Perspective for Replacement Of Hard Chromium by PVD By J.A. Kubinski, CEF, T. Hurkmans, T. Trinh, Dr. W. Fleischer & Dr. G.J. van der Kolk In recent years, hard chromium has such applications as coating of piston in many cases are now substantially been replaced for specific applica- rings and in the textile industry. decreasing. As the cost of hard tions with physical vapor deposition PVD is a technology that has chromium increases, the applications (PVD) films that provide equivalent numerous applications, such as: where PVD can be competitively or superior performance. Unlike applied will increase as well. hard chromium, PVD hard chro- • Conducting metallic layers on mium replacements can be tailored semiconductors, Current PVD Coatings specifically to the application. • Reflecting Al layers on CDs, In Table 1, an overview is given of Coatings of CrN and variants such • Selectively transmitting layers on PVD coatings that are currently used as CrCN exhibit properties that flat glass, in various applications. TiN is also meet or exceed the chromium they • Ceramic wear-resistant layers on shown in the table, because it is the replace, as well as offer additional professional tools. most frequently applied PVD coating properties, such as reduced coeffi- for tools. cient of friction. The family of In the last few years, another group A number of properties are shown. coatings known as metal-containing of coatings is being applied industri- Not shown, however, is adhesion. diamond-like carbon (Me-DLC or ally as well—the anti-wear PVD Adhesion between coating and Me-C:H) exhibits properties of coating. In principle, the PVD coating substrate is vital for a proper coating diamond-like carbon, providing method has the flexibility to tune the performance. As adhesion of the hard outstanding wear protection, coating properties by changing layers to the substrate is relatively toughness and low coefficients of composition, creating multilayers, low, adhesion interlayers, such as pure friction. A survey of some proven adjusting the mechanical properties PVD chromium, can be used. Addi- replacement applications and their and influencing the microstructure. tionally, proper adhesion relies on physical characteristics are dis- Given the flexibility of adjusting proper substrate cleaning and proper cussed here. coating properties, it is possible to substrate etching processes. Another engineer specific coating properties important parameter to coating ard chromium is widely used that perform better than hard chro- performance is internal stress. in many applications. It mium. Intrinsic to PVD-grown coatings is possesses a few characteristic Costs of PVD coatings have been internal stress, which may cause Hproperties, such as relatively high high, relative to hard chromium, but adhesive flaking. As the coating layer hardness, good corrosion resistance and a self-leveling Table 1 effect. Combinations Industrially Applied PVD Anti-wear Coatings of these properties CrN Me-C:H a-C (pure DLC) MoS TiN have led to use in 2 ° applications where the Deposition temperature ( C) 150-300 150-250 150-500 150-250 150-450 conditions require high Hardness (HV 0.05) 1200-2200 800-2200 3000-7000 300-600 2000-2500 hardness, low friction, Internal stress (Gpa/µm) 0.1-1 0.1-1.5 2-6 0.1-1 1-2 low wear and corro- Thickness (µm) 1-50 1-5 1-2 1-10 1-6 sion resistance. Hard Friction coefficient 0.4-0.6 0.1-0.2 0.02-0.1 0.1-0.2 0.5-0.7 chromium is found in a Maximum temperature (°C) 750 350 450 400 450 wide range of applica- Abrasive wear – + +++ ++ – tions, such as hydraulic Adhesive wear – +++ +++ – – parts for aerospace, Corrosion +++ + +++ + + automotive and Practical problems low low adhesive low offshore. Anti-wear flaking shear properties of hard Industrial experience ++++ ++++ – + ++++ chromium are used in 20 PLATING & SURFACE FINISHING grows, high internal stresses are Titanium Nitride overview can be seen in Table 2. generated that limit the thickness of Titanium nitride (TiN) has been The current main application is in the coating. Table 2 indicates achiev- widely used since the late 1970s as a the automotive segment. Major able practical thicknesses. replacement for hard chromium. It has applications are found in the fuel been used because it is generally injection train. The new generation of Chromium Nitride available from various sources. Based very-high-pressure fuel injection Chromium nitride (CrN) is widely on its specific properties, however, it systems has become economically used as an anti-wear coating. Though is not a suitable replacement for hard feasible by use of PVD coatings. A having generally higher coefficients chromium. considerable advantage of a PVD of friction than hard chromium, a coating is that it can be made rela- great advantage of CrN is that the Current PVD Applications tively thin. Because coating thickness internal stresses are very low. Coat- Numerous applications have been requirements are only a few µm ings with thicknesses of more than 40 introduced in the last five years. An (±10%), allowances for coating µm are routinely used in automotive applications. DLC & Metal-containing DLC Diamond-like carbon (DLC) offers a broad potential, as it combines very high hardness with a low friction coefficient. Unfortunately, one of the major shortcomings is that the pure DLC has a very high internal stress, thereby limiting its thickness to about 1 µm. Another disadvantage is that pure DLC coatings show low electri- cal conductivity. DLC coatings, however, become viable when the coating chemistry is enhanced when low concentrations of elements other than carbon are added. Examples are pure metals,1 carbide- forming metals2,3 (indicated with Me- C:H) and other carbide-forming elements, such as silicon4 and boron.5 In principle, nanostructures are formed by addition of between 5 and 15 atomic percent of carbide-forming elements. Small carbide islands exist in a carbon matrix. The hardness is considerably less than that of pure DLC, but other properties such as internal friction and elasticity are greatly improved. These factors improve fatigue life of the coatings. Molybdenum Disulfide Molybdenum disulfide (MoS2) has well-known tribological properties. A major drawback of pure MoS2 is that the shear stress is relatively low. Coatings of pure MoS2 are used in combination with other coatings, such as TiAlN, to produce specific sliding wear properties.6 ______________ Editor’s Note: This is an edited version of a paper presented at AESF’s 1999 Aerospace/Airline Plating and Metal Finishing Forum, which was held April 20-23 in Jacksonville, FL. Free Details: Circle 116 on reader service card. October 1999 21 Fig. 1—Corrosion performance of ap- proximately 3 µm CrN, deposited with Fig. 2—Sticking forces of injection molding of ABS plastics, molds bias voltages of 0 V, coated with CrN and CrN + MoS . -100 V and -200 V, 2 and of 25 µm hard chromium. thickness are not required, and the production. The continuous drive for provide hardness and reduce mechani- part to be coated can be at final improved energy efficiency, weight cal forces by adding a lubricative dimensions. reduction, noise reduction and volume coating during forming are possible Another application to successfully reduction are all favorable conditions by PVD. To get both properties take advantage of PVD coatings is for increased use of low-friction PVD simultaneously, combinations of hard shafts of the turbo compressor. coatings. and soft coatings are used, mainly Considerable growth for PVD is One application that is growing CrN, CrCN and Me-C:H. expected in the next few years in the rapidly is for punching and forming Another application is in molds, automotive industry. Current applica- tools. The goal is to increase the dies and extrusion forms. Initially, tions are mainly for high-performance lifetime of the forming tool and CrN was widely used, but use is car parts. This will certainly have improve the surface quality of the shifting to combinations of hard and spin-offs for the more normal car final formed part. Coatings that soft coatings. Table 2 Overview of Current & Future PVD Applications Current Applications: Segment Substrate PVD Coating Thickness Remarks Automotive Piston ring CrN 40 µm Routinely used by one of the largest Japanese piston ring suppliers Drive rods CrN 3 µm Used for highly loaded race engines, such as Formula 1 Turbo shafts Cr/W-C:H 3 µm For passenger car turbo engines Fuel injection Cr/W-C:H 3 µm Diesel fuel pumps; fuel injectors; DLC applied on >50% of world production Valve train Cr/W-C:H 3 µm Tappet, camshaft; starting to be used in large-scale production Drive train B4C Sun gears Cr/W-C:H Various Forming tools CrN, CrCN 2 µm Molds, dies CrN, CrCN 2 µm & Cr-C:H Future Applications: Segment Substrate PVD Coating Thickness Remarks Hydraulics Pistons CrN-based up to 20 µm Improved field life expectancy multilayers Cutlery Cutting tools CrN-based up to 3 µm Use expected in cutting, slitting, multilayers medical Aeronautical Various Various Textile Various CrN-based up to 10 µm Replacement of Ni/Cr multilayers 22 PLATING & SURFACE FINISHING Fig. 4—Indicative cost price in USD per m2 of 2 µm CrN growth unbalanced magnetron deposition. Fig. 3—Ball grind cross section by Kalo test of CrN + MoS2 multilayer. PVD Coating Development Coating development is focusing on several items: • Corrosion resistance, • Reduced friction, • Proper mechanical properties, such as elasticity and hardness. Corrosion Performance A target of current PVD developments is to reach a corrosion performance identical or better than 20 µm of hard chromium with PVD coatings at considerably lower thicknesses. Figure 1 shows recent results of unbalanced magnetron deposited CrN for various substrate bias voltages in comparison with 25 µm hard chro- mium.7 The figure demonstrates that 25 µm of hard chromium has better corrosion resistance than thinner PVD coatings.