Tribology, the science of friction, wear, and lubrication, plays a key role in machining. It is in the microscopic contact zone between the chip, cutting edge, and workpiece that most of the heat, cutting forces, and micro-damage are generated. Their effects are seen in practice as built-up edge, accelerated tool dulling, surface roughness, or micro-cracks. Conscious control of tribological phenomena makes it possible to improve surface quality, extend tool life, and increase the efficiency of the entire process.

Fundamentals of tribology in the cutting zone

The tool–workpiece contact zone is subject to extreme conditions: pressures in the range of hundreds of MPa, often exceeding 1 GPa, and very high sliding velocities. Usually, boundary or mixed lubrication regimes occur. Friction is generated by adhesion, abrasion, and ploughing caused by cutting-edge rounding and material deformation. On the rake face, an adhesion zone can be observed near the edge and a sliding zone further away, while on the flank face friction is responsible for wear marks and the surface finish of the workpiece.

Merchant’s model – friction and cutting mechanics

In the classical approach, friction is described by the friction angle β and the shear angle φ. Increasing friction reduces the shear angle, which leads to greater chip thickness, higher forces, and elevated temperatures. The friction coefficient can be determined from cutting force components, while in FEM simulations a shear friction model is applied, where higher friction increases heat generation and accelerates tool wear.

Friction and surface quality

Friction affects both geometric and real roughness. Even with the same feed and nose radius, additional ploughing and surface layer deformation worsen the finish. Built-up edge leads to scratches and irregularities, while unstable sliding can cause micro-vibrations. Friction also significantly influences the subsurface structure: it can induce residual stress changes, micro-cracks, or the formation of tempered and hardened layers. In difficult-to-machine materials, it additionally promotes burr formation.

Tool wear mechanisms

The main wear mechanisms, whose intensity depends on tribological conditions, include:

• Adhesive wear – caused by material sticking (e.g., aluminum, stainless steels, titanium). It results in built-up edge and edge chipping. Countermeasures: polished edges, DLC or TiB₂ coatings, MQL lubrication, or high-pressure cooling.
• Abrasive wear – due to hard particles such as carbides or silicon in Al-Si alloys. It causes scratches and accelerated flank wear. Countermeasures: hard coatings (AlCrN, TiAlN) and effective coolant filtration.
• Diffusion and oxidation wear – occurring at high temperatures, leading to crater formation on the rake face. Countermeasures: oxidation-resistant coatings and cryogenic cooling.
• Fatigue wear – caused by cyclic load and temperature changes, resulting in micro-cracks and chipping. Countermeasures: appropriate edge geometry and stable cooling conditions.

Lubrication and cooling

In machining, lubrication usually occurs in the boundary or mixed regime. Additives (sulfur, phosphorus, esters) play a key role by forming protective layers on surfaces. Common methods include:

• MQL (minimum quantity lubrication) – reduces adhesion and improves tool life,
• high-pressure cooling – particularly effective for nickel alloys and stainless steels,
• dry machining – possible with materials of high thermal conductivity, such as cast irons,
• cryogenic cooling – highly useful for titanium alloys and Inconel.

Tool materials and coatings

Tool material selection directly affects tribological behavior. Cemented carbides are the most versatile and, combined with PVD/CVD coatings, provide wear resistance. Cermets ensure excellent surface quality. Ceramics and SiAlON are suitable for cast irons and heat-resistant alloys. PCBN is ideal for hardened steels, while PCD works best for aluminum and copper. Specialized coatings such as DLC or TiB₂ effectively reduce adhesion in aluminum and light alloys.

Edge geometry and preparation

Tool geometry strongly influences friction. A positive rake angle reduces forces and facilitates chip evacuation but weakens the edge. Proper edge honing (rounding of several microns) reduces built-up edge and micro-chipping. Micro-chamfers (K-land, T-land) stabilize the edge under demanding conditions, while laser texturing of rake surfaces further improves lubrication.

Process parameters

• Higher cutting speeds reduce built-up edge but increase crater wear.
• Too small a feed combined with a large edge radius favors friction rather than cutting.
• Depth of cut determines contact area and heat generation.
• Advanced toolpath strategies, such as trochoidal milling, reduce average contact time and stabilize temperature.

Tool life and energy consumption

An increase in friction coefficient raises cutting forces and temperatures, accelerating tool wear and increasing energy consumption. Improving tribological conditions not only extends tool life but also reduces process energy costs.

Diagnostics

Tribological assessment involves measuring cutting forces, chip and surface temperatures, roughness, and analyzing tool wear with microscopy and surface layer studies. In practice, design of experiments (DOE) methods are also used to optimize process conditions with respect to friction and quality.

Recommendations for selected materials

• Aluminum and Al-Si alloys: PCD tools or DLC/TiB₂-coated tools, MQL lubrication.
• Stainless steels: AlCrN/AlTiN coatings, positive rake angle, high-pressure cooling.
• Nickel alloys: AlTiN coatings, chamfered edges, HP or cryogenic cooling.
• Titanium alloys: sharp edges, cryogenic or HP cooling, positive rake angle.
• Hardened steels: PCBN tools, dry machining, fine finishing parameters.

Summary

Friction in machining is a decisive factor for surface quality, tool life, and process costs. With the right choice of tool material and geometry, proper lubrication, and optimized process parameters, the negative impact of tribological phenomena can be significantly reduced. Effective management of friction in machining leads to higher quality, stable production, and improved energy efficiency.