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Coating fracture toughness determined by Vickers indentation: an important parameter in cavitation erosion resistance of WC–Co thermally sprayed coatings
Surface and Coatings Technology 177 – 178 (2004) 489–496
Coating fracture toughness determined by Vickers indentation: an important parameter in cavitation erosion resistance of WC–Co thermally sprayed coatings M.M. Limaa, C. Godoya, P.J. Modenesia, J.C. Avelar-Batistab, *, A. Davisonb, A. Matthewsc a
Department of Metallurgical and Materials Engineering, School of Engineering, Universidade Federal de Minas Gerais, ´ Rua Espırito Santo 35, Belo Horizonte – MG 30160-030, Brazil b Research Centre in Surface Engineering, School of Engineering, University of Hull, Cottingham Road, Hull HU6 7RX, UK c Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK
Abstract The deficient performance of thermally sprayed coatings in cavitation erosion tests is often attributed to the nature of their lamellar microstructure. Poor coatingysubstrate adhesion, low toughness and tensile residual stresses, which are introduced during the deposition process, can also adversely affect their cavitation resistance. In order to improve the cavitation performance of such coatings, it is also important to control some of the coating properties such as elastic modulus (E) and hardness (H). By reducing E and increasing H (i.e. to ensure a higher HyE ratio), a better tribological performance is usually achieved. The erosive environment also plays a decisive role in determining the final chemical composition of the coating. Thermally sprayed WC–Co coatings are well known because of their high hardness and, in such cemented carbides, the corrosion resistance is frequently affected by the susceptibility of the cobalt binder to chemical attack. In this work, an attempt to improve the cavitation resistance of WC–Co coatings was made by either modifying coating composition (and therefore modifying some coating properties such as hardness, elastic modulus and toughness) or by carrying out a ‘melt’ post-deposition treatment in order to disrupt the intrinsic lamellar microstructure of the coating. Four different coatings were deposited onto an AISI 1020 steel substrate: (i) WC–12%Co; (ii) as-sprayed (AS) 50%(WC–12%Co)q50%(NiCr); (iii) post-melted (PM) 50%(WC–12%Co)q50%(NiCr) and (iv) a duplex system comprising a WC–12%Co top layer and a NiCrAl interlayer. The ‘PM’ coating produced from the pre-alloyed powder 50%(WC–12%Co)q50%(NiCr) displayed a higher elastic modulus (measured by Knoop indentation) and a lower hardness (and thus a lower HyE ratio) than the WC–12%Co. Also, the fracture toughness of the latter (measured by Vickers indentation tests) was increased from 1.6"0.9 to 32"12 MPa m1y2 . The worst performance in cavitation erosion tests was achieved by the WC– 12%Co coating, which showed the highest mass loss throughout the test. Conversely, the ‘PM’ 50%(WC–12%Co)q50%(NiCr) coating exhibited the best cavitation resistance and a correlation between coating toughness and cavitation resistance could be established. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Cavitation erosion; Thermal spraying; Duplex coating; Vickers indentation fracture toughness
1. Introduction Solid particle erosion is a wear process that occurs as discrete solid particles strike a surface. Erosive wear is often caused by plastic deformation or brittle fracture, but both mechanisms can also operate simultaneously. Erosion of metals usually involves plastic flow and the predicted wear rate, W, is inversely proportional to H, the hardness of the surface w1x. Conversely, erosive wear *Corresponding author. Tel.: q44-1482-465072; fax: q44-1482466477. E-mail address: [email protected] (J.C. Avelar-Batista).
of more brittle materials is predominantly governed either by flow or fracture depending on the impact conditions. If the impact of an erosive particle leads to brittle fracture, material is removed from the surface by nucleation and intersection of cracks. In this case, the most relevant material property which determines the erosion resistance is fracture toughness (Kc), with hardness (H) being a much less significant parameter w1x. ´ Lopez-Cantera and Mellor w2x investigated erosive wear in WC–Co–Cr thermally sprayed coatings and they concluded that subsurface cracks produced by erosion were similar to those produced by indentation testing.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00917-4
M.M. Lima et al. / Surface and Coatings Technology 177 – 178 (2004) 489–496
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Table 1 Surface roughness (Ra), microhardness, elastic modulus, HyE ratio and elastic property mismatch ((Ecoating yEsubstrate )yEsubstrate ) for WC–12%Cobased coatings and AISI 1020 steel substrate Material
Ra (mm)
WC–12%Co AS 50%(WC–12%Co)q50%(NiCr) PM 50%(WC–12%Co)q50%(NiCr) AISI 1020 steel
6.0"0.4a 6.5"0.6 1.7"0.9 2.8"0.2
a
Microhardness (GPa)
Elastic modulus (GPa)
10.1"0.5 5.7"0.4 8.0"0.4 1.11"0.02
194"39 226"46 378"58 179"33
HyE
((EcoatingyEsubstrate)yEsubstrate) (%)
(52"8)=10y3 (25"7)=10y3 (21"4)=10y3 (6"1)=10y3
8 47 111 –
An Ra-value equal to 13"3 mm was recorded for the duplex WC–12%CoyNiCrAl coating.
Therefore, the fracture toughness determined by the indentation method is an appropriate parameter to be used in mathematical equations to estimate erosion rates. ´ In Lopez-Cantera and Mellor’s work w2x, the coating indentation fracture toughness (Kc) was calculated using erosion models developed by Lawn and Fuller w3x and Wilshaw w4x. Both models predict that cracks will propagate from the indentation corners, where the highest stresses occur. In thermally sprayed coatings, however, the fracture is anisotropic and a lower fracture toughness is reported parallel to the coatingysubstrate interface w5x. Toughness values measured in such a direction are, therefore, the most appropriate ones to be used in models to predict the coating erosive wear, as long as crack formation under erosion conditions is analogous to its formation during indentation. In this work, the cavitation erosion resistance of thermally sprayed coatings, based on a WC–Co powder and deposited onto AISI 1020 steel substrates, was investigated. In this particular erosion process, the cavitation phenomena will lead to material wear. Cavitation phenomena usually arise in fluid systems as a result of strong pressure fluctuations; they basically involve the build-up and subsequent implosion of bubbles in liquids. Cavitation erosion rates are often influenced by surface topography, yield properties (such as hardness and rate of strain-hardening), elastic properties (elastic modulus, resilience, super-elasticity) and toughness. By producing four distinct coatings, different mechanical and elastic properties were achieved and distinct responses in cavitation erosion tests were obtained. Some major coating properties such as hardness, indentation toughness w6x, elastic modulus and density were also evaluated. A statistical correlation between cavitation erosion resistance and indentation fracture toughness could be established, using the model for abrasive erosion resistance ´ in WC–Co coatings proposed by Lopez-Cantera and Mellor w2x. A 50%(WC–12%Co)q50%(NiCr) coating, subjected to a post-melt treatment after deposition, had the highest cavitation resistance, and its low cavitation erosion rate was similar to that recorded for bulk materials.
2. Experimental procedure In this work, four different coating systems were deposited onto AISI 1020 steel substrates to have their cavitation erosion resistance evaluated. The coating systems were produced by either changing coating composition or carrying out a ‘melt’ post-deposition treatment in order to disrupt the intrinsic lamellar microstructure of the coating, as follows: a WC–12%Co coating; a duplex coating consisting of a WC–12%Co top layer and a corrosion resistant NiCrAl interlayer; an as-sprayed (AS) 50%(WC–12%Co)q50%(NiCr) coating, produced by using a homogenous blended powder consisting of 50%(WC–12%Co)q50%(NiCr), with the latter alloy having additions of Si, B, Fe and C; and a post-melted (PM) 50%(WC–Co)q50%(NiCr) coating, which was subjected to a post-melt treatment after deposition, using an oxy-acetylene flame. The WC– 12%Co and 50%(WC–Co)q50%(NiCr) coatings were all deposited by high velocity oxy-fuel (HVOF) using a METCO Diamond Jet equipment, whilst the NiCrAl coating was produced by atmospheric plasma spraying using a METCO plasma spray system. A Future Tech FM-1 microhardness tester was used to evaluate the Vickers hardness of the coatings and substrate. The Vickers microhardness measurements were made at a load of 4.91 N and employing a dwell time of 15 s. The elastic modulus was estimated using the Knoop indentation technique w7x. In order to measure the elastic modulus, 30 Knoop indentations at a load of 2.94 N were performed in each of the coatings and AISI 1020 steel substrate, for the former and at 4.91 N for the latter. The coating and substrate microhardnesses and elastic moduli are shown in Table 1. The surface roughness (Ra-value) of the different coatings and steel substrate was assessed using a Hommelwerke T4000 profilometer. The Ra-values obtained are also shown in Table 1. The cavitation erosion resistance of the coatings was evaluated by cavitation tests according to the ASTM G32-85 standard w8x. Mass loss plots were obtained as a function of time and, from these plots, the mass erosion rate was then estimated. The volume erosion rate was
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Table 2 Coating density and cavitation erosion rates (in mgymin and cm3ymin) for different thermally sprayed coatings and steels
WC–12%Co Duplex WC–12%CoyNiCrAl AS 50%(WC–12%Co)q50%(NiCr) PM 50%(WC–12%Co)q50%(NiCr) AISI 1020 steel
Density (gycm3)
Cavitation erosion rates Mass erosion rate (mgymin)
Volume erosion rate (cm3ymin)
13.7"0.2 13.7"0.2 10.7"0.7 8.8"0.1 7.89"0.01
8.0"0.4 3.08"0.06 1.21"0.03 0.5"0.1 0.42"0.03
(5.8"0.4)=10y4 (2.25"0.08)=10y4 (1.1"0.1)=10y4 (0.6"0.1)=10y4 (0.53"0.04)=10y4
also calculated using the coating density. Coating and substrate densities were determined through gas pycnometry, using helium and a Quanta Chrome SPY-3 equipment. The density values are shown in Table 2. The coating surfaces and cavitation debris, which were collected after the tests, were both evaluated by scanning electron microscopy (SEM). In order to determinate the fracture toughness, Vickers indentations were performed on coating polished crosssections using a Leitz hardness tester. The indentation load varied from 29.5 to 490.5 N. Both Vickers diagonals and crack lengths were measured in a Mitutoyo PJ311 profile projector using a 100= magnification. Thermally sprayed coatings have an anisotropic microstructure typically characterised by lamellae, as an intrinsic result from the deposition process w5,9,10x. Because of such a microstructure, cracks produced under indentation are mainly developed parallel to (along) the lamellae (as interlamellar cracks), since this direction is the one most prone to decohesion. Transverse cracks are usually observed only at very high indentation loads. In this latter case, the translamellar fracture toughness should be considered to describe the degree of crack
propagation. Since an interlamellar crack propagation regime was observed in the WC–Co coatings, only cracks parallel to the interface were taken into account in the analysis. An average of five indentations was carried out at a given load and the total crack length was estimated by: cs
Ž2dIq2dH. 4
q
Žalqar.
(1)
2
where 2d≤ and 2dH are the parallel and perpendicular Vickers diagonals to the coating surface and al and ar are the left and right crack lengths, respectively (Fig. 1). 2d≤ is the Vickers diagonal along (parallel to) the coating lamellae. From the experimental data obtained for indentation load, P, and total crack length, c, the indentation model proposed by Niihara w11x was used to compute Kc: E z2y5w c zy3y2 | x | y Hv ~ yd~ w
Kcs0.0711ŽHvd1y2.x
(2)
where Hv and E are the Vickers hardness and elastic modulus, respectively. This equation is only valid for a ‘Half-Penny’ crack regime, which occurs when cyd02.5, where d is the Vickers half-diagonal. Further details regarding coating fracture toughness determination by the Vickers indentation technique can be found in Refs. w6,9x. The values obtained for Kc using Eq. (2) are listed in Table 3. After spray deposition, the average residual stress in the WC–Co-based coatings was evaluated using the curvature method w9x. Further details regarding residual stress evaluation by the curvature method can be found in Ref. w12x. Table 3 Vickers indentation fracture toughness
Fig. 1. Schematic of the Vickers indentation and crack geometry.
Coating
KC (MPa m1y2)
WC–12%Co AS 50%(WC–12%Co)q50%(NiCr) PM 50%(WC–12%Co)q50%(NiCr)
1.6"0.9 21"14 32"12
492
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Fig. 2. Plot of the cumulative mass loss as a function of cavitation exposure time. The AS and PM acronyms stand for as-sprayed and postmelted, respectively.
3. Results and discussion The addition of the NiCr alloy to the WC–12%Co powder promoted a reduction in the coating hardness (Table 1). An increase in hardness was achieved, however, after carrying out the post-melt treatment, although the hardness of the PM 50%(WC–12%Co)q 50%(NiCr) coating was still somewhat inferior to that of the WC–12%Co coating. Conversely, an increase of 16% in the elastic modulus was achieved by adding the NiCr alloy to the WC–12%Co coating. After the postmelt treatment, the increase in the elastic modulus was even higher (94%) and a difference of 111% was then obtained between the coating and substrate elastic moduli (Table 1). The highest HyE ratio was, however, achieved by the WC–12%Co coating. A high HyE ratio basically means a high resilience and, therefore, a high ability to absorb elastic energy. Under high stresses leading to plastic deformation, a high HyE ratio should indicate that a high resistance to wear caused by impact can be achieved. A relatively low elastic modulus was measured for the AISI 1020 steel substrate w13x. This can be attributed to the Knoop indentation method that was used to determine the elastic modulus, which often yields higher relative errors for materials possessing low values of Hy E w7x. The average residual stress in the coating, measured by the curvature method and obtained using mathematical expressions derived by Clyne and Gill w14x, were compressive for the WC–12%Co coating. When this same coating was deposited onto a NiCrAl interlayer to produce the duplex coating, the average residual stress for the latter was tensile. For this particular system, a
multilayer residual stress model was used to compute the average residual stress in the coating w15x. The average residual stress in the as-sprayed 50%(WC– 12%Co)q50%(NiCr) were compressive. After the postmelt treatment, the average residual stress in the coating became tensile w9x. Statistical analyses revealed that a good fit of the cumulative mass loss as a function of cavitation exposure time could be obtained if a power-law model like ysboØXb1 was used to describe the experimental data (Fig. 2). The application of such power-law model indicates an absence of an incubation period or acceleration stage, with a continuously decreasing erosion rate w16x. In order to calculate the mass loss rate induced by cavitation erosion, it was assumed that the data shown in Fig. 1 could be described by a linear behaviour. The volume loss rate was then computed using the coating density values listed in Table 2. For the duplex WC–12%CoyNiCrAl coating, the WC–12%Co density was used to calculate its volume loss rate, since this top layer would be the one, theoretically, subjected to the cavitation phenomenon. The results shown in Fig. 2 and Table 2 indicated that the PM 50%(WC–12%Co)q50%(NiCr) coating had the lowest mass and volume loss rates among all coatings, whilst the worst performance under cavitation erosion conditions was achieved by the WC–12%Co coating possessing the highest hardness. The addition of the NiCr alloy improved the cavitation resistance of the WC–12%Co coating, although the PM 50%(WC– 12%Co)q50%(NiCr) coating was the only one which displayed cavitation erosion rates similar to those encountered for bulk materials w17,18x and the AISI 1020 steel substrate (Table 2). Nevertheless, the cavi-
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Fig. 3. SEM secondary electron photomicrographs of the (a) AS 50%(WC–12%Co)q50%(NiCr) coating, displaying its lamellar structure and (b) PM 50%(WC–12%Co)q50%(NiCr) coating, showing that the post-melt treatment completely disrupted the intrinsic lamellar microstructure of the coating. The darker areas correspond to WC–Co whilst the lighter ones correspond to NiCr.
tation erosion rates recorded for the WC–12%Co, duplex WC–12%CoyNiCrAl and AS 50%(WC– 12%Co)q50%(NiCr) coatings have the same magnitude of those already reported for thermally sprayed coatings w9,17x. The post-melt treatment carried out in the 50%(WC–12%Co)q50%(NiCr) coating completely disrupted its intrinsic lamellar microstructure, which is characteristic of thermal spray processes (Fig. 3). By disrupting such a microstructure, interlamellar pores and discontinuities that can weaken lamella cohesion and thus make them prone to be removed during the test, were also reduced or completely eliminated. As a result, the PM 50%(WC–12%Co)q50%(NiCr) coating showed a low cavitation erosion rate, only slightly higher than that of the AISI 1020 steel substrate. A plot of cavitation erosion resistance (taken as the inverse of the volume loss rate) as a function of the coating indentation fracture toughness (Table 3) is shown in Fig. 4. It can be seen that the vibratory
cavitation erosion resistance increases proportionally with the coating fracture toughness. The addition of a tougher NiCr alloy increased the indentation fracture toughness of the WC–12%Co coating and the post-melt treatment further improved the indentation fracture toughness of the AS 50%(WC–Co)q50%(NiCr) coating. The best performance in vibratory cavitation erosion tests was achieved by the PM 50%(WC–Co)q 50%(NiCr) coating, having the highest indentation frac´ ture toughness. This result concurs with Lopez-Cantera and Mellor’s model w2x, which assumes that subsurface cracks produced by erosion have the same morphology as those produced by indentation. Although the PM 50%(WC–Co)q50%(NiCr) coating has an average tensile residual stress, lower hardness and higher elastic modulus than the WC–12%Co coating, the former coating still achieved the best performance in vibratory cavitation erosion tests. This result suggests that the coating fracture toughness seems to be the most impor-
Fig. 4. Cavitation erosion resistance vs. indentation fracture toughness for the different coatings.
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Fig. 5. SEM photomicrographs of the coatingysubstrate surface after cavitation testing: (a) WC–12%Co coating, 45 min; (b) duplex coating (WC–12%CoyNiCrAl), 45 min; (c) AS 50%(WC–12%Co)q50%(NiCr) coating, 110 min; (d) PM 50%(WC–12%Co)q50%(NiCr) coating, 118 min.
tant property in determining the cavitation erosion response of thermally sprayed coatings. Another relevant aspect is the low Ra-value (Table 1) presented by the PM 50%(WC–Co)q50%(NiCr) coating, which might have contributed to its high cavitation erosion resistance. The worst cavitation erosion resistance was recorded for the WC–12%Co coating, possessing the lowest indentation fracture toughness. Although this coating had the highest microhardness, lowest elastic modulus and an average compressive residual stress, such desirable properties could not ensure a high cavitation erosion resistance. The duplex WC–12%CoyNiCrAl coating, although having an average tensile residual stress and a higher Ra-value than the WC–12%Co coating, showed a lower cavitation erosion rate than the latter coating. Such a result indicates that the addition of a NiCrAl interlayer, which is metallic and consequently tougher than the WC–12%Co top layer, was able to improve the overall fracture toughness of the coatingysubstrate system and, therefore, its cavitation erosion resistance. The present results indicate that, for thermally sprayed coatings, there is a strong correlation between coating indentation fracture toughness and cavitation erosion resistance.
The SEM photomicrographs of the coatingysubstrate surface after cavitation erosion testing are shown in Fig. 5. The final time of the test is also shown for each of the coating systems. The surface of the PM 50%(WC– Co)q50%(NiCr) coating (Fig. 5d) presented the smallest cavitation craters in comparison to the other coating surfaces (Fig. 5a–c). The cavitation debris collected after the tests is illustrated in Fig. 6. The debris indicated that the main coating failure mechanism during cavitation erosion was lamellar decohesion instead of adhesive failure at the coatingysubstrate interface. The cavitation debris collected from the WC–12%Co coating (Fig. 6a) consisted of agglomerates of smaller particles, as expected for thermally HVOF sprayed coatings, since the particle residence time in the flame is not usually long enough to promote their complete fusion. Conversely, in the duplex WC–12%CoyNiCrAl coating (Fig. 6b), the cavitation debris were composed of small particle agglomerates, corresponding to the WC–Co top layer, and regions having a smoother surface, corresponding to the NiCrAl interlayer. This interlayer was completely fused during the deposition process, corroborating higher temperatures that are achieved by plasma spraying in comparison to the HVOF process. The finding of ‘com-
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Fig. 6. SEM photomicrographs of the cavitation debris from: (a) WC–12%Co coating, 45 min; (b) duplex coating (WC–12%CoyNiCrAl), 45 min; (c) as-sprayed 50%(WC–12%Co)q50%(NiCr) coating, 110 min; (d) PM 50%(WC–12%Co)q50%(NiCr) coating, 118 min.
bined’ WC–CoyNiCrAl debris suggests that the NiCrAl interlayer played a decisive role in the cavitation erosion process of the duplex coating. Probably the induced cracks in the coating propagated at the lamella interfaces rather than at the top coatingyinterlayer interface; this contributed to a better cavitation performance achieved by the duplex coating in comparison to the WC–12%Co coating. For the AS 50%(WC–Co)q50%(NiCr) coating, some rounded particles corresponding to the NiCr lamellae and typical agglomerates corresponding to the WC–Co lamellae could be identified in the cavitation debris (Fig. 6c). The cavitation debris collected from the PM 50%(WC–Co)q50%(NiCr) coating (Fig. 6d) showed a different morphology from those observed in the other coatings. The debris size was also significantly smaller. It seems that smaller debris is generated from coatings which are more resistant to cavitation erosion. 4. Conclusions (1) The best performance in cavitation erosion tests was achieved by a 50%(WC–12%Co)q50%(NiCr) coating which was subjected to a melt treatment after thermal spray deposition, followed by an as-sprayed a 50%(WC–12%Co)q50%(NiCr) coating and a duplex WC–12%CoyNiCrAl coating. The worst cavitation ero-
sion resistance was obtained for a WC–12%Co coating. The highest cavitation resistance achieved by the PM 50%(WC–12%Co)q50%(NiCr) coating was, still, inferior to that of the AISI 1020 steel substrate and other bulk materials. The use of thermal spray processes that (i) generate high temperatures to ensure complete fusion of particles during deposition and (ii) allow particles to acquire high velocities so that well-adherent coatings can be produced, allied to post-melt treatments which disrupt the intrinsic lamellar microstructure of the coating, are promising directions to improve the cavitation resistance of these coatings. (2) For thermally sprayed coatings, a good correlation between fracture toughness and cavitation resistance could be established. The higher the coating fracture toughness, the better the cavitation resistance. (3) The coating fracture toughness was found to be an important property in the selection of thermally sprayed coatings for applications requiring cavitation erosion resistance. The addition of metallic alloys which have good mechanical properties improves the performance of WC–Co-based coatings in cavitation erosion. (4) In these trials, the HyE ratio was not found to be a reliable indicator of wear resistance. This may be due to the nature of the wear mechanisms occurring, and the lamellar nature of the coating that presents a structure
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in which deformation and cracking are more related to morphological effects than ‘averaged’ mechanical property values. Acknowledgments The authors are greatly indebted to CNPq-Conselho ´ Nacional de Desenvolvimento Tecnologico, Brazil, and ¸ ˜ de Amparo a` Pesquisa de to FAPEMIG – Fundacao Minas Gerais, Brazil, for financial support. The authors also thank CETEC-MG for the use of the Thermal Spray Laboratory. References w1x I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, London, 1992, pp. 171–197. w2x E. Lopez-Cantera, ´ B.G. Mellor, Mater. Lett. 37 (1998) 201. w3x H.R. Lawn, E.R. Fuller, J. Mater. Sci. 10 (1975) 2014. w4x A.G. Evans, T.R. Wilshaw, Acta Metall. 24 (1976) 939. w5x S.F. Wayne, S. Sampath, J. Therm. Spray Technol. 1 (1992) 307.
w6x M.M. Lima, C. Godoy, J.C. Avelar Batista, P.J. Modenesi, Mat. Sci. Eng. A 357 (2003) 337–345. w7x D.B. Marshall, T. Noma, A.G. Evans, J. Am. Ceram. Soc. 65 (1982) C175. w8x American Society for Testing and Materials, Standard Method of Vibratory Cavitation Erosion Test, ASTM G 32-85, 1985, pp. 116–121. w9x M.M. Lima, Doctoral Thesis, School of Engineering, Universidade Federal de Minas Gerais, 2002 (in Portuguese). w10x C. Godoy, J.C.A. Batista, J. Therm. Spray Technol. 8 (1999) 531. w11x K. Niihara, J. Mater. Sci. Lett. 2 (1983) 221. w12x C. Godoy, E.A. Souza, M.M. Lima, J.C.A. Batista, Thin Solid Films 420–421 (2002) 438. w13x L.H. Van Vlack, Materials for Engineering: Concepts and Applications, Addison-Wesley, Reading, 1982, p. 588. w14x T.W. Clyne, S.C. Gill, J. Therm. Spray Technol. 4 (1996) 401. w15x Y.C. Tsui, T.W. Clyne, Thin Solid Films 306 (1997) 52. w16x F.J. Heymann, in: S.D. Henry (Ed.), ASM Handbook: Friction Lubrication and Wear Technology, vol. 18, first ed., ASM International, Cincinnati, 1992, pp. 221–232. w17x C.M. Hansson, I.L.H. Hansson, in: S.D. Henry (Ed.), ASM Handbook: Friction Lubrication and Wear Technology, vol. 18, first ed., ASM International, Cincinnati, 1992, pp. 214–220. w18x P.V. Marques, P.J. Modenesi, Ciencia ˆ Engenharia 10 (2001) 103, in Portuguese.
Coating fracture toughness determined by Vickers indentation: an important parameter in cavitation erosion resistance of WC–Co thermally sprayed coatings M.M. Limaa, C. Godoya, P.J. Modenesia, J.C. Avelar-Batistab, *, A. Davisonb, A. Matthewsc a
Department of Metallurgical and Materials Engineering, School of Engineering, Universidade Federal de Minas Gerais, ´ Rua Espırito Santo 35, Belo Horizonte – MG 30160-030, Brazil b Research Centre in Surface Engineering, School of Engineering, University of Hull, Cottingham Road, Hull HU6 7RX, UK c Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK
Abstract The deficient performance of thermally sprayed coatings in cavitation erosion tests is often attributed to the nature of their lamellar microstructure. Poor coatingysubstrate adhesion, low toughness and tensile residual stresses, which are introduced during the deposition process, can also adversely affect their cavitation resistance. In order to improve the cavitation performance of such coatings, it is also important to control some of the coating properties such as elastic modulus (E) and hardness (H). By reducing E and increasing H (i.e. to ensure a higher HyE ratio), a better tribological performance is usually achieved. The erosive environment also plays a decisive role in determining the final chemical composition of the coating. Thermally sprayed WC–Co coatings are well known because of their high hardness and, in such cemented carbides, the corrosion resistance is frequently affected by the susceptibility of the cobalt binder to chemical attack. In this work, an attempt to improve the cavitation resistance of WC–Co coatings was made by either modifying coating composition (and therefore modifying some coating properties such as hardness, elastic modulus and toughness) or by carrying out a ‘melt’ post-deposition treatment in order to disrupt the intrinsic lamellar microstructure of the coating. Four different coatings were deposited onto an AISI 1020 steel substrate: (i) WC–12%Co; (ii) as-sprayed (AS) 50%(WC–12%Co)q50%(NiCr); (iii) post-melted (PM) 50%(WC–12%Co)q50%(NiCr) and (iv) a duplex system comprising a WC–12%Co top layer and a NiCrAl interlayer. The ‘PM’ coating produced from the pre-alloyed powder 50%(WC–12%Co)q50%(NiCr) displayed a higher elastic modulus (measured by Knoop indentation) and a lower hardness (and thus a lower HyE ratio) than the WC–12%Co. Also, the fracture toughness of the latter (measured by Vickers indentation tests) was increased from 1.6"0.9 to 32"12 MPa m1y2 . The worst performance in cavitation erosion tests was achieved by the WC– 12%Co coating, which showed the highest mass loss throughout the test. Conversely, the ‘PM’ 50%(WC–12%Co)q50%(NiCr) coating exhibited the best cavitation resistance and a correlation between coating toughness and cavitation resistance could be established. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Cavitation erosion; Thermal spraying; Duplex coating; Vickers indentation fracture toughness
1. Introduction Solid particle erosion is a wear process that occurs as discrete solid particles strike a surface. Erosive wear is often caused by plastic deformation or brittle fracture, but both mechanisms can also operate simultaneously. Erosion of metals usually involves plastic flow and the predicted wear rate, W, is inversely proportional to H, the hardness of the surface w1x. Conversely, erosive wear *Corresponding author. Tel.: q44-1482-465072; fax: q44-1482466477. E-mail address: [email protected] (J.C. Avelar-Batista).
of more brittle materials is predominantly governed either by flow or fracture depending on the impact conditions. If the impact of an erosive particle leads to brittle fracture, material is removed from the surface by nucleation and intersection of cracks. In this case, the most relevant material property which determines the erosion resistance is fracture toughness (Kc), with hardness (H) being a much less significant parameter w1x. ´ Lopez-Cantera and Mellor w2x investigated erosive wear in WC–Co–Cr thermally sprayed coatings and they concluded that subsurface cracks produced by erosion were similar to those produced by indentation testing.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00917-4
M.M. Lima et al. / Surface and Coatings Technology 177 – 178 (2004) 489–496
490
Table 1 Surface roughness (Ra), microhardness, elastic modulus, HyE ratio and elastic property mismatch ((Ecoating yEsubstrate )yEsubstrate ) for WC–12%Cobased coatings and AISI 1020 steel substrate Material
Ra (mm)
WC–12%Co AS 50%(WC–12%Co)q50%(NiCr) PM 50%(WC–12%Co)q50%(NiCr) AISI 1020 steel
6.0"0.4a 6.5"0.6 1.7"0.9 2.8"0.2
a
Microhardness (GPa)
Elastic modulus (GPa)
10.1"0.5 5.7"0.4 8.0"0.4 1.11"0.02
194"39 226"46 378"58 179"33
HyE
((EcoatingyEsubstrate)yEsubstrate) (%)
(52"8)=10y3 (25"7)=10y3 (21"4)=10y3 (6"1)=10y3
8 47 111 –
An Ra-value equal to 13"3 mm was recorded for the duplex WC–12%CoyNiCrAl coating.
Therefore, the fracture toughness determined by the indentation method is an appropriate parameter to be used in mathematical equations to estimate erosion rates. ´ In Lopez-Cantera and Mellor’s work w2x, the coating indentation fracture toughness (Kc) was calculated using erosion models developed by Lawn and Fuller w3x and Wilshaw w4x. Both models predict that cracks will propagate from the indentation corners, where the highest stresses occur. In thermally sprayed coatings, however, the fracture is anisotropic and a lower fracture toughness is reported parallel to the coatingysubstrate interface w5x. Toughness values measured in such a direction are, therefore, the most appropriate ones to be used in models to predict the coating erosive wear, as long as crack formation under erosion conditions is analogous to its formation during indentation. In this work, the cavitation erosion resistance of thermally sprayed coatings, based on a WC–Co powder and deposited onto AISI 1020 steel substrates, was investigated. In this particular erosion process, the cavitation phenomena will lead to material wear. Cavitation phenomena usually arise in fluid systems as a result of strong pressure fluctuations; they basically involve the build-up and subsequent implosion of bubbles in liquids. Cavitation erosion rates are often influenced by surface topography, yield properties (such as hardness and rate of strain-hardening), elastic properties (elastic modulus, resilience, super-elasticity) and toughness. By producing four distinct coatings, different mechanical and elastic properties were achieved and distinct responses in cavitation erosion tests were obtained. Some major coating properties such as hardness, indentation toughness w6x, elastic modulus and density were also evaluated. A statistical correlation between cavitation erosion resistance and indentation fracture toughness could be established, using the model for abrasive erosion resistance ´ in WC–Co coatings proposed by Lopez-Cantera and Mellor w2x. A 50%(WC–12%Co)q50%(NiCr) coating, subjected to a post-melt treatment after deposition, had the highest cavitation resistance, and its low cavitation erosion rate was similar to that recorded for bulk materials.
2. Experimental procedure In this work, four different coating systems were deposited onto AISI 1020 steel substrates to have their cavitation erosion resistance evaluated. The coating systems were produced by either changing coating composition or carrying out a ‘melt’ post-deposition treatment in order to disrupt the intrinsic lamellar microstructure of the coating, as follows: a WC–12%Co coating; a duplex coating consisting of a WC–12%Co top layer and a corrosion resistant NiCrAl interlayer; an as-sprayed (AS) 50%(WC–12%Co)q50%(NiCr) coating, produced by using a homogenous blended powder consisting of 50%(WC–12%Co)q50%(NiCr), with the latter alloy having additions of Si, B, Fe and C; and a post-melted (PM) 50%(WC–Co)q50%(NiCr) coating, which was subjected to a post-melt treatment after deposition, using an oxy-acetylene flame. The WC– 12%Co and 50%(WC–Co)q50%(NiCr) coatings were all deposited by high velocity oxy-fuel (HVOF) using a METCO Diamond Jet equipment, whilst the NiCrAl coating was produced by atmospheric plasma spraying using a METCO plasma spray system. A Future Tech FM-1 microhardness tester was used to evaluate the Vickers hardness of the coatings and substrate. The Vickers microhardness measurements were made at a load of 4.91 N and employing a dwell time of 15 s. The elastic modulus was estimated using the Knoop indentation technique w7x. In order to measure the elastic modulus, 30 Knoop indentations at a load of 2.94 N were performed in each of the coatings and AISI 1020 steel substrate, for the former and at 4.91 N for the latter. The coating and substrate microhardnesses and elastic moduli are shown in Table 1. The surface roughness (Ra-value) of the different coatings and steel substrate was assessed using a Hommelwerke T4000 profilometer. The Ra-values obtained are also shown in Table 1. The cavitation erosion resistance of the coatings was evaluated by cavitation tests according to the ASTM G32-85 standard w8x. Mass loss plots were obtained as a function of time and, from these plots, the mass erosion rate was then estimated. The volume erosion rate was
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Table 2 Coating density and cavitation erosion rates (in mgymin and cm3ymin) for different thermally sprayed coatings and steels
WC–12%Co Duplex WC–12%CoyNiCrAl AS 50%(WC–12%Co)q50%(NiCr) PM 50%(WC–12%Co)q50%(NiCr) AISI 1020 steel
Density (gycm3)
Cavitation erosion rates Mass erosion rate (mgymin)
Volume erosion rate (cm3ymin)
13.7"0.2 13.7"0.2 10.7"0.7 8.8"0.1 7.89"0.01
8.0"0.4 3.08"0.06 1.21"0.03 0.5"0.1 0.42"0.03
(5.8"0.4)=10y4 (2.25"0.08)=10y4 (1.1"0.1)=10y4 (0.6"0.1)=10y4 (0.53"0.04)=10y4
also calculated using the coating density. Coating and substrate densities were determined through gas pycnometry, using helium and a Quanta Chrome SPY-3 equipment. The density values are shown in Table 2. The coating surfaces and cavitation debris, which were collected after the tests, were both evaluated by scanning electron microscopy (SEM). In order to determinate the fracture toughness, Vickers indentations were performed on coating polished crosssections using a Leitz hardness tester. The indentation load varied from 29.5 to 490.5 N. Both Vickers diagonals and crack lengths were measured in a Mitutoyo PJ311 profile projector using a 100= magnification. Thermally sprayed coatings have an anisotropic microstructure typically characterised by lamellae, as an intrinsic result from the deposition process w5,9,10x. Because of such a microstructure, cracks produced under indentation are mainly developed parallel to (along) the lamellae (as interlamellar cracks), since this direction is the one most prone to decohesion. Transverse cracks are usually observed only at very high indentation loads. In this latter case, the translamellar fracture toughness should be considered to describe the degree of crack
propagation. Since an interlamellar crack propagation regime was observed in the WC–Co coatings, only cracks parallel to the interface were taken into account in the analysis. An average of five indentations was carried out at a given load and the total crack length was estimated by: cs
Ž2dIq2dH. 4
q
Žalqar.
(1)
2
where 2d≤ and 2dH are the parallel and perpendicular Vickers diagonals to the coating surface and al and ar are the left and right crack lengths, respectively (Fig. 1). 2d≤ is the Vickers diagonal along (parallel to) the coating lamellae. From the experimental data obtained for indentation load, P, and total crack length, c, the indentation model proposed by Niihara w11x was used to compute Kc: E z2y5w c zy3y2 | x | y Hv ~ yd~ w
Kcs0.0711ŽHvd1y2.x
(2)
where Hv and E are the Vickers hardness and elastic modulus, respectively. This equation is only valid for a ‘Half-Penny’ crack regime, which occurs when cyd02.5, where d is the Vickers half-diagonal. Further details regarding coating fracture toughness determination by the Vickers indentation technique can be found in Refs. w6,9x. The values obtained for Kc using Eq. (2) are listed in Table 3. After spray deposition, the average residual stress in the WC–Co-based coatings was evaluated using the curvature method w9x. Further details regarding residual stress evaluation by the curvature method can be found in Ref. w12x. Table 3 Vickers indentation fracture toughness
Fig. 1. Schematic of the Vickers indentation and crack geometry.
Coating
KC (MPa m1y2)
WC–12%Co AS 50%(WC–12%Co)q50%(NiCr) PM 50%(WC–12%Co)q50%(NiCr)
1.6"0.9 21"14 32"12
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Fig. 2. Plot of the cumulative mass loss as a function of cavitation exposure time. The AS and PM acronyms stand for as-sprayed and postmelted, respectively.
3. Results and discussion The addition of the NiCr alloy to the WC–12%Co powder promoted a reduction in the coating hardness (Table 1). An increase in hardness was achieved, however, after carrying out the post-melt treatment, although the hardness of the PM 50%(WC–12%Co)q 50%(NiCr) coating was still somewhat inferior to that of the WC–12%Co coating. Conversely, an increase of 16% in the elastic modulus was achieved by adding the NiCr alloy to the WC–12%Co coating. After the postmelt treatment, the increase in the elastic modulus was even higher (94%) and a difference of 111% was then obtained between the coating and substrate elastic moduli (Table 1). The highest HyE ratio was, however, achieved by the WC–12%Co coating. A high HyE ratio basically means a high resilience and, therefore, a high ability to absorb elastic energy. Under high stresses leading to plastic deformation, a high HyE ratio should indicate that a high resistance to wear caused by impact can be achieved. A relatively low elastic modulus was measured for the AISI 1020 steel substrate w13x. This can be attributed to the Knoop indentation method that was used to determine the elastic modulus, which often yields higher relative errors for materials possessing low values of Hy E w7x. The average residual stress in the coating, measured by the curvature method and obtained using mathematical expressions derived by Clyne and Gill w14x, were compressive for the WC–12%Co coating. When this same coating was deposited onto a NiCrAl interlayer to produce the duplex coating, the average residual stress for the latter was tensile. For this particular system, a
multilayer residual stress model was used to compute the average residual stress in the coating w15x. The average residual stress in the as-sprayed 50%(WC– 12%Co)q50%(NiCr) were compressive. After the postmelt treatment, the average residual stress in the coating became tensile w9x. Statistical analyses revealed that a good fit of the cumulative mass loss as a function of cavitation exposure time could be obtained if a power-law model like ysboØXb1 was used to describe the experimental data (Fig. 2). The application of such power-law model indicates an absence of an incubation period or acceleration stage, with a continuously decreasing erosion rate w16x. In order to calculate the mass loss rate induced by cavitation erosion, it was assumed that the data shown in Fig. 1 could be described by a linear behaviour. The volume loss rate was then computed using the coating density values listed in Table 2. For the duplex WC–12%CoyNiCrAl coating, the WC–12%Co density was used to calculate its volume loss rate, since this top layer would be the one, theoretically, subjected to the cavitation phenomenon. The results shown in Fig. 2 and Table 2 indicated that the PM 50%(WC–12%Co)q50%(NiCr) coating had the lowest mass and volume loss rates among all coatings, whilst the worst performance under cavitation erosion conditions was achieved by the WC–12%Co coating possessing the highest hardness. The addition of the NiCr alloy improved the cavitation resistance of the WC–12%Co coating, although the PM 50%(WC– 12%Co)q50%(NiCr) coating was the only one which displayed cavitation erosion rates similar to those encountered for bulk materials w17,18x and the AISI 1020 steel substrate (Table 2). Nevertheless, the cavi-
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Fig. 3. SEM secondary electron photomicrographs of the (a) AS 50%(WC–12%Co)q50%(NiCr) coating, displaying its lamellar structure and (b) PM 50%(WC–12%Co)q50%(NiCr) coating, showing that the post-melt treatment completely disrupted the intrinsic lamellar microstructure of the coating. The darker areas correspond to WC–Co whilst the lighter ones correspond to NiCr.
tation erosion rates recorded for the WC–12%Co, duplex WC–12%CoyNiCrAl and AS 50%(WC– 12%Co)q50%(NiCr) coatings have the same magnitude of those already reported for thermally sprayed coatings w9,17x. The post-melt treatment carried out in the 50%(WC–12%Co)q50%(NiCr) coating completely disrupted its intrinsic lamellar microstructure, which is characteristic of thermal spray processes (Fig. 3). By disrupting such a microstructure, interlamellar pores and discontinuities that can weaken lamella cohesion and thus make them prone to be removed during the test, were also reduced or completely eliminated. As a result, the PM 50%(WC–12%Co)q50%(NiCr) coating showed a low cavitation erosion rate, only slightly higher than that of the AISI 1020 steel substrate. A plot of cavitation erosion resistance (taken as the inverse of the volume loss rate) as a function of the coating indentation fracture toughness (Table 3) is shown in Fig. 4. It can be seen that the vibratory
cavitation erosion resistance increases proportionally with the coating fracture toughness. The addition of a tougher NiCr alloy increased the indentation fracture toughness of the WC–12%Co coating and the post-melt treatment further improved the indentation fracture toughness of the AS 50%(WC–Co)q50%(NiCr) coating. The best performance in vibratory cavitation erosion tests was achieved by the PM 50%(WC–Co)q 50%(NiCr) coating, having the highest indentation frac´ ture toughness. This result concurs with Lopez-Cantera and Mellor’s model w2x, which assumes that subsurface cracks produced by erosion have the same morphology as those produced by indentation. Although the PM 50%(WC–Co)q50%(NiCr) coating has an average tensile residual stress, lower hardness and higher elastic modulus than the WC–12%Co coating, the former coating still achieved the best performance in vibratory cavitation erosion tests. This result suggests that the coating fracture toughness seems to be the most impor-
Fig. 4. Cavitation erosion resistance vs. indentation fracture toughness for the different coatings.
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Fig. 5. SEM photomicrographs of the coatingysubstrate surface after cavitation testing: (a) WC–12%Co coating, 45 min; (b) duplex coating (WC–12%CoyNiCrAl), 45 min; (c) AS 50%(WC–12%Co)q50%(NiCr) coating, 110 min; (d) PM 50%(WC–12%Co)q50%(NiCr) coating, 118 min.
tant property in determining the cavitation erosion response of thermally sprayed coatings. Another relevant aspect is the low Ra-value (Table 1) presented by the PM 50%(WC–Co)q50%(NiCr) coating, which might have contributed to its high cavitation erosion resistance. The worst cavitation erosion resistance was recorded for the WC–12%Co coating, possessing the lowest indentation fracture toughness. Although this coating had the highest microhardness, lowest elastic modulus and an average compressive residual stress, such desirable properties could not ensure a high cavitation erosion resistance. The duplex WC–12%CoyNiCrAl coating, although having an average tensile residual stress and a higher Ra-value than the WC–12%Co coating, showed a lower cavitation erosion rate than the latter coating. Such a result indicates that the addition of a NiCrAl interlayer, which is metallic and consequently tougher than the WC–12%Co top layer, was able to improve the overall fracture toughness of the coatingysubstrate system and, therefore, its cavitation erosion resistance. The present results indicate that, for thermally sprayed coatings, there is a strong correlation between coating indentation fracture toughness and cavitation erosion resistance.
The SEM photomicrographs of the coatingysubstrate surface after cavitation erosion testing are shown in Fig. 5. The final time of the test is also shown for each of the coating systems. The surface of the PM 50%(WC– Co)q50%(NiCr) coating (Fig. 5d) presented the smallest cavitation craters in comparison to the other coating surfaces (Fig. 5a–c). The cavitation debris collected after the tests is illustrated in Fig. 6. The debris indicated that the main coating failure mechanism during cavitation erosion was lamellar decohesion instead of adhesive failure at the coatingysubstrate interface. The cavitation debris collected from the WC–12%Co coating (Fig. 6a) consisted of agglomerates of smaller particles, as expected for thermally HVOF sprayed coatings, since the particle residence time in the flame is not usually long enough to promote their complete fusion. Conversely, in the duplex WC–12%CoyNiCrAl coating (Fig. 6b), the cavitation debris were composed of small particle agglomerates, corresponding to the WC–Co top layer, and regions having a smoother surface, corresponding to the NiCrAl interlayer. This interlayer was completely fused during the deposition process, corroborating higher temperatures that are achieved by plasma spraying in comparison to the HVOF process. The finding of ‘com-
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Fig. 6. SEM photomicrographs of the cavitation debris from: (a) WC–12%Co coating, 45 min; (b) duplex coating (WC–12%CoyNiCrAl), 45 min; (c) as-sprayed 50%(WC–12%Co)q50%(NiCr) coating, 110 min; (d) PM 50%(WC–12%Co)q50%(NiCr) coating, 118 min.
bined’ WC–CoyNiCrAl debris suggests that the NiCrAl interlayer played a decisive role in the cavitation erosion process of the duplex coating. Probably the induced cracks in the coating propagated at the lamella interfaces rather than at the top coatingyinterlayer interface; this contributed to a better cavitation performance achieved by the duplex coating in comparison to the WC–12%Co coating. For the AS 50%(WC–Co)q50%(NiCr) coating, some rounded particles corresponding to the NiCr lamellae and typical agglomerates corresponding to the WC–Co lamellae could be identified in the cavitation debris (Fig. 6c). The cavitation debris collected from the PM 50%(WC–Co)q50%(NiCr) coating (Fig. 6d) showed a different morphology from those observed in the other coatings. The debris size was also significantly smaller. It seems that smaller debris is generated from coatings which are more resistant to cavitation erosion. 4. Conclusions (1) The best performance in cavitation erosion tests was achieved by a 50%(WC–12%Co)q50%(NiCr) coating which was subjected to a melt treatment after thermal spray deposition, followed by an as-sprayed a 50%(WC–12%Co)q50%(NiCr) coating and a duplex WC–12%CoyNiCrAl coating. The worst cavitation ero-
sion resistance was obtained for a WC–12%Co coating. The highest cavitation resistance achieved by the PM 50%(WC–12%Co)q50%(NiCr) coating was, still, inferior to that of the AISI 1020 steel substrate and other bulk materials. The use of thermal spray processes that (i) generate high temperatures to ensure complete fusion of particles during deposition and (ii) allow particles to acquire high velocities so that well-adherent coatings can be produced, allied to post-melt treatments which disrupt the intrinsic lamellar microstructure of the coating, are promising directions to improve the cavitation resistance of these coatings. (2) For thermally sprayed coatings, a good correlation between fracture toughness and cavitation resistance could be established. The higher the coating fracture toughness, the better the cavitation resistance. (3) The coating fracture toughness was found to be an important property in the selection of thermally sprayed coatings for applications requiring cavitation erosion resistance. The addition of metallic alloys which have good mechanical properties improves the performance of WC–Co-based coatings in cavitation erosion. (4) In these trials, the HyE ratio was not found to be a reliable indicator of wear resistance. This may be due to the nature of the wear mechanisms occurring, and the lamellar nature of the coating that presents a structure
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in which deformation and cracking are more related to morphological effects than ‘averaged’ mechanical property values. Acknowledgments The authors are greatly indebted to CNPq-Conselho ´ Nacional de Desenvolvimento Tecnologico, Brazil, and ¸ ˜ de Amparo a` Pesquisa de to FAPEMIG – Fundacao Minas Gerais, Brazil, for financial support. The authors also thank CETEC-MG for the use of the Thermal Spray Laboratory. References w1x I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, London, 1992, pp. 171–197. w2x E. Lopez-Cantera, ´ B.G. Mellor, Mater. Lett. 37 (1998) 201. w3x H.R. Lawn, E.R. Fuller, J. Mater. Sci. 10 (1975) 2014. w4x A.G. Evans, T.R. Wilshaw, Acta Metall. 24 (1976) 939. w5x S.F. Wayne, S. Sampath, J. Therm. Spray Technol. 1 (1992) 307.
w6x M.M. Lima, C. Godoy, J.C. Avelar Batista, P.J. Modenesi, Mat. Sci. Eng. A 357 (2003) 337–345. w7x D.B. Marshall, T. Noma, A.G. Evans, J. Am. Ceram. Soc. 65 (1982) C175. w8x American Society for Testing and Materials, Standard Method of Vibratory Cavitation Erosion Test, ASTM G 32-85, 1985, pp. 116–121. w9x M.M. Lima, Doctoral Thesis, School of Engineering, Universidade Federal de Minas Gerais, 2002 (in Portuguese). w10x C. Godoy, J.C.A. Batista, J. Therm. Spray Technol. 8 (1999) 531. w11x K. Niihara, J. Mater. Sci. Lett. 2 (1983) 221. w12x C. Godoy, E.A. Souza, M.M. Lima, J.C.A. Batista, Thin Solid Films 420–421 (2002) 438. w13x L.H. Van Vlack, Materials for Engineering: Concepts and Applications, Addison-Wesley, Reading, 1982, p. 588. w14x T.W. Clyne, S.C. Gill, J. Therm. Spray Technol. 4 (1996) 401. w15x Y.C. Tsui, T.W. Clyne, Thin Solid Films 306 (1997) 52. w16x F.J. Heymann, in: S.D. Henry (Ed.), ASM Handbook: Friction Lubrication and Wear Technology, vol. 18, first ed., ASM International, Cincinnati, 1992, pp. 221–232. w17x C.M. Hansson, I.L.H. Hansson, in: S.D. Henry (Ed.), ASM Handbook: Friction Lubrication and Wear Technology, vol. 18, first ed., ASM International, Cincinnati, 1992, pp. 214–220. w18x P.V. Marques, P.J. Modenesi, Ciencia ˆ Engenharia 10 (2001) 103, in Portuguese.