Hardness and modulus of CrN x coatings
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Hardness and modulus of CrNx coatings G.C.A.M.Janssen1 R.Hoy2, J.-D.Kamminga2 1
Department of Material Science and Technology, Delft University of Technology, the Netherlands, e-mail: [email protected] 2 Netherlands Institute for Metals Research, Delft, the Netherlands
ABSTRACT Two series of CrNx coatings have been deposited in an industrial PVD set-up. It is shown that for an increased ion bombardment on the growing film the hardness (H) and Young's modulus (E) are independent on composition. For these hard coatings the elastic strain to failure or plasticity limit (H/E) is 0.11. The coating fails long before it is stretched to its plasticity limit. Deformation of the coating by 1% already leads to crack formation. It is argued that the cause of this failure is the low work of fracture of the coating combined with flaws in the coating. INTRODUCTION Hardness of coatings is the prime factor determining the resistance against abrasive wear [1]. Tabor [2] derived the relation H=3σy for metals, relating hardness to yield stress. Recently it has been argued that for brittle ceramic materials the relation H = σy may be a better description of the relation between hardness and yield stress [3,4]. A second parameter used to characterize wear resistance of coatings is the ratio of hardness over Young's modulus, H/E. This parameter was introduced by Oberle [5] to characterize the fatigue wear of metals. In that case it was shown that the higher H/E the less fatigue wear. Recently the ratio H/E and its relation to wear of coatings has been discussed by Leyland and Matthews [6]. They discuss the notion that a high value of H/E will allow the coating to deform elastically when the substrate deforms plastically. In a recent paper by Veprek and Argon [7] a summary of hardness values and Young's moduli for hard coatings is given. Values of H/E range from 0.1 to 0.2. In that paper it is discussed that atomic bonds can be stretched to 20%. It is not implied by Veprek and Argon that actual coatings can be stretched that far. Using the identity H = σy it can be readily shown that elastic strain to failure or plasticity limit (εy) is:
εy =
H E
(1)
Another approach to determining conditions for the failure of a coating is to look at the condition for crack growth.The Griffith criterion [8] relates the maximum flaw in a material (a) to the maximum sustainable stress (σc) or strain (εc) , given Young's modulus and the work of fracture of the material (γf):
εc =
2γ
f
πEa
(2)
Y1.2.1
Y1.2.2
Failure will occur either through violation of the Griffith criterion or through surpassing of the plasticity limit. In the present paper we demonstrate that for our coatings not the elastic strain to failure (εy) but the the critical strain (εc) determines failure of the coating. We present two series of CrNx coatings, one deposited under slight ion bombardment, the other deposited under increased ion bombardment. The H/E values of the coatings are presented. The highest hardness (30 GPa) and the highest ratio H/E (0.11) are bo
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