ICMCTF2014 Session G4: Coatings for Machining Advanced Materials and for use in Advanced Manufacturing Methods

Stamping dies used for AHSS (Advanced High Strength Steel) component production are conventionally coated by carbide coatings such as TiC or VC by thermal process for wear protection. As the strength of the steel increased, it becomes evident that wear and tear of dies with conventional surface treatment proceeds rapidly more than expected. An in-depth failure analysis of a worn die revealed that significant surface oxidation was taking place for conventional carbide coating. This indicated that surface temperature of the stamping mold was increased due to friction generated heat even it was a cold stamping process. Based on this failure analysis, a new coating for was developed for exclusively dies and molds for cold stamping. The new coating has a two layer structure; an inter-layer which increase the adhesion strength and resistance to external stress, and a top layer which is highly oxidation and wear resistant TiAlN based coating. Whole coating system is deposited by arc ion plating process at enough low temperature so virtually there is no dimensional change before-and after the deposition. A high temperature wear test was conducted to simulate oxidation wear process during the stamping process and found that severe oxidation and even oxygen diffusion up to several hundreds of nanometers was observed for VC coating deposited by thermal diffusion process. Whereas only very thin (ca. 10nm) oxide layer was observed in top layer of the developed coating. This difference in oxidation behavior affects wear amount of the coating and VC coating showed 2 to 3 times larger. Other critical properties of the developed coating such as mechanical property, adhesion and load bearing capability will be reported. Finally, it is noteworthy that the developed coating is now widely used for mass-production stamping dies for AHSS more than 500MPa and die life improvement factors are in general 2 to 3, 10 times at best.

Plastics processing is an enormous and expanding commercial sector since the worldwide manufacturing of plastics products breaks through the 280 mio. tons mark in 2011. Injection molding and extrusion are common techniques for efficient mass production of plastics components with high shape accuracy and high surface quality for a broad variety of applications. In the framework of a world affected by globalization, it is fundamental to develop a sustainable production strategy to assure the production in high-wage countries. The cost-effective manufacturing of individualized optical products with improved functionality by reducing the number of process steps and shortening the cycle times offers a promising approach to meet the challenges of production in a globalized world but places high demands on molding processes. Adhesive and abrasive wear as well as corrosion taking place during the production of plastics products and high deforming forces lead to the necessity of developing new material concepts. Ternary nitride hard coatings deposited by PVD (Physical Vapor Deposition) find widespread application as hard protective coatings against wear and corrosion due to their outstanding tribological, mechanical and chemical properties. Besides ternary nitrides, quaternary chromium based oxy-nitride coating systems as (Cr_xAl_1‑x)ON have gained much attention revealing high potential for decreasing adhesion and deforming forces between PVD coated tool and plastics melt during the production, e. g. of optical components or microstructered parts. The present work deals with the development of chromium based oxy-nitride hard coatings (Cr_xAl_1-x)ON on stainless tool steel ASTM 420 (X42Cr13, 1.2083) using middle frequency (mf) pulsed magnetron sputtering (MS) PVD technology. The aluminum content of the (Cr_xAl_1-x)ON coatings was varied in the range of 5 at.-% and 80 at.-%. By means of optical emission spectroscopy (OES) the deposition process was monitored regarding Cr/Cr⁺ and Al/Al⁺ ratios and the working points were varied via oxygen gas flow. Morphology, mechanical properties, phase and chemical composition were analyzed. Adhesion behavior between (Cr_xAl_1-x)ON coatings towards polymers by high temperature contact angle measurements revealed a significant impact of the coatings’ chemical composition. Tribological model tests in a pin-on-disk-tribometer verified a positive influence of (Cr_xAl_1-x)ON coatings on the adhesion behavior towards polymers which is directly linked to lowering of deforming forces. This makes (Cr_xAl_1-x)ON coatings a promising candidate for the production of optical components or microstructured parts by injection molding or extrusion.

Manufacturing industry is craving for more productivity, versatility and reliability while machining more demanding materials. The challenge faced by tool manufacturers is thus to deliver products with longer tool life, and predictable performances, which requires a deep understanding of the wear mechanisms involved: abrasive, adhesive, chemical or thermomechanical. Each of the wear mechanisms results in specific wear types (flank wear, crater wear, thermal crack to cite a few) which, when combined, may results in sporadic and catastrophic failure of the tool.

In this work, the resistance of different PVD coatings to specific wear types has been systematically studied, and related to the physical properties of the films (composition, nano-hardness, E module, crytal structure, architecture, etc). This knowledge can then be applied to tailor coating solutions for specific applications. Examples to illustrate this work will be given.

In the past years many tool manufacturers are offering novel so called thick film bonded tools (TFC tools). These are tools where a very thick bond (300 – 1000 µm) usually made of diamond is produced by CVD on a wafer first, is cut afterwards into small inserts and then brazed on the tip of the tool. The advantage of this novel kind of bond using for cutting tools is that the favorable effect of the bond is not gone when tool wear occurs. Furthermore, due to the large thickness it is possible to machine the bonds in a final step to sharpen the tool edge. Against the background that the bond is made completely out of diamond and no bond phase is existent, like in PCD, diamond thick film tools are much more resistant towards abrasive wear. Thus, the tool life should be significantly higher compared to common TiN coated cemented carbides. However, as diamond turns into graphite at 700°C the suitability of such tools in high performance cutting is questionable. It is unknown if diamond thick film tools are able to substitute TiN coated cemented carbides. Therefore it is investigated in this paper if TFC tools are suitable for high performance cutting of Ti6Al4V super alloys. Experiments were conducted to compare the tool life of TFC tools with common TiN coated cemented carbide tools. Besides this it is investigated which effect the thermal load has on the wear resistance of the diamond coating. This was done by using different kind of cooling strategies and cooling pressures (1 bar flood cooling - 80 bar direct injection cooling). Investigations show that TFC tools have a significantly higher tool life than common TiN coated cemented carbide tools. But, investigations also show that a sufficient high pressure cooling is required. Otherwise thermal loads become too high and cause early tool/ bond failure.

The development of hard coatings for cutting applications starting with the early CVD TiC coatings to the nowadays used multilayer CVD and PVD coating systems will be reviewed. Especially in high demanding cutting applications, solutions for difficult to cut materials like high alloyed steels or titanium alloys are needed. A detailed understanding is indispensable to increase the performance of tool systems regarding wear resistivity and toughness by fine tuning of material properties. Not only carbide and coating are of major importance, also the tool system has a high impact on the performance and productivity. An example of the automotive industry will demonstrate this effect in the crankshaft milling process. The optimized interaction of tool system, geometry, carbide and coating is necessary to fulfil the needs of future challenges in metal cutting operations.