A Fourier Transform Infrared Photoacoustic Study of Zinc Phosphate Coatings on Iron

Abstract

The nondestructive depth profiling of molecular state of zinc phosphate coatings on iron has been made by using infrared photoacoustic spectroscopy. The advantages of this spectroscopy are : (1) the escape depth of photoacoustic signals can be varied with different measuring parameters; (2) highly light-scattering materials and powdered samples are also measurable, and (3) the sample surface area required for the measurements is relatively small. Distinctive five peaks in the photoacoustic spectrum are assigned to phosphate ions and the coordinated water molecules. The escape depth of signal is estimated as about 2.2 micrometers for this sample system but it is possible to make the escape depths longer by impinging primary beams modulated with lower frequencies. TO and LO splitting of phosphate ion vibration is observed with the thin coating layer. Coordinated water is deficient in the region adjacent to metal and substrate interface in comparison to the outer layer of the coatings. Chemical composition and structure at the interface are briefly discussed.