Introduction:
Materials and deposition techniques for producing coatings that are resistant to abrasive wear, moisture, and other environmental stresses have been discussed to various degrees in past Coating Materials News (CMN) articles. Numerous readers have expressed a continuing interest in the topic, therefore more discussion is presented here.

Multilayer coatings are often fully exposed to harsh environments and handling, and are expected to resist the effects of this exposure. Examples are ophthalmic AR coatings, architectural thermal control coating windows, windows on high speed military vehicles, automobile windows and instrument panels, lamp reflectors, medical assay probes, bar code windows in supermarkets, etc. Each application presents unique coating challenges. For example, polymer substrates (AR's for eyeglasses and instrument panel covers, and metal coatings) cannot be subjected to temperatures near 100° C; glass and IR window materials require different surface pre-treatment and coating layers than metal substrates, etc. The characteristics of the coating as well as its operational application demand different coating materials and processes. A few of these applications are covered here.

Basics for Producing Tougher Coatings:
Bond Strength
Previous CMN  issues have emphasized the importance of achieving a strong bond to the substrate as the first step in producing durable films. Substrate surface preparation was discussed in the Jan.-Mar. 1996 issue (V6, I1). Thorough cleaning to the atomic level is often adequate surface preparation for some types of substrate, but other substrate and coating material combinations require a prerequisite layer to promote bonding. Examples of the latter are the adhesion of gold to fused silica, glass, or plastics. A thin layer of chromium, tin oxide, bismuth oxide, magnesium fluoride and others can promote adhesion. The adhesion layer can have a thickness between ~20 Å and 300 Å, depending on whether transmission or other properties are affected.

More than one theory has been presented to explain how the process works to achieve strong chemical (covalent, ionic, or metallic) bonds ¹. The thin metal binding layer is oxidized by residual oxygen in the coating chamber, but an excess of the metal might be present. If the metal is soluble in gold, a chemical bond can form. In the case of dielectric layer deposition on polymers to make AR coatings, the above materials might be used, and the binding mechanism appears to be reaction sites on the polymer molecular chain. One theory holds that electrostatic stress created by anion or cation vacancies enhance diffusion to produce a graded interface between metal and insulator layers ².

Surface adhesion can be promoted by the deposition of foreign materials as mentioned above, or, in the case of ion bombardment, by the creation of nucleation sites at surface defects through interface reaction. Glow discharge cleaning or energetic ion bombardment are two common methods in which these processes are involved. The film grows laterally from these sites until it forms a continuous film layer. We proceed layer by layer through a multilayer stack to build an adherent, strong coating.

Coating Layer Properties
When the first layer of the coating is firmly bonded to the substrate, its mechanical properties must be considered. The layer must have high strength, i.e., coherence, and low stress. To achieve this, the microstructure must be dense, not highly populated with voids. Generally an amorphous structure is preferred because its packing density is greater than that of a columnar microstructure, and the intrinsic stress is smaller. The microstructure is strongly influenced by the deposition parameters and technique. Arriving evaporant atoms must have sufficient kinetic energy to be mobile enough to find and condense on low energy sites. When possible, high substrate temperature, >250° C, provides the energy required for growing dense oxide and fluoride compound layers. Ion bombardment through ion assisted deposition (IAD), sputter deposition, ion plating, and other plasma-dependent processes are alternate low temperature methods. Some materials, notably IRB (N_vis = 1.4), IRX® (N_vis = 1.63), silicon monoxide (N_vis = 1.89), yttrium oxide (N_vis = 1.75), and zirconium dioxide (N_vis = 2.05), build strong layers even at low substrate temperatures (<150° C). New materials composed of mixtures have been developed that grow with amorphous rather than crystalline structures. IRX® and IRB, patented materials from CERAC, are examples. Yttria-doped ZrO₂ is another.

The next layer in the stack is a material of either higher or lower refractive index than the first. It must form a strong adhesive bond to the first layer and possess high internal strength with an appropriate mutual stress level. Intrinsic and thermal expansion stresses must either balance or be overcome by the adhesive force between layers if the multilayer structure is to be able to resist applied mechanical forces such as abrasion. These stresses can be either tensile or compressive in nature. Most film layers are in tension, but the sign of the intrinsic stress can be manipulated in the sputtering process, and often is to produce a multilayer coating of low total resultant stress. Strain introduced by thermal excursions can produce stress levels in excess of the adhesive or cohesive forces. When cohesion is overpowered, the coating can craze, i.e., break into small brittle platelets. When adhesion is exceeded, the coating might break loose from the substrate or from a neighboring layer. Stress cracks indicate tensile stress relief; wrinkling indicates compressive stress relief. As examples, fluorides are nearly always in tension, oxides can be in either form, but are generally also in tension.

The diffusion of moisture can alter the interface force balance, generally weakening it. It is important that the layers be dense and impermeable. Maximum packing density is crucial, but the material choice is also key. For example, silicon monoxide films, even when deposited cold, provide dense barriers to water and oxygen. A common application is the coating of transparent flexible plastic containers for food storage. Evaporated or sputtered silicon dioxide, however, does not provide an effective diffusion barrier.

Low Coefficient Of Sliding Friction
The next essential component of durable coatings, especially those subjected to mechanical forces such as abrasive wear, is low coefficient of sliding friction. The surface of the outermost layer must be able to deflect the applied force along the surface to prevent it from being coupled into the layer. Low coefficient films are produced by choice of material and deposition process. As before, amorphous glassy surfaces provide the smoothness required. Materials that grow with columnar structure, as is the case for most high index dielectrics, require extreme measures such as IAD or special sputtering parameters to obtain smooth surfaces. For example, aluminum oxide films do not have an impermeability or low coefficient of friction comparable to sapphire. Mixed fluorides, such as IRX®, grow with a low coefficient of sliding friction. Diamond-like carbon, an amorphous, glassy form of carbon, is a commonly used protective outer layer for infrared optics. It is not produced by evaporation, but generally through plasma and chemical reaction.

Deposition Process Control:
Deposition technique as well as its parameters affects layer microstructure, as discussed above and in previous CMN's. In addition to the influence of substrate temperature, adatom energy, deposition rate and background pressure, growth angle of incidence is important. Low incidence angle, i.e. near 90° to the surface, is preferred for the strongest structure. Therefore, the coating geometry must be designed appropriately.

Unlike high vacuum physical deposition processes (PVD), in low vacuum PVD such as sputtering or reactive evaporation, background gas can be incorporated in the film leaving voids or unreacted gas molecules. The same can be true with CVD (compound formation and film deposition by reaction of chemical vapors). This condition leads to environmentally unstable coatings. To some degree the effects of high pressure on packing density can be reduced. For example, higher sputtering plasma energy or current density can be used. The danger of excessively high voltage is implantation of the working gas in the film. Often several percent argon is found in sputtered films. Process temperatures above 400° C in CVD form dense films.

Among sputtered materials that deposit with high packing density, low stress, and strong adherence are titanium nitride, silicon nitride, and titanium or tungsten carbides. Of these, only Si₃N₄ is transparent. The others are used as coatings for wear-resistant tool surfaces. High optical quality films of silicon dioxide are produced by reactive sputtering from silicon targets in a partial atmosphere of oxygen in argon. The nitride compounds are sputtered in a partial atmosphere of nitrogen in argon. These materials can also be deposited by CVD.

Summary:
The basic requirements for the layers and interfaces, and our progress in understanding the processes involved in producing strong, adherent thin film coatings was briefly reviewed. For additional information on any of the materials mentioned in this issue, contact CERAC for availability or product data.

Reference:
1. D. M. Mattox, Thin Solid Films 53, 81 (1978).

2. J. Salem and F. Sequeda, J. Vac. Sci Technol., 18(2), 149 (1981).

Samuel Pellicori, Principal Contributor
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922

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