Protective over-layer coating preventing cracking of thin films deposited on flexible substrates

The article reports on a special over-layer which protects brittle coatings deposited on flexible substrates against cracking during bending. It is shown that the over-layer with (i) the low effective Young's modulus E⁎ = E/(1 − ν2) resulting in a high ratio H/E⁎ ≥ 0.1, (ii) the high elastic recovery We ≥ 60% and (iii) the compressive macrostress (σ < 0) deposited on a brittle coating can prevent its cracking during bending; ν is the Poison's ratio and H is the hardness of the over-layer. The possibility of how to prevent cracking of the brittle coatings is demonstrated experimentally. The over-layer was deposited on the brittle coating which easily cracks when the flexible substrate is bended. In this experiment the Zr-Si-O coating with low ratio H/E⁎ < 0.1, low value of We ≈ 50% and tensile macrostress (σ > 0) was used as the brittle coating and the highly elastic Zr-Si-O coating with high ratio H/E⁎ ≈ 0.1, high value of We ≈ 70% and compressive macrostress (σ < 0) was used as the over-layer. Both the brittle coating and the over-layer were prepared by the reactive magnetron sputtering using a dual magnetron. Besides, the mechanical behavior of the coating composed of three and four layers is described in detail. Obtained results can be used in the development of flexible coatings, strengthening of the surface of brittle materials, elimination of cracking of functional coatings and cracking at surfaces of bended materials.