Structural and mechanical properties of TiC/Ti and TiC/B 4 C multilayers deposited by pulsed laser deposition
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J.J. Nainaparampil AFRL / MBLT, Wright-Patterson Air Force Base, Ohio 45433 (Received 18 July 2001; accepted 18 March 2002)
Multilayers of TiC/Ti and TiC/B4C have been deposited by pulsed laser deposition. Ti, B4C, and TiC targets were used to deposit multilayer films onto 440C steel and silicon substrates at 40 °C. The structural, compositional, and mechanical properties of the multilayers were examined by x-ray diffraction, x-ray photoelectron spectroscopy, transmission electron microscopy, and nanoindentation techniques. Tribological properties were also evaluated using a pin-on-disc friction and wear test. The TiC/Ti films were found to have a crystalline structure, and both (200)TiC/(100)Ti and (111)TiC/(101)Ti orientation relationships were found in these films. In the TiC/B4C films, only the sample with the largest bilayer thickness (25 nm) had significant crystallinity and only the TiC layer was crystalline. X-ray photoelectron spectroscopy depth profiles confirmed the presence of composition modulations in these films. Nanoindentation tests of the TiC/Ti multilayers showed hardness levels exceeding that predicted by the rule-of-mixtures. The TiC/B4C multilayers showed increasing hardness with decreasing bilayer thickness but reached only 22 GPa. The pin-on-disc tests gave friction values ranging from 0.3 to 0.9 for both sets of films. These results were correlated with the degree of crystallinity and grain structure of the films.
I. INTRODUCTION
The mechanical properties of multilayer thin films have been the subject of numerous investigations over the past two decades. Hardness and yield strength enhancements for multilayers with composition modulations on the nanometer scale have been reported by a number of investigators who have examined metallic,1–5 nitride,6–8 and metal/nitride9,10 multilayer structures. For example, TiN/VN multilayer films have shown hardness levels of up to 4800 kgf/mm2, much higher than the constituent layers.8 The primary strengthening mechanisms cited in these studies include coherency strengthening, modulus hardening, Hall–Petch strengthening, and the presence of slip-limiting interfaces.11–13 In general, these theories are based on mechanisms that impede dislocation motion due to multilayer interfaces or coherency stresses. Hardness enhancement has also been achieved by combining layers with very different mechanical properties, such as metal/ceramic multilayers of Ti/TiN.9 The mechanical properties of multilayers are closely related to their microstructural characteristics, such as a)
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J. Mater. Res., Vol. 17, No. 6, Jun 2002 Downloaded: 17 Mar 2015
grain structure, the extent of epitaxial growth, and the roughness and compositional sharpness of the interfaces as well as bilayer thickness. Therefore, characterization of multilayer structure and growth mechanisms is essential to understanding their mechanical properties. In addition to hardness enhancements, it m
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