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Microstructure and properties of nanostructured thick CrN coatings
Materials Letters 89 (2012) 55–58
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microstructure and properties of nanostructured thick CrN coatings Jianliang Lin a,n, William D. Sproul a,b, John J. Moore a a b
Advanced Coatings and Surface Engineering Laboratory (ACSEL), Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA Reactive Sputtering, Inc, 2152 Goya Place, San Marcos, CA 92078, USA
a r t i c l e i n f o
abstract
Article history: Received 22 June 2012 Accepted 15 August 2012 Available online 25 August 2012
Thick polycrystalline CrN coatings (20 mm) were deposited at a high deposition rate (10 mm/h) using a highly ionized plasma generated by modulated pulsed power magnetron sputtering. The microstructure of the thick CrN coating was characterized using transmission electron microscopy. The coating exhibited a super dense microstructure with a grain size in the nanometer range of 100–150 nm. The thick CrN coating showed an average high hardness of 24 GPa, excellent adhesion and wear resistant. Published by Elsevier B.V.
Keywords: CrN coating Thick coating Modulated pulsed power magnetron sputtering (MPPMS) High power pulsed magnetron sputtering (HPPMS) Wear
1. Introduction Chromium nitride (CrN) is an important hard coating material useful for its good corrosion, oxidation and wear resistance [1–3]. Magnetron sputtering is a widely used physical vapor deposition technique for CrN film depositions because of its low deposition temperature, and the capability to control the growth and microstructure of the films. In spite of much research and development in the traditional magnetron sputtering of CrN films (e.g., dc magnetron sputtering, pulsed dc magnetron sputtering), a common limitation is that it is difficult to deposit thick coatings (e.g., 20 mm and above) due to stress induced coating delamination and significant grain growth as the coatings grow thicker. In recent years, high power pulsed magnetron sputtering (HPPMS) [4] and modulated pulsed power magnetron sputtering (MPPMS) techniques [5] have been developed for obtaining high density and ionization degree plasmas to improve the quality and adhesion of the coatings. The MPPMS technique has shown the capabilities to shape the high power pulse to achieve a high degree of ionization of the sputtered material and a high deposition rate at the same time [6]. The high ion to neutral ratio of the MPP plasma provides conditions that favor a high surface mobility of the adatoms, which can diffuse longer and further on the film surface therefore densifing the film and decreasing the grain size.
n
Corresponding author. Tel.: þ1 303 273 3178; fax: þ1 303 273 3795. E-mail address: [email protected] (J. Lin).
0167-577X/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2012.08.060
Recently, the MPPMS technique has been successfully used to deposit thick and dense metallic and compound coatings with thicknesses up to 100 mm [7,8]. Nevertheless, the detailed microstructure of these thick coatings needs further clarifications. In this study, the microstructure of a 20 mm thick CrN coating has been characterized using transmission electron microscopy (TEM). The adhesion and wear resistance of the thick MPP CrN coatings are also reported.
2. Experimental The thick CrN coatings were deposited on AISI 440C stainless steel substrates in a two-cathode closed field unbalanced magnetron sputtering system by sputtering a metal Cr target (99.9% purity) using an MPPMS generator (Zpulser LLC). A detailed description of the deposition system was reported earlier [7]. The distance between the substrates to the Cr target was 50 mm. The substrates were sputter etched in pure Ar plasma at a pressure of 1.33 Pa using a pulsed dc substrate bias voltage of 650 V (200 kHz and 80% duty cycle) for 40 min. For the CrN coating depositions, the working pressure was 0.67 Pa. The Ar to N2 flow ratio was 1:1 at a total flow rate of 62 sccm. A 50 V dc substrate bias was used for all depositions. The MPP pulse shape (1000 ms pulse length) and pulsing parameters have been reported in Ref [7]. The average target power, peak target power and current during the high ionization pulse period for the CrN coating depositions were 3 kW, 95 kW, and 190 A, respectively. A high deposition rate of 10 mm/h has been achieved.
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J. Lin et al. / Materials Letters 89 (2012) 55–58
The crystal structure of the coatings was characterized using X-ray diffraction (XRD) (Phillips X’pert diffractometer) with CuKa radiation in the conventional Bragg–Brentano mode. The residual stress of the coatings was measured by glancing incident angle XRD (GIXRD). The cross-sectional microstructure of the coatings was characterized by transmission electron microscopy (TEM) (Phillips CM200) using an accelerating voltage of 200 kV. The adhesion of the coatings was determined by the Rockwell-C indentation adhesion test (HRC) using a standard Rockwell-C hardness tester. A Rockwell C diamond stylus (cone apex angle 1201, tip radius R¼0.2 mm) was used to perform the tests. The applied load on the stylus was 150 kg. After the test, the morphology of the indentation was examined using scanning electron microscopy (SEM) to evaluate the cracks and degree of coating delamination around the indentation. The damage of the coating was compared with a HF adhesion strength quality as standardized in the VDI guidelines 3198, (1991) [9]. The mean hardness and Young’s modulus values of the coatings were measured by an MTS nanoindenter equipped with a diamond Berkovich indenter. The indentation depth was kept below 10% of the coating thickness to minimize the substrate effect. A dry sliding wear test was
Fig. 1. XRD pattern and cross-sectional SEM micrograph of the 20 mm CrN coating.
conducted using a ball-on-disk microtribometer (Center for Tribology, Inc.) in the ambient atmosphere (25–26 1C, 32–35% RH). The sliding counterpart is a 6 mm WC-Co ball. The applied load was
Fig. 3. (a) TEM micrograph showing a grain boundary in the thick CrN coating, and (b) HRTEM micrograph of the coating.
Fig. 2. TEM micrographs of the 20 mm CrN coating deposited using MPPMS: (a) bright field TEM image, (b) selected area electron diffraction pattern, (c) dark field TEM image taken with the (1 1 1) diffraction, and (d) dark field image taken with the (2 0 0) diffraction.
J. Lin et al. / Materials Letters 89 (2012) 55–58
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Fig. 4. (a) SEM micrograph of a HRC indent and (b) The 3D and 2D profilometer scan across the wear track of the thick MPPMS CrN coating after sliding against a WC-Co ball at 10 N and 20 cm/s for 10 km (the wear of the ball is showing on top of the image).
10 N. The sliding speed was 20 cm/s. The total sliding distance was 10 km.
3. Results and discussion Fig. 1 shows the XRD pattern of the 20 mm MPP CrN coating. The coating exhibited a face centered cubic (FCC) structure with strong (1 1 1) and (2 0 0) peaks and weak (2 2 0), (3 1 1) and (2 2 2) peaks. The residual stress of the coating was 4.8 GPa. The inserted SEM micrograph confirmed the coating has an extremely dense microstructure with a smooth coating surface. Fig. 2 shows the bright field (BF) and dark field (DF) TEM micrographs and the selected area electron diffraction (SAED) pattern taken near the top region of the 20 mm CrN coating. As shown in Fig. 2a, the coating exhibited a dense columnar grain structure. The SAED pattern shown in Fig. 2b is indexed according to the FCC structure with (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) reflections. The DF images shown in Fig. 2c and d were obtained by selecting the (1 1 1) and (2 0 0) diffraction segments as circled in Fig. 2b. The bright regions confirm the existence of the (1 1 1) and (2 0 0) columnar grains with grain sizes in the range of 50 to 100 nm. This result clearly demonstrated that the grain size of the thick CrN coating is in the nanometer scale. Fig. 3a shows the typical grain boundary morphology between two columnar grains in the thick CrN coating. The grain boundary is thin and dense with no porosity, which confirms the dense structure of the thick coating. A high resolution TEM (HRTEM) micrograph showing the lattice planes of the coating is presented in Fig. 3b. The interlayer spacing is about 0.235 nm, which corresponds to the interlayer distance of the {1 1 1} crystal planes of the FCC CrN. The corresponding Fast Fourier transform (FFT) images of several selected areas were marked in Fig. 3b. Most regions showed an ordered lattice. Nevertheless, regions with slightly distorted lattices can also been identified, which should be related to relatively higher local micro-strain in these regions. The mean hardness and Young’s modulus values of the 20 mm CrN coating are 24 and 270 GPa, respectively. These values are comparable to various early reported results for CrN thin films (several mm thick) deposited by different physical vapor deposition techniques, e.g., pulsed dc MS [2], ion beam enhanced deposition [10], cathodic arc evaporation [11], HPPMS [12]. The MPPMS thick CrN coating exhibited a very dense microstructure as confirmed by the TEM characterizations. In addition, the size of the disordered columnar grains were maintained in the nanometer scale even near the top region of the coating, as shown in Fig. 2. These nanoscale microstructural features are responsible
for the excellent mechanical properties of the thick MPPMS CrN coating. The adhesion of the 20 mm MPPMS CrN coating was evaluated by the HRC method. Fig. 4a shows the SEM micrograph of the indent after the HRC test at a 150 kg load. Circular cracks within the indent and along the indent edge can be clearly seen, which is related to the brittle nature of the ceramic coatings. Nevertheless, no delamination of the coating was observed. As compared to the VDI guidelines, this indent morphology is associated to with the HRC1 category, which indicates good adhesion of the coating to the stainless steel substrate. The wear resistance of the thick MPPMS CrN coating was evaluated using the ball-on-disk test. Fig. 4b shows an optical micrograph and the 2 dimensional depth profile of the wear track. The wear scar of the WC-Co counterpart is also presented in Fig. 4b. The wear track showed a smooth grove with only a wear depth of 1 mm after high speeding sliding at 10 N for 10 km. The width of the wear track is about 0.9 mm which is consistent with the diameter of the wear scar of the WC-Co ball. The wear rate of the coating was 1 10 7 mm3/(N m). This wear rate is 10 times lower than the thin CrN coatings as deposited on the same steel substrate by traditional magnetron sputtering techniques [2].
4. Conclusions The microstructure, adhesion, mechanical properties and wear resistance of a 20 mm CrN coating deposited by the modulated pulsed power magnetron sputtering technique are reported. The thick MPP CrN coating exhibited a dense and disordered columnar grain microstructure. The width of the columnar grains is in the nanometer range of 100–150 nm. The nanostructured thick CrN coating exhibited high hardness of 24 GPa, good adhesion, and excellent wear resistance.
Acknowledgement The authors are grateful for financial support of this research program from U.S. Department of Energy (DOE) Advanced Technology International (ATI), and the North American Die Casting Association (NADCA). References [1] Fu Y, Zhu X, Tang B, Hu X, He J, Xu K, Batchelor AW. Wear 1998;217:159–66. [2] Lin J, Wu ZL, Zhang XH, Mishra B, Moore JJ, Sproul WD. Thin Solid Films 2009;517:1887–94.
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J. Lin et al. / Materials Letters 89 (2012) 55–58
[3] Navinˇsek B, Panjan P. Surf Coat Technol 1995;74:919–26. [4] Kouznetsov V, Maca´k K, Schneider JM, Helmersson U, Petrov I. Surf Coat Technol 1999;122:290–3. [5] Chistyakov R, Abraham B, Sproul WD, Proceedings of the 49th annual SVC technical conference, Washington, DC, April 23–27, 2006:88–91. [6] Lin J, Sproul WD, Moore JJ, Wu ZL, Lee SL, Chistyakov R, Abraham B. JOM 2011;63:48–58. [7] Lin J, Sproul WD, Moore JJ, Lee S. Surf Coat Technol 2011;205:3226–34.
[8] Lin J, Moore JJ, Sproul WD, Lee SL, Wang J. IEEE Trans Plasma Sci 2010;38:3071–8. [9] Heinke W, Leyland A, Matthews A, Berg G, Friedrich C, Broszeit E. Thin Solid Films 1995;270:431–8. [10] Fu YQ, Zhu XD, Tang B, Hu XF, He JW, Xu KW. Mater Lett 1999;40:192–7. [11] Kim SS, Han JG, Lee SY. Thin Solid Films 1998;334:133–9. [12] Paulitsch J, Schenkel M, Th Zufrass, Mayrhofer PH, Muenz WD. Thin Solid Films 2010;518:5558–64.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microstructure and properties of nanostructured thick CrN coatings Jianliang Lin a,n, William D. Sproul a,b, John J. Moore a a b
Advanced Coatings and Surface Engineering Laboratory (ACSEL), Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA Reactive Sputtering, Inc, 2152 Goya Place, San Marcos, CA 92078, USA
a r t i c l e i n f o
abstract
Article history: Received 22 June 2012 Accepted 15 August 2012 Available online 25 August 2012
Thick polycrystalline CrN coatings (20 mm) were deposited at a high deposition rate (10 mm/h) using a highly ionized plasma generated by modulated pulsed power magnetron sputtering. The microstructure of the thick CrN coating was characterized using transmission electron microscopy. The coating exhibited a super dense microstructure with a grain size in the nanometer range of 100–150 nm. The thick CrN coating showed an average high hardness of 24 GPa, excellent adhesion and wear resistant. Published by Elsevier B.V.
Keywords: CrN coating Thick coating Modulated pulsed power magnetron sputtering (MPPMS) High power pulsed magnetron sputtering (HPPMS) Wear
1. Introduction Chromium nitride (CrN) is an important hard coating material useful for its good corrosion, oxidation and wear resistance [1–3]. Magnetron sputtering is a widely used physical vapor deposition technique for CrN film depositions because of its low deposition temperature, and the capability to control the growth and microstructure of the films. In spite of much research and development in the traditional magnetron sputtering of CrN films (e.g., dc magnetron sputtering, pulsed dc magnetron sputtering), a common limitation is that it is difficult to deposit thick coatings (e.g., 20 mm and above) due to stress induced coating delamination and significant grain growth as the coatings grow thicker. In recent years, high power pulsed magnetron sputtering (HPPMS) [4] and modulated pulsed power magnetron sputtering (MPPMS) techniques [5] have been developed for obtaining high density and ionization degree plasmas to improve the quality and adhesion of the coatings. The MPPMS technique has shown the capabilities to shape the high power pulse to achieve a high degree of ionization of the sputtered material and a high deposition rate at the same time [6]. The high ion to neutral ratio of the MPP plasma provides conditions that favor a high surface mobility of the adatoms, which can diffuse longer and further on the film surface therefore densifing the film and decreasing the grain size.
n
Corresponding author. Tel.: þ1 303 273 3178; fax: þ1 303 273 3795. E-mail address: [email protected] (J. Lin).
0167-577X/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2012.08.060
Recently, the MPPMS technique has been successfully used to deposit thick and dense metallic and compound coatings with thicknesses up to 100 mm [7,8]. Nevertheless, the detailed microstructure of these thick coatings needs further clarifications. In this study, the microstructure of a 20 mm thick CrN coating has been characterized using transmission electron microscopy (TEM). The adhesion and wear resistance of the thick MPP CrN coatings are also reported.
2. Experimental The thick CrN coatings were deposited on AISI 440C stainless steel substrates in a two-cathode closed field unbalanced magnetron sputtering system by sputtering a metal Cr target (99.9% purity) using an MPPMS generator (Zpulser LLC). A detailed description of the deposition system was reported earlier [7]. The distance between the substrates to the Cr target was 50 mm. The substrates were sputter etched in pure Ar plasma at a pressure of 1.33 Pa using a pulsed dc substrate bias voltage of 650 V (200 kHz and 80% duty cycle) for 40 min. For the CrN coating depositions, the working pressure was 0.67 Pa. The Ar to N2 flow ratio was 1:1 at a total flow rate of 62 sccm. A 50 V dc substrate bias was used for all depositions. The MPP pulse shape (1000 ms pulse length) and pulsing parameters have been reported in Ref [7]. The average target power, peak target power and current during the high ionization pulse period for the CrN coating depositions were 3 kW, 95 kW, and 190 A, respectively. A high deposition rate of 10 mm/h has been achieved.
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J. Lin et al. / Materials Letters 89 (2012) 55–58
The crystal structure of the coatings was characterized using X-ray diffraction (XRD) (Phillips X’pert diffractometer) with CuKa radiation in the conventional Bragg–Brentano mode. The residual stress of the coatings was measured by glancing incident angle XRD (GIXRD). The cross-sectional microstructure of the coatings was characterized by transmission electron microscopy (TEM) (Phillips CM200) using an accelerating voltage of 200 kV. The adhesion of the coatings was determined by the Rockwell-C indentation adhesion test (HRC) using a standard Rockwell-C hardness tester. A Rockwell C diamond stylus (cone apex angle 1201, tip radius R¼0.2 mm) was used to perform the tests. The applied load on the stylus was 150 kg. After the test, the morphology of the indentation was examined using scanning electron microscopy (SEM) to evaluate the cracks and degree of coating delamination around the indentation. The damage of the coating was compared with a HF adhesion strength quality as standardized in the VDI guidelines 3198, (1991) [9]. The mean hardness and Young’s modulus values of the coatings were measured by an MTS nanoindenter equipped with a diamond Berkovich indenter. The indentation depth was kept below 10% of the coating thickness to minimize the substrate effect. A dry sliding wear test was
Fig. 1. XRD pattern and cross-sectional SEM micrograph of the 20 mm CrN coating.
conducted using a ball-on-disk microtribometer (Center for Tribology, Inc.) in the ambient atmosphere (25–26 1C, 32–35% RH). The sliding counterpart is a 6 mm WC-Co ball. The applied load was
Fig. 3. (a) TEM micrograph showing a grain boundary in the thick CrN coating, and (b) HRTEM micrograph of the coating.
Fig. 2. TEM micrographs of the 20 mm CrN coating deposited using MPPMS: (a) bright field TEM image, (b) selected area electron diffraction pattern, (c) dark field TEM image taken with the (1 1 1) diffraction, and (d) dark field image taken with the (2 0 0) diffraction.
J. Lin et al. / Materials Letters 89 (2012) 55–58
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Fig. 4. (a) SEM micrograph of a HRC indent and (b) The 3D and 2D profilometer scan across the wear track of the thick MPPMS CrN coating after sliding against a WC-Co ball at 10 N and 20 cm/s for 10 km (the wear of the ball is showing on top of the image).
10 N. The sliding speed was 20 cm/s. The total sliding distance was 10 km.
3. Results and discussion Fig. 1 shows the XRD pattern of the 20 mm MPP CrN coating. The coating exhibited a face centered cubic (FCC) structure with strong (1 1 1) and (2 0 0) peaks and weak (2 2 0), (3 1 1) and (2 2 2) peaks. The residual stress of the coating was 4.8 GPa. The inserted SEM micrograph confirmed the coating has an extremely dense microstructure with a smooth coating surface. Fig. 2 shows the bright field (BF) and dark field (DF) TEM micrographs and the selected area electron diffraction (SAED) pattern taken near the top region of the 20 mm CrN coating. As shown in Fig. 2a, the coating exhibited a dense columnar grain structure. The SAED pattern shown in Fig. 2b is indexed according to the FCC structure with (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) reflections. The DF images shown in Fig. 2c and d were obtained by selecting the (1 1 1) and (2 0 0) diffraction segments as circled in Fig. 2b. The bright regions confirm the existence of the (1 1 1) and (2 0 0) columnar grains with grain sizes in the range of 50 to 100 nm. This result clearly demonstrated that the grain size of the thick CrN coating is in the nanometer scale. Fig. 3a shows the typical grain boundary morphology between two columnar grains in the thick CrN coating. The grain boundary is thin and dense with no porosity, which confirms the dense structure of the thick coating. A high resolution TEM (HRTEM) micrograph showing the lattice planes of the coating is presented in Fig. 3b. The interlayer spacing is about 0.235 nm, which corresponds to the interlayer distance of the {1 1 1} crystal planes of the FCC CrN. The corresponding Fast Fourier transform (FFT) images of several selected areas were marked in Fig. 3b. Most regions showed an ordered lattice. Nevertheless, regions with slightly distorted lattices can also been identified, which should be related to relatively higher local micro-strain in these regions. The mean hardness and Young’s modulus values of the 20 mm CrN coating are 24 and 270 GPa, respectively. These values are comparable to various early reported results for CrN thin films (several mm thick) deposited by different physical vapor deposition techniques, e.g., pulsed dc MS [2], ion beam enhanced deposition [10], cathodic arc evaporation [11], HPPMS [12]. The MPPMS thick CrN coating exhibited a very dense microstructure as confirmed by the TEM characterizations. In addition, the size of the disordered columnar grains were maintained in the nanometer scale even near the top region of the coating, as shown in Fig. 2. These nanoscale microstructural features are responsible
for the excellent mechanical properties of the thick MPPMS CrN coating. The adhesion of the 20 mm MPPMS CrN coating was evaluated by the HRC method. Fig. 4a shows the SEM micrograph of the indent after the HRC test at a 150 kg load. Circular cracks within the indent and along the indent edge can be clearly seen, which is related to the brittle nature of the ceramic coatings. Nevertheless, no delamination of the coating was observed. As compared to the VDI guidelines, this indent morphology is associated to with the HRC1 category, which indicates good adhesion of the coating to the stainless steel substrate. The wear resistance of the thick MPPMS CrN coating was evaluated using the ball-on-disk test. Fig. 4b shows an optical micrograph and the 2 dimensional depth profile of the wear track. The wear scar of the WC-Co counterpart is also presented in Fig. 4b. The wear track showed a smooth grove with only a wear depth of 1 mm after high speeding sliding at 10 N for 10 km. The width of the wear track is about 0.9 mm which is consistent with the diameter of the wear scar of the WC-Co ball. The wear rate of the coating was 1 10 7 mm3/(N m). This wear rate is 10 times lower than the thin CrN coatings as deposited on the same steel substrate by traditional magnetron sputtering techniques [2].
4. Conclusions The microstructure, adhesion, mechanical properties and wear resistance of a 20 mm CrN coating deposited by the modulated pulsed power magnetron sputtering technique are reported. The thick MPP CrN coating exhibited a dense and disordered columnar grain microstructure. The width of the columnar grains is in the nanometer range of 100–150 nm. The nanostructured thick CrN coating exhibited high hardness of 24 GPa, good adhesion, and excellent wear resistance.
Acknowledgement The authors are grateful for financial support of this research program from U.S. Department of Energy (DOE) Advanced Technology International (ATI), and the North American Die Casting Association (NADCA). References [1] Fu Y, Zhu X, Tang B, Hu X, He J, Xu K, Batchelor AW. Wear 1998;217:159–66. [2] Lin J, Wu ZL, Zhang XH, Mishra B, Moore JJ, Sproul WD. Thin Solid Films 2009;517:1887–94.
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J. Lin et al. / Materials Letters 89 (2012) 55–58
[3] Navinˇsek B, Panjan P. Surf Coat Technol 1995;74:919–26. [4] Kouznetsov V, Maca´k K, Schneider JM, Helmersson U, Petrov I. Surf Coat Technol 1999;122:290–3. [5] Chistyakov R, Abraham B, Sproul WD, Proceedings of the 49th annual SVC technical conference, Washington, DC, April 23–27, 2006:88–91. [6] Lin J, Sproul WD, Moore JJ, Wu ZL, Lee SL, Chistyakov R, Abraham B. JOM 2011;63:48–58. [7] Lin J, Sproul WD, Moore JJ, Lee S. Surf Coat Technol 2011;205:3226–34.
[8] Lin J, Moore JJ, Sproul WD, Lee SL, Wang J. IEEE Trans Plasma Sci 2010;38:3071–8. [9] Heinke W, Leyland A, Matthews A, Berg G, Friedrich C, Broszeit E. Thin Solid Films 1995;270:431–8. [10] Fu YQ, Zhu XD, Tang B, Hu XF, He JW, Xu KW. Mater Lett 1999;40:192–7. [11] Kim SS, Han JG, Lee SY. Thin Solid Films 1998;334:133–9. [12] Paulitsch J, Schenkel M, Th Zufrass, Mayrhofer PH, Muenz WD. Thin Solid Films 2010;518:5558–64.