Evaluation of residual stress levels in plasma electrolytic oxidation coatings using a curvature method
Evaluation of residual stress levels in plasma electrolytic oxidation coatings using a curvature method
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DOI：
10.1016/j.surfcoat.2014.11.006
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发表时间：
2015-05
影响因子：
5.4
通讯作者：
J. Dean;T. Gu;T. Clyne
中科院分区：
文献类型：
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作者：
J. Dean;T. Gu;T. Clyne
实验估计了在铝和镁合金基底上形成的等离子体电解氧化（PEO）涂层的典型残余应力水平。这是通过测量仅在一侧涂覆的薄条样品所显示的曲率来完成的，使用当前工作中获得的涂层刚度值。为了获得足够大的曲率以精确测量，有必要在相对较薄（~ 300-500 μm）的基底上生产相对较厚（~ 100 μm）的涂层。在这种情况下，应力水平在两种成分中都是显著的，并且存在显著的应力通过厚度梯度。因此，相变的相关特征（主要是基体的氧化）最好表示为失配应变。我们发现，Al衬底的应变大小约为0.6-0.9毫应变，Mg衬底的应变大小约为2-3毫应变，且有一个正的符号（因此涂层的无应力平面内尺寸大于残余衬底的尺寸）。这使得涂层处于残余压缩状态，在较厚的基材上，Al的典型应力水平约为40-50 MPa， Mg的典型应力水平约为130-150 MPa。这些值应被视为近似值，尽管它们的数量级可能是可靠的。它们比以前使用这种技术进行的（非常有限的）工作的结果要高。另一方面，它们比用x射线衍射得到的许多值要低。对这些差异提出了解释。
Experimental estimates have been made of typical levels of residual stress in plasma electrolytic oxidation (PEO) coatings formed on aluminium and magnesium alloy substrates. This has been done via measurement of the curvature exhibited by thin strip samples, coated on one side only, using coating stiffness values obtained in the current work. In order to obtain curvatures that were sufficiently large to be accurately measurable, it was necessary to produce relatively thick (~ 100 μm) coatings on relatively thin (~ 300–500 μm) substrates. In such cases, stress levels are significant in both constituents, and there are significant through-thickness gradients of stress. The relevant characteristics of the transformation (largely oxidation of the substrate) are therefore best expressed as a misfit strain. This was found to have a magnitude of about 0.6–0.9 millistrain for the Al substrate and 2–3 millistrain for Mg, with a positive sign (so that the stress-free in-plane dimensions of the coating are larger than those of the residual substrate). This puts the coating into residual compression and, on a thick substrate, typical stress levels would be around 40–50 MPa for Al and 130–150 MPa for Mg. These values should be regarded as approximate, although their order of magnitude is probably reliable. They are higher than those from the (very limited) previous work carried out using this type of technique. On the other hand, they are lower than many values obtained using X-ray diffraction. Explanations are proposed for these discrepancies.