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LASER MELTING OF PLASMA SPRAYEDCERAMIC COATINGS

R. Sivakumar, B. Mordike

To cite this version:R. Sivakumar, B. Mordike. LASER MELTING OF PLASMA SPRAYED CERAMIC COATINGS.Journal de Physique Colloques, 1987, 48 (C7), pp.C7-123-C7-126. <10.1051/jphyscol:1987721>.<jpa-00227023>

JOURNAL DE PHYSIQUE Colloque C7, supplement au n012, Tome 48, decembre 1987

LASER MELTING OF PLASMA SPRAYED CERAMIC COATINGS

R. SIVAKUMAR and B.L. MORDIKE

Institut fiir Werkstoffkunde und Werkstofftechnik, Technische Universitat Clausthal, D-3392 Clausthal-Zellerfeld, F.R.G.

Abstract:

The porosity in the plasma sprayed ceramic coatings limits its life in many applications. A viable approach to overcome this problem is to remelt the coating using a laser beam. The melted layer cools non-uniformaly and the resulting tensile stresses generally lead to cracking. In this investigation, laser melting of ZrO , A1203 and Ti02 caotings was carried out. Laser operating paramezers were optimized to obtain melted layers with as few cracks as possible. The conditions that lead to complete elimination of cracks have been identified and successfully demonstrated for A1203 and Ti02 coatings.

Introduction:

Plasma sprayed ceramic coatings on hot section components of gas turbines offer significant advafitages by increasing the operating temperatures and thereby the efficiency (1). Most of these coatings are based on ZrOZ compounds. One of the life limiting factors of these coatings is the oxidation of the metallic bond coat by corro- sive gases and molten salt that can penetrate through the inter- connected porosity of the sprayed coatings (2). A possible method of overcoming this problem is to melt a part of the coating by which the combination of a porous coating with a fully dense top layer can be obtained. The laser offers distinct advantages in remelting the layer with minimal input of heat into the surrounding areas. A few investigations on the efficacy of melting ceramic coatings using laser have been reported (3-6). These clearly point out the major problem in the laser process to be the formation of cracks.

The objective of this investigation was to develop an understanding of the effect of laser processing variables on the nature and ex- tent of cracking. The problem of cracking has been analysed by melting different ceramic coatings based on Zr02, A1203 and Ti02.

Experimental Procedure:

A number of partially stabilized and fully stabilized ZrO coatings with CaO, Y203 and Mg0 as stabilizers, A1203 and Ti02 coagings were melted using a C02 l$ser operating in the pulsed mode. The peak power density was 10 to 10 watt/cm2 with pulse widths varying from 0,03 to 0,4 millisecond and frequencies from 50 to 1000 HZ. The samples were moved relative to the beam at scan speeds of 5 to 200 cm/min. Some experiments were carried out with samples pre- heated upto 800°C. Laser melted surfaces and cross-sections were characterized.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987721

C7-124 JOURNAL DE PHYSIQUE

Results and Discussion:

a) Heat Transfer Calculation:

The laser power absorption value was estimated to be 50 % for the ceramic coatings. This was based on a thermodynamic calculation of the ratio of the energy required to melt an experimantally observed volume element to the input energy. With this value, the depth of melting was calculated using a one-dimensional heat conduction model (7). This model is applicable for ceramics as the beam radius is much larger than the heat diffusion distance. The calulated values agreed well with the experimental ones.

b) Melting at Room Temperature:

Fig. 1 shows the typical surface appearance of laser melted 7Y203 Zr02 coatings. A number of cracks can be seen, on the surface. When a thin layer on top is melted, the cracks are transverse in nature. When the melt thickness is increased, both longitudinal and transverse cracks are produced. These observations can be rational- ised in terms of tensile stresses produced during cooling of the solidified layer. All other ZrO coatings also cracked in a similar fashion. Further details on melzing of ZrOZ coatings can be found elsewhere (8) . The extent of cracking in A1203 was considerably less than that of ZrO coatings (Fig. 2). A range of laser operating parameters were use6 to reduce the extent of cracking. It was found that the extent of cracking was a minimum, when the pulse frequency was low and the scan speed was high, such that each individual spot was melted and cooled before the next spot was melted. The pulse width was chosen such that the peak power was sufficient to melt a thin layer on top. The degree of cracking in Ti02 was between Zr02 and A1203.

c) Melting of Preheated Coatings:

When melting was carried out on A1 O3 and Ti02 coatings which were preheated to 800°C in a furnace, t88re were no cracks on the sur- face (Fig. 3). Under the same conditions, the Zr02 coatings cracked (Fig. 4 ) . The extent of cracking was reduced as compared to the samples melted at room temperature. The above observations can be rationalised by calculating the stresses that arise during cooling of a circular spot. Assuming the conditions of non-uniform cooling of a thin disk, the maximum tensile stress was calculated to be a ET/2, where a is the coefficient of thermal expansion, E, Young's modulus and T the maximum temperature (9). As A1 0 is plastic above 2 3 1000°C, T has the value 980°C for specimens at room temperature. In this case, the maximum stress is about 1500 MPa. It is very high compared to the tensile fracture strength of 220 - 310 MPa. When the surface heated to 800°C, T is only 200°C and the corresponding stress is 305 MPa. Thus a preheated sample does not crack during cooling. A similar calculation for Zr02 coatings indicates that they have to be heated to 1050°C to obtain a crack free melted layer. This temperature is too high for present substrate materials.

Acknowledgement:

Dr. Sivakumar who is on leave from Defence Metallurgical Research Lab. Hyderabad, India wishes to acknowledge Alexander von Humboldt Foundation for providing a fellowship to carry out this work.

References:

1. S. R. Levine and P. E. Hodge, SAMPE Quarterly, 12, 1, 1980, 20

2. R. Sivakumar and M. P. Srivastava, Oxidation of Metals, 20, 3/4, 1983, 67

3. I. Zaplatiynsky, Thin Solid Films, 95, 3 1982, 275

4. A. Adamski and R. McPherson, in "Advances in Thermal Spraying", 11 th Intl. Conf., Pergamon Press, 1986, 555

5. M. Havrada et al., ibid, 569

6. N. Iwamoto et dl., ibid, 563

7. 33. M. Breinen and B. H. Kear in Laser Materials Processingst, ed. M. Bass, North-Holland Pub. Co., Amsterdam, 1983, 237

8. B. L. Mordike and R. Sivakumar, "Laser Surface Melting of Zr02 Protective Layersn, European Conferece on Laser Treatment of Materials, Bad Nauheim, DGM, W. Germany, Sep. 1986

9. S. P. Timoshenko and J. N. Goodier, Theory of Elasticity, McGraw-Hill Kogakusha Ltd., Tokyo, 1970, 441

a) Surface Morphology b) Cross-Section

Fig. 1 Laser Melted 7Y203 Zr02 Coatings

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Fig. 2 Laser MelSed A1203, Fig. 3 A1203, 8 0 0 ° C Room Temperature

Fig. 4 7Y203- Zr02, 800°C