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Substrate temperature effects on the splat formation, microstructure development and properties of plasma sprayed coatings
Materials Science and Engineering A272 (1999) 189 – 198 www.elsevier.com/locate/msea
Substrate temperature effects on the splat formation, microstructure development and properties of plasma sprayed coatings Part II: case study for molybdenum X. Jiang *, J. Matejicek, S. Sampath Center for Thermal Spray Research, Department of Materials Science and Engineering, State Uni6ersity of New York at Stony Brook, Stony Brook, NY 11794 -2275 USA Received 12 January 1999; received in revised form 11 May 1999
Abstract The effects of substrate temperature on plasma sprayed molybdenum splat morphology, deposit microstructure development, and properties have been investigated. The molybdenum splat morphology changes from a fragmented (splashed) to a more contiguous disk shape with increasing substrate or deposit surface temperature. Furthermore, the flattening ratio of the splats prepared on glass is significantly higher than those prepared on steel or molybdenum substrates. With an increase in substrate temperature the deposit exhibits improved lamellar structure with less interlayer pores and fine particle debris (arising from splashing). The oxygen content in the deposit increases with substrate temperature associated with increased surface oxidation subsequent to deposition. The thermal conductivity, hardness and intesplat contact area are enhanced with increased substrate temperature. The fracture characteristics change gradually from interlamellar to translamellar, indicating enhanced interlamellar bonding among splats. The residual stress changes gradually from tensile to compressive with increasing temperature, its magnitude depending on the substrate materials and associated thermal mismatch stresses. A higher substrate temperature dramatically improves the adhesion and bonding of the splats and, therefore, enhances the physical and mechanical properties of the coatings. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Thermal spray; Molybdenum; Microstructure; Substrate temperature
1. Introduction Thermal spraying involves the melting of powder particles in a flame and the subsequent impact of molten droplets on to a substrate to form single splats. The successive impingement and accumulation of these splats forms an integrated deposit. The deposit formation dynamics and microstructure development depend on the droplet interactions with the substrate and/or the previously deposited layers, including their spreading and solidification behavior. The chemical reaction
This paper is dedicated to Professor Herbert Herman on the occasion of his 65th birthday. * Corresponding author. Tel.: + 1-516-632-8515; fax: + 1-516-6328440. E-mail address: [email protected] (X. Jiang)
of in-flight droplets with environmental gases and/or the substrate will affect bonding to the substrate and among the splats themselves. These factors have a critical influence on the microstructure and properties of the deposited coatings. Residual stresses are generated during the deposit build-up, originating from the splat quenching and thermal expansion mismatch between the deposit and the substrate, both of which affect the mechanical response of the coating. Splat morphology plays a crucial role in deposit microstructure and properties. Contiguous disk-shaped splats can improve the bonding between splats through the elimination of porosity among them. Oxidation can be minimized by eliminating the splashing induced debris, which have larger specific surface areas, and, therefore, higher oxide content. Porosity can be reduced by avoiding the formation of pores between the debris
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X. Jiang et al. / Materials Science and Engineering A272 (1999) 189–198
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and the overlaying molten droplets or by making the droplet achieve superior penetration with the surface ‘valleys’. It has been reported that substrate temperature strongly affects the morphology of splats [1,2]. Bianchi et al. [1] observed that the morphology of Al2O3 and ZrO2 splats changes from fragmented to a contiguous disk-like shape with an increase of substrate temperature. Fukumoto et al. [3] observed that in several systems (Ni, Cu, Mo, Cr, etc.) splat morphology changes with substrate temperature. They suggested that there exists a transition temperature of the substrate above which contiguous splats are obtained. The mechanism of splat morphology change with substrate temperature is not fully understood at this time. Vardelle et al. [4] postulated that above the transition substrate temperature, nucleation is delayed and the splat remains molten until the spreading is completed. Fukomoto et al. [3] speculated that the solidification is slowed down above the transition temperature, resulting in more uniform solidification, which does not disturb the droplet spreading. A recent investigation by Li et al. [5] suggested the potential role played by the adsorbed gases at the droplet – substrate interface on the droplet splashing. The splat thickness determines the total interface area per unit volume of the deposit and is affected by particle parameters as well as droplet flattening and spreading. Solidification velocity of the splats, microstructure development and phase selection are very sensitive to the splat thickness. Additionally, splat thickness is determined by the flattening and spreading process of the molten droplet. Analytical and numerical models [6–11] have been proposed to describe the spreading process and to predict the flattening ratio, j, of a splat (defined as the ratio of splat diameter and the diameter of the original droplet). However, these models assume ideal splat spreading behavior on well polished surfaces and, therefore, limit their applicability. Experimental analysis in combination with analytical interpretations is required to fully comprehend the process complexities. Coating adhesion/cohesion and residual stress are also strongly dependent on substrate condition, and are closely interconnected as demonstrated by Dallaire [12]. Gawne et al. [13] calculated the temperature gradients in single splats due to rapid quenching upon impact on the substrate and the resulting thermal stresses. At lower
substrate temperatures, there are larger temperature gradients, and therefore larger stresses, which undermine splat adhesion. Clyne and Gill [14] reviewed the effects of residual stress on the interfacial adhesion on a macroscopic level. In this fracture mechanics approach, residual stress (as well as applied stresses) have direct influences on the strain energy release rate for interfacial debonding. The interrelation could be also viewed from the other side, i.e. the coating/substrate adhesion is the limiting factor giving an upper bound to possible magnitude of stress [15]. Substrate temperature is a key factor in determining the magnitude of quenching stress and thermal stress [16,17]. Although there are number of papers dealing with certain individual aspects of thermal spray coatings, comprehensive, integrated studies are limited owing to the complexity of the process and deposit microstructure Here, splat formation, deposit build-up, microstructure as well as residual stresses are investigated in an integrated manner so as to correlate the mechanical and physical properties of molybdenum coatings with processing through the microstructure development. The results show that microstructure development and properties of the coatings are very sensitive to deposition substrate temperature. This relation can be better elucidated by a fundamental understanding of splat formation, splat pile-up and their interactions, which is the purpose of this investigation. 2. Materials and experimental methods Plasma-densified spherical molybdenum powder in the range 5–44 mm and a mean size of 30 mm was provided by Osram Sylvania (designation SD 152). The oxygen content of the starting powder was approximately 0.1%. Processing was carried out with a Sulzer Metco PTF4 plasma gun under ambient atmosphere. Mild steel, molybdenum and glass substrates were used in the splat experiments. The substrates were fixed in a fixture rotating at 160 rpm, 110 mm away from the exit of the torch and heated with plasma flame. The surface temperature was monitored with an optical pyrometer and calibrated with K-type thermocouple. The substrate temperature was controlled by plasma flame preheating and air-cooling during the deposition. Details of the plasma and spray condition are listed in Table 1.
Table 1 Plasma spray parameters for splats and coatings Current Voltage Nozzle dia- Plasma gas and flow Carrier gas and (A) (V) meter (mm) rate (l min−1) flow rate (l min−1)
Powder injector diameter (mm)
Feed rate Torch traverse Nozzle/substrate (g min−1) speed (mm s−1) distance (mm)
500
1.8
38 (deposit)
70
8
Ar: 50, H2: 10
Ar: 3.5
30
110
X. Jiang et al. / Materials Science and Engineering A272 (1999) 189–198 Table 2 Overview of the specimen types and substrate temperaturesa Specimen type
Splat I
Low temperature (°C) 50 Medium temperature (°C) 200 High temperature (°C) 440 a
Thin II
Thick III
60 200 340
190 260 440
Surface temperature as measured by pyrometer.
Three sets of specimens were prepared for the studies of substrate condition effects: (I) single splats on polished substrates, (II) thin coatings (approximately 3 mm) and (III) thick coatings (approximately 1 mm). In each set, three different substrate temperatures were used, as well as different substrate materials (see Table 2). Substrates for the single splat studies were ground and polished to a roughness less than 0.1 mm in order to eliminate the possible roughness effects. For the study of temperature effects on stress, thin coatings (approximately a monolayer of splats) were deposited on lightly ground substrates in order to eliminate the effects of substrate compliance and through-thickness stress gradients. Coatings (1 mm thick ) were produced at three different temperatures, on steel, aluminum and copper substrates. The splats were observed with optical microscopy and scanning electron microscopy (SEM). The dimensions of splats were measured with a Zygo non-contact surface profiler (a scanning white-light interferometer) [18]. Thermal conductivity measurements of the coatings were carried out on free-standing specimens by a
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laser flash technique, using a Holometrix thermal properties instrument. Open porosity was measured by the mercury intrusion technique, using the Quantachrome Autoscan 33 porosimeter. Murakami’s reagent was used for etching the deposits. Residual stresses were measured in (a) thin coatings by X-ray diffraction (‘sin2 c’ technique), using a Siemens D500 v-diffractometer with Ni-filtered Cu radiation [16], reflection from the (321) crystal planes with elastic constants E= 313 GPa and n=0.31; (b) individual splats by microdiffraction, using a Bruker GADDS microdiffractometer with Cr radiation [19] and from reflection from the (211) planes, with the same elastic constants. Microhardness was measured with a Buehler Micromet II microhardness tester using a Vickers indentor and a 500 g load. The elastic modulus was measured by means of four-point bending.
3. Results
3.1. Effects of substrate temperature and substrate materials on splat morphology Fig. 1(a) illustrates the typical morphology of molybdenum splats on mild steel prepared at room temperature. The splats fractured into several large fragments, which can be roughly fitted into a single piece. The uneven edge suggests that the break-up occurred while the splat was in the solid or semi-solid state. There is
Fig. 1. Morphologies of molybdenum splats formed on mild steel substrate at different temperatures and the corresponding 3-D surface profiles. (a, c) Room temperature, (b, d) 200°C.
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Fig. 2. Morphology of molybdenum splats formed on molybdenum substrate and glass substrate at different temperatures. Molybdenum substrate: (a) room temperature, (b) 340°C; glass substrate: (c) room temperature, (d) 340°C.
evidence of liquid-like flow pattern in the ‘valley’ between the fragments, which suggests that substrate melting occurred. Furthermore, the surface topographical measurement revealed that the ‘valley’ between the fragments is lower than the substrate plane (see Fig. 1(c)). Due to the lift-over of the fragments near the outer periphery, they look much thicker than the central sections. Almost all splats on the room temperature sample have this feature. The sample prepared at 200°C, as shown in Fig. 1(b), is a mixture of broken splats and disk shaped splats. Fig. 1(d) shows the surface profile of a typical disk-shaped splat. The morphology of the splats prepared at about 400°C is similar to that of the splats prepared at 200°C, although the proportion of disk-shaped splats is higher. In the deposit build-up process, the droplets spread and solidify on molybdenum after the first layer is deposited. Therefore, the splats were prepared on a molybdenum substrate in order to examine the role of the substrate chemistry. As shown in Fig. 2, at room temperature, the splat surface is not flat and shows a tendency for splashing at the periphery. At 340°C, the splats are disk-like, although their rims are not as smooth as those of splats made on a hot steel substrate. The splats were also formed on a glass substrate because of its vastly different thermo-physical properties, especially thermal conductivity. The splats made at room temperature exhibited a fragmented disk shape, with projection along the periphery of central disk. The
splats formed at 340°C showed contiguous disk-like shapes with a distinguishable rim. These results show that the substrate materials and temperature dramatically affect the morphology of the splats. For splats produced at room temperature, their morphology exhibits a very different pattern: fragmented on the steel substrate and splashed on the glass substrate, respectively.
3.2. Flattening ratio of molybdenum splats The flattening ratio is the ratio of the splat diameter and the diameter of the droplet, which forms the splat. By using a 3-D surface profilometer, the thickness and diameter of the disk-shaped splats formed on a hot substrate were measured and the volume of the splats and the diameter of the droplet were calculated from these data. The flattening ratio and the thickness of the
Fig. 3. Flattening ratio and thickness of molybdenum splats prepared on steel, molybdenum and glass substrates. Steel at 200°C, molybdenum at 340°C, glass at 340°C.
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Fig. 4. Splat morphologies of molybdenum on glass (a) and on grit-blasted steel substrate (b, c). (a) Glass at 200°C, (b) steel at 115°C, (c) steel at 250°C (roughness of the steel substrate, RA =3.4 mm).
splats produced on hot steel, molybdenum and glass substrates are listed in Fig. 3. The flattening ratio of the splats formed on the steel substrate is between 2.0 and 2.8 with an average of 2.4, and with an average splat thickness of 2.4 mm. The flattening ratio and thickness of the splats made on a hot molybdenum substrate is similar to those of the splats made on the hot steel substrate. The flattening ratio of the splat made on glass is from 3.0 to 6.5 with an average of 4.5, and the splat thickness is from 0.2 to 1.4 mm with an average of 0.7 mm. The large range of flattening ratio is not a measurement error, but is rather associated with particle size, temperature and velocity variations as well as the deposition characteristics.
3.3. Microstructure de6elopment during coating buildup The deposit is generated by the successive impingement of droplets and accumulation of individual splats. Therefore, the interaction between the solidified splat and the subsequent droplet is a key step in determining microstructure development. In this study, it was found that the molybdenum droplet in general, can spread evenly on top of the hot, previously deposited splats to form lamellar structures in the deposit. Fig. 4(a) demonstrates this effect and shows that the droplets can spread and slip down the splat surface, with no splashing or fragmenting on the molybdenum surface. The spreading can follow the substrate or previously deposited layer surface ‘terrain’ in its spreading path reasonably well, suggesting that there is some morphology heritage in the coating build-up. This fact suggests that the morphology of the first layer of splats does play a role in the overall deposit lamellar structure. Fig. 4(b) and (c) show typical splats formed on a grit-blasted (roughened) steel substrate. At lower temperatures there is severe splashing on the rough substrate, presumably due to asperities and air pockets trapped underneath the liquid droplet, which expands by the input of heat brought by the droplet and breaksup the splat into small pieces. At higher temperatures the central part of the splat, which accounts for most of
the droplet volume, is contiguous and significantly smoother than the substrate surface. Only the uneven splat rim and the debris scattered near the periphery of the splat indicates that some splashing has occurred. The ability to reduce splat fragmentation at higher substrate temperatures (i.e. above 200°C), even on a roughened substrate illustrates the significance of this effect. This allows a more efficient lamellar microstructure development through enhanced spreading and contact. In some cases as shown in Fig. 4(c), the initial layers can provide ‘surface smoothening’ through the filling of valleys by the droplets. This allows a more uniform lamellar microstructure to develop within the deposit. Fig. 5 shows the cross sections of deposits produced on a pre-heated substrate with and without cooling, with measured substrate temperatures on the surface stabilized around 260 and 440°C, respectively (the 190°C microstructure is similar to the 260°C microstructure). Both deposits show lamellar morphologies, which arise from the spreading and flattening of droplets on the previously deposited splats. The layer boundaries can be seen clearly. At the locations where the splat edges meet one another, breakdown of the lamellar structure is observed. In these ‘non-lamellar’ areas, there are numerous fine particle debris and small pores. The columnar grain structure can be discerned within each layer after etching (see inset Fig. 5). It suggests that at the moment the droplets impacted on the top of the deposit, the splat beneath was completely solidified; the droplets spread on the pre-deposited splats, nucleating and solidifying from the bottom in a columnar fashion. This can be further explained by the fact that it took about 10 min to deposit a 1 mm coating, therefore, taking an average splat thickness of 2.5 mm, the time interval for the second droplet to arrive at the same location is around 1 s. Assuming a cooling rate of 106 K s − 1, the previous splat is solidified long before the arrival of the second droplet. The deposit made at high substrate temperature (440°C) has a uniform lamellar structure with a few discrete, small, shallow pores at the conjunctions of the splats; these
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pores are probably due to the pull-outs of small oxidized debris. The 260°C deposit has more porosity and clusters of debris which originated from the splashing or break-up of the splats at a lower substrate temperature. The deposit made at a substrate temperature of about 190°C has a microstructure similar to that of the 260°C deposit indicating a transition temperature in the 300–400°C range for this system. The average thickness of the splats were measured for the three coatings, by the linear intercept method. The values are 2.7, 2.7 and 2.3, respectively, for the coatings made at 190, 260 and 440°C. The values are similar to that of average thickness of the single splats discussed earlier. The somewhat thinner high temperature sample splats is indicative of greater flattening. If we assume 2.5 mm as the average layer thickness, it indicates that there are 400 layer interfaces in a 1 mm thick coating, 0.4 m2 interface area per cm3 coating volume. Such a large interface area implies that the inter-lamellar adhesion has a significant effect on the coating properties. The slight difference in average thickness comes probably from the fact that the flattening ratio is affected by the substrate roughness [20]. At high substrate temperature, the splats have contiguous shape, therefore the following droplets spread on a smoother surface; whereas the splats deposited on substrates at lower temperature have a fragmented morphologies. The splats on top of those will have a smaller flattening ratio, therefore a greater layer thickness. Fig. 6 shows the fracture surface of deposits produced at 190 and 440°C. The fracture changes from inter-lamellar to trans-lamellar with an increase in substrate temperature. For the deposit prepared at 190°C, a significant portion of the fracture took place by delamination along the interface between individual splats and through the poorly bonded regions. In the 440°C coating, fracture took place mostly through trans-lamellar crack propagation, and much less along the splat interfaces. This suggests that a coating de-
posited at a higher temperature has significantly improved bonding between the splats (layers) compared to a coating made at lower temperature as confirmed by the thermal conductivity results (see below). Deposit properties are improved with a increase in the substrate temperature as shown in Table 3. The hardness, modulus and thermal conductivity increase is particularly notable from the 260°C deposit to the 440°C deposit, illustrating the sharp transition in substrate temperature effect. The hardness of the deposits associated with oxidation is higher than that of sintered molybdenum, which is typically 200–300 HV. Thermal conductivity data provide quantitative information on the contact area between individual layers (the ratio of the true contact area to the overall interface area). By comparison with the thermal conductivity of the bulk material and using the model of McPherson [21], the contact area ratios were calculated. This result indicates nearly 3-fold improvement in interlayer bonding at high temperature in spite of a doubling of the oxide content.
3.4. Residual stresses Residual stress is strongly affected by substrate temperature. Fig. 7 shows the temperature dependence of residual stress in thin coatings (3 mm in thickness — approximately one layer of splats) on steel and aluminum substrates. It can be seen that for both substrates, the stresses change from tensile to compressive as the temperature rises. This can be explained by different proportions of quenching and thermal stress which have opposite signs [14]. The quenching stress is tensile, whereas the thermal stress is, in this case, compressive, since the thermal expansivity of molybdenum is smaller than that of steel and aluminum. When the substrate temperature increases, the thermal stress upon cooling to room temperature is higher, therefore the residual stress (being the sum of the quenching and thermal stresses), shifts towards compression. This effect is more pronounced for the aluminum substrate, where the thermal mismatch is larger.
Fig. 5. Cross section micrographs of the deposits with etched inset showing grain structures: p, pore; d, debris; I, inter-lamellae crack. (a) Substrate temperature 260°C, (b) substrate temperature 440°C.
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Fig. 6. SEM micrographs of the fracture surface of the deposits. (a) Substrate temperature 190°C, (b) substrate temperature 440°C, d: delaminated regions, s: splat fracture regions.
Experimentally observed residual stress values are significantly lower than those that could be calculated from the temperature history and elastic properties. The actual stress magnitudes are limited by the coating’s adhesion to the substrate and bonding between the individual splats [15]. Higher magnitudes of residual stresses at increased temperature suggest that the bonding is improved, in comparison with depositions on low-temperature deposits, as evidenced by the hardness, modulus, thermal conductivity and contact area results. Also, the change of stress with temperature is steeper in the higher temperature range. Residual stresses in the coatings deposited on copper, glass and polished steel substrates had near-zero values, indicating that the adhesion strength in these cases are very low and that the bonding to rough surfaces is mostly of mechanical nature. One coating deposited on steel at 340°C was reheated to 200°C by four passes of the plasma torch without any powder. The resulting stress, −170 MPa, as compared to − 50 MPa of the as-sprayed specimen, indicates that the quenching stress was significantly reduced by the heat from the flame (recovery) and that the residual stress present is mainly due to thermal mismatch. Similar processes take place during actual processing of thicker deposits, although the heat input from newly deposited layers may differ from the flame alone. In thicker coatings, there exist significant stress gradients. Therefore, a single value is insufficient to describe the stress state. The effect of coating thickness on residual stress is the subject of a separate investigation [22].
4. Discussion
4.1. Splat morphologies and flattening ratio The mechanisms of molybdenum splat fragmentation are different on steel, molybdenum and glass substrates.
It appears that the breaking-up of a splat on the steel substrate is related to the localized substrate melting as found in this study and implied by Houben [23]. One dimensional heat transfer modeling for molybdenum splats on steel confirms this phenomenon [24]. At a higher substrate temperature, with an enhanced contact area, the heat flow might spread out more evenly, reducing (or possibly eliminating) the severity of the substrate melting. For molybdenum and glass substrates, at a higher substrate temperature, in addition to a possible wetting improvement at the interface, the gas absorbed is less. Less gas dissociated from the interface between the splat and the substrate at the input of heat from the molten droplet imposes smaller disturbance to the spreading of the droplet and is favorable for the formation of contiguous splats. This is consistent with the result obtained by Li et al. [5]. This is subject to further investigation. In the preparation of molybdenum splats, the same powder and process conditions were used except for the substrate materials. The striking difference in the average flattening ratio and splat thickness on metal and glass indicates that on metal, the droplet spreading was not as complete as on glass. It has been shown experimentally and analyzed theoretically that in the case of dynamic spreading, equilibrium surface tension is negliTable 3 Selected properties of the deposits Parameters
Oxygen content (wt.%) Porosity (%) Microhardness HV (cross section) Elastic modulus (GPa) Thermal conductivity (W K−1 m−1) Contact area ratio (%)
Substrate temperature (°C) 190
260
440
1.3 8.6 91 365 931
1.9 7.5 9 1 358 9 34
2.5 4.7 91 544937
49 9 12 14.7 91
40 911 194 9 25 15.9 91 36.59 8
9
9
26
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Fig. 7. Residual stresses in thin molybdenum deposits on steel and aluminum substrates, deposited at different temperatures.
gible in the determination of the contact angle [25]. This strongly suggests that on metal substrate, solidification is the arresting factor for the droplet spreading. Steel has much higher thermal diffusivity than glass. The melting of the steel substrate indicates that the splats were in good contact with the substrate, therefore the interface heat transfer coefficient is high, which greatly accelerates the heat extraction from the splat to the substrate. The solidification rate of molybdenum on steel is much higher than that on glass according to the Stefan model [26]. Therefore, the solidification time scale will be dramatically shortened, compared to the case for the glass substrate. An analytical model is being developed to further describe the effect of substrate material properties on droplet spreading [27].
4.2. Microstructure build-up and properties This study shows that even on a rough substrate, a uniform lamellar microstructure can be developed. The droplets spread on the previously deposited splats in ways similar to that on a flat substrate, except at the periphery where splashing occurs and results in inhomogeneities. In general, with an increase of substrate temperature, two processes are enhanced which affect the deposit microstructure build-up and properties in opposite ways. At a higher temperature the splats fragment less; deposit porosity is reduced, and the adhesion between adjacent splats is enhanced by enhancing the diffusion at the interface. On the other hand, oxidation is more severe at high substrate temperatures for metals as shown in Table 3. Although the oxide particles dispersed in the splat or the supersaturated oxygen can strengthen the splat (which is why a higher hardness is observed than that of the sintered materials), the oxide layer on the interface as a less ductile phase can potentially reduce the
strength and ductility of the deposit. The overall deposit properties depend on which effect is decisive. Through thickness thermal conductivity is controlled by the effective bonding area and voids morphology [28]. The increase of oxide content in general can decrease the thermal conductivity of the splat. The increase of thermal conductivity with substrate temperature suggests that there is a dramatic improvement in adhesion/effective contact area despite the sharp increase in oxygen content. The thermal conductivity of the deposits produced at 190, 260 and 440°C is 10, 11 and 26% of that of sintered pure molybdenum (142 W K − 1 m − 1), respectively. The presence of lamellar pores at the splat–splat interface and the high oxygen content in the coating are responsible for the overall thermal conductivity reduction of the sprayed materials, the former being the more dominant component. The high hardness of sprayed molybdenum can be attributed to the fine grain structure associated with the deposits and to the presence of oxides. X-ray diffraction, transmission electron microscopy and the Xray photoelectron spectroscopy studies of the deposits and splats reveal that the main oxide phases are MoO2 and Mo4O11 and they are present as fine particles dispersed inside the lamella [29]. The improvement of adhesion with an increase of substrate temperature is not fully understood at this time but may be due to several considerations: (1) Wetting is improved at higher temperature, which leads to better splat–splat contact [30]. (2) Less air is trapped in the air pocket underneath the splats at higher temperature because of the smaller air density, resulting in lower porosity. (3) A smaller temperature gradient through the splats reduces curling of the splats, etc, improves the splat–splat contact. Another possible mechanism of adhesion improvement comes from the fact that the melting point of MoO3 is only about 800°C. This surface oxide layer may dissolve into the impinging droplet and form a strong bond between the splats. A higher temperature is beneficial to this process. It is evident that more ‘regular’ splat shape and better contact at high substrate temperatures improve inter-lamellar adhesion. Good inter-lamellar adhesion, along with the higher oxide content leads to higher hardness and thermal conductivity of the deposit.
5. Conclusions This paper presents an integrated approach towards examining and developing processing–microstructure – property relationships associated with thermal spraying. Understanding and controlling splat formation
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provides a vital link to microstructure development and thus coating properties. Substrate materials and temperatures have significant effects on the morphology of molybdenum splats formed in the thermal spray process. With a temperature increase from room temperature to 400°C, the molybdenum splat morphology changes from fragmented to predominantly disk shaped. This has also been shown for a number of other systems although the exact mechanism of changes at the interface is not fully understood. The average flattening ratio of molybdenum splats on glass and steel substrates is 4.5 and 2.4 and the average splat thickness is 0.7 and 2.4 mm, respectively. The differences indicate that the larger solidification velocity of molybdenum splats on the steel substrate (which has a higher thermal diffusivity than that of the glass substrate) arrests droplet spreading through stronger interactions. With an increase in substrate temperature, and associated improvements in splat morphology, the deposits exhibit a more uniform lamellar morphology with less interlayer porosity and debris as well as improved bonding among the layers. The fracture characteristics change from interlamellar to translamellar fracture illustrating this enhanced adhesion among the layers. With an increase of the substrate temperature the mechanical properties and the through thickness thermal conductivity increase and the porosity decreases. Significantly enhanced thermal conductivity at a high substrate temperature is confirmation of the adhesion improvement. Residual stress in the thin coatings changes from tensile to compressive as the substrate temperature increases for certain substrates. This is attributed to the bigger role of the thermal mismatch stress in the case of steel and aluminum and enhanced interlayer adhesion. This change of sign was observed over a reasonably achievable range of temperatures, which makes this parameter an effective means of controlling the stress level. This study confirms that substrate temperature is a critical factor in plasma spray processes and can be used effectively to modify the microstructure with implications for enhanced properties and performance.
Acknowledgements This work was supported by the MRSEC Program of the National Science Foundation under Award Number DMR-9632570. Osram Sylvania Inc. is gratefully acknowledged for providing the feedstock powder. Stimulating discussions with Drs R. Goswami, G.-X Wang and A. Vardelle are appreciated. We thank Pro-
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fessor H. Herman for his inspiration and guidance in our activities at the Center. The science base for this industrial technology is where it is today due to his untiring efforts.
References [1] L. Bianchi, F. Blein, P. Lucchese, M. Vardelle, A. Vardelle, P. Fauchais, in: C. Berndt, S. Sampath (Eds.), Thermal Spray Industrial Applications, ASM International, Metals Park, OH, 1994, p. 569. [2] A.C. Leger, M. Vardelle, A. Vardelle, et al., in: C. Berndt (Ed.), Practical Solution for Engineering Problem, ASM International, Metals Park, OH, 1996, p. 623. [3] M. Fukumoto, S. Katoh, I. Okane, in: A. Ohmori (Ed.), Proceeding of the Integrated Thermal Spray Conference, Kobe Japan, May, 1995, Japan High Temperature Society, Japan, 1995, pp. 353 – 358. [4] A. Vardelle, N.J. Themelis, B. Dussoubs, et al., High Temp. Process. 3 (1997) 295. [5] C.J. Li, J.L. Li, W.B. Wang, in: C. Coddet (Ed.), Thermal Spray, Meeting the Challenge of the 21st Century, ASM International, Metals Park, OH, 1998, p. 473. [6] J. Madejski, Int. J. Heat Mass Transf. 19 (1976) 1009. [7] T. Bennett, D. Poulikos, J. Mater. Sci. 28 (1993) 963. [8] G. Trapaga, E.F. Matthys, J.J. Valencia, J. Szekely, Metall. Trans. B23 (1992) 701. [9] H. Liu, E. Lavernia, R.H. Rangel, J. Phys. D: Appl. Phys. 26 (1993) 1900. [10] R.C. Dykhuizen, J. Therm. Spray Technol. 3 (1994) 351. [11] M. Vardelle, A. Vardelle, A.C. Leger, P. Fauchais, in: C. Berndt, S. Sampath (Eds.), Thermal Spray Industrial Applications, ASM International, Metals Park, OH, 1994, pp. 555 – 562. [12] S. Dallaire, Thin Solid Films 95 (1982) 237. [13] D.T. Gawne, Y. Bao, T. Zhang, Proceedings United Thermal Spray Conf., Indianapolis, USA, 14 – 18 September 1997, ASM International, Metals Park,OH, 1997, p. 467. [14] T.W. Clyne, S.C. Gill, J. Therm. Spray Technol. 5 (1996) 401. [15] S. Kuroda, T. Dendo, S. Kitahara, J. Therm. Spray Technol. 4 (1995) 75. [16] J. Matejicek, S. Sampath, J. Dubsky, in: C. Berndt (Ed.), A United Forum for Scientific and Technological Advances, ASM International, Metals Park, OH, 1997, p. 855. [17] P. Scardi, M. Leoni, L. Bertamini, Thin Solid Films 278 (1996) 96. [18] P. De Groot, L. Deck, J. Mod. Opt. 42 (1995) 389. [19] B.B. He, K.L. Smith, in: T. Ericsson (Ed.), Proceedings of the 5th International Conference On Residual Stresses, University of Linkoping, Sweden, 1997, p. 634. [20] A.C. Leger, M. Vardelle, A. Vardelle, B. Dussoubs, P. Fauchais, in: C. Berndt, S. Sampath (Eds.), Advances in Thermal Spray Science and Technology, ASM International, Metals Park, OH, 1995, p. 167. [21] R. McPherson, Thin Solid Films 112 (1984) 89. [22] O. Kesler, J. Matejicek, S. Sampath, et al., Mater. Sci. Eng. A257 (1998) 215. [23] M. Houben, Relationship of the Adhesion of Plasma Sprayed Coatings to the Process Parameters: Size, Velocity and Heat Content of the Spray Particles, PhD Thesis, Technische Universiteit Eindhoven, Eindhoven, Netherlands, December 1988 [24] S.P. Wang, G.-X Wang, E.F. Matthys, Mater. Sci, Eng. A 262 (1999) 25.
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[25] A. Passerone, in: B.N. Chapman, J.C. Anderson (Eds.), Science and Technology of Surface Coating, Academic Press, London, 1974, pp. 194 – 207. [26] F. Crank, Free and Moving Boundary Problems, Oxford Science Publications, Oxford, 1984, p. 101. [27] Y.P. Wang, H. Zhang, X.Y. Jiang, S. Sampath, V. Prasad, Proceedings of 33rd national Heat Transfer conference, August
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15 – 17, 1999 Albuquerque, New Mexico, Cd-ROM paper No. NHTC 99 – 121.. [28] K. Hollis, R. Neiser, J. Therm. Spray Technol. 4 (1995) 403. [29] R. Goswami, X.Y. Jiang, H. Herman, S. Sampath, Y. Tian, G. Halada, (unpublished results). [30] V.N. Eremenko, Y.V. Naidich, I.A. Lavrinenko, Liquid Phase Sintering, Consultants Bureau, New York, 1970, p. 50.
Substrate temperature effects on the splat formation, microstructure development and properties of plasma sprayed coatings Part II: case study for molybdenum X. Jiang *, J. Matejicek, S. Sampath Center for Thermal Spray Research, Department of Materials Science and Engineering, State Uni6ersity of New York at Stony Brook, Stony Brook, NY 11794 -2275 USA Received 12 January 1999; received in revised form 11 May 1999
Abstract The effects of substrate temperature on plasma sprayed molybdenum splat morphology, deposit microstructure development, and properties have been investigated. The molybdenum splat morphology changes from a fragmented (splashed) to a more contiguous disk shape with increasing substrate or deposit surface temperature. Furthermore, the flattening ratio of the splats prepared on glass is significantly higher than those prepared on steel or molybdenum substrates. With an increase in substrate temperature the deposit exhibits improved lamellar structure with less interlayer pores and fine particle debris (arising from splashing). The oxygen content in the deposit increases with substrate temperature associated with increased surface oxidation subsequent to deposition. The thermal conductivity, hardness and intesplat contact area are enhanced with increased substrate temperature. The fracture characteristics change gradually from interlamellar to translamellar, indicating enhanced interlamellar bonding among splats. The residual stress changes gradually from tensile to compressive with increasing temperature, its magnitude depending on the substrate materials and associated thermal mismatch stresses. A higher substrate temperature dramatically improves the adhesion and bonding of the splats and, therefore, enhances the physical and mechanical properties of the coatings. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Thermal spray; Molybdenum; Microstructure; Substrate temperature
1. Introduction Thermal spraying involves the melting of powder particles in a flame and the subsequent impact of molten droplets on to a substrate to form single splats. The successive impingement and accumulation of these splats forms an integrated deposit. The deposit formation dynamics and microstructure development depend on the droplet interactions with the substrate and/or the previously deposited layers, including their spreading and solidification behavior. The chemical reaction
This paper is dedicated to Professor Herbert Herman on the occasion of his 65th birthday. * Corresponding author. Tel.: + 1-516-632-8515; fax: + 1-516-6328440. E-mail address: [email protected] (X. Jiang)
of in-flight droplets with environmental gases and/or the substrate will affect bonding to the substrate and among the splats themselves. These factors have a critical influence on the microstructure and properties of the deposited coatings. Residual stresses are generated during the deposit build-up, originating from the splat quenching and thermal expansion mismatch between the deposit and the substrate, both of which affect the mechanical response of the coating. Splat morphology plays a crucial role in deposit microstructure and properties. Contiguous disk-shaped splats can improve the bonding between splats through the elimination of porosity among them. Oxidation can be minimized by eliminating the splashing induced debris, which have larger specific surface areas, and, therefore, higher oxide content. Porosity can be reduced by avoiding the formation of pores between the debris
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and the overlaying molten droplets or by making the droplet achieve superior penetration with the surface ‘valleys’. It has been reported that substrate temperature strongly affects the morphology of splats [1,2]. Bianchi et al. [1] observed that the morphology of Al2O3 and ZrO2 splats changes from fragmented to a contiguous disk-like shape with an increase of substrate temperature. Fukumoto et al. [3] observed that in several systems (Ni, Cu, Mo, Cr, etc.) splat morphology changes with substrate temperature. They suggested that there exists a transition temperature of the substrate above which contiguous splats are obtained. The mechanism of splat morphology change with substrate temperature is not fully understood at this time. Vardelle et al. [4] postulated that above the transition substrate temperature, nucleation is delayed and the splat remains molten until the spreading is completed. Fukomoto et al. [3] speculated that the solidification is slowed down above the transition temperature, resulting in more uniform solidification, which does not disturb the droplet spreading. A recent investigation by Li et al. [5] suggested the potential role played by the adsorbed gases at the droplet – substrate interface on the droplet splashing. The splat thickness determines the total interface area per unit volume of the deposit and is affected by particle parameters as well as droplet flattening and spreading. Solidification velocity of the splats, microstructure development and phase selection are very sensitive to the splat thickness. Additionally, splat thickness is determined by the flattening and spreading process of the molten droplet. Analytical and numerical models [6–11] have been proposed to describe the spreading process and to predict the flattening ratio, j, of a splat (defined as the ratio of splat diameter and the diameter of the original droplet). However, these models assume ideal splat spreading behavior on well polished surfaces and, therefore, limit their applicability. Experimental analysis in combination with analytical interpretations is required to fully comprehend the process complexities. Coating adhesion/cohesion and residual stress are also strongly dependent on substrate condition, and are closely interconnected as demonstrated by Dallaire [12]. Gawne et al. [13] calculated the temperature gradients in single splats due to rapid quenching upon impact on the substrate and the resulting thermal stresses. At lower
substrate temperatures, there are larger temperature gradients, and therefore larger stresses, which undermine splat adhesion. Clyne and Gill [14] reviewed the effects of residual stress on the interfacial adhesion on a macroscopic level. In this fracture mechanics approach, residual stress (as well as applied stresses) have direct influences on the strain energy release rate for interfacial debonding. The interrelation could be also viewed from the other side, i.e. the coating/substrate adhesion is the limiting factor giving an upper bound to possible magnitude of stress [15]. Substrate temperature is a key factor in determining the magnitude of quenching stress and thermal stress [16,17]. Although there are number of papers dealing with certain individual aspects of thermal spray coatings, comprehensive, integrated studies are limited owing to the complexity of the process and deposit microstructure Here, splat formation, deposit build-up, microstructure as well as residual stresses are investigated in an integrated manner so as to correlate the mechanical and physical properties of molybdenum coatings with processing through the microstructure development. The results show that microstructure development and properties of the coatings are very sensitive to deposition substrate temperature. This relation can be better elucidated by a fundamental understanding of splat formation, splat pile-up and their interactions, which is the purpose of this investigation. 2. Materials and experimental methods Plasma-densified spherical molybdenum powder in the range 5–44 mm and a mean size of 30 mm was provided by Osram Sylvania (designation SD 152). The oxygen content of the starting powder was approximately 0.1%. Processing was carried out with a Sulzer Metco PTF4 plasma gun under ambient atmosphere. Mild steel, molybdenum and glass substrates were used in the splat experiments. The substrates were fixed in a fixture rotating at 160 rpm, 110 mm away from the exit of the torch and heated with plasma flame. The surface temperature was monitored with an optical pyrometer and calibrated with K-type thermocouple. The substrate temperature was controlled by plasma flame preheating and air-cooling during the deposition. Details of the plasma and spray condition are listed in Table 1.
Table 1 Plasma spray parameters for splats and coatings Current Voltage Nozzle dia- Plasma gas and flow Carrier gas and (A) (V) meter (mm) rate (l min−1) flow rate (l min−1)
Powder injector diameter (mm)
Feed rate Torch traverse Nozzle/substrate (g min−1) speed (mm s−1) distance (mm)
500
1.8
38 (deposit)
70
8
Ar: 50, H2: 10
Ar: 3.5
30
110
X. Jiang et al. / Materials Science and Engineering A272 (1999) 189–198 Table 2 Overview of the specimen types and substrate temperaturesa Specimen type
Splat I
Low temperature (°C) 50 Medium temperature (°C) 200 High temperature (°C) 440 a
Thin II
Thick III
60 200 340
190 260 440
Surface temperature as measured by pyrometer.
Three sets of specimens were prepared for the studies of substrate condition effects: (I) single splats on polished substrates, (II) thin coatings (approximately 3 mm) and (III) thick coatings (approximately 1 mm). In each set, three different substrate temperatures were used, as well as different substrate materials (see Table 2). Substrates for the single splat studies were ground and polished to a roughness less than 0.1 mm in order to eliminate the possible roughness effects. For the study of temperature effects on stress, thin coatings (approximately a monolayer of splats) were deposited on lightly ground substrates in order to eliminate the effects of substrate compliance and through-thickness stress gradients. Coatings (1 mm thick ) were produced at three different temperatures, on steel, aluminum and copper substrates. The splats were observed with optical microscopy and scanning electron microscopy (SEM). The dimensions of splats were measured with a Zygo non-contact surface profiler (a scanning white-light interferometer) [18]. Thermal conductivity measurements of the coatings were carried out on free-standing specimens by a
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laser flash technique, using a Holometrix thermal properties instrument. Open porosity was measured by the mercury intrusion technique, using the Quantachrome Autoscan 33 porosimeter. Murakami’s reagent was used for etching the deposits. Residual stresses were measured in (a) thin coatings by X-ray diffraction (‘sin2 c’ technique), using a Siemens D500 v-diffractometer with Ni-filtered Cu radiation [16], reflection from the (321) crystal planes with elastic constants E= 313 GPa and n=0.31; (b) individual splats by microdiffraction, using a Bruker GADDS microdiffractometer with Cr radiation [19] and from reflection from the (211) planes, with the same elastic constants. Microhardness was measured with a Buehler Micromet II microhardness tester using a Vickers indentor and a 500 g load. The elastic modulus was measured by means of four-point bending.
3. Results
3.1. Effects of substrate temperature and substrate materials on splat morphology Fig. 1(a) illustrates the typical morphology of molybdenum splats on mild steel prepared at room temperature. The splats fractured into several large fragments, which can be roughly fitted into a single piece. The uneven edge suggests that the break-up occurred while the splat was in the solid or semi-solid state. There is
Fig. 1. Morphologies of molybdenum splats formed on mild steel substrate at different temperatures and the corresponding 3-D surface profiles. (a, c) Room temperature, (b, d) 200°C.
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Fig. 2. Morphology of molybdenum splats formed on molybdenum substrate and glass substrate at different temperatures. Molybdenum substrate: (a) room temperature, (b) 340°C; glass substrate: (c) room temperature, (d) 340°C.
evidence of liquid-like flow pattern in the ‘valley’ between the fragments, which suggests that substrate melting occurred. Furthermore, the surface topographical measurement revealed that the ‘valley’ between the fragments is lower than the substrate plane (see Fig. 1(c)). Due to the lift-over of the fragments near the outer periphery, they look much thicker than the central sections. Almost all splats on the room temperature sample have this feature. The sample prepared at 200°C, as shown in Fig. 1(b), is a mixture of broken splats and disk shaped splats. Fig. 1(d) shows the surface profile of a typical disk-shaped splat. The morphology of the splats prepared at about 400°C is similar to that of the splats prepared at 200°C, although the proportion of disk-shaped splats is higher. In the deposit build-up process, the droplets spread and solidify on molybdenum after the first layer is deposited. Therefore, the splats were prepared on a molybdenum substrate in order to examine the role of the substrate chemistry. As shown in Fig. 2, at room temperature, the splat surface is not flat and shows a tendency for splashing at the periphery. At 340°C, the splats are disk-like, although their rims are not as smooth as those of splats made on a hot steel substrate. The splats were also formed on a glass substrate because of its vastly different thermo-physical properties, especially thermal conductivity. The splats made at room temperature exhibited a fragmented disk shape, with projection along the periphery of central disk. The
splats formed at 340°C showed contiguous disk-like shapes with a distinguishable rim. These results show that the substrate materials and temperature dramatically affect the morphology of the splats. For splats produced at room temperature, their morphology exhibits a very different pattern: fragmented on the steel substrate and splashed on the glass substrate, respectively.
3.2. Flattening ratio of molybdenum splats The flattening ratio is the ratio of the splat diameter and the diameter of the droplet, which forms the splat. By using a 3-D surface profilometer, the thickness and diameter of the disk-shaped splats formed on a hot substrate were measured and the volume of the splats and the diameter of the droplet were calculated from these data. The flattening ratio and the thickness of the
Fig. 3. Flattening ratio and thickness of molybdenum splats prepared on steel, molybdenum and glass substrates. Steel at 200°C, molybdenum at 340°C, glass at 340°C.
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Fig. 4. Splat morphologies of molybdenum on glass (a) and on grit-blasted steel substrate (b, c). (a) Glass at 200°C, (b) steel at 115°C, (c) steel at 250°C (roughness of the steel substrate, RA =3.4 mm).
splats produced on hot steel, molybdenum and glass substrates are listed in Fig. 3. The flattening ratio of the splats formed on the steel substrate is between 2.0 and 2.8 with an average of 2.4, and with an average splat thickness of 2.4 mm. The flattening ratio and thickness of the splats made on a hot molybdenum substrate is similar to those of the splats made on the hot steel substrate. The flattening ratio of the splat made on glass is from 3.0 to 6.5 with an average of 4.5, and the splat thickness is from 0.2 to 1.4 mm with an average of 0.7 mm. The large range of flattening ratio is not a measurement error, but is rather associated with particle size, temperature and velocity variations as well as the deposition characteristics.
3.3. Microstructure de6elopment during coating buildup The deposit is generated by the successive impingement of droplets and accumulation of individual splats. Therefore, the interaction between the solidified splat and the subsequent droplet is a key step in determining microstructure development. In this study, it was found that the molybdenum droplet in general, can spread evenly on top of the hot, previously deposited splats to form lamellar structures in the deposit. Fig. 4(a) demonstrates this effect and shows that the droplets can spread and slip down the splat surface, with no splashing or fragmenting on the molybdenum surface. The spreading can follow the substrate or previously deposited layer surface ‘terrain’ in its spreading path reasonably well, suggesting that there is some morphology heritage in the coating build-up. This fact suggests that the morphology of the first layer of splats does play a role in the overall deposit lamellar structure. Fig. 4(b) and (c) show typical splats formed on a grit-blasted (roughened) steel substrate. At lower temperatures there is severe splashing on the rough substrate, presumably due to asperities and air pockets trapped underneath the liquid droplet, which expands by the input of heat brought by the droplet and breaksup the splat into small pieces. At higher temperatures the central part of the splat, which accounts for most of
the droplet volume, is contiguous and significantly smoother than the substrate surface. Only the uneven splat rim and the debris scattered near the periphery of the splat indicates that some splashing has occurred. The ability to reduce splat fragmentation at higher substrate temperatures (i.e. above 200°C), even on a roughened substrate illustrates the significance of this effect. This allows a more efficient lamellar microstructure development through enhanced spreading and contact. In some cases as shown in Fig. 4(c), the initial layers can provide ‘surface smoothening’ through the filling of valleys by the droplets. This allows a more uniform lamellar microstructure to develop within the deposit. Fig. 5 shows the cross sections of deposits produced on a pre-heated substrate with and without cooling, with measured substrate temperatures on the surface stabilized around 260 and 440°C, respectively (the 190°C microstructure is similar to the 260°C microstructure). Both deposits show lamellar morphologies, which arise from the spreading and flattening of droplets on the previously deposited splats. The layer boundaries can be seen clearly. At the locations where the splat edges meet one another, breakdown of the lamellar structure is observed. In these ‘non-lamellar’ areas, there are numerous fine particle debris and small pores. The columnar grain structure can be discerned within each layer after etching (see inset Fig. 5). It suggests that at the moment the droplets impacted on the top of the deposit, the splat beneath was completely solidified; the droplets spread on the pre-deposited splats, nucleating and solidifying from the bottom in a columnar fashion. This can be further explained by the fact that it took about 10 min to deposit a 1 mm coating, therefore, taking an average splat thickness of 2.5 mm, the time interval for the second droplet to arrive at the same location is around 1 s. Assuming a cooling rate of 106 K s − 1, the previous splat is solidified long before the arrival of the second droplet. The deposit made at high substrate temperature (440°C) has a uniform lamellar structure with a few discrete, small, shallow pores at the conjunctions of the splats; these
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pores are probably due to the pull-outs of small oxidized debris. The 260°C deposit has more porosity and clusters of debris which originated from the splashing or break-up of the splats at a lower substrate temperature. The deposit made at a substrate temperature of about 190°C has a microstructure similar to that of the 260°C deposit indicating a transition temperature in the 300–400°C range for this system. The average thickness of the splats were measured for the three coatings, by the linear intercept method. The values are 2.7, 2.7 and 2.3, respectively, for the coatings made at 190, 260 and 440°C. The values are similar to that of average thickness of the single splats discussed earlier. The somewhat thinner high temperature sample splats is indicative of greater flattening. If we assume 2.5 mm as the average layer thickness, it indicates that there are 400 layer interfaces in a 1 mm thick coating, 0.4 m2 interface area per cm3 coating volume. Such a large interface area implies that the inter-lamellar adhesion has a significant effect on the coating properties. The slight difference in average thickness comes probably from the fact that the flattening ratio is affected by the substrate roughness [20]. At high substrate temperature, the splats have contiguous shape, therefore the following droplets spread on a smoother surface; whereas the splats deposited on substrates at lower temperature have a fragmented morphologies. The splats on top of those will have a smaller flattening ratio, therefore a greater layer thickness. Fig. 6 shows the fracture surface of deposits produced at 190 and 440°C. The fracture changes from inter-lamellar to trans-lamellar with an increase in substrate temperature. For the deposit prepared at 190°C, a significant portion of the fracture took place by delamination along the interface between individual splats and through the poorly bonded regions. In the 440°C coating, fracture took place mostly through trans-lamellar crack propagation, and much less along the splat interfaces. This suggests that a coating de-
posited at a higher temperature has significantly improved bonding between the splats (layers) compared to a coating made at lower temperature as confirmed by the thermal conductivity results (see below). Deposit properties are improved with a increase in the substrate temperature as shown in Table 3. The hardness, modulus and thermal conductivity increase is particularly notable from the 260°C deposit to the 440°C deposit, illustrating the sharp transition in substrate temperature effect. The hardness of the deposits associated with oxidation is higher than that of sintered molybdenum, which is typically 200–300 HV. Thermal conductivity data provide quantitative information on the contact area between individual layers (the ratio of the true contact area to the overall interface area). By comparison with the thermal conductivity of the bulk material and using the model of McPherson [21], the contact area ratios were calculated. This result indicates nearly 3-fold improvement in interlayer bonding at high temperature in spite of a doubling of the oxide content.
3.4. Residual stresses Residual stress is strongly affected by substrate temperature. Fig. 7 shows the temperature dependence of residual stress in thin coatings (3 mm in thickness — approximately one layer of splats) on steel and aluminum substrates. It can be seen that for both substrates, the stresses change from tensile to compressive as the temperature rises. This can be explained by different proportions of quenching and thermal stress which have opposite signs [14]. The quenching stress is tensile, whereas the thermal stress is, in this case, compressive, since the thermal expansivity of molybdenum is smaller than that of steel and aluminum. When the substrate temperature increases, the thermal stress upon cooling to room temperature is higher, therefore the residual stress (being the sum of the quenching and thermal stresses), shifts towards compression. This effect is more pronounced for the aluminum substrate, where the thermal mismatch is larger.
Fig. 5. Cross section micrographs of the deposits with etched inset showing grain structures: p, pore; d, debris; I, inter-lamellae crack. (a) Substrate temperature 260°C, (b) substrate temperature 440°C.
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Fig. 6. SEM micrographs of the fracture surface of the deposits. (a) Substrate temperature 190°C, (b) substrate temperature 440°C, d: delaminated regions, s: splat fracture regions.
Experimentally observed residual stress values are significantly lower than those that could be calculated from the temperature history and elastic properties. The actual stress magnitudes are limited by the coating’s adhesion to the substrate and bonding between the individual splats [15]. Higher magnitudes of residual stresses at increased temperature suggest that the bonding is improved, in comparison with depositions on low-temperature deposits, as evidenced by the hardness, modulus, thermal conductivity and contact area results. Also, the change of stress with temperature is steeper in the higher temperature range. Residual stresses in the coatings deposited on copper, glass and polished steel substrates had near-zero values, indicating that the adhesion strength in these cases are very low and that the bonding to rough surfaces is mostly of mechanical nature. One coating deposited on steel at 340°C was reheated to 200°C by four passes of the plasma torch without any powder. The resulting stress, −170 MPa, as compared to − 50 MPa of the as-sprayed specimen, indicates that the quenching stress was significantly reduced by the heat from the flame (recovery) and that the residual stress present is mainly due to thermal mismatch. Similar processes take place during actual processing of thicker deposits, although the heat input from newly deposited layers may differ from the flame alone. In thicker coatings, there exist significant stress gradients. Therefore, a single value is insufficient to describe the stress state. The effect of coating thickness on residual stress is the subject of a separate investigation [22].
4. Discussion
4.1. Splat morphologies and flattening ratio The mechanisms of molybdenum splat fragmentation are different on steel, molybdenum and glass substrates.
It appears that the breaking-up of a splat on the steel substrate is related to the localized substrate melting as found in this study and implied by Houben [23]. One dimensional heat transfer modeling for molybdenum splats on steel confirms this phenomenon [24]. At a higher substrate temperature, with an enhanced contact area, the heat flow might spread out more evenly, reducing (or possibly eliminating) the severity of the substrate melting. For molybdenum and glass substrates, at a higher substrate temperature, in addition to a possible wetting improvement at the interface, the gas absorbed is less. Less gas dissociated from the interface between the splat and the substrate at the input of heat from the molten droplet imposes smaller disturbance to the spreading of the droplet and is favorable for the formation of contiguous splats. This is consistent with the result obtained by Li et al. [5]. This is subject to further investigation. In the preparation of molybdenum splats, the same powder and process conditions were used except for the substrate materials. The striking difference in the average flattening ratio and splat thickness on metal and glass indicates that on metal, the droplet spreading was not as complete as on glass. It has been shown experimentally and analyzed theoretically that in the case of dynamic spreading, equilibrium surface tension is negliTable 3 Selected properties of the deposits Parameters
Oxygen content (wt.%) Porosity (%) Microhardness HV (cross section) Elastic modulus (GPa) Thermal conductivity (W K−1 m−1) Contact area ratio (%)
Substrate temperature (°C) 190
260
440
1.3 8.6 91 365 931
1.9 7.5 9 1 358 9 34
2.5 4.7 91 544937
49 9 12 14.7 91
40 911 194 9 25 15.9 91 36.59 8
9
9
26
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Fig. 7. Residual stresses in thin molybdenum deposits on steel and aluminum substrates, deposited at different temperatures.
gible in the determination of the contact angle [25]. This strongly suggests that on metal substrate, solidification is the arresting factor for the droplet spreading. Steel has much higher thermal diffusivity than glass. The melting of the steel substrate indicates that the splats were in good contact with the substrate, therefore the interface heat transfer coefficient is high, which greatly accelerates the heat extraction from the splat to the substrate. The solidification rate of molybdenum on steel is much higher than that on glass according to the Stefan model [26]. Therefore, the solidification time scale will be dramatically shortened, compared to the case for the glass substrate. An analytical model is being developed to further describe the effect of substrate material properties on droplet spreading [27].
4.2. Microstructure build-up and properties This study shows that even on a rough substrate, a uniform lamellar microstructure can be developed. The droplets spread on the previously deposited splats in ways similar to that on a flat substrate, except at the periphery where splashing occurs and results in inhomogeneities. In general, with an increase of substrate temperature, two processes are enhanced which affect the deposit microstructure build-up and properties in opposite ways. At a higher temperature the splats fragment less; deposit porosity is reduced, and the adhesion between adjacent splats is enhanced by enhancing the diffusion at the interface. On the other hand, oxidation is more severe at high substrate temperatures for metals as shown in Table 3. Although the oxide particles dispersed in the splat or the supersaturated oxygen can strengthen the splat (which is why a higher hardness is observed than that of the sintered materials), the oxide layer on the interface as a less ductile phase can potentially reduce the
strength and ductility of the deposit. The overall deposit properties depend on which effect is decisive. Through thickness thermal conductivity is controlled by the effective bonding area and voids morphology [28]. The increase of oxide content in general can decrease the thermal conductivity of the splat. The increase of thermal conductivity with substrate temperature suggests that there is a dramatic improvement in adhesion/effective contact area despite the sharp increase in oxygen content. The thermal conductivity of the deposits produced at 190, 260 and 440°C is 10, 11 and 26% of that of sintered pure molybdenum (142 W K − 1 m − 1), respectively. The presence of lamellar pores at the splat–splat interface and the high oxygen content in the coating are responsible for the overall thermal conductivity reduction of the sprayed materials, the former being the more dominant component. The high hardness of sprayed molybdenum can be attributed to the fine grain structure associated with the deposits and to the presence of oxides. X-ray diffraction, transmission electron microscopy and the Xray photoelectron spectroscopy studies of the deposits and splats reveal that the main oxide phases are MoO2 and Mo4O11 and they are present as fine particles dispersed inside the lamella [29]. The improvement of adhesion with an increase of substrate temperature is not fully understood at this time but may be due to several considerations: (1) Wetting is improved at higher temperature, which leads to better splat–splat contact [30]. (2) Less air is trapped in the air pocket underneath the splats at higher temperature because of the smaller air density, resulting in lower porosity. (3) A smaller temperature gradient through the splats reduces curling of the splats, etc, improves the splat–splat contact. Another possible mechanism of adhesion improvement comes from the fact that the melting point of MoO3 is only about 800°C. This surface oxide layer may dissolve into the impinging droplet and form a strong bond between the splats. A higher temperature is beneficial to this process. It is evident that more ‘regular’ splat shape and better contact at high substrate temperatures improve inter-lamellar adhesion. Good inter-lamellar adhesion, along with the higher oxide content leads to higher hardness and thermal conductivity of the deposit.
5. Conclusions This paper presents an integrated approach towards examining and developing processing–microstructure – property relationships associated with thermal spraying. Understanding and controlling splat formation
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provides a vital link to microstructure development and thus coating properties. Substrate materials and temperatures have significant effects on the morphology of molybdenum splats formed in the thermal spray process. With a temperature increase from room temperature to 400°C, the molybdenum splat morphology changes from fragmented to predominantly disk shaped. This has also been shown for a number of other systems although the exact mechanism of changes at the interface is not fully understood. The average flattening ratio of molybdenum splats on glass and steel substrates is 4.5 and 2.4 and the average splat thickness is 0.7 and 2.4 mm, respectively. The differences indicate that the larger solidification velocity of molybdenum splats on the steel substrate (which has a higher thermal diffusivity than that of the glass substrate) arrests droplet spreading through stronger interactions. With an increase in substrate temperature, and associated improvements in splat morphology, the deposits exhibit a more uniform lamellar morphology with less interlayer porosity and debris as well as improved bonding among the layers. The fracture characteristics change from interlamellar to translamellar fracture illustrating this enhanced adhesion among the layers. With an increase of the substrate temperature the mechanical properties and the through thickness thermal conductivity increase and the porosity decreases. Significantly enhanced thermal conductivity at a high substrate temperature is confirmation of the adhesion improvement. Residual stress in the thin coatings changes from tensile to compressive as the substrate temperature increases for certain substrates. This is attributed to the bigger role of the thermal mismatch stress in the case of steel and aluminum and enhanced interlayer adhesion. This change of sign was observed over a reasonably achievable range of temperatures, which makes this parameter an effective means of controlling the stress level. This study confirms that substrate temperature is a critical factor in plasma spray processes and can be used effectively to modify the microstructure with implications for enhanced properties and performance.
Acknowledgements This work was supported by the MRSEC Program of the National Science Foundation under Award Number DMR-9632570. Osram Sylvania Inc. is gratefully acknowledged for providing the feedstock powder. Stimulating discussions with Drs R. Goswami, G.-X Wang and A. Vardelle are appreciated. We thank Pro-
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fessor H. Herman for his inspiration and guidance in our activities at the Center. The science base for this industrial technology is where it is today due to his untiring efforts.
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