Gas Turbine Tribology

Department of Mechanical Engineering

University of Wisconsin-Milwaukee

1. Introduction

Modern land-based turbine airfoils operate in severe environments with high temperatures and

near critical stresses. Highly turbulent combustor exit flows, with significant thermal gradients

spew hot combustion products and other airborne particulates at the turbine surfaces for in excess

of 20,000 hours before regularly scheduled maintenance [1]. Due to this harsh operating

environment turbine surfaces experience significant degradation with service Technical

advancements in the design and manufacture of gas path turbomachinery components over the

past two decades have only heightened the significance of understanding the effects of flowpath

degradation on gas turbine operation. For example, surface coatings in both the compressor and

turbine, more aggressive airfoil shapes, advanced rotor tip and passage endwall designs, and an

increased number of bleeds to feed more intricate film cooling hole geometries are among the

technologies that have created an increased urgency for fundamental research into the root

causes and effects of degradation. As evidence of this increased emphasis, Figure 1. shows the

number of degradation-related journal articles that have been published each year in three of the

leading publications for gas turbine research.

Figure 1. Journal and conference articles published in the field of Gas Turbine Roughness,

Turbomachinery, Gas Turbines and Power, Propulsion and Power, and Aircraft from 1970 to

2010. [2]

The rising trend evidenced in the figure is likely to continue for the foreseeable future as gas

turbines continue to command an impressive market share in both the power generation and

transportation industries. Wind turbines have had various limitations to their mechanical system

reliability owing to tribological problems over the past few decades [3]. Although current turbine

designs have addressed and solved a number of the problems plaguing their predecessors,

tribological issues still exist. This is evident from the data of Bell (2006), shown in figure 1,

which indicate that the number of failures per turbine per year in Denmark and Germany

dropped from 1 and 2.5,

Figure 2. Data from Bell (2006) indicate that the number of failures per turbine per year in

Denmark (grey circles) and Germany (black triangles) dropped from 1 and 2.5, respectively, in

1994 to 0.5 and 0.8 in 2004 [4]

2. Lubrication

Mineral oils are very popular for lubrication. It has several positive features such as easy

availability and relatively low cost. On the other hand mineral oils also have several serious

defects, such as oxidation and viscosity loss at high temperatures, combustion or explosion in the

presence of strong oxidizing agents and solidification at low temperatures. These effects are

prohibitive in gas turbine engines where a high temperature lubricant is required, but

occasionally very low temperatures must be sustained. [5]

To overcome some of the shortcomings of the mineral oils, synthetic lubricants were developed.

Synthetic lubricants were originally introduced early this century by countries lacking a reliable

supply of mineral oil. These lubricants were expensive and initially did not gain general

acceptance. The use of synthetic oils increased gradually, especially in more specialized

applications for which mineral oils were inadequate.

In other applications such as vacuum pumps and jet engines, low vapour pressure lubricant is

needed; in food processing and the pharmaceutical industry low toxicity lubricant is required,

etc. In recent years the strongest demand has been for high performance lubricants, especially for

applications in the aviation industry with high performance gas turbine engines.

Gearing power transmission is a very important mechanism in any machine having rotating

components. Snidle et al. presents a brief review of developments in understanding of gear tooth

contact lubrication in relation to problems of surface durability and distress [6]. Gear tooth

contacts tend to operate under conditions where the lubricating oil film is thin compared to

surface roughness.

Figure 3. Gear tooth mesh mechanics (left) and performance of gear of two different surface

finish (right)

3. Wear in Gas Turbine

3.1 Erosive Wear

Erosive wear is caused by the impact of particles of solid or liquid against the surface of an

object. Erosive wear occurs in a wide variety of machinery and typical examples are the damage

to gas turbine blades when an aircraft flies through dust clouds, and the wear of pump impellers

in mineral slurry processing systems. Figure 3. shows two severely eroded gas turbine blades.

Figure 4. Severely eroded turbine blades

Impingement angles play a vital role in erosion mechanics. At zero impingement angle there is

negligible wear because the eroding particles do not impact the surface, although even at

relatively small impingement angles of about 20°, severe wear may occur if the particles are hard

and the surface is soft. Wear similar to abrasive wear prevails under these conditions. If the

surface is brittle then severe wear by fragmentation of the surface may occur reaching its

maximum rate at impact angles close to 90°.

Figure 5. Impingement angle of a particle causing erosion of surface.

The relationship between the wear rate and impingement angle for ductile and brittle materials is

shown in Figure 5.

Figure 6. Schematic representation of the effect of impingement angle on wear rates of ductile

and brittle materials.

The rate and mechanism of erosive wear are influenced by temperature. The primary effect of

temperature is to soften the eroded material and increase wear rates. The effects of temperature

on erosion of stainless steel are shown in Figure 5 [7]. It is not until temperatures higher than

600°C are reached that the erosion rate shows significant increase.

Figure 7. Effect of temperature on the erosive wear rate of stainless steel

In common with other forms of wear, mechanical strength does not guarantee wear resistance

and a detailed study of material characteristics is required for wear minimization. The properties

of the eroding particle are also significant and are increasingly being recognized as a relevant

parameter in the control of this type of wear.

New blade coatings and materials are continuously being developed to meet the challenging

requirements of modern gas turbine engines.Because of the serious consequences of erosion on

gas turbine life and performance, it is necessary to gain a better understanding of the blade

surface degradation mechanisms. [8]

Turbine vane/blade surface deterioration is strongly dependent on the turbine geometry, blade

surface material, and particle characteristics. Hamed et al. conducted simultaneous experiment

and computer simulation on turbine blade erosion. Their experimental results for blade and

coating material erosion indicate that both erosion rate and surface roughness increase with the

eroding particle impact velocities and impingement angles and that larger particles produce

higher surface roughness. Table 1 shows the measured surface roughness of the blades that they

used for the experiments.

Table 1. Roughness parameters for pressure surface roughened blades. [6]

Location Ra Rq Rp Rt

Suction side, leading edge 5.75 7.30 13.82 44.10

Pressure side, leading edge 3.93 5.72 11.84 36.37

Pressure side, mid chord 3.28 4.51 9.63 31.92

Pressure side, trailing edge 4.04 5.61 12.60 33.24

3.2 Hot Corrosion in Gas Turbine

During combustion in the gas turbine, sulfur from the fuel reacts with sodium chloride from

ingested air at elevated temperatures to form sodium sulfate. The sodium sulfate then deposites

on the hot-section components, such as nozzle guide vanes and rotor blades, resulting in

accelerated oxidation (or sulfidation) attack. This is commonly referred as “hot corrosion.” [9]

High-temperature alloys that suffered hot corrosion attack were generally found to exhibit both

oxidation and sulfidation.

Figure 8. Scanning electron back scattered image showing thecrosssectionofacorrodedIC-218

nickel aluminide specimen after hot corrosion burner rig testing at 900°C

Abradable coatings are provided to resist thermal corrosion. Figure 7. shows coated and

uncoated turbine blade under high temperature corrosion and a heavily corroded turbine blade.

Figure 9. Coated and uncoated turbine blade under high temperature corrosion (left) heavily

corroded turbine blade (right).

Due to extreme temperatures on the turbine side of gas turbine engines, brazed or welded

metallic structures made from nickel (Ni) based alloys have typically been used to provide gas

path sealing. When thermal shock resistant ceramics became available, oxide ceramic, mostly

zirconia based high temperature abradable seals for the high pressure turbine stages were

developed. Turbine designers need to decide on a case-by-case basis whether metallic or ceramic

abradables should be used and whether they can be cut by bare or hard tipped blades [10].

4. Tribology and Gas Turbine Performance

In last two decades, air traffic volume has increased considerably, whereas the total quantity of

fuel consumed has remained almost unchanged. The jet engine manufacturers strongly

contributed towards this by increasing engine efficiency and power generation. This was

achieved by raising the operating temperatures, by the use of efficient aerodynamic design and

by the use of lightweight materials. Because all of these are mature technologies, one of the last

means to further increase the efficiency is the reduction of the clearance distance between the

blade tip and casing.

4.1 Use of Advanced Materials

Development in advanced materials, more than anything else, have contributed to the spectacular

progress in thrust-to-weight ratio of the aero gas turbine. This has been achieved in the main

through the substitution of titanium and nickel alloys for steel (Figure 8.) [11].A modern turbine

blade alloy is complex in that it contains up to ten significant alloying elements, but its

microstructure is very simple. In the alloy case the ‘block’ are an intermetallic compound with

the approximate composition Ni3(Al,Ta), whereas the ‘cement’ is a nickel solid solution

containing chromium, tungsten and rhenium.

Abradables are not restricted to aeroengines. They can be used in most rotating machinery such

as stationary gas turbines, turbo compressors, radial compressors, turbo charges, and pumps. The

reduction of the blade tip to casing clearance can result in the blades rubbing against the shroud.

By coating the shroud with abradables, however, this interaction can be tolerated [12].

Figure 10. Evolution of materials used in aero gas turbine

Figure 11. High temperature strength and hot corrosion trade-off with conventionally cast

turbine airfoil alloys [13]

Turbine airfoil alloy development typically emphasized high temperature strength and creep

resistance at the expense of reduced ductility and resistance to shock, oxidation and hot

corrosion. The effects of reduced chromium and increased stress rupture life with a hot corrosion

resistance debit are shown in Figure 9. [13].

Yun et al. conducted performance tests in a low-speed, single-stage, axial flow turbine with

roughened blades. Figure 10. shows roughened turbine and the performance plot [14]. They

concluded that blade surface roughness severely degrades turbine efficiency.

Figure 12. Roughened test turbine (left) and efficiency plots.

The study of turbine component roughness and its associated shear drag and convective heat

transfer characteristics is important to increasing turbine component lifespan and even increasing

overall engine efficiency by characterizing how much turbine surface degradation can cost the

engine performance [15].

5. Conclusion

The various roughness mechanisms had very distinct signatures. Air borne contaminants were

typically acts as surface spikes while hot corrosion resulted in surface pitting. The surface

statistical skewness is a natural delineator between these two roughness forms and may be an

important parameter for roughness modeling. Erosion due to the combination of airborne

contaminants and corrosion resulted in much more irregular surfaces. Spallation was the most

significant form of surface roughness measured, with huge variations in surface character.

References

[1] J. P. Bons, R. P. Taylor, S. T. McClain and R. B. Rivir, “The many faces of turbine surface

roughness,” Journal of Turbomachinery, 123, pp. 739-748, 2001

[2] Bons, J. P., 2010, "A Review of Surface Roughness Effects in Gas Turbines," J. Turbomach.,

132, pp. 021004 (2010), DOI:10.1115/1.3066315

[3] M. N. Kotzalas and G. L. Doll, “Tribological advancements for reliable wind turbine

performance,” Phil. Trans. R. Soc., 368, pp. 4829-4850, 2010

[4] Bell, B. 2006 Wind turbine reliability and service improvements. In 2006 Wind Turbine

Reliability Workshop, Albuquerque, NM, 3–4 October 2006. Albuquerque, NM: Sandia

National Laboratories.

[5] Gwidon W. Stachowiak and Andrew W. Batchelor, “Engineering Tribology” Butterworth

Heinemann publications

[6] R. W. Snidle, H. P. Evans and M. P. Alanou, “Gearing lubrication,” Tribological Research

and Design for Engineering Systems, 41, pp. 575-588, 2003

[7] A. A. Hamed, W. Tabakoff, R. B. Rivir, K. Das, P. Arora, “Turbine blade surface

deterioration by erosion,” AGARD (NATO) 83rd Symposium of Propulsion and Energetics

Panel on Turbines, Rotterdam, The Netherlands, 25-28 April 1994

[8] H. Tomizawa and T.E. Fischer, Friction and Wear of Silicon Nitride and Silicon Carbide in

Water: Hydrodynamic Lubrication at Low Sliding Speed Obtained by Tribochemical Wear,

ASLE Transactions, Vol. 30, 1987, pp. 41-46.

[9] “Hot Corrosion in Gas Turbine,” High Temperature Corrosion and Materials, Application

ASM International

[10] D. Sporer, S. Wilson, I. Giovannetti, R. Refke and M. Giannozzi, “On the potential of metal

and ceramic based abradables in turbine seal applications,” Proceedings of the thirty-sixth

turbomachineryh symposium, Turbomachinery Laboratory, Texas A & M Univeristy, 2007

[11] S. Miller, “Advanced materials mean advanced engines,” Materials World, 5, pp.446-49,

1996

[12] F. Ghasripoor, R. Schmid and M. Dorfman, “Abradable coatings increase gas turbine engine

efficiency,” Materials World, 5:6, pp.328-30, June 1997

[13] J. W. Fairbanks and R. J. Hecht, “The durability and performance of coatings in gas turbine

and diesel engines,” Material Science and Engineering, 88, pp. 321-330, 1987

[14] Y. I. Yun, I. Y. Park and S. J. Song, “Performance degradation due to blade surface

roughness in a single stage axial turbine,” Journal of Turbomachinery, 127, pp. 137-143,

2005

[15] J. W. Drab, “Turbine blade surface roughness effects on shear drag and heat transfer,”

Thesis, Department of Aeronautical Engineering, Air Force Institute of Technology, Wright-

Patterson Air Force Base, Ohio, 2001