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Tungsten carbide coatings deposited by high-velocity oxy-fuel spraying on a metallized polymeric substrate
Surface
and Coatings
Technology
90 (1997)
82-90
Tungsten carbide coatings deposited by high-velocity oxy-fuel spraying on a metallized polymeric substrate’ I. Grimberg a$*, K. Soifer a, B. Bouaifi b, U. Draugelates b, B.Z. Weiss a a Department of Matesials Engineering, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel b Institute fkr Schweisstechnik ttnd Trennende Fertigungsuerfahren, Technical University Clausthal, 38678 Clausthal-Zellerfeld, Germany
Received17May 1996;accepted12August 1996
Abstract A triplex systemconsistingof WC(Co) hard coating, metallic bond layers (Ni/Cu/Ni), and acrylonitrile-butadiene-styreneas substrate,was investigated. The WC (88 wt.%)-Co (12 wt.%) coating was depositedby the high-velocity oxy-fuel spraying techniquemodified by the addition of COZgasflows. The microstructure,the composition,and the phasespresentin the coating were studiedby meansof transmissionand scanningelectronmicroscopiesboth combinedwith energydispersiveanalysis,X-ray diffraction, and Auger electron spectroscopy.It was found that during the spraying a fraction of the WC phasepartially decomposes into cc-and jSW,C and /?-WC,-x and reactswith cobalt to form the ternary carbide,Co,W,C. The decomposition occurred on the surfaceof the WC particle to a thicknessof severaltenths of a nanometer.The microstructure of the coating consistedof tungstencarbideparticlesbondedby a binder matrix composedmainly of Co. The microstructureof the binder phase was found to be dependenton the cooling rate. As a result of the variation in the particles’velocitiesand temperatures,different interface morphologiesaswell asdifferent fractions of binder phaseand pore densitieswereobservedin the coating. A theoretical model describingthe dynamic and the thermal performanceof the powder particles in high-velocity oxy-fuel spraying was developed,correlating betweenthe particles’properties(size, density, emissivity)and their velocity and temperature.The factors that influencethe powder velocity and temperatureand thereby the coating microstructureand compositionare discussed. Keywords:
Tungstencarbidecoatings;Polymer; High-velocity oxy-fuel spraying;Microstructure; Mathematical model
1. Introduction
Metallization of plastics is a well-established commercial process that is used in a variety of applications, ranging from microelectronics (for packaging devices) to the aircraft industries. The metals used in plastic metallization are nickel, chromium, copper, and zinc. In many cases these materials do not meet special technical requirements such as high wear resistance. Since, in principle, wear resistance is controlled by the hardness of the metallized layer, it would be beneficial to coat plastics with hard metal compounds to improve their wear resistance. A coated system of that type will be characterized by a combination of the properties of polymers and those of ceramic compounds. Such a composite may be very attractive to industry because of its virtually unique * Corresponding ‘Paper presented 0257~8972/97/$17.00
author. at the ICMCTF’96. 0 1997 Elsevier
PZI SO257-8972(96)03098-S
Science S.A. All rights reserved
properties (combination of high wear resistance, low density, and low cost). This will open new and innovative technical possibilities for the design of components with special thermal, chemical, and mechanical properties, which otherwise would have to be produced at high cost. Their potential fields of application are enormously diverse: decorative applications on a broad range of commercially significant products, medical and surgical parts, and engineering components such as gear wheels and rollers. For some of the applications a thin coating of up to 5 pm is required, while for others, such as rollers, thicker coatings (up to 2 mm) are needed. When depositing a hard metal compound on a polymer substrate, two main problems must be solved: (a) depositing coatings with a minimum of heat impact, so that the substrate will not be degraded; (b) assuring the adhesion of the coat layer. The difference in properties between the hard coating and the polymer causes stresses at the interface, leading to delimitation and crack formation. These factors can be substantially diminished by
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the deposition of an intermediate metallic layer, strongly bound to the polymeric substrate as well as to the coating [ 11. In the present work high velocity oxy-fuel (HVOF) spraying modified by CO, cooling was applied to deposit WC(Co) on the pre-metallized acrylonitrile butadiene styrene (ABS) substrate. This high-rate deposition technique provides a dense coating and produces coatings of thickness the order of a few tenths of a micron [24]. To obtain a dense coating with high adhesive strength the sprayed particles must be delivered to the cooled substrate at high temperatures and high velocities. Since the experimental determination of these values is both expensive and difhcult, a theoretical model describing the influence of the powder parameters on the dynamic and thermal state of the powder was developed. The main objective of this work was to study the effect of the process parameters on the microstructure and the composition of the coating in order to develop a new technology to obtain plastics coated with hardmetal compounds.
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before entering the combustion chamber. The hot gas jet leaving the combustion chamber was deflected and bundled by the nozzle. The average velocity of the jet outside the nozzle was about 1800 m s-l. The flame temperature at the nozzle outlet was 3500°C. The spraying powder was injected axially and centrally into the jet by a carrier gas (nitrogen). The average powder velocity in the jet was 850 m s-l, which is characteristic for the system used (Hypersonic Uni-Spray-Jet TopGun). The distance nozzle-to-substrate was 350 mm and the distance CO2 nozzle-substrate was 200 mm. The microstructure and morphology of the coatings were studied by scanning electron microscopy (SEM) and transmission electron microscopies (TEM), both combined with energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) was employed to the identify the phase, and Auger electron spectroscopy (AES) was employed for the study of the distribution of the elements in the coatings. 3. Results 3. I. Powder characteristics
2. Experimental
The WC(Co) powder consisted of agglomerated spherical particles of sizes ranging from 15 to 45 l.un (see Fig. 2) of composition: WC (88 wt.%) and Co (12 wt.%). The X-ray spectrum of the powder is shown in Fig. 3. The phase analysis indicates that the powder was mainly composed of WC phase. In addition, it contained elemental CL-CO,P-Co, a-W&, and traces of /?-WC1-x.
The polymeric substrate used for WC(Co) coatings was acrylonitrile butadiene styrene (ABS) (BASF, Terman@ KR 2889) metallized by electroplating [5]. The bond layer was composed of several metallic layers: Ni at the interface with the ABS, then Cu, and an Ni overlayer. The WC(Co) coatings were deposited by high-velocity oxy-fuel spraying modified by the addition of COZ gas flows (see Fig. 1). By regulating the COZ gas the substrate temperature could be lowered to below 100°C. Hydrogen supplied at a pressure of 6 bars served as the combustion gas. Oxygen was fed into the combustion chamber under high pressure, and the combustion gas was sucked into the oxygen jet and mixed there with
n
Cooling (COZ)
3.2. Characterization of tungsten carbide cobalt coating
The characteristic microstructure of the triplex system, WC(Co) coating, bond layer, and polymeric substrate is shown in Fig. 4. The composition of the coating and the intermediate layers was verified by means of EDS. gas
combustion chamber oxidant (02
I
I
II
A,
gas
) 11
powder mser t and carrier gas (N2)
1 II combustion gas (Hz) cooling
qas
(CO2)
nozzle
Fig. 1. Schematic presentation of the HVOF spraying modified by the addition of CO2 cooling.
I. Grimberg
Fig. 2. SEM micrograph powder.
et al. 1 Surface
showing the microstructure
and Coatings
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of WC(Co)
I
I
Fig. 4. (a) SEM micrograph of metallized ABS after WC(Co) coating; (b) high-magnification micrograph showing the morphology of the NijWC (Co) interface.
30
40
50
60 angle
70
80
90
28
Fig. 3. X-ray diffraction spectrum of the WC(Co) powder.
The bond layer is composed of three metallic layers: a Ni layer (- 1 urn thick) next to the interface with the polymer, a Cu layer (-20 urn thick), and an additional Ni layer (- 8 ,um thick). After the WC(Co) deposition the interface between the metallic layers remained smooth, and no degradation of the polymer was observed. As a result of the modmed spaying process coating/bond layer interface of different morphologies are formed. A wavy interface is shown in Fig. 4(b). The overheating of particles combined with their high velocities and the high cooling rates evolved in the spraying give rise to the formation of various phases and the development of several microstructures within the coating. Fig. 5 shows the X-ray diffraction spectrum of the WC(Co) coating. A mixture of phases is observed in the coating. The WC phase decomposes to a-W&, p-W& and P-WC, -x. The Co reacts with the tungsten carbide and Co3W,C is formed. Auger electron spectroscopy depth distribution profile for the investigated coating is presented in Fig. 6(a). The results reveal that tungsten, carbon, and cobalt are distributed homogeneously in the coating. The oxygen
content in the coating is lower than 3 at.%. Fig. 6(b) presents carbon KLL Auger diV(E)/dE mode spectra taken from the center of the coating. It can be seen that the prose of the carbon spectrum corresponded to that of carbon in the bonded state (tungsten carbide) [6]. This measurement indicates that there is no carbon loss during the spraying. A typical microstructure of the WC(Co) coating is shown in Fig. 7. The WC grain (marked A) is surrounded by the binder phase (marked B). The latter is composed of a mixture of amorphous and nanocrystalline Co. The grain size of the Co is less than 3 nm. This structure results from the rapid solidification rate from the liquid state. The microstructure of the Co binder depends on the cooling rate. Fig. 8 shows the formation of microcrystalline Co as a result of a lower cooling rate than that shown in Fig. 7. The binder phase which surrounds the WC grains was also found to be a mixture of two phases, viz. /?-WC1-, and /?-Co, as presented in Fig. 9. A WC grain (marked A) is surrounded by the microcrystalline binder phase (marked B). During the powder processing and spraying a fraction of the WC particles decompose. Fig. 10 represents a typical micrograph showing the WC decomposition. Three regions marked A, B, and C are seen. In region A, the Co binder surrounding the WC grains is seen. In region B, a WC grain is presented. In region C, W& has been identified. The decomposition of the WC to W2C occurs at the surface of the WC particle to a thickness of several tenths of a nanometer. The appear-
I. Grimberg
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a
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85
I 6
t
-‘C.__
I
(,,,-,.,.
lb
0
W A.“‘-“”
-
C
4 .. IA
I 3
t 6
I
1
---,,‘~--=Q/
//,-c-----
c3 2
WC(Co)
/----“.L-,
..,.-.,,-
,..(-““-
-WC..,_._ , 9
SPUTTER
1-1 12
15
TIME
I
18
21
I 24
( MIN.)
7 b 6
b
a-w,c
8
(101). p-W,C
(121) (102)
9
Co3W3C
(422)
10
Co3W&
(51 I)
11
P-WC,.,
WJ)
(b)
KINETIC ENERGY,
EV
Fig. 6. (a) Auger electron spectroscopy profile of the WC(Co) coating; (b) Carbon KLL Auger dN(E)/dE mode spectrum measured from the WC(C0) coating.
tion heat transfer being taken into account [7,8]. The equation is of the form
(1) 21
I a-W$
22 23
WC(ll1) WC (200), Cl-W2C (I 12), p-W&Z
24
a-W&
25 26
1
(201), p-w,c
1
29
(302) (240) (223)
(142) (104)
WC (102) 8-W&
27 28
WC (002) (103), fNV&(321)
WJ)
where pP is the density, cP is the heat capacity at constant pressure, A, is the thermal conductivity, T is the absolute temperature, and rP is the radius of the powder particle. The particle temperature is a function of tune, z, and radial coordinate, I’. The subscript p refers to the particle and f to the flame flow. Eq. (1) is supplemented by the following boundary conditions:
WC (201) p-w,c (204)
(2)
Fig. 5. (a) X-ray diffraction spectrum of the WC(Co) coating; (b) analysis of the diffraction spectrum.
ante of the network of dislocations in the WpC grain can be attributed to volume changes following the decomposition of the WC phase to W2C. 3.3. Heat transfer momentum velocity oxy-fuel spraying
transfer models for high-
3.3.1. The heat transfer model The thermal history of a particle in the flame flow is determined by the heat conduction equation, the radia-
%, ~(rP,*)=q,[T~-T(rp,?)]+e~SB(T~-T;) ai
(3)
where Tf is the absolute temperature of the medium in which the particle is suspended and which in principle also depends on the spatial variables r and Z, LX, is the heat transfer coefficient, E is the particle emissivity, and cSB is the Stephan-Boltzmann constant. Boundary condition Eq. (2) follows from the spherical symmetry of the particle. Boundary condition Eq. (3) states that the heat flux through the particle surface is proportional to the difference between the temperature
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et al. 1 Surface
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a
Ring’s number 1 2 3 4 5
I
Fig. 8. TEM micrograph showing microcrystalline Co binder.
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C
Fig. 7. (a) Typical structure of a WC(Co) coating. The WC grains are surrounded by amorphous and nanocrystahine Co. (b) Diffraction pattern of the WC grain; (c) diffraction pattern of Co binder.
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Phases p-wqm, (I 11) co (Ill), @-WC,-, (200) co (ZOO) p-wcl+ (220) co (2201, p-wclex (311) co (311) P-WC,& (331)
Fig. 9. (a) TEM micrograph of the WC(Co) coating. The WC grain is surrounded by /?-WC1-x and /?-Co. (b) Diffraction pattern of /?-WC,-r and P-Co; (c) analysis of the diffraction pattern.
Fig. 10. (a) TEM micrograph showing the partial decomposition of WC grain W&.
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et al. /Surface
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of the particle surface and the local temperature of the gas flow with the PrOpOrtiOnafity Coefficient c(h, The initial condition: T(r,O) = T,,
(5)
where dP is the particle diameter, Reynolds number, Re,
dP=2~,,
and by the
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i.e. Z(T) = Jo’ V,(z) dz
(12)
(6)
where V, is the flame flow velocity and V, is the particle velocity. The bars over Af and pLf in Eq. (5) and Eq. (6) denote, respectively, the values of thermal conductivity and viscosity of the fluid averaged over a temperature interval [8]. The forced-convection heat transfer is described as the correlation between the Nusselt and the Reynolds numbers, e.g., the Ranz-Marshall equation 171:
N, = 2 + O.GRe*Pr* where the Prandtl
and introducing the effective convective-radiative transfer coefficient, A
heat
A=aiz+~osB(T;+T;Tf+TpT;+T;)
(14)
Eq. (1) converts into
where CD is the drag coefficient. When 0.15
-0.5Re-‘,I)
d TP = - A(T, - Tf)
(15)
x
The solution of Eq. (15) is Tp=Tf+(Tpo-Tf)exp
In order to use this formal solution for the calculation of real changes in the particle temperature in the hightemperature flow, the following procedure was adopted. At a first step the heat exchange coefficient, A, is calculated as
(8)
(9)
V,(O) = VP0
(13)
Assuming that the time z = AZ is small, the exponent in Eq. (16) can be expanded into a power series. Taking into account first the two terms yields:
From the physical point of view the dynamic behaviour of the particle is described as a problem of the motion of the particle under a Stokes viscous force. The equation of motion of a spherical particle in a viscous flow is of the form
condition
(T;-T;)=(T,-T,)(T;+T~T,+T,T;+T;)
A = a,, + EoSB(T;, + T;,, Tf + T,, T$ + T;)
3.3.2. The momentum transfer model
Re
The solution of Eq. (1) is based on a linearization procedure for the nonlinear term in Eq. (3) [9]. Presenting it in the form
(7)
number, Pr, is defined as:
Pr = cfp’s3Lf-1
23.707
3.3.3. Numerical solution
PPCPl’P
Re = dPimP - - Vf)
The initial
90 (1997)
(4)
dei?nes the temperature of the particle at the initial moment (~=0). Since the velocities of the particle and of the flame flow differ, the heat transfer is effected through forced convection. Two basic characteristics are used to describe the heat transfer by forced convection: the Nusselt number, N,, representing a dimensionless heat flux defined by
CD=-
Technology
(10)
for Eq. (9) at z = 0 is: (11)
The axial location, z, of the particle at any given moment can be obtained by integrating the solution of Eq. (9),
(17)
A Tp = Tpo - Vpo - Tf) -AT+...
(18)
PPCPYP
The time step AZ can always be chosen small enough to minimize the error introduced by using TpO in calculating A instead of the real value of Tp. In practice, Ar is decreased during numerical calculations until the temperature become insensitive to its value. The numerical procedure is implemented in a stepwise manner T$) = Tf + (pi - ‘) - T,) exp
(
-
&;I
(19)
Together with the standard solution of Eq. (9), i.e., vp = vj” + 17.78(1 +O.l65Re* -0.5Re-‘.‘)G exp ( PPG Solution
(20)
z 1
Eq. (20) is treated in the same manner
as
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et al. /Surface
and Coatings
solution Eq. (19). Since powder particles move a field of temperatures ranging from 20 to 35OO”C, the dependence of the transport coefficients on the temperature must be taken into account. The temperature dependence of the thermal conductivity and dynamic viscosity of the gas flow were taken from [ 10,l I]. The composite particles consisted of WC and Co. The melting point of Co equals 1495°C [12], so that when the temperature of a particle reaches that temperature, its growth is halted and the energy delivered from the environment is consumed for melting. This is taken into account in the numerical procedure. From the moment, Tp = Tmco > the particle temperature remains constant, and the heat flux delivered to the particle is summed up yielding the energy absorbed by the particle, which equals that required to melt cobalt, i.e., t
A(T,-
T,)Az,, = %
d,q,
(21)
i=l
where Cc, is the cobalt fraction (12 wt.%) and qm is the latent heat of fusion of cobalt. When condition Eq. (21) is satisfied, the numerical procedure according to Eq. (19) is applied until the particle impacts on the substrate. The density of WC(Co) was taken as 14.32 x lo3 kg mm3 [ 131. The velocity of the particle depends on its size. The correlation between particle size and velocity is shown in Fig. 11. The velocity of particles at the impact with the bond layer varies between 900 and 300m s-l for particle sizes of 15 and 45 pm, respectively. Fig. 12 presents the temperature of particles of different sizes. That temperature varies between 3000 and 1500°C for particle sizes of 15 and 45 p, respectively. In the present calculation the radiation heat transfer is included. Its influence on the large particles is stronger than on the small ones. Its contribution to the 45 pm particles seems to be specially important. The calculation shows that the particle temperature on the substrate is 1484”C, i.e. it is below the melting temperature of cobalt (1495”(Z), when emissivity is equal zero.
particle Fig. 11. The velocity different sizes.
of a particle
coordinate, as a function
m of its position
for
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I
0.1
I
I
I
,
0.2 particle
Fig. 12. The temperature of a particle different sizes at an emissivity of 0.3.
I
0.3 coordinate,
0.4
(
m
as a function
of its position
for
At an emissivity of 0.3 (which appears to be realistic) the particle temperature at the substrate is 15OO”C, i.e., higher than the melting temperature of Co. 4. Discussion The mathematical model describing particle behaviour during HVOF spraying showed that the particles’ velocities and temperatures are connected with their size and density. The calculations showed that particles of small size arrive at the substrate at higher velocities and temperatures than the bigger ones. According to the X-ray data the spraying powder was composed mainly of WC. In addition, it contained the a-W& phase and elemental u- and P-Co. Traces of ,B-WC,-, were also identified in the powder. The phases observed in the coating were identified as: WC, u-W& p-W&, /3-WC1-x, P-Co, a-Co, and Co,W,C. As a result of the partial decomposition of the WC phase, and the volume fractions of the W& and the p-WC1-, phases increased, and new phases, not present in the original powder, were formed. The decomposition occurred at the surface of the WC grains to a thickness of several tenths of a nanometer. The coating thus consists not only of thermodynamically stable, but also of metastable phases. The presence of the latter can be explained by the high cooling rate of the deposited particles. The typical microstructure of the WC(Co) coating was found to be composed of tungsten carbide grains surrounded by Co binder, the structure of which is heterogeneous. There are regions where the Co phase is of an amorphous and nanocrystalline structure (Fig. 7) and others where the Co phase is microcrystalline (Fig. 8). The formation of these different structures can be explained by different solidification rates. According to the theoretical model the particles’ temperature varied between 3000 and 1500°C and the velocity between 900 and 300 m s-l, for particles of sizes 15 and 45 Frn,
I. Grimberg
et al. / Surface
and Coatings
respectively. In other words, the smaller particles reach high temperatures and strike the substrate at high speeds, which results in a high solid&cation rate and leads to the formation of amorphous or nanocrystalline structure. As to the larger particles, in consequence of their lower temperatures and velocities, their solidification rate is lower, resulting in the formation of a microcrystalline structure. Another factor that controls the formation of the microstructure is the thermal behaviour within the coating during the deposition. The coating is built up by the progressive deposition of splats that produce a continuous layer. A deposited splat increases the temperature in the previously deposited layer and may cause nucleation and grain growth within the Co binder [ 14,151. TEM analysis showed that the ,B-WC,-, phase is polycrystalline with grains of various sizes and in some regions it is found within the binder phase. According to the mathematical model, particles of size 15 um are heated to 3000°C. The melting temperature of tungsten carbide being 2870 + 50°C [ 121, the external surface of these small particles can melt during the particles’ flight in the gun and the combustion chamber. The resulting liquid phase contains Co, W, and C. The rapid cooling rate at the moment of impact leads to the nucleation of p-WC,-, with a nanocrystalline structure within the areas surrounding the carbides. The ternary compound, Co,W,C, can precipitate from the liquid mentioned, or it can be formed from a reaction between the Co and WC. The effect of the variation in the temperature of the deposited particles is that some of the Co particles strike the substrate while they are in the liquid state, while others (a fraction of the 45 pm particles) are solid. This results in different interface morphologies, binder fraction, and pore densities. The mathematical model has shown that the particles strike the substrate with a high force. The density of the tungsten carbide particles is higher than the liquid Co phase. Therefore, when a WC particle in the splat strikes the substrate or the previously deposited layer, the Co liquid is squeezed out, and direct contact is made between the WC particles and the Ni bond layer. A wavy interface is assumed to be result of the high-energy impact of heavy solid tungsten carbide particles, which caused plastic deformation to the relatively “soft” nickel bond layer. The wavy interface provides mechanical interlocking and thus improves the adhesive strength of the WC(Co) coating. The quality of the HVOF sprayed coating, especially as regards porosity, mechanical properties, and adhesive strength, is largely dependent on the extent of particle melting and particle velocity at the moment of impact on the substrate. The control of the temperature during the spraying process is very important. The higher the temperature of the Co binder above its melting temperature, the denser the coating and the higher the adhesive
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strength. The viscosity of the particles at the moment of their impingement also strongly influences the coating quality, since the lower the viscosity of the cobalt, the better the particles will splat on the substrate. Also, splats impinging on previously deposited particles will more readily accommodate all the irregularities of the surface and thus provide a denser coating. However, a high temperature may result in the degradation of the polymeric substrate, may cause ternary compounds to form, and may even lead to the decomposition of the carbides and the appearance of elemental tungsten and graphite [16-191. The decomposition of tungsten carbide is thought to proceed in three stages [4]: (1) 2wc -+ w,c+c (2) w,c+ l/20, --f W,(C,O) (3) W,(C,O) -+ 2w+co The degree of the powder’s resistance to decarburization depends on the amount of WC or W,C in the powder and on the oxygen content. Decomposition is much more intense in the case of W,C, since only the last two decomposition stages are required. Due to the CO, cooling the quantity of oxygen in the contact zone and in the coating is low. As a result carbide decomposition is reduced.
5. Summary and conclusions
A triplex system consisting of WC(Co) coating, Ni/Cu/Ni bond layer, and acrylonitrile butadiene styrene (ABS) substrate was investigated. The main conclusions are summarized as follows: (1) During high velocity oxy-fuel spraying a fraction of the WC phase partially decomposes into a-W&, /3-W&, p-WC,-X, and the ternary compound, Co,W$. (2) The microstructure of the WC(Co) coating is typical for high-velocity oxy-fuel spraying and is composed of tungsten carbide particles bonded by a cobalt matrix binder. Traces of P-WC-, were also identified in the binder phase. The microstructure of the Co binder depends on the cooling rate and on the thermal behaviour within the coating. The small particles have a high solidification rate, resulting in the formation of an amorphous and nanocrystalline structure. The lower solidification rates of the big particles (45 urn) result in the formation of a microcrystalline structure. In addition, the thermal interaction between neighboring layers can cause nucleation and grain growth.
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The variation in the particles’ temperatures and velocities results in the formation of different interface morphologies, variations in the binder phase fraction, and pore density within the coating. A high temperature and velocity of the particles reduces the pore density and leads to the formation of a wavy interface, which provides mechanical interlocking and improves adhesion. A mathematical model for calculating the velocity (4) and the temperature distribution of the powder particles in high velocity oxy-fuel spraying was developed. The model accounts for the heating of the particles by hydrogen-oxygen combustion and their cooling by the carbon dioxide flow added to the flame flow. It addition, it takes account of the contribution of the radiation to the heat transfer. The effect of radiation heat transfer is an important factor in the case of large particles. To the authors’ best knowledge this is the first time that radiation heat transfer has been applied in the simulation of the HVOF spraying process. The dependence of particle velocity and temperature (5) on particle size is consistent with the results of other models known from the literature. An increase in the particle size leads to a reduction in the particle velocity and temperature. (6) For WC(Co) particles with size distribution between 15 and 45 pm the velocity varies between 900 and 300 m s-l and the temperature between 3000 and 15OO”C, respectively. The experimental and the theoretical results show (7) that WC(Co) can be deposited on polymers, provided that there is an adherent metallic bond layer between the coating and the plastic substrate and the deposition temperature is controlled. The control of the particles’ temperature and velocity in the combustion chamber and upon impact on the substrate was found to be crucial for obtaining coatings with high adhesive strength. (3)
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Acknowledgement
The financial support of the Ministry of Science and Art of Niedersachsen is sincerely appreciated.
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Tungsten carbide coatings deposited by high-velocity oxy-fuel spraying on a metallized polymeric substrate’ I. Grimberg a$*, K. Soifer a, B. Bouaifi b, U. Draugelates b, B.Z. Weiss a a Department of Matesials Engineering, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel b Institute fkr Schweisstechnik ttnd Trennende Fertigungsuerfahren, Technical University Clausthal, 38678 Clausthal-Zellerfeld, Germany
Received17May 1996;accepted12August 1996
Abstract A triplex systemconsistingof WC(Co) hard coating, metallic bond layers (Ni/Cu/Ni), and acrylonitrile-butadiene-styreneas substrate,was investigated. The WC (88 wt.%)-Co (12 wt.%) coating was depositedby the high-velocity oxy-fuel spraying techniquemodified by the addition of COZgasflows. The microstructure,the composition,and the phasespresentin the coating were studiedby meansof transmissionand scanningelectronmicroscopiesboth combinedwith energydispersiveanalysis,X-ray diffraction, and Auger electron spectroscopy.It was found that during the spraying a fraction of the WC phasepartially decomposes into cc-and jSW,C and /?-WC,-x and reactswith cobalt to form the ternary carbide,Co,W,C. The decomposition occurred on the surfaceof the WC particle to a thicknessof severaltenths of a nanometer.The microstructure of the coating consistedof tungstencarbideparticlesbondedby a binder matrix composedmainly of Co. The microstructureof the binder phase was found to be dependenton the cooling rate. As a result of the variation in the particles’velocitiesand temperatures,different interface morphologiesaswell asdifferent fractions of binder phaseand pore densitieswereobservedin the coating. A theoretical model describingthe dynamic and the thermal performanceof the powder particles in high-velocity oxy-fuel spraying was developed,correlating betweenthe particles’properties(size, density, emissivity)and their velocity and temperature.The factors that influencethe powder velocity and temperatureand thereby the coating microstructureand compositionare discussed. Keywords:
Tungstencarbidecoatings;Polymer; High-velocity oxy-fuel spraying;Microstructure; Mathematical model
1. Introduction
Metallization of plastics is a well-established commercial process that is used in a variety of applications, ranging from microelectronics (for packaging devices) to the aircraft industries. The metals used in plastic metallization are nickel, chromium, copper, and zinc. In many cases these materials do not meet special technical requirements such as high wear resistance. Since, in principle, wear resistance is controlled by the hardness of the metallized layer, it would be beneficial to coat plastics with hard metal compounds to improve their wear resistance. A coated system of that type will be characterized by a combination of the properties of polymers and those of ceramic compounds. Such a composite may be very attractive to industry because of its virtually unique * Corresponding ‘Paper presented 0257~8972/97/$17.00
author. at the ICMCTF’96. 0 1997 Elsevier
PZI SO257-8972(96)03098-S
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properties (combination of high wear resistance, low density, and low cost). This will open new and innovative technical possibilities for the design of components with special thermal, chemical, and mechanical properties, which otherwise would have to be produced at high cost. Their potential fields of application are enormously diverse: decorative applications on a broad range of commercially significant products, medical and surgical parts, and engineering components such as gear wheels and rollers. For some of the applications a thin coating of up to 5 pm is required, while for others, such as rollers, thicker coatings (up to 2 mm) are needed. When depositing a hard metal compound on a polymer substrate, two main problems must be solved: (a) depositing coatings with a minimum of heat impact, so that the substrate will not be degraded; (b) assuring the adhesion of the coat layer. The difference in properties between the hard coating and the polymer causes stresses at the interface, leading to delimitation and crack formation. These factors can be substantially diminished by
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the deposition of an intermediate metallic layer, strongly bound to the polymeric substrate as well as to the coating [ 11. In the present work high velocity oxy-fuel (HVOF) spraying modified by CO, cooling was applied to deposit WC(Co) on the pre-metallized acrylonitrile butadiene styrene (ABS) substrate. This high-rate deposition technique provides a dense coating and produces coatings of thickness the order of a few tenths of a micron [24]. To obtain a dense coating with high adhesive strength the sprayed particles must be delivered to the cooled substrate at high temperatures and high velocities. Since the experimental determination of these values is both expensive and difhcult, a theoretical model describing the influence of the powder parameters on the dynamic and thermal state of the powder was developed. The main objective of this work was to study the effect of the process parameters on the microstructure and the composition of the coating in order to develop a new technology to obtain plastics coated with hardmetal compounds.
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before entering the combustion chamber. The hot gas jet leaving the combustion chamber was deflected and bundled by the nozzle. The average velocity of the jet outside the nozzle was about 1800 m s-l. The flame temperature at the nozzle outlet was 3500°C. The spraying powder was injected axially and centrally into the jet by a carrier gas (nitrogen). The average powder velocity in the jet was 850 m s-l, which is characteristic for the system used (Hypersonic Uni-Spray-Jet TopGun). The distance nozzle-to-substrate was 350 mm and the distance CO2 nozzle-substrate was 200 mm. The microstructure and morphology of the coatings were studied by scanning electron microscopy (SEM) and transmission electron microscopies (TEM), both combined with energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) was employed to the identify the phase, and Auger electron spectroscopy (AES) was employed for the study of the distribution of the elements in the coatings. 3. Results 3. I. Powder characteristics
2. Experimental
The WC(Co) powder consisted of agglomerated spherical particles of sizes ranging from 15 to 45 l.un (see Fig. 2) of composition: WC (88 wt.%) and Co (12 wt.%). The X-ray spectrum of the powder is shown in Fig. 3. The phase analysis indicates that the powder was mainly composed of WC phase. In addition, it contained elemental CL-CO,P-Co, a-W&, and traces of /?-WC1-x.
The polymeric substrate used for WC(Co) coatings was acrylonitrile butadiene styrene (ABS) (BASF, Terman@ KR 2889) metallized by electroplating [5]. The bond layer was composed of several metallic layers: Ni at the interface with the ABS, then Cu, and an Ni overlayer. The WC(Co) coatings were deposited by high-velocity oxy-fuel spraying modified by the addition of COZ gas flows (see Fig. 1). By regulating the COZ gas the substrate temperature could be lowered to below 100°C. Hydrogen supplied at a pressure of 6 bars served as the combustion gas. Oxygen was fed into the combustion chamber under high pressure, and the combustion gas was sucked into the oxygen jet and mixed there with
n
Cooling (COZ)
3.2. Characterization of tungsten carbide cobalt coating
The characteristic microstructure of the triplex system, WC(Co) coating, bond layer, and polymeric substrate is shown in Fig. 4. The composition of the coating and the intermediate layers was verified by means of EDS. gas
combustion chamber oxidant (02
I
I
II
A,
gas
) 11
powder mser t and carrier gas (N2)
1 II combustion gas (Hz) cooling
qas
(CO2)
nozzle
Fig. 1. Schematic presentation of the HVOF spraying modified by the addition of CO2 cooling.
I. Grimberg
Fig. 2. SEM micrograph powder.
et al. 1 Surface
showing the microstructure
and Coatings
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of WC(Co)
I
I
Fig. 4. (a) SEM micrograph of metallized ABS after WC(Co) coating; (b) high-magnification micrograph showing the morphology of the NijWC (Co) interface.
30
40
50
60 angle
70
80
90
28
Fig. 3. X-ray diffraction spectrum of the WC(Co) powder.
The bond layer is composed of three metallic layers: a Ni layer (- 1 urn thick) next to the interface with the polymer, a Cu layer (-20 urn thick), and an additional Ni layer (- 8 ,um thick). After the WC(Co) deposition the interface between the metallic layers remained smooth, and no degradation of the polymer was observed. As a result of the modmed spaying process coating/bond layer interface of different morphologies are formed. A wavy interface is shown in Fig. 4(b). The overheating of particles combined with their high velocities and the high cooling rates evolved in the spraying give rise to the formation of various phases and the development of several microstructures within the coating. Fig. 5 shows the X-ray diffraction spectrum of the WC(Co) coating. A mixture of phases is observed in the coating. The WC phase decomposes to a-W&, p-W& and P-WC, -x. The Co reacts with the tungsten carbide and Co3W,C is formed. Auger electron spectroscopy depth distribution profile for the investigated coating is presented in Fig. 6(a). The results reveal that tungsten, carbon, and cobalt are distributed homogeneously in the coating. The oxygen
content in the coating is lower than 3 at.%. Fig. 6(b) presents carbon KLL Auger diV(E)/dE mode spectra taken from the center of the coating. It can be seen that the prose of the carbon spectrum corresponded to that of carbon in the bonded state (tungsten carbide) [6]. This measurement indicates that there is no carbon loss during the spraying. A typical microstructure of the WC(Co) coating is shown in Fig. 7. The WC grain (marked A) is surrounded by the binder phase (marked B). The latter is composed of a mixture of amorphous and nanocrystalline Co. The grain size of the Co is less than 3 nm. This structure results from the rapid solidification rate from the liquid state. The microstructure of the Co binder depends on the cooling rate. Fig. 8 shows the formation of microcrystalline Co as a result of a lower cooling rate than that shown in Fig. 7. The binder phase which surrounds the WC grains was also found to be a mixture of two phases, viz. /?-WC1-, and /?-Co, as presented in Fig. 9. A WC grain (marked A) is surrounded by the microcrystalline binder phase (marked B). During the powder processing and spraying a fraction of the WC particles decompose. Fig. 10 represents a typical micrograph showing the WC decomposition. Three regions marked A, B, and C are seen. In region A, the Co binder surrounding the WC grains is seen. In region B, a WC grain is presented. In region C, W& has been identified. The decomposition of the WC to W2C occurs at the surface of the WC particle to a thickness of several tenths of a nanometer. The appear-
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a
and Coatings
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I 6
t
-‘C.__
I
(,,,-,.,.
lb
0
W A.“‘-“”
-
C
4 .. IA
I 3
t 6
I
1
---,,‘~--=Q/
//,-c-----
c3 2
WC(Co)
/----“.L-,
..,.-.,,-
,..(-““-
-WC..,_._ , 9
SPUTTER
1-1 12
15
TIME
I
18
21
I 24
( MIN.)
7 b 6
b
a-w,c
8
(101). p-W,C
(121) (102)
9
Co3W3C
(422)
10
Co3W&
(51 I)
11
P-WC,.,
WJ)
(b)
KINETIC ENERGY,
EV
Fig. 6. (a) Auger electron spectroscopy profile of the WC(Co) coating; (b) Carbon KLL Auger dN(E)/dE mode spectrum measured from the WC(C0) coating.
tion heat transfer being taken into account [7,8]. The equation is of the form
(1) 21
I a-W$
22 23
WC(ll1) WC (200), Cl-W2C (I 12), p-W&Z
24
a-W&
25 26
1
(201), p-w,c
1
29
(302) (240) (223)
(142) (104)
WC (102) 8-W&
27 28
WC (002) (103), fNV&(321)
WJ)
where pP is the density, cP is the heat capacity at constant pressure, A, is the thermal conductivity, T is the absolute temperature, and rP is the radius of the powder particle. The particle temperature is a function of tune, z, and radial coordinate, I’. The subscript p refers to the particle and f to the flame flow. Eq. (1) is supplemented by the following boundary conditions:
WC (201) p-w,c (204)
(2)
Fig. 5. (a) X-ray diffraction spectrum of the WC(Co) coating; (b) analysis of the diffraction spectrum.
ante of the network of dislocations in the WpC grain can be attributed to volume changes following the decomposition of the WC phase to W2C. 3.3. Heat transfer momentum velocity oxy-fuel spraying
transfer models for high-
3.3.1. The heat transfer model The thermal history of a particle in the flame flow is determined by the heat conduction equation, the radia-
%, ~(rP,*)=q,[T~-T(rp,?)]+e~SB(T~-T;) ai
(3)
where Tf is the absolute temperature of the medium in which the particle is suspended and which in principle also depends on the spatial variables r and Z, LX, is the heat transfer coefficient, E is the particle emissivity, and cSB is the Stephan-Boltzmann constant. Boundary condition Eq. (2) follows from the spherical symmetry of the particle. Boundary condition Eq. (3) states that the heat flux through the particle surface is proportional to the difference between the temperature
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Ring’s number 1 2 3 4 5
I
Fig. 8. TEM micrograph showing microcrystalline Co binder.
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Fig. 7. (a) Typical structure of a WC(Co) coating. The WC grains are surrounded by amorphous and nanocrystahine Co. (b) Diffraction pattern of the WC grain; (c) diffraction pattern of Co binder.
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Phases p-wqm, (I 11) co (Ill), @-WC,-, (200) co (ZOO) p-wcl+ (220) co (2201, p-wclex (311) co (311) P-WC,& (331)
Fig. 9. (a) TEM micrograph of the WC(Co) coating. The WC grain is surrounded by /?-WC1-x and /?-Co. (b) Diffraction pattern of /?-WC,-r and P-Co; (c) analysis of the diffraction pattern.
Fig. 10. (a) TEM micrograph showing the partial decomposition of WC grain W&.
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of the particle surface and the local temperature of the gas flow with the PrOpOrtiOnafity Coefficient c(h, The initial condition: T(r,O) = T,,
(5)
where dP is the particle diameter, Reynolds number, Re,
dP=2~,,
and by the
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i.e. Z(T) = Jo’ V,(z) dz
(12)
(6)
where V, is the flame flow velocity and V, is the particle velocity. The bars over Af and pLf in Eq. (5) and Eq. (6) denote, respectively, the values of thermal conductivity and viscosity of the fluid averaged over a temperature interval [8]. The forced-convection heat transfer is described as the correlation between the Nusselt and the Reynolds numbers, e.g., the Ranz-Marshall equation 171:
N, = 2 + O.GRe*Pr* where the Prandtl
and introducing the effective convective-radiative transfer coefficient, A
heat
A=aiz+~osB(T;+T;Tf+TpT;+T;)
(14)
Eq. (1) converts into
where CD is the drag coefficient. When 0.15
-0.5Re-‘,I)
d TP = - A(T, - Tf)
(15)
x
The solution of Eq. (15) is Tp=Tf+(Tpo-Tf)exp
In order to use this formal solution for the calculation of real changes in the particle temperature in the hightemperature flow, the following procedure was adopted. At a first step the heat exchange coefficient, A, is calculated as
(8)
(9)
V,(O) = VP0
(13)
Assuming that the time z = AZ is small, the exponent in Eq. (16) can be expanded into a power series. Taking into account first the two terms yields:
From the physical point of view the dynamic behaviour of the particle is described as a problem of the motion of the particle under a Stokes viscous force. The equation of motion of a spherical particle in a viscous flow is of the form
condition
(T;-T;)=(T,-T,)(T;+T~T,+T,T;+T;)
A = a,, + EoSB(T;, + T;,, Tf + T,, T$ + T;)
3.3.2. The momentum transfer model
Re
The solution of Eq. (1) is based on a linearization procedure for the nonlinear term in Eq. (3) [9]. Presenting it in the form
(7)
number, Pr, is defined as:
Pr = cfp’s3Lf-1
23.707
3.3.3. Numerical solution
PPCPl’P
Re = dPimP - - Vf)
The initial
90 (1997)
(4)
dei?nes the temperature of the particle at the initial moment (~=0). Since the velocities of the particle and of the flame flow differ, the heat transfer is effected through forced convection. Two basic characteristics are used to describe the heat transfer by forced convection: the Nusselt number, N,, representing a dimensionless heat flux defined by
CD=-
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(10)
for Eq. (9) at z = 0 is: (11)
The axial location, z, of the particle at any given moment can be obtained by integrating the solution of Eq. (9),
(17)
A Tp = Tpo - Vpo - Tf) -AT+...
(18)
PPCPYP
The time step AZ can always be chosen small enough to minimize the error introduced by using TpO in calculating A instead of the real value of Tp. In practice, Ar is decreased during numerical calculations until the temperature become insensitive to its value. The numerical procedure is implemented in a stepwise manner T$) = Tf + (pi - ‘) - T,) exp
(
-
&;I
(19)
Together with the standard solution of Eq. (9), i.e., vp = vj” +
(20)
z 1
Eq. (20) is treated in the same manner
as
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and Coatings
solution Eq. (19). Since powder particles move a field of temperatures ranging from 20 to 35OO”C, the dependence of the transport coefficients on the temperature must be taken into account. The temperature dependence of the thermal conductivity and dynamic viscosity of the gas flow were taken from [ 10,l I]. The composite particles consisted of WC and Co. The melting point of Co equals 1495°C [12], so that when the temperature of a particle reaches that temperature, its growth is halted and the energy delivered from the environment is consumed for melting. This is taken into account in the numerical procedure. From the moment, Tp = Tmco > the particle temperature remains constant, and the heat flux delivered to the particle is summed up yielding the energy absorbed by the particle, which equals that required to melt cobalt, i.e., t
A(T,-
T,)Az,, = %
d,q,
(21)
i=l
where Cc, is the cobalt fraction (12 wt.%) and qm is the latent heat of fusion of cobalt. When condition Eq. (21) is satisfied, the numerical procedure according to Eq. (19) is applied until the particle impacts on the substrate. The density of WC(Co) was taken as 14.32 x lo3 kg mm3 [ 131. The velocity of the particle depends on its size. The correlation between particle size and velocity is shown in Fig. 11. The velocity of particles at the impact with the bond layer varies between 900 and 300m s-l for particle sizes of 15 and 45 pm, respectively. Fig. 12 presents the temperature of particles of different sizes. That temperature varies between 3000 and 1500°C for particle sizes of 15 and 45 p, respectively. In the present calculation the radiation heat transfer is included. Its influence on the large particles is stronger than on the small ones. Its contribution to the 45 pm particles seems to be specially important. The calculation shows that the particle temperature on the substrate is 1484”C, i.e. it is below the melting temperature of cobalt (1495”(Z), when emissivity is equal zero.
particle Fig. 11. The velocity different sizes.
of a particle
coordinate, as a function
m of its position
for
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0.1
I
I
I
,
0.2 particle
Fig. 12. The temperature of a particle different sizes at an emissivity of 0.3.
I
0.3 coordinate,
0.4
(
m
as a function
of its position
for
At an emissivity of 0.3 (which appears to be realistic) the particle temperature at the substrate is 15OO”C, i.e., higher than the melting temperature of Co. 4. Discussion The mathematical model describing particle behaviour during HVOF spraying showed that the particles’ velocities and temperatures are connected with their size and density. The calculations showed that particles of small size arrive at the substrate at higher velocities and temperatures than the bigger ones. According to the X-ray data the spraying powder was composed mainly of WC. In addition, it contained the a-W& phase and elemental u- and P-Co. Traces of ,B-WC,-, were also identified in the powder. The phases observed in the coating were identified as: WC, u-W& p-W&, /3-WC1-x, P-Co, a-Co, and Co,W,C. As a result of the partial decomposition of the WC phase, and the volume fractions of the W& and the p-WC1-, phases increased, and new phases, not present in the original powder, were formed. The decomposition occurred at the surface of the WC grains to a thickness of several tenths of a nanometer. The coating thus consists not only of thermodynamically stable, but also of metastable phases. The presence of the latter can be explained by the high cooling rate of the deposited particles. The typical microstructure of the WC(Co) coating was found to be composed of tungsten carbide grains surrounded by Co binder, the structure of which is heterogeneous. There are regions where the Co phase is of an amorphous and nanocrystalline structure (Fig. 7) and others where the Co phase is microcrystalline (Fig. 8). The formation of these different structures can be explained by different solidification rates. According to the theoretical model the particles’ temperature varied between 3000 and 1500°C and the velocity between 900 and 300 m s-l, for particles of sizes 15 and 45 Frn,
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et al. / Surface
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respectively. In other words, the smaller particles reach high temperatures and strike the substrate at high speeds, which results in a high solid&cation rate and leads to the formation of amorphous or nanocrystalline structure. As to the larger particles, in consequence of their lower temperatures and velocities, their solidification rate is lower, resulting in the formation of a microcrystalline structure. Another factor that controls the formation of the microstructure is the thermal behaviour within the coating during the deposition. The coating is built up by the progressive deposition of splats that produce a continuous layer. A deposited splat increases the temperature in the previously deposited layer and may cause nucleation and grain growth within the Co binder [ 14,151. TEM analysis showed that the ,B-WC,-, phase is polycrystalline with grains of various sizes and in some regions it is found within the binder phase. According to the mathematical model, particles of size 15 um are heated to 3000°C. The melting temperature of tungsten carbide being 2870 + 50°C [ 121, the external surface of these small particles can melt during the particles’ flight in the gun and the combustion chamber. The resulting liquid phase contains Co, W, and C. The rapid cooling rate at the moment of impact leads to the nucleation of p-WC,-, with a nanocrystalline structure within the areas surrounding the carbides. The ternary compound, Co,W,C, can precipitate from the liquid mentioned, or it can be formed from a reaction between the Co and WC. The effect of the variation in the temperature of the deposited particles is that some of the Co particles strike the substrate while they are in the liquid state, while others (a fraction of the 45 pm particles) are solid. This results in different interface morphologies, binder fraction, and pore densities. The mathematical model has shown that the particles strike the substrate with a high force. The density of the tungsten carbide particles is higher than the liquid Co phase. Therefore, when a WC particle in the splat strikes the substrate or the previously deposited layer, the Co liquid is squeezed out, and direct contact is made between the WC particles and the Ni bond layer. A wavy interface is assumed to be result of the high-energy impact of heavy solid tungsten carbide particles, which caused plastic deformation to the relatively “soft” nickel bond layer. The wavy interface provides mechanical interlocking and thus improves the adhesive strength of the WC(Co) coating. The quality of the HVOF sprayed coating, especially as regards porosity, mechanical properties, and adhesive strength, is largely dependent on the extent of particle melting and particle velocity at the moment of impact on the substrate. The control of the temperature during the spraying process is very important. The higher the temperature of the Co binder above its melting temperature, the denser the coating and the higher the adhesive
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strength. The viscosity of the particles at the moment of their impingement also strongly influences the coating quality, since the lower the viscosity of the cobalt, the better the particles will splat on the substrate. Also, splats impinging on previously deposited particles will more readily accommodate all the irregularities of the surface and thus provide a denser coating. However, a high temperature may result in the degradation of the polymeric substrate, may cause ternary compounds to form, and may even lead to the decomposition of the carbides and the appearance of elemental tungsten and graphite [16-191. The decomposition of tungsten carbide is thought to proceed in three stages [4]: (1) 2wc -+ w,c+c (2) w,c+ l/20, --f W,(C,O) (3) W,(C,O) -+ 2w+co The degree of the powder’s resistance to decarburization depends on the amount of WC or W,C in the powder and on the oxygen content. Decomposition is much more intense in the case of W,C, since only the last two decomposition stages are required. Due to the CO, cooling the quantity of oxygen in the contact zone and in the coating is low. As a result carbide decomposition is reduced.
5. Summary and conclusions
A triplex system consisting of WC(Co) coating, Ni/Cu/Ni bond layer, and acrylonitrile butadiene styrene (ABS) substrate was investigated. The main conclusions are summarized as follows: (1) During high velocity oxy-fuel spraying a fraction of the WC phase partially decomposes into a-W&, /3-W&, p-WC,-X, and the ternary compound, Co,W$. (2) The microstructure of the WC(Co) coating is typical for high-velocity oxy-fuel spraying and is composed of tungsten carbide particles bonded by a cobalt matrix binder. Traces of P-WC-, were also identified in the binder phase. The microstructure of the Co binder depends on the cooling rate and on the thermal behaviour within the coating. The small particles have a high solidification rate, resulting in the formation of an amorphous and nanocrystalline structure. The lower solidification rates of the big particles (45 urn) result in the formation of a microcrystalline structure. In addition, the thermal interaction between neighboring layers can cause nucleation and grain growth.
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The variation in the particles’ temperatures and velocities results in the formation of different interface morphologies, variations in the binder phase fraction, and pore density within the coating. A high temperature and velocity of the particles reduces the pore density and leads to the formation of a wavy interface, which provides mechanical interlocking and improves adhesion. A mathematical model for calculating the velocity (4) and the temperature distribution of the powder particles in high velocity oxy-fuel spraying was developed. The model accounts for the heating of the particles by hydrogen-oxygen combustion and their cooling by the carbon dioxide flow added to the flame flow. It addition, it takes account of the contribution of the radiation to the heat transfer. The effect of radiation heat transfer is an important factor in the case of large particles. To the authors’ best knowledge this is the first time that radiation heat transfer has been applied in the simulation of the HVOF spraying process. The dependence of particle velocity and temperature (5) on particle size is consistent with the results of other models known from the literature. An increase in the particle size leads to a reduction in the particle velocity and temperature. (6) For WC(Co) particles with size distribution between 15 and 45 pm the velocity varies between 900 and 300 m s-l and the temperature between 3000 and 15OO”C, respectively. The experimental and the theoretical results show (7) that WC(Co) can be deposited on polymers, provided that there is an adherent metallic bond layer between the coating and the plastic substrate and the deposition temperature is controlled. The control of the particles’ temperature and velocity in the combustion chamber and upon impact on the substrate was found to be crucial for obtaining coatings with high adhesive strength. (3)
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Acknowledgement
The financial support of the Ministry of Science and Art of Niedersachsen is sincerely appreciated.
References [ 11 I. Grimberg, B. Bouaifi, U. Draugelates, K. Soifer and B.Z. Weiss, SW. Coat. Technol., 68169 (1994) 166. [2] D.W. Parker and G.L. Kutner, Adu. Mater. Progr., 138 (1991) 68. [3] A. Karimi, Ch. Verdon and G. Barbezat, Szl$ Coat. Technol., 57 (1993) 81. [4] M.E. Vinayo, F. Kassabji, J. Guyonnet and P. Fauchais, J. Vat. Sci. Technol.,
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C.B. BucknaIl, Toughened Plastics, Applied Science Publishers, London, 1971, p. 333. [6] S. Craig and G.L. Harding, Sufl Sci., 124 (1983) 591. [7] D. Apelian, R. Paliwal, R.W. Smith and W.F. Schilling, Int. Met. Rev., 28 (1983) 271. [8] V.V. Sobolev, J.M. Guilemany, J.C. Garmier and J.A. Calero, Surf: Coat. Technol., 63 (1994) 181. [9] V. Bakhvalov, Numerical Methods, Nauka Publishers, Moscow, 1972. [lo] Handbook of Chemistry and Physics, 55th edition, CRC Press, 1975. [ 1 l] T. Irvine, Steam aud Air Tables in CI Units, Hemisphere Publishing Corp., 1976. [12] T.B. Massalski, H. Okamoto, P.R. Subramanian and L. Kacprzak, Binary Alloy Phase Diagrams, 2th Edition, ASM International, 1990. [13] S.V. Joshi and R. Sivakumar, Surf: Coat. Technol., 50 (1991) 67. [14] J. Nutting, J.M. Guilemany and Z. Dong, Mater. Sci. Technol., I1 (1995) 961. [ 151 V.V. Sobolev, J.M. Guilemany and J.A. Calero, Mater. Sci. Techno/., 11 (1995) 810. 1161 V. Rammath and N. Jayaraman, Mater. Sci. Technol., 5 (1989) 382. [ 171 D. Tu, S. Chang, C. Chano and C. Lin, J. Vat. Sci. Technol., A 3 (1985) 2479. [18] A. Karimi, Ch. Verdon, and G. Barbezat, Surj Coat. Technol., 57 (1993) 81. [19] S.V. Yoshi, Mater. Left., 14 (1992) 31. [S]