Advanced Ceramics

The Best of Both Worlds

December 3, 2012
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Organic polymers have a variety of desirable characteristics that make them the materials of choice for many applications. Organic polymers are affordable, easily processable and possess an attractive combination of mechanical attributes. As a class, these durable materials exhibit reasonably high tensile and flexural strengths, good toughness, environmental stability, and tolerance of abuse. On the downside, they lack particularly high compressive strengths and are not hard, abrasion resistant or stable at even moderately high temperatures (i.e., they burn).

Inorganic polymers (those that contain metals in their backbones) are rarely the first choice for manufacturers because they are often environmentally unstable materials and tend to be intractable. Inorganic polymers are more likely to be insoluble, unmeltable, and air or moisture sensitive. However, certain inorganic polymers, including glass, concrete, and clay-based ceramics, have historically had widespread use. It would be hard to imagine what the world would be like without glass windows, bottles, labware, ceramic dinner plates and cookware, and/or concrete skyscrapers or sidewalks.

In such applications, these materials are chosen for their extremely high strength and stiffness, high compressive strengths, resistance to high temperature degradation, non-burning characteristics, high hardness and scratch resistance. On the downside, they are typically brittle materials that are extremely flaw-sensitive and can catastrophically fail when subjected to even modest tensile or flexural stresses.

An Improved Combination

Many applications, such as polymer-reinforced concretes and shatterproof vehicle windshields, benefit from a combination of the beneficial properties exhibited by both organic and inorganic polymers. In the coatings field, extremely high-temperature-stable, corrosion-resistant coatings that have high impact strength, hardness, and flame resistance would be desirable—especially when the coatings are both cost effective and easily applied.

The development of such coatings would require careful attention to the chemistry of materials. Any candidate material would combine molecular assemblages that incorporate both organic and inorganic polymer segments. Two approaches to such “hybrid” organic/inorganic polymer systems can be envisioned. The first is to use a hybrid polymer that, in and of itself, combines many of the attractive features of both an organic and an inorganic polymer.

The second approach, and one that is well-suited to ultra-high-temperature applications, is to use a hybrid polymer that, upon exposure to high temperatures, converts to a material that exhibits the beneficial characteristics of both polymer types. Such an approach requires an in-depth knowledge of the transfigurational chemistries of these polymer systems, as well as the performance attributes of their final compositions.

Enhanced Properties

Coatings have been developed that are cost effective, easily applied, and chemically modeled to provide coating systems that combine the attributes of both an organic coating and an inorganic coating in the same coating system.* This line of hybrid coatings uses proprietary polymer chemistries wherein highly crosslinked structural networks based on silicon and carbon are generated at temperatures as low as ambient.

Such pseudo-ceramic structures provide the benefits of both an organic and an inorganic coating, but are applied and cured at modest temperatures from liquid “paint” systems that are simply spray-applied and allowed to air or heat dry. When properly applied, these coatings are extremely durable.

For example, enhanced abrasion resistance is one attribute that benefits from such an approach. ASTM D 4060 covers the resistance of organic coatings to abrasion as produced by the Taber Abraser on coatings applied to a metal surface. In Figure 1, a standard hybrid organic/inorganic polymer system is compared to typical non-hybrid or non-polymer-based coatings. The hybrid polymer demonstrates twice the abrasion resistance as the next-best product. Such a performance enhancement can be attributed to the highly crosslinked inorganic polymer network that is produced upon curing the hybrid coating.

In the coatings industry, the ASTM test methods listed in Table 1 are the most widely reported. However, exceptional performance over the entire range of these test procedures is seldom obtained in any one coating. Organic coatings typically perform well in some of these tests, while inorganic coatings excel in others. For a number of situations, good performance is contingent upon unique combinations of properties that are often difficult to achieve from either a wholly organic or wholly inorganic coating.

For instance, a firearm coating must have strong adhesion, scratch resistance, abrasion resistance and corrosion/chemical resistance. Traditional coatings are lacking because they provide an “either/or” situation for the above specifications.

Similarly, other coating applications such as sunglasses, watches, and climbing valves require strong adhesion, impact, chemical protection and scratch resistance to achieve good performance. The use of coatings in those applications requires these specifications not only be met, but met by a micron-thin single coat that can maintain the integrity and look of a newly applied coating at all times.

  *Cerakote® coatings, developed by NIC Industries, Inc.

Additional Benefits

Another benefit provided by the use of a hybrid polymer is the potential for an increase in IR reflectivity. Other coatings that do not make use of an organic/inorganic polymer tend to leave a noticeable IR signature. A line of hybrid polymers has been developed that is capable of mimicking the IR signature reflectivity of the natural environment and reducing thermal buildup and sun glint.**

In applications such as firearms and glasses, this provides the ability to provide night-time concealment. Figure 2 details an example of the difference between an uncoated firearm and one covered with an IR signature reflective coating.

Another development is a line of ultra-high-temperature coatings that are based on these organic/inorganic hybrid polymers and which, when heated to relatively modest temperatures (500°F), convert to nanostructured ceramic coatings that are stable to > 2,000°F.** In these coatings, ceramic particulates generated by the organic/inorganic polymer as it is heated to higher and higher temperatures become homogeneously dispersed at the molecular level, all within a coating that has excellent corrosion resistance, impact resistance, high hardness, and outstanding gloss and color intensity.

  **Developed by NIC Industries, Inc.

Improved Thermal Barriers

Ceramic nanostructured coatings based on such “convertible” organic/inorganic hybrid polymers are not only able to withstand ultra-high temperatures, but also act as efficient thermal barriers. This ability is necessary for a number of applications. For instance, a motorcycle exhaust system requires performance coatings that can reduce the internal temperature while providing corrosion resistance, impact resistance and scratch resistance. For many of these characteristics, the coating must excel the ASTM standard while still retaining its color and gloss, even at extremely high temperatures.

One such coating uses technology based on a “convertible” organic/inorganic hybrid polymer and is now being commercialized in exhaust system coatings for applications ranging from trucks, motorcycles and gun suppressors. The attributes of this coating are listed in Table 2; it can retain these properties at temperatures as high as 2,000°F.

Most importantly, an exhaust system must perform as an efficient thermal barrier. Custom equipment has been designed to simulate the conditions that a coated exhaust system encounters in a standard engine. Figure 3 shows a typical exhaust pipe, indicating where collector and muffler temperature gauges are connected to gather data.

In typical exhaust systems, it is desirable to reduce the internal temperature at the muffler from a temperature of 1,200°F to an external temperature below what one would normally see on a bare exhaust pipe. Typical coatings that do not make use of convertible hybrid polymers are only able to provide a 20% temperature drop on the muffler, and can only withstand temperatures up to 1,300°F. Figure 4 shows the surface temperature and internal temperature of a pipe coated at the collector and muffler using the convertible hybrid polymer technology. On average, the collector exhibits a temperature drop of 48%, and the muffler a temperature drop of 38%.

In addition to this temperature reduction deficiency, traditional coatings that target this application are also lacking because they fail to combine the most critical performance characteristics required into one coating—or require multiple coats to provide the necessary resistance or protection. Not only does the hybrid coating excel in many individual attributes, but it exhibits the key feature of being able to combine good impact resistance, excellent corrosion resistance, exceptional thermal barrier protection, and ease of application all in a single coating.

As with any property, the amount of thermal protection varies depending on the nanostructure of the polymers being used. Figure 5 shows various nanostructured polymer coatings and the degree of thermal protection provided (compared to bare metal) when these coatings are applied at a thickness of 1 micron.

  Black Velvet® coating, developed by NIC Industries, Inc.

Having It All

Some people say “you can’t have it both ways.” That has historically been true in the area of coating technology. You have either had to settle for an inexpensive, easily applied organic polymer coating that was deficient in many ways, or you have had to send your part out to have a rather expensive ceramic coating applied by an exotic process that often provides certain characteristics such as high temperature stability and high hardness at the expense of other attributes such as toughness and corrosion resistance.

Today, however, coatings that provide the best of both worlds are now available. Outstanding performance and affordability are no longer mutually exclusive.

For more information, contact NIC Industries at (541) 826-1922 or info@nicindustries.com.

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