Machining requires high standards for cutting tools, long tool life, and low pollution.

High Tool Requirements, Long Lifespan, and Low Pollution in Machining

Approximately 70% of premature failures in various electromechanical products are caused by wear and corrosion. Both failure modes are closely related to the surface condition of materials, including their physical, chemical, and stress states. Therefore, the key to improving the performance of such materials lies in enhancing their surface properties.

With advancements in technology, the demand for superior surface properties in materials is increasing. In recent decades, the rise of various vapor deposition technologies has significantly accelerated research and applications in surface engineering. These technologies not only meet mechanical performance requirements such as wear resistance, friction reduction, and corrosion resistance but also play a crucial role in functional materials related to electromagnetism, optics, optoelectronics, thermodynamics, superconductivity, and biology. Surface engineering not only maximizes the performance and economic benefits of inexpensive metal materials but has also become an essential means for developing new coating and thin-film materials, offering immense application potential.

New Requirements for Cutting Tools in Advanced Machining

As the level of machining technology advances, new demands are placed on cutting tools. In addition to improving tool lifespan, there is a growing need to reduce pollution generated during cutting, with a strong preference for dry cutting. When it is not feasible to completely eliminate cutting fluids, efforts should be made to use only rust inhibitors without organic compounds. This approach significantly reduces recycling costs.

The diversity of cutting tools and their distinct working conditions necessitate different choices of tool coatings. Turning and drilling have different requirements, while milling tools must account for intermittent impact loads. Early coating developments primarily focused on wear resistance, with hardness being the main indicator. Titanium nitride (TiN) coatings, for example, have relatively high friction coefficients (0.4–0.6). During machining, continuous friction between the tool and the workpiece generates excessive heat. To prevent overheating, deformation, and subsequent loss of machining accuracy, cutting fluids are commonly used.

Reducing or Eliminating Cutting Fluids: The Role of Self-Lubricating Coatings

To address issues related to cutting fluid reduction or elimination, tool coatings must not only extend tool lifespan but also provide self-lubricating properties. Diamond-like carbon (DLC) coatings have demonstrated advantages in machining certain materials such as aluminum (Al), titanium (Ti), and their composites. However, years of research have identified three major drawbacks of DLC coatings: high internal stress, poor thermal stability, and catalytic effects with ferrous metals that cause the transformation of the SP3 structure into SP2. These limitations have restricted DLC coatings primarily to non-ferrous metal machining, thereby limiting their broader application in machining.

However, recent studies indicate that DLC coatings with a predominantly SP2 structure, also known as graphitic carbon coatings, can achieve hardness levels of 20–40 GPa. Unlike traditional DLC coatings, these coatings do not exhibit catalytic effects with ferrous metals. They also feature extremely low friction coefficients and excellent moisture resistance, making them suitable for both coolant-assisted and dry cutting applications. These coatings have demonstrated tool lifespans multiple times longer than uncoated tools, with no issues in machining steel materials. As a result, they have garnered significant interest from coating companies and tool manufacturers. With continued advancements, these novel DLC coatings are expected to see widespread application in cutting tool technology.

The Importance of Surface Engineering in Extending Product Lifespan

Approximately 70% of premature failures in electromechanical products result from wear and corrosion, both of which are closely linked to the surface condition of materials. Enhancing surface performance is the key to improving material durability.

As technological advancements continue, the demands for improved surface properties in materials are becoming increasingly stringent. Over the past few decades, the emergence of vapor deposition techniques has propelled significant progress in surface engineering research and applications. These techniques not only fulfill mechanical performance requirements such as wear resistance, friction reduction, and corrosion protection but also play vital roles in electromagnetic, optical, optoelectronic, thermal, superconducting, and biological functional materials. Surface engineering enhances the performance and cost-effectiveness of inexpensive metal materials, establishing itself as a critical method for developing innovative coatings and thin-film materials with vast application potential.

New Demands on Cutting Tools in Advanced Machining

As machining technology progresses, cutting tools face new requirements. Beyond improving tool lifespan, there is an increasing emphasis on reducing environmental pollution during cutting, with a preference for dry cutting wherever possible. When the complete elimination of cutting fluids is not feasible, efforts are being made to use only rust inhibitors while avoiding organic compounds. This measure significantly lowers the cost of recycling and disposal.

The diverse nature of cutting tools and their specific working conditions dictate the choice of appropriate coatings. The requirements for turning differ from those for drilling, and milling tools must accommodate intermittent impact loads. Early coatings focused primarily on wear resistance, with hardness as the key performance indicator. Titanium nitride (TiN) coatings, for instance, have relatively high friction coefficients (0.4–0.6). Continuous friction between the coated tool and the workpiece generates substantial heat during machining. To prevent tool overheating, deformation, and accuracy loss, cutting fluids are traditionally employed.

Advancements in Self-Lubricating Coatings for Dry Cutting

To reduce or eliminate the use of cutting fluids, tool coatings must not only enhance tool longevity but also provide self-lubricating properties. The introduction of diamond-like carbon (DLC) coatings has demonstrated advantages in machining certain materials such as aluminum (Al), titanium (Ti), and their composites. However, extensive research has identified three primary limitations of DLC coatings: high internal stress, poor thermal stability, and catalytic interactions with ferrous metals that cause the transformation of the SP3 structure into SP2. These drawbacks have confined DLC coatings to non-ferrous metal machining, restricting their broader industrial adoption.

Recent studies have shown that DLC coatings with a primarily SP2 structure, also known as graphitic carbon coatings, can achieve hardness levels of 20–40 GPa while avoiding catalytic effects with ferrous metals. These coatings exhibit ultra-low friction coefficients and excellent moisture resistance, making them suitable for both coolant-assisted and dry cutting applications. Compared to uncoated tools, these coatings significantly extend tool lifespan and are effective in machining steel materials. Consequently, they have attracted considerable attention from coating companies and tool manufacturers. With further development, these advanced DLC coatings are expected to gain widespread use in cutting tool technology.