Hardness-depth profiling on nanometer scale

Abstract

Three methods for hardness-depth profiling were applied to a 200-nm-thick TiC film deposited onto a Ti-6Al-4V substrate. The methods were evaluated and compared with regard to their ability to detect local changes in hardness with a depth resolution on the order of a few nanometers. In the cross section method (CSM), the indentations are performed on a cross section of the sample, i.e., in a direction perpendicular to the hardness gradient. In the load-variation method (LVM) and in the constant-load method (CLM), the indentations are made in the direction of the hardness gradient, i.e., perpendicular to the as-prepared surface. In the LVM, the hardnessdepth profile is assessed using a stepwise increase of the maximum load applied to the surface. In the CLM, sublayers are removed successively by ion sputtering and hardness measurements are performed at each newly exposed surface using the same low maximum load. The CSM revealed a nearly constant hardness across the TiC film and a sharp transition from the TiC film to the Ti-6Al-4V substrate. The hardness profile obtained using the LVM and the CLM is smeared out because the recorded response is due to both the TiC film and the underlying softer Ti-6Al-4V substrate.

Hardness–depth profile of a carbon-implanted Ti–6Al–4V alloy and its relation to composition and microstructure

“…The interpretation of such hardness-depth profiles can be complicated due to the roles of surface roughness and sample inhomogeneities. 4,17 First, such effects will be discussed for the present results.…”

“…The better depth resolution of the CSM as compared to the LVM and the CLM is due to measuring the hardness perpendicular to the hardness gradient, i.e., in a direction of a homogeneous hardness. 4 This avoids the main caveat of the LVM and the CLM, the influence of an underlying layer with different properties on the measured values. It is noted that also the CSM measures mean values of hardness and modulus.…”

“…The implanted region investigated here differs from the model system used in Ref. 4 (a thin TiC film on a Ti-6Al-4V substrate): (i) the implanted region consists of several zones over the implanted depth range, which (ii) are not separated by sharp interfaces. In particular, as indicated by all three methods (see Fig.…”

“…To counteract surface roughness effects, the indentation depth must be large relative to the roughness. 4,17 The rms surface roughness of the ␣-phase of the Ti-6Al-4V alloy increased upon carbon implantation from 0.14 nm (aspolished) to 0.47 nm (as-implanted). These roughness values are relevant for the hardness measurements according to the LVM.…”

“…Only the CSM, where the indentations are made perpendicular to the hardness gradient, allows a direct determination of local changes in hardness with depth, with a depth resolution of a few nanometers, as shown previously. 4 In the LVM and the CLM, the measurements are performed in the direction of the hardness gradient. Therefore, the depth profiles obtained using these methods become more (LVM) or less (CLM) smeared, due to a combined response of the surface-engineered region (film, implanted region) and the underlying (softer) substrate.…”