Influence of Growth Defects on the Corrosion Resistance of Sputter-Deposited TiAlN Hard Coatings
, Aljaž Drnovšek 1, Peter Gselman 1,2, Miha Čekada 1, Matjaž Panjan 1, Tonica Bončina 3 and Darja Kek Merl 1,†Round 1
Reviewer 1 Report
The thesis discusses the effects of layer growth defects on the corrosion properties of layer-substrate composites. Disturbances and inclusions in the base material are very well represented. The pictures shown are very vivid and well described.
Unfortunately, the paper is limited to the presentation of the growth defects and their effects. At the begiinig it is mentionned that HiPIMS is suitable to improve, reduce defect in coatings. But this is not investigated and discussed. Thus concepts that could serve to avoid the growth defects are unfortunately not mentioned or investigated. Due to the good representation the pictures have textbook character.
The results of impedance measurement shown in Fig. 7 are somewhat lost in the article. It seems that there is no connection to the shown coating defects. The measurement acts only as the load the coatings was subjected to. Did the different coating failure behave all the same? Was it only one coating that was investigated?
Author Response
Dear reviewer,
The paper was corrected in accordance to your comments. We would like to thank you for valuable comments and suggestions. The corrections are explained bellow (blue color) and highlighted in the manuscript with red color.
Corrections on suggestions of Referee #1:
Remark #1: Unfortunately, the paper is limited to the presentation of the growth defects and their effects. At the begiinig it is mentionned that HiPIMS is suitable to improve, reduce defect in coatings. But this is not investigated and discussed. Thus concepts that could serve to avoid the growth defects are unfortunately not mentioned or investigated. Due to the good representation the pictures have textbook character.
A1: The main objective of this work was to examine which growth defects most influence corrosion resistance. In the introductory part we have also described all known methods for increasing the corrosion resistance of hard coatings. One of these approach is also the high power impulse magnetron sputtering (HiPIMS) deposition technique. Unfortunatly, at this moment we are not able to prepare the hard coatings with HiPIMS deposition tehnique in our lab. HiPIMS provides an increased coating density and smooth surface with lower defect density. However HIPIMS deposited coatings can still suffer from surface imperfections, i.e. coating defects which are associated with external factors, such as substrate pits and the flakes generated from the chamber components and are independent of deposition technique. Only some smaller pinholes can be closed by higher adatom mobility. Anyway the corrosion study performed by Biswas et al. (Surf. & Coat. Technol. 344 (2018) 383-393, doi:10.1016/j.surfcoat.2018.03) confirmed the positive influence of dense microstructure on the barrier properties of the coatings.
Remark #2: The results of impedance measurement shown in Fig. 7 are somewhat lost in the article. It seems that there is no connection to the shown coating defects. The measurement acts only as the load the coatings was subjected to. Did the different coating failure behave all the same? Was it only one coating that was investigated?
A1: The reviewer's comment is understandable. However we would like to stress that the focus of this work is the pitting corrosion attack at the growth defects after corrosion test using electrochemical impedance spectroscopy. The impedance data during immersion in 0.5M NaCl solution are presented in form of complex plane diagrams (Nyquist plots). Such diagrams offers some information regarding the performance of coatings with small defect deposited on less noble metal substrate. Thus, an initially high impedance (low corrosion rate) decreases with immersion time (this means that the corrosion rate increases). Higher corrosion rate can be explained with the fact that the coating defects open out, exposing more surface area of substrate to the NaCl solution. These impedance data are consistent with the results of FIB analysis.
Of course an additional information can be obtained by quantitative analysis of impedance data (e.g. the resistance of pores in the coating (Rpo), polarization resistance of the substrate (Rs), the electrical double-layer capacitance at the electrolyte/substrate interface (Cs)). However we add impedance plots only for illustration, while detail quantitative analysis should be a subject of separate paper. Anyway we added in the manuscript (see section 3.2) some additional explanations.
Of course we investigated the corrosion properties of series of samples. In this work we present EIS measurements only on the sample to which FIB measurements are related.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper deals with the investigation of defects genesis and morphology in TiAlN hard coatings as a result of the specific type of superficial imperfections. The topic is well introduced, the characterizations complete for the aim of the work and well discussed.
My only remark is about really minor grammar and spelling issues in English Language like "extent" (line 35), "was appeared" (168), "growth" (188), line 193, etc...
Author Response
Dear reviewer,
The paper was corrected in accordance to your recommendation. We would like to thank you for valuable suggestions. The corrections are explained bellow (blue color) and highlighted in the manuscript with red color.
Corrections on suggestions of Referee #2:
Reviewer 2: The paper deals with the investigation of defects genesis and morphology in TiAlN hard coatings as a result of the specific type of superficial imperfections. The topic is well introduced, the characterizations complete for the aim of the work and well discussed.
Remark #1: My only remark is about really minor grammar and spelling issues in English Language like "extent" (line 35), "was appeared" (168), "growth" (188), line 193, etc...
A1: The reviewer suggestion was considered. We improved the English according to referee's comments.
Author Response File: Author Response.docx
Reviewer 3 Report
The title have to be improved. Instead "Resitance" please put in "Resistance".
The reference list have to be improved. In the text is 32 references and in the chapter reference even 35. Please see Ref. [29].
How the sentence "It is assumed that ion etching leads to a denser coating..."can be understand? It is known that ion etching and deposition process are two different operationsand during ion etching the coating is not deposited.
The bias voltage was really 100 V, not -100 V?
Chapter "Experiment" should also include the data about Al2O3/TiAlN coating deposition parameters.
Chemical composition of D2 steel include max 0.03% of sulphur and max 0.4% of manganese. For this reason, the creation of MnS and its significance in the formation of defects is rather marginal.
In Fig. 11 description of part c) is needed.
Author Response
Dear reviewer,
The paper was corrected in accordance to your comments. We would like to thank you for valuable comments and suggestions. The corrections are explained bellow (blue color) and highlighted in the manuscript with red color.
Corrections on suggestions of Referee #3:
Remark #1: The title have to be improved. Instead "Resitance" please put in "Resistance".
A1: The reviewer suggestion was considered.
Remark #2: The reference list have to be improved. In the text is 32 references and in the chapter reference even 35. Please see Ref. [29].
A2: The list of reference in the original manuscript was correct. A mistake happened during formating of the text in the editorial office. References 21 (M. Herranen et al., Sur. Coat. Technol. 99 (1998) and Ref. 28 (C. Reinhard et al., Thin Solid Films 515 (2007) were given as double.
Remark #3: How the sentence "It is assumed that ion etching leads to a denser coating..."can be understand? It is known that ion etching and deposition process are two different operations and during ion etching the coating is not deposited.
A3: The intermediate etching process of the growing coating is widely used. Also the TiAlN hard coating we analysed in this work was prepared in such a way. After certain deposition time the deposition process was interrupted for an intermediate plasma etching process. Such an intermediate etching results in a high number of defects and is thought to create new nucleation sites for the subsequently deposited. The intermediate etching provides fine grained and less porous microstructure (less pronounced columnar microstructure). During deposition process the intermediate etching can be repeated several times.
For a better understanding, we added some additional explanation in the chapter "Experimental".
Remark #4: The bias voltage was really 100 V, not -100 V?
A4: The mistake was corrected as reviewer suggested.
Remark #5: Chapter "Experiment" should also include the data about Al2O3/TiAlN coating deposition parameters.
A5: In the manuscript (see section 3.1.2) we added some key information about Al2O3/TiAlN coating deposition.
Remark #6: Chemical composition of D2 steel include max 0.03% of sulphur and max 0.4% of manganese. For this reason, the creation of MnS and its significance in the formation of defects is rather marginal.
A6: All steels contain non-metallic inclusions to a greater or lesser extent. The type and appearance of these non-metallic inclusions depends on factors such as grade of steel, steel making process, secondary metallurgy etc. We found that the surface density of MnS, oxide and other non-metallic inclusions on the conventional D2 tool steel substrate surface are in the range of 450±50 mm–2, 800±200 mm–2 and 1400±300 mm–2. Their sizes are in the range of 0.1–10 µm. The higher is their size the larger is the detrimental effect on corrosion resitance.
Anyway, it is true that the impact of non-metallic inclusions on the corrosion resistance is lower than that caused by growth defects, but it is not negligible, because it cause some degree of porosity.
Remark #7: In Fig. 11 description of part c) is needed.
A7: The text was corrected according to the reviewer's suggestion.
Author Response File: Author Response.docx
