- Email: [email protected]
HVOF thermal sprayed coatings on aluminium alloys and aluminium matrix composites
Surface & Coatings Technology 200 (2005) 1178 – 1181 www.elsevier.com/locate/surfcoat
HVOF thermal sprayed coatings on aluminium alloys and aluminium matrix composites Josep A. Picasa,T, Antonio Forna, Ramiro Rillab, Enric Martı´na a
Material Science Department, Technical University of Catalonian, Avda. Vı´ctor Balaguer, s/n 08800 Vilanova i la Geltru´, Spain b RALSA (Rilla, Alvarez y Lo´pez, S.A.), Langreo, Spain Available online 8 April 2005
Abstract This project is concerned with the investigation of the capability of HVOF thermal spraying to improve the tribological and mechanical properties of aluminium alloys and aluminium matrix composites (AMCs). The present study describes and compares the mechanical and tribological properties of CCr–NiCr and WC–CoCr thermal sprayed HVOF coatings deposited on three different substrates: steel, aluminium alloy and aluminium matrix composite. The coating microstructures were characterised by SEM and optical microscopy. Differences in roughness have been determined by profilometry. The ultra-microindentation technique was applied to measure the hardness and the elastoplastic properties of the coatings. Experiments using a tribometer (pin on disc configuration) under lubricated and dry conditions have been performed in order to evaluate the friction and wear properties of the different systems. The morphology of the wear tracks after pin on disc tests was characterised using SEM microscopy and profilometry with the aim of studying the wear mechanism. D 2005 Elsevier B.V. All rights reserved. Keywords: HVOF thermal spraying; Wear resistance coatings; Aluminium alloys; Tribology
1. Introduction High Velocity Oxy-Fuel (HVOF) coatings are required to protect the surface of critical components used in high erosive environments [1,2]. The main goal of the proposed research is to investigate the possibility of using thermal sprayed HVOF CrC–NiCr and WC–CoCr coatings to improve the wear resistance of aluminium alloys to be used in the injection moulds for the plastic industry. It is proposed that the steel moulds and tools used in the production of plastic components could be replaced with lightweight aluminium alloys coated with wear resistant coatings. This combination of aluminium alloys with specially designed coatings will permit a reduction in injection moulding cycle time and an increase in the life of the tools, as a result of the improvement in the thermal conductivity (aluminium vs. steel) and wear resistance of the cermet coatings. The low density of the aluminium will imply a considerable advantage during the T Corresponding author. Tel.: +34 93 896 77 33; fax: +34 93 896 77 00. E-mail address: [email protected] (J.A. Picas). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.124
manufacture, assembly and maintenance, furthermore, the good thermal conductivity of the aluminium (four times superior than steel) allows the mould to tolerate a rapid and uniform heat distribution and dissipation, providing a better control of the temperature, improving the quality of the product and reducing the cycle times of injection. However, light metals in general exhibit very poor tribological properties resulting in a severe superficial wear. To avoid this disadvantage, the implementation of wear resistance coatings deposited by HVOF is proposed. The characteristics of the High Velocity Oxy-Fuel (HVOF) process (low temperature and supersonic speed of the particles) avoid overheating of the substrate and make the deposition possible in materials with relatively low melting temperatures. Moreover, the HVOF thermal spraying has shown to be one of the better methods for deposition of WC–CoCr and CrC–NiCr feedstock powders [3], because the higher velocities and lower temperatures experienced by the powder particles (as compared to plasma based routes) result in less decomposition of the carbides during spraying [4]. Consequently, this results in higher
J.A. Picas et al. / Surface & Coatings Technology 200 (2005) 1178–1181 Table 1 Spraying parameters used to produce HVOF coatings Fuel (kerosene) flow rate [l/h] Hydrogen flow rate [m3/h] Oxygen flow rate [m3/h] Powder feed rate [l/min] Carrier gas nitrogen [l/min] Spray distance [mm] Feedstock powder size [Am]
CrC75NiCr25
WC86Co10Cr4
8 3 40 25 2*6 250 45 + 15
11 3 45 25 2 * 4.5 250 30 + 15
quality, more wear-resistant coatings, with higher levels of retained reinforced material and less porosity. The objective of the proposed work is demonstrating the feasibility and viability of preparing HVOF coatings on aluminium substrates and determining the influence of the coating microstructure and the substrate on the tribological and mechanical behaviour of the system (coating + substrate).
2. Experimental details The coatings studied in this research have been CrC75(NiCr20)25 and WC86 Co10 Cr4 coatings, deposited on three different substrates: 40 CrMnNiMo steel, Al-7022 aluminium alloy and Al-6061 aluminium alloy reinforced
1179
with 22 vol.% of Al2O3 particles (average size of 13.6 Am), using the HVOF thermal spray process (High Velocity OxyFuel). The feed stock powder was a commercially available powder produced by Thermico. A C-CJS HVOF gun with k4.2 e nozzle configuration was used for thermal spraying of the powders (Ralsa). The spraying parameters used to produce the coatings on the steel and aluminium substrates are summarised in Table 1. The coating microstructure was examined using a JEOL JSM-5600 scanning electron microscope (SEM). Backscattered electron images were obtained, and EDS analysis was conducted. Samples for SEM observation of the coatings were sectioned from a transverse section and prepared by standard metallographic techniques. The coating hardness has been determined with an ultramicrohardness testing system, capable of measuring continuously force and displacement, using a Vickers diamond indentor. The dynamic hardness measurements were carried out with a maximum applied load of 1 N. Each average value has been calculated from 15 different measurements, discarding the anomalous values as consequence of the coating defects. Tribological evaluation of coated substrates was performed using a pin on disc tribometer, according to ASTM wear testing standard (G-99) and DIN standard (DIN
Fig. 1. SEM micrographs showing (a) the cross-section of WC86Co10Cr4 coating on steel substrate and (b) microstructure detail of the coating.
Fig. 2. SEM micrographs showing the cross-sections of (a) CrC75NiCr25 coating on aluminium substrate and (b) microstructure detail of the coating.
1180
J.A. Picas et al. / Surface & Coatings Technology 200 (2005) 1178–1181
Table 2 Characteristic parameters of HVOF sprayed coatings CrC75–NiCr25
WC86–Co10Cr4
Substrate
Steel
A7022
6061/Al2O3
Steel
A7022
6061/Al2O3
Coating thickness [Am] Universal/plastic hardness [GPa] Ra [Am] Friction coefficient, lubricated/dry Specific wear rate [m3/md N 10 14], lubricated /dry
250 7.6/12.7 4.57 0.09/0.2 4 0.5/1.8
230 7.4/12.4 4.52 0.09/0.26 0.9/1.8
260 7.2/12.4 4.49 0.10/0.24 1.6/2.8
120 8.5/13.6 4.35 0.12/0.24 0.5/0.9
100 8.2/14.8 4.72 0.10/0.22 0.8/1.2
100 7.9/13.5 4.44 0.09/0.25 1.2/2.6
50324). Friction and wear properties were evaluated sliding a WC-6%Co 6 mm diameter ball against the coated specimens. All tests were carried out at the same linear speed (0.10 m/s) under lubricated (Repsol 15 W40 oil) and dry conditions with an applied load of 10 N. The tests were performed at room temperature. When testing was completed, the amount of material loss was evaluated by measuring the cross-sectional area of the wear tracks using a rugosimeter-perfilometer. The wear volume and specific wear rate were calculated according to the classic wear theory [5]. The friction coefficient (l = friction force/applied load) was plotted as a function of the number of laps in the test. At least three different tests were conducted for each test conditions and material.
3. Results and discussion SEM micrographs of the polished cross-section of the WC86 Co10 Cr4 and CrC75 NiCr25 coatings, deposited by HVOF thermal spraying process on steel and A7022 substrates, are shown in Figs. 1a and 2a, respectively. The coating thickness of different coatings is shown in Table 2. The coatings exhibit a typical splat pattern-like structure, which consists of islands that are elongated in directions parallel to the substrate. The coating microstructures are shown in Figs. 1b and 2b, and consisted of a metallic binder Co–Cr and Ni–Cr and dispersed carbide phases WC and CrC (bright and dark particles, respectively). During the
HVOF spraying process, the powder particles are accelerated and heated as they travel through the flame. The external part of the particles melts, except for the carbide particles, and the coating built up by the piling up of the impacting droplets that are flattened by the acceleration forces and rapidly cooled down. The splats within the coating result from the collapse of the droplets on the substrate. Fig. 3 shows the WC86 Co10 Cr4 coating, deposited by HVOF thermal spraying process on the aluminium reinforced material. Fig. 3a presents a SEM micrograph of the polished cross-section of the coating and Fig. 3b shows an interface coating–substrate detail. The A6061/(Al2O3)p presents a homogeneous distribution of the alumina particles embedded in the aluminium matrix. Moreover, one can observe the presence of some cracked alumina particles. This fact could be related with the residual stresses in the material caused by the plastic deformation of the aluminium matrix during the extrusion process. Some characteristic parameters of the different coatings are given in Table 2. As reported by other studies [6,7] in all cases, the coatings sprayed from WC–CoCr powders are significantly harder than CrC–NiCr coatings. On the other hand, the tribological behaviour of the coatings under lubricated and dry conditions was evaluated using a pin on disc tribometer. During the test, the values of the friction coefficients increased due to highly adhesive microcontacts produced between the pin and the coating and the production of bthird bodyQ hard particles in the wear track that involves an abrasive wear mechanism which
Fig. 3. SEM micrographs showing the cross-sections of (a) WC86Co10Cr4 coating on reinforced aluminium substrate and (b) interface coating–substrate detail.
J.A. Picas et al. / Surface & Coatings Technology 200 (2005) 1178–1181
influenced quite considerably the final wear rate of the studied coatings. However, no significant differences of the mechanical and tribological properties of the coatings deposited on the different substrates have been observed and it can be noticed that all coatings seem to exhibit a good adherence on the different substrates. In the HVOF coatings deposited on the reinforced aluminium material, one can observe that the coating presents a good continuity over the substrate and it fits in the reinforced particles that are present in the substrate surface (see Fig. 3b).
1181
aluminium components used in high erosive environments, such as die mould, piston liners and others.
Acknowledgements The authors would like to take this opportunity to thank the Spain Government, which founded this work through the Plan Nacional de Investigacio´n Cientı´fica, Desarrollo e Innovacio´ n Tecnolo´ gica 2000–2003: DPI2002-04581-C02-01.
4. Conclusions References CrC–NiCr and WC–CoCr coatings have been successfully prepared by HVOF spraying on steel, aluminium and reinforced aluminium substrates. The substrate seems not to have a significant effect on the mechanical and tribological properties of the coatings. All coatings are dense and homogeneous and seem to exhibit a good continuity and adherence on the different substrates. The HVOF process can produce very dense coatings with a higher control of coating composition and can be used for the production of functional gradient materials deposited on aluminium and aluminium reinforced materials. Therefore, HVOF coatings have the potential to protect the surfaces of
[1] S. Zimmermann, H. Kreye, Proc. Intern. Therm. Spray Conf. 1 (1998) 187 – 192. [2] K.J. Stein, B.S. Schorr, A. Marder, Wear 224 (1999) 153 – 159. [3] J. Nerz, B. Kushner, A. Rotolico, Mater. Sci. Monogr. 67 (1991) 27 – 32. [4] R. Schwetzke, H. Kreye, J. Therm. Spray Technol. 8 (3) (1999) 433 – 439. [5] B. Bhushan, B.K. Gupta, Handbook of Tribology, McGraw Hill, NY, USA, 1997, p. 1168. [6] L.M. Berger, P. Ettmayer, P. Vuoristo, T. Mantyla, W. Kunert, J. Therm. Spray Technol. 10 (2) (2001) 311 – 325. [7] A. Forn, J.A. Picas, G. Matth7us, Mat. Sci. Forum 426/432 (2003) 2623 – 2628.
HVOF thermal sprayed coatings on aluminium alloys and aluminium matrix composites Josep A. Picasa,T, Antonio Forna, Ramiro Rillab, Enric Martı´na a
Material Science Department, Technical University of Catalonian, Avda. Vı´ctor Balaguer, s/n 08800 Vilanova i la Geltru´, Spain b RALSA (Rilla, Alvarez y Lo´pez, S.A.), Langreo, Spain Available online 8 April 2005
Abstract This project is concerned with the investigation of the capability of HVOF thermal spraying to improve the tribological and mechanical properties of aluminium alloys and aluminium matrix composites (AMCs). The present study describes and compares the mechanical and tribological properties of CCr–NiCr and WC–CoCr thermal sprayed HVOF coatings deposited on three different substrates: steel, aluminium alloy and aluminium matrix composite. The coating microstructures were characterised by SEM and optical microscopy. Differences in roughness have been determined by profilometry. The ultra-microindentation technique was applied to measure the hardness and the elastoplastic properties of the coatings. Experiments using a tribometer (pin on disc configuration) under lubricated and dry conditions have been performed in order to evaluate the friction and wear properties of the different systems. The morphology of the wear tracks after pin on disc tests was characterised using SEM microscopy and profilometry with the aim of studying the wear mechanism. D 2005 Elsevier B.V. All rights reserved. Keywords: HVOF thermal spraying; Wear resistance coatings; Aluminium alloys; Tribology
1. Introduction High Velocity Oxy-Fuel (HVOF) coatings are required to protect the surface of critical components used in high erosive environments [1,2]. The main goal of the proposed research is to investigate the possibility of using thermal sprayed HVOF CrC–NiCr and WC–CoCr coatings to improve the wear resistance of aluminium alloys to be used in the injection moulds for the plastic industry. It is proposed that the steel moulds and tools used in the production of plastic components could be replaced with lightweight aluminium alloys coated with wear resistant coatings. This combination of aluminium alloys with specially designed coatings will permit a reduction in injection moulding cycle time and an increase in the life of the tools, as a result of the improvement in the thermal conductivity (aluminium vs. steel) and wear resistance of the cermet coatings. The low density of the aluminium will imply a considerable advantage during the T Corresponding author. Tel.: +34 93 896 77 33; fax: +34 93 896 77 00. E-mail address: [email protected] (J.A. Picas). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.124
manufacture, assembly and maintenance, furthermore, the good thermal conductivity of the aluminium (four times superior than steel) allows the mould to tolerate a rapid and uniform heat distribution and dissipation, providing a better control of the temperature, improving the quality of the product and reducing the cycle times of injection. However, light metals in general exhibit very poor tribological properties resulting in a severe superficial wear. To avoid this disadvantage, the implementation of wear resistance coatings deposited by HVOF is proposed. The characteristics of the High Velocity Oxy-Fuel (HVOF) process (low temperature and supersonic speed of the particles) avoid overheating of the substrate and make the deposition possible in materials with relatively low melting temperatures. Moreover, the HVOF thermal spraying has shown to be one of the better methods for deposition of WC–CoCr and CrC–NiCr feedstock powders [3], because the higher velocities and lower temperatures experienced by the powder particles (as compared to plasma based routes) result in less decomposition of the carbides during spraying [4]. Consequently, this results in higher
J.A. Picas et al. / Surface & Coatings Technology 200 (2005) 1178–1181 Table 1 Spraying parameters used to produce HVOF coatings Fuel (kerosene) flow rate [l/h] Hydrogen flow rate [m3/h] Oxygen flow rate [m3/h] Powder feed rate [l/min] Carrier gas nitrogen [l/min] Spray distance [mm] Feedstock powder size [Am]
CrC75NiCr25
WC86Co10Cr4
8 3 40 25 2*6 250 45 + 15
11 3 45 25 2 * 4.5 250 30 + 15
quality, more wear-resistant coatings, with higher levels of retained reinforced material and less porosity. The objective of the proposed work is demonstrating the feasibility and viability of preparing HVOF coatings on aluminium substrates and determining the influence of the coating microstructure and the substrate on the tribological and mechanical behaviour of the system (coating + substrate).
2. Experimental details The coatings studied in this research have been CrC75(NiCr20)25 and WC86 Co10 Cr4 coatings, deposited on three different substrates: 40 CrMnNiMo steel, Al-7022 aluminium alloy and Al-6061 aluminium alloy reinforced
1179
with 22 vol.% of Al2O3 particles (average size of 13.6 Am), using the HVOF thermal spray process (High Velocity OxyFuel). The feed stock powder was a commercially available powder produced by Thermico. A C-CJS HVOF gun with k4.2 e nozzle configuration was used for thermal spraying of the powders (Ralsa). The spraying parameters used to produce the coatings on the steel and aluminium substrates are summarised in Table 1. The coating microstructure was examined using a JEOL JSM-5600 scanning electron microscope (SEM). Backscattered electron images were obtained, and EDS analysis was conducted. Samples for SEM observation of the coatings were sectioned from a transverse section and prepared by standard metallographic techniques. The coating hardness has been determined with an ultramicrohardness testing system, capable of measuring continuously force and displacement, using a Vickers diamond indentor. The dynamic hardness measurements were carried out with a maximum applied load of 1 N. Each average value has been calculated from 15 different measurements, discarding the anomalous values as consequence of the coating defects. Tribological evaluation of coated substrates was performed using a pin on disc tribometer, according to ASTM wear testing standard (G-99) and DIN standard (DIN
Fig. 1. SEM micrographs showing (a) the cross-section of WC86Co10Cr4 coating on steel substrate and (b) microstructure detail of the coating.
Fig. 2. SEM micrographs showing the cross-sections of (a) CrC75NiCr25 coating on aluminium substrate and (b) microstructure detail of the coating.
1180
J.A. Picas et al. / Surface & Coatings Technology 200 (2005) 1178–1181
Table 2 Characteristic parameters of HVOF sprayed coatings CrC75–NiCr25
WC86–Co10Cr4
Substrate
Steel
A7022
6061/Al2O3
Steel
A7022
6061/Al2O3
Coating thickness [Am] Universal/plastic hardness [GPa] Ra [Am] Friction coefficient, lubricated/dry Specific wear rate [m3/md N 10 14], lubricated /dry
250 7.6/12.7 4.57 0.09/0.2 4 0.5/1.8
230 7.4/12.4 4.52 0.09/0.26 0.9/1.8
260 7.2/12.4 4.49 0.10/0.24 1.6/2.8
120 8.5/13.6 4.35 0.12/0.24 0.5/0.9
100 8.2/14.8 4.72 0.10/0.22 0.8/1.2
100 7.9/13.5 4.44 0.09/0.25 1.2/2.6
50324). Friction and wear properties were evaluated sliding a WC-6%Co 6 mm diameter ball against the coated specimens. All tests were carried out at the same linear speed (0.10 m/s) under lubricated (Repsol 15 W40 oil) and dry conditions with an applied load of 10 N. The tests were performed at room temperature. When testing was completed, the amount of material loss was evaluated by measuring the cross-sectional area of the wear tracks using a rugosimeter-perfilometer. The wear volume and specific wear rate were calculated according to the classic wear theory [5]. The friction coefficient (l = friction force/applied load) was plotted as a function of the number of laps in the test. At least three different tests were conducted for each test conditions and material.
3. Results and discussion SEM micrographs of the polished cross-section of the WC86 Co10 Cr4 and CrC75 NiCr25 coatings, deposited by HVOF thermal spraying process on steel and A7022 substrates, are shown in Figs. 1a and 2a, respectively. The coating thickness of different coatings is shown in Table 2. The coatings exhibit a typical splat pattern-like structure, which consists of islands that are elongated in directions parallel to the substrate. The coating microstructures are shown in Figs. 1b and 2b, and consisted of a metallic binder Co–Cr and Ni–Cr and dispersed carbide phases WC and CrC (bright and dark particles, respectively). During the
HVOF spraying process, the powder particles are accelerated and heated as they travel through the flame. The external part of the particles melts, except for the carbide particles, and the coating built up by the piling up of the impacting droplets that are flattened by the acceleration forces and rapidly cooled down. The splats within the coating result from the collapse of the droplets on the substrate. Fig. 3 shows the WC86 Co10 Cr4 coating, deposited by HVOF thermal spraying process on the aluminium reinforced material. Fig. 3a presents a SEM micrograph of the polished cross-section of the coating and Fig. 3b shows an interface coating–substrate detail. The A6061/(Al2O3)p presents a homogeneous distribution of the alumina particles embedded in the aluminium matrix. Moreover, one can observe the presence of some cracked alumina particles. This fact could be related with the residual stresses in the material caused by the plastic deformation of the aluminium matrix during the extrusion process. Some characteristic parameters of the different coatings are given in Table 2. As reported by other studies [6,7] in all cases, the coatings sprayed from WC–CoCr powders are significantly harder than CrC–NiCr coatings. On the other hand, the tribological behaviour of the coatings under lubricated and dry conditions was evaluated using a pin on disc tribometer. During the test, the values of the friction coefficients increased due to highly adhesive microcontacts produced between the pin and the coating and the production of bthird bodyQ hard particles in the wear track that involves an abrasive wear mechanism which
Fig. 3. SEM micrographs showing the cross-sections of (a) WC86Co10Cr4 coating on reinforced aluminium substrate and (b) interface coating–substrate detail.
J.A. Picas et al. / Surface & Coatings Technology 200 (2005) 1178–1181
influenced quite considerably the final wear rate of the studied coatings. However, no significant differences of the mechanical and tribological properties of the coatings deposited on the different substrates have been observed and it can be noticed that all coatings seem to exhibit a good adherence on the different substrates. In the HVOF coatings deposited on the reinforced aluminium material, one can observe that the coating presents a good continuity over the substrate and it fits in the reinforced particles that are present in the substrate surface (see Fig. 3b).
1181
aluminium components used in high erosive environments, such as die mould, piston liners and others.
Acknowledgements The authors would like to take this opportunity to thank the Spain Government, which founded this work through the Plan Nacional de Investigacio´n Cientı´fica, Desarrollo e Innovacio´ n Tecnolo´ gica 2000–2003: DPI2002-04581-C02-01.
4. Conclusions References CrC–NiCr and WC–CoCr coatings have been successfully prepared by HVOF spraying on steel, aluminium and reinforced aluminium substrates. The substrate seems not to have a significant effect on the mechanical and tribological properties of the coatings. All coatings are dense and homogeneous and seem to exhibit a good continuity and adherence on the different substrates. The HVOF process can produce very dense coatings with a higher control of coating composition and can be used for the production of functional gradient materials deposited on aluminium and aluminium reinforced materials. Therefore, HVOF coatings have the potential to protect the surfaces of
[1] S. Zimmermann, H. Kreye, Proc. Intern. Therm. Spray Conf. 1 (1998) 187 – 192. [2] K.J. Stein, B.S. Schorr, A. Marder, Wear 224 (1999) 153 – 159. [3] J. Nerz, B. Kushner, A. Rotolico, Mater. Sci. Monogr. 67 (1991) 27 – 32. [4] R. Schwetzke, H. Kreye, J. Therm. Spray Technol. 8 (3) (1999) 433 – 439. [5] B. Bhushan, B.K. Gupta, Handbook of Tribology, McGraw Hill, NY, USA, 1997, p. 1168. [6] L.M. Berger, P. Ettmayer, P. Vuoristo, T. Mantyla, W. Kunert, J. Therm. Spray Technol. 10 (2) (2001) 311 – 325. [7] A. Forn, J.A. Picas, G. Matth7us, Mat. Sci. Forum 426/432 (2003) 2623 – 2628.