Abstract
Surface properties are important in any field such as the problems of corrosion in chemical aspects, bonding strength of resin composite on teeth in physical aspects, and new bone adhesion on implant in biological aspects.  These properties are affected by various factors. However, surface roughness is one of the most fundamental factors in any situation. To analyze the effect of roughness on surface properties, conventionally the parameter of arithmetic mean roughness, Ra, has most often and widely been used. However the results are not always unambiguous. Some researches conclude that the binding force is larger, as roughness is larger, but others do not have the same reults. Ra is the evaluation index in the depth direction of roughness. There may be also the influence of the horizontal direction of topography, the periodicity (wavelength) of roughness. To reveal which roughness topography is the most responsible for the certain surface properties, Fourier transform analysis was applied in this study. Fourier transform can perform frequency analysis and is effective for the analysis of complicated roughness curves, coverlapped with multiple waves. As one example, the influence of surface topographic structure on bonding strength of dental zirconia was investigated.  Recently the CAD/CAM fabricated zirconia has been more and more used in dentistry, because zirconia has the advantages of large fracture toughness, fine esthetics and few allergies. Two plate specimens of zirconia were adhered each other with dental resin cement after different polishing/abrasion treatments.  Then surface roughness curves were measured, and shear bond strength test was conducted. The relation between surface roughness and bonding strength was investigated with Fourier transform analysis of roughness curves using fast Fourier transform (FFT) software. The dependence manner of shear bond strength turned out to be irregular to Ra, which represents the topography in vertical direction of roughness. When the data were aligned in the order of wavelength, topography in the horizontal direction, they showed the tendency to become higher as wavelength was smaller, that is, strong (14MPa) for a short wavelength (12 mm), and weak (4MPa) for a long wavelength (1600 mm). Surface sensitive properties depend not only on Ra but also wavelength. Application of Fourier transform was effective for various characteristic examinations of surface-related properties and in this study shear bond strength could be better understood by wavelength dependency.