Effect of Ternary Gas Mixtures on the Microstructure of Refractory Metals and Alloys Deposited by Low Pressure Plasma Spraying

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1 Effect of Ternary Gas Mixtures on the Microstructure of Refractory Metals and Alloys Deposited by Low Pressure Plasma Spraying Rajan Bamola, Santa Fe Springs, U.S.A. and Albert Sickinger, Irvine, U.S.A. To overcome the problem of depositing dense refractory coatings, a study was undertaken on the effect of using three plasma gases simultaneously when depositing tungsten and tungsten alloys utilizing Low Pressure Plasma Spraying (LPPS). A greater degree of control of the plasma flame temperature, jet velocity, and heat transfer capability is believed to occur when using ternary gas mixtures. Samples were prepared and coated using Argon, Helium, and Hydrogen in different ratios. Variations of chamber pressures were used as an additional parameter to control and optimize the deposits. The samples were sectioned and analyzed. Microstructural features such as porosity, unmelted particles, and grain size, were characterized using optical and Scanning Electron Microscopy (SEM). Fractography was used to determine lamellar thickness and distribution. Mechanical properties were evaluated by measuring the microhardness of the different coatings in comparison to one another. 1 Introduction Tungsten possesses many properties; high melting point, high thermionic emission, and high density, desirable in industrial, nuclear and military applications, not found in other metals and alloys. Unfortunately, these properties limit the use of conventional forming and joining techniques when fabricating structures. Much research has been devoted to low pressure plasma spray techniques to form tungsten coatings and structures, however, it is accepted that densities above 92-95% of theoretical are difficult to achieve [1, 2]. A major factor contributing to the lower densities found in reduced pressure sprayed tungsten is less efficient heat transfer between the plasma and the powder particles at a lower pressure environment compared with plasma spraying at ambient pressures. It can be reasoned that denser deposits may be achieved by increasing the chamber pressure and increasing the substrate temperatures to allow better flowability of the particles as they impact the substrate. However, increasing the chamber pressure will also decrease the plasma velocity hence the particles velocities which may negate the advantages of the processing route theorized above. This paper lists the results of using high enthalpy, high velocity plasmas achieved by simultaneously mixing argon, hydrogen, and helium to spray tungsten at various ratios and various chamber pressures. Planche et.al. have reported that particle velocities were higher in the case of Ar-He mixtures when compared to Ar-H 2 at similar isotherms [3]. Particle surface temperatures were higher in the case of Ar-H 2 mixtures then in Ar-He mixtures. They also studied the effects of a ternary gas, Ar-He-H 2, on particle velocities and found an increase in particle velocities compared to binary mixtures of Ar-H 2 and Ar-He. However, axial velocities showed a faster decrease in the ternary mixture. This was attributed to a greater turbulent flow in this case. It must be kept in mind that this study was done in air; therefore, plasma dynamics may be different at reduced pressures. 2 Experimental Procedure A modified Electro Plasma Inc. (EPI) Low Pressure Plasma Spray (LPPS) system equipped with an 03CA torch and a modified anode was used to apply the coating. Initial studies at different chamber pressure levels of 40 (30), 80 (60), 133, 200 (150), and 266 (200) mbar (Torr) used reverse arc sputter cleaning followed by coating application. The substrates consisted of mild steel specimen measuring 25 x 50 x 5 mm grit blasted using 350 µm (36 grit) aluminum oxide. The tungsten powder was obtained from Osram Sylvania, with a particle size distribution of -44 to +5 µm, Figure 1. The majority of particles were between 10 µm and 35 µm, which enabled us to study the melting behavior as a function of the various processing parameters. The particle morphology is shown in Figure 2. Acumulated [%] Particle Size Distribution Particle Size [um] Figure 1: Powder Particle Size Distribution Chart 1

2 An automatic polisher was used to grind and polish all samples in the same manner and the same time. Microstructural studies consisted of electrolytic etching in a NaOH solution at 10V for 12 seconds [4], and examining using an optical microscope. Microstructural features such as splat diameters and thicknesses, resolidified or unmolten particles were analyzed using image analysis. Splat diameters and thicknesses were measured at 1000X magnification. The volume of the splat was used to calculate the diameter of the equivalent powder sphere. Porosity was determined on unetched samples using a grid system and black and white contrast. This allowed better differentiation between pores and artifacts. Figure 2: SEM Photograph of the Tungsten Powder Initial studies at different chamber pressures showed that the modified anode configuration used for the ternary plasma gas parameters worked better at lower pressures of <133 mbar. Also, using reverse arc sputter cleaning applied too much heat to the substrates, which modified the microstructure in the interfacial regions. To produce samples for comparative purposes reverse arc cleaning was omitted and samples were produced using the parameters listed in Table 1. The deposition procedure consisted of evacuating the chamber to mbar ( mtorr), backfilling with argon to the operating pressure of 80 mbar and applying the coating using the same powder feed rate and preprogrammed passes for each sample. Substrate temperature was measured at the end of every coating cycle using an optical pyrometer to evaluate the effect of the different gas mixtures on heat input to the substrate. Table 1: Parameters used for Sample Production Sample #1 #2 #3 #4 #5 Argon [l/min] Helium [l/min] Hydrogen [l/min] (200) 47 (50) 140 (150) Amp Volts Power [kw] Feedrate [g/min] Sub. Temp. [ C] Chamber pressure [mbar] Since reverse arc cleaning was omitted all coatings detached from the substrate. The detached pieces were mounted so the cross-section could be polished. Microhardness was measured using a Tukon Microhardness tester with a 100 gram load and a Knoop indenter. A minimum of five readings were taken on each sample. For ease of comparison the average values were converted in HV using graphical techniques from which the hardness in MPa were calculated. Fractography was conducted by breaking the detached coatings across their cross-section and examining at 200, 400, and 4000X using a JEOL 820 Digital Scanning Electron Microscope (SEM). 3 Results and Discussion Microstructure Representative micro-sections from the samples produced in this study are shown in Figure 3. Distinct differences in microstructures were observed between samples sprayed using binary gas mixtures and those sprayed using ternary mixtures. Samples sprayed using Ar-H 2 (#1) had coating microstructures comprised of roughly spherical particles similar to those in the as-received powder, embedded in a matrix of splats. The size of the spherical particles measured approximately 25 µm and below. The equivalent sphere (original particle size) of the splats ranged from 12 to 22 µm. Of interest was the low fraction of particles molten or otherwise of sizes greater then 30 µm. Samples sprayed with Ar He (#2) revealed a denser structure than those sprayed with Ar-H 2. Equivalent diameter calculations indicated particles with original diameters of µm and below formed splats, these can be observed in the lower section of sample 2 in Figure 3. Unmolten particles measured from 25 µm and below. Very small spherical particles from 10 µm and below could also be observed. When compared with sample 1, a lower fraction of unmolten particles were observed in the samples sprayed with Ar-He. As in the case of the Ar-H 2 sprayed samples, particles measuring greater then 30 µm were not observed. This is probably due to the larger particles having low heat input and higher mass bouncing off the coating. 2

3 Samples sprayed with l/min argon, 47 l/min helium, and l/min hydrogen (# 3) revealed a blocky microstructure of mostly partially molten particles interdispersed with splats originating from particles averaging about 16 µm. The blocky structures were created by two or more partially molten particles sintering together during coating build-up. During metallographic preparation this sample exhibited more particle pull-out then the others indicating lower cohesive strength in this case. Sample 1 (un Samples produced from spraying with l/min argon, l/min helium, and l/min hydrogen (# 4) revealed dense microstructures with a high percentage of splats. The average size of the original particles were µm. The microstructure at 1000X also showed spherical particles 5 µm and less. These appeared to have originated from the powder, where discrete particles or agglomerations of these size particles were observed, see Figure 2. Samples produced from spraying with l/min argon, 140 l/min helium, and l/min hydrogen (#5) showed a dense structure. An interesting feature of these samples were the larger splat widths and lengths that could be measured. The average original diameters of these splats were 22 µm. The largest splats observed resulted in a calculation of 34 µm for the original particle diameter. As in the case of the sample 3, blocky structures from the sintering of two or more particles were observed, but not to that extent, which indicates a higher degree melting of larger particles at higher power levels. Sample 2 (un ( Porosity The major cause of porosity of the various specimen can be related to the fraction of resolidified, unmolten and semi-molten particles in the coatings. Pores originate from interstices created between these types of particles, shown in Figure 4. Area A shows porosity created between various partially molten particles, whereas B shows porosity created between a resolidified particle, a splat, and a partially molten particle. Another cause of porosity that can be observed are those caused by particle fragments preventing flow of partially molten particles into the interstices; area C. Sample 3 (un C A Sample 4 (un B Sample 5 (un ( Figure 4: Metallographical Micro-Section, etched, at 1000X magnification The porosity measurements are presented in Table 2. Figure 3: Metallographical Micro-Sections at 200X (un and 400X ( magnification Table 2: Results of Porosity Measurements Sample #1 #2 #3 #4 #5 Porosity [%]

4 Sample 1 contained the highest fraction of partially molten and unmolten particles; this resulted in it having the highest porosity level. Samples 3 and 5 contained more blocky formations, which are due to agglomeration of two or more partially molten particles sintered together. In the case of sample 5, the higher enthalpy plasma of 132 kw allowed heating of larger particles enabling them to get incorporated into the coating. Sample 4 showed the lowest porosities and the lowest fraction of unmolten particles and agglomerations of partially molten sintered particles. It must be noted that porosity measurements are within experimental error of each other. However, they are useful to indicate a trend in the coating microstructure. Microhardness The microhardness of the coatings are directly related to the porosity and hence the percentage of unmolten, partially molten, and resolidified particles in the coatings. It is worth noting that even though sample 2 had the second highest hardness, the scatter in this case was more then in the other coatings. This can be attributed to the wide variance in structures found in this coating; fine well spread splats and smaller unmolten and partially molten particles. Sample 4 had the least variance in microhardness values this is reflected in the fairly uniform microstructure observed in this sample when compared to the others. Average microhardness values using a Knoop indenter with a 100 gram load are presented in Table 3. Table 3: Knoop Micro Hardness measurements with calculated values for HV and MPa as comparison Fractography The fractographic study using SEM revealed a key difference between samples sprayed using hydrogen, whether in binary or ternary gas mixtures, and those sprayed using just argon-helium. In samples sprayed using hydrogen a high volume of extremely small particles about 1 2 µm were observed throughout the cross-section. These particles were found to originate from the powder (Figure 2), where they appear as discrete particles or agglomerations of 10 µm or more. Samples sprayed using Ar-He contained a much smaller percentage of these particles. A possible explanation could be that in argon-helium plasmas these extremely small particles have trajectories, which cause a high fraction of them to be expelled from the spray pattern hitting the target. However, cinematic studies would need to be performed to verify this theory. In coatings produced using the binary Ar-H 2 parameters, powder particles larger than 25 µm did not melt. According to the sieve analysis in Figure 1, the powder had 17% of particles above 25 µm, in the coating were less then 5% of these particles remaining, which indicates that the majority of these sized particles are not sufficiently heated and did not appear in the coating. Particles between 10 µm and 25 µm formed splats, particles less then 10 µm appear resolidified. About 30% of the area studied in sample 1 appeared as partially molten or superheated particles, capable of being deformed but not forming the traditional pancake shaped splats, which is shown in Figure Microhardness [HK100] # 1 # 2 # 3 # 4 # 5 Sample Number Sample #1 #2 # 3 #4 #5 Knoop Hardness [HK 100 ] (measured) Vickers Hardness [HV 100 ] (calculated) Indentation force [MPa] (calculated) Figure 5: SEM Fractomicrograph of Sample 1 (Ar-H 2 ) The major difference between sample 1 (Ar-H 2 ) and sample 2 (Ar-He) are the smaller splats and the low fraction of unmolten particles in sample 2, shown in Figure 6. This can be explained by the higher velocity plasma with Ar-He allowing heating and deformation of smaller particles but insufficient heating of the larger particles leading to their bouncing off the coating. 4

5 Figure 6: SEM Fractomicrograph of Sample 2 (Ar-He) Samples 3, 4, and 5, produced with Ar-He-H 2 ternary gas mixtures had similar structures with a trend towards increased heating with higher power levels. The microstructure of sample 3 showed increased bonding between partially molten particles forming agglomerated structures along with splats. Sample 4, which is shown in Figure 7, had sufficient power to allow deformation of partially molten and superheated particles to minimize porosity, whereas, in sample 5 the higher power level lead to incorporation of larger partially molten and unmolten particles. These larger particles were not found in the other coatings. acceleration/deceleration rates and peak velocities. Alumina particles, for example, showed a difference of 100m/s between µm particles and µm particles at 67 mbar. The difference in mass between a 25 µm particle and a 45 µm particle is almost a factor of 7. In the case of tungsten, the difference in mass from 15 µm to 25 µm particles is almost a factor of 5. Therefore, one would expect a dramatic decrease in particle velocity of the larger tungsten particles. Also one can calculate using heat capacity data that it takes about five times as much energy to obtain a T of 3000ºK for a 25 µm particle than for a 15 µm particle. This explains why the larger particles bounce off at lower power levels in some samples, but adhere at higher power levels in other samples. In this study, the gun nozzle faced downwards to minimize the effects of trajectory differences between particles of different masses. Realistically, one would still expect different mass particles to occupy different positions in the plasma stream and thus experience different kinetic and thermal histories. 4 Conclusion Measurable differences in LPPS tungsten coating microstructures were observed in samples sprayed using binary gases compared to those using ternary gas mixtures. Chamber pressures of less than 133 mbar (<100 Torr) offer greater degree of control of the plasma characteristics when using ternary gases of Ar-He-H 2 Ternary gas mixtures of Ar-He-H 2 require more control than binary gas mixtures to produce optimized microstructures. Powder particle size was found to be more influential on microstructure optimization above a certain power level. 5 References Figure 7: SEM Fractomicrograph of Sample 4 (Ar-He-H 2 ) Particle plasma interaction is a complex relationship in an LPPS system. Research has shown that varying the arc gas flows and compositions in LPPS does not show significant effects on particle velocity, but these changes can greatly influence particle heating [5]. The same study revealed that the particle mass was a significant factor. As the particle mass increases there is a transition to lower [1] Neiser R.A, Watson R.D., Smolik G.R., Hollis K.J., An Evaluation of Plasma Sprayed Tungsten for Fusion Reactors, Proc. of NTSC Anaheim, CA 19, p [2] McKechnie T., Near Net Shape Forming- Metals, Proc. of 1 st ITSC Montreal, Canada, 2000, p [3] Planche M.P., Fauchais P., Coudert J.F., Betoule O., Valletoux H, Comparison of D.C. Plasma Jet Velocity Distribution for Different Plasma Gas Mixtures: Ar-H 2, Ar-He, Ar-He-H 2, Proc. of 7 th NTSC Boston, MA, 1994, [4] Petzow G., Metallographic Etching, ASM 1978, p. 55. [5] Smith M., Laser Measurement of Particle Velocities in Vacuum Plasma Spray Deposition, 1 st Plasma Technik Symposium Proceedings, 1988, V1, p