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Surface Treatment 20 MoldMaking Technology —— APRIL 2017 Tensiometer measurements of a liquid droplet on a nanocomposite diamond-like coated surface show a low-energy surface, which helps reduce a resin's ability to stick to the surface. spread on a surface and maximize contact area. If the water contact angle is greater than 90 degrees, the solid surface is considered hydrophobic, the tendency for water to bead up and be repelled, thereby minimizing contact area. DLC coat- ings have water contact angles as great as 110 degrees, per- mitting low surface energy that results in reduced bond sites in which polymers can stick to the mold surface. Hardness. DLCs can produce extraordinarily high hard- ness relative to the substrate materials. Hardness is the ability of a material to resist plastic deformation when exposed to a compressive force. Often the hardness is an over-emphasized material parameter when considering wear and efficiency appli- cations. Hardness should always be considered in context to the substrate material and the toughness properties of the DLC. Toughness. The toughness of a DLC can be tailored to the substrate material and application environment. Toughness is the ability of the coating to absorb energy and plastically deform without fracturing. This ideally happens without deformation to the substrate materials. Corrosion resistance. Corrosion is a diffusive process (the intermingling of particles of two or more substances as a result of a thermal motion), which relies on exposure of the surface to sources of oxygen (most typically) or sulfur. DLC corrosion resistance is rooted in the fact that the refined metal substrates are masked from gradual reduction into their more stable forms, typically by electrochemical oxida- tion. Basically, the DLC blocks access to reaction sites via the nanocomposite thin film structure being bonded to the surface. Thermal transfer. High thermal conductivity means better control of mold and core temperatures. For example, when comparing the heat transfer properties of a mold with 0.25- inch of steel between the internal cavity and its cooling water jacket and a DLC film, a simple heat transfer calculation shows that the DLC layer will conduct heat 250 times faster than the thick tool steel. Thickness. The typical DLC coating thickness for mold applications is between 3 and 4 microns (0.00012 and 0.00016 inch), which is extremely thin and allows it to be added to molds that may not have been specifically designed to include a coating. The thickness of the coating is based on the substrate material and surface parameter optimizations. For example, different thicknesses may benefit different applications based on the substrate and desired combination of hardness, tough- ness and thermal transfer properties. Because the coatings are typically much thinner than 5 microns, they are much lower than most mold machining tolerances. This permits integration of the coating on many molds that are properly built to drawing tolerances. Temperature. DLCs also have a low-deposition tempera- ture that helps to eliminate tempering and thermal-related distortions. With common mold materials such as D2 steel, the temperature is less than 300ºF, and with QC-10 alumi- num it is less than 250ºF. Inadvertent tempering of the mate- rial while applying a DLC changes the hardness of the sub- strate. By maintaining much lower deposition temperatures, the potential for tempering is dramatically reduced. The higher temperature DLC process makes thin film coating of theses molds impossible due to the changes in the substrate material properties. This crater, shown at a high magnification, features DLC coatings applied in layers to help control film stress and maximize durability.