Determination of the Adhesive Strength of Film-Substrate Interfaces Using the Constant Depth Scratch Test
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Mat. Res. Soc. Symp. Proc. Vol. 390 01995 Materials Research Society
the precise detection of the initial debonding event, and unlike the other tests, is capable of yielding interfacial adhesion data as function of position on the sample. In the following, modifications and extensions of analyses reported earlier [22], along with experimental results obtained with a number of film-substrate combinations, are reported. ANALYTICAL When an indentor is loaded on to a film attached to a substrate, a shear stress is produced at the film-substrate interface, which increases with increasing depth of penetration of the indentor into the sample. Three indentation configurations (Types I, II and III) are possible [16]. Most film-substrate interfaces of practical importance undergo damage only under Type II or III loadings, and therefore these are the configurations considered here. Type II indentation results in elastic and plastic deformation of the film with the indentor penetrating only part of the film. Type III indentation consists of complete penetration of the film by the indentor, which also plastically deforms the underlying substrate.
The maximum interfacial shear stress in Type II or Type III configuration which is generated at the periphery of the indentor along the elastic-plastic boundary can be expressed as [23,24] =(1)
0
ki1 (z)
ri Vt
2 0 kI(z) + b J7-) where ; ar is the radial stress generated in the film by the indentor; v is the Poisson's ratio of the film; t is the film thickness; 0 = [6(1-v)/(4+v)]l/2; z = Ob/t ; kl(z) is a modified Bessel function of the second kind of the first order, and kl'(z) is its derivative with respect to z. If the maximum shear stress Tiv is equal to, or greater than, the interfacial shear strength Ti, the film debonds from the substrate. In the CDST, the depth of penetration of the indentor into the sample is not adequate for "v to be equal to r, and the film does not debond with only the vertical load (W) applied. A horizontal force (Fh) is then applied so that the indentor scratches the sample. This causes adjustment of civ to a new value tihv. Further, an additional shear stress (Tieff) is now superimposed on tihv. When tieff and Tihv together equal -i, interface failure occurs, and the film is scratched off the substrate. Replacing Cr in equation 1 by appropriate quantities, the maximum interfacial shear stress due to the vertical force only, during scratching, may be expressed as:
[8 W 3c2Hs
-0.6875 3
3 b2- c2)
1IWI "tihv
or
k-1 (z)
III ihv
(2)
vt
k/, (z)
2 0kI(z) + b /ý -
Pk,(z)
Vt b-2
where Hs = indentation hardness of the substrate, and 2b and 2c represent the dimensions of the edges of the square imprints of the pyramidal indentor on the film and the substrate, respectively. "tieffcan be obtained by a force balance in the horizontal direction as follows for Types II and III: Fh Pf +T ff A or FhI = Ps + Pf + tieff A (3) where Ps represents the force required to plough through the substrate [Ps = Hs c2 cot (0/2), 0 being the apex angle of the indento
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