Introduction
Colored coatings that display metallic reflection appear everywhere in the commercial world. Decorative coatings are applied to jewelry, personalized objects such as eye glass frames, watches and pens, automobile and home fixtures, glass art, architectural windows and even sports equipment, multimedia and fashion. These applications require that the coatings possess a substantial amount of abrasive wear and chemical corrosion resistance as well as visual appeal. Their functions, besides aesthetic appeal, include thermal control and energy savings in buildings, and application to surfaces subjected to abrasive wear and high temperature, as in high-speed cutting tools. The industrial production base is expanding as better deposition processes are developed. Professional societies are including technical symposia devoted to decorative and industrial coatings (example: Society of Vacuum C oaters*). This article reviews the general topic.

Color Mechanisms
The coloration of decorative coatings results from selective absorption and reflection. It is therefore a bulk phenomenon rather than an interference effect. However, interference is sometimes involved to enhance colors. The aesthetic appearance of a coating is determined by the color, its saturation, and by the surface finish of the base material. Deep color with high gloss is the goal. Both of these properties are process dependent. Reproducible, uniform, stable color is required in the consumer market, as in the mass production of watch bands and eyeglass frames.

The gold color of TiN is a familiar sight on the windows of modern buildings. In this application, environmental control is aided by limiting transmitted solar light and radiated thermal energy. Adding a second metal or creating an oxide or carbide broadens the spectrum of colors possible and improves the mechanical properties compared with basic TiN. A wide range of colors can be produced by sputtering alloys of the metals Cr, Ni, Au, Cu, etc. or by reactive plasma deposition of compounds such as TiN, ZrN, Cr_xN, ZrC_xN_y, etc. The TiN system has evolved to Ti-Al-N, Ti-Zr-N, Ti-Al-Zr-N, and Ti-Al-V-N systems, each with specific advantages in wear and corrosion resistance. Electrochemical deposited coatings include transition metals, noble metals, alloys, and compounds. These coatings are grown to thicknesses near 12 µm. The table on page 2 lists some common decorative coatings materials and their colors. [1]

Table 1. Common Colors and Compositions of Decorative Coatings

Color

Composition

Common Name

Deposition Processes
Physical and chemical deposition techniques are used to make hard decorative coatings. The physical methods include sputtering, ion plating, cathodic arc plasma and electrochemical deposition. Reactive chemical and electrochemical methods are also used. Magnetron sputtering has the advantage of being able to control alloy composition and sputtered coatings are generally more durable. In contrast, electrochemical deposition is very economical and yields smoother coatings which exhibit higher gloss. Often the two processes are combined. The oxide materials can be produced by sputtering and the metal alloys can be produced by sputtering or electrochemical deposition. The gold coatings listed in the table are three examples of the Swiss gold standards governing jewelry.

Composite coatings are used in jewelry. For example, TiN, TiZrN or TiCN are often the underlayer that provides wear resistance when coated on stainless steel. An alloy of gold is then overcoated or plated to produce the brilliance of gold. In this way an economical process is achieved.

The bulk absorption that is responsible for coloration is influenced by both the structure and the composition of the dispersed impurity ion or metallic particle. These properties determine the electronic absorption of the layer. For example, the color of (TiAl)N is controlled by the relative percentages of Al and N in the composition. Typical sputter targets are composed of 50:50 Ti:Al and the amount of nitrogen is varied. Control of the microstructure and composition of the dopant material can be accomplished with chemical techniques or in sputtering.

When binary materials such as TiN, TiC and ZrN are reacted with O, N, or C, or alloyed with another metal, the range of color possibilities is expanded. With reactive sputtering and cathodic arc, for example, colors can be tuned at will. Sputtered gold-vanadium alloys of varied composition can simulate pure gold and colors ranging from yellow to red. In the TiN and ZrN system, color is varied with nitrogen percentage [2]. Colors can be stabilized by adding carbon, which probably modifies the microstructure of the layer.

Hardness
The compound and alloy materials of decorative coatings exhibit extraordinary micro-hardnesses, a desirable quality in resisting environmental wear. In addition, the energetic deposition processes involved encourage high packing density which leads to increased hardness. The result is hardness values that exceed 2200 kgf m -2 for nitrides and carbonitrides, and at least 1500 kgf m-2 for carbides. CMN Volume 7, Issue 2 (April-June, 1997) covered hard coatings for cutting tool surfaces and optical applications. A number of compounds were discussed there, including TiN, TiC and TiC x N y and low temperature deposition processes. Thicknesses of several µm of coatings capable of high service temperatures are obtained by plasma spray or CVD.

Other Materials and Processes
Aluminum and some steel surfaces are traditionally passivated by anodization and colored by dyes. The stability and range of colors available is limited, however. A cathodic procedure based on molybdate - phosphate aqueous solutions is able to produce stable colors ranging from blue to golden to purple [3]. Variations between solution pH and exposure time control corrosion resistance and color. Longer time increases the thickness and determines color. Higher pH increases corrosion resistance, but reduces the saturation of the color.

Rare-earth hexaboride coatings can exhibit colors ranging from purple-red for LaB₆ to blue for CeB₆, SmB₆, and YB₆ [4]. Sputtering (r. f. magnetron) from sintered or hot-pressed targets is the deposition technique. Sputtering must be done with the precautions of avoiding oxidation and target overheating (as with all sintered or hot-pressed targets). These particular compounds are not widely used in the decorative coating area to date.

Dichroic coatings are another form of decorative coating. The colors created are caused by interference in multilayers that are deposited by electron beam onto glass, plastics, and non-transparent substrates. Dichroic coatings on transparent substrates reflect some colors and transmit the complimentary color. They are used in making jewelry, fashionable eyeglasses, stained glass for decorative windows, art objects such as plates, and high-end sculptured glass objects selling for thousands of dollars. Glass coated with multilayers of zirconia and silica can be fused between compatible transparent glass for earrings, beads, plates, etc. The high-temperature process can cause the coating to fracture into minute flakes, adding sparkle to the colors. Plastic objects such as eyeglasses and wind screens are coated with titania and silica for selective reflection / transmission.

This brief introduction to decorative hard coatings describes a unique example of coating materials and processes that not only have significant industrial application but simultaneously provide aesthetic appeal.

References

1. George Reiners, Uwe Beck, Hermann A. Jehn, Thin Solid Films 253, 33 (1994).
2. H. Randhawa, Surf. and Coatings Tech. 36, 829 (1988).
3. K. P. Han and J. L. Fang, Surf. and Coatings Tech. 88, 178 (1996).
4. C. Mitterer, W. Waldhauser, U. Beck, and G. Reiners, Surf. and Coatings Tech. 86-87, 715 (1996).

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