Engineers Developing 'Superhard' Coatings on Metals
Cal Poly Pomona engineers are finding a way to combine what they say will be the best of both worlds: merging a soft metal with a hard ceramic coating that is wear-resistant.
Developing a hard and strong coating on a relatively softer and ductile metal base opens up a range of engineering possibilities, according to Professor Vilupanur Ravi. "Superhard" surfaces offer better resistance to wear and erosion for applications such as drill bits, grinders or abrasives.
Ravi and his students are working on a process that applies a boride coating, which is a ceramic material, onto a metallic surface to create the hard surface. One such superhard material, rhenium diboride, has been shown to scratch even diamonds.
"Ceramics by nature tend to be hard and wear-resistant. So when you make a metal into a boride, the surface behaves like a ceramic while the base is still metallic," Ravi says. "And then you have the best of both worlds. You have a metal that's soft and a surface that's hard. That means you can shape it into anything you want and then you can apply the coating."
Ravi, who teaches chemical and materials engineering, is working with a $36,500 grant from UCLA, funded by the National Science Foundation, to develop different combinations of boride coatings on metals.
"At Cal Poly Pomona, we have unique expertise and the ability to apply coatings to a variety of different materials," Ravi says. "It's a process we've been developing experimentally, while simultaneously building a sophisticated computer model to guide the experimental design."
Undergraduate research assistants begin by building a pack, in which a blend of powders is placed in a ceramic crucible with a metallic sample in the middle. After the pack is sealed with a ceramic binder, it is placed into a furnace and heated to high temperatures for different periods of time, depending on the experiment.
After the coating process is completed, Ravi and his students use optical microscopes, hardness testers, an X-ray diffraction machine and a scanning electron microscope to evaluate the boride coating applied onto the metal. With the electron microscope, researchers can literally see which elements and in what concentrations are on the metal's surface.
"I like the idea of making a whole new material, and this is kind of like that," says chemical engineering senior Jordan Koch. "We're making a new coating and a new coating process each time."
"The fact that we can get metals to the hardness of a ceramic or a diamond is really good. Because instead of using a diamond, you could use a ceramic-coated metal," Koch adds.
Ravi's approach to creating a harder metal complements efforts by his colleague at UCLA, Professor Richard Kaner of the chemistry and biochemistry department. Kaner is one of the world's leading researchers in developing superhard materials made entirely of boride.
"We have the capability at Cal Poly Pomona to coat a broad range of metallic materials with customized coatings. Very few researchers have the ability to successfully produce these types of boride coatings," Ravi says.
An important tool in these efforts is a computer program that Ravi and chemical engineering Professor Thuan K. Nguyen developed. They continue to work on the program, which can predict optimum conditions to create a desired coating.
(Top photo: To create superhard coatings on metals, Professor Vilupanur Ravi and his students heat the samples to high temperatures in a furnace in the Engineering Lab Building. Bottom photo: Chemical engineering senior Jordan Koch prepares a pack, which is a blend of powders and a metallic sample in a ceramic crucible.)