IMPLEMENTAÇÃO DA TÉCNICA DE FOTOACÚSTICA DE CÉLULA ABERTA PARA OBTENÇÃO DA DIFUSIVIDADE TÉRMICA DE METAIS

Abstract

The photo-acoustic technique has become a standard photo-thermal method for carrying out diverse studies on solids materials. Photo-thermal phenomena in solids results from a combination of thermal expansion, thermal diffusion, and thermo-elastic bending effects and can be obtained by c k . r a = where k is the thermal conductivity, r is the density, and c is the specific heat at constant pressure. For that reason thermal diffusivity is a parameter that measures the rate of heat diffusion in a sample. It is a genuine and important bulk property of the material which is directly related with the materials structure and stabilization conditions. In this work, we propose to build a system to measure the thermal diffusivity of a metal sample, in special the aluminum and the AISI 304 steel with nitrating and non-nitrating. In the AISI 304 steel modifications in surface metals are made to improve its mechanicals and tribological properties as well also the corrosion resistance in metal alloys. These materials surface modifications can be made by using different ion beams processes and the most commonly employed to modify surface properties of steels are ion implantations, glow discharge and plasma immersion ion implantation. In this work we applied the Open-Photo-Acoustic Cell (OPC) to determine the variation in the thermal diffusivity of the glow discharge nitriding in Commercial AISI 304 stainless steel material. Because the ion beams processes modified region, different nitride precipitates are formed and they will improve the mechanical and tribological properties changing the thermal diffusivity. Aluminum samples were cutted from a rod, polished and prepared to measurement. After this treatment the thermal diffusivity were measured and the value 72.9 μm2 /s was obtained. Then the samples were putted at temperature of 120 0C and 240oC for 45 minutes and then the temperature was decreased rapidly back to room temperature. A new set of measured were obtained and a new value of the thermal diffusivity was obtained 84.2 μ m2/s and 90.1 μm2/s, respectively. From the aluminum we were able to show that OPC technique can detect variation in the thermal diffusivity produced by mechanical tensions produced by mechanical treatment, as cut and polish, in the superficies of the metal materials. For commercial AISI 304 stainless steel three samples was cutted in around (20×20×3) mm dimension. Then they were mechanically polished until 1/4 μm diamond paste in order to obtain a clean and mirror finishing. The sample was prepared for measurement and the result for thermal diffusivity for the non-nitrating sample was 4.09 μm2/s. This value agrees well with the literature 4.05 μm2/s. In the second sample we didn’t know the nitrating process and the result for thermal diffusivity was 6.58 μm2/s. From this result we can show that OPC technique can distinguish between a nitrating and a non-nitrating metal surface. And to the third sample, we know the sample nitrating process and the thermal diffusivity obtained was 11.43 μm2/s. This new result confirms our supposition. We are able to distinguish between surfaces of two samples. From this samples results three new conclusion were obtained: (1) the exciting surface, nitrating and the non-nitrating, permit us to obtain the thermal property, (2) the intensity of nitrating process is higher from the centre the border and, (3) the OPC technique can distinguish the thermal diffusivity before and after a surface thermal treatment. In conclusion we show that OPC technical is a new useful tool for characterization studies of metals surfaces. This technique was in the “Laboratório de Óptica and Espectroscopia”.