金相浸蚀

Metallographic Etching and optic contrasting methods

Whether a microstructure is visible under polarized light (with a sensitive tint plate in the optical path) or not, strongly depends on the phases of the analyzed material. In this context the material's Bravais lattice type is of great importance. While BCC and FCC metals like steel, aluminum, copper, or chromium need to be etched to see a microstructure, HCP metals and alloys like α-titanium, cobalt, hafnium, zinc alloys or the orthorhombic bismuth show their microstructure in the polished state.

There are various techniques of optic contrasting. Birefringent crystals, for example, (like spherulites in partially crystalline polymers like polyamides or high-density polyethylene), can be visualized via phase contrast methods. Fluorescence microscopy and dark field microscopy are also important contrasting methods for failure analysis. However, they are usually applied to visualize microcracks, pores, or similar material inhomogeneities.

In many cases however, the incident light from the microscope is reflected quite uniformly. Therefore, contrast must be generated in another way to enable any kind of optical evaluation, e. g. by metallographic etching.  

在金相学中的化学蚀刻

化学蚀刻工艺在金相学中是最常见的。由于其出色的成本效益和简单的操作方式，它们非常受欢迎。大多数情况下，蚀刻是通过浸泡进行的：将需要蚀刻的样品表面完全浸入相应的介质中并移动。另一种适用于某些应用的技术是使用棉签：在这种方法中，将棉垫或非常柔软的纸巾浸湿蚀刻剂后擦拭样品表面。当由于材料对蚀刻剂的敏感性而无法进行浸泡时，通常采用这种方法。需要注意的是，不要划伤已准备好的表面。大多数情况下，化学蚀刻是一种选择性腐蚀或氧化，这被称为结构蚀刻。 在氧化蚀刻的情况下，介质中的某个成分（通常是H+/H2）与固体（通常是金属）之间发生氧化还原反应。根据晶体学取向（晶粒表面蚀刻）和晶体畸变（晶界蚀刻），这种反应会以更高的反应速度发生。相的成分也会导致不同的电化学电位，从而产生不同的氧化速率。这会导致形成浮雕结构，在显微镜下表现为阴影对比。 在某些体系中，例如“Kalling 2”试剂，蚀刻样品上会沉积还原金属或盐。这些沉积物可以用棉花去除，只有这样，蚀刻后的微观结构才能显现出来。纯结构蚀刻过程的信息价值有限，因为上述机制会重叠，并且关于晶粒取向的信息不多。其主要目标更多是确定某些材料的晶粒尺寸分布和相组成。 这使得显微镜方法的发展（如自动确定晶粒取向）几乎不可能。同样的情况在一定程度上也适用于不均匀性。需要注意的是，非金属夹杂物可以通过纯结构过程可靠地呈现出来。 这一点可以通过低合金钢的例子很好地说明。铁素体的电位低于沉淀的渗碳体或石墨，这导致该相氧化得更快。畸变的晶界被去除得较慢，并在这种情况下形成突出的区域。由于珠光体的层状结构，这一相在晶粒中产生均匀的浮雕蚀刻，可通过深灰色条纹识别。根据晶粒相对于研磨平面的取向，这些条纹的可见度会有所不同。在这种情况下，Nital或V2A蚀刻剂是典型的试剂。 使用不同的金相蚀刻剂，可以进行所谓的彩色蚀刻或沉淀蚀刻。这种技术可以提供更多的微观结构信息，但要以可重复的方式执行则困难得多。

除了介质对相和取向的选择性侵蚀外，属于氧化还原系统的层也会沉积。这一层的厚度因局部反应速度不同而异，导致入射光的干涉现象。这表现为晶粒表面的强取向依赖性变色，在偏振光下可见。如果样品过度蚀刻，由于层厚过大，干涉现象会消失。

在低合金钢上应用的一种著名的金相彩色蚀刻技术是根据Klemm的方法。针对钢材，基于阳极形成硫化物膜的不同彩色试剂被应用。Behara和LePera的试剂在添加剂和使用的亚硫酸盐载体上有所不同。根据要分析的特定合金系统选择不同的试剂。

彩色蚀刻工艺在无机非金属应用中也很常见，例如用于水泥熟料。许多这些过程的机制尚未完全理解。然而，在定量分析相时，它们通常能可靠地工作。

_{AlFe10，面心立方（Fcc）铝基体中含有FeAl3针状结构，通过Barker试剂进行电解蚀刻}

_{AlMg 4.5合金采用7%氢氧化钠（NaOH）溶液蚀刻}

_{奥氏体V2A钢（通常指的是18/8不锈钢）采用Beraha 2蚀刻剂进行蚀刻}

_{α-β黄铜使用10%水溶性硝酸铁溶液进行蚀刻}

在金相学中的电解蚀刻

Like the chemical process, electrolytic etching is based on the formation of numerous galvanic elements on the polished sample's surface. In this case, it is necessary to apply an external voltage to the sample to force the desired redox reaction.

In addition to the factors listed above, locally varying electrical conductivity and the set voltage or current of the electrolysis cell influence the removal rate. When an automatic metallographic etch machine is used, flow rates and cell geometry also have an impact on the displayed microstructure. Electrolytic methods usually show higher removal rates than chemical methods, which is why they can be used as metallographic polishing processes, too. This is the greatest advantage of the process, since the replacement of metallographic polishing steps makes it possible to produce completely deformation-free surfaces and reveal the true microstructure, which is not possible otherwise.
The transition between electrolytic polishing and etch processes is mainly determined by the applied current density. Electrolytic etch processes are almost exclusive to the field of metallography. Since they are automatically controlled, they provide a higher reproducibility than purely chemical ones. These are still performed manually and require a higher level of user experience. Electrolysis in metallography can also be described as anodizing a metal. Generally, more noble, or at least equivalent, metals are connected as cathodes, while the sample serves as the anode.