Vacuum plasma spray quality control

Thin Solid Films, 118 (1984) 409-420

PREPARATION AND CHARACTERIZATION 409

VACUUM PLASMA SPRAY QUALITY C O N T R O L *

H. GRUNER

Plasma- Technik AG, Rigackerstrasse 21, 5610 Wohlen (Switzerland)

(Received April 10, 1984; accepted April 13, 1984)

The change in the technology of plasma spraying from air to vacuum has led to substantial improvements in spraying parameters and coating properties. A great variety of reactive and oxygen-sensitive materials with interesting properties can now be applied by vacuum plasma spraying and used as coatings.

However, these materials can only be used in connection with a very time- consuming optimization of coating parameters. Until now this optimization process has been the link between spraying experiments with parameter variations and the resulting microstructure. A new method of quality control, the wipe test, can accelerate the process of optimization and results in additional information. The plasma jet with the injected powder particles is moved across the substrate surface at a very high speed like a wiper. This produces a pictorial distribution of the powder particles in the plasma jet and, if a scanning electron microscope is also used, provides a description of the degree of melting that occurs during spraying.

The test demonstrates the manner in which the quality of a coating is influenced by the degree of melting of the plasma spray powder. The number of unmelted powder particles and their distribution across the plasma jet cross section are determined by the plasma energy and the powder injection parameters. Control of these parameters using the wipe test results in optimal coating structures for specific applications. Titanium and TiB 2 are given as different examples to illustrate this.

1. INTRODUCTION

The large-scale industrial use of vacuum plasma spraying (VPS) technology was possible only after an in-depth study of certain vacuum processing techniques 1. The techniques ensure the reproducibility of the coating quality and a high economic efficiency in coating production 2.

Modern VPS installations are evacuated down to 10 3 mbar to reduce the partial pressure of the reactive residual gases to the same impurity level as the plasma gases. The same purity requirements apply to the spray powder which, in

* Paper presented at the International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April9 13, 1984.

0040-6090/84/$3.00 © Elsevier Sequoia/Printed in The Netherlands

410 n. GRUNER

VPS units, is carefully dried and vacuum degassed in the powder feed unit before spraying. Microprocessors have been used for a long time to monitor and control the coating parameters. Robotic systems specially modified for use in vacuum are used to obtain the required coating thickness distribution on complex workpiece surfaces a. These surfaces are sputter cleaned before spraying using the transferred- arc metho&. This approach significantly improves the adherence of VPS coatings compared with coatings produced by air plasma spraying (APS) 5.

The high energy density in the VPSjet together with the vacuum process means that almost all materials available in powder form can be used as coatings. The quality of the coating is determined by the degree of optimization achieved between the spray parameters and the powder quality. However, spray powders available today are oriented more towards APS applications. Thus, with the increasing importance of the VPS technology, powder manufacturers are starting to take into account the special requirements of VPS powder quality 6.

Spray parameter optimization of new sorts of powder calls for time-consuming experiments with spray parameter variations and the determination of their effects on coating quality. In spite of modern non-destructive test methods for coatings v, microstructural control using modern metallography techniques remains the only true method for absolute quality assurance. Therefore a new method to determine the degree of melting of powder particles and their distribution across the plasma jet would accelerate the optimization process required with new coatings. Without cross section preparation, additional information on coating structure is provided and quality assurance with regard to coating production systems is increased.

2. QUALITY CONTROL BY USING THE WIPE TEST

The degree of melting of all spray particles as they strike the substrate surface has a strong influence on coating quality. The wipe test (Fig. 1) is used to show the state of the powder particles (solid, softened, semiliquid or liquid) on impact and the particle distribution in a cross section through the plasma jet.

With the plasma parameters predefined, powder is injected into the plasma jet using preselected injection parameters. The plasma jet is passed over a strip of substrate material like a wiper after parameter stabilization in the horizontal or

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(a) (b) Fig. 1. Schematic diagram of the wipe test: (a) plasma torch; (b) substrate holder with four specimens

VACUUM PLASMA SPRAY QUALITY CONTROL 411

vertical direction. The plasma torch is rotated at the maximum sweep velocity of 95 ° s-~ so that particles land without forming a continuous layer even at the maximum powder feed rate. The length of the strip of substrate material is chosen according to the cross section of the plasma jet. Particles in the periphery of the jet are included as well, as these are also present in the layer that forms during component spraying.

The substrate material surface is polished to increase definition. This surface is made up of several individual pieces which can be investigated with a scanning electron microscope immediately after the wipe test. The substrate holder comprises four strips of material arranged at 90 ° to each other. This arrangement allows four different sets of plasma parameters to be tested during the same vacuum cycle (Fig. l(b)).

(a) '20 um' (b)

(c) (d) ~ 20 ~tm' Fig. 2. Scanning electron micrographs of Ni-Co-Cr-A1-Y powder particles (a) before and (b)-(d) after the wipe test with various degrees of melting.

412 H. GRUNER

3. SPRAY PARAMATER OPTIMIZATION

The plasma gas energy required to melt powder particles can be calculated for a certain powder feed rate if it is assumed that the physical properties of the powder are known (particle size and distribution, melting point, heat conductivity and heat capacity). Melting occurs during the coating process as a function of the vacuum in the chamber because the plasma jet characteristics are pressure dependent. The plasma jet becomes longer and the gas molecule and powder particle velocities increase at higher vacuums. Therefore the dwell time of powder particles in regions of the plasma jet where thermal energy for melting can occur is a function of the vacuum used.

First the degree of melting of the powder particles is determined by the wipe test as a function of the plasma energy (Fig. 2). Figure 3 shows N i - C o - C r - A I - Y coatings of various qualities sprayed with the same energy as was used for Fig. 2. From the powder injection parameters available (Fig. 4), only the angle of injection has been varied. It can be seen that a higher porosity in the coating microstructure (polished) is due to numerous entrained unmelted particles (microstructure etched). The influence of the injection angle on the density and therefore on the residual porosity of a VPS coating has been documented 8. For the conditions given in Fig.

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Fig. 3. Ccoss sections of N i - C o - C r A1-Y coatings sprayed with the same plasma parameters (not heat treated; vertical injection, 0"): (a) polished (angle of injection, 25°); (b) polished (angle of injection, 0°); (c) polished and etched (angle of injection, - 25 ) ; (d) polished and etched (angle of injection, 0'3.


Fig. 4. Nozzle configuration o fa VPS torch with the available powder injection variations.

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Fig. 5. Scanning electron micrographs of the substrate surface after the wipe test for Ni Co-Cr -A1-Y powder ( - 400 mesh; powder injection conditions the same as in Fig. 3(b)).

414 H. GRUNER

3(b), the wipe test clearly shows that nearly 10% of the sprayed particles were insufficiently molten before impact (Fig. 5).

Measurement and evaluation of the substrate surface after the wipe test provides the following information: (i) the degree of melting of the powder particles; (ii) the percentage of insufficiently melted particles; (iii) the position of the spray centre in relation to the geometric centre of the plasma jet; (iv) the distribution of unmelted particles in the plasma jet cross section.

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Fig. 6. Percentage ofunmelted spray powder particles through the plasmajet cross section determined as a function of carrier gas flow rate for Ni-Co-Cr AI-Y powder ( - 400 mesh; coating pararneters set up as for Fig. 3(a)).

Figure 6 shows the number of unmelted particles through the whole cross section measured as a function of the flow rate of the powder carrier gas. All other parameters were as above. After the powder feed rate and the geometric injection conditions have been adjusted (see Fig. 4), the carrier gas flow has to be optimized for powder particle transportation in the centre of the plasma jet. Optimization of the injection parameters is a question not only of keeping the number of unmelted particles as low as possible but also of distributing the unmelted particles symmetrically throughout the jet cross section. This avoids the possibility that inhomogeneities occur in coatings of several layers because a particular section of the jet contains a very high number of unmelted particles (Fig. 7). After the adaptation of coating parameters for a particular coating structure, the wipe test can also be used for quality control in coating production.

4. APPLICATIONS

The performance capabilities of the wipe test for parameter optimization have been documented for two extremely different spray powders (titanium and TiB2). Figure 8 shows the VPS coating of the low melting point (relative to TiB2) extremely reactive titanium after determination of the melting parameters (Fig. 9) and


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Fig. 7. Micrographs of a VPS N i - C o - C r - A I - Y coating sprayed in several layers with an unsymmetrical distribution of unmelted particles in the plasma jet (polished and etched, not heat treated).

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Fig. 8. Structure o fa VPS double-layer coating of titanium on tantalum (parameters as in Table I): (a) scanning electron micrograph; (b) optical micrograph, mirror image of (a).

optimization of the injection parameters by the wipe test (Fig. 10). In contrast, Fig. 11 shows a TiB2 coating. This is an extremely interesting coating with great potential in many areas of application as a result of its high hardness and

4 1 6 H. GRUNER

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(c) (d) 40 lam Fig. 9. Scanning electron micrographs of t i tanium powder (a) before and (b)-(d) after the wipe test with various degrees of melting.

TABLE I OPTIMIZED SPRAY PARAMETERS FOR T I T A N I U M A N D T i B 2 P O W D E R S

Parameter Values for Ti Values for TiB 2

Particle size (p.m) < 60 5-25 Melting point (°C) 1690 3225 Heat conductivity (cal cm i s - 1 ° C - 1) 0.04 0.06 Specific heat (cal g - 1) 0.14 Knoop hardness (kgf m m - 2 ) 2600 Plasma Primary gas (Ar) flow rate (1 min 1) 60 22 Secondary gas (H2) flow rate (1 min 1) 8.5 12 Plasma power (kW) 38 60 Vacuum (mbar) 60 250 Torch-to-substrate distance (mm) 350 200 Injection a Coordinate x (mm) 44 37 Coordinate y (mm) 4.7 3.8 Injector diameter d (mm) 3 2 Angle of injection q~ (deg) - 25 - 25 Vertical indication 6 (deg) 0 0 Feed rate (g min 1) 37 24 Carrier gas (Ar) flow rate (1 min - 1) 2 3

a See Fig. 4.


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Fig. 10. Results of the wipe test with titanium spray powder (parameters as in Table I).

temperature stability. The degree of melting is shown in Fig. 12. Figure 13 shows the spray powder particle distribution obtained by the wipe test. The spray parameters optimized for the two powders are given in Table I.

5. CONCLUSIONS

The wipe test is an extremely rapid method for determining the plasma jet energy required to melt particles of all types of spray powder. In addition, it allows

4 1 8 H. GRUNER

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20 pm 10 gtm Fig. 11. Scanning electron micrographs of a VPS TiB 2 coating (parameters as in Table I; Vickers hardness, 300 HV).

(a) 10 ~tm (b) 10 p_m

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Fig. 12. Scanning electron micrographs of TiB 2 spray powder (a) before and (b)-(d) after the wipe test with various degrees of melting.

an optimization of powder injection parameters to be carried out for a given set of plasma parameters. Thus the test can directly determine the quality of VPS coatings or adjust their structure. The optimization of powder injection parameters not only ensures that the number of unmelted particles is kept as low as possible but also

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guarantees a symmetrical distribution throughout the cross section of the plasma jet. Therefore the wipe test is a useful method for quality control in the production of coatings.

REFERENCES

1 H. Gruner, Thermische Spritzverfahren, Lehrgang 6520/45.032, 1983 (Technische Akademie Esslingen, Esslingen).

2 R. Henne and H. Nussbaum, 4th Int. Conf. on Solid Surf.aces, Cannes, 1980. 3 K.D. Borbeck, Proc. lOth Int. Thermal Spraying ConiC, Essen, 1983, in D VS Ber., 80 (1983) 99. 4 E. Muehlberger and R. D. Kremith, U.S. Patent 4,328,257, 1982. 5 H.D. Steffens and H. M. H6hle, Proc. 9th Int. Thermal Spraying Conf., Nederlands Instituut voor

Lastechniek, The Hague, 1980, p. 420. 6 H. Eschnauer and E. Lugscheider, Proc. Int. Conf. on Metallurgical Coatings, San Diego, CA, 1984,

in Thin Solid Films, 118 (1984) 421. 7 H.D. Steffens and H. A. Crostack, Proc. 9th Int. Thermal Spraying Conf., Nederlands Instituut voor

Lastechniek, The Hague, 1980, p. 120. 8 J.R. Rairden, M. R. Jackson and M. F. Henry, Proc. lOth Int. Thermal Spraying Conf., Essen, 1983,

in D VS Ber., 80 (1983) 205.

Documents

Vacuum plasma spray quality control