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The aerospace industry makes use of a wide range of materials for its operations. These materials are selected based on a number of characteristics, such as strength, density, and rigidity. Nickel-based superalloys and composites are examples of such materials. However, they are difficult to cut and require specific machining techniques. As a result, diamond is used for applications in which conventional tools cannot be utilized to reach the final objective of the manufacturing process.
Using Diamond for Longer Tool Life and Improved Cutting Speeds
Diamond has the highest value on the Mohs scale of mineral hardness. This scale allows the classification of the resistance to scratching of a material and has a range comprised between 1 (softest) and 10 (hardest). Diamond is characterized by a hardness of 10 Mohs, which corresponds to a hardness that is four times greater than Corundum (Al2O3), which has 9 Mohs.
The success of using diamond tools in the aerospace industry is mainly due to their properties, leading to longer tool life and improved cutting speeds in comparison with conventional materials. Diamond tools can be created using different types of the same material, depending on the specific application and product requirements. Diamond materials are categorized as either natural or synthetic. Synthetic diamond materials could either be grains and powders, polycrystalline diamond (PCD), or chemical vapor deposition (CPD). Natural diamonds are those that have not undergone any chemical and transformative process.
Synthetic Diamond Grains
Synthetic diamond grains (grits) and powders are used as abrasives that are bonded to the surface of a tool. This can be achieved in different ways depending on the required process characteristics, such as tool rotational velocity. Diamond particles can be deposited on the tool surface by using an electroplating technique using a nickel solution or by sintering them in a tungsten-based matrix. Another technique known as braze bonding works by brazing the diamond particles to the tool's metal surface.
Each of these bonding methods has associated benefits and drawbacks related to the tool manufacturing time and the capabilities of the binder in retaining the diamond particles, depending on the final operational characteristics. Recent studies on this area have found that sintering synthetic diamond grains could produce the greatest impact on the grain-metallic phase-bond system. Moreover, a study found that grain stresses are higher in areas located around the metallic phase. These findings suggest the possibility to increase the overall efficiency and applicability of the material.
Polycrystalline Diamond (PCD) Tools
In PCD tools or inserts, segments of the diamond are bonded, typically using brazing, to a carbide substrate. PCD elements are produced by sintering micron-sized synthetic diamond powders to bond particles together in a process characterized by high temperatures and pressures.
PCD tools are manufactured using cemented carbide baking which provides the necessary metal source (e.g. cobalt) in order to carry out the sintering process. During the production process, the metal from the carbide substrate penetrates between the diamond grains, allowing them to be bonded together.
The metallic content of PCD provides the material with electrical conductivity properties. This allows the use of a machining process such as electrical discharge machining (EDM) in order to cut diamond elements.
Chemical Vapor Deposition (CVD) Diamond Tools
Another type of synthetic diamond is produced by using a technique known as chemical vapor deposition. CVD diamond is deposited in thin layers on the surface of a tool in a tightly-controlled growth condition process.
One of the main advantages of using this kind of synthetic diamond is the possibility to obtain addition geometries and cutting edges using a material characterized by highly-predictable properties.
Diamond in Aerospace Manufacturing Processes
Diamond is currently-used in a variety of manufacturing processes in the aerospace industry mainly due to its high wear resistance and ability to machine difficult-to-cut materials. It can be used in processes which employ tools with defined (turning, milling and drilling) and non-defined (grinding and dressing) cutting edges.
Diamond is characterized by its strong affinity for iron and nickel, especially at high temperatures. As such, the use of diamond in machining is limited to non-ferrous materials. Particular attention needs to be given in processes involving maximum temperature, in order to avoid the transformation of diamond in some metastable materials such as graphite.
Specifically, at low pressure, the graphitization of diamond surfaces has been observed at temperatures lower than 1700°C, while the conversion temperature can be lowered up to 400°C if iron elements are present.
As a result, carbide tools are currently used when machining titanium alloys, nickel alloys, and stainless steels using defined cutting edges tools. Conversely, in case of abrasive processes, materials like cubic boron nitride (CBN), aluminum oxide, and silicon carbide (SiC) are used in machining steels, nickel alloys, ceramics, and titanium alloys. Another alternative to diamonds in machining ferrous materials is known as polycrystalline cubic boron nitride (PCBN), a composite material manufactured by sintering micron-sized CBN powders with various ceramics.
The Grinding Process in the Aerospace Industry
One of the many processes employed in aerospace manufacturing is grinding. This operation allows the required surface finish and geometrical accuracy for specific components (e.g. turbine blades) to be obtained by using an abrasive rotary tool with a characteristic profile. Diamond abrasives play a fundamental role in maintaining the grinding wheels’ performance to optimal levels.
Diamond abrasive grains, both natural and synthetic, are used in a group of abrasive tools known as dressers. These tools are used in the preparation of grinding wheels. These tools are used to re-establish the abrasive conditions of a grinding wheel (dressing), remove the clogged debris from its abrasive surface (cleaning), and restore the required profile (truing).
When talking about conventional grinding tools such as aluminum oxide wheels, the term ‘dressing’ refers to both the actions of truing and dressing. The two main categories of dressing tools can be distinguished by their movement with respect to the grinding wheel: stationary and rotary dressers. Rotary dressers are typically used in the manufacture of engine components in the aerospace industry, due to the possibility to dress wheels with complex profiles, such as fir tree root forms in turbine blades.
Diamond in Grinding and Drilling Tools
Diamond, as an abrasive material, is also used in grinding tools. For example, it has been proven that single-layer electroplated diamond grinding wheels can be used to obtain good results when machining a wide range of disc slot forms and sizes for mounting turbine blades in aerospace engines.
Another application of diamonds in the aerospace manufacturing industry is in core drilling tools. Brazed diamond grit cored drills can be utilized in the machining of both carbon fiber reinforced polymers (CFRP) and titanium. These two categories of materials are used to create aerospace components with high strength-to-weight ratios and rigidity, often in combination in order to harness the most favorable characteristics of each material.
The number of aerospace manufacturing processes which make use of diamond is part of an ever growing list. In fact, the high cost of a diamond is justified by the longer tool life and cutting velocities, which allow an increased production rate and lower costs associated with tool replacement.
Sources and Further Reading
- Abrasive Machining Methods for Composites - Composites World
- Synthetic Diamond Grits & Powders - Element Six
- Synthetic Polycrystalline Diamond (PCD) - Element Six
- CVD Synthetic Diamond - Element Six
- Synthetic Diamonds for use in the Aerospace Industry - Element Six
- Aspinwall, D. K. et al., 2007. Profiled Superabrasive Grinding Wheels for the Machining of a Nickel Based Superalloy. CIRP Annals - Manufacturing Technology, 56(1), pp.335–338.
- Jackson, M. J. et al., 2013. High Performance Grinding and Advanced Cutting Tools. Springer-Verlag New York.
- Jackson, M. J. et al., 2011. Machining with Abrasives. Springer US.
- Khmelnitsky, R. A. et al., 2014. Transformation of diamond to graphite under heat treatment at low pressure. Phase Transitions: A Multinational Journal, 87(2), pp. 175 – 192.
This article was updated on 14th February, 2019.
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