G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.
Soft Matter. 2017 Jun 7;13(22):4035-4046. doi: 10.1039/c7sm00340d.
Existing analyses predict that thin metal films deposited on compliant substrates are subject to a variety of surface instabilities, such as wrinkles, folds, creases, etc., that become more prominent with increased compressive residual stress. Under compressive stress, cracks have been assumed to form only when the interfacial strength is weak, allowing the film to detach from the substrate. In this work, we demonstrate that cracks also form on surfaces under compressive mismatch strain when the interface is strong. In particular, we consider metal alloy films sputter deposited under bias on elastomers with different thicknesses, curing temperatures or surface treatments. The deposition parameters created residual compressive strains and strong adhesion in the bilayers. Samples without surface treatment formed wrinkles and through-thickness cracks at 0.25-0.4% mismatch strains. Only through-thickness cracks were observed in UV treated samples. The crack spacing was found to decrease by a factor of 4 when the surface was UV treated and by a factor of 3 as the elastomer thickness decreased from 30 to 6 μm. Cracks penetrated through the elastomer, 15-30 times deeper than the film thickness, and formed in all samples with a brittle coating. A numerical model was developed to explain the formation of through-thickness cracks and wrinkles under applied compressive mismatch strains. The model suggests that cracks can initiate from the peak of wrinkles when the critical fracture strength of the coating is exceeded. For the UV treated samples, through-thickness cracks are possibly impacted by the formation of an embrittled near surface PDMS layer.
现有的分析预测，沉积在柔性衬底上的薄金属膜会受到各种表面不稳定性的影响，如皱纹、褶皱、折痕等，随着压缩残余应力的增加，这些不稳定性会变得更加明显。在压缩应力下，只有当界面强度较弱时，才会假设形成裂纹，从而允许薄膜从衬底上脱离。在这项工作中，我们证明了即使在界面强度较强的情况下，在压缩失配应变下，表面也会形成裂纹。具体来说，我们考虑了在不同厚度、固化温度或表面处理的弹性体上偏置溅射沉积的金属合金薄膜。沉积参数在双层膜中产生了残余压缩应变和强附着力。未经表面处理的样品在 0.25-0.4%失配应变下形成了皱纹和贯穿厚度的裂纹。仅在经过 UV 处理的样品中观察到贯穿厚度的裂纹。当表面经过 UV 处理时，裂纹间距减小了 4 倍，当弹性体厚度从 30 微米减小到 6 微米时，裂纹间距减小了 3 倍。裂纹穿透了弹性体，深度是薄膜厚度的 15-30 倍，并且在所有具有脆性涂层的样品中都形成了裂纹。开发了一个数值模型来解释在施加的压缩失配应变下贯穿厚度裂纹和皱纹的形成。该模型表明，当涂层的临界断裂强度超过时，裂纹可以从皱纹的峰值处开始形成。对于经过 UV 处理的样品，贯穿厚度的裂纹可能受到形成的脆化近表面 PDMS 层的影响。
