Physical vapor deposition (PVD) includes vacuum PhysicalPhysical VaporVapor Deposition—Deposition— evaporation, sputtering and ion plating. PVD pro- cesses are used to apply functional hard coatings Wear-resistantWear-resistant and decorative thin films. The most widely used coating is titanium nitride (TiN). Other coatings & Decorative Coatings successfully commercialized include titanium & Decorative Coatings carbonitride (TiCN), chromium carbide (CrC), chro- mium nitride (CrN), zirconium nitride (ZrN), and ByBy MarkMark PodobPodob titanium aluminum nitride (TiAlN). PVD coatings are very hard, have a low coefficient of friction, are chemically resistant and environmentally safe. When applied to different components, they act as chemical and thermal barriers. Coating applications for TiN include cutting and forming tools, plastic molds, surgical instruments and prostheses. CrC and CrN can be a direct re- placement for chromium plating for decorative applications, such as watch components, eyeglass frames, pen components, door and window hard- ware and plumbing fixtures. PVD coating pro- A plastic mold application for PVD processing is for the protection of compact disc cesses, properties of coatings and commercialized mold surfaces. applications are reviewed here.

hysical vapor deposition To produce compound coatings, the prevent dust from depositing on the describes a family of coating ions are reacted with a gas (e.g., cleaned surfaces. While most pre- P processes that produce surface nitrogen or methane) to produce a coating cleaning methods are satisfac- layers that are the result of the nitride or carbide. This process is tory, some parts cannot be adequately deposition of individual atoms or known as reactive ion plating. cleaned. If components have been molecules. All PVD processes are Functional hard coatings are treated with a plastic rust preventive, carried out under a hard vacuum. typically deposited by evaporative for example, or salt-bath heat-treated Workpieces are heated to tempera- coating processes. Thin films for where residual salt remains on the tures generally < 500 °C (930 °F) to decorative applications are applied by surfaces, special cleaning methods enhance coating adhesion. sputtering processes, although either may be required. During PVD sputtering, argon is process may be adapted for either admitted into a vacuum chamber at category of coatings. All PVD Properties of Hard Coatings relatively low pressure. A high DC processes are line-of-sight, so to PVD allows for the deposition of a voltage is applied to the target, which obtain coating uniformity, it is wide variety of elements and com- creates a gas discharge between the necessary to use specially shaped pounds. Refractory materials may be target and substrate holder. Argon targets or multiple vaporization deposited at temperatures below their ions, created during the discharge, sources, and to rotate or manipulate melting points. For sputtering strike the target with high energy and the substrate uniformly to expose all processes, virtually any material may free atoms from the target, which are areas to the coating source. be deposited (provided that it is then transported to the substrate and available in the form of a target). deposited as a thin film. Cleaning Methods Compounds deposited by evaporative PVD evaporative processes consist To achieve good coating adherence, reactive ion plating are limited by the of heating a source material in a high substrate surfaces must be free from ability of the source metal to be vacuum, which produces a vapor that the slightest amount of contamination conveniently vaporized. condenses on the substrate as a thin (grease, dust, grinding burn or finger For functional applications, the film. Ion plating—an evaporative prints). Parts to be PVD-coated are coatings are deposited to a thickness PVD process—uses an electron beam normally bulk-cleaned in multi-station gun to melt the source. The introduc- ultrasonic cleaning lines, which ______tion of an inert gas at low pressure include a series of wet chemical Editor’s Note: This is an edited version ® creates a plasma discharge between rinses—usually detergents or sol- of a paper presented at SUR/FIN ’96, the source and the substrate. The ions vents. Ultrasonics are used to activate Cleveland, OH. It has been expanded by impinge the substrate surface during the surface of the components being the author to include photographs and deposition, resulting in changes in the cleaned, and to assist in removing case studies. The original article ap- interface region and improved film contaminants. The parts are dried to peared in the Society of Vacuum properties. A negative bias on the eliminate water spots. Hot-air dryers, Coaters’ 39th Annual Technical Con- substrate enhances coating adherence. equipped with micropore filters, ference Proceedings (1996).

42 PLATING & SURFACE FINISHING Table 1 Table 2 Table 3 Common Compounds Reacted Hardness of Recommended Machining From Metallic Elements, Functional Coatings Applications Nitrogen & Carbon Material Hardness (HVN) PVD Recommended Element Material Film Color Titanium carbide 3400 Coating Application Nitrogen TiN Gold (yellow) Vanadium carbide 2800 TiN Free machining steels, high- ZrN Gold (green) Titanium nitride 2600 strength steels, plastics, TiAlN Violet/black Niobium nitride 2400 hard rubber CrN Silver Chromium carbide 2200 TiCN Tough machining steels, Carbon TiC Gray-black Chromium nitride 1600 cast iron, abrasive materials CrC Silver Cemented carbide 1600 CrC Aluminum, titanium alloys Carbon + TiCN Bronze/gray Tool steel 900 TiAlN Cast irons, abrasive Nitrogen materials, dry machining of 2–5 µm. (A greater thickness does commercially popular as a functional Dovetail forms Circular forms not usually enhance coating perfor- hard coating, it is used as a decorative Threading tools Routers mance.) Decorative coatings are coating—particularly on brass Counter-sinks Counter-bores generally 0.2–0.7 µm thick. hardware and plumbing fixtures. The structure of deposited thin Coatings are applied as either These coatings provide abrasion films determines their properties, discrete single or multiple layers to resistance, adhesion resistance, including color (Table 1). Properly provide different benefits, depending excellent lubricity and corrosion applied PVD coatings replicate the on the application. CrC is silver in resistance. PVD coatings are also surface onto which they are deposited. color. When titanium or aluminum used to protect cemented carbide A coating deposited onto a smooth, alloys are machined with tools coated milling and turning inserts. Perfor- highly polished surface will have a with TiN, there is a tendency of the mance-enhancing features include: mirror-like appearance, while the aluminum to chemically react with the same coating deposited on a matte- nitrogen in the coating. This causes • A smooth surface finish, coupled finished surface (e.g., one that has the tool to pick up metal. CrC is with thermal insulating proper- been vapor-honed) will have a matte effective for machining aluminum ties, generates less heat during finish. Table 2 lists the hardness of because it is difficult to form alumi- machining, preventing cutting commonly deposited PVD coatings. num carbide. Titanium alloys, such as edges from breaking down. (The harder a material, the better its Ti-6Al-4V, contain aluminum, so CrC • The low temperature of deposi- resistance to abrasion.) is also effective for machining these tion preserves the transverse materials. The chromium in the rupture strength of the carbide, as Most Common Hard Coatings coating does not diffuse into the well as the formation of eta phase TiN is shiny gold in color and has a titanium surface as it does with TiN. (a carbon-lean, brittle phase). hardness of 2600 HVN. The coating CrN is also silver in color, and can be Both conditions can lead to acts as a chemical and thermal barrier an excellent substitute for chromium premature tool failure. between the surface onto which it is plating, with no associated environ- • Sharp corners can be coated, deposited and the environment. TiN mental concerns. resulting in lower cutting forces. has a low coefficient of friction that TiAlN is violet/black in color and, helps prevent galling and metal pickup. when used on cutting tools, it is Properly designed cutting tools can Among the hardest of the PVD believed that the aluminum in the fail by adhesive or abrasive wear. coatings, TiC is silver-gray in color, coating oxidizes, providing high- Because PVD coatings act as a often resembling the metallic surface temperature protection to the tool and chemical and thermal barrier between of the uncoated component onto extending tool life. the tool and workpiece, the hardness which it is deposited. of the coating provides abrasive wear TiCN ranges in color from bronze PVD Coating Applications protection, and the chemical stability to silver-gray, according to the Cutting Tools protects the piece from adhesive wear. amount of carbon in the compound. Among commercial applications for In addition to extended life, PVD The more carbon, the greater the PVD coatings, cutting tools are some coatings offer increased productivity tendency of the coating to appear gray of the most significant. Improvements and economic benefits. In the case of in color. As a decorative coating, in tool life for PVD-coated tools automotive gears, for example, PVD TiCN was developed for its close range from two to eight times when coatings on gear hobs, shapers and resemblance to bronze. For functional compared to uncoated tools. The broaches have significantly reduced applications, it is used for machining additional improvement in productiv- tooling costs. Several automotive stainless steel, high-hardness steels ity is substantial. companies have installed their own and high-alloy materials. Tools routinely PVD-coated include: PVD equipment to take advantage of ZrN ranges from yellow-gold to Taps Milling inserts recoating tools, which can recapture brass in color, depending upon its Drills & saws Gear-cutting hobs some of the tool life that is lost when stochiometry, and has mechanical Reamers Shaper-cutters the coating is removed from the properties similar to TiN. While not End mills Broaches cutting edges during sharpening.

November 1996 43 Table 4 5. Increased up-time— The use of TiN and TiCN for the TiAlN-coated Tools, Coated tools last longer production of clear plastic lenses has Machining Case Study and can stay in the provided a tool life performance of up Operation: Rough machining nickel base machine longer than to eight times longer than other jet engine turbine blades uncoated tools. coatings. When TiN has worn away, it Machine: 5-axis CNC machining 6. Part tolerances—Tools is evident to the operator, which is not center do not wear as quickly, so the case with chromium plating. TiN Cutting tool: 4 flute 3/8-in. diameter solid they hold tolerances and TiCN may be non-destructively carbide ball nose end mill longer. removed, allowing the tool to be Chip load: 0.015 in./tooth repolished, and then recoated for Speed: 4700 RPM Tables 4 and 5 define case longer wear. The coating perfectly Finish requirement: 80–120 RMS study findings for TiAlN- replicates the high polish required for Uncoated: 11 pieces vs. Cr-coated tools. the optical finish associated with the TiAlN-coated: 44 pieces molded products, and the lubricity of Metal Forming the coating enhances mold release. (Punches & Dies) An additional plastic mold applica- Table 5 Coatings are commonly tion is for the protection of compact Cr-coated Tools, applied to the surfaces of disc mold surfaces. The coatings Machining Case Study punches and dies to extend replicate the surface onto which they Operation: Finish machining aluminum the life of these products. are deposited. The molding of cylinder heads The low coefficient of compact discs requires an extremely Machine: Multi-pallet CNC machining friction associated with high surface finish. Careful polishing center the coatings allows for the produces a flat, parallel surface, with Cutting tool: 4 flute solid carbide step reduction or elimination of a finish close to 0 RMS. Visual reamer the use of die lubrication inspection with low-powered micro- Feed: 0.005 IPR and draw compounds, scopes ensures that all surface Speed: 2400 RPM which reduces the cost imperfections have been removed. Coolant: Water-soluble, pressure-fed associated with degreasing Compact disc mirrors and other Uncoated: 42 RMS required for environmental associated mold components are CrC-coated: 14 RMS reasons. normally coated by reactive ion Table 6 is a listing of plating. This process ensures that the tool life improvement for coating is free of porosity and defects. An additional feature of the TiN-coated tools used in the metal The use of TiCN in place of TiN for coatings is that they are generally stamping industry. As is the case with this application is growing. The applied in compression. Compressive metal cutting, improvements in tool higher hardness of TiCN provides stresses associated with the thin films life and productivity are significant. better abrasion resistance for the serve to increase the fatigue strength The chemical resistance of the polycarbonate plastics that flow onto of the components. It is common to coatings provides protection from the mold cavity surfaces. The reported apply PVD coatings as multilayered galling and metal pickup, which can improvement in life for CD mirror films to take advantage of the lead to premature tool failure. PVD blocks is twice as long with TiCN, differences in compressive stresses of coatings are applied at low tempera- and better release allows for less the individual layers. These differ- tures, normally below the tempering downtime associated with required ences allow cracks to blunt at inter- temperature of steels used to make mold maintenance. faces, rather than propagate through punches and dies, so tool dimensions the coating, delaying the onset of tool do not change as a result of the Aluminum Die Casting Molds failure. coating process. Fine blanking tools CrC is an effective coating for Benefits of PVD coatings on and other tightly toleranced stamping protecting aluminum die casting cutting tools is summarized as: tools are excellent candidates for PVD molds, because the corrosion resis- coatings. tance of this coating serves to delay 1. Extended tool life the onset of soldering (a common 2. More regrinds—The coating does Plastic Molds failure mode for die casting molds). not flake but maintains adhesion TiN and TiCN are used extensively in Reported improvements in tool life throughout the tool life, even under the plastic mold industry, protecting for CrC-coated molds, core pins and severe wear conditions, and molds, gates, screws, tips, core rods related hardware is on the order of remains secure on rake faces and and other associated mold compo- three times that of uncoated compo- wear lands. At least 30 percent less nents. Many plastics are abrasive, nents. The high oxidation temperature material is removed per sharpen- particularly those using glass-filled of the coating (1400 °F), coupled with ing. resins. The hardness of the coating its hardness, also aids in improving 3. Increased cutting speeds & feeds provides abrasion resistance to mold mold component life. 4. Improved surface finish—The surfaces, enhancing performance. chemical resistance of the coatings Polycarbonate plastics, such as Wear Components prevents welding of the tool to the those used for optical applications Applications for thin-film PVD workpiece. Galling and tearing are (see Fig. 1), are particularly corrosive. coatings as wear coatings include effectively eliminated.

44 PLATING & SURFACE FINISHING coating stainless steel jewelry provides the desired color and, because stainless steel has high hardness and excellent corrosion resistance, it is also durable.

The Future for PVD Coatings PVD is being used to deposit func- tional and decorative thin films. A wide variety of coatings can be deposited. As environmental concerns regarding plating processes become The use of TiN and TiCN for the production of clear plastic lenses has provided a tool life performance of up to eight times longer than other coatings. greater, the use of PVD coatings will grow, finding new applications for the coatings and processes. P&SF gears and bearing surfaces. Additional 1. The ability to compete favorably applications exist in the automotive on an economic basis with decora- References industry for antilock brake systems tive electroplating processes, 1. D.M. Mattox, “Ion Plating,” ASM and fuel injector components. PVD particularly in light of environmen- Handbook, Vol. 5, Surface coatings have improved properties tal considerations. Engineering, 582 (1994). over traditional steel surface treat- 2. A broad range of available colors. 2. Daido Steel Co., Ltd., Machinery ments such as carburizing and 3. Corrosion resistance equivalent to, Div., “Technical Trends in Color nitriding. The large quantity of or better than that of the coating Coatings,” Leybold AG Decorative components available for coating has processes being replaced. Hard Coatings Symp. (April 1989) also served to reduce the price-per- 4. Use of less expensive substrates. 3. A.T. Santhanam & D.T. Quinto, piece for the coating. Additional uses “Surface Engineering of Carbide, are chemical ball valves and seats to Decorative applications include: Cermet and Ceramic Cutting provide corrosion and abrasion Watch components and bands, Tools,” ASM Handbook, Vol. 5, resistance, and jet engine compressor jewelry, eyeglass frames, cigarette Surface Engineering, 900 (1994). blades where the coatings provide lighter cases, pen parts (barrels, clips), 4. R. Badger & M. Podob, “PVD enhanced erosion resistance. knives and other accessories. Brackets Hard Coatings for Improving the used for orthodontic braces, where Performance of Broaches,” Medical Applications gold is a desired alternative to the Broaching Technology, SME The combination of mechanical silver-colored stainless steel typically Technical Seminar (April 1995). properties of TiN, particularly wear used for these components, is another 5. J.H. Richter & M. Podob, “CVD and corrosion protection, has led successful application. and PVD Hard Coatings for medical researchers to apply TiN and One of the more interesting and Extending the Life of Tools Used other coatings onto prosthetic widespread applications is for door in the Stamping Industry,” Manu- implants. Because the coatings are and window hardware and faucet facturing Solutions, Vol. 2, applied in compression, the increased parts. While brass hardware is Metalform ’92 Tech. Symp. Proc., compressive strength of components attractive, the corrosion resistance of 337 (February 1992). results in a corresponding improve- brass is somewhat limited. Unlike ment in low cycle fatigue life. When copper, which develops an attractive About the Author coated, prosthetic implants (e.g., hips, and uniform green patina on weather- Mark Podob is vice president, knees and other joints) have shown up ing, brass turns a dull, dark brown, marketing and sales, for Richter to three times improvement in life. and requires continual polishing. The Precision, Inc., 1021 Commercial This is significant when considering introduction of ZrN PVD coatings Ave., East Petersburg, PA 17520, the age of patients who undergo joint onto brass hardware has allowed responsible for sales and marketing of replacement surgery. suppliers of these components to offer PVD and CVD coating services and Other applications in the medical “lifetime” guarantees on surface equipment. He has a bachelor’s of industry include surgical tools where finishes. When deposited to the engineering and an MS in materials the combination of high hardness and appropriate stochiometry, ZrN has a sciences from New York University. corrosion protection play a key role in finish that perfectly matches that of He has been associated with the extending the life of these tools. brass, and it offers excellent corrosion functional and decorative CVD and Surgical tools must also be non- resistance. PVD thin film industries since 1980, reflective (because of glare in the Another interesting decorative and is an author/presenter of numer- operating theater), and these coatings application for PVD coatings is for ous articles on the processes. meet that requirement. jewelry. TiN has an appearance that resembles gold, and has the further Decorative Applications advantage of abrasion resistance. The use of PVD for decorative Substituting stainless steel for gold applications is increasing for a offers a significant cost savings. TiN- number of reasons, such as:

November 1996 45