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Mass loss and wear mechanisms of HVOF-sprayed multi-component white cast iron coatings
Wear 274–275 (2012) 162–167
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Mass loss and wear mechanisms of HVOF-sprayed multi-component white cast iron coatings O. Maranho a,∗ , D. Rodrigues b , M. Boccalini Jr. b , A. Sinatora c a b c
Welding and Thermal Spraying Laboratory, Federal University of Technology – Paraná, Av. Sete de Setembro 3165, 80230-901 Curitiba, Pr, Brazil Institute for Technological Research, Av. Prof. Almeida Prado 532, 05508-901 São Paulo, SP, Brazil Surface Phenomena Laboratory, Polytechnic School of the University of São Paulo, Av. Prof. Mello Moraes 2231, 05508-900 São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 19 March 2011 Received in revised form 12 August 2011 Accepted 19 August 2011 Available online 26 August 2011 Keywords: Mass loss Wear mechanisms Thermal spray coating Multi-component white cast iron Rubber wheel test
a b s t r a c t In this work, multi-component white cast iron was applied by HVOF thermal spray process as alternative to other manufacture processes. Effects of substrate type, substrate pre-heating and heat treatment of coating on mass loss have been determined by rubber wheel apparatus in accordance with ASTM G65. Furthermore, influence of heat treatment of coating on wear mechanisms was also determined by scanning electron microscopy analysis. Heat-treated coatings presented mass loss three times lower than as-sprayed coatings. Furthermore, wear mechanisms of as-sprayed coating are micro-cutting associated with cracks close to unmelted particles and pores. In heat-treated coating, lesser mass loss is due to sintering. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Thermal spraying is one method used to apply coatings on components of several industries as aerospace, automotive, steel, nuclear, and power generation. HVOF thermal spray is one of the most indicated process to apply coatings wear resistant [1–8]. Normally, coatings are NiCr alloy, WC-Co cermets, aluminum oxide and chromium oxide [4,9]. However, recent works have studied the addition of vanadium carbide and titanium carbide in WC-Co cermets, FeCr and NiCr alloys, respectively, to increase formation of hard particles and, consequently, increase wear resistance [10–12]. On the other hand, multi-component white cast iron is used extensively in the manufacture of hot rolling mills rolls [13] due to its excelled wear resistance and capacity to maintain high hardness at high temperatures. This material is similar to AISI M2 steel but it has higher percentage of carbon and vanadium in its composition. Increase of percents of carbon and vanadium facilitates to obtain higher volumetric fraction of MC eutectic carbide and, consequently, more wear resistance [14,15]. But, the relationship between carbon and vanadium percentage must be adequate to enable the secondary hardening of the matrix by decomposition of retained austenite or bainite and precipitation of secondary carbides after heat treatments of quench and tempering [16].
∗ Corresponding author. Tel.: +55 41 33104830; fax: +55 41 33104652. E-mail address: [email protected] (O. Maranho). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.08.024
However, multi-component white cast iron is currently processed by casting and, less often, by powder metallurgy and spray forming [17,18]. So, there are no studies of wear for these alloys in coatings applied by thermal spraying. Basically, wear mechanisms are due to plastic deformation and brittle fracture. Micro-cutting and micro-ploughing are events characterized by plastic deformation of materials. Micro-cutting occurs when hardness of the abrasive is higher than hardness of materials. Micro-ploughing occurs when hardness of materials is higher than the abrasive. On the other hand, events characterized by brittle fracture present cracks formation under surface with detachment of fragments [19–21]. Wear mechanisms of HVOF coatings are mainly micro-polishing, micro-cutting and micro-ploughing. WC-Co coatings tested in rubber wheel apparatus using SiO2 as abrasive, the wear mechanism was micro-polishing, and micro-cutting or micro-ploughing when Al2 O3 abrasive was used [22,23]. Studies showed that, in tests using SiO2 abrasive, the mechanisms are also associated with the degradation of the lamellae due to pre existing micro cracks and porosity [22]. Others suggest that, in tests using SiO2 or Al2 O3 abrasive, cracks form under surface and, then, they spread by W-enriched matrix [23]. For Fe(Cr)–TiC, Fe(Cr)–TiB2 and NiCr–CrC composites, suggested mechanism was also subsurface micro-cracking, but during the ceramic phase with posterior detachment of lamellae [24,25]. Micro-ploughing or micro-cutting occurs in these materials tested with alumina, followed by subsurface cracking and detachment of lamellae [24,25].
O. Maranho et al. / Wear 274–275 (2012) 162–167
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Table 1 Spray parameters to coating deposition. Coating
Substrate
Thickness [m]
Preheating [◦ C]
Heat treatment
1 2 3 4 5
AISI 1020 steel AISI 1020 steel White cast iron White cast iron AISI 1020 steel
200 200 200 200 200
– 150 – 150 –
– – – – Quenching and tempering
Mass loss of thermal spray coatings are influenced by pores, oxides and unmelted particles. The hardness decreases with the increases in porosity and, consequently, coatings present greater mass loss [26,27]. Some works have demonstrated that heat treatment is benefit to avoid pores and to homogenize or modify the structure obtained in the deposition process. These works show that the wear resistance of coatings increases significantly with heat treatments, but they do not show the influence that these heat treatments have in wear mechanisms [28–32]. Therefore, this work aims at setting mass loss and to assist in characterizing wear mechanisms of a new wear resistant alloy applied by thermal spraying over different substrates with or without preheating or posterior thermal treatment.
2. Experimental procedure The METCO® Diamond Jet gun was used to apply with the same parameters used in authors’ previous work [33]. Powder of multicomponent white cast iron with chemical composition (wt%) of 2.5 C, 4 Cr, 4 Mo, 2 W and 8 V was obtained by gas atomization process. It was used as the starting spray powder with a particle size range between 20 and 40 m. Its complete characterization can be found in previous work published by authors [33]. Prior to applying, samples of 12.7 mm × 25.4 mm × 76.2 mm have been degreased and, subsequently, grit blasted using No. 24 alumina grit. This abrasive has been sufficient to obtain surface roughness of about Ra 6.0 m in substrates of AISI 1020 steel and of hypoeutectic alloy of multicomponent white cast iron. Moreover, preheating the substrate at 150 ◦ C was carried out using the thermal spray gun, and it was measured by a contact pyrometer with type K (NiCr/NiAl) thermocouple. After applying, samples were heat treated by quenching and tempering. The quenching was conducted with heating at 1100 ◦ C by 1 h in furnace with argon atmosphere and air cooling. The tempering was done in two steps: heating at 540 ◦ C by 1 h with air cooling, and heating at 530 ◦ C by 1 h with air cooling as well. Summary of applying conditions is described in Table 1. Coatings 1 and 2 were applied on AISI 1020 substrates without pre heating and with pre heating, respectively. Similarly, coatings 3 and 4 were applied also on multi-component white cast iron substrates without and with preheating, respectively. Coating 5 was applied with similar conditions as coating 1, however after deposition it was quenched and tempered. All coatings present nominal thickness of 200 m. The mass loss of samples was determined in accordance with ASTM G-65 [34] in the rubber wheel apparatus. This standard suggests different procedures to perform tests according to materials. E procedure is more indicated for coatings to avoid the influence of substrate in results. In this procedure, it is recommended rotations of 200 rpm, normal load of 130 N and test time of 300 s (1000 wheel revolutions). The mass loss for the three samples for each applying condition was determined by difference between weight of sample prior and after test. An analytical balance with precision of 0.0001 g was used to determine the weight. The wear mechanisms were determined in samples of coatings 1 and 5. Prior the test, coating surfaces were polished with
aluminum oxide solution with grain size up to 1 m. After preparation, the samples were tested in wear test apparatus similar those used previously, but the test time was of 3 s (10 wheel revolutions). This minor test time facilitated the observation of mechanism in the initial stage of wear. Characterizations of the wear surface of the deposited coatings were conducted using scanning electron microscopy (SEM). The phases of coating were determined by Phillips MPD 1880 X-ray diffractometer using CuK␣ radiation ( = 0.154 nm). The hardness was measured on the coating cross section by means of a HVM2 Shimadzu equipment hardness tester, using load of 300 g. The porosity was evaluated by Image Analysis technique. This technique was employed to measure the porosity content in the coatings by converting the micrographs into binary images and quantifying the percentage of dark areas in these images. 3. Results and discussion 3.1. Mass loss Fig. 1 shows typical wear scar of samples tested in rubber wheel apparatus. It is representative of all coatings of this work. Fig. 2 shows the average value and standard deviation of mass loss.
Fig. 1. Typical wear scar of samples tested in rubber wheel apparatus.
Fig. 2. Mass loss of coatings applied with spray parameters summarized in Table 1.
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3.1.1. Influence of material type and pre heating of substrate The results summarized in Fig. 2 show that the mass loss was similar for coatings applied on substrates of AISI 1020 and multicomponent white cast iron. Similarly, mass loss was identical also in coatings applied on the two substrates preheated or without preheating. These results have demonstrated that there is no interference of the substrate preheating on mass loss of coating. In other works published by authors was determined that the substrate preheating have significantly influence on bond strength of coatings [35]. As the bond strength is measured in coating/substrate interface and the mass loss in coating surface, it is possible that preheating temperature was unable to change the properties of coating surface. 3.1.2. Influence of heat treatment As seen previously, there are no influences of type and of preheating substrate in mass loss of coatings. Thus, it has been analyzed only the performance of heat treatment in coatings applied on the substrates of AISI 1020. Fig. 2 presents mass loss results of coatings without heat treatment (coating 1) and with heat treatment (coating 5). The results show better performance in coating with heat treatment because this coating presented mass loss three times inferior to as-sprayed coating. Several authors have demonstrated, in works with other coatings, that heat treatments reduces the mass loss due to transformation austenite into martensite and partial elimination of
Fig. 3. X-ray diffraction curves of coatings: (a) as-sprayed and (b) heat treated.
Fig. 4. Hardness of coatings applied with spray parameters summarized in Table 1.
pores [23,26–28]. In this work, X-ray diffraction also shows the total elimination of austenite in heat treated coatings (Fig. 3a and b). But there is no significant change in hardness of these coatings (Fig. 4). On the other hand, porosity reduces to 0.48 ± 0.03 in heat treated coatings while in as-sprayed coatings remains in 1.28 ± 0.11. Furthermore, SEM analysis (Fig. 5) shows greater modification in morphology and distribution of pores between the coatings. Thus, mass loss reduction can be associated more to quantity, distribution, and morphology of pores than with transformation of austenite to martensite.
Fig. 5. Surface of coatings prior to abrasion tests: (a) as-sprayed and (b) heat treated.
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Fig. 6. Surface of coatings after wear test: (a) as-sprayed coating showing microcutting and plastic deformation and (b) heat treated coating showing micro-cutting.
165
Fig. 7. Surface of as-sprayed coatings after wear test showing displacement of unmelted particles and cracks: (a) overview and (b) detail a.
3.2. Wear mechanisms Fig. 5 shows surface of coatings prior to abrasion tests. Surface of as-sprayed coatings (Fig. 5a) presents pores and unmelted particles inherent to coatings applied by HVOF thermal spray process. In this case, pores present irregular morphology due to their formation related to incorrect filling of spaces between the lamellae and unmelted particles. Surface of heat-treated coating (Fig. 5b) also presents pores and unmelted particles. However, pores are rounded and better distributed around unmelted particles. This is a characteristic of sinterized materials. Thus, it is possible that coatings have been sinterized by quenching and tempering heat treatments. Fig. 6a and b shows the surface of coating after wear tests. Surface of as-sprayed coatings (Fig. 6a) presents areas with microcutting and plastic deformation next of edge of pores. On the other hand, surface of heat-treated coatings (Fig. 6b) also presents microcutting but there are no plastic deformations. These mechanisms are similar those suggested by Bozzi and De Mello [22] and Jones et al. [24] in study about wear of WC-Co and Fe(Cr)–NiCr, tested with aluminum oxide and silicate carbide. In these works, authors suggest that the wear mechanisms were micro-cutting because the abrasive presents better hardness than coatings. Therefore, as in present work the abrasive used in tests presents better hardness than coatings, the wear mechanisms are similar to those suggested by previously mentioned authors. However, surface micrography with higher magnification has showed that there are other wear mechanisms. Fig. 7a shows displacement of unmelted particles into the coating. With the displacement of the particles occurs cracks in its edge (Fig. 7b) and, consequently, increases removal of surface materials.
Furthermore, the increase or decrease of the displacement can be associated with the porosity and the cohesion of lamella of the coatings. Higher porosity or lesser cohesion must diminish sustentation area under particles. On the other hand, several authors have suggested that cracks if form under surface and then it spreads by ceramic phase or by W enriched matrix [23–25]. However there are no ceramic phases in ductile matrix, in this work and, consequently, the material is more homogeneous as can be observed by similar micro-cutting in surface. Therefore, the displacement of unmelted particles, in this work, was responsible for cracks formation. On the other hand, in heat treated coating there is no displacement of unmelted particles and, consequently, there are no cracks in edge of particles (Fig. 8) but only similar micro-cutting on matrix and on unmelted particles. This behavior can be explained by sintering of coating due to heat treatment that promotes the increase of contact between particles with formation of necks. Fig. 9 shows other wear mechanism of coating applied by HVOF thermal spray process. This wear is characterized by formation of cracks in lamellae that are close to pores or to regions of previously detached unmelted particles. Wirojanupatump et al. [25] showed similar wear mechanisms analyzing the cross section of thermal spraying coating, and they named this wear as delamination. However, delamination is used to explain phenomena that take place in bulk materials. Thus, this wear mechanism was denominated as detachment in this work. Therefore, in this work it was determined that the wear of coatings applied by HVOF thermal spray is due to different mechanisms. Besides micro-cutting mechanisms, cracks in coatings occur also. These cracks are formed in regions next to displaced and
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formation close to unmelted particles and pores. In heat-treated coating there is not formation of cracks due to sintering and the wear mechanisms are only micro-cutting. References
Fig. 8. Surface of heat treated coatings after wear test showing only micro-cutting and not displacement or cracks: (a) overview and (b) detail a.
Fig. 9. Surface of as-sprayed coating with (a) cracks in lamellae close to pores.
detached unmelted particles and pores. On the other hand, in heat treated coating there is no crack formation due to sintering of coating. 4. Conclusion There is no influence of substrate type and preheating in the mass loss of coating applied by HVOF thermal spray process. However, heat treatment of coating changed significantly the mass loss. Heat treated coating presents approximately three times lower mass loss than as-sprayed coatings. The higher mass loss of assprayed coating is associated with removal of material by cracks
[1] E. Lugscheider, H. Eschnauer, U. Muller, Th. Weber, Quo vadis thermal spray technology, Powder Metall. 23 (1991) 33–39. [2] R.W. Smith, R. Novak, Advanced and applications in US thermal spray technology, Powder Metall. 23 (1991) 147–154. [3] AWS, Thermal Spray Technology, first ed., 1985, Miami. [4] ASM, Handbook of Thermal Spray Technology, first ed., ASM International, Materials Park, 2004. [5] B. Irving, B. Knight, R.W. Smith, The hvof process – the hottest topic in the thermal spray industry, Weld. J. 7 (1993) 25–30. [6] M.C. Nestler, G. Prensel, T. Sitz, HVOF spraying vs. hard chrome plating – coatings characteristics and aircraft applications, in: C. Coddet (Ed.), Proceedings of the 15th International Thermal Spray Conference, ASM International, Nice, 1998, pp. 1073–1078. [7] M.L. Thorpe, Thermal spraying – industry in transitions, Adv. Mater. Process. 5 (1993) 50–61. [8] M. Friis, C. Persson, J. Wigren, Influence of particle in flight characteristics on the microstructure of atmospheric plasma sprayed yttria stabilized ZrO2 , Surf. Coat. Technol. 141 (2001) 115–127. [9] L. Pawlowski, The Science and Engineering of Thermal Spray, first ed., Wiley, New York, 1995. [10] C.N. Machio, G. Akdogan, M.J. Witcombb, S. Luyckxa, Performance of WC-VC-Co thermal spray coatings in abrasion and slurry erosion tests, Wear 258 (2005) 434–442. [11] M. Jones, A.J. Horlock, P.H. Shipway, D.G. Mccartney, J.V. Wood, Microstructure and abrasive wear behaviour of FeCr–TiC coatings deposited by hvof spraying of shs powder, Wear 249 (2001) 246–253. [12] B. Lotfi, P.H. Shipway, D.G. Mccartney, H. Edris, Abrasive wear behaviour of Ni(Cr)–TiB2 coatings deposited by HVOF spraying of SHS derived cermet powders, Wear 254 (2003) 340–349. [13] H.Q. Wu, N. Sasaguri, Y. Matsubara, M. Hashimoto, Solidification of multialloyed white cast iron: type and morphology of carbides, Trans. AFS 140 (1996) 103–108. [14] Y. Matsubara, M. Hashimoto, Metallurgical aspect of multi-component white cast irons for hot rolling mill roll, in: Proceedings of 44th Mechanical Working Steel Processing Proceedings Conference, Orlando, 2002, pp. 1223– 1230. [15] M. Boccalini Jr., H. Goldenstein, Solidification of high speed steels, Int. Mat. Rev. 46 (2001) 92–115. [16] M. Boccalini Jr., A. Sinatora, Microstructure and wear resistance of high speed steel for rolling mill roll, in: Proceedings of 6th International Tooling Conference, Sweden, 2002, pp. 426–437. [17] A.H. Kasama, A.J. Mourisco, C.S. Kiminami, W.J. Bota Fo, C. Bolfarini, Microstructure and wear resistance of spray formed high chromium white cast iron, Mater. Sci. Eng. A 375–377 (2004) 589–594. [18] T. Koseki, K. Ichino, Y. Kataoka, Development of centrifugal cast roll with high wear resistance for finishing stands of hot strip mill, Kawasaki steel technical report 37 (1997) 13. [19] K.H. Zum Gahr, Microstructure and Wear of Materials, Elsevier ed., New York, 1987. [20] I.M. Hutchings, Tribology-Friction and Wear of Engineering Materials, Edward Arnol ed., London, 1992. [21] J.H. Tylczak, A. Orengon, Abrasive wear, in: P.J. Blau (Ed.), ASM Handbook: Friction, Lubrication and Wear Technology, 1992, pp. 184–190, New York. [22] A.C. Bozzi, J.D.B. de Mello, Wear resistance and wear mechanisms of WC12%Co thermal sprayed coating in three-body abrasion, Wear 233–235 (1999) 575–587. [23] D.A. Stewart, P.H. Shipway, D.G. Mccartney, Abrasive wear behavior of conventional and nanocomposite HVOF-sprayed WC-Co coatings, Wear 225–229 (1999) 789–798. [24] M. Jones, A.J. Horlock, P.H. Shipway, D.G. Mccartney, J.V. Wood, A comparison of the abrasive wear behaviour of HVOF sprayed titanium, Wear 251 (2001) 1009–1016. [25] S. Wirojanupatump, P.H. Shipway, D.G. Mccartney, The influence of HVOF powder feedstock characteristics on the abrasive wear behaviour of CrxCy–NiCr coatings, Wear 249 (2001) 829–837. [26] G. Sundararajan, D. Sen, G. Sivakumar, The tribological behaviour of detonation sprayed coatings: the importance of coatings process parameters, Wear 258 (2005) 377–391. [27] L. Zhao, M. Maurer, F. Fischer, R. Dicks, E. Lugscheider, Influence of spray parameters on the particle in-flight properties and the properties of hvof coatings of WC-Co-Cr, Wear 257 (2004) 41–46. [28] R. Colac¸o, R. Vilar, On the influence of retained austenite in the abrasive wear behaviour of a laser surface melted tool steel, Wear 258 (2005) 225– 231. [29] C.H. Lee, K.O. Min, Effects of heat treatment on the microstructure and properties of HVOF – sprayed NiCrWMoB alloy coatings, Surf. Coat. Technol. 132 (2000) 49–57.
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[33] O. Maranho, D. Rodrigues, M. Boccalini Jr., A. Sinatora, Influence of parameters of the HVOF thermal spray process on the properties of multicomponent white cast iron coatings, Surf. Coat. Technol. 202 (2008) 3494–3500. [34] American Society for Testing and Materials, ASTM G-65, 2001. [35] O. Maranho, D. Rodrigues, M. Baccalini Jr., A. Sinatora, Bond strength of multicomponent white cast iron coatings applied by HVOF thermal spray process, J. Therm. Spray Technol. 18 (2009) 708–713.
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Mass loss and wear mechanisms of HVOF-sprayed multi-component white cast iron coatings O. Maranho a,∗ , D. Rodrigues b , M. Boccalini Jr. b , A. Sinatora c a b c
Welding and Thermal Spraying Laboratory, Federal University of Technology – Paraná, Av. Sete de Setembro 3165, 80230-901 Curitiba, Pr, Brazil Institute for Technological Research, Av. Prof. Almeida Prado 532, 05508-901 São Paulo, SP, Brazil Surface Phenomena Laboratory, Polytechnic School of the University of São Paulo, Av. Prof. Mello Moraes 2231, 05508-900 São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 19 March 2011 Received in revised form 12 August 2011 Accepted 19 August 2011 Available online 26 August 2011 Keywords: Mass loss Wear mechanisms Thermal spray coating Multi-component white cast iron Rubber wheel test
a b s t r a c t In this work, multi-component white cast iron was applied by HVOF thermal spray process as alternative to other manufacture processes. Effects of substrate type, substrate pre-heating and heat treatment of coating on mass loss have been determined by rubber wheel apparatus in accordance with ASTM G65. Furthermore, influence of heat treatment of coating on wear mechanisms was also determined by scanning electron microscopy analysis. Heat-treated coatings presented mass loss three times lower than as-sprayed coatings. Furthermore, wear mechanisms of as-sprayed coating are micro-cutting associated with cracks close to unmelted particles and pores. In heat-treated coating, lesser mass loss is due to sintering. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Thermal spraying is one method used to apply coatings on components of several industries as aerospace, automotive, steel, nuclear, and power generation. HVOF thermal spray is one of the most indicated process to apply coatings wear resistant [1–8]. Normally, coatings are NiCr alloy, WC-Co cermets, aluminum oxide and chromium oxide [4,9]. However, recent works have studied the addition of vanadium carbide and titanium carbide in WC-Co cermets, FeCr and NiCr alloys, respectively, to increase formation of hard particles and, consequently, increase wear resistance [10–12]. On the other hand, multi-component white cast iron is used extensively in the manufacture of hot rolling mills rolls [13] due to its excelled wear resistance and capacity to maintain high hardness at high temperatures. This material is similar to AISI M2 steel but it has higher percentage of carbon and vanadium in its composition. Increase of percents of carbon and vanadium facilitates to obtain higher volumetric fraction of MC eutectic carbide and, consequently, more wear resistance [14,15]. But, the relationship between carbon and vanadium percentage must be adequate to enable the secondary hardening of the matrix by decomposition of retained austenite or bainite and precipitation of secondary carbides after heat treatments of quench and tempering [16].
∗ Corresponding author. Tel.: +55 41 33104830; fax: +55 41 33104652. E-mail address: [email protected] (O. Maranho). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.08.024
However, multi-component white cast iron is currently processed by casting and, less often, by powder metallurgy and spray forming [17,18]. So, there are no studies of wear for these alloys in coatings applied by thermal spraying. Basically, wear mechanisms are due to plastic deformation and brittle fracture. Micro-cutting and micro-ploughing are events characterized by plastic deformation of materials. Micro-cutting occurs when hardness of the abrasive is higher than hardness of materials. Micro-ploughing occurs when hardness of materials is higher than the abrasive. On the other hand, events characterized by brittle fracture present cracks formation under surface with detachment of fragments [19–21]. Wear mechanisms of HVOF coatings are mainly micro-polishing, micro-cutting and micro-ploughing. WC-Co coatings tested in rubber wheel apparatus using SiO2 as abrasive, the wear mechanism was micro-polishing, and micro-cutting or micro-ploughing when Al2 O3 abrasive was used [22,23]. Studies showed that, in tests using SiO2 abrasive, the mechanisms are also associated with the degradation of the lamellae due to pre existing micro cracks and porosity [22]. Others suggest that, in tests using SiO2 or Al2 O3 abrasive, cracks form under surface and, then, they spread by W-enriched matrix [23]. For Fe(Cr)–TiC, Fe(Cr)–TiB2 and NiCr–CrC composites, suggested mechanism was also subsurface micro-cracking, but during the ceramic phase with posterior detachment of lamellae [24,25]. Micro-ploughing or micro-cutting occurs in these materials tested with alumina, followed by subsurface cracking and detachment of lamellae [24,25].
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Table 1 Spray parameters to coating deposition. Coating
Substrate
Thickness [m]
Preheating [◦ C]
Heat treatment
1 2 3 4 5
AISI 1020 steel AISI 1020 steel White cast iron White cast iron AISI 1020 steel
200 200 200 200 200
– 150 – 150 –
– – – – Quenching and tempering
Mass loss of thermal spray coatings are influenced by pores, oxides and unmelted particles. The hardness decreases with the increases in porosity and, consequently, coatings present greater mass loss [26,27]. Some works have demonstrated that heat treatment is benefit to avoid pores and to homogenize or modify the structure obtained in the deposition process. These works show that the wear resistance of coatings increases significantly with heat treatments, but they do not show the influence that these heat treatments have in wear mechanisms [28–32]. Therefore, this work aims at setting mass loss and to assist in characterizing wear mechanisms of a new wear resistant alloy applied by thermal spraying over different substrates with or without preheating or posterior thermal treatment.
2. Experimental procedure The METCO® Diamond Jet gun was used to apply with the same parameters used in authors’ previous work [33]. Powder of multicomponent white cast iron with chemical composition (wt%) of 2.5 C, 4 Cr, 4 Mo, 2 W and 8 V was obtained by gas atomization process. It was used as the starting spray powder with a particle size range between 20 and 40 m. Its complete characterization can be found in previous work published by authors [33]. Prior to applying, samples of 12.7 mm × 25.4 mm × 76.2 mm have been degreased and, subsequently, grit blasted using No. 24 alumina grit. This abrasive has been sufficient to obtain surface roughness of about Ra 6.0 m in substrates of AISI 1020 steel and of hypoeutectic alloy of multicomponent white cast iron. Moreover, preheating the substrate at 150 ◦ C was carried out using the thermal spray gun, and it was measured by a contact pyrometer with type K (NiCr/NiAl) thermocouple. After applying, samples were heat treated by quenching and tempering. The quenching was conducted with heating at 1100 ◦ C by 1 h in furnace with argon atmosphere and air cooling. The tempering was done in two steps: heating at 540 ◦ C by 1 h with air cooling, and heating at 530 ◦ C by 1 h with air cooling as well. Summary of applying conditions is described in Table 1. Coatings 1 and 2 were applied on AISI 1020 substrates without pre heating and with pre heating, respectively. Similarly, coatings 3 and 4 were applied also on multi-component white cast iron substrates without and with preheating, respectively. Coating 5 was applied with similar conditions as coating 1, however after deposition it was quenched and tempered. All coatings present nominal thickness of 200 m. The mass loss of samples was determined in accordance with ASTM G-65 [34] in the rubber wheel apparatus. This standard suggests different procedures to perform tests according to materials. E procedure is more indicated for coatings to avoid the influence of substrate in results. In this procedure, it is recommended rotations of 200 rpm, normal load of 130 N and test time of 300 s (1000 wheel revolutions). The mass loss for the three samples for each applying condition was determined by difference between weight of sample prior and after test. An analytical balance with precision of 0.0001 g was used to determine the weight. The wear mechanisms were determined in samples of coatings 1 and 5. Prior the test, coating surfaces were polished with
aluminum oxide solution with grain size up to 1 m. After preparation, the samples were tested in wear test apparatus similar those used previously, but the test time was of 3 s (10 wheel revolutions). This minor test time facilitated the observation of mechanism in the initial stage of wear. Characterizations of the wear surface of the deposited coatings were conducted using scanning electron microscopy (SEM). The phases of coating were determined by Phillips MPD 1880 X-ray diffractometer using CuK␣ radiation ( = 0.154 nm). The hardness was measured on the coating cross section by means of a HVM2 Shimadzu equipment hardness tester, using load of 300 g. The porosity was evaluated by Image Analysis technique. This technique was employed to measure the porosity content in the coatings by converting the micrographs into binary images and quantifying the percentage of dark areas in these images. 3. Results and discussion 3.1. Mass loss Fig. 1 shows typical wear scar of samples tested in rubber wheel apparatus. It is representative of all coatings of this work. Fig. 2 shows the average value and standard deviation of mass loss.
Fig. 1. Typical wear scar of samples tested in rubber wheel apparatus.
Fig. 2. Mass loss of coatings applied with spray parameters summarized in Table 1.
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3.1.1. Influence of material type and pre heating of substrate The results summarized in Fig. 2 show that the mass loss was similar for coatings applied on substrates of AISI 1020 and multicomponent white cast iron. Similarly, mass loss was identical also in coatings applied on the two substrates preheated or without preheating. These results have demonstrated that there is no interference of the substrate preheating on mass loss of coating. In other works published by authors was determined that the substrate preheating have significantly influence on bond strength of coatings [35]. As the bond strength is measured in coating/substrate interface and the mass loss in coating surface, it is possible that preheating temperature was unable to change the properties of coating surface. 3.1.2. Influence of heat treatment As seen previously, there are no influences of type and of preheating substrate in mass loss of coatings. Thus, it has been analyzed only the performance of heat treatment in coatings applied on the substrates of AISI 1020. Fig. 2 presents mass loss results of coatings without heat treatment (coating 1) and with heat treatment (coating 5). The results show better performance in coating with heat treatment because this coating presented mass loss three times inferior to as-sprayed coating. Several authors have demonstrated, in works with other coatings, that heat treatments reduces the mass loss due to transformation austenite into martensite and partial elimination of
Fig. 3. X-ray diffraction curves of coatings: (a) as-sprayed and (b) heat treated.
Fig. 4. Hardness of coatings applied with spray parameters summarized in Table 1.
pores [23,26–28]. In this work, X-ray diffraction also shows the total elimination of austenite in heat treated coatings (Fig. 3a and b). But there is no significant change in hardness of these coatings (Fig. 4). On the other hand, porosity reduces to 0.48 ± 0.03 in heat treated coatings while in as-sprayed coatings remains in 1.28 ± 0.11. Furthermore, SEM analysis (Fig. 5) shows greater modification in morphology and distribution of pores between the coatings. Thus, mass loss reduction can be associated more to quantity, distribution, and morphology of pores than with transformation of austenite to martensite.
Fig. 5. Surface of coatings prior to abrasion tests: (a) as-sprayed and (b) heat treated.
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Fig. 6. Surface of coatings after wear test: (a) as-sprayed coating showing microcutting and plastic deformation and (b) heat treated coating showing micro-cutting.
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Fig. 7. Surface of as-sprayed coatings after wear test showing displacement of unmelted particles and cracks: (a) overview and (b) detail a.
3.2. Wear mechanisms Fig. 5 shows surface of coatings prior to abrasion tests. Surface of as-sprayed coatings (Fig. 5a) presents pores and unmelted particles inherent to coatings applied by HVOF thermal spray process. In this case, pores present irregular morphology due to their formation related to incorrect filling of spaces between the lamellae and unmelted particles. Surface of heat-treated coating (Fig. 5b) also presents pores and unmelted particles. However, pores are rounded and better distributed around unmelted particles. This is a characteristic of sinterized materials. Thus, it is possible that coatings have been sinterized by quenching and tempering heat treatments. Fig. 6a and b shows the surface of coating after wear tests. Surface of as-sprayed coatings (Fig. 6a) presents areas with microcutting and plastic deformation next of edge of pores. On the other hand, surface of heat-treated coatings (Fig. 6b) also presents microcutting but there are no plastic deformations. These mechanisms are similar those suggested by Bozzi and De Mello [22] and Jones et al. [24] in study about wear of WC-Co and Fe(Cr)–NiCr, tested with aluminum oxide and silicate carbide. In these works, authors suggest that the wear mechanisms were micro-cutting because the abrasive presents better hardness than coatings. Therefore, as in present work the abrasive used in tests presents better hardness than coatings, the wear mechanisms are similar to those suggested by previously mentioned authors. However, surface micrography with higher magnification has showed that there are other wear mechanisms. Fig. 7a shows displacement of unmelted particles into the coating. With the displacement of the particles occurs cracks in its edge (Fig. 7b) and, consequently, increases removal of surface materials.
Furthermore, the increase or decrease of the displacement can be associated with the porosity and the cohesion of lamella of the coatings. Higher porosity or lesser cohesion must diminish sustentation area under particles. On the other hand, several authors have suggested that cracks if form under surface and then it spreads by ceramic phase or by W enriched matrix [23–25]. However there are no ceramic phases in ductile matrix, in this work and, consequently, the material is more homogeneous as can be observed by similar micro-cutting in surface. Therefore, the displacement of unmelted particles, in this work, was responsible for cracks formation. On the other hand, in heat treated coating there is no displacement of unmelted particles and, consequently, there are no cracks in edge of particles (Fig. 8) but only similar micro-cutting on matrix and on unmelted particles. This behavior can be explained by sintering of coating due to heat treatment that promotes the increase of contact between particles with formation of necks. Fig. 9 shows other wear mechanism of coating applied by HVOF thermal spray process. This wear is characterized by formation of cracks in lamellae that are close to pores or to regions of previously detached unmelted particles. Wirojanupatump et al. [25] showed similar wear mechanisms analyzing the cross section of thermal spraying coating, and they named this wear as delamination. However, delamination is used to explain phenomena that take place in bulk materials. Thus, this wear mechanism was denominated as detachment in this work. Therefore, in this work it was determined that the wear of coatings applied by HVOF thermal spray is due to different mechanisms. Besides micro-cutting mechanisms, cracks in coatings occur also. These cracks are formed in regions next to displaced and
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formation close to unmelted particles and pores. In heat-treated coating there is not formation of cracks due to sintering and the wear mechanisms are only micro-cutting. References
Fig. 8. Surface of heat treated coatings after wear test showing only micro-cutting and not displacement or cracks: (a) overview and (b) detail a.
Fig. 9. Surface of as-sprayed coating with (a) cracks in lamellae close to pores.
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