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Plasma spray deposition of graded metal-ceramic coatings
Plasma spray deposition of graded metal-ceramic
(Received April 16, 1991; accepted
in final form December
coatings
12, 1991)
Abstract Plasma spraymg of graded coatings is described and the metalLceramic interface of the graded intermediate zone is analysed in terms of a simple physical model. Special attention is devoted to the dominant deposition parameters, powder characteristics and the injector configuration for powder feeding, which play a fundamental role in graded coating deposition with controlled formation of a metal-ceramic intermediate zone. On the basis of a knowledge of these parameters, a new and original formula for the coefficient of homogeneity for simultaneous deposition of metal and ceramic particles at the same spot on the substrate is derived. Furthermore, very interesting topotactical relations are described for the metal-ceramic interface of the graded zone. Various techniques of structural analysis (X-ray diffraction, scanning electron microscopy, optical microscopy) and simple thermodynamic calculations allow a new interpretation to be given of the bonding between the metal and ceramic components. The cohesion of graded metalceramic coatings is predicted to be higher than that of ceramic coatings with a metallic bond layer. The results are illustrated by a NiCrrZrO,(MgO) graded coating.
1. Introduction
Plasma-sprayed coatings exhibit many useful properties such as resistance to hot corrosion and oxidation, biocompatibility and other remarkable electronic, electrochemical and mechanical characteristics [l-3]. To a large extent the success of a plasma-spraying technology depends not only on the coating materials and deposition conditions used but also on the compatibility between the properties of the deposited material and those of the substrate. The major limitation to the continued development of these coatings as well as to the expansion of their range of applications lies in their poor fracture toughness and consequently in their poor adhesion to the substrate [4]. Furthermore, large differences between the coefficients of thermal stress are often the very reason for premature failure of such coatings C5, 61. One way of overcoming these problems is to build up the coating by successive plasma deposition of thin layers of gradually varying composition. Such coatings are known as “graded coatings”. Graded coatings are sprayed as multilayered coatings with the composition varying from 100% metal applied directly to the substrate to 100% ceramic for the top coat [7, S]. A rather poor gradation of a coating composition is shown in Fig. I(a), while a continuous gradation is shown in Fig. l(b).
0257-X972,92:$5.00
Graded metal-ceramic coatings, in particular those optimized with respect to the sequence of materials, show above all excellent hot corrosion and oxidation resistance and mechanical properties. Together with plasma-sprayed whisker-reinforced ceramic coatings [9, lo] and cermet coatings [1 11, graded coatings optimized with respect to material and microstructure represent a new and promising perspective in plasma-spraying technology. Many studies have been published exploring the relationships between controllable process variables and identifiable features of the deposit microstructure and properties [ 12- 151. In this paper it is confirmed that the sensible approach is to find how to reproducibly control the deposition of the graded metal-ceramic (GMC) zone. The aim is to achieve a topotaxial intergrowth of crystallites based on a match of the lattices of the respective substances. The desired reproducible deposition of the GMC zone is controlled by the dominant deposition parameters using a microprocessor unit on the basis of the derived concept of the coefficient of homogeneity (COH) value. The cohesion of the graded coating is predicted by structural analyses and thermodynamic calculations. The cohesive mechanical strength of the graded coatinggsubstrate has been tested by scratch tester measurements.
N(‘,1992 ~. Elsevier Sequoia.
All rights reserved
2. Experimental
details
The graded coatings were prepared by atmospheric plasma 13!;, Cr steel substrates spraying on (jzj30 x 7 mm2). All substrates were blasted by alumina sand and cleaned ultrasonically in trichlorethylene before the plasma spray deposition. In our experiments NiCI and ZrO,(MgO) powders were used for plasma spraying. The NiCr coatings are strongly bonded to the substrate. The f.c.c. crystal structure of this nickel-based alloy is favoured for many different applications because of its nearly filled third electron shell which results in high phase stability even in the presence of alloying elements. ZrO, ranks among ionic materials showing polymorphism (monoclinic. tetragonal and cubic modifications) and very interesting physical and mechanical properties [I6 IX].
The graded coatings were produced by means of a microprocessor controlled d.c. plasmatron with a watercooled thoriated tungsten cathode and a water-cooled copper anode (inner diameter 6 mm) with a thoriated tungsten insert. The powder feed rate was controlled by a microprocessor-controlled powder feeder. The substrate temperature distribution was measured by a multichannel measuring instrument and was held constant during spraying in all experiments 1191. The required particle melting was monitored pyrometrically and controlled by the choice of deposition parameters using dual-adaptive system control instruments [IO]. Part of each coated specimen was heated in a controlled atmosphere with the aim of bringing their quenched structure to an equilibrium state. These specimens were then used for measuring contact angles. To determine the influence of the dominant deposition
J. Musil, J. Fiala
/ Plasma spraying
3. Results and Discussion 3.1. Deposition process The goal of our experiments was to determine the influence of the following on the flight and impingement of metal and ceramic particles and their mutual mixing and distribution in the growing GMC zone.
1. Deposition
Deposition step
parameters
Deposition 4 Ar
for individual
213
(1) The dominant deposition parameters: electrical power, P, (kW); plasma gas flow rate of argon, $Ar (L mini’), and hydrogen, & (L mini ‘); spraying difference of carrier gas distance, d,_, (m); magnitude flow rates of metal and ceramic powder feeding, A& (L min - ’ ). (2) The powder characteristics: magnitude differences of melting temperatures and thermal conductivities of metal and ceramic powders, AT (K) and A% (W m-l K _ ’ ) respectively. (3) The injector configuration and geometry of powder feeding (after Fig. 2(c)). From the measurements performed, the following conclusions can be drawn. (1) The required simultaneous deposition of metal and ceramic particles at the same spot on the substrate correlates with: (a) an increase in electrical power, PCz 30-40 kW (an increase in discharge current I,); (b) an increase in the gas flow rate of argon, dAr % 455 (c) a decrease in spraying distance, 50 L min-‘; &r = 150-130 mm. (2) An increase in the gas flow rate of hydrogen, does not influence the required 4 uZ = lo-15 L min-‘, spraying of metal and ceramic particles on the same spot, but increases the focusing of separate impingement particle spots and is responsible for undesirable separation of overheated metal particles during their spreading. (3) The injector configuration plays an important role in the simultaneous deposition of metal and ceramic particles at the same spot on the substrate, as shown schematically in Fig. 2. On the basis of the experiments done, the correlation between the dominant deposition parameters and the impingement of the particles, their spreading, mutual mixing and distribution is described by the coefficient of homogeneity (COH). From the simplified empirical model of ref. 22 the COH can be expressed in the form
parameters, powder characteristics and the injector arrangement for powder feeding on the flight and impingement of melted particles and their mutual mixing and distribution, plasma spraying of (1) metal particles, (2) ceramic particles and (3) metal and ceramic particles steel and glass plates carried out on was (170 mm x 170 mm x 4 mm) with a marking coordinate system X-x A powder collection experiment was also performed [21]. In each experiment (where the plasma torch and substrate are not traversed) one parameter was changed and the others were kept constant. Deposition times were 0.5, 1 and 2 s. By means of microscopical observation, the impingement position of regular melted particles and the degree of their mutual mixing and distribution on the plates were determined. X-ray diffraction (XRD) analysis was done on Mikrometa 2 and DRON 3M diffraction instruments in a Bragg-Brentano (semi)focusing arrangement. The surface morphology and microstructure of the NiCrZrO,(MgO) graded coatings were observed by an optical microscope (Zeiss Neophot I) and a scanning electron microscope (Jeol JSM 840). Energy-dispersive analysis by X-rays (EDAX, Link 860) was used to determine the concentration profile of the elements. The cohesion of the coatings was measured by means of scratch testing (automatic scratch tester CSEM-Revetest). The deposition parameters used for plasma spraying are summarized in Table 1. Some general comments concerning the plasma-spraying characteristics will be given in what follows.
TABLE
of gradedcoatings
steps of NiCrPZrO,(MgO)
graded
coating”
deposition
paramete& ti H,
1,
p,
d S-T
1 2 3 4
42 45 50 50
8 10 10 10
600 600 650 700
28 32 34 36
130 130 130 130
35 _ _ _
5
50
12
700
38
130
_
a Chemical composition of NiCr powder is 80 wt.%Ni, NiCr and ZrO,(MgO) is between 50 and 30 pm. b m (g min-I), powder feed rate; h (lm), layer thickness.
20 wt.%Cr
GMC zone
NiCr layer h
ml
m2
h
180 _ _ _
25 18 10 _
_ 15 20 32 _
_ 200 150 100
and of ZrO,(MgO)
powder
is 75 wt.%ZrO,,
_
ZrO,
top coat
m2
h _
_ _ _ 44 25 wt.%MgO:
200 particle
size of
I”1
C
tL
M
1
---&---
S
: I
I
C I I
I --_ - _
I _L__
---
7
P
~
I
J
I I
Fig. 2. lnjcctor
configuration
feedlng: (b) individual
and scheme of simultaneous
parallel injection
dcpoGtion
ports (from ref. 4); (c) individual
of metal and ccram~c partIclea on the \uhstratc: scr~es InJection ports (praent
(a) prcmlhcd powder
work) (M. metallic poudcr:
C‘. ccramlc
powder; P. plasma torch: S. substrate).
and cl= X.5 x IO 3 m. the COH
where k is a constant (k = niki) including limiting deposition and scale factors of individual parameters i. the term I ,“c tan p (/j is the angle between the ceramic powder injector and the metal powder injector; c (m) is the distance between the injector mouth and the torch nozzle from Fig. 2) represents the influence of the injector arrangement and geometry of powder feeding, the term P4,,‘& TA&o represents the influence of the dominant spraying parameters and the term l:‘jAi,l IATl represents the influence of the basic sprayed powder properties (the size range is identical for metal and ceramic particles). For our experiments. when k = 6.5 x 1O- ’ and the inlector arrangement is fixed at /j = 20 . (’ = 5 x IO 3 m
is
(3) The results of comparison of selected experimentally obtained values of the COH and the corresponding calculated values are given in Fig. 3. The experimentally obtained results are reproducible and arc seen to be in good agreement with the values calculated from eqn. (2). 3.2. Analvsis 3.2.1.
of’grtrded
Microscopical
u~~tings ohserrci~ions
The typical surface morphology of a plasma-sprayed coating (in this case the GMC zone) with characteristic stratification of the spread particles is shown in Fig. 4(a).
J. Mud,
J. Fiala
/
Plasma spraying
ofgraded
coatings
215
Figures 4(b) and 4(c) show X-ray image micrographs of the distribution maps for zirconium (ZrO, component) and nickel (NiCr component) respectively. The results shown in Fig. 4 correspond to the deposition conditions of step 4 in Table 1. Figure 5(a) shows the cross-section of the substrate/NiCrZrO,(MgO) graded coating, Fig. 5(b) shows the fracture section of this system and Figs. 5(c) and 5(d) are the distribution maps for zirconium and nickel respectively. In contrast to the bounded interface of an NiCr/ZrO,(MgO) sandwich coating, the GMC zone of the graded coating strongly suppresses the incidence of defects and eliminates sudden changes in thermal and elastic material constants, thus increasing the coating cohesion and the adhesive mechanical strength of the graded coating-substrate system. 3.2.2. XRD analysis By successive removal of the top layers, the depth profile of the phase composition of the coatings has been investigated using X-ray diffraction. Twenty layers have been removed in total, each being 30 urn thick, so that 600 pm of the coating has been mapped in this way. The composition changed from ZrO,(MgO) at the top, through the GMC zone in the middle to the NiCr bond layer at the cr-Fe substrate. As can be seen from Table 2, conspicuous topotactic relations [23325] exist between the individual phases forming the structure of the coating (equal interplanar spacings d of various crystallographic lattice families for different phases). This is manifested in the XRD patterns by multiple superpositions of lines corresponding to the matched crystallographic lattice families. There are distinct structural matches between the crystals of cl-Fe and NiCr on the one hand and those of NiCr and ZrO,(MgO) on the other hand (Fig. 6). Such a structural matching constitutes a basis for coherent intergrowth of the individual components in the coating.
(b)
TABLE 2. Topotactic relations ZrO,(MgO) graded coating cc-Fe substrate
NiCr a = 3.55 A
a = 2.90 A
hkl
110 15
Cc)
36 eluctr.lc
38
37 power-
3 9
P
hkl
d (A)
2.05
in the system
I
ZrWMgO) a = 5.02 A
d (A)
Ill 200
2.050 1.775
220
1.255
311 222
1.070 1.025
40
[KWI
Fig. 3. Plots of COH t’s, dominant spraying parameters for simultaneous deposition of metal and ceramic particles at the same spot on the substrate: (a) COH us. spraying distance; (b) COH us. gas flow rate of argon; (c) COH vs. electrical power (- -, experiment; -3 calculation).
200
220
1.450
1.025
steel (substrate)iNiCr-
hkl
d (A)
Ill 200
2.898 2.510
220 311 222 400 331 420
1.775 1.514 1.449 1.225 1.152 1.123
422
1.025
Coherency between the particular crystals (grains) entails in its turn minimum boundary energy, maximum cohesion strength and a minimum number of defects. which could lead to rupture of the coating upon mechanical loading 126, 271. In this sense we may speak of a topotactic mechanism of bonding. In order to establish quantitatively the relation between the bonding strengths of different components, we shall have to count the number of all matched lattice families. taking into consideration the so-called multiplicity factor m [B] which gives the number of crystallographically equivalent lattice families of a given kind (Table 3). In these terms, for example. the bond between ZrO,(MgO) and NiCr is 1.3 times stronger than the bond between Zr02(MgO) and Q-Fe:
conclusions are completed and simple thermodynamic
considerations
J..?._<. %J7/dP tllc~r.rlloll!‘rll7rllic. ItIdP/ cJf c’Cd7C\i(H7 Two dissimilar materials form a thcrmodynamicully stable interface if the interfacial free energy of the bonded structure is less than the sum of the free energies of the separate interfaces. The driving force for the formation of a metal- ceramic interface is the yield in energy arising when an intimate contact is established between the metal and ceramic surfaces [20]. The simplest description of the physical interaction between a metal and a ceramic is the work of adhesion. WI,,, [30]. Specifically, when clean, defect-free surfaces arc brought into contact. energy is released in accordance with the Dupr6 equation [?I] II:,,, = ;‘, + .’i \I
The total cohesion of a graded metal-ceramic coating is higher than the total cohesion of a ceramic coating with a metallic bond layer. because in the extensive GMC zone of the graded coating the probability of the above-mentioned binding interactions is essentially higher than the probability of bond existence in the narrow interface of the ceramic coating with the metal bond coat. Simplified calculations performed on the geometrical structure models of a graded coating and a ceramic coating with a metallic bond layer confirm this. These
by subscyucnt calculationa.
-
;‘cTl
(3)
where ;‘, and ;‘h, are the free energies of the relaxed surfaces of the ceramic and the metal respectively and ;‘( h, represents the energy of the relaxed intcrfacc between the metal and the ceramic. In practice. W,,, is deduced by measuring the contact angle 0 established by a solid metal in contact with a ceramic: M.:d= ;.b,( I + cos 0 )
(5)
Adequate measurements of 8 and ;tn constitute a non-trivial experimental task. Measurements on small particles arc preferred, although contamination during thermal annealing is always a problem.
J. Mud,
J.
Fiala
)// Plasmaspraying
of‘ graded coating.5
217
(b)
(4 Fig. 5. (a) Cross-section maps for (c) zirconium
and (b) fracture and (d) nickel.
section
micro; graphs
of the substrate:NiCr-ZrO,(MgO)
The cohesion of a metal-ceramic system, I< is characterized by the sum of the interface energies. If the total interface energy decreases, then the growl :h structure stabilizes and consequently the cohesion of the coating increases. For its simplified calculation it is Inecessary to
create a geometrical energy is
graded
coating
structure
‘E = &M;IMM + &,c;1~c + &,Y,, where S,,
and the corresponding
EC )AX
model. The total interf ace
(6)
(yMM),SMc (yM,-) and Sc, (7,-c) are the surf ace
F,P
d
do,,
;11oi,,
(?26&
I’ig. 6. Topotactic bonding scheme for the system ZrO,(MgO)-NiCr) r-Fe ((/&I),, denotes a crystallographic lattice family; hkl and m are Miller indicts and the multiplicity factor respectively), TABLE 3. Bond strengths between the individual components of ZrO,(MgO)-NiCr:r-Fe expressed in terms of cumulative multiplicity factor\ (IHI) of the matched crystallographic lattice families Bond
Crystallographic lattice family
Multiplicity factor ,?I
(220) (400) (421)
I3 h 24
32
LrO,(MgO)+r-Fc
(22) (421)
H 74
31
r-F‘c + NiCr
(I IO) (23))
12 12
24
2-l;e + ZrO,( MgO)
(200) (120)
h I?
Iti
7r02(MgO)+NiCr
areas (m’) (interface energies (J m-l)) of the metalmetal. metal-ceramic and ceramic-ceramic systems respectively. The values of ,&,, S,, and S,, are calculated from the geometrical structure model, which was obtained by scanning electron microscopy and optical microscopy observations of heat-treated specimens. A schematic illustration of the structural arrangement in the GMC zone microsection is given in Fig. 7. The interface energies ;I~~ (yhlM= 0.4;,), yMC and ycc are calculated from the formulae
\ i /
(7)
from where the angles 0, and O2 were measured microphotographs of horizontal and angular GMC zone microsections [32]. Calculations were carried out for the NiCr:Zr02 sandwich coating and the NiCr ZrOz graded coating. The results are as follows. (I) It can easily be shown that ;I~~ = O.S;l,,, and “1 ,(-h, = o+,,. (2) In good agreement with the conclusion of Scction 3.22, it is possible to predict that the total cohesion of the graded metal-ceramic coating, W,, is higher than the total cohesion of the ceramic coating with the metal bond layer. W,: bKL= 0.8 W,. (3) For the results to be reliable, it is necessary to execute a large number of measurements.
A determination of the cohesive mechanical strength of the coating was obtained from the measurement of fracture toughness by analysis of the half-cone fracture formation during scratch tester measurements [33]. A scratch test is performed perpendicularly to a crosssection of the coating, with typical scratch formation from the substrate to the surface of the top coat. At a critical distance L, from this surface, scratching leads to half-cone-shaped cracking (Fig. 8); the variation in the critical length L, with the normal force F,, is shown in Fig. 9. On the basis of the original method 1333, the stress intensity factor K, characterizing the fracture toughness of the NiCr -ZrO,(MgO) graded coating and of the ZrO, coating with the NiCr bond layer was determined. Results are given in Table 4 and show the following. (I) The total cohesion of the graded NiCrZrO, (MgO) coating (K, = 6.2 MN m ‘:l) is higher than the total cohesion of the ZrO, coating with the NiCr bond coat (K, = 4 MN mm3’2). (2) The effect of the GMC zone is significant in the enhancement of the coating cohesion. A GMC zone with continuous gradation shows better cohesion behaviour than a GMC zone with 50% NiCr and 50% Zr02(MgO).
4. Conclusions
Fig. 7. Scheme of structural (M. metal: C, ceramic).
arrangement
in GMC
zone microsection
(I) An original model describing the correlation between the dominant deposition parameters on the one hand and the impingement of melted metallic and
J. Mud.
J. Fiala
i
Plasma spraying
(a)
of gradedcoatings
219
(b)
Fig. 8. Test of fracture toughness in a plasma-sprayed graded coating (after ref. 33): (a) scratching leads to half-cone fracture; (b) normal and tangential forces (F, and F,) L’S.scratch length.
from the substrate
into the coating
until it
(2) As a result of our structural analyses (X-ray diffraction, scanning electron microscopy and optical microscopy), scratch tester measurements and simple thermodynamic calculations, it has been definitely proved that graded coatings are better than ceramic coatings with a metal bond layer in terms of their cohesion and thus structural stability.
Acknowledgments The authors wish to thank Dr. J. Kasl Mrs. M. Ctvrtnikovi for the SEM observations.
10
I 1
I
I
I
10
5
-
Fig. 9. Variation
(log-log
scale)
50
20 F,
6
toughness
b’i
in critical length L, with normal
K, of coatings K, (MN rnm3:*)
Coating ZrO,(MgO)
4
with NiCr bond coat
References
100
force F,. TABLE 4. Fracture
and
NiCr-ZrO,(MgO) graded coating (GMC zone 50% NiCr + 50% ZrO,(MgO))
5.2
NiCr-ZrO,(MgO) graded coating (GMC zone continuous gradation)
6.2
I E. Lugscheider, in H. Eschnauer, P. Huber, A. R. Nicoll and S. Sandmeier (eds.), Proc. 1st Plasma-Tech&-Symp.. Lucerne, Ma) 19X8, Plasma-Technik AG, Wohlen, Vol. I. p. 23. 2 C. C. Li, Thin Solid Films, 73 (1980) 59. 3 C. X. Ding and B. T. Huang, Preprints 12th Int. Conf. on Thermal Spraying, London, June 1989, The Welding Institute, Cambridge, 1989, Vol. I, p. 18. 4 R. Kaczmarek, W. Robert. .I. Jurewicz. M. I. Boulos and S. Dallaire, MRS Symp. Proc., 30 (1984) 207. 5 H. Holleck and H. Schulz, Thin Solid Films, 153 (1987) I I. 6 0. Knotek, R. Elsing and N. Strompen, Thin Solid Films. 118 (I 984) 457. 7 R. L. Newman and W. C. Spicer. AIAA J., I1 (1973) 401. 8 M. Fukumoto, N. Ueyama and I. Okane, in E. Broszeit, W. D. Miinz, H. Oochsner and K.-T. Rie (eds.). Plasma SurJace Engineering, DGM, Oberursel, 1988, p. 327. 9 N. Iwamoto, N. Umesaki, S. Endo and T. Morimura, J. Mater. Sri.. 22 (1987) 1113.
mixing and distribution on the substrate surface on the other hand has been developed in terms of a newly introduced coefficient of homogeneity. ceramic
particles,
their
spreading,
mutual
10 C. C. Berndt and J. H. Yi. Surf. Coat. Technol., 37 (1989) 89. II C. C. Berndt and J. H. Yi, Preprints 12th Int. Con/: on Thermal Spraying, London, June 1989, The Welding Institute, Cambridge, 1989, Vol. I, p. 2. 12 R. McPherson, Thin Solid Films, 83 (1981) 297.
I? A. R. Nicoll. Surf: Cwt. Tec~lnzol., 30 I 1987) 223. I-1 P. Fauchais, A. Vardelle, M. Vardelle, J. F. Coudert and D. Bernard. Ifrcct Trrcttment ‘87. Prw. 1~. Con/.. Institute of Metals. London, 198X. p. I I. I5 S. Sampath and H. Herman, Preprint& 12tk Int. Con\: on Thmcd Sprclyrrlg, London, Junv /Y#Y. The Welding Institute. Cambridge. 1989. Vol. I, p. 53. I6 D. R. McKenzie, D. J. H. Cockayne, M. G. Sceats, P. J. Martin. W. G. Sainty and R. P. Netterfield, J. Mater. Sci.. .?_?(1987) 3715. 17 J. H. R. Hannink, J. Malw. Sci., 13 (1988) 2487. IX J. Musil and J. Fiala, Proc,. Cxch. Smlinrrr OHX-rrry djfrac~tlon irt Pkcvi, IWO, Czech. Science Technical Matcricd Twhnologies. Society. Czech. Crystal Society. Pilsen. 1990, p. IO. 19 V. Pilous and J. Musil, Metall. Mntcr., 3 (1990) 317 (in Czech). 20 J. P&ha. P1t.D. Thesis. Institute of Technology and Reliability of Structures. Czechoslovak Academy of Sciences. Plzen, 1990. ?I M. Paliwal and D. Apelian. MRS Swp. Proc,., 30 (1984) 1X7. ?? J. Musil. Ph.D. Thesis. Institute of Technology and Reliability of
23 ?4 75 26 17 3X 79 ?O 31 3? 33
Structures, Czechoslovak Academy of Sciences, Plzeti. 1991. Th. Armbruster, New Jh. Miner. Mh., H3 (1979) 109. I. Bonev, Acta Crystallop-. A, 28 (1972) 508. H. G. Schneider (ed.), Epitaxie, Endota.uir. VEB. Leipzig, 1969. W. Bollmann, Crystcll Defects and Crystalline Inrqfirce.~, Springer. Berlin, 1970. F. R. N. Nabarro (ed.), Dishcutions in Solids, Elsevier, Amsterdam, 1989. J. S. Kasper and K. Lonsdale (eds.). Inrernurioncll Tchlcs for X-ru) C‘rystullography, Vol. 2. Kynoch, Birmingham, 1967. K. L. Mittal (ed.). Adhesion Measuremrnt of’ Thin Films. Thic,k Films. und Bulk Coatings, ASTM Spec. Tech. Puhl. 640. 1978. R. E. Lochman. Cerrrm. BuK. 68 (I 989) 891. M. Riihle and A. G. Evans, ,!4ntur. Sci. Eng. 4, 107 (1989) 187. B. V. Derjaguim, Recent Adcunces in Adhesion, Gordon and Breach, New York. 1971. p. 513. F. Beltzung, G. Zambelh. E. Lopez and A. R. Nicoll. Thirl Solid Films, /XI (1989) 407.
(Received April 16, 1991; accepted
in final form December
coatings
12, 1991)
Abstract Plasma spraymg of graded coatings is described and the metalLceramic interface of the graded intermediate zone is analysed in terms of a simple physical model. Special attention is devoted to the dominant deposition parameters, powder characteristics and the injector configuration for powder feeding, which play a fundamental role in graded coating deposition with controlled formation of a metal-ceramic intermediate zone. On the basis of a knowledge of these parameters, a new and original formula for the coefficient of homogeneity for simultaneous deposition of metal and ceramic particles at the same spot on the substrate is derived. Furthermore, very interesting topotactical relations are described for the metal-ceramic interface of the graded zone. Various techniques of structural analysis (X-ray diffraction, scanning electron microscopy, optical microscopy) and simple thermodynamic calculations allow a new interpretation to be given of the bonding between the metal and ceramic components. The cohesion of graded metalceramic coatings is predicted to be higher than that of ceramic coatings with a metallic bond layer. The results are illustrated by a NiCrrZrO,(MgO) graded coating.
1. Introduction
Plasma-sprayed coatings exhibit many useful properties such as resistance to hot corrosion and oxidation, biocompatibility and other remarkable electronic, electrochemical and mechanical characteristics [l-3]. To a large extent the success of a plasma-spraying technology depends not only on the coating materials and deposition conditions used but also on the compatibility between the properties of the deposited material and those of the substrate. The major limitation to the continued development of these coatings as well as to the expansion of their range of applications lies in their poor fracture toughness and consequently in their poor adhesion to the substrate [4]. Furthermore, large differences between the coefficients of thermal stress are often the very reason for premature failure of such coatings C5, 61. One way of overcoming these problems is to build up the coating by successive plasma deposition of thin layers of gradually varying composition. Such coatings are known as “graded coatings”. Graded coatings are sprayed as multilayered coatings with the composition varying from 100% metal applied directly to the substrate to 100% ceramic for the top coat [7, S]. A rather poor gradation of a coating composition is shown in Fig. I(a), while a continuous gradation is shown in Fig. l(b).
0257-X972,92:$5.00
Graded metal-ceramic coatings, in particular those optimized with respect to the sequence of materials, show above all excellent hot corrosion and oxidation resistance and mechanical properties. Together with plasma-sprayed whisker-reinforced ceramic coatings [9, lo] and cermet coatings [1 11, graded coatings optimized with respect to material and microstructure represent a new and promising perspective in plasma-spraying technology. Many studies have been published exploring the relationships between controllable process variables and identifiable features of the deposit microstructure and properties [ 12- 151. In this paper it is confirmed that the sensible approach is to find how to reproducibly control the deposition of the graded metal-ceramic (GMC) zone. The aim is to achieve a topotaxial intergrowth of crystallites based on a match of the lattices of the respective substances. The desired reproducible deposition of the GMC zone is controlled by the dominant deposition parameters using a microprocessor unit on the basis of the derived concept of the coefficient of homogeneity (COH) value. The cohesion of the graded coating is predicted by structural analyses and thermodynamic calculations. The cohesive mechanical strength of the graded coatinggsubstrate has been tested by scratch tester measurements.
N(‘,1992 ~. Elsevier Sequoia.
All rights reserved
2. Experimental
details
The graded coatings were prepared by atmospheric plasma 13!;, Cr steel substrates spraying on (jzj30 x 7 mm2). All substrates were blasted by alumina sand and cleaned ultrasonically in trichlorethylene before the plasma spray deposition. In our experiments NiCI and ZrO,(MgO) powders were used for plasma spraying. The NiCr coatings are strongly bonded to the substrate. The f.c.c. crystal structure of this nickel-based alloy is favoured for many different applications because of its nearly filled third electron shell which results in high phase stability even in the presence of alloying elements. ZrO, ranks among ionic materials showing polymorphism (monoclinic. tetragonal and cubic modifications) and very interesting physical and mechanical properties [I6 IX].
The graded coatings were produced by means of a microprocessor controlled d.c. plasmatron with a watercooled thoriated tungsten cathode and a water-cooled copper anode (inner diameter 6 mm) with a thoriated tungsten insert. The powder feed rate was controlled by a microprocessor-controlled powder feeder. The substrate temperature distribution was measured by a multichannel measuring instrument and was held constant during spraying in all experiments 1191. The required particle melting was monitored pyrometrically and controlled by the choice of deposition parameters using dual-adaptive system control instruments [IO]. Part of each coated specimen was heated in a controlled atmosphere with the aim of bringing their quenched structure to an equilibrium state. These specimens were then used for measuring contact angles. To determine the influence of the dominant deposition
J. Musil, J. Fiala
/ Plasma spraying
3. Results and Discussion 3.1. Deposition process The goal of our experiments was to determine the influence of the following on the flight and impingement of metal and ceramic particles and their mutual mixing and distribution in the growing GMC zone.
1. Deposition
Deposition step
parameters
Deposition 4 Ar
for individual
213
(1) The dominant deposition parameters: electrical power, P, (kW); plasma gas flow rate of argon, $Ar (L mini’), and hydrogen, & (L mini ‘); spraying difference of carrier gas distance, d,_, (m); magnitude flow rates of metal and ceramic powder feeding, A& (L min - ’ ). (2) The powder characteristics: magnitude differences of melting temperatures and thermal conductivities of metal and ceramic powders, AT (K) and A% (W m-l K _ ’ ) respectively. (3) The injector configuration and geometry of powder feeding (after Fig. 2(c)). From the measurements performed, the following conclusions can be drawn. (1) The required simultaneous deposition of metal and ceramic particles at the same spot on the substrate correlates with: (a) an increase in electrical power, PCz 30-40 kW (an increase in discharge current I,); (b) an increase in the gas flow rate of argon, dAr % 455 (c) a decrease in spraying distance, 50 L min-‘; &r = 150-130 mm. (2) An increase in the gas flow rate of hydrogen, does not influence the required 4 uZ = lo-15 L min-‘, spraying of metal and ceramic particles on the same spot, but increases the focusing of separate impingement particle spots and is responsible for undesirable separation of overheated metal particles during their spreading. (3) The injector configuration plays an important role in the simultaneous deposition of metal and ceramic particles at the same spot on the substrate, as shown schematically in Fig. 2. On the basis of the experiments done, the correlation between the dominant deposition parameters and the impingement of the particles, their spreading, mutual mixing and distribution is described by the coefficient of homogeneity (COH). From the simplified empirical model of ref. 22 the COH can be expressed in the form
parameters, powder characteristics and the injector arrangement for powder feeding on the flight and impingement of melted particles and their mutual mixing and distribution, plasma spraying of (1) metal particles, (2) ceramic particles and (3) metal and ceramic particles steel and glass plates carried out on was (170 mm x 170 mm x 4 mm) with a marking coordinate system X-x A powder collection experiment was also performed [21]. In each experiment (where the plasma torch and substrate are not traversed) one parameter was changed and the others were kept constant. Deposition times were 0.5, 1 and 2 s. By means of microscopical observation, the impingement position of regular melted particles and the degree of their mutual mixing and distribution on the plates were determined. X-ray diffraction (XRD) analysis was done on Mikrometa 2 and DRON 3M diffraction instruments in a Bragg-Brentano (semi)focusing arrangement. The surface morphology and microstructure of the NiCrZrO,(MgO) graded coatings were observed by an optical microscope (Zeiss Neophot I) and a scanning electron microscope (Jeol JSM 840). Energy-dispersive analysis by X-rays (EDAX, Link 860) was used to determine the concentration profile of the elements. The cohesion of the coatings was measured by means of scratch testing (automatic scratch tester CSEM-Revetest). The deposition parameters used for plasma spraying are summarized in Table 1. Some general comments concerning the plasma-spraying characteristics will be given in what follows.
TABLE
of gradedcoatings
steps of NiCrPZrO,(MgO)
graded
coating”
deposition
paramete& ti H,
1,
p,
d S-T
1 2 3 4
42 45 50 50
8 10 10 10
600 600 650 700
28 32 34 36
130 130 130 130
35 _ _ _
5
50
12
700
38
130
_
a Chemical composition of NiCr powder is 80 wt.%Ni, NiCr and ZrO,(MgO) is between 50 and 30 pm. b m (g min-I), powder feed rate; h (lm), layer thickness.
20 wt.%Cr
GMC zone
NiCr layer h
ml
m2
h
180 _ _ _
25 18 10 _
_ 15 20 32 _
_ 200 150 100
and of ZrO,(MgO)
powder
is 75 wt.%ZrO,,
_
ZrO,
top coat
m2
h _
_ _ _ 44 25 wt.%MgO:
200 particle
size of
I”1
C
tL
M
1
---&---
S
: I
I
C I I
I --_ - _
I _L__
---
7
P
~
I
J
I I
Fig. 2. lnjcctor
configuration
feedlng: (b) individual
and scheme of simultaneous
parallel injection
dcpoGtion
ports (from ref. 4); (c) individual
of metal and ccram~c partIclea on the \uhstratc: scr~es InJection ports (praent
(a) prcmlhcd powder
work) (M. metallic poudcr:
C‘. ccramlc
powder; P. plasma torch: S. substrate).
and cl= X.5 x IO 3 m. the COH
where k is a constant (k = niki) including limiting deposition and scale factors of individual parameters i. the term I ,“c tan p (/j is the angle between the ceramic powder injector and the metal powder injector; c (m) is the distance between the injector mouth and the torch nozzle from Fig. 2) represents the influence of the injector arrangement and geometry of powder feeding, the term P4,,‘& TA&o represents the influence of the dominant spraying parameters and the term l:‘jAi,l IATl represents the influence of the basic sprayed powder properties (the size range is identical for metal and ceramic particles). For our experiments. when k = 6.5 x 1O- ’ and the inlector arrangement is fixed at /j = 20 . (’ = 5 x IO 3 m
is
(3) The results of comparison of selected experimentally obtained values of the COH and the corresponding calculated values are given in Fig. 3. The experimentally obtained results are reproducible and arc seen to be in good agreement with the values calculated from eqn. (2). 3.2. Analvsis 3.2.1.
of’grtrded
Microscopical
u~~tings ohserrci~ions
The typical surface morphology of a plasma-sprayed coating (in this case the GMC zone) with characteristic stratification of the spread particles is shown in Fig. 4(a).
J. Mud,
J. Fiala
/
Plasma spraying
ofgraded
coatings
215
Figures 4(b) and 4(c) show X-ray image micrographs of the distribution maps for zirconium (ZrO, component) and nickel (NiCr component) respectively. The results shown in Fig. 4 correspond to the deposition conditions of step 4 in Table 1. Figure 5(a) shows the cross-section of the substrate/NiCrZrO,(MgO) graded coating, Fig. 5(b) shows the fracture section of this system and Figs. 5(c) and 5(d) are the distribution maps for zirconium and nickel respectively. In contrast to the bounded interface of an NiCr/ZrO,(MgO) sandwich coating, the GMC zone of the graded coating strongly suppresses the incidence of defects and eliminates sudden changes in thermal and elastic material constants, thus increasing the coating cohesion and the adhesive mechanical strength of the graded coating-substrate system. 3.2.2. XRD analysis By successive removal of the top layers, the depth profile of the phase composition of the coatings has been investigated using X-ray diffraction. Twenty layers have been removed in total, each being 30 urn thick, so that 600 pm of the coating has been mapped in this way. The composition changed from ZrO,(MgO) at the top, through the GMC zone in the middle to the NiCr bond layer at the cr-Fe substrate. As can be seen from Table 2, conspicuous topotactic relations [23325] exist between the individual phases forming the structure of the coating (equal interplanar spacings d of various crystallographic lattice families for different phases). This is manifested in the XRD patterns by multiple superpositions of lines corresponding to the matched crystallographic lattice families. There are distinct structural matches between the crystals of cl-Fe and NiCr on the one hand and those of NiCr and ZrO,(MgO) on the other hand (Fig. 6). Such a structural matching constitutes a basis for coherent intergrowth of the individual components in the coating.
(b)
TABLE 2. Topotactic relations ZrO,(MgO) graded coating cc-Fe substrate
NiCr a = 3.55 A
a = 2.90 A
hkl
110 15
Cc)
36 eluctr.lc
38
37 power-
3 9
P
hkl
d (A)
2.05
in the system
I
ZrWMgO) a = 5.02 A
d (A)
Ill 200
2.050 1.775
220
1.255
311 222
1.070 1.025
40
[KWI
Fig. 3. Plots of COH t’s, dominant spraying parameters for simultaneous deposition of metal and ceramic particles at the same spot on the substrate: (a) COH us. spraying distance; (b) COH us. gas flow rate of argon; (c) COH vs. electrical power (- -, experiment; -3 calculation).
200
220
1.450
1.025
steel (substrate)iNiCr-
hkl
d (A)
Ill 200
2.898 2.510
220 311 222 400 331 420
1.775 1.514 1.449 1.225 1.152 1.123
422
1.025
Coherency between the particular crystals (grains) entails in its turn minimum boundary energy, maximum cohesion strength and a minimum number of defects. which could lead to rupture of the coating upon mechanical loading 126, 271. In this sense we may speak of a topotactic mechanism of bonding. In order to establish quantitatively the relation between the bonding strengths of different components, we shall have to count the number of all matched lattice families. taking into consideration the so-called multiplicity factor m [B] which gives the number of crystallographically equivalent lattice families of a given kind (Table 3). In these terms, for example. the bond between ZrO,(MgO) and NiCr is 1.3 times stronger than the bond between Zr02(MgO) and Q-Fe:
conclusions are completed and simple thermodynamic
considerations
J..?._<. %J7/dP tllc~r.rlloll!‘rll7rllic. ItIdP/ cJf c’Cd7C\i(H7 Two dissimilar materials form a thcrmodynamicully stable interface if the interfacial free energy of the bonded structure is less than the sum of the free energies of the separate interfaces. The driving force for the formation of a metal- ceramic interface is the yield in energy arising when an intimate contact is established between the metal and ceramic surfaces [20]. The simplest description of the physical interaction between a metal and a ceramic is the work of adhesion. WI,,, [30]. Specifically, when clean, defect-free surfaces arc brought into contact. energy is released in accordance with the Dupr6 equation [?I] II:,,, = ;‘, + .’i \I
The total cohesion of a graded metal-ceramic coating is higher than the total cohesion of a ceramic coating with a metallic bond layer. because in the extensive GMC zone of the graded coating the probability of the above-mentioned binding interactions is essentially higher than the probability of bond existence in the narrow interface of the ceramic coating with the metal bond coat. Simplified calculations performed on the geometrical structure models of a graded coating and a ceramic coating with a metallic bond layer confirm this. These
by subscyucnt calculationa.
-
;‘cTl
(3)
where ;‘, and ;‘h, are the free energies of the relaxed surfaces of the ceramic and the metal respectively and ;‘( h, represents the energy of the relaxed intcrfacc between the metal and the ceramic. In practice. W,,, is deduced by measuring the contact angle 0 established by a solid metal in contact with a ceramic: M.:d= ;.b,( I + cos 0 )
(5)
Adequate measurements of 8 and ;tn constitute a non-trivial experimental task. Measurements on small particles arc preferred, although contamination during thermal annealing is always a problem.
J. Mud,
J.
Fiala
)// Plasmaspraying
of‘ graded coating.5
217
(b)
(4 Fig. 5. (a) Cross-section maps for (c) zirconium
and (b) fracture and (d) nickel.
section
micro; graphs
of the substrate:NiCr-ZrO,(MgO)
The cohesion of a metal-ceramic system, I< is characterized by the sum of the interface energies. If the total interface energy decreases, then the growl :h structure stabilizes and consequently the cohesion of the coating increases. For its simplified calculation it is Inecessary to
create a geometrical energy is
graded
coating
structure
‘E = &M;IMM + &,c;1~c + &,Y,, where S,,
and the corresponding
EC )AX
model. The total interf ace
(6)
(yMM),SMc (yM,-) and Sc, (7,-c) are the surf ace
F,P
d
do,,
;11oi,,
(?26&
I’ig. 6. Topotactic bonding scheme for the system ZrO,(MgO)-NiCr) r-Fe ((/&I),, denotes a crystallographic lattice family; hkl and m are Miller indicts and the multiplicity factor respectively), TABLE 3. Bond strengths between the individual components of ZrO,(MgO)-NiCr:r-Fe expressed in terms of cumulative multiplicity factor\ (IHI) of the matched crystallographic lattice families Bond
Crystallographic lattice family
Multiplicity factor ,?I
(220) (400) (421)
I3 h 24
32
LrO,(MgO)+r-Fc
(22) (421)
H 74
31
r-F‘c + NiCr
(I IO) (23))
12 12
24
2-l;e + ZrO,( MgO)
(200) (120)
h I?
Iti
7r02(MgO)+NiCr
areas (m’) (interface energies (J m-l)) of the metalmetal. metal-ceramic and ceramic-ceramic systems respectively. The values of ,&,, S,, and S,, are calculated from the geometrical structure model, which was obtained by scanning electron microscopy and optical microscopy observations of heat-treated specimens. A schematic illustration of the structural arrangement in the GMC zone microsection is given in Fig. 7. The interface energies ;I~~ (yhlM= 0.4;,), yMC and ycc are calculated from the formulae
\ i /
(7)
from where the angles 0, and O2 were measured microphotographs of horizontal and angular GMC zone microsections [32]. Calculations were carried out for the NiCr:Zr02 sandwich coating and the NiCr ZrOz graded coating. The results are as follows. (I) It can easily be shown that ;I~~ = O.S;l,,, and “1 ,(-h, = o+,,. (2) In good agreement with the conclusion of Scction 3.22, it is possible to predict that the total cohesion of the graded metal-ceramic coating, W,, is higher than the total cohesion of the ceramic coating with the metal bond layer. W,: bKL= 0.8 W,. (3) For the results to be reliable, it is necessary to execute a large number of measurements.
A determination of the cohesive mechanical strength of the coating was obtained from the measurement of fracture toughness by analysis of the half-cone fracture formation during scratch tester measurements [33]. A scratch test is performed perpendicularly to a crosssection of the coating, with typical scratch formation from the substrate to the surface of the top coat. At a critical distance L, from this surface, scratching leads to half-cone-shaped cracking (Fig. 8); the variation in the critical length L, with the normal force F,, is shown in Fig. 9. On the basis of the original method 1333, the stress intensity factor K, characterizing the fracture toughness of the NiCr -ZrO,(MgO) graded coating and of the ZrO, coating with the NiCr bond layer was determined. Results are given in Table 4 and show the following. (I) The total cohesion of the graded NiCrZrO, (MgO) coating (K, = 6.2 MN m ‘:l) is higher than the total cohesion of the ZrO, coating with the NiCr bond coat (K, = 4 MN mm3’2). (2) The effect of the GMC zone is significant in the enhancement of the coating cohesion. A GMC zone with continuous gradation shows better cohesion behaviour than a GMC zone with 50% NiCr and 50% Zr02(MgO).
4. Conclusions
Fig. 7. Scheme of structural (M. metal: C, ceramic).
arrangement
in GMC
zone microsection
(I) An original model describing the correlation between the dominant deposition parameters on the one hand and the impingement of melted metallic and
J. Mud.
J. Fiala
i
Plasma spraying
(a)
of gradedcoatings
219
(b)
Fig. 8. Test of fracture toughness in a plasma-sprayed graded coating (after ref. 33): (a) scratching leads to half-cone fracture; (b) normal and tangential forces (F, and F,) L’S.scratch length.
from the substrate
into the coating
until it
(2) As a result of our structural analyses (X-ray diffraction, scanning electron microscopy and optical microscopy), scratch tester measurements and simple thermodynamic calculations, it has been definitely proved that graded coatings are better than ceramic coatings with a metal bond layer in terms of their cohesion and thus structural stability.
Acknowledgments The authors wish to thank Dr. J. Kasl Mrs. M. Ctvrtnikovi for the SEM observations.
10
I 1
I
I
I
10
5
-
Fig. 9. Variation
(log-log
scale)
50
20 F,
6
toughness
b’i
in critical length L, with normal
K, of coatings K, (MN rnm3:*)
Coating ZrO,(MgO)
4
with NiCr bond coat
References
100
force F,. TABLE 4. Fracture
and
NiCr-ZrO,(MgO) graded coating (GMC zone 50% NiCr + 50% ZrO,(MgO))
5.2
NiCr-ZrO,(MgO) graded coating (GMC zone continuous gradation)
6.2
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mixing and distribution on the substrate surface on the other hand has been developed in terms of a newly introduced coefficient of homogeneity. ceramic
particles,
their
spreading,
mutual
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