In a groundbreaking study published in the journal *Metals*, researchers have uncovered new insights into the behavior of hard coatings on titanium alloys, with significant implications for industries ranging from aerospace to energy. The research, led by Emanuele Ghio from the Department of Engineering for Industrial Systems and Technologies at the University of Parma, Italy, delves into the intricate failure modes of Diamond-Like Carbon (DLC)-based and AlCrN coatings on Ti6Al4V substrates, using both Rockwell indentation and scratch tests.
The study reveals that the adhesion class of these coatings can be misinterpreted at lower magnifications, potentially leading to incorrect classifications. “At the magnification required to capture the entire Rockwell imprint, the adhesion class of the investigated DLC-based and AlCrN coatings could be incorrectly classified as HF1,” Ghio explains. “However, higher-magnification observations revealed numerous radial cracks and non-uniformly distributed small delamination areas, changing the adhesion class to HF3.” This finding underscores the importance of detailed, high-magnification analysis in accurately assessing coating performance.
One of the most compelling discoveries is the impact of surface roughness on the detectability of cracks. Roughness values higher than 1 μm can obscure the presence of radial cracks, which align parallel to the deep valleys and high peaks of the roughness profile. This phenomenon was meticulously investigated using Scanning Electron Microscopy (SEM). “Likewise, in the scratch test, the rough surface also made the smallest cracks, formed at the critical load LC1, undetectable,” Ghio notes. This highlights the need for careful consideration of surface finish in coating evaluations.
The study also found that the critical loads for spallation of the coating in the scratch test (LC2, LC3) did not show significant correlation with the number of radial cracks formed during Rockwell indentations. This suggests that a quick Rockwell indentation cannot predict the scratch test results, emphasizing the necessity of comprehensive testing protocols.
For the energy sector, these findings are particularly relevant. Hard coatings are crucial for protecting components in harsh environments, such as those found in turbines and other high-performance machinery. Understanding the failure modes and adhesion properties of these coatings can lead to more durable and reliable components, ultimately reducing maintenance costs and improving operational efficiency.
Ghio’s research also revealed that both DLC-based and AlCrN coatings exhibited good adhesion to Ti6Al4V substrates, regardless of the microstructure and surface finish of the titanium substrates. SEM-FIB observations showed that the cracks formed during Rockwell indentation and scratch tests were deflected longitudinally within the underlying layers of the DLC-based coating and in the bottom part of the AlCrN coating, where the N concentration was higher.
This study not only advances our understanding of coating behavior but also paves the way for future developments in materials science and engineering. As industries continue to push the boundaries of performance and durability, the insights gained from this research will be invaluable in designing and implementing more robust coating solutions.
In summary, Ghio’s work published in *Metals* offers a nuanced look at the complexities of hard coatings on titanium alloys, providing critical data that could shape the future of materials science and engineering. The findings underscore the importance of detailed analysis and comprehensive testing in ensuring the reliability and performance of coated components in demanding applications.