PAD: An Alternative Process to Coat Surfaces

Coatings are applied to metal surfaces to provide protection against corrosion and wear. Two of the most common techniques used are chemical vapor deposition (CVD) and physical vapor deposition (PVD).

CVD involves the exposure of a particular surface to a gaseous material that will either react or decompose when contacting the surface to form a thin film. The gas contains multiple components that will react prior to reaching the surface. Typically, a volatile metal compound will react with hydrogen or hydrocarbon gases to form the coating.

In contrast, the vapor used in PVD is generated either by heating the source of the material utilized in the coating or by an excitation process in which the source is bombarded by energetic particles. The latter is known as sputtering.

One type of application for CVD and PVD is to apply a coating onto the surface of an iron alloy. The resulting process leads to a more effective cutting tool that displays improved hardness and a lower coefficient of friction. This increases the life and effectiveness of the cutting tool. Examples of coatings used are titanium nitride (TiN) and titanium aluminum nitride (TiAlN).

While effective, both CVD and PVD must be conducted under vacuum conditions, and the equipment needed to generate the gaseous material can be expensive. In addition, the chemicals used in CVD are generally corrosive; scrubber systems must be used to remove them, which can also add to the cost of the technique.

There is a need for an alternative method to produce a coating that can be done under less rigorous and restrictive conditions. CVD and PVD also are limited in just being able to coat flat surfaces.

Polymer-assisted deposition

Quanxi Jia, Anthony Burrell and Mark McCleskey of Los Alamos National Laboratory (LANL) in Los Alamos, N.M., have developed an alternative procedure for coating a surface that is known as polymerassisted deposition (PAD). Jia, the principal investigator of the project from LANL’s Superconductivity Technology Center of Materials Science and Technology Division, says, “Our method uses a water-based solution that contains a mixture of a metal precursor and a soluble polymer. The polymer actively binds and encapsulates the metal precursor, which serves to prevent decomposition of the metal and ensures a uniform distribution of metal in solution. Application of the solution is carried out by either spin-coating or dipping.”

Most of the work done with PAD has involved the preparation of metal oxide coatings. The solutions used in the coating process are aqueous mixtures prepared at room temperature that are stable for months prior to preparation of the coating and can be readily mixed to make complex metal combinations. After coating the substrate at room temperature, the material is heated to between 600 C and 1,100 C, which results in the decomposition of the polymer and the formation of the metal oxide.

For example, Jia has reported on the preparation of a titanium dioxide film using polyethylenimine (PEI) as the polymer. A metal polymer complex can be formed either through the treatment of a titanium complex known as titanium triscatecholate or through the direct treatment of titanium and carboxylated polyethylenimine (PEIC). The structure of the latter complex is shown in Figure 3.

Figure 3.

The structure of the complex formed from titanium and carboxylated polyethylenimine (PEIC). A water-based PAD solution is applied to a silicon wafer mounted on a spin-coater.

Figure 3 also shows how the coating can be applied to a surface. ]ia adds, “The process is very similar to painting a wall.” The PAD method can be utilized to coat any type of surface including those that are irregular.

During the heating process, PEI or PEIC will decompose at a temperature between 450 C and 500 C. |ia says, “It is undesirable to have to heat up the coating, but the process is very controllable at temperatures above 500 C. In the case of preparing metal oxides, the heating step can be conducted in an oxygen environment. But for other coatings it is necessary to use an inert atmosphere.”

Use of the right polymer molecular weight also can help the user achieve the proper solution viscosity and, as a consequence, the proper coating thickness. One other parameter that affects film thickness is the concentration of the metal precursor. Burrell and McCleskey indicate, “We have prepared films with thicknesses that range from 10 nanometers to 600 nanometers.”

Besides titanium dioxide, this group has used the PAD process to successfully develop coatings for other metal oxides such as europium oxide, indium tin oxide, lanthanum aluminate, strontium titanate and zinc oxide. Most of the main applications to date have been for use in microelectronics. Titanium dioxide coatings are used as photocatalysts for wastewater treatment and energy conversion media for solar cells.

Much of the coating work to date has been on silicon and ceramic- like substrates, but several metal oxides have been used to coat other substrates for different applications.

Lubricant applications

A limited amount of research to date has been done in showing how PAD can be used to develop new lubricant coatings. Burrell and McCleskey say, “We prepared a zirconia oxide coating on an aluminum oxide membrane to improve its resistance to decomposition at a pH of 13. An uncoated version of this membrane is destroyed after 15 minutes of exposure to potassium hydroxide. One coating of zirconia oxide provides sufficient corrosion protection to enable the membrane to be unaffected for at least 12 hours. We believe that the corrosion resistance will be further improved if the process is optimized.” This group also has done some work with titanium nitride and aluminum nitride coatings.

In discussing other solid lubricants, this research group believes this technique can be used to prepare molybdenum disulfide coatings. |ia says, “We have prepared cadmium sulfide coatings for use in photovoltaic devices and firmly believe that we can also prepare coatings based on molybdenum disulfide.”

“The successful demonstration of both simple and complex epitaxial metal-oxide films using PAD shows the significant advancement of the present invention,” says Dean Peterson, director of the LANL’s Superconductivity Technology Center. “This technology itself provides a cost-effective approach to grow electronic and optical materials. It will find wide applications in many fields where the material is needed in the film form.”

The PAD technique may provide researchers in the lubricant industry with another option for preparing solid lubricant coatings. The ease of use and the flexibility of the technique make it well worth pursing in the future.

Further information about PAD can be found in a paper published in 2004 by Jia and his co-workers.”

The process is very similar to painting a wall. The PAD method can be utilized to coat any type of surface including those that are irregular.

The PAD technique mayprovide researchers in the lubricant industry with another option for preparing solid lubricant coatings.

Reference

(1) Jia, Q.X., McCleskey, T.M., Burrell, A.K., Lin, Y., Collis, G.E., Wang, H., Li, A.D.Q. and Foltyn, S.R. (2004), “Polymer- assisted deposition of metal-oxide films,” Nature Materials, 3 (8), pp. 529-532.

Neil Canter heads his own consulting company, Chemical Solutions, Inc. in Willow Grove, Pa. Submissions to Tech Beat can be sent to him at neilcanter@comcast.net

Copyright Society of Tribologists and Lubrication Engineers Jul 2005