|Characterisation of surfaces and coatings||
This page is incomplete as it is at an early stage of construction
De Chiffre et al  have reviewed the methods used to quantitatively characterise surface texture. They consider conventional 2D/3D roughness parameters with emphasis on recent international standards and developments. They present new texture characterisation methods, including fractals, wavelets and change trees, with the respective parameters and applications.
Reflectance Transformation Imaging (RTI) [2, 3] involves taking multiple high-resolution images with light originating from different sources over a series of exposures. The system is a suite of technologies and procedures using photometric stereo to generate an amalgamation of the surface reflectance information. In a joint project between the universities at Southampton and Oxford, the technique was used to enhance features of cultural/historical artefacts.
Fractal dimension. In conventional notation, a point has dimension 0, a line has dimension 1, an area has dimension 2 and a volume has dimension 3. If a square is decomposed into four (2 x 2 = 22) self-similar smaller squares then it has magnification 2, and if decomposed into nine (3 x 3 = 32) smaller squares, it has magnification 3. The dimension is simply the exponent of the number of self-similar pieces into which the figure has been decomposed . The total number of objects, N, is a function of the magnification factor, r, and the dimension, D, as given by N = rD. The fractal dimension, D, measures the complexity of a self-similar object.
Labrosse et al  proposed a novel procedure for the assessment of the surface finish of in-mould gel-coated fibre-reinforced polymer matrix composites. The technique involves photographing the reflection from a coated surface on an opaque screen and analysing the image using fractal dimensions. The ranking of the quality of surface finish from this technique sensibly aligned with that obtained by human observation and by the commercial Wavescan DOI system. The investment required for the new system was primarily a high-resolution digital camera, and hence was below 5% of the cost of the commercial instrument.
Wavelet Texture Analysis (WTA). Palmer et al  reanalysed the plates considered by Labrosse et al  using the WTA on the digital images to derive an instrumental measure of surface finish quality. The rank correlation between the human expert surface finish quality ratings and those from the WTA image analysis process was found to be positive, large and statistically significant. WTA could form the basis of an inexpensive and practical instrumental method for the ranking of fibre-reinforced composite surface finish quality.
Non-destructive measurement of coating thickness.
Sargent  used a small, purpose-designed thermal wave system to make non-contact measurements of primer layer thickness on grit-blasted steel surfaces. The surface phase of a thermal wave (generated using a modulated semiconducting laser) was measured relative to the phase of a laser heating source. The method had an intrinsic accuracy of ∼±0.1 μm and an absolute accuracy that was limited to ∼±2 μm because of the uncertainty in the paint thermal conductivity. Thermal wave measurements of thickness agreed with those obtained using an independent contact measurement device within the experimental errors. The variation in accuracy was estimated as a result of stand-off error, integration time and power levels.
Ostiguy et al  compared commercial solutions for coating thickness measurement (Table 1). They also used time-of-flight measurements of low frequency (500 kHz) S0 ultrasonic guided waves to estimate primer/coating thickness on aluminium plate, or unidirectional carbon fibre epoxy composite with an integrated metalic mesh, substrates. The technique is insensitive to surface periodicity or roughness. Experimental precision was <10 μm.
|metallic||composite||minimum (nm)||maximum (μm)|
|Magnetic flux/Eddy current||✔||1000||3000||✔|||