Critical Issues in Measuring the Mechanical Properties of Hard Films on Soft Substrates by Nanoindentation Techniques
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H = P(1) A and Eff
E (1-E
V•S 2Fv)
(2)
where H is the hardness, Pn= is the peak indentation load, A is the projected contact area, E is the Young's modulus, v is Poisson's ratio for the indented material, and S is the elastic contact stiffness. Therefore, when the contact area is overestimated, the hardness is underestimated by a factor of similar scale, and the Young's modulus is underestimated by a factor which scales as the square root of the error in contact area. EXPERIMENTAL The system selected for these exploratory studies was NiP on soft annealed copper. This is a 'model' system in which the hardness of the film (7-8 GPa) and the hardness of the substrate (0.75 GPa) vary by roughly a factor of ten. An important consideration in selecting these materials was the similarity of their elastic properties, which minimizes Young's modulus mismatch as a possible factor in the interpretation of results. The Young's modulus of copper is about 129 GPa and that of the NiP is about 160 GPa. High-purity (99.999%) copper disks 19 mm in diameter and 10 mm in height were used as substrate materials. The samples were rough ground with silicon carbide paper to a 600 grit, achieving nearly parallel faces. A subsequent vacuum anneal at 3500 for one hour resulted in a very soft copper with grains as large as 1 mm. The oxidized surface layer was then removed by light grinding using fine grit silicon carbide papers, and final polishing resulted in a near-mirror finish. An electroless NiP plating process (MacDermid, Inc., Bay City, MI) was used to produce hard amorphous NiP films of two thicknesses, 700 nm and 10 rtm, on the copper coupons. The Young's modulus and hardness of the NiP/Cu system were studied using a Nanoindenter II (Nano Instruments, Inc., Oak Ridge, TN) which is capable of accurately measuring both the indentation load and depth during an experiment. The experiments were performed to a range of peak loads with a Berkovich diamond tip, producing indentations which varied in total depth from 15 nm to approximately 2000 nm. Although these depths only penetrate a fraction of the 10 4tm NiP film, penetrations in the 700 nm NiP film ranged from 5% to 300% of the film thickness. Thus, substrate-independent measurements of the NiP were possible in the 10 4tm sample, while the 700 nm sample allowed for the study of the substrate influences on the measured properties. The Oliver-Pharr method was used to determine the contact depth and projected contact area at each indentation load from measurable quantities including the contact stiffness, S, the maximum load, P., and the maximum penetration depth, h.a.. The properties evaluated from the contact depth determined by the Oliver-Pharr method and Equations 1 and 2 will be defined as E na.o' H na.o. and A ..ano.Subsequent imaging of the indentations by SEM and AFM revealed the actual contact area, Aa~uwl which was used with the measured stiffness and Equation 2
to obtain a second measurement of the Young's modulus,
Eactual,
and hardness,
Hactual.
While the
hardness meas
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