Nadcap approved thermal spray coatings

Aerospace thermal barrier coatings

Thermal barrier coatings represent perhaps the most promising and exciting development in superalloy coatings research in recent years. Given the driving force to increase the efficiency and/or output of gas turbines (which inevitably means an increase in turbine inlet temperature), any mechanism by which temperature limits can be raised by overcoming hot-section material restraints is of significant interest. Thermal barrier coatings offer this potential.
A thermal barrier coating, or TBC, is a multilayer coating system that consists of an insulating ceramic outer layer (top coat) and a metallic inner layer (bond coat) between the ceramic and the substrate. In most cases the top coat and bond coat are applied by plasma spraying; sputtering and EBPVD have also been used. Typically, the ceramic top coat is 5-15 mils (0.127-0.381 mm) thick while the metallic bond

Aerospace protective coatings

The function of the ceramic layer is to insulate the metallic substrate from higher surface temperatures than it might otherwise be able to tolerate. Depending on the thermal conductivity of the ceramic, coating thickness, and the heat flux created by the design and cooling configuration of the particular component, temperature gradients of several hundred degrees can be created through the coating. Zirconium oxide (Zr02) has been the material of choice because of its very low thermal conductivity and its relatively high (for ceramics) coefficient of thermal expansion. When heated to above about 2140 °F (1170 °C), however, the Zr02 structure changes from monoclinic to tetragonal; the accompanying volume change of 4-6% can result in severe spalling of the ceramic layer. Stabilization of the tetragonal phase to room temperature or below can be accomplished by the addition of MgO, CaO, Y203, or other rare-earth oxides to the Zr02. Typical state-of-the-art TBCs utilize Zr02 partially stabilized with 6-8 wt. % Y203.
While the zirconia top coat provides an excellent barrier to heat, it is a sieve with respect to oxygen transport. A major function of the metallic bond coat is thus to impart environmental resistance to the substrate, since the gross formation of oxides at the metal-ceramic interface can cause spallation of the ceramic. The roughness of a bond coat applied by plasma spraying aids in adhesion of the plasma-sprayed ceramic top coat by providing some mechanical interlocking. Air plasmasprayed MCrAlY's were originally used for most thermal spray coatings; low-pressure plasma spray is also used today.
Performance/Reliability…By their nature, metal oxides are relatively strain-intolerant. Unfortunately, sources of strain abound in thermal spray coatings, resulting from residual stresses from the coating process, thermal expansion mismatch between the ceramic and metal layers, oxidation/ corrosion of the bond coat, phase transformations in the ceramic layer caused by thermal cycling, and thermal gradients typical of hot-section components in service. Some components also see mechanically induced strains. Consequently, the ceramic layer is prone to spalling which most often occurs just adjacent to its interface with the bond coat.

In response, much recent work has focused on processing techniques that produce a more strain-tolerant ceramic structure and on the development of bond coats with improved environmental resistance, mechanical properties, and metallurgical stability. Approaches for the development of more strain-tolerant structures have included closer control of the as-deposited phase structure and the intentional incorporation of defects during processing. The as-deposited phase structure, which is critical to the performance of the top coat, has been shown to be very sensitive to the composition and structure of the starting powder as well as to variations of plasma spray parameters (substrate temperature, gun-to-workpiece distance, etc.). The introduction of defects into the ceramic layer has been accomplished by careful control of these parameters in order to produce a controlled amount of porosity and/or microcracking in the deposit . Post-deposition processing, including annealing and quenching, has also met with some success in producing the desired defect structures.

PVD coatings

Sputtering and PVD coatings have been used to produce segmented structures consisting of a number of fine cracks perpendicular to the substrate surface. This division of the ceramic layer into a network of small individual segments is thought to improve its strain tolerance; improved cyclic lives have been reported for such structures.
Use of improved oxidation- and corrosion-resistant bond coats also leads to improved performance, as do bond coats applied in protective environments. As an aid to protection of the bond coat, techniques such as laser glazing of the ceramic or adjustment of plasma spray parameters near the end of the coating process have been used to produce a dense surface layer that precludes the absorption of corrosive salts.

Commerical application of thermal spray coatings in turbine engines was initially restricted primarily to the stationary components of the combustion and exhaust systems (e.g., burner cans and transition pieces). Significant reductions in metal substrate temperatures have been achieved, leading to the elimination of creep deflection problems in those components. TBCs are also in limited use on stationary components in the turbine section. A very high potential payoff exists if thermal spray coatings can be used successfully on stationary and rotating turbine airfoils.
The future of thermal spray coatings use on turbine airfoils depends on the success of current efforts to improve characterization capabilities and to enhance reliability. If maximum performance advantage is to be gained, airfoil surface temperatures (i.e., the surface of the ceramic outer layer) will be above the maximum allowable metal temperature of the underlying substrate, making the continued presence of the ceramic top coat critical. Thus, the need for realistic cyclic testing must be carefully addressed and engine testing will be at a premium. Airfoil thermal spray coatings will be a subject of intense interest in the next few years.