How do we machine the world's hardest materials to the highest precision?

Story behind the paper "Laser Processing of Hard and Ultra-Hard Materials for Micro-Machining and Surface Engineering Applications"
How do we machine the world's hardest materials to the highest precision?

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Laser Processing of Hard and Ultra-Hard Materials for Micro-Machining and Surface Engineering Applications

Polycrystalline diamonds, polycrystalline cubic boron nitrides and tungsten carbides are considered difficult to process due to their superior mechanical (hardness, toughness) and wear properties. This paper aims to review the recent progress in the use of lasers to texture hard and ultra-hard materials to a high and reproducible quality. The effect of wavelength, beam type, pulse duration, fluence, and scanning speed is extensively reviewed, and the resulting laser mechanisms, induced damage, surface integrity, and existing challenges discussed. The cutting performance of different textures in real applications is examined, and the key influence of texture size, texture geometry, area ratio, area density, orientation, and solid lubricants is highlighted. Pulsed laser ablation (PLA) is an established method for surface texturing. Defects include melt debris, unwanted allotropic phase transitions, recast layer, porosity, and cracking, leading to non-uniform mechanical properties and surface roughness in fabricated textures. An evaluation of the main laser parameters indicates that shorter pulse durations (ns—fs), fluences greater than the ablation threshold, and optimised multi-pass scanning speeds can deliver sufficient energy to create textures to the required depth and profile with minimal defects. Surface texturing improves the tribological performance of cutting tools in dry conditions, reducing coefficient of friction (COF), cutting forces, wear, machining temperature, and adhesion. It is evident that cutting conditions (feed speed, workpiece material) have a primary role in the performance of textured tools. The identified gaps in laser surface texturing and texture performance are detailed to provide future trends and research directions in the field.

How do we machine the hardest materials in the world to the highest precision? This question prompted the development of our review paper.

Ultra-hard materials include natural diamonds, synthetic diamonds and boron nitride, the hardest man-made material. The hardness value exceeds 40GPa on the Vickers scale [1]. Hard materials include titanium alloys, tungsten carbide, and other composites with hardness values exceeding 15GPa [2].

The superior mechanical, toughness and wear properties of these materials justify why their use in the manufacturing industry, particularly in cutting tools, has skyrocketed.  However, it's these same properties that made them extremely difficult to machine or process using conventional manufacturing techniques for their desired use. Laser processing provides a solution. It is a non-contact, flexible and fast method that can be optimized to process hard and ultra-hard materials accurately and precisely.

Laser processing can either be photo-thermal, where the heating, melting then vaporization occurs for material removal or photo-chemical, where molecular bond breaking occurs with minimal thermal conduction. 

Pulsed laser ablation (PLA) is a photo-thermal and photo-chemical process. Laser energy is pulsed at a set duration (ranging from femtoseconds to microseconds) to maximize the energy density of the laser on the material. Each material has a threshold value that dictates the absorption behavior during PLA (Fig 1).

Fig 1. (a) PLA process, (b) Energy density close to material ablation threshold, (c) Energy density much greater than material ablation threshold. 

Despite the advantages of using PLA on hard and ultra-hard materials, the laser parameters need to be selected carefully to achieve the highest precision and avoid damage to the material's structural integrity [3], [4]. A review of the current trends in laser processing of hard and ultra-hard materials is presented to understand how to accomplish this. 

We also discuss the key laser parameters needed to optimize processing of these materials and summarize the ideal range of each parameter within the context of cutting tools. Finally, it highlights knowledge gaps in research and recommended future directions in the field.

If you are interested in our work, please refer to the review “Laser Processing of Hard and Ultra-Hard Materials for Micro-Machining and Surface Engineering Applications” published in Micromachines:




[1] Smith, G.T. Cutting Tool Technology; Springer: London, UK, 2008.

[2] Crompton, D.; Hirst, W.; Howse, M.G.W. The wear of diamond. Proc. R. Soc. London. Ser. A Math. Phys. Sci. 1973, 333, 435–454.

[3] Pacella M, Nekouie V, Badiee A. Surface engineering of ultra-hard polycrystalline structures using a nanosecond Yb fibre laser: Effect of process parameters on microstructure, hardness and surface finish. J Mater Process Technol 2019;266:311–28.

[4] Hazzan KE, Pacella M, See TL. Understanding the surface integrity of laser surface engineered tungsten carbide. Int J Adv Manuf Technol 2022;118:1141–63.

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