characteristics, and mechanical properties of ZrNx films fabricated by direct current magnetron sputtering. The study shows that the film structure synthesized at N2 flow ratio of 0.5 possessed crystalline ZrN and o-Zr3N4 phases which gradually changed to o-Zr3N4 dominant with further increasing the N2 flow ratio up to >0.85. Additionally, the bonding characteristics of the ZrNx films were consisted of Zr–N bonds of ZrN and Zr3N4 compounds. More interestingly, their nanoindentation hardness and Young’s modulus varied in a range between 16.8 and 17.7 GPa and between 190 and 211 GPa, respectively. The work is interesting and can be published in Coatings if the following issues can be addressed:

1. The authors should cite the papers “Effect of nitrogen flow ratio on structure and properties of zirconium nitride films on Si(100) prepared by ion beam sputtering” (Bulletin of Materials Science, 2012, 35, 885-887) and “High-performance carbon fiber/gold/copper composite wires for lightweight electrical cables” (Journal of Materials Science & Technology, 2020, 42, 46-53) in the introduction section for better reviewing the metal coating methods.

Thanks for the suggestion! The reference (Bulletin of Materials Science, 2012, 35, 885-887) was added as the new Ref. 28. The “Introduction” was improved. New sentences were added in lines 27–28 and 39–44. New references 11–14 were added.

2. Why was N2 flow ratio of 0.5-0.85 chosen? Can flow ratio of more than 1 or below 0.4 be used for film synthesis?

The illustrations in lines 58–62 was revised as “As the deposition rate decreased with increasing N₂ flow ratio (Table 1), both guns 1 and 2, with powers of 150 W, were utilized for raising the deposition rate of the process with f = 1.00. Zr₆₀N₄₀ films prepared with an N₂ flow ratio of 0.4, gun 1 power of 300 W and sputter time of 60 min that were reported in a previous study [30] are discussed for comparison.”

“The decrease in deposition rate with the increasing f level was caused by the reduced ionization and sputtering yield of N₂ related to Ar gas [34]. Moreover, the well-known target poisoning effect in reactive sputtering reduced the deposition rate [34–36].” was added in lines 104–107. New references 34–36 were added.

3. Why did the stoichiometric variable x increase with increasing N2 flow ratio f?

“Previous studies [12,16,28] have reported that interstitial N is incorporated into the ZrN structure to form a Zr₃N₄ phase as the nitrogen flow ratio was higher than the critical level to form stoichiometric ZrN.” was added in lines 39–41.

“Ramana et al. [35] reported that ZrN films prepared with an f <0.28 through DCMS were crystalline. The stoichiometric variable x (N/Zr) increased by raising the f level either in DCMS [35] or hollow cathode discharge ion-plating [37].” was added in lines 113–116. New Ref. 37 was added.

4. Why did the intensity ratio of ZrN(111):Zr3N4(320) reduce with increasing ratio f?

The sentence in lines 125–128 was revised as “The intensity ratio of ZrN(111):Zr₃N₄(320) varied from 54:46 to 49:51, 44:56, 37:63, and 33:67 as the f value increased from 0.50 to 0.65, 0.75, 0.85, and 1.00, which implied that the phase varied from ZrN to Zr₃N₄ dominant as the f value increased.”

5. How did the free surface have serious oxygen pollution?

The sentence was revised as “The profiles at the free surface exhibited serious oxygen pollution because of a high affinity of Zr and O, …” as shown in lines 151–152.

6. Why did the Zr50N50(0.65) have the highest hardness and Young’ modulus ?

The mechanical properties of ZrN_x films in this study were measured again using an indentation depth of 50 nm. (Line 79)

New sentences were added in

Lines 192–195, “The Zr₄₅N₅₅(0.85) films exhibited an underestimated hardness of 17.3 GPa, which was attributed to the substrate effect because the indentation depth was 50 nm and the films possessed a low thickness of 348 nm (Table 1), which did not follow the 1/10 rule for accurate examination [43].” New Ref. 43 was added.

Lines 208–212, “In summary, the hardness values of crystalline ZrN films were related to their stressed conditions. The mechanical properties of ZrN_x(f) films (f = 0.50–1.00) were dominated by the o-Zr₃N₄ phase in a nanocrystalline form accompanied with a low compressive stress level; therefore, the Young’s modulus exhibited a slight decreasing trend and the hardness maintained a constant level as the f value increased.”

Reviewer 3 Report

The article presents by Yi-En Ke and Yung-I Che, regarding the effect of nitrogen flow on the properties of ZrN films, shows an overall interesting study. Although the pertinence and need of the study are not shown, the results are well-conducted and can be of interest to the community. A few comments and concerns are provided here.

1. The introduction is very short. It does not make an effort of showing the current status of the field. Recent and important studies on nitrogen influence on nitride coatings or the need for this current study in the literature. The introduction needs to be improved and much more current and relevant literature on ZrN (and other transition metal nitride coatings) needs to be included and discussed.

Thanks for the suggestion! The “Introduction” was improved. New sentences were added in lines 27–28 and 39–44. New references 11–14 and 28 were added.

2. Table 1 needs to include the carbon content, I assume that the carbon peak was used to calibrate the signal.

The chemical compositions listed in Table 1 were examined by EPMA as indicated in lines 71–72, not by XPS.

The sentence in lines 84–86 was revised as “The chemical states of the constituent elements were examined by using an X-ray photoelectron spectroscope (XPS, PHI 1600, PHI, Kanagawa, Japan) with an Mg Ka X-ray beam and calibrated with the C 1s line at 284.6 eV.”

3. Roughness values need to be provided. As well as the indentation depth and a number of tests performed for each mechanical value. I would recommend include the load vs displacement curves for all the samples as image 7.

Roughness values were added in Table 3. The mechanical properties of ZrN_x films in this study were measured again using an indentation depth of 50 nm. Because all the ZrN_x films exhibited insignificant variations in mechanical properties, a typical load-displacement curve of Zr₅₂N₄₈(0.50) films was shown in the new Figure 8. The mechanical properties of Zr₆₀N₄₀(0.40) films were added in Table 3 for comparison.

“The hardness and Young’s modulus values of films … were determined form 8 measurements based on the Oliver and Pharr method [31].” was revised as shown in line 78.

New sentences were added in

Lines 88–90, “The average surface roughness (Ra) of the films determined from 3 measurements was evaluated by using an atomic force microscope (AFM, Dimension 3100 SPM, NanoScope IIIa, Veeco, Santa Barbara, CA, USA) with a scanned area of 5 × 5 μm² [33].”

Lines 192–196, “The Zr₄₅N₅₅(0.85) films exhibited an underestimated hardness of 17.3 GPa, which was attributed to the substrate effect because the indentation depth was 50 nm and the films possessed a low thickness of 348 nm (Table 1), which did not follow the 1/10 rule for accurate examination [43]. Figure 8 displays a representative curve of displacement against load in the nanoindentation test.” New Ref. 43 was added.

Lines 212–217, “Moreover, the evaluation of thin film mechanical properties by nanoindentation technique was affected by the surface roughness [47]. Surface roughness reduced averages and enlarged deviations of nanoindentation hardness and Young’s modulus. All the ZrN_x films exhibited a low average surface roughness of 0.7–1.2 nm (Table 3); therefore, the error in the determination of mechanical properties was negligible.” New Ref. 47 was added.

4. There is no clear indication of how did the authors calculate the residual stress on the films. The explanation and calculation method is needs.

The equation and illustration were added in lines 81–84.

5. TEM experiments aren’t discussed in detail, beyond the apparition of nanocrystals (nanocomposites). Why did the authors choose to examine the 0.75 sample?. Do the authors have other samples to compare the crystallinity and microstructure?.

The 0.50, 0.65, and 0.75 samples exhibited similar XRD pattern as illustrated in lines 116–117 and same mechanical properties as discussed in Section 3.3. A new TEM observation on the 1.00 sample was added as the new Figure 4. Related modifications were shown in lines 136–140.

6. In figure 3(b), please also include the FFT pattern of the area.

Thanks for the suggestion! FFT patterns were added in Figure 3b.

7. Conclusions are hastily included, please expand this section. Also, the compressive stress plays an important role in this section, which clearly shows that it needs to be addressed properly in the text.

Thanks for the suggestion! The”Conclusions” was rewritten.

The compressive stress issue was illustration in Lines 208–212,

“In summary, the hardness values of crystalline ZrN films were related to their stressed conditions. The mechanical properties of ZrN_x(f) films (f = 0.50–1.00) were dominated by the o-Zr₃N₄ phase in a nanocrystalline form accompanied with a low compressive stress level; therefore, the Young’s modulus exhibited a slight decreasing trend and the hardness maintained a constant level as the f value increased.”