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Physical Vapor Deposition (PVD) Includes Vacuum Physicalphysical Vaporvapor Deposition—Deposition— Evaporation, Sputtering and Ion Plating

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Physical Vapor Deposition (PVD) Includes Vacuum Physicalphysical Vaporvapor Deposition—Deposition— Evaporation, Sputtering and Ion Plating 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
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