PVD Hard Coatings at the Cutting Edge

The commercial acceptance of hard coatings for cutting tools is driven by demands on machining productivity, environmental mandates, and increased usage of new difficult-to-cut materials. Improved cutting performance is derived from synergies of machine tool system and cutting tool development. The latter strives for an optimized combination of tool material, hard coating and cutting edge geometry.

The integration of hard coatings in cutting tools has reached a mature stage after more than three decades of proven performance and productivity benefits in industrial metal cutting. Chemical vapor deposition (CVD) was the first technology used, which advanced from single layer to current multilayer types combining TiC, TiN, TiCN and Al2O3. Some 25 years later it is evident that novel physical vapor deposition (PVD) compositions have surpassed the limited set of available CVD coatings. The relative economics are debatable but it is accepted that total life cycle consideration and environmental friendliness favor PVD technology. Succeeding generations of PVD hard coatings have become commercially available, notably TiCN, TiAlN and AlCrN, with demonstrated success in expanding areas of application.

Intensive development of the superhard coatings diamond (diamond-like-carbon DLC) and cubic boron nitride (CBN) peaked in the 90’s. It seems that an insurmountable technical barrier – extremely high residual stress during deposition – has not been overcome and still prevents successful commercialization.

In mulitlayer coatings, particular properties suitable to the cutting wear conditions can be engineered by creative combinations of coating layers. For example, AlCrN-TiSiN performs well in high speed carbide drilling of 1045 steel. Although AlCrN has low thermal conductivity, it has been shown that the multilayer construction with TiSiN which has high thermal conductivity effectively raises the composite conductivity, minimizing the heat concentration at the critical drill outer corner. This apparently accounts for better performance of certain multilayer coatings with carbide drills. Another example is the TiAlN-WC/C coating designed for drills used for dry machining.

The figure: coating selection guide based on field performance and commercial acceptance.

An excellent historical overview of the development of PVD coatings leading up the latest innovations written by Dennis Quinto. Jump to page 17 for more... http://online.qmags.com/SVC1007/