Evaluation Methods for Degradation and Adhesiveness of Coatings using Microscopic Photo-acoustic (MPA) Method

基于光声法和交流阻抗法，发展了一种新的评价钢表面涂层劣化和腐蚀的技术。将斩波的红外激光或可见Ar激光引入显微镜聚焦在样品上，然后通过位于样品另一侧的压电聚合物传感器检测光声信号。新仪器，基于显微光声（MPA）的方法，指示一个地图，表示退化区域或少附着面积的涂层。使用MPA方法，降解的有机和TiN陶瓷涂层的低碳钢和不锈钢上清楚地检测在5至10 μ m的空间分辨率。结果表明，对于有机涂层，MPA信号的灵敏度取决于基体的腐蚀性。另一方面，陶瓷涂层钢检测到膜下腐蚀。激光的波长对腐蚀产物和膜下腐蚀的检测有影响。这种效应可以用涂层和腐蚀产物对激光的吸收率和反射率的差异来解释。对人工缺陷的涂层钢的交流阻抗分析表明，对于涂层的窄间隙分层，必须考虑电流分布。对于薄膜与基体之间的间隙很窄的情况，采用传输线模型对薄膜的等效电路进行了模拟。用曲线拟合方法模拟了阻抗法的实验结果，并由铜氧络石元素的值拟合出退化面积。

New evaluation technique for degradation and adhesiveness of coatings on steels have been developed based on photo acoustic method and AC impedance method. Chopped infrared laser light or visible Ar laser light was led into a microscope focusing on a sample, then photo-acoustic signal was detected by piezo polymer sensor on the other side of the sample. The new instrument, based on microscopic photo-acoustic (MPA) method, indicates a map which denotes degraded area or less adhesive area of coatings.Using MPA method, degradation of organic and TiN ceramics coatings on a mild steel and a stainless steel were clearly detected in a 5 to 10 mum spatial resolution. It is found that for the organic coatings the sensitivity of MPA signal depends upon the corrosivity of the substrate. On the other hand, under film corrosion is detected for the ceramics coated steels. Wave length of the laser light has an influence on the detection of the corrosion products and under film corrosion. This effect is interpreted by the difference in the absorbency and reflectivity of the laser light for coatings and corrosion products.AC impedance analysis on coated steels with an artificial defects indicated that the current distribution has to be considered especially for narrow gap delamination of coatings. For very narrow gap between the coating film and the substrate, the electrical equivalent circuit is simulated by a transmission line model. Experimental results from impedance method are simulated by curve fitting method, and the area of the degradation is estimated from the values of cuircuite elements by fitting.