2012 Supported by: www.ubmaviationnews.com
Advances in thermal barrier coatings Since gas turbine jet engines were developed more than 70 years ago they have made significant, continuous improvements today s engines are more powerful, more fuel efficient and more reliable than ever. Advances in engine design, components, materials and other factors, including thermal barrier coatings and other applied coatings incorporated onto critical engine parts, have resulted in today s exceptional power systems, as Lucy Liu, Komal Laul and Ravi Shankar of Chromalloy explain. As the internal operating temperatures of turbines have increased to provide more power and improvements in engine operation, the need for new advanced coatings also has increased. A closer look at coatings and the turbine components they insulate in the engine hot section shows how important these applied materials are in the performance of today s aircraft powerplants. Chromalloy s newest coating, the patented Low K RT-35 for aircraft engines, further enhances engine performance. Development and introduction of the new coating was a multi-year process that culminated with strong results and certification for the commercial aircraft engine. Advanced coatings Manufacturers produce high-performance engines whose simple cycle thermal efficiency has increased significantly during the last few decades. These higher thermal efficiencies translate to higher thrust in the aircraft and are achieved through higher operating temperatures. The higher temperatures are achieved due to the use of super- 42
Chromalloy s Low K RT-35 Coating Chromalloy recently announced its newest thermal barrier coating, designed to enhance the performance of gas turbine engines. Chromalloy s new thermal barrier coating the RT-35 Low K coating provides lower thermal conductivity, which allows higher engine temperatures, said Peter Howard, VP technology and quality assurance at Chromalloy. The RT-35 Low K coating was patented in 2006 and certified by the FAA in 2010 for use on the PW4000 second-stage high pressure turbine blade after a series of tests confirmed its low thermal conductivity, high thermal cycle durability and other attributes. The coating is currently in use by a commercial airline in Asia. The RT-35 Low K coating provides a layer of insulation to the base metal component and underlying bond coating surface of a turbine blade from the extreme heat of the combustion gases during engine during operation. The coating provides 50 per cent lower thermal conductivity, allowing engines to perform at higher temperatures. Engines produce greater thrust when operating at a higher temperature and they can operate on the same amount of fuel as powerplants that operate at lower temperatures, said Howard. Chromalloy s RT-35 Low K coating is a critical driver for the engine to deliver greater fuel efficiency to the operator, he added. Chromalloy s EBPVD centre in Orangeburg. alloys and coatings in the gas path or engine hot section. For every 0.001 inch thermal barrier coating thickness on a high pressure turbine (HPT) vane or blade, the temperature drops about 25 F. For a thermal barrier coating of 0.005 inches, that will equal a 125 F cooler metal below the coating. The thermal barrier coating allows the parent metal to operate cooler for a constant operating temperature. There are two types of coatings for the gas turbine engine diffusion and overlay. In the diffusion process, a portion of the coating diffuses into the parent metal structure. Coatings such as precious metal or diffusion aluminide coatings are sacrificial, providing protection against high temperature oxidation and low temperature corrosion. In the HPT blade section of gas turbines, overlay coatings are applied using electron beam physical vapor disposition (EBPVD) or plasma spraying. Metallic overlay coatings such as MCrAlY coatings are applied by EBPVD or by lowpressure plasma spraying. They provide oxidation and corrosion protection and can be used as a stand-alone coating or a bond coating for the overlay ceramic thermal barrier coatings applied by EBPVD or air plasma spraying. Use of thermal barrier coatings has allowed the operating temperatures of the HPT vanes and blades to increase significantly, minimising deleterious effects on the parent material. As a result the efficiency of the gas turbine has increased. Other advantages include increases in the time required between overhaul and maintenance, resulting in significant cost savings to the turbine operator. The leading edge Chromalloy has been a pioneer in the development of innovative ceramic coatings for turbine hot section components for six decades. The company developed the industry s first EBPVD coatings with ceramic materials in the 1980s. Since then it has continued to develop coatings for aerospace, aero-derivative, marine and industrial gas turbine components. The company produces a variety of vacuum plasma and diffused precious metal or aluminide coatings for all hot section engine components. The company is a supplier to aircraft operators for new and repair components, as well as to the main engine original equipment manufacturers (OEMs). 44
Chromalloy s Low K RT-35 coating on a high-pressure turbine blade. When operating temperatures climb in advanced gas turbine engines especially when they rise above 2400?F the conventional 7YSZ thermal barrier coating shows rapid deterioration due to insufficient thermal protection, its own sintering, which reduces the thermal barrier coating s compliance, and from additional stresses resulting from volume changes due to phase transformation at these higher temperatures. To address this, Chromalloy and other developers produced new thermal barrier coatings to provide lower thermal conductivity to more effectively insulate thermal transfer to the components, as well as to provide a coated component with longer service life based on increased coating durability. Research and development began in the 1970s using rare-other stabilisers and other compositions to achieve lower thermal conductivities. During the last 10 years, turbine OEMs that produce aircraft powerplants began introducing components with even lower thermal conductivity coatings than produced earlier. Low thermal conductivity coatings are used on components for the V2500 and PW4000 commercial aircraft engines as well as some military aircraft engines. Chromalloy s Low K RT-35 coating was certified by the Federal Aviation Administration (FAA) in 2010 for use on the PW4000 second-stage HPT blade. Certification followed a series of tests confirming the low thermal conductivity, high thermal cycle durability, high sintering resistance, high thermal-chemical stability and good phase stability of the coating. Currently the Low K RT-35 coating is in use by a commercial airline in Asia. It is an EBPVDapplied coating that was successfully flight tested and demonstrated to enhance thermal conductivity and provide greater protection for erosion and thermal cycling on coupons and pins. Low K RT-35 provides a layer of protection to the base metal component and underlying bond coating surface of a turbine blade from the extreme heat of the combustion gases during engine during operation. The coating provides about 50 per cent lower thermal conductivity, allowing engines to perform at higher temperatures. In addition, Low K RT-35 increases the oxidation and corrosion resistance of the underlying bond coating as it is cooler, thus extending the life of engine components another cost saving for the operator. During development, since the new Chromalloy coating is a different composition than the Low K coating applied by the engine OEM, FAA Higher thermal efficiencies translate to higher thrust in the aircraft and are achieved through higher operating temperatures. The higher temperatures are achieved due to the use of super-alloys and coatings in the gas path or engine hot section. 45
The component selected to use an OEM Low K coating had to be simple in geometry so samples could be easily extracted for testing. Research and development began in the 1970s using rare-other stabilisers and other compositions to achieve lower thermal conductivities. During the last 10 years, turbine OEMs that produce aircraft powerplants began introducing components with even lower thermal conductivity coatings than produced earlier. Designated Engineering Representative (DER) requirements dictated further scale-up comparisons and determinations. The following technical analysis shows how the coating was demonstrated during development. Component selection The selection of the component to be used as a possible candidate for scale-up commenced. The component selected to use an OEM Low K coating had to be simple in geometry so samples could be easily extracted for testing. The second-stage blade of the PW4000-100 engine was selected. The PW4000 engine used on long-haul flights has two general variants the 94 and 100 engine. The PW4000-94 engine has been in service with relatively few changes since the mid 1990s. The second-stage blade in the PW4000-94 has been used with the industry standard seven weight per cent YSZ coating for over a decade, whereas the PW4000-100 was introduced by the OEM with a Low K gadolinia-zirconia coating. Further analysis of engine run PW4000-100 blades indicated that the Chromalloy Low K coating met key coating criteria for thermal conductivity, erosion and thermal cycling compared to the gadolinia based original manufacturer coating. Once the coating optimisation was complete, a matrix of components was coated. The matrix of components coated across several EBPVD runs ensured that a representative sample of the coating thickness and its equivalent weight gain range critical for establishing the components in production could be established. Enhanced turbine components Following successful competition of comparative testing on components, the coating was approved through the DER process, allowing successful application of Low K coatings on PW4000 second blade engines. Following successful demonstration of coating application the blades were applied on PW4000-100 second blade engines. The blades have been constantly in service by an airline and represent a significant milestone towards full production of the Chromalloy Low K RT-35 coating. The Low K coating is now being marketed to other aircraft operators for application in the industry, as well as to industrial gas turbine operators. As its latest development, the Low K RT-35 the company s newest thermal barrier coating offers even lower thermal conductivity to effectively insulate thermal transfer to the engine components, and provides coated components with longer service lives based on increased coating durability. At Chromalloy Komal Laul is repair development engineer; Lucy Liu is senior material scientist and processing engineer; and Ravi Shankar is director, coating and process technologies. 46