Experimental Study of the Influence of Deposition of Multilayer CrN/CrCN PVD Coating on Austenitic Steel on Resistance to Cavitation Erosion

Round 1

Reviewer 1 Report

This study is based on the published literature [17]. Published literature [17] has studied the cavitation resistance of CrN / crcn multilayers under the condition of inlet pressure of 700KPA and outlet pressure of 125kpa. In this paper, the influence of multilayer CrN / crcn-pvd coating on the cavitation resistance of austenitic steel under the condition of inlet pressure of 600kpa and outlet pressure of 123kpa has been studied. Although the research results of this paper are compared with those of literature [17], the content of comparative analysis is too few, and more details are needed to reflect the difference between this paper and published literature [17] and the value of this paper.

Author Response

This study is based on the published literature [17]. Published literature [17] has studied the cavitation resistance of CrN / crcn multilayers under the condition of inlet pressure of 700KPA and outlet pressure of 125kpa. In this paper, the influence of multilayer CrN / crcn-pvd coating on the cavitation resistance of austenitic steel under the condition of inlet pressure of 600kpa and outlet pressure of 123kpa has been studied. Although the research results of this paper are compared with those of literature [17], the content of comparative analysis is too few, and more details are needed to reflect the difference between this paper and published literature [17] and the value of this paper.

Thank you for your comment. In the introduction, a sentence summarizing the results of an earlier article has been added. “It was shown there, that cavitation loading caused rapid degradation of this coating due to its low resistance to plastic deformation (high H/E ratio) and a brittle mode of the fractures.”

The purpose of this article has also been improved in relation to your comment.

“The purpose of this article is to examine an effect of a decrease of cavitation intensity on the cavitation erosion resistance of the CrN/CrCN coating – X6CrNiTi18-10 steel system, especially on the incubation period duration and on the final erosion rate. Because the inlet pressure affects cavitation intensity [21], the CrN / CrCN coating deposited on austenitic steel was tested at the inlet pressure of 600 kPa and the outlet pressure 123 kPa. The results obtained were compared with those from Ref. [20]. Because in hydraulic machinery cavitation intensity is lower than in experimental investigations in laboratories, the aim of this paper is important for practical applications of CrN/CrCN multilayer coating. In addition, the degradation process was investigated in detail, especially the state of the coating after 180 minutes of erosion was compared with the coating state after 600 minutes of erosion.”

Author Response File: Author Response.docx

Reviewer 2 Report

The content of this paper is very similar to Ref [17] published by the authors, except it has a lower inlet pressure of 600 kPa. The paper presents several flaws in the organization, in the methodological approach and in the presentation of the results. The conclusions cannot be justified based on the measurement of surface roughness. Therefore, I do not recommend this article to be published. My comments are listed below:

1. The numbering of the sections for experimental techniques and results and discussion are not in sequence.
2. The result of scratch testing (on line 100) should not be included in the experimental section.
3. The description for Fig. 1 is not clear, which area is low eroded zone and which one shows high eroded zone?
4. On line 177, the erosion rate of the CrN/CrCN-X6CrNiTi19-10 between two inlet pressures of 700 kPa and 600 KPa is very insignificant. Why is the inlet pressure reduced to 600 kPa for this paper, any specific reason?
5. The font size for figure caption in Fig. 4 is not consistent, in comparison to caption in Fig. 5.
6. Spelling errors are found on line 300 and 301, “zone stars from” should be “zone starts from”.
7. Is the SI unit for the roughness on line 337, 343 and 348 correct? Ra= 3.5 m? Ra =3.78 m? Ra= 7.74 m? How is the surface roughness of the uncoated and coated surfaces affecting the cavitation erosion?

Author Response

The content of this paper is very similar to Ref [17] published by the authors, except it has a lower inlet pressure of 600 kPa.

This article refers to the previous one that has been mentioned many times. So nothing was hidden. In the present article, the inlet pressure was changed. The results obtained indicate that a slight change in inlet pressure had a significant impact on the cavitation erosion resistance. This is very important for practical applications of this coating. Such explanation has also been added.

In addition, a thorough analysis of coating damage after 180 and 600 minutes of testing was carried out. This analysis allows learning about the development of coating damage. Such an analysis was not included in previous works. In addition, Figures 2 and 3 show the contribution of the deposited coating in volume loss and erosion rate of the tested system

The paper presents several flaws in the organization, in the methodological approach and in the presentation of the results.

The manuscript organization is the same as in all publications. At the beginning there is an introduction, then the research methodology and results obtained with the discussion. Finally, there are conclusions.

The conclusions cannot be justified based on the measurement of surface roughness.

The conclusions were not justified by roughness measurements. Roughness measurements were used to demonstrate compliance with the results obtained in the cavitation test and during microscopic observations. Nevertheless, the conclusions have been improved. Now they are as follows:

• Deposition of CrN/CrCN coating on X6CrNiTi18-10 austenitic steel increased incubation period and decreased cumulative volume loss. Until 300 minutes of erosion, only the coating in the CrN/CrCN - X6CrNiTi18-10 system was eroded. Starting from 500 minutes of erosion, the erosion rate of the system was higher in comparison to austenitic steel. Thus, in long lasting erosion test, brittle fractures of the coating increased the erosion rate.
• The decrease in the inlet pressure of 100 kPa resulted in a 22 % improvement in the cavitation erosion resistance of the CrN/CrCN - X6CrNiTi18-10 system. This improvement was obtained despite a large difference in H/E ratio between the coating and the substrate.
• Microscopic observations show that on the surface of X6CrNiTi18-10 steel, there were formed deformation twins covered by fatigue slip band agglomerates during the first 180 minutes of test. Slip bands are formed mainly in the easy slip system. However, in some places, slip bands were formed in two intersecting slip systems, indicating the very high impact energy delivered.
• Microscopic study of the CrN/CrCN coating - X6CrNiTi18-10 steel system showed the removal of the micro-droplets and formation of pits / craters by micro-jets during initial 180 minutes of the test. Cracks were formed on the tops of the undulated coating and developed along the pits after the removed micro-droplets, which decreased coating toughness.
• After initial 180 minutes of the test, surface roughness of the coating (Ra = 3.78 mm) was similar to that of the substrate (Ra = 3.5 mm) despite different development of surface damage.
• The mechanisms of formation and development of surface damage were related to the ability to withstand plastic deformation. In the case of X6CrNiTi18-10 steel, elongation of erosion test up to 600 minutes caused generation of pits and craters as well as the removal of surface particles. In the case of the CrN/CrCN coating - X6CrNiTi18-10 steel system, due to high H/E ratio, the coating was fractured in a brittle mode, punctured and removed in some places. The mechanisms of surface degradation affected a rate of change of surface damage.
  
• The measurements of surface roughness showed that at the beginning of the tests surface roughness increased at the highest rate when the volume loss was very low and afterwards the rate of an increase in surface roughness (Ra parameter) decreased with the test duration. This decrease in the rate of a surface roughness increase was greater for X6CrNiTi18-10 steel mainly due to overlapping damage.

Therefore, I do not recommend this article to be published.

My comments are listed below:

1. The numbering of the sections for experimental techniques and results and discussion are not in sequence.

The numbering has been corrected.

2. The result of scratch testing (on line 100) should not be included in the experimental section.

The scratch test result must be included in the experimental section, as they have already been presented in the previous work.

3. The description for Fig. 1 is not clear, which area is low eroded zone and which one shows high eroded zone?

Fig. 1 does not show zones with low and high erosion. Due to your comment, this figure has been modified.

4. On line 177, the erosion rate of the CrN/CrCN-X6CrNiTi19-10 between two inlet pressures of 700 kPa and 600 KPa is very insignificant. Why is the inlet pressure reduced to 600 kPa for this paper, any specific reason?

Thank you for your comment. It shows that the presented research is very important. The results presented in the previous article have not shown this key aspect of the impact of test conditions on coating resistance to cavitation load. Therefore, this article was prepared. The followed explanation has been added

“According to Refs [21,27,28], the inlet pressure in the cavitation chamber affects the flow velocity and cavitation intensity. Thus, a decrease in the inlet pressure of 100 kPa was sufficient to decrease the number of pulses and their impact speed to achieve a 22 % improvement in the cavitation erosion resistance of the CrN/CrCN - X6CrNiTi18-10 system. This increase in resistance to cavitation erosion was obtained despite a large difference in H/E ratio between the coating and the substrate. Considering that the cavitation intensity in real machines is lower than in laboratory devices and that the harder steel (e.g. martensitic steel) is often used as a substrate, this coating is expected to exhibit even better protection against cavitation erosion.”

5. The font size for figure caption in Fig. 4 is not consistent, in comparison to caption in Fig. 5.

Thank you for your comment. The font size has been corrected.

6. Spelling errors are found on line 300 and 301, “zone stars from” should be “zone starts from”.

Thank you. It has been corrected.

7. Is the SI unit for the roughness on line 337, 343 and 348 correct? Ra= 3.5 m? Ra =3.78 m? Ra= 7.74 m? How is the surface roughness of the uncoated and coated surfaces affecting the cavitation erosion?

Thank you for your comment. The SI unit of Ra parameter has been corrected. However, according to your comment, the conclusions have been changed.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors have done a good work that could provide valuable information in this field.

However, some issues should be resolved prior to the next steps.

In the hardness measurement, the maximum indentation depth was controlled to 300 nm, which is about 42% of the total coating thickness. Please note that when the indentation depth is more than 10% of the coating thickness, the substrate could influence the measured hardness [F. Cai et al., Wear 324-325 (2015) 27-35] [C. Berg et al., Surf.Coat.Technol.86–87 (1996)184–191] [D.M. Mattox, 1998, Film Characterization and Some Basic Film Properties, in: D.M. Mattox(Ed.), Handbook of Physical Vapor Deposition(PVD)Processing]. In this case, how can you be sure your hardness measurements were not affected by the substrate?

Also, you have measured the hardness and Young’s modulus, however, I have not seen from your work to indicate the correlation between these mechanical properties and the erosion behaviors of the coating systems. 

Erosion rates are evaluated using volume loss in the unit of mm3. However, this is a kind of absolute volume loss that cannot reflect specific erosion for specific materials and conditions such as on what material, with how much amount of erodent (water), and in how long a time period. Please consider this issue and update your results and discussions.  

There are also some issues in the writing and format. I am going to point out some but not everyone, and suggest the authors have professional editing and proofreading in the revision.

Lines 2 to 4, the title could be modified as (but not limited) "Experimental study of ......".

Line 20, delete "at".

Line 24, add an article "a" for "higher increase". Please check throughout the manuscript for similar problems and fix them all.

Line 31, "significant" is not properly used here, please revise.

Line 34, in "such situation" add an article. Please check throughout the manuscript for similar problems and fix them all.

Line 40, "in" can be changed to "in" here.

Line 44, what is "ca."? Please check "...of its action...".

Line 46 to 47, please check "... not several months or years...".

Line 56 to 57, please check "... with increasing 56 hardness increases brittleness,...".

Line 65, the last "of" can be changed to "in" here.

Line 67, "1." should be changed to "2.", please also check the following number and fix them all.

Line 67, "techniques" can be deleted here.

Line 69, "for" should be changed to "as" in this case.

Line 71, please check the temperature symbol.

Line 72, "Tab. 1" should be written as "Table 1".

Line 72 to 73, the density can be moved into Table 1.

Line 75, "after quenching" should be deleted here.

Table 2 should be moved after where it is first time mentioned in the text.

Line 77, "by" can be changed to "using" or "by using".

Line 81, "Tab. 2" should be written as "Table 2".

Line 85, please check " 10 at %".

Line 88 to 90, there are two thicknesses 4.3 µm and 700 nm, very confused. Please check and clarify.

Line 121, "Fig. 1" has a different format of that in the caption of Figure 1 (Line 123). Please make them written in the same format. Please check throughout the manuscript for similar problems and fix them all.

Line 131, please check why use "-".

Line 144, "that" should be deleted here.

Line 155, as pointed early in my comments, please accordingly check the unit "mm3".

In Figure 2, please check the unit for volume loss.

In Figure 3, please check the unit for volume loss rate.

In the text, both "min"  and "minute" are used. I suggest making them in a consistent format throughout the manuscript.

In the text, both "slip-band"  and "slip band" are used. Please make them in a consistent format throughout the manuscript.

Line 187, "by" should be changed to "with" in this case.

Line 189, delete "on".

Line 196, "by" should be changed to "with" in this case.

Line 275, 308, " approx." should be written as "approximately". Please check throughout the manuscript for the same issue and fix them all.

Line 327, check the title number for "Conclusions".

Line 340, please check "due to that are formed".

Line 337, 343, 348, check the unit "m".

Author Response

The authors have done a good work that could provide valuable information in this field.

Thank you for your feedback on this article and all comments that allow improving it.

However, some issues should be resolved prior to the next steps.

In the hardness measurement, the maximum indentation depth was controlled to 300 nm, which is about 42% of the total coating thickness. Please note that when the indentation depth is more than 10% of the coating thickness, the substrate could influence the measured hardness [F. Cai et al., Wear 324-325 (2015) 27-35] [C. Berg et al., Surf.Coat.Technol.86–87 (1996)184–191] [D.M. Mattox, 1998, Film Characterization and Some Basic Film Properties, in: D.M. Mattox(Ed.), Handbook of Physical Vapor Deposition(PVD)Processing]. In this case, how can you be sure your hardness measurements were not affected by the substrate?

Because the entire thickness of CrN/CrCN coating was 4.3 ± 0.2 mm, the 300 nm indentation depth was less than 10% of the coating thickness. thus, the used indentation depth was correct. The following sentence has been added:

“In order to eliminate an impact of the substrate on coating hardness measurements, the indentation depth must not exceed 10% of the entire coating thickness [23,24]. Due to the coating thickness of 4.3 ± 0.2 µm, the maximum indentation depth of 300 nm has been applied. Hardness of the CrN/CrCN multilayer coatings is 24 ± 2 GPa and Young’s modulus of the coating is 235 ± 5 GPa. Hardness and Young’s modulus depend on the bilayer module thickness [25]. However, increasing thickness of bilayer module above 150 nm has no effect on hardness and Young’s modulus [25].”

Also, you have measured the hardness and Young’s modulus, however, I have not seen from your work to indicate the correlation between these mechanical properties and the erosion behaviors of the coating systems. 

Thank you for your comment. The following descriptions have been added to 3.1. section about Cavitation erosion test:

“The presence of the incubation period of CrN/CrCN coating is associated with its high hardness and Young’s modulus. Both of these properties affect critical stress to initiate fracture. In the case of X6CrNiTi18-10 steel, both of the mentioned properties are low, so material loss occurred at the beginning of cavitation erosion test.”

“Considering that the resistance to plastic deformation of the CrN/CrCN coating was H/E = 0.102, this coating can be considered very stiff in comparison to the steel substrate (H/E = 0.0085). Despite the large difference in the ability to withstand plastic deformation, the deposited coating was characterised by good protection against cavitation erosion.”

“This rate of erosion affected the degradation of the CrN/CrCN coating. Because of high H/E ratio of the coating, critical stress required to initiate fracture is also high. However, after initiation, the cracks develops rapidly. Thus, an increase in the erosion rate of the coating was associated with initiation and development of the cracks.”

“This increase in resistance to cavitation erosion was obtained despite a large difference in H/E ratio between the coating and the substrate. Considering that the cavitation intensity in real machines is lower than in laboratory devices and that the harder steel (e.g. martensitic steel) is often used as a substrate, this coating is expected to exhibit even better protection against cavitation erosion.”

Erosion rates are evaluated using volume loss in the unit of mm3. However, this is a kind of absolute volume loss that cannot reflect specific erosion for specific materials and conditions such as on what material, with how much amount of erodent (water), and in how long a time period. Please consider this issue and update your results and discussions.  

In cavitation erosion, not water, but cavitation micro-jets and shock waves are the factors that cause surface damage. According to vocabulary for solid particle erosion, cavitation micro-jets and shock waves are erodent. How much this “erodent” is, depends on flow conditions, e.g. water pressure and flow rate, properties of water, i.e. its temperature and amount of undissolved air and the construction of the device. A lot of research is still being done to assess the amount of erodent in cavitation erosion, i.e. amount of micro-jets for given cavitation conditions.

According to ASTM G32 standard, the followed definitions are used:

3.1.4 cavitation erosion, n—progressive loss of original material from a solid surface due to continued exposure to cavitation.
3.1.5 cumulative erosion, n—the total amount of material lost from a solid surface during all exposure periods since it was first exposed to cavitation or impingement as a newly finished surface. (More specific terms that may be used are cumulative mass loss, cumulative volume loss, or cumulative mean depth of erosion. See also cumulative erosion-time curve.)
3.1.5.1 Discussion—Unless otherwise indicated by the context, it is implied that the conditions of cavitation or impingement have remained the same throughout all exposure periods, with no intermediate refinishing of the surface.
3.1.6 cumulative erosion rate, n—the cumulative erosion at a specified point in an erosion test divided by the corresponding cumulative exposure duration; that is, the slope of a line from the origin to the specified point on the cumulative erosion-time curve. (Synonym: average erosion rate)
3.1.7 cumulative erosion-time curve—a plot of cumulative erosion versus cumulative exposure duration, usually determined by periodic interruption of the test and weighing of the specimen. This is the primary record of an erosion test. Most other characteristics, such as the incubation period, maximum erosion rate, terminal erosion rate, and erosion rate-time curve,
are derived from it

Thus, in cavitation erosion, loss of material from a solid surface is measured using the mass loss or volume loss. In the presented work, volume loss was used due to different density of the tested coating and steel substrate. The SI unit of volume is m³. Because of the size of volume loss, the unit of mm³ can also be used. The used units of volume loss and erosion rates are correct.

Nevertheless, your comment inspired to show separately the loss of volume of the CrN/CrCN coating and the entire CrN/CrCN - X6CrNiTi18-10 system. The volume losses of the coating and the entire CrN/CrCN - X6CrNiTi18-10 system have been recalculated based on the observed eroded area. Thus, Figs 2 and 3 have been changed.

There are also some issues in the writing and format. I am going to point out some but not everyone, and suggest the authors have professional editing and proofreading in the revision.

Lines 2 to 4, the title could be modified as (but not limited) "Experimental study of ......".

Thank you for your suggestion. The title has been modified.

Line 20, delete "at".   “at” has been deleted.

Line 24, add an article "a" for "higher increase". Please check throughout the manuscript for similar problems and fix them all.

Thank you for your comment. Similar errors have been corrected.

Line 31, "significant" is not properly used here, please revise.         It has been corrected.

Line 34, in "such situation" add an article. Please check throughout the manuscript for similar problems and fix them all.          It has been corrected.

Line 40, "in" can be changed to "in" here.

Line 44, what is "ca."? Please check "...of its action...".       It has been corrected.

Line 46 to 47, please check "... not several months or years...".       This sentence has been corrected.

Line 56 to 57, please check "... with increasing 56 hardness increases brittleness,...".

Thank you for your comment. This sentence has been modified into:

Line 65, the last "of" can be changed to "in" here.

Line 67, "1." should be changed to "2.", please also check the following number and fix them all.          Thank you for comment. It has been corrected.

Line 67, "techniques" can be deleted here.   This word has been deleted.

Line 69, "for" should be changed to "as" in this case.          Thank you. “for” has been changed to “as”.

Line 71, please check the temperature symbol.         It has been corrected.

Line 72, "Tab. 1" should be written as "Table 1".     It has been corrected.

Line 72 to 73, the density can be moved into Table 1.         The density has been moved.

Line 75, "after quenching" should be deleted here.  It has been corrected.

Table 2 should be moved after where it is first time mentioned in the text.

Table 2 has been moved next to the paragraph describing the deposition of the coating.

Line 77, "by" can be changed to "using" or "by using".        It has been corrected.

Line 81, "Tab. 2" should be written as "Table 2".     It has been corrected.

Line 85, please check " 10 at %".     

Line 88 to 90, there are two thicknesses 4.3 µm and 700 nm, very confused. Please check and clarify.

These sentences have been rewritten. Now they are: “The thicknesses of the CrN + CrCN bilayer (so called bilayer period) and the multilayer coating were determined using ball cratering test (Calotest) method. The thickness of the entire CrN/CrCN coating was 4.3 ± 0.2 µm, while the thickness of the bilayer period was approximately 700 nm”

Line 121, "Fig. 1" has a different format of that in the caption of Figure 1 (Line 123). Please make them written in the same format. Please check throughout the manuscript for similar problems and fix them all.             It has been corrected.

Line 131, please check why use "-".  It has been corrected.

Line 144, "that" should be deleted here.       It has been corrected.

Line 155, as pointed early in my comments, please accordingly check the unit "mm3".

In Figure 2, please check the unit for volume loss.

The unit of volume is mm3, so this unit has been used for volume loss.

In Figure 3, please check the unit for volume loss rate.

In order to determine the rate of volume loss, the volume loss obtained in a given period was divided by this time.

In the text, both "min"  and "minute" are used. I suggest making them in a consistent format throughout the manuscript.

Thank you for your comment. Only the term 'minute' is now used throughout the manuscript.

In the text, both "slip-band"  and "slip band" are used. Please make them in a consistent format throughout the manuscript.

It has been corrected

Line 187, "by" should be changed to "with" in this case.     It has been corrected

Line 189, delete "on". It has been corrected

Line 196, "by" should be changed to "with" in this case.     It has been corrected

Line 275, 308, " approx." should be written as "approximately". Please check throughout the manuscript for the same issue and fix them all.         It has been corrected

Line 327, check the title number for "Conclusions". It has been corrected

Line 340, please check "due to that are formed".     This sentence has been corrected

Line 337, 343, 348, check the unit "m".        It has been corrected

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

According to the author's modification, the purpose of this paper is more clear, and the value of this paper is also reflected.

Reviewer 2 Report

The authors have satisfactorily responded to all my questions. There is a minor comment.

On line 125, it should be 2.2 Cavitation erosion test

Reviewer 3 Report

The authors have made a significant improvement in this revision. Please check some of the minor issues and resolved them.  

Line 47, in "... not several months or years...", did you intend to say "...rather than several months or years..."?

Thanks to the author for the explanation about the erosion unit. It will be better to introduce, in Section 2.1. Cavitation erosion test, the test followed ASTM G32 standard, and cite it (with full title and publication information) as a reference to support you.

Line 59, about "H/E" and "H3/E2"...more earlier references are suggested to be cited,

[18] S.-C. Lee, W.-Y. Hob, F.D. Lai, Effect of Substrate Surface Roughness on the Characteristics of CrN Hard Film, Mater. Chem. Phys. 43 (1996) 266.

[19] A. Leyland, A. Matthews, On the Significance of the H/E Ratio in Wear Control: A Nanocomposite Approach to Optimised Tribological Behaviour, Wear 246 (2000) 1.

[20] K.L. Johnson, Contact Mechanics, Cambridge University Press, London, 1985.

Please also rearrange the orders of references in the text and the list of references.

Table 2 should be moved after where it is first time mentioned in the text.

Line 92 to 93, "by use of" should be changed to "by using".

 Line 100, " 10 at %" should be written as "10 at.%" in this case.

Figure 1 should be moved after where it is first mentioned in the text. Please check throughout the manuscript for similar problems and fix them all.

Still some of both "Fig. x" and "Figure x" are used in the manuscript. Please check throughout the manuscript and writing them in a consistent format.