等离子喷涂层原生性孔隙几何结构的分形及统计特性

孔隙是等离子喷涂涂层原生性结构, 对涂层的耐磨损、耐腐蚀、耐高温等性能具有显著影响, 是涂层参数优化的重要指标之一. 因此, 对涂层孔隙结构特征参数的全面表征对于更加精确地评价涂层质量具有重要意义. 本文将概率统计方法、分形方法与数字图像分析技术相结合, 研究了等离子喷涂涂层原生性孔隙数量、形态、尺寸及其分布等结构特征参数表征方法及孔隙的成形机理. 首先通过改变喷涂功率得到不同孔隙状态的Fe基合金涂层, 随后采用数字图像分析技术对涂层截面孔隙的扫描电子显微形貌图进行处理, 最后通过Weibull统计模型分析了孔隙周长及面积的尺寸分布特征, 并利用基于分形思想的面积-周长幂率研究了孔隙不规则形态的定量表征方法. 在实验过程中, 为了分析涂层孔隙的成形机理, 采用Spraywatch在线监测喷涂粒子的飞行状态. 结果表明: 分形维数能够表征孔隙的不规则形态, 分形维数越大, 孔隙面积越大或边界形态越复杂, 并且其与孔隙的成形机理之间存在良好的映射关系; 孔隙面积及周长的尺寸分布均服从明显的两项Weibull分布特征, 孔隙尺寸较小时, 形状参数β 较大, 而孔隙尺寸较大时, 则反之; 喷涂功率对孔隙尺寸的聚集特点产生不同程度的影响, 随着喷涂功率的增加, 粒子的融化状态逐渐改善, 孔隙的尺寸明显降低; 而当孔隙面积(周长)小于特征值时, 相同尺寸的孔隙概率密度值则越来越接近, 说明孔隙功率的变化对小尺寸孔隙出现的概率影响较小.

Fractal and statistical properties of the geometrical structure of natural pores within plasma sprayed coatings

Pores, which have remarkable influence on many properties of coatings such as wear resistance, anti-corrosion, thermostability, etc. are the natural structure formed in plasma sprayed coatings, and have been regarded as one of the most important indexes in spraying parameter refining. Hence, it is of great significance to comprehensively characterize the structural parameters of pores in the coatings, especially for the accurate evaluation of the coating quality. In this paper, probability statistics method, fractal method and digital image analysis technique are used to investigate the number, shape, size and distribution of the pores. Besides, the formation mechanism of the coatings is discussed. First, Fe-based coatings with different porous structures are fabricated at different spraying powers. Second, the digital image analysis technique is used to process the scanning electron microscope micrographs of coatings with different pores. Finally, the Weibull statistical model is utilized to analyze the distribution law of perimeter and area of the pores. The power law of area-perimeter which originats from fractal theory is employed to quantitatively characterize the irregular morphology of the pores. In order to investigate the formation mechanism of the pores, the Spraywatch is used to monitor the flying condition of the spraying particles in the whole experimental process. The result shows that fractal dimension (FD) can characterize the irregular morphology of pores. The area becomes bigger and the border becomes more complex when the FD is larger, and there is a good relationship between FD and the formation mechanism of the pores. Besides, the areas and perimeters of the pores obey the binomial Weibull distribution obviously, namely, the shape parameter (β) turns larger as pore size becomes smaller. The spraying power has a different effect on the distribution law of pore size. With the increase of the spraying power, the molten state is improved. As a result, the size of the pore decreases obviously. When the area or the perimeter is less than their corresponding characteristic values, the probability density of the pores with the same area or perimeter becomes closer to each other, which indicates that the effect of spraying power on the pores of small size is much lower.