Rapid solidification of a plasmasprayed ceramic coating melted by a CO2 laserJuneHua Shieh and ShinnTyan Wu Citation: Applied Physics Letters 59, 1512 (1991); doi: 10.1063/1.106296 View online: http://dx.doi.org/10.1063/1.106296 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/59/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Evaluation of fracture behavior in a plasma-sprayed ceramics coating by acoustic emission using laserinterferometer AIP Conf. Proc. 497, 27 (1999); 10.1063/1.1301979 Polarization effects of a highpower CO2 laser beam on aluminum alloy weldability J. Appl. Phys. 79, 8917 (1996); 10.1063/1.362623 Formation of In x Ga1−x As/GaAs heteroepitaxial layers using a pulsed laser driven rapid meltsolidificationprocess Appl. Phys. Lett. 56, 1844 (1990); 10.1063/1.103065 Effects of As impurities on the solidification velocity of Si during pulsed laser annealing Appl. Phys. Lett. 47, 244 (1985); 10.1063/1.96233 Solidification kinetics of pulsed laser melted silicon based on thermodynamic considerations Appl. Phys. Lett. 46, 644 (1985); 10.1063/1.95514

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Rapid solidification of a miasma-sprayed cera by a CO2 laser

June-Hua Shieh and Shinn-Tyan Wu Department of Materials Science and Engineering, National Tsing Hua University Hsinchu, Taiwan, Republic of China

(Received 10 April 199 1; accepted for publication 19 June 1991)

Successful modeling of the solidification process during laser melting has been achieved by numerical solution of a one-dimensional heat transfer equation. Good agreement with experiments performed on Zr&-8 wt % Y,03 coating is demonstrated. Radiation heat loss from the surface to the environment is found to be important to the microstructure of the solidified coating. The calculation provides a plausible explanation of the experimental observation that a remelted layer usually consists of an equiaxed layer on top of a columnar zone.

Porosity has been a limiting factor of plasma sprayed ceramic coatings in aggressive atmospheres.tl* Laser remelting has been tried to seal the porosities; the results look promising.3” In order to optimize the process, the relationship between process parameters and the micro- structures of the modified layer needs to be established. In this letter, some successful results are reported. Speciii- tally, the remelted layers are usually found to consist of zone of equiaxed grains on top of a columnar grain zone as shown in Fig. 1. Sometimes, the equiaxed zone is absent as in Fig. 2. If the columnar structure has weak grain bound- aries, then through-thickness cracking would be a danger. Therefore, control of the microstructure is highly desir- able. It was found, through a numerical solution of a heat transfer equation, that the microstructure is closely related to the radiation heat loss of the remelted surface.’

Zirconia powder (with 8 wt % Y,03) was plasma- sprayed to a thickness of 300 pm in air on top of a bond coat of NiCrAlY alloy ( 100 ,um) which was itself sprayed on the surface of a test piece of nickel ahoy (Hastelloy X) with dimensions of 1.5 X25X 100 mm3. Remelting was done with a continuous wave 3 kW CO2 laser. Multimode was chosen to minimize the lateral variation of heat flux so

that the heat transfer was as close to a one-dimensional model as possible. The SEM micrograph of Fig. 3 shows that the heat transfer is indeed highly one dimensional, by the flatness of the bottom of the remelted zone and its parallelism to the surface.* The traverse speed of the laser was 4.5 m per minute and the overfocus is 30 mm. The laser powers were 1.2 and 0.6 kW for Figs. I and 2, re- spectively, and the corresponding beam diameters were 4.61 and 3.75 mm. If 100% of the laser was absorbed, then the energy flux into the coating could be calculated by dividing the laser power with beam diameter and travers- ing speed. The results are 3.47 and 2.13 J/mm2 for the two powers. Therefore, the energy input into the ceramic coat- ing of Fig. 1 is 160% that of the coating of Fig. 2. This calculation suggests that the zone structure of Figs. 1 and 2 is related to the heat flux during melting; a higher flux results in two zone structures, a lower flux in one zone structure.

To understand the experimental results, a theoretical modeling was attempted. The heat transfer equation is the well-known one-dimensional transient model for flat semi- infinite body

FIG. 1. SEM of a fractured cross section of plasma sprayed and laser remelted Zr02-8 wt % Y203 (thickness 110 pm; laser power 1.2 kW; traverse speed 4.5 M/min; overfocus 30 mm). FIG. 2. Same as in Fig. 1 (thickness 30 pm; laser power 0.6 kW).

1512 Appt. Phys. tett. 59 (12), 16 September 1991 0003-6951/91/371532-04$02.00 @ 1991 American fnstitute of Physics 1512 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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FIG. 3. Same as in Fig. 1 having lower magnification showing the one-dimensional nature of heat transfer.

where His the enthalpy, p the material density, K the ther- mal conductivity, and T(x,t) the temperature. The rele- vant physical quantities are given in Table I.3t9 The bound- ary condition during the cooling period is

-K~=C&- c),

where o is the Stefan-Boltzmann constant, E is the emis- sivity, T, the surface temperature, and To the ambient tem- perature. The heat transfer equation is solved accurately by a numerical method,” the details of which will be pub- lished soon. Some results are displayed in Figs. 4 and 5. In these figures the depths of the solid-liquid interface are plotted as functions of the solidification time. Notice that there are two curves in Fig. 4. This implies that there are two solidification fronts, one from the melt substrate inter- face and another from the melt air interface. The lower curve is quite similar to conventional calculations, while the upper curve is unique to the present work. It means that radiation heat loss results in surface cooling and sur- face solidification much like ice on a lake during the win- ter. When the two fronts meet, solidification is complete. In Fig. 4 this happens at 0.042 after the laser passes. The depth at which the two fronts meet is 23 pm which is to be compared with 3 1 f 3 pm thickness of the equiaxed grain zone in Fig. 1. In both Figs. 1 and 4 the laser power is 1.2 kW. In contrast, when the laser power is 0.6 kW, only one solidification front is obtained from numerical solution of heat transfer equation, as shown in Fig. 5. The correspond- ing micrograph of Fig. 2 does not show any equiaxed zone. This agreement between the experimental results and the model prediction lead us to propose that the equiaxed zone results from surface cooling caused by radiation heat loss. When the laser power is low and the melt depth shallow,

TABLE I. Thermophysical properties of ZtQ-8 wt % Y,O, (Refs. 3 and 9).

Thermal conductivity (W/m K)

Density (kg/m’)

Specific heat

kJ/‘kg K Thermal diff.

m2/s

Heat of fusion

kJ/mol.

1.39 5400 0.6 4.3x10-7 83.6

FIG. 4. Calculated depth of solid-liquid interface vs time: A and B are interface locations. The melt extends from A to B, below A and above B the melt has solidified. The dots at the intersection are extrapolation (laser power 1.2 kW; traverse speed 4.5 M/mini overfocus 30 mm).

1513 Appl. Phys. Lett., Vol. 59, No. 12, 16 September 1991 J.-H. Shieh and S.-T. Wu 1513

the conductive heat loss is so fast that surface solidification does not have time to develop. In contrast, when the laser power is high, then surface solidification will happen before the columnar zone reaches the surface. In Fig. 4 this hap- pens at 0.013 s after the solidification starts from the bot- tom of the melt. When the surface solidifies and the inte- rior is still liquid, the maximum temperature is at a location below the surface in the melt; therefore, the tem- perature gradient is small. This leads to nondirectional so- lidification and hence an equiaxed grain structure. It is to be noted that the surface temperatures calculated by the well-known method’ are 5006 and 3506 K for Figs. 1 and 5, respectively. The substantial temperature difference would most probably result in different radiative heat loss and solidification process.

In summary, the agreement between experiment and theory strongly suggests that the equiaxed zone on the top surface of a laser-remelted ceramic coating is the conse- quence of radiation loss from the melted surface.

z 3 s i 2 z IA. Y 5 si 0’

30 -

60 -

120 -

150 I I I I a I 1 0 10 20 30 40 50

TIME (S X103)

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0

2 =t 30

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The authors are grateful to R. Y. Tzong and L. W. Tsai for their discussions and technical assistance. The re- search was supported by the Republic of China National Science Council. The grant number is NSC-80-0206-E007- 16.

-I

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‘R. A. Miller and C. E. Loweli, Thin Solid Films 95, 265 ( 1982). *R. Sivakumar and M, P. Srivastava, Qxid. Met. 20, 67 ( 1983). 3R. Sivakumar and B. L. Mordike, Surf. Eng. 4, 127 (1988). 4K Mohammed Jasmin, D. R. F. West, and W. M. Steen, J. Mater. Sci.

Lit. 7, 1307 (1988). 5A. Petitbon, D. Guignot, U. Fischer, and J. M. Guillemot, Mater. Sci.

Eng. A 121, 545 (1989). 6N. lwamoto and N. Umesaki, Surf. Coat. Technol. 34, 59 ( 1988). ‘D. H. Matthiesen and W. T. Petuskey, J. Am. Ceram. Sot. 68, C-114

(1985). ‘V. G. Gregson, in Laser Materiah Processinn, edited bv M. Bass (Am- sterdam, North-Holland, 1983), p. 201. - -

9J. R. Brandon and R, Taylor, Surf. Coat. Technol. 39/40, 143 ( 1989). ‘OS. L. Lee and R. Y. Tzong, Int. J. Heat Mass Transfer 34, 1491 (1991).

FIG. 5. Same as Fig. 4 with lower laser power (0.6 kW). B is absent from the numerical calculation results.

1514 Appl. Phys. Lett., Vol. 59, No. 12, 16 September 1991 J.-H. Shieh and S-T. Wu 1514 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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