A class of high-performance ceramic materials based on titanium carbide-coated graphite, is particularly applicable for unusually demanding applications. This article describes these coatings.

Titanium carbide-coated components are designed for extreme wear applications in a variety of industries.

Graphite parts are refractory, lightweight and corrosion resistant. These properties are critical for many applications, such as dies for continuous casting, rocket nozzles, and heat exchangers for the chemical industry. However, the relatively poor resistance of graphite to wear and oxidation limits its use. The addition of titanium carbide (TiC) coatings, which possess excellent resistance to wear, oxidation and corrosion, as well as having other desirable properties, greatly extends the use of graphite.

Properties of TiC are shown in Table I. Data for TiC-coated graphite, as developed by Lanxide Coated Products, and successful trials project a rapid evolution of applications for these materials (see Fig. 1).

Fig. 2 Photomicrograph of TiC coating microstructure.

Description of Coatings

Figure 2 shows a typical TiC coating of 150 Km on Poco AXF-5Q graphite. These TiC coatings feature both an equiaxed microstructure adjacent to the coating-substrate interface, and columnar grains away from the interface (Fig. 3). Additionally, these coatings adhere well to their graphite substrates even after fracture. Thickness for these coatings can be controlled from a few microns to 250 Km.

Fig. 3 Cross-sectional photomicrograph of TiC-coated graphite.
The surface of the TiC coating (see Fig. 3) is inherently very smooth and a significant improvement over the starting graphite substrate. In fact, one of the striking features of the as-coated TiC coating is its highly polished appearance. The surface finish of these coatings is also dependent upon that of the graphite substrate. For example, the surface finish of a 150 Km TiC coating applied to graphite with a surface finish of 1.4 Km becomes 0.6 Km.

By comparison, an identical 150 Km TiC coating on graphite with a surface finish of 0.5 Km becomes 0.2 Km. In addition, TiC-coated graphite can be polished to a surface finish of less than 10 angstroms (A).

These coatings are crack-free when applied to graphite substrates with coefficients of thermal expansion (CTE) that are within 1.0 ppm/K of that of the carbide coating. Therefore, the substrates of choice are premium, high-density, fine-grain, high CTE graphites.

Fig. 4 Abrasion resistance of TiC coating compared to that of other materials.

Wear Resistance

One of the hardest known compounds, titanium carbide exhibits excellent wear resistance. In Fig. 4, the wear resistances of several materials are compared to that of TiC. A sand/water slurry was employed in a sliding abrasion trial (Miller test) to generate this wear data. The application of TiC coatings has been shown to improve the wear resistance of graphite by a factor of 2000.

Fig. 5 Corrosion rates of various materials in boiling 40% HCl.

Corrosion Resistance

Titanium carbide is known for its excellent corrosion resistance, even in hot concentrated acids. In Fig. 5, the corrosion rate of TiC is compared to that of several other materials in boiling concentrated hydrochloric acid.

Fig. 6 TiC coatings increase the use of temperature of graphite.

Oxidation Resistance

TiC coatings raise the use temperature of graphite in air by over 300ÝC. Unlike graphite, which starts to lose weight at temperatures greater than 450ÝC by converting to gaseous carbon dioxide, TiC gains weight during oxidation due to the formation of titanium dioxide which acts as a protective layer against further oxidation. This improvement in oxidation resistance is shown in Fig. 6.

Examples of Successful Usage

Titanium carbide can achieve many times the life of conventional ceramics, metals or polymers. For example, a TiC-coated graphite glass fiber guide made by Lanxide Coated Products has shown an estimated 500% lifetime improvement over the typical alumina ceramic guides that were used in the process. In addition to reduced downtime, an improvement in the quality of the glass fiber was observed.

In a foundry test, a TiC-Coated graphite baffle showed superior resistance to molten aluminum, lasting over 50 heats. By comparison, conventional refractory materials can fail after eight heats.

To illustrate the broad applications for this material, in a development for the computer industry, it is possible to make a hard-drive substrate that is 3 times stiffer and substantially harder that the aluminum typically used.