THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed In its publications. Discussion Is piinted only if the paper is published in an ASME Journal. Papers are available from ASME for 15 months after the meeting. Printed in U.S.A. Copyright 1994 by ASME 94 GT 449 ADVANCES IN ABRADABLE COATINGS FOR GAS TURBINES R. Schmid Sulzer Innotec Winterthur, Switzerland A. R. Nicoll Sulzer Surface Technology Wohlen, Switzerland ABSTRACT ,1E Gas turbine engine development continues to accelerate, creating more demanding requirements for abradabie seal coatings. These coatings are necessary to provide very small clearances between the rotating and stationary parts in order to minimize gap losses and so Increase efficiency. The relatively few abradable coating materials developed over the last 20 years still perform well in many blade tip seal and labyrinth seal applications. However, rising operating temperatures, corrosion and other environmental changes, longer overhaul times and even better tip clearances are dictating the design of new coating materials which requires a strong scientific approach. For example, ways are being Investigated to replace Nickel-Graphite and other flame sprayed coatings being used between 450 and 700 C respectively because of steady state/corrosion/oxidation/erosion and wear problems respectively. New plasma and HVOF sprayed coatings have been developed using a systematic approach based on material response to operating conditions, minimizing trial and error. The major steps in the programme were: 1. Selection of constituent materials able to withstand service temperatures up to 325 (AISI-Polyester or Polyimide), 450 (A131 base), 700 (MCrAlY base) and 1100 C (ceramic base) respectively. 2. Powder particle manufacture and coating deposition to guarantee highly reproducible coatings. 3. Coating optimization based on wear tests carried out using a fully instrumented abradability test rig and wear mechanism analysis. 4. An investigation of blade tipping systems for high temperature applications. This paper discusses the results of plasma sprayed coatings developed for use at 450 and 700 C 1. INTRODUCTION Increasing the power and efficiency of gas turbines is an on-going process. Improved designs and the availability of lighter, stronger and more temperatureresistant materials make this possible. A consequence of these developments is that turbine gas temperatures and pressures have increased, and will continue to do so. If the desired engine efficiency is to be obtained and maintained during the service life of an engine, adequate sealing between stator and rotor must take place. Thermally sprayed abradable coatings are a simple and weight-saving solution to limiting the gap losses between blade tips or labyrinth fins and the shroud. The abradable seal materials must be able to survive the hot gas environment and resist erosion. Above all, however, they must react in a safe manner when in contact with the rotating blade by easily giving way without damaging the blade tip, while maintaining an acceptable surface roughness. As available abradable coatings did not fulfil these requirements over a range of temperatures, a materials development programme was started with the aim of creating a family of abradable coatings for the gas turbine engines of the present and future. A characteristic of the project is to use a systematic approach aimed at understanding abradability through the analysis of wear mechanisms in-service and those generated by laboratory simulation and not rely on trial and error methods. The present technology status is given in Fig FINDING A SOLUTION TO COMPLEX TRIBOLOGICAL PROBLEMS Abrading is a process during which two surfaces are in contact and interact and thus is clearly a tribological event. The Sulzer Innotec Tribology Department has developed an approach which has proved successful in the solving of complex tribological problems. The following systematic approach was used and has lead to the development and usage of new classes of abradable: Presented at the International Gas Turbine and Aeroangine Congress and Exposition The Hague, Netherlands June 13-16, 1994

2 Investigation of parts taken from service. Determination of the relevant mechanisms. Laboratory trials with different parameters and materials combinations (require a test bed capable of simulating the appropriate parameters) Formulation of a materials model which will provide the desired results under given operating conditions Development of new materials in accordance with a derived working model Prototype trials/engine tests. This was applied and is described below: 2.1 Identification of Wear Mechanisms on Used Components Turbine components that had been worn In-service were analysed in order to determine the predominant wear mechanisms [1] and related to engine operating data. This is necessary if correlations between laboratory and in-service results are to be carried out. The wear mechanisms detected in the compressor stage of turbines (2,3) and confirmed on the test rig are briefly presented here Seal Wear Without Blade Contact These mechanisms are directly related to the environment in which the abradable has to survive such as corrosion, oxidation and erosion (2). 2A.2 Seal Wear due to the Blade-Coating Interaction These mechanisms relate to the reaction between the blade and the coating and are referred to as cutting, smearing, crushing, tribo-oxidation, rupture, melting wear caused by hot-spotting and adhesive transfer. The latter can occur in two ways with material being transferred either from or to the blade. The most desired mechanism is when zero or very slight material transfer occurs to the blade. in the case of AlSi alloys applied by thermal spraying, melting wear occurs at 450 C due to ho spotting Examination of Mechanisms using Parametric Wear Tests In order to understand the wear mechanisms ocurring during abrading, knowledge about the conditions necessary for their occurrence must be gained. To be able to generate the wear mechanisms seen to occur in service [2], a test rig was used that covered the entire range of compressor and turbine operating conditions. The rig has the following capabilities: Blade tip velocity: 150 to 500m/s Incursion rate: 1.5 to 3000 micron/s Specimen temperatures: 20 to 1200 C The results are interpreted with the aim of drawing wear maps as a function of the major operating parameters; temperature, blade tip velocity and incursion rate. Important data for engine designers are those which assist in understanding what physically happens to the blade, and the surface roughness of the abradable coating after interaction. Blade damage is determined by examining the blade mass and/or height variation before and after testing. The abradable surface roughness is measured perpendicular to the rubbing direction. A quantitative evaluation of the blade mass variation and abradable surface roughness is possible by plotting these quantities individually in 3-D graphs, as a function of the blade tip velocity and incursion rate, for a given temperature. These graphs can be projected into the 2-D plane by plotting the incursion rate against the blade tip velocity for a given temperature with the blade mass variation and abradable surface roughness being Indicated by intensity contours. The qualitative information gained by metallographic investigation and expressed in terms of a dominating wear mechanism, for a given parameter set, can be integrated into the 2-D graphs. This is obtained by allocating a letter to the appropriate wear mechanism and plotting this letter at the respective parameter set. Using the intensity contours and the wear mechanism information "wear mechanism regions" are defined. The above procedure results in maps (Fig. 2) which clearly identify the dominant wear mechanisms and their intensity as a function of the major operating conditions; temperature, blade tip velocity and incursion rate; in contrast to the conventional method used by Ashby, where a single wear map is drawn for a tribological system consisting of two similar interacting materials [4]. In the case of abradables, the blade material can be quite different in nature to the abradable material thereby necessitating the construction of two wear maps; one for the blade arid one for the abradable coating. 2.3 Abradability Model and Materials Selection The microstructure of abradable coatings has been found to be one of the most important factors influencing abradability (Fig. 3). The wear maps enable the influence of coating microstructure variations (shape factor, phases and distribution factors) on abradability to be determined quickly and using this information, abradability models may be created (2,3). Integrated into these models are the physical and chemical properties of the matrix and filler phase as a function of temperature. Increasing the temperature of the matrix means that the ductile-brittle transition temperature has to be taken into account in order to restrict matrix plasticity. The function of the filler is two fold (a)it stops adhesive transfer to the blade tip and (b) it acts as arelease agent for material from its own 2

3 surface whilst abrading. The microstructure of the coating is included using phase form and distribution factors. The chemical properties are important as they largely determine if an abradable will survive in its environment. The first step entails the selection of candidate materials. Further material screening takes place when the feasibility of converting these materials into spray powders takes place. The most promising powders are prepared and sprayed. Modifications to the spraying and powder manufacturing process are made until a quality coating Is obtained. The effect of further processing such as machining Is also Investigated. The coating is rig tested and wear maps are drawn to determine the coatings' abradability Ever/mental Procedure Materials Using the above approach, materials and coatings based on an AlSi matrix with polyester and polyimide fillers using plasma and HVOF spraying have been developed and introduced to the marketplace respectively (5). For temperatures up to 450 C, materials based on AISI with Boron Nitride and polyester were developed (6). This type of coating replaces conventional coatings based on NI-graphite which were flame sprayed and AISI-C(g) that suffers from severe corrosion problems. Plasma spraying was carried using a Plasma-Technik F4 plasma gun running on an argon/hydrogen plasma. Samples were degreased and grit-blasted prior to spraying. The use of plasma spraying In replacing flame spraying increases the cohesive strength of the coating thus increasing the useful lifetime, reducing the possibility of coating spallation and increasing erosion resistance. Spray parameters were adjusted according to experience gained previously in order to obtain the most ideal type of microstructure (model). The second material tested was based on CoNiCrAIY with Boron nitride with polyimide for use at 700 C. This coating was also applied using plasma spraying as above. Test Rig The test rig (Flg.4) consists of two major components. A rotor onto which two dummy blades are mounted and a specimen stage which Is heated using a high temperature gas Jet to the required temperature level. The abradable specimen is attached to the stage which is driven by a step motor, thereby enabling the fine tuning of incursion rates. Force sensors on the stage record the cutting forces occurring during blade tip specimen interaction. The rig is so designed that a rapid exchange of specimen and blades Is possible. This, associated with computer control of the rig parameters, leads to short testing times and hence the ability to conduct many tests within a short period of time; a prerequisite for conducting parameter studies. The testing procedure consists of the following steps. The rotor is accelerated to the required blade tip velocity. During this time, the specimen is heated to the required testing temperature which Is measured using a thermocouple attached to the' reverse side of the sample. Increases In temperature are observed during testing these being typically up to 70 at the test temperature of 450 and between 20 and 40 at 700 respectively depending on the conditions, blade thickness and width and wear mechanism. The temperature of the tip is not controlled and generally reaches about 100 C due to air friction. Depending on the incursion rate, tests can last between 0.5 seconds and 10 minutes and the tip can melt In some cases based on the abradable and tip materials, and incursion rate. Engine tests under real conditions indicate the same wear mechanisms as found In rig testing. When temperature and velocity are constant, the test is started by moving the abradable towards the rotor at a set incursion rate. Abrading takes place until the required incursion depth Is attained, at which stage the specimen is rapidly retracted. The specimen is continuously heated, even during the rub. Plasma sprayed coatings were tested at 450 C under the conditions shown below against a Ti6A14V dummy blade of 0.7mm thickness as previous experience testing blade thicknesses up to 3mm had shown that blade damage Is always occurs above 0.8mm. Velocity (m/s) Incursion rate (microns/s) 350 5; , 500 Coatings tested at 700 C were run with a velocity of 450 m/s and an incursion speed of 10 microns/s. Different blade materials were tested (IN718; Ti6AI4V; X42Cr13 Steel). A Ti6AI4V blade was also plasma sprayed with aluminium oxide and run in the test rig at 700 C. The exact position of the specimen, specimen temperature, blade tip velocity, Incursion rate, cutting force and sparking (video camera) are continuously monitored and recorded during testing. 3. RESULTS AND DISCUSSION The results of testing at 450 and 700 C are shown In Figs. 5 and 6 respectively. At 450 C, all conditions were positive except for testing at an incursion rate of 500 microns/s and a velocity of 450 m/s. In contrast to testing at the same Incursion rate at 350 m/s, the abraded surface in this case shows microrupturing. Testing is continuing to determine the effect to incursion depth and the possibility of local melting occuring in the coating during the abradable rub. 3

4 At 700 C, the IN718 blade was successful in abrading the coating whereas the Ti6A14V and steel blades were worn by the coating. Fig.6 shows wear scars of different lengths based on the degree of abradability. In the case of the Ti and steel blades, both act by abrading the coating which with increasing depth stops with wear then occuring on the blade. One observes the low width of the scar and the corresponding darker areas caused by material transfer (blade to coating). The use of a plasma sprayed A1 20,4 coating on the blade tip improved the abradability considerable. Further tests will be carried out to determine the effect of blade thickness and the effectiveness of blade tip coating. 4. CONCLUSIONS Wear mechanism mapping and the analysis of correlations between sample microstructure and wear behaviour for different material systems lead to the formulation of a general abradability model which has been used to design and develop a family of engineered abradables to cover the basic clearance control requirements for gas turbine engines of today and tomorrow. The interaction of powder processing methods, spray process control and advanced abradability testing together with the understanding of engine demands and the mechanisms of abradability has resulted in a new family of coating solutions. The two families of results reported here (for 450 and 700 C) show promise for the future in providing more consistently applied and abradable materials using plasma spraying instead of the flame spray process used in conjunction with Ni-Graphite coatings. The results presented show that the selected materials based on the matrix and filler concept according to the proposed model are working satisfactorily. This work is being extended to cover higher temperatures. 5. REFERENCES [1] D.L. Clingmann, B. Schechter, KR. Cross and J.R. Cavanagh, Lubr. Eng., 39 (1983) 712 [2] M.O. Bore!, A.R. Nicoll, H.W. Schlaepfer and R.K Schmid, Surface and Coating Tech., 39/40 (1989) [3] M.O. Borel, Investigation of Wear Mechanisms Occurring in Abradable Seals of Turbomachines In-Service, SULZER Report No. 514, Winterthur 1988, 1-22 [41 S.C. Lim and M.F. Ashby, Wear-Mechanism Maps, Overview No. 55, Acta metal, Vol. 35, No. 1, 1987, 1-24 [5] R.A. Miller, S. Rangaswamy, M.O. Bore!, AR. Nicoll, Thermal Spray Research and Applications, Proc. 3rd NTSC, Long Beach, CA, USA; May Publ. ASM INTL USA. p [6] To be published; NTSC in Boston; June

5 ABRADABLE FAMILIES LU cc 1000 C TIPPED BLADES Laser SIC Plasma Zr02 A1203 Porous Ceramics re LU LU CoNiCrAIY-BN-PE t APS&CDS XPT C traist I- SOLID BRICANT m AlSi-BN-PE XPT339 (500 C) AlSi-C (450 C) Ni-C (480 C) APS&Flame APS&CDS AMDRY2000 (350 C) AMDRY2010 (325 C) APS&CDS TECHNOLOGY LEVEL FOR ABRADABILITY Fig. 1: Technology status of abradable coatings for gas turbines. 5

6 Coating 20 C I Blade Tip cc Weighted Roughness (P m) a Ire Blade Mass Varia tion (mg) Blade Tip Velocity (WS) Blade Tip Velocity (m/s) Coating 300 C Blade Tip ghted Roughness W M/ Blade Mass Variation (mg Blade Tip Velocity (mis) Blade Tip Velocity (rdis) Left: wear mechanism map for the coating with R c contours. S = smearing C = cutting R = particle rupture M = melting Right: wear mechanism map for the blade tip with dg d contours. A = aluminium transfer (pick-up) C = cutting M = melting Fig. 2: Wear maps for AlSi - Polyester plasma sprayed coatings for 20 and 300 C respectively. 6

7 Fig. 3: Typical microstructure of an aluminium silicon polyester coating.

8 igh Velocity Flame Generator Rotor Flame Guide High Velocity Gas Stream Vinc Thermocouple Abradable Specimen Cooling Plate Dummy Blade Stepper Motor Cutting Force Transducers Fig. 4:Schematic of the test rig for abradability testing. 8

9 V = 350 m/s IR = 5 microns/s Pla:A;c4;getr. minim 112 V = 350 m/s IR = 500 microns/s ninny!!lilt II lingo iv.. 31/4 V = 400 m/s IR = 50 microns/s OM II j2131 V = 450 m/s IR = 5 microns/s ; ij2113. V = 450 m/s IR = 500 microns/s It Fig. 5: Results of testing at 450 C using a Ti6A14V blade of 0.7mm thickness. V = Velocity; IR = Incursion Rate 9

10 1N718 Blade; 0.7 mm thick; Result Positive T16AI4V Blade; 0.7 mm thick; Result Negative. IIiIIIiIIflhIIIII 111J I II X42Cr13 Steel Blade 0.7 mm thick Result negative Ili ' 1N718 Blade; 0.7 mm thick; Air plasma sprayed with A1203; Result Positive Fig. 6: Results of testing at 700 C; blade velocity 450 m/s and 10 microns/s incursion rate. 10