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Investigation of oxidation behaviour of AlCrN and AlTiN coatings deposited by arc enhanced HIPIMS technique
Journal Pre-proofs Full Length Article Investigation of oxidation behaviour of AlCrN and AlTiN coatings deposited by arc enhanced HIPIMS technique Abhishek Singh, S. Ghosh, S. Aravindan PII: DOI: Reference:
S0169-4332(19)33629-3 https://doi.org/10.1016/j.apsusc.2019.144812 APSUSC 144812
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
13 September 2019 5 November 2019 20 November 2019
Please cite this article as: A. Singh, S. Ghosh, S. Aravindan, Investigation of oxidation behaviour of AlCrN and AlTiN coatings deposited by arc enhanced HIPIMS technique, Applied Surface Science (2019), doi: https:// doi.org/10.1016/j.apsusc.2019.144812
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Investigation of oxidation behaviour of AlCrN and AlTiN coatings deposited by arc enhanced HIPIMS technique Abhishek Singh, S. Ghosh, S. Aravindan Department of Mechanical Engineering, Indian Institute of Technology Delhi, Hauz Khas, India110016
Abstract The current study investigates the oxidation behaviour of the AlTiN and AlCrN coatings deposited using the novel arc enhanced HIPIMS technique at elevated temperature. The obtained coatings are not only a successor of the conventional coatings but can prove to be a viable option in terms of sustainable machining owing to this improved surface and sub-surface characteristics. Various techniques were utilized to provide an insight on the surface morphology, surface roughness, and variation in elemental composition with varying temperature during deposition. X-ray diffraction analysis was carried out to determine the oxides formed and change in the phases due to heating. Microscopic studies on the cross-section of the deposited samples were carried out to determine the effect of oxidation on the sub-layers. Surface morphology of the AlTiN coatings revealed the formation of flower like structures at localized sites over the film surface due to oxidation. While no such behaviour was observed in case of the AlCrN based coatings. Formation of the stable and uniform chromium oxides in comparison to sporadic aluminum-based oxides proved to be superior in terms of preventing oxidation of the coating material in the outer environment and as well as oxidation of the substrate material. Keywords: HIPIMS, High Strength Materials, Cutting Tools, Oxidation
1. Introduction With evolution of the cutting tool market and with the development of numerous cutting tool materials, there still exists a need of coatings which can improve the performance of cutting tools. Though several cutting tool materials can provide efficient machining, but the cost associated with them is a major concern. The use of hard nitride-based coatings over the WC-Co substrate gained popularity three decades ago. However, the problem associated with the deposition techniques caused a major concern for its application in current scenario. The major flaws concerned with the conventional physical vapour deposition techniques can be summed up as poor surface finish, surface defects, low ionisation rate, low adhesion etc[1,2]. In order to overcome these problems, new coating techniques were developed. One such technique is arc enhanced high impulse magnetron sputtering technique. The technique utilizes the benefits of both the cathodic arc evaporation as well as magnetron sputtering technique, such synergic adoption of these two techniques eliminate drawbacks associated with each of them. The new technique results in
achieving coatings of improved adhesion , reduced surface defects in reduced time of deposition[3,4]. Among all the nitride-based coatings, AlTiN and AlCrN coatings are widely used in the cutting tool industry as well as protective layer in various other applications[5,6]. Owing to their unique properties and ability to withstand high temperature many researchers have studied their oxidation behaviour at various temperatures. Kawate et al.[7] reported that there is no change in the surface morphology even after annealing of CrAlN film at 800 oC for 14 hours. However, in the case of TiAlN films whiskers were observed near the droplets. In another study it has been reported that high aluminium content AlCrN based coatings exhibit short term oxidation resistance for temperature upto 1100 oC [8]. Vaz et al.[9] reported that oxidation of TiAlN films upto a temperature forms a homogenous mixture of oxides with 55 % of oxygen and 10 % nitrogen while oxidation above this temperature lead to formation of oxide layer without any nitrogen. Most of the studies carried out in the past have shown effectiveness of both the coatings during oxidation studies, however the formation of the surface defect and ambiguousness in the presented results in terms of morphology of surface oxides as well as the other parameters i.e. surface roughness, hardness etc demands thorough study of the phenomenon involved as well as that of the coating deposited by the newly developed technique. As the coatings deposited using newly developed technique have shown significant improvement in its physical and mechanical properties in comparison to coatings deposited using conventional technique in the previous studies by the authors. The current study investigates the oxidation behaviour of the AlTiN and AlCrN films deposited using the novel arc enhanced HIPIMS technique at elevated temperature. The study focusses on the aspects such as change in the morphology, formation of oxides as well as the kinetics of the oxidation phenomenon. Apart from this, the study also brings out the effects of annealing on the coating parameters such as hardness and surface roughness. The obtained results can prove to be beneficial for selection of coating material in the high-speed machining of hard to cut materials where large amount of heat is generated. 2. Experimental Details Coatings were deposited on the WC-Co inserts having geometry CNMA 120408 using the arc enhanced HIPIMS technique. In the deposition process, AlTiN coatings were deposited using AlTi targets while the AlCrN based coatings were deposited using AlCr targets. High purity nitrogen gas (99.99 %) and argon gas (99.99%) were utilized as the reactive and sputtering gas respectively for the process. All the substrates were sequentially cleaned in the ultrasonic bath and were dried up before loading to the vacuum chamber. All the samples were loaded in the planetary substrate holder, which rotates at fixed rpm for ensuring uniform film deposition. The coating chamber was evacuated to a pressure of 5 x 10-3 Pa prior to the coating deposition. Pre-sputtering of the targets and plasma cleaning of the substrates was carried out to remove the impurities present on the surfaces in the argon-filled environment. The deposition of a 1.45 µm film took 51 minutes at a deposition temperature in the range of 420-4500C. During the deposition process, the nitrogen flow rate was maintained around 270 sccm. The deposition conditions and parameters for the coating is same as mentioned in the previous study[4]. Oxidation tests of the coated carbide samples were performed inside a muffle furnace at varying temperature in the air environment. In order to study the effect of annealing temperature on the
physical and mechanical characteristics of the coatings, the samples were maintained at the desired temperature i.e. 400oC and 800oC for 1 hour. The microanalysis of the film morphology and elemental composition before and after oxidation was studied utilizing scanning electron microscope (SEM), equipped with energy dispersive Xray spectroscopy (EDS). Atomic force microscopy was utilized for 3D topographic images of the films as well as measurement of surface roughness (Ra) using tapping mode for scanning range of a few microns. Crystal structure and phase composition of the coatings were analyzed using XRay diffraction (XRD) θ-2θ technique utilizing CuKα radiation. The variation in the mechanical characteristics of the coating due to oxidation was measured in terms of hardness and elastic modulus. Nano-indentation tests were carried out to determine the value of the hardness and elastic moduli using Hysitron TI950 TriboIndenter at a peak load of 5000 µN. 3. Results and Discussion 3.1 Microstructural Analysis Microstructures for both the coatings revealed defect free surfaces. AlCrN coating is much denser than the AlTiN coating as reduced grinding marks can be seen over the top surface even though the surface conditions for the substrates were identical (Figure 1). The same was later confirmed with the intensity of the diffraction peaks for coatings during the XRD analysis. The superficial hardness for the coatings is of ~29 GPa for AlTiN coating and ~30 GPa for the AlCrN based coating. The elemental mapping of the coating revealed uniform deposition of the film over the substrate surface. AlTiN Coating
AlCrN Coating
No surface defects
Denser coating structure
Figure 1. Scanning electron micrographs for the AlTiN and AlCrN coatings in as deposited condition
AlTiN Coating
AlTiN Coating Oxides of substrate material
400 ºC
800 ºC
AlCrN Coating
AlCrN Coating
400 ºC
800 ºC
Figure 2. Scanning electron micrographs for the AlTiN and AlCrN coatings after oxidation at two different temperatures.
The oxidation of the coatings at varying temperature resulted in change of the morphology (Figure 2). These changes became more prominent at higher temperature as compared to 400 oC where the film morphology is very much similar to as deposited condition. The types of oxides that were formed due to the oxidation was later identified with the help of the X-ray diffraction technique. Among the two coatings that were studied it is notable that AlCrN film proved to be much superior in terms of oxidation resistance as compared to AlTiN coating at both the temperatures. The high temperature during the annealing process provides sufficient amount of energy for the synthesis, transition and development of new compounds on the film surface. The oxidation of the AlTiN film at 800oC lead to the formation of the flower like structures at localized sites, which is nothing but the oxides of tungsten and cobalt, which is the substrate material. For the AlTiN coating at temperature of 400 oC small oxides can be seen at localized sites instead of a continuous layer. In the AlCrN film, no such blemishes were observed. The film underwent oxidation at higher
temperature and formed a uniform layer of oxide. Similar observations can be drawn from the EDX mapping shown below in Figure 3.
WC-Co
Contain elements from coatings
Oxidized Substrate
Oxidized Substrate
Oxide Layer/Oxidized Scale Tungsten Oxide Residues at localized sites AlTiN Coating
AlCrN Coating
(a) Cross section for AlTiN and AlCrN film showing substrate and film oxidation.
(b) EDX mapping for AlTiN and AlCrN film after oxidation at 800 oC. Figure 3. SEM and EDX images for AlTiN and AlCrN film after oxidation.
During the cross-sectional study it was found that the main constituents of the coatings were present nearby to the surface with large amount of oxygen content in case of AlTiN films. While in the case of AlCrN film, the formed scales of (Al, Cr)2O3 acted as a barrier layer restricting the inward diffusion of oxygen and outward diffusion of Al and Cr ions. 3.2 Oxidation behaviour of coatings at high temperature The variation in the micro crystallite structure of the film due to the heat treatment was analyzed using X-ray diffraction technique. Figure 4 shows the XRD pattern for the as deposited AlTiN and AlCrN coatings onto the cemented carbide substrates. Due to the small film thickness, the intensity of the substrate peaks is at maximum. However, the sharpness of film peaks confirms its high crystallinity. For the AlTiN coating, main diffraction peaks were obtained at 35.30o for TiN (111) phase (JSPDS 01-074-1214) while for AlN phase (200), (220), (311) peak positions were 43.91o, 63.83o and 76.66o respectively (JSPDS 00-025-1495). In the case of AlCrN coating, main diffraction peaks were obtained at 35.59o, 41.33o and 75.36o for AlN (111), (200) and (222) phase respectively (JSPDS 01-080-0010) while for CrN phase (111), (200) and (220) peak positions were at 37.60o, 43.69o and 63.50o respectively (JSPDS 01-076-2494). Figure 5. shows the evolution of the micro crystallite structure for the AlTiN and AlCrN based coatings due to annealing treatment. The annealing /of both the coatings at 400oC resulted in same diffraction peaks as that of as deposited condition, representing its resistance to oxidation at this temperature. During the analysis of AlTiN film, it was found that the intensities of the AlN peaks has decreased in comparison to as deposited condition due to the formation of aluminum oxide at higher temperatures. The analysis also revealed formation of the oxides from substrate material i.e. cobalt and tungsten which has a high tendency of diffusion at elevated temperature. However, the existence of AlN and TiN peaks even after the annealing represents the excellent oxidation resistance of the coating. The TiN phases in AlTiN film transformed into TiO2 at 800 oC while, their peak intensities were low in comparison to the formed aluminum oxides at the same
temperature. The aluminum oxide formed at this temperature is generally amorphous in nature. During the annealing process the metastable AlTiN phase transforms into thermodynamic metastable phases of TiN and AlN through the cubic precipitation of AlN[10]. XRD analysis of AlCrN based coating revealed formation of chromium oxides at elevated temperature. The peak intensity of the Cr2O3 is lower at certain diffraction position either due to lower content formation or its quasibinary miscibility of Cr2O3 and Al2O3. The formation mechanism of Cr2O3 is different at different temperature as when the temperature is less than 400oC, the formed film is in few nanometers and it grows at logarithmic rate with rising temperature. While at temperature more than 400oC the growth is parabolic in nature[11]. The analysis also revealed the formation of oxide from film material and substrate material i.e. aluminum tungsten oxide for both the coatings. The formed oxide is a combination of tetrahedral WO3 and octahedral AlO6. These oxides are generally formed in W rich phase and are stable upto 830oC.
Figure 4. X-ray diffraction peaks for AlTiN and AlCrN film deposited using arc enhanced HIPIMS technique.
Figure 5. X-ray diffraction peaks for AlTiN and AlCrN film after oxidation at different temperatures.
3.3 Mechanism of oxide formation and its kinetics Both the coatings exhibited excellent resistance against oxidation at elevated temperatures due to the formation of aluminum oxide and chromium oxide continuous layers all over the surface. However, their stability is hindered with the increased exposure time for oxidation. The formation of oxides occur due to the chemisorption of the oxygen onto the film surface. The formed layers are in the range of few nanometers and its thickness increases with increase in oxidation temperature or time upto a certain critical value. At low temperature the formation of layered structure over the films are not possible due to low activation energy values i.e. around 170 kJ/mol for aluminum oxide formation. However, the higher activation energy values for diffusion of oxygen into the aluminum oxide layer limits the oxidation of aluminum titanium nitride films at elevated temperatures. Similar to the AlTiN based coating the oxide formation in AlCrN based coating occur due to the oxidation of the highly mobile Cr at the surface of the coating. However, the higher stability of the Cr2O3 formed by further oxidation of CrO2 lead to significant improvement in oxidation resistance of these coating. When the content of chromium sesquioxide is less it forms quasi-binary mixture of Cr2O3-Al2O3 to exhibit full miscibility[12,13]. The presence of this mixture can be identified with low intensity XRD peaks of Cr2O3. The non-columnar dense structure of AlCrN coatings as compared to columnar structure of AlTiN coatings hinders the diffusion of the oxygen atoms towards the substrate direction resulting in better oxidation
resistance of the AlCrN film. Figure 6. illustrates the oxidation phenomenon for AlTiN and AlCrN films. TiN + O2(chemisorption of oxygen) → TiO2(Rutile) + ½ N2 ………... (1) Large volume expansion and surface defects formation. Al2Ti7O15 (decomposition at higher temperature) → α-Al2O3 + TiO2(Rutile) ………. (2) Act as a passive layer against oxidation at low temperature. However, if the complete oxidation of the film takes place then the reaction can be modified as [14]. AlxTi1-xNy + (2-x)/2O2 → 2(1-x) TiO2 + xAl2O3 + yN2 ……………. (3) The formed oxides are immiscible in nature below a temperature of 1423K. The oxidation of AlCrN films causes phase transformation and subsequently lead to formation of chromium sesquioxide which is an exothermic reaction. 2CrN + 3/2 O2 → Cr2O3 +N2 ………………. (4)
Figure 6. Schematic representation of AlTiN and AlCrN film before and after oxidation.
3.4 Effect of oxidation on the surface roughness and hardness of the coating Characterization of surface roughness over the film was carried out using atomic force microscopy. Formation of the oxide layer resulted an in increase in the roughness value. However, this effect is more prominent at temperature around 800oC. From the AFM images for both the coatings mentioned can be observed. In case of the AlTiN coating the sporadic nature of the oxide layer give non-uniformity of the surface resulting an increase in the overall surface roughness. The AlTiN film surface at temperature corresponding to 800oC also represents the lump of substrate oxides at localized sites which is in coherence with the results obtained during microstructural analysis. The roughness for both the coating at 400oC is very much similar to as deposited condition due to absence of the oxide layers. In the case of AlCrN coatings the increase in the surface roughness with oxidation temperature is not as prominent as compared to AlTiN coating due to the formation of uniform surface oxide layer, which is more illustrious through the AFM images shown in Figure 7. The obtained results are dependent on the surface topography of the samples before oxidation.
.
200
Surface Roughness, Ra (nm)
180 160 140 120 100 80 60 40 20 0 As Deposited
400 deg. C AlTiN
800 deg. C
AlCrN
Figure 7. AFM topography as well the surface roughness variation for the AlTiN and AlCrN film with the oxidation temperature.
Figure 8. represents the variation of film hardness with the oxidation temperature for both the coatings. There was not any significant change in the hardness values for oxidation temperature of 400oC. However, with an increase in the oxidation temperature a slight decrease in the hardness values for both the coatings was observed as the oxygen content increased to 40.90 % and 16.96 % for the AlTiN and AlCrN coating respectively. This decrease in the hardness value can be contributed to the formation of the oxide layer over the film surface and the transformation of harder phases into softer phases at these temperatures. Hardness measurement for the AlCrN coating have shown variable results at certain localized sights where an improvement in film hardness was observed as compared to as deposited state. This may be due to the formation of denser phases of Cr2O3 and Cr2O5 at elevated temperature. Similar observations were made by other researchers in the past, where Chim et al. [15] observed that oxidation of CrN and CrAlN films resulted in increase in nano-hardness. The elastic modulus for both the coatings i.e. AlTiN and AlCrN at the room temperature was found to be 342.5 GPa and 289.9 GPa respectively. During oxidation at temperature corresponding to 400 oC the elastic modulus values remain stable for both the coatings. However, after undergoing oxidation at a temperature of 800 oC a slight change in these values was observed. The elastic modulus for the AlTiN and AlCrN coating decreased to 326.42 GPa and 260.8 GPa respectively. This decrease in the elastic modulus of the coatings can be attributed to contamination of oxygen onto the coating surface and sub-surface as well as formation of other unwanted metal oxides. This variation in elastic modulus is very much in coherence with the hardness variation.
Hardness Variation Hardness (GPa)
31 30 29 28 27 26 25 24 27 deg. C
400 deg. C
800 deg. C
Oxidation Temperature AlTIN
AlCrN
Figure 8. Variation of film hardness with the oxidation temperature.
4. Conclusions The current study investigates the oxidation behaviour of the AlTiN and AlCrN coatings deposited using the novel arc enhanced HIPIMS technique at elevated temperature to know the underlying phenomenon. Followings are the conclusions drawn from the study. 1. Both the coatings exhibited excellent resistance to oxidation at lower temperature however at higher temperature sporadic nature of alumina oxide layer proved detrimental for AlTiN film in comparison to the uniform oxide of Cr2O5 in AlCrN coating. 2. XRD studies revealed formation of Cr2O3 and Al2O3 layers over the film surface along with the oxides of other film constituents and that of the substrate. 3. The denser structure of AlCrN coatings as compared to AlTiN coatings hinders the diffusion of the oxygen atoms towards the substrate direction resulting in better oxidation resistance of the AlCrN film. 4. Oxidation of the film results an increase in surface roughness and decrease in hardness value. 5. Formation of the hard phases in chromium oxide layers results an increase in film hardness at localized sites. References [1]
S. Kadlec, J. Musil, V. Valvoda, W.D. Münz, H. Petersein, J. Schroeder, TiN films grown by reactive magnetron sputtering with enhanced ionization at low discharge pressures, Vacuum. 41 (1990) 2233–2238. doi:10.1016/0042-207X(90)94233-G.
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J.L. Endrino, G.S. Fox-Rabinovich, A. Reiter, S. V. Veldhuis, R. Escobar Galindo, J.M. Albella, J.F. Marco, Oxidation tuning in AlCrN coatings, Surf. Coatings Technol. 201 (2007) 4505–4511. doi:10.1016/j.surfcoat.2006.09.089.
[9]
F. Vaz, L. Rebouta, M. Andritschky, M.F. Silvab, J.C. Soaresc, Thermal Oxidation of T &, Al , N Coatings in, Science (80-. ). 11 (1997) 1971–1977.
[10] L. Chen, J. Paulitsch, Y. Du, P.H. Mayrhofer, Thermal stability and oxidation resistance of Ti-Al-N coatings, Surf. Coatings Technol. 206 (2012) 2954–2960. doi:10.1016/j.surfcoat.2011.12.028. [11] S.C. Tsai, A.. Huntz, C. Dolbin, Growth mechanism of Cr2O3 scales: Oxygen and chromium diffusion, oxidation kinetics and effect of yttrium, Mater. Sci. Eng. A. 212 (1996) 6–13. doi:10.1016/0921-5093(96)10173-8. [12] Z.B. Qi, Z.T. Wu, Z.C. Wang, Improved hardness and oxidation resistance for CrAlN hard coatingswith Y addition by magnetron co-sputtering, Surf. Coatings Technol. 259 (2014) 146–151. doi:10.1016/j.surfcoat.2014.02.034. [13] S.S. Kim, T.H. Sanders, Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3-Cr2O3 and V2O3-Cr2O3 Systems, J. Am. Ceram. Soc. 84 (2004) 1881–1884. doi:10.1111/j.1151-2916.2001.tb00930.x. [14] P. Panjan, B. Navinšek, M. Čekada, A. Zalar, Oxidation behaviour of TiAlN coatings sputtered at low temperature, Vacuum. 53 (1999) 127–131. doi:10.1016/S0042207X(98)00407-2. [15] Y.C. Chim, X.Z. Ding, X.T. Zeng, S. Zhang, Oxidation resistance of TiN , CrN , TiAlN and CrAlN coatings deposited by lateral rotating cathode arc, Thin Solid Films. 517 (2009) 4845–4849. doi:10.1016/j.tsf.2009.03.038.
Highlights First time an attempt has been made to study the oxidation behaviour of the AlTiN and AlCrN coatings deposited using the novel arc enhanced HIPIMS technique. Formation of flower like structures occurred during the oxidation of AlTiN coatings. Uniformity of the surface oxides in case of AlCrN coatings. Oxidation of the film results an increase in surface roughness and decrease in hardness value.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Authors Contribution Statement
Abhishek Singh: Methodology, Investigation, Visualization, Formal analysis, Writing- Original draft preparation. S. Ghosh: Conceptualization, Supervision, Validation, Writing - Review & Editing. S. Aravindan: Conceptualization, Supervision, Validation, Writing - Review & Editing.
S0169-4332(19)33629-3 https://doi.org/10.1016/j.apsusc.2019.144812 APSUSC 144812
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
13 September 2019 5 November 2019 20 November 2019
Please cite this article as: A. Singh, S. Ghosh, S. Aravindan, Investigation of oxidation behaviour of AlCrN and AlTiN coatings deposited by arc enhanced HIPIMS technique, Applied Surface Science (2019), doi: https:// doi.org/10.1016/j.apsusc.2019.144812
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Investigation of oxidation behaviour of AlCrN and AlTiN coatings deposited by arc enhanced HIPIMS technique Abhishek Singh, S. Ghosh, S. Aravindan Department of Mechanical Engineering, Indian Institute of Technology Delhi, Hauz Khas, India110016
Abstract The current study investigates the oxidation behaviour of the AlTiN and AlCrN coatings deposited using the novel arc enhanced HIPIMS technique at elevated temperature. The obtained coatings are not only a successor of the conventional coatings but can prove to be a viable option in terms of sustainable machining owing to this improved surface and sub-surface characteristics. Various techniques were utilized to provide an insight on the surface morphology, surface roughness, and variation in elemental composition with varying temperature during deposition. X-ray diffraction analysis was carried out to determine the oxides formed and change in the phases due to heating. Microscopic studies on the cross-section of the deposited samples were carried out to determine the effect of oxidation on the sub-layers. Surface morphology of the AlTiN coatings revealed the formation of flower like structures at localized sites over the film surface due to oxidation. While no such behaviour was observed in case of the AlCrN based coatings. Formation of the stable and uniform chromium oxides in comparison to sporadic aluminum-based oxides proved to be superior in terms of preventing oxidation of the coating material in the outer environment and as well as oxidation of the substrate material. Keywords: HIPIMS, High Strength Materials, Cutting Tools, Oxidation
1. Introduction With evolution of the cutting tool market and with the development of numerous cutting tool materials, there still exists a need of coatings which can improve the performance of cutting tools. Though several cutting tool materials can provide efficient machining, but the cost associated with them is a major concern. The use of hard nitride-based coatings over the WC-Co substrate gained popularity three decades ago. However, the problem associated with the deposition techniques caused a major concern for its application in current scenario. The major flaws concerned with the conventional physical vapour deposition techniques can be summed up as poor surface finish, surface defects, low ionisation rate, low adhesion etc[1,2]. In order to overcome these problems, new coating techniques were developed. One such technique is arc enhanced high impulse magnetron sputtering technique. The technique utilizes the benefits of both the cathodic arc evaporation as well as magnetron sputtering technique, such synergic adoption of these two techniques eliminate drawbacks associated with each of them. The new technique results in
achieving coatings of improved adhesion , reduced surface defects in reduced time of deposition[3,4]. Among all the nitride-based coatings, AlTiN and AlCrN coatings are widely used in the cutting tool industry as well as protective layer in various other applications[5,6]. Owing to their unique properties and ability to withstand high temperature many researchers have studied their oxidation behaviour at various temperatures. Kawate et al.[7] reported that there is no change in the surface morphology even after annealing of CrAlN film at 800 oC for 14 hours. However, in the case of TiAlN films whiskers were observed near the droplets. In another study it has been reported that high aluminium content AlCrN based coatings exhibit short term oxidation resistance for temperature upto 1100 oC [8]. Vaz et al.[9] reported that oxidation of TiAlN films upto a temperature forms a homogenous mixture of oxides with 55 % of oxygen and 10 % nitrogen while oxidation above this temperature lead to formation of oxide layer without any nitrogen. Most of the studies carried out in the past have shown effectiveness of both the coatings during oxidation studies, however the formation of the surface defect and ambiguousness in the presented results in terms of morphology of surface oxides as well as the other parameters i.e. surface roughness, hardness etc demands thorough study of the phenomenon involved as well as that of the coating deposited by the newly developed technique. As the coatings deposited using newly developed technique have shown significant improvement in its physical and mechanical properties in comparison to coatings deposited using conventional technique in the previous studies by the authors. The current study investigates the oxidation behaviour of the AlTiN and AlCrN films deposited using the novel arc enhanced HIPIMS technique at elevated temperature. The study focusses on the aspects such as change in the morphology, formation of oxides as well as the kinetics of the oxidation phenomenon. Apart from this, the study also brings out the effects of annealing on the coating parameters such as hardness and surface roughness. The obtained results can prove to be beneficial for selection of coating material in the high-speed machining of hard to cut materials where large amount of heat is generated. 2. Experimental Details Coatings were deposited on the WC-Co inserts having geometry CNMA 120408 using the arc enhanced HIPIMS technique. In the deposition process, AlTiN coatings were deposited using AlTi targets while the AlCrN based coatings were deposited using AlCr targets. High purity nitrogen gas (99.99 %) and argon gas (99.99%) were utilized as the reactive and sputtering gas respectively for the process. All the substrates were sequentially cleaned in the ultrasonic bath and were dried up before loading to the vacuum chamber. All the samples were loaded in the planetary substrate holder, which rotates at fixed rpm for ensuring uniform film deposition. The coating chamber was evacuated to a pressure of 5 x 10-3 Pa prior to the coating deposition. Pre-sputtering of the targets and plasma cleaning of the substrates was carried out to remove the impurities present on the surfaces in the argon-filled environment. The deposition of a 1.45 µm film took 51 minutes at a deposition temperature in the range of 420-4500C. During the deposition process, the nitrogen flow rate was maintained around 270 sccm. The deposition conditions and parameters for the coating is same as mentioned in the previous study[4]. Oxidation tests of the coated carbide samples were performed inside a muffle furnace at varying temperature in the air environment. In order to study the effect of annealing temperature on the
physical and mechanical characteristics of the coatings, the samples were maintained at the desired temperature i.e. 400oC and 800oC for 1 hour. The microanalysis of the film morphology and elemental composition before and after oxidation was studied utilizing scanning electron microscope (SEM), equipped with energy dispersive Xray spectroscopy (EDS). Atomic force microscopy was utilized for 3D topographic images of the films as well as measurement of surface roughness (Ra) using tapping mode for scanning range of a few microns. Crystal structure and phase composition of the coatings were analyzed using XRay diffraction (XRD) θ-2θ technique utilizing CuKα radiation. The variation in the mechanical characteristics of the coating due to oxidation was measured in terms of hardness and elastic modulus. Nano-indentation tests were carried out to determine the value of the hardness and elastic moduli using Hysitron TI950 TriboIndenter at a peak load of 5000 µN. 3. Results and Discussion 3.1 Microstructural Analysis Microstructures for both the coatings revealed defect free surfaces. AlCrN coating is much denser than the AlTiN coating as reduced grinding marks can be seen over the top surface even though the surface conditions for the substrates were identical (Figure 1). The same was later confirmed with the intensity of the diffraction peaks for coatings during the XRD analysis. The superficial hardness for the coatings is of ~29 GPa for AlTiN coating and ~30 GPa for the AlCrN based coating. The elemental mapping of the coating revealed uniform deposition of the film over the substrate surface. AlTiN Coating
AlCrN Coating
No surface defects
Denser coating structure
Figure 1. Scanning electron micrographs for the AlTiN and AlCrN coatings in as deposited condition
AlTiN Coating
AlTiN Coating Oxides of substrate material
400 ºC
800 ºC
AlCrN Coating
AlCrN Coating
400 ºC
800 ºC
Figure 2. Scanning electron micrographs for the AlTiN and AlCrN coatings after oxidation at two different temperatures.
The oxidation of the coatings at varying temperature resulted in change of the morphology (Figure 2). These changes became more prominent at higher temperature as compared to 400 oC where the film morphology is very much similar to as deposited condition. The types of oxides that were formed due to the oxidation was later identified with the help of the X-ray diffraction technique. Among the two coatings that were studied it is notable that AlCrN film proved to be much superior in terms of oxidation resistance as compared to AlTiN coating at both the temperatures. The high temperature during the annealing process provides sufficient amount of energy for the synthesis, transition and development of new compounds on the film surface. The oxidation of the AlTiN film at 800oC lead to the formation of the flower like structures at localized sites, which is nothing but the oxides of tungsten and cobalt, which is the substrate material. For the AlTiN coating at temperature of 400 oC small oxides can be seen at localized sites instead of a continuous layer. In the AlCrN film, no such blemishes were observed. The film underwent oxidation at higher
temperature and formed a uniform layer of oxide. Similar observations can be drawn from the EDX mapping shown below in Figure 3.
WC-Co
Contain elements from coatings
Oxidized Substrate
Oxidized Substrate
Oxide Layer/Oxidized Scale Tungsten Oxide Residues at localized sites AlTiN Coating
AlCrN Coating
(a) Cross section for AlTiN and AlCrN film showing substrate and film oxidation.
(b) EDX mapping for AlTiN and AlCrN film after oxidation at 800 oC. Figure 3. SEM and EDX images for AlTiN and AlCrN film after oxidation.
During the cross-sectional study it was found that the main constituents of the coatings were present nearby to the surface with large amount of oxygen content in case of AlTiN films. While in the case of AlCrN film, the formed scales of (Al, Cr)2O3 acted as a barrier layer restricting the inward diffusion of oxygen and outward diffusion of Al and Cr ions. 3.2 Oxidation behaviour of coatings at high temperature The variation in the micro crystallite structure of the film due to the heat treatment was analyzed using X-ray diffraction technique. Figure 4 shows the XRD pattern for the as deposited AlTiN and AlCrN coatings onto the cemented carbide substrates. Due to the small film thickness, the intensity of the substrate peaks is at maximum. However, the sharpness of film peaks confirms its high crystallinity. For the AlTiN coating, main diffraction peaks were obtained at 35.30o for TiN (111) phase (JSPDS 01-074-1214) while for AlN phase (200), (220), (311) peak positions were 43.91o, 63.83o and 76.66o respectively (JSPDS 00-025-1495). In the case of AlCrN coating, main diffraction peaks were obtained at 35.59o, 41.33o and 75.36o for AlN (111), (200) and (222) phase respectively (JSPDS 01-080-0010) while for CrN phase (111), (200) and (220) peak positions were at 37.60o, 43.69o and 63.50o respectively (JSPDS 01-076-2494). Figure 5. shows the evolution of the micro crystallite structure for the AlTiN and AlCrN based coatings due to annealing treatment. The annealing /of both the coatings at 400oC resulted in same diffraction peaks as that of as deposited condition, representing its resistance to oxidation at this temperature. During the analysis of AlTiN film, it was found that the intensities of the AlN peaks has decreased in comparison to as deposited condition due to the formation of aluminum oxide at higher temperatures. The analysis also revealed formation of the oxides from substrate material i.e. cobalt and tungsten which has a high tendency of diffusion at elevated temperature. However, the existence of AlN and TiN peaks even after the annealing represents the excellent oxidation resistance of the coating. The TiN phases in AlTiN film transformed into TiO2 at 800 oC while, their peak intensities were low in comparison to the formed aluminum oxides at the same
temperature. The aluminum oxide formed at this temperature is generally amorphous in nature. During the annealing process the metastable AlTiN phase transforms into thermodynamic metastable phases of TiN and AlN through the cubic precipitation of AlN[10]. XRD analysis of AlCrN based coating revealed formation of chromium oxides at elevated temperature. The peak intensity of the Cr2O3 is lower at certain diffraction position either due to lower content formation or its quasibinary miscibility of Cr2O3 and Al2O3. The formation mechanism of Cr2O3 is different at different temperature as when the temperature is less than 400oC, the formed film is in few nanometers and it grows at logarithmic rate with rising temperature. While at temperature more than 400oC the growth is parabolic in nature[11]. The analysis also revealed the formation of oxide from film material and substrate material i.e. aluminum tungsten oxide for both the coatings. The formed oxide is a combination of tetrahedral WO3 and octahedral AlO6. These oxides are generally formed in W rich phase and are stable upto 830oC.
Figure 4. X-ray diffraction peaks for AlTiN and AlCrN film deposited using arc enhanced HIPIMS technique.
Figure 5. X-ray diffraction peaks for AlTiN and AlCrN film after oxidation at different temperatures.
3.3 Mechanism of oxide formation and its kinetics Both the coatings exhibited excellent resistance against oxidation at elevated temperatures due to the formation of aluminum oxide and chromium oxide continuous layers all over the surface. However, their stability is hindered with the increased exposure time for oxidation. The formation of oxides occur due to the chemisorption of the oxygen onto the film surface. The formed layers are in the range of few nanometers and its thickness increases with increase in oxidation temperature or time upto a certain critical value. At low temperature the formation of layered structure over the films are not possible due to low activation energy values i.e. around 170 kJ/mol for aluminum oxide formation. However, the higher activation energy values for diffusion of oxygen into the aluminum oxide layer limits the oxidation of aluminum titanium nitride films at elevated temperatures. Similar to the AlTiN based coating the oxide formation in AlCrN based coating occur due to the oxidation of the highly mobile Cr at the surface of the coating. However, the higher stability of the Cr2O3 formed by further oxidation of CrO2 lead to significant improvement in oxidation resistance of these coating. When the content of chromium sesquioxide is less it forms quasi-binary mixture of Cr2O3-Al2O3 to exhibit full miscibility[12,13]. The presence of this mixture can be identified with low intensity XRD peaks of Cr2O3. The non-columnar dense structure of AlCrN coatings as compared to columnar structure of AlTiN coatings hinders the diffusion of the oxygen atoms towards the substrate direction resulting in better oxidation
resistance of the AlCrN film. Figure 6. illustrates the oxidation phenomenon for AlTiN and AlCrN films. TiN + O2(chemisorption of oxygen) → TiO2(Rutile) + ½ N2 ………... (1) Large volume expansion and surface defects formation. Al2Ti7O15 (decomposition at higher temperature) → α-Al2O3 + TiO2(Rutile) ………. (2) Act as a passive layer against oxidation at low temperature. However, if the complete oxidation of the film takes place then the reaction can be modified as [14]. AlxTi1-xNy + (2-x)/2O2 → 2(1-x) TiO2 + xAl2O3 + yN2 ……………. (3) The formed oxides are immiscible in nature below a temperature of 1423K. The oxidation of AlCrN films causes phase transformation and subsequently lead to formation of chromium sesquioxide which is an exothermic reaction. 2CrN + 3/2 O2 → Cr2O3 +N2 ………………. (4)
Figure 6. Schematic representation of AlTiN and AlCrN film before and after oxidation.
3.4 Effect of oxidation on the surface roughness and hardness of the coating Characterization of surface roughness over the film was carried out using atomic force microscopy. Formation of the oxide layer resulted an in increase in the roughness value. However, this effect is more prominent at temperature around 800oC. From the AFM images for both the coatings mentioned can be observed. In case of the AlTiN coating the sporadic nature of the oxide layer give non-uniformity of the surface resulting an increase in the overall surface roughness. The AlTiN film surface at temperature corresponding to 800oC also represents the lump of substrate oxides at localized sites which is in coherence with the results obtained during microstructural analysis. The roughness for both the coating at 400oC is very much similar to as deposited condition due to absence of the oxide layers. In the case of AlCrN coatings the increase in the surface roughness with oxidation temperature is not as prominent as compared to AlTiN coating due to the formation of uniform surface oxide layer, which is more illustrious through the AFM images shown in Figure 7. The obtained results are dependent on the surface topography of the samples before oxidation.
.
200
Surface Roughness, Ra (nm)
180 160 140 120 100 80 60 40 20 0 As Deposited
400 deg. C AlTiN
800 deg. C
AlCrN
Figure 7. AFM topography as well the surface roughness variation for the AlTiN and AlCrN film with the oxidation temperature.
Figure 8. represents the variation of film hardness with the oxidation temperature for both the coatings. There was not any significant change in the hardness values for oxidation temperature of 400oC. However, with an increase in the oxidation temperature a slight decrease in the hardness values for both the coatings was observed as the oxygen content increased to 40.90 % and 16.96 % for the AlTiN and AlCrN coating respectively. This decrease in the hardness value can be contributed to the formation of the oxide layer over the film surface and the transformation of harder phases into softer phases at these temperatures. Hardness measurement for the AlCrN coating have shown variable results at certain localized sights where an improvement in film hardness was observed as compared to as deposited state. This may be due to the formation of denser phases of Cr2O3 and Cr2O5 at elevated temperature. Similar observations were made by other researchers in the past, where Chim et al. [15] observed that oxidation of CrN and CrAlN films resulted in increase in nano-hardness. The elastic modulus for both the coatings i.e. AlTiN and AlCrN at the room temperature was found to be 342.5 GPa and 289.9 GPa respectively. During oxidation at temperature corresponding to 400 oC the elastic modulus values remain stable for both the coatings. However, after undergoing oxidation at a temperature of 800 oC a slight change in these values was observed. The elastic modulus for the AlTiN and AlCrN coating decreased to 326.42 GPa and 260.8 GPa respectively. This decrease in the elastic modulus of the coatings can be attributed to contamination of oxygen onto the coating surface and sub-surface as well as formation of other unwanted metal oxides. This variation in elastic modulus is very much in coherence with the hardness variation.
Hardness Variation Hardness (GPa)
31 30 29 28 27 26 25 24 27 deg. C
400 deg. C
800 deg. C
Oxidation Temperature AlTIN
AlCrN
Figure 8. Variation of film hardness with the oxidation temperature.
4. Conclusions The current study investigates the oxidation behaviour of the AlTiN and AlCrN coatings deposited using the novel arc enhanced HIPIMS technique at elevated temperature to know the underlying phenomenon. Followings are the conclusions drawn from the study. 1. Both the coatings exhibited excellent resistance to oxidation at lower temperature however at higher temperature sporadic nature of alumina oxide layer proved detrimental for AlTiN film in comparison to the uniform oxide of Cr2O5 in AlCrN coating. 2. XRD studies revealed formation of Cr2O3 and Al2O3 layers over the film surface along with the oxides of other film constituents and that of the substrate. 3. The denser structure of AlCrN coatings as compared to AlTiN coatings hinders the diffusion of the oxygen atoms towards the substrate direction resulting in better oxidation resistance of the AlCrN film. 4. Oxidation of the film results an increase in surface roughness and decrease in hardness value. 5. Formation of the hard phases in chromium oxide layers results an increase in film hardness at localized sites. References [1]
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Highlights First time an attempt has been made to study the oxidation behaviour of the AlTiN and AlCrN coatings deposited using the novel arc enhanced HIPIMS technique. Formation of flower like structures occurred during the oxidation of AlTiN coatings. Uniformity of the surface oxides in case of AlCrN coatings. Oxidation of the film results an increase in surface roughness and decrease in hardness value.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Authors Contribution Statement
Abhishek Singh: Methodology, Investigation, Visualization, Formal analysis, Writing- Original draft preparation. S. Ghosh: Conceptualization, Supervision, Validation, Writing - Review & Editing. S. Aravindan: Conceptualization, Supervision, Validation, Writing - Review & Editing.