Comprehensive topography characterization of polycrystalline diamond coatings

TL;DR

This study introduces a comprehensive multi-scale topography characterization method for polycrystalline diamond coatings, revealing how surface roughness varies across scales and its relation to grain size, with implications for surface property optimization.

Contribution

It combines advanced measurement techniques to analyze surface topography across all length scales, providing new insights into the multi-scale roughness and its connection to grain structure.

Findings

Multi-scale topography varies significantly across scales.

Surface roughness is linked to grain size and exhibits fractal-like behavior.

Measurement across all scales is crucial for accurate surface characterization.

Abstract

The surface topography of diamond coatings strongly affects surface properties such as adhesion, friction, wear, and biocompatibility. However, the understanding of multi-scale topography, and its effect on properties, has been hindered by conventional measurement methods, which capture only a single length scale. Here, four different polycrystalline diamond coatings are characterized using transmission electron microscopy to assess the roughness down to the sub-nanometer scale. Then these measurements are combined, using the power spectral density (PSD), with conventional methods (stylus profilometry and atomic force microscopy) to characterize all scales of topography. The results demonstrate the critical importance of measuring topography across all length scales, especially because their PSDs cross over one another, such that a surface that is rougher at a larger scale may be smoother at a smaller scale and vice versa. Furthermore, these measurements reveal the connection between multi-scale topography and grain size, with characteristic scaling behavior at and slightly below the mean grain size, and self-affine fractal-like roughness at other length scales. At small (subgrain) scales, unpolished surfaces exhibit a common form of residual roughness that is self-affine in nature but difficult to detect with conventional methods. This approach of capturing topography from the atomic- to the macro-scale is termed comprehensive topography characterization, and all of the topography data from these surfaces has been made available for further analysis by experimentalists and theoreticians. Scientifically, this investigation has identified four characteristic regions of topography scaling in polycrystalline diamond materials.