Microstructure and Surface Topography Study of Nanolayered TiAlN/CrN Hard Coating
, Peter Gselman 1,2, Matjaž Panjan 1, Tonica Bončina 3, Aljaž Drnovšek 1, Mihaela Albu 4, Miha Čekada 1 and Franc Zupanič 3Round 1
Reviewer 1 Report
On the example of TiAlN/CrN multilayer coatings, the authors illustrated the influence of substrate rotation mode, type of the substrate material and the method of ion etching on microstructure, surface topography, layer periodicity, interlayer roughness and formation of growth defects. The manuscript presents novel results, which are worth publishing.
Still, there are some awkward sentences, which should be reformulated, and open questions, which should be answered, before the manuscript will be published.
1. Page 2 lines 72-77: Please explain why the substrate surface roughness and the angle of incidence of the arriving atoms can increase the roughness of the coating surface, while the deposition temperature and the intensity of energetic particle bombardment can decrease its roughness. In my opinion, the coating roughness can also decrease by reducing substrate surface roughness and changing the incidence of the arriving atoms. Also, decreasing the deposition temperature and intensity of energetic particle bombardment should increase its roughness.
2. Page 2 lines 90-92: The authors assume that the epitaxial growth of multilayer are be strongly dependent on the crystal structure and lattice mismatch of constituent layers as well as coating thickness. This assumption must be justified, e.g., by citing a reference. Furthermore, the modulation periods and modulation ratios of multilayer may also affect the formation of coherent interfaces [Yu X. Xu et al., Acta Materialia 130 (2017) 281-288]. The similar discussion part on page 9 lines 307-310 also lacks related evidence.
3. Section 3.1: Please provide the chemical composition of TiAlN sublayer. In general, the change in Al content can affect the crystal structure and surface roughness of the coating. Further, the lattice mismatch between TiAlN and CrN sublayer is also correlated with the content of Al. Please comment on this issue.
4. Page 7 lines 274-275: The authors say that “The thickness of the CrN and TiAlN layers is about 51 nm and 62 nm, respectively.” However, the BF cross-sectional SEM in Fig. 5 reveals the increasing modulation period and modulation ratios (tTiAlN:tTiN) from the substrate-coating boundary to the top surface. Thus at which position is the thickness measured?
5. Page 8 lines 279-280: “It increases more after the deposition of each TiAlN layer, while the upper CrN layer slightly smooths it.” However, in my opinion, there is no distinct roughness difference between TiAlN and CrN sublayers in Fig. 5. Besides, could the larger interlayer roughness near the coating surface than that near the substrate is caused by the geometrical shadowing effect?
6. Page 8 lines 281-282: Why the crystal grains of the TiAlN layer are coarser than CrN ones? Please comment on this issue through further experiments or relevant literature.
7. Page 11 lines 387-388: Can the authors please further comment on the surface topography of the nl-TiAlN/CrN coating prepared by different rotation modes? What phenomenon does the AFM observation in Fig. 8 demonstrate?
8. Page 12 lines 414-415: “The larger clusters in Fig. 10a and 10c have grown on the sites of carbide inclusions in the ASP and D2 substrates.” I cannot find Fig. 10a and 10c in Figure 10.
9. Many discussion parts in Sections 3.3 and 3.4 obviously lack a lot of literature to prove. e.g. “Page 10 lines 361-362: During polishing, the removal rate of harder inclusions (e.g. carbides, oxides) and softer ones (e.g. sulfides) is lower and higher than the matrix, respectively.” “Page 10 lines 365-367: After polishing and ion etching, the geometrical extension from the matrix level (typically up to a few hundred nanometers either in a positive or a negative direction) at the site of inclusions depends on the net removal rate. ” “Page 13 lines 427-429: All steels contain non-metallic inclusions to some extent as they precipitate during the cooling and solidification of the steel.” “Page 13 lines 436-440: Namely, a high density of coarse inclusions can cause the formation of cracks and thus initiates fracture and decreases the toughness of steels. Surface defects caused by them deteriorate mechanical properties like corrosion resistance, deformability, brittle fracture and fatigue strength. They also affect the weldability, polishability, and machinability of the steel.” Please comment on these issues.
Minor comments
Page 4 line 156: Replace "1 mm2" by "1 mm2"
Page 4 line 175: Replace "Z (atomic number) contrast imaging (HAADF-STEM) " by " high angle annular dark field (HAADF-STEM)"
Page 7 line 272: Replace "BSE" by " backscattered electron (BSE)"
Page 10 line 358: Replace “the same coating surface area” by “the same surface area”
Page 11 line 393: Replace “the bare substrates” by “the substrate after polishing”. Also, the corresponding legend in Fig. 9 should be revised.
Author Response
see attached file
Author Response File: 
Author Response.docx
Reviewer 2 Report
Manuscript ID: Coatings- 1995254
The introduced manuscript under the title of “Microstructure and Surface Topography Study of Nanolayered TiAlN/CrN Hard Coating” shows a study of the microstructure, surface topography, layer periodicity, interlayer roughness and formation of growth defects at the sites of non-metallic inclusions and foreign seeds of nanolayered TiAlN/CrN hard coating deposited by DC sputtering system on various substrates materials.
These comments might improve the manuscript
1- The effects of studied topographical and microstructural properties on the mechanical properties like hardness, fracture toughness, and adhesion of deposited nanolayered TiAlN/CrN hard coatings should be indicated.
2- The rotation parameters are not clear in the experimental section to easily differentiate among the 1-,2-, and 3-fold modes, which should be pointed out
3- The substrate materials (WC-Co) should be stated rather than the company name (HM) in the figures' caption and results discussions
4- The orientation investigated SAD pattern and its effects on films properties should be included.
5- The crystal size and lattice structures estimated from HRTEM measurements should be pointed out and compared for the 1-, 2-, and 3-fold modes
Author Response
see attached file
Author Response File: Author Response.docx
Reviewer 3 Report
The manuscript reports the study of the growth and the morphology of nanolayred TiAlN/CrN coatings. The materials were deposited by PVD technique on different steels and cemented substrates. The study was performed according to three different rotation mode. With the single rotation, the coatings have periodic and columnar structure while the nanolayers prepared by 3-fold rotation are aperiodic. The roughness and the effect of the preparation of the substrates surface were discussed and detailed.
The topic is for sure of high interest for the readership of Coatings.
Author Response
We would like to thank the reviewer for his time and effort in accessing the manuscript.
Reviewer 4 Report
The article meets the requirements to be published in Coatings. The article may be published as is.
Author Response
We would like to thank the reviewer for his time and effort in assessing the manuscript.
Round 2
Reviewer 1 Report
The paper can be accepted without any further changes.
Author Response
We took into account both of the reviewer's suggestions:
- we replaced the abbreviation"nl" with "NL" and
- - the term "hard metal" was replaced with the "cemented carbide", while the abbreviation "HM" was replaced with "WC-Co"
Reviewer 2 Report
Manuscript ID: Coatings- 1995254 -V2
The introduced manuscript under the title of “Microstructure and Surface Topography Study of Nanolayered TiAlN/CrN Hard Coating” shows a study of the microstructure, surface topography, layer periodicity, interlayer roughness and formation of growth defects at the sites of non-metallic inclusions and foreign seeds of nanolayered TiAlN/CrN hard coating deposited by DC sputtering system on various substrates materials.
These comments might improve the manuscript
1- Why are you studying the effects of topographical and microstructural properties on the nanolayered TiAlN/CrN hard coatings? The effects of this study on the mechanical properties like hardness, fracture toughness, and adhesion of deposited films should be indicated.
2- The rotation parameters are not clear in the experimental section to easily differentiate among the one-, two-, and three-fold modes, which should be pointed out because these parameters are essential to the reader to understand the novelty of the papers.
3- The substrate materials (WC-Co) should be stated rather than the company name (HM) in the figures' caption and results discussions
4- The investigated orientation by SAD pattern and its effects on films properties should be included.
5- The crystal size and lattice structures estimated from HRTEM measurements should be pointed out and compared for the one-, two-, and three-fold modes
Author Response
Author's reply to the review report (Reviewer 2)
We wish to express our appreciation for your comments, which have helped us improve the paper.
NOTE : black color: reviewer’s comments, blue color: authors’ responses and highlighted in the manuscript with red color (track changes).
Reviewer’s comments and authors’ responses
Reviewer remark 1: The effects of studied topographical and microstructural properties on the mechanical properties like hardness, fracture toughness, and adhesion of deposited nanolayered TiAlN/CrN hard coatings should be indicated.
Answer: The mechanical and tribological properties of the nl-TiAlN/CrN layer have been not studied in this work. In general, the coating topography influences tribological properties, while the microstructure and layer structure of the coating has a large influence on hardness and fracture toughness. There is no direct influence of surface topography and microstructure on the adhesion of the coating.
The multilayer hard coatings have a more fine-grained microstructure than the single-layered ones because the layer interfaces stop the growth of some columnar grains, and at the same time enable the nucleation of new crystal grains. The consequence of the smaller grain size is an increase in the hardness and strength of the coating as follows from the Hall-Petch equation. The stresses formed at the interfaces due to different lattice constants contributes also to the higher hardness of a nanolayered coating in comparison to monolayer coating. Hardness differences between different types of rotation are not attributed to the thickness of the individual layers but to the microstructure of the coating.
Reviewer remark 2: The rotation parameters are not clear in the experimental section to easily differentiate among the 1-,2-, and 3-fold modes, which should be pointed out.
Answer: The relevant reference is added, where a detailed description of the rotation geometry is given. The first axis of rotation is in the center of the turntable, while the substrate towers rotate around the second axis, which is positioned 137 mm away from the first axis. The substrate towers rotate around the first and the second axis. The rotation around the third axis is not continuous but is achieved by a switch fixed on the rod. For every rotation of the substrate tower around the second axis, the switch turns the sample for a specific angle. The distance from the second axis to the third axis is 58 mm.
Reviewer remark 3: The substrate materials (WC-Co) should be stated rather than the company name (HM) in the figures' caption and results discussions
Answer: Cemented carbides can also be referred as to ”hardmetals” or ”hard metals”. In German only the term "Hartmetall" exists (see Ref. H. Ortner, H. Kolaska, P. Ettmayer. The history of the technological progress of hardmetals, Int. J. Refract. Met. Hard Mater. 44 (2014) 148-159).
Reviewer remark 4: The orientation investigated SAD pattern and its effects on films properties should be included.
Answer: Selected area electron diffraction (SAED) analysis of the nl-TiAlN/CrN layer was already discussed in our previous paper “Panjan M.; Šturm S.; Panjan P.; Čekada M. TEM investigation of TiAlN/CrN multilayer coatings prepared by magnetron sputtering, Surf. Coat. Technol., 2007, 202, 815« (see Ref 12 in the manuscript).
Reviewer remark 5: The crystal size and lattice structures estimated from HRTEM measurements should be pointed out and compared for the 1-, 2-, and 3-fold modes.
Answer: HRTEM analysis of the nl-TiAlN/CrN layer was already discussed in more detail in our previous paper “Panjan M.; Šturm S.; Panjan P.; Čekada M. TEM investigation of TiAlN/CrN multilayer coatings prepared by magnetron sputtering, Surf. Coat. Technol., 2007, 202, 815« (see Ref 12 in the manuscript).
Reviewer remark 6: English language and style are fine/minor spell check required
Answer: The revised manuscript was grammatically checked by an English language editing service.
