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The influence of substrate temperature on properties of APS and VPS W coatings
SCT-19578; No of Pages 8 Surface & Coatings Technology xxx (2014) xxx–xxx
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The influence of substrate temperature on properties of APS and VPS W coatings O. Kovářík a,⁎, P. Haušild a, J. Siegl a, T. Chráska b, J. Matějíček b, Z. Pala b, M. Boulos c a b c
Czech Technical University in Prague, Trojanova 13, 120 00 Prague 2, Czech Republic Institute of Plasma Physics ASCR, Za Slovankou 1782/3, 182 00 Prague 8, Czech Republic University of Sherbrooke, 2500, boulevard de l'Université, Sherbrooke, Québec J1K 2R1, Canada
a r t i c l e
i n f o
Article history: Received 28 February 2014 Accepted in revised form 12 July 2014 Available online xxxx Keywords: Plasma spray Substrate temperature W Hardness Coating modulus Thermal conductivity
a b s t r a c t The effect of substrate temperature on integral properties of W coatings was investigated. Cooled sample holder enabling simultaneous deposition on substrates with different temperatures was used. The spraying was performed by atmospheric plasma spray (water stabilized torch) and vacuum plasma spray (RF inductively coupled torch). Three torch power levels were used for vacuum plasma spray to obtain different spray particle in-flight properties. It was observed that the substrate temperature influenced the porosity, coating modulus, hardness and thermal conductivity of the deposits. The strongest influence of the substrate temperature was observed for the optimal spray condition resulting in the lowest porosity deposit. For the other deposits, the presence of W oxides, condensed W phase, unmelted feedstock particles and splashed splats decreased the values of all abovementioned coating properties. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Refractory metals are characterized by their extremely high melting point and are used in demanding applications requiring hightemperature strength and corrosion resistance [1]. Tungsten is a refractory metal with the highest melting point, good thermal conductivity, low thermal expansion and high density. It is commonly manufactured into self-standing parts as for example electrodes, counterweights and penetrators. W coatings are used to improve surface properties of thermionic pipes (see [2]), shielding parts for the fusion reactor (see [3–5]), etc. Other applications of W coatings also call for dense W deposit with favorable crystallographic structure. Suitable technologies for deposition of refractory coatings include cold spray [6,7] and plasma spray [2–4,8–10]. In atmospheric plasma spray (APS), care must be taken to address the problem of oxidation, see e.g. [10,11]. For the W– Cu composite coatings, HVOF can also be used as demonstrated for example in [12]. For fusion applications, however, the properties of plasma sprayed W need to be improved [3] to ensure sufficient lifetime of the coated components, e.g. by using functionally graded coatings or improving W coating quality. During the optimization of W deposition, substrate temperature (Ts) received limited attention due to experimental difficulties associated with its measurement and control. Also, the separation of the substrate temperature effect from the effect of ⁎ Corresponding author at: Faculty of Nuclear Sciences and Physical Engineering,Czech Technical University in Prague, Trojanova 13, 120 00,Prague 2, Czech Republic. E-mail address: Ondrej.Kovarik@fjfi.cvut.cz (O. Kovářík).
other spray conditions is difficult. The optimization of W deposition involving substrate temperature is mentioned e.g. in [9], but spray parameter values and substrate temperatures are not presented. Therefore we tried to systematically observe the influence of substrate temperature on deposit properties for different spray conditions. The fundamental knowledge gained in this research is directly applicable to deposition of W or development of more complex W-based functionally graded materials. Numerous papers deal with the optimization of W deposition by plasma spray, for example see [2,9,13–16]. In this paper, instead of relating the deposit properties to the number of standard process conditions such as torch power and gas flow rates, we focused on more fundamental factors such as particle velocity and temperature at the point of impact and substrate temperature. This description of the spray process is not complete as for example the melting state of the particles is not addressed. On the other hand, it is sufficient to illustrate the basic phenomena involved in coating buildup. The splat formation mode is one of the key factors controlling the properties of the deposited materials. The splat formation itself is influenced by several factors as described below. The influence of substrate temperature on splat formation during plasma spray was already observed experimentally [17–19] and simulated numerically [20,21] for various materials. The resulting splat shapes are described as splash (or flower-like) splats, fragmented splats and disc splats. There exist numerous theories relating the splat shape to the substrate state. These theories are based on the presence of adsorbed gas [22–24], the time sequence of splat flattening and solidification [25,26] or wetting of the substrate by the melted particles [27].
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Regardless of whichever mechanism plays the most important role, the transition temperature of the substrate that marks a boundary between disc shaped and splash splat is usually found in the range of 150–500 °C for plasma sprayed materials [19,22,28]. The transition temperature, encountered by single splat experiments is expected to significantly influence the deposit properties. Other factors influencing the splat formation are particle viscosity and velocity at impact [29]. There exist several criteria based on Reynolds and Weber numbers, e.g. [30,31]. For single liquid droplets impacting on a flat surface, a criterion for impact splashing is based on a threshold value of the Sommerfeld parameter [30]. The Sommerfeld parameter can be computed from the properties of the impacting droplet, namely from density ρ, diameter ρ3d3v5 d, surface tension σ, velocity v and dynamic viscosity μ as K ¼ σ4 1μ4 14 . 2 4 From the above variables, σ and μ strongly depend on droplet temperature. The threshold value K was first estimated by Mundo [30] on free falling alcohol droplets. The values of K N 57.7 lead to splashing, K b 57.7 to deposition of continuous splats. For thermal spray, the threshold between splashing and deposition is considered over a wider range. Escure [32] showed that 5 b K b 58 may result in both splashing and deposition. Values of K N 58 typically lead to splashing. During deposition of coatings, the splats impinge on thermally and geometrically inhomogeneous surface. Thus, taking into account the mechanisms summarized above, the splat splashing usually takes place together with disc splat formation. The splat formation is a random process controlled by the local substrate state. In contrast to the single splat approaches, this paper investigates the influence of substrate temperature on the properties of the whole W deposit. Microstructure, density, hardness, coating modulus and thermal conductivity of deposits prepared using different technologies and different process parameters are investigated. For each combination of deposition technology and process setting, the effect of substrate temperature is studied independently.
2. Experimental Tungsten deposits were prepared by radio frequency (RF) plasma spray in a deposition chamber and by water stabilized plasma (WSP) torch in ambient air. The process conditions are summarized in Tables 1 and 2. Detailed process parameters and description of spray equipment may be found in papers [33,34]. For each spray condition used, six specimens were sprayed at once using a special water cooled holder. The holder ensures that all specimens are sprayed using exactly the same spray condition, including the torch scanning pattern. The holder is a rotating water cooled hollow hexagonal prism of diameter d = 61 mm carrying 6 different specimens of 170 mm × 25.4 mm footprint (see Fig. 1). Sheets of 304L stainless steel were used for the preliminary experiment, and low carbon steel sheet of 5 mm thickness was used in the following experiments. The heat received by the specimens during spraying was transferred Table 1 Process parameters of used plasma technologies. Parameter
RF
WSP
Torch Powder Nominal powder size Feed rate Holder rotation Holder translation # of translations Spray distance Feed distancea Power Chamber pressure
TEKNA PL-50 Amperite Am 140.2 45–90 μm 40 g/min 20 rpm 0.26 mm/s 4, 1 and 3b 210 mm −60 mm 60, 80, 90 kW 40 kPa
PAL WSP® 160 Alldyne W 63–80 μm 240 g/min 60 rpm 180 mm/s 96 350 mm 40 mm 160 kW –
a b
From nozzle, negative values inside torch. VPS-lo, VPS-mi, and VPS-hi samples; the resulting coating thickness is in Table 2.
Table 2 In-flight properties and average coating thicknesses of investigated specimen sets. TV50, v V50 and KV50 characterize average impact temperature, velocity and Sommerfeld parameter of particles with median diameter dV50. The value of dV50 divides the mass of all powder particles in two equal parts. The in-flight temperature of APS set is underestimated probably due to the vapor or oxide formation [34]. The average coating thickness h is also included. Set
Technologya
Torch power (kW)
TV50 (°C)
vV50 (m/s)
KV50
dV50 (μm)
h (mm)
APS VPS-lo VPS-mi VPS-hi
WSP RF RF RF
160 60 80 90
2900 3650 3925 3900
80 32 40 45
630 186 264 303
73 65 65 65
0.83 3.02 0.47 2.09
a
See Table 1.
through silicone grease to the holder. Fine grease pattern was created at the sample holder contact, with the ratio of greased surface to total contact surface area defining the heat flow. Four different specimen sets were deposited by different technologies and under different conditions (see Tables 1 and 2). The achieved substrate temperatures are summarized in Table 3. The sets VPS-lo, VPS-mi, and VPS-hi were sprayed by RF plasma using a deposition chamber at three different torch powers. The selected spray conditions were based on previous research [16,33] and represent incomplete melting of feedstock (set VPS-lo), optimum deposition condition (set VPS-mi) and condition with increased torch power (set VPS-hi). The HF-100 spray system (Tekna Plasma Systems Inc., Sherbrooke, Québec, Canada) with inductively coupled Tekna HF-50 torch was used. The APS set was deposited by WSP® 500 (IPP, Prague, Czech Republic) direct current (DC) plasma torch in ambient air, but using Ar/H2 mixture as carrier gas and as a shield around the samples to suppress oxidation. The substrate temperature was defined as the temperature of specimen surface just after passing through the plasma plume. Pyrometer calibrated against thermocouple using the tungsten coatings as reference samples was used for the VPS-mi set. Substrate temperatures of APS, VPS-lo and VPS-hi sets were measured by thermocouples, connected to a battery operated datalogger encapsulated in water cooled envelope. The maximum sample temperature recorded during one pass of the torch was evaluated. The deposition of coatings with uniform thickness was achieved by rotation and simultaneous translation of the holder (see Fig. 1). In order to prevent overheating of APS substrates due to the high WSP torch power, the translation speed of WSP deposition was chosen significantly higher than for RF deposition and pauses between scanning cycles were applied (see Table 1). Particle in-flight properties were measured by a DPV-2000 particle sensor (Tecnar, St. Bruno, QC, Canada). The details of the measurement together with the results in the form of zp/dp maps are summarized in [16]. The zp/dp map provides average temperature and velocity for each combination of spray distance zp and particle diameter dp. Modulus of elasticity of the deposit was measured using four-point bending apparatus mounted on electro-mechanical tensile testing machine Inspekt 100 (Hegewald Peschke, Nossen, Germany). The apparatus and force/displacement data processing are described elsewhere [35,36]. The loading was performed with coating in compression. Ten force controlled loading cycles (100–1000 N) at loading rate of 10 N/s were applied. Before the measurement, the substrate thickness was reduced to 2 mm in order to increase the method sensitivity. The sample was clamped in a special holder to prevent bending and part of the substrate was removed by milling. The average coating modulus was computed from linear interpolation of a complete stress–strain curve and is referred to as coating modulus later in the text. Coating microstructures were observed on both longitudinal and transversal cross-sections. The samples were ground up to 1500 grit,
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Fig. 1. The geometry of the cooled holder. The datalogger, thermocouples and shielding rods apparent on the inset photo are not drawn. Double arrows show the water flow, and dashed line shows the grease pattern defining the contact condition between the samples and the holder.
polished with 3 μm diamond paste and electrolytically treated in 1.25% KOH water solution (15 V for 5 s and 1 V for 30 s). Light microscope Neophot 32 (Carl Zeiss Jena, Jena, Germany) was used. Additional cross sectional samples for high resolution SEM were prepared by ion milling using Gatan PIPS (Gatan Inc., Pleasanton, CA, USA). The micromorphology of the coating surfaces and fracture surfaces parallel to coating–substrate interface was characterized using JEOL JSM-840 (JEOL Ltd., Tokyo, Japan) scanning electron microscope. High resolution observation was performed on JEOL JSM-7500 and FEI Quanta 3D (FEI, Hillsboro, OR, USA) field emission gun SEMs. EDX analysis was performed by IXRF 5000 analyzer (IXRF Systems Inc., Austin, TX, USA). D8 Discover powder X-ray diffractometer (Bruker Corporation, Billerica, MA, USA) equipped with 1D detector and copper anode tube was employed for determination of phase composition of deposit surfaces. After the identification of crystalline phases, Rietveld refinement procedure was performed in TOPAS 4.2 software. The indispensable structural information about the phases, i.e. cubic tungsten (space group Im-3m), orthorombic WO3 (SG Pmnb) and monoclinic WO3 (SG P21/n), was retrieved from ICSD database. Both processes of measurement and subsequent Rietveld refinement followed the guidelines from [37]. The coating hardness HV1000 was determined using a Nexus 4504 (Innovatest BV, Maastricht, The Netherlands) tester with Vickers indentor. For each specimen, 7 measurements were made across the thickness of the deposit. The indentor load was 1 kg. The density of the deposit was measured by the Archimedes method using the ABT-A01 kit and Kern 220-5DM scales (Kern & Sohn GmbH, Balingen, Germany). The measured samples had footprint of approx. 10 × 10 mm. The estimated deposit density ρc is expressed as a fraction of bulk density by ρc/ρbulk. The bulk properties of deposit and substrate
materials at room temperature are taken from [38,39] and summarized in Table 4. The thermal diffusivity and specific heat capacity were measured for selected samples by FlashLine™ 3000 (Anter Corporation, Pittsburgh, PA, USA) using the flash method. Thermal conductivity can be computed from measured values of thermal diffusivity using the specific heat obtained by a comparative measurement with a standard in the same run.
Table 3 The achieved substrate temperatures Ts at different holder positions. Ts in °C; the highest value from a given set is in bold letters, the lowest value in italics.
Table 4 Bulk properties of materials used: α — coefficient of thermal expansion, E — elastic modulus, k — thermal conductivity, HV — Vickers hardness. Data compiled from [38,39].
3. Results and discussion 3.1. Process condition optimization and deposition The torch parameters were chosen based on previous experiments [16,33,40]. In order to further optimize the spray distance zs, the results of [16] were complemented with analogous measurements for the VPShi. Based on the obtained data, spray distance of 210 mm was chosen for the VPS sets. The in-flight temperature measurements of the APS process were unsuccessful probably due to the tungsten peroxide atmosphere formation around particles [34,39]. Therefore a spray distance zs of 350 mm was chosen for APS deposition based on previous results [34]. The in-flight properties measured at spray distance without substrate are summarized in Table 2 for all tested spray conditions. Sommerfeld parameter K describing the particle state was estimated using data of [41]. Splashing of impinging particles during thermal spray should occur for K N 58 [32]. The obtained K values indicate that some impact splashing is to be expected in all cases. The average coating thicknesses h of all sprayed specimen sets are in Table 2. The thickness of individual coating depends on the torch scanning pattern (translation speed, rotation speed, number of torch passes), achieved spray
Set\holder position
1
2
3
4
5
6
Property
W [39]
304L [38]
l.c. steel [38]
APS VPS-lo VPS-mi VPS-hi
142 246 572 271
152 226 676 361
268 249 578 454
192 380 528 405
378 464 348 496
299 549 514 618
α (μm/m/K) E (GPa) k (W m−1 K−1) HV
4.35 410 150 350–450
17.3 193 16.3 –
10.2 210 43 –
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efficiency, coating porosity, etc. For VPS deposits, different number of holder translations resulted in different coating thicknesses. The influence of coating thickness on its properties was studied e.g. in [42] for plasma sprayed Mo. The results indicate that the thickness effect on microhardness becomes negligible for thickness higher than approx. 50 μm. For the thick coatings investigated in this paper, only a minor influence of coating thickness on measured properties can therefore be expected.
through vapor phase. The vapor subsequently condensates in flight and is propelled toward the specimen by the mechanism described in [43]. The particular mechanism of tungsten transport through vapor phase needs further investigation. It should be noted that the reduced pressure of the deposition chamber should not lower the W boiling point significantly as the vapor pressure of 50 kPa is achieved at approx. 5550 °C based on the extrapolation of data from [39].
3.2. Splat morphology
3.3. Deposit microstructure
The observation of coating surface enables direct qualitative study of splat morphology of the deposited coatings. Splashing was detected for all observed specimens, as expected. The quantification of splash/disc splat ratio was not attempted due to complicated surface morphology, however, qualitative observations are presented. The micrographs on Fig. 2 reveal that splash splats cover a larger part of the surface for APS and VPS-lo sets. For the APS set, the amount of splashing slightly decreases with increasing Ts. The increased oxide content at higher Ts (see Fig. 4) can slow particle cooling and result in formation of less distorted splats. However, the role of the oxide can be more complicated, due to its low melting point and easy volatilization [39]. The splashing observed on VPS-lo samples can be attributed to obstruction of liquid flow by unmelted or resolidified particles or their segments. On VPS-mi and VPS-hi samples, the amount of disc splats increases with increasing substrate temperature, as expected. Above 500 °C, significant number of disc splats is formed, whereas below 500 °C mostly splashing occurs. At high magnification, fine particles were detected on splat surfaces (Fig. 3). On APS samples, these particles covered the majority of specimen surface and were identified by X-ray diffraction as WO3 in monoclinic and orthorhombic forms. The concentration of W oxides estimated by Rietveld method is included in Fig. 4. Increased amount of oxides at higher Ts indicates that the oxides may form both in-flight and after impact on substrate. Some oxides are present also at splat interfaces (Fig. 3) as indicated by EDX analysis and form a layer that prevents splat sintering. On VPS samples, no oxides were detected by XRD. However, small W particles of morphology similar to the oxides were observed (Fig. 2). These particles originate from W transported
Typical microstructure of the deposit (Fig. 6) consists of splats, their interfaces, unmelted or resolidified spherical particles, condensed particles, oxides, linear and globular porosity and crystal grains. The thickness of the splats is given by the used spray technology, powder size and particle melting state at impact. APS samples exhibit splat thickness of around 10 μm, while VPS samples have the thickness around 8 μm. The rotation combined with periodic translation movement of the sample holder leads to formation of sublayers in the deposit. During one rotation of the holder a sublayer denoted as rotational is formed below the torch. In other areas of the sample, only condensate, oxides or fine particles get deposited, forming interfaces between translational sublayers. The translational sublayers are formed during one translation of the torch and consist of multiple rotational sublayers (Fig. 5). The thickest rotational sublayers are at the half thickness of translational sublayer as they are deposited from the center of the spray plume. Rotational sublayers formed on VPS samples consisted of approximately 5 layers of splats in this area. On APS samples, due to higher rotational speed, multiple holder rotations were necessary to deposit one layer of splats and thus rotational sublayers could not be identified. The splat interfaces of VPS specimens contain fine particles of W condensate; W oxides are present between splats of APS deposits. The amount of these fine particles is higher at sublayer interfaces, as the splat surface exposure to the surrounding is longer. The intersplat sintering often took place at higher substrate temperature as indicated in Fig. 3. However, the sintering does not take place between splats covered with thick layer of fine particles, namely for the APS set and at sublayer interfaces of the VPS-hi set.
Fig. 2. Typical surface morphologies of samples from the investigated sets. Disc splats can be identified for the VPS-mi and VPS-hi sets, to a lesser extent also for the VPS-lo set.
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Fig. 3. Samples with the highest substrate temperature from the APS, VPS-mi and VPS-hi sets, SEM micrographs. Left: Typical micromorphology of splat surfaces. Oxides are visible on the APS samples, and condensate is present on the VPS-mi and the VPS-hi samples. Right: Cross sections, polished, electrolytically etched. Splat interfaces with oxides for the APS deposit, well sintered interfaces of the VPS-mi sample and interfaces with condensate for the VPS-hi deposits.
Spherical particles are observed in APS and VPS-lo samples; very limited number of spherical particles is observed for VPS-mi and VPShi samples (Fig. 6). The spherical particles indicate incompletely molten or resolidified feedstock at the point of impact. Their presence is given by low torch power or non-optimal trajectory of the particles through the plasma plume. The grain size of the W deposit is predetermined by the porosity and thermal history of the specimen (Fig. 6). In APS samples, the porosity and oxide layers stop grain growth at splat interfaces. For VPS-lo and VPS-mi samples, grains extending through splat interfaces and stopping at sublayer interfaces were observed. The condensate and high intersplat porosity of VPS-hi splat interfaces stop the grain growth and limit the heat transfer. Therefore, the grains grow preferably inside the splats and cross the intersplat interfaces at higher substrate temperature. The intersplat porosity of VPS-hi substrate is promoted by impact splashing caused by high particle impact temperature and velocity (Table 2). The influence of substrate temperature on grain size was observed for all VPS sets. The most obvious increase of grain size with substrate temperature can be seen for VPS-mi set (see Fig. 6).
independent on Ts, but strong correlation can be found for other sets (e.g. thermal conductivity in Fig. 7). It should be noted that the measured hardness reflects both in-plane and through-thickness properties, with pronounced effect of the through-thickness properties. Coating modulus was measured in the in-plane direction by four-point bending, whereas thermal conductivity was measured in the through-thickness direction. 3.4.1. Hardness For APS specimens, the substrate temperature was too low to influence the coating hardness and no trend was observed. In the same Ts range, i.e. below 400 °C, the VPS sets showed no hardness increase either. However, for Ts above 400 °C, hardness increase was observed for all VPS sets. According to the Hall–Petch relation, the increase of grain size with temperature should lead to decrease of hardness. The opposite effect was observed, however, indicating that the effect of intersplat sintering is stronger than the effect of grain size. For all substrate temperatures, hardness value of VPS-hi was slightly lower than those of other two VPS sets. This was caused by limited intersplat sintering due to the condensate on intersplat boundaries.
3.4. Properties of the deposits The dependence of relative density, coating modulus, hardness and thermal conductivity on substrate temperature is summarized in Fig. 7. It is clearly indicated that for some sets, certain properties appear
Fig. 4. XRD patterns (Cu anode) of coating surfaces. APS samples deposited at different Ts with increasing oxide content and oxide free VPS-hi sample. The oxide concentrations in wt.% estimated by Rietveld analysis are shown in brackets. Intensity is in arbitrary units.
3.4.2. Density The relative density ρ/ρ bulk of VPS specimens deposited for T s b 400 °C was nearly constant. The APS deposits showed significant increase of ρ/ρbulk with Ts, suggesting the oxides increase the sensitivity of ρ/ρbulk to Ts. The slower cooling of W splats deposited on thicker oxide layer can improve splat formation and eliminate porosity. The oxide layer between splats however will degrade the mechanical properties despite the porosity decrease. Therefore the decreased porosity is not accompanied with mechanical property increase. The deposition by APS for Ts N 400 °C was not performed due to expected heavy oxidation of W deposit. The ρ/ρbulk of VPS deposits increased rapidly at Ts above 400 °C. This behavior may be connected to transition temperature effect taking place above 400 °C and leading to disc splat formation. The relative density of the coating is a natural indicator of the quality of the plasma spray process and is related to other measured properties as well. An example is the hardness relation to relative density shown in Fig. 8. Two distinct regions are separated by a threshold density. For ρ/ρbulk b 90%, the hardness shows no trend. Above 90% of the hardness follows an increasing linear relation to relative density. 3.4.3. Coating modulus of elasticity Only weak influence of substrate temperature on coating modulus E4pb was observed for samples from the same set. However the change of spray technology and torch power significantly influences the coating modulus. The porous microstructure and weak splat bonding of APS deposits resulted in very low modulus with an average value of 20 GPa that is comparable with the measurement error. The mean value of
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Fig. 5. Sublayers formed in VPS deposits. Metallographic cut in a plane marked by gray color in the inset drawing. Polished, electrolytically etched, was observed in a light microscope. Rotational sublayer is formed by one rotation of the holder. Translational sublayer is formed by one translation of the holder and consists of multiple rotational sublayers.
the coating modulus was 136.2, 192.2 and 122 for the VPS-lo, VPS-mi and VPS-hi sets, respectively. The VPS-lo and VPS-hi deposits exhibited slight increase of modulus with Ts that can be characterized by a linear relation with slope 60 MPa/K for VPS-lo and 110 MPa/K for VPS-hi. The low thickness of the VPS-mi coatings resulted in a scatter of measured modulus that does not allow the modulus on temperature relation to be established.
and intersplat sintering. In the VPS-hi set, the condensate and porosity at intersplat boundaries prevent intersplat sintering and thus limit the heat transfer. As a result, thermal conductivity does not show a relation to substrate temperature despite the fact that the coating density increases with substrate temperature.
4. Conclusions 3.4.4. Thermal conductivity Thermal conductivity k was measured on samples with sufficient coating thickness, i.e. on the VPS-lo and VPS-hi sets. The increase of thermal conductivity with substrate temperature was detected for the VPS-lo set, resulting from combined effect of splat morphology change
An experimental methodology was developed that enables to deposit coatings on substrates of different temperatures while keeping other deposition conditions constant. Four sets of W deposits were prepared and the deposit properties were measured.
Fig. 6. Microstructures of the deposits as functions of temperature and spray technology. White arrows indicate unmelted/resolidified spherical particles.
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Fig. 7. The dependence of relative density, microhardness, coating modulus by four point bending, and thermal conductivity on substrate temperature.
The results indicate that changing the substrate temperature is an efficient way for tailoring deposit properties. Splat morphology, grain size, and intersplat sintering are properties that are influenced by substrate temperature Ts. The measured values of density, hardness, coating modulus, and thermal conductivity indicate that dense W deposits with good mechanical properties can be obtained for substrate temperature above 400 °C in the protective atmosphere of a deposition chamber. Apart from substrate temperature, the torch power and resultant particle in-flight properties play an important role. For low torch power, the melting state of the particles is insufficient for regular splat formation. For high torch power, on the other hand, the vaporization and
condensation of W were observed to decrease the values of all measured coating properties. When optimal torch power is applied, only a low amount of condensate is present, and the melting state of the particles is favorable to obtain dense and well sintered deposit. For this spray condition, the substrate temperature can efficiently control the grain size, hardness, coating modulus and thermal conductivity of the deposit. Acknowledgment This research was supported by the Czech Science Foundation project No. 108/12/1872. The deposition by RF plasma was performed under the support of Tekna Plasma Systems Inc. and Centre de Recherche en Énergie, Plasma et Électrochimie (CREPE) de l'Université de Sherbrooke in Sherbrooke, Québec. Aleš Jäger from the Institute of Physics, ASCR is acknowledged for the preparation, observation and analysis of the HR-SEM samples in the scope of project KAN300100801. References
Fig. 8. The influence of relative density on microhardness (1000 g load).
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
The influence of substrate temperature on properties of APS and VPS W coatings O. Kovářík a,⁎, P. Haušild a, J. Siegl a, T. Chráska b, J. Matějíček b, Z. Pala b, M. Boulos c a b c
Czech Technical University in Prague, Trojanova 13, 120 00 Prague 2, Czech Republic Institute of Plasma Physics ASCR, Za Slovankou 1782/3, 182 00 Prague 8, Czech Republic University of Sherbrooke, 2500, boulevard de l'Université, Sherbrooke, Québec J1K 2R1, Canada
a r t i c l e
i n f o
Article history: Received 28 February 2014 Accepted in revised form 12 July 2014 Available online xxxx Keywords: Plasma spray Substrate temperature W Hardness Coating modulus Thermal conductivity
a b s t r a c t The effect of substrate temperature on integral properties of W coatings was investigated. Cooled sample holder enabling simultaneous deposition on substrates with different temperatures was used. The spraying was performed by atmospheric plasma spray (water stabilized torch) and vacuum plasma spray (RF inductively coupled torch). Three torch power levels were used for vacuum plasma spray to obtain different spray particle in-flight properties. It was observed that the substrate temperature influenced the porosity, coating modulus, hardness and thermal conductivity of the deposits. The strongest influence of the substrate temperature was observed for the optimal spray condition resulting in the lowest porosity deposit. For the other deposits, the presence of W oxides, condensed W phase, unmelted feedstock particles and splashed splats decreased the values of all abovementioned coating properties. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Refractory metals are characterized by their extremely high melting point and are used in demanding applications requiring hightemperature strength and corrosion resistance [1]. Tungsten is a refractory metal with the highest melting point, good thermal conductivity, low thermal expansion and high density. It is commonly manufactured into self-standing parts as for example electrodes, counterweights and penetrators. W coatings are used to improve surface properties of thermionic pipes (see [2]), shielding parts for the fusion reactor (see [3–5]), etc. Other applications of W coatings also call for dense W deposit with favorable crystallographic structure. Suitable technologies for deposition of refractory coatings include cold spray [6,7] and plasma spray [2–4,8–10]. In atmospheric plasma spray (APS), care must be taken to address the problem of oxidation, see e.g. [10,11]. For the W– Cu composite coatings, HVOF can also be used as demonstrated for example in [12]. For fusion applications, however, the properties of plasma sprayed W need to be improved [3] to ensure sufficient lifetime of the coated components, e.g. by using functionally graded coatings or improving W coating quality. During the optimization of W deposition, substrate temperature (Ts) received limited attention due to experimental difficulties associated with its measurement and control. Also, the separation of the substrate temperature effect from the effect of ⁎ Corresponding author at: Faculty of Nuclear Sciences and Physical Engineering,Czech Technical University in Prague, Trojanova 13, 120 00,Prague 2, Czech Republic. E-mail address: Ondrej.Kovarik@fjfi.cvut.cz (O. Kovářík).
other spray conditions is difficult. The optimization of W deposition involving substrate temperature is mentioned e.g. in [9], but spray parameter values and substrate temperatures are not presented. Therefore we tried to systematically observe the influence of substrate temperature on deposit properties for different spray conditions. The fundamental knowledge gained in this research is directly applicable to deposition of W or development of more complex W-based functionally graded materials. Numerous papers deal with the optimization of W deposition by plasma spray, for example see [2,9,13–16]. In this paper, instead of relating the deposit properties to the number of standard process conditions such as torch power and gas flow rates, we focused on more fundamental factors such as particle velocity and temperature at the point of impact and substrate temperature. This description of the spray process is not complete as for example the melting state of the particles is not addressed. On the other hand, it is sufficient to illustrate the basic phenomena involved in coating buildup. The splat formation mode is one of the key factors controlling the properties of the deposited materials. The splat formation itself is influenced by several factors as described below. The influence of substrate temperature on splat formation during plasma spray was already observed experimentally [17–19] and simulated numerically [20,21] for various materials. The resulting splat shapes are described as splash (or flower-like) splats, fragmented splats and disc splats. There exist numerous theories relating the splat shape to the substrate state. These theories are based on the presence of adsorbed gas [22–24], the time sequence of splat flattening and solidification [25,26] or wetting of the substrate by the melted particles [27].
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Regardless of whichever mechanism plays the most important role, the transition temperature of the substrate that marks a boundary between disc shaped and splash splat is usually found in the range of 150–500 °C for plasma sprayed materials [19,22,28]. The transition temperature, encountered by single splat experiments is expected to significantly influence the deposit properties. Other factors influencing the splat formation are particle viscosity and velocity at impact [29]. There exist several criteria based on Reynolds and Weber numbers, e.g. [30,31]. For single liquid droplets impacting on a flat surface, a criterion for impact splashing is based on a threshold value of the Sommerfeld parameter [30]. The Sommerfeld parameter can be computed from the properties of the impacting droplet, namely from density ρ, diameter ρ3d3v5 d, surface tension σ, velocity v and dynamic viscosity μ as K ¼ σ4 1μ4 14 . 2 4 From the above variables, σ and μ strongly depend on droplet temperature. The threshold value K was first estimated by Mundo [30] on free falling alcohol droplets. The values of K N 57.7 lead to splashing, K b 57.7 to deposition of continuous splats. For thermal spray, the threshold between splashing and deposition is considered over a wider range. Escure [32] showed that 5 b K b 58 may result in both splashing and deposition. Values of K N 58 typically lead to splashing. During deposition of coatings, the splats impinge on thermally and geometrically inhomogeneous surface. Thus, taking into account the mechanisms summarized above, the splat splashing usually takes place together with disc splat formation. The splat formation is a random process controlled by the local substrate state. In contrast to the single splat approaches, this paper investigates the influence of substrate temperature on the properties of the whole W deposit. Microstructure, density, hardness, coating modulus and thermal conductivity of deposits prepared using different technologies and different process parameters are investigated. For each combination of deposition technology and process setting, the effect of substrate temperature is studied independently.
2. Experimental Tungsten deposits were prepared by radio frequency (RF) plasma spray in a deposition chamber and by water stabilized plasma (WSP) torch in ambient air. The process conditions are summarized in Tables 1 and 2. Detailed process parameters and description of spray equipment may be found in papers [33,34]. For each spray condition used, six specimens were sprayed at once using a special water cooled holder. The holder ensures that all specimens are sprayed using exactly the same spray condition, including the torch scanning pattern. The holder is a rotating water cooled hollow hexagonal prism of diameter d = 61 mm carrying 6 different specimens of 170 mm × 25.4 mm footprint (see Fig. 1). Sheets of 304L stainless steel were used for the preliminary experiment, and low carbon steel sheet of 5 mm thickness was used in the following experiments. The heat received by the specimens during spraying was transferred Table 1 Process parameters of used plasma technologies. Parameter
RF
WSP
Torch Powder Nominal powder size Feed rate Holder rotation Holder translation # of translations Spray distance Feed distancea Power Chamber pressure
TEKNA PL-50 Amperite Am 140.2 45–90 μm 40 g/min 20 rpm 0.26 mm/s 4, 1 and 3b 210 mm −60 mm 60, 80, 90 kW 40 kPa
PAL WSP® 160 Alldyne W 63–80 μm 240 g/min 60 rpm 180 mm/s 96 350 mm 40 mm 160 kW –
a b
From nozzle, negative values inside torch. VPS-lo, VPS-mi, and VPS-hi samples; the resulting coating thickness is in Table 2.
Table 2 In-flight properties and average coating thicknesses of investigated specimen sets. TV50, v V50 and KV50 characterize average impact temperature, velocity and Sommerfeld parameter of particles with median diameter dV50. The value of dV50 divides the mass of all powder particles in two equal parts. The in-flight temperature of APS set is underestimated probably due to the vapor or oxide formation [34]. The average coating thickness h is also included. Set
Technologya
Torch power (kW)
TV50 (°C)
vV50 (m/s)
KV50
dV50 (μm)
h (mm)
APS VPS-lo VPS-mi VPS-hi
WSP RF RF RF
160 60 80 90
2900 3650 3925 3900
80 32 40 45
630 186 264 303
73 65 65 65
0.83 3.02 0.47 2.09
a
See Table 1.
through silicone grease to the holder. Fine grease pattern was created at the sample holder contact, with the ratio of greased surface to total contact surface area defining the heat flow. Four different specimen sets were deposited by different technologies and under different conditions (see Tables 1 and 2). The achieved substrate temperatures are summarized in Table 3. The sets VPS-lo, VPS-mi, and VPS-hi were sprayed by RF plasma using a deposition chamber at three different torch powers. The selected spray conditions were based on previous research [16,33] and represent incomplete melting of feedstock (set VPS-lo), optimum deposition condition (set VPS-mi) and condition with increased torch power (set VPS-hi). The HF-100 spray system (Tekna Plasma Systems Inc., Sherbrooke, Québec, Canada) with inductively coupled Tekna HF-50 torch was used. The APS set was deposited by WSP® 500 (IPP, Prague, Czech Republic) direct current (DC) plasma torch in ambient air, but using Ar/H2 mixture as carrier gas and as a shield around the samples to suppress oxidation. The substrate temperature was defined as the temperature of specimen surface just after passing through the plasma plume. Pyrometer calibrated against thermocouple using the tungsten coatings as reference samples was used for the VPS-mi set. Substrate temperatures of APS, VPS-lo and VPS-hi sets were measured by thermocouples, connected to a battery operated datalogger encapsulated in water cooled envelope. The maximum sample temperature recorded during one pass of the torch was evaluated. The deposition of coatings with uniform thickness was achieved by rotation and simultaneous translation of the holder (see Fig. 1). In order to prevent overheating of APS substrates due to the high WSP torch power, the translation speed of WSP deposition was chosen significantly higher than for RF deposition and pauses between scanning cycles were applied (see Table 1). Particle in-flight properties were measured by a DPV-2000 particle sensor (Tecnar, St. Bruno, QC, Canada). The details of the measurement together with the results in the form of zp/dp maps are summarized in [16]. The zp/dp map provides average temperature and velocity for each combination of spray distance zp and particle diameter dp. Modulus of elasticity of the deposit was measured using four-point bending apparatus mounted on electro-mechanical tensile testing machine Inspekt 100 (Hegewald Peschke, Nossen, Germany). The apparatus and force/displacement data processing are described elsewhere [35,36]. The loading was performed with coating in compression. Ten force controlled loading cycles (100–1000 N) at loading rate of 10 N/s were applied. Before the measurement, the substrate thickness was reduced to 2 mm in order to increase the method sensitivity. The sample was clamped in a special holder to prevent bending and part of the substrate was removed by milling. The average coating modulus was computed from linear interpolation of a complete stress–strain curve and is referred to as coating modulus later in the text. Coating microstructures were observed on both longitudinal and transversal cross-sections. The samples were ground up to 1500 grit,
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Fig. 1. The geometry of the cooled holder. The datalogger, thermocouples and shielding rods apparent on the inset photo are not drawn. Double arrows show the water flow, and dashed line shows the grease pattern defining the contact condition between the samples and the holder.
polished with 3 μm diamond paste and electrolytically treated in 1.25% KOH water solution (15 V for 5 s and 1 V for 30 s). Light microscope Neophot 32 (Carl Zeiss Jena, Jena, Germany) was used. Additional cross sectional samples for high resolution SEM were prepared by ion milling using Gatan PIPS (Gatan Inc., Pleasanton, CA, USA). The micromorphology of the coating surfaces and fracture surfaces parallel to coating–substrate interface was characterized using JEOL JSM-840 (JEOL Ltd., Tokyo, Japan) scanning electron microscope. High resolution observation was performed on JEOL JSM-7500 and FEI Quanta 3D (FEI, Hillsboro, OR, USA) field emission gun SEMs. EDX analysis was performed by IXRF 5000 analyzer (IXRF Systems Inc., Austin, TX, USA). D8 Discover powder X-ray diffractometer (Bruker Corporation, Billerica, MA, USA) equipped with 1D detector and copper anode tube was employed for determination of phase composition of deposit surfaces. After the identification of crystalline phases, Rietveld refinement procedure was performed in TOPAS 4.2 software. The indispensable structural information about the phases, i.e. cubic tungsten (space group Im-3m), orthorombic WO3 (SG Pmnb) and monoclinic WO3 (SG P21/n), was retrieved from ICSD database. Both processes of measurement and subsequent Rietveld refinement followed the guidelines from [37]. The coating hardness HV1000 was determined using a Nexus 4504 (Innovatest BV, Maastricht, The Netherlands) tester with Vickers indentor. For each specimen, 7 measurements were made across the thickness of the deposit. The indentor load was 1 kg. The density of the deposit was measured by the Archimedes method using the ABT-A01 kit and Kern 220-5DM scales (Kern & Sohn GmbH, Balingen, Germany). The measured samples had footprint of approx. 10 × 10 mm. The estimated deposit density ρc is expressed as a fraction of bulk density by ρc/ρbulk. The bulk properties of deposit and substrate
materials at room temperature are taken from [38,39] and summarized in Table 4. The thermal diffusivity and specific heat capacity were measured for selected samples by FlashLine™ 3000 (Anter Corporation, Pittsburgh, PA, USA) using the flash method. Thermal conductivity can be computed from measured values of thermal diffusivity using the specific heat obtained by a comparative measurement with a standard in the same run.
Table 3 The achieved substrate temperatures Ts at different holder positions. Ts in °C; the highest value from a given set is in bold letters, the lowest value in italics.
Table 4 Bulk properties of materials used: α — coefficient of thermal expansion, E — elastic modulus, k — thermal conductivity, HV — Vickers hardness. Data compiled from [38,39].
3. Results and discussion 3.1. Process condition optimization and deposition The torch parameters were chosen based on previous experiments [16,33,40]. In order to further optimize the spray distance zs, the results of [16] were complemented with analogous measurements for the VPShi. Based on the obtained data, spray distance of 210 mm was chosen for the VPS sets. The in-flight temperature measurements of the APS process were unsuccessful probably due to the tungsten peroxide atmosphere formation around particles [34,39]. Therefore a spray distance zs of 350 mm was chosen for APS deposition based on previous results [34]. The in-flight properties measured at spray distance without substrate are summarized in Table 2 for all tested spray conditions. Sommerfeld parameter K describing the particle state was estimated using data of [41]. Splashing of impinging particles during thermal spray should occur for K N 58 [32]. The obtained K values indicate that some impact splashing is to be expected in all cases. The average coating thicknesses h of all sprayed specimen sets are in Table 2. The thickness of individual coating depends on the torch scanning pattern (translation speed, rotation speed, number of torch passes), achieved spray
Set\holder position
1
2
3
4
5
6
Property
W [39]
304L [38]
l.c. steel [38]
APS VPS-lo VPS-mi VPS-hi
142 246 572 271
152 226 676 361
268 249 578 454
192 380 528 405
378 464 348 496
299 549 514 618
α (μm/m/K) E (GPa) k (W m−1 K−1) HV
4.35 410 150 350–450
17.3 193 16.3 –
10.2 210 43 –
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efficiency, coating porosity, etc. For VPS deposits, different number of holder translations resulted in different coating thicknesses. The influence of coating thickness on its properties was studied e.g. in [42] for plasma sprayed Mo. The results indicate that the thickness effect on microhardness becomes negligible for thickness higher than approx. 50 μm. For the thick coatings investigated in this paper, only a minor influence of coating thickness on measured properties can therefore be expected.
through vapor phase. The vapor subsequently condensates in flight and is propelled toward the specimen by the mechanism described in [43]. The particular mechanism of tungsten transport through vapor phase needs further investigation. It should be noted that the reduced pressure of the deposition chamber should not lower the W boiling point significantly as the vapor pressure of 50 kPa is achieved at approx. 5550 °C based on the extrapolation of data from [39].
3.2. Splat morphology
3.3. Deposit microstructure
The observation of coating surface enables direct qualitative study of splat morphology of the deposited coatings. Splashing was detected for all observed specimens, as expected. The quantification of splash/disc splat ratio was not attempted due to complicated surface morphology, however, qualitative observations are presented. The micrographs on Fig. 2 reveal that splash splats cover a larger part of the surface for APS and VPS-lo sets. For the APS set, the amount of splashing slightly decreases with increasing Ts. The increased oxide content at higher Ts (see Fig. 4) can slow particle cooling and result in formation of less distorted splats. However, the role of the oxide can be more complicated, due to its low melting point and easy volatilization [39]. The splashing observed on VPS-lo samples can be attributed to obstruction of liquid flow by unmelted or resolidified particles or their segments. On VPS-mi and VPS-hi samples, the amount of disc splats increases with increasing substrate temperature, as expected. Above 500 °C, significant number of disc splats is formed, whereas below 500 °C mostly splashing occurs. At high magnification, fine particles were detected on splat surfaces (Fig. 3). On APS samples, these particles covered the majority of specimen surface and were identified by X-ray diffraction as WO3 in monoclinic and orthorhombic forms. The concentration of W oxides estimated by Rietveld method is included in Fig. 4. Increased amount of oxides at higher Ts indicates that the oxides may form both in-flight and after impact on substrate. Some oxides are present also at splat interfaces (Fig. 3) as indicated by EDX analysis and form a layer that prevents splat sintering. On VPS samples, no oxides were detected by XRD. However, small W particles of morphology similar to the oxides were observed (Fig. 2). These particles originate from W transported
Typical microstructure of the deposit (Fig. 6) consists of splats, their interfaces, unmelted or resolidified spherical particles, condensed particles, oxides, linear and globular porosity and crystal grains. The thickness of the splats is given by the used spray technology, powder size and particle melting state at impact. APS samples exhibit splat thickness of around 10 μm, while VPS samples have the thickness around 8 μm. The rotation combined with periodic translation movement of the sample holder leads to formation of sublayers in the deposit. During one rotation of the holder a sublayer denoted as rotational is formed below the torch. In other areas of the sample, only condensate, oxides or fine particles get deposited, forming interfaces between translational sublayers. The translational sublayers are formed during one translation of the torch and consist of multiple rotational sublayers (Fig. 5). The thickest rotational sublayers are at the half thickness of translational sublayer as they are deposited from the center of the spray plume. Rotational sublayers formed on VPS samples consisted of approximately 5 layers of splats in this area. On APS samples, due to higher rotational speed, multiple holder rotations were necessary to deposit one layer of splats and thus rotational sublayers could not be identified. The splat interfaces of VPS specimens contain fine particles of W condensate; W oxides are present between splats of APS deposits. The amount of these fine particles is higher at sublayer interfaces, as the splat surface exposure to the surrounding is longer. The intersplat sintering often took place at higher substrate temperature as indicated in Fig. 3. However, the sintering does not take place between splats covered with thick layer of fine particles, namely for the APS set and at sublayer interfaces of the VPS-hi set.
Fig. 2. Typical surface morphologies of samples from the investigated sets. Disc splats can be identified for the VPS-mi and VPS-hi sets, to a lesser extent also for the VPS-lo set.
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Fig. 3. Samples with the highest substrate temperature from the APS, VPS-mi and VPS-hi sets, SEM micrographs. Left: Typical micromorphology of splat surfaces. Oxides are visible on the APS samples, and condensate is present on the VPS-mi and the VPS-hi samples. Right: Cross sections, polished, electrolytically etched. Splat interfaces with oxides for the APS deposit, well sintered interfaces of the VPS-mi sample and interfaces with condensate for the VPS-hi deposits.
Spherical particles are observed in APS and VPS-lo samples; very limited number of spherical particles is observed for VPS-mi and VPShi samples (Fig. 6). The spherical particles indicate incompletely molten or resolidified feedstock at the point of impact. Their presence is given by low torch power or non-optimal trajectory of the particles through the plasma plume. The grain size of the W deposit is predetermined by the porosity and thermal history of the specimen (Fig. 6). In APS samples, the porosity and oxide layers stop grain growth at splat interfaces. For VPS-lo and VPS-mi samples, grains extending through splat interfaces and stopping at sublayer interfaces were observed. The condensate and high intersplat porosity of VPS-hi splat interfaces stop the grain growth and limit the heat transfer. Therefore, the grains grow preferably inside the splats and cross the intersplat interfaces at higher substrate temperature. The intersplat porosity of VPS-hi substrate is promoted by impact splashing caused by high particle impact temperature and velocity (Table 2). The influence of substrate temperature on grain size was observed for all VPS sets. The most obvious increase of grain size with substrate temperature can be seen for VPS-mi set (see Fig. 6).
independent on Ts, but strong correlation can be found for other sets (e.g. thermal conductivity in Fig. 7). It should be noted that the measured hardness reflects both in-plane and through-thickness properties, with pronounced effect of the through-thickness properties. Coating modulus was measured in the in-plane direction by four-point bending, whereas thermal conductivity was measured in the through-thickness direction. 3.4.1. Hardness For APS specimens, the substrate temperature was too low to influence the coating hardness and no trend was observed. In the same Ts range, i.e. below 400 °C, the VPS sets showed no hardness increase either. However, for Ts above 400 °C, hardness increase was observed for all VPS sets. According to the Hall–Petch relation, the increase of grain size with temperature should lead to decrease of hardness. The opposite effect was observed, however, indicating that the effect of intersplat sintering is stronger than the effect of grain size. For all substrate temperatures, hardness value of VPS-hi was slightly lower than those of other two VPS sets. This was caused by limited intersplat sintering due to the condensate on intersplat boundaries.
3.4. Properties of the deposits The dependence of relative density, coating modulus, hardness and thermal conductivity on substrate temperature is summarized in Fig. 7. It is clearly indicated that for some sets, certain properties appear
Fig. 4. XRD patterns (Cu anode) of coating surfaces. APS samples deposited at different Ts with increasing oxide content and oxide free VPS-hi sample. The oxide concentrations in wt.% estimated by Rietveld analysis are shown in brackets. Intensity is in arbitrary units.
3.4.2. Density The relative density ρ/ρ bulk of VPS specimens deposited for T s b 400 °C was nearly constant. The APS deposits showed significant increase of ρ/ρbulk with Ts, suggesting the oxides increase the sensitivity of ρ/ρbulk to Ts. The slower cooling of W splats deposited on thicker oxide layer can improve splat formation and eliminate porosity. The oxide layer between splats however will degrade the mechanical properties despite the porosity decrease. Therefore the decreased porosity is not accompanied with mechanical property increase. The deposition by APS for Ts N 400 °C was not performed due to expected heavy oxidation of W deposit. The ρ/ρbulk of VPS deposits increased rapidly at Ts above 400 °C. This behavior may be connected to transition temperature effect taking place above 400 °C and leading to disc splat formation. The relative density of the coating is a natural indicator of the quality of the plasma spray process and is related to other measured properties as well. An example is the hardness relation to relative density shown in Fig. 8. Two distinct regions are separated by a threshold density. For ρ/ρbulk b 90%, the hardness shows no trend. Above 90% of the hardness follows an increasing linear relation to relative density. 3.4.3. Coating modulus of elasticity Only weak influence of substrate temperature on coating modulus E4pb was observed for samples from the same set. However the change of spray technology and torch power significantly influences the coating modulus. The porous microstructure and weak splat bonding of APS deposits resulted in very low modulus with an average value of 20 GPa that is comparable with the measurement error. The mean value of
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Fig. 5. Sublayers formed in VPS deposits. Metallographic cut in a plane marked by gray color in the inset drawing. Polished, electrolytically etched, was observed in a light microscope. Rotational sublayer is formed by one rotation of the holder. Translational sublayer is formed by one translation of the holder and consists of multiple rotational sublayers.
the coating modulus was 136.2, 192.2 and 122 for the VPS-lo, VPS-mi and VPS-hi sets, respectively. The VPS-lo and VPS-hi deposits exhibited slight increase of modulus with Ts that can be characterized by a linear relation with slope 60 MPa/K for VPS-lo and 110 MPa/K for VPS-hi. The low thickness of the VPS-mi coatings resulted in a scatter of measured modulus that does not allow the modulus on temperature relation to be established.
and intersplat sintering. In the VPS-hi set, the condensate and porosity at intersplat boundaries prevent intersplat sintering and thus limit the heat transfer. As a result, thermal conductivity does not show a relation to substrate temperature despite the fact that the coating density increases with substrate temperature.
4. Conclusions 3.4.4. Thermal conductivity Thermal conductivity k was measured on samples with sufficient coating thickness, i.e. on the VPS-lo and VPS-hi sets. The increase of thermal conductivity with substrate temperature was detected for the VPS-lo set, resulting from combined effect of splat morphology change
An experimental methodology was developed that enables to deposit coatings on substrates of different temperatures while keeping other deposition conditions constant. Four sets of W deposits were prepared and the deposit properties were measured.
Fig. 6. Microstructures of the deposits as functions of temperature and spray technology. White arrows indicate unmelted/resolidified spherical particles.
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Fig. 7. The dependence of relative density, microhardness, coating modulus by four point bending, and thermal conductivity on substrate temperature.
The results indicate that changing the substrate temperature is an efficient way for tailoring deposit properties. Splat morphology, grain size, and intersplat sintering are properties that are influenced by substrate temperature Ts. The measured values of density, hardness, coating modulus, and thermal conductivity indicate that dense W deposits with good mechanical properties can be obtained for substrate temperature above 400 °C in the protective atmosphere of a deposition chamber. Apart from substrate temperature, the torch power and resultant particle in-flight properties play an important role. For low torch power, the melting state of the particles is insufficient for regular splat formation. For high torch power, on the other hand, the vaporization and
condensation of W were observed to decrease the values of all measured coating properties. When optimal torch power is applied, only a low amount of condensate is present, and the melting state of the particles is favorable to obtain dense and well sintered deposit. For this spray condition, the substrate temperature can efficiently control the grain size, hardness, coating modulus and thermal conductivity of the deposit. Acknowledgment This research was supported by the Czech Science Foundation project No. 108/12/1872. The deposition by RF plasma was performed under the support of Tekna Plasma Systems Inc. and Centre de Recherche en Énergie, Plasma et Électrochimie (CREPE) de l'Université de Sherbrooke in Sherbrooke, Québec. Aleš Jäger from the Institute of Physics, ASCR is acknowledged for the preparation, observation and analysis of the HR-SEM samples in the scope of project KAN300100801. References
Fig. 8. The influence of relative density on microhardness (1000 g load).
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