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Adhesion evaluation of multilayered based WC–Co–Cr thermally sprayed coatings
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Surface & Coatings Technology 202 (2008) 4399 – 4405 www.elsevier.com/locate/surfcoat
Adhesion evaluation of multilayered based WC–Co–Cr thermally sprayed coatings M. Hadad a,⁎, M. Hockauf b , L.W. Meyer b , G. Marot c , J. Lesage d , R. Hitzek e , S. Siegmann a EMPA — Materials Science and Technology, 3602 Thun, Switzerland Chemnitz University of Technology, Straße der Nationen 62, 09107 Chemnitz, Germany Hautes Etudes d'Ingénieur, Pôle de Recherche Structure et Matériaux, 13 rue de Toul, 59046 Lille Cedex, France d Laboratoire de Mécanique de Lille, UMR CNRS 8107, U.S.T. Lille, IUT A GMP, Villeneuve d'Ascq, France e Stellba Schweisstechnik AG, 5244 Birrhard, Switzerland a
b
c
Available online 9 April 2008
Abstract This paper describes the adhesion evaluation of different interlayers such as Co–Cr, Ni–Cr 80–20 HVOF (High Velocity Oxy-Fuel) thermally sprayed coatings and Ni-plating between the cermet based WC–Co–Cr coatings. Three adhesion measurement methods for these different multilayered based thermal spray coatings, namely tensile adhesive strength (according to EN 582), interfacial indentation and solid impact tests were conducted. The distinguished coating properties include: i) the adhesive strength, ii) the interfacial toughness, iii) the depth of impact. The metallographic and experimental results show that the electrochemically deposited interlayer Ni-plating provides the highest adhesion to cermet coating within the multilayered structured coatings. This is not only due to the chemical affinity between the Niplating and the cermet coating, but also to its homogeneous microstructure, since the electrochemically deposition does not provide splat formation. © 2008 Elsevier B.V. All rights reserved. Keywords: Adhesion; HVOF; Multilayered structure; WC–Co–Cr; Ni-plating; Interface toughness; Impact energy
1. Introduction Cermet based WC–Co–Cr thermally sprayed coating is considered to be potential wear resistant coating materials since WC grain in the metallic matrix provides very good bonding. The hard WC particles in the coatings lead to high coating hardness and high wear resistance, while the metal binder Co– Cr supplies the necessary coating toughness [1,2]. Part of coating surface of component is highly subjected to aggressive environment, such as erosion, corrosion, and accidental mechanical impact. Ni–Cr 80–20 coating is well known for protection against aqueous corrosion, the HVOF spraying is able to make a denser and less-oxidized coating compared with other methods such as plasma spraying because of the cohesive
⁎ Corresponding author. Tel.: +41 33 228 29 63; fax: +41 33 228 44 90. E-mail address: [email protected] (M. Hadad). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.016
strength of the individual splats due to the high impact velocity of particles [3]. Therefore, the microhardness of this coating was shown to be improved [4]. Nickel alloy of electroplating is one of the common and well studied surface engineering techniques used for corrosion protection in many industries [5] where the equipment setup is cost-effective, simple and readily available to industries. Multilayered structure has very good mechanical properties because of its potential improvement of toughness where the cracks might be deflected through the interfaces rather than within the layers [6–9]. However, different deformation zones have been determined caused by the impact of single solid particles on multilayered structure [10]. Other observation were also performed by impact solid object at the macro scale [11]. Different methods have been devoted for evaluating the adhesion of coating to substrate. Among them, a significant number is based on the linear elastic fracture mechanics (LEFM) approach [12–14]. Each method is related to a certain type of coating, loading condition, application of the coating since there are no universal tests for
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M. Hadad et al. / Surface & Coatings Technology 202 (2008) 4399–4405
Fig. 1. Interfacial toughness obtained for the different combinations of coatings.
measuring coating's adhesion. This can be explained by the variety of coatings systems which represent different types of dissimilar material interfaces that are present in many industrial applications (metal/metal, metal/ceramic, polymer/metal, polymer/ceramic, etc). The tests that work with one coating system may not necessarily work with another [15–17]. Though, there is no standard adhesion test for coating system which can suite all materials. The adhesion of coating is very important property to evaluate the performance and reliability of coatings in engineering applications. Many interesting coatings can not be used for some applications because of their low and insufficient adhesive strength. Among the most widespread methods used are indentation tests [18,19], shear tests [20–23] and tensile adhesive strength. This later is widely used in industry like ASTM C633, ASTM F1147, ISO 14916, EN 582 [24–26]. We should also note that adhesion is not a constant in practical applications, but rather a complicated property that depends on loading conditions, coating thickness [22] and different parameters such as grit blasting to roughen the substrate surface [27–31]. Furthermore, the residual stresses due to the mismatch in thermal and mechanical properties between coatings and substrate are of importance [32–36]. The tribological investigations on these coatings materials were previously published [37]. Therefore, this study explores the different interlayer coatings as for novel application to enhance the adhesion to cermet multilayered coatings. Three main tests were performed: Tensile adhesive strength, interfacial indentation and impact
Fig. 2. Cross section of a sample showing some oxide layers between splats.
tests in order to characterize the behaviour of the multilayered materials in different conditions of loading. Metallographic observations were performed to examine the role of the Niplating-X layer in connection to these mechanical tests. 2. Experimental methods 2.1. Preparation and treatment of the materials The grain size and morphology of WC–Co–Cr powder were characterized and reported in the previous study [37]. All cermet based WC–Cr–Co coatings were deposited by HVOF (High Velocity Oxy-Fuel) system on the steel substrate DIN 1.4313 that has been grit blasted with Al2O3 grit of no. 36 mesh size to increase the substrate surface roughness to approximately 5.5 μm Ra to promote a good coating adhesion. Therefore, specimens were degreased immediately prior to deposition. Details of the HVOF operation of the system are given in previous work [37]. Different interlayers were deposited on the first cermet coating, then a cermet coating was sprayed as the upper coating to form the multilayered structure as shown in Fig. 1. Interlayer coatings such as Ni–Cr 80–20 and Co–Cr were deposited by HVOF on the first cermet coating. Ni was electrochemically plated on the cermet coating at 55 °C using
Table 1 Mechanical characteristics of substrate, cermet and interlayer coatings materials Combination number and nomenclature
1. Cer/steel 2. Cer/Ni–Cr/Cer 3. Cer/Co–Cr/Cer 4. Cer/Ni pl/Cer 5. Cer/Ni pl-X/Cer 6. Substrate DIN-1.4313 7. Cermet WC–Cr–Co coating
Characteristics of substrate and interlayer coatings Interlayer
– Ni–Cr 80–20 Co–Cr Ni-Plating Ni-Plating-X – –
Coating system
HVOF HVOF HVOF Electroplating Electroplating – HVOF
Thickness
Young modulus
μm
GPa
500 110 140 50 50 – 250
Hardness Hv 0.3
Mean
St. Dev.
Mean
St. Dev.
100 144 243 243 210 266
14 13 15 15 5 12
387 697 516 516 260 1432
42 63 31 31 18 141
M. Hadad et al. / Surface & Coatings Technology 202 (2008) 4399–4405
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Fig. 3. Indentation at the interface a) between interlayer and cermet coating (combination 5— Cer/Ni-plating-X/Cer), b) between WC–Co–Cr and Co–Cr with crack propagation through the Co–Cr coating.
optimized parameters: 1.3 A and 5 V for 7 min, to achieve an uniform and continuous coverage of the Ni layer. The thickness of the Ni layer was approximately 60 μm, where the cermet coating surface previously exhibits a Ra = 5 μm of roughness. After having deposited the Ni-plating, only the half of the Nielectroplated samples were grit blasted to reach Ra up to 4.5 μm of roughness so-called Ni-plating-X (combination 5 in Table 1). The other Ni-electroplated samples were not grit blasted to perform the combination 4 as shown in the cross section in Fig. 2. Last cermet coating as the upper one, has been deposited on all interlayer coatings with the same spraying conditions as the first cermet layer. 2.2. Mechanical tests The mechanical properties of the steel substrate, WC–Co–Cr and interlayer coatings such as hardness and Young's modulus have been determined by indentation techniques [38,39] and are summarised in Table 1 where each single value was averaged from 10 measurements. Test coated specimen of 25 mm diameter were joined with the cylindrical counter parts using an adhesive agent according to the standard test EN 582 [40]. These specimens have been cured at elevated temperature up to 210 °C. The tensile load was applied with the Universal Epprecht-Multitest tensile machine where the force is applied perpendicularly to the splats staking direction. However, the mean adhesive strength values were calculated from three tests performed under the same conditions. The tensile adhesive strength was calculated by: rmax ¼ F=A
interface is used to calculate the interfacial toughness using a recently developed method by Chicot, Démarecaux and Lesage [41–43]. The interfacial toughness is expressed by: 1=2 PC E ð2Þ KIC ¼ 0:015 3=2 d H i a C
PC and aC denote load and crack length, respectively. The square root of the ratio of the elastic modulus (E) divided by the Vickers hardness (H) at the interface is expressed by: 1=2
ð E=H Þi ¼
ð E=H Þ1=2 s 1 þ ðHs =Hc Þ
þ 1=2
ð E=H Þ1=2 c 1 þ ðHc =Hs Þ1=2
ð3Þ
where the subscripts i, s and c stand for interface, substrate and coating, respectively. In our experimentation, we indent at the interface between the upper layer of the cermet and the interlayer coatings as shown in Fig. 3a. The impact behaviour of the coatings was tested with an experimental shooting device that allows the acceleration of impactors on a very short distance as shown by Reisel [11]. The barrel having a calibre of 8 mm extends into an impact chamber. For the investigation the
ð1Þ
where F is the maximum load at rupture in N, and A is the normal section of specimen in mm2. Interfacial indentation experiments were made using the approach involving a direct measurement the radial crack length initiated by Vickers indentation at the interface of a coated material in a cross section. The measured crack length at the
Fig. 4. Adhesive strength obtained for the different combinations of coatings.
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Fig. 5. Macrographs of the upper view of sandwich combinations at angles of impact, 30° and 90°.
M. Hadad et al. / Surface & Coatings Technology 202 (2008) 4399–4405
Fig. 6. Micrographs of the cross sectioned sandwich combinations at 30° and 90°.
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coated samples where mounted tightly onto a rigid steel block in the chamber in a distance of 100 mm in front of the muzzle. Two different impact angles inclined to the horizontal axis were used: perpendicular (α = 90°) as well as oblique loading (α = 30°). As impactor a 8 mm ball made of a low alloyed QT-steel (60 HRC, 2.1 g) was used. The impact speed of 350 m/s gives a total kinetic energy of 130 J. With the presented parameters relatively hard impacts causing massive deformations on both the coating and the substrate are possible as well as ricochet like impacts causing additional smaller deformations.
highly differentiated. In order to understand the impact behaviour of these combinations, cross sections were performed and observed at the microscopic scale as semi quantitative evaluations. Fig. 6 shows that the combination with Ni-platingX revealed better adhesive behaviour at the interfaces between interlayer and cermet coatings since this combination exhibits less cracks propagation, whereas, the other coatings revealed cracks mainly transversally propagated and this might lead to delaminate the whole multilayered coatings. 4. Conclusion
3. Results and discussion The results of the interfacial indentation experiments are summarised in Fig. 1, where electrochemical combinations with Ni-plating interlayer show higher interfacial toughness than the sprayed combinations with Ni–Cr and Co–Cr interlayer. This is due to the chemical affinity between the Ni-plating and the cermet coating on one hand, and to its homogeneous microstructure on the other hand since the electrochemically deposition process does not provide neither interfaces of splats formation nor an oxide layer during spraying. In case of spraying, the microstructure can help easier crack propagation through the splats and oxide layers as shown in Fig. 2. In Fig. 3a, the combination 5 with the roughened Ni-plating-X interlayer shows a dense and strong interface to cermet coating whereas, the combination 3 with Co–Cr as interlayer exhibit highly oxide layers where many microcracks propagated through the coating during indentation as shown in Fig. 3b. The interfacial toughness of combination 5 with the roughened Ni-plating-X interlayer is higher than that of combination 4, this was mainly attributed to the increasing surface energy by roughening the interface. The interlayer Ni–Cr 80–20 (combination 2) shows a low interfacial toughness value comparing to Ni-plating interlayer combinations. The results of tensile adhesive strength are shown in Fig. 4 give a similar trend as in interfacial toughness results. The interlayer Ni-plating-X showed the highest adhesion value to the cermet coating. The failure of most of coatings was occurred adhesively within the glue and the substrate except combination 3 with the interlayer Co–Cr was failed cohesively within the interlayer coatings. This can be explained by presence of the oxide layer within this coating where the adhesive energy at the interface of interlayer and cermet coating was greater than that in between splats at the microstructure scale. The difference in tensile strength between electrochemical combinations 4 and 5 was lower than that in interfacial indention, may that due to the loading and stress intensity at the interface by interfacial indentation which could drive more easily the crack in the smooth interface than that roughened one. Otherwise for such tensile test, under normal loading, the roughened surface might has a lower impact on tensile strength. Regarding the impact test behaviour, the plastic deformation was so important, as the difference in impact depth of all combinations was not significant. Fig. 5 shows the upper view of coating failure after the impact test. Initially the combination with Co–Cr showed a drastic delamination of coating whereas the other coating at this scale could not be
To enhance the adhesion and the damping of solid particle impact, different interlayer coatings were deposited to perform a multilayered structure. Three methods were studied to characterise the adhesion: tensile adhesive, interfacial indentations and impact tests. The electrochemical Ni-plating-X interlayer shows the best impact behaviour among the other interlayers, as well as it revealed the highest interfacial toughness up to 7 MPa m0.5 and the highest adhesive strength up to 64 MPa. This was mainly attributed to its mechanical properties and to its homogeneous microstructure since the electrochemically deposition process does not provide interfaces within the splats formation, where the absence of interfaces and oxide layers can lead to limit the crack propagation within the interlayer and at the interface of coatings. Acknowledgement We would like to thank the CTI commission for the support under Lewis project 5942.3 as well as Dr. M. Leparoux for the English correction, A. De Meuron for the metallographic observations and R. Egli and B. Von Gunten for the sample spraying. References [1] L. Zhao, M. Maurer, F. Fischer, R. Dicks, E. Lugscheider, Wear 257 (1–2) (2003) 41. [2] D. Toma, W. Brandl, G. Marginean, Surf. Coat. Technol. 138 (2–3) (2001) 149. [3] A. Scrivani, S. Ianelli, A. Rossi, R. Groppetti, F. Casadei, G. Rizzi, Wear 250 (1–12) (2001) 107. [4] N.F. Ak, C. Tekmen, I. Ozdemir, H.S. Soykan, E. Celik, Surf. Coat. Technol. 174–175 (1) (2003) 1070. [5] P. Niranatlumpong, H. Koiprasert, Surf. Coat. Technol. 194 (2005) 96. [6] M. Hadad, G. Blugan, J. Kübler, E. Rosset, L. Rohr, J. Michler, Wear (2006) 634. [7] C.a. Wang, Y. Huang, Q. Zan, L. Zou, S. Cai, J. Am. Cer. Soc. 85 (2002) 2457–2461. [8] J.L. Huang, F.C. Chou, H.H. Lu, J. Mater. Res. 12 (9) (1997) 2357. [9] G. Blugan, M. Hadad, J. Janczak, J. Kuebler, T. Graule, J. Am. Cer. Soc. 88 (4) (2005) 926. [10] X. Chen, R. Wang, N. Yao, A.G. Evans, J.W. Hutchinson, R.W. Bruce, Mater. Sci. Eng., A 352 (1–2) (2003) 221. [11] G. Reisel, S. Steinhäuser, B. Wielage, M. Hockauf, L.W. Meyer, Proceedings of ITSC 2005 Thermal Spray connects: Explore Its Surfacing Potential!, Basel, Switzerland, 2005, p. 742. [12] Y.S. Cheng, W.H. Douglas, A. Versluis, D. Tantbirojn, Eng. Fract. Mech. 64 (1) (1999) 117. [13] M. Menningen, H. Weiss, Surf. Coat. Technol. 76–77 (2) (1995) 835.
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[31] S. Siegmann, C.A. Brown, Proceedings of 17th International SAMPE Europe Conference: Success of Materials by Combination, Basel, CH, 1996, p. 149. [32] T. Keller, N. Margadant, T. Pirling, M.J. Riegert-Escribano, W. Wagner, Mater. Sci. Eng., A 373 (1–2) (2004) 33. [33] Ö. Ünal, D.J. Sordelet, Scr. Mater. 42 (7) (2000) 631. [34] O. Kesler, M. Finot, S. Suresh, S. Sampath, Acta Mater. 45 (8) (1997) 3123. [35] O. Kesler, J. Matejicek, S. Sampath, S. Suresh, T. Gnaeupel-Herold, P.C. Brand, H.J. Prask, Mater. Sci. Eng., A 257 (2) (1998) 215. [36] M.D. Drory, M.D. Thouless, A.G. Evans, Acta Metall. 36 (8) (1998) 2019. [37] M. Hadad, R. Hitzek, P. Buergler, L. Rohr, S. Siegmann, Wear (2007) 1. [38] T. Ohmura, S. Matsuoka, Surf. Coat. Technol. 169–170 (1) (2003) 728. [39] DIN EN ISO 14577-1: Instrumented indentation test for hardness and materials parameters — Part 1: Test method, 2003–05. [40] DIN EN 582: 1993. — Thermal spraying; determination of tensile adhesive strength. [41] J. Lesage, P. Demarecaux, D. Chicot, G. Mesmacque, Proceedings of 14th International Thermal Spray Conference: Thermal Spraying-Current Status and Future Trends, Kobe, Japan, VOL. 2, 1995, p. 891. [42] J. Lesage, P. Demarecaux, O. Bartier, G. Mesmacque, Rev. Métall. 90 (12) (1993) 1655. [43] M. Hadad, G. Marot, P. Démarécaux, J. Lesage, J. Michler, S. Siegmann, Proceedings of ITSC 2005 Thermal Spray connects: Explore Its Surfacing Potential!, Basel, Switzerland, 2005, p. 759.
Surface & Coatings Technology 202 (2008) 4399 – 4405 www.elsevier.com/locate/surfcoat
Adhesion evaluation of multilayered based WC–Co–Cr thermally sprayed coatings M. Hadad a,⁎, M. Hockauf b , L.W. Meyer b , G. Marot c , J. Lesage d , R. Hitzek e , S. Siegmann a EMPA — Materials Science and Technology, 3602 Thun, Switzerland Chemnitz University of Technology, Straße der Nationen 62, 09107 Chemnitz, Germany Hautes Etudes d'Ingénieur, Pôle de Recherche Structure et Matériaux, 13 rue de Toul, 59046 Lille Cedex, France d Laboratoire de Mécanique de Lille, UMR CNRS 8107, U.S.T. Lille, IUT A GMP, Villeneuve d'Ascq, France e Stellba Schweisstechnik AG, 5244 Birrhard, Switzerland a
b
c
Available online 9 April 2008
Abstract This paper describes the adhesion evaluation of different interlayers such as Co–Cr, Ni–Cr 80–20 HVOF (High Velocity Oxy-Fuel) thermally sprayed coatings and Ni-plating between the cermet based WC–Co–Cr coatings. Three adhesion measurement methods for these different multilayered based thermal spray coatings, namely tensile adhesive strength (according to EN 582), interfacial indentation and solid impact tests were conducted. The distinguished coating properties include: i) the adhesive strength, ii) the interfacial toughness, iii) the depth of impact. The metallographic and experimental results show that the electrochemically deposited interlayer Ni-plating provides the highest adhesion to cermet coating within the multilayered structured coatings. This is not only due to the chemical affinity between the Niplating and the cermet coating, but also to its homogeneous microstructure, since the electrochemically deposition does not provide splat formation. © 2008 Elsevier B.V. All rights reserved. Keywords: Adhesion; HVOF; Multilayered structure; WC–Co–Cr; Ni-plating; Interface toughness; Impact energy
1. Introduction Cermet based WC–Co–Cr thermally sprayed coating is considered to be potential wear resistant coating materials since WC grain in the metallic matrix provides very good bonding. The hard WC particles in the coatings lead to high coating hardness and high wear resistance, while the metal binder Co– Cr supplies the necessary coating toughness [1,2]. Part of coating surface of component is highly subjected to aggressive environment, such as erosion, corrosion, and accidental mechanical impact. Ni–Cr 80–20 coating is well known for protection against aqueous corrosion, the HVOF spraying is able to make a denser and less-oxidized coating compared with other methods such as plasma spraying because of the cohesive
⁎ Corresponding author. Tel.: +41 33 228 29 63; fax: +41 33 228 44 90. E-mail address: [email protected] (M. Hadad). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.016
strength of the individual splats due to the high impact velocity of particles [3]. Therefore, the microhardness of this coating was shown to be improved [4]. Nickel alloy of electroplating is one of the common and well studied surface engineering techniques used for corrosion protection in many industries [5] where the equipment setup is cost-effective, simple and readily available to industries. Multilayered structure has very good mechanical properties because of its potential improvement of toughness where the cracks might be deflected through the interfaces rather than within the layers [6–9]. However, different deformation zones have been determined caused by the impact of single solid particles on multilayered structure [10]. Other observation were also performed by impact solid object at the macro scale [11]. Different methods have been devoted for evaluating the adhesion of coating to substrate. Among them, a significant number is based on the linear elastic fracture mechanics (LEFM) approach [12–14]. Each method is related to a certain type of coating, loading condition, application of the coating since there are no universal tests for
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M. Hadad et al. / Surface & Coatings Technology 202 (2008) 4399–4405
Fig. 1. Interfacial toughness obtained for the different combinations of coatings.
measuring coating's adhesion. This can be explained by the variety of coatings systems which represent different types of dissimilar material interfaces that are present in many industrial applications (metal/metal, metal/ceramic, polymer/metal, polymer/ceramic, etc). The tests that work with one coating system may not necessarily work with another [15–17]. Though, there is no standard adhesion test for coating system which can suite all materials. The adhesion of coating is very important property to evaluate the performance and reliability of coatings in engineering applications. Many interesting coatings can not be used for some applications because of their low and insufficient adhesive strength. Among the most widespread methods used are indentation tests [18,19], shear tests [20–23] and tensile adhesive strength. This later is widely used in industry like ASTM C633, ASTM F1147, ISO 14916, EN 582 [24–26]. We should also note that adhesion is not a constant in practical applications, but rather a complicated property that depends on loading conditions, coating thickness [22] and different parameters such as grit blasting to roughen the substrate surface [27–31]. Furthermore, the residual stresses due to the mismatch in thermal and mechanical properties between coatings and substrate are of importance [32–36]. The tribological investigations on these coatings materials were previously published [37]. Therefore, this study explores the different interlayer coatings as for novel application to enhance the adhesion to cermet multilayered coatings. Three main tests were performed: Tensile adhesive strength, interfacial indentation and impact
Fig. 2. Cross section of a sample showing some oxide layers between splats.
tests in order to characterize the behaviour of the multilayered materials in different conditions of loading. Metallographic observations were performed to examine the role of the Niplating-X layer in connection to these mechanical tests. 2. Experimental methods 2.1. Preparation and treatment of the materials The grain size and morphology of WC–Co–Cr powder were characterized and reported in the previous study [37]. All cermet based WC–Cr–Co coatings were deposited by HVOF (High Velocity Oxy-Fuel) system on the steel substrate DIN 1.4313 that has been grit blasted with Al2O3 grit of no. 36 mesh size to increase the substrate surface roughness to approximately 5.5 μm Ra to promote a good coating adhesion. Therefore, specimens were degreased immediately prior to deposition. Details of the HVOF operation of the system are given in previous work [37]. Different interlayers were deposited on the first cermet coating, then a cermet coating was sprayed as the upper coating to form the multilayered structure as shown in Fig. 1. Interlayer coatings such as Ni–Cr 80–20 and Co–Cr were deposited by HVOF on the first cermet coating. Ni was electrochemically plated on the cermet coating at 55 °C using
Table 1 Mechanical characteristics of substrate, cermet and interlayer coatings materials Combination number and nomenclature
1. Cer/steel 2. Cer/Ni–Cr/Cer 3. Cer/Co–Cr/Cer 4. Cer/Ni pl/Cer 5. Cer/Ni pl-X/Cer 6. Substrate DIN-1.4313 7. Cermet WC–Cr–Co coating
Characteristics of substrate and interlayer coatings Interlayer
– Ni–Cr 80–20 Co–Cr Ni-Plating Ni-Plating-X – –
Coating system
HVOF HVOF HVOF Electroplating Electroplating – HVOF
Thickness
Young modulus
μm
GPa
500 110 140 50 50 – 250
Hardness Hv 0.3
Mean
St. Dev.
Mean
St. Dev.
100 144 243 243 210 266
14 13 15 15 5 12
387 697 516 516 260 1432
42 63 31 31 18 141
M. Hadad et al. / Surface & Coatings Technology 202 (2008) 4399–4405
4401
Fig. 3. Indentation at the interface a) between interlayer and cermet coating (combination 5— Cer/Ni-plating-X/Cer), b) between WC–Co–Cr and Co–Cr with crack propagation through the Co–Cr coating.
optimized parameters: 1.3 A and 5 V for 7 min, to achieve an uniform and continuous coverage of the Ni layer. The thickness of the Ni layer was approximately 60 μm, where the cermet coating surface previously exhibits a Ra = 5 μm of roughness. After having deposited the Ni-plating, only the half of the Nielectroplated samples were grit blasted to reach Ra up to 4.5 μm of roughness so-called Ni-plating-X (combination 5 in Table 1). The other Ni-electroplated samples were not grit blasted to perform the combination 4 as shown in the cross section in Fig. 2. Last cermet coating as the upper one, has been deposited on all interlayer coatings with the same spraying conditions as the first cermet layer. 2.2. Mechanical tests The mechanical properties of the steel substrate, WC–Co–Cr and interlayer coatings such as hardness and Young's modulus have been determined by indentation techniques [38,39] and are summarised in Table 1 where each single value was averaged from 10 measurements. Test coated specimen of 25 mm diameter were joined with the cylindrical counter parts using an adhesive agent according to the standard test EN 582 [40]. These specimens have been cured at elevated temperature up to 210 °C. The tensile load was applied with the Universal Epprecht-Multitest tensile machine where the force is applied perpendicularly to the splats staking direction. However, the mean adhesive strength values were calculated from three tests performed under the same conditions. The tensile adhesive strength was calculated by: rmax ¼ F=A
interface is used to calculate the interfacial toughness using a recently developed method by Chicot, Démarecaux and Lesage [41–43]. The interfacial toughness is expressed by: 1=2 PC E ð2Þ KIC ¼ 0:015 3=2 d H i a C
PC and aC denote load and crack length, respectively. The square root of the ratio of the elastic modulus (E) divided by the Vickers hardness (H) at the interface is expressed by: 1=2
ð E=H Þi ¼
ð E=H Þ1=2 s 1 þ ðHs =Hc Þ
þ 1=2
ð E=H Þ1=2 c 1 þ ðHc =Hs Þ1=2
ð3Þ
where the subscripts i, s and c stand for interface, substrate and coating, respectively. In our experimentation, we indent at the interface between the upper layer of the cermet and the interlayer coatings as shown in Fig. 3a. The impact behaviour of the coatings was tested with an experimental shooting device that allows the acceleration of impactors on a very short distance as shown by Reisel [11]. The barrel having a calibre of 8 mm extends into an impact chamber. For the investigation the
ð1Þ
where F is the maximum load at rupture in N, and A is the normal section of specimen in mm2. Interfacial indentation experiments were made using the approach involving a direct measurement the radial crack length initiated by Vickers indentation at the interface of a coated material in a cross section. The measured crack length at the
Fig. 4. Adhesive strength obtained for the different combinations of coatings.
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Fig. 5. Macrographs of the upper view of sandwich combinations at angles of impact, 30° and 90°.
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Fig. 6. Micrographs of the cross sectioned sandwich combinations at 30° and 90°.
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coated samples where mounted tightly onto a rigid steel block in the chamber in a distance of 100 mm in front of the muzzle. Two different impact angles inclined to the horizontal axis were used: perpendicular (α = 90°) as well as oblique loading (α = 30°). As impactor a 8 mm ball made of a low alloyed QT-steel (60 HRC, 2.1 g) was used. The impact speed of 350 m/s gives a total kinetic energy of 130 J. With the presented parameters relatively hard impacts causing massive deformations on both the coating and the substrate are possible as well as ricochet like impacts causing additional smaller deformations.
highly differentiated. In order to understand the impact behaviour of these combinations, cross sections were performed and observed at the microscopic scale as semi quantitative evaluations. Fig. 6 shows that the combination with Ni-platingX revealed better adhesive behaviour at the interfaces between interlayer and cermet coatings since this combination exhibits less cracks propagation, whereas, the other coatings revealed cracks mainly transversally propagated and this might lead to delaminate the whole multilayered coatings. 4. Conclusion
3. Results and discussion The results of the interfacial indentation experiments are summarised in Fig. 1, where electrochemical combinations with Ni-plating interlayer show higher interfacial toughness than the sprayed combinations with Ni–Cr and Co–Cr interlayer. This is due to the chemical affinity between the Ni-plating and the cermet coating on one hand, and to its homogeneous microstructure on the other hand since the electrochemically deposition process does not provide neither interfaces of splats formation nor an oxide layer during spraying. In case of spraying, the microstructure can help easier crack propagation through the splats and oxide layers as shown in Fig. 2. In Fig. 3a, the combination 5 with the roughened Ni-plating-X interlayer shows a dense and strong interface to cermet coating whereas, the combination 3 with Co–Cr as interlayer exhibit highly oxide layers where many microcracks propagated through the coating during indentation as shown in Fig. 3b. The interfacial toughness of combination 5 with the roughened Ni-plating-X interlayer is higher than that of combination 4, this was mainly attributed to the increasing surface energy by roughening the interface. The interlayer Ni–Cr 80–20 (combination 2) shows a low interfacial toughness value comparing to Ni-plating interlayer combinations. The results of tensile adhesive strength are shown in Fig. 4 give a similar trend as in interfacial toughness results. The interlayer Ni-plating-X showed the highest adhesion value to the cermet coating. The failure of most of coatings was occurred adhesively within the glue and the substrate except combination 3 with the interlayer Co–Cr was failed cohesively within the interlayer coatings. This can be explained by presence of the oxide layer within this coating where the adhesive energy at the interface of interlayer and cermet coating was greater than that in between splats at the microstructure scale. The difference in tensile strength between electrochemical combinations 4 and 5 was lower than that in interfacial indention, may that due to the loading and stress intensity at the interface by interfacial indentation which could drive more easily the crack in the smooth interface than that roughened one. Otherwise for such tensile test, under normal loading, the roughened surface might has a lower impact on tensile strength. Regarding the impact test behaviour, the plastic deformation was so important, as the difference in impact depth of all combinations was not significant. Fig. 5 shows the upper view of coating failure after the impact test. Initially the combination with Co–Cr showed a drastic delamination of coating whereas the other coating at this scale could not be
To enhance the adhesion and the damping of solid particle impact, different interlayer coatings were deposited to perform a multilayered structure. Three methods were studied to characterise the adhesion: tensile adhesive, interfacial indentations and impact tests. The electrochemical Ni-plating-X interlayer shows the best impact behaviour among the other interlayers, as well as it revealed the highest interfacial toughness up to 7 MPa m0.5 and the highest adhesive strength up to 64 MPa. This was mainly attributed to its mechanical properties and to its homogeneous microstructure since the electrochemically deposition process does not provide interfaces within the splats formation, where the absence of interfaces and oxide layers can lead to limit the crack propagation within the interlayer and at the interface of coatings. Acknowledgement We would like to thank the CTI commission for the support under Lewis project 5942.3 as well as Dr. M. Leparoux for the English correction, A. De Meuron for the metallographic observations and R. Egli and B. Von Gunten for the sample spraying. References [1] L. Zhao, M. Maurer, F. Fischer, R. Dicks, E. Lugscheider, Wear 257 (1–2) (2003) 41. [2] D. Toma, W. Brandl, G. Marginean, Surf. Coat. Technol. 138 (2–3) (2001) 149. [3] A. Scrivani, S. Ianelli, A. Rossi, R. Groppetti, F. Casadei, G. Rizzi, Wear 250 (1–12) (2001) 107. [4] N.F. Ak, C. Tekmen, I. Ozdemir, H.S. Soykan, E. Celik, Surf. Coat. Technol. 174–175 (1) (2003) 1070. [5] P. Niranatlumpong, H. Koiprasert, Surf. Coat. Technol. 194 (2005) 96. [6] M. Hadad, G. Blugan, J. Kübler, E. Rosset, L. Rohr, J. Michler, Wear (2006) 634. [7] C.a. Wang, Y. Huang, Q. Zan, L. Zou, S. Cai, J. Am. Cer. Soc. 85 (2002) 2457–2461. [8] J.L. Huang, F.C. Chou, H.H. Lu, J. Mater. Res. 12 (9) (1997) 2357. [9] G. Blugan, M. Hadad, J. Janczak, J. Kuebler, T. Graule, J. Am. Cer. Soc. 88 (4) (2005) 926. [10] X. Chen, R. Wang, N. Yao, A.G. Evans, J.W. Hutchinson, R.W. Bruce, Mater. Sci. Eng., A 352 (1–2) (2003) 221. [11] G. Reisel, S. Steinhäuser, B. Wielage, M. Hockauf, L.W. Meyer, Proceedings of ITSC 2005 Thermal Spray connects: Explore Its Surfacing Potential!, Basel, Switzerland, 2005, p. 742. [12] Y.S. Cheng, W.H. Douglas, A. Versluis, D. Tantbirojn, Eng. Fract. Mech. 64 (1) (1999) 117. [13] M. Menningen, H. Weiss, Surf. Coat. Technol. 76–77 (2) (1995) 835.
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