Scanning Scratch Tests for Evaluating the Adhesion of Thin Oxide Films on Stainless Steel
- PDF / 941,984 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 27 Downloads / 238 Views
INTRODUCTION The protection of several materials against environmental attack is generally related to
the presence of adherent and protective films. These films should be inert in aggressive environments. Oxide scales, such as alumina or silica have shown a sufficient chemical resistance against aggressive compounds [1,2]. Besides the chemical properties the mechanical properties are also of considerable importance [3,4]. At present, there is an increasing interest in using indentation and scratch test techniques to study mechanical properties, such as the adhesion strength of thin oxide films to the substrate. In this study, experiments were performed to obtain some understanding of the adhesion characteristics of thin alumina films on AISI 304 deposited by low pressure metal-organic chemical vapour deposition. The adhesion properties were determined as a function of the film thickness, the growth rate, the deposition temperature, and the thermal treatment. THEORETICAL BACKGROUND The total critical stress to induce spallation can be expressed as the sum of the thermal stress (aitherm.1 ), the intrinsic (growth) stress ((Ti,.in), and an externally applied stress, following: 0
C criica
ralt m
+
itrinsic +
r external
(1)
The thermal stress is caused by the thermal mismatch, the intrinsic stress by, for example, phase transformation, and the externally applied stress by the scanning scratch tester. The thermal stress can be calculated by the simplified equation: 863
Mat. Res. Soc. Symp. Proc. Vol. 356 @1995 Materials Research Society
-Ef.AT.AcL
(2)
. aotal = ,thermal ..
where Ef is Young's modulus of the film, AT difference between deposition and actual temperature, Acx difference between thermal expansion coefficients, and gf the Poisson ratio of the film. The growth stress is a function of several process parameters and material properties and cannot be calculated in an easy way: .,
=
f (process parameters)
(3)
The externally applied stress near the film / substrate interface, caused by the scanning scratch tester, is a function of extrinsic and intrinsic parameters [5,6]. The intrinsic parameters are for example, the loading rate, the scratching speed, the amplitude, the frequency, and the indenter tip radius. The extrinsic parameters are film and substrate hardness, film thickness and roughness, friction force and coefficient. For example, generally the critical stress showed a linear increase with increasing film thickness and film hardness [7,8,9], which was explained [8] by the fact that a thicker film requires a higher shear stress on the surface to induce a given shear stress at the interface. The external stress can be expressed by: ,ee,,er~l= f (intrinsic, extrinsic parameters)
(4)
Not much information is available on the scratch tests in relation to the externally induced stress near the interface, causing cracking and delamination of thin films. The critical stress for scale failure, neglecting the Poisson ratio, is given by: _Cyi=fimd [i(7,
+
=
[KicId
(5)
with KI, the fracture tou
Data Loading...
