Have you ever wondered how airplanes, cars, oil and gas pipelines or medical devices are made? It’s not just the materials they’re composed of that’s so important, but also the high-speed machining that shapes them. Improving those processes can improve the industries that use them and the products they make.
Aerospace, automotive, medical devices and oil and gas industries all require materials that resist corrosion and have low thermal conductivity, meaning they don’t transfer heat easily. That’s why materials like austenitic stainless steels, titanium alloys and Inconel super-alloys are crucial to these industries.
But the same properties that make these materials so useful also make them difficult to machine at high speeds, leading to rapid tool wear and shortening the lifespan of cutting tools. Machining refers to a manufacturing process where material is selectively removed from a work piece — typically a raw material in the form of a bar, sheet or block — using cutting tools to achieve the desired shape, dimensions and surface finish.
An innovation in tool coating could solve these machining challenges. The development of what’s known as a bi-layer AlTiN PVD coating enhances cutting-tool performance, improves wear resistance and extends the life of the tool life during ultra-high-speed machining of hard-to-machine materials.
This breakthrough won’t just benefit manufacturers. The development of advanced cutting tool coatings can significantly enhance tool performance under extreme machining conditions and improve the surface quality of the finished work piece. Let’s dive into what makes this discovery so important.
Why it matters
Traditionally, tools have been coated with an AlTiN layer — a hard ceramic coating composed of aluminum (Al), titanium (Ti), and nitrogen (N) — to enhance wear resistance during machining. The coating is applied as an extremely thin film (typically three to five micrometres) through a process called physical vapour deposition (PVD), in which the coating material is vapourized in a vacuum chamber and condensed onto the tool surface.
A single AlTiN layer can improve oxidation resistance and make tools more durable, but these coatings often struggle to balance the hardness, toughness and frictional properties required for demanding machining environments.
The bi-layer coating used in this study overcomes these limitations by optimizing the mechanical properties of each layer. This approach enables the coating to withstand the extreme heat and mechanical loads during the machining of stainless steel.
How does the bi-layer coating work?
A novel coating system was designed: a bi-layer consisting of two AlTiN layers with different ratios of aluminum and titanium. The bi-layer AlTiN coating stands out due to its unique combination of properties.
The top layer, with a higher ratio of aluminum to titanium, reduces friction and improves oxidation resistance. The sub-layer, with an equal ratio of aluminum to titanium, enhances hardness and provides better adhesion to the tungsten carbide substrate used in cutting tools. This combination enables the tool to withstand higher temperatures and mechanical stresses, resulting in longer tool life and more efficient machining.
This bi-layer coating was tested against single-layer coatings on tungsten carbide cutting tools under ultra-high-speed turning of austenitic stainless steel 304 (SS304) — a high-performance material commonly used in the automotive and aerospace industries. The bi-layer coating demonstrated remarkable results, increasing tool life by 33 per cent.
The improved wear resistance is due to the combination of the two layers. It reduced the type of wear caused by high temperatures — known as crater wear — as well as the type of wear caused by mechanical stress — known as flank wear. This balance of properties resulted in longer tool life during high-speed machining.
Better cutting conditions between tool and workpiece
One of the standout features of the bi-layer coating was its improvement in friction, wear and lubrication — three key properties studied in the science of tribology. During machining, these effects were evident in the way chips were formed. Chip formation — the process by which small pieces of material are removed from the whole workpiece by the cutting tool — serves as an important indicator of friction and cutting conditions at the tool–workpiece interface.
In this study, the bi-layer tool produced chips with a smoother surface and a more regular shape compared to the chips produced by single-layer tools.
The smoother chips indicate better frictional conditions, meaning that the cutting tool experienced less resistance as it machined the stainless steel. This reduced friction not only extended tool life but also contributed to a more efficient cutting process, as less energy was required to perform the machining.
The bi-layer coating’s ability to reduce friction was evident in the lower cutting forces recorded during tests. The bi-layer tool consistently showed lower forces, indicating it required less energy to cut through material. This efficiency could lead to energy savings in industrial settings where high-speed machining is frequently used, making the process more cost-effective and sustainable.
Evidence of superior wear resistance
The study used several advanced techniques to analyze the wear mechanisms affecting the tools, which showed how the bi-layer coating effectively reduced both crater and flank wear.
Crater wear occurs on the tool’s rake face — the surface of the cutting tool that comes into direct contact with the chip as it is formed — due to the intense heat generated in the cutting zone, while flank wear happens on the tool’s side, typically as a result of mechanical abrasion. The combination of properties in the bi-layer coating helped reduce both forms of wear. This allows the tool to last longer even under the harsh conditions of ultra-high-speed turning.
The impact of high-speed machining
The development of this bi-layer AlTiN coating represents a significant advancement in cutting tool technology. By enhancing wear resistance and reducing friction, the coating extends tool life and improves the efficiency of machining difficult materials like SS304. For industries that rely on high-speed, precision machining, this innovation could lead to cost savings, reduced downtime and greater productivity.
By enhancing wear resistance and reducing friction, the bi-layer AlTiN coating extends tool life and improves the efficiency of machining difficult materials like austenitic stainless steel 304 (SS304). SS304 is widely used in products that require high strength, corrosion resistance and a smooth surface finish — such as automotive exhaust systems, aerospace components, food-processing equipment and medical instruments. For industries that rely on high-speed, precision machining, this innovation could translate into significant cost savings, reduced downtime and greater productivity.
This research highlights the exciting possibilities of advanced coatings in machining and manufacturing technologies. Innovations like this demonstrate how materials science and mechanical engineering can drive progress across industries such as aerospace, automotive, energy, and medical device manufacturing — where precision, durability and efficiency are critical to performance.
Qianxi He does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
This article was originally published on The Conversation. Read the original article.
