Development of Thick Nanocomposite
Coatings for Erosion
Inclusive Dates: 01/01/04 - Current
Background - Solid particle erosion or abrasion often occurs to many key components of modern machinery in various fields, including compressor blades of turbine engines, vanes and rotor blades of advanced aircraft, impellers of fluid pumps, and piston rings of heavy-duty diesel engines. To increase the erosion and abrasion resistance of the components, better materials such as superalloys are being developed and adopted. Hard coatings are increasingly considered for use in minimizing wear to components. However, with the increasing demand for higher loads and speeds, damage becomes more and more severe. In some cases, failure of these components leads to severe or even fatal disasters. Superalloys are expensive and heavy, while conventional coatings using present commercial technologies are too thin to meet the increasing demand. Therefore, the objective of this project is to develop a novel deposition technology, using thick, extremely hard and tough, nanocomposite coatings that can be deposited on light alloys such as Ti-6Al-4V or commercial steels for various applications.
Approach - During this project, a plasma-enhanced magnetron sputtering (PEMS) system is being developed at SwRI. The technique utilizes magnetrons to sputter metallic materials such as titanium (Ti) onto tools and components. During the deposition, a reactive gas such as nitrogen is fed into the system to form hard nanocrystalline coatings such as titanium nitride (TiN). Deposition of TiN or the like using PVD processes is routinely practiced in tooling and component industry. However, the uniqueness of this internal research project lies in the use of intensive ion bombardment. Together with the introduction of Si in an N2 environment, the coatings thus formed contain nano-size TiN particles embedded in an amorphous matrix of Si3N4. This nanocomposite has a very high hardness and toughness; hence, a very high erosion resistance is anticipated. Another unique aspect of the SwRI technology is that it can be used to deposit thick coatings. Using conventional coating technologies, the typical coating thickness is limited to approximately 10 to 15 micrometers caused by stress buildup and increase in defects. In contrast, because of the intensive ion bombardment in this project, a low stress can be achieved. As a result, thick (more than 25 micrometers) nanocomposite coatings can be obtained.
Accomplishments - Figure 1 shows a coating being deposited using the PEMS technology. To date, we have prepared thick single layers and multilayers of TiN, TiN/Si3N4, and CrN from 40 to 60 micrometers thick on Ti-6Al-4V and stainless steels. Shown in Figure 2 is a cross-sectional SEM micrograph of a 41.5-micrometer thick TiN coating on steel. These are some of the thickest TiN coatings ever made to our knowledge. It should be pointed out that we used only two relatively small magnetrons (6.75 inches in diameter). However, this work demonstrates the possibility of large-scaled production even at a much higher rate, if large magnetrons and power supplies are utilized. Presently, we are refining the deposition process and evaluating coating microstructure, nanohardness, and erosion resistance.