Present status of low pressure and atmospheric plasma spraying

Vacuum/volume 41/numbers 7-9/pages 2209 to 2212/1990 Printed in Great Britain

0042-207X/9063.00+.00 © 1990 Pergamon Press plc

Present status of low pressure and atmospheric plasma spraying H - D S t e f f e n s and M M a c k , Institute of Materials Technology, University of Dortmund, Dortmund, FRG

New developments in the fields of spray gun engineering, powder engineering, process control and plasma technology are seen to stimulate each other. As a result, plasma spraying extends its application field. We report recent achievements in spray equipment, powders and plasma models: strongly vortex-stabilized guns, guns with adjustable arc length, matched supersonic expansion of low pressure plasma jets and spraying in controlled atmosphere are discussed in some detail. Today's status of microstructural control is illustrated with selected application examples: protective layers against chemical attack, advanced ceramics, (electro)catalytic layers and fibre-reinforced materials with sprayed matrix.

Introduction Plasma spraying technology is widely used in industrial practice since the mid-sixties. In the past, research and development was mainly directed to the improvement of a single coating property, e.g. still hotter jets or still higher particle velocities for increased bonding and reduced porosity. Today, the main goal is to achieve a better microstructural control of the layers produced. This means consideration of the various interdependencies of process parameters and spraying system components. Current lines of development may be summarized under the headings: spray gun engineering, plasma-particle interaction, process control and automation, powder engineering, microstructural control.

Spray gun engineering Improved heating of the powder particles is required as is improved confinement of powder particles to the hot core of the plasma jet, which means as well reliable powder injection. Two different concepts are currently seen: strongly vortex-stabilized arc jets (mainly for use in atmospheric plasma spraying (APS) and laminar flow designs for use in low pressure plasma spraying (LPPS). Bernecki ~ has presented a further development of a cascaded constrictor-type arc which uses a movable cathode to control the arc voltage by variation of arc length. By this feature, power levels up to 80 kW are achievable with single gas (i.e. argon) operation only. Stabilization of the arc and rotation of the anode root is augmented by injection of argon gas with a tangential swirl upstream of the nozzle exit, in addition to the usual vortex injector of main plasma gas upstream the cathode. The gun is reported to produce a straight and well-confined plasma plume with a more fully developed temperature profile. At the Japanese Welding Institute, Arata and coworkers

have developed the so-called gas tunnel-type plasma jet 2"3. A special vortex generator nozzle situated in between an intermediate and a final anode creates a low pressure ('sucking') vortex gas tunnel which constricts the plasma jet to the axis (thermal pinch). The advantage is twofold: from a lengthened, well constricted arc, a stable jet with straight symmetrical contours and raised energy density is obtained. Secondly, the sucking pressure allows the use of central injection through a cathode bore. Feed rates as high as 6 kg h-~ for alumina are reported requiring, however, 2001min -~ (STP) Ar-gas flow rate to maintain the gas tunnel. In LPPS, matched supersonic expansion by use of an optimized LAVAL nozzle results in nearly perfect laminar plasma flow, as has been demonstrated by Mayr and Henne 4. This leads to--given the proper injection m o d e - - a nearly ideal centering of powder particle flow to a narrow region around the axis. The powder particle flow stays confined along the whole jet, which ensures uniform heat transfer and a narrow final velocity distribution (only + 10% variation in final velocity for 95% of entrained ( - 2 2 / + 6 / ~ m ) alumina particles). The heat transfer capability of a plasma jet is a strongly varying function of plasma temperature. Therefore, radial temperature profiles with a broad central plateau and steep temperature gradients at the fringes ('flat' profiles in contrast to 'bell-shaped' profiles) are required for more homogeneous melting. In trial calculations, Lee and Pfender 5 have shown that, for flat temperature 'inlet' profiles (i.e. at the nozzle exit), a slower decrease in temperature along the jet will result as well, even if the absolute value of central temperature is slightly lower. Similarly, flat velocity profiles are advantageous. It would be interesting to study the temperature profiles of the vortex designs described above, especially if spraying without H 2 is intended (H 2 may be detrimental in spraying carbides or advanced oxides). Usually, addition of H 2 strongly increases plasma heat transfer capabilities (e.g. adding 20 vol% H2 to Ar improves heat flux at 10,000 K plasma temperature by a factor of 10) 6. There remains some doubt whether a comparably high 2209

H-D Steffens and M Mack: Present status of plasma spraying

heat flux is achieved with the new vortex design concepts, and investigations concerning powder confinement, dwell times and plasma temperature seem to be indicated. APS under controlled atmosphere has been extensively investigated quite recently 7,s. When spraying in argon, the plasma isotherms are stretched considerably (resulting in a lengthened visible jet) as compared with spraying in air. In the latter case, the turbulent flow along the jet fringes mixes considerable amounts of N 2 and 02 to the central parts of the plasma 9. A corresponding fraction of plasma enthalpy is spent to dissociate the molecular gases and can no longer be used to heat the plasma. Thus, spraying under Ar-enclosure significantly enhances heat transfer without altering the velocity field substantially 5. For the vortex concepts discussed above, there may result an improved shielding of the jet by the vortex flow of argon gas ('shrouded jet'), reducing the mixing with air. It may well be that, for the vortex designs described above, this effect i s - - a t least in part--responsible for the claimed improvement in plasma temperature profiles and jet contours.

Plasma-particle interaction Based on investigations in APS, ongoing particle velocity measurements in LPPS ~°'H contribute to an improved understanding of particle acceleration under non-continuum mEconditions. Knowledge of the plasma's drag properties is indispensable for the modelling of powder injection. Interesting trial calculations are reported 5"12 which depict real situations (dispersed trajectories due to turbulent flow) quite closely. The effects of high powder load on plasma properties have been studied as well: Lee and Pfender have demonstrated that there is a significant decrease in plasma velocity and temperature, if powder mass flow exceeds one third of plasma gas mass flow. Figuring that 1001min -~ STP argon corresponds to about 11 kg h - ~ mass flow, one readily recognizes that this limit will apply to many cases in industrial practice. When spraying advanced oxides (like e.g. high T¢ superconductors) in LPPS, depletion of oxygen poses serious problems. Addition of oxygen to the plasma flow seems to be helpful only for some selected choice of main plasma gas composition 13. Efforts should be made to model the mass transfer (oxygen loss/oxidation) between plasma and powder in the framework of plasma chemistry. This would help understanding of the various interactions of oxygen addition with plasma gas constituents which have been reported ~3.

Process control and automation In some respects, the improved understanding of plasma parameter interdependencies has opened the way for closedloop concepts which are now widely applied. It is not until the response behaviour of the plasma jet to parameter changes is known in outline that one can determine the proper time constants and damping factors. The only parameter which used to be operated outside closed loop control up until recently was powder feed rate. Here indeed, short-term stability (with actual response time of 1 s or less) is indispensable, given the high traversing speed of the spray gun during operation. Intense development has been devoted to this field ~4, and several new designs for closed-loop powder feeders are right now being introduced into the market. 2210

Powder engineering Advanced powder processing methods ~5 allow an increased control of chemistry and phases. The powder morphology, composition and phase content must be chosen to be appropriate to the requirements of the coating, and the specifics of the spray equipment used. The following example may illustrate how powder process engineering may contribute to improved spray results. Usually, one experiences problems with homogeneous alloying during melting for spray-dried, agglomerated multi-component powders. On the other hand, the spherical shape of such powders ensures good flowability and facilitates homogeneous melting. For yttria-stabilized zirconia, Court and coworkers ~6 have reported a new type of spray-dried, agglomerated powder: on melting in the plasma jet, the spherical powder particle turns to a hollow sphere. This is brought about by proper choice of density and inner structure of the agglomerated powder. Strong convection of the molten material inside the spherical shell ensures a nearly perfect fusion of the two components. Distinct improvements in coating performance and coating phase content--superior to the use of fused and crushed powder--have been reported.

Microstructural control The microstructure of sprayed layers depends on the microstructure of the single spray lamella (grain growth, phase content, microstresses) and the structural features related to the layer build-up (pore(size) distribution, interlamellar contact, temperature gradients and resulting residual stress profile). Several recent investigations have contributed to the understanding of the basic mechanisms involved: dynamics of freezing and spreading ~7, grain growth ~8 and development of residual stresses 19,2°. Future applications for APS and LPPS are anticipated, if one succeeds in using the potential of plasma spray deposition as a Rapid Solidifying Technique.

Application examples Protective layers against chemical attack. Very dense, essentially pore-free and pure layers are required (inclusions of oxides, nitrides and hydrides are to be avoided), therefore, LPPS is applied. However, even in LPPS devices with low residual gas content, the contamination of powder particles by adsorbed gas is a limiting factor. In part, powder contamination is caused by the specifics of powder production, but in part it is simply due to the high specific surface. With regard to this, coarser powders would be advantageous, but with regard to density, finer powders show better results. Indispensable for the production of dense layers, however, is a sufficiently narrow powder size distribution. Proper injection and careful matching of particle impact velocity to ensure homogeneous spreading and excellent interlamellar contact are further optimization steps. On a laboratory scale, distinct improvements have been achieved in our institute for titanium. The polarization behaviour in In-H2SO4 shows a wide passivation range, very close to Ti-plate values. Electrncatalytic materials, including advanced oxides. Electrocatalytic materials are required to reduce cell overvoltage in water electrolysis for H2-production. As the cathode one uses porous Ni-structures (Raney-nickel) produced by leaching out

H - D Steffens and M Mack: Present status of plasma spraying

AI from sprayed NiAI layers 21,22. Spray engineering problem is to shape porosity as sprayed, lamellae size and interlock, adhesion, residual stresses and the distribution of the Al-rich phases in such a way as to guarantee large effective surfaces combined with excellent mechanical and thermal stability of the active layer. Henne et a121 have demonstrated, how confined particle flow and raised particle acceleration brought about by a LAVAL nozzle may improve results. As anodes, Co-based mixed oxides of the spinel and perovskite type are used. Spraying these materials under reduced pressure in order to ensure purity, one faces the problem of oxygen depletion ~3. Thus, the possibility to spray with 0 2 addition (using A r - H e instead of Ar-H2) is investigated. In a similar case, viz. when spraying ZrN, Derradji and coworkers 23 used a controlled A r - N 2 atmosphere at 900 mbar in order to prevent the nitride powder from N2 depletion. Last but not least, the precious oxide material forces one to optimize spray deposition efficiency distinctly above the 50-60% value reported in previous publications 24. Fibre-reinforced materials. Production of fibre-reinforced composites m a y - - i n the near future--form a further and likewise extensive application of plasma spraying. Plasma spraying excels by its versatility to produce any combination of ceramicmetal, metal-metal and ceramic-ceramic compound. Here, adhesion of the sprayed matrix to the fibres has to be optimized by a careful choice of spray conditions and matrix powder size distributions (as compared to fiber diameter a n d - - f o r discontinous fibres--fibre aspect ratio). Figure 1 gives an impression how the splashes of molten material cover the fibre. Berndt and Yi 25 have investigated the coating microstructure for different systems, dealing extensively with the engineering of composite powders, containing cladded discontinuous ceramic fibres. Various fibre diameter distributions (0.5-20 #m) were used. The fibre morphology is found to be retained in the sprayed structure. In our laboratory, we have dealt with continous fibres 26'27 (range 10-100#m). One layer of fibres is wrapped around a cylindrical mandrel with prescribed fibre distances. In the subsequent spray deposition to the cylindrical body the spreading characteristics of the matrix droplets has to be optimized in order to overcome shadowing effects. There results a mono-

Figure 2. Fibre-reinforced composite: three-layer tape, matrix NiCrAI

(+ 22/- 5.6 #m), fibres austenitic steel (100/~m diameter).

layer tape; after surface finishing--if necessary--wrapping and spraying may be repeated to produce multilayer tapes. Figure 2 shows an example. An alternative approach is to join several monotapes by hot isostatic pressing. The use of still thinner continuous fibres ( < 5 #m) will be a forthcoming application of rf-spraying:8: such fibres may be vulnerable to high impact velocities. Therefore, the low velocities of the usually strongly superheated particles (leading to fine spreading) in rf-spraying will be advantageous. Concluding remarks

For the current and future development of plasma spraying, extended microstructural control by improved process control remains one of the keywords. In spray gun engineering, the performance of the APS vortex type designs should be investigated in terms of temperature and velocity profiles, including turbulence-induced transport processes. (Such studies are currently performed at our laboratory for LPPS guns, especially those of the laminar flow type.) For the plasma processing step there is a need to investigate plasma properties under high powder load and the mass transfer plasma-powder more thoroughly. Improving powder deposition efficiencies requires an improved modelling of powder injection and powder confinement. The existing models for spreading and freezing dynamics should be extended in order to include the results of TEM-investigations concerning grain microstructure and interiamellar contact. Similarly, the mechanisms governing adhesion should be investigated more closely, with special regard to fibre-reinforced materials. Finally, extended use of expert systems, dedicated to the design of composites 29, will help to widen the application fields of both APS and LPPS.

References

Figure 1. Splashes of molten ZrO2-20MgO spray droplets on SiCfibre.

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H-D Steffens and M Mack: Present status of plasma spraying

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