The Cracking and Decohesion of Thin Brittle Films
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THE CRACKING AND DECOHESION OF THIN BRITTLE FILMS MU-SAN HU Materials Department, College of Engineering, University of California, Santa Barbara, CA 93106 ABSTRACT Film cracking and decohesion are two typical failure modes observed in thin coatings. In this investigation, failures were initiated in brittle Cr films deposited on both ductile and brittle substrates to study these phenomena. Fracture mechanics models were proposed to interpret the experimental observations. Excellent agreement between theory and experiment has been demonstrated. INTRODUCTION Thin films are widely used in applications, such as electronic devices, magnetic recording media, cutting tools and optical lenses. Besides the specific material properties needed for each technical application, the mechanical integrity of the film is the key to a high manufacturing yield and reliable service. In this article, film cracking (splitting) and film decohesion, the two typical failure modes when films are stressed in tension, will be described and analyzed. To observe the sequence of failure events, brittle Cr films were deposited onto various substrate materials by electron-beam evaporation. Cracks were then initiated and monitored through bending tests [1] and by controlled stress corrosion cracking [2]. The results are used to validate the analytical predictions. FILM CRACKING
Theor ical BackgrmUnd For a steady state, long crack in an elastic system (Fig. 1), nondimensional analysis shows that the mode I stress intensity factor at the crack front, KI, is independent on the crack length, and can be expressed as [1,2,3]: K I = fc(Y-) o-
(1)
where ; is the film stress, h is the film thickness, Y, is the Young's modulus ratio of the film to the substrate, ie. Ef/Es, and Qc(Y") is a dimensionless quantity. When ductile substrates are used, the stress concentration at the interface crack tip can cause substrate plasticity and blunt the tip (Fig. 5). In this case, the function fc is also dependent on the ratio of the film stress to the substrate yield strength, KI= Qc(L'a/Y) yfhBy rearranging and imposing the critical condition, thickness hc can be derived as:
Mat.Res. Soc. Symp. Proc. Vol.130. c1989 Materials Research Society
(2) a critical film
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2
hc=
2
nc (Y, aY/Y)
(3)
In this expression, KIc is the mode I fracture toughness of the film, which is a material constant. Below the critical thickness, hc, film cracking is completely suppressed. Furthermore, since the film stress a can be controlled or measured in experiments, the only unknown needed to calculate hc is f2 c. Therefore, the following discussion will concentrate on Kc and how it changes with respect to changes in material properties. Note that a higher i~c results in lower hc, ie. the film is more likely to crack.
Figure 1 Schematic illustration of a steady state film crack in an elastic system Calculation of (?c based on a shear lag model [1] (Fig. 2) reveals the enhancement of film cracking with a low yield strength substrate. This can be visualized as follows: as Y decrease
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