Overlay Coatings Research Articles

Some effects of structure and composition on the properties of electron beam vapor deposited coatings for gas turbine superalloys

Multi-element overlay coatings applied by electron beam vapor deposition to gas turbine superalloys have been the subject of intensive research and development programs for the past eight years. Such coatings have proven to be superior to diffusion coatings in a number of critical applications primarily because of the ease with which compositional variations can be effected to meet unique protection and mechanical requirements. Overlay coating structural features have been identified (i.e., open columnar structure, vapor source spit marks, flakes and pits), along with present and future methods for their control. Laboratory observations have indicated that in a line-of-sight process, the open columnar structure is the result primarily of shadowing effects and the limited surface diffusion of the depositing atoms. Point defects (spits, flakes, and pits) result from pool behavior and substrate contamination. Methods of minimizing these defects range from in-process controls to post-coating processing. Current development areas for improved processing techniques such as gas randomization and ion plating should result in further improvements and elimination of some present post-coating processing requirements.

The sulphidation of metals and alloys : K. N. Strafford, Metals Mater. (Met. Rev. No. 138), 3 (10) (1969) 153–174; 17 figs., 1 table, 107 refs.

Sulphidation is a serious problem in many energy conversion systems. Sulphidation attack is particularly severe in environments of low oxygen (pO2~10−18 Pa) and high sulphur (pS2 ~ 10−1_10−3 Pa) potentials at temperatures above 700°C. Recognizing that refractory metals have a high resistance to sulphidation in reducing environments, their use as overlay coatings in sulphur-containing environments deserves serious consideration. In the adoption of such an approach, Mo has been recognized as a highly sulphidation-resistant metal with a Kp value of 10−12 g2 cm−4 s−1 in an atmosphere of pS2 ~ 10−1 Pa at 750°C.In this study, Mo was deposited on two superalloys, Inconel 600 and Nimonic PE11, using air plasma spraying. The coated specimens were tested at 750°C for up to 168 h in an environment comprising pS2 ~ 10−1 Pa and pO2 ~ 10−18 Pa. The sulphidation kinetics (this is not real through-layer kinetics, since the specimens had open edges) were determined by a discontinuous gravimetric method. The exposed samples were characterized and analysed using SEM, EDX and XRD techniques. For uncoated Inconel 600, a duplex scale was formed which consisted of an outer Ni3S2 layer and an inner Cr2S3 layer. The scale developed on uncoated Nimonic PE11 comprised three sub-scale layers—an outer (Fe, Ni)9S8 layer, a mid Cr2S3 layer and an inner MoS2 layer. However, after prolonged exposure, the outer layer contained both (Fe, Ni)9S8 and Ni3S2 on the surface of the uncoated Nimonic PE11.The kinetics results demonstrated that Mo coating enhanced the corrosion resistance of both alloys, and particularly so for Nimonic PE11. For both substrates, Mo coating led to the development of two sub-scale layers at the early stages of exposure (e.g. up to 5 h). The outer layer consisted of Ni3S2 for Inconel 600 and (Fe, Ni)9S8 for Nimonic PE11. A MoS2 layer constituted the inner layer for both materials. After prolonged sulphidation, a Cr2S3 layer gradually developed between the outer layer and the inner MoS2 layer. It is apparent that the use of Mo coating hindered the formation of the Cr2S3 layer.

Development of efficient high-temperature marine gas turbines (ship builder and marine enpaine-builder, sept., 1946)

The oxidation resistance of turbine airfoils was enhanced by the use of diffused aluminides beginning in the mid-1950s; these coatings are still extensively used today. In the late 1960s, research began on developing overlay coatings which formed thin protective CrO2, SiO2 or Al2O3 scales on the coating surface. The Al2O3 scale forming coatings systems or MCrAlY coatings can operate at higher temperatures than other systems and can protect turbine airfoils for engines used in electric power generation, naval surface ship and aircraft propulsion, particularly in commercial and transport service.Most engine manufacturers agree that the best near-term candidate for major engine performance improvement would be to develop successfully thermal barrier coatings for turbine airfoil applications. Thermal barrier coatings could allow up to a 149°C (300°F) increase in turbine inlet temperature without a penalty in service life. Alternatively, at current operating conditions they could significantly extend engine hot-section life by reducing the airfoil operating temperature and eliminating hot-streaking problems.Diesel engines are the most fuel-efficient production heat engines yet devised. This level of performance has been achieved with very modest use of advanced materials. Thus, as far as performance improvements are dependent on materials, the diesel engine holds an exciting potential.The largest diesel performance gains in a decade have been achieved with turbocharging advancements. The major gains for the next decade appear to be in the low heat rejection engine concept, generally referred to as the “adiabatic diesel concept”. The major efficiency advances obtainable with the adiabatic diesel concept result from converting the increased exhaust gas energy into work. The most viable energy extraction method is to install another turbine in the exhaust that is mechanically connected to the engine output shaft. This is termed “turbocompounding”. The greatest improvement comes from a bottoming cycle. The problems of low heat rejection diesel engine waste heat utilization are cost, complexity and durability.