Pulsed laser treatment of plasma sprayed coatings
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PLASMAsprayed
coatings are widely used in order to increase the resistance of materials to wear and corrosion. A problem faced by this technique is the large fraction of both open and closed porosity of the coating which lowers its strength and decreases its resistance to corrosion. Heat treatment of the coated surface in order to reduce the porosity is sometimes precluded by the necessity to avoid melting or deformation of a thin substrate or the modification of the substrate-coating interface. Even a quick scanning of the coating by a CW, CO2 laser leads to unsatisfactory results in the case of air-sprayed coatings, because the expansion of the gas-filled pores within the coating generates a spongelike structure. ~ Better results are obtained with a pulsed TEA laser, which melts only the upper surface portion of the coating. 2 In this paper an analysis of the different parameters of the pulsed laser treating process is made. The effect of the energy, the length, and the shape of the pulse on the heat transfer is evaluated and results on the surface modifications and the porosity reduction produced by the laser treatment are given. In particular, the choice of a slightly spiked pulse shape is shown to produce a better efficiency of absorption by the treated surface. If surface porosity is to be reduced, a small layer of surface material must be melted and kept in the liquid phase for a period of time long enough so that surface tension will smooth out the discontinuities of the surface. On the other hand, surface vaporization must be avoided and the pulse energy required to melt a given depth must be minimized. In order to reach these objectives, the following laser parameters must be optimized: total energy of the pulse, pulse length, and waveform. The last two parameters are strictly related to the CO2/(CO2 + N2) volumetric ratio in a CO2-TEA. As shown in Figure 1, the pulse length goes from about 50/~s to 10/xs as the volumetric ratio goes from 0.01 to 0.15. At the same time, the pulse shape goes from a roughly triangular to a sharp-spiked form. The initial spike becomes more and more important as the volumetric ratio increases. It will be first investigated how the energy and the shape of the pulse affect the surface temperature and the melt depth. An analysis of the pulse length requirements will follow. In order to evaluate the evolution of the surface temperature and the heat affected zone, the model of a semiinfinite solid exposed to one-dimensional variable heat flux is considered. The heat conduction problem with S. DALLAIRE and P. CIELO are both Research Officers, National Research Council Canada, Industrial Materials Research Institute, Montreal, Qurbec, Canada H4C 2K3. Manuscript submitted October 20, 1981. METALLURGICALTRANSACTIONS B
time-dependent boundary conditions is solved by using Duhamel's theorem. For different pulse shapes the analytical expression for the transient surface temperature and temperature distribution within the solid before melting is derived. When the surface reaches the mel
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