Investigations on the mechanical behaviour of rough surfaces of TiNi thin films by nano indentation studies
Introduction
During the last decade, studies on the mechanical properties of thin films by nano indentation have been receiving added thrust due to the various refinements and improvement of the methods adopted earlier [1], [2]. Central to the standard Oliver–Pharr [3] data analysis procedure that is widely followed for interpreting nano indentation test results, is the presumption that the film surface is ideally smooth compared to the indenter penetration depth. In reality, all surfaces are characterised by some amount of roughness, when magnified to a commensurate degree. Hence the actual area of contact will be different from the geometrical area that is presumed for the Oliver–Pharr hypothesis. In such nano indentation experiments, where the indentation depth is comparable to the roughness on the surface, the fundamental understanding of the analysis is prone to influences of the rough surface and any attempt to extract some general inferences from the results requires a careful assessment of various factors. One effective method of avoiding the effect of surface roughness on the indentation results is to eliminate the geometric shape function from the calculation [4]. In such a method, the absolute value of hardness and modulus of elasticity cannot be determined, but the ratio of load/(stiffness)2 (P/S2) can be determined which gives an estimate of the variation of hardness with load. On an actual rough surface, containing a large number of irregularities (asperities), the stiffness of multiple asperity contacts does not add linearly and so the relationship between the contact area and stiffness is lost and on that account the P/(S)2 analysis becomes implausible. The results of the measured hardness of such films will hence be significantly affected and the problem now rests on insulating the ostensible influence of the surface roughness and to construe the mechanical behaviour of the surface alone.
In the present investigation, thin films of Titanium–Nickel (TiNi) alloy were studied by nano indenting the samples to depths that were comparable to their surface roughness scale. Titanium–Nickel alloys have found extensive commercial applications in both engineering and medical sections as they are classified as smart materials due to their shape transformation property [5], [6]. The technical importance of TiNi as a smart material has increased with its utility as a shape memory thin film that behaves as a thermal actuator. The prospect of integrating TiNi thin films into Micro Electro Mechanical Systems (MEMS) and its compatibility for use as an actuator in micro systems has led to recent research explorations into thin film forms of the alloy [7], [8], [9]. Active work on the tribological properties and wear resistance of TiNi thin films has been in progress to establish its applicability in corrosive and biological environments [10], [11]. Hence, the need for a detailed study on the surface level properties of such thin films has become imperative.
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Experimental details
TiNi thin films with the thickness of 0.85 μm were prepared by conventional DC magnetron sputtering. The inherent difference in the sputtering yields of Titanium and Nickel leads to nickel-rich TiNi films. Various workers have adopted novel techniques like introducing a Ti mesh to compensate for the loss of Ti in the film during sputtering from an equiatomic target [12], [13]. To offset this imbalance in the composition of the films, a composite target composed of sectors of Ti and Ni in the
Discussions on the experimental results
Compositional analysis by Rutherford Back Scattering (RBS) revealed the film composition to be Ti47Ni53. The spectral profile along with the fitted curve is shown in Fig. 1.
X-ray diffractogram of the annealed samples, scanned through a range of 10° to 80°, strongly hinted at the presence of the austenite B2 phase, TiNi3 precipitates, and a relatively low percentage of martensite, B19′ (Fig. 2).
The films were observed to have high surface roughness, as was shown by the AFM images (Fig. 3). An
Estimation of hardness of the film from nano indentation of a rough surface
We follow the method of Bobji and Biswas [14], to extract the mechanical hardness of the film alone that lies convoluted in the result because of the high roughness of the surface. Though the original method adopted by the authors [14] is applied to a surface with fractal geometry in their estimates, we adopt the same to a Gaussian statistical distribution of asperity heights over the surface. The calculated hardness coincides well with the hardness expected from the TiNi thin film.
Ideally,
Conclusion
The present study elucidates the nano indentation behaviour of a rough surface of TiNi thin film. Hardness variation as a function of indentation depth was found to exhibit its behaviour in three stages. The actual property of the film was found to lie in the intermediate plateau region bounded by regions of very high hardness and of extremely low hardness on either side. The high hardness values that are observed in region 3 are expected to be from the substrate, while the low hardness values
Acknowledgements
The authors are grateful to the Department of Science and Technology, India for the financial grant supplied to carry out this research work and to the management of PSG College of Technology, India, for providing the infrastructure.
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