Effect of residual stress on the fracture strength of columnar TiN films
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R. Gunda Department of Mechanical Engineering, Indian Institute of Science, Bangalore-560012, India
V. Jayarama) Materials Research Centre and Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India
S.K. Biswas Materials Research Centre and Department of Mechanical Engineering, Indian Institute of Science, Bangalore-560012, India (Received 24 May 2007; accepted 30 August 2007)
The intercolumnar fracture strength of a columnar TiN film was deconvoluted by nanoindentation load-displacement curves. The columnar boundary strength of such films strongly depends on the amount of residual stress present in the film. The residual stress of the coating was modified by plastically stretching the substrate. Indentation studies on the columnar TiN coating as a function of residual stress reveals that the column sliding stress diminishes with residual stress, suggesting that the presence of a high compressive stress substantially improves the fracture strength of a columnar TiN film. True film hardness on the other hand does not change.
I. INTRODUCTION
The residual stress in nitride and other coatings is believed to influence mechanical properties such as hardness, fracture toughness, adhesion, wear resistance, and so forth.1–6 However, it is often not easy to separate its role from those of other microstructural changes that take place concurrently when film deposition variables are changed to vary the residual stress. For example, in columnar coatings made by physical vapor methods such as sputtering, arc evaporation, and so forth the structure of column boundaries, texture, grain size, the degree of atomic packing, and any pores or excess volume in the intercolumnar region are all strongly determined by the same process variables that also change the residual stress such as substrate bias, deposition temperature, sputtering/evaporation conditions, gas pressure, and so on.6–14 Residual stress is an important engineering parameter since the maximum useful film thickness is limited by the generally high compressive stress in these films that arises from ion bombardment.12 Thus, many of the conclusions that have been reported5,15,16 on the role of residual stress in improving adhesion, hardness, and
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0442 J. Mater. Res., Vol. 22, No. 12, Dec 2007
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wear resistance are open to question, as the changes might have been due to changes in the other parameters that are inevitably altered in the attempt to change the residual stress. For columnar TiN, it has been reported earlier that the deformation response to contact loading of film-substrate systems is determined by the shear stress required to cause intercolumnar sliding.17 For sufficiently thin films on hard substrates, when other modes of bending or shear are negligible, it is possible to deconvolute this critical sliding stress from nanoindentation measurements.17–19 It may be
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