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Hydroabrasive wear behaviour of high velocity oxyfuel thermally sprayed WC-M coatings
Surface and Coatings Technology, 62 (1993) 493—498
493
Hydroabrasive wear behaviour of high velocity oxyfuel thermally sprayed WC—M coatings A. Karimi and Ch. Verdon Inst itut de Genie Atomique, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne (Switzerland)
Abstract Hydroabrasive wear tests were conducted on the coatings of tungsten carbide—metal matrix cermets of type WC—M, where M Co, CoCr and Ni. The coatings were deposited using the high velocity oxyfuel (HVOF) spraying process. Cermets of the cobalt binder phase showed 50%—200% higher strength than those of the nickel matrix depending on the erosion conditions, and the addition of chromium improved the erosion resistance several times. Analytical techniques including X-ray diffraction and energy-dispersive spectroscopy (EDS) as well as transmission electron microscopy were used to characterize microstructures formed during powder processing and spraying. Owing to overheating of powder particles within the spray torch, the HVOF cermets develop complex microstructures. The hard phase consists basically of tungsten monocarbide (WC) as well as ditungsten carbide (W 2C), while the binder phase obtains a nanocrystalline structure of the size between 4 and 8 nm, containing ternary carbides and mixed compounds. The erosion results are discussed with respect to the microstructural features developed during spraying.
1. Introduction The high hardness of tungsten carbide in conjunction with good toughness due to the binder phase has made WC—Co cermets one of the most widely used wearresistant materials in a variety of industrial applications such as aerospace, automotive, mining and power generation systems [1—3].These protective coatings are widely applied on the surface of components using various thermal spray processes including plasma spraying, cornbustion (high velocity oxyfuel (HYOF) and flame) and the detonation gun (D-gun). The high temperature of the spray torch, the turbulent character of the gas flow and the rapid solidification of deposits associated with these processes result in complex chemical transformations and lead to metastable phase formation within the coatings [4, 5]. Several experimental studies have dealt with these transformations and with the influence of the powder characteristics and spray conditions on the microstructures and mechanical properties of WC—Co coatings [6, 7]. The main phenomena which have been found to occur are the partial melting of powder and the substantial thermal decomposition of WC, giving rise to the formation of W2C, mixed Co~W~C~ compounds as well as metallic tungsten [8, 9]. Furthermore, these studies have revealed that such transformations are in general unfavourable for most wear applications [10]. Loss of wear performance due to decomposition and decarburization of tungsten monocarbide (WC) is often related to reduction in coating hardness and formation of brittle secondary carbides.
0257—8972/93/S6.0O
Little information is available on the mechanism of structural transformations during powder processing and spraying and, in particular, how, when and where the decomposition of WC occurs, which mixed cornpounds are formed, and what may be their effect on the wear behaviour of coatings. It is the objective of this work to characterize the microstructures developed during the HVOF process of tungsten carbide—metal matrix of type WC—M (M Co, CoCr and Ni) and to relate them to their hydroabrasive wear performance. For this study, electron microscopy observations including scanning electron microscopy (SEM), transmission electron microscopy (TEM) coupled with energydispersive spectroscopy (EDS) microanalysis, and X-ray diffraction (XRD) analysis were used. The results are discussed with respect to erosion testing.
2. Erosion test results Coatings of tungsten carbide cermets having a nominal composition of WC—12 wt.% Co, WC—l2 wt.%Co—4 wt.%Cr and WC—12 wt.%Ni were tested. These coatings were deposited on the austenitic stainless steel substrates using a HYOF spray system from Plasma-Technik AG. The following operating conditions were used to spray the coatings: oxygen flow rate, 4201 min 1, propane flow rate, 55 1 min 1; powder carrier gas, nitrogen; carrier gas flow rate, 35 1 min 1; powder feed rate, 40 g min 1; spraying distance, 300 mm; powder size, 15—45 jim.
©
1993
—
Elsevier Sequoia. All rights reserved
494
A. Karimi, Ch. Verdon
/
Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
Under these conditions, powder particles are accelerated ((105—106)g) to velocities up to 800 m s~. The associated high kinetic energy of particles in conjunction with thermal effects provides compact layers which show less than 0.5% porosity. Measurement of the coating density and image analysis of polished surface did not present a significant difference between the porosities and crack formation features of different cermets. The thickness of coatings was approximately 0.3 mm. The average Vickers microhardnesses (load, 300 kgf) were 1141, 1269 and 993 kgfmm2 respectively, The range of microhardnesses was rather wide because of the large phase heterogeneity and porosity of the coatings. The above values are the average of 20 measurements taken on each coatings. Hydroabrasive tests were conducted in a laboratory device which produces a rotating tangential flow of a mixture of abrasive particles and water over the specimens using a helicalgear-type impeller [3]. Two different WC sizes were used for each coating including fine carbide with d ~ 1—2 jim and coarse carbide with d ~ 3—4 jim. In this study, tests were performed in tap water under the following conditions: flow velocity, 25 m s1 abrasive size, 20—200 jim; abrasive concentration, 0.3 wt.%. The cumulative erosion vs. exposure time is reported in Fig. 1. The values obtained for stainless steel 13—4 and ceramic Cr 203 (air plasma sprayed) are also added to give a comparison of the erosion behaviour of the HYOF sprayed cemented carbides. It is clear from this diagram that cermets of Co matrix resist better than those of Ni matrix, and the addition of chromium improves the erosion resistance of WC—Co coatings by about four to six times. The beneficial action of chromium appeared to be more pronounced at lower and intermediate impingement velocities, normally below 100 m s~’ [11, 12]. The reason for such an improvement due to chromium is not yet clear. Nevertheless, several effects may be consid15
ered to play major roles, e.g. enhancement of the corrosion resistance of the matrix, an optimal proportion of carbides within the binder resulting in a more efficient distribution of carbides, and the formation of favourable microstructures offering a higher toughness to the binding phase or providing an extra strengthening to the cohesion of carbide particles by the matrix [12]. Under the low aggressive erosion conditions used in Fig. 1, higher erosion rates were observed for WC—Co cermets prepared by the powder metallurgy (PM) method compared with those deposited using the HVOF thermal spraying process. Such a result can be attributed to the morphology and behaviour of the matrix, because carbides behave roughly the same in both specimens. The formation of mixed Co~W~C~ compounds during HVOF spraying leads to a substantially higher hardness of the matrix compared with that of the Co matrix obtained by PM. This can probably improve the corrosion resistance and toughness of the matrix as well as compensate for the effect of relative loss of WC in the sprayed coatings. The scanning electron photomicrograph of the eroded surface (Fig. 2) showed that, in general, the carbide grains are tough enough to survive the direct particle impacts. Hence, the mode of material loss in cermets appears to be strongly dependent on the behaviour of their matrix. The effect of the carbide morphology seems to be more pronounced under tangential erosion. In general, an eroding particle will scratch the softer binder phase, whereas carbides interrupt the scratch development and deflect the eroding particles, thereby protecting the matrix. When the surrounding binder phase is sufficiently worn, then the carbide can be removed easily under simultaneous action of impacting particles and shearing of circulating flow. The higher erosion resistance provided by the coarse carbides may be due to their greater
~~‘—‘——‘—
S
13-4 St. St.
1’H~Fiit~
_____
0
~
Time liii
Fig. 1. Cumulative erosion loss vs. exposure time for materials tested. 1 indicates coarse carbide, 2 indicates fine carbide, PM is powder metallurgy, and APS is air plasma spraying.
Fig. 2. SEM observation of the eroded surface of WC—Co cermet after an exposure time of 30 h.
A. Karimi, Ch. Verdon
/
Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
effectiveness in deflecting the eroding particles. As fine carbides are physically too small to deflect the erosive particles, they are worn together with the surrounding matrix. Their contribution to erosion resistance would be limited only to lending hardness to the matrix.
495
In order to analyse the erosion behaviour of the coatings, microstructural characterizations were performed using XRD spectrometry and TEM coupled with EDS microanalysis. The XRD patterns were obtained on a Rigaku diffractometer using Cu K~radiation and a graphite crystal diffracted-beam monochromator, operating at 40 kV and 30 mA. The data were collected, generally, for 26=20—140°. Typical examples
have disappeared. In addition to the distinct peaks corresponding to the identified phases mentioned above, one can easily distinguish two large and diffused humps situated at 26~40°and 70°,as indicated on the diagram. These humps were attributed to the nanocrystalline binder phase developed during HVOF spraying of the cermets [11]. The composition of this nanocrystalline phase consists essentially of ternary carbides and W~C~,CO~ mixed compounds. For a quantitative estimation of WC decomposition and formation of the secondary mixed phases within the coatings, the Rietveld [13] XRD analysis method for multiphase mixtures was used. This technique involves fitting the entire experimental diffraction pattern of the coating as obtained in Fig. 3(b) with a calculated profile using a model that includes the crystal structure parameters of the constituent phases (such as WC, W2C, Co and C06W6C), peak shapes and
of intensity spectrum for 26 = 30—90° are shown for WC—Co powder before spraying in Fig. 3(a) and after being coated in Fig. 3(b). On the X-ray diagram of the powder, one can note, in addition to tungsten carbide and cobalt patterns, the appearance of weak peaks corresponding to the presence of small quantity of W2C and Co6W6C. These secondary carbides have been produced during powder processing. At this stage of transformation the metallic cobalt still shows an f.c.c. structure. However, the spectrum in Fig. 3(b) corresponding to the coating reveals that the main transformations take place during the spraying process. Apart from the retained WC, significant amounts of W2C and metallic tungsten were formed, while traces of metallic cobalt
background [14]. The superposition of the calculated profile onto the experimental pattern allows quantification of the relative abundance of each phase within the coating. Using this method, the results reported in Table 1 have been obtained for coatings tested. From the values mentioned in Table 1, it appears that a substantial fraction of WC decomposed to W2C and metallic W or reacted with the cobalt to form ternary carbides and other W—C--Co mixed compounds. Differences in the contents of retained WC and in the chemical compositions of the coatings can be related to the spraying conditions or powder parameters. For example, fine carbides are more susceptible to overheating and decarborization and release a higher amount of metallic tungsten.
3. Coating microstructures
. •wc OW2C
a)
E Co (fcc)
V Co6W6C
. Oj~0~
~~Vo°
~
.
?
•
.
~
•wc
V
b)
-
OW2C
—
H ~
nanociystalline phase
~ 30
4))
5))
9~°~ 6))
70
00
0)
20 angle Ideg]
Fig. 3. XRD spectrum fOr a WC—Co cermet with indication of the related phases: (a) initial powder; (b) as coated.
496
A. Karimi, Ch. Verdon
/
Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
TABLE I. Phase composition of the coatings estimated using the Rietveld method Sample
metric phases such as Co6W6C or other mixed W—C—Co compounds would be formed within the binder phase. The exact identification of these phases is very difficult
Amount (wt.%)
_________________________ WC w2c w WC-Co (fine) WC—Co (coarse) WC—CoCr
52
23 43
49
36
15
WC-Ni (fine)
65
6
29
54
because of their nanocrystalline character. The size of nanocrystallites measured using high resolution TEM and estimated from XRD peak broadening [11] has been found to vary between 4 and 8 nm. Region C in Fig. 4 denotes an area where some of the WC particles
23
were melted and decomposed into the secondary phases. The flickering contrast indicates distortion of carbide grains due to volume change following decomposition
_____________________________________________________
The values in Table 1 should be considered as an indication of the major phases developed within the coatings. In fact, because of the nanocrystalline character of the binder phase, no distinct diffraction peaks corresponding to the ternary carbides or metallic cobalt appear on the XRD diagrams. For this reason, their contribution to the calculated profile was neglected in this work. In spite of such an approximation the relative values of the principal phases, i.e. WC, W2C and W, do not seem to be changed fundamentally. These values appear to be close to the chemical compositions reported by Ramnath and Jayaraman [15] for tungsten carbide— cobalt coatings deposited using plasma spraying. TEM observation of the coatings confirmed the formation of various phases and development of multiple microstructures detected previously by X-ray analysis. An example of a typical photomicrograph is illustrated in Fig. 4. In this picture, region A corresponds to the WC grain and region B relates to the binder phase possessing the nanocrystalline structure. The use of EDS—TEM and Auger electron spectrometry [11] showed systematically the presence of tungsten, cobalt and carbon in region B. This confirms that the stoichio________
~
_____________ ______
or decarburization.
4. WC decomposition During thermal spraying of cermets, locally overheating occurs and results in partial melting of tungsten carbides which interact with the metal matrix to form ternary carbides or other mixed compounds. Consequently, the final coatings are not the ideal cermets made up of WC particles bound in the metal matrix, but they contain more complex microstructures developed depending on the temperature of the spray partides. At a higher degree of overheating a splat-like microstructure appears owing to impingement of melted carbides, while a lower degree of overheating favours only the formation of nanocrystalline binder phase as observed previously. A further deleterious effect may be produced when overheating takes place in an oxidizing atmosphere which leads to a loss of carbon from WC. One of the consequences of this reaction is the formation of brittle secondary carbides such as W2C, which are unfavourable for different wear applications [7, 8]. To demonstrate the process of WC decarburization, examples of microstructures observed by SEM and TEM are presented in Fig. 5. The carbides WC and W2C were identified using the microdiffraction technique in the transmission electron micrograph.
_______________________ ~ _____
-~
200 nm
Fig. 4. Transmission electron micrograph showing typical microstructures in the WC—Co coating. Region A is WC, and region B is the nanocrystalline matrix containing W, C and Co. The region C shows WC partially melted and decomposed.
The scanning electron micrograph (Fig. 5(a)) shows an entire WC particle within the matrix where a surrounding layer of about 100 nm has been decarburized and changed to W2C. The boundaries between W2C and retained WC as well as between W2C and matrix as visible on the scanning electron micrograph have been revealed by etching. As can be seen from the curvature of the boundaries, the transformation front extends from the carbide particle into the matrix. This suggests that the outer shell of the carbide particle has been overheated during its passage through the spray torch and the thermal effect of future passes allows transformation to occur. One can also note that, because of partial melting of the particle, the sharp angles of the crushed carbide have disappeared, giving rise to a rather rounded partide. TEM observation of the decarburized layers
A.
Karimi,
Ch. Verdon
/
Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
_____
_________________________
______
spraying of cermets, overheating takes places and tung..
____
matrix to form ternary carbides or other mixed compounds. There are several stoichiometric phases in the Co—W—C system. The decarburization of WC inherent in these reactions results in the formation of substantial amounts of W2C and metallic tungsten. In such a case
______
_____
_____________________________________________________
~ ____ _______ _____
500 (a)
~
size of abrasive particles, angle of incidence and velocity of impacts, optimum values may be obtained for binder content and carbide size in order to achieve the highest
_____
____
497
~--~‘
~ WC -~ -
II
~
~
~
______
L
50
nm
~h) Fig. 5. Observation of a carbide particle showing decarburization of WC and formation of W2C: (a) SEM image; (b) transmission electron micrograph.
(Fig. 5(b)) showed that the boundaries between WC and W2C as well as between W2C and matrix are often abrupt. On the scale of conventional TEM no diffused boundaries were detected. High resolution TEM investigations may be necessary to study such interfaces and to analyse the defects present, in particular, at the boundaries between WC and W2C.
5. Discussion The erosive wear behaviour of tungsten carbide—metal matrix cermets have been found to be governed mainly by the carbide grain sizes and by the binder volume fractions. These two parameters determine, in particular, the distribution of the carbides within the cermet which in its turn characterizes the effectiveness of the carbides to deflect the abrasives and hence to inhibit scratching of the matrix. Depending on the erosion conditions, i.e.
the final coating develops more complex microstructures than an ideal cermet made up of WC particles bound in the metal matrix. In addition, rapid solidification of the deposits provides a nanocrystalline character to the binder phase. In spite of the various properties evidenced above for the HVOF sprayed coatings, their erosion resistance appeared to be higher than that of cermets made using the PM technique. The explanation of this result lies in the morphology of metal matrix, because carbides can be assumed to behave in the same way in both cermets. The nanocrystalline binder phase in thermally sprayed coatings is harder than the metallic cobalt phase in PM coatings, which compensates for deleterious effects due to relative loss of WC in sprayed cermets. In addition, the nanocrystalline compounds may provide more toughness to the matrix and improve its corrosion resistance. When the effect of the carbide size is considered, cermets of fine carbides showed a relatively lower erosion resistance than those of coarse carbide. At the same time, fine carbides have been found to undergo more WC decomposition. This probably is due to the higher degree of overheating related to the greater surface-tovolume ratio. The ratio of surface to volume doubles when the size of particles is halved. In such a case, the question of the relative weak strength of fine carbides remains still open. In this regard, two hypotheses based on the microstructural effect or purely mechanical action may be considered. One can expect that a higher degree of decomposition would result in more deterioration of mechanical strength or suppose that fine carbides are physically too small to resist and deflect the abrasive particles. Their contribution would be limited only to lending hardness to the matrix to reduce scratch depth and related material loss. Microscopy observations are ongoing to determine the role of each process and to understand better the mechanisms of wear loss. The addition of chromium decreased erosion rate several times as observed in Fig. 1. This effect is in agreement with the observation of Ninham and Levy [17] under 60 m s’ gas-blast erosion and those of Schmid [12] under small-angle impingement erosion. The reason for such an improvement was attributed to
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/
Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
the solid solution formed by the two metals and possible precipitation of chromium carbides [17]. In the present work the X-ray analysis showed that chromium does not change the amount of WC decomposition significantly. Hence, its beneficial action may be related rather to the formation of favourable microstructures which provide toughness to the matrix or improve the binding of the carbide particles. Another explanation of this behaviour could be the presence of the higher content of binder phase in WC—CoCr dermet compared with WC—Co. It results in an optimal proportion of the carbide-binder phase which offers significant erosion resistance under these conditions. Acknowledgments The authors wish to thank G. Barbezat of Plasma Technik AG for fabricating the coatings, and R. K. Schmid and P. Desponds of Sulzer-Innotec for their keen interest and stimulating discussions in this work. The project was supported by the Commission d’Encouragement a la Recherche Scientifique, the Fonds National Suisse de la Recherche Scientifique and Sulzer AG. References 1 D. J. Varcalle, Jr., Smolik, G. C. Wilson, G. Iron and J. A. WaIter, in R. M. Yazici (ed), Protective Coatings: Processing and
2
3 4 5 6 7
Characterization, Proc. Metallurgical Society of the AIME Northeast Regional Meeting, Hoboken, NJ, May 3—5, 1989, Minerals, Metals and Materials Society, Warrendale, PA, 1990, ~,. l21134. J. A. Sue and R. C. Tucker, Surf. Coat. Technol., 32(1987) 237—248. A. Karimi and R. K. Schmid, Wear, 156 (1992) 33—47. D. Apelian, M. Paliwal, R. W. Smith and W. F. Schilling, mt. Metall. Rev., 28(5) (1983) 27 1—294. J. H. Zaat, Annu. Rev. Mater. Sci., 13 (1983) 9—42. M. E. Vinayo, F. Kassabji, J. Guyonnet and P. Fanchais, J. Vac Sci. Technol. A, 3(6) (1985) 2483—2489. G. Barbezat, E. Muller and B. Walser, Rev. Tech. Sulzer, 4
(1988) 4—10. 8 B. A. Detering, J. R. Knibloe and T. L. Eddy, in T. F. Bernecki (ed), Thermal Research and 3rd Natl. Thermal Spray Plasma Conf., Long Beach, CA,Applications, May 20—25, Proc. 1990, American Society for Metals, Metals Park, OH, 1991, pp. 27—31. 9 D. Tu, S. Chang, C. Chao and C. Lin, J. Vac. Sci. Technol. A, 3(6) (1985) 2479—2482. 10 W. J. Lenling, M. F. Smith and J. A. Henfling, in T. F. Bernecki (ed), Thermal Plasma Research and Applications, Proc. 3rd NatI. Thermal Spray Conf, Long Beach, CA, May 20—25, 1990, American Society for Metals, Metals Park, OH, 1991, pp. 451—455. II A. Karimi, Ch. Verdon and G. Barbezat, Surf. Coat. Technol., 57 (1993) 81—89. 12 R. K. Schmid, Wear of tungsten carbides cobalt coatings, Rep. 527, 1989 (Tribology and Surface Laboratory 1507, Sulzer-Innotec, Winterthur). 13 H. M. Rietveld, J. App!. Crystallogr., 2 (1969) 65—71. 14 R. J. Hill, Powder Diffr., 6 (2) (1991) 74—77. 15 V. Ramnath and N. Jayaraman, Mater. Sc!. Technol., 5 (1989) 382—388. 16 M. K. Keshavan, Met. Powder Rep., 42 (12) (1987) 866—869. 17 A. J. Ninham and A. V. Levy, Wear, 121 (1988) 347—361.
493
Hydroabrasive wear behaviour of high velocity oxyfuel thermally sprayed WC—M coatings A. Karimi and Ch. Verdon Inst itut de Genie Atomique, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne (Switzerland)
Abstract Hydroabrasive wear tests were conducted on the coatings of tungsten carbide—metal matrix cermets of type WC—M, where M Co, CoCr and Ni. The coatings were deposited using the high velocity oxyfuel (HVOF) spraying process. Cermets of the cobalt binder phase showed 50%—200% higher strength than those of the nickel matrix depending on the erosion conditions, and the addition of chromium improved the erosion resistance several times. Analytical techniques including X-ray diffraction and energy-dispersive spectroscopy (EDS) as well as transmission electron microscopy were used to characterize microstructures formed during powder processing and spraying. Owing to overheating of powder particles within the spray torch, the HVOF cermets develop complex microstructures. The hard phase consists basically of tungsten monocarbide (WC) as well as ditungsten carbide (W 2C), while the binder phase obtains a nanocrystalline structure of the size between 4 and 8 nm, containing ternary carbides and mixed compounds. The erosion results are discussed with respect to the microstructural features developed during spraying.
1. Introduction The high hardness of tungsten carbide in conjunction with good toughness due to the binder phase has made WC—Co cermets one of the most widely used wearresistant materials in a variety of industrial applications such as aerospace, automotive, mining and power generation systems [1—3].These protective coatings are widely applied on the surface of components using various thermal spray processes including plasma spraying, cornbustion (high velocity oxyfuel (HYOF) and flame) and the detonation gun (D-gun). The high temperature of the spray torch, the turbulent character of the gas flow and the rapid solidification of deposits associated with these processes result in complex chemical transformations and lead to metastable phase formation within the coatings [4, 5]. Several experimental studies have dealt with these transformations and with the influence of the powder characteristics and spray conditions on the microstructures and mechanical properties of WC—Co coatings [6, 7]. The main phenomena which have been found to occur are the partial melting of powder and the substantial thermal decomposition of WC, giving rise to the formation of W2C, mixed Co~W~C~ compounds as well as metallic tungsten [8, 9]. Furthermore, these studies have revealed that such transformations are in general unfavourable for most wear applications [10]. Loss of wear performance due to decomposition and decarburization of tungsten monocarbide (WC) is often related to reduction in coating hardness and formation of brittle secondary carbides.
0257—8972/93/S6.0O
Little information is available on the mechanism of structural transformations during powder processing and spraying and, in particular, how, when and where the decomposition of WC occurs, which mixed cornpounds are formed, and what may be their effect on the wear behaviour of coatings. It is the objective of this work to characterize the microstructures developed during the HVOF process of tungsten carbide—metal matrix of type WC—M (M Co, CoCr and Ni) and to relate them to their hydroabrasive wear performance. For this study, electron microscopy observations including scanning electron microscopy (SEM), transmission electron microscopy (TEM) coupled with energydispersive spectroscopy (EDS) microanalysis, and X-ray diffraction (XRD) analysis were used. The results are discussed with respect to erosion testing.
2. Erosion test results Coatings of tungsten carbide cermets having a nominal composition of WC—12 wt.% Co, WC—l2 wt.%Co—4 wt.%Cr and WC—12 wt.%Ni were tested. These coatings were deposited on the austenitic stainless steel substrates using a HYOF spray system from Plasma-Technik AG. The following operating conditions were used to spray the coatings: oxygen flow rate, 4201 min 1, propane flow rate, 55 1 min 1; powder carrier gas, nitrogen; carrier gas flow rate, 35 1 min 1; powder feed rate, 40 g min 1; spraying distance, 300 mm; powder size, 15—45 jim.
©
1993
—
Elsevier Sequoia. All rights reserved
494
A. Karimi, Ch. Verdon
/
Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
Under these conditions, powder particles are accelerated ((105—106)g) to velocities up to 800 m s~. The associated high kinetic energy of particles in conjunction with thermal effects provides compact layers which show less than 0.5% porosity. Measurement of the coating density and image analysis of polished surface did not present a significant difference between the porosities and crack formation features of different cermets. The thickness of coatings was approximately 0.3 mm. The average Vickers microhardnesses (load, 300 kgf) were 1141, 1269 and 993 kgfmm2 respectively, The range of microhardnesses was rather wide because of the large phase heterogeneity and porosity of the coatings. The above values are the average of 20 measurements taken on each coatings. Hydroabrasive tests were conducted in a laboratory device which produces a rotating tangential flow of a mixture of abrasive particles and water over the specimens using a helicalgear-type impeller [3]. Two different WC sizes were used for each coating including fine carbide with d ~ 1—2 jim and coarse carbide with d ~ 3—4 jim. In this study, tests were performed in tap water under the following conditions: flow velocity, 25 m s1 abrasive size, 20—200 jim; abrasive concentration, 0.3 wt.%. The cumulative erosion vs. exposure time is reported in Fig. 1. The values obtained for stainless steel 13—4 and ceramic Cr 203 (air plasma sprayed) are also added to give a comparison of the erosion behaviour of the HYOF sprayed cemented carbides. It is clear from this diagram that cermets of Co matrix resist better than those of Ni matrix, and the addition of chromium improves the erosion resistance of WC—Co coatings by about four to six times. The beneficial action of chromium appeared to be more pronounced at lower and intermediate impingement velocities, normally below 100 m s~’ [11, 12]. The reason for such an improvement due to chromium is not yet clear. Nevertheless, several effects may be consid15
ered to play major roles, e.g. enhancement of the corrosion resistance of the matrix, an optimal proportion of carbides within the binder resulting in a more efficient distribution of carbides, and the formation of favourable microstructures offering a higher toughness to the binding phase or providing an extra strengthening to the cohesion of carbide particles by the matrix [12]. Under the low aggressive erosion conditions used in Fig. 1, higher erosion rates were observed for WC—Co cermets prepared by the powder metallurgy (PM) method compared with those deposited using the HVOF thermal spraying process. Such a result can be attributed to the morphology and behaviour of the matrix, because carbides behave roughly the same in both specimens. The formation of mixed Co~W~C~ compounds during HVOF spraying leads to a substantially higher hardness of the matrix compared with that of the Co matrix obtained by PM. This can probably improve the corrosion resistance and toughness of the matrix as well as compensate for the effect of relative loss of WC in the sprayed coatings. The scanning electron photomicrograph of the eroded surface (Fig. 2) showed that, in general, the carbide grains are tough enough to survive the direct particle impacts. Hence, the mode of material loss in cermets appears to be strongly dependent on the behaviour of their matrix. The effect of the carbide morphology seems to be more pronounced under tangential erosion. In general, an eroding particle will scratch the softer binder phase, whereas carbides interrupt the scratch development and deflect the eroding particles, thereby protecting the matrix. When the surrounding binder phase is sufficiently worn, then the carbide can be removed easily under simultaneous action of impacting particles and shearing of circulating flow. The higher erosion resistance provided by the coarse carbides may be due to their greater
~~‘—‘——‘—
S
13-4 St. St.
1’H~Fiit~
_____
0
~
Time liii
Fig. 1. Cumulative erosion loss vs. exposure time for materials tested. 1 indicates coarse carbide, 2 indicates fine carbide, PM is powder metallurgy, and APS is air plasma spraying.
Fig. 2. SEM observation of the eroded surface of WC—Co cermet after an exposure time of 30 h.
A. Karimi, Ch. Verdon
/
Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
effectiveness in deflecting the eroding particles. As fine carbides are physically too small to deflect the erosive particles, they are worn together with the surrounding matrix. Their contribution to erosion resistance would be limited only to lending hardness to the matrix.
495
In order to analyse the erosion behaviour of the coatings, microstructural characterizations were performed using XRD spectrometry and TEM coupled with EDS microanalysis. The XRD patterns were obtained on a Rigaku diffractometer using Cu K~radiation and a graphite crystal diffracted-beam monochromator, operating at 40 kV and 30 mA. The data were collected, generally, for 26=20—140°. Typical examples
have disappeared. In addition to the distinct peaks corresponding to the identified phases mentioned above, one can easily distinguish two large and diffused humps situated at 26~40°and 70°,as indicated on the diagram. These humps were attributed to the nanocrystalline binder phase developed during HVOF spraying of the cermets [11]. The composition of this nanocrystalline phase consists essentially of ternary carbides and W~C~,CO~ mixed compounds. For a quantitative estimation of WC decomposition and formation of the secondary mixed phases within the coatings, the Rietveld [13] XRD analysis method for multiphase mixtures was used. This technique involves fitting the entire experimental diffraction pattern of the coating as obtained in Fig. 3(b) with a calculated profile using a model that includes the crystal structure parameters of the constituent phases (such as WC, W2C, Co and C06W6C), peak shapes and
of intensity spectrum for 26 = 30—90° are shown for WC—Co powder before spraying in Fig. 3(a) and after being coated in Fig. 3(b). On the X-ray diagram of the powder, one can note, in addition to tungsten carbide and cobalt patterns, the appearance of weak peaks corresponding to the presence of small quantity of W2C and Co6W6C. These secondary carbides have been produced during powder processing. At this stage of transformation the metallic cobalt still shows an f.c.c. structure. However, the spectrum in Fig. 3(b) corresponding to the coating reveals that the main transformations take place during the spraying process. Apart from the retained WC, significant amounts of W2C and metallic tungsten were formed, while traces of metallic cobalt
background [14]. The superposition of the calculated profile onto the experimental pattern allows quantification of the relative abundance of each phase within the coating. Using this method, the results reported in Table 1 have been obtained for coatings tested. From the values mentioned in Table 1, it appears that a substantial fraction of WC decomposed to W2C and metallic W or reacted with the cobalt to form ternary carbides and other W—C--Co mixed compounds. Differences in the contents of retained WC and in the chemical compositions of the coatings can be related to the spraying conditions or powder parameters. For example, fine carbides are more susceptible to overheating and decarborization and release a higher amount of metallic tungsten.
3. Coating microstructures
. •wc OW2C
a)
E Co (fcc)
V Co6W6C
. Oj~0~
~~Vo°
~
.
?
•
.
~
•wc
V
b)
-
OW2C
—
H ~
nanociystalline phase
~ 30
4))
5))
9~°~ 6))
70
00
0)
20 angle Ideg]
Fig. 3. XRD spectrum fOr a WC—Co cermet with indication of the related phases: (a) initial powder; (b) as coated.
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Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
TABLE I. Phase composition of the coatings estimated using the Rietveld method Sample
metric phases such as Co6W6C or other mixed W—C—Co compounds would be formed within the binder phase. The exact identification of these phases is very difficult
Amount (wt.%)
_________________________ WC w2c w WC-Co (fine) WC—Co (coarse) WC—CoCr
52
23 43
49
36
15
WC-Ni (fine)
65
6
29
54
because of their nanocrystalline character. The size of nanocrystallites measured using high resolution TEM and estimated from XRD peak broadening [11] has been found to vary between 4 and 8 nm. Region C in Fig. 4 denotes an area where some of the WC particles
23
were melted and decomposed into the secondary phases. The flickering contrast indicates distortion of carbide grains due to volume change following decomposition
_____________________________________________________
The values in Table 1 should be considered as an indication of the major phases developed within the coatings. In fact, because of the nanocrystalline character of the binder phase, no distinct diffraction peaks corresponding to the ternary carbides or metallic cobalt appear on the XRD diagrams. For this reason, their contribution to the calculated profile was neglected in this work. In spite of such an approximation the relative values of the principal phases, i.e. WC, W2C and W, do not seem to be changed fundamentally. These values appear to be close to the chemical compositions reported by Ramnath and Jayaraman [15] for tungsten carbide— cobalt coatings deposited using plasma spraying. TEM observation of the coatings confirmed the formation of various phases and development of multiple microstructures detected previously by X-ray analysis. An example of a typical photomicrograph is illustrated in Fig. 4. In this picture, region A corresponds to the WC grain and region B relates to the binder phase possessing the nanocrystalline structure. The use of EDS—TEM and Auger electron spectrometry [11] showed systematically the presence of tungsten, cobalt and carbon in region B. This confirms that the stoichio________
~
_____________ ______
or decarburization.
4. WC decomposition During thermal spraying of cermets, locally overheating occurs and results in partial melting of tungsten carbides which interact with the metal matrix to form ternary carbides or other mixed compounds. Consequently, the final coatings are not the ideal cermets made up of WC particles bound in the metal matrix, but they contain more complex microstructures developed depending on the temperature of the spray partides. At a higher degree of overheating a splat-like microstructure appears owing to impingement of melted carbides, while a lower degree of overheating favours only the formation of nanocrystalline binder phase as observed previously. A further deleterious effect may be produced when overheating takes place in an oxidizing atmosphere which leads to a loss of carbon from WC. One of the consequences of this reaction is the formation of brittle secondary carbides such as W2C, which are unfavourable for different wear applications [7, 8]. To demonstrate the process of WC decarburization, examples of microstructures observed by SEM and TEM are presented in Fig. 5. The carbides WC and W2C were identified using the microdiffraction technique in the transmission electron micrograph.
_______________________ ~ _____
-~
200 nm
Fig. 4. Transmission electron micrograph showing typical microstructures in the WC—Co coating. Region A is WC, and region B is the nanocrystalline matrix containing W, C and Co. The region C shows WC partially melted and decomposed.
The scanning electron micrograph (Fig. 5(a)) shows an entire WC particle within the matrix where a surrounding layer of about 100 nm has been decarburized and changed to W2C. The boundaries between W2C and retained WC as well as between W2C and matrix as visible on the scanning electron micrograph have been revealed by etching. As can be seen from the curvature of the boundaries, the transformation front extends from the carbide particle into the matrix. This suggests that the outer shell of the carbide particle has been overheated during its passage through the spray torch and the thermal effect of future passes allows transformation to occur. One can also note that, because of partial melting of the particle, the sharp angles of the crushed carbide have disappeared, giving rise to a rather rounded partide. TEM observation of the decarburized layers
A.
Karimi,
Ch. Verdon
/
Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
_____
_________________________
______
spraying of cermets, overheating takes places and tung..
____
matrix to form ternary carbides or other mixed compounds. There are several stoichiometric phases in the Co—W—C system. The decarburization of WC inherent in these reactions results in the formation of substantial amounts of W2C and metallic tungsten. In such a case
______
_____
_____________________________________________________
~ ____ _______ _____
500 (a)
~
size of abrasive particles, angle of incidence and velocity of impacts, optimum values may be obtained for binder content and carbide size in order to achieve the highest
_____
____
497
~--~‘
~ WC -~ -
II
~
~
~
______
L
50
nm
~h) Fig. 5. Observation of a carbide particle showing decarburization of WC and formation of W2C: (a) SEM image; (b) transmission electron micrograph.
(Fig. 5(b)) showed that the boundaries between WC and W2C as well as between W2C and matrix are often abrupt. On the scale of conventional TEM no diffused boundaries were detected. High resolution TEM investigations may be necessary to study such interfaces and to analyse the defects present, in particular, at the boundaries between WC and W2C.
5. Discussion The erosive wear behaviour of tungsten carbide—metal matrix cermets have been found to be governed mainly by the carbide grain sizes and by the binder volume fractions. These two parameters determine, in particular, the distribution of the carbides within the cermet which in its turn characterizes the effectiveness of the carbides to deflect the abrasives and hence to inhibit scratching of the matrix. Depending on the erosion conditions, i.e.
the final coating develops more complex microstructures than an ideal cermet made up of WC particles bound in the metal matrix. In addition, rapid solidification of the deposits provides a nanocrystalline character to the binder phase. In spite of the various properties evidenced above for the HVOF sprayed coatings, their erosion resistance appeared to be higher than that of cermets made using the PM technique. The explanation of this result lies in the morphology of metal matrix, because carbides can be assumed to behave in the same way in both cermets. The nanocrystalline binder phase in thermally sprayed coatings is harder than the metallic cobalt phase in PM coatings, which compensates for deleterious effects due to relative loss of WC in sprayed cermets. In addition, the nanocrystalline compounds may provide more toughness to the matrix and improve its corrosion resistance. When the effect of the carbide size is considered, cermets of fine carbides showed a relatively lower erosion resistance than those of coarse carbide. At the same time, fine carbides have been found to undergo more WC decomposition. This probably is due to the higher degree of overheating related to the greater surface-tovolume ratio. The ratio of surface to volume doubles when the size of particles is halved. In such a case, the question of the relative weak strength of fine carbides remains still open. In this regard, two hypotheses based on the microstructural effect or purely mechanical action may be considered. One can expect that a higher degree of decomposition would result in more deterioration of mechanical strength or suppose that fine carbides are physically too small to resist and deflect the abrasive particles. Their contribution would be limited only to lending hardness to the matrix to reduce scratch depth and related material loss. Microscopy observations are ongoing to determine the role of each process and to understand better the mechanisms of wear loss. The addition of chromium decreased erosion rate several times as observed in Fig. 1. This effect is in agreement with the observation of Ninham and Levy [17] under 60 m s’ gas-blast erosion and those of Schmid [12] under small-angle impingement erosion. The reason for such an improvement was attributed to
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Hydroabrasive wear of HVOF thermally sprayed WC—M coatings
the solid solution formed by the two metals and possible precipitation of chromium carbides [17]. In the present work the X-ray analysis showed that chromium does not change the amount of WC decomposition significantly. Hence, its beneficial action may be related rather to the formation of favourable microstructures which provide toughness to the matrix or improve the binding of the carbide particles. Another explanation of this behaviour could be the presence of the higher content of binder phase in WC—CoCr dermet compared with WC—Co. It results in an optimal proportion of the carbide-binder phase which offers significant erosion resistance under these conditions. Acknowledgments The authors wish to thank G. Barbezat of Plasma Technik AG for fabricating the coatings, and R. K. Schmid and P. Desponds of Sulzer-Innotec for their keen interest and stimulating discussions in this work. The project was supported by the Commission d’Encouragement a la Recherche Scientifique, the Fonds National Suisse de la Recherche Scientifique and Sulzer AG. References 1 D. J. Varcalle, Jr., Smolik, G. C. Wilson, G. Iron and J. A. WaIter, in R. M. Yazici (ed), Protective Coatings: Processing and
2
3 4 5 6 7
Characterization, Proc. Metallurgical Society of the AIME Northeast Regional Meeting, Hoboken, NJ, May 3—5, 1989, Minerals, Metals and Materials Society, Warrendale, PA, 1990, ~,. l21134. J. A. Sue and R. C. Tucker, Surf. Coat. Technol., 32(1987) 237—248. A. Karimi and R. K. Schmid, Wear, 156 (1992) 33—47. D. Apelian, M. Paliwal, R. W. Smith and W. F. Schilling, mt. Metall. Rev., 28(5) (1983) 27 1—294. J. H. Zaat, Annu. Rev. Mater. Sci., 13 (1983) 9—42. M. E. Vinayo, F. Kassabji, J. Guyonnet and P. Fanchais, J. Vac Sci. Technol. A, 3(6) (1985) 2483—2489. G. Barbezat, E. Muller and B. Walser, Rev. Tech. Sulzer, 4
(1988) 4—10. 8 B. A. Detering, J. R. Knibloe and T. L. Eddy, in T. F. Bernecki (ed), Thermal Research and 3rd Natl. Thermal Spray Plasma Conf., Long Beach, CA,Applications, May 20—25, Proc. 1990, American Society for Metals, Metals Park, OH, 1991, pp. 27—31. 9 D. Tu, S. Chang, C. Chao and C. Lin, J. Vac. Sci. Technol. A, 3(6) (1985) 2479—2482. 10 W. J. Lenling, M. F. Smith and J. A. Henfling, in T. F. Bernecki (ed), Thermal Plasma Research and Applications, Proc. 3rd NatI. Thermal Spray Conf, Long Beach, CA, May 20—25, 1990, American Society for Metals, Metals Park, OH, 1991, pp. 451—455. II A. Karimi, Ch. Verdon and G. Barbezat, Surf. Coat. Technol., 57 (1993) 81—89. 12 R. K. Schmid, Wear of tungsten carbides cobalt coatings, Rep. 527, 1989 (Tribology and Surface Laboratory 1507, Sulzer-Innotec, Winterthur). 13 H. M. Rietveld, J. App!. Crystallogr., 2 (1969) 65—71. 14 R. J. Hill, Powder Diffr., 6 (2) (1991) 74—77. 15 V. Ramnath and N. Jayaraman, Mater. Sc!. Technol., 5 (1989) 382—388. 16 M. K. Keshavan, Met. Powder Rep., 42 (12) (1987) 866—869. 17 A. J. Ninham and A. V. Levy, Wear, 121 (1988) 347—361.